The Graovac-Pisanski index of connected bipartite graphs with applications to hydrocarbon molecules

TL;DR

This paper investigates the Graovac-Pisanski index of connected bipartite graphs, proving it is always an integer for certain graphs, and applies these findings to analyze hydrocarbon molecules and specific graph classes.

Contribution

It establishes that the Graovac-Pisanski index is an integer for connected bipartite graphs and graphs with an even number of vertices, and explores its applications to hydrocarbon molecules.

Findings

The index is always an integer for connected bipartite graphs.

Graphs with up to nine vertices mostly have integer indices, with some exceptions.

An infinite class of unicyclic graphs with non-integer indices is identified.

Abstract

The Graovac-Pisanski index, also called the modified Wiener index, was introduced in 1991 and represents an extension of the original Wiener index, because it considers beside the distances in a graph also its symmetries. Similarly as Wiener in 1947 showed the correlation of the Wiener indices of the alkane series with the boiling points, in 2018 the connection between the Graovac-Pisanski index and the melting points of some hydrocarbon molecules was established. In this paper, we prove that the Graovac-Pisanski index of any connected bipartite graph as well as of any connected graph on an even number of vertices is an integer number. These results are applied to some important families of hydrocarbon molecules. By using a computer programme, the graphs with a non-integer Graovac-Pisanski index on at most nine vertices are counted. Finally, an infinite class of unicyclic graphs with a non-integer Graovac-Pisanski index is described.