The Poisson boundary of $\text{Out}(F_N)$

Abstract

Let $\mu$ be a probability measure on $\text{Out}(F_N)$ with finite first logarithmic moment with respect to the word metric, finite entropy, and whose support generates a nonelementary subgroup of $\text{Out}(F_N)$. We show that almost every sample path of the random walk on $(\text{Out}(F_N),\mu )$, when realized in Culler and Vogtmann's outer space, converges to the simplex of a free, arational tree. We then prove that the space $\mathcal{F}\mathcal{I}$ of simplices of free and arational trees, equipped with the hitting measure, is the Poisson boundary of $(\text{Out}(F_N),\mu )$. Using Bestvina-Reynolds' and Hamenstädt's description of the Gromov boundary of the complex ${\mathcal{FF}}_N$ of free factors of $F_N$, this gives a new proof of the fact, due to Calegari and Maher, that the realization in $\mathcal{F}{\mathcal{F}}_{\mathcal{N}}$ of almost every sample path of the random walk converges to a boundary point. We get in addition that $\partial {\mathcal{FF}}_N$, equipped with the hitting measure, is the Poisson boundary of $(\text{Out}(F_N),\mu )$.

23 pages

References in corpus (1)

• Generalized north-south dynamics on the space of geodesic currents

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