The maximum sum of the sizes of all intersections within $m$-size families

Abstract

For a family of sets $\mathcal{F}$, let $\omega(\mathcal{F}):=\sum_{\{A,B\}\subset \mathcal{F}}|A\cap B|$. In this paper, we prove that provided $n$ is sufficiently large, for any $\mathcal{F}\subset \binom{[n]}{k}$ with $|\mathcal{F}|=m$, $\omega(\mathcal{F})$ is maximized by the family consisting of the first $m$ sets in the lexicographical ordering on $\binom{[n]}{k}$. Compared to the maximum number of adjacent pairs in families, determined by Das, Gan and Sudakov in 2016, $\omega(\mathcal{F})$ distinguishes the contributions of intersections of different sizes. Then our results is an extension of Ahlswede and Katona's results in 1978, which determine the maximum number of adjacent edges in graphs. Besides, since $\omega(\mathcal{F})=\frac{1}{2}\left(\sum_{x\in [n]}|\{F\in \mathcal{F}:x\in F\}|^2-km\right)$ for $k$-uniform family with size $m$, our results also give a sharp upper bound of the sum of squares of degrees in a hypergraph.