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Finite equal-load theorem — two-sided bound

Proved
ABKUFinite.finite_balanced_allocation

by Yuxuan Xu · Oct 29, 2026 · Mathlib 0df444a (Lean v4.33.1)

For every natural number ddd, there exists a nonnegative real CCC such that for every positive real ε\varepsilonε, there exists a natural number NNN such that, for every natural number n≥Nn\ge Nn≥N, the uniform probability that the maximum load differs from log⁡log⁡(n+1)/log⁡(d+2)\log\log(n+1)/\log(d+2)loglog(n+1)/log(d+2) by at most CCC is at least 1−ε1-\varepsilon1−ε. The process allocates n+1n+1n+1 balls sequentially to n+1n+1n+1 initially empty bins, each ball making d+2d+2d+2 independent uniform choices with replacement and joining the earliest minimum-load choice. The constant CCC may depend on ddd but not on ε\varepsilonε or nnn; NNN may depend on ddd and ε\varepsilonε.

Preamble
import Definitions.Def_ABKUFinite_Model
Formal statement
namespace ABKUFinite
theorem finite_balanced_allocation (d : ℕ) : TwoSided d := by sorry
end ABKUFinite
Source
Azar, Broder, Karlin, Upfal (1999), Balanced Allocations, https://cs.brown.edu/people/eupfal/papers/SICOMP29.pdf, p.181; Theorems 3.2 and 3.3 combined; equal-load clause of Theorem 1.1
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What the Lean code literally says, in plain math · gpt-6.1-sol

For every natural number ddd, there exists a nonnegative real CCC such that for every positive real ε\varepsilonε, there exists a natural number NNN such that, for every natural number n≥Nn\ge Nn≥N, the uniform probability that the maximum load differs from log⁡log⁡(n+1)/log⁡(d+2)\log\log(n+1)/\log(d+2)loglog(n+1)/log(d+2) by at most CCC is at least 1−ε1-\varepsilon1−ε. The process allocates n+1n+1n+1 balls sequentially to n+1n+1n+1 initially empty bins, each ball making d+2d+2d+2 independent uniform choices with replacement and joining the earliest minimum-load choice. The constant CCC may depend on ddd but not on ε\varepsilonε or nnn; NNN may depend on ddd and ε\varepsilonε.

Human review
  • Endorsed by Shuze Chen · Oct 29, 2026

    Confirmed by the moderator at approval.

  • Endorsed by Yuxuan Xu · Oct 29, 2026

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

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