The de Bruijn-Erdos Theorem for Hypergraphs

Abstract

Fix integers $n \ge r \ge 2$. A clique partition of ${[n] \choose r}$ is a collection of proper subsets $A_1, A_2, \ldots, A_t \subset [n]$ such that $\bigcup_i{A_i \choose r}$ is a partition of ${[n] \choose r}$. Let $\cp(n,r)$ denote the minimum size of a clique partition of ${[n] \choose r}$. A classical theorem of de Bruijn and Erd\H os states that $\cp(n, 2) = n$. In this paper we study $\cp(n,r)$, and show in general that for each fixed $r \geq 3$, \[ \cp(n,r) \geq (1 + o(1))n^{r/2} \quad \quad \mbox{as}n \rightarrow \infty.\] We conjecture $\cp(n,r) = (1 + o(1))n^{r/2}$. This conjecture has already been verified (in a very strong sense) for $r = 3$ by Hartman-Mullin-Stinson. We give further evidence of this conjecture by constructing, for each $r \ge 4$, a family of $(1+o(1))n^{r/2}$ subsets of $[n]$ with the following property: no two $r$-sets of $[n]$ are covered more than once and all but $o(n^r)$ of the $r$-sets of $[n]$ are covered. We also give an absolute lower bound $\cp(n,r) \geq {n \choose r}/{q + r - 1 \choose r}$ when $n = q^2 + q + r - 1$, and for each $r$ characterize the finitely many configurations achieving equality with the lower bound. Finally we note the connection of $\cp(n,r)$ to extremal graph theory, and determine some new asymptotically sharp bounds for the Zarankiewicz problem.