# Inner product on the Hyperbolic half plane

I think this is the dual question to my previous question.

From Gudmundsoon notes, page 60:

We can model the hyperbolic space $$\mathbb H^m$$ as the super half plane space $$\mathbb R^+ \times \mathbb R^{m-1}$$ with the metric

$$g_p(X, Y) = \frac{1}{x_1^2} \langle X, Y \rangle_{\mathbb R^m},$$

where $$x = (x_1, \dots, x_m) \in H^m$$.

My question is: what is the relationship between the metric and the inner product? What is the "right" way of multiplying those vectors in a space that has this curvature? Clearly the "flat" one inherited from $$\mathbb R ^m$$ won't make much sense here.

Follow up question: more generally, what's the right way of constructing the inner product on $$(\mathbb R^ m, g)$$ for an arbitrary metric?

• I'm no expert in Riemannian geometry, but I would assume that the inner product here is not of points from the manifold, but of tangent vectors in the tangent space at point $x$. Since the idea of Riemannian geometry is that the surface is locally Euclidean, using the regular inner product on those makes a lot of sense to me. – MvG Feb 13 at 20:24

You seem to be missing some key concepts regarding tangent spaces and Riemannian metrics.

In general, a Riemannian metric on a manifold $$M$$ is a family of inner products, one for each point $$p \in M$$.

Given $$p,q \in M$$, the tangent spaces at $$p$$ and $$q$$ are disjoint vector spaces $$T_p M$$ and $$T_q M$$. The disjoint union of all these vector spaces is called the tangent bundle and is denoted $$TM = \coprod_{p \in M} T_p M$$

What a Riemannian metric does is to assign an inner product on each $$T_p M$$. There is not just one inner product.

In the example of $$\mathbb H^n$$, $$T \mathbb H^n = \mathbb H^n \times \mathbb R^n$$ and $$T_p \mathbb H^n = \{p\} \times \mathbb R^n$$. The vector space structure on $$T_p \mathbb H^n$$ is $$(p,v) + (p,w) = (p,v+w)$$, and $$r (p,v) = (p,rv)$$. Given $$p = (x_1,...,x_n) \in \mathbb H^n$$, and given $$\vec v,\vec w \in \mathbb R^n$$, what $$g_p$$ does is to assign the inner product $$\langle (p,\vec v) , (p,\vec w) \rangle = \frac{\langle \vec v, \vec w \rangle_{\mathbb R^n}}{x_1^2}$$

• So you are right I was skipping some details. I finally found what I was looking for: the Gyrovector space, a way of carrying over to the hyperbolic space some sort of euclidean space vector space structure. – meto Feb 14 at 5:29
• Then my follow up question would be: is there a standard way of carrying over this construct to an arbitrary Riemann manifold? – meto Feb 14 at 5:29
• The best thing to do with a followup question is to close out the current question and post a new question. No-one but me, or someone who reads these comments, will be aware of a followup question buried in the comments of my answer. – Lee Mosher Feb 14 at 13:35