The Genus, Regional Number, and Betti Number of a Graph

Abstract

Let the genus of an orientable 2-manifold M be denoted by γ(M). The genus, γ(G), of a graph G is then the smallest of the numbers γ(N) for orientable 2-manifolds N in which G can be embedded. An embedding of G in M is called minimal if γ(G) = γ(M). When each component of the complement of G in M is an open 2-cell, the embedding of G in M is called a 2-cell embedding. In (3), J. W. T. Youngs has shown that each minimal embedding is a 2-cell embedding. It follows from the results of (3) that for each graph G, there is a number d(G), called the regional number, such that for any 2-cell embedding of G in an orientable 2-manifold, the number of (2-cell) complementary domains of G is ⩽d(G), with equality holding if and only if the embedding is minimal.