 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 nontrivial 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 nontrivial path . 
 A vertex v of a directed graph is said to be reachable from another vertex u when <a href='dey_it' title='1'><mark>there</mark></a> <a href='uninflect' title='2'><mark>exists</mark></a> a path <a href='null_relcl' title='3'><mark>that</mark></a><a href='uninflect' title='4'><mark>starts</mark></a> at u and <a href='uninflect' title='5'><mark>ends</mark></a> 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 nontrivial 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 nontrivial path . 
A vertex v of a directed graph is said to b reachable from anotha vertex u when it exist a path start at u and end at v. As a special case , every vertex is considered to be reachable from itself ( by a path w/ zero edges ) . If a vertex can reach itself via a nontrivial path ( a path with one or more edges ) , then that path is a cycle , so aanothaway to define directed acyclic graphs is that they are the graphs in which no vertex can reach itself via a nontrivial path .