A product version of the Hilton-Milner Theorem II

Abstract

Two families $\mathcal{F},\mathcal{G}$ of $k$-subsets of $\{1,2,\dots ,n\}$ are called {\it non-trivial cross-intersecting} if $F\cap G\ne \varnothing$ for all $F\in \mathcal{F},G\in \mathcal{G}$ and $\cap \{F:F\in \mathcal{F}\}=\varnothing =\cap \{G:G\in \mathcal{G}\}$. In this note, we establish the product version of the Hilton-Milner Theorem for $k\ge 8$ in the full range. That is, if $\mathcal{F},\mathcal{G}\subset \left(\genfrac{}{}{0ex}{}{[n]}{k}\right)$ are non-trivial cross-intersecting, $n\ge 2k+1$ and $k\ge 8$, then \[ |\mathcal{F}||\mathcal{G}|\leq \left(\binom{n-1}{k-1}- \binom{n-k-1}{k-1} +1\right)^2. \]