A spectral Erdős-Rademacher theorem

Abstract

A classical result of Erdős and Rademacher (1955) indicates a supersaturation phenomenon. It says that if $G$ is a graph on $n$ vertices with at least $\lfloor n^2/4\rfloor +1$ edges, then $G$ contains at least $\lfloor n/2\rfloor$ triangles. We prove a spectral version of Erdős--Rademacher's theorem. Moreover, Mubayi [Adv. Math. 225 (2010)] extends the result of Erdős and Rademacher from a triangle to any color-critical graph. It is interesting to study the extension of Mubayi from a spectral perspective. However, it is not apparent to measure the increment on the spectral radius of a graph comparing to the traditional edge version (Mubayi's result). In this paper, we provide a way to measure the increment on the spectral radius of a graph and propose a spectral version on the counting problems for color-critical graphs.

27 pages, 5 figures. Any comments and suggestions are welcome