English

Can you take Akemann--Weaver's $\diamondsuit_{\aleph_1}$ away?

Logic 2022-11-15 v3 Operator Algebras

Abstract

By Glimm's dichotomy, a separable, simple C\textrm{C}^*-algebra has continuum-many unitarily inequivalent irreducible representations if, and only if, it is non-type I while all of its irreducible representations are unitarily equivalent if, and only if, it is type I. Naimark asked whether the latter equivalence holds for all C\textrm{C}^*-algebras. In 2004, Akemann and Weaver gave a negative answer to Naimark's problem, using Jensen's diamond axiom 1\diamondsuit_{\aleph_1}, a powerful diagonalization principle that implies the Continuum Hypothesis (CH\mathsf{CH}). By a result of Rosenberg, a separably represented simple C\textrm{C}^*-algebra with a unique irreducible representation is necessarily of type I. We show that this result is sharp by constructing an example of a separably represented, simple C\textrm{C}^*-algebra that has exactly two inequivalent irreducible representations, and therefore does not satisfy the conclusion of Glimm's dichotomy. Our construction uses a weakening of Jensen's 1\diamondsuit_{\aleph_1}, denoted Cohen\diamondsuit^\mathsf{Cohen}, that holds in the original Cohen's model for the negation of CH\mathsf{CH}. We also prove that Cohen\diamondsuit^\mathsf{Cohen} suffices to give a negative answer to Naimark's problem. Our main technical tool is a forcing notion that generically adds an automorphism of a given C\textrm{C}^*-algebra with a prescribed action on its space of pure states.

Keywords

Cite

@article{arxiv.2006.06886,
  title  = {Can you take Akemann--Weaver's $\diamondsuit_{\aleph_1}$ away?},
  author = {Daniel Calderón and Ilijas Farah},
  journal= {arXiv preprint arXiv:2006.06886},
  year   = {2022}
}