English

On the largest degrees in intersecting hypergraphs

Combinatorics 2025-11-20 v1

Abstract

Let ([n]k)\binom{[n]}{k} denote the collection of all kk-subsets of the standard nn-set [n]={1,2,,n}[n]=\{1,2,\ldots,n\}. Let n>2kn>2k and let F([n]k)\mathcal{F}\subset \binom{[n]}{k} be an {\it intersecting} kk-graph, i.e., FFF\cap F'\neq \emptyset for all F,FFF,F'\in \mathcal{F}. The number of edges FFF\in \mathcal{F} containing x[n]x\in [n] is called the {\it degree} of xx. Assume that d1d2dnd_1\geq d_2\geq \ldots\geq d_n are the degrees of F\mathcal{F} in decreasing order. An important result of Huang and Zhao states that for n>2kn>2k the minimum degree dnd_n is at most (n2k2)\binom{n-2}{k-2}. For n6k9n\geq 6k-9 we strengthen this result by showing d2k+1(n2k2)d_{2k+1}\leq \binom{n-2}{k-2}. As to the second and third largest degrees we prove the best possible bound d3d2(n2k2)+(n3k2)d_3\leq d_2\leq \binom{n-2}{k-2}+\binom{n-3}{k-2} for n>2kn>2k. Several more best possible results of a similar nature are established.

Keywords

Cite

@article{arxiv.2511.15508,
  title  = {On the largest degrees in intersecting hypergraphs},
  author = {Peter Frankl and Jian Wang},
  journal= {arXiv preprint arXiv:2511.15508},
  year   = {2025}
}
R2 v1 2026-07-01T07:45:30.047Z