By P. M. H. Wilson

This self-contained textbook provides an exposition of the well known classical two-dimensional geometries, reminiscent of Euclidean, round, hyperbolic, and the in the neighborhood Euclidean torus, and introduces the elemental innovations of Euler numbers for topological triangulations, and Riemannian metrics. The cautious dialogue of those classical examples presents scholars with an advent to the extra basic thought of curved areas built later within the booklet, as represented through embedded surfaces in Euclidean 3-space, and their generalization to summary surfaces built with Riemannian metrics. topics operating all through comprise these of geodesic curves, polygonal approximations to triangulations, Gaussian curvature, and the hyperlink to topology supplied via the Gauss-Bonnet theorem. various diagrams support carry the main issues to lifestyles and necessary examples and workouts are incorporated to assist knowing. through the emphasis is put on specific proofs, making this article excellent for any scholar with a simple heritage in research and algebra.

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**Additional resources for Curved Spaces: From Classical Geometries to Elementary Differential Geometry**

**Example text**

Proof x+iy 1−z and π (P) = π(P) π (P) = x2 + y2 = 1. 1 − z2 If P = (x, y, z), then π(P) = x+iy 1+z , and hence Thus the map π ◦ π −1 : C∞ → C∞ is just inversion in the unit ¯ circle, ζ → 1/ζ . 18 We shall however consistently adopt the convention that we project from the north pole. For future use, we observe the simple relationship between the images under π of antipodal points. If P = (x, y, z) ∈ S 2 , then π(P) = ζ = −P = (−x, −y, −z) has π(−P) = − x+iy 1+z x+iy 1−z . The antipodal point and so π(P) π(−P) = − x2 + y2 = −1.

For this, we note that any such isometry f may be extended to a map g : R 3 → R 3 ﬁxing the origin, which for non-zero x is deﬁned via the recipe g(x) := x f (x/ x ). Letting ( , ) denote the standard inner-product on R 3 , we have, for any x, y ∈ R 3 , that (g(x), g(y)) = (x, y). For x, y non-zero, this follows since (g(x), g(y)) = x y f (x/ x ), f (y/ y ) = x y x/ x , y/ y ) = (x, y), using the property that f preserves the angles between unit vectors and the bilinearity of the inner-product.

The only solutions here are: • • ( p, q, r) = (2, 2, n) with n ≥ 2. The area of ( p, q, r) = (2, 3, 3). The area of is π/6. is π/n. 34 SPHERIC AL GEOMETRY • • ( p, q, r) = (2, 3, 4). The area of ( p, q, r) = (2, 3, 5). The area of is π/12. is π/30. The fact that S 2 (of area 4π ) is tessellated by the images of under G then implies that G has order 4n, 24, 48 and 120 in these cases. It is then straightforward to check that in the ﬁrst case G is C2 × D2n , and in the remaining cases it is the full symmetry group of the tetrahedron, cube and dodecahedron, respectively.