Tukey-idempotency and strong p-points

Abstract

We characterize strong $p$-point ultrafilters by showing that they are exactly those $p$-points that are not Tukey above $(\ddot{I}{‰}^{\ddot{I}}‰,\le )$; or equivalently, those $p$-points that are not Tukey-idempotent. Moreover, we show that there are no Canjar ultrafilters on measurable cardinals. We make use of tools which were motivated by topological Ramsey spaces, developed in \cite{Benhamou/Dobrinen24}, and furthermore, show that ultrafilters arising from most of the known topological Ramsey spaces are Tukey-idempotent. Our results answer questions of Hrušák and Verner \cite[Question 5.7]{Hrusak/Verner11}, Brook-Taylor \cite[Question 3.6]{QuestionGeneralized}, and partially Benhamou and Dobrinen \cite[Question 5.6]{Benhamou/Dobrinen24}.

15 pages