# Question on Hilbert Manifolds

I have a very basic question on Hilbert manifolds.

Consider the Hilbert space $$\mathcal{H}:= L^2(S^1)$$ with $S^1$ the unit circle. On $\mathcal{H}$ let us introduce the equivalence relation $$f\sim g : \Leftrightarrow f(\cdot ) = g(\cdot + \alpha)\quad \mbox{for some }\alpha \in S^1.$$ Now define the factor space $$\overline{\mathcal{H}}:= \mathcal{H}/\sim.$$ What is the structure of $\overline{\mathcal{H}}$? Is it a Hilbert manifold? If so, how to construct the smooth structure? I am particularly interested in computing a (Riemannian) distance between two elements of $\overline{\mathcal{H}}$.

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Sorry, I has an answer that was wrong, using Fourier transform. If it can be corrected, I will repost. –  BS. Jun 21 '12 at 13:40
I would be equally interested in the answer for $L^2(G)/G$ if $G$ is a (compact) topological group. –  Spice the Bird Jun 21 '12 at 23:58
See Andrew Stacy's answer to the question mathoverflow.net/questions/10200/… –  Spice the Bird Jun 22 '12 at 0:10
Not a Hilbert manifold. As the answer(s) have pointed out, you get problems at fixed points of the circle action (indeed, at any point where the circle action has a non-trivial stabiliser). There's another problem which is that the circle does not act continuously on the Hilbert space - how much of a problem this is will depend on how you want to fix the first problem (for example, you could go for a stratified space). –  Loop Space Jun 25 '12 at 9:28

## 1 Answer

It probably is NOT a smooth manifold. I think finding a chart around the point corresponding to constants, namely, the fixed points of the action of the group of rotation, is problematic.More precisely, at a fixed point, there is not a well-defined tangent space.

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