Projective Coordinates and Compactification in Elliptic, Parabolic and Hyperbolic 2-D Geometry
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13 Circles and Cross Ratio
13 Circles and Cross Ratio Following up Klein’s Erlangen Program, after defining the group of linear transformations, we study the invariant properites of subsets in the Riemann Sphere under this group. Appolonius’ Circle Recall that a line or a circle on the complex plane, their corresondence in the Riemann Sphere are all circles. We would like to prove that any linear transformation f maps a circle or a line into another circle or line. To prove this, we may separate 4 cases: f maps a line to a line; f maps a line to a circle; f maps a circle to a line; and f maps a circle to a circle. Also, we observe that f is a composition of several simple linear transformations of three type: 1 f1(z)= az, f2(z)= a + b and f3(z)= z because az + b g(z) f(z)= = = g(z)φ(h(z)) (56) cz + d h(z) 1 where g(z) = az + b, h(z) = cz + d and φ(z) = z . Then it suffices to separate 8 cases: fj maps a line to a line; fj maps a line to a circle; fj maps a circle to a line; and fj maps a circle to a circle for j =1, 2, 3. To simplify this proof, we want to write a line or a circle by a single equation, although usually equations for a line and for a circle are are quite different. Let us consider the following single equation: z − z1 = k, (57) z − z2 for some z1, z2 ∈ C and 0 <k< ∞. -
Projective Geometry: a Short Introduction
Projective Geometry: A Short Introduction Lecture Notes Edmond Boyer Master MOSIG Introduction to Projective Geometry Contents 1 Introduction 2 1.1 Objective . .2 1.2 Historical Background . .3 1.3 Bibliography . .4 2 Projective Spaces 5 2.1 Definitions . .5 2.2 Properties . .8 2.3 The hyperplane at infinity . 12 3 The projective line 13 3.1 Introduction . 13 3.2 Projective transformation of P1 ................... 14 3.3 The cross-ratio . 14 4 The projective plane 17 4.1 Points and lines . 17 4.2 Line at infinity . 18 4.3 Homographies . 19 4.4 Conics . 20 4.5 Affine transformations . 22 4.6 Euclidean transformations . 22 4.7 Particular transformations . 24 4.8 Transformation hierarchy . 25 Grenoble Universities 1 Master MOSIG Introduction to Projective Geometry Chapter 1 Introduction 1.1 Objective The objective of this course is to give basic notions and intuitions on projective geometry. The interest of projective geometry arises in several visual comput- ing domains, in particular computer vision modelling and computer graphics. It provides a mathematical formalism to describe the geometry of cameras and the associated transformations, hence enabling the design of computational ap- proaches that manipulates 2D projections of 3D objects. In that respect, a fundamental aspect is the fact that objects at infinity can be represented and manipulated with projective geometry and this in contrast to the Euclidean geometry. This allows perspective deformations to be represented as projective transformations. Figure 1.1: Example of perspective deformation or 2D projective transforma- tion. Another argument is that Euclidean geometry is sometimes difficult to use in algorithms, with particular cases arising from non-generic situations (e.g. -
Hyperbolic Geometry
Hyperbolic Geometry David Gu Yau Mathematics Science Center Tsinghua University Computer Science Department Stony Brook University [email protected] September 12, 2020 David Gu (Stony Brook University) Computational Conformal Geometry September 12, 2020 1 / 65 Uniformization Figure: Closed surface uniformization. David Gu (Stony Brook University) Computational Conformal Geometry September 12, 2020 2 / 65 Hyperbolic Structure Fundamental Group Suppose (S; g) is a closed high genus surface g > 1. The fundamental group is π1(S; q), represented as −1 −1 −1 −1 π1(S; q) = a1; b1; a2; b2; ; ag ; bg a1b1a b ag bg a b : h ··· j 1 1 ··· g g i Universal Covering Space universal covering space of S is S~, the projection map is p : S~ S.A ! deck transformation is an automorphism of S~, ' : S~ S~, p ' = '. All the deck transformations form the Deck transformation! group◦ DeckS~. ' Deck(S~), choose a pointq ~ S~, andγ ~ S~ connectsq ~ and '(~q). The 2 2 ⊂ projection γ = p(~γ) is a loop on S, then we obtain an isomorphism: Deck(S~) π1(S; q);' [γ] ! 7! David Gu (Stony Brook University) Computational Conformal Geometry September 12, 2020 3 / 65 Hyperbolic Structure Uniformization The uniformization metric is ¯g = e2ug, such that the K¯ 1 everywhere. ≡ − 2 Then (S~; ¯g) can be isometrically embedded on the hyperbolic plane H . The On the hyperbolic plane, all the Deck transformations are isometric transformations, Deck(S~) becomes the so-called Fuchsian group, −1 −1 −1 −1 Fuchs(S) = α1; β1; α2; β2; ; αg ; βg α1β1α β αg βg α β : h ··· j 1 1 ··· g g i The Fuchsian group generators are global conformal invariants, and form the coordinates in Teichm¨ullerspace. -
The Riemann Mapping Theorem Christopher J. Bishop
The Riemann Mapping Theorem Christopher J. Bishop C.J. Bishop, Mathematics Department, SUNY at Stony Brook, Stony Brook, NY 11794-3651 E-mail address: [email protected] 1991 Mathematics Subject Classification. Primary: 30C35, Secondary: 30C85, 30C62 Key words and phrases. numerical conformal mappings, Schwarz-Christoffel formula, hyperbolic 3-manifolds, Sullivan’s theorem, convex hulls, quasiconformal mappings, quasisymmetric mappings, medial axis, CRDT algorithm The author is partially supported by NSF Grant DMS 04-05578. Abstract. These are informal notes based on lectures I am giving in MAT 626 (Topics in Complex Analysis: the Riemann mapping theorem) during Fall 2008 at Stony Brook. We will start with brief introduction to conformal mapping focusing on the Schwarz-Christoffel formula and how to compute the unknown parameters. In later chapters we will fill in some of the details of results and proofs in geometric function theory and survey various numerical methods for computing conformal maps, including a method of my own using ideas from hyperbolic and computational geometry. Contents Chapter 1. Introduction to conformal mapping 1 1. Conformal and holomorphic maps 1 2. M¨obius transformations 16 3. The Schwarz-Christoffel Formula 20 4. Crowding 27 5. Power series of Schwarz-Christoffel maps 29 6. Harmonic measure and Brownian motion 39 7. The quasiconformal distance between polygons 48 8. Schwarz-Christoffel iterations and Davis’s method 56 Chapter 2. The Riemann mapping theorem 67 1. The hyperbolic metric 67 2. Schwarz’s lemma 69 3. The Poisson integral formula 71 4. A proof of Riemann’s theorem 73 5. Koebe’s method 74 6. -
Elliot Glazer Topological Manifolds
Manifolds and complex structure Elliot Glazer Topological Manifolds • An n-dimensional manifold M is a space that is locally Euclidean, i.e. near any point p of M, we can define points of M by n real coordinates • Curves are 1-dimensional manifolds (1-manifolds) • Surfaces are 2-manifolds • A chart (U, f) is an open subset U of M and a homeomorphism f from U to some open subset of R • An atlas is a set of charts that cover M Some important 2-manifolds A very important 2-manifold Smooth manifolds • We want manifolds to have more than just topological structure • There should be compatibility between intersecting charts • For intersecting charts (U, f), (V, g), we define a natural transition map from one set of coordinates to the other • A manifold is C^k if it has an atlas consisting of charts with C^k transition maps • Of particular importance are smooth maps • Whitney embedding theorem: smooth m-manifolds can be smoothly embedded into R^{2m} Complex manifolds • An n-dimensional complex manifold is a space such that, near any point p, each point can be defined by n complex coordinates, and the transition maps are holomorphic (complex structure) • A manifold with n complex dimensions is also a real manifold with 2n dimensions, but with a complex structure • Complex 1-manifolds are called Riemann surfaces (surfaces because they have 2 real dimensions) • Holoorphi futios are rigid sie they are defied y accumulating sets • Whitney embedding theorem does not extend to complex manifolds Classifying Riemann surfaces • 3 basic Riemann surfaces • The complex plane • The unit disk (distinct from plane by Liouville theorem) • Riemann sphere (aka the extended complex plane), uses two charts • Uniformization theorem: simply connected Riemann surfaces are conformally equivalent to one of these three . -
The Classical Groups and Domains 1. the Disk, Upper Half-Plane, SL 2(R
(June 8, 2018) The Classical Groups and Domains Paul Garrett [email protected] http:=/www.math.umn.edu/egarrett/ The complex unit disk D = fz 2 C : jzj < 1g has four families of generalizations to bounded open subsets in Cn with groups acting transitively upon them. Such domains, defined more precisely below, are bounded symmetric domains. First, we recall some standard facts about the unit disk, the upper half-plane, the ambient complex projective line, and corresponding groups acting by linear fractional (M¨obius)transformations. Happily, many of the higher- dimensional bounded symmetric domains behave in a manner that is a simple extension of this simplest case. 1. The disk, upper half-plane, SL2(R), and U(1; 1) 2. Classical groups over C and over R 3. The four families of self-adjoint cones 4. The four families of classical domains 5. Harish-Chandra's and Borel's realization of domains 1. The disk, upper half-plane, SL2(R), and U(1; 1) The group a b GL ( ) = f : a; b; c; d 2 ; ad − bc 6= 0g 2 C c d C acts on the extended complex plane C [ 1 by linear fractional transformations a b az + b (z) = c d cz + d with the traditional natural convention about arithmetic with 1. But we can be more precise, in a form helpful for higher-dimensional cases: introduce homogeneous coordinates for the complex projective line P1, by defining P1 to be a set of cosets u 1 = f : not both u; v are 0g= × = 2 − f0g = × P v C C C where C× acts by scalar multiplication. -
Lie Group and Geometry on the Lie Group SL2(R)
INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR Lie group and Geometry on the Lie Group SL2(R) PROJECT REPORT – SEMESTER IV MOUSUMI MALICK 2-YEARS MSc(2011-2012) Guided by –Prof.DEBAPRIYA BISWAS Lie group and Geometry on the Lie Group SL2(R) CERTIFICATE This is to certify that the project entitled “Lie group and Geometry on the Lie group SL2(R)” being submitted by Mousumi Malick Roll no.-10MA40017, Department of Mathematics is a survey of some beautiful results in Lie groups and its geometry and this has been carried out under my supervision. Dr. Debapriya Biswas Department of Mathematics Date- Indian Institute of Technology Khargpur 1 Lie group and Geometry on the Lie Group SL2(R) ACKNOWLEDGEMENT I wish to express my gratitude to Dr. Debapriya Biswas for her help and guidance in preparing this project. Thanks are also due to the other professor of this department for their constant encouragement. Date- place-IIT Kharagpur Mousumi Malick 2 Lie group and Geometry on the Lie Group SL2(R) CONTENTS 1.Introduction ................................................................................................... 4 2.Definition of general linear group: ............................................................... 5 3.Definition of a general Lie group:................................................................... 5 4.Definition of group action: ............................................................................. 5 5. Definition of orbit under a group action: ...................................................... 5 6.1.The general linear -
Math 372: Fall 2015: Solutions to Homework
Math 372: Fall 2015: Solutions to Homework Steven Miller December 7, 2015 Abstract Below are detailed solutions to the homeworkproblemsfrom Math 372 Complex Analysis (Williams College, Fall 2015, Professor Steven J. Miller, [email protected]). The course homepage is http://www.williams.edu/Mathematics/sjmiller/public_html/372Fa15 and the textbook is Complex Analysis by Stein and Shakarchi (ISBN13: 978-0-691-11385-2). Note to students: it’s nice to include the statement of the problems, but I leave that up to you. I am only skimming the solutions. I will occasionally add some comments or mention alternate solutions. If you find an error in these notes, let me know for extra credit. Contents 1 Math 372: Homework #1: Yuzhong (Jeff) Meng and Liyang Zhang (2010) 3 1.1 Problems for HW#1: Due September 21, 2015 . ................ 3 1.2 SolutionsforHW#1: ............................... ........... 3 2 Math 372: Homework #2: Solutions by Nick Arnosti and Thomas Crawford (2010) 8 3 Math 372: Homework #3: Carlos Dominguez, Carson Eisenach, David Gold 12 4 Math 372: Homework #4: Due Friday, October 12, 2015: Pham, Jensen, Kolo˘glu 16 4.1 Chapter3,Exercise1 .............................. ............ 16 4.2 Chapter3,Exercise2 .............................. ............ 17 4.3 Chapter3,Exercise5 .............................. ............ 19 4.4 Chapter3Exercise15d ..... ..... ...... ..... ...... .. ............ 22 4.5 Chapter3Exercise17a ..... ..... ...... ..... ...... .. ............ 22 4.6 AdditionalProblem1 .............................. ............ 22 5 Math 372: Homework #5: Due Monday October 26: Pegado, Vu 24 6 Math 372: Homework #6: Kung, Lin, Waters 34 7 Math 372: Homework #7: Due Monday, November 9: Thompson, Schrock, Tosteson 42 7.1 Problems. ....................................... ......... 46 1 8 Math 372: Homework #8: Thompson, Schrock, Tosteson 47 8.1 Problems. -
Math 311: Complex Analysis — Automorphism Groups Lecture
MATH 311: COMPLEX ANALYSIS | AUTOMORPHISM GROUPS LECTURE 1. Introduction Rather than study individual examples of conformal mappings one at a time, we now want to study families of conformal mappings. Ensembles of conformal mappings naturally carry group structures. 2. Automorphisms of the Plane The automorphism group of the complex plane is Aut(C) = fanalytic bijections f : C −! Cg: Any automorphism of the plane must be conformal, for if f 0(z) = 0 for some z then f takes the value f(z) with multiplicity n > 1, and so by the Local Mapping Theorem it is n-to-1 near z, impossible since f is an automorphism. By a problem on the midterm, we know the form of such automorphisms: they are f(z) = az + b; a; b 2 C; a 6= 0: This description of such functions one at a time loses track of the group structure. If f(z) = az + b and g(z) = a0z + b0 then (f ◦ g)(z) = aa0z + (ab0 + b); f −1(z) = a−1z − a−1b: But these formulas are not very illuminating. For a better picture of the automor- phism group, represent each automorphism by a 2-by-2 complex matrix, a b (1) f(z) = ax + b ! : 0 1 Then the matrix calculations a b a0 b0 aa0 ab0 + b = ; 0 1 0 1 0 1 −1 a b a−1 −a−1b = 0 1 0 1 naturally encode the formulas for composing and inverting automorphisms of the plane. With this in mind, define the parabolic group of 2-by-2 complex matrices, a b P = : a; b 2 ; a 6= 0 : 0 1 C Then the correspondence (1) is a natural group isomorphism, Aut(C) =∼ P: 1 2 MATH 311: COMPLEX ANALYSIS | AUTOMORPHISM GROUPS LECTURE Two subgroups of the parabolic subgroup are its Levi component a 0 M = : a 2 ; a 6= 0 ; 0 1 C describing the dilations f(z) = ax, and its unipotent radical 1 b N = : b 2 ; 0 1 C describing the translations f(z) = z + b. -
WA 4: Solutions
WA 4: Solutions Problem 1. Show that for any z such that |Im z| ≥ δ, with some δ > 0, 1 1 1 2 1 2 |tan z| ≤ 1 + , |cot z| ≤ 1 + . sinh2 δ sinh2 δ Solution. We have | sin z|2 sin2 x + sinh2 y 1 + sinh2 y 1 | tan z|2 = = ≤ = 1 + . | cos z|2 cos2 x + sinh2 y sinh2 y sinh2 y Since sinh2 y is an even function, it suffices to show that sinh2 y ≥ sinh2 δ when y ≥ δ. This follows from the fact that (sinh2 y)0 = 2 sinh y cosh y > 0 when y > 0, and thus sinh2 y is increasing for positive y. Therefore, 1 1 | tan z|2 ≤ 1 + ≤ 1 + . sinh2 y sinh2 δ The second inequality is proved analogously. −1 −1 π Problem 2. (a) Show that the identity sin z +cos z = 2 holds for all z ∈ C and the principal values of sin−1 z and cos−1 z. (b) Is this identity true for sin−1 z and cos−1 z considered as multivalued func- tions? Solution. (a) For z ∈ (−1, 1), the principal values of sin−1 z and cos−1 z coincide with the values of these functions as defined in trigonometry. Indeed, for x ∈ (−1, 1) we have p sin−1 x = −iLog[ix + (1 − x2)1/2] = −iLog[ix + 1 − x2] p p = −i(ln 1 + iArg(ix + 1 − x2)) = Arg(ix + 1 − x2), √ 2 π π −1 and since Arg(ix+ 1 − x ) ∈ (− 2 , 2 ), the statement for sin x follows. Similarly, p cos−1 x = −iLog[x + i(1 − x2)1/2] = −iLog[x + i 1 − x2] p p = −i(ln 1 + iArg(x + i 1 − x2)) = Arg(x + i 1 − x2), √ and since Arg(x + i 1 − x2) ∈ (0, π), the statement for cos−1 x follows. -
Concept of Symmetry in Closure Spaces As a Tool for Naturalization of Information
数理解析研究所講究録 29 第2008巻 2016年 29-36 Concept of Symmetry in Closure Spaces as a Tool for Naturalization of Information Marcin J. Schroeder Akita Intemational University, Akita, Japan [email protected] 1. Introduction To naturalize information, i.e. to make the concept of information a subject of the study of reality independent from subjective judgment requires methodology of its study consistent with that of natural sciences. Section 2 of this paper presents historical argumentation for the fundamental role of the study of symmetries in natural sciences and in the consequence of the claim that naturalization of information requires a methodology of the study of its symmetries. Section 3 includes mathematical preliminaries necessary for the study of symmetries in an arbitrary closure space carried out in Section 4. Section 5 demonstrates the connection of the concept of information and closure spaces. This sets foundations for further studies of symmetries of information. 2. Symmetry and Scientific Methodology Typical account of the origins of the modern theory of symmetry starts with Erlangen Program of Felix Klein published in 1872. [1] This publication was of immense influence. Klein proposed a new paradigm of mathematical study focusing not on its objects, but on their transformations. His mathematical theory ofgeometric symmetry was understood as an investigation of the invariance with respect to transformations of the geometric space (two‐dimensional plane or higher dimensional space). Klein used this very general concept of geometric symmetry for the purpose of a classification of different types of geometries (Euclidean and non‐Euclidean). The fundamental conceptual framework of Kleins Program (as it became function as a paradigm) was based on the scheme of(1) space as a collection of points \rightarrow (2) algebraic structure (group) of its transformations \rightarrow (3) invariants of the transformations, i.e. -
GEOMETRIC PROPERTIES of LINEAR FRACTIONAL MAPS 1. Introduction Linear
GEOMETRIC PROPERTIES OF LINEAR FRACTIONAL MAPS C. C. COWEN, D. E. CROSBY, T. L. HORINE, R. M. ORTIZ ALBINO, A. E. RICHMAN, Y. C. YEOW, B. S. ZERBE Abstract. Linear fractional maps in several variables generalize classical lin- ear fractional maps in the complex plane. In this paper, we describe some geometric properties of this class of maps, especially for those linear fractional maps that carry the open unit ball into itself. For those linear fractional maps that take the unit ball into itself, we determine the minimal set containing the open unit ball on which the map is an automorphism which provides a means of classifying these maps. Finally, when ϕ is a linear fractional map, we describe the linear fractional solutions, f, of Schroeder’s functional equation f ϕ = Lf. ◦ 1. Introduction Linear fractional maps are basic in the theory of analytic functions in the complex plane and in the study of those functions that map the unit disk into itself. We would expect the analogues of these maps in higher dimensions to play a similar role in the study of analytic functions in CN and in the study of those functions that carry the unit ball in CN into itself. In particular, the linear fractional maps of the unit disk can be classified, most analytic maps of the unit disk into itself inherit one of these classifications, and the classification informs the understanding of the map, for example, of its iteration or the properties of the composition operators with it as symbol. Based on the classification, the solutions of Schroeder’s functional equation can be determined and the more general maps of the disk inherit solutions of Schroeder’s functional equation.