In a previous post, we considered Poincaré’s half-plane model for hyperbolic geometry in two dimensions. The half-plane model comprises the upper half plane together with a metric

It is remarkable that the entire structure of the space follows from the metric.

In the earlier post, we derived the total curvature by evaluating the Riemann tensor. Here, we compute the curvature directly, using Gauss’s “Remarkable Theorem”.

** The Principal Curvatures of a Surface **

We define the principal curvatures and of a surface as the maximum and minimum values of the curvature of a curve formed by the intersection of with planes containing the normal to the surface. Clearly, and require measurements external to the surface itself; they are *extrinsic* quantities. In a remarkable theorem, the *Theorema Egregium*, Gauss showed that the total curvature can be evaluated *intrinsically*, using only quantities that can be measured within the surface itself.

**An Easy Way to Evaluate Curvature**

We assume that the surface is specified in terms of two parameters and as

or The First Fundamental Quadratic Form is

where the functions , and can be evaluated within the surface. Then the total curvature may be expressed in terms of these three functions and their first and second derivatives. In the case of orthogonal coordinates, , we get:

(see [1], pg. 183). For the Poincaré half-plane, the First Fundamental Quadratic Form is

so we have and . Then the expression for can be evaluated and yields the result:

This is simpler and more direct than evaluating the entire fourth-order Riemann tensor.

**Coming Soon**

In the next post [ Poincaré’s Half-plane Model (ter) ] we will look at some geodesic triangles in the hyperbolic half-plane and see how the angle deficit is related to the area. This will introduce the key topic of the Gauss-Bonnet Theorem.

** Sources **

[1] Lanczos, Cornelius, 1979: *Space through the ages: The evolution of geometric ideas from Pythagoras to Hilbert and Einstein*. Academic Press. ISBN 0124358500.

[2] Lynch, P, 2013: Curvature of the Poincaré Half-Plane. ( PDF )