A graph is a pair G graph = (V;E) of sets satisfying E µ [V ]2; thus, the elements
of E are 2-element subsets of V . To avoid notational ambiguities,
we shall always assume tacitly that V E = ;. The elements of V are the
vertex vertices (or nodes, or points) of the graph G, the elements of E are its
edge edges (or lines). The usual way to picture a graph is by drawing a dot for
each vertex and joining two of these dots by a line if the corresponding
two vertices form an edge. Just how these dots and lines are drawn is
considered irrelevant: all that matters is the information which pairs of
vertices form an edge and which do not.