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Part (c): randomly scored sets, fair coin on ties

Proved
GameSetMatch.part_c

by willma · Sep 4, 2026 · Mathlib c5ea003 (Lean v4.30.0)

label-aggregationmajority-voteprobabilityrandom-walktennis

Each completed set is scored to its winner with probability aaa, ignored with probability bbb, or scored to its loser with probability ccc (a,b,c≥0a,b,c\ge0a,b,c≥0, a+b+c=1a+b+c=1a+b+c=1, a>ca>ca>c). The scoring rule is applied once per completed set, at the outermost layer only: games and points inside a set are decided by plain majorities with no randomness, and only the set's final result is scored. For q∈(1/2,1)q\in(1/2,1)q∈(1/2,1), a nonempty list of odd set sizes >1>1>1, and n>1n>1n>1: the fair-coin probability of winning from nnn scored nested sets is strictly less than from nm1⋯mknm_1\cdots m_knm1​⋯mk​ scored single points (each point is a set and receives the same random scoring). No condition on bbb.

Preamble
import Definitions.Def_GameSetMatchInternal
Formal statement
namespace GameSetMatch

/-- Part (c). -/
theorem part_c (a b c q : ℝ) (ms : List ℕ) (n : ℕ)
    (ha : 0 ≤ a) (hb : 0 ≤ b) (hc : 0 ≤ c) (habc : a + b + c = 1) (hca : c < a)
    (hq0 : 1 / 2 < q) (hq1 : q < 1)
    (hms : ∀ m ∈ ms, Odd m ∧ 1 < m) (hne : ms ≠ []) (hn1 : 1 < n) :
    fairCoinWinProb (scoredSetLaw a b c (nestedBestOfWinProb ms q)) n
      < fairCoinWinProb (scoredSetLaw a b c q) (n * ms.prod) := by
  sorry

end GameSetMatch
Source
Mission statement 'Don't Label Twice: Game, Set, Match', part (c). Generalises Dorner & Hardt, "Don't Label Twice: Quantity Beats Quality when Comparing Binary Classifiers on a Budget", ICML 2024, arXiv:2402.02249v3 (8 Apr 2026): Section 3, Proposition 1 and the displayed finite-sample inequality; Section 5, Conjecture 1 (independent, homogeneous-label case).
Read-back

What the Lean code literally says, in plain math · claude-opus-5

Blind read-back by an independent auditor agent (Opus 5) given only the Lean code and the platform auditor instructions; docstrings and comments ignored; identifier names treated as opaque; no access to the informal statement, the source proof, or the captain's intent.

A single theorem, proof sorry. Explicit arguments: four real numbers a, b, c, q, a list ms of naturals, and a natural n. Hypotheses, in order:

  1. 0 ≤ a (non-strict);
  2. 0 ≤ b (non-strict);
  3. 0 ≤ c (non-strict);
  4. a + b + c = 1;
  5. c < a (strict; together with 1 and 3 this forces a > 0 and 0 ≤ c < a ≤ 1);
  6. 1/2 < q (strict);
  7. q < 1 (strict);
  8. every member m of the list ms is odd and satisfies 1 < m;
  9. ms ≠ [];
  10. 1 < n (no oddness requirement on n).

Conclusion:

W( ν[a,b,c; Nest(ms, q)] , n )   <   W( ν[a,b,c; q] , n · ∏ms )

Fully unfolded: for a real parameter r, let ν_r : ℤ → ℝ be the three-point weight function

ν_r(+1) = r·a + (1 − r)·c,    ν_r(−1) = r·c + (1 − r)·a,    ν_r(0) = b,
ν_r(z)  = 0 for every integer z outside {−1, 0, +1};

note ν_r(+1) + ν_r(−1) + ν_r(0) = a + b + c, which the hypotheses fix at 1. Let L^{ν_r}_N be defined by L^{ν_r}_0 = the indicator of 0 and

L^{ν_r}_{N+1}(z) = ν_r(−1)·L^{ν_r}_N(z+1) + ν_r(0)·L^{ν_r}_N(z) + ν_r(+1)·L^{ν_r}_N(z−1),

and let W(ν_r, N) = Σ_{z = 1}^{N} L^{ν_r}_N(z) + L^{ν_r}_N(0)/2, the sum running over integers z from 1 to N inclusive plus half the value at 0. The theorem asserts

[ Σ_{z=1}^{n} L^{ν_R}_n(z) + L^{ν_R}_n(0)/2 ]
      <  [ Σ_{z=1}^{n·∏ms} L^{ν_q}_{n·∏ms}(z) + L^{ν_q}_{n·∏ms}(0)/2 ]

where R = Nest(ms, q) = B_{m₁}(B_{m₂}(⋯B_{m_k}(q)⋯)) with the head of ms outermost and B_M(x) = Σ_{i ≤ M, 2i > M} C(M, i) x^i (1 − x)^(M − i), and ∏ms is the product of the list entries. Key structural points: the two sides use the same a, b, c; they differ in the parameter fed to ν (the nested value R on the left, the bare q on the right) and in the number of convolution steps (n on the left, n·∏ms on the right, the latter being at least 3n). The summation range on each side is tied to that side's own step count. The inequality is strict, with the nested/fewer-steps expression on the left. Degenerate values are reachable under the hypotheses: b may be 0, and c may be 0. No hypothesis asserts that R lies in [0, 1] or that ν_R has nonnegative values; these would have to be derived. The hypotheses are satisfiable (a = 0.5, b = 0.3, c = 0.2, q = 0.6, ms = [3], n = 2), so the statement is not vacuous. This theorem matches conjunct 3 of file 2.


Human review
  • Endorsed by Shuze Chen · Sep 4, 2026

  • Endorsed by willma · Sep 4, 2026

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

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