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Let $\mathbb{H}$ be the Poincare upper half-plane, seen as a Riemannian manifold with the metric $$\frac{dx^2+dy^2}{y^2}.$$ Moreover, we consider the action of $\text{SL}_2(\mathbb{R})$ on $\mathbb{H}$ by fractional linear transformations.

The notes I had on the subjet prove that for $z,z'\in \mathbb{H}$ with $d(z,z')=r$, then the hyperbolic sphere is given by $$S(z,r)=\text{stab}_{\text{SL}_2(\mathbb{R})}(z).z'.$$

However, the proof contains a unfixable mistake and I have not been able to find another one (nor to find one myself). Is there any book where I could find that, or would you have any good idea to do that ?

Note that the inclusion $S(z,r)\supset\text{stab}_{\text{SL}_2(\mathbb{R})}(z).z'$ is obvious since $\text{SL}_2(\mathbb{R})$ acts by isometries.

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1 Answer 1

I like to switch between three models of $\mathbb H$ depending on the situation:

  • half-space model when there is a distinguished boundary point
  • disk model when there is a distinguished interior point
  • infinite strip model when there is a distinguished geodesic

So in this case I would switch to the disk model and map $z$ to the center of the disk. Then the orbit of $z'$ under rotations about the origin is clearly $S(z,r)$.

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Thank you! But then, I have to determine the shape of "spheres" in the disk model. Any good reference for that ? Anyway, if $g: \mathbb{H}\to D$ maps $z$ to $0$, then $gS(z,r)=S(gz,r)$ and $$g(\text{stab}(z).z')=\text{stab}(gz).(gz')=\text{stab}(0).(gz'),$$ so the group acting is $\left(\begin{smallmatrix}\pm 1&0\\c&\pm 1\end{smallmatrix}\right)$ and I don't see rotations... –  Klaus Jan 25 '13 at 9:16

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