Survey on the Tukey Theory of Ultrafilters
نویسنده
چکیده
This article surveys results regarding the Tukey theory of ultrafilters on countable base sets. The driving forces for this investigation are Isbell’s Problem and the question of how closely related the Rudin-Keisler and Tukey reducibilities are. We review work on the possible structures of cofinal types and conditions which guarantee that an ultrafilter is below the Tukey maximum. The known canonical forms for cofinal maps on ultrafilters are reviewed, as well as their applications to finding which structures embed into the Tukey types of ultrafilters. With the addition of some Ramsey theory, fine analyses of the structures at the bottom of the Tukey hierarchy are made.
منابع مشابه
Tukey Classes of Ultrafilters on Ω
Motivated by a question of Isbell, we show that ♦ implies there is a non-P-point U ∈ βω \ ω such that neither 〈U ,⊇〉 nor 〈U ,⊇∗〉 is Tukey equivalent to 〈[c],⊆〉. We also show that 〈U ,⊇∗〉 ≡T 〈[c] ,⊆〉 for some U ∈ βω \ ω, assuming cf(κ) = κ ≤ p = c. We also prove two negative ZFC results about the possible Tukey classes of ultrafilters on ω.
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