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Display (8), p. 16 — D^µ(≿_ab, ≿⁺_ab) = µ{b}

Proved
PrefSemimetric.TopDiff.eq_8

by mikedeng1 · Oct 20, 2026 · Mathlib 0df444a (Lean v4.33.1)

p2o-batch-pfp2ap2o-gran-per-chapterp2o-plan-paperp2o-v1preference-relationstop-difference-semimetric

Let XXX be a finite set, μ\muμ a positive measure on 2X2^X2X, and a,b∈Xa,b\in Xa,b∈X distinct. The relations ≿ab\succsim_{ab}≿ab​ (every alternative other than a,ba,ba,b strictly above both aaa and bbb, nothing else) and ≿ab+\succsim^+_{ab}≿ab+​ (the same, plus aaa strictly above bbb) are acyclic orders, and

Dμ(≿ab,≿ab+)=μ{b}.D^\mu(\succsim_{ab},\succsim^+_{ab})=\mu\{b\}.Dμ(≿ab​,≿ab+​)=μ{b}.

The value shows that d(≿ab,≿ab+)d(\succsim_{ab},\succsim^+_{ab})d(≿ab​,≿ab+​) plays the role of the point mass μ{b}\mu\{b\}μ{b} in Axiom 3, which is how the measure is recovered in the "only if" direction of Theorem 3.1.

Formalization Note μ{b}=w(b)\mu\{b\}=w(b)μ{b}=w(b) for the point weights w≥0w\ge0w≥0. The two acyclicity conjuncts record that both relations lie in A(X)\mathcal A(X)A(X), as the page's "partial orders" asserts.

Preamble
import Mathlib
import Definitions.Def_PrefSemimetric_TopDiff_Model
Formal statement
namespace PrefSemimetric.TopDiff

theorem eq_8 {X : Type*} [Fintype X] [DecidableEq X]
    (w : X → ℝ) (hw : ∀ x, 0 ≤ w x) (a b : X) (hab : a ≠ b) :
    IsAcyclicOrder (prefAB a b) ∧ IsAcyclicOrder (prefABplus a b) ∧
      Dmu w (prefAB a b) (prefABplus a b) = w b := by sorry

end PrefSemimetric.TopDiff
Source
Nishimura & Ok, A Class of Dissimilarity Semimetrics for Preference Relations, arXiv:2203.04418v1, p. 16, display (8); ≿ab, ≿+ab defined p. 14
Human review
  • Endorsed by Shuze Chen · Oct 20, 2026

    Confirmed by the moderator at approval.

  • Endorsed by mikedeng1 · Oct 20, 2026

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

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