English

Four-vertex traces of finite sets

Combinatorics 2023-01-18 v1

Abstract

Let [n]=X1X2X3[n]=X_1\cup X_2\cup X_3 be a partition with n3Xin3\lfloor\frac{n}{3}\rfloor \leq |X_i|\leq \lceil\frac{n}{3}\rceil and define G={G[n] ⁣:GXi1,1i3}\mathcal{G}=\{G\subset [n]\colon |G\cap X_i|\leq 1, 1\leq i\leq 3\}. It is easy to check that the trace GY:={GY ⁣:GG}\mathcal{G}_{\mid Y}:=\{G\cap Y\colon G\in \mathcal{G}\} satisfies GY12|\mathcal{G}_{\mid Y}|\leq 12 for all 4-sets Y[n]Y\subset [n]. For n25n\geq 25 it is proven that whenever F2[n]\mathcal{F}\subset 2^{[n]} satisfies F>G|\mathcal{F}|>|\mathcal{G}| then FC13|\mathcal{F}_{\mid C}|\geq 13 for some C[n]C\subset [n], C=4|C|=4. Several further results of a similar flavor are established as well.

Keywords

Cite

@article{arxiv.2301.05830,
  title  = {Four-vertex traces of finite sets},
  author = {Peter Frankl and Jian Wang},
  journal= {arXiv preprint arXiv:2301.05830},
  year   = {2023}
}