Continuous and other finitely generated canonical cofinal maps on ultrafilters

Abstract

This paper concentrates on the existence of canonical cofinal maps of three types: continuous, generated by finitary monotone end-extension preserving maps, and generated by monotone finitary maps. The main theorems prove that every monotone cofinal map on an ultrafilter of a certain class of ultrafilters is actually canonical when restricted to some filter base. These theorems are then applied to find connections between Tukey, Rudin-Keisler, and Rudin-Blass reducibilities on large classes of ultrafilters. The main theorems are the following. The property of having continuous Tukey reductions is inherited under Tukey reducibility, under a mild assumption. It follows that every ultrafilter Tukey reducible to a p-point has continuous Tukey reductions. If U is a Fubini iterate of p-points, then each monotone cofinal map from U to some other ultrafilter is generated (on a cofinal subset of U) by a finitary map on the base tree for U which is monotone and end-extension preserving - the analogue of continuous in this context. This is applied to prove that every ultrafilter which is Tukey reducible to some Fubini iterate of p-points has finitely generated cofinal maps. Similar theorems also hold for some other classes of ultrafilters.