In Section 1.3.4 we introduced the idea of a directed graph. Graphs consist of vertices and edges. We describe vertices and edges in much the same way as we describe points and lines in geometry: we d...In Section 1.3.4 we introduced the idea of a directed graph. Graphs consist of vertices and edges. We describe vertices and edges in much the same way as we describe points and lines in geometry: we don’t really say what vertices and edges are, but we say what they do. We just don’t have a complicated axiom system the way we do in geometry. A graph consists of a set V called a vertex set and a set E called an edge set. Each member of V is called a vertex and each member of E is called an edge.