English

Two extremal problems on intersecting families

Combinatorics 2018-05-01 v1

Abstract

In this short note, we address two problems in extremal set theory regarding intersecting families. The first problem is a question posed by Kupavskii: is it true that given two disjoint cross-intersecting families A,B([n]k)\mathcal{A}, \mathcal{B} \subset \binom{[n]}{k}, they must satisfy min{A,B}12(n1k1)\min\{|\mathcal{A}|, |\mathcal{B}|\} \le \frac{1}{2} \binom{n-1}{k-1}? We give an affirmative answer for n2k2n \ge 2k^2, and construct families showing that this range is essentially the best one could hope for, up to a constant factor. The second problem is a conjecture of Frankl. It states that for n3kn \ge 3k, the maximum diversity of an intersecting family F([n]k)\mathcal{F} \subset \binom{[n]}{k} is equal to (n3k2)\binom{n-3}{k-2}. We are able to find a construction beating the conjectured bound for nn slightly larger than 3k3k, which also disproves a conjecture of Kupavskii.

Keywords

Cite

@article{arxiv.1804.11269,
  title  = {Two extremal problems on intersecting families},
  author = {Hao Huang},
  journal= {arXiv preprint arXiv:1804.11269},
  year   = {2018}
}
R2 v1 2026-06-23T01:40:14.531Z