Rigidity of a class of smooth singular flows on $\mathbb T^2$

Abstract

We study joining rigidity in the class of von Neumann flows with one singularity. They are given by a smooth vector field $\mathcal{X}$ on $\mathbb T^2\setminus \{a\}$, where $\mathcal{X}$ is not defined at $a\in \mathbb T^2$. It follows that the phase space can be decomposed into a (topological disc) $D_\mathcal{X}$ and an ergodic component $E_\mathcal{X}=\mathbb T^2\setminus D_\mathcal{X}$. Let $\omega_\mathcal{X}$ be the 1-form associated to $\mathcal{X}$. We show that if $|\int_{E_{\mathcal{X}_1}}d\omega_{\mathcal{X}_1}|\neq |\int_{E_{\mathcal{X}_2}}d\omega_{\mathcal{X}_2}|$, then the corresponding flows $(v_t^{\mathcal{X}_1})$ and $(v_t^{\mathcal{X}_2})$ are disjoint. It also follows that for every $\mathcal{X}$ there is a uniquely associated frequency $\alpha=\alpha_{\mathcal{X}}\in \mathbb T$. We show that for a full measure set of $\alpha\in \mathbb T$ the class of smooth time changes of $(v_t^\mathcal{X_\alpha})$ is joining rigid, i.e. every two smooth time changes are either cohomologous or disjoint. This gives a natural class of flows for which the answer to a problem of Ratner (Problem 3 in \cite{Rat4}) is positive.