# Section 15 Factor-group computation and simple groups

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1 Section 15 Factor-group computation and simple groups Instructor: Yifan Yang Fall 2006

2 Outline Factor-group computation Simple groups

3 The problem Problem Given a factor group G/H, find an isomorphic group G that is more familiar to us (so that to study the structure of G/H, we can work on the more familiar group G instead). Some simple facts 1. G/{e} is isomorphic to G itself. 2. G/G is isomorphic to the trivial group with one element. 3. If G is cyclic, then G/H is also cyclic. 4. If G = G 1 G 2, then G/({e 1 } G 2 ) is isomorphic to G If G = G 1 G 2, H 1 G 1, and H 2 G 2, then G/(H 1 H 2 ) is isomorphic to (G 1 /H 1 ) (G 2 /H 2 ).

4 Proof of (3) (5) Proof of (3). If G = a, then every coset of H is of the form a k H = (ah) k for some integer k. Thus, G/H = ah. Proof of (4). Let φ : G 1 G 2 G 1 be defined by φ(g 1, g 2 ) = g 1 (the projection map). It is easy to see that φ is a homomorphism with Ker(φ) = {(e 1, g 2 ) : g 2 G 2 } = {e 1 } G 2 and Im(φ) = G 1. Then by Theorem 14.11, G/({e 1 } G 2 ) G 1. Proof of (5). Let φ : G 1 G 2 (G 1 /H 1 ) (G 2 /H 2 ) be defined by φ(g 1, g 2 ) = (g 1 H 1, g 2 H 2 ). Then apply Theorem

5 Examples Example 1. Let G = R be the additive group of real numbers, and H = nr = {nr : r R}. What is the group structure of G/H? Solution. The subgroup H is in fact the whole group G, since every real number x is equal to n(x/n). Thus, R/nR is the trivial group of one element. Example 2. Let G = Z 12 and H = 4 = {0, 4, 8}. What is the group structure of G/H? Solution. The group Z 12 is cyclic. Thus, G/H is cyclic of order (G : H) = 4, i.e., G/H Z 4. In fact, G/H = {H, 1 + H, 2 + H, 3 + H} = 1 + H.

6 Examples Example 3. Classify the factor group Z 4 Z 6 /( 2 3 ). Solution. By the fact G 1 G 2 /(H 1 H 2 ) (G 1 /H 1 ) (G 2 /H 2 ), we see that Z 4 Z 6 /( 2 3 ) (Z 4 / 2 ) (Z 6 / 3 ) Z 2 Z 2.

7 Examples Example 4. Classify the factor group Z 4 Z 6 / (2, 3). (Notice the difference between (2, 3) and 2 3.) Solution. The order of (2, 3) in Z 4 Z 6 is 2. Thus Z 4 Z 6 / (2, 3) is abelian of order 24/2 = 12. There are two abelian groups of order 12 (up to isomorphism), namely, Z 2 Z 2 Z 3 and Z 4 Z 3. To determine which one, we use the property that Z 4 Z 3 has an element of order 4, but Z 2 Z 2 Z 3 does not. Consider (1, 0) + (2, 3). The order of (1, 0) + (2, 3) is the smallest positive integer n such that n(1, 0) (2, 3) = {(0, 0), (2, 3)}. The smallest such positive integer is 4. Thus, the order of (1, 0) + (2, 3) is 4, and Z 4 Z 6 / (2, 3) is isomorphic to Z 4 Z 3.

8 General method 1 Approach. If G/H is a finite group, we may try to use our knowledge in finite groups to classify the structure of G/H. For example, 1. If G/H is equal to a prime, then G/H is isomorphic to Z p. 2. If G/H = 4, then G/H is isomorphic to either Z 4 or Z 2 Z 2. The former has an element of order 4, while the latter has not. 3. If G/H = 2p, where p is a prime, then G/H is isomorphic to Z 2p or D p. 4. If G/H = 8 and abelian, then G/H is isomorphic to Z 8, Z 4 Z 2, or Z 2 Z 2 Z 2. The first group has an element of order 8. The second has an element of order 4, but no element of order 8. The last has only elements of order 2.

9 General method 2 Approach. Construct a group homomorphism φ : G G such that the kernel is H. Find the image of φ, and then apply Theorem (the first isomorphism theorem). Example. Classify the factor group Z Z/ (1, 1). Solution. Let φ : Z Z Z be defined by φ(a, b) = a b. We check 1. φ is a homomorphism: We have φ((a 1, b 1 ) + (a 2, b 2 )) = φ(a 1 + a 2, b 1 + b 2 ) = (a 1 + a 2 ) (b 1 + b 2 ) = (a 1 b 1 ) + (a 2 b 2 ) = φ(a 1, b 1 ) + φ(a 2, b 2 ). 2. Ker(φ) = (1, 1) : Ker(φ) = {(a, b) : a b = 0} = {(a, a) : a Z} = (1, 1). 3. Im(φ) = Z: For all n Z, n = φ(n, 0) Im(φ). Then by Theorem 14.11, Z Z/ (1, 1) Z.

10 Example Example. Classify the group Z Z/ (2, 2). Solution. The set (2, 2) is characterized by the property that (a, b) (2, 2) a b = 0 and a 0 mod 2. Thus, we define φ : Z Z Z Z 2 by φ(a, b) = (a b, a mod 2). It is easy to check that φ is a group homomorphism with kernel (2, 2) and Im(φ) = Z Z 2. By Theorem 14.11, we conclude that Z Z/ (2, 2) Z Z 2.

11 In-class exercises Classify the following factor groups. 1. (Z Z)/ (1, 2). 2. (Z Z Z 4 )/ (3, 0, 0). 3. S 4 /{e, (1, 2)(3, 4), (1, 3)(2, 4), (1, 4)(2, 3)}.

12 Simple groups Definition A group G is simple if it is non-trivial and has no proper non-trivial normal subgroups, i.e., G has more than one elements and {e} and G are the only normal subgroups of G. Example Let p be a prime. Then Z p is a simple group. In fact, these are the only abelian simple groups. Example For n 5, the alternating group A n is a simple group. (Exercise 39.) (The simplicity of A n is the reason why in general the roots of a polynomial of degree n 5 can not be expressed as radicals.)

13 Remark Simple groups can be thought of as building blocks of groups. That is, to study the structure of G, we may first find a normal subgroup H such that G/H is simple. Then we study the properties of the smaller groups H and G/H to obtain those of G. However, note that G in general is not isomorphic to (G/H) H. For instance, G = S 3 and H = {(1, 2, 3)}.

14 When is G/H simple? Definition A maximally normal subgroup of G is a normal subgroup M not equal to G such that if N is a normal subgroup of G satisfying M < N, then N = M or N = G. Theorem (15.18) Let M be a normal subgroup of G. Then G/M is simple if and only if M is a maximally normal subgroup of G.

15 Correspondence of normal subgroups Theorem (15.16) Let φ : G G be a group homomorphism. If N is a normal subgroup of G, then φ[n] is a normal subgroup of φ[g]. Conversely, if N is a normal subgroup of φ[g], then φ 1 [N ] is a normal subgroup of G Proof. Exercises 35 and 36. Remark The theorem only says that φ[n] is a normal subgroup of φ[g]. It does not say anything about whether φ[n] is a normal subgroup of G. For example, let φ : Z 2 S 3 be defined by φ(0) = e and φ(1) = (1, 2). Clearly, Z 2 is a normal subgroup of itself, but φ[z 2 ] = {e, (1, 2)} is not a normal subgroup of S 3.

16 Proof of Theorem ( ) Assume that G/M is simple. Let N be a normal subgroup of G such that M < N < G. We need to show that either N = M or N = G. We claim that N/M is a normal subgroup of G/M, that is, (gm)(nm)(gm) 1 N/M for all g G and all n N. Now (gm)(nm)(gm) 1 = (gng 1 )M. Since N G, we have gng 1 N and thus (gng 1 )M N/M. Having proved that N/M G/M, by the assumption that G/M is simple, we conclude that N/M is either trivial or equal to G/M, that is, N = M or N = G.

17 Proof of Theorem ( ) Conversely, assume that M is a maximally normal subgroup. Define γ : G G/M by γ(g) = gm. Assume that G/M is not simple, say, N is a proper non-trivial normal subgroup of G/M. Then by Theorem 15.16, φ 1 [N ] is a normal subgroup of G. This subgroup φ 1 [N ] is a proper normal subgroup of G and not equal to M (since M is the kernel of γ). This contradicts to the assumption that M is maximal.

18 Center Definition Let G be a group. Then the subgroup Z (G) = {x G : xg = gx for all g G} is called the center of G. Proposition Let G be a group. Then Z (G) is a normal subgroups of G.

19 Commutator subgroups Definition Let G be a group. An element of the form aba 1 b 1 is a commutator of the group. The subgroup generated by all the commutators is called the commutator subgroup, and denoted by [G, G]. Remark The definition of commutator subgroups arises naturally when we try to determine when G/N is abelian. Suppose that N is a normal subgroup of G such that G/N is abelian. Then we have (an)(bn) = (bn)(an), or equivalently, aba 1 b 1 N. Thus, N must contain all the commutators.

20 Commutator subgroups Theorem (15.20) The commutator subgroup [G, G] is a normal subgroup. Moreover, suppose that N is a normal subgroup of G. Then G/N is abelian if and only if C < N. Proof. We first show that [G, G] G. Since [G, G] is generated by commutators. It suffices to prove that g(aba 1 b 1 )g 1 [G, G] for all a, b, g G. Now we have g(aba 1 b 1 )g 1 = gaba 1 g 1 b 1 (bgb 1 g 1 ) = [(ga)b(ga) 1 b 1 ](bgb 1 g 1 ). Thus, g(aba 1 b 1 )g 1 [G, G], and [G, G] G. The proof of the second statement has already been seen in the last remark.

21 Center and commutator subgroups Remark The center and the commutator subgroup of a group G are kind of opposite of each other. For instance, when G is abelian, we have Z (G) = G, but [G, G] = {e}. Example Let G = S 3. We know that G has only three normal subgroups {e}, (1, 2, 3), and S 3 itself. We now determine Z (S 3 ) and [S 3, S 3 ]. By a direct observation, we see that Z (S 3 ) = {e}. To determine [G, G], we notice that [G, G] = {e} if and only if G is abelian. Thus, [S 3, S 3 ] {e}. [S 3, S 3 ] cannot be equal to S 3 either because every commutator is clearly an even permutation, but S 3 also contains odd permutations. We conclude that [S 3, S 3 ] = (1, 2, 3).

22 Homework Do Problems 4, 8, 12, 13, 29, 35, 36, 37, 42 of Section 15.

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