Summability and speed of convergence in an ergodic theorem

Abstract

Given an irrational vector $\alpha$ in $\mathbb{R}^{d}$, a continuous function $f(x)$ on the torus $\mathbb{T}^{d}$ and suitable weights $\Phi(N,n)$ such that $\sum_{n=-\infty}^{+\infty}\Phi(N,n)=1$, we estimate the speed of convergence to the integral $\int_{\mathbb{T}^{d}}f(y)dy$ of the weighted sum $\sum_{n=-\infty}^{+\infty}\Phi(N,n) f(x+n\alpha)$ as $N\rightarrow +\infty$. Whereas for the arithmetic means $N^{-1}\sum_{n=1}^{N}f(x+n\alpha)$ the speed of convergence is never faster than $cN^{-1}$, for other means such speed can be accelerated. We estimate the speed of convergence in two theorems with different flavor. The first result is a metric one, and it provides an estimate of the speed of convergence in terms of the Fourier transform of the weights $\Phi(N,n)$ and the smoothness of the function $f(x)$ which holds for almost every $\alpha$. The second result is a deterministic one, and the speed of convergence is estimated also in terms of the Diophantine properties of the given irrational vector $\alpha\in\mathbb R^d$.