Can you take Akemann--Weaver's $\diamondsuit_{\aleph_1}$ away?
Abstract
By Glimm's dichotomy, a separable, simple -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 -algebras. In 2004, Akemann and Weaver gave a negative answer to Naimark's problem, using Jensen's diamond axiom , a powerful diagonalization principle that implies the Continuum Hypothesis (). By a result of Rosenberg, a separably represented simple -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 -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 , denoted , that holds in the original Cohen's model for the negation of . We also prove that 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 -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}
}