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Abstract Group Theory Preprint typeset in JHEP style - HYPER VERSION Chapter 1: Abstract Group Theory Gregory W. Moore Abstract: Very Abstract.March 30, 2018 Contents 1. Introduction 2 2. Basic Definitions 3 3. Homomorphism and Isomorphism 11 3.1 Group Actions On Sets 13 4. The Symmetric Group. 14 4.1 Cayley's Theorem 16 4.2 Cyclic Permutations and cycle decomposition 17 4.3 Transpositions 18 4.4 Diversion and Example: Card shuffling 22 5. Generators and relations 25 5.1 Fundamental Groups In Topology 30 6. Cosets and conjugacy 35 6.1 Equivalence Relations 35 6.2 Lagrange Theorem 35 6.3 Conjugacy 38 6.4 Conjugacy Classes In Sn 41 6.4.1 Conjugacy Classes In Sn And Harmonic Oscillators 43 6.4.2 Conjugacy Classes In Sn And Partitions 45 6.5 Centralizer And Counting Conjugacy Classes 47 7. Kernel, Image, And Exact Sequence 52 7.1 The Finite Heisenberg Group And The Quantum Mechanics Of A Particle On A Discrete Approximation To A Circle 58 8. Group theory And Elementary Number Theory 61 8.1 Reminder On gcd And The Euclidean Algorithm 61 8.2 The Direct Product Of Two Cyclic Groups 63 8.3 Application: Expressing elements of SL(2; Z) as words in S and T 68 9. The Group Of Automorphisms 70 9.1 The group of units in ZN 74 9.2 Group theory and cryptography 79 10. Products And Semidirect Products 81 { 1 { 11. Group Extensions and Group Cohomology 88 11.1 Group Extensions 88 11.2 Central Extensions 94 11.2.1 Example: Charged Particle On A Circle Surrounding A Solenoid 109 11.3 Heisenberg extensions 113 11.4 General Extensions Of G By An Abelian Group A 121 11.4.1 Crystallographic Groups 124 11.4.2 Time Reversal 126 11.5 General Extensions 131 11.6 Group cohomology in other degrees 135 11.6.1 Definition 136 11.6.2 Interpreting the meaning of H0+! 140 11.6.3 Interpreting the meaning of H1+! 140 11.6.4 Interpreting the meaning of H2+! 141 11.6.5 Interpreting the meaning of H3 141 11.7 Some references 143 12. Overview of general classification theorems for finite groups 143 12.1 Brute force 144 12.2 Finite Abelian Groups 149 12.3 Finitely Generated Abelian Groups 152 12.4 The classification of finite simple groups 154 13. Categories: Groups and Groupoids 161 13.1 Groupoids 171 13.2 The topology behind group cohomology 172 14. Lattice Gauge Theory 176 14.1 Some Simple Preliminary Computations 177 14.2 Gauge Group And Gauge Field 178 14.3 Defining A Partition Function 182 14.4 Hamiltonian Formulation 187 14.5 Topological Gauge Theory 187 15. Example: Symmetry Protected Phases Of Matter In 1 + 1 Dimensions 192 1. Introduction Historically, group theory began in the early 19th century. In part it grew out of the problem of finding explicit formulae for roots of polynomials. 1. Later it was realized that 1See the romantic stories of the life of Galois { 2 { groups were crucial in transformation laws of tensors and in describing and constructing geometries with symmetries. This became a major theme in mathematics near the end of the 19th century. In part this was due to Felix Klein's very influential Erlangen program. In the 20th century group theory came to play a major role in physics. Einstein's 1905 theory of special relativity is based on the symmetries of Maxwell's equations. The general theory of relativity is deeply involved with the groups of diffeomorphism symmetries of manifolds. With the advent of quantum mechanics the representation theory of linear groups, particularly SU(2) and SO(3) came to play an important role in atomic physics, despite Niels Bohr's complaints about \die Gruppenpest." One basic reason for this is the connection between group theory and symmetry, discussed in chapter ****. The theory of symmetry in quantum mechanics is closely related to group representation theory. Since the 1950's group theory has played an extremely important role in particle theory. Groups help organize the zoo of subatomic particles and, more deeply, are needed in the very formulation of gauge theories. In order to formulate the Hamiltonian that governs interactions of elementary particles one must have some understanding of the theory of Lie algebras, Lie groups, and their representations. Now, in the late 20th and early 21st century group theory is essential in many areas of physics including atomic, nuclear, particle, and condensed matter physics. However, the beautiful and deep relation between group theory and geometry is manifested perhaps most magnificently in the areas of mathematical physics concerned with gauge theories (especially supersymmetric gauge theories), quantum gravity, and string theory. It is with that in the background that I decided to cover the topics in the following chapters. 2. Basic Definitions We begin with the abstract definition of a group. Definition 2.1:A group G is a set with a multiplication: 8a; b 2 G there exists a unique element in G, called the product, and denoted a · b 2 G The product is required to satisfy 3 axioms: 1. Associativity: (a · b) · c = a · (b · c) 2. Existence of an identity element: 9e 2 G such that: 8a 2 G a · e = e · a = a (2.1) 3. Existence of inverses: 8a 9a−1 2 G a · a−1 = a−1 · a = e Remarks 1. We will often denote e by 1, or by 1G, when discussing more than one group at a time. The identity element is also often called the unit element, although the term \unit" can have other meanings when dealing with more general mathematical structures such as rings. { 3 { 2. Also, we sometimes denote the product of a and b simply by ab. 3. We can drop some axioms and still have objects of mathematical interest. For ex- ample, a monoid is defined by dropping the existence of inverses. Nevertheless, the definition of a group seems to be in that Goldilocks region of being not too sparse to give too little structure, but not too rigid to allow only limited examples. It is just right to have a deep and rich mathematical theory. Exercise a.) Show that e unique. b.) Given a is a−1 unique? c.) Show that in axiom (2) above we need only say a · e = a, or e · a = a. It is not necessary to postulate both equations. Example 2.1: As a set, G = Z; R, or C. The group operation is ordinary addition, a + b. Check the axioms. Example 2.2: A simple generalization is to take n-tuples for a positive integer n: G = n n n Z ; R ; C , with the operation being vector addition: (x1; : : : ; xn) · (y1; : : : ; yn) := (x1 + y1; : : : ; xn + yn) (2.2) ∗ ∗ Example 2.3: G = R := R − f0g or G = C := C − f0g operation = ×. Definition 2.2: If G is a group, a subset H ⊆ G which is also a group is called, naturally enough, a subgroup. Exercise Subgroups a.) Z ⊂ R ⊂ C with operation +, define subgroups. ∗ b.) Is the subset Z − f0g ⊂ R a subgroup? ∗ c.) Let R± denote the positive and negative real numbers, respectively. Which of these ∗ are subgroups of R ? Definition 2.3: The order of a group G, denoted jGj, is the cardinality of G as a set. Roughly speaking this is the same as the \number of elements in G." A group G is called a finite group if jGj < 1, and is called an infinite group otherwise. The groups in Examples 1,2,3 above are of infinite order. Here are examples of finite groups: { 4 { Example 2.4: The residue classes modulo N, also called \The cyclic group of order N." Choose a natural number 2 N. As a set we can take G = f0; 1;:::;N − 1g. 3 If n is an integer then we can write n = r + Nq in a unique way where the quotient q is integral and the remainder or residue modulo N is the integer r 2 G. The group operation on G, 4 denoted abstractly as r1 · r2, is defined to be the residue of (r1 + r2) modulo N. This group, which appears frequently in the following, will be denoted as Z=NZ or ZN . For example, telling time in hours is arithmetic in Z12, or in Z24 in railroad/military time. Exercise 5 Does Z137 have any nontrivial subgroups? Exercise In example 4 show that if N is even then the subset of classes of even integers forms a proper subgroup of ZN . What happens if N is odd? Already, with the simple concepts we have introduced, we can ask nontrivial questions. For example: Does every infinite group necessarily have proper subgroups of infinite order? It is actually not easy to think of counterexamples, but in fact there are infinite groups all of whose proper subgroups are finite. 6 So far, all our examples had the property that for any two elements a; b a · b = b · a (2.3) 2The natural numbers are the same as the positive integers. 3It is conceptually better to think of G as the integers modulo N, using the notation of equivalence relation of x6.2 below. Then we denote elements by 0¯; 1¯; 2¯; ··· . Thus, e.g. if N = 2 then 1¯ = 3.¯ The group operation is simply r1 + r2 := r1 + r2. 4 It is also possible to define a ring structure where one multiplies r1 and r2 as integers and then takes the residue. This is NOT what is meant here by r1 · r2 !! 5Answer: We will give an elegant answer below.
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