English

Haar-smallest sets

Functional Analysis 2019-04-19 v4

Abstract

In this paper we are interested in the following notions of smallness: a subset AA of an abelian Polish group XX is called Haar-countable/Haar-finite/Haar-nn if there are a Borel hull BAB\supseteq A and a copy CC of 2ω2^\omega such that (C+x)B(C+x)\cap B is countable/finite/of cardinality at most nn, for all xXx\in X. Recently, Banakh et al. have unified the notions of Haar-null and Haar-meager sets by introducing Haar-I\mathcal{I} sets, where I\mathcal{I} is a collection of subsets of 2ω2^\omega. It turns out that if I\mathcal{I} is the σ\sigma-ideal of countable sets, the ideal of finite sets or the collection of sets of cardinality at most nn, then we get the above notions. Moreover, those notions have been studied independently by Zakrzewski (under a different name -- perfectly κ\kappa-small sets). We study basic properties of the corresponding families of small sets, give suitable examples distinguishing them (in all abelian Polish groups of the form R×X\mathbb{R}\times X) and study σ\sigma-ideals generated by compact members of the considered families. In particular, we show that Haar-countable sets do not form an ideal. Moreover, we answer some questions concerning null-finite sets, asked by Banakh and Jab{\l}o\'nska, and pose several open problems.

Keywords

Cite

@article{arxiv.1711.09753,
  title  = {Haar-smallest sets},
  author = {Adam Kwela},
  journal= {arXiv preprint arXiv:1711.09753},
  year   = {2019}
}