 A vertex v of a directed graph is said to be reachable from another vertex u when there exists a path that starts at u and ends at v. As a special case , every vertex is considered to be reachable from itself ( by a path with zero edges ) . If a vertex can reach itself via a <unk> path ( a path with one or more edges ) , then that path is a cycle , so another way to define directed acyclic graphs is that they are the graphs in which no vertex can reach itself via a <unk> path . 
