Minimal covers of hypergraphs

Abstract

For a hypergraph $H=(V,\mathcal E)$, a subfamily $\mathcal C\subseteq \mathcal E$ is called a cover of the hypergraph if $\bigcup\mathcal C=\bigcup\mathcal E$. A cover $\mathcal C$ is called minimal if each cover $\mathcal D\subseteq\mathcal C$ of the hypergraph $H$ coincides with $\mathcal C$. We prove that for a hypergraph $H$ the following conditions are equivalent: (i) each countable subhypergraph of $H$ has a minimal cover; (ii) each non-empty subhypergraph of $H$ has a maximal edge; (iii) $H$ contains no isomorphic copy of the hypergraph $(\omega,\omega)$. This characterization implies that a countable hypergraph $(V,\mathcal E)$ has a minimal cover if every infinite set $I\subseteq V$ contains a finite subset $F\subseteq I$ such that the family of edges $\mathcal E_F:=\{E\in\mathcal E:F\subseteq E\}$ is finite. Also we prove that a hypergraph $(V,\mathcal E)$ has a minimal cover if $\sup\{|E|:E\in\mathcal E\}<\omega$ or for every $v\in V$ the family $\mathcal E_v:=\{E\in\mathcal E:v\in E\}$ is finite.