Axiomatic $KK$-theory for Real C*-algebras

TL;DR

This paper develops axiomatic characterizations of K-theory and KK-theory for real C*-algebras, establishing conditions under which functors factor through KK and are isomorphic to K-theory.

Contribution

It provides new axiomatic criteria for real C*-algebra K-theory and KK-theory, including factorization and isomorphism results under specific invariance and exactness conditions.

Findings

Functor F is homotopy invariant, stable, and split exact iff it factors through KK.

Natural isomorphism between K and F for a broad class of real C*-algebras.

Isomorphism of functors when A is real, given they are isomorphisms for complex algebras.

Abstract

We establish axiomatic characterizations of $K$-theory and $KK$-theory for real C*-algebras. In particular, let $F$ be an abelian group-valued functor on separable real C*-algebras. We prove that if $F$ is homotopy invariant, stable, and split exact, then $F$ factors through the category $KK$. Also, if $F$ is homotopy invariant, stable, half exact, continuous, and satisfies an appropriate dimension axiom, then there is a natural isomorphism $K(A) \to F(A)$ for a large class of separable real C*-algebras $A$. Furthermore, we prove that a natural transformation $F(A) \to G(A)$ of homotopy invariant, stable, half-exact functors which is an isomorphism when $A$ is complex is necessarily an isomorphism when $A$ is real.