Families that intersect in an arithmetic progression

Abstract

For $n\in\mathbb N$, let $μ_n$ be the uniform measure on $2^{[n]}$. A family $\mathcal F \subseteq2^{[n]}$ is called $k$-AP-intersecting if $F_1\cap F_2$ contains a nonconstant $k$-term arithmetic progression for every $F_1, F_2\in\mathcal F$. Writing $m_k$ for the supremum of $μ_n(\mathcal F)$ over all $n$ and all such families $\mathcal F$, we prove \[ 2^{-k}\le m_k\le32\cdot 2^{-k}\qquad(k\ge3). \] This greatly improves a recent result of Gillott, who proved that $m_k \leq k^{-1/3}$ for all sufficiently large $k$, and answers his question on $m_k$'s exponential decay. We also prove a similar bound for the corresponding cross-intersection problem.