A Novel/Old Modification of the First Zagreb Index

Abstract

In the paper [I. Gutman, N. Trinajstić, Chem. Phys. Lett. 17 (1972), 535], it was shown that total $\pi$-electron energy ($E$) of a molecule $M$ depends on the quantity ${\sum }_{v\in V(G)}{d}_v^2$ (nowadays known as the "first Zagreb index"), where $G$ is the graph corresponding to $M$, $V(G)$ is the vertex set of $G$ and $d_v$ is degree of the vertex $v$. In the same paper, the graph invariant ${\sum }_{v\in V(G)}{d}_v{\tau }_v$ (where ${\tau }_v$ is the connection number of $v$, that is the number of vertices at distance 2 from $v$) was also proved to influence $E$, but this invariant was never restudied explicitly. We call it "modified first Zagreb connection index" and denote it by $Z{C}_1^{\ast }$. In this paper, we characterize the extremal elements with respect to the graph invariant $Z{C}_1^{\ast }$ among the collection of all $n$-vertex chemical trees.

12 Pages