Nordhaus-Gaddum and other bounds for the sum of squares of the positive eigenvalues of a graph

Abstract

Terpai [22] proved the Nordhaus-Gaddum bound that $μ(G) + μ(\overline{G}) \le 4n/3 - 1$, where $μ(G)$ is the spectral radius of a graph $G$ with $n$ vertices. Let $s^+$ denote the sum of the squares of the positive eigenvalues of $G$. We prove that $\sqrt{s^{+}(G)} + \sqrt{s^+(\overline{G})} < \sqrt{2}n$ and conjecture that $\sqrt{s^{+}(G)} + \sqrt{s^+(\overline{G})} \le 4n/3 - 1.$ We have used AutoGraphiX and Wolfram Mathematica to search for a counter-example. We also consider Nordhaus-Gaddum bounds for $s^+$ and bounds for the Randić index.

9 pages