MT4003 Groups MRQ April 27, 2018 Contents Introduction 3 Prerequisite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Overview of course structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Textbooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Standard notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1 Definition and Examples of Groups 6 The axioms of a group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Examples of groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Cayley tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Advantages of Cayley tables . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Disadvantages of Cayley tables . . . . . . . . . . . . . . . . . . . . . . . . . 12 What we try to do in group theory . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Basic properties of group elements . . . . . . . . . . . . . . . . . . . . . . . . . . 13 The order of an element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2 Subgroups 17 Generating sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 The generation algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Dihedral groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Cosets and Lagrange’s Theorem. . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3 Normal Subgroups, Quotient Groups and Homomorphisms 27 Normal subgroups and quotient groups . . . . . . . . . . . . . . . . . . . . . . . . 27 Homomorphisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Kernels and images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 The Isomorphism Theorems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4 Cyclic Groups 39 Subgroups of cyclic groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 5 Constructing Groups 43 Permutation Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Direct Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 6 Finite Abelian Groups 49 7 Simple Groups 56 Simplicity of the alternating groups. . . . . . . . . . . . . . . . . . . . . . . . . . 56 1 8 Further Tools: The centre, commutators and conjugation 60 The centre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Commutators and the derived subgroup . . . . . . . . . . . . . . . . . . . . . . . 61 Soluble groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Conjugacy classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Centralisers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Conjugation of subgroups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Normalisers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 9 Sylow’s Theorem 69 10 Classification of Groups of Small Order 76 Groups of order p2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 Groups of order 2p . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Groups of order 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 Groups of order pq . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Groups of order 12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 2 Introduction The purpose of this course is to introduce the detailed study of groups. These algebraic structures occur throughout mathematics and the physical sciences since they are the natural way to encode symmetry. They have been introduced in some previous courses (MT2505 primarily, but also MT1003), but here we shall pursue a much more in-depth study. In this course we shall encounter how to analyse the structure of a group — and what the term “structure” signifies — and consider how to classify groups. Prerequisite • MT2505 As Honours students, a higher level of mathematical maturity will be expected com- pared to the study of abstract algebra at 2000-level. Some of the content of MT2505 will be assumed (particularly an ability to manipulate permutations effectively), but some material will be discussed again in order to take the study deeper. Overview of course structure Groups: Definitions and examples. Subgroups: Finding groups inside larger groups. How to describe new groups via gen- erating sets. Theoretical restrictions on subgroups. This provides the first example of basic structure within groups. Homomorphisms & quotient groups: Further examples of basic structure within groups. Quotients enable us to “factorise” a group into smaller groups. The Corre- spondence Theorem explains how the structure of a quotient group is related to that of the original group, and in particular why is it more simplified. The Isomorphism Theorems describe how the three aspects of structure (subgroups, homomorphisms, and quotients) relate to each other. Constructing groups: How to build new groups from old. We shall use one of these constructions to describe a classification of all finite abelian groups. Simple groups: These are the smallest building blocks from which to construct groups. We shall establish that the alternating group A is simple whenever n (cid:62) 5. n The centre, conjugation, and commutators: Three useful tools which enable us to examinethestructureofagroupinmoredetail. Wedefinetwonewclassesofgroups: soluble groups and nilpotent groups. These are more general than abelian groups but still more tractable to in-depth study than arbitrary groups. (My lecture notes 3 for MT5824 Topics in Groups cover these concepts in more detail. Other lecturers may cover other material instead in that module.) Sylow’s Theorem: The most important theorem in finite group theory. We shall illus- trate this with numerous applications. Final applications: Classification of groups of small order, namely order p, p2, pq (p, q distinct primes) and order (cid:54) 15. Textbooks Almost any textbook on group theory or abstract algebra is likely to cover the material in this course and provide a good supplement to the lecture course. The following are particularly recommended and are available for consultation in the library: • R.B.J.T. Allenby, Rings, Fields and Groups: An Introduction to Abstract Algebra, Chapters 5 and 6. Second Edition: Butterworth-Heinemann, 1991; QA162.A6F91. First Edition: Edward Arnold, 1983; QA162.A6: Nicely written, reasonably modern, and fits this course well. • W. Ledermann, Introduction to Group Theory, Oliver and Boyd, 1973; QA171.L43: Reasonably good fit to the course. • John S. Rose, A Course on Group Theory, Dover, 1994, up to Chapter 6; QA171.R7: Reasonably detailed at the level we want; quite cheap. • T.S. Blyth & E.F. Robertson, Algebra Through Practice: A Collection of Problems in Algebra with Solutions, Book 5: Groups, CUP, 1985; QA157.B6R7;5 • Thomas W. Hungerford, Algebra, Holt, Rinehart and Winston, 1974, Chapters 1 and 2; QA155.H8 • I. N. Herstein, Topics in Algebra, Second Edition, Wiley, 1975, Chapter 2; QA159.H4F76: Aclassictextongeneralalgebra,thoughlesseasyformodernreaders, also a little idiosyncratic in some definitions and notations. • Derek J.S. Robinson, A Course in the Theory of Groups, Second Edition, Springer, 1996; QA171.R73: An advanced text, so goes quite rapidly through the material we cover in its early chapters. • JosephJ.Rotman,AnIntroductiontotheTheoryofGroups, FourthEdition,Springer, 1995; QA171.R7: Similarly advanced, though perhaps slightly less speedy in its cov- erage of our material. Standard notation The following are standard pieces of mathematical notation that will be used throughout the notes. x ∈ A: x is an element of the set A. A = {x | ...}: A is the set of those elements x that satisfy the condition present in the second part of the bracket (replacing “...”). Also written A = {x: ...} in some textbooks. The definitions that follow give examples of the use of this notation. 4 A∩B: The intersection of A and B, defined by A∩B = {x | x ∈ A and x ∈ B}. A∪B: The union of A and B, defined by A∪B = {x | x ∈ A or x ∈ B}. A\B: The complement of B in A, defined by A\B = {x | x ∈ A and x (cid:54)∈ B}. This set therefore consists of all those elements of A which do not belong to B. A×B: The set of all ordered pairs (a,b) where a ∈ A and b ∈ B; that is, A×B = {(a,b) | a ∈ A,b ∈ B}. φ: A → B: φ is a function (or mapping) with domain A and codomain B. This means that we can apply φ to elements in A to produce as output elements of B. In this course, we write maps on the right, so that if a is an element of A, then the output when φ is applied to a is denoted by aφ (rather than φ(a) as would be common in other branches of mathematics, such as analysis, etc.). 5 Chapter 1 Definition and Examples of Groups The axioms of a group Abstract algebra is the study of sets with operations defined upon them. These opera- tions in some sense mimic addition or multiplication of numbers. The following definition provides the operations we work with. Definition 1.1 Let G be any (non-empty) set. A binary operation on G is a function G×G → G. We usually denote the image of a pair (x,y) under a binary operation by a notation such as x∗y, x◦y, x+y, xy, etc. This has the advantage of encouraging us to view binaryoperationsasgeneralisationsofourfamiliararithmeticoperations. Formostofthis course, we shall use multiplicative notation for our binary operations and so we write xy for the image of (x,y) under the operation; that is, xy denotes the effect of combining two elements x and y using the operation. Definition 1.2 A group is a set G together with a binary operation such that (i) the binary operation is associative, that is, x(yz) = (xy)z for all x,y,z ∈ G; (ii) there is an identity element 1 in G having the property x1 = 1x = x for all x ∈ G; (iii) every element x in G possesses an inverse x−1 also belonging to G having the prop- erty xx−1 = x−1x = 1. Remarks: (i) Some authors specify “closure” as an axiom for a group; that is, for all x,y ∈ G, the product xy belongs to G. Note, however, that this is built into the definition of a binary operation as a function G×G → G that takes values back in G. 6 Nevertheless, when verifying that a particular example is a group, we should not ignore that establishing we have a binary operation is part of the steps. So, to be explicit, when checking the axioms of a group, we need to check for (1) closure (that we do indeed have a binary operation defined on our set), (2) associativity, (3) existence of identity, and (4) existence of inverses. (ii) As we are using multiplication notation, we often refer to the binary operation as the group multiplication. There are, however, some groups where it is more natural to use addition as the binary operation (see, for example, Example 1.5 below) and we adjust our terminology appropriately. (iii) Some authors use e to denote the identity element in a group. These lectures follow the more common (and perhaps slightly more sophisticated) approach of using 1. We shall rely on the experience of the student to be able to distinguish from the context between the identity element of a group and the integer 1. It follows by repeated use of the associativity axiom that any bracketing of a product x x ...x ofelementsx ,x ,...,x inagroupGcanbeconvertedtoanyotherbracketing 1 2 n 1 2 n withoutchangingthevalueoftheproduct. Inviewofthis, wecansafelyomitthebrackets in any such product and still know that we have specified a unique element in our group by the product. For example, (cid:0) (cid:1) (cid:0) (cid:1) x (x x ) x = (x x )x x = (x x )(x x ). 1 2 3 4 1 2 3 4 1 2 3 4 What should be noticed is that the order of the elements in a product does matter. The axioms of a group do not tell us any clear link between the products x x x , x x x , x x x , etc. 1 2 3 1 3 2 2 1 3 and there are examples of groups where such products are all different. We give a special name for groups where we can reorder the elements without changing the value of a product. Definition 1.3 Anabelian groupisagroupGwherethebinaryoperationiscommutative, that is, xy = yx for all x,y ∈ G. We shall often make statements such as “G is a group” to mean that G is a set with a binary operation defined upon it satisfying the axioms of Definition 1.2. This illustrates how we often do not distinguish between a group and the underlying set upon which the binary operation is defined. We take this into account when making the following definition. Definition 1.4 Let G be a group. The order of G, denoted by |G|, is the number of elements in the underlying set on which our group is defined. A finite group is a group whose order is a finite number, while an infinite group is a group G for which |G| = ∞. Examples of groups We now illustrate how groups arise in multiple settings across mathematics by providing a range of examples. 7 Example 1.5 (Groups of numbers) Consider the set Z of all integers. This forms a group under addition +, as we shall now show. The sum of two integers is an integer, so addition is a binary operation on Z. Our familiarity with addition of numbers tells us x+(y+z) = (x+y)+z for all x,y,z ∈ Z. The identity element is 0, since x+0 = 0+x = x for all x ∈ Z. The inverse of x is −x, since x+(−x) = (−x)+x = 0 for all x ∈ Z. Moreover, Z is an abelian group under addition, since x+y = y+x for all x,y ∈ Z. In the same way, the rational numbers Q forms an abelian group under addition, the real numbers R forms an abelian group under addition, and the complex numbers C forms an abelian group under addition. However, none of Z, Q, R and C form groups under multiplication. [Exercise: Show that 0 has no inverse in any of these sets with respect to multiplication. Indeed, in Z only ±1 have multiplicative inverses.] Removing 0 from some of these sets does yield a multiplicative group. The set of non-zero rationals Q\{0} is a group with respect to multiplication. For we know that the product of two non-zero rational numbers is a rational number, so multiplication is a binary operation on Q\{0}. Multiplication is associative: x(yz) = (xy)z for all x,y,z ∈ Q\{0}. The identity element is 1: x1 = 1x = x for all x ∈ Q\{0}. The inverse of x = m/n is 1/x = n/m for x (cid:54)= 0: x· 1 = 1 ·x = 1. x x Moreover, Q \ {0} forms an abelian group under multiplication since xy = yx for all x,y ∈ Q, x,y (cid:54)= 0. In the same way, the non-zero real numbers R\{0} and the non-zero complex num- bers C\{0} form abelian groups under multiplication. Example 1.6 (Symmetric groups) A common way to produce groups is as sets of bijective functions (often those satisfying nice properties) under composition. In algebra, it is most common to write functions on the right so that when composing them we can read from left to right. Thus if X is a set, the composite of two functions α: X → X and β: X → X is the function αβ: X → X given by x(αβ) = (xα)β for each x ∈ X. Lemma 1.7 Composition of functions is associative. 8 Proof: Let α,β,γ: X → X be functions defined on X. We calculate the effect of α(βγ) and (αβ)γ on an element of X: (cid:0) (cid:1) (cid:0) (cid:1) x α(βγ) = (xα)(βγ) = (xα)β γ and (cid:0) (cid:1) (cid:0) (cid:1) (cid:0) (cid:1) x (αβ)γ = x(αβ) γ = (xα)β γ. Hence α(βγ) = (αβ)γ. (cid:3) A permutation of the set X is a function σ: X → X that is bijective; that is, it is injective: if x,y ∈ X and xσ = yσ, then x = y; and surjective: if y ∈ X, then there is some x ∈ X such that xσ = y. This guarantees we can find an inverse σ−1 of σ that has the effect of “undoing” the application of σ. Thus σσ−1 = ε = σ−1σ (1.1) where ε = ε is the identity map on X (ε: x (cid:55)→ x for all x ∈ X). X The set of all permutation on X forms a group under the composition of permuta- tions. This is called the symmetric group and is denoted S or Sym(X). Associativity of X composition is provided by Lemma 1.7; the identity element is the identity map ε = ε : X εσ = σε = σ for all σ ∈ S ; X and the inverse of a permutation σ as a function is the inverse in the group (see Equa- tion (1.1) above). Although we have been careful to use ε to denote our identity element in S to em- X phasise that it is a mapping, we will frequently follow the usual convention of writing 1 for the identity element in our group. (Sometimes even when 1 happens also to denote an element of X. Though in this case, context will always make the difference clear!) We are particularly interested in the symmetric group S in the case when X = X {1,2,...,n}. We then write S for the symmetric group of degree n consisting of all n permutations of X = {1,2,...,n}. A permutation of this X can be written in two-row notation where the elements of X are listed in the top row and below i we write the image iσ. For example, (cid:18) (cid:19) (cid:18) (cid:19) 1 2 3 4 1 2 3 4 σ = and τ = 3 1 2 4 4 1 2 3 are two permutations from S . Their product στ is calculated by first applying σ to an 4 element of X = {1,2,3,4} and then applying τ. (Remember that we are writing maps on the right!) Thus (cid:18) (cid:19) 1 2 3 4 στ = . 2 4 1 3 The inverse of σ is calculated by undoing the effect of applying σ; i.e., interchanging the rows of σ and reordering the columns to get the result in the correct form: (cid:18) (cid:19) (cid:18) (cid:19) 3 1 2 4 1 2 3 4 σ−1 = = . 1 2 3 4 2 3 1 4 9