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Differential Geometry Notes Mathematics 433/533; Class Notes Richard Koch March 24, 2005 ii Contents Preface vii 1 Curves 1 1.1 Introduction .................................... 1 1.2 Arc Length .................................... 2 1.3 Reparameterization ................................ 3 1.4 Curvature ..................................... 5 1.5 The Moving Frame ................................ 7 1.6 The Frenet-Serret Formulas ........................... 8 1.7 The Goal of Curve Theory ............................ 12 1.8 The Fundamental Theorem ........................... 12 1.9 Computers .................................... 14 1.10 The Possibility that κ = 0 ............................ 16 1.11 Formulas for γ(t) ................................. 20 1.12 Higher Dimensions ................................ 21 2 Surfaces 25 2.1 Parameterized Surfaces .............................. 25 2.2 Tangent Vectors ................................. 28 2.3 Directional Derivatives .............................. 31 2.4 Vector Fields ................................... 32 2.5 The Lie Bracket .................................. 32 2.6 The Metric Tensor ................................ 34 2.7 Geodesics ..................................... 38 2.8 Energy ....................................... 40 2.9 The Geodesic Equation ............................. 42 2.10 An Example .................................... 46 2.11 Geodesics on a Sphere .............................. 49 2.12 Geodesics on a Surface of Revolution ...................... 50 2.13 Geodesics on a Poincare Disk .......................... 58 iii iv CONTENTS 3 Extrinsic Theory 61 3.1 Differentiating the Normal Vector ........................ 61 3.2 Vector Fields in R3 ................................ 63 3.3 Differentiating Vector Fields ........................... 65 3.4 Basic Differentiation Facts ............................ 67 3.5 The Normal Field ................................ 69 3.6 Decomposing Vectors .............................. 70 3.7 The Fundamental Decomposition ........................ 71 3.8 The Crucial Results ............................... 73 3.9 The Principal Axis Theorem ........................... 73 3.10 Principal Curvatures ............................... 76 3.11 A Formula for b .................................. 76 3.12 A Formula for B ................................. 78 3.13 Justification of the Definition .......................... 79 3.14 Gaussian Curvature and Mean Curvature ................... 81 3.15 Examples ..................................... 82 3.16 Algebraic Postscript ............................... 86 4 The Covariant Derivative 87 4.1 Introduction .................................... 87 4.2 Review ....................................... 88 4.3 The Debauch of Indices ............................. 88 4.4 Fundamental Properties ............................. 89 4.5 A Formula for ∇X Y ............................... 91 4.6 Tangent Fields Along Curves .......................... 93 4.7 Acceleration .................................... 95 4.8 Surface Decomposition of Acceleration ..................... 96 4.9 Geodesic Curvature ................................ 98 4.10 Example ...................................... 99 5 The Theorema Egregium 103 5.1 Introduction .................................... 103 5.2 The Fundamental Theorem ........................... 104 5.3 Gaussian Curvature ............................... 106 5.4 The Curvature Tensor .............................. 108 5.5 The Curvature of the Poincare Disk ...................... 111 5.6 The Codazzi-Mainardi Equations ........................ 113 5.7 Where To Go From Here ............................. 116 6 The Gauss-Bonnet Theorem 117 6.1 Introduction .................................... 117 CONTENTS v 6.2 Polygons in the Plane .............................. 118 6.3 The Gauss-Bonnet Theorem ........................... 122 6.4 Initial Applications ................................ 123 6.5 Non-Euclidean Geometry ............................ 125 6.6 Book I of Euclid ................................. 134 6.7 Compact Surfaces ................................ 135 6.8 Constant Curvature ............................... 140 6.9 Surfaces of Constant Curvature ......................... 142 6.10 Frames ....................................... 148 6.11 Differential Forms ................................ 151 6.12 The Correct Interpretation ........................... 152 6.13 Forms Acting on Vectors ............................. 155 6.14 The Moving Frame ................................ 156 6.15 The Structural Equation ............................. 158 6.16 Proof of Gauss-Bonnet .............................. 160 7 Riemann’s Counting Argument 161 7.1 Riemann’s Career ................................. 161 7.2 The Counting Argument ............................. 162 7.3 The Main Theorems ............................... 163 7.4 Surfaces of Revolution .............................. 169 vi CONTENTS Preface Differential geometry began in 1827 with a paper of Gauss titled General Investigations of Curved Surfaces. In the paper, Gauss recalled Euler’s definition of the curvature of such a surface at a point p. Then he completely transformed the subject by asking a profound question. Here is Euler’s definition of curvature. Suppose p is a point on a surface S. We can translate and rotate this surface without changing the curvature at p. Let us translate and rotate until p lies at the origin in the xy-plane and its tangent plane is this xy-plane. -2 2 0 2 1 0 0.5 -2 0 -0.55 0 -11 -2-2 -2 2 0 -1 -0.50 2 0.5 1 After this motion, we can imagine that the new surface is given by z = f(x, y). Expanding in a Taylor series about the origin, we have ∂f ∂f 1 ∂2f ∂2f ∂2f z = f(0, 0) + x + y + x2 + 2 xy + y2 + ... ∂x ∂y 2! ∂x2 ∂x∂y ∂y2 where all partial derivatives are computed at the origin. But since the tangent plane at the ∂f ∂f origin is the xy-plane, both partials ∂x and ∂y vanish and the Taylor expansion reduces to 1 ∂2f ∂2f ∂2f f(x, y) = x2 + 2 xy + y2 + ... 2! ∂x2 ∂x∂y ∂y2 vii viii PREFACE It is convenient to call these three second partials A, B, and C and thus write 1 f(x, y) = Ax2 + 2Bxy + Cy2 + ... 2! We are still free to rotate the surface about the z-axis, since such rotation leaves the tangent plane unchanged. A famous algebraic result asserts that we can eliminate the xy quadratic term by rotating the function appropriately. Many people are familiar with this result in a different context. The solution of the equation Ax2 + 2Bxy + Cy2 = D is an ellipse in general position. If we rotate this ellipse until it is parallel to the x and y axes, its equation x2 y2 changes to the form a2 + b2 = 1. Imagine that we have eliminated the xy term in our formula. The resulting numbers A and C are then called κ1 and κ2 in the literature, and the surface is given by the Taylor expansion κ κ f(x, y) = 1 x2 + 2 y2 + ... 2 2 All of this was done by Euler. The numbers κ1 and κ2 describe the curvature of the surface at p. Euler could compute these numbers directly from the formula z = F (x, y) of the original surface without physically rotating and translating. Notice that the numbers κ1 and κ2 depend on p and thus become functions on the surface. They are called the principal curvatures of the surface. The pictures below show the surfaces κ κ z = 1 x2 + 2 y2 2 2 for both κ’s positive, for one positive and one negative, for one positive and one zero, and for both zero. 1 1 1 1 0.5 0.5 0.5 0.5 0 0 0 0 -0.5 -0.5 -0.5 -1 -1 -1 -0.5 1 1 1 -13 0.5 0.75 0.5 0 0.5 0 2 -0.5 0.25 -0.5 1 -1 0 -1 0 -1 -1 -1 -1 -0.5 -0.5 -0.5 -0.5 0 0 0 0 0.5 0.5 0.5 0.5 1 1 1 1 Consider the special case of a torus, shown on the next page. Along the outside rim, we will have κ1 > 0 and κ2 > 0 and the surface looks like a paraboloid, while along the inside rim we have κ1 > 0 and κ2 < 0 and the surface looks like a saddle. ix We now come to Gauss’ profound question: Can a two-dimensional person living on the surface determine κ1 and κ2, if that person is unable to see into the third dimension? Let us provide our two-dimensional person with tools: an infinitesimal ruler and an in- finitesimal protractor. Using the ruler, our worker can determine the lengths of curves on the surface. Rigorously, this means that our worker can integrate to determine the length of curves. Using the protractor, our worker can determine the angle between two curves. Rigorously this means that our worker can compute the angle between the tangent vectors to any two curves. Can κ and κ be computed with this information? 1 -2 2 0 2 1 0.5 0 -0.55 -11 -2 0 2 The surprising answer to Gauss’ question is no! To see this, consider the surfaces f(x, y) = 1 2 2 x and g(x, y) = 0, shown on the next page. For the first we have κ1 = 1 and κ2 = 0, while for the second we have κ1 = κ2 = 0. However, it is possible to bend the first surface until it is flat without changing the lengths of curves in the surface or the angles of curves at intersection points. So a two-dimensional worker could not tell the difference between these surfaces. x PREFACE But Gauss did not give up. Before Gauss, mathematicians often wrote in an expansive way. Numerical experiments were mixed with conjectures and half-proved theorems. Gauss introduced a more austere style, which has been adopted by mathematicians ever since. He calmly stated theorems and proofs, letting his results speak for themselves. But when Gauss came to the main result in his paper on surfaces, he allowed himself to write “a remarkable theorem.” And the theorem has been known as the theorema egregium ever since: Theorem Egregium A two-dimensional person can compute the product κ1 · κ2. The number κ = κ1 · κ2 is called the Gaussian curvature of the surface at p. Consider the four surfaces below again. The Gaussian curvature is positive in the first case, negative in the second case, and zero in the third and fourth cases. So a two-dimensional person can distinguish between the first two surfaces, but not between the last two. 1 1 1 1 0.5 0.5 0.5 0.5 0 0 0 0 -0.5 -0.5 -0.5 -1 -1 -1 -0.5 1 1 1 -13 0.5 0.75 0.5 0 0.5 0 2 -0.5 0.25 -0.5 1 -1 0 -1 0 -1 -1 -1 -1 -0.5 -0.5 -0.5 -0.5 0 0 0 0 0.5 0.5 0.5 0.5 Gauss1 gave several proofs of the theorema1 egregium.
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