Directed Graphs (Digraphs)
A directed graph or digraph consists of a set of vertices and a set of edges.
V = { v1, v2, . . . , vn }
E = { e1, e2, . . . , em } ei = ? va(i), vb(i) ?
In Sahni, self loops (e.g., ? vk, vk ? ) are not permitted.
However, in many applications, they are permitted (for example, finite-state automata).
In general a digraph is similar to a binary relation.