English

The comparability numbers and the incomparability numbers

Logic 2026-01-30 v2

Abstract

We introduce new cardinal invariants of a poset, called the comparability number and the incomparability number. We determine their value for well-known posets, such as ωω\omega^\omega, P(ω)/fin\mathcal{P}(\omega)/\mathrm{fin}, the Turing degrees D\mathcal{D}, the quotient algebra Borel(2ω)/null\mathsf{Borel}(2^\omega)/\mathsf{null}, the ideals meager\mathsf{meager} and null\mathsf{null}. Moreover, we consider these invariants for the Rudin-Keisler ordering of the nonprincipal ultrafilters on ω\omega. We also consider these invariants for ideals on ω\omega and on ω1\omega_1.

Keywords

Cite

@article{arxiv.2302.08040,
  title  = {The comparability numbers and the incomparability numbers},
  author = {Tatsuya Goto},
  journal= {arXiv preprint arXiv:2302.08040},
  year   = {2026}
}

Comments

14 pages

R2 v1 2026-06-28T08:41:24.236Z