Norm-variation of triple ergodic averages for commuting transformations

Abstract

We prove an $r$-variation estimate, $r>4$, in the norm for ergodic averages with respect to three commuting transformations. It is not known whether such estimates hold for all $r\ge 2$ as in the analogous cases for one or two commuting transformations, or whether such estimates hold for any $r<\infty$ for more than three commuting transformations.

49 pages; final version

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• Power-type cancellation for the simplex Hilbert transform
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