## Group actions on Polish spaces

### Volume 125 / 2023

#### Abstract

In this paper we investigate the action of Polish groups (not necessary abelian) on uncountable Polish spaces. We consider two main situations. First, when the orbits given by group action are small and the second when the family of orbits are at most countable. We have found some subgroups which are not measurable with respect to a given $\sigma $-ideal on the group and the action on some subsets gives a completely nonmeasurable sets with respect to some $\sigma $-ideals with a Borel base on the Polish space. In most cases the general results are consistent with ZFC theory and are strictly connected with cardinal coefficients. We give some suitable examples, namely the subgroup of isometries of the Cantor space where the orbits are sufficiently small. In the opposite case we give an example of the group of homeomorphisms of a Polish space in which there is a large orbit and we have found the subgroup without Baire property and a subset of the space such that the action of this subgroup on this set is completely nonmeasurable with respect to the $\sigma $-ideal of subsets of first category.