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ε\varepsilonε-orders have no endpoints when ε\varepsilonε is not eventually constant

Proved
Erdos195.epsLt_no_endpoints

by Ethan Cheung Wang · Oct 22, 2026 · Mathlib 0df444a (Lean v4.33.1)

arithmetic-progressionscombinatoricserdos-problems

Let ε:N→{true,false}\varepsilon:\mathbb{N}\to\{\mathrm{true},\mathrm{false}\}ε:N→{true,false} be not eventually constant. Then ≺ε\prec_\varepsilon≺ε​ has neither a least nor a greatest element:

∀x∈Z  ∃y,y′∈Z: y≺εx≺εy′.\forall x\in\mathbb{Z}\ \ \exists y,y'\in\mathbb{Z}:\ y\prec_\varepsilon x\prec_\varepsilon y'.∀x∈Z  ∃y,y′∈Z: y≺ε​x≺ε​y′.

By contrast, the two's-complement order (ε≡true\varepsilon\equiv\mathrm{true}ε≡true) has least element −1-1−1, which blocks Ho's method on Z\mathbb{Z}Z. Orders without endpoints are the candidate replacement.

Formalization Note epsLt ε x y is x ≠ y ∧ (x / 2^k % 2 = 1 ↔ ε k = true) with k = padicValInt 2 (y - x), i.e. k=ν2(y−x)k=\nu_2(y-x)k=ν2​(y−x); integer division is floor division, so x / 2^k % 2 is the kkk-th two's-complement digit of xxx.

Preamble
import Mathlib
import Definitions.Def_Erdos195_defs
Formal statement
namespace Erdos195

theorem epsLt_no_endpoints (ε : ℕ → Bool) (hε : ¬ ∃ N : ℕ, ∃ b : Bool, ∀ n ≥ N, ε n = b)
    (x : ℤ) : (∃ y, epsLt ε y x) ∧ (∃ y, epsLt ε x y) := by sorry

end Erdos195
Source
New to this mission (motivated by the obstruction for ε ≡ true); setting from B. S. Ho, arXiv:2609.12780 §2
Human review
  • Endorsed by Shuze Chen · Oct 23, 2026

    Confirmed by the moderator at approval.

  • Endorsed by Ethan Cheung Wang · Oct 23, 2026

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

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