# 120A LECTURE OUTLINES

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1 120A LECTURE OUTLINES RUI WANG CONTENTS 1. Lecture 1. Introduction An algebraic object to study Group Isomorphic binary operations 2 2. Lecture 2. Introduction The multiplication over C The abelian group (R/2 Z, + R/2 Z ) Finite abelian group (U n (1), ) 3 3. Lecture 3, 4, 5. Binary Operations Definition Isomorphic binary operations Examples Associativity and Commutativity The identity element 4 4. Lecture 5, 6, 7. The Definition of Group Inverse Definition of a group Key properties of a group Table of finite groups 5 5. Lecture 8, 9. Subgroups Definition of a subgroup Cyclic subgroups 7 6. Lecture 10, 11. Cyclic groups Definition Properties Some applications 8 7. Lecture 12, 13. The permutation groups Symmetric group Permutation groups Cayley s theorem 9 8. Lecture 14, 15. The concepts for group actions in general Group G action on a set X Free action, faithful action and transitive action Orbits Orbits of an element g 2 G G-Invariant subset Lecture 17, 18 (Lecture 16 is midterm). The symmetric group S n Cycles Even and odd permutations 11 Date: March 22,

2 2 RUI WANG 10. Lecture 18, 19. Cosets Cosets The Lagrange theorem Lecture 20, 21. Structures of finite abelian groups Direct product of groups Direct product of finite abelian groups Finitely generated abelian groups Lecture 22, 23, 24, 25. Group homomorphisms Definition of group homomorphism Normal subgroups Fundamental theorem of group homomorphism Lecture 26. More on normal subgroups Simple groups and Centers Simple groups Centers and the commutator subgroup Semidirect-products and split extention semidirect-products Splitting extension LECTURE 1. INTRODUCTION An algebraic object to study. An algebraic object includes three ingredients: (1) A set S; (2) A binary operation, which is map from S S to S; (3) Certain properties. Example 1.1. (1) The set N = {0, 1, 2, }assigned with a binary operation. For example, (a) a b := a + b; (b) a b := a b; (c) a b := a b are binary operations. (a)(b) are associative and commutative. (c) is neither. While (a) a b := a b; (b) a b := a/b are NOT binary operations. (2) Similarly, consider S = Z, Q, R, C for the as above Group. A group is a set G assigned with a binary operation, which (1) is associative; (2) has identity; (3) every element has an inverse. If it is also commutative, it is called a commutative group or an abelian group. Example 1.2. (1) (Z, +) is an abelian group with identity 0; (2) (R, ) is not a group, but (R := R \{0}, ) is a group with identity 1; 1.3. Isomorphic binary operations. Assume (S 1, 1 ) and (S 2, 2 ) are two sets assigned with binary operations respectively. If there is a bijective map between S 1 and S 2 such that 1 can be identified with 2 via this map, then they are called isomorphic to each other. Moreover, if both are groups, then they are called isomorphic groups. For example, the abelian group (C, +) is isomorphic to (R R, +).

3 2.1. The multiplication over C. MATH 120A 3 2. LECTURE 2. INTRODUCTION 2 (C, ) is NOT isomorphic to (R R, ). Use logarithm coordinates, we can make (C, ) isomorphic to (R (R/2 Z), +), where + is defined as the addition on R together with + R/2 Z on R/2 Z, which we explain later. This isomorphism can be proved by Euler s formula The abelian group (R/2 Z, + R/2 Z ). As a set, R/2 Z is the set of equivalence classes of R with respect to the equivalence relation : a b iff a b 2 2 Z. The addition + on R is well-defined on this set of equivalence classes, so + R/2 Z is a binary operation over R/2 Z. In fact, it is a group and isomorphic to (U(1) := {z 2 C z =1}, ). (In standard notation, the 1 in U(1) denotes 1-dimensional linear space C over C. Such groups are called unitary groups in general. ) 2.3. Finite abelian group (U n (1), ). For any positive integer n =1, 2,, define the finite set For example, U n (1) = {z k := e 2 ik/n k =0, 1,,n 1}. U 4 (1) = {z 0 =1,z 1 = e i/2,z 2 = e i,z 3 = e 3 i/2 }. In general, (U n (1), + R/2 Z ) is an abelian group of n elements. It is isomorphic to (Z n, + n ), where Z n := {0, 1,,n 1} = Z/nZ and + n is the addition induced from the addition over Z Definition. 3. LECTURE 3, 4, 5. BINARY OPERATIONS Assume S is a set. A binary operation on S is a map : S S! S, (a, b) 7! a b. Table representation for sets with finite elements. Assume S is assigned with a binary operation. A subset H S is called closed, if the image (S S) S. In this case, we call the binary operation S S =: S is the restriction of to S Isomorphic binary operations. Assume (S i, i ),i =1, 2 are two sets with binary operations. They are called isomorphic if there exists bijective map : S 1! S 2 such that (a) 2 (b) = (a 1 b), a, b 2 S 1. Isomorphism of binary operation is an equivalence relation.

4 4 RUI WANG 3.3. Examples. (1) (C, +), (C, ), etc. ; (2) (Z n, +); (3) (M m n (R), +) is a group; (4) Denote by gl n (R) =M n n (R). (gl n (R), ) is not a group; (5) Assume V is a n-dimensional linear space. Then (End(V ), ) is a set with binary operation, where End(V ) denotes the set of linear transformations from V to itself and denotes the composition of maps; (6) (End(V ), ) = (gl n (R), ) if V is a n-dimensional linear space over R; (7) (GL n (R), ) is a group; (8) Assume V is a n-dimensional linear space. Then (Aut(V ), ) is a group, where Aut(V ) denotes the set of invertible linear transformations of V ; (9) (Aut(V ), ) = (GL n n (R), ) if V is a n-dimensional linear space over R; (10) Assume (S, ) is a set assigned with binary operation and X is an arbitrary set. Denote by C(X, S) the set of all maps from the set X to S. Then induces a binary operation X on C(X, S) Associativity and Commutativity. Call (S, ) is associative, if (a b) c = a (b c) for any a, b, c 2 S. Call (S, ) is commutative, if a b = b a for any a, b 2 S. Remark 3.1. Associativity gives a canonical way of extending the binary operator to an operator acting on arbitrarily many ordered elements. Moreover, if is also commutative, this is independent of orders. Example 3.2. Assume (S, ) is a set assigned with binary operation and X is an arbitrary set. We have known it induces (C(X, S), X ). If (S, ) is associative (or commutative), (C(X, S), X ) is also associative (or commutative) The identity element. Assume (S, ) is a set with a binary operation. An element e 2 S is called an (the) identity, if e a = a, b e = b, for any a, b 2 S. Identity may not exist (e.g. (N, +) has no identity), but if it exists, it must be the unique. Examples: (1) (N, +) has the identity 0; (2) (N, ) has the identity 1. Isomorphism of two binary structures must maps identity to identity. 4. LECTURE 5, 6, 7. THE DEFINITION OF GROUP 4.1. Inverse. Assume (S, ) is a set with a binary operation and has identity e. An element a 2 S has left (right) inverse, if there exists some a 0 2 S such that a 0 a = e (a a 0 = e). An element a 2 S has inverse, if it has both left inverse and right inverse, and they are the same, i.e., there exists some a 0 2 S such that a 0 a = a a 0 = e. Assume (S, ) is associative. If an element has both left inverse and right inverse, then these two inverses must be the same.

5 MATH 120A 5 If (S, ) is associative, then the inverse of an element is unique. We denote it by a 1 in general, but sometimes by a if is commutative Definition of a group. A group is a set (G, ) with binary operation satisfying It is associative; It has identity; Every element has an inverse. Example 4.1. Use definition to check (Z n, + n ) is a group. In fact, to check (G, ) is a group, we only need the following Proposition 4.2. Assume (G, ) is a set with binary operation. It is a group if It is associative; It has left identity (i.e., there exists some e 2 G such that e a = a for any a 2 G); Every element has a left inverse. Proof. We first show e is also a right inverse, i.e., for every a 2 G, a e = a. Take a 0 as a left inverse of a, then a 0 (a e) =(a 0 a) e = e e = e. On the other hand, a 0 a = e. So assume a 00 is a left inverse of a 0, and then a 00 (a 0 (a e)) = a 00 (a 0 a). Use the associativity again, we get a e = a. Next we show if a 0 is a left inverse of a, then it is also a right inverse of a. a 0 (a a 0 )=a 0 = a 0 e. We multiply the left inverse of a 0 from left and get a a 0 = e Key properties of a group. The cancellation rule. Existence and uniqueness of the equations a x = b, x a = b. Examples in linear algebra Table of finite groups. Basic rules: (1) the row/column of e; (2) no repeating element in each row/column, so every element must show once. Example 4.3. If G =1, then G = {e}; If G =2, then G = Z 2 ; If G =3, then G = Z 3 ; If G =4, then G = Z 4 or K 4 = Z 2 Z 2.

6 6 RUI WANG Order Number Abelian Non- Abelian LECTURE 8, 9. SUBGROUPS 5.1. Definition of a subgroup. A subset H of a group (G, ) is called a subgroup, if H is closed under and the induced binary operation on H makes (H, H ) be a group. Denote by H<G. If H<G, then the identity of G must be the identity of H. Any element a 2 H, the inverse a 1 of a in G is also the inverse of a in H. Example 5.1. (1) (Z, +) < (R, +); (2) (U n (1), ) < (C, ); (3) (nz, +) < (Z, +); (4) (Z n, + n ) is not a subgroup of (Z, +)! Proposition 5.2. Assume G is a group. A subset H of G is a subgroup if and only if ab 1 2 H for any a, b 2 H. Proposition 5.3. If H, K are two subgroups of G, then H \ K is also a subgroup of G.

7 Example 5.4. mz \ nz = lcm(m, n)z as subgroups of Z. MATH 120A 7 Remark 5.5. (1) H [ K is not a subgroup in general, but it is a subgroup if G is abelian. (A more general condition is either H or K is normal in G. We are going to introduce the concept of normal subgroup later. ) (2) HK is not a subgroup in general. Example 5.6. Assume G is a group. (1) For any a 2 G, C(a) :={x 2 G ax = xa} is called the set of commutators of a. C(a) <G. (2) For any subset S G, C(S) :={x 2 G xs = sx, any s 2 S} is called the centralizer of S in G. C(S) <G. (3) In particular, C(G) =:Z(G) is called the center of G. It is a normal abelian subgroup of G Cyclic subgroups. Assume G is a group. Take any a 2 G, it generates a subgroup <a> named a cyclic subgroup. It is the smallest subgroup containing a. This gives another definition of <a>as <a>= \ H<G,a2H H. Order of a is defined as the minimal positive integer such that a n = e. It is the same as <a>. E.g., In Z 6, 2 has order 3 which is the same as < 2 > = {0, 2, 4} =3. In a finite group, every element has a finite order. Example of element of finite order in an infinite group: 1. e 2 iq, q 2 Q, in C ; 2. I n 2 GL n (R); 3. J 2 GL 2 (R). If there exists some a 2 G such that G =< a>, then G is called a cyclic group. E.g., Z =< 1 >=< 1 >; 3Z =< 3 >. A cyclic group is obviously an abelian group Definition. 6. LECTURE 10, 11. CYCLIC GROUPS A group G is called a cyclic group, if there exists some element a 2 G such that G =< a>. The element a is called a generator of G. (a n ) 1 = a n, a m a n = a m+n. Examples: (1) (Z, +), Z =< 1 >=< 1 >; (2) Z n =< 1 > Properties. Proposition 6.1. A cyclic group must be an abelian group. Proposition 6.2. All subgroups of a cyclic group are cyclic. Proposition 6.3. <a> =min n2z +{a n = e}. Proposition 6.4 (Classification of cyclic groups). Up to group isomorphisms, cyclic groups have only two types: (Z, +); (Z n, + n ), n =1, 2, 3, (where Z 1 = {0}).

8 8 RUI WANG 6.3. Some applications. The concept of g.c.d. can be generalized to any cyclic groups. (In fact, this concept can be introduced to more general algebraic objects that satisfy the division algorithm. ) : G! H is a group homomor- Proposition 6.5. If G, H are two cyclic groups, and phism, then g.c.d( (a), (b)) = (g.c.d(a, b)), for any a, b 2 G. This can be used to calculate g.c.d.s for any cyclic group, whenever you know a group homomorphism Z! G. All subgroups of Z are of the form nz, n =0, 1, 2,. All subgroups of Z n are of the form <k>, where k =0, 1,,n 1. n Proposition 6.6. (1) The order of k, <k> = = l.c.m.(k,n). g.c.d(k,n) k (2) <k>=< `>if and only if g.c.d(k, n) =g.c.d(`, n). (3) <k>= Z n if and only if g.c.d(k, n) =1, i.e., k and n are co-prime. Proof. (1) Denote by d = g.c.d(k, n). First, notice that nk = n k is a multiple of n, it d d follows min {mk =0}applen m>0 d. On the other hand, from the definition of d, we know n and k are coprime. It d d follows n divides nk = kn implies n divides n. Thus d d d d min {mk =0} n m>0 d. Hence this proves min m>0 {mk =0} = n. Since we have shown <k> = d min m>0 {mk =0} and we are done. (2) Denote by d = g.c.d(k, n) =g.c.d(`, n). Let s show in fact <d>=< k>=< `>. Because d k, then it follows <d> < k>. Notice <d> = n d = <k>, so it follows <d>=< k>. The other equality is exactly the same. (3) Immediately follows from (2). The above properties hold for any cyclic group from the classification of cyclic groups. 7. LECTURE 12, 13. THE PERMUTATION GROUPS 7.1. Symmetric group. Definition of S n and notations. (Here we take n =1, 2.) S n is a finite group of n! s elements. S n is abelian if and only if n =1, Permutation groups. Any set X, define S X as the set of all bijective maps from X to itself. If X is ordered and X = n, which means there is a bijective map ord : {1, 2,,n}!X, then ord induces a group isomorphism S X! S n.

9 7.3. Cayley s theorem. Definition of group homomorphism. Concept of subgroups. MATH 120A 9 Theorem 7.1 (Cayley s). Any group G can be considered as a subgroup of S G via a canonical injective group homomorphism. Proof. Construct : G! S G as (g) =xg. This is named a (right) regular representation of G. (Convention: My convention here is different from the book that I define the binary operation : S X S X! S X as f g := g f. In the previous statement, we omit and write fg = f g = g f.) Remark 7.2. In general, isomorphisms of G. (g) /2 Aut(G), where Aut(G) is defined as the group of group Definition of a group representation in general, and why we can consider the regular representation as a presentation. (Taking V = C(G, R) for example. ) 8. LECTURE 14, 15. THE CONCEPTS FOR GROUP ACTIONS IN GENERAL 8.1. Group G action on a set X. We always assume X 6= ;. Definition 8.1. Call a group G acting on a set X (from right), if there exists a map A : G X! X, satisfying the following two requirements: denote by A g (x) :=A(g, x), (1) A e = id X ; (2) A g2 A g1 = A g1 g 2. If a set X admitting a G-action, we call X a G-set. Proposition 8.2. Assume G is a group, the map is a group action of G on X if and only if is group homomorphism. Example 8.3. A : G X! X, A : G! S X, A(g) :=A g (1) Any set X admitting S X -action defined as A (x) = (x). (Ex: Check this is compatible with our convention here. ) (2) Any group G admitting G-action of the right multiplication as R g (x) :=xg, g 2 G, and this is named as the right action. The Cayley s theorem tells us that, by regarding G as a subgroup of S X by the regular representation, the G-action on G is induced from the S G action on G. (3) Any group G admitting G-action of left inverse multiplication as and this is named as the left action. L g (x) :=g 1 x, g 2 G,

10 10 RUI WANG (4) Any group G admitting an G action defined as Ad g = L g R g, for any g 2 G. This is called the adjoint action. (5) For any H<G, the G action on X induces H action on X. In particular, any element g 2 G, the G-action on X induces the cyclic group <g>on X Free action, faithful action and transitive action. Assume G acts on a set X, for some x 2 X, denote by G x := {g 2 G A g (x) =x}, and this is called the isotropy group at x. For any x 2 X, G x <G. Definition 8.4. (1) An action G on X is called a free action, if G x = {e} for any x 2 X. (2) An action G on X is called a faithful (or effective) action, if \ x2x G x = {e}. From definition, a free action must be faithful, but a faithful action may not be free. Proposition 8.5. A group action G on X is faithful if and only if the group homomorphism is injective. A : G! S X Remark 8.6. In fact, ker A = \ x2x G x is a normal subgroup of G, which we are going to prove in later lectures. Definition 8.7. An action G on X is called transitive, if for any x, y 2 X, there exists some g 2 G, such that A g (x) =y. Example 8.8. (1) The left and right actions of G on itself are both free and transitive. (2) For the adjoint action of G on itself, we have G x = C(x), i.e., the commutator of x 2 G. ker Ad = C(G), i.e., the center of G. Hence the adjoint action is faithful if and only if G has trivial center C(G) Orbits. Assume G acts on X. Then we can define an equivalence relation on X as: x y if and only if there exists some g 2 G such that A g (x) =y. Usually, we use [x] to denote the equivalence class including x. In particular, for the group action case, we also call [x] the orbit of G containing x, sometimes we use orb G (x) to denote it. Proposition 8.9. G has only one orbit if and only if the action is transitive. Remark There is a very interesting result, which is, if G acts on X transitively and both G and X are finite, then G = g2g X g, where X g is the fixed point set of X, i.e., X g = {x 2 X A g (x) =x}. For example, take G = S 3 acting on N = {1, 2, 3}. It is a transitive action. We know S 3 =3!=6. On the other hand, N 0 =3, N (12) = N (23) = N (31) =1, and =6. This is a special case of the Burnside s formula, which we can prove after we learn more about finite groups Orbits of an element g 2 G. Assume group G acting on the set X, for any g 2 G, we know <g>is a subgroup of G and hence <g>acts on X. For any x 2 X, we call the orbit of <g>containing x, denote by orb g (x) the orbit of g containing x. Example (1) Consider = Then orb (1) = orb (3) = orb (6) = {1, 3, 6} orb (2) = orb (8) = {2, 8} orb (4) = orb (5) = orb (7) = {4, 5, 7}.

11 8.5. G-Invariant subset. MATH 120A 11 Definition Assume the group G acting on a set X. A subset Y X is called G-invariant, if A g (y) 2 Y for any y 2 Y, g 2 G. If Y X is G-invariant subset, then it becomes a G-set by the group action induced from X. Proposition Assume the group G acting on a set X. Then for any x 2 X, orb G (x) is G-variant, and the induced G action on orb G (x) is transitive. 9. LECTURE 17, 18 (LECTURE 16 IS MIDTERM). THE SYMMETRIC GROUP S n This section, we focus on S n. The finiteness of n plays an essential role Cycles. Definition of a cycle: 2 S n is called a cycle if it has at most one orbit containing more than one element. Length of a cycle. A cycle of length 2 is called a transposition. Example. Notation for a cycle, e.g., in S µ =(1, 3, 6) = =(3, 6, 1) = (6, 1, 3) Disjoint cycles. Any permutation can be written into a product of disjoint cycles. Any two disjoint cycles are commutative. (Though not any two permutation commute. ) Example 9.1. (1) =(1, 6)(2, 5, 3) = (2, 5, 3)(1, 6) (2) (1, 4, 5, 6)(2, 1, 5) = 9.2. Even and odd permutations. Theorem 9.2. Assume n For example, (1, 2, 3, 4) = (1, 4)(1, 3)(1, 2) = (2, 1, 5)(1, 4, 5, 6) Any permutation is a product of transpositions. Theorem 9.3. If a permutation is a product of even number of transpositions, then it can not be written as a product of odd number of transpositions. Proof. Consider the S n action on GL n (R) and the map : S n!±1, 7! det(a (I n )). This theorem makes us be able to define even/odd permutations. Theorem 9.4. In S n, the number of odd permutations are the same as the number of even permutations. Denote by A n S n the subset of even permutations. Theorem 9.5. A n is a subgroup of S n of n! 2 elements, which is called the alternating group.

12 12 RUI WANG Proof. (1) Method 1: Direct proof. (2) Method 2: Construct group homomorphism S n! Z Cosets. 10. LECTURE 18, 19. COSETS Definition H<G, a left coset containing a is defined as ah := {ah h 2 H} G. Similarly, a right coset containing a 2 G is defined as Ha := {ha h 2 H} G. Remark (1) In general, ah is not a subgroup of G. But if G is abelian, ah = Ha < G. (In fact, ah = Ha if and only if H C G. ) (2) Consider the right action of H on G, ah = orb H (a). Proposition For each coset ah, there a canonical bijective map from H to it given as a : H! ah, h 7! ah. Remark This is a general fact that G acts on X freely, then there exits a bijective map from G to each orbit orb G (x) as x : G! orb G (x), Corollary If G is finite, ah = H. g 7! A g (x). Lemma (1) For any a, b 2 G, either ah = bh or ah \ bh = ;. (2) ah = bh if and only if b 1 a 2 H; Lemma Define a H b as b 1 a 2 H. Then H is an equivalence relation on G. Denote by [G : H] the set of such equivalence classes. Hence Proposition Can decompose G = t [a]2[g:h] ah as disjoint union The Lagrange theorem. Theorem 10.9 (Lagrange). Assume G is a finite group and H<G. Then H divides G. In fact, we know G = H [G : H]. Denote by (G : H) := [G : H] and call it the index of H in G. Now we list some applications of the Lagrange theorem on finite groups. Assume G is a finite group. (1) If G is prime, then G is cyclic. (2) Any a 2 G, <a> must divide G. (3) If K<G, H<G, K<H, then (G : K) =(G : H)(H : K). 11. LECTURE 20, 21. STRUCTURES OF FINITE ABELIAN GROUPS Direct product of groups. n i=1g i is called the direct product of groups G i s, i =1,,n. If every G i is abelian, then n i=1g i is also abelian. For this case, usually write as n i=1 G i. If every G i is finite, then n i=1g i = n i=1 G i.

13 11.2. Direct product of finite abelian groups. MATH 120A 13 Proposition Z m Z n is isomorphic to Z mn if and only if gcd(m, n) =1. More generally, Theorem m i=1 Z ni is isomorphic to Z n1 n 2 n m if and only if gcd(n i,n j )=1for any i 6= j. Proof. Show : Z n1 n 2 n m! m i=1z ni, a 7! ([a] n1, [a] n2,, [a] nm ) is a group homomorphism. It is injective if and only if gcd(n i,n j )=1for any i 6= j. Now given n =(p 1 ) n 1 (p 2 ) n2 (p m ) nm, for p 1,,p m are disjoint prime numbers, then using the previous theorem, we know For example, Z 72 = Z8 Z 9. Z n = Z(p1 ) n 1 Z (p2 ) n 2 Z (pm) nm. Theorem Assume a i 2 G i has order r i, then the order of the element a =(a 1,a 2,,a n ) 2 n i=1g i is the least common multiple of r i s, i =1,,n. Proof. We show n =2now and for higher n s, you may use induction to see. Assume a = (a 1,a 2 ) 2 G 1 G 2 with ord(a 1 )=r 1, ord(a 2 )=r 2. We show a has order r := lcm(r 1,r 2 ) now. First, it is clear that a r = e, so r apple ord(a). On the other hand, any a k = e, and it follows a k 1 = e 1, a k 2 = e 2. We have r 1 k, r 2 k, so k lcm(r 1,r 2 ). Then we are done. Example The order of (8, 4, 10) 2 Z 12 Z 60 Z 24 is 60. That is because the order of Z 12 has order =3, the order of 4 2 Z 60 gcd(8,12) 60 has order =15, the order of gcd(4,60) Z 24 has order =12. The least common multiple of 3, 15, 12 is 60. gcd(10,24) Theorem Every finite abelian group is isomorphic to some direct product Z (p1 ) r 1 Z (p2 ) r 2 Z (pn) rn, where p i are prime numbers. It is cyclic if and only if all p i s are disjoint. Proof. We prove if all p i s are disjoint prime numbers, then this direct product is cyclic. It is enough to find an element of order n i=1(p i ) r i. Take a i as the generator of Z (pi ) r i. From Theorem 11.3, we know it has order n i=1(p i ) r i Finitely generated abelian groups. Definition Assume G is a group. We say it is generated by a subset S of it, if G is the only subgroup of G which contains S. A group G is called finitely generated, if there exists a finite subset S of G which generates G. The following theorem is one the most important results about abelian groups. Theorem If G is a finitely generated abelian group, then G is isomorphic to the direct product Z Z Z Z (p1 ) r 1 Z (p2 ) r 2 Z (pn) rn, where p i are prime numbers. Moreover, this decomposition is unique up to orders. This number of copies of Z s is called the rank (or Betti number or dimension) of G. The finite abelian group part Z (p1 ) r 1 Z (p2 ) r 2 Z (pn) rn is called the torsion part of G. Remark Finitely generated abelian groups naturally appear as (singular) (co)homology groups of CW complexes.

14 14 RUI WANG 12. LECTURE 22, 23, 24, 25. GROUP HOMOMORPHISMS Definition of group homomorphism. : G! H is a group homomor- Definition Assume G and H are two groups. A map phism if (g 1 g 2 )= (g 1 ) (g 2 ), for any g 1,g 2 2 G. Here are some basic properties: There exists some injective homomorphism G! H if and only if G<H; There exists some bijective homomorphism G! H if and only if G = H. A group homomorphism maps identity to identity, inverse to inverse, subgroup to subgroups, preimage of a subgroup is a subgroup. In particular, the preimage of {e H } = ker is a normal subgroup of the domain group. Composition of two group homomorphisms is a group homomorphism. Aut(G) is a group. (Caution, S G 6= Aut(G) in general. e.g., Aut(Z 3 ) = Z 2, but S Z3 = S 3.) Proposition A group homomorphism is injective if and only if ker = {e}. Example (1) Trivial homomorphism. (2) The inclusion of G 1! G 1 G 2 ; (3) The projection of G 1 G 2! G 1. (4) G action on a set X induces a homomorphism G! S X. (5) Z! Z n. (6) S n! Z 2. (7) det : GL n (F)! F, F = Q, R, C or other fields. (8) A homomorphism from a cyclic group completely determined by the image of the generator Normal subgroups. A subgroup N<Gis called normal, if g 1 ag 2 N for any a 2 N, g 2 G. N<Gis normal if and only if N is Ad-action invariant. N C G if and only if an = Na for any a 2 G. In this case, {an a 2 G} becomes a group. This is called the quotient group of G by N, which is denoted by G/N. Any subgroup of an abelian group is normal. Proposition Assume N C G, then the map is a group homomorphism, whose kernel is N. : G! G/N, (g) =gn Definition A group G is called simple, if there is no proper nontrivial normal subgroup Fundamental theorem of group homomorphism. Theorem Assume : G! H is a group homomorphism. Then induces a group isomorphism from the quotient group G/ ker to the image (G). 13. LECTURE 26. MORE ON NORMAL SUBGROUPS SIMPLE GROUPS AND CENTERS Simple groups. Definition A group is called simple if there is no nontrivial, property normal subgroups. Example The alternating group A n is simple for n 5.

15 MATH 120A 15 Definition A maximal normal subgroup of a group G is a proper normal subgroup of G such that there is no proper normal subgroup N of G properly containing M. Lemma M is a maximal normal subgroup of G if and only if G/M is simple Centers and the commutator subgroup. The subset Z(G) :={g 2 G gx = xg, for any x 2 G} is called the center of G, which is a normal subgroup of G. Lemma G = Z(G) if and only if G is abelian. If Z(G) is trivial, i.e., Z(G) ={e}, we call the group G has trivial center. This indicates this group is most noncommutative. Example S 3 has trivial center. Lemma Z(G 1 G 2 )=Z(G 1 ) Z(G 2 ). Lemma The kernel of the surjective homomorphism c : G! Inn(G) is Z(G). To better characterize non-commutativity, we consider another normal subgroup. Definition The elements of the form a 1 b 1 ab for some a, b 2 G are called commutators of the group G. We use [G, G] to denote the subgroup generated by all commutators of the group G. The subgroup [G, G] measures how much the group G is non-commutative. For example, Lemma G is abelian if and only if [G, G] is trivial. The following observation in essential that people introduce this commutator subgroup. Lemma [G, G] C G. Moreover, the quotient group G/[G, G] is abelian. 14. SEMIDIRECT-PRODUCTS AND SPLIT EXTENTION semidirect-products. Assume G and Q are two groups, and : Q! Aut(G) is a group homomorphism, we can creates a new group structure on the set Q G using. Define (q 1,g 1 ) (q 2,g 2 ):=(q 1 q 2, ( (q 2 )g 1 )g 2 ). Lemma Q G becomes a group with respect to the binary operation, if and only if is a group homomorphism. The identity element is (e Q,e G ) and the inverse of (q, g) is (q, g) 1 =(q 1, (q 1 )(g 1 )). Usually we use Q n G to denote this group and call it a semidirect-product of Q and G. Example (1) The direct product Q G is a special case of semidirect-product as id G. (2) Take Q = Z 2 and G = Z 3. Define (0) = id Z3, (1) = ( ) 1. Direct checking shows that : Z 2! Aut(Z 3 ) is a group homomorphism. The semidirect-product Z 2 n Z 3 in fact is isomorphic to S 3, which is different from Z 2 Z 3 (to see they are different, e.g., the inverse of (1, 1) in Z 2 Z 3 is (1, 2), but in Z 2 nz 3 is (1, 1)). (3) In fact, for all n 2, we can similarly construct Z 2 n Z n, which turns to be the same as D n, the symmetry group of regular n-polygons (the Dihedral group). In particular, D 2 = K 4, D 3 = S3. (You can check, all D n with n 3 are nonabelian. )

16 16 RUI WANG Splitting extension. Assume : H! Q is a surjective group homomorphism. Definition A map s : Q! H satisfying s = id H, is called a section of : H! Q. Remark In general, we can not expect to find a section which is a group homomorphism. Define : Q! Aut(H) as follows: For each q, define s (q)(x) =Ad s(q) (x). Denote by G := ker. Since G C H, G is invariant under the adjoint action. So we obtain a map s : Q! Aut(G). Lemma If s is a group homomorphism, then s is a group homomorphism. We know if s is a group homomorphism, then it defines the semidirect-product Q n s G. Hence for this situation, we have Proposition If the group extension H has a section s which is a homomorphism, then it must be isomorphic to Q n s G. (Of course, by this, the semidirect-product Q n s G is independent of choices of sections s whenever they are homomorphisms. ) Proof. Consider s : Q G! H, (q, g) 7! s(q)g Direct checking shows that when s is a homomorphism, s is a group isomorphism. If there exists such homomorphic section s, the homomorphism : H! Q is called split. We have shown that, for this case, H must be a semidirect-product. Conversely, given a semidirect-product Q n G, we can construct a group extension Q n G! Q, using the projection : Q G! Q, (q, g) =q. Then the section s : Q! Q G defined as s(q) =(q, e) is a group homomorphism by direct checking. We summarize it as Proposition A surjective group homomorphism semidirect-product of Q and ker. : H! Q splits if and only if H is a Remark Notation: We define here (q 1,g 1 ) (q 2,g 2 )=(q 1 q 2, (q 2 )(g 1 )g 2 ). So, if follows, (q, e G ) (e Q,g)=(q, g) but (e Q,g) (q, e G )=(q, (q)(g)) which is not the same as (q, g) in general when is not trivial map. However, if you define the semidirect-product (you can check this defines a group too) as (q 1,g 1 ) 0 (q 2,g 2 )=(q 1 q 2,g 1 (q 1 )(g 2 )), then (e Q,g) 0 (q, e G )=(q, g) which is somehow wired. To resolve it, we can write G Q instead. Hence usually, this way is referred as G o Q.

17 MATH 120A 17 I didn t use this way, because when we take a section s and define s (q) =Ad s(q), you will find, the second convention leads to s : Q! Aut(G) is homomorphism if and only if s(q 1 q 2 )=s(q 2 )s(q 1 ), which is not consistent with the notations we have used for this whole quarter. When we identity S 3 with Z 2 n Z 3, you may take for example. Then we map =(1, 2), =(1, 2, 3) 7! (1, 0) 2 Z 2 Z 3, 7! (0, 1) 2 Z 2 Z 3. Then by the semidirect-product we used, there is and := 7! (1, 0) (0, 1) = (1, 1) := 7! (0, 1) (1, 0) = (1, 2) = (1, 1).

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