Abstract
An exotic crossed product is a way of associating a C ∗-algebra to each C ∗-dynamical system that generalizes the well-known universal and reduced crossed products. Exotic crossed products provide natural generalizations of, and tools to study, exotic group C ∗-algebras as recently considered by Brown-Guentner and others. They also form an essential part of a recent program to reformulate the Baum-Connes conjecture with coefficients so as to mollify the counterexamples caused by failures of exactness.In this paper, we survey some constructions of exotic group algebras and exotic crossed products. Summarising our earlier work, we single out a large class of crossed products—the correspondence functors—that have many properties known for the maximal and reduced crossed products: for example, they extend to categories of equivariant correspondences, and have a compatible descent morphism in K K-theory. Combined with known results on K-amenability and the Baum-Connes conjecture, this allows us to compute the K-theory of many exotic group algebras. It also gives new information about the reformulation of the Baum-Connes Conjecture mentioned above. Finally, we present some new results relating exotic crossed products for a group and its closed subgroups, and discuss connections with the reformulated Baum-Connes conjecture.
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Notes
- 1.
We always assume inner products on Hilbert spaces to be linear in the second variable.
- 2.
Added in proof: these ingredients recently appeared for second countable G in a paper of Brodzki, Cave, and Li, arXiv.1603.01829.
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Acknowledgements
Part of the work on this paper took place during a visit of the first and third authors to the Westfälische Wilhelms-Universität, Münster. We would like to thank this institution for its hospitality.
This work has been supported by Deutsche Forschungsgemeinschaft (SFB 878, Groups, Geometry & Actions), by CNPq/CAPES—Brazil, and by the US NSF (DMS 1401126).
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Buss, A., Echterhoff, S., Willett, R. (2016). Exotic Crossed Products. In: Carlsen, T.M., Larsen, N.S., Neshveyev, S., Skau, C. (eds) Operator Algebras and Applications. Abel Symposia, vol 12. Springer, Cham. https://doi.org/10.1007/978-3-319-39286-8_3
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