The maximum product of sizes of cross-\(t\)-intersecting families

Abstract

Two families of sets \(\mathcal{A}\) and \(\mathcal{B}\) are called \emph{cross-\(t\)-intersecting} if \(|A \cap B| \geq t\) for all \(A \in \mathcal{A}\) and \(B \in \mathcal{B}\). Determining the maximum product of sizes for such cross-\(t\)-intersecting families is an active problem in extremal set theory. In this paper, we verify the following cross-\(t\)-intersecting version of the Erd\H{o}s-Ko-Rado theorem: For \(k\geq l \geq t \geq 3\) and \(\min\{m,n\} \geq (t+1)(k-t+1)\), the maximun value of \(|\mathcal{A}||\mathcal{B}|\) for two cross-\(t\)-intersecting families \(\mathcal{A}\subseteq \binom{[n]}{k}\) and \(\mathcal{B} \subseteq \binom{[m]}{l}\) is \( \binom{n-t}{k-t}\binom{m-t}{l-t}\). Moreover, we characterize the extremal families attaining the upper bound. Our result confirms a conjecture of Tokushige for \(t \geq 3\), and actually proves a more general result.