Elementwise conservative actions and boomerang subgroups

by Waltraud Lederle (Universität Bielefeld)

Monday 16 Feb 2026, 14:00 → 15:00 Europe/Brussels

B/2nd floor-B.203 - Seminar room (Marc de Hemptinne (chemin du Cyclotron, 2, Louvain-la-Neuve))

We propose to study actions of countable groups on measure spaces such that every group element individually acts as a conservative transformation, that is, a transformation such that no set of positive measure is disjoint from all its translates. We construct such actions of the free group, using the measurable full group of a hyperfinite equivalence relation. The motivation for our work was to find interesting examples of boomerang subgroups. Based on joint work with Y. Glasner and T. Hartnick.