Two conjectured strengthenings of Tur\'an's theorem

Abstract

We investigate two conjectured spectral graph theoretic strengthenings of Tur\'an's theorem. Let $\mu_1 \ge \ldots \ge \mu_n$ denote the eigenvalues of a graph $G$ with $n$ vertices, $m$ edges and clique number $\omega(G)$. The concise version of Tur\'an's theorem is that $n/(n - d)$ is a lower bound for the clique number $\omega(G)$, where $d$ is the average degree. Our first conjecture is that $d$ can be replaced in this bound with $\sqrt{s^+}$, where $s^+$ is the sum of the squares of the positive eigenvalues. We prove this conjecture for triangle-free, weakly perfect and Kneser graphs and for almost all graphs. We have also used various software tools to search for a counter-example. Nikiforov proved a spectral version of Tur\'an's theorem that \[ \mu_1^2 \le \frac{2m(\omega(G) - 1)}{\omega(G)}, \] and Bollob\'as and Nikiforov conjectured that for $G \not = K_n$ \[ \mu_1^2 + \mu_2^2 \le \frac{2m(\omega(G) - 1)}{\omega(G)}. \] For our second conjecture, we propose that for all graphs $(\mu_1^2 + \mu_2^2)$ in this inequality can be replaced by the sum of the squares of the $\omega(G)$ largest eigenvalues, provided they are positive. We prove the conjecture for weakly perfect, Kneser, and classes of strongly regular graphs. We also provide experimental evidence and describe how the bound can be applied. Liu and Ning published a wide-ranging paper entitled ``Unsolved Problems in spectral graph theory'', and these two conjectures were placed second and fourth in their list of such problems.