Cartan subalgebras and the UCT problem

The question whether every separable, nuclear C*-algebra satisfies Rosenberg-Schochet's universal coefficient theorem (UCT) is a major open problem in C*-algebra theory. Currently, renewed interest in this so-called UCT problem arises from the recent breakthrough results in the classification program for separable, simple, nuclear C*-algebras, where the UCT plays a rather mysterious role. In this talk, connections between Cartan subalgebras, that is, MASAs admitting faithful conditional expectations and generating the ambient C*-algebras in a suitable sense, and the UCT problem will be illustrated. Using remarkable results of Renault and Tu, we will see that separable, nuclear C*-algebras with Cartan subalgebras satisfy the UCT. Moreover, I will try to explain the close connection between the UCT problem on the one hand and Cartan subalgebras and finite order automorphisms of the Cuntz algebra O_2 on the other. This is joint work with Xin Li.