# Exercises MAT2200 spring 2013 Ark 4 Homomorphisms and factor groups

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1 Exercises MAT2200 spring 2013 Ark 4 Homomorphisms and factor groups This Ark concerns the weeks No. (Mar ) and No. (Mar ). Plans until Eastern vacations: In the book the group theory included in the curriculum is divided in two parts. The first part ends with section 16 Group actions on sets, thesection17 Application of G-sets to counting isnotinthecurriculum, and the second part consists of the two sections 36 and 37 Sylow theorems and Applications of the Sylow theory. Ifinditbesttofinishallthegrouptheory,andthen start on rings and fields. This means that we after section 16 do the sections 36 and 37. I think that is about what we ll have time to do before the eastern vacation. The plans for coming two weeks are as follows: On Wednesday Mar : We llfinishsection 15 Factor-Group computations and Simple Groups and do Section 16 GroupActionsonsets. On Friday Mar Exercises + buffer On Wednesday Mar We start on Section 36 Sylow Theorems On Friday Mar Exercises + buffer The exercises on this sheet cover Section 13, Section 14 and Section 15 in the book. They are ment for the groups on Friday Mar and Mar and Thursday Mar and with the following distribution: Friday, Mar 8: No.: 5, 6, 7, 11, 12, 14, 16 Friday, Mar 15: No.: 17, 19, 20, 23, 28, 29, 32, 33 Thursday, Mar 7: No.: 1, 2, 3, 4, 8, 9, 10, 13, 15 Thursday, Mar 14: No.: 18, 21, 22, 24, 25, 26, 27, 30, 31 Key words: Homomorphisms,normalsubgroupsandconjugation,factorgroups,the center and the commutator subgroup. Homomorphisms Problem 1. ( Section 13, No.: 3 and 4 on page 133 in the book). Which of the following two maps are group homomorphism? : R! R, (x) = x : R! R, (x) =2 x. In case they are, describe the kernel and the image. Problem 2. (Basically Section 13, No.: 19 and 21 on page 134 in the book). Let the following two homomorphisms and both from Z to S 8 be given by: (1) = (1, 4, 2, 6)(2, 5, 7) (1) = (2, 5)(1, 4, 6, 7) Determine Ker and Ker. Compute (20) and (14).

2 Problem 3. ( Section 13, No.: 24 on page 134 in the book). Let : Z Z! S 10 be given by (1, 0) = (3, 5)(2, 4) and (0, 1) = (1, 7)(6, 10, 8, 9). Determinethekernel Ker and compute (3, 10). Problem 4. Let G, H and K be three groups and let two homomorphisms. a) Show that the composition is a homomorphism. b) Show that is surjective if both and are. c) Show that is injective if both and are. : G! H and : H! K be d) What can you say about the the composition if one of and is surjective and the other one injective? Give examples. e) Show that if both and are isomorphisms, then is an isomorphism. Problem 5. Let G be a group and let Aut(G) be the set of isomorphisms : G! G. The elements of Aut(G) are called automorphisms of G. a) Show that Aut(G) with composition as binary operation is a group. Hint: Aut(G) is contained in Sym(G). Showthatitisasubgroup. b) Show that a homomorphism : Z n! Z n is uniquely determined by (1). Show that is an automorphism if and only if (1) is relatively prime to n. Hint: If k is relatively prime to n, onemayfindak 0 with kk 0 1 mod n. c) Show that there are isomorphisms Aut(Z 2 ) '{e} Aut(Z 4 ) ' Z 2 Aut(Z 3 ) ' Z 2. Problem 6. Let G be a group and let g 2 G be an element. a) Show that c g (x) =gxg 1 defines a group homomorphism c g : G! G. b) Show that c gh = c g c h and that c g 1 is an inverse for c g.hencec: G! Aut(G) defined by g 7! c g is a group homomorphism. c) Show that Ker c equals the center Z(G) of G. RecallthatZ(G) ={ x 2 G xy = yx for all y 2 G }. Problem 7. Let : G! H be a homomorphism. Show that if x 2 G is of finite order, then (x) is of finite order and ord( (x)) divides ord(x). Showbygivingexamplesthat if ord(x) is infinite, then (x) can be of any order. Problem 8. What are the homomorphisms from Z 77 to Z 78? And what are the ones from Z 78 to Z 77? Problem 9. Show that for any integer n 2, thereareneithernontrivialhomomorphisms from Z n to Z n+1 nor from Z n to Z n 1. 2

3 Problem 10. Show that there is just one nontrivial homomorphism from Z 2 Z 3 to Z 2,andonefromZ 2 to Z 2 Z 3. Problem 11. Let p be a prime and r an integer. Let C be a cyclic group of order p r generated by a. a) Show that D = h a pr 1 i is a cyclic subgroup of C of order p. Hint: You may of course use theorems from the book. b) Show that the map sending x to x pr 1 defines a surjective homomorphism C! D. What is the kernel? c) Formulate and prove the additive version of this problem. Hint: Try the map a 7! p r 1 a. Problem 12. Show that if A is an abelian group and p is a prime number, then there is a surjective homomorphisms A! Z p if and only if p divides the order A of A. Show by examples that the hypothesis that p be prime can not be suppressed. Problem 13. Define a map Z 10! Z 22 by sending [x] 10 to [11x] 22 where [x] n is a temporary notation for the residue class of the integer x modulo the integer n. Show that this indeed is well defined and that it is a homomorphism. What is the order of the kernel and the image? For which other integers a will [x] 10 7! [ax] 22 be well defined and a homomorphism? Problem 14. Show that the map z 7! z 39 of complex numbers defines a homomorphism from µ 6 to µ 26.Findthekernelandtheimage.Showthat is the only nontrivial homomorphism from µ 6 to µ 26. Problem 15. Let (n, m) =2 n 5 m for (n, m) 2 Z Z.Showthat : Z Z! R.Is injective? Normal subgroups and conjugation is a homomorphism Problem 16. Show that the subgroup h r i of D 8 is normal where r denotes the rotation by 90. Ifs is one of the reflections, is the subgroup h s i normal? Problem 17. Let X A 4 be the set of the elements of order two. We have earlier shown that X has three elements, namely the elements (1, 2)(3, 4), (1, 3)(2, 4) and (1, 4)(2, 3). a) Show that X together with the unity forms a subgroup H of S 4 isomorphic to Z 2 Z 2. Hint: It suffices to see that the product of two of the elements of X equals the third. Why? b) Let 2 S 4.Showthatc ( )= 1 2 X whenever 2 X. c) Show that H is a normal subgroup of S 4. d) Show that 7! c defines a homomorphism c: S 4! Sym(X) ' S 3 e) Show that c is surjective and that the kernel equals H. 3

4 Problem 18. a) Show that the center Z(G) of a group G is a normal subgroup. b) Let 2 Aut(G) be any automorphism of the group G. Showthat (Z(G)) = Z(G). c) Give an example of two groups G and H and a group homomorphism : G! H such that (Z(G)) is not contained in Z(G). Problem 19. Let G be a group and N G anormalsubgroup. a) Show c g (x) =gxg 1 defines an automorphism of N and that g 7! c g defines a homomorphism c: G! Aut(N). (Cfr. problem 6.) b) Show that if N is of order 2, thenn is cental, i.e., every element in N commutes with every element of G. Hint: Problem 5 c) mightbeuseful. c) Show that if G is of odd order and N is of order 3, thenn is central. Problem 20. Let n and k be a natural numbers with 2 apple k apple n. a) Assume that 2 S n is a permutation such that (i) =a i for i 2{1,...,k}. Show that the following equality holds true: (1, 2,...,k) 1 =(a 1,a 2,...,a k ). Hint: Feed the left side with a i and check that it returns a i+1. b) If two subgroups both are generated by a k-cycle, show that they are conjugate. c) Is the subgroup of S n generated by the cycle (1, 2,...,k) normal? Problem 21. If H G and K G are normal subgroups, show that HK is a normal subgroup. Problem 22. Show that if H G and K G are two normal subgroups such that H \ K = {e}, thenelementsfromh and K commute. Hint: If x 2 H and y 2 K, take a look at the commutator c = xyx 1 y 1. Problem 23. ( Section 14, No.: 35 on page 143 in the book). Show that if H and N are subgroups of G and N is normal in G, thenn \ H is normal in H. Showbygiving an example, that N \ H is not necessarily normal in G. Factor groups Problem 24. ( Section 14, No.: 1, 5 and 6 on page 142 in the book). Find the order of the following factor groups Z 6 /3Z 6 Z 2 Z 4 /h (1, 1) i Z 12 Z 18 /h (4, 3) i Problem 25. ( Section 14, No.: 9, 10, 11 and 14 on page 142 in the book). Find the order of the indicated elements: 5 in Z 12 /4Z in Z 60 /12Z 60 (2, 1) in /Z 3 Z 6 /h (1, 1) i (3, 3) in Z 4 Z 8 /h (1, 2) i. 4

5 Problem 26. ( Section 15, No.: 3 and 5 on page 151 in the book). Describe the following abelian groups as products of indecomposable abelian groups: Z 2 Z 4 /h (1, 2) i Z 4 Z 4 Z 8 /h (1, 2, 4) i. Problem 27. Let : G! K be a group homomorphisms with kernel H. Recallthe equivalence relation defined by is given as x y, (x) = (y). Show that the equivalence classes of this relation is precisely the cosets of H. Problem 28. Show that if A and B are two subgroups of a group G, thenthesubset AB = { ab a 2 A and b 2 B } is a subgroup if and only if AB = BA. Problem 29. By numbering the corners of a square from 1 to 4 we may consider the dihedral group D 8 as a subgroup of the symmetric group S 4,thatisD 8 S 4. a) Show that D 8 is not a normal subgroup. Hint: Renumber the corners of the square. b) How many conjugates does D 8 have in S 4? c) Determine the intersection D 8 \ A 4. The center and the commutator subgroup Problem 30. ( Section 15, No.: 13 on page 152 in the book). Find both the center and the commutator subgroup of the dihedral group D 8. (The group D 4 in the book is what we call D 8,thesymmetrygroupofthesquare). Problem 31. Assume that G and H are two groups. Show that Z(G H) =Z(G) Z(H). DeterminethecenterofZ 5 D 8.Isthisacyclicgroup? Problem 32. (Basically Section 15, No.: 37 on page 153 in the book). Let G be a group with center Z = Z(G). AssumethatG/Z is a cyclic group, and let a 2 G be an element such that the coset az is a generator for G/Z. a) Show that any element x in G can be written as x = a i z where z 2 Z and i is some integer. b) Show that G is abelian. Problem 33. ( Section 15, No.: 38 on page 153 in the book). Let p and q be two different primes and assume that G is a non-abelian group of order pq. Showthatthe center of G is trivial. Hint: Exercise 32 might help. Versjon: Monday, February 18, :26:33 PM 5

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