Gruff Ultrafilters

Abstract

We investigate the question of whether $\mathbb Q$ carries an ultrafilter generated by perfect sets (such ultrafilters were called gruff ultrafilters by van Douwen). We prove that one can (consistently) obtain an affirmative answer to this question in three different ways: by assuming a certain parametrized diamond principle, from the cardinal invariant equality $\mathfrak d=\mathfrak c$, and in the Random real model. [Edit: replace "the Randor real model" with "the model obtained by adding $\omega_1$ Cohen reals to a model of $\mathsf{CH}$, and subsequently forcing with the Random algebra"; this is clarified in the corrigendum attached at the end of the paper.]