A new class of Ramsey-classification theorems and their applications in the Tukey theory of ultrafilters

Abstract

Motivated by Tukey classification problems and building on work in \cite{Dobrinen/Todorcevic11}, we develop a new hierarchy of topological Ramsey spaces $\mathcal{R}_{\alpha}$, $\alpha<\omega_1$. These spaces form a natural hierarchy of complexity, $\mathcal{R}_0$ being the Ellentuck space, and for each $\alpha<\omega_1$, $\mathcal{R}_{\alpha+1}$ coming immediately after $\mathcal{R}_{\alpha}$ in complexity. Associated with each $\mathcal{R}_{\alpha}$ is an ultrafilter $\mathcal{U}_{\alpha}$, which is Ramsey for $\mathcal{R}_{\alpha}$, and in particular, is a rapid p-point satisfying certain partition properties. We prove Ramsey-classification theorems for equivalence relations on fronts on $\mathcal{R}_{\alpha}$, $2\le\alpha<\omega_1$. These are analogous to the Pudlak-\Rodl\ Theorem canonizing equivalence relations on barriers on the Ellentuck space. We then apply our Ramsey-classification theorems to completely classify all Rudin-Keisler equivalence classes of ultrafilters which are Tukey reducible to $\mathcal{U}_{\alpha}$, for each $2\le\alpha<\omega_1$: Every ultrafilter which is Tukey reducible to $\mathcal{U}_{\alpha}$ is isomorphic to a countable iteration of Fubini products of ultrafilters from among a fixed countable collection of rapid p-points. Moreover, we show that the Tukey types of nonprincipal ultrafilters Tukey reducible to $\mathcal{U}_{\alpha}$ form a descending chain of order type $\alpha+1$.