Note on VDB Topological Indices of k-Cyclic Graphs

Abstract

Let $G$ be a connected graph with $n$ vertices and $m$ edges. The vertex-degree-based topological index (VDB) (or graphical function-index) $TI(G)$ of $G$ with edge-weight function $I(x,y)$ is defined as $$TI(G)=\sum _{uv\in E(G)}I(d_u,d_v),$$ where $I(x,y)>0$ is a symmetric real function with $x\ge 1$ and $y\ge 1$, $d_u$ is the degree of vertex $u$ in $G$. In this note, we deduce a number of previously established results, and state a few new. For a VDB topological index $TI$ with the property $P^{\ast }$, we can obtain the minimum $k$-cyclic (chemical) graphs for $k\ge 3$, $n\ge 5(k-1)$. These VDB topological indices include the Sombor index, the general Sombor index, the $p$-Sombor index, the general sum-connectivity index and so on. Thus this note extends the results of Liu et al. [H. Liu, L. You, Y. Huang, Sombor index of c-cyclic chemical graphs, MATCH Commun. Math. Comput. Chem. 90 (2023) 495-504] and Ali et al. [A. Ali, D. Dimitrov, Z. Du, F. Ishfaq, On the extremal graphs for general sum-connectivity index $(\ddot{I}{‡}_{\hat{I}}\pm )$ with given cyclomatic number when $\hat{I}\pm >1$, Discrete Appl. Math. 257 (2019) 19-30].

7 pages, 1 figure