When a digraph and its line digraph are connected and cospectral

In this paper we characterize all digraphs each one of which is cospectral with its line digraph and both the digraph and its line digraph are connected. Some related enumeration problems are also considered. From these results we can see that there are arbitrarily large sets of cospectral digraphs. (C) 1998 Elsevier Science B.V. All rights reserved