DOCSLIB.ORG

Lecture 1: Group C*-Algebras and Actions of Finite Groups on C*-Algebras 11–29 July 2016

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Lecture 1: Group C*-Algebras and Actions of Finite Groups on C*-Algebras 11–29 July 2016 The Second Summer School on Operator Algebras and Noncommutative Geometry 2016 East China Normal University, Shanghai Lecture 1: Group C*-algebras and Actions of Finite Groups on C*-Algebras 11{29 July 2016 Lecture 1 (11 July 2016): Group C*-algebras and Actions of Finite N. Christopher Phillips Groups on C*-Algebras Lecture 2 (13 July 2016): Introduction to Crossed Products and More University of Oregon Examples of Actions. Lecture 3 (15 July 2016): Crossed Products by Finite Groups; the 11 July 2016 Rokhlin Property. Lecture 4 (18 July 2016): Crossed Products by Actions with the Rokhlin Property. Lecture 5 (19 July 2016): Crossed Products of Tracially AF Algebras by Actions with the Tracial Rokhlin Property. Lecture 6 (20 July 2016): Applications and Problems. N. C. Phillips (U of Oregon) Group C*-Algebras, Actions of Finite Groups 11 July 2016 1 / 28 N. C. Phillips (U of Oregon) Group C*-Algebras, Actions of Finite Groups 11 July 2016 2 / 28 A rough outline of all six lectures General motivation The material to be described is part of the structure and classification The beginning: The C*-algebra of a group. theory for simple nuclear C*-algebras (the Elliott program). More Actions of finite groups on C*-algebras and examples. specifically, it is about proving that C*-algebras which appear in other Crossed products by actions of finite groups: elementary theory. parts of the theory (in these lectures, certain kinds of crossed product C*-algebras) satisfy the hypotheses of known classification theorems. More examples of actions. Crossed products by actions of finite groups: Some examples. To keep things from being too complicated, we will consider crossed The Rokhlin property for actions of finite groups. products by actions of finite groups. Nevertheless, even in this case, one Examples of actions with the Rokhlin property. can see some of the techniques which are important in more general cases. Crossed products of AF algebras by actions with the Rokhlin property. Crossed product C*-algebras have long been important in operator Other crossed products by actions with the Rokhlin property. algebras, for reasons having nothing to do with the Elliott program. It has The tracial Rokhlin property for actions of finite groups. generally been difficult to prove that crossed products are classifiable, and there are really only three cases in which there is a somewhat satisfactory Examples of actions with the tracial Rokhlin property. theory: actions of finite groups on simple C*-algebras, free minimal actions Crossed products by actions with the tracial Rokhlin property. d of groups which are not too complicated (mostly, not too far from Z ) on Applications of the tracial Rokhlin property. compact metric spaces, and \strongly outer" actions of such groups on simple C*-algebras. N. C. Phillips (U of Oregon) Group C*-Algebras, Actions of Finite Groups 11 July 2016 3 / 28 N. C. Phillips (U of Oregon) Group C*-Algebras, Actions of Finite Groups 11 July 2016 4 / 28 Background Do the exercises! These lectures assume some familiarity with the basic theory of C*-algebras, as found, for example, in Murphy's book. K-theory will be There will be many exercises given, with varying levels of difficulty. To occasionally used, but not in an essential way. A few other concepts will really get to understand this material, please do them! be important, such as tracial rank zero. They will be defined as needed, and some basic properties mentioned, usually without proof. Various side comments will assume more background, but these can be skipped. (Many I am happy to talk to people about the exercises. side comments which should be made will be omitted entirely, for lack of time.) N. C. Phillips (U of Oregon) Group C*-Algebras, Actions of Finite Groups 11 July 2016 5 / 28 N. C. Phillips (U of Oregon) Group C*-Algebras, Actions of Finite Groups 11 July 2016 6 / 28 The beginning: The group ring The beginning: The group ring (continued) Let G be a finite group. Its group ring C[G] (a standard construction in G is a discrete group, and algebra) is, as a vector space, the set of formal linear combinations ( ) X X C[G] = ag · g : ag 2 C, ag = 0 for all but finitely many g 2 G : ag · g (1) g2G g2G Multiplication: (a · g)(b · h) = (ab) · (gh), extended linearly. of group elements with coefficients ag 2 C. (Formally: the free C-module on G.) The multiplication is (a · g)(b · h) = (ab) · (gh) for g; h 2 G and Recall the usual polynomial ring C[x]. Let hSi denote the ideal generated a; b 2 C, extended linearly. That is, the product comes from the group. by a set S. Also, abbreviate Z=nZ to Zn. (We won't use p-adic integers.) G need not be finite (but must be discrete), provided that in (1) one uses Example ∼ 2 only finite sums (ag = 0 for all but finitely many g 2 G). Take G = Z2. Then C[G] = C[x]=hx − 1i. The identity of the group is 1 Exercise and the nontrivial element is x. Prove that C[G] is an associative unital algebra over C. Exercise (easy) Check the statements made in the previous example. One can use any field (even ring) in place of C. The algebraists actually do this. Exercise (easy) Motivation: Representation theory (brief comments below). Generalize the previous example to Zn for n 2 N. N. C. Phillips (U of Oregon) Group C*-Algebras, Actions of Finite Groups 11 July 2016 7 / 28 N. C. Phillips (U of Oregon) Group C*-Algebras, Actions of Finite Groups 11 July 2016 8 / 28 The beginning: The group ring (continued) The beginning: The C*-algebra of a group G is a discrete group, and G is a discrete group, and ( ) X ( ) C[G] = ag · g : ag 2 C, ag = 0 for all but finitely many g 2 G : X C[G] = ag · g : ag 2 C, ag = 0 for all but finitely many g 2 G : g2G g2G Multiplication: (a · g)(b · h) = (ab) · (gh), extended linearly. ∗ −1 Multiplication: (a · g)(b · h) = (ab) · (gh), extended linearly. Make C[G] a *-algebra by making the group elements unitary: g = g . C*-algebraists thus usually write ug for the element g 2 C[G]. So ∼ 2 Recall: C[Z2] = C[x]=hx − 1i, with the nontrivial group element being x. ( ) X Example C[G] = ag · ug : ag 2 C, ag = 0 for all but finitely many g 2 G : g2G ∼ −1 Take G = Z. Then C[G] = C[x; x ], the ring of Laurent polynomials in k The multiplication is (a · ug )(b · uh) = (ab) · ugh for g; h 2 G and a; b 2 C, one variable. The group element k 2 Z is x . ∗ extended linearly, and the adjoint is (a · ug ) = aug −1 . Exercise (easy) Exercise Show that this adjoint in [G] is well defined, conjugate linear, reverses Check the statements made in the previous example. C multiplication: (xy)∗ = y ∗x∗, and satisfies (x∗)∗ = x. N. C. Phillips (U of Oregon) Group C*-Algebras, Actions of Finite Groups 11 July 2016 9 / 28 N. C. Phillips (U of Oregon) Group C*-Algebras, Actions of Finite Groups 11 July 2016 10 / 28 The beginning: The C*-algebra of a group (continued) The beginning: The C*-algebra of a group (continued) G is a discrete group, and G is a discrete group. Multiplication in C[G]: (a · ug )(b · uh) = (ab) · ugh. ∗ ( ) Adjoint: (a · u ) = au −1 . X g g C[G] = ag · ug : ag 2 C, ag = 0 for all but finitely many g 2 G : g2G If w is a unitary representation of G on a Hilbert space H (a group ∗ Multiplication: (a · ug )(b · uh) = (ab) · ugh. Adjoint: (a · ug ) = aug −1 . homomorphism g 7! wg from G to the group U(H) of unitary operators on H), then the unital *-homomorphism π : [G] ! L(H) is We still need a norm. We will assume G is finite; things are otherwise w C ! more complicated. First, a bit of representation theory. X X πw ag · ug = ag · wg : Let G be a discrete group, and let H be a Hilbert space. Let w be a g2G g2G unitary representation of G on H: a group homomorphism g 7! wg from G to the group U(H) of unitary operators on H. Then we define a linear Theorem map πw : [G] ! L(H) by C The assignment w 7! πw is a bijection from unitary representations of G ! X X on H to unital *-homomorphisms C[G] ! L(H). πw ag · ug = ag · wg : g2G g2G Exercise Exercise Prove this theorem. (To recover w from πw , look at πw (ug ).) Prove that πw is a unital *-homomorphism. N. C. Phillips (U of Oregon) Group C*-Algebras, Actions of Finite Groups 11 July 2016 11 / 28 N. C. Phillips (U of Oregon) Group C*-Algebras, Actions of Finite Groups 11 July 2016 12 / 28 The left regular representation The norm on C ∗(G) G is a discrete group. Multiplication in C[G]: (a · ug )(b · uh) = (ab) · ugh. G is a discrete group. If w is a unitary representation of G on H, then ∗ Adjoint: (a · u ) = au −1 . P P g g πw : C[G] ! L(H) is πw g2G ag · ug = g2G ag · wg . Take H = l2(G), and let z be the left regular representation: If w is a unitary representation of G on H, then πw : [G] ! L(H) is the C −1 P P (zg ξ)(h) = ξ(g h).
Load more

Recommended publications
  • Physics.Hist-Ph] 15 May 2018 Meitl,Fo Hsadsadr Nepeainlprinciples
    Why Be Regular?, Part I Benjamin Feintzeig Department of Philosophy University of Washington JB (Le)Manchak, Sarita Rosenstock, James Owen Weatherall Department of Logic and Philosophy of Science University of California, Irvine Abstract We provide a novel perspective on “regularity” as a property of representations of the Weyl algebra. We first critique a proposal by Halvorson [2004, “Complementarity of representa- tions in quantum mechanics”, Studies in History and Philosophy of Modern Physics 35(1), pp. 45–56], who argues that the non-regular “position” and “momentum” representations of the Weyl algebra demonstrate that a quantum mechanical particle can have definite values for position or momentum, contrary to a widespread view. We show that there are obstacles to such an intepretation of non-regular representations. In Part II, we propose a justification for focusing on regular representations, pace Halvorson, by drawing on algebraic methods. 1. Introduction It is standard dogma that, according to quantum mechanics, a particle does not, and indeed cannot, have a precise value for its position or for its momentum. The reason is that in the standard Hilbert space representation for a free particle—the so-called Schr¨odinger Representation of the Weyl form of the canonical commutation relations (CCRs)—there arXiv:1805.05568v1 [physics.hist-ph] 15 May 2018 are no eigenstates for the position and momentum magnitudes, P and Q; the claim follows immediately, from this and standard interpretational principles.1 Email addresses: [email protected] (Benjamin Feintzeig), [email protected] (JB (Le)Manchak), [email protected] (Sarita Rosenstock), [email protected] (James Owen Weatherall) 1Namely, the Eigenstate–Eigenvalue link, according to which a system has an exact value of a given property if and only if its state is an eigenstate of the operator associated with that property.
    [Show full text]
  • Fourier Analysis Using Representation Theory
    FOURIER ANALYSIS USING REPRESENTATION THEORY NEEL PATEL Abstract. In this paper, it is our goal to develop the concept of Fourier transforms by using Representation Theory. We begin by laying basic def- initions that will help us understand the definition of a representation of a group. Then, we define representations and provide useful definitions and the- orems. We study representations and key theorems about representations such as Schur's Lemma. In addition, we develop the notions of irreducible repre- senations, *-representations, and equivalence classes of representations. After doing so, we develop the concept of matrix realizations of irreducible represen- ations. This concept will help us come up with a few theorems that lead up to our study of the Fourier transform. We will develop the definition of a Fourier transform and provide a few observations about the Fourier transform. Contents 1. Essential Definitions 1 2. Group Representations 3 3. Tensor Products 7 4. Matrix Realizations of Representations 8 5. Fourier Analysis 11 Acknowledgments 12 References 12 1. Essential Definitions Definition 1.1. An internal law of composition on a set R is a product map P : R × R ! R Definition 1.2. A group G, is a set with an internal law of composition such that: (i) P is associative. i.e. P (x; P (y; z)) = P (P (x; y); z) (ii) 9 an identity; e; 3 if x 2 G; then P (x; e) = P (e; x) = x (iii) 9 inverses 8x 2 G, denoted by x−1, 3 P (x; x−1) = P (x−1; x) = e: Let it be noted that we shorthand P (x; y) as xy.
    [Show full text]
  • Factorization of Unitary Representations of Adele Groups Paul Garrett [email protected]
    (February 19, 2005) Factorization of unitary representations of adele groups Paul Garrett [email protected] http://www.math.umn.edu/˜garrett/ The result sketched here is of fundamental importance in the ‘modern’ theory of automorphic forms and L-functions. What it amounts to is proof that representation theory is in principle relevant to study of automorphic forms and L-function. The goal here is to get to a coherent statement of the basic factorization theorem for irreducible unitary representations of reductive linear adele groups, starting from very minimal prerequisites. Thus, the writing is discursive and explanatory. All necessary background definitions are given. There are no proofs. Along the way, many basic concepts of wider importance are illustrated, as well. One disclaimer is necessary: while in principle this factorization result makes it clear that representation theory is relevant to study of automorphic forms and L-functions, in practice there are other things necessary. In effect, one needs to know that the representation theory of reductive linear p-adic groups is tractable, so that conversion of other issues into representation theory is a change for the better. Thus, beyond the material here, one will need to know (at least) the basic properties of spherical and unramified principal series representations of reductive linear groups over local fields. The fact that in some sense there is just one irreducible representation of a reductive linear p-adic group will have to be pursued later. References and historical notes
    [Show full text]
  • 23 Infinite Dimensional Unitary Representations
    23 Infinite dimensional unitary representations Last lecture, we found the finite dimensional (non-unitary) representations of SL2(R). 23.1 Background about infinite dimensional representa- tions (of a Lie group G) What is an infinite dimensional representation? 1st guess Banach space acted on by G? This is no good for the following reasons: Look at the action of G on the functions on G (by left translation). We could use L2 functions, or L1 or Lp. These are completely different Banach spaces, but they are essentially the same representation. 2nd guess Hilbert space acted on by G? This is sort of okay. The problem is that finite dimensional representations of SL2(R) are NOT Hilbert space representations, so we are throwing away some interesting representations. Solution (Harish-Chandra) Take g to be the Lie algebra of G, and let K be the maximal compact subgroup. If V is an infinite dimensional representation of G, there is no reason why g should act on V . The simplest example fails. Let R act on L2(R) by left translation. Then d d 2 R d the Lie algebra is generated by dx (or i dx ) acting on L ( ), but dx of an L2 function is not in L2 in general. Let V be a Hilbert space. Set Vω to be the K-finite vectors of V , which are the vectors contained in a finite dimensional representation of K. The point is that K is compact, so V splits into a Hilbert space direct sum finite dimensional representations of K, at least if V is a Hilbert space.
    [Show full text]
  • CHAPTER 2 Representations of Finite Groups in This Chapter We Consider
    CHAPTER 2 Representations of Finite Groups In this chapter we consider only finite-dimensional representations. 2.1. Unitarity, complete reducibility, orthogonality relations Theorem 1. A representation of a finite group is unitary. c Proof. Let (π, V ) be a (finite- dimensional) representation of a finite group G = { g1, g2, . gn }. Let h·, ·i1 be any inner product on V . Set n X hv, wi = hπ(gj)v, π(gj)wi1, v, w ∈ V. j=1 Then it is clear from the definition that hπ(g)v, π(g)wi = hv, wi, v, w ∈ V, g ∈ G. Pn Note that if v ∈ V , then hv, vi = j=1hπ(gj)v, π(gj)vi1 and if v 6= 0, then π(gj)v 6= 0 for all j implies hπ(gj)v, π(gj)vi1 > 0 for all j. Hence v 6= 0 implies hv, vi > 0. The other properties of inner product are easy to verify for h·, ·i, using the fact that h·, ·i1 is an inner product, and each π(gj) is linear. The details are left as an exercise. qed The following is an immediate consequence of Theorem 1 and a result from Chapter I stating that a finite-dimensional unitary representation is completely reducible. Theorem 2. A representation of a finite group is completely reducible. Example. Let G be a finite group acting on a finite set X. Let V be a complex vector space having a basis { vx1 , . , vxm } indexed by the elements x1, . , xm of X. If g ∈ G, let π(g) be the operator sending vxj to vg·xj , 1 ≤ j ≤ m.
    [Show full text]
  • On a Representation of the Group GL (K; R) and Its Spherical Functions
    Mem Fac ut & scl Shrmane Univ., Nat. Sci. 4, pp. Io 06 March 15 1971 On a RepreSentation of The Group GL (k; R) and ItS Spherical FunctionS Yasuhiro ASOo (Department of Mathematlcs shlmane Unrversity, Mat**ue, Japan) (Received October 12, 1970) In 1961, A. T. James [1] introduced the zonal polynomial of real positive definite matrix and he described some properties and a method of calculation of it. Lately, he [2] also showed that it is ' an eigenfunction of the Laplace-~eltrami operator. The '_onal polynomial plays significant roles in di-~*tribution problem** of eigenvalues related to the normal multivariate distri.bution [3]. In the present paper, we will describe a representation of the group G'L (k ; R) and its spherical functions guided by N. J. Vilenkin [4]. Our assumptions I and 2 in the following may be satisfied with zonal polynomials. We also give a deLinition of (zonal) spherical function of the group GL (k ; R) guided by K. Maurin [5] and we show that our zonal spherical function is in agreement with the latter definition. S I . Definitiola of spherical functions and son~_e properties related to the representation A(2f) (g) of th:e group GL (k ; R) Let { {g}(2)}(f), g E GL (k ; R) be a representation of the group GL (k ; R) on the space Vf of hornogeneous polynomials of degree f of the matrix ~;t ~ S, where ~;t is the space of real positive definite matrices of order k. Owing to Thrall [6]-Hua [71 's result, the representation space space Vf is completely decomposable mto rrreducrble mvanant subspaces V(f) on which representation A(2f,,.
    [Show full text]
  • Expander Graphs and Kazhdan's Property (T) Giles Gardam
    Expander Graphs and Kazhdan's Property (T) Giles Gardam An essay submitted in partial fulfillment of the requirements for the degree of B.Sc. (Honours) Pure Mathematics University of Sydney October 2012 Contents Introduction ........................................................................ iv Acknowledgements ................................................................. vi Chapter 1. Expander Graphs.................................................... 1 1.1. Graph Theory Background . 1 1.2. Introduction to Expander Graphs . 5 1.3. Diameter in Expanders. 8 1.4. Alternative Definitions of Expansion . 10 1.5. Existence of Expanders . 13 1.6. Cayley Graphs . 14 1.7. Some Negative Results for Cayley Graph Expansion. 15 1.8. Expansion in SL(2; Z=pZ)..................................................... 17 Chapter 2. Random Walks on Expanders ...................................... 19 2.1. Random Walks and the Graph Spectrum . 19 2.2. Spectral Expansion . 20 2.3. Efficient Error Reduction for RP .............................................. 23 Chapter 3. Kazhdan's Property (T) ............................................ 27 3.1. Unitary Representations of Locally Compact Groups . 27 3.2. Property (T). 33 3.3. Compact Groups Have Property (T) . 37 3.4. Kazhdan Sets and Generation . 41 3.5. Lattices and Property (T). 44 3.6. Some Non-compact Kazhdan Groups . 46 Chapter 4. Constructions of Expanders ........................................ 51 4.1. Kazhdan Expanders. 51 4.2. Margulis's Construction of Expanders . 54 Appendix A. Asymptotic Representations
    [Show full text]
  • Hyperspherical Functions and Harmonic Analysis on the Lorentz
    Hyperspherical Functions and Harmonic Analysis on the Lorentz Group V. V. Varlamov Department of Mathematics, Siberia State University of Industry, Kirova 42, Novokuznetsk 654007, Russia. Abstract Matrix elements of spinor and principal series representations of the Lorentz group are studied in the basis of complex angular momentum (helicity basis). It is shown that matrix elements are expressed via hyperspherical functions (relativistic spherical functions). In essence, the hyperspherical functions present itself a four-dimensional (with respect to a pseudo-euclidean metrics of Minkowski spacetime) generalization of the usual three-dimensional spherical functions. An explicit form of the hyperspher- ical functions is given. The hypespherical functions of the spinor representations are represented by a product of generalized spherical functions and Jacobi functions. It is shown that zonal hyperspherical functions are expressed via the Appell functions. The associated hyperspherical functions are defined as the functions on a two-dimensional complex sphere. Integral representations, addition theorems, symmetry and recur- rence relations for hyperspherical functions are given. In case of the principal and supplementary series representations of the Lorentz group, the matrix elements are ex- pressed via the functions represented by a product of spherical and conical functions. The hyperspherical functions of the principal series representations allow one to apply methods of harmonic analysis on the Lorentz group. Different forms of expansions of square integrable functions on the Lorentz group are studied. By way of example, an expansion of the wave function, representing the Dirac field (1/2, 0) (0, 1/2), is given. ⊕ arXiv:math-ph/0308039v2 21 May 2004 1 Introduction Numerous applications of three-dimensional spherical functions in different areas of mathe- matical and theoretical physics are well known.
    [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]
  • Unitary Representations of Topological Groups
    (July 28, 2014) Unitary representations of topological groups Paul Garrett [email protected] http:=/www.math.umn.edu/egarrett/ We develop basic properties of unitary Hilbert space representations of topological groups. The groups G are locally-compact, Hausdorff, and countably based. This is a slight revision of my handwritten 1992 notes, which were greatly influenced by [Robert 1983]. Nothing here is new, apart from details of presentation. See the brief notes on chronology at the end. One purpose is to isolate techniques and results in representation theory which do not depend upon additional structure of the groups. Much can be done in the representation theory of compact groups without anything more than the compactness. Similarly, the discrete decomposition of L2(ΓnG) for compact quotients ΓnG depends upon nothing more than compactness. Schur orthogonality and inner product relations can be proven for discrete series representations inside regular representations can be discussed without further hypotheses. The purely topological treatment of compact groups shows a degree of commonality between subsequent treatments of Lie groups and of p-adic groups, whose rich details might otherwise obscure the simplicity of some of their properties. We briefly review Haar measure, and prove the some basic things about invariant measures on quotients HnG, where this notation refers to a quotient on the left, consisting of cosets Hg. We briefly consider Gelfand-Pettis integrals for continuous compactly-supported vector valued functions with values in Hilbert spaces. We emphasize discretely occurring representations, neglecting (continuous) Hilbert integrals of representa- tions. Treatment of these discrete series more than suffices for compact groups.
    [Show full text]
  • Discretely Decomposable Restrictions of $(\Mathfrak {G}, K) $-Modules For
    DISCRETELY DECOMPOSABLE RESTRICTIONS OF (g,K)-MODULES FOR KLEIN FOUR SYMMETRIC PAIRS OF EXCEPTIONAL LIE GROUPS OF HERMITIAN TYPE HAIAN HE Abstract. Let (G,GΓ) be a Klein four symmetric pair. The author wants to classify all the Klein four symmetric pairs (G,GΓ) such that there exists at least one nontrivial unitarizable simple (g,K)-module πK that is discretely decomposable as a (gΓ,KΓ)-module. In this article, three assumptions will be made. Firstly, G is an exceptional Lie group of Hermitian Γ type, i.e., G = E6(−14) or E7(−25). Secondly, G is noncompact. Thirdly, there exists an element σ ∈ Γ corresponding to a symmetric pair of anti-holomorphic type such that πK is discretely decomposable as a (gσ ,Kσ)-module. 1. Introduction Let G be a noncompact simple Lie group, and G′ a reductive subgroup of G. Write g and g′ for the complexified Lie algebras of G and G′ respectively. Take a maximal compact subgroup K of G such that K′ := K G′ is a maximal compact subgroup of G′. An important problem in branching theory is to classify∩ the reductive Lie group pairs (G, G′) such that there exists at least one nontrivial unitarizable simple (g,K)-module that is discretely decomposable as a (g′,K′)-module. If G′ is compact, this always holds obviously. Hence in this article, the author will only considers the cases when G′ is noncompact. If (G, G′) is a symmetric pair, the problem was solved completely by Toshiyuki KOBAYASHI and Yoshiki OSHIMA¯ in [11, Corollary 5.8].
    [Show full text]
  • An Irreducible Unitary Representation of a Compact Group Is Finite Dimensional
    712 W. T. REID [August Bibliography 1. S. P. Diliberto, On systems of ordinary differential equations, Contributions to the Theory of Nonlinear Oscillations, Annals of Mathematics Studies, no. 20, Prince- ton, 1950, pp. 1-38. 2. -, A note on linear ordinary differential equations, Proc. Amer. Math. Soc. vol. 8 (1957) pp. 462-464. 3. O. Perron, Uber eine Matrixtransformation, Math. Zeit. vol. 32 (1930) pp. 465- 473. 4. W. T. Reid, Some results on the growth of solutions of differential systems, Bull. Amer. Math. Soc. Abstract 48-11-319. Northwestern University AN IRREDUCIBLE UNITARY REPRESENTATION OF A COMPACT GROUP IS FINITE DIMENSIONAL PAUL KOOSIS This note contains a proof of the statement included in the title. The result is certainly known; Anna Hurevitsch [l] proves it using the theorem of Peter and Weyl, under the assumption that the group satisfies the second axiom of countability. It would perhaps neverthe- less still be desirable to have at hand a short proof which uses the minimum amount of machinery, then the Peter-Weyl theorem, as well as the rest of the representation theory for compact groups [2, Ch. IV] would follow directly from the classical work of Gelfand and Raikov [3] on the unitary Hilbert space representations of a general locally compact group. Since Cartan and Godement [4] have already shown how the theory of abelian locally compact groups follows from the general one of Gelfand and Raikov, one would have available a rather unified treatment of the two branches of topological group theory which have been most extensively studied, a unity which is not so apparent in existing expositions of the subject (cf.
    [Show full text]