Introduction

We are here concerned with a special class of graphs called chemical graphs, that is, graphs representing chemical structures. If chemical structures under consideration are molecules, we call this type of chemical graph a molecular graph. They are generated by replacing atoms and bonds with vertices and edges, respectively (Trinajstić, 1983, 1992; Gutman and Polansky, 1986). Hydrogen atoms are ordinarily neglected. A picture of a simple molecular graph G1 representing the hydrogendepleted carbon skeleton of 1-ethyl-2-methylcyclobutane is given in Figure 1.1.