Cofinal types of ultrafilters
نویسندگان
چکیده
We study Tukey types of ultrafilters on ω, focusing on the question of when Tukey reducibility is equivalent to Rudin-Keisler reducibility. We give several conditions under which this equivalence holds. We show that there are only c many ultrafilters that are Tukey below any basically generated ultrafilter. The class of basically generated ultrafilters includes all known ultrafilters that are not Tukey above [ω1] <ω . We give a complete characterization of all ultrafilters that are Tukey below a selective. A counterexample showing that Tukey reducibility and RK reducibility can diverge within the class of P-points
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ورودعنوان ژورنال:
- Ann. Pure Appl. Logic
دوره 163 شماره
صفحات -
تاریخ انتشار 2012