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Hypergraphs without non-trivial intersecting subgraphs

Part of: Extremal combinatorics Graph theory

Published online by Cambridge University Press:  09 June 2022

Xizhi Liu
Xizhi Liu*
Affiliation:
Department of Mathematics, Statistics, and Computer Science, University of Illinois, Chicago, IL, 60607 USA
*
Email: xliu246@uic.edu
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Abstract

A hypergraph $\mathcal{F}$ is non-trivial intersecting if every pair of edges in it have a nonempty intersection, but no vertex is contained in all edges of $\mathcal{F}$ . Mubayi and Verstraëte showed that for every $k \ge d+1 \ge 3$ and $n \ge (d+1)k/d$ every $k$ -graph $\mathcal{H}$ on $n$ vertices without a non-trivial intersecting subgraph of size $d+1$ contains at most $\binom{n-1}{k-1}$ edges. They conjectured that the same conclusion holds for all $d \ge k \ge 4$ and sufficiently large $n$ . We confirm their conjecture by proving a stronger statement.

They also conjectured that for $m \ge 4$ and sufficiently large $n$ the maximum size of a $3$ -graph on $n$ vertices without a non-trivial intersecting subgraph of size $3m+1$ is achieved by certain Steiner triple systems. We give a construction with more edges showing that their conjecture is not true in general.

Keywords

hypergraphsextremal set theoryintersecting family

MSC classification

Primary: 05D05: Extremal set theory
Secondary: 05C65: Hypergraphs 05C69: Dominating sets, independent sets, cliques
Type
Paper
Information
Combinatorics, Probability and Computing , Volume 31 , Issue 6 , November 2022 , pp. 1076 - 1091
Copyright
© The Author(s), 2022. Published by Cambridge University Press

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Footnotes

*

Research partially supported by NSF award DMS-1763317.

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