Infinite primitive directed graphs

Abstract

A group $G$ of permutations of a set $\Omega$ is {\em primitive} if it acts transitively on $\Omega$, and the only $G$-invariant equivalence relations on $\Omega$ are the trivial and universal relations. A graph $\Gamma$ is {\em primitive} if its automorphism group acts primitively on its vertex set. A graph $\Gamma$ has {\em connectivity one} if it is connected and there exists a vertex $\alpha$ of $\Gamma$, such that the induced graph $\Gamma \setminus \{\alpha\}$ is not connected. If $\Gamma$ has connectivity one, a {\em block} of $\Gamma$ is a connected subgraph that is maximal subject to the condition that it does not have connectivity one. The primitive undirected graphs with connectivity one have been fully classified by Jung and Watkins: the blocks of such graphs are primitive, pairwise-isomorphic and have at least three vertices. When one considers the general case of a directed primitive graph with connectivity one, however, this result no longer holds. In this paper we investigate the structure of these directed graphs, and obtain a complete characterisation.