Invertible on with invertible exists iff (Erdős 624 cross-review)
ProvedErdos624.linear_ingredient_iffcombinatoricserdos-problems
For a natural number there is a linear map such that both and are bijective if and only if . (For the space is trivial; for the only invertible map is , and then .)
This is the linear ingredient of the cross-review's local counterexample (blocks , , ), used there with .
Context: Erdős Problem 624 asks whether , where is the least such that some with satisfies for all . This is an auxiliary finite statement from Sol's Round 12 Step 17 cross-review / resilience analysis of the two-track (PAIR) case. It does not settle Erdős Problem 624, PAIR or Step 17.
Preamble
import Mathlib open Finset
Formal statement
namespace Erdos624
theorem linear_ingredient_iff (d : ℕ) :
(∃ T : (Fin d → ZMod 2) →ₗ[ZMod 2] (Fin d → ZMod 2),
Function.Bijective T ∧ Function.Bijective (fun v => v + T v)) ↔ d ≠ 1 := by sorry
end Erdos624
Source
Erdős Problem 624 (https://www.erdosproblems.com/624); Round 12: Sol's 'Step 17: First-Round Cross-Review' and 'Step 17: resilient relations below the top layer'; derived from GPT 6.1 Sol's analysis, relayed by Ethan Cheung; hand-checked and Lean-verified by Grok Bot for Ethan Cheung (Oct 24 2026). Auxiliary result only; does NOT settle PAIR / Step 17 / #624.