Two Results on Union-Closed Families

Abstract

We show that there is some absolute constant $c>0$, such that for any union-closed family $\mathcal{F}\subseteq 2^{[n]}$, if \mbox{$|\mathcal{F}|\ge (\frac{1}{2}-c)2^n$}, then there is some element $i\in [n]$ that appears in at least half of the sets of $\mathcal{F}$. We also show that for any union-closed family $\mathcal{F}\subseteq 2^{[n]}$, the number of sets which are not in $\mathcal{F}$ that cover a set in $\mathcal{F}$ is at most $2^{n-1}$, and provide examples where the inequality is tight.

13 pages