The equivariant Cuntz semigroup

Eusebio Gardella, Luis Santiago

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1 Citation (Scopus)


We introduce an equivariant version of the Cuntz semigroup that takes an action of a compact group into account. The equivariant Cuntz semigroup is naturally a semimodule over the representation semiring of the given group. Moreover, this semimodule satisfies a number of additional structural properties. We show that the equivariant Cuntz semigroup, as a functor, is continuous and stable. Moreover, cocycle conjugate actions have isomorphic associated equivariant Cuntz semigroups. One of our main results is an analog of Julg's theorem: the equivariant Cuntz semigroup is canonically isomorphic to the Cuntz semigroup of the crossed product. We compute the induced semimodule structure on the crossed product, which in the abelian case is given by the dual action. As an application of our results, we show that freeness of a compact Lie group action on a compact Hausdorff space can be characterized in terms of a canonically defined map into the equivariant Cuntz semigroup, extending results of Atiyah and Segal for equivariant
K-theory. Finally, we use the equivariant Cuntz semigroup to classify locally representable actions on direct limits of one-dimensional NCCW-complexes, generalizing work of Handelman and Rossmann.
Original languageEnglish
Pages (from-to)189-241
Number of pages53
JournalProceedings of the London Mathematical Society
Issue number2
Early online date11 Jan 2017
Publication statusPublished - Feb 2017

Bibliographical note

Funded by
SFB 878 Groups, Geometry and Actions
Humboldt Foundation


  • 46L55 (primary)
  • 46L35
  • 46L80 (secondary)


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