English

Borel complexity of sets of ideal limit points

General Topology 2025-04-21 v2 Classical Analysis and ODEs Functional Analysis

Abstract

Let XX be an uncountable Polish space and let I\mathcal{I} be an ideal on ω\omega. A point ηX\eta \in X is an I\mathcal{I}-limit point of a sequence (xn)(x_n) taking values in XX if there exists a subsequence (xkn)(x_{k_n}) convergent to η\eta such that the set of indexes {kn:nω}I\{k_n: n \in \omega\}\notin \mathcal{I}. Denote by L(I)\mathscr{L}(\mathcal{I}) the family of subsets SXS\subseteq X such that SS is the set of I\mathcal{I}-limit points of some sequence taking values in XX or SS is empty. In this paper, we study the relationships between the topological complexity of ideals I\mathcal{I}, their combinatorial properties, and the families of sets L(I)\mathscr{L}(\mathcal{I}) which can be attained. On the positive side, we provide several purely combinatorial (not dependind on the space XX) characterizations of ideals I\mathcal{I} for the inclusions and the equalities between L(I)\mathscr{L}(\mathcal{I}) and the Borel classes Π10\Pi^0_1, Σ20\Sigma^0_2, and Π30\Pi^0_3. As a consequence, we prove that if I\mathcal{I} is a Π40\Pi^0_4 ideal then exactly one of the following cases holds: L(I)=Π10\mathscr{L}(\mathcal{I})=\Pi^0_1 or L(I)=Σ20\mathscr{L}(\mathcal{I})=\Sigma^0_2 or L(I)=Σ11\mathscr{L}(\mathcal{I})=\Sigma^1_1 (however we do not have an example of a Π40\Pi^0_4 ideal with L(I)=Σ11\mathscr{L}(\mathcal{I})=\Sigma^1_1). In addition, we provide an explicit example of a coanalytic ideal I\mathcal{I} for which L(I)=Σ11\mathscr{L}(\mathcal{I})=\Sigma^1_1. On the negative side, we show that there are no ideals I\mathcal{I} such that L(I)=Π20\mathscr{L}(\mathcal{I})=\Pi^0_2 or L(I)=Σ30\mathscr{L}(\mathcal{I})=\Sigma^0_3. We conclude with several open questions.

Keywords

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}
}