Cacti with Extremal PI Index

TL;DR

This paper characterizes the extremal cactus graphs with the maximum and minimum vertex PI indices, providing sharp bounds and extending previous results in the study of this molecular descriptor.

Contribution

It completely determines the extremal cacti with respect to the vertex PI index, establishing sharp bounds and extending prior findings.

Findings

Identified cacti with extremal PI indices

Established sharp bounds for PI indices in cacti

Extended known results in molecular graph theory

Abstract

The vertex PI index $PI(G) = \sum_{xy \in E(G)} [n_{xy}(x) + n_{xy}(y)]$ is a distance-based molecular structure descriptor, where $n_{xy}(x)$ denotes the number of vertices which are closer to the vertex $x$ than to the vertex $y$ and which has been the considerable research in computational chemistry dating back to Harold Wiener in 1947. A connected graph is a cactus if any two of its cycles have at most one common vertex. In this paper, we completely determine the extremal graphs with the largest and smallest vertex PI indices among all the cacti. As a consequence, we obtain the sharp bounds with corresponding extremal cacti and extend a known result.