A Model in Which Every Boolean Algebra Has Many Subalgebras

We show that it is consistent with ZFC (relative to large cardinals) that every infinite Boolean algebra B has an irredundant subsetA such that 2|A| = 2|B|. This implies in particular that B has 2|B| subalgebras. We also discuss some more general problems about subalgebras and free subsets of an algebra. The result on the number of subalgebras in a Boolean algebra solves a question of Monk from [6]. The paper is intended to be accessible as far as possible to a general audience, in particular we have confined the more technical material to a “black box” at the end. The proof involves a variation on Foreman and Woodin’s model in which GCH fails everywhere. Research supported by a Postdoctoral Fellowship at the Mathematics Institute, Hebrew University of Jerusalem Research partially supported by the Basic Research Fund of the Israel Academy of Science. Paper number 530.