A strongly connected digraph is a directed graph in which it is possible to reach any node starting from any other node by traversing edges in the direction(s) in which they point. The nodes in a strongly connected digraph therefore must all have indegree of at least 1. The numbers of nonisomorphic simple strongly connected digraphs on n = 1, 2, ... nodes are 1, 1, 5, 83, 5048, 1047008, ... (OEIS A035512). A directed graph can be tested to see if it is strongly connected using ConnectedGraphQ[g].