Borel complexity of sets of ideal limit points
Abstract
Let be an uncountable Polish space and let be an ideal on . A point is an -limit point of a sequence taking values in if there exists a subsequence convergent to such that the set of indexes . Denote by the family of subsets such that is the set of -limit points of some sequence taking values in or is empty. In this paper, we study the relationships between the topological complexity of ideals , their combinatorial properties, and the families of sets which can be attained. On the positive side, we provide several purely combinatorial (not dependind on the space ) characterizations of ideals for the inclusions and the equalities between and the Borel classes , , and . As a consequence, we prove that if is a ideal then exactly one of the following cases holds: or or (however we do not have an example of a ideal with ). In addition, we provide an explicit example of a coanalytic ideal for which . On the negative side, we show that there are no ideals such that or . We conclude with several open questions.
Cite
@article{arxiv.2411.10866,
title = {Borel complexity of sets of ideal limit points},
author = {Rafal Filipow and Adam Kwela and Paolo Leonetti},
journal= {arXiv preprint arXiv:2411.10866},
year = {2025}
}