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Theorem 14, p. 14 — an allocation meeting Theorem 8's three conditions gives every agent at least MMS_i(n, M)/(2 − |P|/n)

Proved
LittleCharity.MMS.theorem_14

by mikedeng1 · Oct 8, 2026 · Mathlib 0df444a (Lean v4.33.1)

efxenvy-graphfair-divisionindivisible-goodsmaximin-sharep2o-batch-pfp2ap2o-gran-per-chapterp2o-plan-paperp2o-v1

Let N=[n]N=[n]N=[n] be a set of nnn agents with additive valuations and MMM a set of mmm goods. Let X=⟨X1,…,Xn⟩X=\langle X_1,\dots,X_n\rangleX=⟨X1​,…,Xn​⟩ be a partial allocation with pool of unallocated goods P=M∖⋃iXiP=M\setminus\bigcup_i X_iP=M∖⋃i​Xi​ and k=∣P∣k=|P|k=∣P∣, satisfying the three conditions of Theorem 8:

  1. XXX is EFX;
  2. vi(Xi)≥vi(P)v_i(X_i)\ge v_i(P)vi​(Xi​)≥vi​(P) for all agents iii;
  3. ∣P∣|P|∣P∣ is less than the number of sources (unenvied agents) of the envy graph GXG_XGX​.

Then for every agent i∈Ni\in Ni∈N,

vi(Xi) ≥ MMSi(n,M)2−kn.v_i(X_i)\ \ge\ \frac{\mathrm{MMS}_i(n,M)}{2-\frac{k}{n}} .vi​(Xi​) ≥ 2−nk​MMSi​(n,M)​.

The guarantee interpolates between the factor 12\tfrac1221​ for an empty pool and the factor (1+1n)−1\bigl(1+\tfrac1n\bigr)^{-1}(1+n1​)−1 when ∣P∣=n−1|P|=n-1∣P∣=n−1: the more goods the algorithm leaves to charity, the closer every agent gets to her full maximin share.

Formalization Note The paper's Theorem 14 asserts that there exists an allocation satisfying Theorem 8's conditions, a ½-Nash-social-welfare guarantee, and this bound. Existence of such an allocation is Theorem 8 (the goal of mission I of this series); its proof of the third bullet uses only the three conditions, so the goal is posed as the implication for every such allocation, and combining it with Theorem 8 gives the printed bullet. The Nash-social-welfare bullet is not posed: it rests on Lemma 12, which initialises the algorithm with the allocation of [12], a black box not proved in the paper. Condition 3 forces n≥1n\ge 1n≥1 and k<nk<nk<n, so the denominator 2−k/n2-k/n2−k/n (computed in R\mathbb{R}R) exceeds 1 and MMSi(n,M)\mathrm{MMS}_i(n,M)MMSi​(n,M) uses n≥1n\ge 1n≥1 bundles. Agents are Fin n, 0-based.

Preamble
import Mathlib
import Definitions.Def_LittleCharity_MMS_Setting
Formal statement
namespace LittleCharity.MMS

/-- Theorem 14, p. 14, third bullet: every allocation `X` with LittleCharity.EFX.pool `P = M \ ⋃ X_i` that meets
the three conditions of Theorem 8 (EFX; nobody envies `P`; `|P|` < number of LittleCharity.EFX.sources) gives every
agent `i` at least `MMS_i(n, M)/(2 − k/n)`, where `k = |P|`. -/
theorem theorem_14 {n m : ℕ} (v : Fin n → Finset (Fin m) → ℝ) (hadd : IsAdditive v)
    (X : Fin n → Finset (Fin m)) (hX : LittleCharity.EFX.IsPartialAllocation X) (hEFX : IsEFX v X)
    (hpool : ∀ i, v i (LittleCharity.EFX.pool X) ≤ v i (X i))
    (hsrc : (LittleCharity.EFX.pool X).card < (LittleCharity.EFX.sources v X).card) (i : Fin n) :
    mms (v i) n Finset.univ / (2 - ((LittleCharity.EFX.pool X).card : ℝ) / (n : ℝ)) ≤ v i (X i) := by sorry

end LittleCharity.MMS
Source
Chaudhury, Kavitha, Mehlhorn & Sgouritsa, A Little Charity Guarantees Almost Envy-Freeness, arXiv:1907.04596v3, p. 14, Theorem 14 (third bullet); conditions of Theorem 8, pp. 11–12; proof p. 15
Human review
  • Endorsed by Shuze Chen · Oct 8, 2026

    Confirmed by the moderator at approval.

  • Endorsed by mikedeng1 · Oct 8, 2026

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

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