Quantum CW-complexes in a Waldhausen category for unital C*-algebras

Abstract

Using the ring structure of the K-groups of finite CW-complexes, Atiyah and Todd unravelled the K-theory of complex projective spaces $CP^n$. Herein, in the realm of noncommutative topology, we develop a new framework of finite quantum CW-complexes using the language of Waldhausen categories, which allows us to enrich the class of standard morphisms between unital C*-algebras by adding inverses of *-homomorphisms that are isomorphisms in K-theory. Our concept of quantum CW-complexes subsumes earlier constructions and enjoys a plethora of examples. Moreover, the framework allows us to reduce problems concerning the multipushout quantum complex projective space $CP^n_H$ to the much more approachable setting of the Vaksman-Soibelman quantum complex projective space $CP^n_q$ enjoying the availability of graph-algebraic methods. In particular, these methods permit us to transport the ring structure from $K^0(CP^n)$ to $K^0(CP_q^n)$. Finally, we adapt the formalism of Waldhausen categories to determine a natural set of free generators of $K^0(CP_H^n)$ from a natural set of free generators of $K^0(CP_q^n)$, and to transport the ring structure from $K^0(CP_q^n)$ to $K^0(CP_H^n)$.