HvdH machine lemma CR.stA_cost (Parameter computation, part 4: costs (namespace `NLogN.CR`))

Proved

NLogN.CR.stA_cost

by xuanji · Oct 13, 2026 · Mathlib 0df444a (Lean v4.33.1)

harvey-van-der-hoeven-tminteger-multiplicationturing-machines

The following holds (Lean statement, in the scope of module NLogN.HCrPar4):

theorem NLogN.CR.stA_cost (K N nb : ℕ) (hK : 1 ≤ K) (hN : 1 ≤ N) (hnb : N < 2 ^ nb) (s : IState HS)
    (LN LNb : List Item) (hsN : s.G rN = .int N :: LN) (hsNb : s.G rNb = .int nb :: LNb)
    (B : ℕ) (hV : 16 * N + 4 * nb + 4 * K + 16 < 2 ^ B) :
    (stA K).cost s ≤ (2 * nb + 40) * 10 ^ 9 * (B + 2) ^ 2

Context. The lemma belongs to the part Parameter computation, part 4: costs (namespace NLogN.CR).

Formalization Note. This is a lemma of the Lean development NLogN, a multitape-Turing-machine implementation of the Harvey–van der Hoeven $O(n\log n)$ integer multiplication algorithm, used in the proof of NLogN.FIN.theorem_1_1_nlogn : IntMul.MulTimeBound fun n => n * Real.log n. All names refer to the definition bundles IntMul_HvdH_M_Defs1–IntMul_HvdH_M_Defs8 (namespace NLogN), built on the platform model Def_IntMul_MultitapeModel and the published reference Def_IntMul_HvdH_MachineRef. The machine layer: IState HS is a state of 64 item stacks; RM programs have a semantics p.run s, a definedness predicate p.ok M s and a cost p.cost M s relative to a multiplication spec M : MulSpec; zsz z is the bit length of $|z|$ .

Preamble

-- Harvey–van der Hoeven multitape-TM development (NLogN), generated by skeleton subtraction.

import Definitions.Def_IntMul_HvdH_M_Defs8

set_option autoImplicit false
set_option relaxedAutoImplicit false
set_option maxHeartbeats 1000000

set_option linter.unusedSimpArgs false
set_option linter.unusedVariables false
open NLogN
open NLogN.CR
open IntMul RS IntMul.HvdH IntMul.HvdH.Ref
set_option maxHeartbeats 4000000

Formal statement

theorem NLogN.CR.stA_cost (K N nb : ℕ) (hK : 1 ≤ K) (hN : 1 ≤ N) (hnb : N < 2 ^ nb) (s : IState HS)
    (LN LNb : List Item) (hsN : s.G rN = .int N :: LN) (hsNb : s.G rNb = .int nb :: LNb)
    (B : ℕ) (hV : 16 * N + 4 * nb + 4 * K + 16 < 2 ^ B) :
    (stA K).cost s ≤ (2 * nb + 40) * 10 ^ 9 * (B + 2) ^ 2 := by sorry

Source

D. Harvey, J. van der Hoeven, Integer multiplication in time O(n log n), Ann. of Math. 193 (2021), 563–617 (Theorem 1.1: multitape Turing machine multiplication in O(n log n)). Lemma `NLogN.CR.stA_cost` of the Lean development NLogN/HCrPar4.lean (lines 46–244), written for the prove2.me integer-multiplication campaign.

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