English

Counting sunflowers with restricted matching number

Combinatorics 2026-04-24 v1

Abstract

For a family H([n]k)\mathcal{H} \subseteq \binom{[n]}{k}, a subset {A1,A2,,Am}H\{A_1, A_2, \ldots, A_m\} \subseteq \mathcal{H} is called a \textit{matching} of size~mm if the sets A1,A2,,AmA_1, A_2, \ldots, A_m are pairwise disjoint. The \textit{matching number} of H\mathcal{H}, denoted by ν(H)\nu(\mathcal{H}), is the largest integer~mm for which such a matching exists. {A1,A2,,Al}([n]k)\{A_1,A_2,\ldots,A_l\}\subseteq \binom{[n]}{k} is said to be a \textit{kk-uniform sunflower} with ll \textit{petals}, if there exists a core set C[n]C\subseteq[n] contained in every AiA_i and AiCA_i\setminus C are pairwise disjoint, for 1il1\leq i\leq l. Let Sk,lk1S_{k,l}^{k-1} denote the kk-uniform sunflower with ll petals and the core set of size k1k-1. The \textit{codegree} of EE in H\mathcal{H}, denoted by dH(E)d_{\mathcal{H}}(E), is defined as dH(E)={FH:EF}d_{\mathcal{H}}(E) =|\{F\in \mathcal{H}:E\subseteq F\}|. Let the \textit{p\ell_p-norm} of H\mathcal{H} be cop(H)=E([n]k1)(dH(E))pco_p(\mathcal{H})= \sum_{E\in \binom{[n]}{k-1}}(d_{\mathcal{H}}(E))^p. For sufficiently large nn, we determine the maximum p\ell_p-norm and the maximum number of sunflowers Sk,lk1S_{k,l}^{k-1} for a family F([n]k)\mathcal{F} \subseteq \binom{[n]}{k} with matching number ν(F)=s\nu(\mathcal{F}) = s. These results can be viewed as a Tur\'an-type problem (specifically exk(n,Sk,lk1,Ms)\mathrm{ex}_k(n, S_{k,l}^{k-1}, M_s)) and a generalization of the Erd\H{o}s Matching Conjecture. Furthermore, for the case k=3k = 3, we establish a linear threshold for nn.

Keywords

Cite

@article{arxiv.2604.21855,
  title  = {Counting sunflowers with restricted matching number},
  author = {Haixiang Zhang and Mengyu Cao and Mei Lu},
  journal= {arXiv preprint arXiv:2604.21855},
  year   = {2026}
}

Comments

17 pages

R2 v1 2026-07-01T12:32:47.040Z