# On the Unbounded Picture of $KK$-Theory

## TL;DR

This paper characterizes the kernel of the bounded transform in unbounded $KK$-theory, introduces an equivalence relation on unbounded Kasparov modules, and establishes a topological unbounded $KK$-theory isomorphic to $KK$-theory.

## Contribution

It defines an equivalence relation on unbounded Kasparov modules and introduces topological unbounded $KK$-theory, clarifying the structure of the kernel of the bounded transform.

## Key findings

- The kernel of the bounded transform is described via an equivalence relation.
- A new notion of topological unbounded $KK$-theory is introduced.
- The topological unbounded $KK$-theory is shown to be isomorphic to $KK$-theory.

## Abstract

In the founding paper on unbounded $KK$-theory it was established by Baaj and Julg that the bounded transform, which associates a class in $KK$-theory to any unbounded Kasparov module, is a surjective homomorphism (under a separability assumption). In this paper, we provide an equivalence relation on unbounded Kasparov modules and we thereby describe the kernel of the bounded transform. This allows us to introduce a notion of topological unbounded $KK$-theory, which becomes isomorphic to $KK$-theory via the bounded transform. The equivalence relation is formulated entirely at the level of unbounded Kasparov modules and consists of homotopies together with an extra degeneracy condition. Our degenerate unbounded Kasparov modules are called spectrally decomposable since they admit a decomposition into a part with positive spectrum and a part with negative spectrum.