A product version of the Hilton-Milner Theorem II

TL;DR

This paper proves a product version of the Hilton-Milner Theorem for non-trivial cross-intersecting families of k-subsets, extending the range of parameters for which the bound holds.

Contribution

It establishes the product bound for non-trivial cross-intersecting families for all k≥8 and n≥2k+1, expanding previous results.

Findings

Proves the product bound for non-trivial cross-intersecting families for k≥8.

Extends the Hilton-Milner Theorem to a broader parameter range.

Provides a new combinatorial inequality for families of subsets.

Abstract

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