Forcing minimal extensions of Boolean algebras

Abstract

Abstract. We employ a forcing approach to extending Boolean algebras. A link between some forcings and some cardinal functions on Boolean algebras is found and exploited. We find the following applications: 1) We make Fedorchuk's method more flexible, obtaining, for every cardinal λ of uncountable cofinality, a consistent example of a Boolean algebra A λ whose every infinite homomorphic image is of cardinality λ and has a countable dense subalgebra (i.e., its Stone space is a compact S-space whose every infinite closed subspace has weight λ). In particular this construction shows that it is consistent that the minimal character of a nonprincipal ultrafilter in a homomorphic image of an algebra A can be strictly less than the minimal size of a homomorphic image of A, answering a question of J. D. Monk.2) We prove that for every cardinal of uncountable cofinality it is consistent that 2 ω = λ and both A λ and Aω 1 exist.3) By combining these algebras we obtain many examples that answer questions of J.D. Monk. 4) We prove the consistency of MA + ¬CH + there is a countably tight compact space without a point of countable character, complementing results of A. Dow, V. Malykhin, and I. Juhasz. Although the algebra of clopen sets of the above space has no ultrafilter which is countably generated, it is a subalgebra of an algebra all of whose ultrafilters are countably generated. This proves, answering a question of Arhangel skii, that it is consistent that there is a first countable compact space which has a continuous image without a point of countable character. 5) We prove that for any cardinal λ of uncountable cofinality it is consistent that there is a countably tight Boolean algebra A with a distinguished ultrafilter ∞ such that for every a ∞ the algebra A|a is countable and ∞ has hereditary character λ.

An Efimov space with character less than s

“…Finally, we introduce the remarkable generalization of linear coherence to a coherence structure based on binary trees. These are the T-algebras introduced in [13]. However for this paper we restrict to subalgebras of P(ω) and subtrees of 2 <ω1 .…”

“…Lemma 8. [13] Let A Γ = a σ : σ ∈ Γ be a T-algebra. Let x ∈ X(Γ) and let U be any ultrafilter on the Boolean subalgebra, B Γ , of P(ω) generated by A Γ .…”

Compact Spaces with a $P$-base

“…It was first shown in [6] that the celebrated Moore-Mrowka problem was independent of Martin's Axiom plu c = ω 2 . The methods in [6] are indeed based on the paper [11] using the notion of T-algebras first formulated in [13]. The example in [11] is itself a space generated by a T-algebra but is not explicitly formulated as such because of its simpler structure.…”

Minimally generated Boolean algebras and the Nikodym property

“…Measure-theoretic properties of minimally generated Boolean algebras were investigated by Borodulin-Nadzieja [3], who, i.a., showed that they can carry only measures of countable Maharam type. Forcing aspects of minimally generated Boolean algebras were deeply studied by Koszmider [29].…”