Random walks on $\mathrm{Homeo}(S^1)$

Abstract

In this paper, we study random walks $g_n=f_{n-1}\cdots f_0$ on the group $\mathrm{Homeo}(S^1)$ of the homeomorphisms of the circle, where the homeomorphisms $f_k$ are chosen randomly, independently, with respect to a same probability measure $\nu$. We prove that under the only condition that there is no probability measure invariant by $\nu$-almost every homeomorphism, the random walk almost surely contracts small intervals. It generalizes what has been known on this subject until now, since various conditions on $\nu$ were imposed in order to get the phenomenon of contractions. Moreover, we obtain the surprising fact that the rate of contraction is exponential, even in the lack of assumptions of smoothness on the $f_k$'s. We deduce next various dynamical consequences on the random walk $(g_n)$: finiteness of ergodic stationary measures, distribution of the trajectories, asymptotic law of the evaluations, etc. The proof of the main result is based on a modification of the \'Avila-Viana's invariance principle, working for continuous cocycles on a space fibred in circles.