On a theorem of Nosal

Abstract

Let $G$ be a graph with $m$ edges and spectral radius ${\lambda }_1$. Let $bk\left(G\right)$ stand for the maximal number of triangles with a common edge in $G$. In 1970 Nosal proved that if ${\lambda }_1^2>m,$ then $G$ contains a triangle. In this paper we show that the same premise implies that \[ bk\left( G\right) >\frac{1}{12}\sqrt[4]{m}. \] This result settles a conjecture of Zhai, Lin, and Shu. Write ${\lambda }_2$ for the second largest eigenvalue of $G$. Recently, Lin, Ning, and Wu showed that if $G$ is a triangle-free graph of order at least three, then \[ λ_{1}^{2}+λ_{2}^{2}\leq m, \] thereby settling the simplest case of a conjecture of Bollobás and the author. We give a simpler proof of their result.

12 pages