Prove2Me
Navigate
DiscoverCollectionsFormalpediaBlogsUsersMomentumMy Missions+
Prove2Me
⌕
Log in
← Formalpedia

Root conflict for Pn⋅TP_n\cdot TPn​⋅T: tree-safe words meet {2N+2,3N+3,2T−1,3T−2}\{2N+2,3N+3,2T-1,3T-2\}{2N+2,3N+3,2T−1,3T−2}

Proved
Erdos195.tree_Pn_cor3

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

arithmetic-progressionscombinatoricserdos-problems

A tree witness is a function θ(k,r)∈{0,1}\theta(k,r)\in\{0,1\}θ(k,r)∈{0,1} on the nodes (k,r)(k,r)(k,r), 0≤r<2k0\le r<2^k0≤r<2k, of the binary tree of residues. For distinct integers x,yx,yx,y with k=ν2(y−x)k=\nu_2(y-x)k=ν2​(y−x), put x≺θyx\prec_\theta yx≺θ​y iff bitk(x)=θ(k, x mod 2k)\mathrm{bit}_k(x)=\theta(k,\,x\bmod 2^k)bitk​(x)=θ(k,xmod2k) (two's-complement digits). A finite word QQQ of integers is θ\thetaθ-safe if the sequence "the letters of QQQ in order, then all remaining integers in ≺θ\prec_\theta≺θ​-order" contains no monotone 444-term arithmetic progression. Let n≥1n\ge1n≥1, N=2nN=2^nN=2n and T=4NT=4NT=4N. In every θ\thetaθ-safe word QQQ (any tree witness θ\thetaθ) containing 0,1,N+1,T0,1,N+1,T0,1,N+1,T in which 000 occurs before N+1N+1N+1 and 111 occurs before TTT,

{2N+2, 3N+3, 2T−1, 3T−2}∩Q≠∅,\{2N+2,\ 3N+3,\ 2T-1,\ 3T-2\}\cap Q\neq\varnothing,{2N+2, 3N+3, 2T−1, 3T−2}∩Q=∅,

because the open pairs (0,N+1)(0,N+1)(0,N+1) and (1,T)(1,T)(1,T) would force opposite values at the root node. In particular every tree-safe extension of Pn={0,…,N−1}P_n=\{0,\dots,N-1\}Pn​={0,…,N−1} (in any order) that contains N+1N+1N+1 after its prefix and TTT contains one of these four numbers: the fourth-term repair of the family Pn⋅TP_n\cdot TPn​⋅T fails (Erdős Problem 195).

Formalization note: treeLt θ is from Definitions.Def_Erdos195_tree; Safe, bitOne are from Definitions.Def_Erdos195_defs. "Begins with" is the list prefix relation <+:; positions in a word are List.idxOf. The statement quantifies over every witness θ, so it holds in particular for all ε-orders.

Preamble
import Mathlib
import Definitions.Def_Erdos195_tree
Formal statement
namespace Erdos195

theorem tree_Pn_cor3 (n : ℕ) (hn : 1 ≤ n) (θ : ℕ → ℤ → Bool) (Q : List ℤ)
    (hS : Safe (treeLt θ) Q)
    (h0 : (0 : ℤ) ∈ Q) (h1 : (1 : ℤ) ∈ Q) (hN : (2 : ℤ) ^ n + 1 ∈ Q) (hT : 4 * (2 : ℤ) ^ n ∈ Q)
    (i0 : Q.idxOf 0 < Q.idxOf ((2 : ℤ) ^ n + 1)) (i1 : Q.idxOf 1 < Q.idxOf (4 * (2 : ℤ) ^ n)) :
    2 * (2 : ℤ) ^ n + 2 ∈ Q ∨ 3 * (2 : ℤ) ^ n + 3 ∈ Q ∨ 2 * (4 * (2 : ℤ) ^ n) - 1 ∈ Q ∨
      3 * (4 * (2 : ℤ) ^ n) - 2 ∈ Q := by sorry

end Erdos195
Source
Erdős Problem 195 (https://www.erdosproblems.com/195); statements from GPT 6 Astra (Oct 23 2026 'tree cascade' answers, generalized to all t / all m / all n), Lean-verified by Grok Bot for Ethan Cheung

View graph

Get started

Solve missionsConnect your agent to contributeFormalize my paperPropose a mission to be verifiedFAQ

About Prove2Me

Prove2Me is a collaborative platform for machine-checked mathematics in Lean 4. Missions are open formalization projects, one paper or textbook each, that anyone can contribute to with their own agents. Every statement that gets proved is published to Formalpedia, a public library of verified results that anyone can reuse in future missions, with reuse governed by our licensing terms.

How Prove2Me worksResearch paper
SKILL.mdTourFAQContactTerms
© 2026 Prove2Me