DIFFERENTIAL GEOMETRY: A First Course in Curves and Surfaces

90 Chapter 3. Surfaces: Further TopicsWe conclude with an interesting application. As we saw in the previous section, the Gauss-Bonnet Theorem gives a deep relation between the total curvature of a surface and its topologicalstructure (Euler characteristic). We know that if a compact surface M is topologically equivalent toa sphere, then its total curvature must be that of a round sphere, namely 4π. IfM is topologicallyequivalent to a torus, then (as the reader checked in Exercise 3.1.4) its total curvature must be0. We know that there is no way of making the torus in R 3 in such a way that it has constantFigure 2.4Gaussian curvature K =0(why?), but we can construct a flat torus in R 4 by takingx(u, v) =(cos u, sin u, cos v, sin v), 0 ≤ u, v ≤ 2π.(We take a piece of paper and identify opposite edges, as indicated in Figure 2.4; this can be rolledinto a cylinder in R 3 but into a torus only in R 4 .) So what happens with a 2-holed torus? Inthat case, χ(M) =−2, so the total curvature should be −4π, and we can reasonably ask if there’sa 2-holed torus with constant negative curvature. Note that we can obtain a 2-holed torus byidentifying pairs of edges on an octagon, as shown in Figure 2.5.dabcadcbFigure 2.5This leads us to wonder whether we might have regular n-gons R in H. Bythe Gauss-Bonnetformula, we would have area(R) =(n−2)π − ∑ ι j ,soit’s obviously necessary that ∑ ι j < (n−2)π.This shouldn’t be difficult so long as n ≥ 3. First, let’s convince ourselves that, given any pointP ∈ H, 0