Extremal values of vertex-degree-based topological indices over hexagonal systems with fixed number of vertices

Let G be a graph with n vertices. A vertex-degree-based topological index is defined from a set of real numbers {psi(ij)} as TI(G) = Sigma(1 <= i <= j <= n-1) m(ij)psi(ij) where m(ij) is the number of edges of G connecting a vertex of degree i with a vertex of degree j. Many of the well-known topological indices are particular cases of this expression, including all Randic-type connectivity indices. In this work we determine extremal values for TI over the set HSn of hexagonal systems with a fixed number of vertices. (C) 2014 Elsevier Inc. All rights reserved.