Key words Subalgebras of Boolean algebras, cardinal functions.
MSC (2000) 06E05, 03G05We consider eight special kinds of subalgebras of Boolean algebras. In Section 1 we describe the relationships between these subalgebra notions. In succeeding sections we consider how the subalgebra notions behave with respect to the most common cardinal functions on Boolean algebras.
Definitions and relationshipsIn notation we follow Koppelberg [4] and Monk [9], but also we give key definitions that we will work with.AThis is illustrated by the following diagram:This is the definition of projective subalgebra given in Koppelberg [5, p. 752].A ≤ rc B iff A is a relatively complete subalgebra of B; see Koppelberg [4, p. 123]. This means that for each b ∈ B there is a smallest a ∈ A with b ≤ a, and also a greatestA ≤ s B iff B is a simple extension of A. This means that B is obtained from A by adding just one element, and all elements then necessary to form a BA; we denote B by A(x), where x is the new element.A ≤ m B iff B is a minimal extension of A. Thus B is a simple extension of A and there is no BA C such thatA ≤ mg B iff B is a minimally generated extension of A. Thus B can be obtained from A by a sequence of minimal extensions, with unions taken at limit steps.A ≤ π B iff A is a dense subalgebra of B. Here dense means that for every b ∈ B + there is an a ∈ A + such that a ≤ b. *