Quadratic inequalities between the largest eigenvalues of a graph

Abstract

We prove a sharp quadratic inequality between the largest two eigenvalues $λ_1 \ge λ_2$ of a graph with $n$ vertices. We also prove a quadratic inequality between the second and third largest eigenvalues $λ_2 \ge λ_3$. These results in particular imply the bounds $λ_1 + λ_2 \le \frac{8}{7} n - 2$, $λ_3 \le \frac{n}{3} - 1$ and $λ_2 + λ_3 \le \frac{2}{3} n - 2$. In fact we determine the closure of the set of possible $(\frac{λ_1+1}{n}, \frac{λ_2+1}{n}) \in \mathbb{R}^2$ and $(\frac{λ_2+1}{n}, \frac{λ_3+1}{n}) \in \mathbb{R}^2$. More generally, we prove quadratic bounds in the case of symmetric matrices in $[0,1]^{n \times n}$, and we also give a quadratic bound for two eigenvalues of a symmetric matrix in $[-1,1]^{n \times n}$. These bounds are proved by transforming the problem into extremal geometric questions in $\mathbb{R}^3$ and $\mathbb{R}^2$. We use the method of Lagrange multipliers to reduce to special cases with at most five points, and we deal with these special cases directly.