Graph Formalism
G = (V, E) where G is a graph, V a set of vertices and E a set of edges, such that e ? E iff e = (v1, v2), v1, v2 ? V.
If G is undirected, then e = (v1, v2) implies e = (v2, v1), i.e. vertices are unordered.
If G is directed (digraph) then (v1, v2) are ordered. v1 is the origin, v2 is the terminus or destination.