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Fourier transforms below $n\log n$
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Difficulty:🧠🧠🧠🧠🧠 Ages:13 - ∞
Skills:algorithms, speed Levels:2
Category:Theoretical computer science Lean version:YES! ✔
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Exact Fourier transforms below $n\log n$. Gives a deterministic length-n discrete Fourier transform algorithm using $O(n(\log n)^{1-\delta})$ operations for every n, with explicit $\delta=10^{-13}$. The model uses exact complex arithmetic, unrestricted coefficients and a supplied root of unity, and counts scalar preparation and logarithmic-word indexing.

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released 2026-09-25  |  3 theorems · 12 lemmas · 18 proofs · 14,003 words  |  PLAY LEVEL 1 »  (pdf)
We give a deterministic algorithm that computes the discrete Fourier transform at every length n in $O(n(\log n)^{1-10^{-13}})$ operations. The model uses exact complex arithmetic, unrestricted coefficients, specified Fourier roots, and unit-cost logarithmic-size indexing; scalar preparation and array organization are included.
released 2026-09-25  |  6 theorems · 17 lemmas · 25 proofs · 19,619 words  |  PLAY LEVEL 2 »  (pdf)
We construct exact nonuniform Fourier circuits of size $o(n\log n)$ along an unbounded sequence of lengths, counting every addition, subtraction, and scalar multiplication. This refutes the $\Omega(n\log n)$ lower bound in the unrestricted complex linear-circuit model. The construction uses a finite tensor saving: a tensor power of some invertible nonmonomial complex matrix can be computed with fewer matrix calls than the standard tensor-axis algorithm on the same coordinates, when invertible monomial maps are allowed freely between calls.

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