On the inverse sum indeg index

Discrete Adriatic indices have been defined by Vukicevic and Gasperov (2010), as a way of generalizing well-known molecular descriptors defined as the sum of individual bond contributions, such as the second Zagreb index or the Randic index. Among the 148 discrete Adriatic indices analyzed by Vukicevic and Gasperov on the benchmark datasets of the International Academy of Mathematical Chemistry, 20 indices were selected as significant predictors of physicochemical properties. Here we study graph-theoretical properties of the Inverse Sum Indeg index, which was selected as a significant predictor of total surface area of octane isomers, and determine extremal values of this index across several graph classes, including connected graphs, chemical graphs, trees and chemical trees.