Quadratic bounds for intersecting hypergraphs: An affirmative answer to a question of Duke and Erdős

Abstract

Let $m(k,b)$ denote the minimum number of edges in an intersecting $k$-uniform hypergraph in which every $(k-1)$-set of vertices is disjoint from at least $b$ edges, with $m(k,b)=+\infty$ if no such hypergraph exists. Duke and Erdős [Congressus Numerantium (1980)] asked whether $m(k,k)=o(k^4)$ is possible. In this paper, we answer this question affirmatively and determine the order of growth of $m(k,k)$ by proving that $k^2+1\leq m(k,k)\leq Ck^2$ for all sufficiently large $k$, where $C$ is an absolute constant. More generally, we show that $m(k,b)=Θ(kb)$ for all sufficiently large odd $k$ and every integer $b$ with $k\leq b\leq (k-1)(k-3)/2$, where the implied constants are independent of $k$ and $b$.