Spectral gap and edge universality of dense random regular graphs

TL;DR

This paper establishes optimal eigenvalue bounds and edge universality for the spectral distribution of dense random regular graphs, revealing Tracy-Widom fluctuations at the spectral edge.

Contribution

It proves optimal eigenvalue rigidity and Tracy-Widom edge universality for dense random regular graphs in a new regime of degree growth.

Findings

Eigenvalues are tightly concentrated near the spectral edge.

Extreme eigenvalues follow Tracy-Widom distribution.

Results hold for degrees growing faster than N^{2/3}.

Abstract

Let $\mathcal A$ be the adjacency matrix of a random $d$-regular graph on $N$ vertices, and we denote its eigenvalues by $\lambda_1\geq \lambda_2\cdots \geq \lambda_{N}$. For $N^{2/3}\ll d\leq N/2$, we prove optimal rigidity estimates of the extreme eigenvalues of $\mathcal A$, which in particular imply that \[ \max\{|\lambda_N|,\lambda_2\} <2\sqrt{d-1} \] with overwhelming probability. In the same regime of $d$, we also show that \[ N^{2/3}\bigg(\frac{\lambda_2+d/N}{\sqrt{d(N-d)/N}}-2\bigg) \overset{d}{\longrightarrow} \mathrm{TW}_1\,, \]where $\mathrm{TW}_1$ is the Tracy-Widom distribution for GOE; analogues results also hold for other non-trivial extreme eigenvalues.