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Hilbert's fifth problem inquires whether every locally Euclidean group is necessarily a Lie group. Von Neumann demonstrated that this is indeed true for the compact case.

The definition of a quantum group is not universally agreed upon. For some, it specifically refers to deformations of Lie groups, whereas for others, it encompasses the broader concept of a Hopf algebra.

Is there a quantum analogue of von Neumann's result? Specifically, under what conditions can a Hopf algebra be viewed as a deformation of a Lie group?

I am not aware of such a result, even if we restrict to quasitriangular Hopf algebras. Any insights or references on possible quantum analogues would be greatly appreciated.

An initial step could involve focusing on compact quantum groups and developing a non-commutative generalization of the concept of "locally Euclidean" that applies to them.

In contemporary terminology, the concept of a locally Euclidean space is understood as a topological manifold. One approach to the notion of noncommutative topological manifold is explored here; however, it is not compatible with the property of compactness.

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  • $\begingroup$ I guess one issue is that a Hopf algebra does not come with a topology... $\endgroup$ Commented Oct 24 at 13:36
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    $\begingroup$ @SamHopkins One solution to this issue could be to assume the existence of a $C^*$-algebra structure that is compatible with the Hopf algebra structure. For instance, consider the notion of a compact quantum group. $\endgroup$ Commented Oct 24 at 15:07
  • $\begingroup$ Well, there is a quite standard definition of a compact quantum group. And it contains examples such as the free orthogonal quantum group (and other "free" quantum groups), which (as far as I know) do not admit differential structure and are not duals of some sort of universal envelopes of (quantum) Lie algebras, so they do not really behave like Lie groups whatever this is supposed to mean. $\endgroup$
    – Daniel
    Commented Oct 24 at 15:08
  • $\begingroup$ @Daniel I do not claim that the concept of a compact quantum group is the definitive solution; rather, I propose focusing on this specific case. Naturally, additional assumptions will be necessary. $\endgroup$ Commented Oct 24 at 15:22
  • $\begingroup$ @Daniel A colleague privately noted that differential calculi exist for free orthogonal Hopf algebras. See doi.org/10.1007/s10468-015-9518-y $\endgroup$ Commented Oct 27 at 3:25

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