Non-uniform Cross-intersecting Families

TL;DR

This paper determines the maximum total size of multiple non-empty cross-intersecting families of subsets with varying sizes, generalizing previous results and fully characterizing the extremal configurations.

Contribution

It extends known bounds for cross-intersecting families to non-uniform cases with multiple families, providing a complete characterization of extremal families.

Findings

Maximum sum of sizes for non-uniform cross-intersecting families determined

Generalizes previous uniform case results by Shi, Frankl, and Qian

Extremal families are fully characterized

Abstract

Let $m\geq 2$, $n$ be positive integers, and $R_i=\{k_{i,1} >k_{i,2} >\cdots> k_{i,t_i}\}$ be subsets of $[n]$ for $i=1,2,\ldots,m$. The families $\mathcal{F}_1\subseteq \binom{[n]}{R_1},\mathcal{F}_2\subseteq \binom{[n]}{R_2},\ldots,\mathcal{F}_m\subseteq \binom{[n]}{R_m}$ are said to be non-empty cross-intersecting if for each $i\in [m]$, $\mathcal{F}_i\neq\emptyset$ and for any $A\in \mathcal{F}_i,B\in\mathcal{F}_j$, $1\leq i<j\leq m$, $|A\bigcap B|\geq1$. In this paper, we determine the maximum value of $\sum_{j=1}^{m}|\mathcal{F}_j|$ for non-empty cross-intersecting family $\mathcal{F}_1, \mathcal{F}_2,\ldots,\mathcal{F}_m$ when $n\geq k_1+k_2$, where $k_1$ (respectively, $k_2$) is the largest (respectively, second largest) value in $\{k_{1,1},k_{2,1},\ldots,k_{m,1}\}$. This result is a generalization of the results by Shi, Frankl and Qian \cite{shi2022non} on non-empty cross-intersecting families. Moreover, the extremal families are completely characterized.