1. A graph G is a pair, G = (V, E), in which V is a finite nonempty set, called the set of vertices of G, and E ? V X V, called the set of edges. 2. In an undirected graph G = (V, E), the elements of...

1. A graph G is a pair, G = (V, E), in which V is a finite nonempty set, called the set of vertices of G, and E ? V X V, called the set of edges.

2. In an undirected graph G = (V, E), the elements of E are unordered pairs.

3. In a directed graph G = (V, E), the elements of E are ordered pairs.

4. Let G be a graph. A graph H is called a subgraph of G if every vertex of H is a vertex of G and every edge in H is an edge in G.