Counting multigeodesics and the Selberg-Delange method

Abstract

We study the counting function $\mathcal{P}^m(X,x)$ for $m$-multigeodesics of length at most $x$ on a compact hyperbolic surface $X$ where $m$ is a positive integer. Building on the Selberg-Delange method, originally developed for counting integers with exactly $m$ prime factors in the interval $[1,x]$, we prove a large $x$ asymptotic for $\mathcal{P}^m(X,x)$. We show that for fixed $m$ as $x\to\infty$ the asymptotic is independent of the surface, whereas for $\frac{m}{\log x}$ bounded away from zero, the asymptotic depends on an infinite Euler product over the lengths of primitive closed geodesics. We also study the distribution of the function $ω_1$, defined on multigeodesics $Γ$ as $ω_1(Γ)=k-1$ for $Γ=(γ_1,...,γ_k)$ a $k$-multigeodesic. We show that as $x\to\infty$, the distribution of $ω_1$ in the set of multigeodesics of length $\leq x$ is close to that of a Poisson random variable with parameter $\log x$. Both of these theorems rely on studying the generating function of the random variable $ω_1$ which we relate to the Selberg zeta function.