English

Stability of intersecting families

Combinatorics 2022-05-12 v1

Abstract

The celebrated Erd\H{o}s-Ko-Rado theorem \cite{EKR1961} states that the maximum intersecting kk-uniform family on [n][n] is a full star if n2k+1n\ge 2k+1. Furthermore, Hilton-Milner \cite{HM1967} showed that if an intersecting kk-uniform family on [n][n] is not a subfamily of a full star, then its maximum size achieves only on a family isomorphic to HM(n,k):={G([n]k):1G,G[2,k+1]}{[2,k+1]}HM(n,k):= \Bigl\{G\in {[n] \choose k}: 1\in G, G\cap [2,k+1] \neq \emptyset \Bigr\} \cup \Bigl\{ [2,k+1] \Bigr\} if n>2kn>2k and k4k\ge 4, and there is one more possibility in the case of k=3k=3. Han and Kohayakawa \cite{HK2017} determined the maximum intersecting kk-uniform family on [n][n] which is neither a subfamily of a full star nor a subfamily of the extremal family in Hilton-Milner theorm, and they asked what is the next maximum intersecting kk-uniform family on [n][n]. Kostochka and Mubayi \cite{KM2016} gave the answer for large enough nn. In this paper, we are going to get rid of the requirement that nn is large enough in the result by Kostochka and Mubayi \cite{KM2016} and answer the question of Han and Kohayakawa \cite{HK2017}.

Keywords

Cite

@article{arxiv.2205.05394,
  title  = {Stability of intersecting families},
  author = {Yang Huang and Yuejian Peng},
  journal= {arXiv preprint arXiv:2205.05394},
  year   = {2022}
}

Comments

28 pages, 1 figure

R2 v1 2026-06-24T11:14:04.562Z