Some Vertex-Degree-Based Topological Indices of Graphs

Abstract

Many topological indices which are being used by researchers in the quantitative structure-property relationship (QSPR) and quantitative structure-activity relationship (QSAR) studies to predict the physico-chemical properties of molecules, are based on vertex degrees of the corresponding molecular graphs. When a new topological index is introduced in chemical graph theory, one of the important questions that need to be answered is for which members of a certain class of n-vertex graphs this index assumes minimal and maximal values? On the other hand, there are many well known graph families and vertex-degree-based topological indices in the literature for which this question remains open. The main purpose of current study is to address this open question for some well known families of graphs. Firstly, the collection of all k-polygonal chains (for k = 3, 4, 5) with fixed number of k-polygons is considered and the extremal elements from this family are characterized with respect to several well known bond incident degree (BID) indices (BID indices form a subclass of the class of all vertex-degree-based topological indices). From the derived results, many already reported results are obtained as corollaries. Furthermore, the extremal 4-polygonal (polyomino) chains for some renowned vertex-degree-based topological indices (other than BID indices) are also determined. Next, the problem of characterizing the extremal cacti over the certain classes of cacti (tree-like polyphenylene systems, spiro hexagonal systems and general cacti) with some fixed parameters is addressed for various well known vertex-degree-based topological indices. Finally, some mathematical properties of the atom-bond connectivity index and augmented Zagreb index are explored