Dynamical obstructions to classification by (co)homology and other TSI-group invariants

Allison, Shaun; Panagiotopoulos, Aristotelis

doi:10.1090/tran/8475

Dynamical obstructions to classification by (co)homology and other TSI-group invariants
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by Shaun Allison and Aristotelis Panagiotopoulos PDF

Trans. Amer. Math. Soc. 374 (2021), 8793-8811

Abstract:

In the spirit of Hjorth’s turbulence theory, we introduce “unbalancedness”: a new dynamical obstruction to classifying orbit equivalence relations by actions of Polish groups which admit a two-sided invariant metric (TSI). Since abelian groups are TSI, unbalancedness can be used for identifying which classification problems cannot be solved by classical homology and cohomology theories.

In terms of applications, we show that Morita equivalence of continuous-trace $C^*$-algebras, as well as isomorphism of Hermitian line bundles, are not classifiable by actions of TSI groups. In the process, we show that the Wreath product of any two non-compact subgroups of $S_{\infty }$ admits an action whose orbit equivalence relation is generically ergodic against any action of a TSI group and we deduce that there is an orbit equivalence relation of a CLI group which is not classifiable by actions of TSI groups.

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