# Q N id β. 2. Let I and J be ideals in a commutative ring A. Give a simple description of

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1 Additional Problems 1. Let A be a commutative ring and let 0 M α N β P 0 be a short exact sequence of A-modules. Let Q be an A-module. i) Show that the naturally induced sequence is exact, but that 0 Hom(P, Q) β Hom(N, Q) α Hom(M, Q) Hom(N, Q) α Hom(M, Q) is not necessarily surjective. ii) Show that the naturally induced sequence is exact. Q M id α Q N id β Q P 0 2. Let I and J be ideals in a commutative ring A. Give a simple description of (A/I) (A/J). 3. Let M and N be cyclic modules over a principal ideal domain R. The module tor R (M, N) is defined as follows. (This is a special case of a more general construction.) Take an exact sequence 0 R R M 0 Then tor R (M, N) is defined by the exactness of the sequence 0 tor R (M, N) N R N R N M 0 (Note: N R = N.) i) Describe tor R (M, N) in simple terms. ii) Show that tor R (M, N) tor R (N, M). 4. Let k be a field and let S : k n k n and T : k m k m be k-linear endomorphisms. i) Find the determinant of S T : k n k m k n k m. ii) Find the trace of S T : k n k m k n k m. MATH 8000 Homework 14, due Monday 12/07/09. 1

2 2 1. Read the definition of composite field on page 528. Let N/F and E/F be algebraic field extensions, with N normal. Let ψ : E C be an algebraic closure. Let φ : N C be a homomorphism such that ψ F = φ F. i) Show that, up to isomorphism, the composite field φ(n)ψ(e) is independent of the choice of C, ψ and φ. We call it the composite of N and E, and denote it NE. ii) Show that the intersection φ(n) ψ(e) is also independent of the choices. We denote it N E. 2. Read proposition Let N/F and E/F be finite extensions, with N Galois. Assume that N E = F. Show that Gal(NE/F ) Gal(NE/N) Gal(NE/E) 3. Let n be a nonnegative integer. Let 0 a Q and let θ C such that θ n = a. Let ω C be a primitive n th root of unity. Assume Q(ω) Q(θ) = Q. Let d be the degree of the minimal polynomial of θ over Q. Prove that d n, and that furthermore, Gal Q (x n a) = Z d Z n. (Z n acts on Z d as follows. Identify Z n with Aut(Z n ). Since there is a unique copy of Z d inside Z n, Aut(Z n ) acts on it.) 4. Give generators and relations for Gal Q (x 8 3). 5. Give generators and relations for Gal Q (x 8 9). 6. Here is yet another proof of the Cayley-Hamilton theorem. i) The Cayley-Hamilton theorem for an n n matrix is a collection of n 2 polynomial identities, over Z, in n 2 indeterminates, namely the entries of the matrix. Show therefore that if the Cayley-Hamilton theorem holds for every n n matrix over your favorite field of characteristic zero, it holds for every n n matrix over an arbitrary commutative ring. ii) Fix a positive integer n, and let A be an n n matrix over C. Give an easy argument that proves the Cayley-Hamilton theorem holds if A has distinct eigenvalues. iii) Prove that there is a polynomial p(a i,j ) in the entries of A such that A has distinct eigenvalues if and only if p(a i,j ) 0. (Hint: See page 610.) Therefore, if f A is the characteristic polynomial of A, then p(a i,j )f A (A) = 0. iv) Conclude that f A (A) = 0. DF 14.2: 21,22, : 27 MATH 8000 Homework 13, due Friday 11/20/ Sept. 92, #5

3 3 2. Sept. 93, #3 3. May 95, #4 4. Fall 02, #5 5. Let λ C, λ 5 = 1. Let f(z) C[z] and suppose that for all z, f(λz) 2 z 5 f(λz) = f(z) 2 z 5 f(z). Show that f(λz) = f(z). 6. Write t t t 3 3 as a polynomial in the elementary symmetric polynomials in t 1, t 2, t 3. DF 14.1: : 6, 13, 14 MATH 8000 Homework 12, due Friday 11/13/ Let F L N be a tower of fields, such that N is normal over F. Prove that every F -homomorphism φ : L N extends to a homomorphism ψ : N N. (Hint: Let C/N be an algebraic closure. First show that φ extends to a homomorphism ψ : N C and then show that ψ(n) N.) 2. Let f and g be polynomials over a field F. Prove that there is a field extension E/F in which f and g have a common root if and only if f and g have a common factor in F [x]. 3. Find F ix(gal(q[2 1/3 ]/Q)). 4. Let E/F be an algebraic extension, and let φ : F C be a homomorphism from F to an algebraically closed field C. Prove that the cardinality of the set Hom φ (E, C) is independent of the choice of φ. 5. Let E/F be a field extension, and let E sep E denote the set of elements α E such that α is separable over F. Prove that E sep is a field. 6. Let E be the splitting field of separable polynomial f F [x]. Prove that E/F is separable. DF 13.4: 1, : 7 MATH 8000 Homework 11, due Monday 11/09/09. DF 13.1: 1, : 1, 5, 7, 10, 13, 17, 18 MATH 8000 Homework 10, due Wednesday 11/04/ Let R be a domain. Given an R-module M and a prime p R, the p-torsion

4 4 part of M is the submodule p-tor(m) = {m M n N, p n m = 0} p-tor(m) is also called the p-primary part of M. module over a PID R, then M = p p-t or(m) Prove that if M is a torsion 2. Let M be a finitely generated torsion module over a PID R. Let d 1, d 2,..., d k be the invariant factors obtained from a presentation matrix of M. Here is a proof that the invariant factors depend only on M and not on the presentation. If one factors each d i into primes, one may apply the Chinese Remainder Theorem to each R/d i to obtain the decomposition of R/d i into its p-primary parts, p prime. For a fixed prime p, we then get an invariant factor decomposition of the p-primary part of M by adding the contributions from each R/d i. One can recover d 1,..., d k from all the invariant factor decompositions of the p-primary parts of M as p varies over all the primes in M. So to recover the invariant factors in general, it suffices to consider the case that M itself is p-primary for some p. Thus we may assume M = R/p n 1 R/p n 2... with n 1 n Let F denote the field R/p. i) Show that for all nonnegative integers k, p k M/p k+1 M is an F -vector space. ii) Let r k denote the dimension of the F -vector space p k M/p k+1 M. (Notice that these numbers depend only on M and not on the presentation matrix.) Show that n 1, n 2,... can be recovered from r 1, r 2,.... (This should remind you of a problem from homework 8.) 3. Find the invariant factor decomposition and the primary decomposition of the abelian group presented as follows: 10a + 270c + 20d = 0 15a + 540c + 30d = 0 40a + 30b + 60d = 0 60a + 60b + 90d = 0 4. Find the rational canonical form of the following matrix:

5 5 5. Let A be a square matrix with entries in a principal ideal domain R. Let M A denote the R-module with presentation matrix A. Prove that M A is isomorphic to M A t, where A t is the transpose of A. MATH 8000 Homework 9, due Friday 10/23/ Let R be a commutative ring and let A 1,..., A k be pairwise relatively prime ideals in R. Let M be an R-module such that Π k i=1 A i ann(m). Let M i M denote the submodule annihilated by A i. Prove that M k i=1 M i. Hint: Let e j Π k i=1 R/A i denote the k-tuple with 1 in position j and zeroes elsewhere. Let φ : R Π k i=1 R/A i be the natural homomorphism. Use the Chinese Remainder Theorem to establish the existence of α i R such that a) φ(α i ) = e i b) α i 1mod Π k i=1 A i c) α i Π k j i A j Then for all m M, let m i = α i m. 2. Let k be a field. Let A M n (k) be a square matrix such that the characteristic polynomial of A factors into linear factors over k. For each eigenvalue λ of A, define the generalized eigenspace to be the subspace M λ = {v k n : j N (A λi) j v = 0} Use the Cayley-Hamilton theorem together with problem 1 to show that k n = λ M λ. Thus k n is the direct sum of the generalized eigenspaces for A. (Hint: Regard k n as a k[x]-module.) DF 10.1: 8bc, : 6, : 2, 5, : 1bc, 2, 14 MATH 8000 Homework 8, due Friday 10/16/ ( mod p cancellation law ) Let p be a prime number. Prove that if n and j are nonnegative integers, and we set n = ap + b and j = cp + d, with 0 b < p and 0 d < p, then ( ) n j (Hint: Think about (1 + x) n mod p.) ( a c )( b d ) mod p 2. Let A be a nilpotent matrix over a field. For each positive integer j, let b(j) denote the number of Jordan blocks of size j in the Jordan form of A, and let n(j) denote the nullity of A j. Establish an explicit formula for the b(j) s in terms of the n(j) s. 3. Let f = z 3 + z 2 x 2 + z 2 y 3 + z 2 y + zyx 5 + x + xz 3 + z 2 x 3 + xz 2 y 3 + xz 2 y + zyx 6 + x 2

6 6 i) Find the irreducible factors of f as an element of Z[x, y, z]. ii) Find the irreducible factors of f as an element of Q(x, y)[z]. suggestion: Write f as a polynomial in z with coefficients in Z[x, y]. DF 9.2: 7 9.3: 1,4 9.4: 1, 6, 9, 11, 12, 15, 16 MATH 8000 Homework 7, due Friday 10/02/ Let S Z 18 be the set of powers of 2. Then the ring S 1 Z 18 is a familiar ring. Which one? 2. Show by example that if R is a commutative ring and S R is a multiplicative subset, then the relation on R S given by (r, s) (r, s ) rs r s = 0 need not be an equivalence relation. The next two problems use problem 3 from the last homework. 3. (The Cayley-Hamilton Theorem) Let k be a field and let A M n (k). Let ξ A (x) denote the characteristic polynomial, ξ A (x) = det(xi A) k[x]. Theorem (Cayley-Hamilton): ξ A (A) = 0 (You can t prove this by plugging in A for x in the definition!) Here is one of many proofs: Regard k n as a k[x]-module, by letting a polynomial p(x) act by p(a). The goal then is to prove that ξ A (x) acts by 0. Do this by verifying the hypotheses of HW 6 problem 3, using the matrix xi A regarded as an element of M n (k[x]). 4. (Integral closure) Let R be a subring of a commutative ring S and let s S. Then we say that s is integral over R if there is a monic polynomial p(x) R[x] such p(s) = 0. (For example, 2 is integral over Z, while 1 2 is not.) The integral closure of R in S is the set of elements s S integral over R. Theorem: The integral closure of R in S is a subring. The proof is as follows: Let s and t be integral over R, and let R[s, t] S be the R-subalgebra generated by s and t. We must show that for all u R[s, t], u is integral over R. i) Show that R[s, t] is a finitely generated R-module. ii) Let v 1,..., v n R[s, t] generate R[s, t] as an R-module. Then there is a matrix (A i,j ) over R such that uv j = i A i,jv i for all j. (Say why.) Use HW 6 problem 3 to prove that u is a root of the polynomial det(xi A). (This is not quite a corollary of the Cayley-Hamilton theorem, because v 1,..., v n need not be a basis.) iii) Find a monic polynomial in Z[x] satisfied by a + b, given that a 2 a 1 = 0 b 2 2b 1 = 0

7 7 DF 7.4: 36, : 5,6 8.1: 8 8.2: 8 MATH 8000 Homework 6, due Friday 9/25/ Let R be a ring, and let M be a free, finitely generated R-module. Prove that End R (M) is isomorphic to a ring of matrices. (Note that R is not assumed to be commutative.) 2. (Cramer s rule) The solution to this exercise may be found in many texts, but you should know the proof. Let R be a commutative ring, and let A = (A i,j ) be an n n matrix over R. The indexing convention being used here is that if e 1,..., e n denotes the standard basis for R n, then Ae j = i A i,je i. Let a 1,..., a n denote the columns of A. The adjugate of A, denoted A adj, is the matrix (B i,j ) defined by B i,j = det(a 1,..., a i 1, e j, a i+1,..., a n ) That is, substitute e j in place of a i and take the determinant. i) Prove that that BA = det(a)i. ii) Write an equivalent definition of A adj using the rows of A, and use it to conclude that AB = det(a)i. 3. Here is a useful application of the adjugate. Let R be a commutative ring, and let M be a finitely generated R-module. Then the ring of matrices M n (R) acts by matrix multiplication on M n, regarded as column vectors. Let A M n (R) and suppose there exists a set of generators m 1,..., m n for M, such that Prove that det(a) annihilates M. m 1 A. = 0 m n DF 7.1: 23, 26, : 3, 5, 7 7.3: 29, 33, 34cd 7.4: 33 MATH 8000 Homework 5, due Friday 9/18/ Let G denote the group of affine linear automorphisms of R n, i.e, functions of the form f(x) = Ax + b, where A Gl n (R) and x, b R n. The group law on G is composition of functions. Prove that G R n φ Gl n (R) for an appropriate choice of φ. 2. Let H and K be groups. Let Hom(K, Aut(H)) be the set of homomorphisms from K to Aut(H). Let φ Hom(K, Aut(H)).

8 8 i) Find an action of Aut(H) on Hom(K, Aut(H)) such that for all β Aut(H), the function ψ : H K H K ψ(h, k) = (β(h), k) is an isomorphism ψ : H φ K H β φ K ii) Find an action of Aut(K) on Hom(K, Aut(H)) such that for all α Aut(K), the function ψ : H K H K ψ(h, k) = (h, α(k)) is an isomorphism ψ : H φ K H α φ K 3. Let σ S n for some n. Let k be a field. Consider the linear transformation T σ T σ : k n k n x 1 x 2. x n = x σ 1 (1) x σ 1 (2). x σ 1 (n) i) Let P σ denote the standard matrix of T σ. Describe P σ. (A convincing example will be sufficient proof.) Matrices of the form P σ for some σ S n are called permutation matrices. ii) Prove that the function S n Gl n (k) sending σ P σ is a homomorphism. iii) Prove that sign(σ) = det(p σ ). (Some authors take this as the definition of the sign homomorphism. That would be cheating.) 4. (apropos Spring 04,#9) Let A M 2 2 (R) be a 2 2 matrix. Let v C 2 be an eigenvector for A, with nonreal eigenvalue λ. i) Prove that v, v form a basis for C 2 and that this basis diagonalizes A. ii) Prove that Re(v), Im(v) form a basis for R 2. iii) Letting λ = re iθ, find the matrix for the linear transformation defined by A, in the basis Re(v), Im(v). 5. Construct a nonabelian group of order 75. DF , , , MATH 8000 Homework 4, due Monday 9/14/09.

9 9 1. Let K be a field and let V be a finite-dimensional vector space over K. A complete flag over V is a sequence of subspaces {0} = W 0 W 1 W 2 W n = V such that dim(w i ) = i. i) Let F n (K) denote the set of all complete flags over the vector space K n. Show that Gl n (F ) acts transitively on F n (K). ii) Describe the stabilizer of the standard flag, W i =span(e 1,..., e i ), where e 1,..., e n is the standard basis. 2. Let p be a prime integer, and n an integer. Let H Gl n (F p ) be the subgroup consisting of matrices with ones on the diagonal and zeroes below the diagonal. Prove that H is a Sylow p-subgroup of Gl n (F p ). 3. Find a Sylow 3-subgroup for S i, i = 1,..., 9. (You may give your answer by providing a set of generators for the group.) DF 4.2.7b, 4.3.5, , , , MATH 8000 Homework 3, due Friday 9/04/ Let G be a group and let H G be a subgroup. Let X denote the set of right H-cosets. Then G acts transitively on X. Therefore, as a G-set, X is isomorphic to some left coset space. Which one, and what is the isomorphism? 2. Here is a nice characterization of cyclic groups. Theorem: Let G be a finite group, and let n = G. equivalent: Then the following are 1. For every d n, G has at most one cyclic subgroup of order d. 2. For every d n, G has at least one cyclic subgroup of order d. 3. G is cyclic. Prove this theorem. HINT: Let C be the set of all cyclic subgroups of G. Let π : G C be the map sending every element x G to the cyclic group it generates. Let m d denote the number of cyclic subgroups of G, of order d. Using π to count the elements of G, establish the formula n = d n m d φ(d). (1) where φ is Euler s φ-function. (I.e., φ(d) is the number integers 0 < k < d such that k and d are relatively prime.) Get a second formula by applying (1) to the

10 10 special case of a cyclic group of order n. Prove the theorem by subtracting the two formulas. DF , , , , , , , , 3.2.4, 3.2.9ef, , 3.3.4, 3.3.7, MATH 8000 Homework 2, due Friday 8/28/ Let G be the group generated by elements x i, i = 1, 2, subject to the relations x 2 1 = x 2 2 = e, x 2 x 1 x 2 = x 1 x 2 x 1 G is a familiar group. Which one? (With proof) 2. Same as problem 1, with three elements x i, i = 1, 2, 3, subject to relations x 2 1 = x 2 2 = x 2 3 = e, x 1 x 3 = x 3 x 1, x 2 x 1 x 2 = x 1 x 2 x 1, x 3 x 2 x 3 = x 2 x 3 x 2 3. Prove that Aut(Q) Q. 4. Prove that Aut(C) C. DF , 1.4.2, 1.6.4, , , , , , 2.1.6, def, MATH 8000 Homework 1, due Friday 8/21/ Let X, Y and Z be sets, and let f : X Y and g : X Z be surjections. Say that f and g are equivalent if there exists a bijection h : Y Z such that h f = g. i) Given a surjection f : X Y, write down an equivalence relation naturally associated to f. ii) Given an equivalence relation R X X, write down a partition naturally associated to R. iii) Given a partition of P of X, write down a surjection naturally associated to P. iv) Prove that if you if you start with a surjection f : X Y and and go around the loop just described, you return to an equivalent surjection. DF: 0.2.1f, 0.3.8, , , ,

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