Invariants in Noncommutative Dynamics

TL;DR

This paper investigates the limitations of invariants in noncommutative dynamics, particularly in resolving noncommutative Borsuk-Ulam conjectures, revealing cases where invariants fail to distinguish certain free coactions.

Contribution

It demonstrates that for some finite-dimensional quantum groups, no invariant can universally solve the Type 1 conjecture, contrasting with the abelian case, and explores invariants' behavior under deformation.

Findings

No well-behaved invariant solves the Type 1 conjecture for certain finite-dimensional H.

All iterated joins of H can be cleft as comodules over the associated Hopf algebra.

Invariants like local-triviality dimension and spectral count can change under $ heta$-deformation.

Abstract

When a compact quantum group $H$ coacts freely on unital $C^*$-algebras $A$ and $B$, the existence of equivariant maps $A \to B$ may often be ruled out due to the incompatibility of some invariant. We examine the limitations of using invariants, both concretely and abstractly, to resolve the noncommutative Borsuk-Ulam conjectures of Baum-Dabrowski-Hajac. Among our results, we find that for certain finite-dimensional $H$, there can be no well-behaved invariant which solves the Type 1 conjecture for all free coactions of $H$. This claim is in stark contrast to the case when $H$ is finite-dimensional and abelian. In the same vein, it is possible for all iterated joins of $H$ to be cleft as comodules over the Hopf algebra associated to $H$. Finally, two commonly used invariants, the local-triviality dimension and the spectral count, may both change in a $\theta$-deformation procedure.