Path-Minimality of $p$-Energy for Connected Graphs

Abstract

Let $G$ be a simple connected graph on $n$ vertices, and let $\hat{I}{»}_1(G),\hat{I}{»}_2(G),\dots ,\hat{I}{»}_n(G)$ be the eigenvalues of its adjacency matrix $A(G)$. For $p>0$, define the $p$-energy of $G$ by ${\mathcal{E}}_p(G)={\sum }_{i=1}^n|\hat{I}{»}_i(G){|}^p$. We prove that, for every real number $p\ge 2$ and every simple connected graph $G$ on $n$ vertices, $${\mathcal{E}}_p(G)\ge {\mathcal{E}}_p(P_n),$$ where $P_n$ denotes the path on $n$ vertices. Moreover, for each fixed $p>2$, equality holds if and only if $G\cong P_n$. Together with the previously known star-minimality results, this completes the solution of two questions of Nikiforov. The proof combines two different comparison principles. For $2<p<4$, we use a bipartite reduction, a Mellin representation of fractional powers, and a determinant comparison involving matching generating polynomials and tree shifts. For $p\ge 4$, we prove a second-order stop-loss comparison for the squared singular values of bipartite graphs. This comparison is established by rank-one spectral-shift estimates, deletion-minimal counterexamples, and a finite certified analysis of the terminal sparse-sun configurations.

65 pages, 2 figures. This is the submitted version