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On the elementary equivalence of automorphism groups of Boolean algebras; downward Skolem Löwenheim theorems and compactness of related quantifiers

Published online by Cambridge University Press:  12 March 2014

Matatyahu Rubin
Hebrew University, Jerusalem, Israel University of Colorado, Boulder, Colorado 80309
Saharon Shelah
Ben Gurion University, Beer Sheva, Israel University of Colorado, Boulder, Colorado 80309


Theorem 1. (◊ℵ1,) If B is an infinite Boolean algebra (BA), then there is B1, such that ∣ Aut (B1) ≤∣B1∣ = ℵ1 and 〈B1, Aut (B1)〉 ≡ 〈B, Aut(B)〉.

Theorem 2. (◊ℵ1) There is a countably compact logic stronger than first-order logic even on finite models.

This partially answers a question of H. Friedman. These theorems appear in §§1 and 2.

Theorem 3. (a) (◊ℵ1) If B is an atomic ℵ-saturated infinite BA, Ψ Є Lω1ω and 〈B, Aut (B)〉 ⊨Ψ then there is B1, Such that ∣Aut(B1)∣ ≤ ∣B1∣ =ℵ1, andB1, Aut(B1)〉⊨Ψ. In particular if B is 1-homogeneous so is B1. (b) (a) holds for B = P(ω) even if we assume only CH.

Research Article
Copyright © Association for Symbolic Logic 1980

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