Local Statistics of Random Permutations from Free Products

Abstract

Let $\alpha$ and $\beta$ be uniformly random permutations of orders $2$ and $3$, respectively, in $S_{N}$, and consider, say, the permutation $\alpha\beta\alpha\beta^{-1}$. How many fixed points does this random permutation have on average? The current paper studies questions of this kind and relates them to surprising topological and algebraic invariants of elements in free products of groups. Formally, let $\Gamma=G_{1}*\ldots*G_{k}$ be a free product of groups where each of $G_1,\ldots,G_k$ is either finite, finitely generated free, or an orientable hyperbolic surface group. For a fixed element $\gamma\in\Gamma$, a $\gamma$-random permutation in the symmetric group $S_{N}$ is the image of $\gamma$ through a uniformly random homomorphism $\Gamma\to S_{N}$. In this paper we study local statistics of $\gamma$-random permutations and their asymptotics as $N$ grows. We first consider $\mathbb{E}\left[\mathrm{fix}_{\gamma}\left(N\right)\right]$, the expected number of fixed points in a $\gamma$-random permutation in $S_{N}$. We show that unless $\gamma$ has finite order, the limit of $\mathbb{E}\left[\mathrm{fix}_{\gamma}\left(N\right)\right]$ as $N\to\infty$ is an integer, and is equal to the number of subgroups $H\le\Gamma$ containing $\gamma$ such that $H\cong\mathbb{Z}$ or $H\cong C_{2}*C_{2}$. Equivalently, this is the number of subgroups $H\le\Gamma$ containing $\gamma$ and having (rational) Euler characteristic zero. We also prove there is an asymptotic expansion for $\mathbb{E}\left[\mathrm{fix}_{\gamma}\left(N\right)\right]$ and determine the limit distribution of the number of fixed points as $N\to\infty$. These results are then generalized to all statistics of cycles of fixed lengths.