rv_half (KFU §6: the axis reversal)

Proved

NLogN.KFU.rv_half

by xuanji · Oct 17, 2026 · Mathlib 0df444a (Lean v4.33.1)

complexity-theoryinteger-multiplicationkappa-improvement

The following holds (Lean statement, in the scope of module Solutions.KFURevTop):

theorem NLogN.KFU.rv_half {r : ℕ} {l : List ℕ} (hr : r ≤ l.length) :
    rv r (r / 2) l = (l.take r).reverse ++ l.drop r

Context. The lemma belongs to the part KFU §6: the axis reversal.

Formalization Note. This lemma belongs to layer 5 of the Lean formalization (integer-multiplication-kappa campaign) of the OpenAI preprint Integer multiplication below $n\log n$ : the uniform item-language machine (register programs over the NLogN multitape framework) assembling the Proposition 14 / Lemma 13 / Proposition 18 pieces, the resampling and chirp stages and the cost recurrences into one fixed machine, culminating in NLogN.KFTop.kappa_bound : IntMul.KappaBound (1 / 2 ^ 182) (the bound of Def_IntMul_MultitapeModel). All names refer to the definition bundles up to IntMul_Kappa_K_Defs33.

Preamble

-- OpenAI-preprint / κ-improvement development (integer-multiplication-kappa), generated by skeleton subtraction.

import Definitions.Def_IntMul_Kappa_K_Defs33

set_option autoImplicit false
set_option relaxedAutoImplicit false
set_option maxHeartbeats 1000000

set_option linter.unusedSimpArgs false
set_option linter.unnecessarySeqFocus false
open NLogN
open NLogN.KFU
open IntMul KFC AModel AModelL12

Formal statement

theorem NLogN.KFU.rv_half {r : ℕ} {l : List ℕ} (hr : r ≤ l.length) :
    rv r (r / 2) l = (l.take r).reverse ++ l.drop r := by sorry

Source

OpenAI, Integer multiplication below n log n (preprint, 23 Sep 2026), https://github.com/openai/math/tree/adc7f1241b42e322a6451854ab7e4b4c146bf78a/preprints/Integer-multiplication-below-n-log-n-September-23-2026. Lemma `NLogN.KFU.rv_half` of the Lean development Solutions/KFURevTop.lean (lines 48–61), integer-multiplication-kappa campaign.

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