Pairwise Intersection Profiles of Intersecting Families

Abstract

Let $\mathcal F\subseteq\binom{[n]}{k}$ be a $k$-uniform family, and let $P_m(\mathcal F)$ denote the number of ordered pairs $(A,B)\in\mathcal F\times\mathcal F$ with $|A\cap B|=m$. We study the pairwise intersection profile $(P_0(\mathcal F),P_1(\mathcal F),\ldots,P_k(\mathcal F))$ of intersecting families. We prove that for every $1\leq m\leq k$, if $n\geq\max\{2k,3k-m-1\}$, then $$ P_m(\mathcal F)\leq P_m(\mathcal S_1^k), $$ where $\mathcal S_1^k$ is a full star. Moreover, if $n\geq \max\{2k+1,3k-m\}$, then equality holds if and only if $\mathcal F$ is a star up to permutations. As a corollary, for any function $f:\{0,1,\ldots,k\}\rightarrow \mathbb{R}_{\geq 0}$ and $n\geq 3k-2$, we show that $\Phi_f(\mathcal F)=\sum_{(A,B)\in\mathcal F\times\mathcal F}f(|A\cap B|)$ is maximized by a star. We also establish a stability result showing that, for $n\geq \max\{2k+1,3k-m\}$, if $P_m(\mathcal F)$ is sufficiently close to its maximum, then $\mathcal F$ has an element of degree close to $\binom{n-1}{k-1}$.