A Vertex-Localized Positive Square-Energy Strengthening of Turán's Theorem

Abstract

Let $G$ be a graph of order $n$ with the adjacency eigenvalues ${\lambda }_1(G)\ge \cdots \ge {\lambda }_n(G)$. Let $c(v)$ denote the maximum order of a clique containing vertex $v$. We prove the vertex-localized positive square-energy inequality \[ \sqrt{s_+(G)} \leq \sum_{v\in V}\left(1-\frac1{c(v)}\right), \] where \[ s_+(G)=\sum_{λ_i(G)>0}λ_i(G)^2. \] We also characterize equality. Apart from edgeless graphs, equality holds precisely for graphs obtained from a complete regular multipartite graph by adding an arbitrary number of isolated vertices. This settles a conjecture of Kannan, Kumar and Pragada.

Preliminary version. Comments are welcome