English

Compact actions whose orbit equivalence relations are not profinite

Dynamical Systems 2018-07-17 v1 Logic

Abstract

Let Γ(X,μ)\Gamma\curvearrowright (X,\mu) be a measure preserving action of a countable group Γ\Gamma on a standard probability space (X,μ)(X,\mu). We prove that if the action ΓX\Gamma\curvearrowright X is not profinite and satisfies a certain spectral gap condition, then there does not exist a countable-to-one Borel homomorphism from its orbit equivalence relation to the orbit equivalence relation of any modular action (i.e., an inverse limit of actions on countable sets). As a consequence, we show that if Γ\Gamma is a countable dense subgroup of a compact non-profinite group GG such that the left translation action ΓG\Gamma\curvearrowright G has spectral gap, then ΓG\Gamma\curvearrowright G is antimodular and not orbit equivalent to any, {\it not necessarily free}, profinite action. This provides the first such examples of compact actions, partially answering a question of Kechris and answering a question of Tsankov.

Keywords

Cite

@article{arxiv.1807.05476,
  title  = {Compact actions whose orbit equivalence relations are not profinite},
  author = {Adrian Ioana},
  journal= {arXiv preprint arXiv:1807.05476},
  year   = {2018}
}
R2 v1 2026-06-23T03:01:37.422Z