Proving the optimality of the product construction for the intersection--union problem

Abstract

Let $m(n,p,q)$ be the largest size of a family of subsets of $[n]$ whose members pairwise intersect in at least $p$ vertices and omit at least $q$ common vertices. We prove that a product on disjoint sets attains this maximum whenever $p,q\ge2$ and $n\ge2^{15}(p+q-2)^3$. Together with the second author's result for $p=1$ or $q=1$, this establishes Frankl's Intersection--Union Product Conjecture in a wide range.