### Regular subalgebras of complete Boolean algebras

by Blaszczyk and Shelah. [BzSh:640]

J Symbolic Logic, 2001

It is shown that there exists a complete, atomless,
sigma-centered Boolean algebra, which does not contain any
regular countable subalgebra if and only if there exist a nowhere
dense ultrafilter. Therefore the existence of such algebras is
undecidable in ZFC.

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