We present an overview of the recent developments in the study of the classification problem for automorphisms of C*-algebras from the perspective of Borel complexity theory.

We prove that, under rather general conditions, the 1-cohomology of a von Neumann algebra $M$ with values in a Banach $M$ -bimodule satisfying a combination of smoothness and operatorial conditions vanishes. For instance, we show that, if $M$ acts normally on a Hilbert space ${\mathcal{H}}$ and ${\mathcal{B}}_{0}\subset {\mathcal{B}}({\mathcal{H}})$ is a norm closed $M$ -bimodule such that any $T\in {\mathcal{B}}_{0}$ is smooth (i.e., the left and right multiplications of $T$ by $x\in M$ are continuous from the unit ball of $M$ with the $s^{\ast }$ -topology to ${\mathcal{B}}_{0}$ with its norm), then any derivation of $M$ into ${\mathcal{B}}_{0}$ is inner. The compact operators are smooth over any $M\subset {\mathcal{B}}({\mathcal{H}})$ , but there is a large variety of non-compact smooth elements as well.

In this paper we present a classification, up to equivariant isomorphism, of ${{C}^{*}}$-dynamical systems $(A,\,\mathbb{R},\,\alpha )$ arising as inductive limits of directed systems $\{({{A}_{n}},\,\mathbb{R},\,{{\alpha }_{n}}),\,{{\varphi }_{nm}}\}$, where each ${{A}_{n}}$ is a finite direct sum of matrix algebras over the continuous functions on the unit circle, and the ${{\alpha }_{n}}\text{s}$ are outer actions generated by rotation of the spectrum.

An invariant is presented which classifies, up to equivariant isomorphism, ${{C}^{*}}$-dynamical systems arising as limits from inductive systems of elementary ${{C}^{*}}$-algebras on which the Euclidean motion group acts by way of unitary representations that decompose into finite direct sums of irreducibles.

Certain linear operators from a Banach algebra $A$ into a Banach $A$-bimodule $X$, which are called approximately local derivations, are studied. It is shown that when $A$ is a ${\rm C^*}$-algebra, a Banach algebra generated by idempotents, a semisimple annihilator Banach algebra, or the group algebra of a SIN or a totally disconnected group, bounded approximately local derivations from $A$ into $X$ are derivations. This, in particular, extends a result of B. E. Johnson that ‘local derivations on ${\rm C^*}$-algebras are derivations’ and provides an alternative proof of it.

Let $A$ be a non-commutative normed algebra with centre $Z(A)$. A simple application of the triangle inequality shows that if $a \in A$ is close to $Z(A)$ then $a$ almosts commutes with elements of the unit ball of $A$. The extent to which the converse holds is a fundamental question in the study of non-commutativity and has been previously investigated for various classes of Banach algebras, including von Neumann algebras and other C*-algebras. In this context, $K(A)$ is defined to be the smallest number in $[0, \infty]$ such that $$ {\rm distance}(a, Z(A)) \leq K(A) \Vert {\rm ad}(a) \Vert $$ for all $a \in A$, where ${\rm ad}(a)$ is the inner derivation $x \to xa-ax$ for $(x \in A)$. For a C*-algebra $A$, the constant $K_s(A)$ is defined similarly, restricting to self-adjoint $a \in A$.

For a unital C*-algebra $A$, it is already known that the only possible values for $K_s(A)$ are $ \frac{1}{2}, 1, \frac{3}{2}, \ldots, \infty $, that $K(A) \in \{\frac{1}{2}, \frac{1}{\sqrt{3}}\} \cup [1, \infty]$ and that these cases are distinguished by purely topological conditions on the primitive ideal space ${\rm Prim}(A)$. In this paper, further progress is made on the question of possible values for $K(A)$ in $[1, \infty]$. It is shown that $ K(A) \leq \frac{2}{\sqrt{3}}K_s(A)$ and that either $ K(A) \leq \frac{4}{\sqrt{15}} \sim 1.033$ or $ K(A) \geq \frac{1}{2} + \frac{1}{\sqrt{3}} \sim 1.077$. The values $\frac{4}{\sqrt{15}}$ and $\frac{1}{2} + \frac{1}{\sqrt{3}}$ are attained and each of them arises as the maximal bounding radius of a constrained subset of the plane which is obtained from the structure of ${\rm Prim}(A)$.

It is also shown that if $A$ has trivial centre then $K(A)$ is further restricted: $$ K(A) \in \{\frac{1}{2}, 1, \frac{1}{2} + \frac{1}{\sqrt{3}}\} \cup [\frac{3}{2}, \infty].$$ Again, the various cases are distinguished by purely topological conditions on ${\rm Prim}(A)$, some of which are expressed in terms of primal ideals.

We study conjugacy invariants for completely positive maps that are inspired by the concept of curvature introduced for commuting d-tuples of contractions by Arveson.

An $\text{AF}$ flow is a one-parameter automorphism group of an $\text{AF}$${{C}^{*}}$-algebra $A$ such that there exists an increasing sequence of invariant finite dimensional sub-${{C}^{*}}$-algebras whose union is dense in $A$. In this paper, a classification of ${{C}^{*}}$-dynamical systems of this form up to equivariant isomorphism is presented. Two pictures of the actions are given, one in terms of a modified Bratteli diagram/pathspace construction, and one in terms of a modified ${{K}_{0}}$ functor.

We examine Hilbert bimodules which possess a (generally unbounded) involution. Topics considered include a linking algebra representation, duality, locality, and the role of these bimodules in noncommutative differential geometry

We use some results about stable relations to show that some of the simple, stable, projectionless crossed products of ${{O}_{2}}$ by $\mathbb{R}$ considered by Kishimoto and Kumjian are inductive limits of type I ${{C}^{*}}$-algebras. The type I ${{C}^{*}}$-algebras that arise are pullbacks of finite direct sums of matrix algebras over the continuous functions on the unit interval by finite dimensional ${{C}^{*}}$-algebras.