Abstract

Let κ and λ be infinite cardinals, F a filter on κ, and G a set of functions from κ to κ. The filter F is generated by G if F consists of those subsets of κ which contain the range of some element of G. The set G is $ -closed if it is closed in the $ -topology on κ κ. (In general, the $ -topology on IA has basic open sets all Π i∈ I U i such that, for all $i \in I, U_i \subseteq A$ and $|\{i \in I: U_i \neq A\}| .) The primary question considered in this paper asks "Is there a uniform ultrafilter on κ which is generated by a closed set of functions?" (Closed means $ -closed.) We also establish the independence of two related questions. One is due to Carlson: "Does there exist a regular cardinal κ and a subtree T of $^{ such that the set of branches of T generates a uniform ultrafilter on κ?"; and the other is due to Pouzet: "For all regular cardinals κ, is it true that no uniform ultrafilter on κ is analytic?" We show that if κ is a singular, strong limit cardinal, then there is a uniform ultrafilter on κ which is generated by a closed set of increasing functions. Also, from the consistency of an almost huge cardinal, we get the consistency of CH + "There is a uniform ultrafilter on ℵ 1 which is generated by a closed set of increasing functions". In contrast with the above results, we show that if κ is any cardinal, λ is a regular cardinal less than or equal to κ and P is the forcing notion for adding at least $(\kappa^{ generic subsets of λ, then in V P , no uniform ultrafilter on κ is generated by a closed set of functions