Connectedness of a Directed Graph

Data Structures and Algorithms with Object-Oriented Design Patterns in Java

Connectedness of a Directed Graph

When dealing with directed graphs, we define two kinds of connectedness, strong and weak. Strong connectedness of a directed graph is defined as follows:

Definition (Strong Connectedness of a Directed Graph) A directed graph is strongly connected if there is a path in G between every pair of vertices in .

For example, Figure shows the directed graph given by

Notice that the graph is not connected! For example, there is no path from any of the vertices in to any of the vertices in . Nevertheless, the graph ``looks'' connected in the sense that it is not made of up of separate parts in the way that the graph in Figure is.

This idea of ``looking'' connected is what weak connectedness represents. To define weak connectedness we need to introduce first the notion of the undirected graph that underlies a directed graph: Consider a directed graph . The underlying undirected graph is the graph where represents the set of undirected edges that is obtained by removing the arrowheads from the directed edges in G:

Figure: An weakly connected directed graph and the underlying undirected graph.

Weak connectedness of a directed graph is defined with respect to its underlying, undirected graph:

Definition (Weak Connectedness of a Directed Graph) A directed graph is weakly connected if the underlying undirected graph is connected.

For example, since the undirected graph in Figure is connected, the directed graph is weakly connected. Consider what happens when we remove the edge (b,e) from the directed graph . The underlying undirected graph that we get is in Figure . Therefore, when we remove edge (b,e) from , the graph that remains is neither strongly connected nor weakly connected.