# The Universal Coefficient Theorem for $C_{∗}$-Algebras with Finite Complexity

### Rufus Willett

University of Hawai'i at Mānoa, Honolulu, USA### Guoliang Yu

Texas A&M University, College Station, USA

This book is published *open access.*

A $C_{∗}$-algebra satisfies the Universal Coefficient Theorem (UCT) of Rosenberg and Schochet if it is equivalent in Kasparov's $KK$-theory to a commutative $C_{∗}$-algebra. This paper is motivated by the problem of establishing the range of validity of the UCT, and in particular, whether the UCT holds for all nuclear $C_{∗}$-algebras.

We introduce the idea of a $C_{∗}$-algebra that "decomposes" over a class $C$ of $C_{∗}$-algebras. Roughly, this means that locally there are approximately central elements that approximately cut the $C_{∗}$-algebra into two $C_{∗}$-subalgebras from $C$ that have well-behaved intersection. We show that if a $C_{∗}$-algebra decomposes over the class of nuclear, UCT $C_{∗}$-algebras, then it satisfies the UCT. The argument is based on a Mayerr–Vietoris principle in the framework of controlled $KK$-theory; the latter was introduced by the authors in an earlier work. Nuclearity is used via Kasparov's Hilbert module version of Voiculescu's theorem, and Haagerup's theorem that nuclear $C_{∗}$-algebras are amenable.

We say that a $C_{∗}$-algebra has finite complexity if it is in the smallest class of $C_{∗}$-algebras containing the finite-dimensional $C_{∗}$-algebras, and closed under decomposability; our main result implies that all $C_{∗}$-algebras in this class satisfy the UCT. The class of $C_{∗}$-algebras with finite complexity is large, and comes with an ordinal-number invariant measuring the complexity level. We conjecture that a $C_{∗}$-algebra of finite nuclear dimension and real rank zero has finite complexity; this (and several other related conjectures) would imply the UCT for all separable nuclear $C_{∗}$-algebras. We also give new local formulations of the UCT, and some other necessary and sufficient conditions for the UCT to hold for all nuclear $C_{∗}$-algebras.