Normal trees of digraphs

In this paper, we investigate normal trees of directed graphs, which extend the fundamental concept of normal trees of undirected graphs. We prove that a directed graph $D$ has a normal spanning tree if and only if the topological space $\mathrm{\mid }D\mathrm{\mid }$ is metrizable, which generalises Diestel’s result for undirected graphs. Furthermore, we show that the existence of normal arborescences implies the existence of normal trees in directed graphs, and that the converse is generally not true.