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Lemma 1, Eq.(57): path rounding

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QAlgorithms.Papers.Independent.Intakethird100.P35.path_rounding

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

prove2quantum

Let G′=(V,E′)G'=(V,E')G′=(V,E′) be a finite undirected graph with distinct vertices s,ts,ts,t. Fix positive integers m,km,km,k and finite nonnegative edge capacities cec_ece​, which may depend on m,km,km,k. Let NNN be the number of all simple directed paths from sss to ttt in G′G'G′, and let fs→tmax⁡(c)f_{s\to t}^{\max}(c)fs→tmax​(c) be the maximum net source outflow subject to nonnegative oriented flows, the shared constraint fvw+fwv≤c{v,w}f_{vw}+f_{wv}\le c_{\{v,w\}}fvw​+fwv​≤c{v,w}​, and conservation at other vertices. Replace edge eee by ⌊mkNce⌋\lfloor mkNc_e\rfloor⌊mkNce​⌋ unit edges. There is a family of FFF edge-disjoint sss--ttt paths in this actual multigraph satisfying

F≥mkN(fs→tmax⁡(c)−1k).F\ge mkN\left(f_{s\to t}^{\max}(c)-\frac1k\right).F≥mkN(fs→tmax​(c)−k1​).

This turns fractional single-pair flow into an integral path packing. If no path exists, N=0N=0N=0 and the empty family satisfies the statement.

Preamble
import Definitions.Def_QAlgorithms_Papers_Independent_Intakethird100_P35
Formal statement
noncomputable section

namespace QAlgorithms.Papers.Independent.Intakethird100.P35

open scoped BigOperators

attribute [local instance] Classical.propDecidable Classical.decEq

/-- PDF p.20, Lemma 1, (57): actual edge-disjoint paths in floor(mkNc).
The source's positive integers m,k are explicit; N is the number of ALL simple
s-to-t paths. Capacities are quantified after m,k and may depend on them. -/
theorem path_rounding {n e : ℕ} (G : EdgeGraph n (Fin e))
    (hSimple : IsSimple G) (s t : Fin n) (hst : s ≠ t)
    (m k : ℕ) (hm : 0 < m) (hk : 0 < k)
    (c : Fin e → ℝ) (hc : ∀ a, 0 ≤ c a) :
    ∃ P : Packing G (roundedCopies c m k (pathCount G s t))
      (fun _ : Fin 1 => s) (fun _ : Fin 1 => t),
      ((m * k * pathCount G s t : ℕ) : ℝ) *
        (singlePairFlowValue G c s t - 1 / (k : ℝ)) ≤ (P.count 0 : ℝ) := by sorry

end QAlgorithms.Papers.Independent.Intakethird100.P35

end
Source
Linear programs for entanglement and key distribution in the quantum internet, arXiv:1809.03120v3, PDF p. 20, Lemma 1, Eq.(57): path rounding
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What the Lean code literally says, in plain math · gpt-6.1-sol

For all natural numbers n,en,en,e, let GGG have vertex set V={0,…,n−1}V=\{0,\ldots,n-1\}V={0,…,n−1} and edge set F={0,…,e−1}F=\{0,\ldots,e-1\}F={0,…,e−1}, with distinct endpoints ℓ(a),ρ(a)\ell(a),\rho(a)ℓ(a),ρ(a) for each edge. Assume that whenever two edge labels have the same endpoints in the same or reversed order, those edge labels are equal. Choose vertices s,t∈Vs,t\in Vs,t∈V with s≠ts\ne ts=t, positive natural numbers m,km,km,k, and a function c:F→Rc:F\to\mathbb Rc:F→R with c(a)≥0c(a)\ge0c(a)≥0 for every edge. Let NNN count all paths obtained by choosing a length d<nd<nd<n, an injective sequence v0,…,vdv_0,\ldots,v_dv0​,…,vd​ with v0=sv_0=sv0​=s, vd=tv_d=tvd​=t, and ddd edge labels joining consecutive vertices in either orientation. Let F∗F_*F∗​ be the total real supremum of the set of source outflows Δh(s)\Delta_h(s)Δh​(s) over functions h:F×{0,1}→Rh:F\times\{0,1\}\to\mathbb Rh:F×{0,1}→R satisfying h(a,b)≥0h(a,b)\ge0h(a,b)≥0, h(a,0)+h(a,1)≤c(a)h(a,0)+h(a,1)\le c(a)h(a,0)+h(a,1)≤c(a) for every edge, and Δh(v)=0\Delta_h(v)=0Δh​(v)=0 for every v≠s,tv\ne s,tv=s,t, where Δh(v)=∑a∈F(1ℓ(a)=v(h(a,0)−h(a,1))+1ρ(a)=v(h(a,1)−h(a,0)))\Delta_h(v)=\sum_{a\in F}\bigl(\mathbf1_{\ell(a)=v}(h(a,0)-h(a,1))+\mathbf1_{\rho(a)=v}(h(a,1)-h(a,0))\bigr)Δh​(v)=∑a∈F​(1ℓ(a)=v​(h(a,0)−h(a,1))+1ρ(a)=v​(h(a,1)−h(a,0))). Replace each edge aaa by L(a)=⌊(mkN)c(a)⌋NL(a)=\lfloor(mkN)c(a)\rfloor_{\mathbb N}L(a)=⌊(mkN)c(a)⌋N​ labeled copies with the same endpoints. Then there exists a packing for the single terminal pair (s,t)(s,t)(s,t): it has a natural number qqq of selected paths, each with length less than nnn, injective vertex sequence from sss to ttt, and copied edges joining consecutive vertices in either orientation, such that distinct selected paths share no copied edge, and (mkN)(F∗−1/k)≤q(mkN)(F_*-1/k)\le q(mkN)(F∗​−1/k)≤q, with natural numbers interpreted as reals in this inequality. The assumptions force n≥2n\ge2n≥2 but allow e=0e=0e=0 and allow N=0N=0N=0; if N=0N=0N=0, every L(a)=0L(a)=0L(a)=0, the only possible count is q=0q=0q=0, and the asserted bound is 0≤00\le00≤0. The inequality itself does not require a positive count when its left side is nonpositive.

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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