Pointwise Equidistribution and Translates of Measures on Homogeneous Spaces

Abstract

Let $(X,\mathfrak{B},\mu )$ be a Borel probability space. Let $T_n:X\to X$ be a sequence of continuous transformations on $X$. Let $\nu$ be a probability measure on $X$ such that $\frac{1}{N}{\sum }_{n=1}^N(T_n{)}_{\ast }\nu \to \mu$ in the weak-$\ast$ topology. Under general conditions, we show that for $\nu$ almost every $x\in X$, the measures $\frac{1}{N}{\sum }_{n=1}^N{\delta }_{T_{n}x}$ get equidistributed towards $\mu$ if $N$ is restricted to a set of full upper density. We present applications of these results to translates of closed orbits of Lie groups on homogeneous spaces. As a corollary, we prove equidistribution of exponentially sparse orbits of the horocycle flow on quotients of $SL(2,\mathbb{R})$, starting from every point in almost every direction.

arXiv commentThis is an updated version containing a stronger criterion for the existence of Ratner sequences and a more general sparse equidistribution theorem