English

Yet another ideal version of the bounding number

Logic 2023-08-01 v1

Abstract

Let I\mathcal{I} be an ideal on ω\omega. For f,gωωf,g\in\omega^\omega we write fIgf \leq_{\mathcal{I}} g if f(n)g(n)f(n) \leq g(n) for all nωAn\in\omega\setminus A with some AIA\in\mathcal{I}. Moreover, we denote \mathcal{D}_{\mathcal{I}}=\{f\in\omega^\omega: f^{-1}[\{n\}]\in\mathcal{I} \text{ for every n\in \omega}\} (in particular, DFin\mathcal{D}_{Fin} denotes the family of all finite-to-one functions). We examine cardinal numbers b(I(DI×DI))\mathfrak{b}(\geq_{\mathcal{I}}\cap (\mathcal{D}_{\mathcal{I}} \times \mathcal{D}_{\mathcal{I}})) and b(I(DFin×DFin))\mathfrak{b}(\geq_{\mathcal{I}}\cap (\mathcal{D}_{Fin}\times \mathcal{D}_{Fin})) describing the smallest sizes of unbounded from below with respect to the order I\leq_{\mathcal{I}} sets in DFin\mathcal{D}_{Fin} and DI\mathcal{D}_{\mathcal{I}}, respectively. For a maximal ideal I\mathcal{I}, these cardinals were investigated by M. Canjar in connection with coinitial and cofinal subsets of the ultrapowers. We show that b(I(DFin×DFin))=b\mathfrak{b}(\geq_{\mathcal{I}}\cap (\mathcal{D}_{Fin} \times \mathcal{D}_{Fin})) =\mathfrak{b} for all ideals I\mathcal{I} with the Baire property and that 1b(I(DI×DI))b\aleph_1 \leq \mathfrak{b}(\geq_{\mathcal{I}}\cap (\mathcal{D}_{\mathcal{I}} \times \mathcal{D}_{\mathcal{I}})) \leq\mathfrak{b} for all coanalytic weak P-ideals (this class contains all Π40\Pi^0_4 ideals). What is more, we give examples of Borel (even Σ20\Sigma^0_2) ideals I\mathcal{I} with b(I(DI×DI))=b\mathfrak{b}(\geq_{\mathcal{I}}\cap (\mathcal{D}_{\mathcal{I}} \times \mathcal{D}_{\mathcal{I}}))=\mathfrak{b} as well as with b(I(DI×DI))=1\mathfrak{b}(\geq_{\mathcal{I}}\cap (\mathcal{D}_{\mathcal{I}} \times \mathcal{D}_{\mathcal{I}})) =\aleph_1.

Keywords

Cite

@article{arxiv.2307.16017,
  title  = {Yet another ideal version of the bounding number},
  author = {Rafał Filipów and Adam Kwela},
  journal= {arXiv preprint arXiv:2307.16017},
  year   = {2023}
}
R2 v1 2026-06-28T11:43:29.713Z