Joining measures for horocycle flows on abelian covers

Abstract

A celebrated result of Ratner from the eighties says that two horocycle flows on hyperbolic surfaces of finite area are either the same up to algebraic change of coordinates, or they have no non-trivial joinings. Recently, Mohammadi and Oh extended Ratner's theorem to horocycle flows on hyperbolic surfaces of infinite area but finite genus. In this paper, we present the first joining classification result of a horocycle flow on a hyperbolic surface of infinite genus: a $\mathbb{Z}$ or ${\mathbb{Z}}^2$-cover of a general compact hyperbolic surface. We also discuss several applications.

Final Version. To appear in JMD