# Group Theory. 1. Show that Φ maps a conjugacy class of G into a conjugacy class of G.

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1 Group Theory Jan 2012 #6 Prove that if G is a nonabelian group, then G/Z(G) is not cyclic. Aug 2011 #9 (Jan 2010 #5) Prove that any group of order p 2 is an abelian group. Jan 2012 #7 G is nonabelian nite group of order p 3, prove Z(G) = [G, G]. Aug 2011 #12 G is a nite group, and M G be a maximal subgroup. Show that if M is normal subgroup of G, then G : M is prime. Jan 2011 #1(Aug 2010 #8) Find the element g of order 2 in S 6 with minimal order of the centralizer C(g) = {h G hg = gh}. (Find the numbers of element in S 5 that commute with g S 5 where g has order 6.) Aug 2010 #2 Let G be a nite group and Φ : G G be an automorphism. 1. Show that Φ maps a conjugacy class of G into a conjugacy class of G. 2. Give an example of non-trivial G and Φ such that {e} is the only conjugacy class of G that maps into itself. Explain. 3. Show that if G = S 5, then g and Φ(g) must be conjugate for any g G. Jan 2010 #6 How many conjugacy classes are there in the symmetric group S 5. Jan 2010 #4 Suppose G is a group of order 60 that has 5 conjugacy classes of orders 1,15,20,12,12. Prove that G is a simple group. Aug 2010 #5 Let G be the group of rigid motions(more specically, rotations) in R 3 generated by a =a 90 degree rotation about x axis, and b =a 90 degree rotation about y axis. 1. How many elements does G have? 2. Show that the subgroup generated by a 2 and b 2 is a normal subgroup of G. Jan 2012 #5 Let a, b be elements of a group G. Prove that ab and ba have the same order. Jan 2012 #8 Determine the group of Aut(C 2 C 2 ), calculating its order and identifying it with a familiar group. Aug 2011 #11 Find the cardinality of Hom(Z/20Z,Z/50Z). Jan 2011 #3 Show that every nite group of order 3 has a non-trivial automorphism. Jan 2010 #8 1. Show that Hom(G,H) is an abelian group if H is abelian group. 2. Prove that if G is nite cyclic group, then Hom(G,Q/Z) is isomorphic to G. 3. Find an innite abelian group G such that Hom(G,Q/Z) is not isomorphic to G. 1

2 Aug 2011 #10 Let a G. Prove that a commutes with each of its conjugates in G i a belongs to an abelian normal subgroup of G. Jan 2011 #2 Let G be a group and H 3 and H 5 be normal subgroups of G of index 3 and 5 respectively. Prove that every element of g G can be written in the form g = h 3 h 5 with h 3 H 3 and h 5 H 5. Linear Algebra part 1 Aug 2011 #2 Let V be a nite dimensional real vector space of dimension n. Dene an equivalence relation on the set End R (V ) of R-linear homomorphisms V V as follows: if S, T End R (V ) then S T i there are invertible maps A, B : V V s.t. S = BT A. Determine, as a function of n, the number of equivalence classes. Aug 2010 #3 Let V and W be real vector spaces, and let T : V W be a linear map. If the dimensions of V and W are 3 and 5, respectively, then for any bases B of V and B of W, we can represent T by a 5 3 matrix A T,B,B. Find a set S of 5 3 matrices as small as possible such that for any T : V W there are bases B of V and B of W such that A T,B,B S. Jan 2011 #5 Let T : R 4 R 2 be the linear transformation T (a, b, c, d) = (a + b c, c + d). Find a basis for the null space. Jan 2010 #7 Let G = GL 2 (F 5 ). What is the order of G? Jan 2010 #3 Let A, B be n n complex matrices such that AB = BA. Prove that there exists a vector v 0 in C n which is an eigenvector for A and for B. Jan 2011 #4 The following matrix has four distinct real eigenvalues. Find their sum and their product Jan 2010 #1 Let A be the a n n complex matrix which does not have eigenvalue -1. Show that the matrix A + I n is invertible. Aug 2011 #3 Let A be the n n matrix with zeros on the diagonal and ones everywhere else. Find the characteristic polynomial of A. Ring Theory Aug 2011 #6 Let P be a prime ideal in a commutative ring R with 1, and let f(x) R[x] be a polynomial of positive degree. Prove that following statement: if all but the leading coecient of f(x) are in P and f(x) = g(x)h(x), for some non-constant polynomials g(x), h(x) R[x], then the constant term f(x) is in P 2. Jan 2011 #7 Prove that in a commutative ring with a nite number of elements, prime ideals are maximal. Jan 2011 #9 1. Give an example of a ring R and a unit r R with r 1. 2

3 2. Give an example of a ring R and a nilpotent element r R with r Show that for any ring R and for any element r R, that r is a nilpotent element of R i 1 rx is a unit in the polynomial ring R[x]. Aug 2010 #6 Let R be a ring with 1. Dene a R to be periodic of period k if a, a 2, a 3,..., a k are all dierent, but a k+1 = a. 1. In R = Z/76Z, nd an element a 0, 1 of period In the same ring R = Z/76Z, nd an element that is not periodic. 3. In R = Z/76Z, list the possible periods and the elements of each period. Jan 2010 #12 Determine the following ideals in Z by giving generators: (2)+(3), (4)+(6), (2) (3), (4) (6) Jan 2012 #11 Prove that the rings F 16, F 4 F 4, and Z/16Z are pairwise non-isomorphic. Aug 2010 #7 In this problem, R is a nite commutative ring with 1. Let p(x) R[x], the ring of polynomials over R. 1. Show that a R is a root of p(x) i p(x)can be written as p(x) = (x a)g(x) with g(x) R[x] of degree one less than the degree of g(x). 2. Prove or give a counter example: A polynomial of p(x) R[x] of degree n can have at most n distinct roots in R. Jan 2012 #12(Jan 2010 #9) Find all the maximal ideals in R[x]. (Describe the prime ideals in C[x]), Jan 2010 #13 Let f(x) C[x] be a polynomial of degree n such that f and f (the derivative of f) have no common roots. Show that the quotient ring C[x]/(f ) is isomorphic to C C... C(n times). Aug 2011 #5 Let R = K[x, y, z]/(x 2 yz), where K is a eld. Show that R is an integral domain, but not a unique factorization domain. Aug 2010 #12 For which values of n in Z does the ring Z[x]/(x 3 + nx + 3) have no zero divisors? Aug 2010 #11 Let M be the ring of 3 3 matrices with integer entries. Find all maximal two-sided ideals of M. Linear Algebra part 2 Jan 2012 #3 Find the eigenvalues and a basis for the eigenspace of the matrix B = Jan 2010 #2(Jan 2012 #1) Find invertible matrix P s.t. P 1 AP is diagonal where A = (A = )

4 Jan 2012 #2 Find the matrix A 2001 for A = Aug 2011 #4 Find the Jordan canonical form of Jan 2012 #4 Find [ the matrix ] e C := I + C + C where 1 4 C = Aug 2010 #9 Let A be a 5 5 real matrix of rank 2 having λ = i as one of its eigenvalues. Show that A 3 = A and that A is diagonalizable. Aug 2011 #1 Let A be a matrix in GL n (C). Show that if A has nite order(i.e. A k is the identity matrix for some k 1), then A is diagonalizable. Jan 2011 #6 A 5 5 matrix A satises the equation (A 2I) 3 (A + 2I) 2 = 0. Assume that there are at least two linearly independent vectors v satisfy Av = 2v. What are the possibilities for the Jordan canonical form? Jan 2011 #10 Let M n (C) denote the vector space over C of all n n complex matrices. Prove that if M is a complex n n matrix, then C(M) = {A M n (C) AM = MA} is a subspace of M n (C) if dimension at least n. Algebraic Number Theory Jan 2012 #9 Find all irreducible polynomials of degree 4 in F 2 [x]. Jan 2012 #10 Find the set of polynomials in F 2 [x] which are the minimal polynomials of elements in F 16. Aug 2010 #1 Find all irreducible monic quadratic polynomials in F 3 [x]. Jan 2010 #11 1. Prove that the polynomial x 2 + x + 1 is irreducible over the eld F 2 with two elements. 2. Factor x 9 x into irreducible polynomials in F 3 [x], where F 3 is the eld with three elements. Jan 2011 #8 Let F 4 be the nite eld with 4 elements. Express F 4 [x]/(x 4 + x 3 + x 2 + 1) as a product of elds. Prove your result. Aug 2011 #7 Let p be a prime number and denote by F p the eld with p elements. For a positive integer n, let F p n be the splitting eld of x pn x F p [x]. Prove that the following are equivalent: 1. k n 2. (p k 1) (p n 1) 4

5 3. F p k F p n Aug 2010 #4 Is it possible to nd a eld F with at most 100 elements so that F has exactly ve dierent proper subelds? If so, nd all such elds. If not, prove that no such eld F exists. Aug 2011 #8 1. Show that x 3 2 and x 5 2 are irreducible over Q. 2. How many eld homomorphism are there from Q[ 3 2, 5 2] to C? 3. Prove that the degree of over Q is 15. Jan 2010 #10 Find the degree of the minimal polynomial of α = over Q. Aug 2010 #10 1. Give an example of an irreducible monic polynomial of degree 4 in Z[x] that is reducible in the eld Q[ 2]. Explain why your example has the stated property. 2. Show that there are no irreducible monic polynomial of degree 5 in Z[x] that is reducible in the eld Q[ 2]. Jan 2012 #13 Let R = Z[i] and I R be an ideal. If R/I has 4 elements what are the possibilities for I and R/I. 5

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