On the Relativized Alon Second Eigenvalue Conjecture VI: Sharp Bounds for Ramanujan Base Graphs

TL;DR

This paper proves sharp bounds for the eigenvalues of random covers of Ramanujan graphs, confirming conjectures about their spectral properties and the rarity of eigenvalue deviations outside the Alon bound.

Contribution

It establishes that for Ramanujan base graphs, the algebraic power of the random covering model is infinite, leading to precise bounds on eigenvalue deviations.

Findings

Matching bounds for the probability of eigenvalue deviations

Low probability of eigenvalues outside the Alon bound without tangles

Infinite algebraic power for Ramanujan base graphs

Abstract

This is the sixth in a series of articles devoted to showing that a typical covering map of large degree to a fixed, regular graph has its new adjacency eigenvalues within the bound conjectured by Alon for random regular graphs. In this article we show that if the fixed graph is regular Ramanujan, then the {\em algebraic power} of the model of random covering graphs is $+\infty$. This implies a number of interesting results, such as (1) one obtains the upper and lower bounds---matching to within a multiplicative constant---for the probability that a random covering map has some new adjacency eigenvalue outside the Alon bound, and (2) with probability smaller than any negative power of the degree of the covering map, some new eigenvalue fails to be within the Alon bound without the covering map containing one of finitely many "tangles" as a subgraph (and this tangle containment event has low probability).