On the Relativized Alon Second Eigenvalue Conjecture V: Proof of the Relativized Alon Conjecture for Regular Base Graphs

TL;DR

This paper proves that for regular base graphs, as the covering degree increases, the new eigenvalues mostly stay within the Alon bound, with precise probability bounds depending on algebraic and tangle powers.

Contribution

It establishes probabilistic bounds on eigenvalues of large random covers of regular graphs, extending the relativized Alon conjecture with tight bounds and conjectures on algebraic power.

Findings

Probability of eigenvalue outside the bound is O(1/n).

Bounds depend on algebraic and tangle powers of the model.

Eigenvalues stay within bounds except when certain subgraph tangles occur.

Abstract

This is the fifth 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 use the results of Articles~III and IV in this series to prove that if the base graph is regular, then as the degree, $n$, of the covering map tends to infinity, some new adjacency eigenvalue has absolute value outside the Alon bound with probability bounded by $O(1/n)$. In addition, we give upper and lower bounds on this probability that are tight to within a multiplicative constant times the degree of the covering map. These bounds depend on two positive integers, the \emph{algebraic power} (which can also be $+\infty$) and the \emph{tangle power} of the model of random covering map. We conjecture that the algebraic power of the models we study is always $+\infty$, and in Article~VI we prove this when the base graph is regular and \emph{Ramanujan}. When the algebraic power of the model is $+\infty$, then the results in this article imply stronger results, such as (1) the upper and lower bounds mentioned above are matching to within a multiplicative constant, and (2) with probability smaller than any negative power of the degree, the some new eigenvalue fails to be within the Alon bound only if the covering map contains one of finitely many "tangles" as a subgraph (and this event has low probability).