Yet another ideal version of the bounding number
Logic
2023-08-01 v1
Abstract
Let I be an ideal on ω. For f,g∈ωω we write f≤Ig if f(n)≤g(n) for all n∈ω∖A with some A∈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 denotes the family of all finite-to-one functions). We examine cardinal numbers b(≥I∩(DI×DI)) and b(≥I∩(DFin×DFin)) describing the smallest sizes of unbounded from below with respect to the order ≤I sets in DFin and DI, respectively. For a maximal ideal 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 for all ideals I with the Baire property and that ℵ1≤b(≥I∩(DI×DI))≤b for all coanalytic weak P-ideals (this class contains all Π40 ideals). What is more, we give examples of Borel (even Σ20) ideals I with b(≥I∩(DI×DI))=b as well as with b(≥I∩(DI×DI))=ℵ1.
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}
}