Closed Walks Of Low Dimension And Twisted Moments On Self-Loop Graphs

Abstract

Let $G_S$ be a graph with loops attached at each vertex in $S \subseteq V(G).$ In this article, we develop exact formulae for the number of closed $3$- and $4$-walks on $G_S$ in terms of vertex degrees and certain elementary subgraphs of $G_S.$ We then derive the specific closed walks formulae for several graph families such as complete bipartite self-loop graphs, complete graphs, cycle graphs, etc. We demonstrate that such invariants are non-trivial in $G_S,$ which otherwise may be trivial in the loopless case. Moreover, we study a moment-like quantity $\mathcal{M}_q(G_S)=\sum^n_{i=1} |\lambda_i(G_S) - \frac{\sigma}{n}|^q,$ twisted by the spectral moment $\mathsf{M}_1(G_S)$ for $G_S,$ and show a positivity result. We also establish that the following ratio inequality holds: \[ \frac{\mathcal{M}_{1}}{\mathcal{M}_{0}} \leq \frac{\mathcal{M}_{2}}{\mathcal{M}_{1}} \leq \frac{\mathcal{M}_{3}}{\mathcal{M}_{2}} \leq \frac{\mathcal{M}_{4}}{\mathcal{M}_{3}} \leq \cdots \leq \frac{\mathcal{M}_{n}}{\mathcal{M}_{n-1}} \leq \cdots. \] As a consequence, we obtain lower bounds for the self-loop graph energy $\mathcal{E}(G_S)$ in terms of $\mathcal{M}_i,$ extending some classical bounds.