Abstract
We establish new characterizations for (pseudo)isometric extensions of topological dynamical systems. For such extensions, we also extend results about relatively invariant measures and Fourier analysis that were previously only known in the minimal case to a significantly larger class, including all transitive systems. To bypass the reliance on minimality of the classical approaches to isometric extensions via the Ellis semigroup, we show that extensions of topological dynamical systems can be described as groupoid actions and then adapt the concept of enveloping semigroups to construct a uniform enveloping semigroupoid for groupoid actions. This approach allows to deal with the more complex orbit structures of nonminimal systems.
We study uniform enveloping semigroupoids of general groupoid actions and translate the results back to the special case of extensions of dynamical systems. In particular, we show that, under appropriate assumptions, a groupoid action is (pseudo)isometric if and only if the uniform enveloping semigroupoid is actually a compact groupoid. We also provide an operator theoretic characterization based on an abstract Peter—Weyl-type theorem for representations of compact, transitive groupoids on Banach bundles which is of independent interest.
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Acknowledgements
The authors express their gratitude to Markus Haase, Rainer Nagel, and Jean Renault for ideas and inspiring discussions as well as to the referees for their valuable comments. They also thank Patrick Hermle for reading the article and giving feedback. The first author thanks theMPIM and both authors thank the MFO for providing a stimulating atmosphere for working on this project. The second author was supported by a scholarship of the Friedrich-Ebert-Stiftung while working on this article. Moreover, a preliminary version of this article is included in the second author’s PhD thesis [Kre19].
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Edeko, N., Kreidler, H. Uniform enveloping semigroupoids for groupoid actions. JAMA 148, 739–796 (2022). https://doi.org/10.1007/s11854-022-0243-2
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DOI: https://doi.org/10.1007/s11854-022-0243-2