On $r$-wise $t$-intersecting uniform families

Abstract

We consider families, $\mathcal{F}$ of $k$-subsets of an $n$-set. For integers $r\ge 2$, $t\ge 1$, $\mathcal{F}$ is called $r$-wise $t$-intersecting if any $r$ of its members have at least $t$ elements in common. The most natural construction of such a family is the full $t$-star, consisting of all $k$-sets containing a fixed $t$-set. In the case $r=2$ the Exact Erdős-Ko-Rado Theorem shows that the full $t$-star is largest if $n\ge (t+1)(k-t+1)$. In the present paper, we prove that for $n\ge (2.5t{)}^{1/(r-1)}(k-t)+k$, the full $t$-star is largest in case of $r\ge 3$. Examples show that the exponent $\frac{1}{r-1}$ is best possible. This represents a considerable improvement on a recent result of Balogh and Linz.