1. 1. Springer Undergraduate Mathematics Series Advisory Board M.A.J. Chaplain University of Dundee K. Erdmann University of Oxford A. MacIntyre Queen Mary, University of London L.C.G. Rogers University of Cambridge E. Sli University of Oxford J.F. Toland University of Bath For other titles published in this series, go to www.springer.com/series/3423
2. 2. Andrew Pressley Elementary Differential Geometry Second Edition 123
3. 3. Andrew Pressley Department of Mathematics Kings College London Strand, London WC2R 2LS United Kingdom andrew.pressley@.kcl.ac.uk Springer Undergraduate Mathematics Series ISSN 1615-2085 ISBN 978-1-84882-890-2 e-ISBN 978-1-84882-891-9 DOI 10.1007/978-1-84882-891-9 Springer London Dordrecht Heidelberg New York British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Control Number: 2009942256 Mathematics Subject Classication (2000): 53-01, 53A04, 53A05, 53A35 c Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licenses issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers. The use of registered names, trademarks, etc., in this publication does not imply, even in the absence of a specic statement, that such names are exempt from the relevant laws and regulations and therefore free for general use. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. Cover design: Deblik Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) Springer-Verlag London Limited 2010, Corrected printing 2012
4. 4. Preface The Dierential Geometry in the title of this book is the study of the geometry of curves and surfaces in three-dimensional space using calculus techniques. This topic contains some of the most beautiful results in Mathematics, and yet most of them can be understood without extensive background knowledge. Thus, for virtually all of this book, the only pre-requisites are a good working knowledge of Calculus (including partial dierentiation), Vectors and Linear Algebra (including matrices and determinants). Many of the results about curves and surfaces that we shall discuss are pro- totypes of more general results that apply in higher-dimensional situations. For example, the GaussBonnet theorem, treated in Chapter 11, is the prototype of a large number of results that relate local and global properties of geometric objects. The study of such relationships formed one of the major themes of 20th century Mathematics. We want to emphasise, however, that the methods used in this book are not necessarily those which generalise to higher-dimensional situations. (For readers in the know, there is, for example, no mention of connections in the remainder of this book.) Rather, we have tried at all times to use the simplest approach that will yield the desired results. Not only does this keep the pre- requisites to an absolute minimum, it also enables us to avoid some of the conceptual diculties often encountered in the study of Dierential Geometry in higher dimensions. We hope that this approach will make this beautiful subject accessible to a wider audience. It is a cliche, but true nevertheless, that Mathematics can be learned only by doing it, and not just by reading about it. Accordingly, the book contains over 200 exercises. Readers should attempt as many of these as their stamina permits. Full solutions to all the exercises are given at the end of the book, but v
5. 5. vi Preface these should be consulted only after the reader has obtained his or her own solution, or in case of desperation. We have tried to minimise the number of instances of the latter by including hints to many of the less routine exercises. Preface to the Second Edition Few books get smaller when their second edition appears, and this is not one of those few. The largest addition is a new chapter devoted to hyperbolic (or non- Euclidean) geometry. Quite reasonably, most elementary treatments of this sub- ject mimic Euclids axiomatic treatment of ordinary plane geometry. A much quicker route to the main results is available, however, once the basics of the dierential geometry of surfaces have been established, and it seemed a pity not to take advantage of it. The other two most signicant changes were suggested by commentators on the rst edition. One was to treat the tangent plane more geometrically - this then allows one to dene things like the rst and second fundamental forms and the Weingarten map as geometric objects (rather than just as matrices). The second was to make use of parallel transport. I only partly agreed with this suggestion as I wanted to preserve the elementary nature of the book, but in this edition I have given a denition of parallel transport and related it to geodesics and Gaussian curvature. (However, for the experts reading this, I have stopped just short of introducing connections.) There are many other smaller changes that are too numerous to list, but perhaps I should mention new sections on map-colouring (as an appli- cation of Gauss-Bonnet), and a self-contained treatment of spherical geome- try. Apart from its intrinsic interest, spherical geometry provides the simplest non-Euclidean geometry and it is in many respects analogous to its hyperbolic cousin. I have also corrected a number of errors in the rst edition that were spotted either by me or by correspondents (mostly the latter). For teachers thinking about using this book, I would suggest that there are now three routes through it that can be travelled in a single semester, terminating with one of chapters 11, 12 or 13, and taking in along the way the necessary basic material from chapters 110. For example, the new section on spherical geometry might be covered only if the nal destination is hyperbolic geometry. As in the rst edition, solutions to all the exercises are provided at the end of the book. This feature was almost universally approved of by student commentators, and almost as universally disapproved of by teachers! Being one myself, I do understand the teachers point of view, and to address it
6. 6. Preface vii I have devised a large number of new exercises that will be accessible online to all users of the book, together with a solutions manual for teachers, at www.springer.com. I would like to thank all those who sent comments on the rst edition, from beginning students through to experts - you know who you are! Even if I did not act on all your suggestions, I took them all seriously, and I hope that readers of this second edition will agree with me that the changes that resulted make the book more useful and more enjoyable (and not just longer).
7. 7. Contents Preface Contents 1. Curves in the plane and in space 1.1 What is a curve? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Arc-length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.3 Reparametrization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.4 Closed curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 1.5 Level curves versus parametrized curves . . . . . . . . . . . . . . . . . . . . . 23 2. How much does a curve curve? 2.1 Curvature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.2 Plane curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 2.3 Space curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3. Global properties of curves 3.1 Simple closed curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 3.2 The isoperimetric inequality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 3.3 The four vertex theorem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 4. Surfaces in three dimensions 4.1 What is a surface? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.2 Smooth surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 4.3 Smooth maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 4.4 Tangents and derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 4.5 Normals and orientability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 ix
8. 8. x Contents 5. Examples of surfaces 5.1 Level surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 5.2 Quadric surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 5.3 Ruled surfaces and surfaces of revolution . . . . . . . . . . . . . . . . . . . . 104 5.4 Compact surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 5.5 Triply orthogonal systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 5.6 Applications of the inverse function theorem . . . . . . . . . . . . . . . . . 116 6. The rst fundamental form 6.1 Lengths of curves on surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 6.2 Isometries of surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 6.3 Conformal mappings of surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 6.4 Equiareal maps and a theorem of Archimedes . . . . . . . . . . . . . . . . 139 6.5 Spherical geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 7. Curvature of surfaces 7.1 The second fundamental form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 7.2 The Gauss and Weingarten maps . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 7.3 Normal and geodesic curvatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 7.4 Parallel transport and covariant derivative . . . . . . . . . . . . . . . . . . . 170 8. Gaussian, mean and principal curvatures 8.1 Gaussian and mean curvatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 8.2 Principal curvatures of a surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 8.3 Surfaces of constant Gaussian curvature . . . . . . . . . . . . . . . . . . . . . 196 8.4 Flat surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 8.5 Surfaces of constant mean curvature . . . . . . . . . . . . . . . . . . . . . . . . 206 8.6 Gaussian curvature of compact surfaces . . . . . . . . . . . . . . . . . . . . . 212 9. Geodesics 9.1 Denition and basic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 9.2 Geodesic equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 9.3 Geodesics on surfaces of revolution . . . . . . . . . . . . . . . . . . . . . . . . . 227 9.4 Geodesics as shortest paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 9.5 Geodesic coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 10. Gauss Theorema Egregium 10.1 The Gauss and CodazziMainardi equations . . . . . . . . . . . . . . . . . 247 10.2 Gauss remarkable theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 10.3 Surfaces of constant Gaussian curvature . . . . . . . . . . . . . . . . . . . . . 257 10.4 Geodesic mappings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
9. 9. Contents xi 11. Hyperbolic geometry 11.1 Upper half-plane model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 11.2 Isometries of H . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 11.3 Poincare disc model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 11.4 Hyperbolic parallels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 11.5 BeltramiKlein model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 12. Minimal surfaces 12.1 Plateaus problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 12.2 Examples of minimal surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 12.3 Gauss map of a minimal surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320 12.4 Conformal parametrization of minimal surfaces . . . . . . . . . . . . . . . 322 12.5 Minimal surfaces and holomorphic functions . . . . . . . . . . . . . . . . . 325 13. The GaussBonnet theorem 13.1 GaussBonnet for simple closed curves . . . . . . . . . . . . . . . . . . . . . . 335 13.2 GaussBonnet for curvilinear polygons . . . . . . . . . . . . . . . . . . . . . . 342 13.3 Integration on compact surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346 13.4 GaussBonnet for compact surfaces . . . . . . . . . . . . . . . . . . . . . . . . . 349 13.5 Map colouring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 13.6 Holonomy and Gaussian curvature . . . . . . . . . . . . . . . . . . . . . . . . . . 362 13.7 Singularities of vector elds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365 13.8 Critical points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372 A0. Inner product spaces and self-adjoint linear maps A1. Isometries of Euclidean spaces A2. Mobius transformations Hints to selected exercises Solutions Index
10. 10. 1 Curves in the plane and in space In this chapter, we discuss two mathematical formulations of the intuitive notion of a curve. The precise relation between them turns out to be quite subtle, so we begin by giving some examples of curves of each type and prac- tical ways of passing between them. 1.1 What is a curve? If asked to give an example of a curve, you might give a straight line, say y 2x = 1 (even though this is not curved!), or a circle, say x2 + y2 = 1, or perhaps a parabola, say y x2 = 0. y2x= 1 yx2= 0 x2+y2= 1 Andrew Pressley, Elementary Dierential Geometry: Second Edition, 1 Springer Undergraduate Mathematics Series, DOI 10.1007/978-1-84882-891-9 1, c Springer-Verlag London Limited 2010
11. 11. 2 1. Curves in the plane and in space All of these curves are described by means of their Cartesian equation f(x, y) = c, where f is a function of x and y and c is a constant. From this point of view, a curve is a set of points, namely C = {(x, y) R2 | f(x, y) = c}. (1.1) These examples are all curves in the plane R2 , but we can also consider curves in R3 for example, the x-axis in R3 is the straight line given by y = 0, z = 0, and more generally a curve in R3 might be dened by a pair of equations f1(x, y, z) = c1, f2(x, y, z) = c2. Curves of this kind are called level curves, the idea being that the curve in Eq. 1.1, for example, is the set of points (x, y) in the plane at which the quantity f(x, y) reaches the level c. But there is another way to think about curves which turns out to be more useful in many situations. For this, a curve is viewed as the path traced out by a moving point. Thus, if (t) is the position of the point at time t, the curve is described by a function of a scalar parameter t with vector values (in R2 for a plane curve, in R3 for a curve in space). We use this idea to give our rst formal denition of a curve in Rn (we shall be interested only in the cases n = 2 or 3, but it is convenient to treat both cases simultaneously). Denition 1.1.1 A parametrized curve in Rn is a map : (, ) Rn , for some , with < . The symbol (, ) denotes the open interval (, ) = {t R | < t < }. A parametrized curve, whose image is contained in a level curve C, is called a parametrization of (part of) C. The following examples illustrate how to pass from level curves to parametrized curves and back again in practice. Example 1.1.2 Let us nd a parametrization (t) of the parabola y = x2 . If (t) = (1(t), 2(t)), the components 1 and 2 of must satisfy 2(t) = 1(t)2 (1.2)
12. 12. 1.1 What is a curve? 3 for all values of t in the interval (, ) where is dened (yet to be decided), and ideally every point on the parabola should be equal to (1(t), 2(t)) for some value of t (, ). Of course, there is an obvious solution to Eq. 1.2: take 1(t) = t, 2(t) = t2 . To get every point on the parabola we must allow t to take every real number value (since the x-coordinate of (t) is just t, and the x-coordinate of a point on the parabola can be any real number), so we must take (, ) to be (, ). Thus, the desired parametrization is : (, ) R2 , (t) = (t, t2 ). But this is not the only parametrization of the parabola. Another choice is (t) = (t3 , t6 ) (with (, ) = (, )). Yet another is (2t, 4t2 ), and of course there are (innitely many) others. So the parametrization of a given level curve is not unique. Example 1.1.3 Now we try the circle x2 +y2 = 1. It is tempting to take x = t as in the previous example, so that y = 1 t2 (we could have taken y = 1 t2). So we get the parametrization (t) = (t, 1 t2). But this is only a parametrization of the upper half of the circle because 1 t2 is always 0. Similarly, if we had taken y = 1 t2, we would only have covered the lower half of the circle. If we want a parametrization of the whole circle, we must try again. We need functions 1(t) and 2(t) such that 1(t)2 + 2(t)2 = 1 (1.3) for all t (, ), and such that every point on the circle is equal to (1(t), 2(t)) for some t (, ). There is an obvious solution to Eq. 1.3: 1(t) = cos t and 2(t) = sin t (since cos2 t + sin2 t = 1 for all values of t). We can take (, ) = (, ), although this is overkill: any open interval (, ) whose length is greater than 2 will suce. The next example shows how to pass from parametrized curves to level curves.
13. 13. 4 1. Curves in the plane and in space Example 1.1.4 Take the parametrized curve (called an astroid) (t) = (cos3 t, sin3 t), t R. Since cos2 t + sin2 t = 1 for all t, the coordinates x = cos3 t, y = sin3 t of the point (t) satisfy x2/3 + y2/3 = 1. This level curve coincides with the image of the map . See Exercise 1.1.5 for a picture of the astroid. In this book, we shall be studying parametrized curves (and later, surfaces) using methods of calculus. Such curves and surfaces will be described almost exclusively in terms of smooth functions: a function f : (, ) R is said to be smooth if the derivative dn f dtn exists for all n 1 and all t (, ). If f(t) and g(t) are smooth functions, it follows from standard results of calculus that the sum f(t)+g(t), product f(t)g(t), quotient f(t)/g(t), and composite f(g(t)) are smooth functions, where they are dened. To dierentiate a vector-valued function such as (t) (as in Denition 1.1.1), we dierentiate componentwise: if (t) = (1(t), 2(t), . . . , n(t)), then d dt = d1 dt , d2 dt , . . . , dn dt , d2 dt2 = d2 1 dt2 , d2 2 dt2 , . . . , d2 n dt2 , etc. To save space, we often denote d/dt by (t), d2 /dt2 by (t), etc. We say that is smooth if the derivatives dn /dtn exist for all n 1 and all t (, ); this is equivalent to requiring that each of the components 1, 2, . . . , n of is smooth. From now on, all parametrized curves studied in this book will be assumed to be smooth. Denition 1.1.5 If is a parametrized curve, its rst derivative (t) is called the tangent vector of at the point (t).
14. 14. 1.1 What is a curve? 5 To see the reason for this terminology, note that the vector (t + t) (t) t is parallel to the chord joining the points (t) and (t+t) of the image C of : (t) (t + t) As t tends to zero the length of the chord also tends to zero, but we expect that the direction of the chord becomes parallel to that of the tangent to C at (t). But the direction of the chord is the same as that of the vector (t + t) (t) t , which tends to d/dt as t tends to zero. Of course, this only determines a well- dened direction tangent to the curve if d/dt is non-zero. If that condition holds, we dene the tangent line to C at a point p of C to be the straight line passing through p and parallel to the vector d/dt. The following result is intuitively clear: Proposition 1.1.6 If the tangent vector of a parametrized curve is constant, the image of the curve is (part of) a straight line. Proof If (t) = a for all t, where a is a constant vector, we have, integrating compo- nentwise, (t) = d dt dt = a dt = t a + b,
15. 15. 6 1. Curves in the plane and in space where b is another constant vector. If a = 0, this is the parametric equation of the straight line parallel to a and passing through the point b: (t)ta b a 0 If a = 0, the image of is a single point (namely, b). Before proceeding further with our study of curves, we should point out a potential source of confusion in the discussion of parametrized curves. This is regarding the question what is a point of such a curve? The diculty can be seen in the following example. Example 1.1.7 The limacon is the parametrized curve (t) = ((1 + 2 cost) cos t, (1 + 2 cos t) sin t), t R (see the diagram below). Note that has a self-intersection at the origin in the sense that (t) = 0 for t = 2/3 and for t = 4/3. The tangent vector is (t) = ( sin t 2 sin 2t, cost + 2 cos2t). In particular, (2/3) = ( 3/2, 3/2), (4/3) = ( 3/2, 3/2). So what is the tangent vector of this curve at the origin? Although (t) is well- dened for all values of t, it takes dierent values at t = 2/3 and t = 4/3, both of which correspond to the point 0 on the curve.
16. 16. 1.1 What is a curve? 7 This example shows that we must be careful while talking about a point of a parametrized curve : strictly speaking, this should be the same thing as a value of the curve parameter t, and not the corresponding geometric point (t) Rn . Thus, Denition 1.1.5 should more properly read If is a parametrized curve, its rst derivative (t) is called the tangent vector of at the parameter value t. However, it seems to us that to insist on this distinction takes away from the geometric viewpoint, and we shall sometimes repeat the error committed in the statement of Denition 1.1.5. This should not lead to confusion if the preceding remarks are kept in mind. EXERCISES 1.1.1 Is (t) = (t2 , t4 ) a parametrization of the parabola y = x2 ? 1.1.2 Find parametrizations of the following level curves: (i) y2 x2 = 1; (ii) x2 4 + y2 9 = 1. 1.1.3 Find the Cartesian equations of the following parametrized curves: (i) (t) = (cos2 t, sin2 t); (ii) (t) = (et , t2 ). 1.1.4 Calculate the tangent vectors of the curves in Exercise 1.1.3. 1.1.5 Sketch the astroid in Example 1.1.4. Calculate its tangent vector at each point. At which points is the tangent vector zero? 1.1.6 Consider the ellipse x2 p2 + y2 q2 = 1,
17. 17. 8 1. Curves in the plane and in space where p > q > 0 (see below). The eccentricity of the ellipse is = 1 q2 p2 and the points (p, 0) on the x-axis are called the foci of the ellipse, which we denote by f1 and f2. Verify that (t) = (p cos t, q sin t) is a parametrization of the ellipse. Prove that (i) The sum of the distances from f1 and f2 to any point p on the ellipse does not depend on p. (ii) The product of the distances from f1 and f2 to the tangent line at any point p of the ellipse does not depend on p. (iii) If p is any point on the ellipse, the line joining f1 and p and that joining f2 and p make equal angles with the tangent line to the ellipse at p. ff2 pp ff1 1.1.7 A cycloid is the plane curve traced out by a point on the circum- ference of a circle as it rolls without slipping along a straight line. Show that, if the straight line is the x-axis and the circle has radius a > 0, the cycloid can be parametrized as (t) = a(t sin t, 1 cos t). 1.1.8 Show that (t) = (cos2 t 1 2 , sin t cos t, sin t) is a parametrization of the curve of intersection of the circular cylinder of radius 1 2 and axis the z-axis with the sphere of radius 1 and centre (1 2 , 0, 0). This is called Vivianis Curve see above.
18. 18. 1.2 Arc-length 9 1.1.9 The normal line to a curve at a point p is the straight line passing through p perpendicular to the tangent line at p. Find the tangent and normal lines to the curve (t) = (2 cos t cos 2t, 2 sin t sin 2t) at the point corresponding to t = /4. 1.2 Arc-length We recall that, if v = (v1, . . . , vn) is a vector in Rn , its length is v = v2 1 + + v2 n. If u is another vector in Rn , u v is the length of the straight line segment joining the points u and v in Rn . To nd a formula for the length of a parametrized curve , note that, if t is very small, the part of the image C of between (t) and (t + t) is nearly a straight line, so its length is approximately (t + t) (t) . Again, since t is small, ((t + t) (t))/t is nearly equal to (t), so the length is approximately (t) t. (1.4) If we want to calculate the length of a (not necessarily small) part of C, we can divide it into segments, each of which corresponds to a small increment t in t, calculate the length of each segment using (1.4), and add up the results. Letting t tend to zero should then give the exact length.
19. 19. 10 1. Curves in the plane and in space This motivates the following denition: Denition 1.2.1 The arc-length of a curve starting at the point (t0) is the function s(t) given by s(t) = t t0 (u) du. Thus, s(t0) = 0 and s(t) is positive or negative according to whether t is larger or smaller than t0. If we choose a dierent starting point (t0), the resulting arc-length s diers from s by the constant t0 t0 (u) du because t t0 (u) du = t t0 (u) du + t0 t0 (u) du. Example 1.2.2 For a logarithmic spiral (t) = (ekt cos t, ekt sin t), where k is a non-zero constant, we have = (ekt (k cos t sin t), ekt (k sin t + cos t)), 2 = e2kt (k cos t sin t)2 + e2kt (k sin t + cos t)2 = (k2 + 1)e2kt . Hence, the arc-length of starting at (0) = (1, 0) (for example) is s = t 0 k2 + 1eku du = k2 + 1 k (ekt 1).
20. 20. 1.2 Arc-length 11 The arc-length is a dierentiable function. Indeed, if s is the arc-length of a curve starting at (t0), we have ds dt = d dt t t0 (u) du = (t) . (1.5) Thinking of (t) as the position of a moving point at time t, ds/dt is the speed of the point (rate of change of distance along the curve). This suggests the following denition. Denition 1.2.3 If : (, ) Rn is a parametrized curve, its speed at the point (t) is (t) , and is said to be a unit-speed curve if (t) is a unit vector for all t (, ). We shall see many examples of formulas and results relating to curves that take on a much simpler form when the curve is unit-speed. The reason for this simplication is given in the next proposition. Although this admittedly looks uninteresting at rst sight, it will be extremely useful for what follows. We recall that the dot product (or scalar product) of vectors a = (a1, . . . , an) and b = (b1, . . . , bn) in Rn is a b = n i=1 aibi. If a and b are smooth functions of a parameter t, we shall make use of the product formula d dt (a b) = da dt b + a db dt . This follows easily from the denition of the dot product and the usual product formula for scalar functions, d dt (aibi) = dai dt bi + ai dbi dt . Proposition 1.2.4 Let n(t) be a unit vector that is a smooth function of a parameter t. Then, the dot product n(t) n(t) = 0 for all t, i.e., n(t) is zero or perpendicular to n(t) for all t. In particular, if is a unit-speed curve, then is zero or perpendicular to .
21. 21. 12 1. Curves in the plane and in space Proof Using the product formula to dierentiate both sides of the equation n n = 1 with respect to t gives n n + n n = 0, so 2 n n = 0. The last part follows by taking n = . EXERCISES 1.2.1 Calculate the arc-length of the catenary (t) = (t, cosh t) starting at the point (0, 1). This curve has the shape of a heavy chain suspended at its ends see Exercise 2.2.4. 1.2.2 Show that the following curves are unit-speed: (i) (t) = 1 3 (1 + t)3/ 2 , 1 3 (1 t)3/ 2 , t 2 . (ii) (t) = 4 5 cos t, 1 sin t, 3 5 cos t . 1.2.3 A plane curve is given by () = (r cos , r sin ), where r is a smooth function of (so that (r, ) are the polar coor- dinates of ()). Under what conditions is regular? Find all func- tions r() for which is unit-speed. Show that, if is unit-speed, the image of is a circle; what is its radius? 1.2.4 This exercise shows that a straight line is the shortest curve joining two given points. Let p and q be the two points, and let be a curve passing through both, say (a) = p, (b) = q, where a < b. Show that, if u is any unit vector, u and deduce that (q p) u b a dt. By taking u = (q p)/ q p , show that the length of the part of between p and q is at least the straight line distance q p .
22. 22. 1.3 Reparametrization 13 1.3 Reparametrization We saw in Examples 1.1.2 and 1.1.3 that a given level curve can have many parametrizations, and it is important to understand the relation between them. Denition 1.3.1 A parametrized curve : (, ) Rn is a reparametrization of a parametrized curve : (, ) Rn if there is a smooth bijective map : (, ) (, ) (the reparametrization map) such that the inverse map 1 : (, ) (, ) is also smooth and (t) = ((t)) for all t (, ). (1.6) Note that, since has a smooth inverse, is a reparametrization of : (1 (t)) = ((1 (t))) = (t) for all t (, ). Two curves that are reparametrizations of each other have the same image, so they should have the same geometric properties. Example 1.3.2 In Example 1.1.3, we found that the circle x2 + y2 = 1 has a parametrization (t) = (cos t, sin t). Another parametrization is (t) = (sin t, cos t) (since sin2 t + cos2 t = 1). To see that is a reparametrization of , we have to nd a reparametrization map such that (cos (t), sin (t)) = (sin t, cost). One solution is (t) = /2 t. As we remarked in Section 1.2, the analysis of a curve is simplied when it is known to be unit-speed. It is therefore important to know exactly which curves have unit-speed reparametrizations. Denition 1.3.3 A point (t) of a parametrized curve is called a regular point if (t) = 0; otherwise (t) is a singular point of . A curve is regular if all of its points are regular.
23. 23. 14 1. Curves in the plane and in space Before we show the relation between regularity and unit-speed reparametri- zation, we note two simple properties of regular curves. Although these results are not particularly appealing, they are very important for what is to follow. Proposition 1.3.4 Any reparametrization of a regular curve is regular. Proof Suppose that and are related as in Denition 1.3.1, let t = (t) and = 1 so that t = (t). Dierentiating both sides of the equation ((t)) = t with respect to t and using the chain rule gives d dt d dt = 1. This shows that d/dt is never zero. Since (t) = ((t)), another application of the chain rule gives d dt = d dt d dt , which shows that d/dt is never zero, if d/dt is never zero. Proposition 1.3.5 If (t) is a regular curve, its arc-length s (see Denition 1.2.1), starting at any point of , is a smooth function of t. Proof We have already seen that (whether or not is regular) s is a dierentiable function of t and ds dt = (t) . To simplify the notation, assume from now onwards that is a plane curve, say (t) = (u(t), v(t)), where u and v are smooth functions of t, so that ds dt = u2 + v2.
24. 24. 1.3 Reparametrization 15 The crucial point is that the function f(x) = x is a smooth function on the open interval (0, ). Indeed, it is easy to prove by induction on n 1 that dn f dxn = (1)n1 1.3.5. . . . .(2n 1) 2n x(2n+1)/2 . Since u and v are smooth functions of t, so are u and v and hence is u2 + v2 . Since is regular, u2 + v2 > 0 for all values of t, so the composite function ds dt = f( u2 + v2 ) is a smooth function of t, and hence s itself is smooth. The main result we want is the following. Proposition 1.3.6 A parametrized curve has a unit-speed reparametrization if and only if it is regular. Proof Suppose rst that a parametrized curve : (, ) Rn has a unit-speed reparametrization , with reparametrization map . Letting t = (t), we have (t) = (t) and so d dt = d dt dt dt , d dt = d dt dt dt . Since is unit-speed, d/dt = 1, so d/dt cannot be zero. Conversely, suppose that the tangent vector d/dt is never zero. By Eq. 1.5, ds/dt > 0 for all t, where s is the arc-length of starting at any point of the curve, and by Proposition 1.3.5 s is a smooth function of t. It follows from the inverse function theorem that s : (, ) R is injective, that its image is an open interval (, ), and that the inverse map s1 : (, ) (, ) is smooth. (Readers unfamiliar with the inverse function theorem should accept these statements for now; the theorem will be discussed informally in Section 1.5 and formally in Section 5.6.) We take = s1 and let be the corresponding reparametrization of , so that (s) = (t) (see Eq. 1.6). Then, d ds ds dt = d dt ,
25. 25. 16 1. Curves in the plane and in space d ds ds dt = d dt = ds dt (by Eq. 1.5), d ds = 1. The proof of Proposition 1.3.6 shows that the arc-length is essentially the only unit-speed parameter on a regular curve: Corollary 1.3.7 Let be a regular curve and let be a unit-speed reparametrization of : (u(t)) = (t) for all t, where u is a smooth function of t. Then, if s is the arc-length of (starting at any point), we have u = s + c, (1.7) where c is a constant. Conversely, if u is given by Eq. 1.7 for some value of c and with either sign, then is a unit-speed reparametrization of . Proof The calculation in the rst part of the proof of Proposition 1.3.6 shows that u gives a unit-speed reparametrization of if and only if du dt = d dt = ds dt (by Eq. 1.5), which is equivalent to u = s + c for some constant c. Although every regular curve has a unit-speed reparametrization, this may be very complicated, or even impossible, to write down explicitly, as the fol- lowing examples show. Example 1.3.8 For the logarithmic spiral (t) = (ekt cos t, ekt sin t), we found in Example 1.2.2 that 2 = (k2 + 1)e2kt . This is never zero, so is regular. The arc- length of starting at (1, 0) was found to be s = k2 + 1(ekt 1)/k. Hence, t = 1 k ln ks k2+1 + 1 , so a unit-speed reparametrization of is given by the rather unwieldy formula
26. 26. 1.3 Reparametrization 17 (s) = ks k2 + 1 + 1 cos 1 k ln ks k2 + 1 + 1 , ks k2 + 1 + 1 sin 1 k ln ks k2 + 1 + 1 . Example 1.3.9 The twisted cubic is the space curve given by (t) = (t, t2 , t3 ), t R. We have (t) = (1, 2t, 3t2 ) and so (t) = 1 + 4t2 + 9t4. This is never zero, so is regular. The arc-length starting at (0) = 0 is s = t 0 1 + 4u2 + 9u4 du. This integral cannot be evaluated in terms of familiar functions like logarithms and exponentials, and trigonometric functions. (It is an example of an elliptic integral.) Our nal example shows that a given level curve can have both regular and non-regular parametrizations. Example 1.3.10 For the parametrization (t) = (t, t2 ) of the parabola y = x2 , (t) = (1, 2t) is obviously never zero, so is regular. But (t) = (t3 , t6 ) is also a parametrization of the same parabola. This time, = (3t2 , 6t5 ), and this is zero when t = 0, so is not regular.
27. 27. 18 1. Curves in the plane and in space EXERCISES 1.3.1 Which of the following curves are regular? (i) (t) = (cos2 t, sin2 t) for t R. (ii) The same curve as in (i), but with 0 < t < /2. (iii) (t) = (t, cosh t) for t R. Find unit-speed reparametrizations of the regular curve(s). 1.3.2 The cissoid of Diocles (see below) is the curve whose equation in terms of polar coordinates (r, ) is r = sin tan , /2 < < /2. Write down a parametrization of the cissoid using as a parameter and show that (t) = t2 , t3 1 t2 , 1 < t < 1 is a reparametrization of it. 1.3.3 The simplest type of singular point of a curve is an ordinary cusp: a point p of , corresponding to a parameter value t0, say, is an ordinary cusp if (t0) = 0 and the vectors (t0) and ... (t0) are linearly independent (in particular, these vectors must both be non- zero). Show that: (i) The curve (t) = (tm , tn ), where m and n are positive integers, has an ordinary cusp at the origin if and only if (m, n) = (2, 3) or (3, 2).
28. 28. 1.4 Closed curves 19 (ii) The cissoid in Exercise1.3.2 has an ordinary cusp at the origin. (iii) If has an ordinary cusp at a point p, so does any reparametrization of . 1.3.4 Show that: (i) If is a reparametrization of a curve , then is a reparametrization of . (ii) If is a reparametrization of , and is a reparametrization of , then is a reparametrization of . 1.4 Closed curves It is obvious that some curves close up, like a circle or an ellipse, while some do not, like a straight line or a parabola. If a point moves, say at constant speed, around a curve that closes up, it will return to its starting point after some time interval, and will then trace out the same curve all over again. On the other hand, if a point moves at constant speed along a straight line or a parabola, it will never return to its starting point. But there are some intermediate cases like (t) = (t2 1, t3 t); a point moving at constant speed along this curve may return to its starting point if the starting point is the origin, but will not do so otherwise. So a careful denition of what it means for a curve to close up is needed.
29. 29. 20 1. Curves in the plane and in space Denition 1.4.1 Let : R Rn be a smooth curve and let T R. We say that is T -periodic if (t + T ) = (t) for all t R. If is not constant and is T -periodic for some T = 0, then is said to be closed. Thus, if is T -periodic, a point moving around returns to its starting point after time T , whatever the starting point is. Of course, every curve is 0-periodic. Remark If is T -periodic, it is clear that is determined by its restriction to any interval of length |T |. Conversely, closed curves are often given to us as curves dened on a closed interval, say : [a, b] Rn . If and all its derivatives take the same value at a and b,1 there is a unique way to extend to a (b a)- periodic (smooth) curve : R Rn . Thus, the discussion below can be applied to curves dened on closed intervals. It is clear that if a curve is T -periodic then it is (T )-periodic because (t T ) = ((t T ) + T ) = (t). It follows that if is T -periodic for some T = 0, then it is T -periodic for some T > 0. Denition 1.4.2 The period of a closed curve is the smallest positive number T such that is T -periodic. It is actually not quite obvious that this number T exists (remember that not every set of positive real numbers has a smallest element). A proof that it does exist can be found in the exercises. Example 1.4.3 The ellipse (t) = (p cos t, q sin t) (Exercise 1.1.6) is a closed curve with period 2 because both of its components are (by well-known properties of trigono- metric functions). 1 The derivatives at the endpoints a and b must be dened in the one-sided sense.
30. 30. 1.4 Closed curves 21 If is a regular closed curve, a unit-speed reparametrization of is always closed. To see this, note that since every point in the image of a closed curve of period T is traced out as the parameter t of varies through any interval of length T , for example, 0 t T , it is reasonable to dene the length of to be () = T 0 (t) dt. By the proof of Proposition 1.3.6, using the arc-length s = t 0 (u) du of as the parameter gives a unit-speed reparametrization of (so that (s) = (t)). Note that s(t + T )= t+T 0 (u) du = T 0 (u) du+ t+T T (u) du = () + s(t), since, putting v = uT and using (uT ) = (u) (and hence by dierentiation (u T ) = (u)), we get t+T T (u) du = t 0 (v) dv = s(t). Hence, (s(t)) = (s(t )) (t) = (t ) t t = kT s(t ) s(t) = k(), where k is an integer. This shows that is a closed curve with period (). Note that, since is unit-speed, this is also the length of . In short, we can always assume that a closed curve is unit-speed and that its period is equal to its length. Returning to the curve illustrated at the beginning of this section, it is clearly not closed; nevertheless, if a point starts at the origin and moves at constant speed around the loop in the region x < 0 it will return to its starting point. This suggests the following denition. Denition 1.4.4 A curve is said to have a self-intersection at a point p of the curve if there exist parameter values a = b such that (i) (a) = (b) = p, and (ii) if is closed with period T , then a b is not an integer multiple of T .
31. 31. 22 1. Curves in the plane and in space Example 1.4.5 The limacon in Example 1.1.7 is a closed curve with period 2. It is clear from the picture that it has exactly one self-intersection, at the origin. (This can also be veried analytically cf. Exercise 1.4.1 and its solution.) EXERCISES 1.4.1 Show that the Cayley sextic (t) = (cos3 t cos 3t, cos3 t sin 3t), t R, is a closed curve which has exactly one self-intersection. What is its period? (The name of this curve derives from the fact that its Cartesian equation involves a polynomial of degree 6.) 1.4.2 Give an example to show that a reparametrization of a closed curve need not be closed. 1.4.3 Show that if a curve is T1-periodic and T2-periodic, then it is (k1T1 + k2T2)-periodic for any integers k1, k2. 1.4.4 Let : R Rn be a curve and suppose that T0 is the smallest pos- itive number such that is T0-periodic. Prove that is T -periodic if and only if T = kT0 for some integer k. 1.4.5 Suppose that a non-constant function : R R is T -periodic for some T = 0. This exercise shows that there is a smallest positive T0 such that is T0-periodic. The proof uses a little real analysis. Suppose for a contradiction that there is no such T0. (i) Show that there is a sequence T1, T2, T3, . . . such that T1 > T2 > T3 > > 0 and that is Tr-periodic for all r 1. (ii) Show that the sequence {Tr} in (i) can be chosen so that Tr 0 as r . (iii) Show that the existence of a sequence {Tr} as in (i) such that Tr 0 as r implies that is constant. 1.4.6 Let : R Rn be a non-constant curve that is T -periodic for some T > 0. Show that is closed.
32. 32. 1.5 Level curves versus parametrized curves 23 1.5 Level curves versus parametrized curves We shall now try to clarify the relation between the two types of curves we have considered in previous sections. Level curves in the generality we have dened them are not always the kind of objects we would want to call curves. For example, the level curve x2 + y2 = 0 is a single point. The correct conditions to impose on a function f(x, y) in order that f(x, y) = c, where c is a constant, will be an acceptable level curve in the plane are contained in the following theorem, which shows that such level curves can be parametrized. Note that we might as well assume that c = 0 (since we can replace f by f c). Theorem 1.5.1 Let f(x, y) be a smooth function of two variables (which means that all the par- tial derivatives of f, of all orders, exist and are continuous functions). Assume that, at every point of the level curve C = {(x, y) R2 | f(x, y) = 0}, f/x and f/y are not both zero. If p is a point of C, with coordinates (x0, y0), say, there is a regular parametrized curve (t), dened on an open interval containing 0, such that passes through p when t = 0 and (t) is contained in C for all t. The proof of this theorem makes use of the inverse function theorem (one version of which has already been used in the proof of Proposition 1.3.6). For the moment, we shall only try to convince the reader of the truth of this theorem. The proof will be given later (Exercise 5.6.2) after the inverse function theorem has been formally introduced and used in our discussion of surfaces. To understand the signicance of the conditions on f in Theorem 1.5.1, suppose that (x0 + x, y0 + y) is a point of C near p, so that f(x0 + x, y0 + y) = 0. By the two-variable form of Taylors theorem, f(x0 + x, y0 + y) = f(x0, y0) + x f x + y f y , neglecting products of the small quantities x and y (the partial derivatives are evaluated at (x0, y0)). Hence, x f x + y f y = 0. (1.8)
33. 33. 24 1. Curves in the plane and in space Since x and y are small, the vector (x, y) is nearly tangent to C at p, so Eq. 1.8 says that the vector n = f x , f y is perpendicular to C at p. (x, y) P n C x y The hypothesis in Theorem 1.5.1 tells us that the vector n is non-zero at every point of C. Suppose, for example, that f y = 0 at p. Then, n is not parallel to the x-axis at p, so the tangent to C at p is not parallel to the y-axis. x P x0 y0 y C This implies that vertical lines x = constant near x = x0 all intersect C in a unique point (x, y) near p. In other words, the equation f(x, y) = 0 (1.9)
34. 34. 1.5 Level curves versus parametrized curves 25 has a unique solution y near y0 for every x near x0. Note that this may fail to be the case if the tangent to C at p is parallel to the y-axis (i.e., if f/y = 0): y P xx0 C In this example, lines x = constant just to the left of x = x0 do not meet C near p, while those just to the right of x = x0 meet C in more than one point near p. The italicized statement about f in the last paragraph means that there is a function g(x), dened for x near x0, such that y = g(x) is the unique solution of Eq. 1.9 near y0. We can now dene a parametrization of the part of C near p by (t) = (t, g(t)). If we accept that g is smooth (which follows from the inverse function theorem), then is certainly regular since = (1, g) is obviously never zero. This proves Theorem 1.5.1. x2 + y2 = 1 x2 y2 = 1 It is actually possible to prove slightly more than we have stated in Theorem 1.5.1. Suppose that f(x, y) satises the conditions in the theorem, and assume in addition that the level curve C given by f(x, y) = 0 is connected.
35. 35. 26 1. Curves in the plane and in space For readers unfamiliar with point set topology, this means roughly that C is in one piece. For example, the circle x2 + y2 = 1 is connected, but the hy- perbola x2 y2 = 1 is not (see above). With these assumptions on f, there is a regular parametrized curve whose image is the whole of C. Moreover, if C is not closed can be taken to be injective; if C is closed, then maps some closed interval [, ] onto C, () = () and is injective on the open interval (, ). A similar argument can be used to pass from parametrized curves to level curves: Theorem 1.5.2 Let be a regular parametrized plane curve, and let (t0) = (x0, y0) be a point in the image of . Then, there is a smooth real-valued function f(x, y), dened for x and y in open intervals containing x0 and y0, respectively, and satisfying the conditions in Theorem 1.5.1, such that (t) is contained in the level curve f(x, y) = 0 for all values of t in some open interval containing t0. The proof of Theorem 1.5.2 is similar to that of Theorem 1.5.1. Let (t) = (u(t), v(t)), where u and v are smooth functions. Since is regular, at least one of u(t0) and v(t0) is non-zero, say u(t0). This means that the graph of u as a function of t is not parallel to the t-axis at t0: u u0 t0 t As in the proof of Theorem 1.5.1, this implies that any line parallel to the t-axis close to u = x0 intersects the graph of u at a unique point u(t) with t close to t0. This gives a function h(x), dened for x in an open interval containing
36. 36. 1.5 Level curves versus parametrized curves 27 x0, such that t = h(x) is the unique solution of u(t) = x if x is near x0 and t is near t0. The inverse function theorem tells us that h is smooth. The function f(x, y) = y v(h(x)) has the properties we want. It is not in general possible to nd a single function f(x, y) satisfying the conditions in Theorem 1.5.1 such that the image of is contained in the level curve f(x, y) = 0, for may have self-intersections like the limacon in Example 1.1.7. It follows from the inverse function theorem that no single func- tion f satisfying the conditions in Theorem 1.5.1 can be found that describes a curve near such a self-intersection. EXERCISES 1.5.1 Show that the curve C with Cartesian equation y2 = x(1 x2 ) is not connected. For what range of values of t is (t) = (t, t t3) a parametrization of C? What is the image of this parametrization? 1.5.2 State an analogue of Theorem 1.5.1 for level curves in R3 given by f(x, y, z) = g(x, y, z) = 0. 1.5.3 State and prove an analogue of Theorem 1.5.2 for curves in R3 (or even Rn ). (This is easy.) In the remainder of this book, we shall speak simply of curves, unless there is serious danger of confusion as to which type (level or parametrized) is intended.
37. 37. 2 How much does a curve curve? In this chapter, we associate two scalar functions, its curvature and torsion, to any curve in R3 . The curvature measures the extent to which a curve is not contained in a straight line (so that straight lines have zero curvature), and the torsion measures the extent to which a curve is not contained in a plane (so that plane curves have zero torsion). It turns out that the curvature and torsion together determine the shape of a curve. 2.1 Curvature We are going to try to nd a measure of how curved a curve is, and to simplify matters we shall work with plane curves initially. Since a straight line should certainly have zero curvature, a measure of the curvature of a plane curve at a point p of the curve should be its deviation from the tangent line at p. Suppose then that is a unit-speed curve in R2 . As the parameter t of changes to t + t, the curve moves away from its tangent line at (t) by a distance ((t + t) (t)) n, where n is a unit vector perpendicular to the tangent vector (t) of at the point (t). A n d r e w P r e s s l e y , Elementary Dierential Geometry: Second Edition, 29 S p r i n g e r U n d e r g r a d u a t e M a t h e m a t i c s S e r i e s , D O I 10.1007/978-1-84882-891-9 2, c Springer-Verlag London Limited 2010
38. 38. 30 2. How much does a curve curve? (t) (t +t) n By Taylors theorem, (t + t) = (t) + (t)t + 1 2 (t)(t)2 + remainder, (2.1) where (remainder)/(t)2 tends to zero as t tends to zero. Since n = 0, the deviation of from its tangent line at (t) is 1 2 (t) n(t)2 + remainder. Since is unit-speed, is perpendicular to and therefore parallel to n. Hence, neglecting the remainder terms, the magnitude of the deviation of from its tangent line is 1 2 (t) (t)2 . This suggests the following denition: Denition 2.1.1 If is a unit-speed curve with parameter t, its curvature (t) at the point (t) is dened to be (t) . Note that we make this denition for unit-speed curves in Rn for all n 2. Note also that this denition is consistent with Proposition 1.1.6, which tells us that if = 0 everywhere then is part of a straight line, and so should certainly have zero curvature.
39. 39. 2.1 Curvature 31 Let us see if Denition 2.1.1 is consistent with what we expect for the curvature of circles. Consider the circle in R2 centred at (x0, y0) and of radius R. This has a unit-speed parametrization (t) = x0 + R cos t R , y0 + R sin t R . We have (t) = sin t R , cos t R , and so (t) = sin t R 2 + cos t R 2 = 1, conrming that is unit-speed, and hence (t) = 1 R cos t R, 1 R sin t R , so the curvature (t) = 1 R cos t R 2 + 1 R sin t R 2 = 1 R is the reciprocal of the radius of the circle. This is in accordance with our expectation that small circles should have large curvature and large circles small curvature. So far we have only considered unit-speed curves. If is any regular curve, then by Proposition 1.3.6, has a unit-speed parametrization , say, and we can dene the curvature of to be that of . For this to make sense, we need to know that if is another unit-speed parametrization of , the curvatures of and are the same. To see this, note that will be a reparametrization of (Exercise 1.3.4), so by Corollary1.3.7, (t) = (u), where u = t + c and c is a constant. Then, by the chain rule, d dt = d du , so d2 dt 2 = d du d du = d2 du2 , which shows that and do indeed have the same curvature. Although every regular curve has a unit-speed reparametrization, it may be complicated or impossible to write it down explicitly (see Examples 1.3.8 and 1.3.9), and so it is desirable to have a formula for the curvature of in terms of itself rather than a reparametrization of it. Proposition 2.1.2 Let (t) be a regular curve in R3 . Then, its curvature is = 3 , (2.2) where the indicates the vector (or cross) product and the dot denotes d/dt.
40. 40. 32 2. How much does a curve curve? Of course, since a curve in R2 can be viewed as a curve in the xy-plane (say) in R3 , Eq. 2.2 can also be used to calculate the curvature of plane curves. Proof Let s be a unit-speed parameter for . Then, by the chain rule, = d dt = d ds ds dt , so = d2 ds2 = d ds d/dt ds/dt = d dt d/dt ds/dt ds/dt = ds dt d2 dt2 d2 s dt2 d dt (ds/dt)3 . (2.3) Now, ds dt 2 = 2 = , and dierentiating with respect to t gives ds dt d2s dt2 = . Using this and Eq. 2.3, we get = ds dt 2 d2 s dt2 ds dt (ds/dt)4 = ( ) ( ) 4 . Using the vector triple product identity a (b c) = (a c)b (a b)c (where a, b, c R3 ), we get ( ) = ( ) ( ) . Further, and are perpendicular vectors, so ( ) = . Hence, ( ) ( ) 4 = ( ) 4 = 4 = 3 .
41. 41. 2.1 Curvature 33 Note that formula (2.2) makes sense provided that = 0. Thus, the curvature is dened at all regular points of the curve. Example 2.1.3 A circular helix with axis the z-axis is a curve of the form () = (a cos , a sin , b), R, where a and b are constants. If (x, y, z) is a point on the helix, so that x = a cos , y = a sin , z = b, for some value of , then x2 +y2 = a2 , showing that the helix lies on the cylinder with axis the z-axis and radius |a|; the positive number |a| is called the radius of the helix. As increases by 2, the point (a cos , a sin , b) rotates once round the z-axis and moves parallel to the z-axis by 2b; the positive number 2|b| is called the pitch of the helix. Let us compute the curvature of the helix using the formula in Proposi- tion 2.1.2. Denoting d/d by a dot, we have () = (a sin , a cos , b) so () = a2 + b2. This shows that () is never zero, so is regular (unless a = b = 0, in which case the image of the helix is a single point). Hence, the formula in
42. 42. 34 2. How much does a curve curve? Proposition 2.1.2 applies, and we have = (a cos , a sin , 0) so = (ab sin , ab cos , a2 ) and hence = (ab sin , ab cos, a2 ) (a sin , a cos , b) 3 = (a2 b2 + a4 )1/2 (a2 + b2)3/2 = |a| a2 + b2 . (2.4) Thus, the curvature of the helix is constant. Let us examine some limiting cases to see if this result agrees with what we already know. First, suppose that b = 0 (but a = 0). Then, the helix is simply a circle in the xy-plane of radius |a|, so by the calculation following Denition 2.1.1 its curvature is 1/|a|. On the other hand, the formula (2.4) gives the curvature as |a| a2 + 02 = |a| a2 = |a| |a| 2 = 1 |a| . Next, suppose that a = 0 (but b = 0). Then, the image of the helix is just the z-axis, a straight line, so the curvature is zero. And formula (2.4) gives zero when a = 0 too. EXERCISES 2.1.1 Compute the curvature of the following curves: (i) (t) = 1 3 (1 + t)3/2 1 3 (1 t)3/2 , t 2 . (ii) (t) = 4 5 cos t, 1 sin t, 3 5 cos t . (iii) (t) = (t, cosh t). (iv) (t) = (cos3 t, sin3 t). For the astroid in (iv), show that the curvature tends to as we approach one of the points (1, 0), (0, 1). Compare with the sketch found in Exercise 1.1.5. 2.1.2 Show that, if the curvature (t) of a regular curve (t) is > 0 every- where, then (t) is a smooth function of t. Give an example to show that this may not be the case without the assumption that > 0. 2.2 Plane curves For plane curves, it is possible to rene the denition of curvature slightly and give it an appealing geometric interpretation.
43. 43. 2.2 Plane curves 35 Suppose that (s) is a unit-speed curve in R2 . Denoting d/ds by a dot, let t = be the tangent vector of ; note that t is a unit vector. There are two unit vectors perpendicular to t; we make a choice by dening ns, the signed unit normal of , to be the unit vector obtained by rotating t anticlockwise by /2. ns t By Proposition 1.2.4, t = is perpendicular to t, and hence parallel to ns. Thus, there is a scalar s such that = sns; s is called the signed curvature of (it can be positive, negative or zero). Note that, since ns = 1, we have = = sns = |s|, (2.5) so the curvature of is the absolute value of its signed curvature. The following diagrams show how the sign of the signed curvature is determined (in each case, the arrow on the curve indicates the direction of increasing s); s is negative for the two middle diagrams and positive for the other two. ks> 0 ns ns ns ns < 0 < 0 > 0 t t t t ks ks ks
44. 44. 36 2. How much does a curve curve? I f ( t) i s a r e g u l a r , b u t n o t n e c e s s a r i l y u n i t - s p e e d , c u r v e w e d e n e t h e u n i t t a n g e n t v e c t o r t, s i g n e d u n i t n o r m a l ns a n d s i g n e d c u r v a t u r e s o f t o b e t h o s e o f i t s u n i t - s p e e d p a r a m e t r i z a t i o n ( s) , w h e r e s i s t h e a r c - l e n g t h o f . T h u s , t = d/dt ds/dt = d/dt d/dt , ns i s o b t a i n e d b y r o t a t i n g t a n t i c l o c k w i s e b y /2 , a n d dt dt = dt ds ds dt = s ds dt ns = s d dt ns. T h e s i g n e d c u r v a t u r e h a s a s i m p l e g e o m e t r i c i n t e r p r e t a t i o n i n t e r m s o f t h e r a t e a t w h i c h t h e t a n g e n t v e c t o r r o t a t e s . I f i s a u n i t - s p e e d c u r v e , t h e d i r e c t i o n o f t h e t a n g e n t v e c t o r ( s) i s m e a s u r e d b y t h e a n g l e ( s) s u c h t h a t ( s) = ( c o s ( s) ,s i n ( s) ) . ( 2 . 6 ) T h e a n g l e ( s) i s n o t u n i q u e , h o w e v e r , a s w e c a n a d d t o a n y p a r t i c u l a r c h o i c e a n y i n t e g e r m u l t i p l e o f 2 . T h e f o l l o w i n g r e s u l t g u a r a n t e e s t h a t t h e r e i s a l w a y s a smooth c h o i c e : Proposition 2.2.1 L e t : ( , ) R2 b e a u n i t - s p e e d c u r v e , l e t s0 ( , ) a n d l e t 0 b e s u c h t h a t ( s0) = ( c o s 0,s i n 0) . T h e n t h e r e i s a u n i q u e s m o o t h f u n c t i o n : ( , ) R s u c h t h a t ( s0) = 0 a n d t h a t E q . 2 . 6 h o l d s f o r a l l s ( , ) . Proof L e t ( s) = ( f( s) , g( s) ) ; n o t e t h a t f( s) 2 + g( s) 2 = 1 f o r a l l s ( 2 . 7 ) s i n c e i s u n i t - s p e e d . D e n e ( s) = 0 + s s0 ( f g g f) dt. O b v i o u s l y ( s0) = 0. M o r e o v e r , s i n c e t h e f u n c t i o n s f a n d g a r e s m o o t h , s o i s = f g g f, a n d h e n c e s o i s .
45. 45. 2.2 Plane curves 37 Let F = f cos + g sin , G = f sin g cos . Then, F = ( f + g ) cos + (g f ) sin . But f + g = f(1 g2 ) + fg g = f(f f + g g) = 0, where the second equality used Eq. 2.7 and the last equality used its conse- quence f f + g g = 0. Similarly, g f = 0. Hence, F = 0 and F is constant. A similar argument shows that G is constant. But F(s0) = f(s0) cos 0 + g(s0) sin 0 = cos2 0 + sin2 0 = 1, and similarly G(s0) = 0. It follows that f cos + g sin = 1, f sin g cos = 0 for all s. These equations imply that f = cos , g = sin , and hence that the smooth function satises Eq. 2.6. As to the uniqueness, if is another smooth function such that (s0) = 0 and (s) = (cos (s), sin (s)) for s (, ), there is an integer n(s) such that (s) (s) = 2n(s) for all s (, ). Because and are smooth, n is a smooth, hence continuous, function of s. This implies that n is a constant: otherwise we would have n(s0) = n(s1) for some s1 (, ), and then by the intermediate value theorem the continuous function n(s) would have to take all values between n(s0) and n(s1) when s is between s0 and s1. But most real numbers between n(s0) and n(s1) are not integers! Thus, n is actually independent of s, and since (s0) = (s0) = 0, we must have n = 0 and hence (s) = (s) for all s (, ). Denition 2.2.2 The smooth function in Proposition 2.2.1 is called the turning angle of determined by the condition (s0) = 0. We are now in a position to give the geometric interpretation of the signed curvature that we promised earlier.
46. 46. 38 2. How much does a curve curve? Proposition 2.2.3 Let (s) be a unit-speed plane curve, and let (s) be a turning angle for . Then, s = d ds . Thus, the signed curvature is the rate at which the tangent vector of the curve rotates. As the diagrams following Eq. 2.5 show, the signed curvature is positive or negative accordingly as t rotates anticlockwise or clockwise as one moves along the curve in the direction of increasing s. Proof By Eq. 2.6 the tangent vector t = (cos , sin ), so t = ( sin , cos ). Since ns = ( sin , cos ), the equation t = sns gives the stated result. Example 2.2.4 Let us nd the signed curvature of the catenary (Exercise 1.2.1). Using the parametrization (t) = (t, cosh t) we get = (1, sinh t) and hence s = t 0 1 + sinh2 t dt = sinh t, so if is the angle between and the x-axis, tan = sinh t = s, sec2 d ds = 1, s = d ds = 1 sec2 = 1 1 + tan2 = 1 1 + s2 . Proposition 2.2.3 has an interesting consequence in terms of the total signed curvature of a unit-speed closed curve of length , namely 0 s(s) ds. (2.8) Corollary 2.2.5 The total signed curvature of a closed plane curve is an integer multiple of 2.
47. 47. 2.2 Plane curves 39 Proof Let be a unit-speed closed plane curve and let be its length. By Proposition 2.2.3, the total signed curvature of is 0 d ds ds = () (0), where is a turning angle for . Now, is l -periodic (see Section 1.4): (s + ) = (s). Dierentiating both sides gives (s + ) = (s), and in particular () = (0). Hence, by Eq. 2.6, (cos (), sin ()) = (cos (0), sin (0)), which implies that () (0) is an integer multiple of 2. The next result shows that a unit-speed plane curve is essentially determined once we know its signed curvature at each point of the curve. The meaning of essentially here is up to a direct isometry of R2 , i.e., a map M : R2 R2 of the form M = Ta , where is an anticlockwise rotation by an angle about the origin, (x, y) = (x cos y sin , x sin + y cos ), and Ta is the translation by the vector a, Ta(v) = v + a, for any vectors (x, y) and v in R2 (see Appendix 1). Theorem 2.2.6 Let k : (, ) R be any smooth function. Then, there is a unit-speed curve : (, ) R2 whose signed curvature is k. Further, if : (, ) R2 is any other unit-speed curve whose signed curvature is k, there is a direct isometry M of R2 such that (s) = M((s)) for all s (, ).
48. 48. 40 2. How much does a curve curve? Proof F or the rst part, x s0 (, ) and dene, for any s (, ), (s) = s s0 k(u)du, (cf. Proposition 2.2.3), (s) = s s0 cos (t)dt, s s0 sin (t)dt . Then, the tangent vector of is (s) = (cos (s), sin (s)), which is a unit vector making an angle (s) with the x-axis. Thus, is unit- speed and, by Proposition 2.2.3, its signed curvature is d ds = d ds s s0 k(u)du = k(s). For the second part, let (s) be a smooth turning angle for . Thus, (s) = (cos (s), sin (s)), (s) = s s0 cos (t)dt, s s0 sin (t)dt + (s0). (2.9) By Proposition 2.2.3, k(s) = d /ds so (s) = s s0 k(u)du + (s0). Inserting this into Eq. 2.9, and writing a for the constant vector (s0) and for the constant scalar (s0), we get (s) = Ta s s0 cos((t) + )dt, s s0 sin((t) + )dt = Ta cos s s0 cos (t)dt sin s s0 sin (t)dt, sin s s0 cos (t)dt + cos s s0 sin (t)dt = Ta s s0 cos (t)dt, s s0 sin (t)dt = Ta((s)).
49. 49. 2.2 Plane curves 41 Example 2.2.7 Any regular plane curve whose curvature is a positive constant is part of a circle. To see this, let be the curvature of , and let s be its signed curvature. Then, by Eq. 2.5, s = . A priori, we could have s = at some points of the curve and s = at others, but in fact this cannot happen since s is a continuous function of s (see Exercise 2.2.2), so the intermediate value theorem tells us that, if s takes both the value and the value , it must take all values between. Thus, either s = at all points of the curve, or s = at all points of the curve. In particular, s is constant. The idea now is to show that, whatever the value of s, we can nd a parametrized circle whose signed curvature is s. The theorem then tells us that every curve whose signed curvature is s can be obtained by applying a direct isometry to this circle. Since rotations and translations obviously take circles to circles, it follows that every curve whose signed curvature is constant is (part of) a circle. A unit-speed parametrization of the circle with centre the origin and radius R is (s) = R cos s R , R sin s R . Its tangent vector t = = sin s R , cos s R is the unit vector making an angle /2 + s/R with the positive x-axis: t s=R x s=R Hence, the signed curvature of is d ds 2 + s R = 1 R . Thus, if s > 0, the circle of radius 1/s has signed curvature s.
50. 50. 42 2. How much does a curve curve? If s < 0, it is easy to check that the curve (s) = R cos s R , R sin s R (which is just another parametrization of the circle with centre the origin and radius R) has signed curvature 1/R. Thus, if R = 1/s we again get a circle with signed curvature s. These calculations should be compared to the analogous ones for curvature (as opposed to signed curvature) following Denition 2.1.1. Example 2.2.8 Theorem 2.2.6 shows that we can nd a plane curve with any given smooth function as its signed curvature. But simple curvatures can lead to complicated curves. For example, let the signed curvature be s(s) = s. Following the proof of Theorem 2.2.6, and taking s0 = 0, we get (s) = s 0 udu = s2 2 so (s) = s 0 cos t2 2 dt, s 0 sin t2 2 dt . These integrals cannot be evaluated in terms of elementary functions. (They arise in the theory of diraction of light, where they are called Fresnels inte- grals, and the curve is called Cornus Spiral, although it was rst considered by Euler.) The picture of above is obtained by computing the integrals nu- merically.
51. 51. 2.2 Plane curves 43 y x y x It is natural to ask whether Theorem 2.2.6 remains true if we replace signed curvature by curvature. The rst part holds if (and only if) we assume that k 0, for then can be chosen to have signed curvature k and so will have curvature k as well. The second part of Theorem 2.2.6, however, no longer holds. For, we can take a (smooth) curve that coincides with the x-axis for 1 x 1 (say), and is otherwise above the x-axis. (The reader who wishes to write down such a curve explicitly will nd the solution of Exercise 9.4.3 helpful.) We now reect the part of the curve with x 0 in the x-axis. The new curve has the same curvature as , but obviously cannot be obtained by applying an isometry to . See Exercise 2.2.3 for a version of Theorem 2.2.6 that is valid for curvature instead of signed curvature. EXERCISES 2.2.1 Show that, if is a unit-speed plane curve, ns = st. 2.2.2 Show that the signed curvature of any regular plane curve (t) is a smooth function of t. (Compare with Exercise 2.1.2.) 2.2.3 Let and be two plane curves. Show that, if is obtained from by applying an isometry M of R2 , the signed curvatures s and s of and are equal if M is direct but that s = s if M is opposite (in particular, and have the same curvature). Show, conversely, that if and have the same nowhere-vanishing curvature, then can be obtained from by applying an isometry of R2 . 2.2.4 Let k be the signed curvature of a plane curve C expressed in terms of its arc-length. Show that, if Ca is the image of C under the dilation
52. 52. 44 2. How much does a curve curve? v av of the plane (where a is a non-zero constant), the signed curvature of Ca in terms of its arc-length s is 1 a k(s a ). A heavy chain suspended at its ends hanging loosely takes the form of a plane curve C. Show that, if s is the arc-length of C measured from its lowest point, the angle between the tangent of C and the horizontal, and T the tension in the chain, then T cos = , T sin = s, where , are non-zero constants (we assume that the chain has constant mass per unit length). Show that the signed curvature of C is s = 1 a 1 + s2 a2 1 , where a = /, and deduce that C can be obtained from the catenary in Example 2.2.4 by applying a dilation and an isometry of the plane. 2.2.5 Let (t) be a regular plane curve and let be a constant. The parallel curve of is dened by (t) = (t) + ns(t). Show that, if s(t) = 1 for all values of t, then is a regular curve and that its signed curvature is s/|1 s|. 2.2.6 Another approach to the curvature of a unit-speed plane curve at a point (s0) is to look for the best approximating circle at this point. We can then dene the curvature of to be the reciprocal of the radius of this circle. Carry out this programme by showing that the centre of the circle which passes through three nearby points (s0) and (s0 s) on approaches the point (s0) = (s0) + 1 s(s0) ns(s0) as s tends to zero. The circle C with centre (s0) passing through (s0) is called the osculating circle to at the point (s0), and (s0) is called the centre of curvature of at (s0). The radius of C is 1/|s(s0)| = 1/(s0), where is the curvature of this is called the radius of curvature of at (s0).
53. 53. 2.2 Plane curves 45 2.2.7 With the notation in the preceding exercise, we regard as the parametrization of a new curve, called the evolute of (if is any regular plane curve, its evolute is dened to be that of a unit-speed reparametrization of ). Assume that s(s) = 0 for all values of s (a dot denoting d/ds), say s > 0 for all s (this can be achieved by replacing s by s if necessary). Show that the arc-length of is 1 s(s) (up to adding a constant), and calculate the signed curvature of . Show also that all the normal lines to are tangent to (for this reason, the evolute of is sometimes described as the envelope of the normal lines to ). Show that the evolute of the cycloid (t) = a(t sin t, 1 cos t), 0 < t < 2, where a > 0 is a constant, is (t) = a(t + sin t, 1 + cos t) (see Exercise 1.1.7) and that, after a suitable reparametrization, can be obtained from by a translation of the plane. 2.2.8 A string of length is attached to the point (0) of a unit-speed plane curve (s). Show that when the string is wound onto the curve while being kept taught, its endpoint traces out the curve (s) = (s) + ( s) (s), where 0 < s < and a dot denotes d/ds. The curve is called the involute of (if is any regular plane curve, we dene its involute to be that of a unit-speed reparametrization of ). Suppose that the signed curvature s of is never zero, say s(s) > 0 for all s. Show that the signed curvature of is 1/( s). 2.2.9 Show that the involute of the catenary (t) = (t, cosh t) with l = 0 (see the preceding exercise) is the tractrix x = cosh1 1 y 1 y2. See Section 8.3 for a simple geometric characterization of this curve. 2.2.10 A unit-speed plane curve (s) rolls without slipping along a straight line parallel to a unit vector a, and initially touches at a point p = (0). Let q be a point xed relative to . Let (s) be the point to which q has moved when has rolled a distance s
54. 54. 46 2. How much does a curve curve? along (note that will not usually be unit-speed). Let (s) be the angle between a and the tangent vector . Show that (s) = p + sa + (s)(q (s)), where is the rotation about the origin through an angle . Show further that (s) (s)(q (s)) = 0. Geometrically, this means that a point on moves as if it is rotating about the instantaneous point of contact of the rolling curve with . See Exercise 1.1.7 for a special case. 2.3 Space curves Our main interest in this book is in curves (and surfaces) in R3 , i.e., space curves. While a plane curve is essentially determined by its curvature (see Theorem 2.2.6), this is no longer true for space curves. For example, a circle of radius 1 in the xy-plane and a circular helix with a = b = 1/2 (see Exam- ple 2.1.3) both have curvature 1 everywhere, but it is obviously impossible to change one curve into the other by any isometry of R3 . We shall dene an- other type of curvature for space curves, called the torsion, and we shall prove that the curvature and torsion of a curve together determine the curve up to a direct isometry of R3 . Let (s) be a unit-speed curve in R3 , and let t = be its unit tangent vector. If the curvature (s) is non-zero, we dene the principal normal of at the point (s) to be the vector n(s) = 1 (s) t(s). (2.10) Since t = , n is a unit vector. Further, by Proposition 1.2.4, t t = 0, so t and n are actually perpendicular unit vectors. It follows that b = t n (2.11) is a unit vector perpendicular to both t and n. The vector b(s) is called the binormal vector of at the point (s). Thus, {t, n, b} is an orthonormal basis of R3 , and is right-handed, i.e., b = t n, n = b t, t = n b.
55. 55. 2.3 Space curves 47 n b t Since b(s) is a unit vector for all s, b is perpendicular to b. Now we use the product rule for dierentiating the vector product of vector-valued functions u and v of a parameter s: d ds (u v) = du ds v + u dv ds . (This is easily proved by writing out both sides in component form and using the usual product rule for dierentiating scalar functions.) Applying this to b = t n gives b = t n + t n = t n, (2.12) since by the denition (2.10) of n, t n = n n = 0. Equation 2.12 shows that b is perpendicular to t. Being perpendicular to both t and b, b must be parallel to n, so b = n, (2.13) for some scalar , which is called the torsion of (inserting the minus sign here will reduce the total number of minus signs later). Note that the torsion is only dened if the curvature is non-zero. Of course, we dene the torsion of an arbitrary regular curve to be that of a unit-speed reparametrization of . As in the case of the curvature, to see that this makes sense, we have to investigate how the torsion is aected by a change in the unit-speed parameter of of the form u = s + c, where c is a constant. But this change of parameter clearly has the following eect on the vectors introduced above: t t, t t, n n, b b, b b; it follows from Eq. 2.13 that . Thus, the curvature and torsion are well- dened for any regular curve.
56. 56. 48 2. How much does a curve curve? Just as we did for the curvature in Proposition 2.1.2, it is possible to give a formula for the torsion of a regular space curve in terms of itself, rather than in terms of a unit-speed reparametrization: Proposition 2.3.1 Let (t) be a regular curve in R3 with nowhere-vanishing curvature. Then, denoting d/dt by a dot, its torsion is given by = ( ) ... 2 . (2.14) Note that this formula shows that (t) is dened at all points (t) of the curve at which its curvature (t) is non-zero, since by Proposition 2.1.2 this is the condition for the denominator on the right-hand side to be non-zero. Proof We could derive Eq. 2.14 by imitating the proof of Proposition 2.1.2. But it is easier and clearer to proceed as follows, even though this method has the disadvantage that one must know the formula (2.14) for in advance. We rst treat the case in which is unit-speed. Using Eqs. 2.11 and 2.13, = n b = n (t n)= n (t n + t n) = n (t n). Now, n = 1 t = 1 , so = 1 d dt 1 = 1 1 ... 2 = 1 2 ... ( ), since ( ) = 0 and ( ... ) = ... ( ). This agrees with Eq. 2.14, for, since is unit-speed, and are perpendicular, so = = = . In the general case, let s be arc-length along . Then, d dt = ds dt d ds , d2 dt2 = ds dt 2 d2 ds2 + d2 s dt2 d ds , d3 dt3 = ds dt 3 d3 ds3 + 3 ds dt d2 s dt2 d2 ds2 + d3 s dt3 d ds . Hence, = ds dt 3 d ds d2 ds2 , ... ( ) = ds dt 6 d3 ds3 d ds d2 ds2 .
57. 57. 2.3 Space curves 49 So the torsion of is = d3 ds3 d ds d2 ds2 d ds d2 ds2 2 = ... ( ) 2 . Example 2.3.2 We compute the torsion of the circular helix () = (a cos , a sin , b) studied in Example 2.1.3. We have () = (a sin , a cos , b), () = (a cos, a sin , 0), ... () = (a sin , a cos , 0), = (ab sin , ab cos, a2 ), 2 = a2 (a2 + b2 ), ( ) ... = a2 b, so the torsion = ( ) ... 2 = a2 b a2(a2 + b2) = b a2 + b2 . Note that the torsion of the circular helix in Example 2.3.2 becomes zero when b = 0, in which case the helix is just a circle in the xy-plane. This gives us a clue to the geometrical interpretation of torsion, contained in the next proposition. Proposition 2.3.3 Let be a regular curve in R3 with nowhere vanishing curvature (so that the torsion of is dened). Then, the image of is contained in a plane if and only if is zero at every point of the curve. Proof We can assume that is unit-speed (for this can be achieved by reparametrizing , and reparametrizing changes neither the torsion nor the fact that is, or is not, contained in a plane). We denote the parameter of by s and d/ds by a dot as usual. Suppose rst that the image of is contained in the plane v N = d, where N is a constant vector and d is a constant scalar and v R3 . We can assume that N is a unit vector. Dierentiating N = d with respect to s, we get t N = 0, (2.15) t N = 0 (since N = 0),
58. 58. 50 2. How much does a curve curve? n N = 0 (since t = n), n N = 0 (since = 0). (2.16) Equations 2.15 and 2.16 show that t and n are perpendicular to N. It follows that b = t n is parallel to N. Since N and b are both unit vectors, and b(s) is a smooth (hence continuous) function of s, we must have b(s) = N for all s or b(s) = N for all s. In both cases, b is a constant vector. But then b = 0, so = 0. Conversely, suppose that = 0 everywhere. By Eq. 2.13, b = 0, so b is a constant vector. The rst part of the proof suggests that should be contained in a plane v b = constant. We therefore consider d ds ( b) = b = t b = 0, so b is a constant (scalar), say d. This means that is indeed contained in the plane v b = d. There is a gap in our calculations which we would like to ll. Namely, we know that, for a unit-speed curve, we have t = n and b = n (these were our denitions of n and , respectively), but we have not com- puted n. This is not d Since {t, n, b} is a right-handed orthonormal basis of R3 , t n = b, n b = t, b t = n. Hence, n = b t + b t = n t + b n = t + b. Putting all these together, we get the following theorem. Theorem 2.3.4 Let be a unit-speed curve in R3 with nowhere vanishing curvature. Then, t = n n = t + b (2.17) b = n. Equations 2.17 are called the FrenetSerret equations. Notice that the matrix 0 0 0 0 0
59. 59. 2.3 Space curves 51 which expresses t, n and b in terms of t, n and b is skew-symmetric, i.e., it is equal to the negative of its transpose. This helps when trying to remember the equations. (The reason for this skew-symmetry can be seen in Exercise 2.3.6.) Here is a simple application of FrenetSerret: Proposition 2.3.5 Let be a unit-speed curve in R3 with constant curvature and zero torsion. Then, is a parametrization of (part of) a circle. Proof This result is actually an immediate consequence of Example 2.2.7 and Proposition 2.3.3, but the following proof is instructive and gives more in- formation, namely the centre and radius of the circle and the plane in which it lies. By the proof of Proposition 2.3.3, the binormal b is a constant vector and is contained in a plane , say, perpendicular to b. Now d ds + 1 n = t + 1 n = 0, using the fact that the curvature is constant and the FrenetSerret equation n = t + b = t (since = 0) (the reason for considering + 1 n can be found in Exercise 2.2.6). Hence, + 1 n is a constant vector, say a, and we have a = 1 n = 1 . This shows that lies on the sphere S, say, with centre a and radius 1/. The intersection of and S is a circle, say C, and we have shown that is a parametrization of part of C. If r is the radius of C, we have = 1/r so r = 1/ is also the radius of S. It follows that C is a great circle on S, i.e., that passes through the centre a of S. Thus, a is the centre of C and the equation of is v b = a b. We conclude this chapter with the analogue of Theorem 2.2.6 for space curves.
60. 60. 52 2. How much does a curve curve? Theorem 2.3.6 Let (s) and (s) be two unit-speed curves in R3 with the same curvature (s) > 0 and the same torsion (s) for all s. Then, there is a direct isometry M of R3 such that (s) = M((s)) for all s. Further, if k and t are smooth functions with k > 0 everywhere, there is a unit-speed curve in R3 whose curvature is k and whose torsion is t. Proof Let t, n and b be the tangent vector, principal normal and binormal of , and let t, n and b be those of . Let s0 be a xed value of the parameter s, let be the angle between t(s0) and t(s0) and let be the rotation through an angle around the axis passing through the origin and perpendicular to both of these vectors. Then, (t(s0)) = t(s0); let n = (n(s0)), b = (b(s0)). If is the angle between n and n(s0), let be the rotation through an angle around the axis passing through the origin parallel to t(s0). Then, xes t(s0) and takes n to n(s0). Since {t(s0), n(s0), b(s0)} and {t(s0), n(s0), b(s0)} are both right- handed orthonormal bases of R3 , takes the vectors t(s0), n(s0), b(s0) to the vectors t(s0), n(s0), b(s0), respectively. Now let M be the direct isometry M = T(s0)(s0) . By Exercise 2.3.5, the curve = M() is unit- speed, and if T, N and B denote its unit tangent vector, principal normal and binormal, we have (s0) = (s0), T(s0) = t(s0), N(s0) = n(s0), B(s0) = b(s0). (2.18) The trick now is to consider the expression A(s) = t T + n N + b B. In view of Eq. 2.18, we have A(s0) = 3. On the other hand, since t and T are unit vectors, t T 1, with equality holding if and only if t = T; and similarly for n N and b B. It follows that A(s) 3, with equality holding if and only if t = T, n = N and b = B. Thus, if we can prove that A is constant, it will follow in particular that t = T, i.e., that = , and hence that (s) (s) is a constant. But by Eq. 2.18 again, this constant vector must be zero, so = . For the rst part of the theorem, we are therefore reduced to proving that A is constant. But, using the FrenetSerret equations, A = t T + n N + b B + t T + n N + b B = n T + (t + b) N + ( n) B + t N +n (T + B) + b (N), and this vanishes since the terms cancel in pairs.
61. 61. 2.3 Space curves 53 For the second part of the theorem, we observe rst that it follows from the theory of ordinary dierential equations that the equations T = kN, (2.19) N = kT + tB, (2.20) B = tN (2.21) have a unique solution T(s), N(s), B(s) such that T(s0), N(s0), B(s0) are the standard orthonormal vectors i = (1, 0, 0), j = (0, 1, 0), k = (0, 0, 1), respec- tively. Since the matrix 0 k 0 k 0 t 0 t 0 expressing T, N and B in terms of T, N and B is skew-symmetric, it follows that the vectors T, N and B are orthonormal for all values of s (see Exercise 2.3.6). Now dene (s) = s s0 T(u)du. Then, = T, so since T is a unit vector, is unit-speed. Next, T = kN by Eq. 2.19, so since N is a unit vector, k is the curvature of and N is its principal normal. Next, since B is a unit vector perpendicular to T and N, B = T N where is a smooth function of s that is equal to 1 for all s. Since k = i j, we have (s0) = 1, so it follows that (s) = 1 for all s. Hence, B is the binormal of and by Eq. 2.21, t is its torsion. EXERCISES 2.3.1 Compute , , t, n and b for each of the following curves, and verify that the FrenetSerret equations are satised: (i) (t) = 1 3 (1 + t)3/2 1 3 (1 t)3/2 , t 2 . (ii) (t) = 4 5 cos t, 1 sin t, 3 5 cos t . Show that the curve in (ii) is a circle, and nd its centre, radius and the plane in which it lies. 2.3.2 Describe all curves in R3 which have constant curvature > 0 and constant torsion .
62. 62. 54 2. How much does a curve curve? 2.3.3 A regular curve in R3 with curvature > 0 is called a generalized helix if its tangent vector makes a xed angle with a xed unit vector a. Show that the torsion and curvature of are related by = cot . Show conversely that, if the torsion and curvature of a regular curve are related by = where is a constant, then the curve is a generalized helix. In view of this result, Examples 2.1.3 and 2.3.2 show that a circular helix is a generalized helix. Verify this directly. 2.3.4 Let (t) be a unit-speed curve with (t) > 0 and (t) = 0 for all t. Show that, if is spherical, i.e., if it lies on the surface of a sphere, then = d ds 2 . (2.22) Conversely, show that if Eq. 2.22 holds, then 2 + ( )2 = r2 for some (positive) constant r, where = 1/ and = 1/, and deduce that lies on a sphere of radius r. Verify that Eq. 2.22 holds for Vivianis curve (Exercise 1.1.8). 2.3.5 Let P be an nn orthogonal matrix and let a Rn , so that M(v) = Pv + a is an isometry of R3 (see Appendix 1). Show that, if is a unit-speed curve in Rn , the curve = M() is also unit-speed. Show also that, if t, n, b and T, N, B are the tangent vector, principal normal and binormal of and , respectively, then T = Pt, N = Pn and B = Pb. 2.3.6 Let (aij) be a skew-symmetric 3 3 matrix (i.e., aij = aji for all i, j). Let v1, v2 and v3 be smooth functions of a parameter s satisfying the dierential equations vi = 3 j=1 aijvj, for i = 1, 2 and 3, and suppose that for some parameter value s0 the vectors v1(s0), v2(s0) and v3(s0) are orthonormal. Show that the vectors v1(s), v2(s) and v3(s) are orthonormal for all values of s. For the remainder of this book, all parametrized curves will be assumed to be regular.
63. 63. 3 Globalproperties of curves All the properties of curves that we have discussed so far are local: they depend only on the behaviour of a curve near a given point and not on the global shape of the curve. Proving global results about curves often requires concepts from topology, in addition to the calculus techniques we have used in the rst two chapters of this book. Since we are not assuming that readers of this book have extensive familiarity with topological ideas, we will not be able to give complete proofs of some of the global results about curves that we discuss in this chapter. 3.1 Simple closed curves In this chapter, we shall consider plane curves of the following type. Denition 3.1.1 A simple closed curve in R2 is a closed curve in R2 that has no self-intersections. It is a standard, but highly non-trivial, result of the topology of R2 , called the Jordan Curve Theorem, that any simple closed curve in the plane has an interior and an exterior: more precisely, the complement of the image of Andrew Pressley, Elementary Dierential Geometry: Second Edition, 55 Springer Undergraduate Mathematics Series, DOI 10.1007/978-1-84882-891-9 3, c Springer-Verlag London Limited 2010
64. 64. 56 3. Global properties of curves (i.e., the set of points of R2 that are not in the image of ) is the disjoint union of two subsets of R2 , denoted by int() and ext(), with the following properties: (i) int() is bounded, i.e., it is contained inside a circle of suciently large radius. (ii) ext() is unbounded. (iii) Both of the regions int() and ext() are connected, i.e., they have the property that any two points in the same region can be joined by a curve contained entirely in the region (but any curve joining a point of int() to a point of ext() must cross the curve ). Example 3.1.2 The ellipse (t) = (p cos t, q sin t), where p and q are non-zero constants, is a simple closed curve with period 2. The interior and exterior of are, of course, given by (x, y) R2 | x2 p2 + y2 q2 < 1 and (x, y) R2 | x2 p2 + y2 q2 > 1 , respectively. Not all examples of simple closed curves have such an obvious interior and exterior, however. Is the point p in the interior or the exterior of the simple closed curve shown below? P Example 3.1.3 The limacon in Example 1.1.7 is closed but is not a simple closed curve as it has a self-intersection see Exercise 3.1.1. The fact that a simple closed curve has an interior and an exterior enables us to distinguish between the two possible orientations of . We shall say that is positively-oriented if the signed unit normal ns of (see Section 2.2) points
65. 65. 3.1 Simple closed curves 57 into int() at every point of . This can always be achieved by replacing the parameter t of by t, if necessary. In the diagrams below, the arrow indicates the direction of increasing parameter. Is the simple closed curve shown above positively-oriented? t ns ns t Positively-oriented Not positively-oriented We conclude this section by stating the following importa