DOCSLIB.ORG

Note on the Decomposition of Semisimple Lie Algebras

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Note on the Decomposition of Semisimple Lie Algebras Note on the Decomposition of Semisimple Lie Algebras Thomas B. Mieling (Dated: January 5, 2018) Presupposing two criteria by Cartan, it is shown that every semisimple Lie algebra of finite dimension over C is a direct sum of simple Lie algebras. Definition 1 (Simple Lie Algebra). A Lie algebra g is Proposition 2. Let g be a finite dimensional semisimple called simple if g is not abelian and if g itself and f0g are Lie algebra over C and a ⊆ g an ideal. Then g = a ⊕ a? the only ideals in g. where the orthogonal complement is defined with respect to the Killing form K. Furthermore, the restriction of Definition 2 (Semisimple Lie Algebra). A Lie algebra g the Killing form to either a or a? is non-degenerate. is called semisimple if the only abelian ideal in g is f0g. Proof. a? is an ideal since for x 2 a?; y 2 a and z 2 g Definition 3 (Derived Series). Let g be a Lie Algebra. it holds that K([x; z; ]; y) = K(x; [z; y]) = 0 since a is an The derived series is the sequence of subalgebras defined ideal. Thus [a; g] ⊆ a. recursively by D0g := g and Dn+1g := [Dng;Dng]. Since both a and a? are ideals, so is their intersection Definition 4 (Solvable Lie Algebra). A Lie algebra g i = a \ a?. We show that i = f0g. Let x; yi. Then is called solvable if its derived series terminates in the clearly K(x; y) = 0 for x 2 i and y = D1i, so i is solv- trivial subalgebra, i.e. there exist an n 2 N such that able (by Cartan’s criterion for solvability). But since g Dng = f0g. is semisimple, f0g is the only solvable ideal in g, thus a \ a? = f0g. Definition 5 (Adjoint representation). The adjoint rep- The restriction of K to a is non-degenerate. Let x 2 resentation of a Lie algebra g is the Lie algebra homo- a with x 6= 0. Since K is non-degenerate on g, there morphism exists a y 2 g such that K(x; y) 6= 0. Assume on the contrary that there exists no such y with y 2 a, then x ad : g ! gl(g) ad(x)(y) := [x; y] is orthogonal to all elements of a, so x 2 a \ a? = f0g, from g into the Lie algebra gl(g), which is GL(g) with in contradiction with x 6= 0. A similar argument shows ? the commutator as the Lie bracket. that the restriction of K to a is non-degenerate. Since the restriction of K to a is non-degenerate, n Definition 6 (Killing from). The Killing form of a Lie there exists a K-orthonormal basis feigi=1 of a with algebra of finite dimension over C is the bilinear mapping K(ei; ej) = δij. Define the projection π : g ! a by ij ? π(x) := δ K(x; ei)ej. Obviously x − π(x) 2 a for all K : g × g ! C K(x; y) := tr(ad x ad y): x 2 g. Thus, g = a + a?. We have shown that g = a + a? and a \ a? = f0g, so Proposition 1. Let g be a Lie algebra. Then g is as a vector space it holds that g = a ⊕ a?. semisimple if and only if f0g is the only solvable ideal ? in g. For all x 2 a; y 2 a and z 2 g we have K(x; [y; z]) = 0 (since a? is an ideal), which is equivalent to K([x; y]; z) = Proof. 0. Since K is non-degenerate on g, it follows that [x; y] = ? ? ( An abelian ideal i is solvable since D1i = f0g. 0 for all x 2 a and y 2 a , so g = a ⊕ a also holds in the sense of Lie algebras. ) We show the contraposition: if g has a solvable ideal distinct from f0g, then g contains an non- Corollary 1. Let g be a finite dimensional semisimple trivial abelian ideal. Let a ⊆ g be a solvable ideal Lie algebra over C. Then g is a direct sum of simple Lie with a 6= f0g. If a is abelian, there is nothing to algebras. show. Otherwise, let n 2 N be the smallest num- ber such that Dn+1a = f0g. Then Dna 6= f0g is Proof. Let g be a finite dimensional semisimple Lie al- abelian, since [Dna;Dna] = Dn+1a = f0g. gebra over C. If g is simple, there is nothing to show. Otherwise, g contains a non-abelian ideal a (necessarily different from f0g). By the above proposition, it holds Theorem 1 (Cartan). Let g be a finite dimensional Lie that g = a ⊕ a? where both a and a? are semisimple algebra over R or C and denote by K the Killing form. again (since their Killing-forms are non-degenerate). If Then either of them is not simple, we can apply the same ar- 1. g is solvable if and only if for all x 2 g and all gument to decompose it into a direct sum of semisimple y 2 D1g it holds that K(x; y) = 0. Lie algebras. Since g has finite dimension, this process terminates and we end up with a decomposition of g into 2. g is semisimple if and only if the Killing form K is a direct sum of simple Lie algebras. non-degenerate..
Load more

Recommended publications
  • Lie Algebras and Representation Theory Andreasˇcap
    Lie Algebras and Representation Theory Fall Term 2016/17 Andreas Capˇ Institut fur¨ Mathematik, Universitat¨ Wien, Nordbergstr. 15, 1090 Wien E-mail address: [email protected] Contents Preface v Chapter 1. Background 1 Group actions and group representations 1 Passing to the Lie algebra 5 A primer on the Lie group { Lie algebra correspondence 8 Chapter 2. General theory of Lie algebras 13 Basic classes of Lie algebras 13 Representations and the Killing Form 21 Some basic results on semisimple Lie algebras 29 Chapter 3. Structure theory of complex semisimple Lie algebras 35 Cartan subalgebras 35 The root system of a complex semisimple Lie algebra 40 The classification of root systems and complex simple Lie algebras 54 Chapter 4. Representation theory of complex semisimple Lie algebras 59 The theorem of the highest weight 59 Some multilinear algebra 63 Existence of irreducible representations 67 The universal enveloping algebra and Verma modules 72 Chapter 5. Tools for dealing with finite dimensional representations 79 Decomposing representations 79 Formulae for multiplicities, characters, and dimensions 83 Young symmetrizers and Weyl's construction 88 Bibliography 93 Index 95 iii Preface The aim of this course is to develop the basic general theory of Lie algebras to give a first insight into the basics of the structure theory and representation theory of semisimple Lie algebras. A problem one meets right in the beginning of such a course is to motivate the notion of a Lie algebra and to indicate the importance of representation theory. The simplest possible approach would be to require that students have the necessary background from differential geometry, present the correspondence between Lie groups and Lie algebras, and then move to the study of Lie algebras, which are easier to understand than the Lie groups themselves.
    [Show full text]
  • Matrix Lie Groups
    Maths Seminar 2007 MATRIX LIE GROUPS Claudiu C Remsing Dept of Mathematics (Pure and Applied) Rhodes University Grahamstown 6140 26 September 2007 RhodesUniv CCR 0 Maths Seminar 2007 TALK OUTLINE 1. What is a matrix Lie group ? 2. Matrices revisited. 3. Examples of matrix Lie groups. 4. Matrix Lie algebras. 5. A glimpse at elementary Lie theory. 6. Life beyond elementary Lie theory. RhodesUniv CCR 1 Maths Seminar 2007 1. What is a matrix Lie group ? Matrix Lie groups are groups of invertible • matrices that have desirable geometric features. So matrix Lie groups are simultaneously algebraic and geometric objects. Matrix Lie groups naturally arise in • – geometry (classical, algebraic, differential) – complex analyis – differential equations – Fourier analysis – algebra (group theory, ring theory) – number theory – combinatorics. RhodesUniv CCR 2 Maths Seminar 2007 Matrix Lie groups are encountered in many • applications in – physics (geometric mechanics, quantum con- trol) – engineering (motion control, robotics) – computational chemistry (molecular mo- tion) – computer science (computer animation, computer vision, quantum computation). “It turns out that matrix [Lie] groups • pop up in virtually any investigation of objects with symmetries, such as molecules in chemistry, particles in physics, and projective spaces in geometry”. (K. Tapp, 2005) RhodesUniv CCR 3 Maths Seminar 2007 EXAMPLE 1 : The Euclidean group E (2). • E (2) = F : R2 R2 F is an isometry . → | n o The vector space R2 is equipped with the standard Euclidean structure (the “dot product”) x y = x y + x y (x, y R2), • 1 1 2 2 ∈ hence with the Euclidean distance d (x, y) = (y x) (y x) (x, y R2).
    [Show full text]
  • Lecture 5: Semisimple Lie Algebras Over C
    LECTURE 5: SEMISIMPLE LIE ALGEBRAS OVER C IVAN LOSEV Introduction In this lecture I will explain the classification of finite dimensional semisimple Lie alge- bras over C. Semisimple Lie algebras are defined similarly to semisimple finite dimensional associative algebras but are far more interesting and rich. The classification reduces to that of simple Lie algebras (i.e., Lie algebras with non-zero bracket and no proper ideals). The classification (initially due to Cartan and Killing) is basically in three steps. 1) Using the structure theory of simple Lie algebras, produce a combinatorial datum, the root system. 2) Study root systems combinatorially arriving at equivalent data (Cartan matrix/ Dynkin diagram). 3) Given a Cartan matrix, produce a simple Lie algebra by generators and relations. In this lecture, we will cover the first two steps. The third step will be carried in Lecture 6. 1. Semisimple Lie algebras Our base field is C (we could use an arbitrary algebraically closed field of characteristic 0). 1.1. Criteria for semisimplicity. We are going to define the notion of a semisimple Lie algebra and give some criteria for semisimplicity. This turns out to be very similar to the case of semisimple associative algebras (although the proofs are much harder). Let g be a finite dimensional Lie algebra. Definition 1.1. We say that g is simple, if g has no proper ideals and dim g > 1 (so we exclude the one-dimensional abelian Lie algebra). We say that g is semisimple if it is the direct sum of simple algebras. Any semisimple algebra g is the Lie algebra of an algebraic group, we can take the au- tomorphism group Aut(g).
    [Show full text]
  • Holomorphic Diffeomorphisms of Complex Semisimple Lie Groups
    HOLOMORPHIC DIFFEOMORPHISMS OF COMPLEX SEMISIMPLE LIE GROUPS ARPAD TOTH AND DROR VAROLIN 1. Introduction In this paper we study the group of holomorphic diffeomorphisms of a complex semisimple Lie group. These holomorphic diffeomorphism groups are infinite di- mensional. To get additional information on these groups, we consider, for instance, the following basic problem: Given a complex semisimple Lie group G, a holomor- phic vector field X on G, and a compact subset K ⊂⊂ G, the flow of X, when restricted to K, is defined up to some nonzero time, and gives a biholomorphic map from K onto its image. When can one approximate this map uniformly on K by a global holomorphic diffeomorphism of G? One condition on a complex manifold which guarantees a positive solution of this problem, and which is possibly equiva- lent to it, is the so called density property, to be defined shortly. The main result of this paper is the following Theorem Every complex semisimple Lie group has the density property. Let M be a complex manifold and XO(M) the Lie algebra of holomorphic vector fields on M. Recall [V1] that a complex manifold is said to have the density property if the Lie subalgebra of XO(M) generated by its complete vector fields is a dense subalgebra. See section 2 for the definition of completeness. Since XO(M) is extremely large when M is Stein, the density property is particularly nontrivial in this case. One of the two main tools underlying the proof of our theorem is the general notion of shears and overshears, introduced in [V3].
    [Show full text]
  • Note to Users
    NOTE TO USERS This reproduction is the best copy available. The Rigidity Method and Applications Ian Stewart, Department of hIathematics McGill University, Montréal h thesis çubrnitted to the Faculty of Graduate Studies and Research in partial fulfilrnent of the requirements of the degree of MSc. @[an Stewart. 1999 National Library Bibliothèque nationale 1*1 0fC-& du Canada Acquisitions and Acquisilions et Bibliographie Services services bibliographiques 385 Wellington &eet 395. nie Weüington ôuawa ON KlA ON4 mwaON KlAW Canada Canada The author has granted a non- L'auteur a accordé une Licence non exclusive licence aiiowing the exclusive pennettant à la National Lïbrary of Canada to Brbhothèque nationale du Canada de reproduce, Ioan, distri'bute or sell reproduire, prêter, distribuer ou copies of this thesis m microform, vendre des copies de cette thèse sous paper or electronic formats. la forme de microfiche/filnl de reproduction sur papier ou sur format éIectronique. The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fiom it Ni la thèse ni des extraits substantiels may be printed or otherwise de ceiie-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. Abstract The Inverse Problem ol Gnlois Thcor? is (liscuss~~l.In a spticifiç forni. the problem asks tr-hether ewry finitr gruiip occurs aç a Galois groiip over Q. An iritrinsically group theoretic property caIIed rigidity is tlcscribed which confirnis chat many simple groups are Galois groups ovrr Q.
    [Show full text]
  • Irreducible Representations of Complex Semisimple Lie Algebras
    Irreducible Representations of Complex Semisimple Lie Algebras Rush Brown September 8, 2016 Abstract In this paper we give the background and proof of a useful theorem classifying irreducible representations of semisimple algebras by their highest weight, as well as restricting what that highest weight can be. Contents 1 Introduction 1 2 Lie Algebras 2 3 Solvable and Semisimple Lie Algebras 3 4 Preservation of the Jordan Form 5 5 The Killing Form 6 6 Cartan Subalgebras 7 7 Representations of sl2 10 8 Roots and Weights 11 9 Representations of Semisimple Lie Algebras 14 1 Introduction In this paper I build up to a useful theorem characterizing the irreducible representations of semisim- ple Lie algebras, namely that finite-dimensional irreducible representations are defined up to iso- morphism by their highest weight ! and that !(Hα) is an integer for any root α of R. This is the main theorem Fulton and Harris use for showing that the Weyl construction for sln gives all (finite-dimensional) irreducible representations. 1 In the first half of the paper we'll see some of the general theory of semisimple Lie algebras| building up to the existence of Cartan subalgebras|for which we will use a mix of Fulton and Harris and Serre ([1] and [2]), with minor changes where I thought the proofs needed less or more clarification (especially in the proof of the existence of Cartan subalgebras). Most of them are, however, copied nearly verbatim from the source. In the second half of the paper we will describe the roots of a semisimple Lie algebra with respect to some Cartan subalgebra, the weights of irreducible representations, and finally prove the promised result.
    [Show full text]
  • Algebraic D-Modules and Representation Theory Of
    Contemporary Mathematics Volume 154, 1993 Algebraic -modules and Representation TheoryDof Semisimple Lie Groups Dragan Miliˇci´c Abstract. This expository paper represents an introduction to some aspects of the current research in representation theory of semisimple Lie groups. In particular, we discuss the theory of “localization” of modules over the envelop- ing algebra of a semisimple Lie algebra due to Alexander Beilinson and Joseph Bernstein [1], [2], and the work of Henryk Hecht, Wilfried Schmid, Joseph A. Wolf and the author on the localization of Harish-Chandra modules [7], [8], [13], [17], [18]. These results can be viewed as a vast generalization of the classical theorem of Armand Borel and Andr´e Weil on geometric realiza- tion of irreducible finite-dimensional representations of compact semisimple Lie groups [3]. 1. Introduction Let G0 be a connected semisimple Lie group with finite center. Fix a maximal compact subgroup K0 of G0. Let g be the complexified Lie algebra of G0 and k its subalgebra which is the complexified Lie algebra of K0. Denote by σ the corresponding Cartan involution, i.e., σ is the involution of g such that k is the set of its fixed points. Let K be the complexification of K0. The group K has a natural structure of a complex reductive algebraic group. Let π be an admissible representation of G0 of finite length. Then, the submod- ule V of all K0-finite vectors in this representation is a finitely generated module over the enveloping algebra (g) of g, and also a direct sum of finite-dimensional U irreducible representations of K0.
    [Show full text]
  • LIE GROUPS and ALGEBRAS NOTES Contents 1. Definitions 2
    LIE GROUPS AND ALGEBRAS NOTES STANISLAV ATANASOV Contents 1. Definitions 2 1.1. Root systems, Weyl groups and Weyl chambers3 1.2. Cartan matrices and Dynkin diagrams4 1.3. Weights 5 1.4. Lie group and Lie algebra correspondence5 2. Basic results about Lie algebras7 2.1. General 7 2.2. Root system 7 2.3. Classification of semisimple Lie algebras8 3. Highest weight modules9 3.1. Universal enveloping algebra9 3.2. Weights and maximal vectors9 4. Compact Lie groups 10 4.1. Peter-Weyl theorem 10 4.2. Maximal tori 11 4.3. Symmetric spaces 11 4.4. Compact Lie algebras 12 4.5. Weyl's theorem 12 5. Semisimple Lie groups 13 5.1. Semisimple Lie algebras 13 5.2. Parabolic subalgebras. 14 5.3. Semisimple Lie groups 14 6. Reductive Lie groups 16 6.1. Reductive Lie algebras 16 6.2. Definition of reductive Lie group 16 6.3. Decompositions 18 6.4. The structure of M = ZK (a0) 18 6.5. Parabolic Subgroups 19 7. Functional analysis on Lie groups 21 7.1. Decomposition of the Haar measure 21 7.2. Reductive groups and parabolic subgroups 21 7.3. Weyl integration formula 22 8. Linear algebraic groups and their representation theory 23 8.1. Linear algebraic groups 23 8.2. Reductive and semisimple groups 24 8.3. Parabolic and Borel subgroups 25 8.4. Decompositions 27 Date: October, 2018. These notes compile results from multiple sources, mostly [1,2]. All mistakes are mine. 1 2 STANISLAV ATANASOV 1. Definitions Let g be a Lie algebra over algebraically closed field F of characteristic 0.
    [Show full text]
  • Introuduction to Representation Theory of Lie Algebras
    Introduction to Representation Theory of Lie Algebras Tom Gannon May 2020 These are the notes for the summer 2020 mini course on the representation theory of Lie algebras. We'll first define Lie groups, and then discuss why the study of representations of simply connected Lie groups reduces to studying representations of their Lie algebras (obtained as the tangent spaces of the groups at the identity). We'll then discuss a very important class of Lie algebras, called semisimple Lie algebras, and we'll examine the repre- sentation theory of two of the most basic Lie algebras: sl2 and sl3. Using these examples, we will develop the vocabulary needed to classify representations of all semisimple Lie algebras! Please email me any typos you find in these notes! Thanks to Arun Debray, Joakim Færgeman, and Aaron Mazel-Gee for doing this. Also{thanks to Saad Slaoui and Max Riestenberg for agreeing to be teaching assistants for this course and for many, many helpful edits. Contents 1 From Lie Groups to Lie Algebras 2 1.1 Lie Groups and Their Representations . .2 1.2 Exercises . .4 1.3 Bonus Exercises . .4 2 Examples and Semisimple Lie Algebras 5 2.1 The Bracket Structure on Lie Algebras . .5 2.2 Ideals and Simplicity of Lie Algebras . .6 2.3 Exercises . .7 2.4 Bonus Exercises . .7 3 Representation Theory of sl2 8 3.1 Diagonalizability . .8 3.2 Classification of the Irreducible Representations of sl2 .............8 3.3 Bonus Exercises . 10 4 Representation Theory of sl3 11 4.1 The Generalization of Eigenvalues .
    [Show full text]
  • Arxiv:1208.0416V2 [Math.RT] 6 Sep 2012 Itoutr)Gaut Ore Ntesubject
    REPRESENTATIONS OF COMPLEX SEMI-SIMPLE LIE GROUPS AND LIE ALGEBRAS APOORVA KHARE† Abstract. This article is an exposition of the 1967 paper by Parthasarathy, Ranga Rao, and Varadarajan, on irreducible admissible Harish-Chandra modules over complex semisimple Lie groups and Lie algebras. It was written in Winter 2012 to be part of a special collection organized to mark 10 years and 25 volumes of the series Texts and Readings in Mathematics (TRIM). Each article in this collection is intended to give nonspecialists in its field, an appreciation for the impact and contributions of the paper being surveyed. Thus, the author has kept the prerequisites for this article down to a basic course on complex semisimple Lie algebras. While it arose out of the grand program of Harish-Chandra on admissible representations of semisimple Lie groups, the work by Parthasarathy et al also provided several new insights on highest weight modules and related areas, and these results have been the subject of extensive research over the last four decades. Thus, we also discuss its results and follow-up works on the classification of irreducible Harish-Chandra modules; on the PRV conjecture and tensor product multiplicities; and on PRV determinants for (quantized) affine and semisimple Lie algebras. Contents 1. Notation and preliminaries 3 2. Harish-Chandra modules 5 3. Tensor products, minimal types, and the (K)PRV conjecture 10 4. IrreducibleBanachspacerepresentations 15 5. The rings Rν,Π′ and the PRV determinants 21 6. Representations of class zero 26 7. Conclusion: The classification of irreducible Harish-Chandra modules 27 References 29 arXiv:1208.0416v2 [math.RT] 6 Sep 2012 Lie groups and Lie algebras occupy a prominent and central place in mathematics, connecting differential geometry, representation theory, algebraic geometry, number theory, and theoretical physics.
    [Show full text]
  • Filtrations in Semisimple Lie Algebras, I 11
    TRANSACTIONS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 00, Number 0, Pages 000–000 S 0002-9947(XX)0000-0 FILTRATIONS IN SEMISIMPLE LIE ALGEBRAS, I Y. BARNEA AND D. S. PASSMAN Abstract. In this paper, we study the maximal bounded Z-filtrations of a complex semisimple Lie algebra L. Specifically, we show that if L is simple of classical type An, Bn, Cn or Dn, then these filtrations correspond uniquely to a precise set of linear functionals on its root space. We obtain partial, but not definitive, results in this direction for the remaining exceptional algebras. Maximal bounded filtrations were first introduced in the context of classify- ing the maximal graded subalgebras of affine Kac-Moody algebras, and the maximal graded subalgebras of loop toroidal Lie algebras. Indeed, our main results complete this classification in most cases. Finally, we briefly discuss the analogous question for bounded filtrations with respect to other Archimedean ordered groups. 1. Introduction Let L be a Lie algebra over a field K.A Z-filtration F = {Fi | i ∈ Z} of L is a collection of K-subspaces · · · ⊆ F−2 ⊆ F−1 ⊆ F0 ⊆ F1 ⊆ F2 ⊆ · · · indexed by the integers Z such that [Fi,Fj] ⊆ Fi+j for all i, j ∈ Z. One usually also S T assumes that i Fi = L and i Fi = 0. In particular, F0 is a Lie subalgebra of L and each Fi is an F0-Lie submodule of L. Furthermore, we say that the filtration 0 is bounded if there exist integers ` and ` with F` = 0 and F`0 = L. In this case, it is clear that each Fi, with i < 0, is ad-nilpotent on L.
    [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]