DOCSLIB.ORG

A Note on Orthogonal Similitude Groups

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
A Note on Orthogonal Similitude Groups Linear and Multilinear Algebra, Vol. 54, No. 6, December 2006, 391–396 A note on orthogonal similitude groups C. RYAN VINROOT* Department of Mathematics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai – 400 005, India Communicated by R. Guralnick (Received 18 October 2004; in final form 24 March 2005) Let V be a vector space over the field F such that charðF Þ 6¼ 2, and let V have a symmetric nondegenerate bilinear form. Let GOðV Þ be the orthogonal similitude group for this symmetric form, with similitude character . We prove that if g 2 GOðV Þ with ðgÞ¼ , then g ¼ t1t2 2 where t1 is an orthogonal involution, and t2 is such that t2 ¼ I and ðt2Þ¼ . As an applica- tion, we obtain an expression for the sum of the degrees of the irreducible characters of GOðn, FqÞ for odd q. Keywords: Similitude group; Orthogonal group; Factorization of matrices; Characters of the similitude group over a finite field 2000 AMS Subject Classifications: 15A23; 20G40 1. Introduction This note is an addendum to [1], where we obtain a factorization in the symplectic similitude group. In Theorem 1 below, we obtain a factorization in the group of orthogonal similitudes GOðV Þ, where V is an F-vector space with charðF Þ 6¼ 2, and the similitude character is . The method is the same as in [1], and the notation established there is freely used. In the proof of Theorem 1, we refer to [1] to all parts that immediately apply to the orthogonal case, while any changes that are needed in the proof are given specifically. As an application of Theorem 1, we use a result of Gow [2] on the orthogonal group over a finite field to obtain information on the characters of the orthogonal similitude group over a finite field, as given in Theorem 2 and Corollary 2. In a paper to appear by Adler and Prasad [3], Corollary 1 is used to prove a theorem on p-adic groups. In particular, if V is a vector space over a p-adic field, Adler and Prasad prove that any irreducible admissible representation of GOðV Þ restricted to OðV Þ is multiplicity free, and they also prove the corresponding statement for the symplectic similitude group. *Email: [email protected] Linear and Multilinear Algebra ISSN 0308-1087 print: ISSN 1563-5139 online ß 2006 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/03081080500209588 392 C. R. Vinroot 2. The main theorem Let V be an F-vector space, with charðF Þ 6¼ 2, equipped with a nondegenerate bilinear symmetric form h, i : V  V ! F. Then the orthogonal group of similitudes of V with respect to this form is the group GOðV Þ¼fg 2 GLðV Þ: hgv, gwi¼ðgÞhv, wi for some ðgÞ2F  for all v, w 2 V g. Then : GOðV Þ!F  is a multiplicative character called the similitude character, and the orthogonal group is OðV Þ¼kerðÞ. THEOREM 1 Let g be an element of GOðV Þ satisfying ðgÞ¼ . Then we may factor g as 2 g ¼ t1t2, where t1 is an orthogonal involution and t2 satisfies t2 ¼ I and ðt2Þ¼ . Proof Wonenburger [4] proved that any element of OðV Þ is the product of two orthogonal involutions. So if ðgÞ¼ is a square in F, then the theorem follows directly from Wonenburger’s result. So we assume is not a square. As in [1], for any monic polynomial f 2 F ½x of degree d, define the -adjoint of f to be f^ðxÞ¼f ð0ÞÀ1xdf ð=xÞ, and define a monic polynomial to be self- -adjoint if f^ ¼ f. Then, for any g 2 GOðV Þ, the minimal polynomial of g is self- -adjoint. All of the results in sections 2 and 3 of [1] are valid for transformations g which are self- -adjoint, as the proofs only use this fact. These results reduce us to looking at the case that either g is a cyclic transformation for V, that is V is generated by vectors of the form giv for some v 2 V, or the case that g has minimal polynomial of the form qðxÞs, where q(x) is an irreducible self- -adjoint polynomial. We deal with the cyclic case first. In [1, Proposition 3(i)], we prove that if g 2 GLðV Þ is a cyclic transformation with self- -adjoint minimal polynomial, ignoring any inner 2 2 product structure, then we can factor g ¼ t1t2 such that t1 ¼ I and t2 ¼ I. This is proven as follows. If V is cyclic for the vector v, then we let P be the space spanned by vectors of the form ðgi þ igiÞv and let Q be the space spanned by the vectors of the form ðgi À igiÞv. Then V ¼ P È Q, and the transformation having P as its þ1 eigen- space and Q as its À1 eigenspace is exactly the involution t1 that we seek. For the case i i i that g 2 GOðV Þ, we must show that this t1 is orthogonal. Let ðg þ g Þv 2 P and ðg j À jg jÞv 2 Q. Then we have: ðgi þ igÀiÞv, ðg j À jgÀjÞv ¼ðgi þ igÀiÞv, g jv À giv, jgÀjv À igÀiv, jg Àjv ¼ðgi þ igÀiÞv, g jv À g jv, igÀiv À g jv, giv ¼ðgi þ igÀiÞv, g jv À g jv, ðgi þ igÀiÞv ¼ 0: So P and Q are mutually orthogonal. Now let u and u0 be any two vectors in V ¼ P È Q. 0 0 0 0 0 0 Write u ¼ w þ y, u ¼ w þ y , where w, w 2 P and y, y 2 Q. We compute ht1u, t1u i: 0 ¼ ð þ Þ ð 0 þ 0Þ ¼ À 0 À 0 t1u, t1u t1 w y , t1 w y w y, w y ¼ 0 þ 0 À 0 À 0 w, w y, y y, w w, y ¼ w, w0 þ y, y0 : A note on orthogonal similitude groups 393 While computing hu, u0i gives us: u, u0 ¼ w þ y, w0 þ y0 ¼ w, w0 þ y, y0 þ y, w0 þ w, y0 ¼ w, w0 þ y, y0 : 0 0 Therefore, we have ht1u, t1u i¼hu, u i, and t1 is orthogonal. Since g satisfies ðgÞ¼ , then t2 ¼ t1g satisfies ðt2Þ¼ . We now deal with the case that the minimal polynomial of g is of the form qðxÞs, where q(x) is irreducible and self- -adjoint. It follows from [1, Lemmas 4 and 5] and the cyclic case above that we may assume that V is the sum of two cyclic spaces, and sÀ1 further we may assume that for any u1 2 V satisfying qðgÞ u1 6¼ 0, the cyclic space U1 generated by u1 is degenerate, and for an appropriate u2 2 V,wehave V ¼ U1 È U2 where U2 is the cyclic space generated by u2. We follow the proof of [1, Proposition 3(iii)]. We may write U1 ¼ P1 È Q1 where P1 is spanned by vectors k k Àk k k Àk of the form ðg þ g Þu1 and Q1 is spanned by vectors of the form ðg À g Þu1. There are two different cases, the first is when either q(x) is relatively prime to x2 À 2 ? or qðxÞ¼x À and s is odd. In this case, we find a u2 2 Q1 , and V ¼ U1 È U2 where U2 is cyclically generated by u2. Then U2 ¼ P2 È Q2, where P2 is spanned by vectors k k Àk k k Àk of the form ðg þ g Þu2 and Q2 is spanned by vectors of the form ðg À g Þu2. Letting P ¼ P1 È P2 and Q ¼ Q1 È Q2, we are able to show that if t1 is the involution 2 with P as its þ1 eigenspace and Q as its À1 eigenspace, then ðt1gÞ ¼ I (for the symplectic group, we actually show this in the second case of Proposition 3(iii)). We need to show that t1 is orthogonal. From the cyclic case above, we have Pi ? Qi for i ¼ 1, 2. In the proof of [1, Proposition 3(iii)], we show that Pi ? Qj for i 6¼ j. So now P ? Q, and from the argument in the cyclic case above, we have that t1 is orthogonal, and so t2 ¼ t1g satisfies ðt2Þ¼ . 2 ? In the case that qðxÞ¼x À and s is even, we are able to find a u2 2 P1 such that V ¼ U1 È U2, where U2 is the space cyclically generated by u2. We define P2 and Q2 as before. We let P ¼ P1 È Q2 be the þ1 eigenspace and Q ¼ P2 È Q1 be the À1 eigen- 2 space of an involution t1, and this satisfies ðt1gÞ ¼ I. To show t1 is orthogonal, we need only show that P ? Q and appeal to the cyclic case above. We have already shown Pi ? Qi for i ¼ 1, 2, so now we need P1 ? P2 and Q1 ? Q2. We have: ÀÁÀÁ k k Àk l l Àl g Æ g u1, g Æ g u2 ÀÁ ÀÁ k k Àk l k k Àk l Àl ¼ g Æ g u1, g u2 Æ g Æ g u1, g u2 ÀÁ ÀÁ l Àl k k Àk l k k Àk ¼ g g Æ g u1, u2 Æ g g Æ g u1, u2 ÀÁÀÁ l l Àl k k Àk ¼ g Æ g g Æ g u1, u2 ¼ 0, since ÀÁÀÁ l l Àl k k Àk g Æ g g Æ g u1 ÀÁÀÁ lþk lþk ðlþkÞ k lÀk lÀk ðlÀkÞ ¼ g þ g u1 Æ g þ g u1 2 P1 394 C.
Load more

Recommended publications
  • INTEGER POINTS and THEIR ORTHOGONAL LATTICES 2 to Remove the Congruence Condition
    INTEGER POINTS ON SPHERES AND THEIR ORTHOGONAL LATTICES MENNY AKA, MANFRED EINSIEDLER, AND URI SHAPIRA (WITH AN APPENDIX BY RUIXIANG ZHANG) Abstract. Linnik proved in the late 1950’s the equidistribution of in- teger points on large spheres under a congruence condition. The congru- ence condition was lifted in 1988 by Duke (building on a break-through by Iwaniec) using completely different techniques. We conjecture that this equidistribution result also extends to the pairs consisting of a vector on the sphere and the shape of the lattice in its orthogonal complement. We use a joining result for higher rank diagonalizable actions to obtain this conjecture under an additional congruence condition. 1. Introduction A theorem of Legendre, whose complete proof was given by Gauss in [Gau86], asserts that an integer D can be written as a sum of three squares if and only if D is not of the form 4m(8k + 7) for some m, k N. Let D = D N : D 0, 4, 7 mod8 and Z3 be the set of primitive∈ vectors { ∈ 6≡ } prim in Z3. Legendre’s Theorem also implies that the set 2 def 3 2 S (D) = v Zprim : v 2 = D n ∈ k k o is non-empty if and only if D D. This important result has been refined in many ways. We are interested∈ in the refinement known as Linnik’s problem. Let S2 def= x R3 : x = 1 . For a subset S of rational odd primes we ∈ k k2 set 2 D(S)= D D : for all p S, D mod p F× .
    [Show full text]
  • Unitary Group - Wikipedia
    Unitary group - Wikipedia https://en.wikipedia.org/wiki/Unitary_group Unitary group In mathematics, the unitary group of degree n, denoted U( n), is the group of n × n unitary matrices, with the group operation of matrix multiplication. The unitary group is a subgroup of the general linear group GL( n, C). Hyperorthogonal group is an archaic name for the unitary group, especially over finite fields. For the group of unitary matrices with determinant 1, see Special unitary group. In the simple case n = 1, the group U(1) corresponds to the circle group, consisting of all complex numbers with absolute value 1 under multiplication. All the unitary groups contain copies of this group. The unitary group U( n) is a real Lie group of dimension n2. The Lie algebra of U( n) consists of n × n skew-Hermitian matrices, with the Lie bracket given by the commutator. The general unitary group (also called the group of unitary similitudes ) consists of all matrices A such that A∗A is a nonzero multiple of the identity matrix, and is just the product of the unitary group with the group of all positive multiples of the identity matrix. Contents Properties Topology Related groups 2-out-of-3 property Special unitary and projective unitary groups G-structure: almost Hermitian Generalizations Indefinite forms Finite fields Degree-2 separable algebras Algebraic groups Unitary group of a quadratic module Polynomial invariants Classifying space See also Notes References Properties Since the determinant of a unitary matrix is a complex number with norm 1, the determinant gives a group 1 of 7 2/23/2018, 10:13 AM Unitary group - Wikipedia https://en.wikipedia.org/wiki/Unitary_group homomorphism The kernel of this homomorphism is the set of unitary matrices with determinant 1.
    [Show full text]
  • 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.
    [Show full text]
  • 10 Group Theory
    10 Group theory 10.1 What is a group? A group G is a set of elements f, g, h, ... and an operation called multipli- cation such that for all elements f,g, and h in the group G: 1. The product fg is in the group G (closure); 2. f(gh)=(fg)h (associativity); 3. there is an identity element e in the group G such that ge = eg = g; 1 1 1 4. every g in G has an inverse g− in G such that gg− = g− g = e. Physical transformations naturally form groups. The elements of a group might be all physical transformations on a given set of objects that leave invariant a chosen property of the set of objects. For instance, the objects might be the points (x, y) in a plane. The chosen property could be their distances x2 + y2 from the origin. The physical transformations that leave unchanged these distances are the rotations about the origin p x cos ✓ sin ✓ x 0 = . (10.1) y sin ✓ cos ✓ y ✓ 0◆ ✓− ◆✓ ◆ These rotations form the special orthogonal group in 2 dimensions, SO(2). More generally, suppose the transformations T,T0,T00,... change a set of objects in ways that leave invariant a chosen property property of the objects. Suppose the product T 0 T of the transformations T and T 0 represents the action of T followed by the action of T 0 on the objects. Since both T and T 0 leave the chosen property unchanged, so will their product T 0 T . Thus the closure condition is satisfied.
    [Show full text]
  • Matrix Lie Groups and Their Lie Algebras
    Matrix Lie groups and their Lie algebras Alen Alexanderian∗ Abstract We discuss matrix Lie groups and their corresponding Lie algebras. Some common examples are provided for purpose of illustration. 1 Introduction The goal of these brief note is to provide a quick introduction to matrix Lie groups which are a special class of abstract Lie groups. Study of matrix Lie groups is a fruitful endeavor which allows one an entry to theory of Lie groups without requiring knowl- edge of differential topology. After all, most interesting Lie groups turn out to be matrix groups anyway. An abstract Lie group is defined to be a group which is also a smooth manifold, where the group operations of multiplication and inversion are also smooth. We provide a much simple definition for a matrix Lie group in Section 4. Showing that a matrix Lie group is in fact a Lie group is discussed in standard texts such as [2]. We also discuss Lie algebras [1], and the computation of the Lie algebra of a Lie group in Section 5. We will compute the Lie algebras of several well known Lie groups in that section for the purpose of illustration. 2 Notation Let V be a vector space. We denote by gl(V) the space of all linear transformations on V. If V is a finite-dimensional vector space we may put an arbitrary basis on V and identify elements of gl(V) with their matrix representation. The following define various classes of matrices on Rn: ∗The University of Texas at Austin, USA. E-mail: [email protected] Last revised: July 12, 2013 Matrix Lie groups gl(n) : the space of n
    [Show full text]
  • 1 the Spin Homomorphism SL2(C) → SO1,3(R) a Summary from Multiple Sources Written by Y
    1 The spin homomorphism SL2(C) ! SO1;3(R) A summary from multiple sources written by Y. Feng and Katherine E. Stange Abstract We will discuss the spin homomorphism SL2(C) ! SO1;3(R) in three manners. Firstly we interpret SL2(C) as acting on the Minkowski 1;3 spacetime R ; secondly by viewing the quadratic form as a twisted 1 1 P × P ; and finally using Clifford groups. 1.1 Introduction The spin homomorphism SL2(C) ! SO1;3(R) is a homomorphism of classical matrix Lie groups. The lefthand group con- sists of 2 × 2 complex matrices with determinant 1. The righthand group consists of 4 × 4 real matrices with determinant 1 which preserve some fixed real quadratic form Q of signature (1; 3). This map is alternately called the spinor map and variations. The image of this map is the identity component + of SO1;3(R), denoted SO1;3(R). The kernel is {±Ig. Therefore, we obtain an isomorphism + PSL2(C) = SL2(C)= ± I ' SO1;3(R): This is one of a family of isomorphisms of Lie groups called exceptional iso- morphisms. In Section 1.3, we give the spin homomorphism explicitly, al- though these formulae are unenlightening by themselves. In Section 1.4 we describe O1;3(R) in greater detail as the group of Lorentz transformations. This document describes this homomorphism from three distinct per- spectives. The first is very concrete, and constructs, using the language of Minkowski space, Lorentz transformations and Hermitian matrices, an ex- 4 plicit action of SL2(C) on R preserving Q (Section 1.5).
    [Show full text]
  • The Classical Matrix Groups 1 Groups
    The Classical Matrix Groups CDs 270, Spring 2010/2011 The notes provide a brief review of matrix groups. The primary goal is to motivate the lan- guage and symbols used to represent rotations (SO(2) and SO(3)) and spatial displacements (SE(2) and SE(3)). 1 Groups A group, G, is a mathematical structure with the following characteristics and properties: i. the group consists of a set of elements {gj} which can be indexed. The indices j may form a finite, countably infinite, or continous (uncountably infinite) set. ii. An associative binary group operation, denoted by 0 ∗0 , termed the group product. The product of two group elements is also a group element: ∀ gi, gj ∈ G gi ∗ gj = gk, where gk ∈ G. iii. A unique group identify element, e, with the property that: e ∗ gj = gj for all gj ∈ G. −1 iv. For every gj ∈ G, there must exist an inverse element, gj , such that −1 gj ∗ gj = e. Simple examples of groups include the integers, Z, with addition as the group operation, and the real numbers mod zero, R − {0}, with multiplication as the group operation. 1.1 The General Linear Group, GL(N) The set of all N × N invertible matrices with the group operation of matrix multiplication forms the General Linear Group of dimension N. This group is denoted by the symbol GL(N), or GL(N, K) where K is a field, such as R, C, etc. Generally, we will only consider the cases where K = R or K = C, which are respectively denoted by GL(N, R) and GL(N, C).
    [Show full text]
  • Special Unitary Group - Wikipedia
    Special unitary group - Wikipedia https://en.wikipedia.org/wiki/Special_unitary_group Special unitary group In mathematics, the special unitary group of degree n, denoted SU( n), is the Lie group of n×n unitary matrices with determinant 1. (More general unitary matrices may have complex determinants with absolute value 1, rather than real 1 in the special case.) The group operation is matrix multiplication. The special unitary group is a subgroup of the unitary group U( n), consisting of all n×n unitary matrices. As a compact classical group, U( n) is the group that preserves the standard inner product on Cn.[nb 1] It is itself a subgroup of the general linear group, SU( n) ⊂ U( n) ⊂ GL( n, C). The SU( n) groups find wide application in the Standard Model of particle physics, especially SU(2) in the electroweak interaction and SU(3) in quantum chromodynamics.[1] The simplest case, SU(1) , is the trivial group, having only a single element. The group SU(2) is isomorphic to the group of quaternions of norm 1, and is thus diffeomorphic to the 3-sphere. Since unit quaternions can be used to represent rotations in 3-dimensional space (up to sign), there is a surjective homomorphism from SU(2) to the rotation group SO(3) whose kernel is {+ I, − I}. [nb 2] SU(2) is also identical to one of the symmetry groups of spinors, Spin(3), that enables a spinor presentation of rotations. Contents Properties Lie algebra Fundamental representation Adjoint representation The group SU(2) Diffeomorphism with S 3 Isomorphism with unit quaternions Lie Algebra The group SU(3) Topology Representation theory Lie algebra Lie algebra structure Generalized special unitary group Example Important subgroups See also 1 of 10 2/22/2018, 8:54 PM Special unitary group - Wikipedia https://en.wikipedia.org/wiki/Special_unitary_group Remarks Notes References Properties The special unitary group SU( n) is a real Lie group (though not a complex Lie group).
    [Show full text]
  • 1 Classical Groups (Problems Sets 1 – 5)
    1 Classical Groups (Problems Sets 1 { 5) De¯nitions. A nonempty set G together with an operation ¤ is a group provided ² The set is closed under the operation. That is, if g and h belong to the set G, then so does g ¤ h. ² The operation is associative. That is, if g; h and k are any elements of G, then g ¤ (h ¤ k) = (g ¤ h) ¤ k. ² There is an element e of G which is an identity for the operation. That is, if g is any element of G, then g ¤ e = e ¤ g = g. ² Every element of G has an inverse in G. That is, if g is in G then there is an element of G denoted g¡1 so that g ¤ g¡1 = g¡1 ¤ g = e. The group G is abelian if g ¤ h = h ¤ g for all g; h 2 G. A nonempty subset H ⊆ G is a subgroup of the group G if H is itself a group with the same operation ¤ as G. That is, H is a subgroup provided ² H is closed under the operation. ² H contains the identity e. ² Every element of H has an inverse in H. 1.1 Groups of symmetries. Symmetries are invertible functions from some set to itself preserving some feature of the set (shape, distance, interval, :::). A set of symmetries of a set can form a group using the operation composition of functions. If f and g are functions from a set X to itself, then the composition of f and g is denoted f ± g, and it is de¯ned by (f ± g)(x) = f(g(x)) for x in X.
    [Show full text]
  • 18.704 Supplementary Notes March 23, 2005 the Subgroup Ω For
    18.704 Supplementary Notes March 23, 2005 The subgroup Ω for orthogonal groups In the case of the linear group, it is shown in the text that P SL(n; F ) (that is, the group SL(n) of determinant one matrices, divided by its center) is usually a simple group. In the case of symplectic group, P Sp(2n; F ) (the group of symplectic matrices divided by its center) is usually a simple group. In the case of the orthog- onal group (as Yelena will explain on March 28), what turns out to be simple is not P SO(V ) (the orthogonal group of V divided by its center). Instead there is a mysterious subgroup Ω(V ) of SO(V ), and what is usually simple is P Ω(V ). The purpose of these notes is first to explain why this complication arises, and then to give the general definition of Ω(V ) (along with some of its basic properties). So why should the complication arise? There are some hints of it already in the case of the linear group. We made a lot of use of GL(n; F ), the group of all invertible n × n matrices with entries in F . The most obvious normal subgroup of GL(n; F ) is its center, the group F × of (non-zero) scalar matrices. Dividing by the center gives (1) P GL(n; F ) = GL(n; F )=F ×; the projective general linear group. We saw that this group acts faithfully on the projective space Pn−1(F ), and generally it's a great group to work with.
    [Show full text]
  • Volumes of Orthogonal Groups and Unitary Groups Are Very Useful in Physics and Mathematics [3, 4]
    Volumes of Orthogonal Groups and Unitary Groups ∗ Lin Zhang Institute of Mathematics, Hangzhou Dianzi University, Hangzhou 310018, PR China Abstract The matrix integral has many applications in diverse fields. This review article begins by presenting detailed key background knowledge about matrix integral. Then the volumes of orthogonal groups and unitary groups are computed, respectively. As a unification, we present Mcdonald’s volume formula for a compact Lie group. With this volume formula, one can easily derives the volumes of orthogonal groups and unitary groups. Applications are also presented as well. Specifically, The volume of the set of mixed quantum states is computed by using the volume of unitary group. The volume of a metric ball in unitary group is also computed as well. There are no new results in this article, but only detailed and elementary proofs of existing results. The purpose of the article is pedagogical, and to collect in one place many, if not all, of the quantum information applications of the volumes of orthogonal and unitary groups. arXiv:1509.00537v5 [math-ph] 3 Nov 2017 ∗ E-mail: [email protected]; [email protected] 1 Contents 1 Introduction 3 2 Volumes of orthogonal groups 5 2.1 Preliminary ..................................... ..... 5 2.2 Thecomputationofvolumes . ....... 17 3 Volumes of unitary groups 28 3.1 Preliminary ..................................... ..... 28 3.2 Thecomputationofvolumes . ....... 44 4 The volume of a compact Lie group 55 5 Applications 73 5.1 Hilbert-Schmidt volume of the set of mixed quantum states.............. 73 5.2 Areaoftheboundaryofthesetofmixedstates . ........... 77 5.3 Volumeofametricballinunitarygroup . .......... 79 6 Appendix I: Volumes of a sphere and a ball 85 7 Appendix II: Some useful facts 89 7.1 Matrices with simple eigenvalues form open dense sets of fullmeasure .
    [Show full text]
  • COMPACT LIE GROUPS Contents 1. Smooth Manifolds and Maps 1 2
    COMPACT LIE GROUPS NICHOLAS ROUSE Abstract. The first half of the paper presents the basic definitions and results necessary for investigating Lie groups. The primary examples come from the matrix groups. The second half deals with representation theory of groups, particularly compact groups. The end result is the Peter-Weyl theorem. Contents 1. Smooth Manifolds and Maps 1 2. Tangents, Differentials, and Submersions 3 3. Lie Groups 5 4. Group Representations 8 5. Haar Measure and Applications 9 6. The Peter-Weyl Theorem and Consequences 10 Acknowledgments 14 References 14 1. Smooth Manifolds and Maps Manifolds are spaces that, in a certain technical sense defined below, look like Euclidean space. Definition 1.1. A topological manifold of dimension n is a topological space X satisfying: (1) X is second-countable. That is, for the space's topology T , there exists a countable base, which is a countable collection of open sets fBαg in X such that every set in T is the union of a subcollection of fBαg. (2) X is a Hausdorff space. That is, for every pair of points x; y 2 X, there exist open subsets U; V ⊆ X such that x 2 U, y 2 V , and U \ V = ?. (3) X is locally homeomorphic to open subsets of Rn. That is, for every point x 2 X, there exists a neighborhood U ⊆ X of x such that there exists a continuous bijection with a continuous inverse (i.e. a homeomorphism) from U to an open subset of Rn. The first two conditions preclude pathological spaces that happen to have a homeomorphism into Euclidean space, and force a topological manifold to share more properties with Euclidean space.
    [Show full text]