On the speed of convergence in the ergodic theorem for shift operators

Abstract

Abstract Given a probability space $(X,\mu )$ , a square integrable function f on such space and a (unilateral or bilateral) shift operator T, we prove under suitable assumptions that the ergodic means $N^{-1}\sum _{n=0}^{N-1} T^nf$ converge pointwise almost everywhere to zero with a speed of convergence which, up to a small logarithmic transgression, is essentially of the order of $N^{-1/2}$ . We also provide a few applications of our results, especially in the case of shifts associated with toral endomorphisms.