Weak mixing and sparse equidistribution

Max Auer

Published 2024-08-27Version 1

The celebrated Birkhoff Ergodic Theorem asserts that, for an ergodic map, orbits of almost every point equidistributes when sampled at integer times. This result was generalized by Bourgain to many natural sparse subsets of the integers. On the other hand, the behaviour of orbits of \textbf{all} points in a dynamical system is much less understood, especially for sparse subsets of the integers. We generalize a method introduced by A. Venkatesh to tackle this problem in two directions, general $\mathbb{R}^d$ actions instead of flows, and weak mixing, rather than mixing, actions. Along the way, we also establish some basic properties of weak mixing and show weak mixing for the time 1-map of a weak mixing flow. The main examples are equidistribution along $\{(n^{1+\epsilon_i})_{i=1}^d\}$, which is a direct generalisation of Venkatesh's example, and equidistribution along random subsets of $\mathbb{Z}^d$ for horosperical action.