Instytut Matematyczny Polskiej Akademii Nauk / pl / Wydawnictwa / Czasopisma IMPAN / Fundamenta Mathematicae / Wszystkie zeszyty

Streszczenie

Generalizing Keisler’s notion of regularity for ultrafilters, Taylor introduced degrees of regularity for ideals and showed that a countably complete nonregular ideal on $\omega _1$ must be somewhere $\omega _1$-dense. We prove a dichotomy about degrees of regularity for $\kappa$-complete ideals on successor cardinals $\kappa$ and apply this to show that Taylor’s Theorem does not generalize to higher cardinals. In particular, the existence of a nonregular ideal on $\omega _2$ does not imply the existence of an $\omega _2$-dense ideal on $\omega _2$. We obtain similar results for normal ideals on $\mathcal P _\kappa (\lambda )$.