# Poisson limit for the number of cycles in a random permutation and the number of segregating sites

## TL;DR

This paper proves that the joint distribution of the number of cycles in a random permutation and the number of segregating sites in a genetic sample converges to a Poisson point process, revealing deep connections between combinatorics and population genetics.

## Contribution

It establishes the weak convergence of measures associated with permutation cycles and genetic segregating sites to a Poisson point process, extending previous results on their joint behavior.

## Key findings

- Joint measures converge to a Poisson point process
- Weak convergence of permutation and genetic site measures
- Extension of previous convergence results to joint measures

## Abstract

Consider a random permutation of $\{1, \ldots, \lfloor n^{t_2}\rfloor\}$ drawn according to the Ewens measure with parameter $t_1$ and let $K(n, t)$ denote the number of its cycles, where $t\equiv (t_1, t_2)\in\mathbb [0, 1]^2$. Next, consider a sample drawn from a large, neutral population of haploid individuals subject to mutation under the infinitely many sites model of Kimura whose genealogy is governed by Kingman's coalescent. Let $S(n, t)$ count the number of segregating sites in a sample of size $\lfloor n^{t_2}\rfloor$ when mutations arrive at rate $t_1/2$. We show that $K(n, (t_1/\log n, t_2))-1$ and $S(n, (t_1/\log n, t_2))$ induce unique random measures $\Pi_n^K$ and $\Pi_n^S,$ respectively, on the positive quadrant $[0, \infty)^2.$ Our main result is to show that in the coupling of $S(n, t)$ and $K(n, t)$ introduced in~\cite{Pitters2019} we have weak convergence as $n\to\infty$ \begin{align*} (\Pi_n^K, \Pi_n^S)\to_d (\Pi, \Pi), \end{align*} where $\Pi$ is a Poisson point process on $[0, \infty)^2$ of unit intensity. This complements the work in~\cite{Pitters2019} where it was shown that the process $\{(K(n, t), S(n, t)), t\in [0, 1]^2\},$ appropriately rescaled, converges weakly to the product of the same one-dimensional Brownian sheet.