# Algebra Exercises in group theory

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1 Algebra Exercises in group theory February 2010 Exercise 1*: Discuss the Exercises in the sections in Chapter I of the notes. Exercise 2: Show that an infinite group G has to contain a non-trivial subgroup, i.e. a subgroup G, {e}. Exercise 3: Suppose that a 2 b 2 = (ab) 2 for all a, b in the group G. Show that G is abelian. Exercise 4*: Show that (1, 2, 3, 4), (1, 2) = S 4. Exercise 5: Let a, b and c be elements in a group G. (1) Show that a and a 1 have the same order. (2) Show that ab and ba have the same order. (3) Show that abc and bca have the same order. (Try to generalize this to a general result.) (4) Find three elements a, b, c in the symmetric group S 3, such that abc and bac have different orders. Exercise 6*: (1) Let a and b be commuting elements in G of finite orders m and n, respectively. Show that (ab) mn = e. (2) Suppose in addition that n and m are relatively prime. Prove that if a power a i of a equals a power b j of b, then a i = b j = e. Use this to prove that the order of ab is nm. Exercise 7: Show that the elements of finite order in an abelian group G form a subgroup of G. Exercise 8: Consider the following matrices as elements in the group GL(2, R): 1

2 [ 0 1 a = 1 1 ] b = [ ] Show that a has order 3 and that b has order 4. Show that the order of ab is infinite. Exercise 9: Give examples of the following: (1) An infinite group G with a subgroup H G and G : H finite. (2) An infinite group G with a subgroup H {e} and H finite. Exercise 10*: Suppose that the elements a, b in the group G satisfy the relation aba 1 = b 2, where b e. (1) Show that a 5 ba 5 = b 32. (2) Assume that a = 5. Compute b. Exercise 11: Define an equivalence relation on the elements of the group G by a c b a = b. Here a is the cyclic subgroup of G generated by a. Show that the c - equivalence class of an element a is always finite. Exercise 12*: Let G be a finite group. (1) Show that if G is even then the number of elements of order 2 in G is odd. (Consider the mapping x x 1. Which elements are the fixed points of this map?) (2) Show that the number of elements of order 3 in G is even (possibly 0). (3) Show that the number of elements of order 4 in G is even (possibly 0). (4) What can be said about the number of elements of order p in G, when p is a prime? Exercise 13: Suppose that G = {a 1, a 2,..., a n } is a finite abelian group of order n. Let c = a 1 a 2...a n be the product of all the elements in G. Show that c 2 = 1. 2

3 Exercise 14*: Consider the group (Q, +) and its subgroup (Z, +). Let p, q be two different prime numbers. Show that the cosets 1 p + Z and 1 q + Z in Q are different. Show that the index Q : Z is infinite. Exercise 15: Let S be a subgroup of the group G. Suppose that a, b G satisfies Sa = bs. Thus the left coset of S containing a equals the right coset of S containing b. Show that Sa = as = bs = Sb. Exercise 16: It is wellknown that the subgroups of the group (Z, +) are exactly those on the form (mz, +) for some m Z, m 0. (Discuss this, if necessary.) Let n 1, n 2 Z \ {0} have greatest common divisor n. Show that the smallest subgroup of of Z which contains n 1 and n 2 is nz. Exercise 17: Let H, K be finite subgroups of the group G. Let m be the least common multiple of H and K. Show that m HK. Exercise 18: Suppose that H G and that G : H = p, where p is a prime number. Let K be a subgroup of G. Show that either K H or G = HK, K : K H = p. Exercise 19: Find all subgroups of the dihedral group D 4 and draw a diagram illustrating their mutual positions. Find all normal subgroups of D 4. Exercise 20*: Discuss the Exercises in the sections in Chapter I of the notes. Exercise 21*: Suppose that N 1 and N 2 are normal subgroups in the finite group G. Are the following claims true or false? (Please give either a proof or a counterexample.) (i) If N 1 N 2, then G/N 1 G/N 2. (ii) If G/N 1 G/N 2, then N 1 N 2. Exercise 22*: Suppose that S G, that G is finite and that κ : G G = G/S is the canonical epimorphism. Suppose that g G has an order, which is relatively prime to S. Show that g = κ(g). Exercise 23*: Discuss the Exercises in the sections in Chapter I of the notes. 3

4 Exercise 24: Let G be a group. Show that the following subset T of Aut(G) is a normal subgroup in Aut(G) T = {α Aut(G) α(u) = U for all subgroups U in G} Exercise 25: Show that for any (finite) group G we have: G 2 Aut(G) = {1 G }. Exercise 26*: Show that the only finite group with 2 conjugacy classes is Z 2. Exercise 27: Let G be a group. Suppose that G : Z(G) = n is finite. Show that any conjugacy class of G contains at most n elements. Exercise 28: Show that the matrices 1 a b { 0 1 c a, b, c Z/3Z} with usual matrix multiplication form a group of order 27, where every element e has order 3. Use this to find two non-isomorphic finite groups for which for every t the number of elements of order t coincide for the two groups. Exercise 29: Compute the center of the group of order 27 from the previous exercise. Exercise 30*: Let G be a finite p-group (p a prime). Suppose N G, N {e}. (1) Show that N Z(P ) {e}. Hint: It is possible to modify the proof of Theorem in the notes. Note that N is a union of conjugacy classes of G. 4

5 Exercise 31*: Let G be a finite p-group (p a prime) and N a subgroup of order p. Show that N G if and only if N Z(P ). Exercise 32: Let G be a p-group. Suppose that H G is a subgroup. Show that H N G (H), ie. N G (H) contains H properly. Exercise 33: Let G 4 be the abelian group consisting of all infinite sequences (a 1, a 2, ) where each a i is some element in a cyclic group of order 4. (1) Define an isomorphism φ : G 4 G 4 G 4. (2) Investigate whether the following subsets of G 4 are subgroups: M 1 = {(a 1, a 2, ) G 4 There exists k N such that a i = e for all i k.} M 2 = {(a 1, a 2, ) G 4 Only finitely many a is equal e} {(e, e, )}. Exercise 34: Let G 4 be the same group as in the previous exercise. Consider the (outer) direct product: H 4 = C G 4 where C is a cyclic group of order 2. Show that G 4 is isomorphic to a proper subgroup of H 4 and that H 4 is isomorphic to a proper subgroup of G 4. Show that G 4 and H 4 are not isomorphic. (Thus there is no Bernstein s equivalence theorem for groups.) Exercise 35: Describe all the 2-Sylow subgroups and all the 3-Sylow subgroups of D 6 and of S 3 S 3. Exercise 36*: Show that m p (D n ) = 1 for all n 2 and all odd primes p. Exercise 37*: Which of the following groups are isomorphic? Z 24, Z 4 Z 6, S 4, A 4 Z 2, Z 8 Z 3, D 12, D 6 Z 2. Exercise 38*: Show that a group of order 45 is abelian. Determine the number of isomorphism classes of groups of order 45. Exercise 39*: Show that if G = 80 = then either m 5 (G) = 1 or m 2 (G) = 1. Is G solvable? Give examples of three non-isomorphic nonabelian groups of order 80, each with a normal 5-Sylow subgroup. 5

6 Exercise 40: Show that a group of order 200 has a normal 5-Sylow subgroup and that it is solvable. Exercise 41*: Let G have order 231 = Show that m 7 (G) = m 11 (G) = 1. Show that G has a cyclic subgroup of order 33. Show that if P Syl 11 (G), then P Z(G). Exercise 42: Lad n = pa, where p is a prime and 1 a < p. Compute the order of a p-sylow group of the symmetric group S pa. Show that such a group is abelian. Exercise 43: Consider the permutations α = (1, 2, 3) β = (1, 4, 7)(2, 5, 8)(3, 6, 9) S 9. Show that α and β generate a subgroup of order 81 = 3 4 in S 9. Show that this is a non-abelian 3-Sylow subgroup in S 9. Exercise 44: (The Frattini argument). Suppose that H G and that P Syl p (H). Show that G = HN G (P ). (This result will be presented by the instructor. It is sometimes very useful, for instance in the study of finite solvable groups.) Exercise 45: Show that any group of order 132 = contains subgroups of order 33 = 3 11, 44 = and 12 = (This is an example of P. Hall s generalization of Sylow s theorems for finite solvable groups. Ask your instructor!) Hint: The final Example in the notes discusses groups of order 132. The previous exercise may also be used to show the existence of a subgroup of order 12. Exercise 46*: Discuss the Exercises in the sections in the notes. Exercise 47*: Let G be a subgroup of S n. Suppose that we can find elements τ 2,, τ n G, such that τ i (1) = i for 2 i n. Show that G is transitive. Exercise 48*: Show, eg. using the previous exercise, that for n 3 is A n a transitive subgroup of S n. 6

7 Exercise 49*: For m N let s(m) = min{n N m divides n!}. Thus for example if m = 700 = then s(m) = 10. Show that a simple group of order m cannot have a proper subgroup of index < s(m). Hint: Use Theorem in the notes. Exercise 50*: Let p be the largest prime number dividing the order of the simple group G. Show, e.g. using the previous exercise, that G cannot have a proper subgroup of index < p. Exercise 51*: Let G have order 2552 = Use Burnside s normal complement theorem to show for p {11, 29} we have that either m p (G) = 1 or G has a normal p-complement. 7

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