DOCSLIB.ORG

Curves and Surfaces in the Three Dimensional Sphere Placed in the Space of Quaternions

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Curves and Surfaces in the Three Dimensional Sphere Placed in the Space of Quaternions Transactions on Engineering Sciences vol 15, © 1997 WIT Press, www.witpress.com, ISSN 1743-3533 Curves and surfaces in the three dimensional sphere placed in the space of quaternions Yoshihiko Tazawa Department of Natural Sciences, Tokyo Denki University, 2-2 Kanda-nishikicho, Chiyodaku, Tokyo 101, Japan [email protected] Abstract In this article we will show how to use Mathematica in dealing with curves and surfaces in the three dimensional unit sphere S^ embedded in the four dimensional Euclidian space E\ Since S* is the Lie group of unit quater- nions and at the same time it is a space of constant curvature, the analogy of the theory of curves in E^ holds. We calculate curvature and torsion of curves in S^ by Mathematica. The Gauss map v of a surface in E^ is decomposed into the two maps v+ and z/_. If the surface is contained in 5^, we can define another Gauss map z/g. We use Mathematica to visualize the shapes of the images of these Gauss maps. Finally, the meaning of these images becomes clear through the notion of the slant surface. 1 Curves The space E* is regarded as the space of quaternions with the multiplication ii — jj = kk = — 1, ij = —ji — k, jk — —kj = i, ki = —ik = j The unit sphere S^ C E^ is the Lie group of unit quaternions. Let X\ = (0,1,0,0), Xz = (0,0,1,0), %3 = (0,0,0,1) be the orthonormal frame of the tangent space T\S^ at 1 = (1,0,0,0) and Xi be the left-invariant extension of Xi over S^. By V, we denote the covariant differentiation Transactions on Engineering Sciences vol 15, © 1997 WIT Press, www.witpress.com, ISSN 1743-3533 460 Innovation In Mathematics on S*. In the geometry of S^, left translations play the analogous role to that of parallel displacements in the ordinary Euclidian space E^ (cf. Chen&Tazawa[3]). The unit tangent vector of a curve c(s) parametrized by arc-length s is expressed as The curvature, unit normal vector, unit binomial vector, and torsion are defined respectively by 1 Example 1 sin(sin(^)), cos(sin(f)), sin(f - 4^), cos(f - 4 The curvature and torsion are calculated and plotted by Mathematica in Figure 1. Figure 1: curvature and torsion Example 2 : Geodesic Let a and b be any constants. A geodesic ( great circle ) is given by Cab(t) = ( cos t, ~ sin t cos b sin a, sin t cos 6 cos a, sin t sin b ) The curvature of a geodesic vanishes. Transactions on Engineering Sciences vol 15, © 1997 WIT Press, www.witpress.com, ISSN 1743-3533 Innovation In Mathematics 461 Example 3 : Helix in S* For any constant a and 6, the curve defined by hab(t) = (asinZ, a cost, Vl - a?smbt, Vl - o? cos bt has constant curvature and torsion. Moreover, the angle between its tangent vector and the left invariant vector field Xi is constant. We call this curve a helix in 5^ (cf. Chen&Tazawa[3]). The projections to the three dimensional coordinate spaces of a helix in g^ with a = 1/64 and 6 = 1/512 are shown in Figure 2. 0.94' -0.02^ 0.02 Figure 2: projections of a helix 2 Gauss map The exterior algebra A^E^ of E^ is decomposed into two eigen spaces of the Hodge's star operator Transactions on Engineering Sciences vol 15, © 1997 WIT Press, www.witpress.com, ISSN 1743-3533 462 Innovation In Mathematics corresponding to the eigen values -hi and — 1, respectively. Let 5+, S^_ be the spheres of radius 1/\/2 in A^_E\ A^E^ centered at the origin. Then the Grassmannian manifold G(2, 4) of all oriented 2-planes in E^ is identified with the Riemannian product O(2,4) =^ xS^ of these spheres through the identification V = e\ A 62, where {GI, 62} is an orthonormal frame of a plane V G G(2, 4). For a surface / : D C E^ —> E*, the Gauss map v : D —» h?E* takes its value to be the tangent plane at each point of the surface. It is decomposed into the two maps (cf. Chen&Tazawa[2]). Example 4 (u, v] —> ( u, u^ — 3v, uv, u — v^ ) The two Gauss images of this surface are plotted in Figure 3. Figure 3: Gauss images of z/+ and z/_ Example 5 A surface z —> ( z, z*\ identifying C^ = E^. The Gauss images are shown in Figure 4. Although the image of i/+ cannot be seen in the figure, it is a singleton on the axis of the first coordinate. Transactions on Engineering Sciences vol 15, © 1997 WIT Press, www.witpress.com, ISSN 1743-3533 Innovation In Mathematics 463 0.5 Figure 4: Gauss images Example 6 : Flat torus (u,v) —> —=( cos?z, sin 16, cos?;, sin?; ) The images of z/+ and z/_ in Figure 5 are the great circles perpendicular to the axis of the first coordinate. -0 0.5 Figure 5: Gauss images of a flat torus Transactions on Engineering Sciences vol 15, © 1997 WIT Press, www.witpress.com, ISSN 1743-3533 464 Innovation In Mathematics 3 Another Gauss map If a surface is contained in the 3-sphere, / : D C E^ —> S* C E*, we can define another Gauss map as follows (cf. Chen&Tazawa[3]). Let n : D —> Tf(u,v)S^ be the unit normal vector field in S^ along the surface. Then, the spherical Gauss map 1/3 : D —> S^ C T±S^ is defined by ^ = L~* o n, that is, VQ(U,V) = Lj^^(n(u,v)). Since the left translation in S^ is analogous to the parallel displacement in E^, the spherical Gauss map is a natural generalization of the ordinary Gauss map. Example 7 The image under the spherical Gauss map of the flat torus given in Example 6 is plotted in Figure 6. Figure 6: z/g-image of a flat torus 4 Slant surfaces A surface in the 4-dimensional space can also be regarded as a surface in the 2-dimensional complex space / : D C E^ —> C^ = El We denote by J the complex structure of C^. A surface is called an a—slant surface, if the angle between JX and TpM is constant, i. e., Z (JX, TpM) = a for any X G T^M and any p G M = /(D) (cf. Chen[l]) . Let {ei, 62, 63, 64} be the canonical basis of E*. Then, the following three are equivalent (cf. Ghen&Tazawa[3]). (1) / is an a— slant surface. (2) The angle between z/+(%, f) and 4= (ei A ^2 + 63 A 64) is a. (3) The angle between i/s(u,v) and X]_ is a. A family of slant surfaces contained in the 3-sphere is obtained by the quaternionic product of a geodesic in S^ and a helix in S^ (cf. ChenfeTazawa[2])). Transactions on Engineering Sciences vol 15, © 1997 WIT Press, www.witpress.com, ISSN 1743-3533 Innovation In Mathematics 465 0.! -0.251 Figure 7: projections of a slant surface Example 8 We choose a helix h(s) in S^ parametrized by arc-length s so that its coef- ficient of the unit tangent vector satisfies /i(s) = — sin (yr/lO). We obtain such h(s) by solving a system of ordinary differential equations making use of NDSolve. We also choose a geodesic ^(t) = Cab(t) in Example 2 with a = 7T/10 and 6 = ?r/4 . Define a mapy? by (^(a;,?/, z,w) = (a;,?/, w, z) arid put f(s,t) = (p(^(t] • h(s)), where • is the quaternionic product. The projections of this surface to the coordinate spaces are shown in Figure 7. The surface is a vr/lO-slant surface and its image under v+ is contained in a circle perpendicular to the first coordinate axis as is seen in Figure 8. Transactions on Engineering Sciences vol 15, © 1997 WIT Press, www.witpress.com, ISSN 1743-3533 466 Innovation In Mathematics Figure 8: z/+-image of a slant surface As can be seen from the images under v+ , the surface of Example 4 is not a slant surface. The surface of Example 5 is a 0-slant surface ( i. e., a holomorphic surface ) and the flat torus in Example 6 is an yr/2-slant sur- face ( i. e., a totally real surface) (cf. Chen& Tazawa [2], Chenfe Tazawa [3], Tazawa[5]). References 1. Paper in a journal: 1. Chen, B.-y. Slant immersions, Bull. Austral. Math. Soc., vol 4. pp!35-147,1990. 2. Chen, B.-y. & Tazawa, Y. Slant surfaces with codimension 2, Ann. Foe. &%. TWowae, vol XI, No. 3, pp29-43,1990. 3. Chen, B.-y. & Tazawa, Y. Slant submanifolds in complex Euclidean spaces, Tokyo J. Math., vol 14, No. 1, pp 101-120, 1991. 4. Tazawa, Y. Construction of slant immersions, Bull. Inst. Math. Acad. Amm., 22, No. 2, pp!53-166,1994. 2. Edited book: 5. Tazawa, Y. Construction and visualization of slant immersions, Ge- ometry and Topology of Submanifolds, VII, World Scientific, pp257- 260,1995. .
Load more

Recommended publications
  • Examples of Manifolds
    Examples of Manifolds Example 1 (Open Subset of IRn) Any open subset, O, of IRn is a manifold of dimension n. One possible atlas is A = (O, ϕid) , where ϕid is the identity map. That is, ϕid(x) = x. n Of course one possible choice of O is IR itself. Example 2 (The Circle) The circle S1 = (x,y) ∈ IR2 x2 + y2 = 1 is a manifold of dimension one. One possible atlas is A = {(U , ϕ ), (U , ϕ )} where 1 1 1 2 1 y U1 = S \{(−1, 0)} ϕ1(x,y) = arctan x with − π < ϕ1(x,y) <π ϕ1 1 y U2 = S \{(1, 0)} ϕ2(x,y) = arctan x with 0 < ϕ2(x,y) < 2π U1 n n n+1 2 2 Example 3 (S ) The n–sphere S = x =(x1, ··· ,xn+1) ∈ IR x1 +···+xn+1 =1 n A U , ϕ , V ,ψ i n is a manifold of dimension . One possible atlas is 1 = ( i i) ( i i) 1 ≤ ≤ +1 where, for each 1 ≤ i ≤ n + 1, n Ui = (x1, ··· ,xn+1) ∈ S xi > 0 ϕi(x1, ··· ,xn+1)=(x1, ··· ,xi−1,xi+1, ··· ,xn+1) n Vi = (x1, ··· ,xn+1) ∈ S xi < 0 ψi(x1, ··· ,xn+1)=(x1, ··· ,xi−1,xi+1, ··· ,xn+1) n So both ϕi and ψi project onto IR , viewed as the hyperplane xi = 0. Another possible atlas is n n A2 = S \{(0, ··· , 0, 1)}, ϕ , S \{(0, ··· , 0, −1)},ψ where 2x1 2xn ϕ(x , ··· ,xn ) = , ··· , 1 +1 1−xn+1 1−xn+1 2x1 2xn ψ(x , ··· ,xn ) = , ··· , 1 +1 1+xn+1 1+xn+1 are the stereographic projections from the north and south poles, respectively.
    [Show full text]
  • An Introduction to Topology the Classification Theorem for Surfaces by E
    An Introduction to Topology An Introduction to Topology The Classification theorem for Surfaces By E. C. Zeeman Introduction. The classification theorem is a beautiful example of geometric topology. Although it was discovered in the last century*, yet it manages to convey the spirit of present day research. The proof that we give here is elementary, and its is hoped more intuitive than that found in most textbooks, but in none the less rigorous. It is designed for readers who have never done any topology before. It is the sort of mathematics that could be taught in schools both to foster geometric intuition, and to counteract the present day alarming tendency to drop geometry. It is profound, and yet preserves a sense of fun. In Appendix 1 we explain how a deeper result can be proved if one has available the more sophisticated tools of analytic topology and algebraic topology. Examples. Before starting the theorem let us look at a few examples of surfaces. In any branch of mathematics it is always a good thing to start with examples, because they are the source of our intuition. All the following pictures are of surfaces in 3-dimensions. In example 1 by the word “sphere” we mean just the surface of the sphere, and not the inside. In fact in all the examples we mean just the surface and not the solid inside. 1. Sphere. 2. Torus (or inner tube). 3. Knotted torus. 4. Sphere with knotted torus bored through it. * Zeeman wrote this article in the mid-twentieth century. 1 An Introduction to Topology 5.
    [Show full text]
  • The Sphere Project
    ;OL :WOLYL 7YVQLJ[ +XPDQLWDULDQ&KDUWHUDQG0LQLPXP 6WDQGDUGVLQ+XPDQLWDULDQ5HVSRQVH ;OL:WOLYL7YVQLJ[ 7KHULJKWWROLIHZLWKGLJQLW\ ;OL:WOLYL7YVQLJ[PZHUPUP[PH[P]L[VKL[LYTPULHUK WYVTV[LZ[HUKHYKZI`^OPJO[OLNSVIHSJVTT\UP[` /\THUP[HYPHU*OHY[LY YLZWVUKZ[V[OLWSPNO[VMWLVWSLHMMLJ[LKI`KPZHZ[LYZ /\THUP[HYPHU >P[O[OPZ/HUKIVVR:WOLYLPZ^VYRPUNMVYH^VYSKPU^OPJO [OLYPNO[VMHSSWLVWSLHMMLJ[LKI`KPZHZ[LYZ[VYLLZ[HISPZO[OLPY *OHY[LYHUK SP]LZHUKSP]LSPOVVKZPZYLJVNUPZLKHUKHJ[LK\WVUPU^H`Z[OH[ YLZWLJ[[OLPY]VPJLHUKWYVTV[L[OLPYKPNUP[`HUKZLJ\YP[` 4PUPT\T:[HUKHYKZ This Handbook contains: PU/\THUP[HYPHU (/\THUP[HYPHU*OHY[LY!SLNHSHUKTVYHSWYPUJPWSLZ^OPJO HUK YLÅLJ[[OLYPNO[ZVMKPZHZ[LYHMMLJ[LKWVW\SH[PVUZ 4PUPT\T:[HUKHYKZPU/\THUP[HYPHU9LZWVUZL 9LZWVUZL 7YV[LJ[PVU7YPUJPWSLZ *VYL:[HUKHYKZHUKTPUPT\TZ[HUKHYKZPUMV\YRL`SPMLZH]PUN O\THUP[HYPHUZLJ[VYZ!>H[LYZ\WWS`ZHUP[H[PVUHUKO`NPLUL WYVTV[PVU"-VVKZLJ\YP[`HUKU\[YP[PVU":OLS[LYZL[[SLTLU[HUK UVUMVVKP[LTZ"/LHS[OHJ[PVU;OL`KLZJYPILwhat needs to be achieved in a humanitarian response in order for disaster- affected populations to survive and recover in stable conditions and with dignity. ;OL:WOLYL/HUKIVVRLUQV`ZIYVHKV^ULYZOPWI`HNLUJPLZHUK PUKP]PK\HSZVMMLYPUN[OLO\THUP[HYPHUZLJ[VYH common language for working together towards quality and accountability in disaster and conflict situations ;OL:WOLYL/HUKIVVROHZHU\TILYVMºJVTWHUPVU Z[HUKHYKZ»L_[LUKPUNP[ZZJVWLPUYLZWVUZL[VULLKZ[OH[ OH]LLTLYNLK^P[OPU[OLO\THUP[HYPHUZLJ[VY ;OL:WOLYL7YVQLJ[^HZPUP[PH[LKPU I`HU\TILY VMO\THUP[HYPHU5.6ZHUK[OL9LK*YVZZHUK 9LK*YLZJLU[4V]LTLU[ ,+0;065 7KHB6SKHUHB3URMHFWBFRYHUBHQBLQGG The Sphere Project Humanitarian Charter and Minimum Standards in Humanitarian Response Published by: The Sphere Project Copyright@The Sphere Project 2011 Email: [email protected] Website : www.sphereproject.org The Sphere Project was initiated in 1997 by a group of NGOs and the Red Cross and Red Crescent Movement to develop a set of universal minimum standards in core areas of humanitarian response: the Sphere Handbook.
    [Show full text]
  • Quick Reference Guide - Ansi Z80.1-2015
    QUICK REFERENCE GUIDE - ANSI Z80.1-2015 1. Tolerance on Distance Refractive Power (Single Vision & Multifocal Lenses) Cylinder Cylinder Sphere Meridian Power Tolerance on Sphere Cylinder Meridian Power ≥ 0.00 D > - 2.00 D (minus cylinder convention) > -4.50 D (minus cylinder convention) ≤ -2.00 D ≤ -4.50 D From - 6.50 D to + 6.50 D ± 0.13 D ± 0.13 D ± 0.15 D ± 4% Stronger than ± 6.50 D ± 2% ± 0.13 D ± 0.15 D ± 4% 2. Tolerance on Distance Refractive Power (Progressive Addition Lenses) Cylinder Cylinder Sphere Meridian Power Tolerance on Sphere Cylinder Meridian Power ≥ 0.00 D > - 2.00 D (minus cylinder convention) > -3.50 D (minus cylinder convention) ≤ -2.00 D ≤ -3.50 D From -8.00 D to +8.00 D ± 0.16 D ± 0.16 D ± 0.18 D ± 5% Stronger than ±8.00 D ± 2% ± 0.16 D ± 0.18 D ± 5% 3. Tolerance on the direction of cylinder axis Nominal value of the ≥ 0.12 D > 0.25 D > 0.50 D > 0.75 D < 0.12 D > 1.50 D cylinder power (D) ≤ 0.25 D ≤ 0.50 D ≤ 0.75 D ≤ 1.50 D Tolerance of the axis Not Defined ° ° ° ° ° (degrees) ± 14 ± 7 ± 5 ± 3 ± 2 4. Tolerance on addition power for multifocal and progressive addition lenses Nominal value of addition power (D) ≤ 4.00 D > 4.00 D Nominal value of the tolerance on the addition power (D) ± 0.12 D ± 0.18 D 5. Tolerance on Prism Reference Point Location and Prismatic Power • The prismatic power measured at the prism reference point shall not exceed 0.33Δ or the prism reference point shall not be more than 1.0 mm away from its specified position in any direction.
    [Show full text]
  • Sphere Vs. 0°/45° a Discussion of Instrument Geometries And
    Sphere vs. 0°/45° ® Author - Timothy Mouw Sphere vs. 0°/45° A discussion of instrument geometries and their areas of application Introduction When purchasing a spectrophotometer for color measurement, one of the choices that must be made is the geometry of the instrument; that is, whether to buy a spherical or 0°/45° instrument. In this article, we will attempt to provide some information to assist you in determining which type of instrument is best suited to your needs. In order to do this, we must first understand the differences between these instruments. Simplified schematics of instrument geometries We will begin by looking at some simple schematic drawings of the two different geometries, spherical and 0°/45°. Figure 1 shows a drawing of the geometry used in a 0°/45° instrument. The illumination of the sample is from 0° (90° from the sample surface). This means that the specular or gloss angle (the angle at which the light is directly reflected) is also 0°. The fiber optic pick-ups are located at 45° from the specular angle. Figure 1 X-Rite World Headquarters © 1995 X-Rite, Incorporated Doc# CA00015a.doc (616) 534-7663 Fax (616) 534-8960 September 15, 1995 Figure 2 shows a drawing of the geometry used in an 8° diffuse sphere instrument. The sphere wall is lined with a highly reflective white substance and the light source is located on the rear of the sphere wall. A baffle prevents the light source from directly illuminating the sample, thus providing diffuse illumination. The sample is viewed at 8° from perpendicular which means that the specular or gloss angle is also 8° from perpendicular.
    [Show full text]
  • Recognizing Surfaces
    RECOGNIZING SURFACES Ivo Nikolov and Alexandru I. Suciu Mathematics Department College of Arts and Sciences Northeastern University Abstract The subject of this poster is the interplay between the topology and the combinatorics of surfaces. The main problem of Topology is to classify spaces up to continuous deformations, known as homeomorphisms. Under certain conditions, topological invariants that capture qualitative and quantitative properties of spaces lead to the enumeration of homeomorphism types. Surfaces are some of the simplest, yet most interesting topological objects. The poster focuses on the main topological invariants of two-dimensional manifolds—orientability, number of boundary components, genus, and Euler characteristic—and how these invariants solve the classification problem for compact surfaces. The poster introduces a Java applet that was written in Fall, 1998 as a class project for a Topology I course. It implements an algorithm that determines the homeomorphism type of a closed surface from a combinatorial description as a polygon with edges identified in pairs. The input for the applet is a string of integers, encoding the edge identifications. The output of the applet consists of three topological invariants that completely classify the resulting surface. Topology of Surfaces Topology is the abstraction of certain geometrical ideas, such as continuity and closeness. Roughly speaking, topol- ogy is the exploration of manifolds, and of the properties that remain invariant under continuous, invertible transforma- tions, known as homeomorphisms. The basic problem is to classify manifolds according to homeomorphism type. In higher dimensions, this is an impossible task, but, in low di- mensions, it can be done. Surfaces are some of the simplest, yet most interesting topological objects.
    [Show full text]
  • The Real Projective Spaces in Homotopy Type Theory
    The real projective spaces in homotopy type theory Ulrik Buchholtz Egbert Rijke Technische Universität Darmstadt Carnegie Mellon University Email: [email protected] Email: [email protected] Abstract—Homotopy type theory is a version of Martin- topology and homotopy theory developed in homotopy Löf type theory taking advantage of its homotopical models. type theory (homotopy groups, including the fundamen- In particular, we can use and construct objects of homotopy tal group of the circle, the Hopf fibration, the Freuden- theory and reason about them using higher inductive types. In this article, we construct the real projective spaces, key thal suspension theorem and the van Kampen theorem, players in homotopy theory, as certain higher inductive types for example). Here we give an elementary construction in homotopy type theory. The classical definition of RPn, in homotopy type theory of the real projective spaces as the quotient space identifying antipodal points of the RPn and we develop some of their basic properties. n-sphere, does not translate directly to homotopy type theory. R n In classical homotopy theory the real projective space Instead, we define P by induction on n simultaneously n with its tautological bundle of 2-element sets. As the base RP is either defined as the space of lines through the + case, we take RP−1 to be the empty type. In the inductive origin in Rn 1 or as the quotient by the antipodal action step, we take RPn+1 to be the mapping cone of the projection of the 2-element group on the sphere Sn [4].
    [Show full text]
  • The Sphere Packing Problem in Dimension 8 Arxiv:1603.04246V2
    The sphere packing problem in dimension 8 Maryna S. Viazovska April 5, 2017 8 In this paper we prove that no packing of unit balls in Euclidean space R has density greater than that of the E8-lattice packing. Keywords: Sphere packing, Modular forms, Fourier analysis AMS subject classification: 52C17, 11F03, 11F30 1 Introduction The sphere packing constant measures which portion of d-dimensional Euclidean space d can be covered by non-overlapping unit balls. More precisely, let R be the Euclidean d vector space equipped with distance k · k and Lebesgue measure Vol(·). For x 2 R and d r 2 R>0 we denote by Bd(x; r) the open ball in R with center x and radius r. Let d X ⊂ R be a discrete set of points such that kx − yk ≥ 2 for any distinct x; y 2 X. Then the union [ P = Bd(x; 1) x2X d is a sphere packing. If X is a lattice in R then we say that P is a lattice sphere packing. The finite density of a packing P is defined as Vol(P\ Bd(0; r)) ∆P (r) := ; r > 0: Vol(Bd(0; r)) We define the density of a packing P as the limit superior ∆P := lim sup ∆P (r): r!1 arXiv:1603.04246v2 [math.NT] 4 Apr 2017 The number be want to know is the supremum over all possible packing densities ∆d := sup ∆P ; d P⊂R sphere packing 1 called the sphere packing constant. For which dimensions do we know the exact value of ∆d? Trivially, in dimension 1 we have ∆1 = 1.
    [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]
  • Sphere Packing, Lattice Packing, and Related Problems
    Sphere packing, lattice packing, and related problems Abhinav Kumar Stony Brook April 25, 2018 Sphere packings Definition n A sphere packing in R is a collection of spheres/balls of equal size which do not overlap (except for touching). The density of a sphere packing is the volume fraction of space occupied by the balls. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ In dimension 1, we can achieve density 1 by laying intervals end to end. In dimension 2, the best possible is by using the hexagonal lattice. [Fejes T´oth1940] Sphere packing problem n Problem: Find a/the densest sphere packing(s) in R . In dimension 2, the best possible is by using the hexagonal lattice. [Fejes T´oth1940] Sphere packing problem n Problem: Find a/the densest sphere packing(s) in R . In dimension 1, we can achieve density 1 by laying intervals end to end. Sphere packing problem n Problem: Find a/the densest sphere packing(s) in R . In dimension 1, we can achieve density 1 by laying intervals end to end. In dimension 2, the best possible is by using the hexagonal lattice. [Fejes T´oth1940] Sphere packing problem II In dimension 3, the best possible way is to stack layers of the solution in 2 dimensions. This is Kepler's conjecture, now a theorem of Hales and collaborators. mmm m mmm m There are infinitely (in fact, uncountably) many ways of doing this! These are the Barlow packings. Face centered cubic packing Image: Greg A L (Wikipedia), CC BY-SA 3.0 license But (until very recently!) no proofs. In very high dimensions (say ≥ 1000) densest packings are likely to be close to disordered.
    [Show full text]
  • 17 Measure Concentration for the Sphere
    17 Measure Concentration for the Sphere In today’s lecture, we will prove the measure concentration theorem for the sphere. Recall that this was one of the vital steps in the analysis of the Arora-Rao-Vazirani approximation algorithm for sparsest cut. Most of the material in today’s lecture is adapted from Matousek’s book [Mat02, chapter 14] and Keith Ball’s lecture notes on convex geometry [Bal97]. n n−1 Notation: We will use the notation Bn to denote the ball of unit radius in R and S n to denote the sphere of unit radius in R . Let µ denote the normalized measure on the unit sphere (i.e., for any measurable set S ⊆ Sn−1, µ(A) denotes the ratio of the surface area of µ to the entire surface area of the sphere Sn−1). Recall that the n-dimensional volume of a ball n n n of radius r in R is given by the formula Vol(Bn) · r = vn · r where πn/2 vn = n Γ 2 + 1 Z ∞ where Γ(x) = tx−1e−tdt 0 n−1 The surface area of the unit sphere S is nvn. Theorem 17.1 (Measure Concentration for the Sphere Sn−1) Let A ⊆ Sn−1 be a mea- n−1 surable subset of the unit sphere S such that µ(A) = 1/2. Let Aδ denote the δ-neighborhood n−1 n−1 of A in S . i.e., Aδ = {x ∈ S |∃z ∈ A, ||x − z||2 ≤ δ}. Then, −nδ2/2 µ(Aδ) ≥ 1 − 2e . Thus, the above theorem states that if A is any set of measure 0.5, taking a step of even √ O (1/ n) around A covers almost 99% of the entire sphere.
    [Show full text]
  • CIRCLE and SPHERE GEOMETRY. [July
    464 CIRCLE AND SPHERE GEOMETRY. [July, CIRCLE AND SPHERE GEOMETRY. A Treatise on the Circle and the Sphere. By JULIAN LOWELL COOLIDGE. Oxford, the Clarendon Press, 1916. 8vo. 603 pp. AN era of systematizing and indexing produces naturally a desire for brevity. The excess of this desire is a disease, and its remedy must be treatises written, without diffuseness indeed, but with the generous temper which will find room and give proper exposition to true works of art. The circle and the sphere have furnished material for hundreds of students and writers, but their work, even the most valuable, has been hitherto too scattered for appraisement by the average student. Cyclopedias, while indispensable, are tan­ talizing. Circles and spheres have now been rescued from a state of dispersion by Dr. Coolidge's comprehensive lectures. Not a list of results, but a well digested account of theories and methods, sufficiently condensed by judicious choice of position and sequence, is what he has given us for leisurely study and enjoyment. The reader whom the author has in mind might be, well enough, at the outset a college sophomore, or a teacher who has read somewhat beyond the geometry expected for en­ trance. Beyond the sixth chapter, however, or roughly the middle of the book, he must be ready for persevering study in specialized fields. The part most read is therefore likely to be the first four chapters, and a short survey of these is what we shall venture here. After three pages of definitions and conventions the author calls "A truce to these preliminaries!" and plunges into inversion.
    [Show full text]