Compact actions whose orbit equivalence relations are not profinite
Abstract
Let be a measure preserving action of a countable group on a standard probability space . We prove that if the action 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 is a countable dense subgroup of a compact non-profinite group such that the left translation action has spectral gap, then 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.
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}
}