English

A local version of Katona's intersection theorem

Combinatorics 2022-06-10 v1

Abstract

Katona's intersection theorem states that every intersecting family F[n](k)\mathcal F\subseteq[n]^{(k)} satisfies FF\vert\partial\mathcal F\vert\geq\vert\mathcal F\vert, where F={Fx:xFF}\partial\mathcal F=\{F\setminus x:x\in F\in\mathcal F\} is the shadow of F\mathcal F. Frankl conjectured that for n>2kn>2k and every intersecting family F[n](k)\mathcal F\subseteq [n]^{(k)}, there is some i[n]i\in[n] such that F(i)F(i)\vert \partial \mathcal F(i)\vert\geq \vert\mathcal F(i)\vert, where F(i)={Fi:iFF}\mathcal F(i)=\{F\setminus i:i\in F\in\mathcal F\} is the link of F\mathcal F at ii. Here, we prove this conjecture in a very strong form for n>(k+12)n> \binom{k+1}{2}. In particular, our result implies that for any j[k]j\in[k], there is a jj-set {a1,,aj}[n](j)\{a_1,\dots,a_j\}\in[n]^{(j)} such that F(a1,,aj)F(a1,,aj)\vert \partial \mathcal F(a_1,\dots,a_j)\vert\geq \vert\mathcal F(a_1,\dots,a_j)\vert. A similar statement is also obtained for cross-intersecting families.

Keywords

Cite

@article{arxiv.2206.04278,
  title  = {A local version of Katona's intersection theorem},
  author = {Marcelo Sales and Bjarne Schülke},
  journal= {arXiv preprint arXiv:2206.04278},
  year   = {2022}
}

Comments

6 pages

R2 v1 2026-06-24T11:44:29.796Z