Marcel Berger Geometry I Translated from the French by M. Cole and S. Levy A \ fyj Springer Table of contents Volume I Introduction Chapter 0 . N o t a t i o n and background 0.1 Set theory 0 I Algebra 0.3 Metric spaces 0.4 General topology 0.5 Hyperbolic trigonometry 0.6 Lebesgue measure; integration theory xi 1 1 1 2 2 3 3 Chapter 1. G r o u p actions: examples and applications . . . . 1 1 Group actions . . . 1.2 Examples 1.3 Faithful actions 1 4 Transitive actions 1.5 Stabilizers; homogeneous spaces 1.6 Orbits; the class formula 1.7 Tilings and crystallographic groups '. 1.8 Tilings of the two-sphere and regular polyhedra 1.9 Exercises 4 5 5 6 6 7 9 11 22 28 Chapter 2 . Affine spaces 2.1 Definitions 2.2 Examples. Affine frames 2.3 Affine morphisms 2.4 Affine subspaces 2.5 Geometry at last: Thales, Pappus, Desargues 2.6 The fundamental theorem of affine geometry 2.7 Finite-dimensional real affine spaces 32 33 34 37 42 49 52 56 VI Table of c o n t e n t s 2.8 Exercises C h a p t e r 3. Barycenters; the universal space 3.1 The universal space 3.2 Morphisms and the universal space 3.3 Polynomials over an affine space 3.4 Barycenters 3.5 Relationship with affine maps and subspaces 3.6 Barycentric coordinates 3.7 Exercises 64 . 67 68 71 72 75 79 81 83 C h a p t e r 4. Projective spaces 85 4.0 Introduction 86 4.1 Definition and examples 87 4.2 Structure of projective spaces: charts 88 4.3 Structure of projective spaces: topology and algebraic topology 90 4.4 Projective bases 95 4.5 Morphisms 96 4.6 Subspaces 100 4.7 Perspective; aerial photography 102 4.8 The non-commutative case 105 4.9 Exercises 109 C h a p t e r 5. Affine-projective relationship: a p p l i c a t i o n s . . . . 5.0 Introduction 5.1 The projective completion of an affine space . • 5.2 Examples 5.3 Relationship between affine and projective subspaces. Parallelism 5.4 Sending objects to infinity; applications 5.5 Exercises C h a p t e r 6. Projective lines, cross-ratios, homographies 6.1 Definition of cross-ratios 6.2 Computation of cross-ratios 6.3 Action of permutations 6.4 Harmonic division * 6.5 Cross-ratios and duality; applications 6.6 Homographies of a projective line 6.7 Involutions 6.8 Exercises C h a p t e r 7. Complexifications 7.0 Introduction . . . 7.1 Complexification of a real vector space Ill 112 113 114 115 116 119 . . . 122 123 124 126 127 130 134 137 138 142 143 145 Table of contents 7.2 7.3 7.4 7.5 7.6 7.7 Functoriality; complexification of morphisms Complexification of polynomials Subspaces and complexifications Complexification of a projective space Complexification of an affine space Exercises vii 146 146 147 148 148 150 C h a p t e r 8. Euclidean vector spaces 151 8.1 Definition and basic properties 153 8.2 The orthogonal group: first properties and plan of attack . . 157 8.3 The two-dimensional case 161 8.4 Structure of elements of O(E). Generators for O(E) and O+(E) 165 8.5 Simplicity of O(E) 168 8.6 Angles between lines and half-lines 171 8.7 Oriented angles in the plane 173 8.8 Similarities. Isotropic cone and lines 183 8.9 Quaternions and orthogonal groups 186 8.10 Algebraic topology and orthogonal groups 190 8.11 Canonical volume form, mixed product and cross product . . 193 8.12 Exercises 196 C h a p t e r 9. Euclidean affine spaces 9.1 Definitions. Isometries and rigid motions 9.2 Orthogonal subspaces. Distances between subspaces . . . . 9.3 Structure of isometries. Generators of Is(X) and Is + (X) . . 9.4 Polygonal billiards and the structure of plane isometries . . 9.5 Similarities 9.6 Plane similarities 9.7 Distances between several points 9.8 Stabilizers of subsets 9.9 Length of curves 9.10 Distance and differential geometry. The first variation formula 9.11 The Hausdorff metric 9.12 Canonical measure and volume 9.13 Steiner symmetrization 9.14 Exercises , 200 202 204 207 212 219 228 236 245 248 252 255 258 264 268 C h a p t e r 10. Triangles, spheres and circles 10.1 Triangles: definitions and notation 10.2 Classical results 10.3 Formulary 10.4 Inequalities and problems of minimum 10.5 Polygons _ 10.6 Tetrahedra 279 280 282 284 288 293 294 viii Table of contents 10.7 10.8 10.9 10.10 10.11 10.12 10.13 Spheres Inversion Circles in the plane Pencils of circles Classical problems Parataxis: prelude to sections 18.9, 20.5 and 20.7 Exercises 296 304 308 313 •• • 317 320 323 C h a p t e r 11. Convex sets 11.1 Definition and examples 11.2 The dimension of a convex set 11.3 Topology of convex sets 11.4 Convex sets and hyperplanes. Separation theorems 11.5 Supporting hyperplanes; applications 11.6 The frontier of a convex set 11.7 Helly's theorem and applications 11.8 Convex functions 11.9 Exercises 331 332 340 341 349 353 360 363 369 381 Bibliography 385 Index of n o t a t i o n s 401 Index 405 Volume II , C h a p t e r 12. Polytopes; compact convex sets 12.1-Definitions and examples 12.2 Volume of polytopes 12.3 Area of polytopes 12.4 Regular polygons 12.5 Regular polytopes: definition and examples 12.6 Classification of regular polytopes 12.7 Euler's formula 12.8 Cauchy's theorem 12.9 Approximation of compact convex sets by polytopes . . . . 12.10 Area of compact convex sets 12.11 The isoperimetric inequality 12.12 Exercises . 1 3 13 16 21 24 36 43 50 55 59 70 79 C h a p t e r 13. Quadratic forms 13.1 Definitions and examples 13.2 Isotropy, singularity, radicals and degeneracy 86 88 91 Table of contents 13.3 Orthogonality. The non-singular completion of a subspace . . 13.4 Orthogonalization. Classification of real and complex quadratic forms 13.5 Simultaneous orthogonalization of two quadratic forms . . . 13.6 The group of a quadratic form. Generalities 13.7 Witt's theorem and the theorem of Cartan-Dieudonne . . . 13.8 Isometries of Artinian planes 13.9 Exercises ix 94 97 99 101 105 110 114 Chapter 14. Projective quadrics 14.1 Definitions and examples 14.2 Subspaces of PQ{E). Pencils of quadrics 14.3 Topology of real and complex quadrics 14.4 Quadrics in Art4 14.5 Duality with respect to a proper quadric. Polarity 14.6 Tangential quadrics and tangential equation 14.7 The group of a proper quadric 14.8 Exercises 116 117 122 125 129 134 139 142 143 Chapter 15. Affine quadricB 15.1 Definitions and notation 15.2 Reduction of affine quadratic forms 15.3 Classification of real and complex affine quadrics 15.4 The topology of real and complex affine quadrics 15.5 Polarity with respect to a proper affine quadric 15.6 Euclidean affine quadrics 15.7 Exercises 146 147 149 150 157 159 163 166 Chapter 16. Projective conies 170 16.1 Notation and expressions 171 16.2 Good parametrizations, cross-ratios and Pascal's theorem . . 173 16.3 Homographies and group of a conic. Applications 178 16.4 Intersection of two conies. Bezout's theorem 182 16.5 Pencils of conies 192 16.6 The great Poncelet theorem 203 16.7 Affine conies 210 16.8 Exercises 213 Chapter 17. Euclidean conies 17.1 Descartes's principle 17.2 Metrical properties (elementary) 17.3 Metrical properties (spatial) 17.4 Metrical properties (projective) 17.5 Pencils of conies and the cyclic points 17.6 Tangential pencils of conies. Homofocal conies 218 219 221 227 228 232 239 X Table of contents 17.7 Specific properties of ellipses 17.8 Specific properties of hyperbolas 17.9 Exercises 245 248 250 C h a p t e r 18. T h e sphere for its own sake 18.1 Definitions, special dimensions, charts and projections 18.2 The topology of spheres 18.3 The canonical measure on the sphere 18.4 The intrinsic metric on the sphere 18.5 The isometry group of the sphere 18.6 Spherical triangles 18.7 Convex spherical polygons and Cauchy's lemma 18.8 The three-sphere and Clifford parallelism 18.9 Villarceau circles and parataxy 18.10 The Mobius group . . 18.11 Exercises 255 . . . 257 273 276 279 281 284 292 298 304 305 311 C h a p t e r 19. Elliptic and hyperbolic geometry 19.1 Elliptic geometry 19.2 The projective model and the ball model 19.3 The fundamental formula 19.4 The isometry group 19.5 The canonical measure of hyperbolic space 19.6 The Poincare model 19.7 Other models 19.8 Exercises 318 320 326 329 331 334 336 343 345 C h a p t e r 20. The space of spheres 20.1 The space of generalized spheres 20.2 The fundamental quadratic form 20.3 Orthogonality 20.4 Intersection of two spheres 20.5 Pencils 20.6 The circular group 20.7 Polyspheric coordinates 20.8 Exercises 349 350 351 353 354 356 358 359 361 " Bibliography 363 Index of n o t a t i o n s 379 Index 381
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