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Cofinal types of ultrafilters

2011; Elsevier BV; Volume: 163; Issue: 3 Linguagem: Inglês

10.1016/j.apal.2011.08.002

1873-2461

Dilip Raghavan, Stevo Todorčević,

Mathematical Dynamics and Fractals

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 is also given.