Strongly Connected Spanning Subdigraphs with the Minimum Number of Arcs in Quasi-transitive Digraphs

We consider the problem (MSSS) of nding a strongly connected spanning subgraph with the minimum number of arcs in a strongly connected digraph. This problem is NP-hard for general digraphs since it generalizes the hamil-tonian cycle problem. We show that the problem is polynomially solvable for quasi-transitive digraphs. We describe the minimum number of arcs in such a spanning subgraph of a quasi-transitive digraph in terms of the path covering number. Our proofs are based on a number of results (some of which are new and interesting in their own right) on the structure of cycles and paths in quasi-transitive digraphs and in extended semicomplete digraphs. In particular, we give a new characterization of the longest cycle in an extended semicomplete digraph. Finally, we point out that our proofs imply that the MSSS problem is solvable in polynomial time for all digraphs that can be obtained from strong semicomplete digraphs on at least two vertices by replacing each vertex with a digraph whose path covering number can be decided in polynomial time.