Free sequences in P(ω)/fin

David Chodounský, Vera Fischer, Jan Grebík

Published 2018-08-17Version 1

We investigate maximal free sequences in the Boolean algebra $\mathcal{P}(\omega)/\mathrm{fin}$, as defined by D. Monk. We provide some information on the general structure of these objects and we are particularly interested in the minimal cardinality of a free sequence, a cardinal characteristic of the continuum denoted $\mathfrak{f}$. Answering a question of Monk, we demonstrate the consistency of $\omega_1 = \mathfrak{i} = \mathfrak{f} < \mathfrak{u} = \omega_2$. In fact, this consistency is demonstrated in the model of S. Shelah for $\mathfrak{i} < \mathfrak{u}$. Our paper provides a streamlined and mostly self contained presentation of this construction.