Conjugacy, orbit equivalence and classification of measure-preserving group actions

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We prove that if G is a countable discrete group with property (T) over an infinite subgroup HG which contains an infinite Abelian subgroup or is normal, then G has continuum-many orbit-inequivalent measure-preserving almost-everywhere-free ergodic actions on a standard Borel probability space. Further, we obtain that the measure-preserving almost-everywhere-free ergodic actions of such a G cannot be classified up to orbit equivalence by a reasonable assignment of countable structures as complete invariants. We also obtain a strengthening and a new proof of a non-classification result of Foreman and Weiss for conjugacy of measure-preserving ergodic almost-everywhere-free actions of discrete countable groups.
Original languageEnglish
JournalErgodic Theory and Dynamical Systems
Volume29
Issue number3
Pages (from-to)1033-1049
Number of pages17
ISSN0143-3857
DOIs
Publication statusPublished - 1 Jun 2009

ID: 61335006