Mathematics – Dynamical Systems
Scientific paper
2008-10-09
Mathematics
Dynamical Systems
After a few minor corrections, to appear in Ergodic Theory and Dynamical Systems
Scientific paper
A sequence $(s_n)$ of integers is good for the mean ergodic theorem if for each invertible measure preserving system $(X,\mathcal{B},\mu,T)$ and any bounded measurable function $f$, the averages $ \frac1N \sum_{n=1}^N f(T^{s_n}x)$ converge in the $L^2$ norm. We construct a sequence $(s_n)$ that is good for the mean ergodic theorem, but the sequence $(s_n^2)$ is not. Furthermore, we show that for any set of bad exponents $B$, there is a sequence $(s_n)$ where $(s_n^k)$ is good for the mean ergodic theorem exactly when $k$ is not in $B$. We then extend this result to multiple ergodic averages. We also prove a similar result for pointwise convergence of single ergodic averages.
Frantzikinakis Nikos
Johnson Michael
Lesigne Emmanuel
Wierdl Mate
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