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Almost-Linear-Time Maximum-Cardinality Matching in General Graphs
expertly designed by an internal OpenAI model  ·  released 2026-09-24  ·  original PDF
Theorems: 2 Lemmas: 49 Proofs: 53
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We prove that maximum-cardinality matching in a simple undirected graph with n vertices and m edges can be found by one uniform randomized algorithm in $(n+m)^{1+o(1)}$ time. The bound holds on every computation path in a logarithmic-word model, and the algorithm returns an explicit maximum matching with probability at least 2/3. An explicit reduction gives the same time and probability guarantees for deciding whether a simple host graph has a spanning subgraph with prescribed valid vertex degrees, and for finding one when it exists.

>>> Level Map <<<
  1. Introduction
  2. Statement and computational model
  3. Context and significance
  4. Main ideas and organization
  5. Conventions and static primitives
  6. Representations and fixed-parameter bounds
  7. Exact integral flow
  8. A clocked fallback
  9. Matching states and structural operations
  10. The phase state and its ledgers
  11. The top objective and the pricing primitive
  12. The main actions and exact ledger edits
  13. Maintaining the top barrier
  14. Reset schedule and the exact exchange problem
  15. Tight sets and factor-critical contraction
  16. Bounded searches for a bin pair
  17. Explicit matching materialization
  18. Local projection and the perfect-matching bound
  19. A local exact projection
  20. Debt, structural work, and progress
  21. Implementation and total phase cost
  22. Guarded prefixes and the PM guarantee
  23. Priced logarithmic steps
  24. The interface
  25. A backend with explicit forest paths
  26. Degree reduction and integer pricing
  27. Oriented forest operations
  28. Preserving exact values under forest changes
  29. Drift detection and refresh accounting
  30. Precision, total cost, and completion of the engine proof
  31. A resource-preserving static routing sparsifier
  32. Stages, pieces, and normalized sampling
  33. Median cuts, integral matchings, and an expanding overlay
  34. Recursive routing on the overlay
  35. Filtering real embeddings and verifying a stage
  36. Failure probability and finite computation
  37. Frozen decompositions and cycle quality
  38. Parameters, scores, and the bottom level
  39. Verified covers and a finite family of forests
  40. Fixed marginals and the clipped decomposition
  41. Portal legs and local correction cycles
  42. One joint child graph
  43. Maintaining and exposing the cycle backend
  44. Class changes and immutable assignment snapshots
  45. Small changes to the compressed child
  46. A public forest with prebuilt arms
  47. Explicit recourse and persistent edge lifetimes
  48. Total work, finite arithmetic, and reliability
  49. Explicit output and one uniform program
  50. Witness-preserving reductions
  51. Threshold search and its error budget
  52. Fixed branches and eventual completion
  53. An instruction-level schedule
  54. A common exponent envelope
  55. Prescribed-degree factors

Introduction

A matching in a graph is a set of edges with pairwise disjoint endpoints. The maximum-cardinality matching problem asks for such a set with the largest possible number of edges. We prove an almost-linear bound in the adjacency-list input size for general graphs, with explicit output and a single uniform algorithm. The running-time guarantee includes every unsuccessful random computation, all finite-precision work, and all input and output costs.

Statement and computational model

The input is a simple undirected unweighted graph \(G=([n],E)\), where \(n\ge1\) and \(m=|E|\). It is given by an array of adjacency-list boundaries and two neighbor occurrences per edge. Labels and indices use \(O(\log(n+2))\) bits. The lists may have any valid order. Isolated vertices count toward \(n\), and the graph need not be connected. Write \(p=n+m\), which is within a constant factor of the number of input word records, and write \(\nu(G)\) for the maximum matching size.

We use a classical randomized word-RAM whose word length is \[w=\lceil c_0\log_2(n+2)\rceil,\] for a fixed constant \(c_0\ge2\) chosen with the algorithm. Unit-cost instructions are word reads and writes, comparison, addition, subtraction, multiplication modulo \(2^w\), integer quotient and remainder with nonzero divisor, Boolean operations, and shifts by fewer than \(w\) positions. Each independent fair random bit costs one instruction. Longer arithmetic uses charged multiword implementations. Allocation, initialization, data movement, preprocessing, and output writing are included. There is no oracle or nonuniform advice. Since \(n\le p\le n^2\) for a simple graph, the word length is also \(\Theta(\log(p+2))\).

Theorem 1 (Exact matching in almost-linear input-size time). There are a single uniform randomized word-RAM algorithm, fixed constants \(c_0\ge2\) and \(C\ge1\), and a nonincreasing function \(\eta:\mathbb N_{\ge1}\to\mathbb R_{\ge0}\) with \(\eta(q)\to0\) as \(q\to\infty\) such that the following holds. For every input graph \(G=([n],E)\) with \(m=|E|\) and \(p=n+m\), the algorithm halts on every computation path in at most \[C p^{1+\eta(p)}\] instructions. With probability at least \(2/3\) it outputs an explicit list of edges of a matching of size \(\nu(G)\).

For every fixed integer \(\kappa\ge1\), this gives \(C_\kappa n^{1+\eta(n)}\) instructions on graphs with \(m\le\kappa n\), for some constant \(C_\kappa\). The same algorithm and exponent envelope work for all \(\kappa\), which need not be supplied as an input. At the dense extreme \(m=\Theta(n^2)\), the bound is \(n^{2+o(1)}\).

A perfect matching is not promised. In particular, the perfect-matching procedure for graphs of maximum degree three will be used only after reductions that preserve explicit witnesses. A separate algorithm for each fixed exponent slack would also be insufficient; Section 9 constructs the single finite program and the common envelope in Theorem 1.

A reduction also gives an almost-linear bound for prescribed vertex degrees. Given an integer \(f(v)\) with \(0\le f(v)\le\deg_G(v)\) for every vertex \(v\), an \(f\)-factor is a spanning subgraph \(([n],F)\) with \(\deg_F(v)=f(v)\) for every \(v\). Corollary 53 gives one uniform randomized algorithm that decides whether an \(f\)-factor exists and returns its edge list when it does, with success probability at least \(2/3\) and \((n+m)^{1+o(1)}\) instructions on every computation path. The degree prescription is an array of one-word integers. Section 10 gives the witness-preserving reduction with only logarithmic expansion in the number of edge incidences.

Context and significance

The structure of matchings has long connected combinatorial existence criteria with efficient construction. Hall’s theorem characterizes when a family of sets has distinct representatives (Hall 1935), and Tutte’s perfect-matching theorem identifies the obstruction caused by odd components (Tutte 1947). Berge’s augmenting-path criterion turns maximum cardinality into a question about alternating paths (Berge 1957). Edmonds made this approach algorithmic in general graphs through blossom contraction and expansion (Edmonds 1965b). His matching-polytope theorem gives a complementary explanation of the odd-set constraints (Edmonds 1965a): vertex-degree inequalities alone allow, for example, value \(1/2\) on each edge of a triangle.

Tutte’s short proof of the factor theorem reduces prescribed-degree \(f\)-factors to perfect matchings (Tutte 1954). The reduction in Section 10 follows this incidence-port approach, replacing the local complete connections to auxiliary vertices by the rearrangeable networks used in Section 9. This controls the size of the perfect-matching instance even when a host vertex has high degree. There are also direct algorithms: Gabow finds a maximum-cardinality edge set subject to \(\deg_F(v)\le f(v)\) in \(O(n^{2/3}m)\) time on simple graphs (Gabow 2025, Theorem 4.1). After linear-time input processing, including removal of zero-demand vertices, this gives an \(O(n+n^{2/3}m)\) factor test: an \(f\)-factor exists exactly when the returned set has \(2|F|=\sum_v f(v)\).

Batching shortest augmenting paths led to the bipartite algorithms of Hopcroft and Karp (Hopcroft and Karp 1973) and, independently, Karzanov (Karzanov 1973), and to the Micali–Vazirani algorithm for general graphs (Micali and Vazirani 1980; Vazirani 2024). The latter has running time \(O(m\sqrt n)\) on a RAM; reading the explicit input and removing isolates gives the convenient universal bound \(O(n+m\sqrt n)\). This is \(O_\kappa(n^{3/2})\) when \(m\le\kappa n\). Gabow and Tarjan developed a different route through weighted scaling (Gabow and Tarjan 1991), and Gabow later gave a direct weighted-search formulation at the same cardinality bound (Gabow 2017). On very dense graphs, Goldberg and Karzanov improve the combinatorial bound to \(O(n+m\sqrt n\log(n^2/m)/\log n)\) by combining skew-symmetric flow with graph compression (Goldberg and Karzanov 2004, Theorem 10.3). The weighted problem has its own sequence of improvements: the scaling algorithm of Duan, Pettie, and Su finds an exact maximum-weight matching in \(O(m\sqrt n\log(nW))\) time for integer weights of magnitude at most \(W\) (Duan et al. 2018), with logarithms bounded below by a constant.

Algebraic methods offer another important approach. Random evaluation of the Tutte matrix connects matching to linear algebra (Lovász 1979). Mucha and Sankowski, and subsequently Harvey, obtained randomized algorithms that construct maximum matchings at the matrix-multiplication time scale, conventionally written \(O(n^\omega)\) field operations (Mucha and Sankowski 2004; Harvey 2009). These algorithms already recover explicit witnesses and can work over finite fields of polynomial size. Their matrix-multiplication bound is most competitive on dense graphs. Theorem 1 instead gives an almost-linear bound in the explicit input size throughout the range of densities, including \(n^{2+o(1)}\) at the dense extreme.

For bipartite matching, the unit-capacity flow formulation permits continuous optimization to accelerate exact search. Mądry’s electrical-flow interior-point method (Mądry 2013) and the moderately dense algorithm of van den Brand et al. (Brand et al. 2020) were intermediate advances. The almost-linear exact integral-flow algorithms of Chen et al. (Chen et al. 2022) and van den Brand et al. (Brand, Chen, et al. 2023) then yield almost-linear exact bipartite matching. General graphs require additional control of odd blocks. Here that control is provided by the matching state and its local projections. The deterministic flow theorem is used through the explicit static interface in Lemma 2.

The proof combines several established ideas with interfaces tailored to that state. Its constructive factor-critical blocks belong to the contraction-and-expansion tradition, while Hall’s theorem underlies the reassignment of their exposed vertices. The multiscale allocation and debt bounds are proved directly. The routing helper uses the cut-matching structure of Khandekar, Rao, and Vazirani (Khandekar et al. 2006), together with concentration and conductance estimates (Tropp 2012; Sinclair and Jerrum 1989). Its recursive virtual graphs follow the hierarchical random-walk embeddings of Ghaffari, Kuhn, and Su (Ghaffari et al. 2017, sec. 3.1.2). Here the sampled support and the substituted routes must also preserve supplied resource loads, with their full construction costs charged.

The use of approximate minimum-ratio circulations as optimization directions has its modern antecedent in the almost-linear flow framework (Chen et al. 2022; Brand, Chen, et al. 2023). Chen et al. prove a deterministic fully dynamic approximate minimum-ratio-cycle theorem with flat forest representations (Chen et al. 2023, Definition 2.3 and Theorem 3.5). Our routing hierarchy is related to the origin-aware construction of Rozhoň et al. (Rozhoň et al. 2022); the finite forest family and associated cycle extraction adapt the construction in (Chen et al. 2023, sec. 4.5). The preallocated forest copies used to lift recursive paths follow Kyng, Meierhans, and Probst Gutenberg (Kyng et al. 2023, sec. 3.3, Lemma 3.21 and Algorithm 5). Sections 5 and 8 specify how retained mapped edges preserve accumulated coordinate changes and absolute motion through these representation updates. The recent distance-oracle approach of Kyng et al. (Kyng et al. 2026) provides another development of this method. Sections 7 and 8 supply the complete backend used here, including its adaptive guarantees, explicit changes of representation, stable edge-copy lifetimes, and bounds on traversal length before cancellation. Those guarantees are the inputs to our priced-step analysis.

Main ideas and organization

The central subproblem is to find a perfect matching in a graph of maximum degree three. Section 9 reduces exact cardinality queries to this subproblem while retaining the original edge identifiers, so that every successful answer can be decoded into a matching of the input graph.

The perfect-matching procedure maintains a current matching through a partition of the vertices. Its odd blocks, called bins, come with internal matchings after any prescribed vertex is left exposed. Some outside singleton vertices, called rows, are assigned along original edges to distinct bins. Each assigned bin matches its exposed vertex to its row; an unassigned bin contributes one unmatched vertex. The remaining vertices are stored as matched pairs, apart from unmatched vertices set aside during repairs. This representation permits a change of exposure without immediately expanding a nested block.

The difficulty is to repair this representation after structural changes without repeatedly scanning all its odd blocks. A ledger allocates a fixed amount from each row among its neighboring bins. The difference between a bin’s capacity and its allocated load is its residual. Several ledgers impose residual margins proportional to bin size; a nonnegative debt allows a margin to fail temporarily. At each settled state, the lowest ledger has no debt and certifies a strict Hall surplus for every nonempty set of bins. The highest ledger carries a logarithmic objective that must admit progress when a perfect matching exists and no structural operation is available.

The intermediate ledgers turn that progress into affordable repairs. A repair is an exact flow problem on row–bin incidences, initially explored only at bins whose debt is substantial relative to their size. An unexplored boundary bin temporarily accepts an optimistic amount of flow. If enough arrives, the algorithm explores that bin and permanently locks in an absorbed amount proportional to its size. The total available flow is bounded by the starting debt, so these locks bound the number of physical vertices examined. If the requested residual margin is infeasible, the repair instead removes a set of bins. The gap between adjacent margins makes this removal decrease debt in every preserved coarser ledger by an amount proportional to the removed size. Thus debt pays both for local exploration and for removing blocks from the state. The logarithmic optimization uses exact conservation throughout; integral cut inequalities turn a point whose total bound violation is below one into a certificate of exact feasibility. A final local cut also recovers the entire residual source-reachable set needed by the matching repair.

Both the matching objective and these local flow tests use the same priced-step interface. It searches for a circulation direction with sufficient objective increase relative to its weighted length. When it accepts such a direction, it takes a small exact dyadic step. The direction is represented by individual edges and paths in a changing forest, so a long path need not be expanded for every step. The implementation separately records signed coordinate changes and absolute motion. The latter counts repeated traversals before cancellation and controls when a coordinate must be read and repriced.

The cycle data structure supplying this interface is constructed in full. A static routing sparsifier first provides short routes whose total use of specified resources is bounded. During an interval of updates, a frozen collection of forests then decomposes every current circulation into local correction cycles and a circulation in one smaller graph. A hierarchy of routing snapshots limits the number of new representative edges added to that graph. Finally, prebuilt copies of the paths represented by its edges lift its forest back to the current graph. When recursive occurrences change at otherwise unaffected child labels, these copies retain their identities. This preserves the exact accumulated values used by the priced-step implementation; replacing an affected copy first transfers its saved history.

Let \(S\) be a power-of-two upper bound on the number of vertices in the reduced graph. For each sufficiently large fixed parameter \(a\), these ingredients yield time \(S^{1+O(1/a)+o_a(1)}\) for a subcubic perfect-matching test. The coefficient hidden in \(O(1/a)\) is absolute. Logarithmic powers may depend on the fixed parameter. Deterministic prerequisite failures suspend a parameter branch; exhausted random retries terminate its test with a possibly erroneous negative answer. This distinction permits an every-path asymptotic bound. Finally, instruction-level dovetailing with summable error budgets turns all fixed-parameter guarantees into Theorem 1, without a convergence rate or an input-dependent table of thresholds.

Sections 3 and 4 develop the matching algorithm assuming the priced-step interface. Section 5 proves that interface from the flat-cycle backend. Section 6 gives the static helper, and Sections 7 and 8 prove the backend’s quality and implementation guarantees. Section 9 completes the reductions and the uniform finite-word implementation. Section 10 applies the resulting matching algorithm to prescribed-degree factors through a linearithmic-size reduction.

The principal interfaces and their proof dependencies are summarized below.

Result Role in the proof
Theorem 20 Fixed-parameter subcubic perfect-matching search; Sections 3–4 use Lemma 21.
Lemma 21 Priced logarithmic steps; Section 5 proves the interface from Lemma 22.
Lemma 22 Flat-cycle backend; Sections 7–8 construct it using the static routing helper, Lemma 25.
Theorem 1 Explicit maximum matching with one input-size exponent envelope; Section 9 applies the witness reductions and instruction-level schedule to Theorem 20.

Conventions and static primitives

Representations and fixed-parameter bounds

Auxiliary graphs may have parallel edges or arcs, always distinguished by identifiers. Fix a reference orientation on every variable edge. Traversing an edge backward negates its coefficient. A signed circulation has zero divergence at every vertex; its length norm for positive lengths \(\ell\) is \[\|\ell f\|_1=\sum_e\ell_e|f_e|.\] This norm differs from the raw length of a supplied traversal list: the latter counts every traversal with its multiplicity, before cancellation. Later a forest edge may map either to an actual graph edge, with its endpoint correspondence specified, or to a null edge whose two endpoints have the same image. A null traversal has zero actual projected length, although a reduction may assign it an additional positive bookkeeping length.

All representation sizes include vertices or slots, individual edges, terminal incidences, and explicitly stored lists. Ordinary adjacency lists, heaps, balanced dictionaries, and path-copy persistent maps incur logarithmic overhead per record operation. No argument permits a free scan of all coordinates after an implicit path update. Arithmetic and key-length costs are included separately when they exceed one word.

The matching procedure uses a master bound \(S=2^s\), an integer parameter \(a\ge1000\), and a reliability parameter \(\sigma\). In the final schedule \(\sigma\le a+1\). A bound of the form \[S^{O(1/a)+o_a(1)}\] has an absolute coefficient in \(O(1/a)\); for each fixed \(a,\sigma\), the second exponent tends to zero as \(S\to\infty\). Constants and powers of logarithms may depend on these fixed parameters. All smaller local routines measure their permitted logarithmic factors against the indicated master bound. The parameters are not fixed uniformly while taking \(a\to\infty\).

An implementation may check a deterministic size or precision prerequisite. If that check is inadequate, it suspends the entire parameter branch without an answer. In contrast, exhaustion of a capped randomized routine is a detected failure; the calling perfect-matching test returns a negative answer promptly. The fixed-parameter analysis proves that deterministic suspensions cannot occur on any path once the input is sufficiently large for that fixed parameter. It does not require random attempts always to succeed. These conventions are used in Theorem 20 and the schedule in Section 9.

Exact integral flow

Lemma 2 (Static integral flow). For an explicitly represented directed network of total size at most \(P\), with integral capacities, demands and costs bounded in magnitude by a fixed power of \(P\), exact integral minimum-cost flow can be computed uniformly and deterministically in \(P^{1+o(1)}\) time. Its finite-precision operations can be simulated with polynomially logarithmic overhead on logarithmic-bit words. In particular, integral source–sink maximum flow with nonnegative upper capacities and zero lower capacities returns an explicit integral arc vector and a residual-reachability minimum-cut certificate within that bound.

Proof. Use the deterministic minimum-cost-flow theorem of van den Brand et al. (Brand, Chen, et al. 2023, Theorem 1.1), with its finite fixed-point model and deterministic integral rounding (Brand, Chen, et al. 2023, sec. 2 and Lemma 4.1). The stated logarithmic dependence on integral magnitudes and polynomial logarithmic word lengths is absorbed in \(P^{o(1)}\). A fixed-point word is stored as a charged array of logarithmic-bit words. Longer virtual addresses can be keys in a dictionary of touched cells; no unused address space is initialized.

Maximum flow is obtained by adding a bounded sink-to-source return arc and minimizing its negative flow. Its capacity can be the sum of the original capacities. Feasibility and absence of a residual source–sink path certify maximality. Residual reachability supplies the cut. ◻

Only the nonnegative-upper-capacity maximum-flow specialization is called algorithmically below. Integral lower bounds are used in feasibility arguments through the classical cut criteria for circulation feasibility (Gale 1957; Ford and Fulkerson 1958). The following consequence records the precision margin we use.

Lemma 3 (Integral cuts from a small total violation). Let every lower and upper arc bound be integral, with lower bound no larger than upper bound. If an exactly conserved real arc vector has total bound violation less than one, the bounds admit an exactly feasible integral circulation.

Proof. For a vertex set \(X\), feasibility requires the sum of lower bounds on arcs leaving \(X\) to be at most the sum of upper bounds on arcs entering \(X\). Conservation of the real vector implies that any failure of this inequality is at most its total bound violation. The two bound sums are integers, so a positive discrepancy would be at least one, a contradiction. The circulation cut criterion therefore holds. To obtain that criterion, subtract the lower bounds, introduce a super-source and super-sink for the resulting vertex imbalances, and apply the integral maximum-flow/minimum-cut theorem. Saturating the imbalance arcs gives the asserted integral circulation. ◻

Large capacities relative to a small local graph do not require a different finite program for each magnitude exponent.

Lemma 4 (One polynomial-capacity implementation). For maximum flow with nonnegative integral upper capacities of at most \(b\) bits, a single fixed polynomial-capacity version of Lemma 2 suffices, using \(b\) calls on linear-size residual graphs, together with the charged capacity arithmetic and identifier translations.

Proof. Begin with zero capacities. In each binary digit round write the new capacity vector as \(c'=2c+\xi\), where every \(\xi_e\) is zero or one. If \(f\) was a maximum flow of value \(v\) for \(c\), then \(2f\) is feasible for \(c'\). Let \(A\) be the number of original arcs. An old minimum cut has new capacity at most \(2v+A\), so the largest residual augmentation has integral value \(\Delta\le A\).

Decompose an optimal integral residual augmentation into source–sink paths and cycles and discard the cycles. Its remaining total path value is \(\Delta\), so every forward and reverse residual arc carries at most \(\Delta\le A\). Truncating every residual capacity at \(A\) therefore preserves the optimum augmentation. Compute it with the fixed polynomial-capacity routine. Adding forward usage and subtracting reverse usage updates \(2f\) to a maximum flow for \(c'\). This proves the induction over digits.

Local vertices and arcs can be renumbered, using dictionaries to translate their caller identifiers on input and output. Integer arrays, comparisons, and translations are charged at their actual bit or multiword cost. In the fixed-parameter applications below, \(b\) and all key lengths contribute only the permitted logarithmic powers in the master size. ◻

When paths from an integral flow are needed, pair incoming and outgoing flow units and trace source paths, dropping cycles. We charge the total represented throughput, not merely the number of graph arcs. The routing helper supplies the explicit throughput bounds at each such use.

A clocked fallback

The final schedule needs one fixed polynomial fallback, solely to guarantee halting and polynomial physical addressability at every finite input size. Edmonds’ deterministic algorithm supplies the following interface.

Lemma 5 (Polynomial perfect-matching fallback). There is one uniform deterministic algorithm on explicit loopless graphs of maximum degree three that halts within a fixed polynomial number of finite-word instructions. It returns a checked perfect matching when one exists and a negative answer otherwise.

Proof. Run Edmonds’ deterministic polynomial-time maximum-cardinality matching algorithm (Edmonds 1965b, sec. 2, pp. 451–452). For parallel edge copies, first deduplicate unordered endpoint pairs while retaining one original edge identifier for each pair. This preserves perfect matchability and explicit witness decoding. Return the computed matching if it covers every vertex, and a negative answer otherwise; check every returned identifier and vertex incidence before acceptance.

The classical algorithm, the deduplication, and this check have a fixed polynomial clock without a matchability promise. Their combinatorial records and polynomially bounded integer indices have polynomial bit cost, so charged multiword and touched-address implementations retain a fixed polynomial bound in the stated model. Exact maximum cardinality makes the returned answer correct in both cases. ◻

The two witness-preserving reductions used with this fallback are proved explicitly in Section 9.

Matching states and structural operations

We describe a perfect-matching procedure on a loopless graph of maximum degree three. This section develops its combinatorial state, exact ledger transformations, and structural searches. The local implementation of projection and the phase analysis follow in Section 4. The dynamic optimization interface used here is Lemma 21, proved later.

The graph has \(N\le S\) vertices, where \(S=2^s\) is a master size bound. Disconnected graphs are allowed; parallel edge copies retain separate identifiers, and degree counts incidences. A successful answer is an explicit perfect matching, checked before acceptance. A negative answer may arise from a capped randomized search, whose error is bounded later. A deterministic prerequisite failure instead suspends this parameter branch without an answer. Perfect matchability will prove progress, never a running-time bound.

The phase state and its ledgers

The empty graph is settled directly, and odd \(N\) gives a negative answer. Otherwise start with the empty matching and work in phases. A phase starts with an actual matching \(M\), whose positive deficit is \(K=N/2-|M|\). At deficit zero the procedure is finished. Within a phase we represent the changing matching by odd blocks and single vertices assigned to those blocks.

A bin is a physical odd vertex set with a constructive factor-critical witness: after prescribing any one vertex as its exposure, the others can be matched internally. Its weight is \(w_d=(|d|+1)/2\). The bins form a disjoint family \(\mathcal D\). The rows \(\mathcal A\) are physical singleton vertices outside them. An explicit injection \[\mu:\mathcal A\longrightarrow\mathcal D\] assigns each row along a specified physical edge to its bin. The bipartite incidence graph \(\mathcal B\) contains every physical row–bin edge, with different incidences retained separately. Each row has degree at most three.

An assigned bin exposes the physical endpoint of its assigned edge and is matched there to its row. A free, or unassigned, bin exposes one unmatched vertex. The remaining unblocked vertices form a set \(U\) of explicitly matched pairs. A phase may also set aside individual unmatched vertices as blockers. This state specifies an actual matching, although a changed internal exposure need not be propagated through a bin witness until materialization. Initially the bins are the unmatched singleton vertices, \(U\) contains all matched pairs, and there are no rows or blockers.

Constructive odd blocks, with an exposed vertex chosen during expansion, follow the blossom contraction-and-expansion tradition of Edmonds (Edmonds 1965b, secs. 4.10–4.14 and 7.1–7.2). Here each block carries the witness needed to expand the represented matching; the phase analysis below proves the required progress bound.

For \(X\subseteq\mathcal D\), define its captive rows and deficiency by \[ A(X)=\{r\in\mathcal A:N_{\mathcal B}(r)\subseteq X\}, \qquad h(X)=|X|-|A(X)|. \tag{1}\] Each row has its assigned bin as a neighbor. The injection therefore gives \(h(X)\ge0\), even at intermediate states. Write \(v(X)=\sum_{d\in X}v_d\) for a bin function \(v\).

Fix integers \(a\ge1000\) and \(0\le\sigma\le a+1\), and set \(C=a+1\). The phase seeks \[ g_*=\max\left\{1,\left\lfloor\frac{K}{100C}\right\rfloor\right\}, \qquad L=\left\lceil\frac{10000CN}{K}\right\rceil \tag{2}\] gains, each increasing the matching size by one. The parameter \(L\) will be the amount allocated by each row in the ledgers below. The choices balance the gain target against the work permitted per gain: \(Lg_*=O_a(N)\), including when \(K<100C\), since then \(g_*=1\) and \(L\le10000CN+1\). At a phase boundary we materialize the matching and initialize a new state.

For the reset periods, pricing steps and exact arithmetic, put \[ B=2^{\lceil s/a\rceil},\qquad q=B^{-100},\qquad \delta=S^{-1000},\qquad \varepsilon=S^{-3000}. \tag{3}\] The top ledger uses the grid \(\varepsilon\mathbb Z\); the other ledgers use integers. Their common purpose is to retain enough unallocated capacity at each bin to rearrange the row assignment. Debt records a temporary shortfall from the specified residual floor.

Definition 6 (Ledger invariant). There are ledgers \(j=0,\ldots,a\), with \(c_j=C-j\). Ledger \(j\) has nonnegative incidence allocations \(x^j_e\), with \[\sum_{e\text{ at row }r}x^j_e=L\qquad(r\in\mathcal A).\] Its column residual and nonnegative debt are \[y^j_d=L-\operatorname{load}^j_d,\qquad \operatorname{load}^j_d=\sum_{e\text{ at bin }d}x^j_e,\qquad p^j_d\ge0,\] and they satisfy \[ y^j_d+p^j_d\ge c_jw_d. \tag{4}\] At the top level, \[ x_e=x^0_e>0,\qquad s_d=y^0_d+p^0_d-Cw_d>0 \tag{5}\] are \(\varepsilon\)-grid coordinates. The other ledgers are integral. At each settled main-loop state, the bottom debt \(p^a\) is zero.

Initially allocations are empty and debts vanish. Singleton residuals are \(L\), and \(L>C\), so the top slacks are positive.

Lemma 7 (Bottom surplus). At each settled state and every nonempty \(X\subseteq\mathcal D\), \[ Lh(X)\ge y^a(X)\ge w(X)>0. \tag{6}\] Thus \(h(X)\ge1\), every bin has weight at most \(L\), and a tight set \(h(X)=1\) has total weight at most \(L\).

Proof. Every captive row allocates all \(L\) units inside \(X\), so \(\operatorname{load}^a(X)\ge L|A(X)|\) and \(y^a(X)\le Lh(X)\). Zero bottom debt and \(c_a=1\) give \(y^a(X)\ge w(X)\). Weights are positive integers, proving the assertions. ◻

The ledgers have three roles. At settled states the debt-free bottom ledger supplies the strict Hall surplus just proved; the top ledger supplies a smooth progress objective; and the intermediate integral ledgers make repairs local and amortizable. Adjacent floors differ by one. In a failed projection, this gap provides the margin that pays for the peeled weight in each preserved coarser ledger, as proved in Lemma 10.

The top objective and the pricing primitive

The top ledger chooses its next allocation by increasing \[ \Phi=\sum_{e\in E(\mathcal B)}\log x_e+ \sum_{d\in\mathcal D}\log s_d. \tag{7}\] Orient allocation arcs from rows into bins, and add a slack arc \(s_d\) from one common auxiliary vertex into each bin. For fixed \(p^0,w\), circulation increments preserve exactly the row sums and the column equations \[\sum_{e\text{ at }d}x_e+s_d=L+p^0_d-Cw_d.\] Conversely, respecting these equations gives a circulation: the common slack-source equation follows by summing them and the row equations.

Here is the part of Lemma 21 needed to read the matching algorithm. Let \(f\) denote the vector of its positive allocation and slack coordinates; here the gradient and derivative-sum lengths are \(g_e=\lambda_e=1/f_e\). A successful pricing call makes an exact \(\varepsilon\)-grid circulation increment \(h\) with \(\sum_e |h_e|/f_e\le q/100\), preserves positivity, and increases \(\Phi\) by \(\Omega(q^2)\). Its alternative outcome \(\mathrm{LOW}\) is impossible whenever a circulation direction \(d\ne0\) satisfies \[\sum_e\frac{d_e}{f_e} \ge\frac12\sum_e\frac{|d_e|}{f_e},\] unless the primitive reports a stochastic failure or suspends. The sums here range over both kinds of arc in the top graph.

The primitive stores the exact values implicitly and supplies cached registers \(\widehat f_e\). After settling its notifications they obey \(|f_e-\widehat f_e|\le(q/100)\widehat f_e\). It reports refreshed coordinates in a list, so the client can repair small arguments and update expansion eligibility without scanning all coordinates after a step. The number of these notifications is charged to successful pricing steps by Lemma 21. Explicit point reads and settings remain available. Section 5 proves this interface and its time and reliability bounds; below we verify the positivity and representation conditions required of this client.

Check the loose prerequisites \[ S\ge2^{100},\qquad 100000C\le S^{1/20},\qquad B\le S^{1/20},\qquad q\ge S^{-1}. \tag{8}\] Cap main-loop actions, including structural operations and pricing attempts, at \(S^{20}\) per phase. Separately cap pricing iterations in each local threshold test of Section 4 at \(S^{20}\). Each engine instance has its stated \(S^{100}\) external-operation guard, together with incrementally checked numerical and representation bounds. Failed prerequisites or deterministic caps suspend the branch. Exhaustion of a prescribed randomized search instead returns a negative perfect-matching answer immediately. The guarded-prefix analysis will show that the deterministic checks interrupt no path for sufficiently large \(S\), at each fixed sufficiently large choice of \(a,\sigma\).

The main actions and exact ledger edits

The actions below distinguish progress in matching size from changes to its representation. A gain increases the size by one; contraction and expansion preserve it. A projection may also peel bins, moving their free exposures to the blocker list without changing matching size. Whenever the phase continues, the scheduled projection restores the debt-free bottom ledger before the next settled state.

At a settled state, take the first applicable action in this order.

  1. If a physical edge joins distinct bins \(i,j\), use it. If the injection can be rearranged to leave both free, match their prescribed exposed endpoints along this edge and move their vertices into \(U\). This is a gain. The \(g_*\)-th gain ends the phase immediately, without further ledger repair. Otherwise contract the unique minimal tight \(X\supseteq\{i,j\}\), with \(h(X)=1\), together with its captive rows to one bin. The witnesses and efficient randomized implementation are proved below.

  2. If bin \(d\) has an edge to \(v\in U\) and cached top slack \(\widehat s_d\ge4C\), expand along it. Make \(v\) a row and its current \(U\)-mate a singleton bin, with their old match as assigned edge.

  3. If neither action applies, request a top pricing step from Lemma 21. LOW or explicit sampling failure returns a negative answer; suspension suspends the parameter branch.

After each nonterminal structural operation, perform the scheduled projection below before returning to a settled state.

In a contraction, \(|A(X)|=|X|-1\), so the new block has physical size \(2w(X)-1\) and weight \(w(X)\). Delete captive rows and preserve every other assignment. At most one exterior assignment entered \(X\), because captive rows already assigned \(|X|-1\) of its bins. Retain allocations on surviving incidences and aggregate \(p,y\), and top \(s\), by summing over \(X\). This is exact: removing captive rows deletes \(L(|X|-1)\) load, while \(|X|\) residual columns become one. New incidences, including edges from a surviving row to a swallowed row, start at zero in integral ledgers; the top uses the positivity modifications below.

On expansion, ledger \(j\ge1\) allocates the new row’s whole \(L\) to its assigned child and gives that child debt \(c_j\). Its other new incidences start at zero. At the top, place \(C+1\) units on the trigger incidence into the parent and \(L-C-1\) on the assigned child incidence, with no initial child debt. The child has top slack one. Lemma 9 ensures actual parent slack at least \(3C\), leaving it positive after paying \(C+1\). Other new top incidences receive the small positive padding specified there.

The following deletion transformation applies to gains and projection peels. Given bins \(R\), remove all rows assigned to \(R\). For a peel, materialize each assigned bin with its row as matched pairs in \(U\). For each free bin, place its exposed vertex on the blocker list and its other vertices into matched pairs in \(U\). At a gain the two newly freed bins instead use their joining edge, so no blocker is introduced.

Lemma 8 (Deletion and net debt). In a preserved ledger of floor \(cw\), redirect each allocation of a surviving row into \(R\) to its current assigned edge, adding an equal debt at the recipient bin. This preserves the exterior ledger invariant. If \(O\) is redirected mass, \(I\) is removed rows’ allocation to the exterior, and \(f\) is the number of free bins removed, then \[ \begin{split} O&=I+Lf-y(R),\\ I&\le L(h(R)-f),\\ \Delta\!\left(\sum_d p_d\right)=O-p(R)&\le Lh(R)-cw(R). \end{split} \tag{9}\] The last bound is at most \(2L\) at a gain. A gain decreases the represented unmatched count by two; contractions, expansions, and peels leave it unchanged.

Proof. A surviving row’s assigned bin is outside \(R\). The added allocation and debt cancel in \(y+p-cw\); removing rows only increases exterior slack.

Exactly \(|R|-f\) rows are removed. Their allocation into \(R\) is \(L(|R|-f)-I\), giving \[y(R)=L|R|-\bigl(L(|R|-f)-I+O\bigr)=Lf+I-O.\] Every captive row is removed, as its assigned bin lies in \(R\). Only the other \(h(R)-f\) removed rows contribute to \(I\). Deleting debt \(p(R)\) and adding \(O\) gives the exact net debt change, and (4) gives the inequality. At a gain, \(R\) has two bins and \(h(R)\le2\).

There is one unmatched vertex per free bin or blocker. Expansion adds one bin and one assigned row. Contraction replaces \(|X|\) bins and \(|X|-1\) assigned captive rows by one bin, preserving any exterior assignment. A peel transfers free bins’ unmatched vertices to blockers. A gain matches two free exposures. This proves the unmatched-count assertions. ◻

Maintaining the top barrier

Structural edits change the graph and its column equations, while pricing preserves those equations. We now specify the local positivity repairs that make both operations compatible with the pricing interface.

Lemma 9 (Top maintenance). The top ledger can be maintained on the \(\varepsilon\)-grid with logarithmic arguments above \(\delta\) before pricing and above \(\delta/10\) at refreshes. Apart from structural work, repairs cost constantly many explicit point operations per dirty coordinate. Between structural operations a small-allocation repair decreases \(\Phi\) by at most \(O(\delta)\), and slack repairs never decrease it. Positivity repairs add less than \(S^{-800}\) debt on every guarded phase prefix. Moreover, \[ \begin{split} \widehat s_d\ge4C&\ \Longrightarrow\ s_d\ge3C,\\ \text{no eligible expansion}&\ \Longrightarrow\ s_d\le5C\quad\text{for every bin with a \(U\)-contact}. \end{split} \tag{10}\]

Proof. A new positive allocation receives \(4\delta\) from a large donor in its row. Reading its at most three entries finds such a donor, since their sum is \(L\). Add \(4\delta\) debt at the destination bin. Its increased load is canceled by the debt; the donor bin’s slack increases by \(4\delta\). If both bins coincide, their load is unchanged and their slack still increases by that amount. The two specified large entries of a newly expanded row may be installed first and used as donors for its remaining incidences.

Use the same transfer whenever an existing allocation has cached value at most \(3\delta\). If a cached slack is at most \(3\delta\), increase that slack and its debt by \(4\delta\). Read touched values exactly and re-anchor explicit settings. Check touched slacks after exact re-anchoring too, including values at the small-value threshold.

Between motions or structural changes, each row has at most three entries to repair. A donor initially at least \(L/3\) remains large after these tiny transfers. Both changed allocations are re-anchored at nonsmall values. Transfers never decrease a slack, and a slack repair changes no other barrier coordinate. Engine notifications arising during settlement are queued and served, but price and forest maintenance itself causes no primal motion. Without a new motion or structural change, an already repaired coordinate cannot require another repair. Thus there is no uncharged repair chain.

For a single positive logarithm the register bound gives \[|f-\widehat f|\le(q/100)\widehat f.\] After service, an unrepaired coordinate has cached value above \(3\delta\), while a repaired one has been increased and re-anchored by \(4\delta\). This bound and the step-size bound leave the stated margins before a step and at refreshes after it. Contraction aggregates positive slacks; deletion increases surviving slacks; expansion has the positive margins specified above. New descriptors are inserted only after their positive final values are computed. All settings are exact \(\varepsilon\)-grid operations.

Only the large donor’s logarithm can decrease during a transfer, by \(O(\delta)\); recipient allocation and donor slack increase. A slack repair increases its logarithm. There are \(O(S)\) coordinates at any state, with constantly many repairs per coordinate or changed physical incidence between motions or structural batches. The \(S^{20}\) phase-action cap and the no-motion observation give, very crudely, at most \(S^{30}\) such fixes on a guarded prefix, including the last completed edit batch. Consequently their \(4\delta\) debt additions total less than \(S^{-800}\). This precision estimate does not permit scanning all coordinates after each step: only touched entries and dirty notifications are serviced.

Maintain bin-to-\(U\) contact lists and a cached-slack eligibility index. The relative register inequality, with \(q<1\), gives \(s_d\ge3C\) when \(\widehat s_d\ge4C\); if no eligible contact exists, its cached slack is below \(4C\) and its true slack below \(5C\). ◻

An explicit edit batch reads affected coordinates exactly, retains unchanged allocations on retargeted incidences, and computes positive final settings before insertions or descriptor changes. No pricing occurs before its full row and column equations settle. The identity \(y^0_d=s_d-p^0_d+Cw_d\) supplies a top residual without summing implicitly moving incidences. Old descriptors and edge mappings remain available through removal transactions. Dirty-entry service is completed before selecting the next main action.

Use nominal top-engine lifetime size \(R=SB\). Lifetime size counts vertex and edge births and endpoint insertions, including retargeting; mere value settings, positivity fixes, and repricing on retained topology do not create new incidences. Lemma 21 charges \[\bigl(R+B+\#\{\text{external pricing, point, or edge operations}\}\bigr) S^{O(1/a)+o_a(1)}\] instructions under \(R+B\le S^{1+2/a}\), with polynomial labels renumbered if needed. The phase analysis verifies this nominal size and the numerical bounds on sufficiently large guarded prefixes. Engine notifications are paid by actual pricing motion, not by a global scan.

Reset schedule and the exact exchange problem

A phase has a tick counter initially zero. A contraction or expansion contributes one tick; a gain needing ledger repair contributes \(L\). A projection peel contributes no ticks. For \(j=0,\ldots,a\), put \[ b_j=B^{a-j}. \tag{11}\] After preliminary structural edits, choose the smallest \(j\in\{1,\ldots,a\}\) whose period quotient increased during the event. Such a level exists since \(b_a=1\). Construct a fresh zero-debt \(j\)-ledger of floor \(c_jw\), and give every deeper level a copy with zero debt. Projection may peel one simultaneous bin set. The coarser levels \(0,\ldots,j-1\) are preserved through it using Lemma 8.

For \(j>1\), start from the just-current integral parent \(j-1\), whose debt total is \(P=\sum_dp_d\). For \(j=1\), instead use \(L\) on each current assigned edge and zero elsewhere; no top snapshot is needed. Set \(c=c_j\) and define \[ b(d)=(cw_d-y_d)_+,\qquad u(d)=(y_d-cw_d)_+,\qquad \beta=\sum_db(d). \tag{12}\] The exchange network has source-to-bin capacities \(b(d)\), bin-to-sink capacities \(u(d)\), and, for every incidence \(e=(r,d)\), \[d\longrightarrow r\quad[x_e], \qquad r\longrightarrow d\quad[L+1],\] where brackets indicate integral upper capacities and all lower capacities are zero.

Lemma 10 (Exact exchange projection). An integral maximum flow in the exchange network produces either a zero-debt ledger of floor \(cw\) on the whole state, or a nonempty bin set \(R\) whose peel leaves such a ledger on the remaining state. In the latter case, \[ Lh(R)<c_jw(R). \tag{13}\] Every preserved coarser ledger loses at least \(w(R)\) in net debt, and the number \(f\) of new blockers satisfies \[ f\le h(R)<\frac{Cw(R)}{L}. \tag{14}\]

Proof. For an integral maximum flow \(F\), set \[x'_e=x_e-F_{d,r}(e)+F_{r,d}(e).\] Every row has total incoming exchange capacity \(L\), so its outgoing exchange flow is at most \(L\). Thus \(x'\ge0\), its row sums are \(L\), and no row-to-bin arc of capacity \(L+1\) is saturated. If \(F_{\mathrm{src},d}\) and \(F_{d,\mathrm{snk}}\) denote source and sink arc flows, then \[y'_d=y_d+F_{\mathrm{src},d}-F_{d,\mathrm{snk}}.\] At value \(\beta\), all deficits are filled while a spare loses at most \(u(d)\), proving \(y'\ge cw\).

Otherwise let \(R\) be all residual-reachable bins. For an incidence \(e=(r,d)\), residual capacity from \(d\) to \(r\), combining the original arc and the reverse of its companion, is \[x_e-F_{d,r}(e)+F_{r,d}(e)=x'_e.\] A positive allocation into a reachable bin makes its row reachable. All that row’s neighbors are reachable because its row-to-bin arcs are unsaturated. The rows allocating into \(R\) are therefore exactly its captives, which allocate their full \(L\) there. Hence \(y'(R)=Lh(R)\). Every reachable spare sink arc is saturated, whereas some reachable source deficit remains unsaturated. Summing the residual changes over \(R\) gives (13).

Every deficit outside \(R\) is saturated, as otherwise its source arc would make it reachable; so \(y'\ge cw\) there. A surviving row has no allocation into \(R\), since such a row would be captive and removed. Removing the assigned rows of \(R\) only increases exterior residuals. Restricted \(x'\) is the requested ledger.

For a coarser level \(\ell<j\), one has \(c_\ell\ge c_j+1\). Lemma 8 gives net debt change at most \[Lh(R)-c_\ell w(R)<-w(R).\] Thus at least \(w(R)\) is paid. Every captive is among the removed assigned rows, giving \(f\le h(R)\); now (13) and \(c_j\le C\) give the blocker bound. ◻

At \(j=1\), construct this network globally and use Lemma 2, with Lemma 4 if necessary. Its size is \(O(S)\) and its capacities are polynomial in \(S\), yielding \(S^{1+o(1)}\) time including the arc-flow vector and residual cut. For \(j>1\), the parent inequality \(y+p\ge(c_j+1)w\) permits the local implementation of Lemma 14. It returns these exact allocation modifications and, when needed, the entire residual-reachable peel set in \[(P+B)S^{O(1/a)+o_a(1)}\] time. Its engine calls have their individual conditional failure budgets. A flagged failure ends this perfect-matching test negatively; a deterministic suspension suspends the branch.

Tight sets and factor-critical contraction

Lemma 11 (The tight-set alternative). Suppose (6) holds and an edge joins distinct bins \(i,j\). Either the injection can be rearranged to leave both free, or a unique inclusion-minimal \(X\supseteq\{i,j\}\) satisfies \(h(X)=1\). In the latter case \(w(X)\le L\), and the physical block \(\bigl(\bigcup_{d\in X}d\bigr)\cup A(X)\) is constructively factor-critical using the joining edge, its row–bin incidences, and its child-bin witnesses.

Proof. An injection avoiding forbidden bins \(F\) exists precisely when \[|A(Z)|\le |Z\setminus F| \qquad(Z\subseteq\mathcal D).\] Necessity is immediate. For sufficiency, apply Hall’s condition (Hall 1935, Theorem 1) to any row subset: if \(Z\) is its full neighbor set, those rows belong to \(A(Z)\), and \(Z\setminus F\) is large enough. For \(F=\{i,j\}\), the criterion becomes \[ h(Z)\ge |Z\cap\{i,j\}|. \tag{15}\] By the bottom surplus, failure means exactly a tight set containing both bins.

Each neighborhood-inclusion indicator in \(|A(Z)|\) is supermodular; thus \(h\) is submodular. Two tight sets containing \(i,j\) have nonempty intersection and union, both of deficiency at least one. Submodularity forces their intersection to be tight. Intersecting all such sets gives the unique minimal \(X\). Its weight bound follows from Lemma 7.

To expose a physical vertex \(v\) in child bin \(d\in X\), match all captive rows into \(X\setminus\{d\}\). The Hall inequalities are \(h(Z)\ge1\) when \(d\in Z\) and \(h(Z)\ge0\) otherwise, for \(Z\subseteq X\). Expose \(v\) inside \(d\) and each assigned edge’s physical contact inside every other child.

To expose a captive row \(r\), match \(i,j\) along the joining edge, and match the other captives into \(X\setminus\{i,j\}\). The needed Hall inequalities are \[ h(Z)+\boldsymbol1_{\{r\in A(Z)\}}\ge |Z\cap\{i,j\}| \qquad(Z\subseteq X). \tag{16}\] A proper subset containing both \(i,j\) has \(h(Z)\ge2\) by minimality and integrality. Other nonempty proper subsets use \(h(Z)\ge1\), and the empty set is harmless. For \(Z=X\), the deficiency is one and \(r\) is captive. Thus (16) holds. Each child exposes its actual prescribed physical contact, so the child witnesses lift these injections and the joining edge to the desired internal matchings. ◻

Bounded searches for a bin pair

We implement the alternative without examining the physical interiors of visited bins, first testing for a small tight set and then trying a joint freeing.

A capacity-one witness.

Use a directed reverse graph with split vertices \(d_{\rm in}\to d_{\rm out}\) of capacity one for each bin. The source has capacity-two arcs to \(i_{\rm in},j_{\rm in}\). A free bin’s output has a capacity-two arc to the sink. An assigned bin’s output has capacity-two arcs to the inputs of all neighbors of its assigned row, retaining individual incidences. Original outdegrees are bounded.

For the minimal tight \(X\), its \(|X|-1\) captive rows are assigned to all but one bin \(t\) of \(X\). The set comprising the source, all inputs of \(X\), and all outputs of \(X\setminus\{t\}\) has at most \(2|X|\) vertices and outgoing capacity one: only \(t\)’s split arc leaves it. The original sink is reachable. Otherwise all reachable bins are assigned, and their assigned rows have every neighbor among those bins, contradicting positive deficiency for that nonempty set.

Use doubling vertex budgets \(v_0=1,2,4,\ldots\), through the first budget at least \(8L\). A fresh trial explores the original graph from the source until reaching the sink or visiting \(v_0\) vertices. Send one unit along a predecessor path, ending at the sink if reached, and otherwise at a uniformly sampled visited vertex. Then explore residual reachability with the same budget.

When \(v_0\ge4|X|\), either the endpoint is the sink or exactly \(v_0\) vertices were visited. At most \(2|X|\) of the latter lie in the small cut, so the endpoint is outside with probability at least one half. A simple predecessor path to the exterior uses its sole outgoing arc once and cannot return: it would then have to use that same outgoing arc again to reach its endpoint. It therefore saturates the outgoing capacity and has no positive incoming flow across the cut. The cut has no outgoing residual arc. Since \(v_0>2|X|\), residual search then exhausts without the sink.

Only a fully exhausted, sink-free residual source side is accepted. Its original outgoing arcs are saturated and its incoming flow is zero, so its original outgoing capacity equals net path flow, at most one. That capacity is positive since the original sink was reachable. It is therefore exactly one. No capacity-two arc crosses the source side: the unique crossing is a split arc, both specified inputs are inside, and every included output has all of its assigned row’s neighbors inside.

An included output has its own input inside too. It is reached either through its split arc, or through the residual reverse of an original outgoing arc. In the second case that outgoing arc carried the predecessor-path flow, so its split arc also carried flow and has a residual reverse to its input. Thus the included inputs form a bin set \(X'\) in which all except the crossing-split bin have distinct captive assigned rows. Consequently \(h(X')\le1\), and the bottom surplus makes \(X'\) a tight witness containing \(i,j\). Truncated searches and searches reaching the sink certify nothing.

Refining a witness.

List the captives of \(X'\) through its assigned bins, checking each row’s bounded neighbor list. On these rows and bins, use source-to-row capacities one, row-to-bin capacities \(H_\infty\), bin-to-sink capacities one, and additional source arcs of capacity \(H_\infty\) to \(i,j\), where \(H_\infty=2|X'|+3\) exceeds the finite cut values of interest. Given a bin source side \(Z\supseteq\{i,j\}\), the optimal row source side is \(A(Z)\), with cut capacity \[|A(X')|+h(Z).\] The minimum is \(|A(X')|+1\). Start with the captive assigned edges, a flow of value \(|A(X')|\). One augmentation reaches the maximum, and residual reachability gives its smallest source-side minimum cut. Its bins are the unique minimal tight \(X\). This costs linear work in \(|X'|\), up to dictionaries.

Repeat fresh trials at each budget as specified below. After refinement, if the discovery budget exceeds \(16|X|+16\), abort the perfect-matching test negatively. On a tight pair this requires failure of every trial at the first budget at least \(4|X|\). On every path, however, a continued contraction costs only \(O(|X|\mathop{\mathrm{polylog}}S)\), with fixed-parameter reliability factors; a final abort may cost \(O(L\mathop{\mathrm{polylog}}S)\). The one pushed path creates only linearly many residual reversals, so each bounded exploration has linear budget cost up to dictionaries.

Freeing two bins.

If no tight witness is found, try to free the bins jointly. At an assigned bin, a reverse walk follows an incidence of its assigned row with probability \(x^a_e/L\); free bins absorb. For the transition submatrix \(P_m\) on assigned bins, each destination column \(d\) has sum at most \[\operatorname{load}^a_d/L\le1-w_d/L.\] This remains true after an injection rearrangement: each assigned row still supplies exactly one transition row. Since \(w_d\ge1\), the probability of surviving \(\ell\) steps from any start is at most \[ N\exp(-\ell/L). \tag{17}\] Indeed the matrix column norm is at most \(1-1/L\). Each entry of \(P_m^\ell\) is at most its column sum, and summing a starting row over at most \(N\) columns proves the bound.

First free \(i\) by a successful walk to a free bin: erase loops while keeping the assignment fixed, and then shift each encountered row to the next bin of the resulting path. If \(i\) was already free, no change is needed. With the new injection, joint freeability guarantees a path from \(j\) to another free bin, avoiding \(i\), in the full-neighbor reverse graph. Otherwise the exhausted set \(T\) of reached assigned bins has all its assigned rows captive in \(T\cup\{i\}\). This set contains \(i,j\) and has deficiency at most one, contrary to (15).

Explore until another free bin is reached, or until reaching \(8L\) distinct assigned bins. In the first case use its predecessor path. In the second, sample a reached bin uniformly, retain its prefix path from \(j\), and try a bottom-ledger walk from there. Let \(H_d\) be the probability of eventually hitting \(i\) from an assigned bin, and \(p_i\) the one-step column into \(i\). For the assignment fixed during this attempt, \[ H=P_mH+p_i,\qquad w^{\mathsf T}H\le L\boldsymbol1^{\mathsf T}(I-P_m)H =L\boldsymbol1^{\mathsf T}p_i\le L. \tag{18}\] The first inequality is the destination-column bound with \(H\ge0\); the last uses \(\operatorname{load}^a_i/L\le1\). As weights are at least one, the average hit probability over the \(8L\) reached bins is at most \(1/8\). Together with (17), this gives a constant chance of reaching another free bin within the walk cap while avoiding \(i\). Concatenate the fixed prefix with that walk, erase loops, and shift assignments. This frees \(j\) while leaving \(i\) free.

Loop erasure uses a stack indexed by bin. Each retained outgoing transition follows the row assigned at its origin, since the assignment stays fixed throughout path construction. Hence a path to a free bin certifies the reassignment. An already-free target needs no rearrangement. Exhaustion or failure after the capped repetitions gives a negative answer; successful joint freeing always certifies a genuine gain.

Lemma 12 (A bounded randomized pair action). At a settled state, the preceding searches process a queried distinct-bin edge as follows. A returned contraction is the minimal tight set with a valid factor-critical witness; a returned gain has a valid joint-freeing witness. Conditional on the valid current state, the probability of a premature negative answer, including a late-witness abort, is at most \(2^{-\sigma}S^{-60}\). Continued contractions cost \(O(|X|\mathop{\mathrm{polylog}}S)\); a gain attempt or final abort costs \(O(L\mathop{\mathrm{polylog}}S)\), with fixed-parameter reliability factors. These work bounds hold on every path.

Proof. Use walk length and repetition count \[\ell=16L(1+s),\qquad T_{\rm rep}=1000(\sigma+s+1).\] Use \(T_{\rm rep}\) fresh tight-set trials at each budget, that many attempts at first freeing, and that many sampled second-freeing attempts. A tight-set trial succeeds with probability at least one half once its budget is at least \(4|X|\), so missing the early witness is exponentially unlikely in \(T_{\rm rep}\). For a jointly freeable pair, the first walk has constant success probability by (17). The second has probability at most \(1/8\) of hitting \(i\), by (18), and its nonabsorption probability at the length cap is bounded by (17). Their sum is strictly below one. Unsuccessful trials do not change the injection, so the same conditional estimates apply repeatedly.

A range-uniform draw generates enough independent fair bits to cover the range and rejects out-of-range strings; each elementary attempt succeeds with probability at least one half. Cap its string attempts at \(T_{\rm rep}\), returning negative on exhaustion. An integral allocation draw uses a uniform element of \(\{1,\ldots,L\}\) and the row’s at most three cumulative intervals. Under (8), fewer than \(S^{10}\) such draws occur in one pair query, including every budget and trial. A union bound over constant-probability trial failures and capped fair-bit rejection failures gives \(2^{-\sigma}S^{-60}\), with ample numerical slack. Equivalently, couple unlimited exact rejection to this capped process until its first cap failure.

The witness arguments prove soundness. Doubling budgets sum to a constant multiple of the last, so the post-refinement budget test gives \(O(|X|\mathop{\mathrm{polylog}}S)\) for a continued contraction even on an unfavorable random path. Other attempts use \(O(L)\) vertex caps, \(O(L\log S)\) walk caps, and the fixed repetition counts.

Generate outgoing reverse arcs from bin/assignee data and the assigned row’s physical neighbor list, of degree at most three. Store path-created residual reversals and local visited records in dictionaries. No query scans all physical vertices of a visited bin. Random bits, initialization, loop erasure, record operations, and explicit path or witness output are included in the work. ◻

Explicit matching materialization

Lemma 13 (Materializing a bin). A current bin can be materialized with any prescribed physical exposure in time linear in its physical size up to logarithmic factors. All current bins can be dissolved at an erasure or phase boundary within the corresponding total physical-size bound.

Proof. At a contraction store the child-bin records, captive rows and their physical incidences, joining edge, and captive assignment at contraction time. This gives a contraction tree whose leaves include both child-bin vertices and swallowed row vertices. The stored captive assignment covers all but one child.

If the desired exposure \(v\) is in child \(d\), forbid \(d\) and remove its captive assignment if present. At most one row becomes unmatched. Lemma 11 guarantees an injection of all captives avoiding \(d\), so at most one ordinary row augmentation suffices. If \(d\) was the unused child, none is needed. Expose \(v\) inside \(d\), and the physical assigned-edge contact in every other child. A preserved exterior assignment merely prescribes \(v\) to be its contact endpoint.

If the desired exposure is swallowed row \(r\), delete \(r\) and its assigned edge, reserve \(i,j\) for the stored joining edge, and remove any remaining assignments into those children. At most two remaining rows become unmatched. Inequality (16) guarantees a matching of the remaining rows into \(X\setminus\{i,j\}\), so at most two augmentations suffice. The joining edge prescribes exposures in \(i,j\); every other child gets its row-edge contact.

In either case each child receives exactly one prescribed exposure and is recursively materialized once. The searches scan only this node’s stored abstract incidence graph, of size linear in its arity because rows have degree at most three, rather than its children’s physical interiors.

At the start of dissolving a root, one traversal assigns subtree leaf intervals. The interval containing a prescribed physical vertex identifies its direct child, with logarithmic search overhead if needed. No contraction is unary, and each record has size linear in its arity. The sum of record sizes and leaves is therefore linear in the dissolved root’s physical size. Summing the local searches, endpoint lookups, and single child visits proves the bound. Disjoint roots at an erasure or phase boundary can be handled in the same total physical-size time. ◻

Later structural accounting charges any subsequent return of these vertices to bins as a new stay supplied by initialization or expansion. Thus each dissolved contraction record pays for one materialization, rather than repeated descent through an unchanged nested witness.

Local projection and the perfect-matching bound

We now implement the fine-ledger projection and prove the progress and cost bounds for the phase algorithm of Section 3. We use its state notation and the guards in (8), and the priced-step interface of Lemma 21, whose construction is given later. All estimates in this section with a subscript \(a\) are for a fixed parameter \(a\). Constants in an exponent \(O(1/a)\), in contrast, are absolute.

The threshold tests use the potential-reduction viewpoint of approximate minimum-ratio circulations developed by Chen et al. and its dynamic threshold formulation by van den Brand, Liu, and Sidford (Chen et al. 2022; Brand, Liu, et al. 2023). The logarithmic objective, local exploration and projection guarantees needed here are proved directly.

A local exact projection

The exchange network of Lemma 10 has one vertex for every bin and row. Its source and sink will be denoted by \(s_*,t_*\), to distinguish them from the size exponent in \(S=2^s\). For an integral parent ledger and a requested floor \(cw\), write \[ b(d)=(cw_d-y_d)_+,\qquad u(d)=(y_d-cw_d)_+,\qquad \beta=\sum_d b(d),\qquad P=\sum_d p_d . \tag{19}\] The parent floor is one unit higher: \[ y_d+p_d\ge(c+1)w_d . \tag{20}\] In particular, \(\beta\le P\). A maximum exchange flow, followed when necessary by its full residual source-reachable bin set, is the exact information required by Lemma 10. The following lemma obtains that information without scanning every bin.

Lemma 14 (Local exact projection). Suppose an integral parent ledger satisfies (20) in a partition state of a subcubic graph on at most \(S\) physical vertices. Its allocations, column residuals, weights, and positive debts support point access and sparse indexed updates. Assume \(P=O_a(S)\) for the fixed-parameter asymptotic bound. Then the exact exchange projection can be implemented in \[ (P+B)S^{O(1/a)+o_a(1)} \tag{21}\] total time on every path, at all sufficiently large sizes for fixed sufficiently large parameters.

The implementation either returns the exact exchange-flow changes and, if needed, the full residual reachable set \(R_0\); reports a detected stochastic failure; or suspends on a deterministic prerequisite check. It uses \(O(\log S)\) priced-step instances. Each reached instance with valid prerequisites has its conditional detected-failure allowance \(2^{-\sigma}S^{-60}\). All updates to a fresh child ledger and all necessary physical scans are confined to \(O(P)\) represented incidences and bin weight, apart from the padding and subpolynomial factors in (21). The stated deterministic prerequisites do not cause suspension in the sufficiently large fixed-parameter range.

The construction tests whether the full exchange maximum is at least an integer threshold. An optimistic local network supplies an upper bound on that maximum, and the same explored region with true capacities supplies a lower bound. We first construct and analyze this test, then show how two of its regions recover the full residual cut. The proof of Lemma 14 will assemble these steps.

Optimistic and true regions.

Assume \(\beta>0\) for the threshold construction. Start the exploration with every seed bin satisfying \(p_d\ge w_d/2\). Their total weight is at most \(2P\). The index of positive integral debts has at most \(P\) entries, so listing these seeds and computing their deficits takes local work. Every deficit is a seed: if \(y_d<cw_d\), then (20) gives \(p_d>w_d\).

Expanding a bin exposes all its bin-to-row exchange arcs, its true source and sink capacities, and all row-to-bin arcs and endpoints of the rows thereby discovered. Arc identifiers are retained, so an already represented arc is not inserted a second time. An unexpanded bin reached as an endpoint is a boundary. It has no represented bin-to-row arcs yet and no positive source capacity, since it is not a seed. Give it, optimistically, a sink arc of capacity \(\beta+1\). Its true spare is available by a point lookup, and (20) implies \[ u(d)=y_d-cw_d\ge w_d-p_d>w_d/2 . \tag{22}\] In particular \(u(d)\ge1\), by integrality.

For every region so obtained, its unlocked optimistic maximum flow is at least the full true maximum flow. Indeed, take an integral path decomposition of a full flow and discard its cycles. Each source path starts at a seed. Retain it until its first boundary, if any, and then send it directly to the sink. The retained interior arcs are represented with their true capacities, and the total flow is at most \(\beta\), less than every optimistic boundary sink capacity. The explored true graph has the same represented arcs, but uses the true sink capacity at every boundary. It embeds into the full exchange graph.

Testing a threshold.

The two local networks just defined have zero lower bounds. For optimization we use a third network on the represented arcs: its capacities are scaled, its logarithms have padded arguments, and some sink arcs acquire permanent positive lower bounds. These lower bounds will limit the total explored weight. Lemma 3 will convert its exactly conserved real points into integral feasibility statements, with the total violation including the return arc.

For an integer \(1\le b\le\beta\), begin a fresh exploration and multiply all its ordinary capacities by \(16\). Introduce a return arc \(t_*\to s_*\) with variable \(z\), and set \[ T=16b,\qquad W=20000(P+1). \tag{23}\] The objective is \[ \Phi(F,z)= \sum_{e\ {\rm ordinary}} \left[\log(F_e-\underline c_e+\zeta_e^-) +\log(\overline c_e+\zeta_e^+-F_e)\right] -W\log(T-z). \tag{24}\] Here \(\underline c_e,\overline c_e\) are the current unpadded bounds, initially with lower bound zero, and \(\zeta_e^-,\zeta_e^+>0\) are dyadic pads. At the current point, write \(g=\nabla\Phi\), and let \(\lambda_e\) be the sum of the absolute derivatives of the individual weighted logarithmic terms on coordinate \(e\), as in Lemma 21. In particular, this is not in general \(|g_e|\): the two ordinary-arc derivatives can cancel, whereas on the return arc \(g_z=\lambda_z=W/(T-z)\). All flows start at zero. Added arcs also start at zero, and every subsequent motion is an exact circulation supplied by Lemma 21; no client operation changes an existing flow value. Figure 1 shows the threshold network and the transition from an optimistic boundary to a permanently locked sink.

The local threshold network and the boundary transition in Lemma 14. Labels on ordinary arcs are scaled upper capacities; omitted unpadded lower bounds are zero. The schematic shows one expanded incidence and one boundary incidence. Initially all seeds \(p_d\ge w_d/2\), and therefore all deficit bins, are expanded. A boundary has no outgoing bin-to-row arcs and no positive source capacity until expansion. Padded bounds are used by the engine. Expansion keeps the current flow and installs a permanent lower lock. If the last step succeeds before a new trigger is served, its boundary flow still satisfies \(f<10u(d)<16u(d)\), so the true sink capacity is respected up to padding.

Start each pad at \(4\delta\). If a cached argument is at most \(3\delta\), increase its pad by \(4\delta\) and re-anchor that coordinate. Explicit changes of bounds also re-anchor it. Increasing a pad only increases the objective. The register and step-size guarantees, exactly as in Lemma 9, keep all ordinary arguments above \(\delta\) before a new step and above \(\delta/10\) at a refresh after motion. A serviced side does not need another increase without further motion or a change of its bounds. The repairs can therefore be made on dirty and newly inserted coordinates, without a scan of the represented graph at each step.

The loose iteration and operation caps give, on every guarded prefix, a total sum of pads \[ \alpha:=\sum_{e\ {\rm ordinary}} (\zeta_e^-+\zeta_e^+)<S^{-800}. \tag{25}\] One may bound the number of potential repairs using all \(O(S)\) ordinary arcs of the full exchange graph; locality is not needed for this preliminary numerical estimate. Alternatively, the external-operation cap gives at most a fixed multiple of \(S^{100}\) pad settings, each of size \(O(\delta)\), which is smaller than the right side of (25) for large \(S\). There is no intervening infinite repair chain: a repaired argument is exactly re-anchored, and the current point does not move during the service. All ordinary unpadded-bound violations sum to at most \(\alpha\). Conservation at the source gives \[ -\alpha\le z\le16\beta+\alpha . \tag{26}\]

At each loop test read \(z\) exactly. Stop successfully when \(z\ge T-1/4\). Otherwise the next step cannot cross \(T\): the return contribution to its length is \(W|\Delta z|/(T-z)\), and the step’s total length is at most \(q/100\). Thus its relative return-argument change is at most \(q/(100W)\). In particular even the final successful step leaves \[ T-z\ge1/8 . \tag{27}\]

Boundary locks and explored size.

Before taking another pricing step, expand any boundary whose cached sink flow is at least \(8u(d)\). Replace its unpadded optimistic sink interval by \[ [\,4u(d),16u(d)\,] . \tag{28}\] Retain this lower lock for the rest of the threshold test, and add the newly discovered arcs with flow zero. Pads can be reinitialized when the sink bounds change.

The transition is interior with a constant margin. At the start of a step, an untriggered boundary has actual flow less than \(8.2u(d)\): apply the register-error inequality to its padded-zero lower argument and use \(u(d)\ge1\). During the step its absolute change is at most \((q/100)(F_e+\zeta_e^-)\), so its resulting flow is less than \(10u(d)\). At a first trigger the same cache guarantee gives actual flow greater than \(7u(d)\). Hence the new interval (28) is safely interior. Triggers are checked on dirty or newly supplied entries using their latest registers. Re-anchoring without motion cannot invalidate the estimate. In particular, if success is detected immediately after a step, before a newly triggered boundary has been serviced, that boundary still has \[ F_e<10u(d)<16u(d). \tag{29}\] Thus a successful exit does not require an uncharged final scan or expansion.

Every previously locked sink carries at least \(4u(d)\), apart from its pad violation; each just-triggering sink carries more than \(7u(d)\). All other sink flows are nonnegative apart from their pad violations, and their sum is \(z\). Equations (26) and (22) therefore bound the total weight of nonseed expanded or triggering bins by \(8P+1\). Including seeds, the expanded weight is at most \[ 11(P+1). \tag{30}\] This also bounds physical scanning: an expanded bin has \(2w_d-1\) vertices of degree at most three. Per unit of expanded weight there are at most six bin-to-row incidences, at most eighteen row-to-bin incidences, and at most eighteen boundary occurrences. Including source and sink arcs, there are fewer than \(1000(P+1)\) ordinary arcs. Each is inserted once, the region only grows, and each sink is locked at most once. The same bound thus controls cumulative topological insertions in this test.

Lemma 15 (Threshold certificate). An unfailed, nonsuspended threshold test cannot report \(\mathrm{LOW}\) if the unlocked optimistic maximum is at least \(T\) in scaled units. Its success exit implies that the explored true unscaled maximum is at least \(b\).

Proof. First impose the current integral ordinary bounds, including the locks, and return interval \([0,16\beta]\). The current point is exactly conserved. Its ordinary violation is at most \(\alpha\), and (26) adds at most \(\alpha\) more for the return interval. The total is at most \(2\alpha<1\), not merely the ordinary-arc error. Lemma 3 supplies an exactly feasible integral circulation for these bounds.

Starting from its ordinary flow, one can attain the unlocked optimistic maximum while preserving all sink locks. If the present value is smaller, the ordinary residual network contains a source-to-sink path; erase cycles to make that path simple. The only additional lower bounds are on incoming sink arcs. Their reversals leave the sink and cannot occur on a simple source-to-sink path. Every augmentation therefore preserves the locks, even if the starting flow contained cycles. This constructs, existentially, a feasible comparator \((F',z')\) with \(z'\ge T\).

The difference from the current point is a nonzero circulation. For each ordinary logarithmic term, a negative directional derivative has magnitude at most one, since the comparator is feasible even without the pads. There are fewer than \(2000(P+1)\) such terms. The return term contributes \[\frac{W(z'-z)}{T-z}\ge W\] positively. If \(A_+\) and \(A_-\) are the sums of positive and absolute negative individual contributions, then \[A_+\ge20000(P+1),\qquad A_-<2000(P+1),\qquad \frac{g^{\mathsf T}(F'-F,z'-z)} {\|\lambda(F'-F,z'-z)\|_1} =\frac{A_+-A_-}{A_++A_-}>\frac12 .\] Only a direction is being compared; evaluating the logarithmic objective at \(z'\ge T\) is neither intended nor necessary. Lemma 21 rules out \(\mathrm{LOW}\).

At a success exit, replace the optimistic boundary capacities by the true ones and impose return interval \([T,16\beta]\). Equation (29) shows that no unexpanded boundary violates its true upper capacity, even before trigger service. There are no unexpanded deficits. The ordinary violation is at most \(\alpha\); the additional return violation is at most \(1/4+\alpha\). The total is less than one. Lemma 3 gives a feasible integral scaled flow of value at least \(T=16b\) in the explored true graph. Dividing capacities and flow value by \(16\) proves that its unscaled maximum is at least \(b\). ◻

Continue until success, \(\mathrm{LOW}\), detected stochastic failure, or suspension. On \(\mathrm{LOW}\), verify by a static integral maximum-flow computation that the unlocked optimistic unscaled maximum is below \(b\). On success, verify that the explored true unscaled maximum is at least \(b\). A contradictory exact check causes suspension; Lemma 15 shows that it cannot occur in an unfailed proceeding computation. These calls use Lemmas 2 and 4 with size padded to \(O(P+B)\). Capacity digit scaling in particular avoids a variable polynomial-capacity exponent when \(P\) is small relative to \(S\).

Thus each verified test decides the threshold for the full graph. On \(\mathrm{LOW}\), optimistic domination implies that the full maximum is below \(b\). On success, the explored true graph embeds into the full graph and supplies value at least \(b\). The locked engine network is discarded after the test; the exact checks use the unlocked optimistic or true capacities, respectively.

Termination and cost of one test.

The objective range at loop and service states is \(O_a((P+1)\log S)\). There are \(O(P+1)\) ordinary terms with arguments between \(\delta/10\) and a guarded polynomial upper bound, and the weighted return term has the same total order by (27) and \(W=O(P+1)\). Pad increases never lower the objective. An arc insertion or a lock change costs at most \(O(\log S)\) in objective, and there are only \(O(P+1)\) such touches. The \(\Omega(q^2)\) increase per successful priced step consequently bounds their number by \[ O_a((P+1)q^{-2}\log S). \tag{31}\] Use, for example, nominal engine lifetime size \(8192(P+1)\), including every new endpoint and arc. Point reads of the return arc, local pad changes, boundary notifications, and the final static verification are charged. No global scan is made at a pricing step. Lemma 21 and its notification bound now give (21) for one threshold. The guard \(S^{20}\) on local pricing iterations is eventually larger than (31) because \(q^{-2}=O_a(S^{200/a})\) and \(P=O_a(S)\).

Recovering the exact flow and the full peel.

The threshold tests determine the maximum value, but the matching procedure also needs the exact allocation changes and, when a deficit remains, the entire residual source-reachable bin set. We now recover this information from the small regions explored by the tests.

Lemma 16 (Recovering the full residual cut). Suppose \(\beta>0\), and let \(v\) be the full unscaled exchange maximum. Suppose a true explored region of maximum \(v\) is available when \(v>0\), and an unlocked optimistic region of maximum \(v\) is available when \(v<\beta\). The union of the available regions, using true capacities and original arc identifiers, suffices to compute an exact full exchange flow and, when \(v<\beta\), the full residual source-reachable bin set.

Proof. Call the supplied true region the success graph and the supplied optimistic region the failure graph. Suppose \(v<\beta\), and let \(Q\) be a minimum source side of the failure graph. No boundary belongs to \(Q\), because its sink arc alone has capacity \(\beta+1>v\). Consequently every bin of \(Q\) is expanded and all its outgoing bin-to-row arcs of positive true capacity are represented. Every represented row has all its outgoing row-to-bin arcs, and every positive source arc is present because all deficits are seeded. Thus all arcs leaving \(Q\) in the full true graph occur with the same capacity in the failure graph. The same set \(Q\) is a full-graph cut of capacity \(v\).

The true success graph supplies value \(v\) when \(v>0\). Solve an exact integral maximum flow in the union with true capacities, using the failure graph alone when \(v=0<\beta\), or the success graph alone when \(v=\beta>0\). The union’s value is exactly \(v\), by the cut just proved or by the total source capacity \(\beta\). Extend its flow by zero to the full network.

When \(v<\beta\), equality of its value with the capacity of \(Q\) forces every outgoing cut arc to be saturated and every incoming cut arc to have zero flow. Neither an unsaturated forward arc nor a positive-flow reverse arc can leave \(Q\). All positive-capacity original arcs needed within \(Q\) are represented, and every needed reverse arc is the reverse of an arc carrying positive flow in the union. Residual reachability computed in this local representation is therefore the full residual source reachability. Its bin part is precisely \(R_0\). ◻

Proof of Lemma 14. List the positive parent debts to find the seeds and compute \(\beta\), in \(O(P)\) point operations. If \(\beta=0\), the unchanged allocation already satisfies the requested floor, so giving the fresh child an empty debt map finishes the projection. The parent ledger remains unchanged.

Otherwise binary search, using the verified threshold tests, determines the full unscaled maximum \(v\). Retain a true success graph for \(v\) when \(v>0\). If \(v<\beta\), retain an optimistic failure graph for \(v+1\). Its unlocked maximum is exactly \(v\): it is below \(v+1\) by the test, at least \(v\) by optimistic domination, and integral. At most a constant number of additional tests retrieves these regions. Lemma 16 then returns the exact flow and, when needed, its full residual bin set.

The union has size \(O(P+1)\). Its flow support supplies all allocation changes. The set \(R_0\) has weight \(O(P)\), either by (30) or by applying the parent deletion inequality to the failed floor-\(c\) projection. Listing its assigned rows and their affected allocations requires only that physical weight and bounded row degree. No scan of an unexpanded high-volume bin or of an unchanged parent allocation is necessary. This completes the local construction. There are \(O(\log S)\) thresholds and retrieval tests because \(\beta\le P=O_a(S)\). The per-test bound (21), the final static flow on the union, and these sparse ledger changes give the asserted total time. Each reached priced-step instance has its stated conditional failure allowance; a detected failure or suspension is returned to the caller immediately. The numerical and nominal-size prerequisite checks, including those used above, are justified uniformly on guarded prefixes in Lemma 19. ◻

Debt, structural work, and progress

The following estimates apply before the first deterministic interruption, and do not assume that the graph has a perfect matching. A detected failure terminates the test; a returned structural action or projection satisfies its deterministic invariants.

Lemma 17 (Phase accounting). In a phase with initial deficit \(K\), threshold \(L\), and target \(g_*\), every guarded prefix before completion satisfies \[ \sum_{\rm peels}w(R_0)\le2L(g_*-1)+S^{-800}, \qquad \#\{\text{blockers}\}\le2C(g_*-1). \tag{32}\] There are \(O(N+Lg_*)\) expansions, the sum of contraction arities \(|X|\) is \(O(N+Lg_*)\), and the total tick increase is \(\tau=O_a(N)\).

Proof. Before the terminal \(g_*\)-th gain there are at most \(g_*-1\) gains whose ledgers are repaired. Top contractions preserve total debt, top expansions consume available slack without other debt, and the positivity service of Lemma 9 adds less than \(S^{-800}\) debt. By Lemma 8, each repaired gain adds at most \(2L\) net debt. By Lemma 10, every projection peel consumes at least its weight from the top ledger. Nonnegative remaining debt gives the first bound in (32).

The number of new blockers at a peel is at most \(h(R_0)<Cw(R_0)/L\). Summing this strict inequality and using integrality gives the second bound in (32); the tiny positive remainder is less than one. In particular, before the first gain of a phase with \(g_*=1\), no positive-weight peel can occur.

The bottom certificate of Lemma 7 bounds the weight removed by a gain by \(2L\). A removed bin has \(2w_d-1\) physical vertices, and at most one assigned row per removed bin is materialized with it. Thus the number of vertices returned to the paired remainder is bounded by a constant times the total erased weight. Every expansion consumes a pair from that remainder. Its initial population is at most \(N\), so expansions number \(O(N+Lg_*)\). Initially there are at most \(N\) bins and each expansion creates one new bin. A contraction decreases their number by \(|X|-1\), and \(|X|\ge2\); hence the sum of \(|X|\) over contractions is also \(O(N+Lg_*)\). Expansions and contractions contribute one tick each, and a repaired gain contributes \(L\), giving \(\tau=O(N+Lg_*)\).

Finally \(Lg_*=O_a(N)\). If \(K<100C\), then \(g_*=1\) and \(L=O_a(N)\). Otherwise \[Lg_*\le \left(\frac{10000CN}{K}+1\right)\frac{K}{100C} =O(N).\] This proves the tick bound. ◻

Lemma 18 (No false stall without a stochastic failure). If the subcubic graph has a perfect matching, a settled phase state with fewer than \(g_*\) gains admits neither a valid \(\mathrm{LOW}\) exit nor a permanent lack of progress. More precisely, when neither a distinct-bin contact nor an eligible expansion is available, the top objective has a circulation direction \(d\) with \[g^{\mathsf T}d >\tfrac12\|\lambda d\|_1 .\]

Proof. Let \(i_*\le g_*-1\) be the gains so far. For this argument only, realize the matching represented by the partition. This is an existence comparison, not an instruction to materialize the graph at every pricing step. Taking the symmetric difference with a perfect matching gives \(K-i_*\) vertex-disjoint augmenting paths (Berge 1957). At most the blocker count meet a blocker.

Orient each remaining path from an unmatched endpoint. Its first bin is free and is entered at its exposed base. That bin has no other unmatched vertex and no crossing matching edge. At a stall there is no edge to a distinct bin. If the path exits to a row by a nonmatching edge, its next matching edge is that row’s assigned edge, entering another bin at its base. This new bin is assigned; its only possible crossing matching edge is the one just used on entry. A simple path must therefore next exit it by a nonmatching edge. Continuing this argument, the path must eventually reach \(U\): it avoids blockers, cannot end inside an assigned bin, and cannot enter another free bin through an assigned row edge.

Every such entry uses the unique physical base of its bin. Simplicity and vertex-disjointness consequently imply that the prefixes through their first \(U\)-contacts use distinct bins and rows, both within a prefix and across different prefixes. Reassign each prefix row to the preceding bin in its chain. This is simultaneously injective and frees the last bin of every prefix. If \(Z\) denotes all bins with a \(U\)-contact, one can thus leave simultaneously free a subset of \(Z\) of size \[ r\ge K-i_*-2C(g_*-1)\ge K/2 . \tag{33}\]

Scale this conceptual assignment by \(L\). The desired top column capacities are \[L+p_d^0-Cw_d ,\] which are nonnegative because the current top allocation and positive slack realize them. Clip overloaded columns by removing mass. An assigned column initially carries \(L\), so its removed mass is at most \(Cw_d\); the total removed is at most \(Cw(\mathcal D)\le CN\). The current top allocation proves that all rows can be filled under these capacities. Complete the clipped partial transportation flow by residual source-to-sink augmentations; one can scale the dyadic data and use integral path decomposition for this existence argument. Simple augmenting paths do not leave the sink, so column sink flows do not decrease. The \(r\) initially empty specified columns receive in total at most the removed mass. The resulting allocation \(x'\) has nonnegative slacks \(s'\) and satisfies \[ s'(Z)\ge Lr-2CN . \tag{34}\]

At a stall, the cache-based eligibility guarantee of Lemma 9 gives \(s_d\le5C\) on \(Z\). Take the difference from the current allocation and slack as a direction. Row sums and column equations are unchanged, so it is a circulation in the top engine graph. Every negative individual logarithmic directional contribution has magnitude at most one, since \(x',s'\ge0\). Their total is at most \(|E(\mathcal B)|+|\mathcal D|\le4N\). The positive sum is at least \[\frac{s'(Z)}{5C}-|Z| \ge \frac{Lr-2CN}{5C}-N .\] By (33) and \(L\ge10000CN/K\), this is greater than three times \(4N\). As \(\lambda\) counts the absolute individual contributions, the claimed ratio is greater than one half. Lemma 21 excludes \(\mathrm{LOW}\) unless it explicitly reports failure or suspension. Its positive objective increase, with the bounded objective accounting below, also excludes indefinite pricing without another structural action. ◻

Implementation and total phase cost

We give the representation and cost details needed to sum the local work. The charges are to touched records and actual engine notifications, rather than to a whole graph per event.

Physical structure.

Maintain physical vertices, bin and row identifiers, mates, assignments, incident lists, and edge classifications in dictionaries and linked lists. Maintain a set of distinct-bin contacts and, for each bin, a list of contacts to \(U\) with its first member accessible. A register refresh changes a bin’s eligibility flag without scanning its contacts.

At a contraction retain a largest old bin’s representative. Move the smaller bins’ member lists and the swallowed rows, updating their physical incidences and affected ledger records. Unchanged exterior incidence allocations retain their values and need only endpoint retargeting when necessary. Sum bin data using \(|X|\) point reads per retained ledger; do not scan the largest bin’s members. A moved old member’s bin size at least doubles during its uninterrupted stay in bins. Initial membership, new expansions, and swallowed row entries begin new charges; erasures and later returns are covered by Lemma 17. Deletions inspect only removed bins, their assigned rows, and their affected physical incidences, applying Lemma 8. Subcubicity therefore bounds all physical structural touches, including endpoint retargeting, by \[ O_a((S+Lg_*)\log S). \tag{35}\] Assignment flips in the pair primitive require only abstract assignment updates. By Lemma 12, successful contractions charge their abstract arity, gains charge \(L\) up to logarithmic factors, and a final abort costs at most that latter amount. All bin-pair work is consequently \[ O_a((S+Lg_*+L)\mathop{\mathrm{polylog}}S), \tag{36}\] with the fixed reliability factors included. Materialization uses Lemma 13 and is charged by the same physical erasure work or by one final phase-boundary scan.

Top values and objective.

At a structural edit, read and reset only the affected top coordinates. The top column residual is available from \[y_d^0=s_d-p_d^0+Cw_d ;\] it does not require a separate global sum of implicitly moving allocations. The aggregation, redirection, restriction, and tiny transfers determine affected slacks exactly, preserving column equations without rounding. Compute all final settings of a batch before submitting deletions, point changes, and insertions to the engine. Both old and newly anchored descriptors then have positive arguments, and no pricing call occurs until the completed topology again satisfies its equations. Notifications arising during this service can be queued: there is no implicit primal motion during the service. Settle all dirty small-argument and eligibility checks before choosing the next main action.

Use nominal top-engine lifetime size \(SB\), with an explicit check on the representation. Only actual topological insertions and retargetings consume new incidence slots; repricing, motion, and positivity repairs on retained coordinates do not. Equation (35) makes this nominal bound sufficient eventually. For fixed \(a\), Lemma 17 gives \[L=O_a(S),\qquad \sum_d p_d^0=O_a(Lg_*+1),\qquad s_d\le L+\sum_d p_d^0 .\] Thus at pricing states all top arguments lie in a range whose logarithmic width is \(O_a(\log S)\), and there are only \(O(S)\) coordinates. The top objective range is \(O_a(S\log S)\). Each affected coordinate at a structural edit costs at most \(O_a(\log S)\) in objective, so the cumulative structural loss is \(O_a((S+Lg_*)\log^2 S)\). The donor decrease in a tiny positivity transfer has loss \(O(\delta)\), and its total is already covered by the tiny-repair count. Each successful priced step increases the objective by \(\Omega(q^2)\). Hence their number is \[ O_a(Sq^{-2}\log^2 S). \tag{37}\] Dirty-entry repairs and eligibility work charge a constant number of point operations per notified or structurally touched entry. The notification bound of Lemma 21 and (37) put the total top work in \(S^{1+O(1/a)+o_a(1)}\).

Persistent integral ledgers.

Store allocations by physical incidence and \(y,p\) by bin in persistent point maps, for example path-copy balanced trees. Maintain an index and sum of positive live debts; deleted keys are cleared, and new bins are initialized locally. Retained levels receive their structural edits separately. At a projection from integral parent \(j-1\), retain the parent’s roots from before a possible peel, and modify a copy only along the obtained flow support and deletions. Include the exterior loads freed by removed rows. The new child’s debt map is empty. Apply preserving deletions separately to coarser ledgers, and let all deeper fresh ledgers initially share the new child’s roots. This costs local projection-size work and logarithmic record overhead, not a whole-vector copy at each reset. Level \(1\) may build entire maps after its global static flow computation.

The reset periods of Section 3.5 are \(b_j=B^{a-j}\). Suppose an event selects level \(j>1\). Even if the event advances ticks by \(L\), it crosses no \(b_{j-1}\) boundary: otherwise a coarser reset would have been selected. Its integral parent last reset to zero debt in this same \(b_{j-1}\) period interval; every crossing of such a boundary resets that parent or a coarser ledger. Its debt rises by at most \(C\) per expansion and \(2L\) per gain, and never positively for another action. Since these actions advance ticks by \(1,L\), respectively, the debt at the triggering event obeys \[ P\le(C+2)b_{j-1}. \tag{38}\] This reasoning uses both the pre-event and post-event quotients, so a jump over several periods cannot defeat it. In particular \(P=O_a(S)\) for \(j>1\).

At most \(\lfloor\tau/b_j\rfloor\) events select level \(j\): each can be charged to a distinct crossed \(b_j\) boundary. By \(b_{j-1}/b_j=B\), Lemma 14, and (38), all \(j>1\) projection costs sum to \[ \sum_{j=2}^a \frac{\tau}{b_j}\bigl((C+2)b_{j-1}+B\bigr) S^{O(1/a)+o_a(1)} =S^{1+O(1/a)+o_a(1)} . \tag{39}\] The small-instance padding \(B\) is included here. For level \(1\), the global exact exchange computation has cost \(S^{1+o(1)}\). Since \(b_0=B^a\ge S\) and \(b_1=b_0/B\), its total is at most \[\frac{\tau B}{S}S^{1+o(1)} =S^{1+O(1/a)+o_a(1)} .\] Together with initialization and the preceding charges, this proves the desired phase cost on completed guarded prefixes.

Guarded prefixes and the PM guarantee

Lemma 19 (Eventual adequacy of the guards). For every sufficiently large fixed \(a\) and fixed admissible reliability parameter \(\sigma\), all deterministic guards of the PM construction are adequate for all sufficiently large \(S\), on every computation path. The bounds just proved apply also to a prefix ending in a failed or interrupted attempted operation. No perfect-matching promise is needed for these time and adequacy assertions.

Proof. Consider the prefix through a putative first deterministic interruption. The elementary initial checks, such as \[S\ge2^{100},\qquad 100000C,\ B\le S^{1/20},\qquad q\ge S^{-1},\] are eventually satisfied at each fixed sufficiently large \(a\). The loose \(S^{20}\) iteration caps and the \(S^{100}\) external-operation cap suffice first to bound tiny positivity debt and local pads. A small argument increased and exactly re-anchored does not require another increase without motion or a reset of its bounds. A top donor is chosen exactly from a bounded-degree row with sum \(L\); local repair does not create an unbounded chain of new small donors. Thus these numerical error estimates do not assume the sharper running-time bounds that they are used to prove.

All completed structural actions and peels preserve the deterministic ledger and matching invariants. Their debt payoffs and the bottom certificates prove Lemma 17 independently of objective progress. Include the present structural action if any. For a coarse numerical bound, one has \[L\le10000CS+1,\qquad g_*\le S,\qquad \text{erased weight}\le4Lg_*+1,\] and at most \(N+4Lg_*+2\) expansions, allowing the next one as well. Every retained integral ledger has acquired at most \[ C(N+4Lg_*+2)+2Lg_* \tag{40}\] debt between its resets, even without using (38). Column loads are at most \(L|\mathcal A|\). Together with \(100000C\le S^{1/20}\), these estimates put all top data, exchange capacities, local return weights and scaled capacities, and argument upper bounds well inside \(S^{20}\). Thus numerical adequacy does not depend on parameter-dependent rounding constants in the period bound.

At a local threshold test, the current sink consumption already bounds every lock-trigger expansion by (30), before that expansion is performed. Ready arguments and post-motion refreshes have the required lower margin, and explicit edit batches use positive old and final settings. At the top, (35), with one extra physical-size edit allowance, verifies the nominal lifetime bound \(SB\) at a possible first overflow. At a local test, (38) and its \(8192(P+1)\) nominal size give the engine condition \(R+B\le S^{1+2/a}\) eventually. The same is true for the top nominal size.

The structural and objective estimates (31) and (37) bound all completed iterations. Add at most one final \(\mathrm{LOW}\), failed, or interrupted attempt. Because \(q^{-2}=O_a(S^{200/a})\), these bounds remain below \(S^{20}\) for fixed sufficiently large \(a\) and large \(S\). Up to a hypothetical first external-operation cap, the engine’s notification bound charges dirty coordinates to priced motion. Client repairs add only constantly many point operations per dirty trigger, besides the structural edits, and have no no-motion repair chain. Consequently even this cap-reaching prefix has only \(S^{1+O(1/a)+o_a(1)}\) external work, eventually smaller than \(S^{100}\). The priced-step lemma’s own eventual prerequisite adequacy applies within these hypotheses.

These arguments are uniform over successful verified states and rejected stochastic attempts. The engine lemma bounds attempted operations on every path; its construction uses verified structures and capped failed attempts, not a good-event-only time estimate. The pair primitive likewise either returns a valid action within its charged bound or terminates negatively. A detected stochastic failure promptly ends the PM test, whereas an inadequate deterministic prerequisite suspends the branch. The latter has just been excluded in the fixed-parameter asymptotic range. All local scans and updates, including a final pair abort or incomplete attempt, are charged by their represented work. This proves the claim. ◻

Theorem 20 (Fixed-parameter subcubic perfect-matching search). For the integers \(a\ge1000\) and fixed reliability parameters \(0\le\sigma\le a+1\), the construction defines a uniform randomized algorithm on loopless subcubic graphs with identified edge copies and \(N\le S\) vertices. Every computation halts with a checked perfect matching, a negative answer, or a suspension. It has no false positive. For each fixed input with a perfect matching, the probability of a negative answer is at most \[ 2^{-\sigma}S^{-5}. \tag{41}\] This statement bounds erroneous answers, not suspensions, and holds conditionally for adaptive invocations with fresh randomness and valid prerequisites.

For every sufficiently large fixed \(a\) and fixed admissible \(\sigma\), there is no suspension on any path at all sufficiently large \(S\). The total time on every path is then \[ S^{1+O(1/a)+o_a(1)}, \tag{42}\] where the exponent coefficients in \(O(1/a)\) are absolute. All initialization, physical data movement, arithmetic, random bits, and explicit witness writing are included.

Proof. Settle trivial cases directly; an odd number of vertices has no perfect matching. Start with the empty matching and run the phase construction. When \(K\ge200C\), a completed phase gains at least \(K/(200C)\). Below that threshold, at most \(200C\) further phases are needed, each gaining at least one. Thus there are \(O_a(\log S)\) completed phases, and at most one final negative or suspended phase. Each has the bound established above. Initialization, phase-boundary materialization, and the final edge-identifier and vertex-coverage check cost only the already allowed near-linear work. Lemma 19 makes (42) an every-path bound and excludes suspension in the stated asymptotic range. On arbitrary checked sizes, the explicit iteration, operation, representation, and sampling caps still give halting, with suspension permitted.

On a PM instance, in the absence of a stochastic failed guarantee or suspension, valid pair actions and projections preserve the invariants. Lemma 18 prevents a \(\mathrm{LOW}\) negative answer before the phase target. The objective step bound prevents an infinite sequence of priced steps, so each phase reaches its gains and the algorithm eventually constructs a perfect matching. Checking the proposed edge copies and coverage excludes false positives on every input, including no-instances.

For the probability bound, count invocations using the loose guards rather than conditioning on later noninterruption. There are fewer than \(S\) nontrivial phases, at most \(S^{20}\) main actions per phase, and only logarithmically many threshold tests and retrieval tests per projection. On these prefixes, \[\beta\le CN+L|\mathcal A|,\] so its search range has logarithmic bit length. The total number of pair queries and priced-step instances is, conservatively, less than \(S^{50}\). Each reached pair query has the conditional error allowance in Lemma 12; each reached engine instance has the conditional allowance of Lemma 21. Both are at most \(2^{-\sigma}S^{-60}\). Union over the reached invocations gives at most \(2^{-\sigma}S^{-10}\), which is smaller than (41).

The conditional bounds are applied when the invocation’s input and prerequisites are fixed by its past history, and its new bits are fresh. They are not conditioned on whether the branch later suspends or wins a scheduling race. An additional deterministic suspension produces no negative answer by itself. This proves (41) also at checked sizes where suspension remains possible, and completes the theorem. ◻

Priced logarithmic steps

This section constructs the optimization primitive used by the matching algorithm and the local projection procedure. The primitive maintains exact values implicitly, while exposing sufficiently accurate coordinate registers to its client. Its progress guarantee depends on the length of the actual path representation returned by a cycle data structure. In particular, cancellation in the resulting circulation does not reduce the motion charged to a step or to a register refresh.

Approximate minimum-ratio circulations serve as optimization directions in the almost-linear flow framework of Chen et al. (Chen et al. 2022). The step estimate below is proved for the logarithmic objective and the raw-motion convention used here.

The interface

Fix a master bound \(S=2^s\) and integer parameters \(a\ge1000\) and \(\sigma\ge0\). Recall \[ B=2^{\lceil s/a\rceil},\qquad q=B^{-100},\qquad \delta=S^{-1000},\qquad \varepsilon=S^{-3000}. \tag{43}\] Every asymptotic estimate in this section is for fixed parameters. The constants multiplying \(1/a\) in exponents are absolute; an \(o_a(1)\) term may include logarithmic powers whose exponents depend on \(a\) and \(\sigma\). We use deterministic prerequisite checks, including \(q\ge S^{-1}\). An inadequate prerequisite suspends the parameter branch without producing an answer. A detected stochastic failure is a distinct outcome, with the probability bound stated below.

Let a changing oriented multigraph have real edge variables \(f_e\) on the grid \(\varepsilon\mathbb Z\). Loops and individual parallel edge copies are allowed. Its separable objective \(\Phi(f)=\sum_e\phi_e(f_e)\) has one of the following terms on each edge: \[ \log f,\qquad \log(f-D_1)+\log(D_2-f),\qquad -W\log(T-f),\quad 1\le W\le S^{20}, \tag{44}\] where \(W\) is an integer. The endpoints \(D_1,D_2,T\) lie on the same grid as the values. Write \(g_e=\phi'_e(f_e)\), and let \(\lambda_e\) be the sum of the absolute derivatives of the individual logarithmic terms on that edge, including their weights. Thus, for the two-sided term, \[g_e=\frac1{f_e-D_1}-\frac1{D_2-f_e},\qquad \lambda_e=\frac1{f_e-D_1}+\frac1{D_2-f_e}.\] All data and values have magnitude at most \(S^{20}\), and the positive arguments at pricing and refresh states lie between \(\delta/10\) and \(S^{20}\). Small additional relative margins immediately after a step are harmless in these bounds. The applications maintain the stricter pre-step margins established in the preceding sections.

The client may add, remove, or retarget edges and vertices, change a descriptor or value on one coordinate, and request an exact point value. Its explicit value settings need not be circulations. Between such edits, all increments performed by the primitive must be circulations. The values themselves need not have zero divergence.

Lemma 21 (Priced logarithmic steps). Suppose a nominal bound \(R\) counts the logical vertex and edge births and endpoint insertions over one instance, including retargetings. Mere value or price changes on retained topology need not be counted as new incidences. Assume \[ R+B\le S^{1+2/a}. \tag{45}\] There is a uniform implementation with the following properties.

  1. A pricing call may return Low, but it cannot do so if there is a nonzero circulation \(d\) satisfying \[ g^{\mathsf T}d\ge\tfrac12\|\lambda d\|_1, \tag{46}\] unless it reports a detected failure or suspends.

  2. A call not returning Low, failure, or suspension performs an exact increment \(h\in\varepsilon\mathbb Z^E\) that is a circulation and satisfies \[ \|\lambda h\|_1\le q/100, \qquad g^{\mathsf T}h\ge q^2/1000. \tag{47}\] It leaves all logarithmic arguments positive and increases \(\Phi\) by \(\Omega(q^2)\).

  3. The implementation exposes registers \(\widehat f_e\). After settling updates and before the next pricing call, the descriptors are valid at these registers and \[ \lambda_e(\widehat f_e)|f_e-\widehat f_e|\le q/100. \tag{48}\] An explicit client setting can reanchor the coordinate exactly. Apart from such client settings, the number of refresh notifications is at most the number of successful pricing steps times \(S^{O(1/a)+o_a(1)}\). Notifications are supplied as a list of dirty coordinates. Maintaining the representation without taking a step causes no implicit primal motion.

  4. With \(Q\) external pricing, point, and edge operations, total time, excluding the client’s own work, is \[ (R+B+Q)S^{O(1/a)+o_a(1)}. \tag{49}\] The bound is on every computation path and includes initialization, arithmetic, random bits, and all representation changes. Polynomially many operations are allowed, with a loose guard \(Q\le S^{100}\).

The conditional probability of detected failure in the entire instance is at most \(2^{-\sigma}S^{-60}\), including when the instance is chosen adaptively. Deterministic size, precision, and representation checks may suspend, but none does so for all sufficiently large \(S\) at each sufficiently large fixed \(a\) and each fixed \(\sigma\ge0\) within the stated hypotheses.

The objective conclusion in part (ii) follows directly from its two inequalities once positivity is established. The sum of absolute relative argument changes is at most \(\|\lambda h\|_1\). For a positive logarithm, \(\log(1+z)\ge z-z^2\) when \(|z|\le q/100\); the sum of these squared relative changes is at most \(q^2/10000\). A negative logarithm in (44) is convex and lies above its supporting tangent. Hence the total increase is at least \(q^2/1000-q^2/10000\).

A backend with explicit forest paths

We use the following fully dynamic backend. Its construction, rather than a dynamic optimization oracle, is given in Sections 7 and 8. The minimum-ratio-cycle and hierarchical-routing viewpoint is related to the flat-forest cycle machinery of (Chen et al. 2023). We give a complete construction for the precise interface below, including its raw path length and explicit forest recourse guarantees.

Lemma 22 (Flat-cycle backend). Let \(M=2^u\), where \(u\) is divisible by \(2a^2\), and suppose \[ S^{1/a}\le M\le S^2. \tag{50}\] Consider a dynamic undirected graph with reference orientations and fixed vertex slots, of total current size at most \(M\) and maximum degree at most three. It has positive integral lengths \(\ell_e\) and integral signed gradients \(\gamma_e\), both of magnitude at most \(M^{1200a}\). Insertions and deletions are arbitrary; a price replacement is a deletion followed by an insertion. Put \[\mathop{\mathrm{OPT}}=\max_{d\ne0,\ d\text{ a circulation}} \frac{|\gamma^{\mathsf T}d|}{\|\ell d\|_1},\] with value zero if there is no nonzero circulation.

The backend exposes a forest. Every forest vertex maps to an input vertex slot; every forest edge either maps to a current input edge with specified endpoint correspondence, or is null and joins equal images. The multiplicity over each input slot is at most \(M^{10/a}/4\). A query returns a list of oriented single input edges and oriented forest paths, the latter specified by endpoints. Each piece is used with unit multiplicity, with repeated pieces listed separately. Their signed physical projection is a circulation \(c\). Its raw length \(V\) is the sum of input lengths over all mapped edge traversals with repetition; a null forest traversal contributes zero. If \(\mathop{\mathrm{OPT}}>0\), the answer has \(V>0\) and \[ \frac{|\gamma^{\mathsf T}c|}{V} \ge M^{-10/a}\mathop{\mathrm{OPT}}. \tag{51}\] An empty or missing answer is permitted when \(\mathop{\mathrm{OPT}}=0\). There are \(M^{O(1/a)+o_a(1)}\) pieces in an answer.

Initialization costs \(M^{1+O(1/a)+o_a(1)}\). Amortized edit and query costs, including explicitly exposed forest logs, are \(M^{O(1/a)+o_a(1)}\), with bounds on every path. Retained forest occurrences and edges have stable lifetimes. Endpoint or mapping changes are replacements, and removals are exposed before the old mappings or input data are invalidated. Transactions permit deletion-first old-to-final updates with no intervening path query.

The bounds apply over the polynomial lifespans required by Lemma 21, and against adaptive clients. Reliability can be chosen so that detected failure in an entire engine instance has conditional probability at most \(2^{-\sigma}S^{-60}\). A flagged state has no quality requirement. Deterministic prerequisite checks may suspend, with eventual adequacy as in Lemma 21.

Degree reduction and integer pricing

For each logical edge insertion, create a fresh port at each endpoint; a loop receives two distinct ports. The ports belonging to one logical vertex lifetime form a path, extended on further endpoint insertions. Retargeting uses fresh ports. A retired port may remain on its path, and a retired vertex may leave a wholly inactive path after all its real incidences have disappeared. Distinct vertex lifetimes have distinct paths. Mere value or price changes retain the ports. Give every path connector length one and gradient zero. The resulting graph has maximum degree three and linear lifetime size in \(R\).

Choose \(M\) by rounding \(16(R+B)\) upward to a power of two whose exponent is divisible by \(2a^2\). For fixed \(a\) this introduces only a fixed factor. Reserve, for example, \(M/2\) vertex slots; the ports and any isolated logical-vertex representatives fit these slots, and slots plus edges fit the backend bound. These counts and (50) are checked. They hold eventually by (45). Identifiers can be renumbered using dictionaries.

Summing conservation over one port path cancels its connectors and recovers conservation at the logical vertex. Conversely, a logical circulation \(d\) lifts to the port graph by routing its incidence imbalances along each path. The total absolute connector coefficient is at most \[ 2M\sum_{e\text{ real}}|d_e|. \tag{52}\] Indeed each real incidence contributes its coefficient to at most \(M\) connector positions, and each real edge has two incidences. No connector primal value needs to be stored; connectors represent conservation of increments only.

An anchor of coordinate \(e\) records its exact current value \(\widehat f_e\). Set \(H=S^{100}\) and price real edges by \[ \ell_e=\bigl\lceil H\lambda_e(\widehat f_e)\bigr\rceil, \qquad \gamma_e=\operatorname{round}\bigl(Hg_e(\widehat f_e)\bigr). \tag{53}\] The sum or difference of at most two reciprocals of positive dyadic arguments, with the allowed integer weight, can be priced by exact bounded-length integer quotient arithmetic. No logarithm is evaluated. At anchors, \[ S^{-20}\le\lambda_e(\widehat f_e)\le S^{1022}. \tag{54}\] The small post-step margins do not affect this loose upper estimate. Since \(S\le M^a\), these prices fit the backend range \(M^{1200a}\).

For the moment assume the register estimate (48). Put \(\tau=q/100\). An individual log argument \(r\) with weight \(W\ge1\) changes by a relative amount at most \(\tau/W\le\tau\) between an anchor and a pricing call. Its derivative magnitude changes by at most \(\tau/(1-\tau)\) times its anchored magnitude. Summing these errors term by term remains valid when the signed derivatives nearly cancel. Together with rounding error at most \(1/(2H)\) and \(\ell_e\ge S^{80}\), this gives, at every pricing call, \[ \left|g_e(f_e)-\frac{\gamma_e}{H}\right| \le\frac q{10}\frac{\ell_e}{H},\qquad .9\frac{\ell_e}{H}\le\lambda_e(f_e) \le1.1\frac{\ell_e}{H}. \tag{55}\] The guard \(q\ge S^{-1}\) makes the integer rounding error negligible uniformly in the coordinate. These inequalities require no scan of the exact current vector: anchors are checked locally, and the displacement invariant below supplies the intervening estimate.

If (46) holds, its lift has integer-priced ratio at least \(1/4\). To see this, the numerator in units of \(H\) is at least \[\tfrac12\|\lambda d\|_1 -\frac q{10}\sum_{e\text{ real}}\frac{\ell_e}{H}|d_e|,\] whereas (55) compares the real length sum with \(\|\lambda d\|_1\). The additional connector length in (52) is negligible, since \(2M\le2S^2\) while every real length is at least \(S^{80}\). The bound \(1/4\) follows with room to spare for sufficiently large checked sizes. We also check \[ M^{-10/a}/4>2q. \tag{56}\] It holds eventually: \(M\le S^2\), whereas \(q=2^{-100\lceil s/a\rceil}\le S^{-100/a}\).

Compute the exact gradient sum and raw length of a backend answer using the path aggregates constructed below. Accept it precisely when \[ V>0,\qquad |\gamma^{\mathsf T}c|\ge2qV; \tag{57}\] otherwise return Low. Lemma 22 and (56) show that this cannot miss (46) in an unfailed proceeding computation.

Reverse every piece of an accepted answer if necessary so that \(\gamma^{\mathsf T}c>0\), and delete its connector coordinates from the logical increment. If \(N_e\) counts every real traversal of \(e\) in its pieces, then \(|c_e|\le N_e\). In particular, (55) and (57) imply \[ g^{\mathsf T}c\ge qV/H,\qquad \sum_{e\text{ real}}\lambda_e N_e\le1.1V/H. \tag{58}\] The first bound could be strengthened to \(1.9qV/H\); the weaker version is convenient. With \(c\) now oriented for ascent, choose a positive power of two \(\mu\) with \[ q/400\le \mu V/H\le q/200. \tag{59}\] The interval has ratio two and therefore contains such a power. Applying \(\mu\) along all pieces gives \(h=\mu c\), with \[g^{\mathsf T}h\ge q^2/400, \qquad \|\lambda h\|_1\le1.1q/200.\] These are stronger than (47). Moreover, the second inequality of (58) bounds the sum of absolute partial motions during the entire list of traversals, not just its net motion. Every individual argument consequently stays within relative distance \(1.1q/200\) of its starting value, and within that distance divided by its weight for a weighted term. All arguments remain positive. The whole step is applied before repricing or servicing client changes; individual pieces need not themselves be circulations. Exact grid compatibility of \(\mu\) is verified below.

Oriented forest operations

We implement path sums, path increments, and threshold detection with link-cut trees (Sleator and Tarjan 1983). Signed path updates, separate absolute accumulation, and length-weighted threshold reporting are the dynamic-tree operations in (Chen et al. 2023, Lemma 2.7). Its flow-maintenance construction also transfers implicit forest contributions into explicit edge values when mapped edges change (Chen et al. 2023, Theorem 3.6). For our interface, the signed-value and absolute-motion invariants must survive dormant-copy reuse and repricing transactions. We give the augmentation and amortization here, then specify the copy transfers and reset ordering in Sections 5.5 and 5.6.

Subdivide each forest edge by an edge node. The other nodes are occurrence vertices, and represented roots are always occurrence vertices. On an edge node \(c\) mapping to an oriented actual edge, let \(s_c=+1\) when its represented-parent side is its tail and \(s_c=-1\) when that side is its head. A null edge may receive either reference orientation. A real-variable copy stores two different accumulations:

  • \(h_c\) is signed accumulation since its creation, in the fixed orientation of the mapped actual edge;

  • \(A_c\) is absolute accumulation since the coordinate’s most recent anchor or the copy’s creation, whichever occurred later.

After everting an occurrence vertex \(v\) and exposing the path to \(w\), the signs \(s_c\) describe traversal from \(v\) to \(w\). A path increment uses \[ h_c\leftarrow h_c+\mu s_c, \qquad A_c\leftarrow A_c+|\mu| \tag{60}\] on real-variable copies. Connector and null copies do not need these primal accumulations.

Preferred-path aggregates store the signed sum \(\sum_c s_c\gamma_c\) and the raw sum \(\sum_c\ell_c\) on mapped nonnull edges. They also store a witnessed minimum of \(\theta_c-A_c\) over enabled real copies, where the thresholds are specified in Section 5.6. Ineligible nodes are omitted from this minimum. Disabled real copies still receive the updates in (60).

Evert flips signs on edge nodes of the reversed exposed path, but does not negate their stored \(h_c\). The reversal, signed-addition, and absolute-addition tags have the constant-size algebra \[ (s_c,h_c,A_c)\longmapsto (r s_c,\ h_c+\alpha s_c,\ A_c+\beta),\qquad r\in\{-1,+1\}, \tag{61}\] together with reversal of path order when appropriate. Applying one such transformation and then another composes their signed offsets as \(\alpha_1+r_1\alpha_2\). Thus a later reroot does not retroactively negate an earlier physical increment. The gradient aggregate changes sign on reversal, the raw length does not, and the enabled minimum shifts by the absolute-addition tag with its witness retained. Tags are pushed before modifying children or point fields. Only preferred-path aggregates are combined; no virtual-subtree aggregate is used for these operations.

Access at a node does not change any represented-parent direction; it only changes which represented edges are preferred. Thus it does not alter the signs \(s_c\). Point accesses without evert recover exact individual data. To insert an edge node, link it as a new child on its tail side, initializing sign \(+1\); evert the head endpoint in its separate component and link that endpoint below the edge node. To delete an edge node, inspect its sign by point access, cut its child endpoint from its parent, and then cut the edge node from its parent. No evert at an edge node is needed. Forest transactions give deletions before invalidation and insertions into the final forest, so every link joins different components.

Lemma 23 (Augmented forest cost). On a forest of at most \(N_f\) subdivided nodes, the preceding operations use \(O(\log N_f)\) amortized tree primitives per link, cut, evert, expose, point operation, or lazy path update. Reporting and disabling a witnessed threshold violation costs the same amortized amount per reported node. Each primitive uses a constant number of operations on the stored fields.

Proof. Use auxiliary splay trees in down-path order for preferred paths (Sleator and Tarjan 1985, sec. 6). Their roots keep path-parent pointers to the represented parents of their path tops. Access at \(v\) starts at \(v\) and repeatedly splays the current node \(y\), replaces its right subtree by the path exposed so far, and advances to its previous path parent. The demoted path receives path parent \(y\). A final splay of \(v\) exposes the represented-root-to-\(v\) path. Evert reverses that path. A cut from parent accesses its endpoint and detaches the left ancestor subtree. A link accesses its endpoints and installs a path-parent pointer for a represented root. The stated tags require only bounded work during each pointer or aggregate change.

For the amortization, give a node weight one plus the sum of represented subtree sizes of its nonpreferred children. Let \(s(x)\) be its auxiliary subtree weight and \(r(x)=\log_2 s(x)\), and use \(\Phi=\sum_x r(x)\). Node weights stay fixed during a splay. The usual rotation inequalities from the access lemma (Sleator and Tarjan 1985, Lemma 1) can be recalled briefly. In a double rotation with node \(x\), parent \(y\), and grandparent \(z\), we have \(r'(x)=r(z)\) and \(r(y)\ge r(x)\). For zig-zag, \(r'(y)+r'(z)\le2r'(x)-2\) by disjoint child weights. For zig-zig, \(r(x)+r'(z)\le2r'(x)-2\) by the disjoint old-\(x\) and new-\(z\) subtrees, and \(r'(y)\le r'(x)\). Thus two rotations plus their change in potential cost at most \(3(r'(x)-r(x))\); a final single rotation adds at most one.

At a preferred-path switch, the total weight of a detached or attached auxiliary path equals the represented subtree size of the demoted or promoted child. The change in the splayed root’s own weight compensates exactly, so the switch does not change \(\Phi\). Along an upward access, the next node’s initial auxiliary weight contains the preceding exposed path through its dashed-child weight. The rank differences therefore telescope to \(O(\log N_f)\), apart from a constant per access iteration.

To pay for those iterations, call a represented edge heavy if its child’s subtree has more than half its parent’s size, and add a sufficiently scaled count of dashed heavy edges to the potential. A represented root path has at most \(\log_2 N_f\) light edges. Access promotes every dashed edge it traverses. A heavy off-path edge is demoted only opposite a light preferred child on the accessed path, or at its endpoint. The iteration count plus the change in this additional potential is consequently \(O(\log N_f)\).

It remains to check represented-tree changes. After access, write the weight of a path vertex as one plus the sizes of its off-path dashed subtrees. These weights are unchanged by reversing the complete exposed path. Recursive left-right swapping in its auxiliary splay tree preserves each auxiliary subtree’s vertex set and total weight, hence preserves \(\Phi\). If an off-path child becomes heavy after reversal, the next edge down the reversed path must be light, or the vertex is the path endpoint. There are only \(O(\log N_f)\) such events. Cutting an accessed parent edge similarly leaves the path weights unchanged, does not increase \(\Phi\), and charges newly heavy off-path children to light edges of the shortened path. Linking a root below an accessed vertex increases only that auxiliary root’s weight sum; its rank grows by at most \(\log N_f\). Existing off-path children of its ancestors cannot newly become heavy as ancestor subtrees grow, and the new dashed edge contributes at most one. All potentials are nonnegative and the initial singleton state has zero potential. This proves the bound, also under recycling of a fixed node pool.

For threshold reporting, expose the updated path, read a witnessed nonpositive minimum, access that node, and disable it at a point. Repeating these operations costs the claimed amount per actual disable. ◻

Preserving exact values under forest changes

Set \[ D=M^{10/a}. \tag{62}\] There are at most \(D\) current forest copies of any actual edge. Indeed, the two endpoint occurrence sets have at most \(D/2\) vertices together, and the copies between them form a forest. For a loop the same argument uses just one occurrence set. The constant slack in the vertex multiplicity bound is more than sufficient.

Maintain a list of current copies for each actual edge. For each real coordinate keep an explicit signed base, with invariant \[ f_e=\mathrm{base}_e+ \sum_{c\text{ a current copy of }e} h_c. \tag{63}\] An explicit single-edge traversal changes the base. Before deleting a copy, read its \(h_c\) and flush it into the base; a new copy starts with \(h_c=0\). This procedure is used also on price replacements and retargetings, with old mapped data retained until all removals have been processed. A client value setting to \(f_e^{\rm new}\) sets the base to \(f_e^{\rm new}-\sum_c h_c\). Consequently a point read, point setting, or sweep of all copies costs \(O(D)\) tree point operations, plus dictionary work.

The invariant is independent of the copy’s present role in the backend. In particular, the public-forest construction in Section 8 may reassign a dormant arm slot to a different child occurrence at the same label. Its internal mapped edges retain their lifetimes and physical images. Their \(h_c\) are therefore retained, not restarted. A previous increment need not be a circulation in the current representation of the forest: its already performed physical increment is recorded by (63). Every new step is a circulation in the current mapped graph. Thus representation maintenance alone neither changes a value nor loses an earlier increment.

Drift detection and refresh accounting

Absolute counters have a separate purpose. At an anchor of edge \(e\), reset all its current copy counters \(A_c\) and one additional bucket. This bucket stores absolute motion of explicit single-edge traversals and absolute counts flushed from copies that subsequently die. Choose a power-of-two threshold \(\theta_e\) with \[ \frac{qH}{800D\ell_e}\le\theta_e \le\frac{qH}{400D\ell_e}. \tag{64}\] The interval has ratio two, so it contains such a power. Each copy and the bucket use this same threshold. A new copy starts with \(A_c=0\) and the current threshold. Copy death transfers \(A_c\) to the bucket before removal, as well as transferring \(h_c\) to the base.

On an updated path, report every enabled copy reaching threshold using the witnessed minimum, and disable it pending service. A disabled copy continues to accumulate both signed and absolute motion. The whole circulation update is completed before any client change or reprice is serviced. An explicit traversal or a bucket flush is tested directly. Each threshold event sets a coalesced pending flag on the variable, not just on the copy. The flag survives deletion of the copy that caused it.

Service a pending variable by an exact point read and anchor/price refresh, with a notification to the client. The ordering is important: first reset every current absolute counter and the bucket, reenable all current copies, and clear the old pending flag; then process the backend replacement and its forest logs. The current register, prices, and thresholds are installed consistently with this no-motion reset. Its own old-copy removals therefore flush zero absolute mass for that coordinate. Other coordinates flagged by those logs are queued for later service, after the current transaction finishes. Explicit client settings and descriptor changes reanchor in the same way and either service or subsume an old pending flag. Stale queue entries are skipped or coalesced.

All old mapped edge data remain available through removals. New copies receive their current mapped prices and thresholds before a path can be used again. Drain pending engine flags and return the register-update list; the client can then settle its local changes before asking for another pricing step. Further notifications arising in that settlement are handled by the same rule. No motion occurs during these transactions.

Lemma 24 (Register accuracy and notification bound). After settlement, (48) holds. The total number of engine refresh reports and copy-disable events is \(O(D)\) times the number of successful pricing steps, apart from explicit client operations. The assertion is unaffected by arbitrary forest churn, dormant-slot reuse, or client reanchoring.

Proof. The signed displacement since the most recent anchor is bounded by the sum of current absolute copy counters and the bucket. This is true initially, preserved by path updates and explicit traversals, and preserved by transfer on copy death. Dormant retained copies keep their counters. At a settled state there are at most \(D\) copies and one bucket, each below threshold. Since \(\lambda_e(\widehat f_e)\le\ell_e/H\), we obtain \[\lambda_e(\widehat f_e)|f_e-\widehat f_e| \le \frac{\ell_e}{H}(D+1)\theta_e \le \frac{q(D+1)}{400D}\le q/200.\] This is stronger than (48).

For the cost bound, partition raw real-edge motion into intervals between successive anchors of each coordinate. Its price is fixed on such an interval. An engine report consumes at least \[\theta_e\ell_e/H\ge q/(800D)\] of that interval’s raw priced absolute motion. The consumed intervals are disjoint for one coordinate, and different coordinates consume different real-edge motion. Absolute mass moves between copies and the bucket without duplication. It is reset before price-replacement logs, so it cannot be charged again at a new price. A client anchor may discard old drift credit, but does not create new credit and is charged as an external operation.

Similarly, a disable event can charge the motion of its copy since creation or its latest reset. A copy is disabled at most once during that uninterrupted accumulation; reenabling at an anchor resets it. These disable charges are disjoint within this second charge category. Counting both report charges and disable charges only introduces a constant factor. By (59), a successful step creates at most \(q/200\) of raw priced absolute motion. The claimed \(O(D)\) amortized bound follows.

Finally, a no-motion refresh cascade is finite. A queued variable can consume its extant absolute mass once; after its reset, all its absolute locations are zero and remain so until new motion. Its own replacement logs cannot recreate that mass. Other variables may expose previously accumulated mass, but their reports have the disjoint charges just described. Pending flags are per variable, so copy deletion loses neither the required service nor its charge. This proves both termination of settlement and the notification bound. ◻

Precision, total cost, and completion of the engine proof

All quantities maintained by the engine can be represented exactly with charged multiword arithmetic. The only nonintegral input values and increments are on the common dyadic grid. In particular, a positive logarithmic argument is \(k\varepsilon\) for an integer \(k>0\) with \(O(\log S)\) bits, since the argument is at most \(S^{20}\) and \(\varepsilon=S^{-3000}\). Its reciprocal is \(\varepsilon^{-1}/k\). Each price uses at most two such fractions and one bounded integer weight, so their products, sums, and quotient operations still have \(O(\log S)\)-bit inputs and outputs. Rational comparisons of scores and scale choices likewise use bounded integer operations. No barrier value or ideal real-valued probability is evaluated.

For a concrete precision check, bound the total number of traversals in a returned representation by \(S^{20}\), using its piece count and the forest-size bound rather than expanding its paths. The check holds eventually for each fixed sufficiently large parameter, since \(M\le S^2\) and both piece count and occurrence overhead have exponent \(O(1/a)+o_a(1)\). Equations (54) and (53) then give the loose bound \[ V/H\le S^{1044},\qquad |\mu|\ge S^{-1045}/400. \tag{65}\] For sufficiently large checked \(S\), a power of two satisfying (59) is therefore an exact multiple of \(\varepsilon=S^{-3000}\). Each piece has integral signed traversal coefficients, so every partial update and the final circulation are on the grid, with no rounding of conservation equations. Thresholds fit the same grid: even using \(D\le S^2\) gives \(\theta_e\ge S^{-1025}/800\). These inequalities can be checked directly, with suspension at inadequate sizes.

Since \(V\ge1\), step scalars have magnitude at most \(S^{100}\). There are at most \(Q\le S^{100}\) successful steps, each with at most \(S^{20}\) raw traversals. Thus their total unweighted absolute motion is at most \(S^{220}\). Transfers between copies and bases or buckets do not duplicate this motion; anchors reset absolute counters, and client settings replace values of magnitude at most \(S^{20}\). Signed and absolute tags, sums, buckets, and bases consequently have polynomial magnitudes on the common grid. Their bit lengths are \(O_a(\log S)\) at fixed parameters; exact pricing and key operations add only logarithmic-power overhead. The backend separately accounts for its arithmetic and imported static flow calls. As in the final uniform implementation, larger logical integers in branches with varying parameters are digit arrays whose construction and operations are incrementally charged, not atomic unbounded-word instructions.

Reserve and recycle a pool of \(N_f=O(M^{1+10/a})\) subdivided forest nodes. The initial pool and forest cost fit \(M^{1+O(1/a)+o_a(1)}\). By Lemma 23, processing each exposed forest-log record or path piece costs logarithmic amortized overhead. A coordinate read, reset, or price change uses at most \(O(D)\) point operations for its copy list. By Lemma 24, successful steps cause only \(O(D)\) reports and disables amortized. Repricing those reported coordinates invokes only that many backend replacements, each with the cost and explicit recourse of Lemma 22. Thus the total of backend work, forest-log processing, path operations, copy sweeps, and threshold service is \[(M+Q)M^{O(1/a)+o_a(1)}.\] Using the chosen rounding of \(M\) and (45) gives (49). A Low attempt uses the same query overhead and no scan of all coordinates. Explicit changes and all notifications are settled before the next query; no hidden full-vector update is involved.

Proof of Lemma 21. Apply Lemma 22 to the port graph with prices (53). Exact point operations and forest updates preserve (63). The refresh procedure proves (48), and therefore validates (55) before each pricing call. The lifting, quality margin, and acceptance test prove part (i). Equations (58)–(59), the partial-motion bound, and the grid check prove part (ii), including exact circulation and positivity. The objective estimate following the statement proves its progress assertion. Lemma 24 proves part (iii), and the preceding cost accounting proves part (iv).

Backend failures are explicitly propagated. Its conditional instance budget gives \(2^{-\sigma}S^{-60}\) for the whole engine instance; the remaining operations are deterministic. Representation, anchor-range, and precision prerequisites are checked on their local inputs, with counts maintained incrementally. During a whole step, the already proved raw-motion bound supplies the required relative margins; no global scan is needed to certify them. The operation and record guards are applied to prefixes, and the preceding bounds hold through a putative first guard-reaching operation. For each sufficiently large fixed \(a\) and each fixed \(\sigma\ge0\), these bounds fall below the loose polynomial caps, and the size and quality inequalities hold for all sufficiently large \(S\). Together with the backend’s guarded-prefix guarantees in Section 8, this proves eventual adequacy on every path, independently of whether a stochastic routine ultimately reports failure. Such a failure is propagated promptly and is never reclassified as a deterministic suspension. ◻

A resource-preserving static routing sparsifier

The recursive cycle construction needs a sparse set of representative edges through which all edges of an auxiliary graph can be routed. Its application also associates resources with the original edges: using a representative edge consumes its resources in either direction, and repeated traversals must be charged repeatedly. We prove a static sparsification statement that preserves these loads. The support bound depends on the number of incident labels, even when the number of demands is much larger.

Throughout this section, let \(S=2^s\ge2\) and \(M=2^u\) satisfy \(S^{1/a}\le M\le S^2\), where \(a\ge10\) is an integer and \(u>0\) is divisible by \(2a^2\). Define \[ b=M^{1/a^2},\qquad \Lambda=2^{1000a^2}(1+u),\qquad P_*=\Lambda^{400a^2}, \qquad P_*\le b. \tag{66}\] The last inequality is a checked prerequisite. In particular, \(b\) is an integer. Powers of \(\Lambda\) are logarithmic powers of the master size for each fixed \(a\).

Lemma 25 (Static routing with resources). Let \(F\) be an explicitly listed undirected multigraph with identified edge copies, and let \(v_+\) be its number of incident vertex labels. For each edge \(f\), an explicit list specifies resource counts \(m_{xf}\in\{1,\ldots,10\}\); unlisted counts are zero. Suppose that \[\deg_F(v)\le C_0,\qquad \sum_{f\in F}m_{xf}\le C_0 \quad\text{for every vertex $v$ and resource $x$},\] where \(C_0\ge1\) is an integer, degrees count incidences, and both \(2|F|\) and the total resource-list length are at most \(M^2\). Assume that identifiers and numerical input data have \((\log S)^{O_a(1)}\) bits. There is a uniform randomized algorithm that either reports detected failure or returns a set \(F'\subseteq F\) and, for every \(f=vw\in F\), an explicit walk \(Q_f\) from \(v\) to \(w\) in \(F'\), such that \[ |F'|\le P_*v_+,\qquad |Q_f|\le P_*,\qquad \sum_{f\in F}\ \sum_{e\text{ traversed in }Q_f}m_{xe} \le P_*C_0\quad\text{for every $x$}. \tag{67}\] The inner sum counts traversal multiplicities and ignores orientation. A loop demand may be assigned the empty walk. Every returned result is verified. For each fixed reliability parameter \(\sigma\ge0\), the time on every computation path is \[ (1+\text{input list size})\,b\,M^{o_a(1)}, \tag{68}\] and the probability of detected failure is at most \(2^{-\sigma}S^{-800}\). The same statement holds conditionally when the input is determined by earlier computation and this call uses fresh bits.

We build the representative set in stages, each routing at least half the remaining demands. An attempt samples edges according to their endpoint degrees and gives each retained edge its inverse-probability weight. Introduce a distinct stub for each edge incidence. A flow between two equal halves of the stubs either identifies a cut for recursion or pairs almost all the stubs by walks in the sampled graph. The sampling estimate ensures that the cuts are sparse with high probability. We defer their crossing demands to later stages and recurse on the two induced pieces with fresh samples. When almost all stubs are paired, completing the unpaired stubs with temporary fake edges gives a perfect matching on the stubs. A piece that completes the prescribed rounds without a further cut obtains an overlay made from these matchings; the median rule makes it expanding with high probability. Each nonfake overlay edge stores its sampled-edge walk.

We then route the demand stub pairs through this overlay. Partition its labels into groups and send each demand packet on a short random walk conditioned to reach its target’s group. Additional independent walks, starting and ending within each group, supply the edges of smaller graphs on which routing continues. Recursion reaches singleton groups, and substituting the saved walks gives overlay routes with bounded length and congestion. Discard routes that touch a fake edge or an excessively long real embedding. Congestion bounds the number of lost demands, while the sampling weights bound resource use after the remaining real embeddings are substituted. The final verification checks these walks and their loads before accepting the stage.

We use the following parameters: \[ \begin{gathered} r_m=\Lambda^3,\qquad K_s=\Lambda^{230a^2},\qquad \chi=\Lambda^{110a^2},\qquad \epsilon_0=\Lambda^{-100a^2},\\ D_p=\Lambda^{105a^2},\qquad H_*=\Lambda^{70a^2},\qquad L_0=\Lambda^{200a^2},\qquad t=\Lambda^{10},\qquad q_v=\Lambda^3. \end{gathered} \tag{69}\] Here \(r_m\) counts overlay rounds, \(K_s\) sets the sampling scale, \(\chi\) multiplies flow capacities, \(\epsilon_0\) sets the deficiency scale, and \(D_p\) caps the cut depth. The bounds \(H_*\) and \(L_0\) control overlay routes and retained real embeddings; \(t\) is half the conditioned-walk length. The symbol \(q_v\) prescribes the same number of virtual jobs at each vertex, a fixed logarithmic power for fixed \(a\); it is unrelated to the accuracy parameter \(q\) of the priced engine.

For an unweighted multigraph \(H\), let \(\partial_H X\) be the edge-copy cut of \(X\), and let \(\mathop{\mathrm{vol}}_H(X)=\sum_{v\in X}\deg_H(v)\). Parallel copies count separately; loops count twice in volume and never in a cut. We suppress the subscript when the graph is clear. Its graph conductance, whenever there is a cut with two positive volumes, is \[\phi(H)=\min_{\substack{X\subseteq V(H)\\ \mathop{\mathrm{vol}}_H(X)>0,\ \mathop{\mathrm{vol}}_H(V(H)\setminus X)>0}} \frac{|\partial_H X|} {\min\{\mathop{\mathrm{vol}}_H(X),\mathop{\mathrm{vol}}_H(V(H)\setminus X)\}}.\] We invoke conductance and mixing only on nonsingleton active walk graphs; singleton router instances are settled directly. For the lazy walk used below, the corresponding transition-kernel conductance is \(\phi(H)/2\).

Stages, pieces, and normalized sampling

Remove loop demands by giving them empty walks. Work on the remaining demands in stages. One successful stage must route at least half its input demands while using, separately for support, maximum hop count, and resource load, the bound \(\Lambda^{350a^2}\) in place of \(P_*\) in (67). All these properties, including endpoints and selected edge identities, are checked before a stage is accepted. A stage gets at most \[ T=2(\sigma+1000\log_2 S+10) \tag{70}\] fresh attempts (rounding up if necessary). Exhausting these attempts is a detected failure. There are at most \(2u+2\) successful stages, since \(|F|\le M^2/2\) and the residual number halves each time.

An attempt processes a recursive family of pieces. A piece is an edge list induced by the vertex side of each earlier cut; cut edges are discarded for this stage. Empty lists need no further processing. Pieces at a fixed depth have disjoint edge lists and disjoint incident vertex sets. Process pieces by depth, using fresh samples at each depth. We cap the recursion depth at \(D_p\) and abort an attempt that would exceed the cap. Let a nonempty piece have \(s'\) edges and positive original degrees \(d_i\). Retain each edge \(e=ij\) independently with probability \(p_e\), the smallest number of the form \(2^{-j}\), \(j\ge0\), that is at least \[\min\{1,K_s(1/d_i+1/d_j)\}.\] A retained edge receives integral weight \(\omega_e=1/p_e\). Before any flow call, require \[ \sum_{e\text{ retained}}\omega_e\le2s',\qquad \#\{e\text{ retained}\}\le4K_s\#\{i:d_i>0\}. \tag{71}\] A violation aborts this attempt. Volumes in the following sampling estimate use the original piece degrees, not the sampled degrees.

Lemma 26 (Simultaneous cuts and resource loads). For a sampled piece, except with probability at most \(4s'\exp[-K_s/(100\chi^2)]\), every vertex set \(X\) satisfies \[ \left|\sum_{e\in\partial X\text{ retained}}\omega_e -|\partial X|\right| \le\frac{\min\{\mathop{\mathrm{vol}}(X),\mathop{\mathrm{vol}}(\bar X)\}}{4\chi}. \tag{72}\] This event implies the total-weight check in (71); the sample-count check also holds with exponentially high probability in \(K_s\). Conditional on the input pieces at a fixed depth, for each resource \(x\), \[ \sum_{\substack{e\text{ retained}\\\text{over all pieces at this depth}}} \omega_e m_{xe}\le2C_0 \tag{73}\] except with probability at most \(\exp(-K_s/30)\).

Proof. Put \(D=\operatorname{diag}(d_i)\) on the nonisolates and set \[A_e=D^{-1/2}(\mathbf e_i-\mathbf e_j) (\mathbf e_i-\mathbf e_j)^\mathsf{T}D^{-1/2}.\] These are positive semidefinite rank-one matrices, \(\|A_e\|=1/d_i+1/d_j\), and \(\|\sum_e A_e\|\le2\). If \(\xi_e\) is the retention indicator, write \(Y_e=(\xi_e/p_e-1)A_e\). Deterministically retained edges contribute zero. For any other edge, \(p_e\ge K_s\|A_e\|\), whence \[\|Y_e\|\le K_s^{-1},\qquad \mathbb EY_e^2=(p_e^{-1}-1)A_e^2\preceq K_s^{-1}A_e.\] Thus the variance parameter is at most \(2/K_s\). The self-adjoint matrix Bernstein inequality, applied to both signs (Tropp 2012, Theorem 6.1(ii)), states that independent centered self-adjoint summands of norm at most \(J\) and total variance norm at most \(V\) satisfy \[\Pr\left\{\left\|\sum_eY_e\right\|\ge z\right\} \le2d\exp\left(-\frac{z^2}{2(V+Jz/3)}\right),\] where \(d\) is the matrix dimension. Here \(d\le2s'\), \(J=1/K_s\), \(V\le2/K_s\), and \(z=1/(4\chi)\) give the claimed probability bound. Testing the quadratic form on \(D^{1/2}\mathbf1_X\) gives additive error \(\mathop{\mathrm{vol}}(X)/(4\chi)\) in its cut weight. Testing the complement gives the other bound in (72). Testing a singleton bounds its weighted degree by \((1+1/(4\chi))d_i\); summing degrees proves the weight check.

Dyadic rounding gives \[\sum_ep_e\le2K_s\sum_{ij=e}(1/d_i+1/d_j) =2K_s\#\{i:d_i>0\}.\] The scalar Chernoff inequality therefore proves the sample-count check with failure at most \(\exp(-K_s\#\{i:d_i>0\}/3)\). If the threshold exceeds the total possible edge count, failure is of course impossible.

For (73), condition on all lists at this depth. They are disjoint sublists of the input, so the expected sum for \(x\) is at most \(C_0\). A nondeterministic summand \(\xi_e m_{xe}/p_e\) lies in \([0,10C_0/K_s]\), since piece degrees are at most \(C_0\) and \(1/p_e\le C_0/K_s\). After centering, its absolute value has the same upper bound. The sum of variances is at most \(10C_0^2/K_s\), by bounding each second moment by its maximum value times its expectation. Deterministic summands have zero variance. For an upward deviation of \(C_0\), scalar Bernstein consequently gives \[\Pr\{\text{sum}>2C_0\} \le\exp\left( -\frac{C_0^2}{2(10C_0^2/K_s+10C_0^2/(3K_s))}\right) \le e^{-K_s/30}.\] ◻

Median cuts, integral matchings, and an expanding overlay

The median-projection and matching-overlay argument has the cut-matching structure of Khandekar, Rao, and Vazirani (Khandekar et al. 2006, secs. 3–3.2). Here the vertices are incidence stubs, and the proof separately controls deficient flow, resource loads, exact arithmetic and rejected attempts.

The piece has \(N_p=2s'\) distinct stubs, one for each original edge incidence. Run at most \(r_m\) rounds. Each round divides the stubs into two equal halves by the median rule below. Let \(l_i\) and \(h_i\) count the left and right stubs at original vertex \(i\). Form a flow network with source capacities \(l_i\), sink capacities \(h_i\), and both directed arcs of capacity \(\chi\omega_e\) for every retained edge. Its maximum possible value is \(s'\).

If its deficiency \(\Delta=s'-\text{maximum value}\) is at least \(\epsilon_0s'\), use the source-side vertex set \(X\) of a minimum cut to split the piece and stop its rounds. The cut formula gives \[ l(X)-h(X)=\Delta+ \chi\sum_{e\in\partial X\text{ retained}}\omega_e. \tag{74}\] The left side equals \(h(\bar X)-l(\bar X)\), so both original volumes are at least \(\epsilon_0s'\). It is also at most their minimum. On the event in Lemma 26, \[ |\partial X|\le \frac{2\min\{\mathop{\mathrm{vol}}(X),\mathop{\mathrm{vol}}(\bar X)\}}{\chi}. \tag{75}\] Indeed the weighted cut is at most the minimum volume divided by \(\chi\), and (72) adds only one quarter of that upper bound. Each child’s internal edge count is at most \((1-\epsilon_0/2)s'\), because the opposite side has volume at least \(\epsilon_0s'\). A nonempty branch thus has fewer than \(1+4\epsilon_0^{-1}\log M<D_p\) cuts. Moreover, \(\min\{\mathop{\mathrm{vol}}(X),\mathop{\mathrm{vol}}(\bar X)\}\le s'\), so summing (75) at each depth shows that all cuts discard at most the fraction \[ 2(D_p+1)/\chi \tag{76}\] of the attempt’s initial demands. This charging uses disjoint piece lists at each depth; it does not charge each demand to every piece at that depth.

When \(\Delta<\epsilon_0s'\), extract the integral flow as individually identified pairs of left and right stubs with explicit sampled-edge walks. Pair all remaining left and right stubs arbitrarily and call these pairs fake edges. The result is a perfect matching on the stubs. Save this matching as one round of an overlay multigraph, together with real embeddings for its nonfake edges.

These flow calls use Lemma 2. Padding to \(O((\chi+1)s')\) makes every capacity polynomially bounded in the padded size, since the weight check precedes the call. Alternatively, Lemma 4 provides the same fixed primitive when local capacities are presented with longer bit strings. Path extraction has a separate explicit charge: pair incoming and outgoing flow units at each internal vertex, also pairing the units on source and sink arcs with their stubs, and trace from source units. Remaining cycles are omitted. There are at most \[ 2\chi\sum_{e\text{ retained}}\omega_e\le4\chi s' \tag{77}\] interior arc units per round. Lists of these units and their pairings therefore construct all paths within this bound plus the network size. No uncharged expansion of a large integral flow is used.

Lemma 27 (The median rule). There is a median rule for which a piece completing all \(r_m\) rounds has an \(r_m\)-regular overlay of conductance at least \(1/(4r_m)\), except with probability at most \(4N_p^3\exp[-r_m/(80\Lambda)]\). This guarantee holds for any choice of perfect matching across each chosen pair of halves.

Proof. For a perfect matching let \(A\) be its averaging matrix: on each matched pair it replaces two coordinates by their average. Start with \(P_0=I\), and after round \(j\) set \(P_j=A_jP_{j-1}\). All these matrices are doubly stochastic. At a round’s start, let \(p_i\) be the current rows and choose a fresh vector \(\zeta\) of independent uniform signs. Order the real numbers \(\langle p_i,\zeta\rangle\), resolving ties by stub identifiers, and put their first and last halves on opposite sides.

Write \(\bar p=\mathbf1/N_p\) and \(\Psi=\sum_i\|p_i-\bar p\|_2^2\). The centered projected values \(z_i=\langle p_i-\bar p,\zeta\rangle\) have mean zero over \(i\), and \(Z=\sum_i z_i^2\) satisfies \[\mathbb EZ=\Psi,\qquad \mathbb EZ^2\le3\Psi^2.\] For the second inequality, sign fourth moments give \(\mathbb E\langle v,\zeta\rangle^4\le3\|v\|_2^4\), and Cauchy–Schwarz bounds each cross term in \(Z^2\). The elementary second-moment inequality now gives \(\Pr\{Z\ge\Psi/2\}\ge1/12\) when \(\Psi>0\). The sign tail estimate and a union bound over pairs give, with failure probability at most \(2N_p^2e^{-\Lambda/2}\), \[|\langle p_i-p_j,\zeta\rangle|^2 \le\Lambda\|p_i-p_j\|_2^2\quad\text{for every $i,j$}.\]

Choose a median value \(m\) between the two central centered values \(z_i\). For any pair \(i,j\) on opposite sides, \((z_i-z_j)^2\ge(z_i-m)^2+(z_j-m)^2\). Every stub occurs once in the matching, and the sum of squared distances is minimized at the mean, zero. Consequently, for every matching across the halves, \[\sum_{ij\text{ matched}}(z_i-z_j)^2 \ge\sum_i(z_i-m)^2\ge Z.\] The exact decrease under averaging is \(\frac12\sum_{ij\text{ matched}}\|p_i-p_j\|_2^2\). With conditional probability at least \(1/20\) this is at least \(\Psi/(4\Lambda)\). The parameter values ensure that the sign-tail failure above is smaller than \(1/12-1/20\). If \(\Psi=0\), it stays zero. Thus \[\mathbb E\Psi_{r_m}\le N_p\exp[-r_m/(80\Lambda)].\] For this analysis one can continue with arbitrary median-crossing matchings after a piece has been cut; hence stopping a piece early does not introduce conditioning on successful mixing. If any entry of \(P_{r_m}\) is below \(1/(2N_p)\), then \(\Psi_{r_m}\ge1/(4N_p^2)\). Markov’s inequality proves the stated failure bound.

It remains to identify which walk this product describes. Since \(P_{r_m}=A_{r_m}\cdots A_1\), its row starting at stub \(i\) describes averaging steps in the order \(r_m,r_m-1,\ldots,1\). Place one unit of mass at each stub of a set \(X\) with \(|X|\le N_p/2\), and run this reverse-round walk. All product entries are at least \(1/(2N_p)\), so at least \(|X|/4\) mass finishes outside \(X\). At every intermediate time, double stochasticity keeps mass at each stub at most one. A matching edge \(\{v,w\}\) at its single time step therefore carries total mass at most \((\mathrm{mass}(v)+\mathrm{mass}(w))/2\le1\). The number of overlay edge occurrences crossing \(X\) is at least \(|X|/4\). Every stub has degree \(r_m\), which proves conductance at least \(1/(4r_m)\).

The construction does not form dense products. To compute a new projection, apply the earlier matching averages in chronological order to \(\zeta\), obtaining \(P_{j-1}\zeta\). This costs \(O(jN_p)\) elementary averages. All entries are dyadic rationals with denominator dividing \(2^{j-1}\), so exact projections, comparisons, and tie resolution use \(O(r_m+\log N_p)\) bits per number. ◻

Recursive routing on the overlay

The next task is to route the demand stub pairs through the overlay with bounded path length and total traversal load on each overlay edge. These two bounds will control the lengths and resource loads after the saved real-edge embeddings are substituted.

The recursive sampling of virtual graphs uses the group-conditioned walk construction of Ghaffari, Kuhn, and Su (Ghaffari et al. 2017, sec. 3.1.2). We give the packet routing, congestion, and finite-computation estimates needed here, allowing nonuniform stationary distributions and counting repeated traversals.

Pair the original stubs according to their demand edges and orient each pair arbitrarily, giving one packet with a specified source and final target. Both the initial sources and the final targets are distinct. At recursive calls current packet locations can coincide, but their final targets remain distinct. We route these packets through the overlay, temporarily permitting fake edges.

Every walk graph retains distinct parallel edge copies and uses incidence degrees: a loop has two incidences. Its lazy walk holds with probability \(1/2\) and otherwise chooses a uniform incident edge occurrence. Holding incurs no traversal. An actual traversal of a loop is counted once. The stationary law is \(\pi(v)=d_v/\sum_i d_i\).

We use the standard conductance-to-mixing argument for reversible chains (Sinclair and Jerrum 1989, sec. 3). The proof fixes our graph conductance normalization and allows loops and parallel copies.

Lemma 28 (Mixing from conductance). In a connected undirected multigraph with at least two vertices and conductance \(\phi\), the lazy walk has spectral gap at least \(\phi^2/4\). If the graph has at most \(N_p\) labels, degree ratio at most \(D_c\le\Lambda^{1/20}\), and conductance at least \(1/(4r_m)\), then for every \(h\ge t\), \[ \tfrac12\pi(y)\le P^h(x,y)\le2\pi(y) \quad\text{for all $x,y$}. \tag{78}\]

Proof. For any nonnegative vector \(f\) supported on at most half the degree volume, integrate the boundary sizes of the sets \(\{i:f_i^2>\xi\}\) over \(\xi\ge0\). The result is \[\sum_{ij\text{ edge}}|f_i^2-f_j^2| \ge\phi\sum_i d_i f_i^2.\] Cauchy–Schwarz, together with \(\sum_{ij\text{ edge}}(f_i+f_j)^2\le2\sum_i d_i f_i^2\), implies \[ \sum_{ij\text{ edge}}(f_i-f_j)^2 \ge\frac{\phi^2}{2}\sum_i d_i f_i^2. \tag{79}\] Both formulas remain valid with loops counted by their two incidences. For an arbitrary vector \(g\) of degree-weighted mean zero, subtract a degree-weighted median \(m\). The positive and negative parts of \(g-m\) each have support volume at most half. Their edge energies sum to at most that of \(g\), while their squared degree norms sum to \[\sum_i d_i(g_i-m)^2 =\sum_i d_i g_i^2+m^2\sum_i d_i \ge\sum_i d_i g_i^2.\] Applying (79) to both parts proves the nonlazy gap bound \(\phi^2/2\), by the Dirichlet quotient, and hence the lazy bound \(\phi^2/4\). The nonlazy reversible operator is a contraction in the degree-weighted Euclidean norm; making it lazy places its spectrum in \([0,1]\).

Apply its contraction on mean-zero densities to a point mass. Cauchy–Schwarz in the stationary inner product gives relative pointwise error at most \[\frac{\exp(-h\phi^2/4)}{\sqrt{\pi(x)\pi(y)}} \le\frac{\exp(-h\phi^2/4)}{\pi_{\min}}.\] Here \(\pi_{\min}\ge1/(D_cN_p)\), \(N_p\le M^2\), \(\phi\ge1/(4\Lambda^3)\), and \(t=\Lambda^{10}\). Thus the last expression is at most \(D_cM^2\exp(-\Lambda^4/64)<1/2\), establishing (78). ◻

The grouped recursive virtual-job construction below adapts the hierarchical routing idea of Ghaffari–Kuhn–Su (Ghaffari et al. 2017, Lemma 3.1). The sequential implementation, explicit caps, and resource-load bounds needed here are established below; we do not invoke their distributed-algorithm guarantee.

At a recursive call, a singleton label requires no routing. Otherwise divide its label set into \(\min\{b,\#\text{labels}\}\) groups whose sizes differ by at most one. For each packet, draw a \(2t\)-step walk from its present location conditioned to end in the group of its final target. For each vertex, also draw \(q_v\) independent virtual jobs with the same length, starting there and conditioned to end in that vertex’s own group. The start and endpoint of every virtual job form one undirected edge within its group; its sampled walk is saved as the edge’s embedding. Recurse on each of these virtual graphs with the packets at their newly sampled locations and with their original final targets. On return, substitute the stored embeddings, reversing them when necessary.

All job draws within a call are independent conditional on its input. A conditioned draw is implemented by ordinary walk trials, retaining the first whose endpoint is in the prescribed group. Allow at most \(\Lambda^3b\) trials per job. For a uniform choice among \(d\) incident occurrences, draw \(\lceil\log_2 d\rceil\) fair bits and reject indices outside \(\{0,\ldots,d-1\}\); allow at most \(\Lambda\) strings for one such choice. A cap violation aborts the attempt. These rules also cover graphs with loops and parallel edges. The group-size rule makes the maximum size after \(j\) recursive levels at most \(\lceil N_p/b^j\rceil\). Hence the recursion depth is at most \(2a^2\), since \(b^{2a^2}=M^2\ge N_p\).

Lemma 29 (Packet and virtual-job router). Conditional on an overlay satisfying Lemma 27, the procedure above produces explicit routes for all packets with both maximum length and total traversal congestion on each overlay edge at most \(H_*\), except with probability less than \(M^{-20}\) throughout the checked parameter range. All its data sizes and running times remain bounded by the specified caps even when it fails.

Proof. First analyze ideal unlimited rejection, stopping the analysis when an inductive good event fails. The current degree ratio is allowed to be \(D_c\), initially one, with the allowance multiplied by \(20\) per level. Thus \[D_c\le20^{2a^2}\le\Lambda^{1/20}.\] The inductive conductance lower bound is \(1/(4r_m)\), so Lemma 28 applies. A group \(A\) occupies at least \(1/(2b)\) of the labels; hence \(\pi(A)\ge1/(2D_cb)\). An ordinary trial succeeds with probability at least \(1/(4D_cb)\). The endpoint of a successful trial has distribution \[ \frac{P^{2t}(x,y)}{P^{2t}(x,A)} \in\left[\frac14,4\right]\frac{\pi(y)}{\pi(A)} \subseteq\left[\frac{1}{4D_c|A|},\frac{4D_c}{|A|}\right] \quad(y\in A). \tag{80}\] Distinct final targets mean that at most \(|A|\) packets seek group \(A\). Packet arrivals at a specified child vertex therefore have expectation at most \(4D_c\), and Chernoff gives an upper bound of \(q_v\) with overwhelmingly high probability.

Exactly \(q_v|A|\) virtual jobs seek \(A\). If \(I_z\) counts their endpoints at \(z\), then \(\mathbb EI_z\le4D_cq_v\), and with overwhelmingly high probability \(I_z\le16D_cq_v\) for every \(z\). The child degree is exactly \(q_v+I_z\): a loop contributes its start incidence and its endpoint incidence. Thus every child degree is at least \(q_v\) and at most \((1+16D_c)q_v\le20D_cq_v\), proving the claimed next degree ratio.

For any \(X\subseteq A\) with \(1\le|X|\le|A|/2\), each of the \(q_v|X|\) jobs starting in \(X\) leaves \(X\) with probability at least \(1/(8D_c)\), by (80). Chernoff yields \[ \#\{\text{virtual edges from $X$ to $A\setminus X$}\} \ge\frac{q_v|X|}{16D_c} \tag{81}\] except with probability \(\exp[-q_v|X|/(64D_c)]\). Union over subsets by cardinality costs at most \[ \sum_{j\ge1}N_p^j\exp[-q_vj/(64D_c)]. \tag{82}\] For an arbitrary nontrivial cut, apply (81) to its smaller-cardinality side \(X\). Its volume is at most \(20D_cq_v|X|\), and the smaller of the two cut volumes is no larger than this volume. The child conductance is at least \(1/(320D_c^2)\ge1/(4r_m)\). This establishes all graph and packet invariants required at the next level.

We next bound loads of all jobs at one call before any substitution. Set \(Q=q_v\), \(W=\sum_i d_i\), and \(d_{\min}=\min_i d_i\). At most \(2Q\) jobs start at any vertex: there are \(Q\) virtual jobs and at most \(Q\) packets. At most \((Q+1)|A|\) jobs seek group \(A\), because virtual jobs seeking \(A\) start in \(A\) and packet final targets are distinct.

Fix an oriented incidence \(x\to y\) of a specific edge. At a step in the first half, the remaining suffix has at least \(t\) steps. Conditioning a job on its prescribed terminal group increases the probability of this step’s traversal by at most four: the ratio is \(P^h(y,A)/P^{2t}(z,A)\le2\pi(A)/(\pi(A)/2)\), where \(z\) is its start. Without conditioning, the aggregate initial density is at most \(2Q/\pi_{\min}=2QW/d_{\min}\) relative to stationarity, and the walk preserves this bound. A stationary lazy step traverses this oriented incidence with probability \(1/(2W)\). Therefore its expected conditioned load is at most \[ 4\frac{2QW}{d_{\min}}\frac{1}{2W} =\frac{4Q}{d_{\min}}. \tag{83}\]

In a second-half step, the prefix before traversal has at least \(t\) steps. If \(j\) steps remain after this incidence and the job seeks \(A\), its conditional traversal probability is at most \[\frac{2\pi(x)\,(1/(2d_x))\,P^j(y,A)}{\pi(A)/2} =\frac{2P^j(y,A)}{W\pi(A)}.\] Summing first over jobs seeking \(A\) and then over the groups gives \[\begin{align*} \sum_A\frac{2(Q+1)|A|}{W\pi(A)}P^j(y,A) &=2(Q+1)\sum_A\frac{|A|}{\mathop{\mathrm{vol}}(A)}P^j(y,A)\\ &\le\frac{2(Q+1)}{d_{\min}}\sum_AP^j(y,A) =\frac{2(Q+1)}{d_{\min}}. \tag{84}\end{align*}\] This calculation cancels the rare-conditioning factor group by group. In particular it introduces neither the number of groups nor a degree-ratio factor. Adding the two oriented incidences of an undirected edge, including the two incidences of a loop, bounds its expected traversal load at any step by \(24q_v\).

Conditional on the current input, different job streams are independent. At one fixed step, their indicators of traversing a fixed edge are independent Bernoulli variables, regardless of dependence between different steps of one job. Scalar Chernoff therefore bounds this load by \(\Lambda^6\) with overwhelmingly high probability. A union bound over edges and the \(2t\) steps gives total local edge load at most \(2t\Lambda^6\le\Lambda^{20}\); local walk lengths are also at most \(\Lambda^{20}\).

To account for substitution, suppose child routes have maximum length and edge congestion at most \(Z\). Initial packet walks contribute at most \(\Lambda^{20}\) load to a current edge. Each virtual-edge embedding is then used at most \(Z\) times, so all substitutions contribute at most \(Z\Lambda^{20}\). The same inequality bounds length, using the maximum embedding length. Starting at the leaves and iterating through at most \(2a^2\) levels bounds both quantities by \[ (2\Lambda^{20})^{2a^2}\le H_*. \tag{85}\]

For completeness, all probabilities just used are conditional on the actual recursively specified input. Endpoints chosen at a parent create the child graph, but at that child the new streams are fresh independent streams. This permits induction up to the first failed graph or packet invariant and avoids any assumption that the child graph is independent of its packet starts. The group-cut bound (82), all point-count and edge-step tails, and all cap tails are below \(M^{-40}\) with the parameters above. A conditioned job exceeds its trial cap with probability at most \[\left(1-\frac{1}{4D_cb}\right)^{\Lambda^3b} \le\exp[-\Lambda^3/(4D_c)],\] and a uniform incidence choice exceeds its string cap with probability at most \(2^{-\Lambda}\). Coupling capped rejection to unlimited rejection until the first cap justifies using these bounds with the ideal distributions.

Finally, the bounds on the work of a failed run require no good event. Packets never duplicate; each vertex always initiates exactly \(q_v\) virtual jobs; group sizes shrink by the fixed rule; and every trial and walk has an explicit cap. Across a router depth there are at most \(q_vN_p\) virtual edges, with incidence arrays for constant-index access. Even if congestion bounds fail, let \(\mathcal L_h\) bound a fully expanded packet route with \(h\) levels remaining. The newly sampled prefix has at most \(2t\) steps, and each edge of the child route expands to at most \(2t\) steps. Thus \[\mathcal L_0=0,\qquad \mathcal L_h\le2t(1+\mathcal L_{h-1}) \le\sum_{j=1}^{h}(2t)^j\le2(2t)^h \qquad(1\le h\le2a^2).\] This is a fixed power of \(\Lambda\). These deterministic size bounds also bound all elementary choice counts. Under \(P_*\le b\), a crude total of pieces, calls, edges, steps, and capped choices in an entire attempt is less than \(M^{20}\). A union bound thus proves the asserted router probability, with ample margin. ◻

Filtering real embeddings and verifying a stage

The router’s good event bounds route lengths and congestion in the overlay, where fake edges are still allowed. We must now discard routes using fake edges or excessively long real embeddings. The congestion bound limits how many demands this discards and controls the resource load of the remaining expanded routes.

At a completed piece, discard every packet route using a fake overlay edge or an overlay edge whose real embedding has more than \(L_0\) sampled-edge hops. This filtering is performed on overlay identities before any long real embedding is substituted. In one piece there are fewer than \(\epsilon_0s'r_m\) fake edges. By (77), the total length of all real embeddings is at most \(4r_m\chi s'\), so at most \(4r_m\chi s'/L_0\) overlay edges have a long real embedding. Each such edge is used by at most \(H_*\) packets, since traversal congestion bounds the number of packets using it. The fraction discarded within completed pieces is therefore at most \[ H_*\epsilon_0r_m+\frac{4H_*r_m\chi}{L_0}. \tag{86}\] Adding (76), the total lost fraction is at most \[2(D_p+1)/\chi+ H_*\epsilon_0r_m+4H_*r_m\chi/L_0<\tfrac12.\] Thus more than half the original stage demands survive all cuts and filtering on the good events.

Replace each remaining overlay traversal by its real sampled-edge embedding. Its direction specifies whether to use the embedding or its reverse. The resulting walks have correct endpoints and at most \(H_*L_0\) hops. In each round, an undirected sampled edge \(e\) appears at most \(2\chi\omega_e\) times among the real embeddings, by its two directed capacities. It is therefore traversed at most \[ 2H_*r_m\chi\omega_e \tag{87}\] times by the final walks from its completed piece.

Completed pieces are vertex-disjoint even when they stop at different depths: no completed piece has descendants, and sibling pieces use disjoint vertex sets. Their samples therefore contain at most \(4K_sv_+\) distinct original edges. For resources, use (87) and apply (73) separately at each of the at most \(D_p+1\) depths. This bounds every resource load by \[ 4H_*r_m\chi(D_p+1)C_0. \tag{88}\] The parameters give each required stage margin explicitly: \[ 4K_s\le\Lambda^{350a^2},\qquad H_*L_0=\Lambda^{270a^2}\le\Lambda^{350a^2},\qquad 4H_*r_m\chi(D_p+1)\le\Lambda^{350a^2}. \tag{89}\]

The algorithm accepts a stage only after checking these three bounds with \(\Lambda^{350a^2}\), checking that at least half the demands have been assigned, and checking every returned walk’s edge identities and endpoint sequence. It counts total traversals of each selected edge and multiplies these counts into the edge’s sparse resource list to verify resource loads. In particular, neither expansion nor the probabilistic premises of the construction need to be trusted by a subsequent user of the sparsifier: accepted walks satisfy the deterministic interface.

Failure probability and finite computation

Proof of Lemma 25. At a fixed cut depth, condition on its input lists before making its fresh samples. Lemma 26 applies to these lists even though earlier cuts were random. Likewise the median potential and the recursive routing estimates are conditional on their respective inputs. The sum of all bad-event probabilities in one attempt is less than \(1/2\) in the checked parameter range. Here are quantitative counts for this assertion. There are at most \(M^2(D_p+1)\) pieces and at most \(M^2\) resources having nonzero input data. The matrix and sample-count tails, resource tails, and product-mixing tails displayed above are each below \(M^{-40}\), including their per-piece factors. For the router, the group-cut union is bounded by (82); the point-count and edge-step tails use \(q_v=\Lambda^3\) and threshold \(\Lambda^6\); the rejection tails are the ones proved in Lemma 29. Each is below \(M^{-40}\). Their total number, including capped random-choice positions, is less than \(M^{20}\). These comparisons follow directly from \(\Lambda=2^{1000a^2}(1+u)\), \(a\ge10\), and \(P_*\le b\): every polynomial count in the listed powers of \(\Lambda\) is absorbed by the latter inequality, while every displayed tail has an exponent dominating a constant multiple of \(u\). All good events imply that the stage passes its verification.

After each unsuccessful attempt, the unchanged residual graph is tried again with fresh bits. Conditional failure probability is at most \(1/2\) per attempt; hence failure of all \(T\) attempts has probability at most \(2^{-T}\). Taking a union bound over at most \(2u+2\) stages, using \(u\le2\log_2 S\), gives \[(2u+2)2^{-T}\le2^{-\sigma}S^{-800}.\] This also proves the stated conditional guarantee for adaptive inputs. Taking the union of selected supports over accepted stages and retaining each demand’s first accepted walk yields (67), because \[(2u+2)\Lambda^{350a^2}\le P_*.\] Support and resource loads add over stages; maximum hop count does not increase when supports are united.

We finish with the cost on every path, including rejected attempts. At any fixed cut depth, piece edge counts sum to at most the starting edge count. Sampling scans their lists once. For each piece there are at most \(r_m\) flow calls, each on padded size \(P=O((1+\chi)s')\). Lemmas 2 and 4 bound their exact finite-word cost by padded size times \(M^{o_a(1)}\), uniformly over these local sizes. In particular, \(P\le O((1+\chi)M^2)\) and the flow primitive’s subpolynomial overhead at these sizes is \(M^{o_a(1)}\). The total padding over a depth is linear in its edge count times a fixed power of \(\Lambda\). The weight check (71) bounds flow throughput by (77) even for a sample with inaccurate cuts. Constructing all integral paths is therefore charged explicitly. Projecting sign vectors requires only repeated matching averages, \(O(r_m^2N_p)\) operations per piece, and never requires a dense matrix. Sorting projected values to choose each median costs \(O(N_p\log N_p)\) comparisons and is absorbed in the same logarithmic-power allowance.

At a completed piece, the initial overlay has \(r_mN_p/2\) edges. At each routing depth there are at most \(q_vN_p\) virtual edges and \(O(q_vN_p)\) jobs across its calls. Store all incident-edge arrays explicitly, so a walk step does not scan a neighborhood. A job uses at most \(\Lambda^3b\) trials, each of \(2t\) steps, and each incident choice has at most \(\Lambda\) bit-string trials. These caps supply the sole additional nonlogarithmic factor \(b\). The deterministic recursion-depth and substitution bounds from Lemma 29 control intermediate walk-list sizes even on bad random outcomes. Long real embeddings are filtered before substitution; all remaining expansions have a further factor of at most \(L_0\). Thus explicit output construction is also linear in the input size times \(b\) and a fixed power of \(\Lambda\).

Finally, sparse dictionaries translate original labels, edge identifiers, and resource names to local indices and back. Verification first counts traversals by selected edge and then scans its resource list, so a large resource list is not recopied at every traversal. All comparisons, capacity operations, resource-load products, and dyadic averages use bounded multiword integers. In particular, the projection denominator has at most \(r_m\) binary digits; local path lengths and load counters have the deterministic size bounds just proved; and all input bit strings have the assumed logarithmic-power width. The normalized matrices, potentials, and stationary probabilities are used only for analysis; the algorithm never computes their square roots or eigenvectors. The sampling probabilities are found by exact integer comparisons, and an event of probability \(2^{-j}\) is drawn using \(j\) fair bits. Here \(j\le2u\), since the positive piece degrees are at most \(M^2\). Each random sign uses one fair bit. The ordinary multiword implementations of all arithmetic operations add only logarithmic powers for fixed \(a\). Individual generated bits, initialization of arrays, allocation, and movement of these records are included in the same charges. Multiplication by the capped numbers of cut depths, routing depths, stages, and attempts leaves precisely (68). Since all caps are enforced independently of the good events, this is a bound on every computation path. ◻

Frozen decompositions and cycle quality

We now construct the flat-cycle backend of Lemma 22. This section proves its quality guarantee: one recursive level reduces an arbitrary current circulation to local correction cycles and a circulation in one smaller graph. The construction uses verified covers of a frozen graph, so its conclusion holds for every subsequent state of an epoch, including states chosen adaptively. Section 8 supplies the path assignments, maintains the smaller graph, and publishes a forest in which the returned walks have the required representation and recourse.

Parameters, scores, and the bottom level

Continue with the parameters of Lemma 25: \[ b=M^{1/a^2},\qquad \Lambda=2^{1000a^2}(1+u),\qquad P_*=\Lambda^{400a^2},\qquad M=2^u. \tag{90}\] Here \(u\) is a positive multiple of \(2a^2\) and \(S^{1/a}\le M\le S^2\). All powers in the following display are integers: \[ \begin{gathered} d=2a-4,\qquad k=M^{1/a},\qquad n_i=M^{1-i/(2a)},\qquad G_i=1200a+2i \quad(0\le i\le d),\\ r=(1210a+3)a^2,\qquad \Gamma=K_h=\Lambda^2,\\ A_*=64r\Gamma K_h b,\qquad p_*=(10P_*)^{a^2+1},\qquad Z_*=40(p_*+1). \end{gathered} \tag{91}\] In addition to the helper’s prerequisites, impose the deterministic checks \[ P_*\le b,\qquad 20Z_*A_*\le b^2. \tag{92}\] For each fixed \(a\), these inequalities hold for all sufficiently large \(M\): every factor other than the displayed powers of \(M\) is a fixed power of \(1+u\). A failed prerequisite suspends the parameter branch.

Level \(i\) accepts an undirected multigraph on fixed vertex slots, of total size at most \(n_i\), counting slots and edge copies. Its lengths are positive integers and its signed gradients are integers, with \[1\le\ell_e\le M^{G_i},\qquad |\gamma_e|\le M^{G_i}.\] Each edge has an identifier and a reference orientation. The degree bound needed for the running time is a fixed-parameter logarithmic power; Section 8 proves that this bound persists under recursion. The outer input has degree at most three. Write \(\mathop{\mathrm{OPT}}_i\) for the maximum circulation ratio on the current level-\(i\) graph, with the same zero convention as in Lemma 22.

A certified score is a nonnegative rational lower bound on the absolute gradient divided by the positive raw length of a returned traversal list. A zero score may accompany no candidate. Raw length counts every actual traversal with multiplicity; in the constructions below, assigning length one to an artificial null link only enlarges the length used for an upper bound. When a vector is used solely in an existential comparison, its length norm is instead the sum of length times the absolute net coefficient on each edge. We explicitly distinguish these two uses throughout.

Lemma 30 (Bottom-level search). At level \(d\), a query can return individual oriented edges forming a circulation with certified score at least \(\mathop{\mathrm{OPT}}_d/2\), whenever \(\mathop{\mathrm{OPT}}_d>0\). Initialization and each graph operation or query cost \(M^{O(1/a)+o_a(1)}\), and the answer has at most \(n_d=M^{2/a}\) pieces.

Proof. Replace each undirected edge by its two possible traversals. For each dyadic threshold \(\rho=2^j\) with \[-(G_d+2)u\le j\le (G_d+1)u,\] give these two arcs costs \(\rho\ell_e-\gamma_e\) and \(\rho\ell_e+\gamma_e\), respectively. For every start vertex, tabulate minimum costs of walks with each number of arcs from zero through the number of vertices, retaining predecessor pointers. This detects a negative closed walk whenever one exists, because a negative closed walk contains a negative simple directed cycle. Keep a witness for the largest successful threshold, and return its exact absolute-gradient to raw-length ratio.

To justify the approximation, negate a nonzero signed circulation if necessary to make its gradient nonnegative. Orient its used edges along their coefficient signs and decompose the resulting nonnegative directed circulation into simple directed cycles. Gradient and length add in this decomposition, so one cycle has ratio at least that of the circulation. Conversely, any positive-ratio directed cycle projects to a nonzero signed circulation; its projected length norm is at most its raw length. It follows that \(\mathop{\mathrm{OPT}}_d\) is attained by a directed cycle. Integer gradients and positive integer lengths give \[\frac{1}{n_d M^{G_d}}\le\mathop{\mathrm{OPT}}_d\le M^{G_d} \qquad\text{if }\mathop{\mathrm{OPT}}_d>0.\] The tested thresholds cover this interval with room on both sides. The largest successful threshold is at least \(\mathop{\mathrm{OPT}}_d/2\), and its negative walk has still larger ratio. The witness has at most the number of vertices in the graph.

There are \(O_a(1+u)\) thresholds. The elementary tables take at most \(O(n_d^3)\) arithmetic operations per threshold, since both the vertex and arc counts are \(O(n_d)\). All costs have \(O_a(1+u)\) bits after a common dyadic scaling. This proves the stated bound, including charged multiword arithmetic, without a forest at the bottom level. ◻

For \(i<d\), use epochs of \[ h_i=n_i/k \tag{93}\] single input edits. A price replacement is a deletion followed by an insertion. At the beginning of an epoch, freeze the current nonloop graph \(\bar G\), including its prices and edge identities. An old edge keeps these data until its death. Every later insertion is new, even when its endpoints and prices coincide with those of a dead edge. A heap on all current loops supplies exact single-loop candidates. Loops can otherwise be omitted: their coordinates are independently circulations, and the optimum on a graph is the maximum of its loop optimum and its nonloop optimum.

Fix a current nonloop circulation \(\Delta\), and write \(\Delta=\Delta_s+\Delta_n\) on the surviving frozen and new edges. The surviving part need not be a circulation: its imbalance is the negative of that of \(\Delta_n\), so it is supported at new-edge endpoints. We will express \(\Delta\) as a linear combination of current circulation candidates plus the projection of one circulation in a smaller graph. The coefficient-weighted lengths and the child vector’s length norm must be bounded by a controlled multiple of \(\|\ell\Delta\|_1\). Each candidate, however, needs a bound on its own raw traversal length, because that is the cost charged when the engine uses it.

Verified covers and a finite family of forests

The covers below adapt the shortest-path partitions with random shifts of Miller, Peng, and Xu (Miller et al. 2013, Algorithm 2 and Lemmas 4.1 and 4.4). We prove the required guarantees for weighted distances, geometric delays, verified padding, and capped retries.

For each layer \(j=1,\ldots,r\), let \(R_j=b^j\).

Lemma 31 (Verified padded covers). For every layer \(j\) one can construct \(K_h\) partitions of the frozen vertices into cells, with a shortest-path tree in each cell, such that:

  1. every cell tree has radius at most \(\Gamma R_j\) about its center;

  2. for every vertex, at least one of the partitions has a cell containing its entire closed \(R_j\)-ball in \(\bar G\).

The properties are verified before use. The construction takes \(n_iM^{o_a(1)}\) time on every path for fixed reliability parameter \(\sigma\), with detected-failure probability at most \(2^{-\sigma}S^{-800}\) conditional on any valid frozen input. It uses integer operations and fresh fair random bits.

Proof. Independently for each partition, give every vertex \(c\) an integer delay \(X_c\) with \(\Pr[X_c\ge t]=2^{-t}\) for nonnegative integers \(t\). At \(v\), choose the center maximizing \[ R_jX_c-\mathop{\mathrm{dist}}_{\bar G}(c,v), \tag{94}\] breaking ties by center identifier. Reject the attempt if any delay exceeds \(\Gamma\). A vertex’s own center has nonnegative score, so its winning center is at distance at most \(\Gamma R_j\). Dominance of a center persists along a shortest path towards it: the triangle inequality preserves every score comparison, including the common identifier tie rule. Hence a shortest-path tree to the center lies inside the winning cell.

For each vertex also compute the runner-up among distinct centers. Verify that at each layer every vertex has, in some partition, winning margin greater than \(2R_j\). The difference between the scores of two centers changes by at most twice the distance moved. Thus this check implies containment of the whole closed \(R_j\)-ball. A runner-up in another connected component has score minus infinity and causes no difficulty.

For unlimited geometric draws, fix a vertex and all delays except that of a proposed winner \(c\). The event that \(c\) wins is a lower integer threshold on \(X_c\), truncated below at zero. Exceeding that threshold by three raises its score by \(3R_j\) and gives margin greater than \(2R_j\). The geometric tail assigns probability at least \(1/8\) of the winning-event probability to this overshoot. Summing over the disjoint winning events gives margin probability at least \(1/8\). Independence of the partitions and a union bound therefore bound the probability that the delay or coverage checks fail by \[ rK_hn_i2^{-\Gamma}+rn_i(1-1/8)^{K_h}<\frac12. \tag{95}\] The displayed inequality follows from the chosen scales in the checked parameter range. Repeat with fresh draws at most \(2(\sigma+1000\log_2 S+10)\) times. Exhaustion is a detected failure; its probability is at most \(2^{-\sigma}S^{-800}\).

Generate a geometric delay by fair bits, stopping and rejecting when its cap is exceeded. Shifted distances and the best two distinct center labels are computed by multi-source Dijkstra with initial offsets \(-R_jX_c\), finalizing at most two distinct origins at each vertex. An origin among the best two at a vertex has prefixes among the best two along a shortest path; otherwise two strictly preferred origins at a prefix would remain preferred at the endpoint. Thus this truncation loses neither of the required origins nor a winning tree pointer. Initial offsets can be negative because edge lengths are positive. Each partition needs only near-linear integer graph work. The layer, partition, delay, and repetition counts are fixed powers of \(1+u\) for fixed \(a,\sigma\), and their integer data have fixed-parameter logarithmic bit length. This proves the every-path cost bound. ◻

Let \(H_j\) be the disjoint union of copies of all the cell trees in the \(K_h\) partitions at layer \(j\). A tree edge maps to its frozen actual edge, with the appropriate endpoint correspondence and reference orientation. Introduce additional vertices \(V_j(v)\) for every frozen slot \(v\) and \(0\le j\le r\). The layered system has two kinds of artificial links:

  • an OUT link from \(V_{j-1}(v)\) to the copy of \(v\) in each partition of \(H_j\);

  • an IN link from every cell root \(c\) to \(V_j(c)\).

Each artificial link maps to a null traversal at one physical slot, and is assigned length one and gradient zero for the construction.

The following finite family adapts the bit-selection construction of Chen et al. (Chen et al. 2023, Definition 4.30 and Lemma 4.31).

Lemma 32 (Finite family of routing forests). There is a family of at most \((K_h^2(u+1))^{O(r)}\) forests \(T^0\) with the following property. At each layer, any two distinct lower slots can be prescribed arbitrary OUT options, and some family member realizes all these prescriptions simultaneously across the layers. Each member contains separate copies of all cell trees and IN links, and exactly one OUT link per lower slot.

Proof. At a given layer choose a bit position of the slot index and two partition options, using the chosen option as a function of that bit. Include constant functions as well. Two distinct indices differ in some bit, so any two prescribed options can be realized. Choose these functions independently at the \(r\) layers and include every choice in the family. There are at most \(2K_h^2(u+1)\) choices per layer, giving the claimed bound.

Orient each cell tree towards its root, and the links towards higher layers. Every vertex has outdegree at most one and there is no directed cycle. Such an underlying undirected graph is a forest: an undirected cycle would, by its edge count and outdegree bound, force a directed cycle. This also proves the asserted structure of each \(T^0\). ◻

The chords of each \(T^0\) are the frozen input nonloops on its \(V_0\) vertices, and a null pair chord between every two distinct OUT-option targets of the same lower vertex. The latter has length one and gradient zero, whether or not either OUT option was selected in \(T^0\). Retain only chords whose endpoints are connected in that forest. For such a chord \(f\), let \(C_f^0\) be its signed fundamental cycle, traversing \(f\) forward and the tree path backwards. Let \(w_f\) be its full length, including artificial links, and fix the bucket \[ W_f=2^{\lceil\log_2 w_f\rceil}. \tag{96}\] Its value is found by integer bit-length operations, so logarithms in this notation require no real arithmetic.

Whenever a frozen actual edge dies, delete every tree edge mapping to it, both in the \(T^0\) copies and in the separate raw forests \(H_j\). These deletions are called holes. For a surviving chord \(f\), write \(C_f^{\mathrm{clip}}\) for the chord together with the surviving signed tree traversals of \(C_f^0\). Null chords always survive. The clipped vector need not be a circulation.

Fixed marginals and the clipped decomposition

Let \(\pi\) denote projection onto the frozen actual edge coordinates, ignoring null links and adding signed coefficients of copies. We first define a distribution of an upward route for every vertex. These distributions are used only in the proof, and are not sampled or evaluated by the algorithm.

The normalized boundary-distance weights and retention of each starting vertex’s route law adapt the origin-aware routing of Rozhoň et al. (Rozhoň et al. 2022, sec. 4) and its dynamic development in Chen et al. (Chen et al. 2023, sec. 4.3).

To account for the imbalance of \(\Delta_s\), we will compare a surviving edge with the difference of two upward routes, one from each endpoint. The same vertex must have the same route law when it appears at the endpoint of different edges. With this fixed marginal, the route contributions cancel wherever \(\Delta_s\) is conserved; only its imbalance at new-edge endpoints remains. Thus the route law will be indexed by the starting vertex, even when the route has reached a different representative in a higher layer.

For a cell \(C\) at layer \(j\), including its partition identity, define \[ a_{j,C}(v)= \max\!\left\{0, \min\{R_j,\mathop{\mathrm{dist}}_{\bar G}(v,V(\bar G)\setminus C)\} -\frac{R_j}{2}\right\}. \tag{97}\] Distance to the empty set, or to vertices in other components only, is infinity. Normalize these weights to a distribution \(\mathcal L_j(v)\) on all cells at that layer. A verified covering cell gives a weight of at least \(R_j/2\), so the normalizer is at least \(R_j/2\). A positive-weight cell contains every vertex at distance at most \(R_j/2\) from \(v\).

Lemma 33 (Upward route laws). The independent layer choices \(\mathcal L_j(v)\) define an upward path \(P(v)\) in the layered system with all OUT options. For every frozen edge \(e=vw\), its two path laws admit a coupling such that the closed walk \(e+P(w)-P(v)\), after canceling a common final suffix, has expected full length at most \(A_*\ell_e\). The marginal law of \(P(v)\) is the same for every incident edge used in this assertion.

Proof. The functions in (97) are one-Lipschitz on a connected component. At two vertices, at most \(2K_h\) cell weights can be nonzero, since a positive weight requires membership in the cell. Comparing normalized weights along \(e=vw\) yields \[ \mathop{\mathrm{TV}}(\mathcal L_j(v),\mathcal L_j(w)) \le\frac{4K_h\ell_e}{R_j}. \tag{98}\] Indeed the sum of absolute unnormalized changes is at most \(2K_h\ell_e\), and total variation of the normalized laws is at most that sum divided by the smaller normalizer.

Starting at \(V_0(v)\), use the chosen cell at layer one, then its tree path to its center and the IN link to the corresponding \(V_1\) vertex. Continue likewise. At layer \(j\ge2\) the previous representative is the center of a cell containing \(v\), so its distance from \(v\) is at most \(\Gamma R_{j-1}\). The second check in (92) implies \(b>2\Gamma\), and therefore \[\Gamma R_{j-1}<R_j/2.\] Every positive next-layer choice contains that representative, so its OUT option exists. This defines \(P(v)\) using independent choices across layers and a marginal law fixed by \(v\) alone.

For the endpoints of \(e\), couple their cell choices maximally at each layer, independently between layers. At the top, \(R_r/2\) exceeds every finite component diameter: such a diameter is at most \(n_iM^{G_i}\), whereas \(R_r=M^{1210a+3}\). A top-layer cell with positive weight at a vertex must consequently contain its whole connected component. All such cells have weight \(R_r/2\) at every vertex of that component. The two endpoint laws therefore coincide at the last layer and can be coupled to agree, including the partition identity.

Retain the two arms through one layer after their last cell disagreement; if there is no disagreement, retain layer one. From then on their representative and choices coincide, so their suffixes are identical and cancel. Two arms at layer \(j\), including their OUT and IN links, cost at most \(4\Gamma R_j\). For \(j\ge2\) this layer is retained only if a disagreement occurred at some layer \(l\ge j-1\). Equation (98) and a geometric sum bound that probability by \[\sum_{l\ge j-1}\frac{4K_h\ell_e}{R_l} \le\frac{8K_h\ell_e}{R_{j-1}}.\] Including the original edge and the first layer, the expected length is consequently at most \[\ell_e+4\Gamma b+ \sum_{j=2}^r32\Gamma K_hb\ell_e \le64r\Gamma K_hb\ell_e=A_*\ell_e.\] The coupling changes none of the prescribed per-vertex marginals. ◻

The next decomposition uses the pair-chord construction of (Chen et al. 2023, Definitions 4.28 and 4.30 and Lemma 4.31).

Each coupled walk decomposes into fundamental cycles from the finite family. To see this, split the two trimmed arms at their common representative vertices \(V_j\). Before the first common vertex, the two lower slots at every layer are distinct. Lemma 32 therefore supplies a forest containing the paths that close the input chord \(e\). Between successive common vertices, equal first OUT options give identical tree paths to the next layer and cancel. If those first options differ, replace the two null OUT traversals by the corresponding null pair chord. For the remaining arms in that piece, all simultaneous lower-slot prescriptions concern distinct slots; another family member realizes them. The pair-chord endpoints are connected there, as the arms meet at the next common representative. Simplifying each tree walk preserves its signed tree vector and can only decrease length. Replacing two unit null links by one unit pair chord also cannot increase length. Thus the projected coupled walk is a sum of projected oriented fundamental cycles whose full lengths sum to at most its original length.

Lemma 34 (Frozen decomposition with holes). Fix any state of the epoch and any circulation \(\Delta\) on its current nonloops. Write \(\Delta=\Delta_s+\Delta_n\) for its surviving frozen and new coordinates. There are real coefficients \(\alpha_f\) on live fundamental chords and a vector \(z\) on the unduplicated raw forests \(H_j\) such that, before restricting the frozen coordinates, \[\begin{align*} \sum_f\alpha_f\pi C_f^0&=\Delta_s+\pi z, \tag{99}\\ \sum_f|\alpha_f|w_f&\le A_*\|\ell\Delta_s\|_1, \tag{100}\\ \|z\|_{\mathrm{length}}&\le A_*\|\ell\Delta_s\|_1. \tag{101}\end{align*}\] Moreover \(z\) is the raw-tree part of \[ \sum_v\bigl(\operatorname{in}_{\Delta_s}(v) -\operatorname{out}_{\Delta_s}(v)\bigr)\mathbb E[P(v)]. \tag{102}\] Consequently this description uses only upward paths starting at slots incident to new edges. If \(z_L\) is the restriction of \(z\) to live raw forest edges, then \[ \sum_f\alpha_f\pi C_f^{\mathrm{clip}}-\pi z_L =\Delta_s. \tag{103}\]

Proof. Apply the preceding coupled-walk decomposition separately to each surviving frozen edge, weighted by its signed coefficient in \(\Delta_s\), and take expectations. There are finitely many choices in these conceptual distributions. Collecting coefficients of the oriented family chords gives \(\alpha_f\); collecting the signed route parts on the raw copies gives \(z\). Only surviving input edges and null pair chords are used as chords, so every indexed chord is live. The triangle inequality and Lemma 33 give (100) and (101), as well as the exact identity (99).

Although the couplings may differ between incident edges, the law of each endpoint path is fixed. Summing the route term \(P(w)-P(v)\) over oriented edges therefore gives (102). Cancellation of identical final suffixes does not change this signed vector. Because \(\Delta\) is a circulation, the displayed imbalance of \(\Delta_s\) is the negative of that of \(\Delta_n\) and vanishes away from new-edge endpoints.

Finally delete every coordinate of every tree copy mapped to a dead frozen edge. Projection after this deletion is the same as restriction of the projected physical vector. Restricting (99) therefore proves (103), even when coefficients on a dead edge had canceled between different copies. No circulation property is asserted for an individual clipped cycle. ◻

The identity (103) accounts for every surviving frozen coordinate. Its fundamental-cycle terms need not be current circulations, because holes may have opened them. We next extract current circulation candidates using short routed corrections and compress the remaining forest vectors. The description of \(z\) uses only upward routes starting at new-edge endpoints. The endpoints of their cell-tree legs, together with hole endpoints, will supply the compression terminals for this raw-forest remainder.

Portal legs and local correction cycles

Lemma 35 (Portal grid). A forest admits a set of at most its number of vertices divided by \(k\) portals such that every component remaining after portal removal has fewer than \(k\) vertices. The set is constructed by one postorder traversal.

Proof. Root each tree arbitrarily. Accumulate the number of still-connected unmarked vertices in the subtrees of a vertex, including the vertex itself. If the accumulated count is at least \(k\), mark that vertex as a portal and pass count zero to its parent. Every mark charges a disjoint group of at least \(k\) vertices. In any component off the marks, the residual count at its highest vertex is exactly its size and is below \(k\). ◻

Place such a grid on every \(T^0\) and separately on each raw forest. Call the components off the grid pockets. Grids remain compression terminals for the whole epoch. Classify and assign each live fundamental chord separately in its forest and bucket \(W\).

  1. If its full frozen tree path touches the grid, call the chord regular. At each incidence choose the nearest portal to its endpoint in the full frozen tree metric, breaking ties by a fixed portal order. Use the leg towards that portal, truncated at the first hole and ending on the incidence side of that hole. The full path contains a portal at distance at most \(w_f\le W\) from each endpoint, so each such leg has length at most \(W\).

  2. Otherwise the full tree path lies in one pocket. Root that pocket and measure weighted depth. For this bucket take connected clusters in half-open depth bands of width \(4W\), using both the unshifted bands and their translate by \(2W\). The depth range of the chord path has width at most its path length, hence at most \(W\); one of the two band systems contains that whole range. Assign the chord to its containing connected cluster there. Its tree diameter is at most \(8W\), since the least common ancestor of two vertices in a connected band cluster is in that cluster.

    Until any interior edge of that cluster is cut, use the intact fundamental cycle directly. At its first such cut, activate all surviving assigned chords. Give each of the two resulting sides a class with target its endpoint of the cut. A later cut splits the corresponding live fragment into two fresh classes, with targets the new cut endpoints. All resulting legs stay inside the original cluster and have length at most \(8W\).

A class is a logical vertex whose incidences have a common target tree node. Regular incidences are initially grouped by their target portal; holes subsequently split those classes. Splits always create fresh child labels, including for an empty side. The exact class maintenance is given in Lemma 39. A chord using the leg scheme is called active.

For every active \(f\), the construction in Section 8 supplies a logical path \(\Pi_f\) between its endpoint classes, using selected active chords of the same forest and bucket and containing at most \(p_*\) hops. The assignments may use an empty path for a logical loop. If \(f\) is oriented from its tail to its head, let \(L_f\) traverse from its tail target to its tail, then across \(f\), and then from its head to its head target. Such walks concatenate, also in reverse, to realize logical paths; write \(L_{\Pi_f}\) for the resulting walk. Every logical hop has full length at most \[ 8W+W+8W=17W. \tag{104}\]

The local correction candidate is \[ B_f=\begin{cases} C_f^0,&\text{while the chord is used directly},\\ L_f-L_{\Pi_f},&\text{for an active chord}. \end{cases} \tag{105}\] Here a negative walk is traversed backwards; this formula specifies a traversal list, not a cancellation rule for its denominator. Its projection is a circulation, and its raw actual length is at most \[ 17(p_*+1)W_f\le Z_*W_f. \tag{106}\] The direct case satisfies the same bound because \(w_f\le W_f\). Hence \[ \frac{|\gamma^{\mathsf T}\pi B_f|}{Z_*W_f} \tag{107}\] is a certified score. A positive numerator ensures positive actual raw length: null traversals have zero gradient. The list consists of actual individual chord edges and live forest paths; null chord traversals can be omitted. Its numerator is computed from integer path-gradient sums, with the fixed positive denominator above.

One joint child graph

The smaller instance is one graph \(J\) incorporating all family forests and the separate raw forests. In each live component take the Steiner subtree of its terminals, retain the terminals and branching vertices, and suppress degree-two nonterminals. A one-terminal component contributes its terminal alone; an empty terminal set contributes nothing. The terminal set includes:

  1. every initial portal and both endpoints of every hole;

  2. every class target and the original tree-node endpoints of every selected chord;

  3. in the raw forests, for every slot ever incident to a new edge in this epoch, both forest endpoints of every possible cell-tree leg in its frozen upward route.

All promotions may persist to the end of the epoch. For item (iii), enumerating all sequences of partition options suffices, even if some enumerated routes have zero probability in the conceptual law. Only their endpoints are needed; the cell-tree paths need not be enumerated. There are \(K_h^{O(r)}\) such endpoints per incident slot, a fixed-parameter logarithmic factor.

A compressed segment receives the sum of its constituent lengths and the oriented sum of their gradients. Include each currently selected chord on its original tree-node endpoints, with its actual or null prices; earlier selected chords may also remain while live. For every needed physical slot introduce a hub, and join each compressed vertex to the hub of its physical image by a unit-length, zero-gradient null edge. Include all current new nonloops directly between their hubs. Unused child slots remain isolated.

Every child edge expands into a live parent walk between the physical images of its endpoints, with exactly its gradient and with raw actual length at most its child length. We also write \(\pi\) for the induced projection from child coordinates to current parent coordinates; on surviving frozen edges it agrees with the earlier projection. A compressed segment has at most \(k+1\) constituent edges: it has no internal portal and its internal nodes lie in one pocket. Consequently its length and absolute gradient are at most \[(k+1)M^{G_i}\le M^{G_i+2}=M^{G_{i+1}}.\] The same bound holds for the other child edges. The number of child slots and edits, and their degree bounds, are proved in Section 8; only the stated graph and maps are needed for the following comparison.

Lemma 36 (Compression of a forest vector). If a vector on a forest has divergence only at a specified terminal set, it vanishes off the terminal Steiner subtrees and has constant oriented coefficient along each suppressed segment. Compression therefore preserves its gradient and its length norm exactly.

Proof. Deleting an edge outside a terminal Steiner subtree leaves a side containing no terminal. The sum of divergences on that side is zero, so the edge coefficient is zero. At an internal degree-two nonterminal of a remaining segment, zero divergence forces the two consistently oriented coefficients to agree. Replacing that segment by an edge with summed length and signed gradient consequently preserves both its gradient contribution and its length norm. ◻

For an active \(f\), subtracting its correction from its clipped vector leaves the clipped tree path minus the two legs of \(L_f\), together with \(L_{\Pi_f}\). The first part is a live forest vector with divergence only at hole endpoints and the two targets. The original chord-endpoint divergences cancel, also when a hole is adjacent to one of those endpoints. All remaining divergence points are terminals. The second part uses only selected chords and live paths between terminals. By Lemma 36 there is a child vector \(J_f\), not necessarily a circulation, satisfying \[ \pi J_f=\pi(C_f^{\mathrm{clip}}-B_f),\qquad \|\ell^J J_f\|_1\le Z_*W_f. \tag{108}\] For the length bound, the clipped tree part costs at most \(W_f\), the two removed legs at most \(16W_f\), and the routed selected path at most \(17p_*W_f\). Their sum is at most \(17(p_*+1)W_f\). In the direct case the relevant cluster path is intact, so \(C_f^{\mathrm{clip}}=B_f\) and we take \(J_f=0\).

Likewise, the vector \(z_L\) in Lemma 34 compresses to a raw-forest child vector \(\widehat z_L\) with its length norm preserved. Indeed its description (102) is a combination of cell-tree legs whose endpoints were included in terminal item (iii). Clipping creates additional divergence only at hole endpoints. It is this net forest vector that is compressed; no assertion is made that its individual conceptual routes have disjoint support.

Figure 2 illustrates the compression of one net forest contribution to the joint child graph.

Compression preserves a net tree flow whose divergence is supported on terminals. The shown targets \(t_-,t_+\) and hole endpoints \(h_1,h_2\) are retained; the terminal-free branch has zero flow, and each surviving path becomes one segment with the same coefficient. This is the tree compression used in constructing the remainder for Lemma 37; the pictured contribution need not be closed. Vertices shown here are copied tree nodes. Physical hubs, null connectors, selected chords, and the contributions of every other frozen and raw forest join the same child graph \(J\).

Lemma 37 (One-level quality reduction). In every state of an epoch, suppose the active logical path assignments use selected chords and have at most \(p_*\) hops. Let \(L_{\mathrm{best}}\) be the largest local correction score (107), or zero if there is none. Then, for every nonzero circulation \(\Delta\) on the current nonloops, \[ \frac{|\gamma^{\mathsf T}\Delta|}{\|\ell\Delta\|_1} \le 2Z_*A_*L_{\mathrm{best}}+12Z_*A_*\mathop{\mathrm{OPT}}(J) \le b^2\max\{L_{\mathrm{best}},\mathop{\mathrm{OPT}}(J)\}. \tag{109}\] This conclusion is simultaneous for all current circulations and requires no independence between the state and the verified covers or path assignments.

Proof. Use the coefficients and raw vector of Lemma 34, and form on the child edges \[ \Xi_0=\sum_f\alpha_fJ_f-\widehat z_L+\Delta_n, \tag{110}\] with \(\Delta_n\) carried by the hub-to-hub new edges. Its physical projection is \[\pi\Xi_0=\Delta-\sum_f\alpha_f\pi B_f,\] a physical circulation. Since \(W_f\le2w_f\), the norm estimates give \[\begin{align*} \|\ell^J\Xi_0\|_1 &\le 2Z_*A_*\|\ell\Delta_s\|_1 +A_*\|\ell\Delta_s\|_1+\|\ell\Delta_n\|_1\\ &\le4Z_*A_*\|\ell\Delta\|_1. \tag{111}\end{align*}\] The vector \(\Xi_0\) need not balance at each compressed copy of a physical vertex. Route its imbalance there along the null edge to that vertex’s hub. The hub then balances because the sum of divergences over all copies with its physical image is zero. The total absolute imbalance at the compressed vertices is at most twice the sum of absolute coefficients of \(\Xi_0\), and hence at most twice its length norm, since every child edge has length at least one. The resulting child circulation \(\Xi\) therefore satisfies \[ \pi\Xi=\pi\Xi_0,\qquad \|\ell^J\Xi\|_1\le12Z_*A_*\|\ell\Delta\|_1. \tag{112}\] Its gradient equals that of its projection because all connectors have zero gradient.

For the local part, (107) and (100) give \[\left|\sum_f\alpha_f\gamma^{\mathsf T}\pi B_f\right| \le L_{\mathrm{best}}Z_*\sum_f|\alpha_f|W_f \le2Z_*A_*L_{\mathrm{best}}\|\ell\Delta\|_1.\] The child circulation contributes at most its norm times \(\mathop{\mathrm{OPT}}(J)\), with the same conclusion if it is zero. Adding these inequalities proves the first bound in (109). The second follows from \(14Z_*A_*\le20Z_*A_*\le b^2\).

Every distribution, coupling, coefficient, and comparison vector in this proof is existential. The algorithm constructs only integral graphs, terminal sets, path assignments, and rational scores. Once the covers and assignments have their verified properties, the argument applies to any state and any circulation chosen afterwards. This proves the asserted adaptive scope. ◻

Corollary 38 (Quality through the recursion). Assume the path assignments and public forest lifting of Section 8. The backend at level \(i\) returns a certified score at least \[ \frac12 b^{-2(d-i)}\mathop{\mathrm{OPT}}_i. \tag{113}\] At the outer level this is at least \(M^{-10/a}\mathop{\mathrm{OPT}}\).

Proof. At the bottom use Lemma 30. At a higher level compare the local correction scores, exact current-loop scores, and the recursive child score. Lemma 43 preserves the child gradient under lifting and does not increase raw actual length, so the child’s certified score remains valid. If its approximation factor is \(\eta\le1\), the maximum of its score and \(L_{\mathrm{best}}\) is at least \(\eta\max\{L_{\mathrm{best}},\mathop{\mathrm{OPT}}(J)\}\). Apply Lemma 37 to lose at most \(b^2\) at this level. Exact loop candidates give the same conclusion for the full graph. This proves (113) by induction.

Since \(d=2a-4\), the outer factor is \(\tfrac12M^{-(4a-8)/a^2}\), which exceeds \(M^{-10/a}\) in the checked range. A zero optimum does not require a positive answer. The representation size, explicit forest logs, and total operation cost are established next; the quality argument itself has incurred only one recursive child call at each level. ◻

Maintaining and exposing the cycle backend

We now implement the reduction of Section 7. There are two quantities to control: the work performed locally and the number of edits passed to the one recursive child. The latter must be smaller, since it is multiplied through the recursion. We also construct the public forest, with explicit persistent edge identities, so that a child path can be lifted without traversing all its edges.

Recursive vertex and edge sparsification for dynamic minimum-ratio cycles is developed in (Chen et al. 2023, sec. 5 and 7). Here we establish the snapshot, edit-count, and forest-lifetime guarantees required by Lemma 22.

Retain the parameters of Section 7. Throughout this section a logarithmic factor means \(\log^{O_a(1)}(2+M)\), with a constant depending on the fixed parameters \(a,\sigma\). Different occurrences may denote different factors. In particular \(P_*,p_*\), the number of forest copies, and the number of width buckets are logarithmic factors. We do not replace these factors by powers of \(b\) when accounting for work. At recursion level \(i<d\), an epoch has \(h_i=n_i/k\) single input edits. A price replacement is a deletion followed by an insertion. Queries do not advance this epoch counter.

Class changes and immutable assignment snapshots

Use one logical graph for each frozen tree \(T^0\) and width \(W\), grouping all components of that tree in the same graph. Thus neither rebuilding nor taking the best score incurs a separate overhead for every component. Let \(D_T\) bound the combined tree and chord degree at a frozen tree vertex, and set \[ D_\ell=(k+1)(D_T+1)^2. \tag{114}\] The maximum can be measured when the tree is built. Cell-tree edges use actual incidences; a null pair chord only joins copies of the same slot in different partitions of one layer. At a vertex \(V_j(v)\) there are only the appropriate OUT and IN links, and, at \(V_0(v)\), the old actual chord incidences. Hence \(D_T\) is bounded by the input degree plus \(O(K_h)\) and is a logarithmic factor whenever the input degree is.

Lemma 39 (Class edits). The leg assignments of Section 7 can be maintained with logical degree at most \(D_\ell\). One input edit induces only a logarithmic factor many logical edit units across all tree–width graphs, where a unit is a class split, a pair of fragment-class births at a cluster activation, or a single chord death. Each unit involves at most \(O(D_\ell)\) incidence work. Every change to the leg of a surviving active incidence entails a split of its class.

Proof. Call a chord regular when its frozen fundamental tree path meets the initial portal grid. The nearest-portal regions are connected: along a shortest tree path toward the chosen portal, the distance comparison with every competing portal persists, as does the global index tie rule. A region consists of its portal and parts of the adjacent pockets, and therefore has at most \(1+kD_T\) vertices.

The target of a regular incidence is the first hole on its fixed path toward its original portal, stopped on the incidence side, or the portal if there is no such hole. For a fixed bucket, all current legs crossing a newly cut edge belong to one old class. Indeed that edge lies in one nearest-portal region, and their earlier truncation, if present, is the first hole on their common suffix toward the portal. Split this class by moving the crossing incidences to the incidence-side endpoint of the new cut. This target is determined by the newly cut parent edge toward the portal, so no merging is needed. Give both children fresh labels, even when one is empty or one retains the old target.

For a nonregular chord, use the two depth-band clusterings described in Section 7. The first interior cut activates the cluster and creates two fresh fragment classes. Every newly active chord has both incidences in these new classes, including chords whose two incidences lie on the same side. Thereafter cuts split fragment classes, again with fresh labels on both sides. Classes in different clusters remain distinct even if their targets are the same tree node. Each cluster is contained in an initial pocket. Its number of incidences, and that of any regular region, is bounded by (114). The two incidences of a logical loop are counted separately.

An old actual-edge deletion creates only a logarithmic factor many holes and chord deaths in all copies. In one width bucket, a hole is interior to at most one cluster in each of the two shifted band partitions, and it causes at most the regular split just described. A newly inserted physical edge creates no frozen chord. These facts give the asserted number of units.

Here is an explicit way to list and process them. At freezing time, compute tree ancestor tables, length and oriented-gradient prefix sums, portal mark counts, nearest portals, pockets, and cluster membership. The band index at a pocket vertex is an integer quotient of its weighted depth; connected components of equal band index give the clusters. No record is allocated for an empty band. These computations take near-linear tree work up to logarithmic factors and determine the relevant path sums without scanning each fundamental path. Initial regular legs have at most \(k+1\) tree edges, so their membership lists on tree edges can also be built explicitly. On a hole, inspect the region-bounded lists, discard dead incidences and legs already stopped before this hole, and update the remaining class lists. A cluster cut can inspect the whole small cluster and its chord incidences.

Before activation, keep each direct fundamental-cycle score in a heap using its frozen sums. Delete its entry when the chord dies or its cluster activates. For an input death, physical chord removals can be processed before the splits and activations that it causes. Afterwards every surviving active leg is the unique live tree path to its stored target. A leg whose class has not split retains that path and its gradient. This also proves the last assertion. ◻

We maintain assignments by an inner hierarchy of snapshots. At edit \(h_i\) rebuild the whole outer epoch on the resulting input graph, without updating the obsolete child. For edits \(1\le t<h_i\), put \[ z_0=\lceil\log_b h_i\rceil\le a^2, \qquad s_j=b^{z_0-j}\quad(0\le j\le z_0). \tag{115}\] These are integral powers of two under the divisibility assumption on \(u\). Level zero is the initial frozen assignment. At tick \(t\), choose the smallest \(j\ge1\) for which \(s_j\mid t\). Rebuild from the parent snapshot at \[ t_0=\left\lfloor\frac{t}{s_{j-1}}\right\rfloor s_{j-1}<t, \qquad t-t_0\le b s_j, \tag{116}\] and let all finer levels share the resulting snapshot. Minimality of \(j\) ensures \(s_{j-1}\nmid t\), so the strict inequality in (116) holds. If a coarser level was rebuilt at \(t_0\), its shared snapshot also supplies the indicated parent.

An assigned path is simple; an empty path visits its sole vertex once. The invariants for a level-\(j\) snapshot are \[ p(j)=(10P_*)^{j+1},\qquad C(j)=D_\ell(100P_*)^{j+1}, \tag{117}\] bounding its path hops and the number of assigned paths visiting any one class, respectively. At initialization only regular chords are active and their classes are portals. Apply Lemma 25 with endpoint incidence counts as resources and simplify the returned walks. For nonempty walks, vertex visits are bounded by endpoint incidence charges on their traversed edges; empty paths add at most the demand degree. This proves (117) for \(j=0\). All finer levels initially share this snapshot.

Selections are physical chord identifiers. A selected chord remains in support for the rest of the epoch while alive, even if its logical endpoint classes later split. Repeated selection stores it just once in the child graph.

Lemma 40 (The anchor core). Suppose the interval from a parent snapshot to a rebuild contains \(E_0\) logical edit units in one tree–width graph. The rebuild can restore (117) using a helper instance on \(O(E_0)\) incident labels. It selects at most \(O(P_*E_0)\) additional central chords. The work is \[ O(1+D_\ell E_0)\,bM^{o_a(1)}, \tag{118}\] in addition to scanning the interval’s edit log with logarithmic overhead per tick and explicitly handled incidence.

Proof. Mark labels of the snapshot at \(t_0\) as follows. Every split marks its ancestor at \(t_0\), if one exists; every chord death marks the ancestors of both endpoint classes. Let \(Y\) be the marked set. Then \(|Y|\le2E_0\). The anchors are all current descendants of \(Y\) and all current classes born from activations after \(t_0\), including their later descendants. There are \(O(E_0)\) anchors: initially marked labels contribute at most \(2E_0\), and every subsequent split or activation adds only constantly many class labels. Ancestors absent at \(t_0\) belong to the latter activation families.

Mark every old assigned path that visits \(Y\), including at an endpoint. In particular the path of a deleted demand is marked, as is any path using a deleted support edge. Every unmarked path still uses the same live oriented chords on unchanged labels. Moreover its demand and all support edges on the path retain the same legs and leg gradients, by Lemma 39. Thus its correction \(B_f=L_f-L_{\Pi_f}\) has exactly the stored gradient. A hole outside these legs can change the clipped comparison vector, but does not change this correction. Inactive direct-cycle candidates are handled separately by their activation lists.

Consider a marked old path whose demand \(e\) survives. Follow its prefix from the tail to the first marked label. Every preceding label is unsplit. Every edge of this prefix is live: if one had died, its earlier endpoint’s old label would also have been marked. The final edge now ends at a current descendant of the first marked label. Interpreting the same oriented edges therefore gives a current path \(A_e\) from the current tail to an anchor. If the old path starts marked, \(A_e\) is the empty path at the current tail. Reversing this argument gives a suffix \(Z_e\) from a current anchor to the current head at the last marked label. Repeated splits cause no ambiguity, since the current incidence of the last prefix edge specifies its current descendant. For newly activated demands use empty \(A_e,Z_e\); both endpoints are anchors by the activation rule.

For each such demand put an edge in a core graph on the anchors, associated with the direct logical walk \[ -A_e+e-Z_e. \tag{119}\] Its orientation is from the end of \(A_e\) to the beginning of \(Z_e\); a minus reverses a path. For every current base class use its number of visits on this direct walk as a resource count. Each count is at most two. Indeed the prefix and suffix each come from a simple old path, with at most their final marked label replaced by a descendant; the two resulting vertex lists concatenate across \(e\). This also covers empty prefixes, suffixes, and a loop demand. The total resource count at a class, over all direct walks, is at most \[ C_0=2C(j-1)+D_\ell. \tag{120}\] Prefix visits and suffix visits of old demands charge separately to visits at that class’s old ancestor; newly activated endpoint incidences charge to its current degree. The same bound controls the core demand degree, because its endpoint incidences are among these resource visits.

Apply Lemma 25 to the core with the resources just defined. Lift its assigned walks by substituting (119), and for demand \(e\) prepend \(A_e\) and append \(Z_e\). Simplify the resulting walk to a simple path, erasing the whole walk if its endpoints coincide. Cycle erasure never increases hops or resource visits. The hop count before simplification is at most \[ 2p(j-1)+P_*\bigl(2p(j-1)+1\bigr)\le p(j). \tag{121}\] The substituted helper walks cost at most \(P_*C_0\) visits at each class. The added prefixes and suffixes cost at most \(2C(j-1)+D_\ell\), including the empty paths; the unchanged assignments add at most \(C(j-1)\). Thus a valid upper bound is \[P_*\bigl(2C(j-1)+D_\ell\bigr)+3C(j-1)+D_\ell \le100P_*C(j-1)=C(j).\] All edges in the prefixes and suffixes were already live support edges. Only the central chords of the selected core edges must be added to support. The core has \(O(E_0)\) incident labels, regardless of how many old paths met \(Y\), so the helper selects at most \(O(P_*E_0)\) such chords. When \(E_0=0\) the old snapshot can simply be retained.

For completeness, the implementation does not copy whole snapshots. Each held snapshot consists of immutable balanced-dictionary roots for paths keyed by demand, an index from class to its visiting paths, and an ordered collection of correction-score keys. Path copying changes only the search paths of modified dictionary entries. Begin with the parent roots and replace marked or dead records and add the newly active records. The class index enumerates at most \(C(j-1)|Y|\) old path occurrences before their lists are read; a dictionary removes duplicates. Activation lists enumerate new demands. Every such path has logarithmic-factor length, and the number of newly activated demands is at most \(O(D_\ell E_0)\).

Store split-parent pointers with class birth ticks and jump pointers. To find the ancestor at \(t_0\), follow this history to the class alive at the end of tick \(t_0\). All splits within one tick belong to that tick and snapshots are taken only after settlement, so equal birth ticks do not create a partially settled ancestor. Physical chord identifiers, the two current incidence classes, and their targets are explicit records. They determine the oriented lifted prefixes and suffixes. Frozen tree prefix data gives each live leg’s gradient and length; summing over the new short logical paths recomputes their correction-score keys. Unmodified keys remain valid by the unmarked path argument.

Building the direct lists and the helper’s resource lists, simplifying its output, and changing the indices costs \(O(1+D_\ell E_0)bM^{o_a(1)}\) by Lemma 25. Comparing the maintained best keys over the logarithmic-factor many tree–width graphs has only logarithmic overhead. The helper’s input prerequisites can be checked: twice the demand count and the nonzero resource-list length must each be at most \(M^2\). They hold for all sufficiently large \(M\) at fixed parameters, since even all frozen chords number at most \(O(n_i)\) times a logarithmic factor, and every direct walk has logarithmic-factor length. This proves the work assertion and both snapshot invariants. ◻

Lemma 41 (Cumulative assignment work and support). The initial assignments use at most \(h_i\) times a logarithmic factor many support chords. On every epoch prefix of \(t'\le h_i\) edits, the additional support selections number at most \(bt'\) times a logarithmic factor, and local assignment work is \(t'M^{O(1/a)+o_a(1)}\) after initialization. Initialization costs \(n_iM^{O(1/a)+o_a(1)}\).

Proof. Initially the number of portal classes, summed over all tree–width graphs, is at most \((n_i/k)\) times a logarithmic factor. The helper selects at most \(P_*\) times this many chords. At inner level \(j\) there are at most \(\lfloor t'/s_j\rfloor\) rebuilds, each using an interval of at most \(bs_j\) ticks. By Lemma 39, the sum of their interval edit counts across all tree–width graphs is at most \(bs_j\) times a logarithmic factor per rebuild. Consequently the cumulative support additions at that level are at most \(bt'\) times a logarithmic factor by Lemma 40. There are at most \(a^2\) inner levels. Summing (118) similarly bounds the local work by \(t'D_\ell b^2\) times a logarithmic factor, in addition to smaller constant-per-rebuild and log-scanning costs. Since \(D_\ell=(k+1)\log^{O_a(1)}(2+M)\), this has the asserted exponent with an absolute coefficient. Initial tree preparation, the explicit short regular-leg lists, and initial helper calls fit the stated initialization bound. A physical removal lists the deaths of selected copies in logarithmic-factor work. Keeping earlier live selections therefore requires no additional support budget. ◻

In particular (121) gives \(p(j)\le(10P_*)^{a^2+1}=p_*\), supplying the path assumption in Lemma 37.

Small changes to the compressed child

The snapshot hierarchy now supplies the short selected-support paths required by the quality reduction. We next bound the number of edits needed to maintain their compressed representation in the child graph. Afterwards, the public forest construction will expose child paths while preserving the identities of retained mapped edges.

Lemma 42 (Compressed graph edits). The single child graph \(J\) can be initialized nonempty. Throughout an epoch it uses at most \(h_ib\) times a logarithmic factor many active slots and edges, including slots assigned earlier in that epoch. On an epoch prefix of \(t'\) further input edits it receives at most \(bt'\) times a logarithmic factor individual edits. Its degree is a logarithmic factor, and its integral prices obey the level-\((i+1)\) bounds. The local construction work per parent edit is \(M^{O(1/a)+o_a(1)}\) amortized.

Proof. Initial terminal promotions number at most \(h_i\) times a logarithmic factor. Subsequent promotions come from hole endpoints, new class targets, endpoints of selected support chords, and the raw-hierarchy route endpoints needed for new physical incidences. The first three counts follow from Lemmas 39 and 41. For the fourth, enumerate the \(K_h^{O(r)}\) possible sequences of partition options from the given physical slot. At each step look up the entering copy and its frozen cell root in stored tables. This lists the needed endpoints without traversing the raw legs. It costs a logarithmic factor per incidence. Thus the cumulative additional promotions on a prefix number at most \(bt'\) times a logarithmic factor. Retain all promoted terminals until the epoch ends.

Build each initial Steiner subtree in bulk, using subtree terminal counts. To promote one additional tree node, split its current compressed segment if it is already in the represented subtree. If it is outside, explore its hanging component to the unique attachment and add the joining path, splitting the segment at that attachment if needed. If the live component has no represented subtree, create an isolated terminal. The explored exterior component contains no initial portal, and hence lies within one original pocket of fewer than \(k\) vertices. Existing segments have at most \(k+1\) constituent edges. Membership and segment pointers on these nodes, together with local forest adjacency, therefore suffice for \(O(k+1)\) times degree and logarithmic-factor work. Only \(O(1)\) compressed vertices and edges change per promotion. No whole live component must be relabeled.

Before cutting a frozen tree edge, promote both endpoints. The edge is now itself a compressed segment and can be deleted. On either side, every other represented edge still separates terminals: a terminal formerly reached across the deleted edge is replaced for this purpose by its now-terminal endpoint. Thus there is no pruning or merging cascade. Keep each segment’s full constituent list with orientations. A changed segment or changed realization is removed and replaced by a fresh edge, with deletion first.

Assign each represented frozen tree or raw-forest node its own child slot, without reuse in this invocation, and assign at most one hub per physical label. A selected chord remains attached at its original tree-node endpoints. Its logical leg targets can therefore change in bulk without migrating its child edge. New actual nonloops are inserted directly between hubs and removed at their deaths; actual loops have already been handled by the loop heap. These rules establish the claimed size and edit counts.

Reserve \(\lfloor n_{i+1}/2\rfloor\) child slots, with unused slots isolated, and check both assignment overflow and total slots plus current edges against the child’s capacity. For fixed parameters these checks hold eventually, because \[ n_{i+1}=h_iM^{1/(2a)},\qquad b=M^{1/a^2},\qquad \frac1{a^2}<\frac1{2a}. \tag{122}\] The active-slot and edge bounds above are consequently smaller than either reserved half for all sufficiently large \(M\).

At a compressed node, segment degree is at most its frozen tree degree; add its chord degree and one hub connector. At a hub, the connector degree is bounded by the number of frozen tree and raw-forest copies of its physical label, plus the current new actual incidences. Both are logarithmic factors. Deleting old segments before adding replacements preserves these degree bounds even during the child edit sequence. This proves the degree hypothesis inductively from the degree-three outer input.

A segment sums prices over at most \(k+1\) edges, so its positive integral length and absolute gradient are at most \((k+1)M^{G_i}\le M^{G_i+2}=M^{G_{i+1}}\) for adequate parameters. Other child prices are inherited or are \((\ell,\gamma)=(1,0)\). The local work includes an \(O(k+1)\) factor per promotion or changed segment and the assignment work already bounded. Its exponent is therefore \(O(1/a)\) with an absolute coefficient. Only the smaller edit count proved above is passed into the recursive time bound. ◻

A public forest with prebuilt arms

This representation uses the flat-embedding viewpoint of (Chen et al. 2023, Definition 2.3 and Theorem 3.5), whose minimum-ratio-cycle data structure is fully dynamic. The preallocation and reassignment of unused tree copies follow the flat-forest lifting construction of Kyng, Meierhans, and Probst Gutenberg (Kyng et al. 2023, sec. 3.3, Lemma 3.21 and Algorithm 5). Their construction already provides path-length domination and bounds on changes to the lifted forest. Here the copied trees are the arms of compressed segments. We track the lifetime of every mapped edge and process each parent edit as one old-to-final transaction; these details preserve the engine’s signed and absolute accumulations when an unused copy is reassigned.

Put \[ D_i=b^{2(d-i)}\qquad(0\le i\le d). \tag{123}\] The bottom forest is empty. At a higher level include full live copies of all \(T^0\) and raw \(H_j\) forests as separate public components, with their holes and their actual or null mappings. They represent local correction paths and individual compressed segments. The following additional components, disjoint from the full copies, represent the recursively exposed forest \(F^J\).

For each active child slot \(q\), including hubs, write \(\varphi(q)\) for its physical image. Prebuild a pool of \(D_{i+1}\) arm slots. An arm slot has a center mapping to \(\varphi(q)\). For each tree segment of \(J\) incident to \(q\), attach an independent copy of its entire constituent path, oriented outward from \(q\), with a fresh far tip and all constituent vertex and edge maps. Different arms in one slot meet only at its center; different slots are disjoint. Retain the whole pool, including unassigned, or dormant, slots. Assign a distinct arm slot to each occurrence of \(q\) in \(F^J\). Occurrences over an inactive child slot are ignored publicly: such a slot is isolated in \(J\), so all its forest components have only null edges and project to that one slot.

Add one connector for every child forest edge in the active part:

  1. If its map is a segment \(qz\), choose a fixed endpoint side, say \(q\). Use the corresponding arm in the slot assigned to its occurrence over \(q\), and add a null connector from that arm’s far tip to the center assigned to the other occurrence over \(z\).

  2. If its map is a nonsegment child edge, join the two assigned centers with that edge’s single actual-edge or null map.

  3. If the child forest edge itself is null, join its assigned centers with a null connector.

The endpoints of each null connector have equal physical images. Centers have indexed identifiers, and tips are indexed by arm slot and incident segment identifier. Connector endpoints can thus be looked up without scanning an arm.

Lemma 43 (Flat lifting). The construction is a forest with valid current actual or null edge maps. A child forest-path piece lifts to at most one public forest-path piece, preserving its signed projection and with parent raw length at most its child raw length. An individual child-edge piece lifts to one path or one actual-edge piece, or can be omitted if null. The lifted sum of a child circulation is a circulation with the same gradient. The number of returned pieces is \(M^{O(1/a)+o_a(1)}\).

Proof. Contract every arm-slot tree to its center. On assigned slots the connector quotient is exactly the active subforest of \(F^J\). Dormant slots have no connectors. A cycle in the expanded graph would give a cycle in this quotient, since each contracted part is itself a tree. Together with the disjoint full copies, the result is a forest.

Expand each edge on an oriented child path using the prescribed arm and connector, or its nonsegment connector. The concatenation has the expanded signed projection of that child path. Segment length is the sum of its constituent parent lengths; null child edges contribute zero parent raw length, and other child null realizations can only decrease it. Thus the concatenation’s parent raw length is at most the sum of child-edge lengths counted with their traversal multiplicities.

This concatenation may backtrack, even when the child path is simple, because several connectors can use the same arm in one slot. In a forest, deleting a consecutive edge and its reverse preserves the signed count of every oriented edge and decreases nonnegative raw length. Repeating this deletion leaves the unique forest path between the assigned endpoint centers. It therefore has exactly the needed projection and no larger raw length. We return its endpoints; we do not perform these cancellations by expanding the walk. If the child path has an inactive endpoint, its whole component is one of the ignored null components, so the piece can be omitted.

For a piece given as a single child segment, use its endpoints in the appropriate full live forest copy. For a nonsegment piece, return its actual edge or omit its null realization. Each edge expansion has divergence equal to its mapped endpoint divergence. Summing a child circulation therefore gives zero physical divergence and preserves its gradient. In particular a positive child score gives positive gradient magnitude after lifting and therefore positive parent raw length. Its score remains a valid lower bound for the lifted ratio.

Only endpoint and slot lookups are needed in these conversions; no long lifted path is traversed or priced anew. A local correction uses \(O(p_*)\) pieces in full copies and individual actual chords, with opposite traversals explicitly reversed and null chords omitted. An actual loop uses one piece. A child answer gives at most as many parent pieces as it had child pieces. At each level we return one best candidate, rather than concatenating candidates from several levels. The bottom answer has at most \(n_d=M^{2/a}\) pieces. Taking the maximum of these bounds proves the last assertion. Every listed piece has unit multiplicity; repetitions, if present, are listed and charged. ◻

Figure 3 shows the shared-arm lift and a dormant slot whose internal edge lifetimes persist through reassignment.

A shared-arm lift in Lemma 43. Both child edges map to the same segment \(qz\), but their endpoint occurrences over \(z\) use different slot trees. Each edge adds one null connector from the far tip of the chosen arm to the other assigned center. Contracting all slot trees recovers the child forest. In the child path from \(z_1\) to \(z_2\), the two traversals of the shared arm cancel when the concatenated realization is reduced to the unique forest path; signed projection is preserved and raw length cannot increase. Dormant slots retain their internal edges, so reassignment at an unaffected label changes assignments and connectors without rebuilding those edges.

Lemma 44 (Occurrence multiplicity). For all sufficiently large \(M\) at fixed parameters, level \(i\) has at most \(D_i\) public forest vertices over each input slot. In particular the outer multiplicity is at most \(M^{10/a}/4\).

Proof. Full copies contribute a logarithmic factor. For one compressed frozen forest, segment interiors are disjoint. Thus a node in such an interior occurs in at most two arms per pool index, one from each endpoint of its segment. At a compressed endpoint, its own centers and the far tips of arms from adjacent endpoints contribute at most one plus its tree degree per pool index. These bounds include dormant slots. A physical input label has only a logarithmic factor many frozen-tree and raw-forest copies, and at most one hub. Consequently its total multiplicity is at most \[ (1+D_{i+1})\log^{O_a(1)}(2+M). \tag{124}\] There is no factor of the segment length in this bound. The \(b^2\) gap between \(D_i\) and \(D_{i+1}\) absorbs the logarithmic factor for sufficiently large \(M\). Induction starts with the empty bottom forest. Finally \[D_0=M^{(4a-8)/a^2}\le M^{10/a}/4\] eventually. Both the inductive multiplicities and the last inequality may be checked as deterministic prerequisites. ◻

Explicit recourse and persistent edge lifetimes

An occurrence or forest edge has an immutable lifetime identifier. Changing an occurrence’s label, an edge’s endpoints, or its map requires a replacement with a fresh identifier. A vertex death explicitly logs all incident edge removals first. Additions use present endpoints. The client retains old input labels, prices, and maps until all old public removals have been processed, then installs the new input state and public additions. No path call occurs inside this transaction. A whole epoch boundary can remove the old public forest and install the new one with these rules.

Lemma 45 (Forest recourse). For one parent input edit within an epoch, let \(Q\) be the set of child labels assigned or changed, or incident to a changed child edge or changed segment realization. Apart from local full-copy edits, the public forest can be updated in \[ |Q|D_{i+1}(k+1)\log^{O_a(1)}(2+M) +\mathcal L\log^{O_a(1)}(2+M) \tag{125}\] work and public log records, where \(\mathcal L\) is the number of child log records processed. Retained arm edges at labels outside \(Q\) keep their lifetimes and maps, even if their arm slot is reassigned to a different child occurrence.

Proof. First compute the batch of individual \(J\) edits for this parent operation, without a child query, and determine \(Q\). Its size is bounded by a constant times the exogenous child edit and assignment counts of Lemma 42. Retain access to the old occurrence assignments and old child forest records. Run the child edit sequence while maintaining a passive copy of its logs, indexed by lifetime identifier and label, with individual incidence lists. Track which records were added or removed. Each log changes only its own dictionary and incidence-list entries and costs logarithmic overhead. In particular deleting an occurrence does not trigger an uncharged degree scan: its incident edge deletions have their own records. Persistent roots or saved old records retain the old information until the parent transaction is complete.

At labels in \(Q\), remove all old arm pools and construct the final active pools with final occurrence assignments. Sweep their old and final incident child edges as well, to remove and reconstruct the corresponding connectors. To bound these sweeps, fix a label \(q\) in either the old or the final state and let \(N_J(q)\) be its set of distinct other neighbors. Every child forest edge incident to an occurrence over \(q\) belongs to the forest induced on the occurrence sets over \(\{q\}\cup N_J(q)\). That forest has at most \[ D_{i+1}\bigl(1+|N_J(q)|\bigr) \le D_{i+1}\bigl(1+\deg_J(q)\bigr) \tag{126}\] vertices, and hence no more than this many edges. Parallel child-graph edges only increase the safe degree bound. A child-null edge has both ends over \(q\) and is already included. This proves the bound separately for the old and final sweeps; old lists can be saved before applying the child batch. Constructing each arm takes at most \(k+1\) constituent steps. The degree bound on \(J\) now gives the first term of (125).

At every unaffected active label, retain the arm pools and the assignments of retained occurrences. First free assignments of deleted old occurrences, then assign each final new occurrence from a free-slot stack. There is sufficient space because the final occurrence count is at most \(D_{i+1}\). Remove the old connectors for changed child edge records and add the necessary final ones; also handle the connectors found in the affected sweeps. Additions can wait until all final assignments exist. Deduplicate swept records by identifier. Transient child records that are neither old nor final need not be published by the parent, although their individual processing is charged.

If a child edge and both its assignments survive unchanged at unaffected labels, its connector retains the same map. Every other change at these labels has an explicit child edge record: an occurrence replacement logs its incident edge removals and new edges, and an edge endpoint or map change is itself a replacement. Thus the remaining work is logarithmic per child record, giving the second term in (125). A high-degree child occurrence can have many incident records, but each is processed once; it does not add a degree sweep to each record. Child lists over inactive slots are likewise kept passively until activation, which places their label in \(Q\).

Full-copy hole edits are performed separately. Every public map to a dying parent input edge is removed: it lies in a full copy that receives the cut, in an arm of a changed or removed segment whose endpoints are in \(Q\), or in a connector realizing a removed nonsegment child edge. Remove connectors before their old pool vertices, and all old edges before their vertex deaths. After these removals the retained public graph is a subforest of the final graph, so its final pools, vertices, and connectors can be inserted in a valid order.

During this computation the child sees only its abstract slots, edges, and integral prices. An intermediate child state may still contain the old realization of a parent segment whose physical edge is about to die. Its abstract graph remains an in-range input, and it never asks for parent physical data. Only the final child state must be the reduction of the new parent input. The parent therefore publishes one old-to-final transaction, using retained old data for all removals, rather than a lifted public state for every intermediate child edit.

Finally, a dormant arm slot at an unaffected label retains all its internal edges when assigned to another occurrence. Their identity is determined by the child label, pool index, and incident segment realization, not by the assigned occurrence identifier. Existing signed and absolute accumulations in the client’s path tracker consequently remain attached to the same mapped edges. No \(D_{i+1}\) or \(k\) factor multiplies the processing of general child-internal occurrence changes. ◻

The two marking rules used above serve different purposes. A terminal promotion or segment subdivision may merely reencode an unchanged physical path, so its logical correction need not be marked in Lemma 40. It nevertheless marks the endpoint arm pools in Lemma 45. Conversely, a logical class split changes its leg data while selected central chords remain attached at their original tree nodes. This is why the logical snapshot update and public forest update have the stated separate bounds.

Total work, finite arithmetic, and reliability

The recurrence must charge local arm-pool reconstruction separately from processing the child’s forest logs. A changed child label can require rebuilding its arms, whereas each general child log record incurs only logarithmic overhead. Keeping these charges separate allows the small child edit count to control the recursive cost.

Lemma 46 (Recursive work bounds). For a prefix of \(s\) graph operations, counting queries and edits and including an operation that terminates partway through, level \(i\) has total work and explicit public-log size at most \(n_iA_i+sU_i\), where \[A_i,U_i=M^{O(1/a)+o_a(1)}.\] All coefficients in the \(O(1/a)\) exponents are absolute.

Proof. Give the child its arbitrary nonempty initial graph directly. The parent’s own initialization costs \(n_iM^{O(1/a)+o_a(1)}\). This includes tree preparation, initial assignments and compression, full forest copies, and initial arm pools. Initially there are only \(h_i\) times a logarithmic factor many active child labels; each has \(D_{i+1}\) pools and arms of at most \(k+1\) steps. Since \(D_{i+1}\le M^{4/a}\), this cost has the required absolute exponent. Epoch teardown and its public logs obey the same allowance. Allocations and eventual deallocation are charged; a discarded arena can be reclaimed against its earlier allocations.

By Lemmas 41, 42, and 45, all local incremental work costs \(M^{O(1/a)+o_a(1)}\) amortized per input operation. In particular the cumulative affected-label count in an epoch prefix is only \(b\) times a logarithmic factor per parent edit. Queries use the maintained best keys and lift one answer, with logarithmic overhead per child piece. They need not alter the public forest.

Let \(L=M^{C/a+o_a(1)}\), for an absolute constant \(C\), dominate these local factors and the bottom-level costs. Such an absolute \(C\) exists: all nonlogarithmic local factors above are fixed powers of \(b\), \(k\), and \(D_{i+1}\); at the bottom the elementary negative-cycle computation is a fixed-degree polynomial in \(n_d=M^{2/a}\). Factors involving \(r,a^2,P_*,p_*\) and degree bounds are fixed-parameter logarithmic factors.

Initialization of the child contributes \(n_{i+1}A_{i+1}\), with logarithmic overhead on its logs, and \(n_{i+1}/n_i=M^{-1/(2a)}\). At most \(1+s/h_i\) parent epochs start. After initialization the child receives at most \(bM^{o_a(1)}\) times the parent operation count, including the one recursive query used for a parent query. Only a logarithmic factor multiplies child log-processing work, by Lemma 45. Consequently it suffices to choose dominating upper estimates satisfying \[\begin{align*} A_i&=L+M^{-1/(2a)+o_a(1)}A_{i+1},\tag{127}\\ U_i&=L+kA_i+M^{1/a^2+o_a(1)}U_{i+1}, \tag{128}\end{align*}\] with \(A_d,U_d\le L\). The term \(kA_i\) pays rebuilding every \(h_i=n_i/k\) edits, including its child initialization.

For fixed \(a\), the multiplier in the first recurrence tends to zero, so \(A_i\le2L\) eventually after enlarging \(L\) if necessary. Unrolling the second recurrence through at most \(d\le2a\) levels gives \[U_i\le (d+1)(L+2kL) M^{d/a^2+o_a(1)} =M^{(C+3)/a+o_a(1)}.\] This explicitly shows why no coefficient depending on \(a\) appears in the \(1/a\) exponent. Logarithmic overhead accumulated over the fixed number of levels remains \(M^{o_a(1)}\).

Each attempted cover or helper reconstruction has its own every-path bound, whether it succeeds or is rejected. Charge such a reconstruction to the attempted operation, including if failure or a guard stops it before publication. All previously published states satisfy their verified bounds. The same recurrences therefore hold for these prefixes and unsuccessful computations. ◻

Finite arithmetic.

All maintained data are finite integers, dyadics, explicit lists, or identifiers. Frozen tree prefix sums and segment sums use at most the number of constituent edges times the level price bound. Width buckets are integer powers of two. Correction scores compare ratios of integral gradient magnitudes to the positive integral proxies \(Z_*W_f\) by cross multiplication; bottom scores and inherited scores are likewise represented by bounded integral numerator–denominator pairs. The construction requires no evaluation of the conceptual hierarchy-law probabilities, real flows in the comparison argument, or barrier logarithms. Exact geometric delays, dyadic thresholds, and helper randomness are generated by the finite procedures already specified. Fixed-parameter data have \(\log^{O_a(1)} S\) bit length, and ordinary dictionary and forest record operations have multiword logarithmic-factor overhead. The integral-flow calls inside the helper use the simulation of Lemma 4; its logical addresses are not assumed polynomially bounded in their numerical values.

Lemma 47 (Guards and conditional failure). The construction can enforce all size, multiplicity, arithmetic, and sampling prerequisites by finite checks. For every sufficiently large fixed \(a\) and every fixed \(\sigma\ge0\), those deterministic prerequisites are adequate for all sufficiently large sizes in the permitted engine range, on every random path. Conditional on any adaptively specified history before an invocation, the detected sampling failure probability over an entire engine instance is at most \(2^{-\sigma}S^{-60}\).

Proof. Share counters throughout the backend tree of an engine instance. Allow \(S^{200}\) outer backend graph operations, and guard total records, fresh identifiers, and stochastic invocations by \(S^{500}\). Fresh identifiers can be sequential. Count allocations and record creations as they occur; check child capacity, helper input-list sizes, public multiplicity, price ranges, and quality inequalities before using the corresponding state. Deletion-first transactions maintain the capacity and degree bounds during replacement. The checks \(P_*\le b\) and \(20Z_*A_*\le b^2\), the inequalities in (122), the multiplicity checks, and the price bounds all hold eventually by their positive power gaps over fixed-parameter logarithmic factors.

We justify that the coarse guards do not hide a condition on successful randomness. Published covers satisfy their verified coverage and radius requirements, and published helper outputs satisfy the verified support, path, hop, and resource bounds. Every rejected attempt separately has bounded work: sampled piece lists partition their input edges at a capped recursion depth; sample size and weight checks precede flow extraction; the router has a fixed number of jobs per label, shrinking label groups, no packet duplication, and capped walk trials and substitution depths. Long real embeddings are filtered before expansion. These properties, proved in the routing construction, hold even for unsuccessful samples. Covers likewise have capped delays, attempts, and finite graph computations.

Apply Lemma 46 to a prefix ending just before a putative first coarse guard violation, charging the partially attempted operation as one operation. With \(S^{1/a}\le M\le S^2\), at most \(S^{200}\) outer operations and initial construction cost at most \[M^{1+O(1/a)+o_a(1)} +S^{200}M^{O(1/a)+o_a(1)} =S^{200+O(1/a)+o_a(1)}\] elementary records, invocations, and local operations. This is strictly below \(S^{500}\) for sufficiently large fixed parameters and size, including the attempted guard-reaching operation. The engine’s motion and refresh accounting, independent of forest churn, keeps its outer graph-operation count below \(S^{200}\) under its client bounds, including threshold-driven reprices. Thus the first supposed violation is impossible. This argument uses deterministic bounds for retained states and attempted computations and applies on every random path.

An inadequate deterministic prerequisite causes suspension. This includes a shared guard that interrupts a stochastic invocation before it has completed its prescribed attempts; such an interruption is not reported as a sampling failure. For an in-range helper call, Lemma 25 gives conditional failure probability at most \(2^{-\sigma}S^{-800}\). A cover draw passes jointly with probability greater than \(1/2\) under its checked parameters. Using fresh draws for the prescribed \(2(\sigma+1000\log_2 S+10)\) attempts makes its exhaustion probability at most \(2^{-2\sigma-20}S^{-2000}\le2^{-\sigma}S^{-800}\). The input to either invocation may depend on all earlier outputs: the stated bounds are conditional on that history, and the new randomness is fresh.

There are at most \(S^{500}\) such invocations. Summing their conditional probabilities, or equivalently summing probabilities of the first exhausted invocation, bounds the probability of any sampling failure by \[S^{500}\,2^{-\sigma}S^{-800} \le2^{-\sigma}S^{-60}.\] Exhausting the full allowed retries is explicitly reported as detected failure, never as suspension. Until such failure, all returned states satisfy the deterministic representation, length, and recourse guarantees, regardless of the samples that produced them. ◻

Completion of the proof of Lemma 22. Lemma 40 supplies the simple selected-support paths required by Lemma 37, with at most \(p_*\) hops. Lemma 42 constructs its one child with the required capacity, integral prices, and degree. Lemma 43 preserves certified child scores in the public representation. The bottom computation gives at least half the optimum; applying the reduction at each level and comparing the exact loop candidates therefore gives at level \(i\) a score at least \[\tfrac12 b^{-2(d-i)}\mathop{\mathrm{OPT}}_i.\] At the outer level this is at least \(M^{-10/a}\mathop{\mathrm{OPT}}\) for sufficiently large parameters, as required by the checked quality inequalities. If \(\mathop{\mathrm{OPT}}>0\), the chosen pieces have positive raw length and achieve their certified score. If \(\mathop{\mathrm{OPT}}=0\), an empty answer is allowed.

The pieces are unit-multiplicity actual edges or public forest paths, their signed sum is a circulation, and their number has the required bound by Lemma 43. Vertex multiplicity is at most \(M^{10/a}/4\) by Lemma 44. Lemma 45 gives explicit forest edits, retained immutable lifetimes, and removals before map invalidation. Finally Lemmas 46 and 47 give initialization time \(M^{1+O(1/a)+o_a(1)}\), every-path amortized operation and log cost \(M^{O(1/a)+o_a(1)}\), finite arithmetic, and the stated conditional instance failure bound. These are all the assertions of the flat-cycle interface. ◻

Explicit output and one uniform program

We first reduce exact cardinality to subcubic perfect matching, retaining all witness information. We then combine the fixed-parameter procedures into the single program of Theorem 1. Throughout this section \(p=n+m\) refers to the original explicit input size.

Witness-preserving reductions

Read adjacency-list boundaries and neighbor occurrences once. Emit an edge record, with its endpoints and a fresh identifier, only at the occurrence \((u,v)\) with \(u<v\). Input validity and simplicity imply that this produces each edge exactly once, regardless of list order. Retain all vertex records, including isolates. This takes \(O(n+m)\) word operations, and the resulting read-only data can be shared among branches. If \(n=1\), return the empty list.

Two reductions connect the required output to the subcubic procedure. First, a permutation network turns a cardinality threshold into a perfect-matching instance. Second, a path gadget reduces the degree to three. We give both constructions and their explicit witness decoders. For a target cardinality \(k\), adding \(n-2k\) new vertices adjacent to every original vertex would turn a size-\(k\) matching into a perfect matching, but could add quadratically many edges. The network replaces these complete connections by \(O(n\log(n+2))\) positions and arcs while allowing any \(n-2k\) original vertices to reach the required new vertices.

Lemma 48 (A linearithmic permutation network). For every power of two \(s\ge1\), there is a directed acyclic network with \(s\) distinct input positions and \(s\) distinct output positions, of size \(O(s\log(s+2))\), in which every permutation of inputs to outputs is routable by vertex-disjoint directed paths. A position may be both an input and an output when \(s=1\). The network can be constructed uniformly within its size bound.

Proof. Use the recursive switching construction underlying permutation networks (Beneš 1964). For \(s=1\), take a single position and its length-zero route. For \(s>1\), take \(s\) new input positions, \(s\) new output positions, and disjoint upper and lower networks on \(s/2\) positions. Connect each input in pair \(2i-1,2i\) to input \(i\) of both copies. Connect output \(j\) of either copy to both positions in output pair \(2j-1,2j\). The size recurrence is \(Q(s)=2Q(s/2)+O(s)\), and all arcs respect the resulting input-to-output order.

Given a permutation, form a bipartite multigraph between its input pairs and output pairs, with one edge per assigned input–output pair. Every vertex has degree two, so the components are even cycles, allowing a cycle of two parallel edges. Color each cycle alternately upper and lower. Every input pair and output pair has one edge of each color. The upper edges specify a permutation through the upper network, and the lower edges one through the lower network. Route inductively and attach the corresponding outer arcs. The two copies are disjoint and each outer position occurs once, giving vertex-disjoint routes. The construction of the network itself uses only the stated recursive lists and port identifiers. ◻

Lemma 49 (Exact cardinality to perfect matching). Given \(G\) and an integer \(0\le k\le\lfloor n/2\rfloor\), one can construct in \(O(m+n\log(n+2))\) time a loopless graph \(H_k\) of that size such that \(H_k\) has a perfect matching if and only if \(G\) has a matching of size \(k\). A perfect matching of \(H_k\) yields an explicit size-\(k\) matching of \(G\) in linear time in the constructed size.

Proof. Put \(b=n-2k\) and choose the least power of two \(s\ge n\). Use the network of Lemma 48 and select \(b\) distinct output positions. Retain all original vertices and edges. Split every network position \(v\) into vertices \(v_{\mathrm{in}},v_{\mathrm{out}}\) joined by an idle edge. An arc \(u\to v\) becomes the edge \(u_{\mathrm{out}}v_{\mathrm{in}}\). Join original vertex \(i\) to the input endpoint of network input \(i\). Finally, add one mandatory sink vertex adjacent to the output endpoint of each selected output. Every constructed vertex is required to be covered by a perfect matching.

In any perfect matching, a network pair either uses its idle edge or has both endpoints matched externally. The numbers of externally matched input and output endpoints are therefore equal. Selected transition edges contribute once to each number and cancel. Original-to-network edges contribute one input endpoint, and edges to mandatory sinks contribute one output endpoint. All \(b\) sinks must be covered, so exactly \(b\) original vertices use network edges. The remaining \(n-b=2k\) original vertices are covered by original edges, which give the decoded matching.

Conversely, let a size-\(k\) matching leave \(b\) original vertices uncovered. Map them bijectively to the selected outputs and extend to a permutation of all \(s\) positions. Route the permutation and keep just those \(b\) paths. On each retained path, select its original-to-input edge, all transition edges, and the edge to its sink. Each active split position is covered once at each endpoint. Use idle edges on all other positions and the original matching on its covered vertices. This is a perfect matching of \(H_k\). For a length-zero route, the two outer edges cover the one split position. For \(b=0\), every network pair is idle.

The number of added positions and edges is \(O(n\log(n+2))\). Store each edge’s type and original identifier. Decoding simply selects the original edges from the returned perfect matching. No list ordering is assumed. ◻

The next reduction uses the path replacement for matching described by Dahlhaus and Karpinski (Dahlhaus and Karpinski 1992). We record the construction and the witness conversion needed here.

Lemma 50 (Degree reduction). A loopless graph with \(v\) vertices and \(e\) edges either has an isolated vertex, certifying absence of a perfect matching, or can be replaced in \(O(v+e)\) time by an equivalent perfect-matching instance of maximum degree three and size \(O(v+e)\). Witnesses can be converted in either direction within the same bound.

Proof. Replace a vertex of degree \(r\ge1\) by a path on \(2r-1\) vertices, attaching its \(r\) old incidences to successive odd positions. Retain the original edge identifiers at both ends. There are \(r\) odd and \(r-1\) even positions. Every even position must be matched internally to an odd one. Exactly one odd position therefore remains for an external edge. Hence each gadget selects exactly one old incidence, and any perfect matching projects to one in the original graph.

Conversely, the odd attachment of a selected original matching edge leaves two even-length paths on its sides. Match those internally. This also covers \(r=1\), whose gadget is a single vertex. Each odd position has at most two path neighbors and one external edge, and each even position has degree two. The number of gadget vertices is \[\sum_u(2\deg(u)-1)=4e-v.\] A degree scan, incidence-port assignment, and path construction give the time bound and the explicit conversions. Check for isolates first. ◻

Applying these reductions at a threshold \(k>0\) gives a loopless subcubic instance with \[ N=O(m+n\log(n+2)),\qquad N\ge n, \tag{129}\] unless absence of a perfect matching is already certified by an isolate. The latter inequality holds because every original vertex has at least one gadget vertex. The construction on a simple input can itself use simple graphs: network arcs, attachments and path edges have their specified distinct positions. It is also covered by the intermediate algorithms’ individual-edge-copy convention.

For the perfect-matching procedure set \(S\) to the first power of two at least \(16+n+m+N\). Thus \[ n\le S=O(p\log(p+2)). \tag{130}\] Check every proposed perfect matching by edge identifier and one-time coverage of every gadget vertex. The reductions then decode its actual original matching in the same auxiliary-size order.

Threshold search and its error budget

Each branch runs its own integer binary search. Initially the feasible lower endpoint is zero, with the stored empty matching, and the exclusive upper endpoint is \(\lfloor n/2\rfloor+1\). An accepted test stores its decoded witness and raises the lower endpoint; a negative test lowers the upper endpoint. There are \(O(\log(n+2))\), and certainly at most \(n+1\), tests. Exact-size feasibility is monotone because edges may be dropped from a matching. Consequently, if no feasible test receives a false negative, the final stored witness is maximum.

For each branch index \(t\ge1\), use \[ a=1000+t,\qquad \sigma=t+20 \tag{131}\] in Theorem 20. A deterministic prerequisite suspension suspends the entire branch with no output. A capped stochastic failure terminates that perfect-matching test with an ordinary negative answer. All invocations use fresh independent bits.

On a feasible perfect-matching instance, conditional on its input and the earlier history, the probability of a false negative is at most \(2^{-\sigma}S^{-5}\). This bound does not condition on future noninterruption. Each valid reached randomized invocation has its declared conditional bound, while a deterministic suspension inside an invocation does not itself issue a negative answer. The main guards permit the coarse polynomial union bound in Theorem 20; each backend instance already includes its capped internal invocations in its reliability estimate. There are no false positive perfect-matching answers. By \(S\ge n+1\) at all nontrivial tests and the bound on the number of adaptive thresholds, \[ \Pr[\text{branch $t$ completes with an incorrect matching}] \le 2^{-(t+20)}. \tag{132}\] No guarantee of avoiding suspension on small inputs is needed for this inequality.

Branch zero performs the same reduction and search, using the deterministic procedure of Lemma 5. Every threshold answer is exact, so this branch always completes with a maximum matching. It uses a fixed polynomial number of instructions after finite-word simulation. Indeed every simple input satisfies \(m\le n(n-1)/2\), and Equation (129) makes every fallback instance polynomial in \(n+2\). No perfect-matching promise is used.

Fixed branches and eventual completion

Lemma 51 (Every-path performance of a fixed branch). For every sufficiently large fixed \(t\), branch \(t\) avoids deterministic suspension on all sufficiently large inputs, on every computation path. Its total instruction count, including unsuccessful tests, threshold search, and explicit witness handling, is \[p^{1+O(1/a)+o_a(1)}.\] The coefficient in \(O(1/a)\) is absolute.

Proof. Each test satisfies Equation (130). The guarded-prefix estimates in Theorem 20 and Lemma 22 show eventual adequacy at fixed sufficiently large \(a,\sigma\), independently of matchability and of which verified constructions or sampling failures occurred. Every accepted sampled structure satisfies its checked deterministic invariants, and rejected attempts obey their own unconditional caps. The estimates therefore apply through a purported first deterministic interruption. They do not require a favorable random stream. An exhausted sampling routine terminates the test rather than suspending it.

The fixed-parameter time bound, the logarithmic number of thresholds, and the reductions give the displayed exponent. Logarithmic and fixed-parameter subpolynomial factors absorb the substitution \(S=O(p\log(p+2))\).

All parameters and adequacy checks are computed by finite integer loops: bit lengths, rounded powers of two, products, quotients, and comparisons of scaled dyadics or ratios. Inadequate giant parameters need not pass their checks. The assertion that fixed-parameter logarithmic factors eventually fit size, multiplicity or quality margins requires no encoded table of thresholds. Fixed-parameter bit lengths and arithmetic/dictionary simulation costs are logarithmic powers, or are already charged in the static flow primitive. Individual random-bit costs are included. ◻

An instruction-level schedule

Weighted dovetailing, familiar from universal search (Levin 1973), lets one program compete with every fixed parameter branch. The proof also accounts for summable error allowances and finite-word execution.

Run rounds \(j=0,1,2,\ldots\). In round \(j\), give branches \(t=0,\ldots,j\), in that order, at most \(2^{j-t}\) elementary implementation steps each, resuming a branch at its interruption point. A suspended branch stays inactive. Stop when the first branch completes with its answer. The winning branch constructs its decoded output privately within its charged work; copying the list of at most \(n/2\) original labeled edges adds input-linear work.

Here a scheduled step means an actual bounded word-RAM instruction with constant control overhead, not a whole graph routine, arbitrary-precision operation, or allocation request. The parameter algorithms consist of a fixed program of graph, list, tree and integer subroutines, the one fixed fallback, and the one fixed integral-flow implementation. Store each branch as a coroutine with a program counter and stack. Use charged digit arrays, even unit-bit digits if desired, for longer logical values. Long virtual indices and addresses are keys in dictionaries of touched cells with a default initial value. Comparisons, initialization, recursion, and dictionary access are themselves interruptible implementations.

Allocate and initialize actual records one word at a time. A paused computation retains its state without copying it; its physical storage is proportional to actual executed allocations plus the shared input. Arenas may retain already spent cells. In particular, constructing an enormous power for a large varying parameter is charged and can pause inside its arithmetic or allocation loop. It is not an atomic reservation of an enormous memory interval. For each fixed branch of Lemma 51, these implementations add only the logarithmic-power overheads already allowed. The fixed fallback simulation still has a polynomial bound.

Lemma 52 (Halting and one physical word constant). The schedule is realizable by one finite program with a fixed physical word constant \(c_0\). On every path, including failed paths, it halts after polynomially many instructions and uses polynomially many addressed words, with constants independent of the input.

Proof. First ignore physical-address overflow, and fix the fallback’s finite digit-array and touched-cell simulation. Its logical clock, bit lengths, and dictionary costs give a fixed polynomial bound \(F(n)\) on its actual work and records. This estimate can use unit-bit data and physical pointer operations; it makes no assumption that the varying parameters of other branches fit a word.

Unless the schedule has already stopped, the fallback receives \(2^j\) steps in round \(j\). It therefore finishes within \(J=O(\log(n+2))\) rounds, with a fixed constant. Total branch allowances through round \(J\) are \[\sum_{j=0}^J\sum_{t=0}^j2^{j-t}<2^{J+2}.\] Starting and switching branches, status checks, and loop control add only polynomial logarithmic overhead; input/setup and output are polynomial as well. Since every allocation is charged, all actually touched physical records before halt are bounded by a fixed polynomial in \(n+2\), even if every arena retains spent cells.

Use independent logical indexing in each branch and actual physical addresses only as pointers, for example with sequential allocation. Interleaving cannot change a simulated algorithm. Choose \(c_0\) once, large enough that this polynomial bound covers physical addresses, quota counters, program control, and input indices in the prescribed \(w\)-bit words. No overflow then occurs on any path. Long logical integers remain digit arrays. Smaller logical words can be masked or simulated; long shifts, divisions and other operations use the corresponding charged loops. Thus the choice of \(c_0\) follows the fixed polynomial bound rather than being a premise about unbounded logical arithmetic. ◻

For the probability calculation, couple the schedule to independent infinite fair-bit streams for its branches, revealing bits only when requested. This is equivalent to generating fresh charged bits during execution. Consider each branch’s own complete computation on its stream, including a possible suspension. Interleaving and the identity of the first finisher impose no conditioning on the error estimates. Branch zero is exact, so Equation (132) gives \[ \Pr[\text{the scheduled output is incorrect}] \le\sum_{t\ge1}2^{-(t+20)}<\frac13. \tag{133}\]

A common exponent envelope

Proof of Theorem 1. Halting and the computation model follow from Lemma 52; explicit output and success follow from the reductions, threshold search, and Equation (133).

Fix a sufficiently large branch index \(t\). If it needs \(T\) steps, round \(j=t+\lceil\log_2\max(1,T)\rceil\) alone gives it enough quota, unless another answer has already terminated the schedule. The total budget through that round is at most a constant times \(2^t(T+1)\), in addition to input and logarithmic control overhead. Thus the single schedule competes with every fixed branch with a fixed-index factor. For every \(\xi>0\), choose \(t\) large enough in the analysis that the absolute \(O(1/a)\) exponent in Lemma 51 is smaller than a suitable fraction of \(\xi\). For all sufficiently large \(p\), its \(o_a(1)\) term, constant factor and logarithmic overhead fit the remaining slack. Consequently the same program has every-path time at most \(p^{1+\xi}\) for all sufficiently large inputs. No choice of \(t\) based on the input is made by the program.

To exhibit one envelope, let \(T(G,\omega)\) be the actual total instruction count of the schedule on input \(G\) and random path \(\omega\). For \(x\ge2\) define \[e(x)=\sup_{\substack{G,\omega:\,n(G)+m(G)\ge x}} \max\left\{0, \log_{n(G)+m(G)}\bigl(\max\{1,T(G,\omega)\}\bigr)-1\right\}.\] The polynomial fallback bounds this supremum, and the preceding every-path estimates imply \(e(x)\to0\). Put \(\eta(q)=e(\max\{2,q\})\) for integers \(q\ge1\). This function is nonnegative and nonincreasing, and it tends to zero. If \(p\ge2\), the term for \((G,\omega)\) in the supremum defining \(e(p)\) gives \[T(G,\omega)\le p^{1+\eta(p)}.\] The only input with \(p=1\) is the one-vertex empty graph, which is handled directly. Choose a fixed \(C\ge1\) large enough to cover its instruction count. The all-input bound in Theorem 1 follows.

For the sparse specialization, let \(E=\sup_{q\ge1}\eta(q)<\infty\). If \(m\le\kappa n\), then \(n\le p\le(1+\kappa)n\) and monotonicity gives \[T(G,\omega)\le C p^{1+\eta(p)} \le C(1+\kappa)^{1+E}n^{1+\eta(n)}.\] Thus the same algorithm and envelope apply for every fixed \(\kappa\). When \(m=\Theta(n^2)\), one instead has \(p=\Theta(n^2)\), giving the stated \(n^{2+o(1)}\) dense specialization.

Every operation, random bit, intermediate construction and output copy is part of \(T\), so no further uncharged term remains. The input conversion did not assume connectedness, nonisolation, or a particular list order. This proves all assertions of Theorem 1. ◻

Prescribed-degree factors

Recall that an \(f\)-factor of \(G=([n],E)\) is a set \(F\subseteq E\) with \(\deg_F(v)=f(v)\) for every vertex \(v\). We reduce its existence and construction to one maximum-matching computation, adapting Tutte’s factor-to-perfect-matching reduction (Tutte 1954) with the split network of Lemma 49. The underlying rearrangeable network is the Beneš construction proved in Lemma 48 (Beneš 1964). In the incidence reduction, each vertex has one port per incident edge, and \(\deg_G(v)-f(v)\) of its ports must be matched locally. Complete connections to that many auxiliary vertices can cost \(\deg_G(v)(\deg_G(v)-f(v))\) edges. A separate permutation network at each vertex permits any such subset of ports at logarithmic expansion in its degree.

Corollary 53 (Almost-linear \(f\)-factor search). There are a single uniform randomized word-RAM algorithm, fixed constants \(c_f\ge2\) and \(C_f\ge1\), and a nonincreasing function \(\eta_f:\mathbb N_{\ge1}\to\mathbb R_{\ge0}\) with \(\eta_f(q)\to0\) as \(q\to\infty\) such that the following holds. The input is a simple graph \(G=([n],E)\) with \(n\ge1\), \(m=|E|\), and \(p=n+m\), in the adjacency-list representation of Section 1, together with one-word integers \(f(v)\) satisfying \(0\le f(v)\le\deg_G(v)\) for all \(v\). With the instruction set and costs of Section 1 and word length \(w_f=\lceil c_f\log_2(n+2)\rceil\), the algorithm halts on every computation path in at most \[C_f p^{1+\eta_f(p)}\] instructions. Every output is either an explicit edge list of a valid \(f\)-factor or NO. If an \(f\)-factor exists, it returns one with probability at least \(2/3\); otherwise it always returns NO.

The \(n\) degree records keep the input length within a constant factor of \(p\). The problem selects an edge subset; it does not encode binary edge multiplicities or optimize edge weights.

Lemma 54 (Linearithmic factor reduction). For the input of Corollary 53, one can construct a possibly empty simple graph \(H_f\) with \(|V(H_f)|+|E(H_f)|=O(m\log(n+2))\) such that \(H_f\) has a perfect matching if and only if \(G\) has an \(f\)-factor. The construction takes \(O(n+m\log(n+2))\) word instructions and records. A perfect matching of \(H_f\) yields an explicit \(f\)-factor within the same bound.

Proof. For every host edge \(e=\{u,v\}\), create distinct port vertices \(x_{u,e}\) and \(x_{v,e}\) and join them by a cross edge labeled by \(e\). For a vertex \(v\) write \(d_v=\deg_G(v)\) and \(b_v=d_v-f(v)\). If \(d_v=0\), then \(f(v)=0\) and no local construction is needed. Otherwise, let \(s_v\) be the least power of two at least \(d_v\) and take a separate copy of the recursive network in Lemma 48. Assign distinct input positions to the \(d_v\) incidences at \(v\), and select \(b_v\) distinct output positions.

Split every network position \(a\) into vertices \(a_{\mathrm{in}}\) and \(a_{\mathrm{out}}\) joined by an idle edge. For every directed arc \(a\to a'\) of the network, add the transition edge \(a_{\mathrm{out}}a'_{\mathrm{in}}\). Join \(x_{v,e}\) to the input endpoint of its assigned input position. At each selected output position, add a new sink vertex adjacent only to that position’s output endpoint. All these local constructions use disjoint new vertices. The recursive network has distinct arcs, and the edge types have distinct endpoint pairs, so \(H_f\) is simple. This also covers \(s_v=1\), when the single position is both an input and an output and there is no transition edge.

Consider any perfect matching of \(H_f\). At every split position, either its idle edge is used, or both endpoints are matched by non-idle edges. Therefore among endpoints matched by non-idle edges, the input and output counts in the network at \(v\) are equal. Each selected transition edge contributes once to each number and cancels from this equality. A port attachment contributes only an input endpoint, and a sink attachment contributes only an output endpoint. All \(b_v\) degree-one sinks must be covered, so exactly \(b_v\) port attachments are used. Every port has only its attachment and its cross edge. Hence exactly \(d_v-b_v=f(v)\) ports at \(v\) use cross edges. The labels of all selected cross edges form a set \(F\subseteq E\) with \(\deg_F(v)=f(v)\) for every \(v\).

Conversely, suppose that \(F\) is an \(f\)-factor. At \(v\), exactly \(b_v\) ports belong to edges outside \(F\). Map their input positions bijectively to the selected outputs and extend this map to a permutation of all \(s_v\) input and output positions. Lemma 48 routes this permutation by vertex-disjoint directed paths; retain only the \(b_v\) paths just chosen. Select the cross edge for each \(e\in F\). Along each retained path, select its port attachment, all its transition edges, and its final sink edge. Each split position on these paths is covered once at its input endpoint and once at its output endpoint. The paths are vertex-disjoint, so these choices do not conflict. Use idle edges at every unused position. All ports, split endpoints, and sinks are now covered exactly once. When \(b_v=0\) no path is retained; a length-zero path when \(s_v=1\) uses just its port and sink attachments. Thus the construction gives a perfect matching of \(H_f\).

For \(d_v>0\) we have \(s_v\le2d_v\), so the local network has \(O(d_v\log(d_v+2))\) positions and arcs. Ports, cross edges, attachments, and sinks add \(O(m)\) more records. Since \(d_v\le n-1\) and \(\sum_v d_v=2m\), their total is \(O(m\log(n+2))\). The uniform network construction and a scan of the adjacency lists take \(O(n+m\log(n+2))\) instructions. Scanning the occurrence with the smaller endpoint label assigns an identifier to each host edge without assuming any list order. Two passes over the constructed edge list count degrees and fill the adjacency-list representation of \(H_f\) within the same bound. Every identifier uses \(O(\log(n+2))\) bits because the constructed size is polynomial in \(n\). Store each cross edge’s host identifier; scanning a returned perfect matching and selecting these labels gives the stated decoder bound. ◻

Proof of Corollary 53. If \(m=0\), validity forces every \(f(v)=0\), so return the empty edge list. Otherwise \(H_f\) is nonempty; construct it and apply the single algorithm of Theorem 1. Check the returned edge identifiers and one-time coverage of every vertex, returning NO unless the list is a perfect matching. Section 9’s output implementation copies at most \(|V(H_f)|/2\) edges on every path, so the check takes \(O(|V(H_f)|+|E(H_f)|)\) instructions, including initialization. If it passes, decode the cross-edge labels by Lemma 54, checking their host identifiers and prescribed degrees.

When the matching call returns a maximum matching, the test accepts exactly when \(H_f\) has a perfect matching and then supplies the factor by the reduction. This maximum-matching event has probability at least \(2/3\). The explicit checks ensure that every accepted edge list is a valid factor, regardless of the matching call’s outcome. Thus a failed call can cause a false negative, whereas an input with no factor always yields NO.

Put \(P=|V(H_f)|+|E(H_f)|=O(p\log(p+2))\). Since \(|V(H_f)|\) is polynomial in \(n\), the matching algorithm’s virtual word length \(\lceil c_0\log_2(|V(H_f)|+2)\rceil\) is at most \(\lceil c_f\log_2(n+2)\rceil\) for one fixed \(c_f\). Choose \(c_f\) large enough also for the construction’s indices. Simulate each virtual word instruction by a constant number of physical instructions, masking to the virtual width for modular arithmetic and Boolean operations. Quotients, remainders, comparisons, shifts, and memory addresses then have the same semantics as in the matching call; each fair random bit remains charged. The fixed masks and input conversion take at most the reduction’s work bound. Thus this is one finite program with a fixed physical word constant, not a separate algorithm for each input size or degree prescription.

For fixed \(\xi>0\), Theorem 1 gives \(O_\xi(P^{1+\xi/2})\) every-path instructions, absorbing small \(P\) into the constant. Together with all reduction, simulation, checking, and output work, the bound on \(P\) gives \(O_\xi(p^{1+\xi})\) for large \(p\), for this same program for every \(\xi>0\). Boundedness of \(\eta\) also gives a fixed polynomial every-path bound. The supremum-envelope construction at the end of Section 9 now gives a nonnegative nonincreasing \(\eta_f(q)\to0\); a fixed \(C_f\) covers the small input sizes, including \(p=1\). ◻

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