English

On the $\ell$-th largest degree of an intersecting family

Combinatorics 2026-04-22 v3

Abstract

Let F([n]k)\mathcal{F}\subset\binom{[n]}{k} be an intersecting family. For an element i[n]i\in[n], the degree of ii is the number of sets in F\mathcal{F} that contain ii. Assume that the degrees are ordered as d1d2dnd_{1}\ge d_{2}\ge\cdots\ge d_{n}.Huang and Zhao showed that if n>2kn>2k, then the minimum degree satisfies dn(n2k2)d_{n}\le\binom{n-2}{k-2}, with the maximum attained by the 11-star. We strengthen this result by proving that for n2k+1n\ge 2k+1, the (2k+1)(2k+1)-th largest degree satisfies d2k+1(n2k2)d_{2k+1}\le\binom{n-2}{k-2}, thereby confirming a conjecture of Frankl and Wang. Furthermore, we prove that for large kk and n>12kn>12k, the (k+2)(k+2)-th largest degree dk+2d_{k+2} is already at most (n2k2)\binom{n-2}{k-2}. The techniques we developed also yield a tight upper bound for the (+1)(\ell+1)-th largest degree d+1d_{\ell+1} for εkk\varepsilon k \le \ell \le k and sufficiently large n>Cεkn>C_{\varepsilon} k.

Keywords

Cite

@article{arxiv.2602.01692,
  title  = {On the $\ell$-th largest degree of an intersecting family},
  author = {Hao Huang and Rui Rao},
  journal= {arXiv preprint arXiv:2602.01692},
  year   = {2026}
}