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On invariant subalgebras of group $C^*$ and von Neumann algebras

Part of: Selfadjoint operator algebras Locally compact groups and their algebras

Published online by Cambridge University Press:  04 November 2022

MEHRDAD KALANTAR Open the ORCID record for MEHRDAD KALANTAR [Opens in a new window]  and
NIKOLAOS PANAGOPOULOS
MEHRDAD KALANTAR*
Affiliation:
Department of Mathematics, University of Houston, Houston, TX 77004, USA (e-mail: [email protected])
NIKOLAOS PANAGOPOULOS
Affiliation:
Department of Mathematics, University of Houston, Houston, TX 77004, USA (e-mail: [email protected])
*
e-mail: [email protected]
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Abstract

Given an irreducible lattice $\Gamma $ in the product of higher rank simple Lie groups, we prove a co-finiteness result for the $\Gamma $-invariant von Neumann subalgebras of the group von Neumann algebra $\mathcal {L}(\Gamma )$, and for the $\Gamma $-invariant unital $C^*$-subalgebras of the reduced group $C^*$-algebra $C^*_{\mathrm {red}}(\Gamma )$. We use these results to show that: (i) every $\Gamma $-invariant von Neumann subalgebra of $\mathcal {L}(\Gamma )$ is generated by a normal subgroup; and (ii) given a weakly mixing unitary representation $\pi $ of $\Gamma $, every $\Gamma $-equivariant conditional expectation on $C^*_\pi (\Gamma )$ is the canonical conditional expectation onto the $C^*$-subalgebra generated by a normal subgroup.

Keywords

invariant subalgebrasunitary representationslattices

MSC classification

Primary: 22D25: $C^*$-algebras and $W^*$-algebras in relation to group representations
Secondary: 46L40: Automorphisms
Type
Original Article
Information
Ergodic Theory and Dynamical Systems , Volume 43 , Issue 10 , October 2023 , pp. 3341 - 3353
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Copyright
© The Author(s), 2022. Published by Cambridge University Press

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