ABSTRACT
Directed graphs have already been introduced in the Chapter 8. If G is a digraph and u, v ∈ V(G), we write u → v to indicate that the edge uv is directed from u to v. The in-edges at v are the edges of the form (u,v). The in-degree of v is d− (v), the number of in-edges. Similarly the out-edges at u are all edges of the form (u, v) and the out-degree d+ (u) is the number of out-edges at u. The degree of u is
Deg(w) = d+(u) + d−(u).
