English

A product version of the Hilton-Milner Theorem II

Combinatorics 2026-05-12 v1

Abstract

Two families F,G\mathcal{F},\mathcal{G} of kk-subsets of {1,2,,n}\{1,2,\ldots,n\} are called {\it non-trivial cross-intersecting} if FGF\cap G\neq \emptyset for all FF,GGF\in \mathcal{F}, G\in \mathcal{G} and {F ⁣:FF}=={G ⁣:GG}\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 k8k\geq 8 in the full range. That is, if F,G([n]k)\mathcal{F},\mathcal{G}\subset \binom{[n]}{k} are non-trivial cross-intersecting, n2k+1n\geq 2k+1 and k8k\geq 8, then FG((n1k1)(nk1k1)+1)2. |\mathcal{F}||\mathcal{G}|\leq \left(\binom{n-1}{k-1}- \binom{n-k-1}{k-1} +1\right)^2.

Keywords

Cite

@article{arxiv.2605.09246,
  title  = {A product version of the Hilton-Milner Theorem II},
  author = {Peter Frankl and Jian Wang},
  journal= {arXiv preprint arXiv:2605.09246},
  year   = {2026}
}