DOCSLIB.ORG

Group Representation Theory for Knowledge Graph Embedding

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Group Representation Theory for Knowledge Graph Embedding Group Representation Theory for Knowledge Graph Embedding Chen Cai, Yunfeng Cai, Mingming Sun, Zhiqiang Xu Cognitive Computing Lab Baidu Research No.10 Xibeiwang East Road, Beijing, 10085, China [email protected], {caiyunfeng,sunmingming01,xuzhiqiang04}@baidu.com Abstract Knowledge1 graph embedding has recently become a popular way to model re- lations and infer missing links. In this paper, we present a group theoretical perspective of knowledge graph embedding, connecting previous methods with different group actions. Furthermore, by utilizing Schur’s lemma from group repre- sentation theory, we show that the state-of-the-art embedding method RotatE has the capacity to model relations from any finite Abelian group. 1 Introduction Knowledge graphs are collections of factual triples, where each triple (h; r; t) represents a relation r between a head entity h and a tail entity t. Examples of real-world knowledge graphs include Freebase, Yago [13], and WordNet [9]. Knowledge graphs are potentially useful for a variety of applications such as question-answering [6], information retrieval [18], recommender systems [22], and natural language processing [20]. Research on knowledge graphs is attracting growing interests in both academia and industry communities. Since knowledge graphs are usually incomplete, a fundamental problem for knowledge graphs is predicting the missing links. Recently, extensive studies have been done on learning low-dimensional representations of entities and relations for missing link prediction (a.k.a. knowledge graph em- bedding). These methods have been shown to be scalable and effective compared to previous methods [2, 8, 10, 12, 15, 17]. The general intuition of these methods is to model and infer the connectivity patterns in knowledge graphs according to the observed knowledge facts. For example, some relations are symmetric (e.g., marriage) while others are anti-symmetric (e.g., filiation); some relations are the inverse of other relations (e.g., hypernym and hyponym); and some relations may be composed by others (e.g., my mother’s husband is my father). It is critical to find ways to model and infer these patterns, i.e., symmetry/anti-symmetry, inversion, and composition, from the observed facts in order to predict missing links. We argue that relation patterns, symmetry/anti-symmetry, inversion, and composition, have natural correspondence to notions in group theory. Many existing embedding models can be interpreted as modeling relations as elements of a group acting on a properly chosen space. We believe that the algebraic (group) perspective of knowledge graph embedding is interesting and worth investigating as an attempt to shed light on the power and limitation of knowledge graph embedding. Moreover, it complements existing statistical approaches [11,24] and deepens understanding. Our main contributions in this work can be summarized as follows: • We revisit knowledge graph embedding from the perspective of group representation theory. To the best of our knowledge, this connection has not been made explicitly before. b731afe6-XeAuIkRK1GnSwCkLSQI1eeCZYQrXRz49U6CKDw==-500ddfd78926 1Chen Cai’s work was conducted while interning with Baidu Research from May to the end of August 2019. Chen Cai is a graduate student from the Department of Computer Science & Engineering, Ohio State University. 33rd Conference on Neural Information Processing Systems (NeurIPS 2019), Vancouver, Canada. • We provide a self-contained introduction of concepts and examples from group theory and group representation theory. • We characterize the representation power of a recent embedding method, i.e., RotatE. By utilizing tools from group representation theory, we show that RotatE has the capacity to represent any finite Abelian groups. 2 Background The general methodology of Knowledge Graph Embedding(KGE) is to define a score function for triples. Formally, letting E denote the set of entities and R the set of relations, a knowledge graph is a collection of factual triples (h; r; t), where h; t 2 E and r 2 R. Since entity embeddings are usually represented as vectors, the score function takes the form f(h; r; t), where h and t are head and tail entity embeddings. We list the score functions for a few popular embedding methods as below: • TransE [1]: f(h; r; t) = −kh + r − tk, h; r; t 2 Rk. • TorusE [4]: f(h; r; t) = −kh + r − tk, h; r; t 2 Tk, where Tk is a k-dimensional torus. • DistMul [21]: f(h; r; t) = hT diag(r)t, h; r; t 2 Rk. • ComplEx [16]: f(h; r; t) = Re(hT diag(r)t¯), h; r; t 2 Ck. d • RotatE [14]: f(h; r; t) = −kh ◦ r − tk, h; r; t 2 C ; krik = 1, where ◦ denotes Hadamard product. Most models can capture only a portion of relation patterns. For example, TransE represents each relation as a translation from source entities and target entities, and thus implicitly models inversion and composition of relations. However, it cannot model symmetric relations. ComplEx [7] extends DistMult by introducing complex embeddings so as to better model asymmetric relations, but it cannot infer the composition pattern. Particularly, RotatE is the first model able to cover all the relation patterns. We thus mainly focus on this model in this paper. 2.1 Relation Pattern We formally define common relation patterns that many embedding methods try to model in knowl- edge graphs. Definition 2.1. A relation clause r is symmetric if 8x; y; r(y; x) =) r(y; x): A clause in this form is a symmetry pattern. Definition 2.2. A relation clause r is anti-symmetric if 8x; y; r(x; y) =):r(y; x): A clause in this form is an anti-symmetry pattern. Definition 2.3. A relation r1 is inverse to r2 if 8x; y; r2(x; y) =) r1(y; x): A clause in this form is an inversion pattern. Definition 2.4. A relation r1 is composed of relation r2 and relation r3 if 8x; y; z; r1(x; y) ^ r2(y; z) =) r3(x; z). A clause in this form is a composition pattern. Definition 2.5. A relation r1 the subrelation of r2 if 8x; y; r1(x; y) =) r2(x; y): A clause in this form is a subrelation pattern. 3 Group and Relation Modeling Due to space limitation, we only provide the definition of group here. Readers who are not familiar with group theory are encouraged to read the appendix, where a self-contained introduction of group theory is given. Definition 3.1. Group. Let G be a set. We say that G is a group with law of composition ? if the b731afe6-XeAuIkRK1GnSwCkLSQI1eeCZYQrXRz49U6CKDw==-500ddfd78926 following axioms hold: (G1, closure of ?): the assignment (g; h) ! g ? h 2 G, defines a function G × G ! G. We call g ? h the product of g and h. 2 (G2, existence of identity): there exists a eG 2 G such that for every g 2 G, we have g ? eG = g = eG ? g. We call eG an identity of G. (G3, existence of inverse): for every g 2 G, there exists an h 2 G such that h ? g = eG = g ? h. We call such h an inverse of g. (G4, associativity): for any g; h; k 2 G we have (g ? h) ? k = g ? (h ? k). As we can see, the definitions of inversion and composition pattern of the relation are similar to G3 and G4 in the definition of the group. A symmetry pattern can be modeled as a group element of order 2. (In the case of RotatE, it corresponds to the rotation of magnitude π.) We summarize the correspondence in Table 1. For embedding models like TransE, TorusE, RotatE, and DihEdral [19], relations are modeled as elements of groups acting on certain spaces. In particular, TransE models relations as elements of the translation group acting on Rn. TorusE models relations as elements of the translation group acting on the high dimensional torus Tn. RotatE models relations as elements of the group of diagonal complex matrices (where each entry is of length 1) acting on Cn. DihEdral models relations as elements of the Dihedral group acting on Rn. Table 1: The correspondence between relation modeling and group theory. Relation Group Example −1 Symmetry r(x; y) =) r(y; x) gr = gr Marriage −1 Antisymmetry r(x; y) =):r(y; x) gr 6= gr Filiation r (x; y) =) r (y; x) g = g−1 Inverse 1 2 r1 r2 Hypernym and hyponym Composition r1(x; y) ^ r2(y; z) =) r3(x; z) gr1 ? gr2 = gr3 Mother’s husband is father Subrelation r1(x; y) =) r2(x; y) - Father of and parent of Limitation: Combining composition and subrelation patterns, cases like r1(x; y) ^ r1(y; z) =) r1(x; z) would happen. An example of this type of relation pattern can be sibling of or coworker with. However, in the group formulation, gr1 ? gr1 = gr1 . Therefore, gr1 is modeled as the identity in the group. Due to the uniqueness of identity, all relations that exhibit the above pattern will be modeled as the same element in the group. This certainly limits the modeling capacity and is shared by all models that can be formulated under the group action perspective. 4 Main Theorem Assuming that readers are familiar with concepts defined in the appendix, we are now ready to present two lemmas and the main theorem. Lemma 4.1. Schur’s Lemma. The only self-isomorphisms of a finite dimensional irreducible representation of a group G on the complex vector space are given by scalar multiplication. Proof. Fix an isomorphism φ : V ! V of complex representations of G. The linear map φ must have an eigenvalue λ over C, and also some non-zero eigenvector v. But then the G-linear map (because both φ(x) and λx are both linear maps) V ! V x ! φ(x) − λx has the vector v in its kernel, which is again a representation of G.
Load more

Recommended publications
  • Representation Theory with a Perspective from Category Theory
    Representation Theory with a Perspective from Category Theory Joshua Wong Mentor: Saad Slaoui 1 Contents 1 Introduction3 2 Representations of Finite Groups4 2.1 Basic Definitions.................................4 2.2 Character Theory.................................7 3 Frobenius Reciprocity8 4 A View from Category Theory 10 4.1 A Note on Tensor Products........................... 10 4.2 Adjunction.................................... 10 4.3 Restriction and extension of scalars....................... 12 5 Acknowledgements 14 2 1 Introduction Oftentimes, it is better to understand an algebraic structure by representing its elements as maps on another space. For example, Cayley's Theorem tells us that every finite group is isomorphic to a subgroup of some symmetric group. In particular, representing groups as linear maps on some vector space allows us to translate group theory problems to linear algebra problems. In this paper, we will go over some introductory representation theory, which will allow us to reach an interesting result known as Frobenius Reciprocity. Afterwards, we will examine Frobenius Reciprocity from the perspective of category theory. 3 2 Representations of Finite Groups 2.1 Basic Definitions Definition 2.1.1 (Representation). A representation of a group G on a finite-dimensional vector space is a homomorphism φ : G ! GL(V ) where GL(V ) is the group of invertible linear endomorphisms on V . The degree of the representation is defined to be the dimension of the underlying vector space. Note that some people refer to V as the representation of G if it is clear what the underlying homomorphism is. Furthermore, if it is clear what the representation is from context, we will use g instead of φ(g).
    [Show full text]
  • Representing Groups on Graphs
    Representing groups on graphs Sagarmoy Dutta and Piyush P Kurur Department of Computer Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, India 208016 {sagarmoy,ppk}@cse.iitk.ac.in Abstract In this paper we formulate and study the problem of representing groups on graphs. We show that with respect to polynomial time turing reducibility, both abelian and solvable group representability are all equivalent to graph isomorphism, even when the group is presented as a permutation group via generators. On the other hand, the representability problem for general groups on trees is equivalent to checking, given a group G and n, whether a nontrivial homomorphism from G to Sn exists. There does not seem to be a polynomial time algorithm for this problem, in spite of the fact that tree isomorphism has polynomial time algorithm. 1 Introduction Representation theory of groups is a vast and successful branch of mathematics with applica- tions ranging from fundamental physics to computer graphics and coding theory [5]. Recently representation theory has seen quite a few applications in computer science as well. In this article, we study some of the questions related to representation of finite groups on graphs. A representation of a group G usually means a linear representation, i.e. a homomorphism from the group G to the group GL (V ) of invertible linear transformations on a vector space V . Notice that GL (V ) is the set of symmetries or automorphisms of the vector space V . In general, by a representation of G on an object X, we mean a homomorphism from G to the automorphism group of X.
    [Show full text]
  • A B S T R a C T. World Spinors, the Spinorial Matter (Particles, P-Branes and Fields) in a Generic Curved Space Is Considered. R
    Bulletin T.CXLI de l'Acad´emieserbe des sciences et des arts − 2010 Classe des Sciences math´ematiqueset naturelles Sciences math´ematiques, No 35 SPINORIAL MATTER AND GENERAL RELATIVITY THEORY DJ. SIJAˇ CKIˇ (Inaugural lecture delivered on the 24th of May 2010 at the Serbian Academy of Sciences and Arts in Belgrade) A b s t r a c t. World spinors, the spinorial matter (particles, p-branes and fields) in a generic curved space is considered. Representation theory, as well as the basic algebraic and topological properties of relevant symme- try groups are presented. Relations between spinorial wave equations that transform respectively w.r.t. the tangent flat-space (anholonomic) Affine symmetry group and the world generic-curved-space (holonomic) group of Diffeomorphisms are presented. World spinor equations and certain basic constraints that yield a viable physical theory are discussed. A geometric con- struction based on an infinite-component generalization of the frame fields (e.g. tetrads) is outlined. The world spinor field equation in 3D is treated in more details. AMS Mathematics Subject Classification (2000): 22E46, 22E65, 58B25, 81T20, 83D05 Key Words: General relativity, world spinors, Dirac-like equation, SL(n; R) representations 98 Dj. Sijaˇckiˇ 1. Introduction Point-like matter. The Dirac equation turned out to be one of the most successful equations of the XX century physics - it describes the basic matter constituents (both particles and fields), and very significantly, it paved a way to develop the concept of gauge theories thus playing an important role in description of the basic interactions (forces) as well. It is a Poincar´ecovariant linear field 1 equation which describes relativistic spin 2 matter objects, that a coupled to fundamental interactions in Nature through the gauge (minimal coupling) prescription.
    [Show full text]
  • GROUP REPRESENTATIONS and CHARACTER THEORY Contents 1
    GROUP REPRESENTATIONS AND CHARACTER THEORY DAVID KANG Abstract. In this paper, we provide an introduction to the representation theory of finite groups. We begin by defining representations, G-linear maps, and other essential concepts before moving quickly towards initial results on irreducibility and Schur's Lemma. We then consider characters, class func- tions, and show that the character of a representation uniquely determines it up to isomorphism. Orthogonality relations are introduced shortly afterwards. Finally, we construct the character tables for a few familiar groups. Contents 1. Introduction 1 2. Preliminaries 1 3. Group Representations 2 4. Maschke's Theorem and Complete Reducibility 4 5. Schur's Lemma and Decomposition 5 6. Character Theory 7 7. Character Tables for S4 and Z3 12 Acknowledgments 13 References 14 1. Introduction The primary motivation for the study of group representations is to simplify the study of groups. Representation theory offers a powerful approach to the study of groups because it reduces many group theoretic problems to basic linear algebra calculations. To this end, we assume that the reader is already quite familiar with linear algebra and has had some exposure to group theory. With this said, we begin with a preliminary section on group theory. 2. Preliminaries Definition 2.1. A group is a set G with a binary operation satisfying (1) 8 g; h; i 2 G; (gh)i = g(hi)(associativity) (2) 9 1 2 G such that 1g = g1 = g; 8g 2 G (identity) (3) 8 g 2 G; 9 g−1 such that gg−1 = g−1g = 1 (inverses) Definition 2.2.
    [Show full text]
  • Spinors for Everyone Gerrit Coddens
    Spinors for everyone Gerrit Coddens To cite this version: Gerrit Coddens. Spinors for everyone. 2017. cea-01572342 HAL Id: cea-01572342 https://hal-cea.archives-ouvertes.fr/cea-01572342 Submitted on 7 Aug 2017 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. Spinors for everyone Gerrit Coddens Laboratoire des Solides Irradi´es, CEA-DSM-IRAMIS, CNRS UMR 7642, Ecole Polytechnique, 28, Route de Saclay, F-91128-Palaiseau CEDEX, France 5th August 2017 Abstract. It is hard to find intuition for spinors in the literature. We provide this intuition by explaining all the underlying ideas in a way that can be understood by everybody who knows the definition of a group, complex numbers and matrix algebra. We first work out these ideas for the representation SU(2) 3 of the three-dimensional rotation group in R . In a second stage we generalize the approach to rotation n groups in vector spaces R of arbitrary dimension n > 3, endowed with an Euclidean metric. The reader can obtain this way an intuitive understanding of what a spinor is. We discuss the meaning of making linear combinations of spinors.
    [Show full text]
  • Representation Theory
    M392C NOTES: REPRESENTATION THEORY ARUN DEBRAY MAY 14, 2017 These notes were taken in UT Austin's M392C (Representation Theory) class in Spring 2017, taught by Sam Gunningham. I live-TEXed them using vim, so there may be typos; please send questions, comments, complaints, and corrections to [email protected]. Thanks to Kartik Chitturi, Adrian Clough, Tom Gannon, Nathan Guermond, Sam Gunningham, Jay Hathaway, and Surya Raghavendran for correcting a few errors. Contents 1. Lie groups and smooth actions: 1/18/172 2. Representation theory of compact groups: 1/20/174 3. Operations on representations: 1/23/176 4. Complete reducibility: 1/25/178 5. Some examples: 1/27/17 10 6. Matrix coefficients and characters: 1/30/17 12 7. The Peter-Weyl theorem: 2/1/17 13 8. Character tables: 2/3/17 15 9. The character theory of SU(2): 2/6/17 17 10. Representation theory of Lie groups: 2/8/17 19 11. Lie algebras: 2/10/17 20 12. The adjoint representations: 2/13/17 22 13. Representations of Lie algebras: 2/15/17 24 14. The representation theory of sl2(C): 2/17/17 25 15. Solvable and nilpotent Lie algebras: 2/20/17 27 16. Semisimple Lie algebras: 2/22/17 29 17. Invariant bilinear forms on Lie algebras: 2/24/17 31 18. Classical Lie groups and Lie algebras: 2/27/17 32 19. Roots and root spaces: 3/1/17 34 20. Properties of roots: 3/3/17 36 21. Root systems: 3/6/17 37 22. Dynkin diagrams: 3/8/17 39 23.
    [Show full text]
  • Notes on Representations of Finite Groups
    NOTES ON REPRESENTATIONS OF FINITE GROUPS AARON LANDESMAN CONTENTS 1. Introduction 3 1.1. Acknowledgements 3 1.2. A first definition 3 1.3. Examples 4 1.4. Characters 7 1.5. Character Tables and strange coincidences 8 2. Basic Properties of Representations 11 2.1. Irreducible representations 12 2.2. Direct sums 14 3. Desiderata and problems 16 3.1. Desiderata 16 3.2. Applications 17 3.3. Dihedral Groups 17 3.4. The Quaternion group 18 3.5. Representations of A4 18 3.6. Representations of S4 19 3.7. Representations of A5 19 3.8. Groups of order p3 20 3.9. Further Challenge exercises 22 4. Complete Reducibility of Complex Representations 24 5. Schur’s Lemma 30 6. Isotypic Decomposition 32 6.1. Proving uniqueness of isotypic decomposition 32 7. Homs and duals and tensors, Oh My! 35 7.1. Homs of representations 35 7.2. Duals of representations 35 7.3. Tensors of representations 36 7.4. Relations among dual, tensor, and hom 38 8. Orthogonality of Characters 41 8.1. Reducing Theorem 8.1 to Proposition 8.6 41 8.2. Projection operators 43 1 2 AARON LANDESMAN 8.3. Proving Proposition 8.6 44 9. Orthogonality of character tables 46 10. The Sum of Squares Formula 48 10.1. The inner product on characters 48 10.2. The Regular Representation 50 11. The number of irreducible representations 52 11.1. Proving characters are independent 53 11.2. Proving characters form a basis for class functions 54 12. Dimensions of Irreps divide the order of the Group 57 Appendix A.
    [Show full text]
  • Wave Front Sets of Reductive Lie Group Representations Iii 3
    WAVE FRONT SETS OF REDUCTIVE LIE GROUP REPRESENTATIONS III BENJAMIN HARRIS AND TOBIAS WEICH Abstract. Let G be a real, reductive algebraic group, and let X be a ho- mogeneous space for G with a non-zero invariant density. We give an explicit description of a Zariski open, dense subset of the asymptotics of the tempered support of L2(X). Under additional hypotheses, this result remains true for vector bundle valued harmonic analysis on X. These results follow from an upper bound on the wave front set of an induced Lie group representation under a uniformity condition. 1. Introduction Let G be a real, linear algebraic group with Lie algebra g. Denote the col- lection of purely imaginary linear functionals on g by ig∗ := HomR(g,iR). The ∗ Kirillov-Kostant orbit method seeks to associate a coadjoint orbit Oπ ⊂ ig to every irreducible, unitary representation π of G. This program is not possible in full generality; some irreducible, unitary representations do not naturally corre- spond to coadjoint orbits. However, it has been carried out for irreducible, unitary representations occurring in the Plancherel formula for G [Duf82], and there are encouraging signs of success for other important families of representations (see for instance [Vog00]). The passage from Oπ to π is often referred to as quantization. Let X be a homogeneous space for G with a non-zero invariant density. For each x ∈ X, let Gx denote the stabilizer of x in G, and let gx denote the Lie algebra 2 of Gx. Abstract harmonic analysis seeks to decompose L (X) into irreducible, unitary representations of G.
    [Show full text]
  • The Origin of Representation Theory
    THE ORIGIN OF REPRESENTATION THEORY KEITH CONRAD Abstract. Representation theory was created by Frobenius about 100 years ago. We describe the background that led to the problem which motivated Frobenius to define characters of a finite group and show how representation theory solves the problem. The first results about representation theory in characteristic p are also discussed. 1. Introduction Characters of finite abelian groups have been used since Gauss in the beginning of the 19th century, but it was only near the end of that century, in 1896, that Frobenius extended this concept to finite nonabelian groups [21]. This approach of Frobenius to group characters is not the one in common use today, although some of his ideas that were overlooked for a long period have recently been revived [30]. Here we trace the development of the problem whose solution led Frobenius to introduce characters of finite groups, show how this problem can be solved using classical representa- tion theory of finite groups, and indicate some relations between this problem and modular representations. Other surveys on the origins of representation theory are by Curtis [7], Hawkins [24, 25, 26, 27], Lam [32], and Ledermann [35]. While Curtis describes the development of modular representation theory focusing on the work of Brauer, we examine the earlier work in characteristic p of Dickson. 2. Circulants For a positive integer n, consider an n × n matrix where each row is obtained from the previous one by a cyclic shift one step to the right. That is, we look at a matrix of the form 0 1 X0 X1 X2 :::Xn−1 B Xn−1 X0 X1 :::Xn−2 C B C B .
    [Show full text]
  • Representation Theory of Finite Groups Notes (Loosely) Inspired by a Class Taught by Brian Parshall in Fall 2017
    Representation Theory of Finite Groups Notes (loosely) inspired by a class taught by Brian Parshall in Fall 2017 George H. Seelinger 1. Introduction From one perspective, the representation theory of finite groups is merely a special case of the representation theory of associative algebras by consid- ering the fact that a sufficiently nice finite group ring k[G] is semisimple, and thus the results from Artin-Wedderburn theory apply. However, the ex- tra structure of the group provides a rich connection between k[G]-modules and linear algebra. A representation of a finite group is a way to induce an action of a finite group on a vector space. Sometimes, such a perspective allows mathe- maticians to see structure or symmetries in groups that may not have been readily apparent from a purely group theoretic point of view, much like mod- ules can provide insight into the structure of rings. In fact, a reprsentation of a finite group is fundamentally the same as a module of a finite group algebra, but the representation theoretic perspective allows us to leverage tools from linear algebra to gain additional insights. One of the most oft-cited examples of representation theory in action is Burnside's pq-theorem (see 11), but representations are also often studied in their own right. In general, representation theories seem to follow a general program along the following lines. (a) To what degree do representations decompose into direct sums of subrepresentations? The best answer is when all representations are completely reducible, that is, a representation breaks up into a direct sum of the irreducible subrepresentations.
    [Show full text]
  • Representation Theory
    Representation Theory This is a course in representation theory, which essentially is the study of abstract algebra through (the language of) linear algebra. The ideas and methods from representation theory permeate many disparate areas of mathematics. For example, it is one of the (many) key components in the proof of Wiles Theorem1. No prior knowledge of group theory is needed for this course, though it will certainly be useful. However, a good basis in linear algebra and (abstract) vector spaces will be essential. Material for these notes is derived from these primary sources: (1) Representation Theory of Finite Groups by Martin Barrow, (2) Algebra by Michael Artin, (3) The Symmetric Group by Bruce Sagan, and (4) lecture notes on algebra by Allen Bell2. I should also throw in a shoutout to the Unapologetic Mathematician3. 1. A short (re)introduction to group theory Definition. A group is a pair (G; ·) with G a set and · a binary operation4 on G satisfying (1) Associativity: a · (b · c) = (a · b) · c for all a; b; c 2 G. (2) Identity: there exists e 2 G such that a · e = e · a for all a 2 G. (3) Inverses: for all a 2 G there exists an element a0 2 G such that a · a0 = a0 · a = e. If in addition, a · b = b · a for all a; b 2 G (commutativity) the group is said to be abelian. The number jGj is called order of the group G (this may be infinite). There are two \generic" group operations: addition and multiplication. Almost universally, the addition operation is reserved for abelian groups.
    [Show full text]
  • M3/4/5P12 GROUP REPRESENTATION THEORY Course Arrangements Lecture 1 Send Comments, Questions, Requests Etc. to J.Newton@Imperial
    M3/4/5P12 GROUP REPRESENTATION THEORY JAMES NEWTON Course Arrangements Lecture 1 Send comments, questions, requests etc. to [email protected]. The course homepage is http://wwwf.imperial.ac.uk/˜jjmn07/M3P12.html. Problem sheets and solutions, lecture notes and other information will all be posted here. There is no black- board page for this course. Problems classes. Problems classes will take place in the Tuesday lecture slot of odd weeks from week 3 (January 26) onwards. Assessment. 90% of your mark is from the final exam, 10% from two progress tests (5% each) on Tuesday February 16 and Tuesday March 15. Office hour. There will an office hour, in office 656, every Thursday 4–5pm, starting week 2 (January 21) and ending week 10 (March 17). Other reading. The course webpages for 2014 http://wwwf.imperial.ac.uk/˜epsegal/ repthy.html and 2015 https://sites.google.com/site/matthewtowers/m3p12 contain lecture notes and problem sheets. The course content this year will be very similar, but the material will be reorganised a little. Here are some textbooks: • G. James and M. Liebeck, Representations and Characters of Groups. The first 19 sections of this book cover almost all of the course, apart from the final part on semisimple algebras. You should be able to access an ebook version through the Imperial library website. • J. P. Serre, Linear Representations of Finite Groups. Part I of this book gives a concise and elegant exposition of character theory. • J. L. Alperin, Local Representation Theory. The first couple of sections cover the part of the course on semisimple algebras.
    [Show full text]