Abstract
We prove a version of Furstenberg’s ergodic theorem with restrictions on return times. More specifically, for a measure preserving system (X, B, μ,T), integers 0 ≤j <k, andE ⊂X with μ(E) > 0, we show that there existsn ≡ j (modk) with ώ(E ∩T -nE ∩T -2nE ∩T -3nE) > 0, so long asT k is ergodic. This result requires a deeper understanding of the limit of some nonconventional ergodic averages and the introduction of a new class of systems, the ‘Quasi-Affine Systems’.
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This work was partially carried out while the second author was visiting the Université de Marne la Vallée, supported by NSF grant 9804651.
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Host, B., Kra, B. An odd Furstenberg-Szemerédi theorem and quasi-affine systems. J. Anal. Math. 86, 183–220 (2002). https://doi.org/10.1007/BF02786648
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DOI: https://doi.org/10.1007/BF02786648