Symmetry, Integrability and Geometry: Methods and Applications (SIGMA)

SIGMA 16 (2020), 082, 21 pages      arXiv:1901.05161      https://doi.org/10.3842/SIGMA.2020.082
Contribution to the Special Issue on Noncommutative Manifolds and their Symmetries in honour of Giovanni Landi

On the Unbounded Picture of $KK$-Theory

Jens Kaad
Department of Mathematics and Computer Science, The University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark

Received October 22, 2019, in final form August 05, 2020; Published online August 22, 2020

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.

Key words: $KK$-theory; unbounded $KK$-theory; equivalence relations; bounded transform.

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