Set-theoretic absoluteness for analysts and separable \cstar-algebras without Choice

Abstract

Basic theory of separable C*-algebras can be developed without the Axiom of Choice, and it does not depend on the Continuum Hypothesis, Martin's Axiom, and other standard set-theoretic assumptions. This can be proved in two ways. First, by showing that the standard proofs do not require Choice. Second, by utilizing set-theoretic absoluteness theorems. We provide an introduction to projective complexity and absoluteness for analysts. We also give some limiting examples consistent with ZF, such as a commutative \cstar-algebra concretely represented on a Hilbert space but not isomorphic to $C(X)$ for any compact Hausdorff space $X$ and whose state space is not compact and has no extreme points.

Revised, with more emphasis put on absoluteness. 35 pages