Haar-smallest sets
Abstract
In this paper we are interested in the following notions of smallness: a subset of an abelian Polish group is called Haar-countable/Haar-finite/Haar- if there are a Borel hull and a copy of such that is countable/finite/of cardinality at most , for all . Recently, Banakh et al. have unified the notions of Haar-null and Haar-meager sets by introducing Haar- sets, where is a collection of subsets of . It turns out that if is the -ideal of countable sets, the ideal of finite sets or the collection of sets of cardinality at most , then we get the above notions. Moreover, those notions have been studied independently by Zakrzewski (under a different name -- perfectly -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 ) and study -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}
}