John Oprea Diﬀerential Geometry and Its Applications · 2020. 5. 19. · P1: RTJ dgmain maab004 May 25, 2007 17:5 Proofs Without Words, Roger B. Nelsen Proofs Without Words II,

AMS / MAA CLASSROOM RESOURCE MATERIALS VOL 59

Differential Geometry and Its Applications

John Oprea

| Second Edition |

Differential Geometryand its Applications

This book was previously published by Pearson Education, Inc.

Originally published byThe Mathematical Association of America, 2007.

ISBN: 978-1-4704-5050-2LCCN: 2007924394

Copyright © 2007, held by the Amercan Mathematical SocietyPrinted in the United States of America.

Reprinted by the American Mathematical Society, 2019The American Mathematical Society retains all rightsexcept those granted to the United States Government.

⃝∞ The paper used in this book is acid-free and falls within the guidelinesestablished to ensure permanence and durability.

Visit the AMS home page at https://www.ams.org/

10 9 8 7 6 5 4 3 2 24 23 22 21 20 19

AMS/MAA CLASSROOM RESOURCE MATERIALS

VOL 59

Differential Geometryand its ApplicationsSecond Edition

John Oprea

10.1090/clrm/059

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dgmain maab004 May 25, 2007 17:5

Council on PublicationsJames Daniel, Chair

Classroom Resource Materials Editorial Board

Zaven A. Karian, Editor

Gerald M. BryceDouglas B. Meade

Wayne RobertsKay B. Somers

Stanley E. SeltzerSusan G. Staples

George ExnerWilliam C. BauldryCharles R. HadlockShahriar Shahriari

Holly S. Zullo

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CLASSROOM RESOURCE MATERIALS

Classroom Resource Materials is intended to provide supplementary classroom material forstudents—laboratory exercises, projects, historical information, textbooks with unusual ap-proaches for presenting mathematical ideas, career information, etc.

101 Careers in Mathematics, 2nd edition edited by Andrew Sterrett

Archimedes: What Did He Do Besides Cry Eureka?, Sherman Stein

Calculus Mysteries and Thrillers, R. Grant Woods

Combinatorics: A Problem Oriented Approach, Daniel A. Marcus

Conjecture and Proof, Miklos Laczkovich

A Course in Mathematical Modeling, Douglas Mooney and Randall Swift

Cryptological Mathematics, Robert Edward Lewand

Differential Geometry and Its Applications, John Oprea

Elementary Mathematical Models, Dan Kalman

Environmental Mathematics in the Classroom, edited by B. A. Fusaro and P. C. Kenschaft

Essentials of Mathematics, Margie Hale

Exploratory Examples for Real Analysis, Joanne E. Snow and Kirk E. Weller

Fourier Series, Rajendra Bhatia

Geometry From Africa: Mathematical and Educational Explorations, Paulus Gerdes

Historical Modules for the Teaching and Learning of Mathematics (CD), edited by Victor Katzand Karen Dee Michalowicz

Identification Numbers and Check Digit Schemes, Joseph Kirtland

Interdisciplinary Lively Application Projects, edited by Chris Arney

Inverse Problems: Activities for Undergraduates, Charles W. Groetsch

Laboratory Experiences in Group Theory, Ellen Maycock Parker

Learn from the Masters, Frank Swetz, John Fauvel, Otto Bekken, Bengt Johansson, and VictorKatz

Mathematical Connections: A Companion for Teachers and Others, Al Cuoco

Mathematical Evolutions, edited by Abe Shenitzer and John Stillwell

Mathematical Modeling in the Environment, Charles Hadlock

Mathematics for Business Decisions Part 1: Probability and Simulation (electronic textbook),Richard B. Thompson and Christopher G. Lamoureux

Mathematics for Business Decisions Part 2: Calculus and Optimization (electronic textbook),Richard B. Thompson and Christopher G. Lamoureux

Math Made Visual: Creating Images for Understanding Mathematics, Claudi Alsina and RogerB. Nelsen

Ordinary Differential Equations: A Brief Eclectic Tour, David A. Sanchez

Oval Track and Other Permutation Puzzles, John O. Kiltinen

A Primer of Abstract Mathematics, Robert B. Ash

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Proofs Without Words, Roger B. Nelsen

Proofs Without Words II, Roger B. Nelsen

A Radical Approach to Real Analysis, 2nd edition, David M. Bressoud

Real Infinite Series, Daniel D. Bonar and Michael Khoury, Jr.

She Does Math!, edited by Marla Parker

Solve This: Math Activities for Students and Clubs, James S. Tanton

Student Manual for Mathematics for Business Decisions Part 1: Probability and Simulation,David Williamson, Marilou Mendel, Julie Tarr, and Deborah Yoklic

Student Manual for Mathematics for Business Decisions Part 2: Calculus and Optimization,David Williamson, Marilou Mendel, Julie Tarr, and Deborah Yoklic

Teaching Statistics Using Baseball, Jim Albert

Topology Now!, Robert Messer and Philip Straffin

Understanding our Quantitative World, Janet Andersen and Todd Swanson

Writing Projects for Mathematics Courses: Crushed Clowns, Cars, and Coffee to Go, AnnalisaCrannell, Gavin LaRose, Thomas Ratliff, Elyn Rykken

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To my mother and father,

Jeanne and John Oprea.

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Contents

Preface xiii

Note to Students xix

1 The Geometry of Curves 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Arclength Parametrization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.3 Frenet Formulas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171.4 Non-Unit Speed Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271.5 Some Implications of Curvature and Torsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341.6 Green’s Theorem and the Isoperimetric Inequality . . . . . . . . . . . . . . . . . . . . . . . . . . . 381.7 The Geometry of Curves and Maple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

2 Surfaces 672.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 672.2 The Geometry of Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 772.3 The Linear Algebra of Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 862.4 Normal Curvature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 912.5 Surfaces and Maple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

3 Curvatures 1073.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1073.2 Calculating Curvature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1113.3 Surfaces of Revolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1193.4 A Formula for Gauss Curvature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1233.5 Some Effects of Curvature(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1273.6 Surfaces of Delaunay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1333.7 Elliptic Functions, Maple and Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1373.8 Calculating Curvature with Maple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

4 Constant Mean Curvature Surfaces 1614.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1614.2 First Notions in Minimal Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

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4.3 Area Minimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1704.4 Constant Mean Curvature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1734.5 Harmonic Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1794.6 Complex Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1824.7 Isothermal Coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1844.8 The Weierstrass-Enneper Representations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1874.9 Maple and Minimal Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197

5 Geodesics, Metrics and Isometries 2095.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2095.2 The Geodesic Equations and the Clairaut Relation . . . . . . . . . . . . . . . . . . . . . . . . . . . 2155.3 A Brief Digression on Completeness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2255.4 Surfaces not in R

3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2265.5 Isometries and Conformal Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2355.6 Geodesics and Maple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2415.7 An Industrial Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262

6 Holonomy and the Gauss-Bonnet Theorem 2756.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2756.2 The Covariant Derivative Revisited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2776.3 Parallel Vector Fields and Holonomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2806.4 Foucault’s Pendulum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2846.5 The Angle Excess Theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2866.6 The Gauss-Bonnet Theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2896.7 Applications of Gauss-Bonnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2926.8 Geodesic Polar Coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2976.9 Maple and Holonomy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

7 The Calculus of Variations and Geometry 3117.1 The Euler-Lagrange Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3117.2 Transversality and Natural Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3187.3 The Basic Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3227.4 Higher-Order Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3277.5 The Weierstrass E-Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3347.6 Problems with Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3467.7 Further Applications to Geometry and Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . 3567.8 The Pontryagin Maximum Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3667.9 An Application to the Shape of a Balloon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3717.10 The Calculus of Variations and Maple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380

8 A Glimpse at Higher Dimensions 3978.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3978.2 Manifolds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3978.3 The Covariant Derivative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4018.4 Christoffel Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4098.5 Curvatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4168.6 The Charming Doubleness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430

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A List of Examples 437A.1 Examples in Chapter 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437A.2 Examples in Chapter 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437A.3 Examples in Chapter 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438A.4 Examples in Chapter 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438A.5 Examples in Chapter 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438A.6 Examples in Chapter 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438A.7 Examples in Chapter 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439A.8 Examples in Chapter 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439

B Hints and Solutions to Selected Problems 441

C Suggested Projects for Differential Geometry 453

Bibliography 455

Index 461

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Preface

The Point of this Book

How and what should we teach today’s undergraduates to prepare them for careers in mathemat-ically oriented areas? Furthermore, how can we ameliorate the quantum leap from introductorycalculus and linear algebra to more abstract methods in both pure and applied mathematics?There is a subject which can take students of mathematics to the next level of development andthis subject is, at once, intuitive, calculable, useful, interdisciplinary and, most importantly, inter-esting. Of course, I’m talking here about Differential Geometry, a subject with a long, wonderfulhistory and a subject which has found new relevance in areas ranging from machinery design tothe classification of four-manifolds to the creation of theories of Nature’s fundamental forces tothe study of DNA.

Differential Geometry provides the perfect transition course to higher mathematics and itsapplications. It is a subject which allows students to see mathematics for what it is — notthe compartmentalized courses of a standard university curriculum, but a unified whole mixingtogether geometry, calculus, linear algebra, differential equations, complex variables, the calculusof variations and various notions from the sciences. Moreover, Differential Geometry is not justfor mathematics majors, but encompasses techniques and ideas relevant to students in engineeringand the sciences. Furthermore, the subject itself is not quantized. By this, I mean that there isa continuous spectrum of results that proceeds from those which depend on calculation aloneto those whose proofs are quite abstract. In this way students gradually are transformed fromcalculators to thinkers.

Into the mix of these ideas now comes the opportunity to visualize concepts and constructionsthrough the use of computer algebra systems such as Maple and Mathematica. Indeed, it isoften the case that the consequent visualization goes hand-in-hand with the understanding ofthe mathematics behind the computer construction. For instance, in Chapter 5, I use Mapleto visualize geodesics on surfaces and this requires an understanding of the idea of solving asystem of differential equations numerically and displaying the solution. Further, in this case,visualization is not an empty exercise in computer technology, but actually clarifies variousphenomena such as the bound on geodesics due to the Clairaut relation. There are many otherexamples of the benefits of computer algebra systems to understanding concepts and solving

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problems. In particular, the procedure for plotting geodesics can be modified to show equationsof motion of particles constrained to surfaces. This is done in Chapter 7 along with describingprocedures relevant to the calculus of variations and optimal control. At the end of Chapters1, 2, 3, 5 and 7 there are sections devoted to explaining how Maple fits into the framework ofDifferential Geometry. I have tried to make these sections a rather informal tutorial as opposedto just laying out procedures. This is both good and bad for the reader. The good comes from thelittle tips about pitfalls and ways to avoid them; the bad comes from my personal predilectionsand the simple fact that I am not a Maple expert. What you will find in this text is the sort ofMaple that anyone can do. Also, I happen to think that Maple is easier for students to learn thanMathematica and so I use it here. If you prefer Mathematica, then you can, without too muchtrouble I think, translate my procedures from Maple into Mathematica.

In spite of the use of computer algebra systems here, this text is traditional in the sense ofapproaching the subject from the point of view of the 19th century. What is different aboutthis book is that a conscious effort has been made to include material that I feel science andmath majors should know. For example, although it is possible to find mechanistic descriptionsof phenomena such as Clairaut’s relation or Jacobi’s theorem and geometric descriptions ofmechanistic phenomena such as the precession of Foucault’s pendulum in advanced texts (see[Arn78] and [Mar92]), I believe they appear here for the first time in an undergraduate text. Also,even when dealing with mathematical matters alone, I have always tried to keep some application,whether mathematical or not, in mind. In fact, I think this helps to show the boundaries betweenphysics (e.g., soap films) and mathematics (e.g., minimal surfaces).

The book, as it now stands, is suitable for either a one-quarter or one-semester course inDifferential Geometry as well as a full-year course. In the case of the latter, all chapters may becompleted. In the case of the former, I would recommend choosing certain topics from Chap-ters 1–7 and then allowing students to do projects, say, involving other parts. For example, agood one-semester course can be obtained from Chapter 1, Chapter 2, Chapter 3 and the first“half” of Chapter 5. This carries students through the basic geometry of curves and surfaceswhile introducing various curvatures and applying virtually all of these ideas to study geodesics.My personal predilections would lead me to use Maple extensively to foster a certain geometricintuition. A second semester course could focus on the remainder of Chapter 5, Chapter 6 andChapter 7 while saving Chapter 4 on minimal surfaces or Chapter 8 on higher dimensionalgeometry for projects. Students then will have seen Gauss-Bonnet, holonomy and a kind ofrecapitulation of geometry (together with a touch of mechanics) in terms of the Calculus ofVariations. There are, of course, many alternative courses hidden within the book and I can onlywish “good hunting” to all who search for them.

Projects

I think that doing projects offers students a chance to experience what it means to do research inmathematics. The mixture of abstract mathematics and its computer realization affords studentsthe opportunity to conjecture and experiment much as they would do in the physical sciences.My students have done projects on subjects such as involutes and gear teeth design, re-creationof curves from curvature and torsion, Enneper’s surface and area minimization, geodesics onminimal surfaces and the Euler-Lagrange equations in relativity. In Appendix C, I give five

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suggestions for student projects in differential geometry. (Certainly, experienced instructors willbe able to point the way to other projects, but the ones suggested are mostly “self-contained”within the text.) Groups of three or four students working together on projects such as these cantruly go beyond their individual abilities to gain a profound understanding of some aspect ofgeometry in terms of proof, calculation and computer visualization.

Prerequisites

I have taught differential geometry to students (e.g., mathematics, physics, engineering, chemistry,biology and philosophy majors) with as little background as a complete calculus sequence anda standard differential equations course. But this surely is a minimum! While a touch of linearalgebra is used in the book, I think it can be covered concurrently as long as a student hasseen matrices. Having said this, I think a student can best appreciate the interconnections amongmathematical subjects inherent in differential geometry when the student has had the experienceof one or two upper-level courses.

Book Features

The reader should note two things about the layout of the book. First, the exercises are integratedinto the text. While this may make them somewhat harder to find, it also makes them an essentialpart of the text. The reader should at least read the exercises when going through a chapter —they are important. Also, I have used Exercise∗ to denote an exercise with a hint or solution inAppendix B. Secondly, I have chosen to number theorems, lemmas, examples, definitions andremarks in order as is usually done using LaTeX. To make it a bit easier to find specific examples,there is a list of examples (with titles and the pages they are on) in Appendix A.

Elliptic Functions and Maple Note

In recent years, I have become convinced of the utility of the elliptic functions in differentialgeometry and the calculus of variations, so I have included a simplified, straightforward introduc-tion to these here. The main applications of elliptic functions presented here are the derivation ofexplicit parametrizations for unduloids and for the Mylar balloon. Such explicit parametrizationsallow for the determination of differential geometric invariants such as Gauss curvature as wellas an analysis of geodesics. Of course, part of this analysis involves Maple. These applications ofelliptic functions are distillations of joint work with Ivailo Mladenov and I want to acknowledgethat here with thanks to him for his insights and diligence concerning this work.

Unfortunately, in going from Maple 9 to Maple 10, Maple developers introduced an error (amisprint!) into the procedure for Elliptic E. In order to correct this, give the command

> ‘evalf/Elliptic/Ell_E‘:=parse(StringTools:-Substitute(convert(eval(‘evalf/Elliptic/Ell_E‘),string),"F_0","E_0")):

Maple also seems to have changed its “simplify” command to its detriment. Therefore, theremay be slight differences between the Maple output as displayed in the text and what Maple 10

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gives, but these differences are usually minor. Nevertheless, to partially correct this discrepancyin simplification, define the command

> mysimplify:=a->‘simplify/do‘(subsindets(a,function,x->sqrt(x^2)),symbolic):

and then apply it to any expression you want to simplify further.

These corrections are due to Alec Mihailovs. The rest of the procedures in the book work fine in Maple 10.

Thanks

Since the publication of the First Edition, many people have sent me comments, suggestions and corrections. I have tried to take all of these into account in preparing the present MAA Edition, but sometimes this has proved to be impossible. One reason for this is that I want to keep the book at a level that is truly accessible to undergraduates. So, for me, some arguments simply can’t be made. On the other hand, I have learned a great deal from all of the comments sent to me and, in some sense, this is the real payment for writing the book. Therefore, I want to acknowledge a few people who went beyond the call of duty to give me often extensive commentary. These folks are (in alphabetical order!): David Arnold, David Bao, Neil Bomberger, Allen Broughton, Jack Chen, Rob Clark, Gary Crum, Dan Drucker, Lisbeth Fajstrup, Davon Ferrara, Karsten Grosse-Brauckmann, Sigmundur Gudmundsson, Sue Halamek, Laszlo Illyes, Greg Lupton, Takashi Kimura, Carrie Kyser, Jaak Peetre, John Reinmann, Ted Shifrin and Peter Stiller. Thanks to all of you.

Finally, the writing of this book would have been impossible without the help, advice and understanding of my wife Jan and daughter Kathy.

John OpreaCleveland, Ohio [email protected]

For Users of Previous EditionsThe Maple work found in the present MAA Edition once again focusses on actually doing interesting things with computers rather than simply drawing pictures. Nevertheless, there are many more pictures of interesting phenomena in this edition. The pictures have all been created by me with Maple. In fact, by examining the Maple sections at the ends of chapters, it is usually pretty clear how all pictures were created. The version of Maple used for the Second Edition was Maple 8. The version of Maple used for the MAA Edition is Maple 10. The Maple work in the First Edition needed extensive revision to work with Maple 8 because Maple developers changed the way certain commands work. However, going from Maple 8 to Maple 10 has been much easier and the reader familiar with the Second Edition will have no trouble with the MAA Edition. Everything works the same. Should newer versions of Maple cause problems for the

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Preface xvii

procedures in this book, look at my website for updates. One thing to pay attention to concerningthis issue of Maple command changes is the following. Maple no longer supports the “linalg”package. Rather, Maple has moved to a package called “LinearAlgebra” and I have changed allMaple work in the book to reflect this. This should be stable for some time to come, no matterwhat new versions of Maple arise. Of course, the one thing that doesn’t change is the book’sfocus on the solutions of differential equations as the heart of differential geometry. Because ofthis, Maple plays an even more important role through its “dsolve” command and its ability tosolve differential equations explicitly and numerically.

Maple 8 to 9

Maple 9 appeared in Summer of 2003 and all commands and procedures originally writtenfor Maple 8 work with one small exception. The following Maple 8 code defines a surface ofrevolution with functions g and h.

> h:=t->h(t);g:=t->g(t);

h := hg := g

> surfrev:=[h(u)*cos(v),h(u)*sin(v),g(u)];

surfrev := [h(u) cos(v), h(u) sin(v), g(u)]

This works fine in Maple 8, but Maple 9 complains about defining g and h this way saying thatthere are too many levels of recursion in the formula for “surfrev”. The fix for Maple 9 is simple.Just don’t define g and h at all! Go straight to the definition of “surfrev”. In the present MAAedition, the code has been modified to do just this, but if you are still using Maple 8, then use thecode above.

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Note to Students

Every student who takes a mathematics class wants to know what the real point of the courseis. Often, courses proceed by going through a list of topics with accompanying results andproofs and, while the rationale for the ordering and presentation of topics may be apparent to theinstructor, this is far from true for students. Books are really no different; authors get caught upin the “material” because they love their subject and want to show it off to students. So let’s takea moment now to say what the point of differential geometry is from the perspective of this text.

Differential geometry is concerned with understanding shapes and their properties in termsof calculus. We do this in two main ways. We start by defining shapes using “formulas” calledparametrizations and then we take derivatives and algebraically manipulate them to obtain newexpressions that we show represent actual geometric entities. So, if we have geometry encodedin the algebra of parametrizations, then we can derive quantities telling us something about thatgeometry from calculus. The prime examples are the various curvatures which will be encounteredin the book. Once we see how these special quantities arise from calculus, we can begin to turnthe problem around by restricting the quantities in certain ways and asking what shapes havequantities satisfying these restrictions. For instance, once we know what curvature means, wecan ask what plane curves have curvature functions that are constant functions. Since this is, in asense, the reverse of simply calculating geometric quantities by differentiation, we should expectthat “integration” arises here. More precisely, conditions we place on the geometric quantitiesgive birth to differential equations whose solution sets “are” the shapes we are looking for.

So differential geometry is intimately tied up with differential equations. But don’t get the ideathat all of those crazy methods in a typical differential equations text are necessary to do basicdifferential geometry. Being able to handle separable differential equations and knowing a fewtricks (which can be learned along the way) are usually sufficient. Even in cases where explicitsolutions to the relevant differential equations don’t exist, numerical solutions can often producea solution shape. The advent of computer algebra systems in the last decade makes this feasibleeven for non-experts in computer programming.

In Chapter 1, we will treat the basic building blocks of all geometry, curves, and we will doexactly as we have suggested above. We will use calculus to develop a system of differentialequations called the Frenet equations that determine a curve in three-dimensional space. We willuse the computer algebra system Maple to numerically solve these equations and plot curves in

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xx Note to Students

space. But with any computer program there are inputs, and these are the parameters involved inthe Frenet equations; the curvature and torsion of a curve. So, we are saying that the curvatureand torsion determine a curve in a well-defined sense. This is exactly the program outlined above.Of course, we will also see that, by putting restrictions on curvature and torsion, we can see whatcurves arise analytically as well.

In Chapter 2, we take what we have learned about curves and apply it to study the geometryof surfaces in 3-space. The key definition is that of the shape operator because from it flows allof the rest of the types of curvatures we use to understand geometry. The shape operator is reallyjust a way to take derivatives “in a tangential direction” and is related to the usual directionalderivative in 3-space. What is interesting here is that the shape operator can be thought of as amatrix, and this allows us to actually define curvatures in terms of the linear algebraic invariantsof the matrix. For instance, principal curvatures are simply the eigenvalues of the shape operator,mean curvature is the average of the eigenvalues (i.e., one half the trace of the matrix) and Gausscurvature is the product of the eigenvalues (i.e., the determinant of the matrix). Of course, thechallenge now is twofold: first, show that the shape operator and its curvature offspring reflectour intuitive grasp of the geometry of surfaces and, secondly, show that these curvatures areactually computable. This leads to the next chapter.

In Chapter 3, we show that curvatures are computable just in terms of derivatives of a para-metrization. This not only makes curvatures computable, but allows us to put certain restrictionson curvatures and produce analytic solutions. For instance, we can really see what surfaces arisewhen Gauss curvature is required to be constant on a compact surface or when mean curvatureis required to be zero on a surface of revolution. An important byproduct of this quest for com-putability is a famous result of Gauss that shows that Gauss curvature can be calculated directlyfrom the metric; that is, the functions which tell us how the surface distorts usual Euclideandistances. The reason this is important is that it gives us a definition of curvature that can betransported out of 3-space into a more abstract world of surfaces. This is the first step towards amore advanced differential geometry.

Chapter 4 deals with minimal surfaces. These are surfaces with mean curvature equal to zeroat each point. Our main theme shows up here when we show that minimal surfaces (locally)satisfy a (partial) differential equation known as the minimal surface equation. Moreover, byputting appropriate restrictions on the surface’s defining function, we will see that it is possible tosolve the minimal surface equation analytically. From a more geometric (as opposed to analytic)viewpoint, we focus here on basic computations and results, as well as the interpretations ofsoap films as minimal surfaces and soap bubbles as surfaces where the mean curvature is aconstant function. The most important result along these lines is Alexandrov’s theorem, where itis shown that such a compact surface embedded in 3-space must be a sphere. The chapter alsodiscusses harmonic functions and this leads to a more advanced approach to minimal surfaces,but from a more analytic point of view. In particular, we introduce complex variables as thenatural parameters for a minimal surface. We don’t expect the reader to have any experience withcomplex variables (beyond knowing what a complex number is, say), so we review the relevantaspects of the subject. This approach produces a wealth of information about minimal surfaces,including an example where a minimal surface does not minimize surface area.

In Chapter 5, we start to look at what different geometries actually tell us. A fundamentalquality of a “geometry” is the type of path which gives the shortest distance between points. Forinstance, in the plane, the shortest distance between points is a straight line, but on a sphere this

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Note to Students xxi

is no longer the case. If we go from Cleveland to Paris, unless we are very good at tunnelling, wemust take the great circle route to achieve distance minimization. Knowing that shortest lengthcurves are great circles on a sphere gives us an intuitive understanding of the curvature andsymmetry of the sphere. So this chapter deals with “shortest length curves” (i.e., geodesics) on asurface. In fact, we modify this a bit to derive certain differential equations called the geodesicequations whose solutions are geodesics on the surface. Again, while it is sometimes possible toobtain analytic expressions for geodesics, more often we solve the geodesic equations numericallyand plot geodesics to discover the underlying geometry of the surfaces. The geodesic equationsmay also be transported to a more abstract situation, so we begin to see more general geometriceffects here as well.

Chapter 6 is the culmination of much of what has come before. For in this chapter, we seehow curvature can affect even the most basic of geometric qualities, the sum of the anglesin a triangle. The formalization of this effect, which is one of the most beautiful results inMathematics, is known as the Gauss-Bonnet theorem. We present various applications of thistheorem to show how “abstract” results can produce concrete geometric information. Also inthis chapter, we introduce a notion known as holonomy that has profound effects in physics,ranging from classical to quantum mechanics. In particular, we present holonomy’s effect on theprecession of Foucault’s pendulum, once again demonstrating the influence of curvature on theworld in which we live.

Chapter 7 presents what can fairly be said to be the prime philosophical underpinning of therelationship of geometry to Nature, the calculus of variations. Physical systems often take aconfiguration determined by the minimization of potential energy. For instance, a hanging ropetakes the shape of a catenary for this reason. Generalizing this idea leads yet again to a differentialequation, the Euler-Lagrange equation, whose solutions are candidates for minimizers of variousfunctionals. In particular, Hamilton’s principle says that the motions of physical systems ariseas solutions of the Euler-Lagrange equation associated to what is called the action integral. Aspecial case of this gives geodesics and we once again see geometry arising from a differentialequation (which itself is the reflection of a physical principle).

In Chapter 8, we revisit virtually all of the earlier topics in the book, but from the viewpoint ofmanifolds, the higher-dimensional version of surfaces. This is necessarily a more abstract chapterbecause we cannot see beyond three dimensions, but for students who want to study physics ordifferential geometry, it is the stepping stone to more advanced work. Systems in Nature rarelydepend on only two parameters, so understanding the geometry inherent in larger parameterphenomena is essential for their analysis. So in this chapter, we deal with minimal submanifolds,higher-dimensional geodesic equations and the Riemann, sectional, Ricci and scalar curvatures.Since these topics are the subjects of many volumes themselves, here we only hope to indicatetheir relation to the surface theory presented in the first seven chapters.

So this is the book. The best advice for a student reading it is simply this: look for the rightdifferential equations and then try to solve them, analytically or numerically, to discover theunderlying geometry. Now let’s begin.

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AList of Examples

A.1 Examples in Chapter 1

Example 1.1.1, page 1: First Example of a Curve: a Line in R3.

Example 1.1.5, page 3: The Cusp α(t) = (t2, t3).

Example 1.1.12, page 8: The Circle of Radius r Centered at (0, 0).

Example 1.1.15, page 10: The Astroid.

Example 1.1.18, page 11: The Helix.

Example 1.1.19, page 11: The Suspension Bridge.

Example 1.1.21, page 12: The Catenary.

Example 1.1.23, page 13: The Pursuit Curve.

Example 1.1.24, page 14: The Mystery Curve.

Example 1.2.4, page 16: Helix Re-Parametrization.

Example 1.3.10, page 20: Circle Curvature and Torsion.

Example 1.3.18, page 22: Helix Curvature.

Example 1.3.29, page 26: Rate of Change of Arclength.

Example 1.4.11, page 31: Plane Evolutes.

Example 1.4.12, page 32: The Plane Evolute of a Parabola.

Example 1.6.4, page 39: Green’s Theorem and Area.


Example 2.4.5, page 94: Saddle Surface Normal Curvature.

Example 2.4.8, page 94: Cylinder Normal Curvature.

437

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438 A. List of Examples


Example 3.1.3, page 108: Bending and Gauss Curvature.

Example 3.2.8, page 112: Enneper’s Surface Curvatures.

Example 3.2.11, page 114: Hyperboloid of Two Sheets Curvature.

Example 3.3.2, page 120: Torus Curvature.

Example 3.3.9, page 121: The Pseudosphere.

Example 3.6.2, page 134: The Roulette of an Ellipse.

Example 3.7.3, page 140: The Arclength of an Ellipse.


Example 4.5.3, page 180: Harmonic Conjugates.

Example 4.8.9, page 190: WE Representation of the Catenoid.


Example 5.1.8, page 213: Geodesics on S2R .

Example 5.2.5, page 216: Geodesic Equations on the Unit Sphere S2.

Example 5.2.6, page 218: Geodesic Equations on the Torus.

Example 5.4.3, page 228: The Poincare plane.

Example 5.4.5, page 228: The Hyperbolic Plane H .

Example 5.4.7, page 229: The Stereographic Sphere S2N .

Example 5.4.9, page 229: The Flat Torus Tflat.

Example 5.4.13, page 232: Geodesics on the Poincare Plane P .

Example 5.4.16, page 233: Geodesics on the Hyperbolic Plane H .

Example 5.5.4, page 236: Helicoid Isometry.

Example 5.7.4, page 270: A Shoulder Curve.


Example 6.1.3, page 276: Surface Area of the R-Sphere.

Example 6.1.7, page 277: Total Gauss Curvature.

Example 6.3.8, page 282: Holonomy on a Sphere.

Example 6.5.9, page 289: Angle Excess.

Example 6.7.1, page 293: Closed Geodesics on the Hyperboloid of One Sheet.

Example 6.8.11, page 302: Shortest Length Curves on the Sphere.

Example 6.8.12, page 303: Jacobi Equation on the Sphere.

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A.7. Examples in Chapter 7 439


Example 7.1.2, page 312: Hamilton’s Principle.

Example 7.1.3, page 312: Shortest Distance Curves in the Plane.

Example 7.2.6, page 321: Transversality at Vertical Line.

Example 7.2.7, page 321: Transversality at Horizontal Line.

Example 7.3.1, page 322: The Brachistochrone Problem.

Example 7.3.2, page 324: The Brachistochrone to a Line.

Example 7.3.4, page 324: Shortest Distance Curves in the Plane.

Example 7.3.6, page 325: Shortest Distance from a Point to a Curve.

Example 7.3.7, page 325: Least Area Surfaces of Revolution.

Example 7.3.11, page 326: Hamilton’s Principle Again.

Example 7.4.5, page 330: Buckling under Compressive Loading.

Example 7.4.7, page 333: Natural Boundary Conditions.

Example 7.5.2, page 335: A Field of Extremals.

Example 7.5.12, page 340: Weierstrass E.

Example 7.5.13, page 341: Weierstrass E for the Brachistochrone.

Example 7.5.20, page 342: The Jacobi Equation Revisited.

Example 7.6.2, page 347: Bending Energy and Euler’s Spiral.

Example 7.6.9, page 349: The Isoperimetric Problem.

Example 7.6.20, page 355: Second Order Integrals.

Example 7.7.8, page 360: The Two Body Problem.

Example 7.7.11, page 361: A Pendulum.

Example 7.7.12, page 362: A Spring-Pendulum Combination.

Example 7.7.14, page 363: The Taut String.

Example 7.8.3, page 369: Geodesics.


Example 8.2.1, page 398: The Sphere Sn as a Manifold.

Example 8.2.5, page 401: The Sphere Snas an Orientable Manifold.

Example 8.3.8, page 407: The Shape Operator on the Sphere Sn of Radius R.

Example 8.4.4, page 410: Christoffel Symbols in Dimension 2.

Example 8.5.5, page 418: Sectional Curvature of the Sphere Sn of Radius R.

Example 8.5.10, page 422: Contraction of the Metric.

Example 8.5.20, page 426: The Metric 〈·, ·〉 has Zero Divergence.

Example 8.5.25, page 429: The Schwarzschild Solution.

Example 8.6.3, page 432: Wedge Product.

Example 8.6.7, page 434: Gauss Curvature by Forms.

Example 8.6.8, page 435: Gauss Curvature of the sphere S2 via Forms.

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BHints and Solutions to Selected

Problems

Chapter 1: The Geometry of Curves

Exercise 1.1.2 Let p = (−1, 0, 5) and q = (3, −1, −2) and substitute in the equation α(t) = p + t(q −p).

Exercise 1.1.3 α(t) = (−1 + t, 6 − 5t, 5 − 8t, 9t).

Exercise 1.1.4 The vector (v1, v2, 0) is the hypotenuse of the right triangle whose sides are the vector(v1, 0, 0) and a translation of the vector (0, v2, 0). The vector (v1, v2, v3) is the hypotenuse of the righttriangle whose sides are the vector (v1, v2, 0) and a translation of the vector (0, 0, v3).

Exercise 1.1.6 135◦ intersection (or 45◦ if you prefer).

Exercise 1.1.25 Consider α(0), α( π

2 ), α(π ), and α( 3π

2 ). What is the distance of each of these points fromthe origin? Also note that α(t) · α′(t) = 0 and that α′′(t) = −α(t).

Exercise 1.1.26 Start with the basic parametrization α(t) = (r cos t, r sin t) and recognize that t = 0should correspond to the point (r + a, b) in the xy-plane. Similarly, t = π

2 should correspond to the point(a, r + b).

Exercise 1.1.27 α(t) = (a cos(t), b sin(t)).

Exercise 1.2.2 Use the chain rule on β(s) = α(h(s)).

Exercise 1.2.5 Show that |α′(t)| = r; s(t) = rt ; and t(s) = s

r. Use the definition β(s) = α(t(s)) to obtain

β(s) = (r cos s

r, r sin s

r).

Exercise 1.2.6 Show that |α′(t)| = √a2 sin2 t + b2 cos2 t . Is s(t) = ∫ t

0 |α′(u)|du integrable in closed form?

Exercise 1.3.2 Using the fact that T · e1 = |T ||e1| cos θ and taking derivatives of both sides, first showthat κN · e1 = − sin θ dθ

ds. There are two normals to β at any point on the curve, so we have two cases: (1)

N = N1, where the angle between N1 and e1 is θ + π

2 ; and (2) N = N2, where the angle between N2 and e1

is π

2 − θ . Use κN · e1 = − sin θ dθ

dsand the definition of the dot product to show that, for N = N1, κ = dθ

ds,

and for N = N2, κ = − dθ

ds.

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442 B. Hints and Solutions to Selected Problems

Exercise 1.3.3 Recall that cofactor expansion gives i j k

v1 v2 v3

w1 w2 w3

=[

v2 v3

w2 w3

]i −

[v1 v3

w1 w3

]j +

[v1 v2

w1 w2

]k.

Exercise 1.3.4 Recall that interchanging two rows of a determinant changes the sign of the determinant.What does this imply about w × v and v × w?

Exercise 1.3.5 Try Maple.

Exercise 1.3.6 Rewrite Lagrange’s Identity as

|v × w|2 = |v|2|w|2 − |v|2|w|2 cos2 θ

and simplify the right-hand side to obtain the desired result.

Exercise 1.3.8 The area of a parallelogram is given by bh, where b is the length of the base and h is thealtitude. In the parallelogram spanned by v and w, h is equal to |v| sin θ , where θ is the angle between v andw.

Exercise 1.3.11 (1) Show that β ′(s) =(√

1+s

2 , −√1−s

2 ,√

22

).

(2) Note that T ′ = β ′′ =(

14√

1+s, 1

4√

1−s, 0)

. Also, since T ′ = κN , |T ′| = |κ||N | = κ .

(3) Since T ′ = κN , N = T ′κ

.(4) B ′ = −τN , so |B ′| = | − τ ||N |, or |B ′| = | − τ | = τ .

Exercise 1.3.22 Use the fact that p = β(s) + r(s)β ′(s) for some function r(s). Differentiate both sides toobtain

(1 + r ′(s))T + r(s)β ′′(s) = 0.

Take the dot products of both sides with T to establish that r(s) �= 0. Take the dot products of both sideswith β ′′ to get the contradictory statement r(s) = 0 — unless β ′′ = 0. This then says that β is a line.

Exercise 1.3.23 Let α(s) − p = aT + bN + cB so that T · (α − p) = a, N · (α − p) = b, and B · (α −p) = c. Recognize that (α − p) · (α − p) = R2 since α lies on a sphere of center p and radius R. Takederivatives of both sides of this equation to obtain an expression for T · (α − p). Then take derivatives ofboth sides of T · (α − p) = a to obtain an expression for N · (α − p). Finally, take derivatives of both sidesof N · (α − p) = b to obtain an expression for B · (α − p).

Exercise 1.3.24 Use the previous problem. Let the constant be R2 and show that α + 1κN + 1

τ( 1

κ)′B is a

constant.

Exercise 1.4.4 If the road is not banked, α′′(t) can be resolved into two components: (1) tangentialacceleration = dν

dtT (t) = 0 since the car is traveling at a constant speed; and (2) centripetal acceleration =

κν2N (t). By Newton’s Law, the magnitude of the force due to centripetal acceleration is |mκν2N (t)| =mκν2, which must be balanced by the force due to friction, µmg. If the road is banked, there are threeprimary forces acting on the car: (1) a downward force mg due to gravity; (2) a corresponding normal forceexerted by the road; and (3) a kinetic frictional force preventing the car from flying off the road. The staticfrictional force preventing the car from sliding downward is negligible.

Recall from physics that |f | = µ|N |, yielding fx = µNy and fy = µNx . Summing the vertical forceson the car yields

Ny = mg + fy = mg + µNx.

The total of the horizontal forces, fx + Nx , produces the centripetal acceleration, so we have

mκν2 = fx + Nx = µNy + Nx.

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Hints and Solutions to Selected Problems 443

Solve simultaneously for Nx and Ny to obtain

Nx = m

1 + µ2[κν2 − µg]

Ny = m

1 + µ2[g + µκν2].

An expression for tan θ may now be obtained. Solve this expression for ν and obtain

ν ≤√

g(tan θ + µ)

κ(1 − µ tan θ ).

Exercise 1.4.6 Use κ = |α′×α′′ ||α′ |3 for both parts of this problem. In the case of the general plane curve

α(t) = (x(t), y(t)), we have α′(t) = (x ′(t), y ′(t)) and α′′(t) = (x ′′(t), y ′′(t)), leading to:

κ = x ′(t)y ′′(t) − x ′′(t)y ′(t)

[(x ′(t))2 + (y ′(t))2]32

.

Exercise 1.4.9 As part of Exercise 1.4.8, we have |α′(t)| = √2 cosh t .

Exercise 1.5.2 Since γ (s) is not necessarily a unit speed parametrization, use κγ = |γ ′×γ ′′ ||γ ′ |3 .

(1) Show that γ ′ × γ ′′ = κB − κ cos θ (u × N ).(2) Show that u × N = cos θB − sin θT .(3) Show that |γ ′ × γ ′′| = κ sin θ .(4) Show that |γ ′| = sin θ .

Exercise 1.5.3 (⇒) β a circular helix ⇒ γ a circle ⇒ κγ is constant. A circular helix is a special case of acylindrical helix. Thus, T · u = cos θ is constant. What do these results imply about κ = κγ sin2 θ? Finally,use the fact that for a cylindrical helix, τ

κis a constant.

(⇐) τ and κ constant ⇒ τ

κ= cot θ is constant. Use this to show that κγ is constant. Also show that

τγ = 0 and conclude that γ is a circle.

Exercise 1.5.4 Use the fact that κ = τ = 1√8(1−s2)

(Exercise 1.3.11) to show that cot θ = τ

κis a constant.

Exercise 1.5.5 Use |β ′×β ′′ ||β ′ |3 to compute κ = 1

4 . Use (β ′×β ′′)·β ′′′|β ′×β ′′ |2 to compute τ = − 1

4 . What is true if both τ

and κ are constants?

Exercise 1.5.6 Prove that τ

κconstant ⇔ 4b4 = 9a2 by using MAPLE.

Exercise 1.5.7 WLOG assume that β has unit speed. Show that T ′(s)κ

· u = 0 or, equivalently, 1κ

(T ′(s) · u) =0. Use the fact that (T (s) · u)′ = T ′(s) · u + T (s) · u′ = T ′(s) · u since u is a constant vector.

Chapter 2: Surfaces

Exercise 2.1.1 (⇒) If xu and xv are linearly dependent, then xu = cxv where c is a scalar. Use the factthat xu × xv may be expressed as a determinant and use properties of determinants.

(⇐) xu × xv = 0 ⇒ |xu||xv| sin θ = 0. What does this imply about θ , the angle between xu and xv?

Exercise 2.1.6 Use a Monge patch, x(u, v) = (u, v, u2 + v2); determine the u-parameter curve, x(u, v0),and the v-parameter curve, x(u0, v). Note that each of the parameter curves lies in a coordinate plane of R

3.

Exercise 2.1.8 x(u, v) = (u, v, +√1 − u2 − v2).

Exercise 2.1.19 A ruling patch for a cone is of the form x(u, v) = p + vδ(u) where p is a fixed point.Let p = (0, 0, 0) as the cone emanates from the origin. The line that is to sweep out the surface must thus

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extend from (0, 0, 0) to a point on the circle (a cos u, a sin u, a) lying parallel to and above the xy-plane.(The z-coordinate of any point on the circle is a because z = √

x2 + y2 = √a2 cos2 u + a2 sin2 u = a.)

A ruling patch for a cylinder is of the form x(u, v) = β(u) + vq where q is a fixed direction vector. Thedirectrix β(u) for a standard cylinder is the unit circle in the xy-plane (cos u, sin u, 0). We want a standard,right circlular cylinder, so q = (0, 0, 1).

Exercise 2.1.20 To show that a surface is doubly ruled, we need to identify two ruling patches for thesurface. Since z = xy = f (x, y), we can use a Monge patch x(u, v) = (u, v, uv) and write x(u, v) in termsof β(u) = (u, 0, 0) and δ(u) = (0, 1, u). Alternatively, y(u, v) = (v, u, vu) is also a patch for the surface.

Exercise 2.1.21 A patch for the helicoid is x(u, v) = (0, 0, bu) + v(a cos u, a sin u).

Exercise 2.1.22 A directrix for the hyperboloid of one sheet is the ellipse β(u) = (a cos u, b sin u, 0). Letδ(u) = β ′(u) + (0, 0, c). It can be shown that x(u, v) = β(u) + vδ(u) is indeed a patch for the hyperboloid.Alternatively, let β(u) be as above and let δ(u) = β ′(u) + (0, 0, −c) and verify that this, also, is a patch forthe hyperboloid.

Exercise 2.2.5 By definition,

v[fg] = d

dt(fg(α(t)) |t=0= ∇fg(p) · v.

Express ∇fg as ( ∂(fg)∂x

,∂(fg)

∂y,

∂(fg)∂z

) and recognize that ∂(fg)∂x

= ∂f

∂xg + ∂g

∂xf . Finally, collect like terms to

obtain the desired result, v[fg] = v[f ]g + f v[g].

Exercise 2.2.6 View x as a function f (p1, p2, p3) and write v = (v1, v2, v3). Thus, by definition, v[x] =( ∂x

∂p1, ∂x

∂p2, ∂x

∂p3) · (v1, v2, v3). But since x(p1, p2, p3) = p1, ∂x

∂p2= ∂x

∂p3= 0 and ∂x

∂p1= 1. A similar procedure

may be used for v[y] and v[z].

Exercise 2.2.11 Let α(t) = x(a1(t), a2(t)), β(t) = x(b1(t), b2(t)) with α(0) = p = β(0) and α′(0) = v,β ′(0) = w. Then if γ (t) = x((a1(2t) + b1(2t))/2, (a2(2t) + b2(2t))/2), γ ′(t) = xu

du

dt+ xv

dv

dt= xu(a′

1(2t) +b′

1(2t)) + xv(a′2(2t) + b′

2(2t)). Finding α′(t) and β ′(t) and substituting yields γ ′(0) = v + w. Thus,(v + w)[f ] = γ ′(0)[f ] = ∇f · γ ′(0). By using v[f ] + w[f ] = ∇f · v + ∇f · w, show that (v + w)[f ] =v[f ] + w[f ].

Exercise 2.3.4 To compute the eigenvalues of a matrix S, we set det(λI − S) = 0. This yields, in the caseof the 2 × 2 symmetric matrix [

a b

b c

],

the equation λ2 − (a + c)λ + ac − b2 = 0. Solve for λ1 and λ2 and show that they are both real.

Exercise 2.4.4 For the first part, just use S(α′) = −∇α′U . For the second (which is also an if and only if ),show that both S(α′) and α′ are in P ∩ Tα(t)M for each t .

Chapter 3: Curvatures

Exercise 3.1.2 Since K = k1k2, k1 and k2 must be of opposite sign. Because k1(u) is defined to be themaximum curvature, k1(u) > k2(u), so k1 > 0 and k2 < 0 is the case here.

Exercise 3.1.6 (1) Euler’s formula states that k(u) = cos2 θk1 + sin2 θk2 where u = cos θu1 + sin θu2

(i.e., u is a function of θ ). Thus,

1

2π

∫ 2π

0k(θ )dθ = 1

2π

∫ 2π

0(cos2 θk1 + sin2 θk2)dθ.

Evaluate the integral, remembering that k1 and k2 are constants.(2) Express v1 and v2 in terms of u1 and u2. That is, v1 = cos φu1 + sin φu2 and v2 = cos(φ + π

2 )u1 +sin(φ + π

2 )u2. Use Euler’s formula to obtain k(v1) and k(v2).

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Exercise 3.1.10 M minimal ⇒ H (p) = 0 for every p ∈ M . What does this imply about k1 and k2 and, inturn, about K?

Exercise 3.2.6 Computing (EtGt − F t )2 directly, setting it equal to |xtu × xt

v|2 = (1 − 2Ht +Kt2)2(EG − F 2) and equating coefficients of powers of t produces the identities:

(1) (−2K + 4H 2)EG = G(

2

E+ m2

G

)+ E

(m2

E+ n2

G

);

(2) (4HK)EG = 2n(

2

E+ m2

G

)+

(m2

E+ n2

G

)− 8m2H ;

(3) K2EG =(

2

E+ m2

G

) (m2

E+ n2

G

)− 4m2H 2;

Now use these in the formulas for Ht and Kt to establish the mean and Gauss curvatures for parallelsurfaces.

Exercise 3.2.18 (a) Compute xu, xv , and xu × xv to obtain U = β ′×δ+vδ′×δ

W. Show that (U · xuv)2 = (β ′ ·δ×δ′)2

W 2 .To do so, you will need to recall that a · (b × c) = −b · (a × c). Use the Lagrange Identity to show that

|xu × xv|2 = EG − F 2 = W 2. Finally, show that K = −(U ·xuv )2

EG−F 2 and combine with the above.(b) A ruling patch for the saddle surface is x(u, v) = (u, 0, 0) + v(0, 1, u). Then β(u) = (u, 0, 0) and

δ(u) = (0, 1, u). Use the results of (a) to obtain K = −1(x2+y2+1)2 .

(c) Note that β(u) = (p1, p2, p3) so that β ′(u) = (0, 0, 0).(d) Note that δ(u) = (q1, q2, q3) so that δ′(u) = (0, 0, 0).

Exercise 3.2.20 For one direction, note that U cannot depend on v only when the term v(δ′ × δ) = 0.Thus, δ′ × δ = 0 and the formula for K of a ruled surface shows K = 0. For the other direction, note thatUv = −S(xv) is a tangent vector. Show that Uv · xv = 0 (automatically!) and Uv · xu = 0 by the hypothesisK = 0 (and the formula for K of a ruled surface).

Exercise 3.2.23 If β is a line of curvature, then β ′ · U × U ′ = β ′ · U × cβ ′ = 0 (why?). For the otherway, show that developable implies that U ′ is perpendicular to β ′ × U which is also perpendicular to β ′.Then note that all these vectors are in the tangent plane.

Exercise 3.2.26 (a) and (b) are self-explanatory. In (c), use the results of part (b) in the expressionsK = ln−m2

EG−F 2 and H = Gl+En−2Fm

2(EG−F 2)and simplify. For part (d), recognize (using results of (c)), that D is the

numerator of K evaluated at the critical point (u0, v0). Since the denominator of K is always positive,D = 0 ⇒ K = 0, D < 0 ⇒ K < 0, and D > 0 ⇒ K > 0. What must be true of k1 and k2 when K = 0?When K < 0? What do these results imply about the surface? Two sub-cases correspond to K > 0. Iffuu(u0, v0) is positive, k1 and k2 must both be positive. If fuu(u0, v0) is negative, k1 and k2 must both benegative. What must be true of the surface in each of these sub-cases?

Exercise 3.3.4 F = m = 0 for a surface of revolution. Thus, xu and xv are orthogonal and we canexpress S(xu) in terms of the basis vectors xu and xv . Thus, let S(xu) = axu + bxv . Compute S(xu) · xu andrecognize that this is equal to l. Compute S(xu) · xv and recognize that this is equal to m. Similarly, letS(xv) = cxu + dxv and take dot products with xv and xu.

Exercise 3.3.7 (a) Derive

K = 1 − u2

(1 + u2e−u2 )2

by using the expression for K for a surface of revolution. Algebraically determine when K > 0, K = 0,and K < 0. Part (b) is similar; parametrize the ellipse as α(u) = (R + a cos u, b sin u, 0) to produce thefollowing patch for the elliptical torus: x(u, v) = ((R + a cos u) cos v, b sin u, (R + a cos u) sin v). Usingthe expression for K for a surface of revolution yields

K = ab2 cos u

(R + a cos u)(b2 cos2 u + a2 sin2 u)2.

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Exercise 3.3.11 By separating variables, we obtain the expression

u = −∫ √

1

h2− 1 dh.

Make the substitution h = 1/ cosh w to obtain u = ∫tanh2 wdw. Integrate, then use the fact that

cosh−1(1h) = ln( 1h

+√

1h2 − 1) and recall that tanh x = ex − e−xex + e−x . Simplify to obtain u = ln | 1

h+√

1−h2

h| − √

1 − h2 + C.

Exercise 3.4.4 To verify the expression for Uu, recall that a u-parameter velocity vector applied to afunction of u and v takes the u-partial derivative of that function. Thus,

∇xuU = (xu[u1], xu[u2], xu[u3]) = Uu.

Then, since xu and xv form a basis for Tp(M) and since ∇xuis in Tp(M), we have ∇xu

U = Axu + Bxv .Take the dot product of both sides to obtain ∇xu

U · xu = Axu · xu = AE. Also recognize that 0 = xu[0] =xu[U · xu] = ∇xu

U · xu + U · xuu and use this to show that A = −l/E. Proceed in a similar manner to findB and to obtain an expression for Uv .

Exercise 3.4.5 By finding the two partial derivatives(

Ev

2G

)v

and(

Gu

2G

)u, we obtain, as an equivalent

expression for the right-hand side,

EuGu

4E2G− 2GEvv

4G2E+ EvGv

4G2E+ EvEv

4E2G− 2GGuu

4G2E+ GuGu

4G2E.

Combine these terms over the common denominator 4E2G2. Next, compute

∂

∂v

(Ev√EG

)= Evv√

EG− EvEvG

2(EG)32

− EEvGv2(EG)32

and∂

∂u

(Gu√EG

)= Guu√

EG− GuEuG

2(EG)32

− GuEGu

2(EG)32

.

Substitute into the given expression and write the result over the common denominator 4E2G2 to obtain thesame expression as above.

Exercise 3.4.6 A patch for a sphere of radius R is given by the formula x(u, v) = (R cos u cos v,

R sin u cos v, R sin v). Compute xu, xv , E, and G and substitute into the given expression to obtainK = 1/R2.

Exercise 3.5.1 Since Sp is a linear transformation from Tp(M) to itself, we can write S(xu) = Axu + Bxv .But since p is an umbilic point, we have S(xu) = kxu ⇒ B = 0. Now use

l = S(xu) · xu = AE + BF, m = S(xu) · xv = AF + BG

and solve for B to get B = −F l+Em

EG−F 2 = 0, so −F l + Em = 0 or l/E = m/F . Do the same for S(xv).

Exercise 3.5.9 A patch for a surface of revolution is given by x(u, v) = (u, h(u) cos v, h(u) sin v). ThenK = −h′′

h(1+h′2)2 = 0 ⇒ −h′′ = 0. Thus, h(u) = C1u + C2 (a line). Note that, if C1 = 0, a cylinder is generated

by revolving h(u) about the x-axis; if C1 �= 0, a cone is generated.

Chapter 4: Constant Mean Curvature Surfaces

Exercise 4.2.3 Use a Monge patch x(u, v) = (u, v, f (u, v)) to obtain fu = g′(u), fuu = g′′(u), fv = h′(v),fuv = 0, and fvv = h′′(v). Then

H = 0 ⇔ (1 + h′2(y))g′′(x) + (1 + g′2(x))h′′(y) = 0.

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Separate variables to obtain−g′′(x)

1 + g′2(x)= h′′(y)

1 + h′2(y).

Since x and y are independent, each side is constant relative to the other side. Thus, let

a = −g′′(x)

1 + g′2(x).

Also let w = g′(x) so that g′′(x) = dw

dxand integrate to obtain w = dg

dx= − tan ax. Integrating again gives

g(x) = 1a

ln(cos ax). Apply the same reasoning to the other side of the original differential equation to obtainh(y) = − 1

aln(cos ay). Combining terms yields

f (x, y) = 1

aln

(cos ax

cos ay

).

Exercise 4.2.7 dτβ

du= (β ′ × δ)′ · δ′ + (β ′ × δ) · δ′′. Both terms in this expression are zero.

Exercise 4.3.3 Compute

∂P

∂u= (fuuV + fuVu)(1 + f 2

u + f 2v ) − Vf 2

u fuu − Vfufvfuv

(1 + f 2u + f 2

v )32

and∂Q

∂v= (fvvV + fvVv)(1 + f 2

u + f 2v ) − Vf 2

v fvv − Vfufvfuv

(1 + f 2u + f 2

v )32

.

Then apply Green’s Theorem: ∫v

∫u

∂P

∂u+ ∂Q

∂vdudv =

∫C

P dv − Q du.

Exercise 4.4.1 Compute partial derivatives to obtain

∂P

∂u+ ∂Q

∂v= −Vu · (U × xv) + Vv · (U × xu) + V · [Uv × xu − Uu × xv].

Since U is a function on M , we have Uv = ∇xvU = −S(xv) and Uu = ∇xu

U = −S(xu). Substituting in theabove equation and yields

∂P

∂u+ ∂Q

∂v= Vv · (U × xu) + Vu · (U × xv) + V · (2Hxu × xv).

Now apply Green’s Theorem.

Exercise 4.6.2 For Cauchy-Riemann, z2 = x2 − y2 + i2xy, so

∂φ

∂x= 2x = ∂ψ

∂y

∂φ

∂y= −2y = −∂ψ

∂x.

Thus, f (z) = z2 is holomorphic and f ′(z) = 2x + i2y = 2z as it should.

Exercise 4.6.8

∂f

∂z= 1

2

(∂φ

∂u+ i

∂ψ

∂u+ i

∂φ

∂v+ i2 ∂ψ

∂v

)

= 1

2

(∂φ

∂u− ∂ψ

∂v+ i

∂ψ

∂u+ i

∂φ

∂v

)= 0

by Cauchy-Riemann.

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Exercise 4.8.10 The calculations are exactly as in Example 4.8.9 except that an extra factor of i occurs ineach term. This affects the real parts to produce x1 = sinh u sin v, x2 = − sinh u cos v and x3 = v; thus, ahelicoid.

Exercise 4.8.29 M is minimal with isothermal coordinates, so l = −n and, consequently,

K = ln − m2

EG − F 2= −l2 − m2

E2 − 0= − l2 + m2

E2.

Exercise 4.8.30 A conformal Gauss map implies Uu · Uv = 0. Plugging in the usual expressions for Uu

and Uv , we get 0 = mH . Now consider the two cases, m = 0 and H = 0.

Chapter 5: Geodesics, Metrics and Isometries

Exercise 5.1.12 A parametrization of a right circular cylinder is given by x(u, v) = (R cos u, R sin u, bv).Then a curve on the surface is given by α(t) = (R cos u(t), R sin u(t), bv(t)). Find α′′ by differentiatingtwice and noting that the chain rule gives d

dtcos u(t) = − sin u du

dt. Now, α′′ = α′′

tan + (α′′ · U )U , whereU in this case is (cos u, sin u, 0). Taking the dot product of α′′ with U yields α′′ · U = −R( du

dt)2. From

this and from α′′ = α′′tan + (α′′ · U )U , we know that αtan = 0 results in d2u

dt2 = 0 and d2v

dt2 = 0, yieldingu(t) = k1t + c1 and v(t) = k2t + c2. Thus, α(t) = (R cos(k1t + c1), R sin(k1t + c1), b(k2t + c2)). Finally,consider the following cases: (1) c1 = c2 = 0, k1 = k2 = 1; (2) k1 = 1, k2 = 0, c1 = 0, c2 �= 0; (3) k1 = 1,k2 = 0, c1 = 0, c2 = 1

b.

Exercise 5.2.12 √G sin φ =

√G cos(π/2 − φ) = xv · α′ = Gv′

since α′ = xuu′ + xvv

′. Now use the relation v′ = c/G derived from the second geodesic equation.

Exercise 5.2.13 In polar coordinates, a patch for the plane is given by x(u, v) = (u cos v, u sin v). ComputeE = 1, F = 0, and G = u2, verifying that x is u-Clairut. Then we have

v(u) − v(u0) =∫ u

u0

c√

E√G

√G − c2

du =∫ u

u0

cdu

u√

u2 − c2.

Integrate using the substitution u = c sec x ⇒ du = c sec x tan x dx to obtain v(u) − v(u0) = ± cos−1 c

u, or

u cos(v − v0) = c, the polar equation of a line.

Exercise 5.2.14 Compute E = 2, F = 0, and G = u2, verifying that the patch for the cone is u-Clairut.Then

v(u) − v(u0) =∫ u

u0

c√

E√

G√

G − c2du =∫ u

u0

c√

2du

u√

u2 − c2.

Integrate using the substitution u = c sec x to obtain v(u) − v(u0) = √2 sec−1 u

c.

Exercise 5.4.6 Compute E = 1/

(1 − u2/4)2, F = 0, and G = u2/

(1 − u2/4)2. Then, K =

− 1

2√

EG

(∂

∂v

(Ev√EG

)+ ∂

∂u

(Gu√EG

))= − 1

2√

EG

(∂

∂u

(Gu√EG

)).

Find the required derivatives and work through the algebra to obtain K = −1.

Chapter 6: Holonomy and the Gauss-Bonnet Theorem

Exercise 6.1.4 A patch for the torus is given by x(u, v) = ((R + r cos u) cos v, (R + r cos u) sin v, r sin u).Find xu, xv , and compute |xu × xv| = r(R + r cos u). Then

SA =∫ 2π

0

∫ 2π

0r(R + r cos u)dvdu = 4π 2rR.

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Exercise 6.1.5 (1) A patch for a surface of revolution is given by x(u, v) = (u, h(u) cos v, h(u) sin v).Find xu, xv , and compute |xu × xv| = h(u)(1 + h′2(u))

12 . Recognize that for a surface of revolution, h(u) is

usually written as f (x). Use the expression for surface area to complete the exercise. (2) Define a Mongepatch x(u, v) = (u, v, f (u, v)). Then |xu × xv| = √

1 + f 2u + f 2

v .

Exercise 6.1.8 A patch for the bugle surface (with c = 1) is given by

x(u, v) = (u − tanh u, sech u cos v, sech u sin v).

Compute |xu × xv| = (sech4 u tanh2 u + sech2 u − 2sech4 u + sech6 u)12 and simplify using the identity

tanh2 u = 1 − sech2 u, obtaining |xu × xv| = sech u tanh u. Then

SA =∫ ∞

0

∫ 2π

0sech u tanh u dvdu = 2π.

Exercise 6.1.9 (1) Total Gaussian curvature is given by∫M

K =∫ ∫

cos u

r(R + r cos u)|xu × xv| dudv,

for the torus, where |xu × xv| = r(R + r cos u). (2) A patch for the catenoid is given by x(u, v) =(u, cosh u cos v, cosh u sin v), yielding |xu × xv| = cosh2 u. Also, K = −1

/cosh4 u. Integrate

∫M

K toshow that the total Gaussian curvature is −4π .

Exercise 6.3.1 α′[V · V ] = 2∇α′V · V = 0 since V is parallel.

Exercise 6.3.3 α′[V · W ] = ∇α′V · W + V · ∇α′W . V and W parallel imply α′[V · W ] = 0 and V parallel,α′[V · W ] = 0, α′[W · W ] = 0 imply ∇α′W is perpendicular to both V and W in a plane. Thus, ∇α′W = 0and W is parallel.

Exercise 6.3.10 For the R-sphere, K = 1/R2 and |xu × xv| = R2 cos v. Integrate

∫M

K to show that thetotal Gaussian curvature above v0 is 2π − 2π sin v0. We know that the holonomy around the v0-latitudecurve is −2π sin v0. Thus, the holonomy around a curve is equal to the total Gaussian curvature over theportion of the surface bounded by the curve (up to additions of multiples of 2π ).

Exercise 6.3.11 At the Equator, v0 = 0. What is the holonomy along the Equator and what does this implyabout the apparent angle of rotation of a vector moving along the Equator? What does this signify about theEquator?

Exercise 6.4.4 But, of course, gravity really doesn’t point that way on a planetary torus, does it?

Exercise 6.5.5 The vector must come back to itself, so the total number of revolutions it makes is amultiple of 2π .

Exercise 6.5.10 Note that the Gaussian curvature for both H and P is a constant K = −1. Thus we have∫�

K = −∫

�= −area of �.

But, since the sum of the interior angles of the triangle differs from π by (+ or −) the total Gaussiancurvature, we have

�ij − π = −area of �.

What does this imply about the sum of the angles, noting that area is a strictly positive quantity?

Exercise 6.6.7 If K ≤ 0 and K < 0 at even a single point, then the total Gauss curvature is negative. Butthe Euler characteristic of the torus is zero.

Exercise 6.7.3 A disk has Euler characteristic 1.

Exercise 6.8.17 Their curvatures are not bounded away from zero.

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Chapter 7: The Calculus of Variations and Geometry

Exercise 7.1.9

d

dt

(f − x

∂f

∂x

)= ∂f

∂t+ ∂f

∂xx + ∂f

∂xx − x

∂f

∂x− x

d

dt

∂f

∂x

= 0 + 0 + x

(∂f

∂x− d

dt

∂f

∂x

)= 0

if and only if ∂f

∂x− d

dt

∂f

∂x= 0.

Exercise 7.1.13 x(t) = t − sin t .

Exercise 7.1.15 x(t) = sin t .

Exercise 7.3.3 The Euler-Lagrange equation for the time integral

T =∫ √

1 + y ′2

kydx

is √1 + y ′2

ky− y ′ y ′

ky√

1 + y ′2= c.

Then

1

ky√

1 + y ′2= c

is separable withy√

c2 − y2dy = dx.

The solutions are then (x − a)2 + y2 = c2, circles centered on the x-axis — the geodesics of the Poincareplane!

Exercise 7.5.3 x(t) = t − sin t + b.

Exercise 7.5.5 x(t) = c sin t .

Exercise 7.5.16

E = x sin t + 1

2x2 − x sin t − 1

2p2 + x2 − (x − p)p

= 1

2(x − p)2

≥ 0.

Exercise 7.5.18

E = x2 − x2 − p2 + x2 − (x − p)2p

= x2 − p2 − 2px + 2p2

= (x − p)2

≥ 0.

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Exercise 7.6.5 The Euler-Lagrange equation is

x − λ − d

dt(x + x) = 0

with simplification x = −λ and solution

x(t) = −λ

2t2 + at + b.

The initial conditions give a = λ/2 and b = 0. Applying the constraint, we obtain

7

12= −λ

2

∫ 1

0t2 − t dt = λ

12

so that λ = 7. Finally, x(t) = − 72 t2 + 7

2 t .

Exercise 7.6.13 The equations of motion for the particle in the paraboloid are

(1 + 4u2)u + 4uu2 + 2u − uv2 = 0 v = 1

u2.

Exercise 7.6.17 T = m/2 (Eu2 + Gv2), so (forgetting m)

E = 1/2(Eu2 + Gv2 − Ep21 − Gp2

2) − (u − p1)Ep1 − (v − p2)Gp2

= 1

2(E(u − p1)2 + G(v − p2)2)

≥ 0.

Chapter 8: A Glimpse at Higher Dimensions

Exercise 8.3.4

[f V, gW ] = f V [gW ] − gW [f V ]

= f V [g]W + fgV W − gW [f ]V − gf WV

= fg[V,W ] + f V [g]W − gW [f ]V.

Exercise 8.3.11 Assume xu · xv = 0 and take an orthonormal basis xu = xu/√

E and xv = xv/√

G. Then

(∇xuxu

)N = 1√E

((xu√E

)u

)=(

xuu

√E − xu(

√E)u

E3/2

)N

= l

E.

Similarly, (∇xvxv)N = n/G. Then, the sum is

Gl + En

EG= 2H since F = 0.

Exercise 8.5.17 If we take X/|X| as Ek , then E1, . . . , Ek is a frame for Mk . By definition then,

Ric(X,X) = 〈X, X〉 Ric(Ek, Ek) = 〈X, X〉k−1∑j=1

〈R(Ek, Ej )Ek, Ej 〉

since 〈R(Ek, Ek)Ek, Ek〉 = 0. Then use the definition of sectional curvature.

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Exercise 8.5.21 Let X = ∑j XjEj and df = ∑

j dfj θj where θj is a dual vector space basis element to

Ej (see §6) defined by θj (Ei) = δj

i (where δj

i is the Kronecker delta). Then

df (X) = df (∑

j

XjEj ) =∑

j

Xjdf (Ej ) =∑

j

∑i

Xjdfiθi(Ej ) =

∑j

Xjdfj .

Also. we have by the definition of divergence,

div(f 〈·, ·〉)(X) =∑

j

∇Ej(f 〈·, ·〉)(Ej , X)

=∑

j

Ej [f 〈Ej , X〉] − f 〈∇EjEj , X〉 − f 〈Ej , ∇Ej

X〉

=∑

j

Ej [f ]〈Ej , X〉 + f Ej 〈Ej , X〉

− f 〈∇EjEj , X〉 − f 〈Ej ,∇Ej

X〉=∑

j

Ej [f ]〈Ej , X〉 + f 〈∇EjEj , X〉 + f 〈Ej , ∇Ej

X〉

− f 〈∇EjEj , X〉 − f 〈Ej ,∇Ej

X〉=∑

j

Ej [f ]〈Ej , X〉

=∑

j

df (Ej )Xj

=∑

j

dfjXj

= df (X).

Exercise 8.5.23

κ =k∑

i=1

Ric(Ei , Ei) =k∑

i=1

λ〈Ei , Ei〉 = kλ.

Thus, dκ = kdλ and we also have (by Exercise 8.5.21)

2dλ = 2 div f 〈·, ·〉= 2 div(Ric)

= dκ by Theorem 8.5.22

= kdλ.

Thus, (k − 2)dλ = 0 and, since k ≥ 3, we must have dλ = 0. Hence, λ is a constant.

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CSuggested Projects for Differential

Geometry

The following are suggested projects for Differential Geometry. All problems come from theMAA Edition of Differential Geometry and its Applications, but some material from my bookThe Mathematics of Soap Films ([Opr00]) may also be required.

Project 1: Developable Surfaces

Developable surfaces are the special cases of ruled surfaces (those having a parametrizationx(u, v) = β(u) + v δ(u)) having zero Gauss curvature. These surfaces have industrial applica-tions. The project consists of: Exercise 3.2.20 – Exercise 3.2.25 and either of two choices:

(1) Exercise 5.5.5, Exercise 5.5.7, Exercise 6.9.1 and the Maple procedure holounroll inSection 6.9 and Maple material in Section 5.6:

(2) A synopsis of Section 5.7 together with the Maple work found there and that in Section 2.5for the tangent developable. For the synopsis, present the motivation and important pointsof the discussion as well as the most important calculations.

Project 2: The Gauss Map

The Gauss map is a mapping from a surface to the unit sphere given by the unit normal ofthe surface. The Gauss map can tell you a great deal about the surface. The project consistsof: Exercise 2.3.9 – Exercise 2.3.12, Exercise 4.8.27, Proposition 4.8.28, Exercise 4.8.30 and adescription of Schwarz’s theorem in The Mathematics of Soap Films giving a criterion for areaminimization. Maple should be used to draw Gauss maps for various surfaces.

Project 3: Minimal Surfaces and Area Minimization

Minimal surfaces are those with mean curvature equal to zero at each point. If a surface minimizesarea inside some boundary, it is a minimal surface. So soap films are physical manifestations

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454 C. Suggested Projects for Differential Geometry

of minimal surfaces. This project focusses on some aspects of area minimization and minimalsurfaces. It consists of: Exercise 2.1.16, Exercise 4.2.3, Exercise 4.3.5, Exercise 4.3.6 (the Mapleapproach from The Mathematics of Soap Films), Exercise 4.9.2, Exercise 4.9.3, Exercise 7.3.8,Exercise 7.3.9 and the Maple material in Subsection 4.9.5.

Warning The following two projects do not have as many exercises associated with them, butthey require learning about elliptic functions and explaining what you learn. So, do not thinkthey are easy.

Project 4: Unduloids

One-celled organisms sometimes take the shape of unduloids. These are surfaces of revolution thatarise from minimizing surface area subject to enclosing a fixed volume (read Theorem 7.7.15).Equivalently, unduloids are examples of surfaces of revolution with constant non-zero meancurvature. The project consists of: deriving the differential equation of Theorem 3.6.1, and usingthe equation to parametrize unduloids via elliptic functions. This material is in Section 3.7. Explainthis material, carry out all Maple calculations and plot unduloids. Furthermore, do Exercise 3.7.4.Finally, plot geodesics on unduloids by carrying out the discussion in Subsection 5.6.4 (and using“halfbouncepoint”) and do Exercise 5.6.9.

Project 5: The Shape of a Mylar Balloon

Mylar balloons are found at children’s birthday parties. They are constructed by sewing togethertwo disks of Mylar and inflating. The “sideways” shape is determined by a variational argumentfrom the calculus of variations. The project consists of: explaining the calculus of variationsderivation of the parametrization of the Mylar balloon using elliptic functions. Explain thematerial on elliptic function in Section 3.7 (including Exercise 3.7.4) and the material on theMylar balloon in Section 7.9 (including Exercise 7.9.5). Create a “halfbouncepoint” procedurefor the Mylar balloon similar to the one in Subsection 5.6.4. Finally, do Exercise 5.6.9.

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[Mar88] J. Martin, General Relativity, Ellis Horwood Ltd, 1988.

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[MP77] R. Millman and G. Parker, Elements of Differential Geometry, Prentice-Hall, 1977.

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[Opr95] J. Oprea, Geometry and the Foucault pendulum, Amer. Math. Monthly 102 no. 6 (1995),515–522.

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[Spi79] M. Spivak, A Comprehensive Introduction to Differential Geometry: 5 volumes, Publish orPerish, 1979.

[SS93] E. Saff and A. Snider, Fundamentals of Complex Analysis for Mathematics, Science andEngineering, Prentice-Hall, 1993.

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[Sym71] K. Symon, Mechanics, Addison-Wesley, 1971, 3rd Edition.

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[Zwi63] C. Zwikker, The Advanced Geometry of Plane Curves and their Applications, Dover, 1963.

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Index

1-forms, 4302-forms, 432k-patch, 3972-body problem, 3602-cell embedding, 2912-independent variable EL equation, 316, 3262-variable EL equation, 3152-variable first integral, 316

acceleration components, 210acceleration formulas, 125acceleration vector, 3action integral, 326, 358adjoint, 193

to Henneberg’s surface, 193adjoint of minimal surface, 193Alexandrov’s theorem, 176almost complex structure J, 230angle excess theorem, 289, 301angular momentum, 223arclength, 4, 214, 222

of geodesic on cone, 248arclength parametrization, 14area, 414

minimizationand Maple, 203implies minimal, 172, 416

of bugle surface, 277of pseudosphere, 277of sphere, 276of surface, 164, 276, 277

of torus, 277variation, 171

associated family of minimal surfaces, 193astroid, 10, 11, 33astroid evolute, 33asymptotic curve, 166

banked highway, 29Bat, 207bending energy, 347Bernoulli, 324Bernoulli’s principle, 330Bernstein’s theorem, 197Bianchi identity, 419binormal, 19Bonnet’s theorem, 304

converse, 305Book Cover, 207brachistochrone, 341brachistochrone problem, 322bracket, 403buckled column, 330bump lemma, 314

cantilevered beam, 332Catalan’s surface, 168

WE representation, 190via Maple, 198

Catalan’s theorem, 167catenary, 12, 13, 173, 349catenary evolute, 33

461

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462 Index

catenoid, 73, 120, 132, 325, 370WE representation, 190associated family, 193via Maple, 197, 201

catenoid-helicoid deformation, 240Cauchy-Riemann equations, 183Christoffel symbols, 125, 409

determined by metric, 410circle, 8, 14, 16, 20, 24, 350

characterization of, 24osculating, 26

circular helix, 35, 36Clairaut geodesic equations, 220Clairaut parametrization, 219Clairaut relation, 218, 222, 224, 225, 353

on torus, 219physical viewpoint, 223

closed, 129co-state equations, 368, 369Codazzi-Mainardi equations, 127column

buckled, 330compact, 128complete, 225complex analytic, 182complex conjugate, 183complex differentiable, 182complex integral, 184cone, 75, 117cone unrolling, 236

via Maple, 249conformal Gauss map, 193–195conformal map, 194, 240, 241conformal metric, 227, 360

scaling factor, 227conformality factor, 194conjugate point, 302, 303, 344connected, 69connection 1-forms, 431constant Gauss curvature, 112, 121, 123

via Maple, 153constant mean curvature, 112, 134, 136, 137, 366constant precession curve, 21constant speed relation, 216constrained problem, 346, 347, 349, 365, 382contraction, 422

of metric, 422coordinate chart, 397

covariant derivative, 82, 278, 286, 402, 423, 431properties of, 279, 402

cross product, 18CrossProduct, 43curvature

2-form, 434Einstein, 425Gauss, 107, 109Ricci, 420Riemann, 417average normal, 109constant Gauss, 112, 121, 123constant mean, 112, 134, 137geodesic, 210line of, 93mean, 107, 109, 181nonnegative Gauss, 305normal, 92of curve, 17, 23, 29of plane curve, 17principal, 93, 109scalar, 421sectional, 418total Gauss, 110, 277

curvature of curve, 43, 329, 347curvature of involute, 31curve, 1

arclength of, 4characterization of, 38closed, 38, 69differentiable, 1evolute of, 31non-unit speed, 27of constant precession, 21, 63rectifying, 36regular, 3simple, 39smooth, 1speed of, 3torsion of, 20total torsion of, 22

cusp, 3cycloid, 9, 51, 324cylinder, 75, 94, 117

twisted, 105cylinder unrolling, 237, 249cylindrical helix, 34, 36

characterization of, 34

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Index 463

D’Alembert’s principle, 223, 350Darboux vector, 21, 36Delaunay surface, 133, 137, 365Delaunay theorem, 137derivative map, 89developable surface, 117, 263direction vector, 2directional derivative, 81directrix, 75Dirichlet integral, 326divergence, 426

of Ricci curvature, 427of metric, 426

dot product, 5DotProduct, 43double pendulum, 363doubly ruled, 75Douglas-Rado theorem, 171

eigenvalue, 86Einstein curvature, 425Einstein manifold, 428elastic rod, 348ellipse, 14, 16, 30

arclength of, 140ellipse evolute, 33Elliptic E correction, xv, 145, 393elliptic functions, 138elliptic integral, 138endpoint-curve problem, 318Enneper’s surface, 74, 97, 112, 170

WE representation, 191via Maple, 198

Euler characteristic, 291, 292Euler’s formula, 96Euler’s spiral, 38, 348Euler-Lagrange equation, 314–316, 319, 328, 347,

355, 358, 367, 380, 412evolute, 31, 48

of astroid, 33, 49of catenary, 33, 49of ellipse, 33, 48of parabola, 32

exterior derivative, 432extremal, 315, 334, 356extremize, 315, 356

Fermat’s principle, 324

Feynman quote, 284field of extremals, 334, 345final time fixed, 317first Bianchi identity, 419first integral, 316, 358, 381first structure equation, 432fixed endpoint problem, 312, 316, 341flat, 417flat surface, 110flat surface of revolution, 133flat surfaces of revolution

via Maple, 201flat torus, 229, 230Foucault pendulum, 284Foucault vector field, 285frame, 278frame field, 278, 420, 434Frenet

formulas, 20, 28frame, 19

fundamental frequency, 365fundamental theorem of space curves, 38

Gauss curvature, 107, 109, 227, 418, 434depends only on metric, 124nonnegative, 305of R-sphere, 114, 126of Enneper’s surface, 114of minimal surface, 193of surface of revolution, 119, 121sign of, 108via Maple, 150

Gauss map, 90, 110, 193, 194area of, 110for Enneper, 91for catenoid, 91for cone, 90for cylinder, 90from WE representation, 195

Gauss’s lemma, 298Gauss-Bonnet theorem, 291geodesic, 212, 238, 352, 356, 369, 411

as length minimizer, 214as line of curvature, 214constant speed relation, 216equations, 216

via Maple, 242existence of, 216

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464 Index

geodesic (cont.)has constant speed, 212in conformal metric, 256on Poincare plane, 232on Whirling Witch of Agnesi, 222on cone, 222

via Maple, 247on cylinder, 214, 216on hyperbolic plane, 233, 239on hyperboloid of 1-sheet, 225on paraboloid, 224on plane, 222on sphere, 213, 216

via Frenet formulas, 214on stereographic plane, 234, 256on stereographic sphere, 234on surface of revolution, 221on torus, 218on unduloid, 251parameter curve, 221plane curves, 216via Maple, 243

geodesic curvature, 210, 231, 281, 358depends only on metric, 211

geodesic equations, 369, 413Clairaut, 220

geodesic on hyperboloid of 1-sheet, 293geodesic polar coordinates, 214, 297geodesic torsion, 281, 282geodesically complete, 225geodesics, 353geographical coordinates, 70Goldschmidt discontinuous solution, 173great circle, 213Green’s theorem, 39, 287

Hadamard’s theorem, 296Hamilton’s principle, 312, 326Hamiltonian, 368, 369harmonic conjugate, 183harmonic function, 179helicoid, 74, 75, 116, 166

WE representation, 190helicoid isometry, 236helicoid-catenoid animation, 199helix, 11, 16, 44

circular, 35, 56curvature of, 22

cylindrical, 34hyperbolic, 31involute of, 17

Henneberg’s surface, 168WE representation, 191

Henneberg’s surface adjoint, 193higher-order Euler-Lagrange equation, 329Hilbert’s invariant integral, 337Hilbert’s lemma, 132holomorphic, 182holonomic constraints, 350holonomy, 281, 283

as total Gauss curvature, 288on Poincare plane, 287on cone, 284, 308on sphere, 282preserved by isometry, 281via Maple, 306

Hopf’s conjecture, 179Hopf-Rinow theorem, 225Huygens, 51hyperbolic helix, 31hyperbolic plane, 228

geodesic, 233hyperboloid of 1-sheet, 75, 76, 97, 116hyperboloid of 2-sheets, 114, 305hypersurface, 402, 418

imaginary part, 182immersed, 137interior angles, 288Inverse Function Theorem, 90involute, 16, 47

curvature of, 31of circle, 47of cycloid, 51of helix, 17

isometry, 235global, 236preserves geodesics, 238

isoperimetric inequality, 39isoperimetric problem, 349isothermal coordinates, 181, 185, 195isothermal parametrization, 181

J transformation, 230Jacobi bisection theorem, 295Jacobi elliptic functions, 138

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Index 465

Jacobi equation, 300, 302, 303, 305, 342Jacobi identity, 404Jacobi’s theorem, 360Jacobian matrix, 400, 401

Killing form, 425kinetic energy, 312, 323, 326, 351, 352, 358, 362,

412Kuen’s surface, 116

Lagrange identity, 18, 109, 111Lagrange multiplier, 347, 349Laplace equation, 179, 183Laplace-Young equation, 163lassoing a cone, 237least area, 203, 325, 341, 370, 416Leibniz rule, 81lemniscate, 46, 141level set, 82Lie bracket, 403, 423

properties of, 404Lie group, 424Liebmann’s theorem, 130line, 1, 209

characterization of, 4, 23line of curvature, 93, 112, 114, 117, 120, 214, 281

planar, 114line of striction, 76linear transformation, 84linking number, 22local, 43loxodrome, 261

Mobius strip, 80manifold, 398

Einstein, 428Mann quote, 430map command, 44Maple Elliptic E correction, xv, 145, 393mean curvature, 107, 109, 181, 408

and soap bubbles, 175constant, 173, 175for isothermal coordinates, 186of Enneper’s surface, 114vector field, 408, 416via Maple, 150

mechanical curvature, 361Mercator projection, 241, 259

meridian, 119as geodesic, 213

meromorphic, 188metric, 123, 409, 423

divergence of, 426multiple, 423

metric coefficients, 409minimal ruled surface, 167minimal surface, 110

Gauss curvature of, 193adjoint of, 193associated family, 193examples, 168

minimal surface equation, 165, 172, 326via Maple, 199

minimal surface of revolution, 132modulus, 137, 183Monge patch, 70, 78monkey saddle, 180moving frame, 278multiple metric, 423Myers’s theorem, 422Mylar balloon, 371

characterization of, 378parametrization of, 374

mystery curve, 14

natural boundary conditions, 321, 333natural equations, 21Neil’s parabola, 33Newton’s problem, 4, 370nodary, 136, 148nodoid, 148Norm, 43normal

outward-pointing, 93principal, 18

normal coordinates, 413normal curvature, 92, 120, 210normal vector, 78normal vector field, 402

optimal control problem, 366orbits, 361orientable, 400orientation option, 44osculating circle, 26osculating plane, 26, 36

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466 Index

pair of pants, 152parabola evolute, 32paraboloid, 108, 393

elliptic, 108, 116, 151hyperbolic, 108, 116

parallel, 119parallel postulate, 233, 234, 275parallel surface, 112parallel translation, 412parallel transport, 281parallel vector field, 280, 411

for Foucault pendulum, 285parameter curve, 67parametrization, 1, 68

by arclength, 14, 16of line, 2

patch, 68Monge, 70

path connected, 69pendulum, 361

and spring, 362double, 363

periodic solution, 360plane evolute, 31Plateau’s problem, 170Poincare plane, 228

geodesic, 232Poincare upper half space, 423pole, 299Pontryagin maximum principle, 368, 370potential energy, 312, 323, 326, 351, 353, 358, 362,

412potential function, 338principal curvatures, 93, 109principal normal, 18principal vectors, 93procedure, 43product rule, 6, 81pseudosphere, 121–123pursuit curve, 13

real part, 182recreate procedure, 54recreate3d, 55rectifying curve, 36

characterization of, 36regular, 3regular mapping, 68

reparametrization, 15by arclength, 15

Ricci curvature, 420, 422, 424divergence of, 427for surfaces, 423

Richmond’s surfaceWE representation, 191

Riemann curvature, 417generalizes Gauss curvature, 418symmetries of, 419

Riemannian connection, 280Ros’s theorem, 176roulette, 134, 135ruled minimal surface, 167ruled surface, 74, 117ruling, 75

saddle surface, 75, 94, 116scalar curvature, 421, 423Scherk’s fifth surface, 170Scherk’s surface, 165

WE representation, 191via Maple, 198, 201

Schwarz inequality, 6, 341Schwarzschild solution, 429second Bianchi identity, 419second derivative test, 118second fundamental form, 407

properties of, 407second structure equation, 434second variation, 343secondary variational problem, 343sectional curvature, 418, 424

as Gauss curvature, 418shape operator, 84, 107, 108, 279, 406

and Gauss map, 110as symmetric transformation, 88formulas, 108of cylinder, 85of hypersurface, 418of saddle, 85of sphere, 84, 407of torus, 85zero, 85

shortest distance, 4, 6, 302, 312, 324, 350, 370shoulder, 263

Maple procedure for, 270shrinkable curve, 287

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Index 467

simplify correction, xvismooth, 68soap bubble, 175soap film, 163space evolute, 36spacecurve, 44speed of curve, 3sphere, 84, 398, 401, 407

Gauss curvature offrom forms, 435

sectional curvature of, 418spherical curve, 25spiral of Cornu, 38, 348spring-pendulum, 362state equations, 368stereographic plane

geodesic, 234stereographic projection, 195, 398stereographic sphere, 229structure equation

first, 432second, 434

Sturm-Liouville theorem, 304subs command, 44surface, 68

closed, 296compact, 70convex, 296non-orientable, 80of Delaunay, 133, 137of revolution, 72orientable, 80ruled, 74saddle, 75

surface area, 164, 414surface of revolution, 119surface tension, 161suspension bridge, 11symmetric matrix, 88

tangent developable, 102, 117tangent indicatrix, 64, 294tangent plane, 77–79tangent space, 398tangent vector field, 402tantrix, 64, 294taut string, 363tautochrone, 9, 61

theorem egregium, 124torsion

geodesic, 282of curve, 20, 23, 29total, 93

torsion of curve, 43torus, 73, 120

flat, 229total Gauss curvature, 110, 277, 283total torsion, 93

of curve, 22tractrix, 121transition map, 398, 399transversality condition, 318, 320triangle angle sum, 276trick to remember, 17tubeplot, 45twist, 22twisted cylinder, 105

umbilic, 127, 193umbilic point, 95undetermined time problem, 318undulary, 136unduloid, 136, 142

geodesic, 251mean curvature of, 147parametrization, 144

unit normal, 79, 80, 82unit speed relation, 222unrolling, 236, 237upper half-plane, 228

variation, 313, 414vector field, 81, 82, 337, 402

parallel, 280, 411velocity vector, 1Viviani’s curve, 26, 61

wave equation, 364wedge product of forms, 432Weierstrass condition, 340Weierstrass excess function, 336, 341, 357Weierstrass-Enneper representation, 188, 189, 206

of the bat, 207Weierstrass-Erdmann condition, 344Weingarten map, 84witch of Agnesi, 11, 45

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About the Author

John Oprea was born in Cleveland, Ohio and was educated at Case Western Reserve Universityand at Ohio State University. He received his PhD at OSU in 1982 and, after a post-doc at PurdueUniversity, he began his tenure at Cleveland State in 1985. Oprea is a member of the MathematicalAssociation of America and the American Mathematical Society. He is an Associate Editor of theJournal of Geometry and Symmetry in Physics. In 1996, Oprea was awarded the MAA’s LesterR. Ford award for his Monthly article, “Geometry and the Foucault Pendulum." Besides variousjournal articles on topology and geometry, he is also the author of The Mathematics of Soap Films(AMS Student Math Library, volume 10), Symplectic Manifolds with no Kahler Structure (with A.Tralle, Springer Lecture Notes in Mathematics, volume 1661), Lusternik-Schnirelmann Category(with O. Cornea, G. Lupton and D. Tanre, AMS Mathematical Surveys and Monographs, volume103) and the forthcoming Algebraic Models in Geometry (with Y. Felix and D. Tanre, for OxfordUniversity Press).

469

CLRM/59

AMS / MAA CLASSROOM RESOURCE MATERIALS

| Second Edition | John Oprea

Differential Geometry and Its ApplicationsDifferential geometry has a long, wonderful history. It has found relevance in areas ranging from machinery design to the classification of four-manifolds to the creation of theories of nature's fundamental forces to the study of DNA. This wide range of applications means that differential geometry is not just for mathematics majors. It is also an excellent course of study for students in engineering and the sciences.

This book studies the differential geometry of surfaces with the goal of helping students make the transition from the compartmentalized courses in a standard university curriculum to a type of mathematics that is a unified whole. It mixes together geometry, calculus, linear algebra, differential equations, complex vari-ables, the calculus of variations, and notions from the sciences.

That mix of ideas offers students the opportunity to visualize concepts through the use of computer algebra systems such as Maple. The book emphasizes that this visualization goes hand-in-hand with understanding the mathematics behind the computer construction. Students will not only "see" geodesics on surfaces, they will also observe the effect that an abstract result such as the Clairaut relation can have on geodesics. Furthermore, the book shows how the equations of motion of particles constrained to surfaces are actually types of geodesics.

The book is rich in results and exercises that form a continuous spectrum, from those that depend on calculation to proofs that are quite abstract.

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John Oprea Diﬀerential Geometry and Its Applications · 2020. 5. 19. · P1: RTJ dgmain maab004 May 25, 2007 17:5 Proofs Without Words, Roger B. Nelsen Proofs Without Words II,