Extremal Bounds on the Properties of Sigma and Albertson Indices for Non-Decreasing Degree Sequences

Abstract: In this paper, we establish bounds on the topological index -- the Sigma index -- focusing on analyzing the upper bound of its maximum value, which is known as $\sigma_{\max}(\mathcal{S}) = \max {\sigma(\mathscr{G}) \mid \mathscr{G} \in \mathcal{S}}$, where $\mathcal{S}$ is a class of graphs. We establish precise lower and upper bounds for the Sigma index by leveraging degree sequences $\mathscr{D} = (d_1, d_2, \dots, d_n)$, $\mathscr{R} = (t_1, t_2, \dots, t_m)$, and $\mathscr{A} = (a_1, a_2, \dots, a_r)$. The minimum bound for the Albertson index related to the Sigma index bounds incorporates terms such as $\mathrm{irr}(T)$ and $\left\lfloor \frac{n-2}{a_r - t_m} \right\rfloor $. These results, rooted in extremal graph theory, enhance the understanding of topological indices in molecular and network analysis.