On a Conjecture of Erd{ő}s, Frankl and F{ü}redi

Let $\mathcal{X}$ be an $n$-element set. Assume $\mathscr{F}$ is a collection of subsets of $\mathcal{X}$. We call $\mathscr{F}$ an $r$-cover-free family if $F_0\nsubseteq F_1\cup\cdots\cup F_r$ holds for all distinct $F_0,F_1,...,F_r\in\mathscr{F}$. Given $r$, denote $n(r)$ the minimal $n$ such that there exits an $r$-cover-free family on an $n$-element set with cardinality larger than $n$. Thirty years ago, Erd\H{o}s, Frankl and F{\"u}redi \cite{CFF} proved that $\binom{r+2}{2}\leq n(r) (1+o(1))\frac{5}{6}r^2$, without proof. In this paper, it is proved that $\lim_{r\rightarrow\infty} n(r)/r^2\geq(15+\sqrt{33})/24$, which is a quantity in $[6/7,7/8]$. In particular, their conjecture is proved to be true for all r-cover-free families with uniform $(r+1)$-subsets.