English

Nonregular ideals

Logic 2020-09-04 v2

Abstract

Generalizing Keisler's notion of regularity for ultrafilters, Taylor introduced degrees of regularity for ideals and showed that a countably complete nonregular ideal on ω1\omega_1 must be somewhere ω1\omega_1-dense. We prove a dichotomy about degrees of regularity for κ\kappa-complete ideals on successor cardinals κ\kappa and apply this to show that Taylor's Theorem does not generalize to higher cardinals. In particular, the existence of a nonregular ideal on ω2\omega_2 does not imply the existence of an ω2\omega_2-dense ideal on ω2\omega_2. We obtain similar results for normal ideals on Pκ(λ)\mathcal P_\kappa(\lambda).

Keywords

Cite

@article{arxiv.1901.02822,
  title  = {Nonregular ideals},
  author = {Monroe Eskew},
  journal= {arXiv preprint arXiv:1901.02822},
  year   = {2020}
}