The maximum product of sizes of cross-$t$-intersecting uniform families

Abstract

We say that a set $A$ \emph{$t$-intersects} a set $B$ if $A$ and $B$ have at least $t$ common elements. Two families $\mathcal{A}$ and $\mathcal{B}$ are said to be \emph{cross-$t$-intersecting} if each set in $\mathcal{A}$ $t$-intersects each set in $\mathcal{B}$. For any positive integers $n$ and $r$, let ${[n] \choose r}$ denote the family of all $r$-element subsets of $\{1,2,\dots, n\}$. We show that for any integers $r$, $s$ and $t$ with $1 \leq t \leq r \leq s$, there exists an integer $n_0(r,s,t)$ such that for any integer $n \geq n_0(r,s,t)$, if $\mathcal{A} \subset {[n] \choose r}$ and $\mathcal{B} \subset {[n] \choose s}$ such that $\mathcal{A}$ and $\mathcal{B}$ are cross-$t$-intersecting, then $|\mathcal{A}||\mathcal{B}| \leq {n-t \choose r-t}{n-t \choose s-t}$, and equality holds if and only if for some $T \in {[n] \choose t}$, $\mathcal{A} = \{A \in {[n] \choose r} \colon T \subset A\}$ and $\mathcal{B} = \{B \in {[n] \choose s} \colon T \subset B\}$. This verifies a conjecture of Hirschorn.