A spectral Erd\H{o}s-Rademacher theorem

TL;DR

This paper establishes a spectral analogue of the Erd ext{"o}s-Rademacher supersaturation theorem, extending classical edge-based results to spectral graph theory and providing new tools to analyze color-critical graphs.

Contribution

It introduces a method to measure spectral radius increments and extends supersaturation results to spectral versions for color-critical graphs.

Findings

Spectral version of Erd ext{"o}s-Rademacher theorem proved.

Extension of Mubayi's results to spectral context.

New techniques for spectral measurement in graph counting problems.

Abstract

A classical result of Erd\H{o}s 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 Erd\H{o}s--Rademacher's theorem. Moreover, Mubayi [Adv. Math. 225 (2010)] extends the result of Erd\H{o}s 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.