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Alice-starting exact pointer jumping

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QAlgorithms.Papers.Independent.Intakethird100.P81.alice_starting_pointer_upper

by Gohan · Nov 1, 2026 · Mathlib 0df444a (Lean v4.33.1)

prove2quantum

Let n≥2n\ge2n≥2 and k≥1k\ge1k≥1. Alice and Bob have unrestricted pointer maps on two disjoint sets of nnn labelled vertices, and v1v_1v1​ is a fixed Alice vertex independent of the inputs. Let gkg_kgk​ be the endpoint after k+1k+1k+1 pointer applications. There is an absolute C>0C>0C>0, chosen before k,n,v1k,n,v_1k,n,v1​, and an Alice-starting protocol with at most kkk messages which is correct on every input with probability one and has worst-case bit cost

c≤Cklog⁡2n.c\le Ck\log_2 n.c≤Cklog2​n.

The output is the whole endpoint, so its bit parity is available locally as well. The last pointer application is local to the receiver after the kkk charged messages; announcing the answer is uncharged. The public-mixture representation includes a deterministic protocol as a single seed.

Preamble
import Definitions.Def_QAlgorithms_Papers_Independent_Intakethird100_P81
Formal statement
noncomputable section

open scoped Matrix ComplexConjugate ENNReal

attribute [local instance] Classical.propDecidable

namespace QAlgorithms.Papers.Independent.Intakethird100.P81

open QAlgorithms.Papers.Independent.Information100.P54 (relativeEntropyBits)

open QAlgorithms.Papers.Independent.Bridge50.P44 (ClassicalFirst)

/-- Abstract p.1 and Definition 11 p.12: Alice starts, with exact endpoint output after
k+1 evaluations; the final receiver performs the last evaluation locally. -/
theorem alice_starting_pointer_upper :
    ∃ C : ℝ, 0 < C ∧ ∀ (k n : ℕ), 0 < k → 2 ≤ n → ∀ v1 : Fin n,
      ∃ (c : ℕ) (P : PublicProtocol (PointerMap n) (PointerMap n) (Vertex n)),
        P.bounded .alice k c ∧ P.computes (endpoint k v1) 0 ∧
        (c : ℝ) ≤ C * k * Real.logb 2 n := by sorry

end QAlgorithms.Papers.Independent.Intakethird100.P81

end
Source
On rounds in quantum communication, arXiv:quant-ph/0004100v5, PDF p. 1, Abstract; Definition 11 on PDF p.12
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What the Lean code literally says, in plain math · gpt-6.1-sol

There exists a real constant C>0C>0C>0 such that, for every pair of natural numbers k,nk,nk,n with k>0k>0k>0 and n≥2n\ge2n≥2, and every starting index v1∈{0,…,n−1}v_1\in\{0,\ldots,n-1\}v1​∈{0,…,n−1}, there exist a natural number ccc and a public randomized classical protocol PPP with inputs a,b:{0,…,n−1}→{0,…,n−1}a,b:\{0,\ldots,n-1\}\to\{0,\ldots,n-1\}a,b:{0,…,n−1}→{0,…,n−1} and output in the disjoint union of two copies of {0,…,n−1}\{0,\ldots,n-1\}{0,…,n−1}, satisfying the following conditions. The protocol consists of finitely many deterministic communication trees with nonnegative real weights summing to one. Each internal node sends one Boolean determined solely by the sending party’s input and follows the corresponding branch; a leaf outputs a function of either Alice’s input or Bob’s input. Every tree of strictly positive weight has communication cost at most ccc, where cost is the maximum number of internal nodes on a path, and has at most kkk rounds starting with Alice: an output-only tree has zero rounds, while a nonempty path counts its consecutive speaker blocks, with an additional initial round if its first speaker is Bob. For every a,ba,ba,b, the total weight of trees whose output is succ⁡a,b k+1(inl⁡v1)\operatorname{succ}_{a,b}^{\,k+1}(\operatorname{inl}v_1)succa,bk+1​(inlv1​) is at least 111, where succ⁡a,b(inl⁡v)=inr⁡(a(v))\operatorname{succ}_{a,b}(\operatorname{inl}v)=\operatorname{inr}(a(v))succa,b​(inlv)=inr(a(v)) and succ⁡a,b(inr⁡v)=inl⁡(b(v))\operatorname{succ}_{a,b}(\operatorname{inr}v)=\operatorname{inl}(b(v))succa,b​(inrv)=inl(b(v)). Finally, c≤Cklog⁡2nc\le Ck\log_2 nc≤Cklog2​n. Trees of weight zero are not subject to the cost and round bounds.

Human review
  • Endorsed by Shuze Chen · Nov 1, 2026

    Confirmed by the moderator at approval.

  • Endorsed by Gohan · Nov 1, 2026

    Confirmed by the mission captain (proposal self-audit).

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