English

Naively Haar null sets in Polish groups

Logic 2015-08-11 v1

Abstract

Let (G,)(G,\cdot) be a Polish group. We say that a set XGX \subset G is Haar null if there exists a universally measurable set UXU \supset X and a Borel probability measure μ\mu such that for every g,hGg, h \in G we have μ(gUh)=0\mu(gUh)=0. We call a set XX naively Haar null if there exists a Borel probability measure μ\mu such that for every g,hGg, h \in G we have μ(gXh)=0\mu(gXh)=0. Generalizing a result of Elekes and Stepr\=ans, which answers the first part of Problem FC from Fremlin's list, we prove that in every abelian Polish group there exists a naively Haar null set that is not Haar null.

Keywords

Cite

@article{arxiv.1508.02227,
  title  = {Naively Haar null sets in Polish groups},
  author = {Márton Elekes and Zoltán Vidnyánszky},
  journal= {arXiv preprint arXiv:1508.02227},
  year   = {2015}
}
R2 v1 2026-06-22T10:29:57.198Z