Borel complexity of sets of ideal limit points
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2024
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| author | Filipow, Rafal Kwela, Adam Leonetti, Paolo |
| author_facet | Filipow, Rafal Kwela, Adam Leonetti, Paolo |
| contents | Let $X$ be an uncountable Polish space and let $\mathcal{I}$ be an ideal on $ω$. A point $η\in X$ is an $\mathcal{I}$-limit point of a sequence $(x_n)$ taking values in $X$ if there exists a subsequence $(x_{k_n})$ convergent to $η$ such that the set of indexes $\{k_n: n \in ω\}\notin \mathcal{I}$. Denote by $\mathscr{L}(\mathcal{I})$ the family of subsets $S\subseteq X$ such that $S$ is the set of $\mathcal{I}$-limit points of some sequence taking values in $X$ or $S$ is empty. In this paper, we study the relationships between the topological complexity of ideals $\mathcal{I}$, their combinatorial properties, and the families of sets $\mathscr{L}(\mathcal{I})$ which can be attained.
On the positive side, we provide several purely combinatorial (not dependind on the space $X$) characterizations of ideals $\mathcal{I}$ for the inclusions and the equalities between $\mathscr{L}(\mathcal{I})$ and the Borel classes $Π^0_1$, $Σ^0_2$, and $Π^0_3$. As a consequence, we prove that if $\mathcal{I}$ is a $Π^0_4$ ideal then exactly one of the following cases holds: $\mathscr{L}(\mathcal{I})=Π^0_1$ or $\mathscr{L}(\mathcal{I})=Σ^0_2$ or $\mathscr{L}(\mathcal{I})=Σ^1_1$ (however we do not have an example of a $Π^0_4$ ideal with $\mathscr{L}(\mathcal{I})=Σ^1_1$). In addition, we provide an explicit example of a coanalytic ideal $\mathcal{I}$ for which $\mathscr{L}(\mathcal{I})=Σ^1_1$.
On the negative side, we show that there are no ideals $\mathcal{I}$ such that $\mathscr{L}(\mathcal{I})=Π^0_2$ or $\mathscr{L}(\mathcal{I})=Σ^0_3$. We conclude with several open questions. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2411_10866 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Borel complexity of sets of ideal limit points Filipow, Rafal Kwela, Adam Leonetti, Paolo General Topology Classical Analysis and ODEs Functional Analysis 28A05, 54A20, 03E15, 03E75, 40A05, 40A35 Let $X$ be an uncountable Polish space and let $\mathcal{I}$ be an ideal on $ω$. A point $η\in X$ is an $\mathcal{I}$-limit point of a sequence $(x_n)$ taking values in $X$ if there exists a subsequence $(x_{k_n})$ convergent to $η$ such that the set of indexes $\{k_n: n \in ω\}\notin \mathcal{I}$. Denote by $\mathscr{L}(\mathcal{I})$ the family of subsets $S\subseteq X$ such that $S$ is the set of $\mathcal{I}$-limit points of some sequence taking values in $X$ or $S$ is empty. In this paper, we study the relationships between the topological complexity of ideals $\mathcal{I}$, their combinatorial properties, and the families of sets $\mathscr{L}(\mathcal{I})$ which can be attained. On the positive side, we provide several purely combinatorial (not dependind on the space $X$) characterizations of ideals $\mathcal{I}$ for the inclusions and the equalities between $\mathscr{L}(\mathcal{I})$ and the Borel classes $Π^0_1$, $Σ^0_2$, and $Π^0_3$. As a consequence, we prove that if $\mathcal{I}$ is a $Π^0_4$ ideal then exactly one of the following cases holds: $\mathscr{L}(\mathcal{I})=Π^0_1$ or $\mathscr{L}(\mathcal{I})=Σ^0_2$ or $\mathscr{L}(\mathcal{I})=Σ^1_1$ (however we do not have an example of a $Π^0_4$ ideal with $\mathscr{L}(\mathcal{I})=Σ^1_1$). In addition, we provide an explicit example of a coanalytic ideal $\mathcal{I}$ for which $\mathscr{L}(\mathcal{I})=Σ^1_1$. On the negative side, we show that there are no ideals $\mathcal{I}$ such that $\mathscr{L}(\mathcal{I})=Π^0_2$ or $\mathscr{L}(\mathcal{I})=Σ^0_3$. We conclude with several open questions. |
| title | Borel complexity of sets of ideal limit points |
| topic | General Topology Classical Analysis and ODEs Functional Analysis 28A05, 54A20, 03E15, 03E75, 40A05, 40A35 |
| url | https://arxiv.org/abs/2411.10866 |