1 Number Theory and Graph Theory Chapter 6 Basic concepts and definitions of graph theory By A. Satyanarayana Reddy Department of Mathematics Shiv Nadar University Uttar Pradesh, India E-mail:
[email protected] 2 Module-4: Connectedness of a graph Objectives • Connected, disconnected graphs and connected components • Connectedness in directed graphs • Few properties of connected graphs Let X = (V;E) be a graph. Before proceeding further, we recall the following definitions. Connectedness: A vertex u 2 V is said to be connected to a vertex v 2 V if there is a path in X from u to v. Connected Graph: X is said to connected if any two vertices of X are connected. Note that Complete graphs, complete bipartite graphs, wheel graphs, cyclic graphs, path graphs and Johnson graphs etc. are a few examples of connected graphs. A graph that is not connected is the union of two or more connected subgraphs, each pair of which has no vertex in common. These disjoint connected subgraphs are called the connected components of the graph. The number of components of a graph X is denoted by C(X). If X is connected then C(X) = 1. Let e be an edge of a graph X then it can be easily observed that C(X) ≤ C(X n feg) ≤ C(X) + 1. A direct application of the definition of a connected/disconnected graph gives the following result and hence the proof is omitted. Theorem 1. A graph is disconnected if and only if its vertex set V can be partitioned into two nonempty disjoint subsets V1 and V2 such that there exists no edge in X whose one end vertex is in V1 and other is in V2.