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With one track, TrackBad(n,m,0)  ⟺  H(n)>m\mathrm{TrackBad}(n,m,0)\iff H(n)>mTrackBad(n,m,0)⟺H(n)>m (Erdős 624)

Proved
Erdos624.trackBad_zero_iff

by Ethan Cheung Wang · Oct 23, 2026 · Mathlib 0df444a (Lean v4.33.1)

combinatoricserdos-problems

Let H(n)H(n)H(n) be the least mmm such that some f:P(X)→Xf:\mathcal P(X)\to Xf:P(X)→X, ∣X∣=n|X|=n∣X∣=n, satisfies {f(A):A⊆Y}=X\{f(A):A\subseteq Y\}=X{f(A):A⊆Y}=X for all Y⊆XY\subseteq XY⊆X with ∣Y∣≥m|Y|\ge m∣Y∣≥m (Erdős Problem 624). Say TrackBad(n,m,s)\mathrm{TrackBad}(n,m,s)TrackBad(n,m,s) holds if for every family of maps gB:P(U)→Ug_B:\mathcal P(U)\to UgB​:P(U)→U (∣U∣=n|U|=n∣U∣=n) indexed by the subsets BBB of an sss-set, there are Y⊆UY\subseteq UY⊆U with ∣Y∣=m|Y|=m∣Y∣=m and u∈Uu\in Uu∈U with u∉{gB(A):A⊆Y}u\notin\{g_B(A):A\subseteq Y\}u∈/{gB​(A):A⊆Y} for every BBB. By an earlier result, TrackBad(n,m,s)\mathrm{TrackBad}(n,m,s)TrackBad(n,m,s) implies H(n+s)>m+sH(n+s)>m+sH(n+s)>m+s for n≥1n\ge1n≥1. Theorem. For n≥1n\ge1n≥1 and every mmm,

TrackBad(n,m,0)  ⟺  H(n)>m.\mathrm{TrackBad}(n,m,0)\iff H(n)>m.TrackBad(n,m,0)⟺H(n)>m.

So the s=0s=0s=0 case of the track condition is exactly the statement that no map is mmm-good; together with not_trackBad_of_H_le, the track reformulation loses no strength and gains none.

Formalization Note. U=Fin nU=\mathrm{Fin}\,nU=Finn and the sss-set is Fin s\mathrm{Fin}\,sFins, as in Erdos624.track_amplification.

Preamble
import Mathlib
import Definitions.Def_Erdos624_defs
open Finset
Formal statement
namespace Erdos624

theorem trackBad_zero_iff {n m : ℕ} (hn : 0 < n) :
    (∀ g : Finset (Fin 0) → Finset (Fin n) → Fin n, ∃ Y : Finset (Fin n), #Y = m ∧
      ∃ u : Fin n, ∀ B : Finset (Fin 0), ∀ A ∈ Y.powerset, g B A ≠ u) ↔ m < H n := by sorry

end Erdos624
Source
Erdős Problem 624 (https://www.erdosproblems.com/624); definitions from Formal Conjectures 624.lean; Claim B2a of claim from GPT 6.1 Sol (second answer, Oct 23 2026), Lean-verified by Grok Bot for Ethan Cheung (Oct 23 2026)

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