Eulerian Directed Multigraphs

Abstract

For $\Delta$ a finite connected nontrivial directed multigraph, we prove: 1. $\Delta$ has a directed circuit using each directed edge exactly once if and only if both each pair of distinct vertices of $\Delta$ occur in a common directed circuit and in-degree$({\bf x}) =$ out-degree$({\bf x})$ for every vertex ${\bf x}$. 2. $\Delta$ contains a non-circuit directed path which uses every directed edge exactly once if and only if both every pair of distinct vertices of $\Delta$ occur in a common directed circuit and there are vertices ${\bf b \not= e}$ such that in-degree$({\bf e}) -$ out-degree$({\bf e}) = 1 =$ out-degree$({\bf b}) -$ in-degree$({\bf b})$ but, for every vertex ${\bf x \notin \{b,e\}}$, it happens that in-degree$({\bf x}) =$ out-degree$({\bf x})$.