erdos_847

Let $A \subset \mathbb{N}$ be an infinite set for which there exists some $\epsilon > 0$ such that in any subset of $A$ of size $n$ there is a subset of size at least $\epsilon n$ which contains no three-term arithmetic progression. Is it true that $A$ is the union of a finite number of sets which contain no three-term arithmetic progression? A negative answer was given by Reiher, Rödl, and Sales [RRS24], who proved that, for any $0<\mu<1/2$, there exists $A\subseteq \mathbb{N}$ such that every finite colouring of $A$ contains a three-term arithmetic progression, and yet every subset of $A$ of size $n$ contains a subset of size $\geq \mu n$ without a three-term arithmetic progression.

theorem erdos_847 : answer(False) ↔ ∀ (A : Set ℕ), Infinite A → HasFew3APs A →
    ∃ n, ∃ (S : Fin n → Set ℕ), (∀ i, ThreeAPFree (S i)) ∧ A = ⋃ i : Fin n, S i

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