Lower Bounds for Inverse Sum Indeg Index of Graphs

Abstract

Let G = (V,E), V = {1, 2,…,n}, be a simple connected graph with n vertices and m edges and let d1 ≥ d2 ≥⋅ ⋅⋅≥ dn > 0, be the sequence of its vertex degrees. With i ∼ j we denote the adjacency of the vertices i and j in G. The inverse sum indeg index is defined as ISI = ∑ -didj- di+dj with summation going over all pairs of adjacent vertices. We consider lower bounds for ISI. We first analyze some lower bounds reported in the literature. Then we determine some new lower bounds.