A ratio ergodic theorem for multiparameter non-singular actions

作者：Hochman Michael^*

来源：Journal of the European Mathematical Society, 2010, 12(2): 365-383.

DOI：10.4171/JEMS/201

• Group actions
• measure preserving transformations
• commuting transformations
• non-singular actions
• ergodic theorem
• maximal inequality

We prove a ratio ergodic theorem for non-singular free Z(d) and R-d actions, along balls in an arbitrary norm. Using a Chacon-Ornstein type lemma the proof is reduced to a statement about the amount of mass of a probability measure that can concentrate on (thickened) boundaries of balls in R-d. The proof relies on geometric properties of norms, including the Besicovitch covering lemma and the fact that boundaries of balls have lower dimension than the ambient space. We also show that for general group actions, the Besicovitch covering property not only implies the maximal inequality, but is equivalent to it, implying that further generalization may require new methods.

• 出版日期2010