DOCSLIB.ORG

Hyperbolic Geometry on a Hyperboloid, the American Mathematical Monthly, 100:5, 442-455, DOI: 10.1080/00029890.1993.11990430

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Hyperbolic Geometry on a Hyperboloid, the American Mathematical Monthly, 100:5, 442-455, DOI: 10.1080/00029890.1993.11990430 William F. Reynolds (1993) Hyperbolic Geometry on a Hyperboloid, The American Mathematical Monthly, 100:5, 442-455, DOI: 10.1080/00029890.1993.11990430. (c) 1993 The American Mathematical Monthly 1985 Mathematics Subject Classification 51 M 10 HYPERBOLIC GEOMETRY ON A HYPERBOLOID William F. Reynolds Department of Mathematics, Tufts University, Medford, MA 02155 E-mail: [email protected] 1. Introduction. Hardly anyone would maintain that it is better to begin to learn geography from flat maps than from a globe. But almost all introductions to hyperbolic non-Euclidean geometry, except [6], present plane models, such as the projective and conformal disk models, without even mentioning that there exists a model that has the same relation to plane models that a globe has to flat maps. This model, which is on one sheet of a hyperboloid of two sheets in Minkowski 3-space and which I shall call H2, is over a hundred years old; Killing and Poincar´eboth described it in the 1880's (see Section 14). It is used by differential geometers [29, p. 4] and physicists (see [21, pp. 724-725] and [23, p. 113]). Nevertheless it is not nearly so well known as it should be, probably because, like a globe, it requires three dimensions. The main advantages of this model are its naturalness and its symmetry. Being embeddable (distance function and all) in flat space-time, it is close to our picture of physical reality, and all its points are treated alike in this embedding. Once the strangeness of the Minkowski metric is accepted, it has the familiar geometry of a sphere in Euclidean 3-space E3 as a guide to definitions and arguments. For example, the lines of H2 are its non-empty intersections with the planes through the origin of the Minkowski space M 3. The length of a line segment of H2 is defined by analogy with arc length in 1 calculus; this leads naturally to the hyperbolic functions. As in the spherical case, every isometry of H2 can be extended to a linear transformation of M 3, so that straightforward calculations with matrices can be used to prove theorems and develop the trigonometry of H2. The circles, horocycles, and equidistant curves have a beautiful interpretation: they are precisely the nontrivial intersections of H2 with planes of M 3 that do not pass through the origin. An area function for H2 can be constructed from the volume function on M 3. The aim of this article is to show that H2 can be used to give an intro- duction to hyperbolic geometry to undergraduates who know a little about linear transformations and groups, a bit of special relativity being helpful for motivation. The mixing of rigor and intuition is similar to what is common in calculus courses. The treatment is not axiomatic, since there is no in- trinsic reason to stress axiomatics in hyperbolic geometry any more than in, say, spherical trigonometry. For historical and philosophical reasons, how- ever, many treatments are based on axioms; therefore I shall refer to Moise's axioms [22] at the points where they can be verified from my approach. (I have chosen these axioms since, incorporating real-valued functions for dis- tance and measure of angles, they are closer to my analytic approach than Hilbert's [6], [8]; the latter, being weaker, would be easier to verify.) I will treat them not as axioms, but just as properties of H2. I have included something about the hyperbolic analogues of map projections and about the history of the model. A special feature of my approach, which distinguishes it from Faber's [6, Chapter VII], is its extensive use of orthogonal transformations of M 3, the analogues of the rigid motions of E3 that fix the origin, to move subsets of H2 to convenient positions. 2 I want to thank Alan H. Durfee and Mark E. Kidwell for encouraging me to write up this article. It originated in an undergraduate course at Tufts, and I did some of the work while visiting at Harvard. 2. Minkowski 3-space. By Minkowski 3-space M 3 I mean a 3- dimensional real vector space together with a real-valued function q on it such that X2 − 2 2 2 q( xi Ui) = x0 + x1 + x2 (2:1) i=0 for some basis U = (U0;U1;U2) of the space. More briefly, X X 2 q( xi Ui) = ei xi (2:2) where e0 = −1; e1 = e2 = 1; then q(Ui) = ei. We can define Minkowski n-space M n similarly for any n ≥ 2 (with one minus sign). To relate the 2 3 cases n = 2 and 3, take M to be the subspace of M with basis (U0;U1) together with the restriction of q. If we replace (2.2) by X X 2 qE( xi Ui) = xi ; (2:3) 3 we get ordinary Euclidean 3-space E , with qE giving the square of length. I shall constantly use analogies from E3 to study M 3; watch for such analogies when they are not mentioned. P P 3 For X = xi Ui and Y = yi Ui in M , define 1 p(X; Y ) = [q(X + Y ) − q(X) − q(Y )]: (2:4) 2 (This is the bilinear form or pairing corresponding to the quadratic form q.) Then q(X) = p(X; X) (2:5) 3 and X p(X; Y ) = ei xi yi = −x0 y0 + x1 y1 + x2 y2; (2:6) in particular p(Ui;Uj) equals ei if i = j and 0 otherwise. Observe that p is 3 analogous to the ordinary dot product on E given by pE(X; Y ) = X · Y = P xi yi and U to an orthonormal basis. 3. The hyperboloidal model. In M 3, let H2 be the set of all vectors P X = xi Ui for which q(X) = −1; (3:1) x0 > 0: (3:2) These two conditions can be expressed by the equation q 2 2 x0 = 1 + x1 + x2: (3:3) (3.1) describes a hyperboloid of two sheets and (3.2) picks out one sheet. We can define Hn ⊂ M n+1 similarly for arbitrary n ; for n = 1; take H1 = H2 \ M 2 with M 2 as in the previous section. Fig. 1 may help in thinking about H2. In flat space-time, identified with M 4, H2 appears to each observer as a circle whose radius is increasing at slightly greater (!) than the speed of light. We now begin to construct a model of hyperbolic geometry whose points, or H-points, are the elements of H2. We think of them as either points or vectors when considered in M 3, and as points when considered in H2. The lines or H-lines of the model are defined to be all the nonempty intersections of H2 with 2-dimensional subspaces of M 3; for example, H1 is an H-line. (The prefix \H-" will always be optional.) Incidence of H-points and H-lines and betweenness for points of an H- line are defined in the natural way. Each pair of distinct H-points A and B 4 ! lies on a unique H-line AB; namely the intersection of H2 with the plane OAB of M 3, O being the origin of M 3. The definitions of the H-segment −−! AB and the H-ray AB are straightforward. We can now check the plane incidence axioms of [22, pp. 37-38]. It should be clear that the complement in H2 of each H-line consists of two H-half planes, called its sides. This statement can be made precise as the plane-separation axiom or Pasch's axiom [22, p. 62]. Two distinct H-lines have one or zero H-points in common according as the line of intersection of the planes of M 3 in which they lie intersects H2 or not; this gives the hyperbolic parallel axiom [22, p. 114]. The Archimedean axiom and the axiom of completeness (or continuity) [22, pp. 256 and 265] are also clear. 3 In the analogous situation in E , the equation qE(X) = 1 (cf. (2.3)) defines a sphere S2; this leads to a model of spherical (or double elliptic) geometry whose lines or S-lines are the great circles of S2. 4. Distance. For distinct H-points A and B we want to define the H- distance d(A; B), also called the H-length of AB. The natural way to adapt the usual definition of arc length is to partition AB, as a curve in M 3, by suitable points P0 = A; P1; ··· ;Pm = B and to define Xm q d(A; B) = lim q(Pj − Pj−1); (4:1) m!1 j=1 but first we must check that q(Pj − Pj−1), analogous to a squared length, is positive. The equation q(X) = 0 describes a cone in M 3 and the vectors with q(X) > 0 are the points outside this cone (cf. the spacelike vectors of special relativity). By the mean value theorem there is a point of Pj−1Pj at which 3 the tangent vectors in M to this H-segment are parallel to Pj − Pj−1, so it 5 suffices to show that all vectors V =6 O tangent to H2 have q(V ) > 0. We can show this by turning to advantage a limitation of our intuition. (For a different approach, see Section 7.) Since we are used to Euclidean space, any attempt to visualize M 3 pictorially as in Fig. 1 identifies it with E3, i.e. imposes a Euclidean quadratic form on it.
Load more

Recommended publications
  • Models of 2-Dimensional Hyperbolic Space and Relations Among Them; Hyperbolic Length, Lines, and Distances
    Models of 2-dimensional hyperbolic space and relations among them; Hyperbolic length, lines, and distances Cheng Ka Long, Hui Kam Tong 1155109623, 1155109049 Course Teacher: Prof. Yi-Jen LEE Department of Mathematics, The Chinese University of Hong Kong MATH4900E Presentation 2, 5th October 2020 Outline Upper half-plane Model (Cheng) A Model for the Hyperbolic Plane The Riemann Sphere C Poincar´eDisc Model D (Hui) Basic properties of Poincar´eDisc Model Relation between D and other models Length and distance in the upper half-plane model (Cheng) Path integrals Distance in hyperbolic geometry Measurements in the Poincar´eDisc Model (Hui) M¨obiustransformations of D Hyperbolic length and distance in D Conclusion Boundary, Length, Orientation-preserving isometries, Geodesics and Angles Reference Upper half-plane model H Introduction to Upper half-plane model - continued Hyperbolic geometry Five Postulates of Hyperbolic geometry: 1. A straight line segment can be drawn joining any two points. 2. Any straight line segment can be extended indefinitely in a straight line. 3. A circle may be described with any given point as its center and any distance as its radius. 4. All right angles are congruent. 5. For any given line R and point P not on R, in the plane containing both line R and point P there are at least two distinct lines through P that do not intersect R. Some interesting facts about hyperbolic geometry 1. Rectangles don't exist in hyperbolic geometry. 2. In hyperbolic geometry, all triangles have angle sum < π 3. In hyperbolic geometry if two triangles are similar, they are congruent.
    [Show full text]
  • 1 Euclidean Geometry
    Dr. Anna Felikson, Durham University Geometry, 26.02.2015 Outline 1 Euclidean geometry 1.1 Isometry group of Euclidean plane, Isom(E2). A distance on a space X is a function d : X × X ! R,(A; B) 7! d(A; B) for A; B 2 X satisfying 1. d(A; B) ≥ 0 (d(A; B) = 0 , A = B); 2. d(A; B) = d(B; A); 3. d(A; C) ≤ d(A; B) + d(B; C) (triangle inequality). We will use two models of Euclidean plane: p 2 2 a Cartesian plane: f(x; y) j x; y 2 Rg with the distance d(A1;A2) = (x1 − x2) + (y1 − y2) ; a Gaussian plane: fz j z 2 Cg, with the distance d(u; v) = ju − vj. Definition 1.1. A Euclidean isometry is a distance-preserving transformation of E2, i.e. a map f : E2 ! E2 satisfying d(f(A); f(B)) = d(A; B). Corollary 1.2. (a) Every isometry of E2 is a one-to-one map. (b) The composition of any two isometries is an isometry. (c) Isometries of E2 form a group (denoted Isom(E2)). Example 1.3: Translation, rotation, reflection in a line are isometries. Theorem 1.4. Let ABC and A0B0C0 be two congruent triangles. Then there exists a unique isometry sending A to A0, B to B0 and C to C0. Corollary 1.5. Every isometry of E2 is a composition of at most 3 reflections. (In particular, the group Isom(E2) is generated by reflections). Remark: the way to write an isometry as a composition of reflection is not unique.
    [Show full text]
  • Hyperbolic Geometry
    Flavors of Geometry MSRI Publications Volume 31,1997 Hyperbolic Geometry JAMES W. CANNON, WILLIAM J. FLOYD, RICHARD KENYON, AND WALTER R. PARRY Contents 1. Introduction 59 2. The Origins of Hyperbolic Geometry 60 3. Why Call it Hyperbolic Geometry? 63 4. Understanding the One-Dimensional Case 65 5. Generalizing to Higher Dimensions 67 6. Rudiments of Riemannian Geometry 68 7. Five Models of Hyperbolic Space 69 8. Stereographic Projection 72 9. Geodesics 77 10. Isometries and Distances in the Hyperboloid Model 80 11. The Space at Infinity 84 12. The Geometric Classification of Isometries 84 13. Curious Facts about Hyperbolic Space 86 14. The Sixth Model 95 15. Why Study Hyperbolic Geometry? 98 16. When Does a Manifold Have a Hyperbolic Structure? 103 17. How to Get Analytic Coordinates at Infinity? 106 References 108 Index 110 1. Introduction Hyperbolic geometry was created in the first half of the nineteenth century in the midst of attempts to understand Euclid’s axiomatic basis for geometry. It is one type of non-Euclidean geometry, that is, a geometry that discards one of Euclid’s axioms. Einstein and Minkowski found in non-Euclidean geometry a This work was supported in part by The Geometry Center, University of Minnesota, an STC funded by NSF, DOE, and Minnesota Technology, Inc., by the Mathematical Sciences Research Institute, and by NSF research grants. 59 60 J. W. CANNON, W. J. FLOYD, R. KENYON, AND W. R. PARRY geometric basis for the understanding of physical time and space. In the early part of the twentieth century every serious student of mathematics and physics studied non-Euclidean geometry.
    [Show full text]
  • A. Geodesics in the Hyperboloid Model B. Proof of Theorem 1 C
    Hyperbolic Entailment Cones A. Geodesics in the Hyperboloid Model One can sanity check that indeed the formula from theorem n n 1 satisfies the conditions: The hyperboloid model is (H ; h·; ·i1), where H := fx 2 n;1 R : hx; xi1 = −1; x0 > 0g. The hyperboloid model can be viewed from the extrinsically as embedded in the pseudo- • d (γ(0); γ(t)) = t; 8t 2 [0; 1] n;1 D Riemannian manifold Minkowski space (R ; h·; ·i1) and n;1 inducing its metric. The Minkowski metric tensor gR of signature (n; 1) has the components • γ(0) = x 2−1 0 ::: 03 n;1 6 0 1 ::: 07 gR = 6 7 4 0 0 ::: 05 • γ_ (0) = v 0 0 ::: 1 The associated inner-product is hx; yi1 := −x0y0 + • lim γ(t) := γ(1) 2 @ n Pn t!1 D i=1 xiyi. Note that the hyperboloid model is a Rieman- nian manifold because the quadratic form associated with gH is positive definite. C. Proof of Corollary 1.1 n Proof. Denote u = p 1 v. Using the notations from In the extrinsic view, the tangent space at H can be de- gD(v;v) n n;1 x scribed as TxH = fv 2 R : hv; xi1 = 0g. See Robbin p Thm.1, one has expx(v) = γx;u( gxD(v; v)). Using Eq.3 & Salamon(2011); Parkkonen(2013). and6, one derives the result. Geodesics of Hn are given by the following theorem (Eq D. Proof of Corollary 1.2 (6.4.10) in Robbin & Salamon(2011)): n n Proof.
    [Show full text]
  • Unifying the Hyperbolic and Spherical 2-Body Problem with Biquaternions
    Unifying the Hyperbolic and Spherical 2-Body Problem with Biquaternions Philip Arathoon December 2020 Abstract The 2-body problem on the sphere and hyperbolic space are both real forms of holo- morphic Hamiltonian systems defined on the complex sphere. This admits a natural description in terms of biquaternions and allows us to address questions concerning the hyperbolic system by complexifying it and treating it as the complexification of a spherical system. In this way, results for the 2-body problem on the sphere are readily translated to the hyperbolic case. For instance, we implement this idea to completely classify the relative equilibria for the 2-body problem on hyperbolic 3-space for a strictly attractive potential. Background and outline The case of the 2-body problem on the 3-sphere has recently been considered by the author in [1]. This treatment takes advantage of the fact that S3 is a group and that the action of SO(4) on S3 is generated by the left and right multiplication of S3 on itself. This allows for a reduction in stages, first reducing by the left multiplication, and then reducing an intermediate space by the residual right-action. An advantage of this reduction-by-stages is that it allows for a fairly straightforward derivation of the relative equilibria solutions: the relative equilibria may first be classified in the intermediate reduced space and then reconstructed on the original phase space. For the 2-body problem on hyperbolic space the same idea does not apply. Hyperbolic 3-space H3 cannot be endowed with an isometric group structure and the symmetry group SO(1; 3) does not arise as a direct product of two groups.
    [Show full text]
  • Self-Organization on Riemannian Manifolds
    JOURNAL OF GEOMETRIC MECHANICS doi:10.3934/jgm.2019020 c American Institute of Mathematical Sciences Volume 11, Number 3, September 2019 pp. 397{426 SELF-ORGANIZATION ON RIEMANNIAN MANIFOLDS Razvan C. Fetecau∗ and Beril Zhang Department of Mathematics Simon Fraser University Burnaby, BC V5A 1S6, Canada (Communicated by Darryl D. Holm and Manuel de Le´on) Abstract. We consider an aggregation model that consists of an active trans- port equation for the macroscopic population density, where the velocity has a nonlocal functional dependence on the density, modelled via an interaction potential. We set up the model on general Riemannian manifolds and provide a framework for constructing interaction potentials which lead to equilibria that are constant on their supports. We consider such potentials for two specific cases (the two-dimensional sphere and the two-dimensional hyperbolic space) and investigate analytically and numerically the long-time behaviour and equi- librium solutions of the aggregation model on these manifolds. Equilibria ob- tained numerically with other interaction potentials and an application of the model to aggregation on the rotation group SO(3) are also presented. 1. Introduction. The literature on self-collective behaviour of autonomous agents (e.g., biological organisms, robots, nanoparticles, etc) has been growing very fast recently. One of the main interests of such research is to understand how swarm- ing and flocking behaviours emerge in groups with no leader or external coordina- tion. Such behaviours occur for instance in natural swarms, e.g., flocks of birds or schools of fish [14, 25]. Also, swarming and flocking of artificial mobile agents (e.g., robots) in the absence of a centralized coordination mechanism is of major interest in engineering [37, 38].
    [Show full text]
  • Harmonic Analysis on the Proper Velocity Gyrogroup ∗ 1 Introduction
    Harmonic Analysis on the Proper Velocity gyrogroup ∗ Milton Ferreira School of Technology and Management, Polytechnic Institute of Leiria, Portugal 2411-901 Leiria, Portugal. Email: [email protected] and Center for Research and Development in Mathematics and Applications (CIDMA), University of Aveiro, 3810-193 Aveiro, Portugal. Email [email protected] Abstract In this paper we study harmonic analysis on the Proper Velocity (PV) gyrogroup using the gyrolanguage of analytic hyperbolic geometry. PV addition is the relativis- tic addition of proper velocities in special relativity and it is related with the hy- perboloid model of hyperbolic geometry. The generalized harmonic analysis depends on a complex parameter z and on the radius t of the hyperboloid and comprises the study of the generalized translation operator, the associated convolution operator, the generalized Laplace-Beltrami operator and its eigenfunctions, the generalized Poisson transform and its inverse, the generalized Helgason-Fourier transform, its inverse and Plancherel's Theorem. In the limit of large t; t ! +1; the generalized harmonic analysis on the hyperboloid tends to the standard Euclidean harmonic analysis on Rn; thus unifying hyperbolic and Euclidean harmonic analysis. Keywords: PV gyrogroup, Laplace Beltrami operator, Eigenfunctions, Generalized Helgason- Fourier transform, Plancherel's Theorem. 1 Introduction Harmonic analysis is the branch of mathematics that studies the representation of functions or signals as the superposition of basic waves called harmonics. It investigates and gen- eralizes the notions of Fourier series and Fourier transforms. In the past two centuries, it has become a vast subject with applications in diverse areas as signal processing, quantum mechanics, and neuroscience (see [18] for an overview).
    [Show full text]
  • Numerically Accurate Hyperbolic Embeddings Using Tiling-Based Models
    Numerically Accurate Hyperbolic Embeddings Using Tiling-Based Models Tao Yu Christopher De Sa Department of Computer Science Department of Computer Science Cornell University Cornell University Ithaca, NY, USA Ithaca, NY, USA [email protected] [email protected] Abstract Hyperbolic embeddings achieve excellent performance when embedding hierar- chical data structures like synonym or type hierarchies, but they can be limited by numerical error when ordinary floating-point numbers are used to represent points in hyperbolic space. Standard models such as the Poincaré disk and the Lorentz model have unbounded numerical error as points get far from the origin. To address this, we propose a new model which uses an integer-based tiling to represent any point in hyperbolic space with provably bounded numerical error. This allows us to learn high-precision embeddings without using BigFloats, and enables us to store the resulting embeddings with fewer bits. We evaluate our tiling-based model empirically, and show that it can both compress hyperbolic embeddings (down to 2% of a Poincaré embedding on WordNet Nouns) and learn more accurate embeddings on real-world datasets. 1 Introduction In the real world, valuable knowledge is encoded in datasets with hierarchical structure, such as the IBM Information Management System to describe the structure of documents, the large lexical database WordNet [14], various networks [8] and natural language sentences [24, 5]. It is challenging but necessary to embed these structured data for the use of modern machine learning methods. Recent work [11, 26, 27, 7] proposed using hyperbolic spaces to embed these structures and has achieved exciting results.
    [Show full text]
  • Seminar: ”Geometry Structures on Manifolds” Hyperbolic Geometry
    Seminar: "Geometry Structures on manifolds" Hyperbolic Geometry Xiaoman Wu December 1st, 2015 1 Poincare´ disk model Definition 1.1. (Poincar´e disk model) The hyperbolic plane H2 is homeomorphic to R2, and the Poincar´e disk model, introduced by Henri Poincar´e around the turn of this century, maps it onto the open unit disk D in the Euclidean plane. Hyperbolic straight lines, or geodesics, appear in this model as arcs of circles orthogonal to the boundary @D of D, and every arc is one special case: any diameter of the disk is a limit of circles orthogonal to @D and it is also a hyperbolic straight line. Figure 1: Straight lines in the Poincar´e disk model appear as arcs orthogonal to the boundary of the disk or, as a special case, as diameters. Define the Riemannian metric by means of this construction (Figure 2). To find the length of a tangent vector v at a point x, draw the line L orthogonal to v through x, and the equidistant circle C through the tip. The length of v (for v small) is roughly the hyperbolic distance between C and L, which in turn is roughly equal to the Euclidean angle between C and L where they meet. If we want to exact value, we consider the angle αt of the banana built on tv, for t approaching zero: the length of v is then dαt=dt at t = 0. 1 Figure 2: Hyperbolic versus Euclidean length. The hyperbolic and Euclidean lengths of a vector in the Poincar´e model are related by a constant that depends only on how far the vector's basepoint is from the origin.
    [Show full text]
  • DIFFERENTIAL GEOMETRY I HOMEWORK 8 Models For
    DIFFERENTIAL GEOMETRY I HOMEWORK 8 DUE: WEDNESDAY, NOVEMBER 12 Models for Hyperbolic Geometry 3 Hyperboloid model. Let R be equipped with the following non-degenerate, symmetric bilinear form: hhx; yii = x1y1 + x2y2 − x3y3 : (y) 3 Another gadget we are going to use is the following symmetric (0; 2)-tensor on R : 2 2 2 1 3 2 ∗ 3 (dx1) + (dx2) − (dx3) 2 C (R ; Sym T R ) : (z) Consider one-branch of the two-sheeted hyperboloid: 3 H = x 2 R hhx; xii = −1 and x3 > 0 : 3 Since the only critical (non-regular) value of hhx; xii is 0. The set H is a submanifold of R , and 3 TxH = fv 2 R j hhx; vii = 0g. It is a straightforward computation to check that : 2 ! 3 = 2 × 2 R R pR R u 7! (u; 1 + u2) 2 defines a diffeomorphism from R to H. (i) Let g be the restriction of (z) on H. Show that g is a Riemannian metric on H by checking that ∗g is positive-definite. 2 ∗ (ii) Calculate the Christoffel symbols for (R ; g). Then, write down its geodesic equation, and 2 check that u(t) = sinh(t)v is a geodesic for any v 2 R of unit length. Note that the initial point u(0) = 0, and the initial velocity u0(0) = v. It follows that (u(t)) = (sinh(t)v; cosh(t)) is a geodesic on (H; g). Isometry and geodesic for the hyperboloid model. Introduce the Lorentz group: T O(2; 1) = m 2 Gl(3; R) m L m = L where 2 3 1 0 0 L 6 7 = 40 1 0 5 : 0 0 −1 3 (iii) For any m 2 O(2; 1), regard it as a self-diffeomorphism of R .
    [Show full text]
  • 1 Euclidean Geometry
    Dr. Anna Felikson, Durham University Geometry, 29.11.2016 Outline 1 Euclidean geometry 1.1 Isometry group of Euclidean plane, Isom(E2). A distance on a space X is a function d : X × X ! R,(A; B) 7! d(A; B) for A; B 2 X satisfying 1. d(A; B) ≥ 0 (d(A; B) = 0 , A = B); 2. d(A; B) = d(B; A); 3. d(A; C) ≤ d(A; B) + d(B; C) (triangle inequality). We will use two models of Euclidean plane: p 2 2 a Cartesian plane: f(x; y) j x; y 2 Rg with the distance d(A1;A2) = (x1 − x2) + (y1 − y2) ; a Gaussian plane: fz j z 2 Cg, with the distance d(u; v) = ju − vj. Definition 1.1. A Euclidean isometry is a distance-preserving transformation of E2, i.e. a map f : E2 ! E2 satisfying d(f(A); f(B)) = d(A; B). Thm 1.2. (a) Every isometry of E2 is a one-to-one map. (b) A composition of any two isometries is an isometry. (c) Isometries of E2 form a group (denoted Isom(E2)) with composition as a group operation. Example 1.3: Translation, rotation, reflection in a line are isometries. Definition 1.4. Let ABC be a triangle labelled clock-wise. An isometry f is orientation-preserving if the triangle f(A)f(B)f(C) is also labelled clock-wise. Otherwise, f is orientation-reversing. Proposition 1.5. (correctness of Definition 1.12) Definition 1.4 does not depend on the choice of the triangle ABC.
    [Show full text]
  • Geodesics and Geodesic Spheres in ˜ SL(2,R) Geometry 1. Introduction
    MATHEMATICAL COMMUNICATIONS 413 Math. Commun., Vol. 14, No. 2, pp. 413-424 (2009) Geodesics and geodesic spheres in SL^(2; R) geometry¤ Bla·zenka Divjak1,y, Zlatko Erjavec1, Barnabas¶ Szabolcs2 and Brigitta Szilagyi¶ 2 1 Faculty of Organization and Informatics, University of Zagreb, Pavlinska 2, HR-42 000 Vara·zdin,Croatia 2 Department of Geometry, Budapest University of Technology and Economics, H-1 521 Budapest, Hungary Received May 20, 2009; accepted October 21, 2009 Abstract. In this paper geodesics and geodesic spheres in SL^(2; R) geometry are consid- ered. Exact solutions of ODE system that describes geodesics are obtained and discussed, geodesic spheres are determined and visualization of SL^(2; R) geometry is given as well. AMS subject classi¯cations: 53A35, 53C30 Key words: SL^(2; R) geometry, geodesics, geodesic sphere 1. Introduction SL^(2; R) geometry is one of the eight homogeneous Thurston 3-geometries E3;S3;H3;S2 £ R;H2 £ R; SL^(2; R); Nil; Sol: SL^(2; R) is a universal covering group of SL(2; R) that is a 3-dimensional Lie group of all 2 £ 2 real matrices with determinant one. SL^(2; R) is also a Lie group and it admits a Riemann metric invariant under right multiplication. The geometry of SL^(2; R) arises naturally as geometry of a ¯bre line bundle over a hyperbolic base plane H2: This is similar to Nil geometry in a sense that Nil is a nontrivial ¯bre line bundle over the Euclidean plane and SL^(2; R) is a twisted bundle over H2: In SL^(2; R), we can de¯ne the in¯nitesimal arc length square using the method of Lie algebras.
    [Show full text]