English

Best possible bounds on the double-diversity of intersecting hypergraphs

Combinatorics 2025-02-26 v1

Abstract

For a family F([n]k)\mathcal{F}\subset \binom{[n]}{k} and two elements x,y[n]x,y\in [n] define F(xˉ,yˉ)={FF ⁣:xF, yF}\mathcal{F}(\bar{x},\bar{y})=\{F\in \mathcal{F}\colon x\notin F,\ y\notin F\}. The double-diversity γ2(F)\gamma_2(\mathcal{F}) is defined as the minimum of F(xˉ,yˉ)|\mathcal{F}(\bar{x},\bar{y})| over all pairs x,yx,y. Let L([7]3)\mathcal{L}\subset\binom{[7]}{3} consist of the seven lines of the Fano plane. For n7n\geq 7, k3k\geq 3 one defines the Fano kk-graph FL\mathcal{F}_{\mathcal{L}} as the collection of all kk-subsets of [n][n] that contain at least one line. It is proven that for n13k2n\geq 13k^2 the Fano kk-graph is the essentially unique family maximizing the double diversity over all kk-graphs without a pair of disjoint edges. Some similar, although less exact results are proven for triple and higher diversity as well.

Keywords

Cite

@article{arxiv.2212.11650,
  title  = {Best possible bounds on the double-diversity of intersecting hypergraphs},
  author = {Peter Frankl and Jian Wang},
  journal= {arXiv preprint arXiv:2212.11650},
  year   = {2025}
}
R2 v1 2026-06-28T07:48:38.696Z