The underlying graph of a line digraph (Q1199455)

A restricted line digraph is the line digraph of a digraph without parallel arcs (and hence with at most one loop at any vertex) and in which each vertex has in-degree and out-degree at least two. The author shows that, apart from a few cases, any connected graph can be oriented as a restricted line digraph in at most two ways. He also characterizes the underlying graphs of restricted line digraphs and deduces from this a relation between the automorphism groups of a restricted line digraph and its underlying graph.