Paper

Commutativity of Cofinal Types

Benhamou, Tom

2025

Overview

We continue the study of the pseudo-intersection property with respect to an ideal introduced in TomNatasha2. Our theory applies to the study of the Tukey types of general sums of ultrafilters, which, as evidenced by the results of this paper, can be quite complex. It also applies to construct a large class of ultrafilter \\(C\\) over \\(\\) such that any two ultrafilters \\(U,Vın C\\) commute; that is, \\(U V_T V U\\). The class \\(C\\) class contains most known cofinal types of ultrafilters on \\(\\). This is in sharp contrast to the Rudin-Keisler ordering. In the third part of this paper, we apply our results to study the class of ultrafilters Tukey above \\(^\\). Specifically, we prove that ultrafilters without the \\(I\\)-p.i.p are always above \\(I^\\) and in particular non-\\(p\\)-points are Tukey above \\(^\\). Finally, we introduce the hierarchy of \\(\\)-almost rapid ultrafilters. We prove that it is consistent for them to form a strictly wider class than the rapid ultrafilters, and give an example of a non-rapid \\(p\\)-point ultrafilter which is Tukey above \\(^\\). This addresses and answers several questions from TomNatasha,TomNatasha2,Dobrinen/Todorcevic11,Milovich08.

• Publisher: Cornell University Library, arXiv.org
• Subject: Commutativity