High dimensional Ellentuck spaces and initial chains in the Tukey structure of non-p-points

TL;DR

This paper investigates the Tukey order of certain ultrafilters, establishing their positions using high-dimensional Ellentuck spaces and Ramsey theory, revealing a hierarchical structure of nonprincipal ultrafilters.

Contribution

It introduces a hierarchy of high-dimensional Ellentuck spaces and proves new Ramsey-classification theorems to analyze the Tukey structure of ultrafilters.

Findings

The ultrafilter _G_2 is Tukey minimal over its projected Ramsey ultrafilter.

For each k, the ultrafilters Tukey reducible to _G_k form a chain of length k.

The spaces _E_k form a hierarchy of high-dimensional Ellentuck spaces.

Abstract

The generic ultrafilter $\mathcal{G}_2$ forced by $\mathcal{P}(\omega\times\omega)/($Fin$\otimes$Fin) was recently proved to be neither maximum nor minimum in the Tukey order of ultrafilters (in a recent paper of Blass, Dobrinen, and Raghavan), but it was left open where exactly in the Tukey order it lies. We prove that $\mathcal{G}_2$ is in fact Tukey minimal over its projected Ramsey ultrafilter. Furthermore, we prove that for each $k\ge 2$, the collection of all nonprincipal ultrafilters Tukey reducible to the generic ultrafilter $\mathcal{G}_k$ forced by $\mathcal{P}(\omega^k)/$Fin$^{\otimes k}$ forms a chain of length $k$. Essential to the proof is the extraction of a dense subset $\mathcal{E}_k$ from (Fin$^{\otimes k})^+$ which we prove to be a topological Ramsey space. The spaces $\mathcal{E}_k$, $k\ge 2$, form a hiearchy of high dimensional Ellentuck spaces. New Ramsey-classification theorems for equivalence relations on fronts on $\mathcal{E}_k$ are proved, extending the Pudlak-Rodl Theorem for fronts on the Ellentuck space, which are applied to find the Tukey structure below $\mathcal{G}_k$.