DOCSLIB.ORG

On the Heisenberg Group

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
On the Heisenberg Group HEISENBERG GROUP OVERVIEW ARMIN SCHIKORRA The Heisenberg group is R3 with an interesting metric structure. Usually one denotes points in the Heisenberg-Group by (x, y, t) ∈ R3. 1. Horizontal space The horizontal space at a point (x, y, t) tells us the permissible directions. Usually it is described as a kernel of a one-form α = dt + 2 x1dx2 − x2dx1 . At a point (x, y, t) the horizontal vectors v = (v1, v2, v3) are exactly those which satisfy v1 +2y 2 3 2 1 0 = v · −2x = v + 2(xv − yv ) v3 1 The horizontal vectors form a two-dimensional vector space in R3 with basis (X, Y ) where 1 0 X := 0 ,Y := 1 2y −2x and v = (v1, v2, v3) is horizontal if and only if v = v1X + v2Y q 2 2 The lenght of a (horizontal!) vector v is defined as |v1| + |v2| 2. Curves A horizontal curve γ :[a, b] → R3 with derivativeγ ˙ (t) satisfies (2.1) 0 =γ ˙ 3(t) + 2(γ1(t)γ ˙ 2(t) − γ2(t)γ ˙ 1(t)) The same equation can also be written as γ1(t)γ ˙ 1(t) ! (2.2) 0 =γ ˙ 3(t) + 2 det γ2(t)γ ˙ 2(t) 1 HEISENBERG GROUP OVERVIEW 2 The lenght of a (horizontal!) curve is defined as Z b Z b q 1 2 2 2 L(γ) := |γ˙ (t)|H dt = |γ˙ (t)| + |γ˙ (t)| dt a a It is often useful to consider projections into the (x, y)-coordinate, which we will denote by π : R3 → R2 with π(x, y, t) := (x, y). Observe that by the fundamental theorem of calculus, whenever we have a curveγ ˜ = (˜γ1, γ˜2):[a, b] → R2 in the two-dimensional plane then we can set for any ta ∈ R Z t 3 1 2 2 1 γ (t) := ta + 2(˜γ (s) γ˜˙ (t) − γ˜ (s) γ˜˙ (s))ds a and find that γ := (˜γ1, γ˜2, γ3):[a, b] → R3 is a horizontal curve starting at γ(a) = 1 2 (˜γ (a), γ˜ (a), ta) – this procedure is called Lifting. We can use this Lifting for example in the following way: Letγ ˜ = (˜γ1, γ˜2) : [0, 1] → R2 be a curve in the two-dimensional plane, withγ ˜(0) = (0, 0). We can lift this to a horizontal curve γ := (˜γ1, γ˜2, γ3) : [0, 1] → R3 which satisfies γ(0) = (0, 0, 0), recall that γ3(t) is then given by Z t γ3(t) := 2(˜γ1(s) γ˜˙ 2(t) − γ˜2(s) γ˜˙ 1(s))ds 0 In particular, Z 1 γ3(1) = 2(˜γ1(s) γ˜˙ 2(t) − γ˜2(s) γ˜˙ 1(s))ds 0 Now ifγ ˜ is a closed curve without self-intersection (in particular γ(0) = γ(1)) then Z 1 1 1 2 2 1 (˜γ (s) γ˜˙ (t) − γ˜ (s) γ˜˙ (s))ds = Area(A) 2 0 where A is the area enclosed by γ. This is the Sector formula of Leibniz, (I found only a German link) 3. Scaling and rotation 3 2 Let r > 0 then for a point p = (x, y, t) ∈ R we set δr(p) := (rx, ry, r t). Observe that if 3 3 γ :[a, b] → R is horizontal, then (δr ◦ γ):[a, b] → R is still horizontal. Moreover, 0 0 |(δrγ) (t)|H = r|γ (t)|H , and thus L(δrγ) = rL(γ). Let P be a rotation matrix of R2, i.e. for some α ∈ R let sin(α) − cos(α) ! P = . cos(α) sin(α) HEISENBERG GROUP OVERVIEW 3 3 We then define the rotation of a vector v = (v1, v2, v3) ∈ R , rot(v; P ) as ! ! v1 1 2 1 2 rot(v; P ) := P , v3 ≡ sin(α)v − cos(α)v , cos(α)v + sin(α)v , v3 . v2 One can check (the determinant formula as in (2.2) is here easier) that if v = (v1, v2, v3) is a horizontal vector at the point p = (p1, p2, p3), then for any rotation P the vector rot(v; P ) is horizontal at the point rot(p; P ). This implies if we have a horizontal curve γ :[a, b] → R3 and set for a fixed rotation P , η(t) := rot (γ(t),P ) then η is a horizontal curve from rot (γ(a),P ) to rot (γ(b),P ), with the same lenght, L(η) = L(γ). 4. Group structure Let (x, y, t), (x0, y0, t0) ∈ R3 then we define the group multiplication (x, y, t) ∗ (x0, y0, t0) = (x + x0, y + y0, t + t0 + 2(x0y − xy0)) Observe • (x, y, t)∗(−x, −y, −t) = (0, 0, 0), we thus say for p = (x, y, t) that p−1 = (−x, −y, −t) is the inverse. • this group does not commute: in general p ∗ q 6= q ∗ p (careful!) • Let γ :[a, b] → R3 be a horizontal curve and p ∈ R3, then 3 p ∗ γ :[a, b] → R is a horizontal curve: Indeed, set η(t) := p ∗ γ(t), then 1 2 3 1 2 η(t) = (p1 + γ (t), p2 + γ (t), p3 + γ (t) + 2(γ (t)p2 − p1γ (t)) so 1 2 3 1 2 η˙(t) = (γ ˙ (t), γ˙ (t), γ˙ (t) + 2(γ ˙ (t)p2 − p1γ˙ (t)) so by (2.1) 1 2 1 2 1 2 η˙(t) = (γ ˙ (t), γ˙ (t), 2(γ ˙ (t)(p2 + γ (t)) − (p1 + γ (t))γ ˙ (t)) Thus, 0 =η ˙3(t) + 2(η1(t)η ˙2(t) − η˙1(t)η2) which means that η satisfies (2.1), i.e. η is horizontal. • If γ :[a, b] → R3 is horizontal and for p ∈ R3 we set η(t) := p ∗ γ(t), then |γ˙ (t)|H = |η˙(t)|H . In particular, γ and η have the same length! HEISENBERG GROUP OVERVIEW 4 5. Geodesics (shortest curves) • It suffices to compute shortest curves (“geodesics”) between the origin (0, 0, 0) and a point (x, y, z). If we can do this, then to compute the shortest geodesics between p and q we compute the geodesic γ between (0, 0, 0) and p−1 ∗ q, then η := p ∗ γ is the shortest geodesic between p and q. • The shortest geodesic between (0, 0, 0) and (x, y, 0) is γ(t) := (tx, ty, 0) for t ∈ [0, 1]. • The shortest geodesic between (0, 0, 0) and (0, 0,T ) is non-unique: One example is the spiral q q |T | |T | T η(t) := √ sin(t), √ (cos(t) − 1), (t − sin(t)) t ∈ [0, 2π] 4π 4 π 2π (This weird formula comes from the fact that the projection onto the xy plane must be a circle, then the t-entry can be computed from (2.1)) For α ∈ [0, 2π] also any R2 × {0}-rotation is a geodesic between (0, 0, 0) and (0, 0,T ): 1 2 1 2 3 ηα(t) := sin(α)η (t) − cos(α)η (t), cos(α)η (t) + sin(α)η (t), η (t) • The shortest geodesic between (0, 0, 0) and (x, y, T ) (with (x, y) 6= (0, 0) and T 6= 0) lies on one of these spirals geodesics (to some other point on the t-axis). Since (x, y) 6= (0, 0) there is a unique rotation P such that √ x ! x2 + y2 ! = P y 0 √ Set R := x2 + y2. t If R 6= 0 and we have a geodesic γ(t) from (0, 0, 0) to (1, 0, R2 ) then η(t) := rot(δR(γ(t)),P ) is a geodesic from (0, 0, 0) to (x, y, t). So we need to compute a formula for geodesics from (0, 0, 0) to (1, 0,T ). The idea is that the geodesic from (0, 0, 0) to (1, 0,T ) is part of the (unique) geodesic between (0, 0, 0) and (0, 0, τ) for τ > T (assume that T > 0) passing through (1, 0,T ). After projecting this situation into x, y-axis R2, we are left with the task of computing τ as the area of the circle that passes through (0, 0) and 2 (0, 1) with area of the circle intersected with R− being t. See (2.16) and (2.17) in [Haj lasz and Zimmerman, 2015], HEISENBERG GROUP OVERVIEW 5 According to wikipedia, wikipedia we can compute the (red) area A of the circula 1 segment (which will need to equal to 4 T ) as follows: R2 A = (θ − sin(θ)). 2 We will parametrize it in h ∈ R, then we have 1 R2 = + h2 4 1 1 tan(θ/2) = ⇔ θ = 2 arctan 2h 2h Consequently, for h ∈ R the corresponding A˜(h) is given by 1 + h2 1 1 A˜(h) = 4 2 arctan − sin(2 arctan ) 2 2h 2h This is at least consistent with the notion that π A(0+) = 1/2Area( ball of radius 1/2) = 8 This formula works only for h > 0, so we need to set 1 2 4 +h 1 1 2 2 arctan 2h − sin(2 arctan 2h ) h > 0 A(h) := 1 +h2 2 4 1 1 π(1/4 + h ) − 2 2 arctan −2h − sin(2 arctan −2h ) h < 0 According to Wolfram alpha A˜ : [0, ∞) → [0, π/8] is one-to-one; so A : R → [0, +∞) with limh→∞ A(h) = 0 and limh→−∞ A(h) = +∞. Observe that according to Wolfram alpha, A˜0(h) = 2h cot−1(2h − 1) In particular for any T > 0 there exists h ∈ R such that 4A(h) = T , h = A−1(T/4). So our geodesic is the lifting of the curve along the sphere centered at q the point (1/2, h) of radius 1/4 + h2.
Load more

Recommended publications
  • The Heisenberg Group Fourier Transform
    THE HEISENBERG GROUP FOURIER TRANSFORM NEIL LYALL Contents 1. Fourier transform on Rn 1 2. Fourier analysis on the Heisenberg group 2 2.1. Representations of the Heisenberg group 2 2.2. Group Fourier transform 3 2.3. Convolution and twisted convolution 5 3. Hermite and Laguerre functions 6 3.1. Hermite polynomials 6 3.2. Laguerre polynomials 9 3.3. Special Hermite functions 9 4. Group Fourier transform of radial functions on the Heisenberg group 12 References 13 1. Fourier transform on Rn We start by presenting some standard properties of the Euclidean Fourier transform; see for example [6] and [4]. Given f ∈ L1(Rn), we define its Fourier transform by setting Z fb(ξ) = e−ix·ξf(x)dx. Rn n ih·ξ If for h ∈ R we let (τhf)(x) = f(x + h), then it follows that τdhf(ξ) = e fb(ξ). Now for suitable f the inversion formula Z f(x) = (2π)−n eix·ξfb(ξ)dξ, Rn holds and we see that the Fourier transform decomposes a function into a continuous sum of characters (eigenfunctions for translations). If A is an orthogonal matrix and ξ is a column vector then f[◦ A(ξ) = fb(Aξ) and from this it follows that the Fourier transform of a radial function is again radial. In particular the Fourier transform −|x|2/2 n of Gaussians take a particularly nice form; if G(x) = e , then Gb(ξ) = (2π) 2 G(ξ). In general the Fourier transform of a radial function can always be explicitly expressed in terms of a Bessel 1 2 NEIL LYALL transform; if g(x) = g0(|x|) for some function g0, then Z ∞ n 2−n n−1 gb(ξ) = (2π) 2 g0(r)(r|ξ|) 2 J n−2 (r|ξ|)r dr, 0 2 where J n−2 is a Bessel function.
    [Show full text]
  • Analogue of Weil Representation for Abelian Schemes
    ANALOGUE OF WEIL REPRESENTATION FOR ABELIAN SCHEMES A. POLISHCHUK This paper is devoted to the construction of projective actions of certain arithmetic groups on the derived categories of coherent sheaves on abelian schemes over a normal base S. These actions are constructed by mimicking the construction of Weil in [27] of a projective representation of the symplectic group Sp(V ∗ ⊕ V ) on the space of smooth functions on the lagrangian subspace V . Namely, we replace the vector space V by an abelian scheme A/S, the dual vector space V ∗ by the dual abelian scheme Aˆ, and the space of functions on V by the (bounded) derived category of coherent sheaves on A which we denote by Db(A). The role of the standard symplectic form ∗ ∗ ∗ −1 ˆ 2 on V ⊕ V is played by the line bundle B = p14P ⊗ p23P on (A ×S A) where P is the normalized Poincar´ebundle on Aˆ× A. Thus, the symplectic group Sp(V ∗ ⊕ V ) is replaced by the group of automorphisms of Aˆ×S A preserving B. We denote the latter group by SL2(A) (in [20] the same group is denoted by Sp(Aˆ ×S A)). We construct an action of a central extension of certain ”congruenz-subgroup” Γ(A, 2) ⊂ SL2(A) b on D (A). More precisely, if we write an element of SL2(A) in the block form a11 a12 a21 a22! then the subgroup Γ(A, 2) is distinguished by the condition that elements a12 ∈ Hom(A, Aˆ) and a21 ∈ Hom(A,ˆ A) are divisible by 2.
    [Show full text]
  • Problems in Abstract Algebra
    STUDENT MATHEMATICAL LIBRARY Volume 82 Problems in Abstract Algebra A. R. Wadsworth 10.1090/stml/082 STUDENT MATHEMATICAL LIBRARY Volume 82 Problems in Abstract Algebra A. R. Wadsworth American Mathematical Society Providence, Rhode Island Editorial Board Satyan L. Devadoss John Stillwell (Chair) Erica Flapan Serge Tabachnikov 2010 Mathematics Subject Classification. Primary 00A07, 12-01, 13-01, 15-01, 20-01. For additional information and updates on this book, visit www.ams.org/bookpages/stml-82 Library of Congress Cataloging-in-Publication Data Names: Wadsworth, Adrian R., 1947– Title: Problems in abstract algebra / A. R. Wadsworth. Description: Providence, Rhode Island: American Mathematical Society, [2017] | Series: Student mathematical library; volume 82 | Includes bibliographical references and index. Identifiers: LCCN 2016057500 | ISBN 9781470435837 (alk. paper) Subjects: LCSH: Algebra, Abstract – Textbooks. | AMS: General – General and miscellaneous specific topics – Problem books. msc | Field theory and polyno- mials – Instructional exposition (textbooks, tutorial papers, etc.). msc | Com- mutative algebra – Instructional exposition (textbooks, tutorial papers, etc.). msc | Linear and multilinear algebra; matrix theory – Instructional exposition (textbooks, tutorial papers, etc.). msc | Group theory and generalizations – Instructional exposition (textbooks, tutorial papers, etc.). msc Classification: LCC QA162 .W33 2017 | DDC 512/.02–dc23 LC record available at https://lccn.loc.gov/2016057500 Copying and reprinting. Individual readers of this publication, and nonprofit libraries acting for them, are permitted to make fair use of the material, such as to copy select pages for use in teaching or research. Permission is granted to quote brief passages from this publication in reviews, provided the customary acknowledgment of the source is given. Republication, systematic copying, or multiple reproduction of any material in this publication is permitted only under license from the American Mathematical Society.
    [Show full text]
  • Lie Groups Locally Isomorphic to Generalized Heisenberg Groups
    PROCEEDINGS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 136, Number 9, September 2008, Pages 3247–3254 S 0002-9939(08)09489-6 Article electronically published on April 22, 2008 LIE GROUPS LOCALLY ISOMORPHIC TO GENERALIZED HEISENBERG GROUPS HIROSHI TAMARU AND HISASHI YOSHIDA (Communicated by Dan M. Barbasch) Abstract. We classify connected Lie groups which are locally isomorphic to generalized Heisenberg groups. For a given generalized Heisenberg group N, there is a one-to-one correspondence between the set of isomorphism classes of connected Lie groups which are locally isomorphic to N and a union of certain quotients of noncompact Riemannian symmetric spaces. 1. Introduction Generalized Heisenberg groups were introduced by Kaplan ([5]) and have been studied in many fields in mathematics. Especially, generalized Heisenberg groups, endowed with the left-invariant metric, provide beautiful examples in geometry. They provide many examples of symmetric-like Riemannian manifolds, that is, Riemannian manifolds which share some properties with Riemannian symmetric spaces (see [1] and the references). Compact quotients of some generalized Heisen- berg groups provide isospectral but nonisometric manifolds (see Gordon [3] and the references). Generalized Heisenberg groups also provide remarkable examples of solvmanifolds by taking a certain solvable extension. The constructed solvmani- folds are now called Damek-Ricci spaces, since Damek-Ricci ([2]) showed that they are harmonic spaces (see also [1]). The aim of this paper is to classify connected Lie groups which are locally iso- morphic to generalized Heisenberg groups. They provide interesting examples of nilmanifolds, that is, Riemannian manifolds on which nilpotent Lie groups act iso- metrically and transitively. They might be a good prototype for the study of non- simply-connected and noncompact nilmanifolds.
    [Show full text]
  • Introduction to Representation Theory HW2 - Fall 2016 for Thursday October 20 Meeting
    Introduction to Representation Theory HW2 - Fall 2016 For Thursday October 20 Meeting 1. Finite Symplectic Spaces. A symplectic vector space over a field k is a pair (V, , ) , where V is a finite dimensional vector space over k, and , is a non-degenerate bilinearh i form , on V , which is alternating, i.e., u, v = v,h ui for every u, v V. Such a form ish i also called symplectic form. h i h i 2 (a) Define the category of symplectic vector spaces over k and show that every mor- phism there is injection. (b) Show that if V is symplectic then dim(V ) = 2n, for some n. (c) Show that any two symplectic vector spaces of the same dimension are symplec- tomorphic, i.e., isomorphic by a symplectic morphism. (d) Show that if I V is a subspace on which the symplectic form vanishes then dim(I) n. Such subspace are called isotropic. A maximal isotropic subspace L V is≤ called Lagrangian. For such L show that dim(L) = n. (e) Denote by Lag(V ) the space of Lagrangians in V. Try to compute #Lag(V ) in case k = Fq. 2. Heisenberg Group. Let (V, , ) be a symplectic space over a field k. Let us assume that char(k) = 2. Consider theh seti H = V k equipped with the product 6 1 (v, z) (v0, z0) = (v + v0, z + z0 + v, v0 ). · 2 h i (a) Show that H is a group. It is called the Heisenberg group associated with (V, , ). h i (b) Compute the center Z = Z(H) of H, and the commutator subgroup [H, H].
    [Show full text]
  • Foundations of Harmonic Analysis on the Heisenberg Group
    DIPLOMARBEIT Titel der Diplomarbeit Foundations of Harmonic Analysis on the Heisenberg Group Verfasser David Rottensteiner zur Erlangung des akademischen Grades Magister der Naturwissenschaften (Mag.rer.nat.) Wien, M¨arz2010 Studienkennzahl lt. Studienblatt: A 405 Studienrichtung lt. Studienblatt: Mathematik Betreuer: ao. Univ.-Prof. Dr. Roland Steinbauer Contents Introduction1 1. Some Prerequisites from Lie Group Representation Theory3 1.1. One-Parameter Groups of Operators.....................3 1.1.1. Definitions and Estimates.......................3 1.1.2. The Infinitesimal Generator......................5 1.1.3. Unitary One-Parameter Groups.................... 13 1.1.4. Smooth and Analytic Vectors..................... 17 1.2. Representations of Lie Groups......................... 22 1.2.1. General Groups and Measure..................... 22 1.2.2. Translations Revisited......................... 25 1.2.3. The Integrated Representation.................... 26 1.2.4. Smooth Vectors on G......................... 27 2. Foundations of Harmonic Analysis on the Heisenberg Group 30 2.1. The Heisenberg Group Hn ........................... 30 2.1.1. Motivation............................... 30 2.1.2. Lie Algebras and Commutation Relations.............. 30 2.1.3. The Heisenberg Algebra........................ 32 2.1.4. Construction of the Heisenberg Group................ 33 2.1.5. The Automorphisms of the Heisenberg Group............ 36 2.2. Representations of Hn ............................. 38 2.2.1. The Schr¨odingerRepresentation................... 38 2.2.2. Integrated Representation and Twisted Convolution........ 42 2.2.3. Matrix Coefficients and the Fourier-Wigner Transform....... 50 2.2.4. The Stone-von Neumann Theorem.................. 55 2.2.5. The Group Fourier Transform..................... 58 A. The Bochner Integral 65 A.1. Motivation................................... 65 A.2. Measure Space................................. 65 A.3. Measurability.................................. 65 A.4. Integrability................................... 68 A.5.
    [Show full text]
  • Lie Algebras from Lie Groups
    Preprint typeset in JHEP style - HYPER VERSION Lecture 8: Lie Algebras from Lie Groups Gregory W. Moore Abstract: Not updated since November 2009. March 27, 2018 -TOC- Contents 1. Introduction 2 2. Geometrical approach to the Lie algebra associated to a Lie group 2 2.1 Lie's approach 2 2.2 Left-invariant vector fields and the Lie algebra 4 2.2.1 Review of some definitions from differential geometry 4 2.2.2 The geometrical definition of a Lie algebra 5 3. The exponential map 8 4. Baker-Campbell-Hausdorff formula 11 4.1 Statement and derivation 11 4.2 Two Important Special Cases 17 4.2.1 The Heisenberg algebra 17 4.2.2 All orders in B, first order in A 18 4.3 Region of convergence 19 5. Abstract Lie Algebras 19 5.1 Basic Definitions 19 5.2 Examples: Lie algebras of dimensions 1; 2; 3 23 5.3 Structure constants 25 5.4 Representations of Lie algebras and Ado's Theorem 26 6. Lie's theorem 28 7. Lie Algebras for the Classical Groups 34 7.1 A useful identity 35 7.2 GL(n; k) and SL(n; k) 35 7.3 O(n; k) 38 7.4 More general orthogonal groups 38 7.4.1 Lie algebra of SO∗(2n) 39 7.5 U(n) 39 7.5.1 U(p; q) 42 7.5.2 Lie algebra of SU ∗(2n) 42 7.6 Sp(2n) 42 8. Central extensions of Lie algebras and Lie algebra cohomology 46 8.1 Example: The Heisenberg Lie algebra and the Lie group associated to a symplectic vector space 47 8.2 Lie algebra cohomology 48 { 1 { 9.
    [Show full text]
  • Threads Through Group Theory
    Contemporary Mathematics Threads Through Group Theory Persi Diaconis Abstract. This paper records the path of a letter that Marty Isaacs wrote to a stranger. The tools in the letter are used to illustrate a different way of studying random walk on the Heisenberg group. The author also explains how the letter contributed to the development of super-character theory. 1. Introduction Marty Isaacs believes in answering questions. They can come from students, colleagues, or perfect strangers. As long as they seem serious, he usually gives it a try. This paper records the path of a letter that Marty wrote to a stranger (me). His letter was useful; it is reproduced in the Appendix and used extensively in Section 3. It led to more correspondence and a growing set of extensions. Here is some background. I am a mathematical statistician who, for some unknown reason, loves finite group theory. I was trying to make my own peace with a corner of p-group theory called extra-special p-groups. These are p-groups G with center Z(G) equal to commutator subgroup G0 equal to the cyclic group Cp. I found considerable literature about the groups [2, Sect. 23], [22, Chap. III, Sect. 13], [39, Chap. 4, Sect. 4] but no \stories". Where do these groups come from? Who cares about them, and how do they fit into some larger picture? My usual way to understand a group is to invent a \natural" random walk and study its rate of convergence to the uniform distribution. This often calls for detailed knowledge of the conjugacy classes, characters, and of the geometry of the Cayley graph underlying the walk.
    [Show full text]
  • IN FOURIER ANALYSIS on the HEISENBERG GROUP Daniel Bump, Persi Diaconis, Angela Hicks, Laurent Miclo, Harold Widom
    AN EXERCISE (?) IN FOURIER ANALYSIS ON THE HEISENBERG GROUP Daniel Bump, Persi Diaconis, Angela Hicks, Laurent Miclo, Harold Widom To cite this version: Daniel Bump, Persi Diaconis, Angela Hicks, Laurent Miclo, Harold Widom. AN EXERCISE (?) IN FOURIER ANALYSIS ON THE HEISENBERG GROUP. Annales de la Faculté des Sciences de Toulouse. Mathématiques., Université Paul Sabatier _ Cellule Mathdoc 2017, 26 (2), pp.263-288. hal-02022494 HAL Id: hal-02022494 https://hal.archives-ouvertes.fr/hal-02022494 Submitted on 18 Feb 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. AN EXERCISE (?) IN FOURIER ANALYSIS ON THE HEISENBERG GROUP DANIEL BUMP, PERSI DIACONIS, ANGELA HICKS, LAURENT MICLO, AND HAROLD WIDOM Abstract. Let H(n) be the group of 3 × 3 uni-uppertriangular matrices with entries in Z=nZ, the integers mod n. We show that the simple random walk converges to the uniform distribution in order n2 steps. The argument uses Fourier analysis and is surpris- ingly challenging. It introduces novel techniques for bounding the spectrum which are useful for a
    [Show full text]
  • Contact Structures of Arbitrary Codimension and Idempotents In
    CONTACT STRUCTURES OF ARBITRARY CODIMENSION AND IDEMPOTENTS IN THE HEISENBERG ALGEBRA ERIK VAN ERP Abstract. A contact manifold is a manifold equipped with a distribution of codimension one that satisfies a “maximal non-integrability” condition. A standard example of a contact structure is a strictly pseudoconvex CR manifold, and operators of analytic interest are the tangential Cauchy-Riemann operator ∂¯b and the Szeg¨oprojector onto the kernel of ∂¯b. The Heisenberg calculus is the pseudodifferential calculus originally developed for the analysis of these operators. We introduce a “non-integrability” condition for a distribution of arbitrary codimension that directly generalizes the definition of a contact structure. We call such distributions polycontact structures. We prove that the polycontact condition is equivalent to the existence of nontriv- ial projections in the Heisenberg calculus, and show that generalized Szeg¨oprojections exist on polycontact manifolds. We explore some geometrically interesting examples of polycontact structures. 1. Introduction Geometers have considered various ways to define contact structures in codimension 3. This paper is a contribution to this consideration from an analytic perspective. We propose here a notion of “contact structure” for distributions of arbitrary codimension. Roughly speaking, a polycontact structure is a distribution that is “maximally twisted” or “maximally non-integrable” (the precise definition is in section 2). Our definition is a straightforward generalization of the usual notion to higher codimensions. It is remarkable that this simple-minded generalization is equivalent to a non-trivial analytic property. Theorem 1. Let M be a compact connected manifold with distribution H ⊆ T M. Then there exist projections in the algebra of Heisenberg pseudodifferential operators on M that have infinite dimensional kernel and range if and only if H is polycontact.
    [Show full text]
  • Stone - Von Neumann Theorem
    (April 22, 2015) Stone - von Neumann theorem Paul Garrett [email protected] http:=/www.math.umn.edu/egarrett/ The bibliography indicates the origins of these issues in proving uniqueness, up to isomorphism, of the model for the commutation rules of quantum mechanics. As Mackey and Weil observed, some form of the argument succeeds in quite general circumstances. The argument here is an amalgam and adaptation of those in sources in the bibliography. The proof here applies nearly verbatim, with suitable adjustment of the notion of Schwartz space, to Heisenberg groups over other local fields in place of R. Fix a finite-dimensional R-vectorspace V with non-degenerate alternating form h; i. The corresponding Heisenberg group has Lie algebra h = V ⊕ R, with Lie bracket 0 0 0 [v + z; v + z ] = hv; v i 2 R ≈ f0g ⊕ R ⊂ V ⊕ R Exponentiating to the Lie group H, the group operation is hv; v0i exp(v + z) · exp(v0 + z0) = exp v + v0 + z + z0 + 2 This is explained by literal matrix exponentiation: for example, with v = (x; y) 2 R2, 0 1 0 xy 1 0 x z 1 x z + 2 exp(v + z) = exp @ 0 0 y A = @ 0 1 y A (with v = (x; y)) 0 0 0 0 0 1 We use Lie algebra coordinates on H, often suppressing the exponential on abelian subgroups, but reinstating an explicit exponential notation when necessary to avoid confusion. [0.0.1] Theorem: (Stone-vonNeumann) For fixed non-trivial unitary central character, up to isomorphism there is a unique irreducible unitary representation of the Heisenberg group with that central character.
    [Show full text]
  • Notes on Lie Groups, Lie Algebras, and the Exponentiation Map Mitchell Faulk
    Notes on Lie Groups, Lie algebras, and the Exponentiation Map Mitchell Faulk 1. Preliminaries. In these notes, we concern ourselves with special objects called matrix Lie groups and their corresponding Lie algebras, connected via the exponentiation map. Our main goal is to establish that the Lie algebra corresponding to a matrix Lie group is a real vector space. Thus, in order to further study a Lie group, one might more easily study its corresponding Lie algebra, since vector spaces are well-studied. In these notes, we refer to the set of m × n matrices over the field F by Mm×n(F ). We also use the notation R for field of real numbers and C for the field of complex numbers. We expect that the reader is familiar with the exponential of a matrix. Namely, if A is an n × n matrix, the matrix 1 X Ai eA := (1) i! i=0 is the exponential of A, often called \e to the A". It can be shown that the A exponential function exp : Mn×n(C) ! Mn×n(C) defined by exp(A) = e is well-defined with domain Mn×n(C). In other words, the series in (1) converges for each A 2 Mn×n(C). 2. Matrix Lie Groups. Before we can answer the question of what a matrix Lie group is, we must first define a group. Definition 1. A group is a set G together with a map ∗ : G × G ! G [∗(g1; g2) 7! g1 ∗ g1] such that 1. (g1 ∗ g2) ∗ g3 = g1 ∗ (g2 ∗ g3); g1; g2; g3 2 G; 2.
    [Show full text]