# Homework Problems, Math 200, Fall 2011 (Robert Boltje)

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1 Homework Problems, Math 200, Fall 2011 (Robert Boltje) Due Friday, September 30: ( ) 0 a 1. Let S be the set of all matrices with entries a, b Z. Show 0 b that S is a semigroup under matrix multiplication and show that S has a right identity but no left identity. Determine all right identities. Give an example of a semigroup which has a left identity but no right identity. 2. Let G be a semigroup which has a left identity element e such that every element of G has a left inverse with respect to e, i.e., for every x G there exists an element y G with yx = e. Show that G is a group. 3. Let G, H, and K be groups and let f : G H and g : H K be homomorphisms. (a) Show that g f is a homomorphism. (b) Show that f(1 G ) = 1 H and that f(x 1 ) = f(x) 1 for all x G. (c) Show that if f is an isomorphism then the inverse map f 1 : H G is again a homomorphism (and then of course also an isomorphism). (d) Show that the following are equivalent: (i) f is an isomorphism. (ii) There exists a function f : H G such that f f = id G and f f = id H. (iii) There exists a homomorphism f : H G such that f f = id G and f f = id H. 4. Let (G, ) be a group and let H be a subset such that H together with the map : H H G, (x, y) x y, is again a group. Show that H is a subgroup of G. (Hint: A priory you need to think of the identity element of (H, ) as begin possibly different from the identity element of (G, ). Also, a priori, the inverse of an element x H with respect to 1

2 the group (G, ) might be different from the inverse of x with respect to the group (H, ). Show first that identity elements and inverses coincide.) 5. Let H be a subset of a group G. Show that the following are equivalent: (i) H G. (ii) H and for all x, y H one has xy 1 H. Due Monday, October 10: 6. Let G be a group and let H and K be subgroups of G. (a) Show that HK is a subgroup of G if and only if HK = KH. (b) Now assume that H and K are finite subgroups. Show that HK = H K H K. (Hint: Consider the function f : H K HK, (h, k) hk, show that it is surjective and that every element of HK has precisely H K preimages under f.) 7. Let G and H be groups and let f : G H be a group homomorphism. (a) Let U G. Show that f(u) H. (b) Let V H. Show that the preimage f 1 (V ) := {g G f(g) V } of V is a subgroup of G. (c) Show that f is injective if and only if ker(f) = {1 G }. Here, ker(f) := f 1 (1 H ) := {g G f(g) = 1 H }; this subgroup of G is called the kernel of f. 8. Let n N. For pairwise distinct elements a 1,..., a k in {1,..., n} we denote by (a 1, a 2,..., a k ) the permutation σ Sym(n) given by σ(a 1 ) = a 2, σ(a 2 ) = a 3,..., σ(a k 1 ) = a k, σ(a k ) = a 1, and σ(a) = a for all other a {1,..., n}. Such an element is called a k-cycle. A 2-cycle is also called a transposition. (a) Show that every element in Sym(n) is a product of disjoint cycles. (b) Show that every cycle is a product of transpositions. (c) Show that every transposition is a product of an odd number of simple transpositions, i.e., transpositions of the form (i, i + 1), i = 1,..., n 1. 2

3 (d) Let σ Sym(n). A pair (i, j) of natural numbers i, j with 1 i < j n is called an inversion for σ if σ(j) < σ(i). We denote by l(σ) the number of inversions of σ. Show that for a transposition τ = (a, b) with 1 a < b n one has l(τ) = 2(b a) 1. (e) Show that for every i = 1,..., n 1 one has { 1 if σ 1 (i) < σ 1 (i + 1), l((i, i + 1)σ) l(σ) = 1 if σ 1 (i) > σ 1 (i + 1). (f) Show that if σ Sym(n) can be written as a product of r transpositions then r l(σ) mod 2. Conclude that if σ can also be written as a product of s transpositions then r s mod 2. (g) Show that the map Sym(n) {±1}, σ ( 1) l(σ), is a group homomorphism which coincides with the homomorphism sgn from class and that sgn(τ) = 1 for every transposition τ. 9. With the cycle notation from Problem 3, show that for all σ Sym(n). σ(a 1,..., a k )σ 1 = (σ(a 1 ), σ(a 2 ),..., σ(a k )), 10. Let G and A be groups and assume that A is abelian. Show that the set Hom(G, A) of group homomorphisms from G to A is again an abelian group under the multiplication defined by (f 1 f 2 )(g) := f 1 (g)f 2 (g) for f 1, f 2 Hom(G, A) and g G. 11. Consider the elements σ := (1, 2, 3) and τ := (1, 2) of Sym(3). (a) Show that σ 3 = 1, τ 2 = 1 and τσ = σ 2 τ. (b) Show that {σ, τ} is a generating set of Sym(3). (c) Show that every element of Sym(3) can be written in the form σ i τ j with i {0, 1, 2} and j {0, 1}. (d) Compute all subgroups of Sym(3) and their normalizers and centralizers. (e) Compute the commutator subgroup of Sym(3). 12. Consider the elements σ := (1, 2, 3, 4) and τ := (1, 4)(2, 3) of Sym(4). 3

4 (a) Show that σ 4 = 1, τ 2 = 1, and τσ = σ 3 τ. (b) Determine the subgroup σ, τ of Sym(4). It is called the dihedral group of order 8 and is denoted by D 8. (c) Determine Z(D 8 ). (d) Determine the derived subgroup D 8 of D 8. Due Friday, October 14: 13. (a) Let G be a group, let N be a normal subgroup of G, and let ν : G G/N, g gn, denote the natural epimorphism. Show that, for every group H, the function Hom(G/N, H) {f Hom(G, H) N ker(f)}, α α ν, is bijective. (b) Let G be a group and let A be an abelian group. Let ν : G G ab := G/G denote the canonical epimorphism. Show that, with the group structure from Homework Problem 10 on the homomorphism sets, the map is a group isomorphism. Hom(G ab, A) Hom(G, A), α α ν, 14. Let G and H be groups and let f : G H be an isomorphism (a) Show that G is abelian if and only if H is abelian. (b) Let X be a subset of G and set Y := f(x) H. Show that f( X ) = Y, f(n G (X)) = N H (Y ), f(c G (X)) = C H (Y ). (c) Show that G is cyclic if and only if H is cyclic. (d) Show that f(z(g)) = Z(H). (e) Show that f(g ) = H. (f) Let N be a normal subgroup of G and set M := f(n). Show that M is normal in H and that G/N = H/M. 15. (a) Let f : G H be a group homomorphism and let U G and V H be subgroups. Show that f 1 (f(u)) = U ker(f) and f(f 1 (V )) = V im(f). 4

5 (b) Let G be a group, let N G and let ν : G G/N denote the canonical epimorphism. Show that for every subgroup U of G one has ν(u) = UN/N. 16. Let G be a group. (a) Show that for every subset X G one has C G (X) N G (X). (b) Assume that M and N are normal subgroups of G. Show that then also MN and M N are normal subgroups of G. 17. A subgroup H of a group G is called characteristic in G if f(h) = H for every automorphism f Aut(G). In this case we write H G. Show char that: (a) H G H G. char (b) M char N G M G. char char N G M G. (c) M char (d) M N G M G. (Give a counterexample.) 18. (a) Let p be a prime, let C p = x be a cyclic group of order p and set G := C p C p. Show that G has exactly p + 1 subgroups of order p. (b) A group of 25 mathematicians meets for a 6 day conference. Between the morning and afternoon lectures they have their lunch in a room with 5 round tables and 5 chairs around each table. The organizer would like to assign every day new places at the tables in such a way that each participant has eaten with any other one at least once at the same table. Is this possible? (Hint: Use (a) and use convenient equivalence relations on G.) Due Friday, October 21: 19. (Dedekind s Identity) Let U, V, W be subgroups of a group G with U W. Show that UV W = U(V W ) and W V U = (W V )U. 20. (Butterly Lemma or Zassenhaus Lemma or 3 rd Isomorphism Theorem) Let U and V be subgroups of a group G and let U 0 U and V 0 V. Show that U 0 (U V 0 ) U 0 (U V ), (U 0 V )V 0 (U V )V 0, (U 0 V )(U V 0 ) U V 5

6 and U 0 (U V )/U 0 (U V 0 ) = (U V )/(U 0 V )(U V 0 ) = (U V )V 0 /(U 0 V )V Let G be a group and i N 0. (a) Show that G (i) G. char (b) Show that H (i) G (i) for all H G. (c) Show that (G/N) (i) = G (i) N/N for all N G. 22. (a) Show that Alt(4) is the derived subgroup of Sym(4). (b) Find all composition series of Sym(4). (c) Determine the higher derived subgroups of Sym(4). 23. Show that a group is solvable if and only if it has a normal series with abelian factors. 24. Let G be a cyclic group of order n and let m N be a divisor of n. Show that G has exactly one subgroup of order m. 25. Let G be a group. (a) Show that every transitive G-set is isomorphic to the G-set G/H for some H G. (b) Let H, K G. Show that there are no G-equivariant maps between G/H and G/K unless there exists g G with H gkg 1. Moreover, show that G/H and G/K are isomorphic G-sets if and only if H and K are conjugate subgroups. Due Friday, October 28: 26. Let p be a prime and let G be a p-group. (a) Let N be a non-trivial normal subgroup of G. Show that N Z(G) > {1}. (b) Let H be a group such that H/Z(H) is cyclic. Show that H is abelian. (c) Show that if G has order p 2 then G is abelian. 27. (Frattini Argument) Let G be a finite group, p a prime, H G and P Syl p (H). Show that G = HN G (P ). (Hint: Let g G and consider P and gp g 1. Show that both are Sylow p-subgroups of H.) 6

7 28. (a) Show that every group of order 1000 is solvable. (b) Show that every group G of order p 2 q 2 is solvable. (Hint: You may assume that p > q. Consider the possibilities for n G (p 2 ). The difficult case is when n G (p 2 ) = q 2. In this case, let P Q be Sylow p-subgroups of G. Show that R := P Q has order p, and that R is normal in G. For the latter show that P and Q are contained in N G (R) and show that this forces N G (R) = G.) 29. Show that every group of order smaller than 60 is solvable. 30. Let Q 8 be the subgroup of GL 2 (C) generated by the matrices ( ) ( ) i a := and b :=. 0 i 1 0 (a) Show that a 4 = 1, b 2 = a 2, bab 1 = a 3. (b) Show that Q 8 has order 8 and Q 8 = {a i b j 0 i 3, 0 j 1}. The group Q 8 is called the quaternion group of order 8. (c) Determine all subgroups of Q 8 and their normalizers. Determine the derived subgroup and the center of Q 8. (d) Assume that Q 8 acts faithfully on a set X. Show that X contains an orbit of length 8. (Hint: Show that the kernel of the action is equal to x X G x and use Part (c).) (e) What is the smallest n N such that Q 8 is isomorphic to a subgroup of Sym(n)? (Hint: If Q 8 is isomorphic to a subgroup of Sym(n) then one obtains a faithful action of Q 8 on {1, 2,..., n}. Now use Part (d).) 31. Let G be a finite group, let p be a prime, let P be a Sylow p-subgroup of G and set U := N G (P ). Show that N G (U) = U. Due Monday, November 7 (any 6 of the following 8 problems will give 100% score; if you do more you get extra points): 32. (a) Show that every non-abelian group of order 6 is isomorphic to Sym(3). (b) Show that every abelian group of order 6 is isomorphic to Z/6Z. (c) Show that every group of order 4 is isomorphic to Z/4Z or to the Klein 4-group V 4. 7

8 33. Let G be a finite group and let π be a set of primes. An element x of G is called a π-element if its order involves only primes from π. It is called a π -element if its order involves only primes outside π. (a) Let g G. Assume that we can write g = xy with a π-element x G and a π -element y G satisfying xy = yx. Show that x and y are powers of g. (b) Show that for given g G there exist unique elements x, y G satisfying: x is a π-element, y is a π -element, g = xy and xy = yx. (The element x is called the π-part of g and the element y is called the π -part of g. Notation: x = g π, y = g π.) 34. (a) Let G be a cyclic group of order p and assume that p is a prime. Show that Aut(G) has order p 1. (b) Let G be a group of order pq with primes q < p such that q p 1. Show that G is cyclic. 35. Let G be a group and let H and K be subgroups of G. Show that the following are equivalent: (i) H K = 1, HK = G and for every h H and k K one has hk = kh. (ii) H G, K G, H K = 1 and HK = G. (iii) For every h H and k K one has hk = kh and for every element g G there exist unique elements h H and k K such that g = hk. (iv) The function H K G, (h, k) hk, from the cartesian product group H K to G is a group isomorphism. 36. Let G and H be finite groups. (a) Show that if U G and V H then U V G H. (b) Assume that G and H have coprime order. Show that every subgroup of G H is of the form U V for some subgroups U G and V H. (c) Find a counterexample to (b) when one drops the hypothesis that G and H are coprime. 8

9 37. In the tableau one is allowed to shift any square bordering the empty square to the empty square. Find out if it is possible to obtain the constellation from the first one Let G := Alt(5). (a) Show that the conjugacy classes of G have length 1, 15, 20, 12 and 12. Derive from this that G is a simple group. (Hint: Show that all elements with the same cycle type (except for 5-cycles) are also conjugate in G. For 5-cycles, compute the centralizer in Sym(5) and in G. Derive from there that the 5-cycles break up into 2 conjugacy classes of the same size. Then note tat every normal subgroup of G is a union of conjugacy classes.) (b) Find all subgroups of Alt(5), up to conjugation, and find their normalizers. (Hint: There are no subgroups of order 30, 20, or 15. This needs to be proven. Make use of subgroups you already know: Alt(4), Sylow subgroups for different primes, their normalizers.) 39. (a) Let H and N be groups and assume that H act on N by group automorphisms. Show that the multiplication (a, h) (b, k) := (a hb, h k) for a, b N and h, k H defines a group structure on the set N H. This group is denoted by N ρ H if ρ: H Aut(N) denotes the group homomorphism corresponding to the action of H on N. (b) Let H 1, H 2, N 1, N 2 be groups and assume that H i acts on N i via the homomorphism ρ i : H i Aut(N i ) for i = 1, 2. Assume further that there are group isomorphisms α: N 1 N 2 and κ: H 1 H 2 such that κ(h) α(a) = α( h a) for all a N 1 and h H 1. Show that N 1 ρ1 H 1 = N2 ρ2 H 2. 9

10 Due Monday, November 14: 40. Show that a, b a 4 = 1, a 2 = b 2, bab 1 = a 3 is a presentation of the quaternion group Q 8 of order (a) Let n N. Show that x x n is a cyclic group of order n. (b) Let X = {x} be a set with one element. Show that F (X) = Z. (c) Show that x, y xyx 1 y 1 = Z Z. 42: (a) Show that the set of inner automorphisms of G, is a normal subgroup of Aut(G). Inn(G) := {c g g G}, (b) Compute Out(D 8 ) := Aut(D 8 )/Inn(D 8 ), the outer automorphism group of D 8. (Hint: Use a presentation of D 8 to show that Out(D 8 ) has order 2.) (c) Compute Out(Q 8 ). (Hint: Show that Aut(Q 8 ) acts on the set X consisting of the 3 subgroups of Q 8 of order 4. Use this to construct an isomorphism Out(Q 8 ) Sym(3).) 43. (a) Let R be a finite integral domain. Show that R is a field. (b) Let R be a division ring. Show that Z(R) is a field. 44. Determine Z(H). 45. (a) Determine the unit group of the ring Z[i] := {a + bi a, b Z}, the ring of Gaussian integers. (b) Show that the ring Z[ 2] := {a + b 2 a, b Z} has infinitely many units and find all units of finite order. Due Monday, November 21: 10

11 46. Let G =< a > be a multiplicatively written cyclic group of order n. (a) Show that, for every k N, the order of the element a k is equal to n/ gcd(k, n). (b) Show that G has ϕ(n) generators. (c) Show that d n ϕ(d) = n. (Hint: Use Problem 24 and count elements of order d in G.) (d) Let H be a finite group of order n and assume that H has at most one subgroup of order d for every divisor d of n. Show that H is cyclic. (Hint: Count elements of H of given order and use (c).) 47. Let f : R S be a ring homomorphism. Show the following statements. (a) If T is a subring of R then f(t ) is a subring of S. (b) If U is a subring of S then f 1 (U) is a subring of R. (c) If J is a left (resp. right, resp. two-sided) ideal of S then f 1 (J) is a left (resp. right, resp. two-sided) ideal of R. (d) If f is surjective and I left (resp. right, resp. two-sided) ideal of R then f(i) is a left (resp. right, resp. two-sided) ideal of S. 48. Let R be a ring. An element r R is called nilpotent if there exists n N such that r n = 0. (a) Show that if r R is nilpotent then 1 r is a unit of R. (b) Show that if R is commutative then the nilpotent elements of R form an ideal N of R. (c) Show that if R is commutative and N is the ideal of nilpotent elements then 0 is the only nilpotent element of R/N. 49. (a) Let F be a field. Show that the characteristic of F is either a prime number or 0. (b) Let p be a prime and let R be a ring with p elements. Show that R = Z/pZ as rings. 50. Let R be an integral domain and a, b R. We write a b (a divides b) if there exists c R such that ac = b. Moreover, we call a and b associated (a b), if there exists a unit u U(R) such that b = au. Show the following statements: (a) a b br ar. (b) ar = br a b. (c) ar = R a U(R). 11

12 (d) is an equivalence relation. (e) R is the equivalence class of the element Let D be a division ring and let n N. Show that the matrix ring Mat n (D) is a quasi-simple ring. Due Monday, November 28 (any 6 of the following 7 problems will give 100% score; if you do more you get extra points): 52. Let F be a field and let f(x) F [X] be a polynomial in the indeterminate X with coefficients in F. (a) Let g(x) F [X] be a non-zero polynomial. Show that there exist polynomials q(x), r(x) F [X] such that f(x) = q(x)g(x) + r(x) and deg(r(x)) < deg(g(x)). (Hint: Assume that n := deg(f(x)) deg(g(x)) =: m. Now use induction on n m. First write f(x) = ax n m g(x) + r 1 (X) with an appropriate a F and r 1 (X) F (X) of degree smaller than n. Then use the induction hypothesis for r 1 (X) in place of f(x).) (b) Show that if a F is a root of f(x) then f(x) = (X a)q(x) for some q(x) F [X]. (c) Assume that f(x) 0. Show that f(x) has at most deg(f) roots in F. (Hint: Use induction on deg(f(x)).) (d) Let F be a field and let H F be a finite (multiplicative) subgroup. Show that H is cyclic. (Hint: Use part (c) and 46(d).) (e) Show that (Z/pZ) is cyclic. 53. Let p be an odd prime. Furthermore let r Z such that r + pz generates (Z/pZ) as group. (a) Show that there exist t Z such that (r + tp) p 1 1 mod p 2. (b) Let t Z be as in (a). Show that (r + pt) + p n Z is a generator of (Z/p n Z) for every n N. (Hint: Use induction on n. In order to show that a group element g has order p n 1 (p 1) it suffices to show that p 1 divides o(g) and that g pn 2 (p 1) 1.) 54. (a) Let F be a field and let a F. Show that F [X]/(X a) = F as rings. (Hint: Use the evaluation homomorphism with respect to a and 45(b).) (b) Show that R[X]/(X 2 1) = R R as rings. (Hint: Use the Chinese Remainder Theorem.) 12

13 (c) Show that R[X]/(X 2 + 1) = C as rings. (Hint: Use the evaluation homomorphism with respect to a suitable complex number.) 55. Show that the ideal (2, X) of Z[X] is not principal. 56. Let F := Z/3Z. Compute a greatest common divisor d(x) of f(x) = X 4 X 3 + X 1 and g(x) = X 3 + X 2 + X + 1 in F [X] and find polynomials p(x), q(x) F [X] with d(x) = p(x)f(x) + q(x)g(x). 57. Let R := Z[ 5i] = {a + b 5i a, b Z}. (a) Compute all common divisors of 9 and 3(2 + 5i). (b) Do 9 and 3(2 + 5i) have a greatest common divisor? (c) Consider the ideal I := (2, 1 + 5i). Show that I is not principal. Show that R/I has precisely 2 elements. Show that I 2 = (2). (d) Show that R/2R is a ring with 4 elements which is neither isomorphic to Z/4Z nor to Z/2Z Z/2Z. 58. Let R := Z[i] = {a + bi a, b Z}. (a) Show that R = {1, 1, i, i}. (b) Compute a greatest common divisor of α = 10 and β = 1 5i using the Euclidean Algorithm. Due Monday, December 5, before final exam (all extra points): 59. Let R := Z[i]. Show the following (thus giving an overview over all prime elements in R and giving the prime factorization in R of every prime number in Z): (a) Every prime element π of R divides precisely one prime number of Z. (b) If π R is such that N(π) is a prime number in Z, then π is a prime element in R. (c) 2 = ( i) (1 + i) 2 is a prime factorization of 2 in R. (d) If p is a prime number in Z with p 3 mod 4, then p is also a prime element in R. (e) If p is a prime number in Z with p 1 mod 4, then there exists z Z with z 2 1 mod p. (f) If p is a prime number in Z with p 1 mod 4, then p = π π for a prime element π of R such that π is also a prime element which is not associate to π. 60. Let n N and assume n = p e 1 1 per r following are equivalent: is its prime factorization. Then the 13

14 (i) n = x 2 + y 2 for some x, y N 0. (ii) For every i {1,..., r} such that p i 3 mod 4, the exponent e i is even. Hint: Use Let R be a commutative ring and let S R be a multiplicatively closed subset containing 1. (a) Show that S 1 R = {0} if and only if 0 S. (b) Show that the ring homomorphism ι: R S 1 R, r r 1, is injective if and only if S does not contain a zero-divisor of R. 62. Let R be an integral domain and let S be a multiplicatively closed subset of R with 1 S and 0 / S. View R as a subring of R := S 1 R via the canonical map ι: R R, r r 1. (a) Let I denote the set of ideals of R and denote by I the set of ideals of R. Consider the maps α: I I, I I R, and β : I I, I (I) R. Show that β α = id I. (b) Let P denote the set of prime ideals of R. Show that α defines a bijection between P and the set of prime ideals of R whose intersection with S is empty. 63. Let F := Z/2Z. (a) Show that X 2 + X + 1 is the only irreducible polynomial of degree 2 in F [X]. (b) Show that X 4 + X is irreducible in F [X]. (c) Show that X 4 + X is irreducible in Q[X]. 64. Show that the polynomial X 4 + 7X X X + 7 is irreducible in Q[X]. 14

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