Keywords: sum of eigenvalues; graph energy; random matrix
Summary: Let $G$ be a graph on $n$ vertices and let $\lambda _{1}\geq \lambda _{2}\geq \ldots \geq \lambda _{n}$ be the eigenvalues of its adjacency matrix. For random graphs we investigate the sum of eigenvalues $s_{k}=\sum _{i=1}^{k}\lambda _{i}$, for $1\leq k\leq n$, and show that a typical graph has $s_{k}\leq (e(G)+k^{2})/(0.99n)^{1/2}$, where $e(G)$ is the number of edges of $G$. We also show bounds for the sum of eigenvalues within a given range in terms of the number of edges. The approach for the proofs was first used in Rocha (2020) to bound the partial sum of eigenvalues of the Laplacian matrix.
[1] Das, K. C., Mojallal, S. A., Sun, S.: On the sum of the $k$ largest eigenvalues of graphs and maximal energy of bipartite graphs. Linear Algebra Appl. 569 (2019), 175-194. DOI 10.1016/j.laa.2019.01.016 | MR 3904318 | Zbl 1411.05156
[3] Graovac, A., Gotman, I., Trinajstić, N.: Topological Approach to the Chemistry of Conjugated Molecules. Lecture Notes in Chemistry 4. Springer, Berlin (1977). DOI 10.1007/978-3-642-93069-0 | Zbl 0385.05032