HIGHEST WEIGHT CLASSIFICATION OF THE IRREDUCIBLE REPRESENTATIONS OF THE SPECIAL UNITARY GROUP By Thomas Holtzworth A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science in Mathematics Northern Arizona University May 2016 Approved: Michael Falk, Ph.D., Chair Jim Swift, Ph.D. William Schulz, Ph.D. ABSTRACT HIGHEST WEIGHT CLASSIFICATION OF THE IRREDUCIBLE REPRESENTATIONS OF THE SPECIAL UNITARY GROUP Thomas Holtzworth Representation theory of the special unitary group, SU(n), has a fundamen- tal role in theoretical physics. Therefore it is the purpose of this thesis to pro- vide a detailed exposition of the highest weight classification of the irreducible representations of SU(n). The exposition has two components: provide a com- plete classification of all the irreducible representations, and build corresponding SU(n)-modules to carry one of each. The classification scheme is founded on the bijective correspondence between the representations of SU(n) and the finite-dimensional complex analytic repre- sentations of the special linear group of complex matrices, SL(n; C). These repre- sentations of SL(n; C) are accompanied by the presence of weights, resulting from the analytic nature of the representations. Ultimately, all such representations of SL(n; C) are uniquely identified by their highest weights, which additionally, are in a one to one correspondence with integer partitions of length less than or equal to n − 1. The construction utilizes the representation theory of the symmetric group, Sm, and of the general linear group of complex matrices, GL(n; C). Irreducible representations of Sm are also identified with integer partitions. The irreducible representations of SL(n; C) are subrepresentations of GL(n; C) on the mth tensor ii power of n-dimensional complex space. The symmetric group is used in construct- ing these subrepresentations. The realizations of irreducible representations of SU(n) are constructed as the images of specific operators built from the integer partitions associated with their highest weights. iii Acknowledgements I would like to thank the Department of Mathematics and Statistics at Northern Arizona University for affording me this opportunity. I would especially like to thank Dr. Michael Falk for giving me the creative license in choosing this topic, for providing invaluable feedback that helped shape my thesis, and for investing countless hours meeting with me to discuss my interests in advanced mathematics. I would also like to thank Dr. Jim Swift and Dr. William Schulz for being on my thesis committee, and Dr. John Hagood for teaching real analysis, measure theory, and functional analysis. To my love, Aaris Snyder, for your support, encouragement, praise, and for your patience through the many long days, during which I was away from home, I cannot thank you enough. I will be always grateful you are by my side. Finally, I would like to thank Melville my chinchilla. iv Contents Chapter 1 General representation theory 2 1.1 Definitions and basic concepts . 2 1.2 G−submodules and reducibility . 4 1.3 Homomorphisms of G−Modules and Schur's Lemma . 5 1.4 Multiplicity and isotypic components . 8 1.5 The classification of irreducible G-modules for finite groups . 12 1.5.1 The regular representation and the group algebra . 12 1.5.2 The decomposition of C[G]........................ 13 1.5.3 The number of distinct irreducible group modules . 21 Chapter 2 Representations of the symmetric group 26 2.1 Cycle type and integer partitions . 26 2.2 Tableaux, tabloids and permutation modules . 27 2.3 Specht modules . 29 2.4 Orderings on shapes . 31 2.5 The submodule theorem . 32 2.6 General projection operators . 36 Chapter 3 Irreducible tensor representations of GL(n; C) 40 N m 3.1 V as a GL(V )-module and Sm−module. 41 N m 3.2 The commuting action of GL(V ) and Sm on V : . 41 λ N m 3.3 HomSm (S ;V ): An irreducible GL(V )-module . 46 3.4 Realizations using the general projection operators . 50 N 3.5 (Cn) m as a GL(n; C)-module . 52 Chapter 4 Finite-dimensional representations of SL(n; C), and SU(n) 53 4.1 Elementary matrix Lie group theory . 54 4.1.1 The special linear group and the special unitary group . 54 4.1.2 GL(n; C), SL(n; C), and SU(n) as matrix Lie groups . 54 4.1.3 The matrix exponential and logarithm . 55 4.1.4 The matrix Lie algebra . 58 4.2 Representations of matrix Lie groups and Lie algebras . 61 4.3 The complexification of su(n): .......................... 65 4.4 The correspondence between SL(n; C) and SU(n): . 68 v Chapter 5 Highest weight description of analytic representations 86 5.1 Lie's theorem . 86 5.2 Gauss decomposition of SL(n; C) and GL(n; C) . 90 5.3 Highest weight identification of irreducible representations . 97 5.4 Description of weights . 102 5.4.1 Weights as analytic homomorphisms . 102 5.4.2 Weights and weight spaces for gl(n; C) and sl(n; C) . 106 5.4.3 The lexicographic order on weights and permutation matrices . 110 Chapter 6 Irreducible representations of SL(n; C) and SU(n) 116 6.1 The irreducible tensor representations are complex analytic . 116 6.2 The standard basis as weight vectors . 118 N m 6.3 The image space t V as an irreducible SL(n; C)-module . 124 6.4 Highest weight classification for SU(n) . 132 Bibliography 134 1 Chapter 1 General representation theory In this chapter general group representation theory will be introduced. Decomposition and classification theorems will be of primary importance as they will be necessary in the following chapters addressing representation theory for the symmetric group as well as tensor product representations of the group of complex invertible matrices. Group representation theory can be described in the language of linear group actions or modules. Both are advantageous; however, the shape of this chapter will rest heavily on the latter. The main influences for the treatment here include the expositions given by Sagan [3] and Sternberg [4]. Unless otherwise stated, vector spaces are assumed to be complex and finite-dimensional. 1.1 Definitions and basic concepts In this section the equivalent notions of group representations, matrix representations, and group modules will be defined. If V and W are vector spaces, denote by GL(V ) the group of invertible linear transfor- mations on V , and denote by HomC(V; W ) the space of linear transformations from V to W . In addition, GL(n; C) is the group of complex n × n invertible matrices, and Mn(C) is the space of all complex n × n matrices. Definition 1.1.1. Let G be a group. A representation of G is a group homomorphism ρ : G ! GL(V ); where V is a vector space. If ρ : G ! GL(V ) is a representation of G, one says V carries the representation ρ. Suppose dim V = n. By fixing a basis for V , one identifies ρ(g) with an n×n nonsingular complex matrix, leading to the following definition. Definition 1.1.2. Let G be a group and n be a positive integer. Then, a matrix represen- tation of G is a group homomorphism X : G ! GL(n; C): 2 Definitions 1.1.1 and 1.1.2 are equivalent in the sense that, by use of a fixed basis of V; each gives rise to the other. Alternatively, the operation v 7! ρ(g)v; can be interpreted as multiplication of vectors v 2 V by group elements g 2 G: This allows one to consider V as a left module over a ring, as described below. Let G be a finite group. The group algebra of G (over C), denoted as C[G], is the set of complex-valued functions on G, with its natural vector space structure and ring product defined by X −1 (f1 ∗ f2)(h) := f1(g h)f2(g) g2G for all g 2 G and f1; f2 2 C[G]. Consider G to be contained in C[G]; then ∗ is the unique distributive, bilinear extension to C[G] of the group product on G. Let " be the group identity element, and f 2 C[G]. Then " ∗ f = f ∗ " = f: Consequently, C[G] is a unital associative algebra over C. Note C[G] is commutative if and only if G is abelian. For simplicity, the use of ∗ will be suppressed. Furthermore, f 2 C[G] will typically be expressed using the formal sum, X f = cgg; g2G where cg = f(g) 2 C. In the formal sum notation, X X X X f1f2 := ( cgg)( bhh) = ( cgbg−1h)h: g2G h2G h2G g2G Let R be a ring with unity. A left R-module is an abelian group M endowed with scalar multiplication by elements of R, (r; m) 7! rm; such that, for all m; n 2 M and r; s 2 R, (1) r(m + n) = rm + rn, (2) (r + s)m = rm + sm, (3) (sr)m = s(rm); and (4) 1Rm = m: An R-submodule of an R-module M is an additive subgroup N of M satisfying rx 2 N for all r 2 R, x 2 N. For simplicity, a left C[G]-module is called a G−module. If V is a G−module, then for each g 2 G, the multiplication v 7! gv 3 defines an element ρ(g) 2 GL(V ). The mapping ρ is representation of G. Conversely, if ρ : G ! GL(V ) is a group representation, then the binary operation G × V ! V defined by (g; v) 7! ρ(g)v endows V with a natural G−module structure.