Formal Zeta Function Expansions and the Frequency of Ramanujan Graphs

Abstract

We show that logarithmic derivative of the Zeta function of any regular graph is given by a power series about infinity whose coefficients are given in terms of the traces of powers of the graph's Hashimoto matrix. We then consider the expected value of this power series over random, $d$-regular graph on $n$ vertices, with $d$ fixed and $n$ tending to infinity. Under rather speculative assumptions, we make a formal calculation that suggests that for fixed $d$ and $n$ large, this expected value should have simple poles of residue $-1/2$ at $\pm (d-1{)}^{-1/2}$. We shall explain that calculation suggests that for fixed $d$ there is an $f(d)>1/2$ such that a $d$-regular graph on $n$ vertices is Ramanujan with probability at least $f(d)$ for $n$ sufficiently large. Our formal computation has a natural analogue when we consider random covering graphs of degree $n$ over a fixed, regular "base graph." This again suggests that for $n$ large, a strict majority of random covering graphs are relatively Ramanujan. We do not regard our formal calculations as providing overwhelming evidence regarding the frequency of Ramanujan graphs. However, these calculations are quite simple, and yield intiguing suggestions which we feel merit further study.