On digraphs with a rooted tree structure

Abstract

A special class of reducible digraphs is characterized and polynomial time algorithms are described for their recognition, isomorphism and finding minimum equivalent digraphs. An approximative algorithm is also given for solving this last problem in this general case. The size of the approximation is always less thantwice the exact solution. In addition, isomorphism of depht first search is solved as a special case of isomorphism of this class.