Existential Closure in Uniform Hypergraphs

Abstract

For a positive integer $n$, a graph with at least $n$ vertices is $n$-existentially closed or simply $n$-e.c. if for any set of vertices $S$ of size $n$ and any set $T\subseteq S$, there is a vertex $x\notin S$ adjacent to each vertex of $T$ and no vertex of $S\setminus T$. We extend this concept to uniform hypergraphs, find necessary conditions for $n$-e.c. hypergraphs to exist, and prove that random uniform hypergraphs are asymptotically $n$-existentially closed. We then provide constructions to generate infinitely many examples of $n$-e.c. hypergraphs. In particular, these constructions use certain combinatorial designs as ingredients, adding to the ever-growing list of applications of designs.