## Multiplication of weak equivalence classes may be discontinuous

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- by Anton Bernshteyn PDF
- Trans. Amer. Math. Soc.
**372**(2019), 8091-8106

## Abstract:

For a countably infinite group $\Gamma$, let ${\mathcal {W}}_{\Gamma }$ denote the space of all weak equivalence classes of measure-preserving actions of ${\Gamma }$ on atomless standard probability spaces, equipped with the compact metrizable topology introduced by Abért and Elek. There is a natural multiplication operation on ${\mathcal {W}}_{\Gamma }$ (induced by taking products of actions) that makes ${\mathcal {W}}_{\Gamma }$ an Abelian semigroup. Burton, Kechris, and Tamuz showed that if ${\Gamma }$ is amenable, then ${\mathcal {W}}_{\Gamma }$ is a topological semigroup; i.e., the product map ${\mathcal {W}}_{\Gamma } \times {\mathcal {W}}_{\Gamma } \to {\mathcal {W}}_{\Gamma } \colon (\mathfrak {a}, \mathfrak {b}) \mapsto \mathfrak {a} \times \mathfrak {b}$ is continuous. In contrast to that, we prove that if ${\Gamma }$ is a Zariski dense subgroup of ${\mathrm {SL}}_d({\mathbb {Z}})$ for some $d \geqslant 2$ (for instance, if ${\Gamma }$ is a non-Abelian free group), then multiplication on ${\mathcal {W}}_{\Gamma }$ is discontinuous, even when restricted to the subspace ${\mathcal {FW}}_{\Gamma }$ of all free weak equivalence classes.## References

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## Additional Information

**Anton Bernshteyn**- Affiliation: Department of Mathematics, University of Illinois at Urbana–Champaign, Illinois; and Department of Mathematical Sciences, Carnegie Mellon University, Pittsburgh, Pennsylvania
- MR Author ID: 1104079
- Email: bernsht2@illinois.edu; abernsht@math.cmu.edu
- Received by editor(s): April 4, 2018
- Received by editor(s) in revised form: March 11, 2019
- Published electronically: June 13, 2019
- Additional Notes: This research is supported in part by the Waldemar J., Barbara G., and Juliette Alexandra Trjitzinsky Fellowship.
- © Copyright 2019 Anton Bernshteyn
- Journal: Trans. Amer. Math. Soc.
**372**(2019), 8091-8106 - MSC (2010): Primary 37A15, 22F10, 37A35; Secondary 20G40
- DOI: https://doi.org/10.1090/tran/7847
- MathSciNet review: 4029691