English

Trace spaces of counterexamples to Naimark's Problem

Operator Algebras 2020-04-27 v4 Logic

Abstract

A counterexample to Naimark's problem is a CC^\ast-algebra that is not isomorphic to the algebra of compact operators on some Hilbert space, yet still has only one irreducible representation up to unitary equivalence. It is well-known that such algebras must be nonseparable, and in 2004 Akemann and Weaver used the diamond principle (a set theoretic axiom independent from ZFC) to give the first counterexamples. For any such counterexample AA, the unitary group U(A)U(A) acts transitively on the pure states, which are the extreme points of the state space S(A)S(A). It is conceivable that this implies (as happens for finite-dimensional simplexes) that the action of U(A)U(A) on S(A)S(A) has at most one fixed point, i.e. AA has at most one trace. We give a strong negative answer here assuming diamond. In particular, we adapt the Akemann-Weaver construction to show that the trace space of a counterexample to Naimark's problem can be affinely homeomorphic to any metrizable Choquet simplex, and can also be nonseparable.

Keywords

Cite

@article{arxiv.1711.05845,
  title  = {Trace spaces of counterexamples to Naimark's Problem},
  author = {Andrea Vaccaro},
  journal= {arXiv preprint arXiv:1711.05845},
  year   = {2020}
}

Comments

20 pages; updated version: some significant revision has been done to take care of a gap in the proof of theorem A

R2 v1 2026-06-22T22:47:32.746Z