arXiv subjects
Publications and source records attributed to Eusebio Gardella.
We show that formation of crossed products and passage to fixed point algebras by compact group actions with finite Rokhlin dimension preserve the following regularity properties: finite decomposition rank, finite nuclear dimension, and tensorial absorption of the Jiang-Su algebra, the latter in the formulation with commuting towers.
The group of automorphisms of the Cuntz algebra $\mathcal{O}_{2}$ is a Polish group with respect to the topology of pointwise convergence in norm. Our main result is that the relations of conjugacy and cocycle conjugacy of automorphisms of $\mathcal{O}_{2}$ are complete analytic sets and, in particular, not Borel. Moreover, we show that from the point of view of Borel complexity theory, classifying automorphisms of $\mathcal{O}_{2}$ up to conjugacy or cocycle conjugacy is strictly more difficult than classifying up to isomorphism any class of countable structures with Borel isomorphism relation. In fact the same conclusions hold even if one only considers automorphisms of $\mathcal{O}_{2}$ of a fixed finite order. In the course of the proof we will show that the relation of isomorphism of Kichberg algebras (with trivial $K_{1}$-group and satisfying the Universal Coefficient Theorem) is a complete anaytic set. Moreover, it is strictly more difficult to classify Kirchberg algebras (with trivial $K_{1}$-group and satisfying the Universal Coefficient Theorem) than classifying up to isomorphism any class of countable structures with Borel isomorphism relation.