A connection between decomposable ultrafilters and possible cofinalities. II

Abstract

We use Shelah's theory of possible cofinalities in order to solve a problem about ultrafilters. THEOREM. Suppose that $ \lambda $ is a singular cardinal, $ \lambda ' < \lambda $, and the ultrafilter $D$ is $ \kappa $-decomposable for all regular cardinals $ \kappa $ with $ \lambda ' < \kappa < \lambda $. Then $D$ is either $ \lambda $-decomposable, or $ \lambda ^+$-decomposable. We give applications to topological spaces and to abstract logics.