Characterizing existence of certain ultrafilters

Abstract

Following Baumgartner [J. Symb. Log. 60 (1995), no. 2], for an ideal $\mathcal{I}$ on $ω$, we say that an ultrafilter $\mathcal{U}$ on $ω$ is an $\mathcal{I}$-ultrafilter if for every function $f:ω\toω$ there is $A\in \mathcal{U}$ with $f[A]\in \mathcal{I}$. If there is an $\mathcal{I}$-ultrafilter which is not a $\mathcal{J}$-ultrafilter, then $\mathcal{I}$ is not below $\mathcal{J}$ in the Katětov order $\leq_{K}$ (i.e. for every function $f:ω\toω$ there is $A\in \mathcal{I}$ with $f^{-1}[A]\notin \mathcal{J}$). On the other hand, in general $\mathcal{I}\not\leq_{K}\mathcal{J}$ does not imply that existence of an $\mathcal{I}$-ultrafilter which is not a $\mathcal{J}$-ultrafilter is consistent. We provide some sufficient conditions on ideals to obtain the equivalence: $\mathcal{I}\not\leq_{K}\mathcal{J}$ if and only if it is consistent that there exists an $\mathcal{I}$-ultrafilter which is not a $\mathcal{J}$-ultrafilter. In some cases when the Katětov order is not enough for the above equivalence, we provide other conditions for which a similar equivalence holds. We are mainly interested in the cases when the family of all $\mathcal{I}$-ultrafilters or $\mathcal{J}$-ultrafilters coincides with some known family of ultrafilters: P-points, Q-points or selective ultrafilters (a.k.a. Ramsey ultrafilters). In particular, our results provide a characterization of Borel ideals $\mathcal{I}$ which can be used to characterize P-points as $\mathcal{I}$-ultrafilters. Moreover, we introduce a cardinal invariant which is used to obtain a sufficient condition for the existence of an $\mathcal{I}$-ultrafilter which is not a $\mathcal{I}$-ultrafilter. Finally, we prove some new results concerning existence of certain ultrafilters under various set-theoretic assumptions.