# Problem 4 (Wed Jan 29) Let G be a finite abelian group. Prove that the following are equivalent

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1 Last revised: May 16, 2014 A.Miller M542 miller/ Problem 1 (Fri Jan 24) (a) Find an integer x such that x = 6 mod 10 and x = 15 mod 21 and 0 x 210. (b) Find the smallest positive integer y such that y = 6 mod 10 and y = 15 mod 21 and y = 8 mod 11. Problem 2 (Fri Jan 24) (a) Find integers i, j such that there is no integer x with x = i mod 6 and x = j mod 15. (b) Find all pairs i, j with i = 0, 1,... 5 and j = 0, 1,..., 14 such that there is an integer x with x = i mod 6 and x = j mod 15. Problem 3 (Mon Jan 27) Prove that for any n there is only one abelian group (up to isomorphism) of size n iff n is square-free. Square-free mean that no p 2 divides n for p a prime. Problem 4 (Wed Jan 29) Let G be a finite abelian group. Prove that the following are equivalent 1. For every subgroup H of G there is a subgroup K of G with HK = G and H K = {e}. 2. Every element of G has square-free order. Problem 5 (Fri Jan 31) How many abelian groups of order 144 are there up to isomorphism? Explain. Problem 6 (Mon Feb 3) Suppose G 1, G 2, H 1, H 2 are finite abelian groups, G 1 G 2 H 1 H 2 and G 1 H 1. Prove that G 2 H 2. Give a counterexample if the word finite is dropped, i.e., G 1 G 2 H 1 H 2 and G 1 H 1 but G 2 is not isomorphic to H 2. Problem 7 (Wed Feb 5) Prove or disprove: For any finite abelian groups G 1 and G 2 with subgroups, H 1 G 1 and H 2 G 2 such that H 1 H 2, if G 1 /H 1 G 2 /H 2 then G 1 G 2. Problem 8 (Wed Feb 5) Prove that Stab(ga) = g Stab(a) g 1. 1

2 Problem 9 This is due in lecture on valentines day. It will be graded in class so do not hand-in. (a) Suppose G is a finite abelian group which contains an element which has non-square-free order. Prove that for some prime p it has an element of order p 2. (b) Suppose a is an element of a finite abelian group G with order p 2 let b = a p, let H =< b > be the subgroup generated by b and suppose K is a subgroup of G with K H = {e}. Prove that a is not an element of HK. (c) Suppose G 1, G 2 are finite abelian groups with G 1 and G 2 relatively prime. Show that for any subgroup H G 1 G 2 there are subgroups H 1 G 1 and H 2 G 2 such that H = H 1 H 2. (Warning: the relatively prime hypothesis is necessary.) (d) Suppose G 1, G 2 are finite abelian groups with G 1 and G 2 relatively prime. Show that if G 1 and G 2 both have the CP then G 1 G 2 has CP. 1 (e) Prove that C p C p C p has the CP. (f) Prove Problem 4. Problem 10 (Mon Feb 10) Prove for any n 3 that Z(S n ) = {id}. Problem 11 (Wed Feb 12) (a) Prove that there are no simple groups of order either 575 or 272. (b) For any prime p prove there are no simple groups of order p(p 1) or p(p + 2). Problem 12 (Fri Feb 14) Question (August J.) Suppose every subgroup of finite group G is a normal subgroup. Must G be abelian? Problem 13 (Fri Feb 14) (a) Suppose P is a p-sylow subgroup of G and H a subgroup such that P H and H G. Prove that P G. (b) If K H and H G, does it follow that K G? Show that the answer is No. Consider G = S 4, K = {id, σ} where σ = (12)(34) and H = {id, σ, τ, ρ} where τ and ρ are conjugates of σ. Determine what τ and ρ are and show that K H and H G, but K is not a normal subgroup of G. Problem 14 (Mon Feb 17) Suppose for every x G that x 2 = e. Prove that G is abelian. 1 CP is defined after Problem 4. 2

3 Problem 15 (Mon Feb 17) Suppose H G is subgroup of index 2, i.e., [G : H] = 2. Prove that it is a normal subgroup of G. Problem 16 (Wed Feb 19) For F a finite field call a F a generator of F iff every nonzero element of F is a power of a. (a) Find a generator of Z 7. (b) How many generators does Z 17 have? (c) How many generators does Z 31 have? Problem 17 (Fri Feb 21) Prove that v 1, v 2,..., v n are linearly dependent iff v 1 = 0 or v i+1 span{v 1, v 2,..., v i } for some i with 1 i < n. Problem 18 (Mon Feb 24) Let R be a commutative ring with 1. Let I be a maximal ideal in R. Suppose ab = 0. Prove that a I or b I. Problem 19 (Mon Feb 24) Consider p(x) = x 3 + x + 1 as a polynomial in Z 2 [x]. Suppose p has a root α is in some field extension. Construct the multiplication table for Z 2 [α] = def {a + bα + cα 2 : a, b, c Z 2 } Problem 20 (Wed Feb 26) Let α be transcendental over Z 2. Let F = Z 2 (α) and let p(x) = x 2 α. (a) Prove that p is irreducible over F. (b) Prove that if β is a root of p in some some extension field, then p(x) = (x β) 2. (c) Suppose that F is a finite field of characteristic 2. Prove that for every a F there is a b F such that b 2 = a. (d) Suppose that F is a finite field of odd characteristic. Prove that there exists a F for every b F such that b 2 a. (e) Find a field F and an irreducible polynomial p(x) of degree three such that in any extension field in which p splits there exist a β such that p(x) = (x β) 3. Problem 21 (Fri Feb 28) Prove that the formal derivative for polynomials in F [x] satisfies (a) The power rule: (f n ) = n(f n 1 )f (b) The chain rule: f(g(x)) = f (g(x))g (x) 3

4 Problem 22 ((Mon ) Mar 3) Prove for any prime p and positive integer n p n that p divides for any k with 0 < k < p k n. Problem 23 (Wed Mar 5) p is a prime and n a positive integer. Prove: (a) If F is a field such that F = p n and m is a positive integer then there is a field E with F E and E = p nm. (b) If F E are fields, F = p n and E = p N, then n divides N. Problem 24 (Wed Mar 26) Prove or disprove: Using a straight edge and compass it is possible to construct an equilateral triangle with area 1. Problem 25 (Fri Mar 28) Prove that [F c : Q] is infinite. F c is the field of constructible reals (straight edge and compass). Problem 26 (Fri Mar 28) Prove that if 2 m 1 is prime, then m is prime. Problem 27 (Mon Mar 31) Find the roots of x 3 + 3x 2 + 6x + 5 = 0 using addition, subtraction, multiplication, division, and extraction of roots, i.e., solvability by radicals. Problem 28 (Wed Apr 2) Suppose [F [α] : F ] = n, [F [β] : F ] = m, and gcd(n, m) = 1. Prove that [F [α, β] : F ] = nm. Problem 29 (Fri Apr 4) Prove the following: (a) Suppose α + β is algebraic over F, then α is algebraic over F [β]. (b) Suppose α + β and αβ are both algebraic over F, then α is algebraic over F. Problem 30 (Mon Apr 7) Suppose that F K 1 L and F K 2 L and K 1 and K 2 are splitting fields over F. Prove that K 1 K 2 is a splitting field over F. Problem 31 (Wed Apr 9) Suppose that E is a splitting field over F and p F [x] splits in E as p(x) = (x α 1 )(x α 2 ) (x α n ) where α i α j whenever i j. Prove that the following are equivalent: (1) p is irreducible. (2) for any i, j there is σ aut(e F ) such σ(α i ) = α j. 4

5 Problem 32 (Mon Apr 14) For each of the following polynomials compute its Galois group, i.e., aut(k Q) where K is the splitting field of the polynomial. (a) x 5 1 (b) x 4 2 (c) x 4 2x 2 2 Problem 33 (Wed Apr 16) In the Lemma?? (d) must n be prime? Prove or disprove: {(1, 2, 3, 4), (1, 3)} generates S 4. Problem 34 (Mon Apr 21) σ, τ A 5 are conjugate in A 5 iff there is ρ A 5 such that σ = ρ 1 τρ. Prove that every element of A 5 except the identity is conjugate to exactly one of the following: (a) (1, 2, 3) (b) (1, 2)(3, 4) (c) (1, 2, 3, 4, 5) (d) (2, 1, 3, 4, 5) In particular (c) and (d) are not conjugates. Problem 35 (Wed Apr 23) Let K Q be the splitting field of f(x) = (x 2 + 1) (x 2 2) = x 4 x 2 2 (a) Prove that aut(k Q) Z 2 Z 2 (b) Find an irreducible polynomial p(x) whose splitting field is K. Problem 36 (Mon Apr 28) Suppose A and B are matrices with real entries and there exists a matrix P with complex entries such that A = P BP 1. Prove there exists a matrix P with real entries such that A = P BP 1. Hint: Show {Q : AQ = QB} is a subspace. 5

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