# Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville January 2018

Save this PDF as:

Size: px
Start display at page:

Download "Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville January 2018" ## Transcription

1 Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville January 2018 Do 6 problems with at least 2 in each section. Group theory problems: (1) Suppose G is a group. The commutator subgroup G of G is the subgroup of G generated by {aba 1 b 1 : a G, b G}. (a) Show that G is a normal subgroup. (b) Let N be a normal subgroup of G. Show that G/N is Abelian if and only if G N. (2) (a) Let G be a group of order 12. Prove that not all non-identity elements of G are of order 3. (b) Use (a) to prove that every group of order 12 must have an element of order 2. (3) Let G be a group with the property that G : Z(G) = 2 k for some k > 1. Prove that g 2k 1 Z(G) for all g G. (4) Let G be a group that acts on a set A, and let a A. Recall that the orbit of a under the given action is O a = {g a : g G}, and the stabilizer of a is G a = {h G : h a = a}. Prove that there is a bijection between the left cosets of G a in G (i.e., the set {hg a : h G}) and O a, the orbit of a. Ring theory problems: (1) Prove that (x, y) and (2, x, y) are prime ideals in Z[x, y], but only the latter ideal is a maximal ideal. (2) Let k be a field and consider the subring R = k[x 2, x 3 ] of k[x] generated by 1, x 2 and x 3. Prove that for any f, g R, there exists an unique pair (q, r) R 2 with the property that f(x) = g(x)q(x) + r(x) with deg(r) = deg(g) + 1, or deg(r) < deg(g). (Here we use the convention that deg(0) =.) (3) A ring R with identity 1 satisfies the Ascending Chain Condition on its ideals (or, A.C.C. on ideals) if no infinite strictly increasing chain of ideals exists in R, i.e., whenever I 1 I 2 I 3... is an increasing chain of ideals in R, then there is a positive integer m such that I k = I m for all k m. Prove that if R is a Principal Ideal Domain then R satisfies A.C.C. on its ideals. Hint: Consider k=1 I k. (4) Let R be a commutative ring with 1. (a) Prove that the principal ideal I = (x) in the polynomial ring R[x] is prime if and only if R is an integral domain. (b) Prove that the principal ideal I = (x) in the polynomial ring R[x] is maximal if and only if R is a field. Field theory problems on next page. 1

2 2 Field theory problems: (1) (a) Prove that Q( 2 + 3) = Q( 2, 3). (b) Prove that [Q( 2 + 3) : Q] = 4. (c) Find an irreducible polynomial satisfied by (2) Suppose K is a finite dimensional field extension of F an infinite field, and K contains only finitely many subfields that contain F. (a) Show that if α, β are in K, then there exists θ K such that F (α, β) = F (θ). (b) Use (a) above to show that there exists γ K such that F (γ) = K. (3) Consider the direct product of fields R = Q(i) Q(ω), where i is a primitive fourth root of unity and ω is a primitive cube root of unity. Define Gal Q (R) to be the set of ring automorphisms ψ of R fixing the subring Q Q. Prove that Gal Q (R) is a group and that Gal Q (R) Gal Q (Q(i)) Gal Q (Q(ω)). (4) Determine the Galois group of f(x) = x 4 5x over Q. (Recall that the Galois group of a separable polynomial f(x) F [x] is defined to be the Galois group of the splitting field of f(x) over F.)

3 Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville August 2017 Do 6 problems with at least 2 in each section. Group theory problems: (1) (a) Find all possible orders of the elements of S 5. Explain. Give a concrete element of each order listed. Is S 5 a cyclic group? Explain. (b) Give an element of A 9 of order 12. Explain. (2) Let (G, ) be a group. (a) Define the center Z(G) of G. (b) Prove that Z(G) is a normal subgroup of G. (First prove that it is a subgroup.) (3) A group G is solvable if there exists a series of subgroups H i of G such that {e} = H 0 H 1 H k = G, and each quotient H i /H i 1 is Abelian for 1 i k. A well-known theorem states that given any group G and K G, if G/K and K are both solvable then G is solvable as well. Use this fact to prove that any group of order 126 is solvable. (4) Let G be a group of order 2n (n > 0). Show that G has at least one element of order 2. You may not directly quote Cauchy s Theorem for this exercise. Ring theory problems: (1) Let R be an integral domain. (a) Prove that every prime element in R is irreducible. (b) Let p, q be two associate elements of R. Prove that p is prime if and only if q is prime. (2) Find three prime ideals p 1, p 2 and p 3 in Z[x, y] such that 0 p 1 p 2 p 3 Z[x, y]. Verify primality and proper containment. (3) Let D be a PID and let R be an integral domain containing D as a subring. Recall that an element d in a ring S is said to be a g.c.d. (greatest common divisor) in S of elements a, b S if d a, d b and whenever any element h divides both a and b it must also divide d. Show that if d is a g.c.d. of a, b D then d is also a g.c.d. of a and b in R. (4) Let R be a commutative ring with identity, a, b R and let I = (a 2 b a) be the principal ideal in R generated by a 2 b a. Prove that ab + I is an idempotent in R/I. Field theory problems on the next page:

4 Field theory problems: (1) Let E = Z 3 (t) be the field of rational functions with coefficients in Z 3 in the indeterminant t. Consider the splitting field F over E of the polynomial f(x) = x 3 t. Prove that F is not a Galois extension of E by finding [F : E] and proving that this quantity does not equal Aut E (F), that is, the number of automorphisms of F fixing E. (2) Let E be a finite extension of the field F. Prove that E is algebraic over F. (3) Let F = Q(i, 3), where i = 1. Find the degree of the extension [F : Q] and a (vector space) basis of F over Q. Justify your answer. (4) Given an extension field E of F and elements a, b E \ F, prove that if ab is algebraic over F then b is algebraic over F (a).

5 Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville January 2016 Do 6 problems with at least 2 in each section. Group theory problems: (1) Let G be a group. Consider the commutator subgroup [G, G] of G, the subgroup generated by all commutators, aba 1 b 1 where a, b G. It is known that [G, G] is a normal subgroup of G. Prove that G/[G, G] is Abelian, and if K G such that G/K is Abelian, then [G, G] K. (2) Let (G, ) be a group and Z = Z(G) its center. Suppose that the group G/Z is cyclic. Show that G is Abelian. (3) Prove that any finite group is isomorphic to a subgroup of A n for some n, where A n is the alternating group on n elements. (Consider Cayley s Theorem.) Ring theory problems: (1) Prove that every prime ideal is maximal in a PID. (2) Let R be a commutative ring. Prove that the following are equivalent: (i) R is a field; (ii) R {0} and the only ideals of R are {0} and R. (3) Let R be a commutative ring with 1. For any two ideals I and J of R, let IJ be the ideal generated by all products xy where x I and y J. Suppose I + J = R, prove that IJ = I J. Field theory problems: (1) (a) Find the degree [F : Q] of the extension F = Q(i, 3) over Q. Find a basis of the vector space F over Q. (b) Find a primitive element of the extension F of Q. (2) (a) Identify Gal(Q( 3 2)/Q), the group of all automorphisms of Q( 3 2) which fix Q. (b) Determine if the field extension Q( 3 2)/Q is Galois. (3) Prove that for any prime p, the field F p with p elements contains an element a such that [F p ( 3 a) : F p ] = 3 if and only if 3 divides p 1 (Hint: you may use the fact that for any finite field F, the multiplicative group F is cyclic.) 1

6 Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville August 2015 Do 6 problems with at least 2 in each section. Group theory problems: (1) Let G be an arbitrary group. For any two subgroups H and K of G, define HK = {hk : h H, k K}. Assume H G and K G, where denotes a subgroup and denotes a normal subgroup. (a) Prove that HK G. (b) Prove that H HK and (H K) K. (c) Prove that HK H = K H K. (2) (a) Consider the cyclic group (Z 10, +), where Z 10 = {0, 1,..., 9} and the addition is modulo 10. If f is an automorphism of this group, what are all possible values of f(1)? Explain. Find all automorphisms of this group. (b) A subgroup N of a group G is called a characteristic subgroup if φ(n) = N for all automorphisms φ of G. Prove that every subgroup of the group (Z 10, +) is characteristic. (3) Prove that a group of order 300 cannot be simple. Ring theory problems: (1) A ring R (not necessarily commutative) with identity 1 R 0 in which every nonzero element is a unit is called a division ring. Prove that a ring R with identity 1 R is a division ring if and only if R has no proper left ideal. (2) Let A and B be ideals of a ring R with B A. (a) Show that A/B = {a + B a A} is an ideal of the ring R/B. (b) Show that the map f : R/B R/A, given by f(x + B) = x + A for every x R, is well-defined, onto and a homomorphism. (c) Show that (R/B)/(A/B) is isomorphic to R/A. (3) Let R be a commutative ring and I an ideal of R. Let (I, X) be the ideal of R[X] generated by I {X}. 1

7 2 (a) Prove that (I, X) consists precisely of all the polynomials in R[X] whose constant term belongs to I. (b) For any maximal ideal M of R, prove that (M, X) is a maximal ideal of R[X]. Field theory problems: (1) Let F = Q(i, 3), where i = 1. Find the degree of the extension [F : Q] and a (vector space) basis of F over Q. Justify your answer. (2) Let F be a field and let f(x) F [x]. (a) What is the definition of the splitting field of the polynomial f(x) over the field F? (b) Let a F. Show that f(x) and f(x + a) have the same splitting field over F. (3) Determine the splitting field and its degree over Q for the polynomial X 6 4. Justify your answer.

8 Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville January 2015 This exam consists of three parts. For each part select two problems and work on them. If you work more problems, clearly indicate which two should be graded. Please write on one side of each sheet of paper and put your name in the upper right hand corner. Group theory problems: (1) If N is a normal subgroup of G and H is any subgroup of G, prove that NH := {nh n N, h H} is a subgroup of G. (2) Let G be an arbitrary group with a center Z(G). Prove that the inner automorphism group Inn(G) is isomorphic to G/Z(G). (Recall: by definition, Inn(G) is the group of all automorphisms φ h of the form φ h (g) = h 1 gh, g G). (3) Let G be a finite group and suppose that H G. Prove that for any Sylow p-subgroup P of G, P H is a Sylow p-subgroup of H. (4) Show that a group of order 56 is not simple. Ring theory problems: (1) Let f : R S be a homomorphism of rings, I an ideal in R, and J an ideal in S. (a) Prove that f 1 (J) = {x R : f(x) J} is an ideal in R and that this ideal contains Ker f. (b) Prove that if f is surjective (i.e., onto), then f(i) = {f(y) : y I} is an ideal in S. Give an example to show that f(i) need not be an ideal in S when f is not surjective (i.e., onto). (2) A ring R is called Boolean if every element is idempotent, that is a 2 = a for all a R. Prove that every Boolean ring is commutative. (3) Let R be a commutative ring with unity and let P R be an ideal of R. Prove that the following are equivalent: i. P is prime. ii. For any ideals I, J in R, if IJ P then I P or J P. (4) Show that every prime element in an integral domain is irreducible. 1

9 2 Field theory problems: (1) Let E be a finite extension of the field F. Prove that E is algebraic over F. (2) Find the degree of the extension [F : Q], where F = Q( 1, 3). Is this a simple extension? Why or why not? (3) Prove or provide a counterexample: Galois extensions of Galois extensions are Galois extensions. That is, if K/F is Galois and L/K is Galois, then L/F is Galois. (4) Determine the Galois group of f(x) = x 4 5x over Q. (Recall that the Galois group of a separable polynomial f(x) F [x] is defined to be the Galois group of the splitting field of f(x) over F.)

10 Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville August 2014 Do 6 problems with at least 2 in each section. Group theory problems: (1) Let G be a group of order 56. Prove that G is not simple. (2) Suppose that G is a finite group with even order. Prove that the number of conjugacy classes in G with odd order is even. (3) Let H G. Prove that N G (H)/C G (H) is isomorphic to a subgroup of Aut(H). Recall the normalizer N G (H) = {g G ghg 1 = H} and the centralizer C G (H) = {g G gh = hg, h H}. Ring theory problems: (1) An integral domain D is said to be Artinian if for any descending chain of ideals I 1 I 2 I 3... of D, there is an integer n such that I i = I n for all i n. Prove that an integral domain D is Artinian if and only if it is a field. (2) Let R be a commutative ring with 1. Let e R be an idempotent element (that is, e 2 = e). Show that R = er (1 e)r. (3) A principal ideal ring is a ring in which every ideal has a single generator. (a) Prove that an element of a principal ideal domain is prime if and only if it is irreducible. (b) Provide an example of a principal ideal ring containing an element which is prime but not irreducible. Field theory problems: (1) Let E be a finite extension of the field F. Prove that E is algebraic over F. (2) Let a = and put E = Q(a). (a) Find the minimal polynomial of a over Q. (b) Find [E : Q]. (c) Identify the group Gal(E/Q), the set of all automorphisms of E which fix Q. (3) Let K be an extension field of F and let α K. Prove that if α is transcendental over F, then F (α) F (α 2 ). Provide an example showing the reverse implication false. 1

11 Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville January 2014 Do 7 problems with at least 2 in each section. Group theory problems: (1) There are five nonisomorphic groups of order 12 (you may take this as given). List them all, and show that the groups you listed are mutually nonisomorphic. (2) Let G be a group, and let [G, G] be the subgroup of G generated by {aba 1 b 1 : a, b G}. (So [G, G] is the commutator subgroup of G.) (a) Prove that [G, G] is a normal subgroup of G. (b) Let N be a normal subgroup of G. Prove that G/N is Abelian if and only if [G, G] N. (3) Suppose G is a group acting on a set X. For a X, let O(a) = {g(a) : g G}, the orbit of a under the action of G on X. Also, for a X, let G a = {g G : g(a) = a}, the stabilizer of a in G. Let O = {O(a) : a X} be the set of orbits, and let G/G a = {hg a : h G} be the set of all left cosets of G a in G. (Note that G/G a is not a group since G a is not in general a normal subgroup of G.) (a) Suppose that a X. Prove that G a is a subgroup of G. (b) Explicitly define a bijection φ from G/G a to O(a). You need to justify that φ is well defined, and is both one-to-one and onto. (c) Let G act on itself by conjugation. Suppose p is a prime number and G is a finite p-group, then G has a non-trivial center. Suggestion: By (b), it follows that the size of any orbit is a power of p. (4) Show that A 8 has no subgroup of index 7. (hint: use the fact that A 8 is a simple group) Ring theory problems: (1) Let R be a commutative ring with identity. For any two ideals I and J of R, let IJ be the ideal generated by all products xy where x I and y J. (a) Suppose that P is a prime ideal of R and that IJ P. Show that either I P or J P. (b) Suppose I + J = R, show that IJ = I J. (2) Let R be a ring (not necessarily commutative). Recall that a right ideal I of R is a non-empty subset of R that is closed under, and for all a R, Ra I. (This problem has two parts.) (a) Suppose that R is a ring with identity, and that the only right ideals of R are {0} and R. Prove that R is a division ring. (That is, each 1

12 2 non-0 element a R has a multiplicative inverse a 1.) (b) Suppose that R does not have an identity 1, and that the only right ideals of R are {0} and R. Prove that R has trivial multiplication; that is, for all a, b R, we have ab = 0. (3) Let D be the integral domain Z[ 5]. (a) Show that 2 is an irreducible element of D. (b) Show that 2 is not a prime element of D. (c) Show that D is not a UFD (unique factorization domain). Field theory problems (1) Let L C be a finite Galois extension of Q with Galois group G. Suppose that G has odd order. (a) Show that L R. (b) Is the converse true? In other words, if L R is a finite Galois extension of Q with the Galois group G, then must the order of G be odd? (c) Give an explicit example of an extension L of Q such that L has degree 3 over Q, L C, and L R. (2) Let K N be a field extension, with N algebraic over K. Suppose that L is a subring of N, and that K is contained in L. (a) Prove that L is a field. (b) Find a field extension A C such that C contains a subring B, B contains A, but B is not a field. Provide an element b B whose inverse is not contained in B. (3) Let q(x) = x (a) Show that q(x) is irreducible over Q. (Note that is not enough to state that q(x) has no real root.) (b) Determine the four roots of q(x) = x (c) Let S be the splitting field of q(x) = x over Q. Determine [S : Q]. Justify your answer. (d) Determine the Galois group of q(x) over Q. Justify your answer. (4) Let K F be a finite field extension and assume both fields are of prime characteristic p.

13 3 (a) Prove that [K p : F p ] = [K : F ]. Here F p := {a p : a F } and K p is defined similarly. (b) Use the above to show that K = K p if and only if F = F p. (that is, K is a perfect field if and only if F is perfect)

14 1 Groups Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville August 2012 Do any two problems from this section. 1. For any group G, we denote by G 2 the subgroup of G generated by squares of elements in G. In other words, G 2 = {x 2 : x G}. First, prove that G 2 is a normal subgroup of G. Second, prove that the factor group G/G 2 is abelian. 2. (a) If N is a normal subgroup of G and H is any subgroup of G, prove that NH := {nh n N, h H} is a subgroup of G. (b) Moreover, if H is also a normal subgroup of G, prove that NH is a normal subgroup of G. 3. If G is a group, Z its center, and if G/Z is cyclic, prove that G must be abelian. 4. Show that groups of order 105 are not simple. 2 Rings Do any two problems from this section. 1. Let F [x] be a ring polynomials where the coefficients belong to the field F. Prove that every ideal in F [x] is principal. Is this fact still true if we replace F with any commutative ring R? Justify your answer. 2. Consider the subring R = {a + bi 3 a, b Z} of C. (a) Show that the map ϕ : R Z, given by satisfies the property ϕ(a + bi 3) = a 2 + 3b 2, ϕ(uv) = ϕ(u)ϕ(v) for all u, v R. Show also that ϕ(u) > 3 for every u R, unless u {0, 1, 1}. (b) Use (a) to show that the only units of R are 1 and 1. (c) Use (a) to show that the elements 2, 1 + i 3, 1 i 3 of R are irreducible. Deduce that R is not a unique factorization domain.

15 3. Let D = { r s Q r, s Z, s odd, r, s relatively prime}. (a) Show that D is a subring of Q. (b) Describe the units of D. (c) Show that D is a principal ideal domain. 4. Let R be a commutative ring with 1. Let I 1, I 2,, I n be ideals of R such that I i + I j = R for any i j. Show that there exists an isomorphism 3 Fields n R/( I i ) = (R/I 1 ) (R/I n ). i=1 Do any two problems from this section. 1. Find the minimal polynomial of α = over the field Q of rational numbers, and prove it is the minimal polynomial. 2. (a) Show that the polynomial X 2 + X + 1 is irreducible over Z 2 = {0, 1}. (b) Explain why the ideal X 2 + X + 1 is a maximal ideal of Z 2 [X] and why K = Z 2 [X]/ X 2 + X + 1 is a field. (c) List all the elements of K. (d) Are there two distinct elements α, β of K such that α 2 = β? Explain. 3. Let G = Gal(Q( 3 5, i 3), Q). (a) How many elements are there in G? Describe all of them. (b) Give a proper nontrivial subgroup H of G. Describe the subfield K of Q( 3 5, i 3) consisting of the elements fixed by H. What is [Q( 3 5, i 3) : K]? 4. Assume both E/F and K/E are separable field extensions. Prove that K/F is separable.

16 Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville August 2011 This exam consists of three parts. For each part select two problems and work on them. If you work more problems, clearly indicate which should be graded. Please write on one side of each sheet of paper and put your name in the upper right hand corner. 1 Groups 1. Let Z denote the set of integers and let H denote the set of all 3 3 matrices of the form 1 x y 0 1 z x, y, z Z Prove that H is a noncommutative group under matrix multiplication. 2. Let H and K be normal subgroups of a group G. Assume that HK = G and H G = {1}. (a) Prove that hk = kh for any h H and k K. (b) Prove that G is isomorphic to H K. 3. Determine up to isomorphism all Abelian groups of order Let S n denote the group of permutations of the set {1, 2,... n}. Find all the subgroups of S 3 and determine which of them are normal in S 3. 1

17 2 Rings 1. Let f : R S be a homomorphism of rings, I an ideal in R, and J an ideal in S. (a) Prove that f 1 (J) = {x R : f(x) J} is an ideal in R and that this ideal contains Ker f. (b) Prove that if f is onto, then f(i) = {f(y) : y I} is an ideal in S. Give an example to show that f(i) need not be an ideal in S when f is not onto. 2. Show that every prime element in an integral domain is irreducible. 3. A commutative ring with 1 is said to be Artinian if for every descending chain of ideals I 1 I 2, there is an integer n such that I i = I n for all i n. Let R be an Artinian ring. (a) For every ideal I in R show that R/I is Artinian. (b) Show that every prime ideal in R is also a maximal ideal. 4. Consider the set Γ = {x + yi x, y Z} of complex numbers, called Gaussian integers. (a) Show that Γ is an integral domain. (b) Find all the units of Γ. (c) It is known that Γ is, in fact, a Euclidean domain, and hence a unique factorization domain. Does 13 = (2 + 3i)(2 3i) = (3 + 2i)(3 2i) contradict unique factorization in Γ? (d) Express 5 as a product of irreducible elements of Γ. 2

18 3 Fields 1. Prove that a finite integral domain is a field. 2. Let E denote the extension field Q( 2, 3 2) (a) Find [E : Q], the degree of the extension. (b) Prove that E = Q( ). (c) Let S be the smallest field containing E which is normal over Q. Find [S : E]. 3. (a) Show that the polynomial X 4 2 is irreducible over Q. (b) Find a factorization of X 4 2 into linear factors over C. Find the splitting field F of X 4 2. (c) Find the degree of F over Q. Justify. 4. (a) Explain why Q( 2, 3) is a normal extension of Q. (b) How many elements are there in the Galois group Gal(Q( 2, 3), Q)? Define all of them. (c) Which well-known group is Gal(Q( 2, 3), Q) isomorphic to? (d) Which subgroup of Gal(Q( 2, 3), Q) has Q( 6) as the corresponding fixed subfield of Q( 2, 3)? 3

19 Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville Summer Groups Do any two problems from this section. 1. Let K and H be groups. Consider the group K H, the direct product of K and H. Determine whether the statements below are true or false. If true, provide a proof; otherwise provide a specific counterexample. (a) If K and H are both cyclic, then K H is cyclic. (b) If K and H are both solvable, then K H is solvable. (c) If K and H are both simple, then K H is simple. 2. Let G be a group. (a) Prove that if H is a subgroup of G such that G : H = 2, then H is normal in G. (b) Prove that if G is infinite, then G has infinitely many subgroups. 3. Let G be a group under the operation. Let u be an element of G. Define a new binary operation on G as follows: a b = a u b, for all a, b G. (a) Prove that is associative. (b) Show that G contains an identity element under ; identify this element explicitly. (c) Show that every element of G contains an inverse under. (d) True or False: The groups (G, ) and (G, ) are isomorphic. Justify your answer by either exhibiting an isomorphism or giving a specific counterexample. 4. Show that, if f : G H is a surjective group homomorphism and K is a normal subgroup of G, then f(k) is a normal subgroup of H and H/f(K) = G/K. 1

20 2 Rings Do any two problems from this section. 1. Let F be a field. Answer the following; justify your answers. (a) Describe all maximal ideals of K[x]. (b) Describe all prime ideals of K[x]. (c) Give an example of some field K and some polynomial f K[x] such that K[x]/(f(x)) is not an integral domain. (d) Give an example of some field K and some polynomial f K[x] such that K[x]/(f(x)) is a field. 2. Answer all four parts of this question. (a) State Eisenstein s criterion for an integral domain R. (b) Prove that 2x 5 6x 3 + 9x 2 15 is irreducible in Q[x]. (c) Use the substitution y = x+1 to prove that, if p is a prime integer, then x p 1 + x p x + 1 is irreducible in Q[x]. (d) Prove or disprove that g(x, y) = x 3 +y 3 +1 is irreducible in C[x, y]. 3. Recall that Z[i] := {a + bi : a, b Z} and that Z[ 5] := {a + b 5 : a, b Z}. You may assume that both are integral domains. (a) What are the units of Z[i]? (b) What are the units of Z[ 5]? (c) Is 2 irreducible in Z[i]? Is 2 irreducible in Z[ 5]? (d) The ring Z[i] is a unique factorization domain, a fact you can assume is true. Notice that 5 = (2 + i)(2 i) = (1 + 2i)(1 2i). Does the existence of these two factorizations of 5 into a product of primes in Z[i] contradict that Z[i] is a Unique Factorization Domain? Why? 2

21 3 Fields Do any two problems from this section. 1. Let a and b be distinct prime numbers. (a) Prove that Q( a + b) = Q( a, b). [ (b) Using (a), prove that Q( a + ] b) : Q = Answer the following with your justification. (a) What is the splitting field K of x 8 1 over Q? (b) Find the minimal polynomial of K over Q? (c) Find Gal(K/Q)? 3. Let K be an extension of the field F. Show that the elements in K that are algebraic over F form a subfield of K. 4. Answer the following with your justification. (a) Exhibit an irreducible polynomial of degree 3 in Z 3 [x]. (b) Use (a) to prove that there exists a finite field with 27 elements. 3

22 Ph.D. Qualifying Examination in Algebra 1 Groups Department of Mathematics University of Louisville Spring 2008 Do any two problems from this section. 1. Show that if G is a cyclic group then every subgroup of G is cyclic. 2. Let G be a group and Z(G) be its center. Prove that if G/Z(G) is cyclic then G is Abelian. 3. Let H and N be subgroups of a group G with N normal. Prove that NH = HN and that this set is a subgroup of G. (Here NH = {nh n N, h H}, HN = {hn h H, n N}.) 1

23 2 Rings Do any two problems from this section. 1. Let Z be the ring of integers. Prove that the polynomial ring Z[x] is not a principal ideal domain. Is it a unique factorization domain? 2. Prove that every nonzero prime ideal in a principal ideal domain is a maximal ideal. 3. Show that a proper ideal M in a commutative ring R is maximal if and only if for every r R \ M there exists x R such that 1 rx M. 3 Fields Do any two problems from this section. 1. If F is a finite dimensional extension field of K, then prove that F is finitely generated and algebraic over K. 2. Let f(x) = x 4 2 over Q. (a) Find the splitting field K of f(x) over Q. (b) Find the Galois group G = Gal(K/Q). (c) Draw the diagram of the subgroups of G and the diagram of corresponding fixed fields. 3. Show that the polynomial g(x) = x 2 + x + 1 Z 2 [x] is irreducible in Z 2 [x]. Using this polynomial construct a field of four elements. Construct the addition and the multiplication table for this field. 2

24 Ph.D. Qualifying Examination in Algebra 1 Groups Department of Mathematics University of Louisville Summer 2008 Do any two problems from this section. 1. Show that a group of order 1000 = cannot be simple. 2. Let K be a normal subgroup of a group G and suppose that K has exactly two elements. Show that K is contained in the center Z(G) of G. 3. Prove that if H is a normal subgroup of a group G of prime index p, then for all K G either (i) K H or (ii) G = HK and K : K H = p. 1

25 2 Rings Do any two problems from this section. 1. Let R = C[0, 1] be the set of all continuous real-valued functions on [0, 1]. Define addition and multiplication on R as follows. For f, g R and x [0, 1], (f + g)(x) = f(x) + g(x) (fg)(x) = f(x)g(x) a) Show that R with these operations is a commutative ring with identity. b) Find the units of R. c) If f R and f 2 = f, then show that f = 0 R or f = 1 R. 2. In the ring Z of integers the following conditions on a nonzero ideal I are equivalent: (i) I is prime; (ii) I is maximal; (iii) I = (p) with p prime. 3. Consider the ring Z[i] of Gaussian integers. (a) Show that any prime p Z with p 3 (mod 4) is irreducible over Z[i]. (b) Prove that f(x) = x x x + 6 is irreducible over Z and over Z[i]. 2

26 3 Fields Do any two problems from this section. 1. Let F be a field. Prove that if [F (α) : F ] is odd then F (α) = F (α 2 ). 2. Let f(x) = x 4 5x over Q. (a) Find the splitting field K of f(x) over Q. (b) Find the Galois group G = Gal(K/Q). (c) Draw the diagram of the subgroups of G and the diagram of corresponding fixed fields. 3. Let F be a finite field with multiplicative identity 1 F. Prove that when you multiply all the nonzero elements of F, then the answer is 1 F. (Here 1 F is the additive inverse of 1 F.) 3

27 1 Groups Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville Spring 2007 Do any two problems from this section. 1. Classify up to isomorphism all groups of order Let H and N be subgroups of a group G with N normal. Prove that NH = HN and that this set is a subgroup of G. 3. Do two parts to receive full credit. (a) Let G be the multiplicative [ group ] of all nonsingular 2 [ 2 matrices ] with rational numbers. Show that g = has order 4 and h = has order 3, but that gh has infinite order. (b) Show that the additive group H = (Z/2Z) Z contains nonzero elements a, b of infinite order such that a + b is nonzero and has finite order. 4. Prove that every finitely generated subgroup of the additive group of rational numbers is cyclic. 2 Rings Do two problems from this section. One problem should include either Problem 1 or 2 (but not both). 1. The following is a well known fact: if K is a commutative ring with identity and I is an ideal of K, then K/I is a field if and only if I is a maximal ideal of K. Answer all the parts to receive full credit. (a) Find all the maximal ideals of Z. (You may use the fact that every ideal of Z is of the form nz.) (b) Determine whether the ideal (3, x) is a maximal ideal in Z[x]. (c) Determine whether the ideal (x) is a maximal ideal in Z[x].

28 2. Do all the parts to receive full credit. (a) Define prime ideal and maximal ideal in a commutative ring R with identity. (b) Let R and S be commutative rings with identities 1 R and 1 S, respectively, and let f : R S be a ring homomorphism such that f(1 R ) = 1 S. If P is a prime ideal of S show that f 1 (P ) is a prime ideal of R. (c) Let f be as in part (b). If M is a maximal ideal of S, is f 1 (M) a maximal ideal of R? Prove that it is or give a counter example. 3. Let {[ ] a b A = c d } : a + c = b + d, a, b, c, d Z. It is easy to see that A is a subring of M 2 (Z) (the ring of 2 2 matrices with elements from Z). Do the two parts to receive full credit. [ ] m 0 (a) Let R be the ring of 2 2 lower triangular matrices with elements from Z. n p Consider the map f : R A defined by ([ ]) [ ] m 0 m n m n p f =. n p n n + p Is f a homomorphism of rings? Justify the answer. (b) Are the rings R and A isomorphic? Explain. 4. Show that a proper ideal M in a commutative ring R is maximal if and only if for every r R \ M there exists x R such that 1 rx M. 3 Fields Do any two problems from this section. 1. Prove that Q( 3+ 5) = Q( 3, 5). Describe the lattice of subgroups of Gal(Q( 3, 5)/Q) and the lattice of subfields of Q( 3, 5). 2. Do all the parts to receive full credit. (i) Show that g = X 3 3X 1 is an irreducible polynomial over Q. (ii) It is known that there is a simple extension field Q(u) of Q such that u is a root of g and [Q(u) : Q] = 3. How is Q(u) defined? List the elements of a basis of Q(u) over Q. (iii) Show that there exists a splitting extension K of g with [K : Q] Prove that one of 2, 3 or 6 is a square in the finite field F p for every prime p. Conclude that the polynomial x 6 11x x 2 36 = (x 2 2)(x 2 3)(x 2 6) has a root modulo p for every prime p but has no root in Z.

29 Ph.D. Qualifying Examination in Algebra 1 Groups Department of Mathematics University of Louisville Spring 2006 Do any two problems from this section. 1. Let G be a group of order (a) Using the Sylow Theorems, show that G is Abelian. (b) Determine all isomorphism types of Abelian groups of order Let G be a finite group with exactly three conjucacy classes. Show that exactly one of the following possibilities holds: (a) G = 3. (b) G = 6 and G is non-abelian. 3. Prove that an infinite group is cyclic if and only if it is isomorphic to each of its proper non-trivial subgroups. 4. Let G be a finite group. (a) State the Class Equation for G in a form that involves the center Z(G) of G and the index of centralizers of elements of G. (b) Determine the conjugacy class of S 4 and their orders. Then verify the Class Equation, given in part (a), is valid for S 4. (c) Let p be a prime number and let G be a non-trivial p-group. Use the Class Equation to show that G has a non-trivial center. 5. Prove that if a group G is nilpotent, then it is solvable. 1

30 2 Rings Do any two problems from this section. 1. Do both. (a) Prove that if F is a field, then the polynomial ring F [x] is a Euclidean domain. That is, F [x] is an integral domain and there exists a function φ : F [x] {0} N such that if f, g F [x] and g 0, then there exist q, r F [x] such that f = qg + r with r = 0 or φ(r) < φ(g). (b) Prove or disprove that if R is an integral domain and A is a proper ideal of R, then R/A is an integral domain. 2. Do both. (a) Show that Z[i]/(3 + i) is isomorphic to Z/10Z. (b) Is (3 + i) a maximal ideal of Z[i]? Give a reason for your answer. 3. Let R, S be rings and let f : R S be a surjective ring homomorphism (that is, f(r) = S). Let Ker(f) be the kernel of f. Define f : R/Ker(f) S as follows: f (a + Ker(f)) = f(a). (a) Show that f is well-defined. (b) Show that f : R/Ker(f) S is a bijection. (c) Show that f : R/Ker(f) S is a ring homomorphism. 3 Fields Do two problems from this section with one of the problems being problem 3 1. Do both. [ ] a b (a) Prove that F = { : a, b Q} is a field under matrix 2b a addition and multiplication. (b) Prove that F is isomorphic to the field Q( 2). 2

31 2. Given f(x) = x 3 + x Z 2 [x]. (a) Show that f(x) is irreducible over Z 2. (b) Let F = Z 2 [x]/ f(x) and α be a zero of f(x) in F. Show that for each nonzero element β of F, there exists a polynomial q(x) Z 2 [x] of degree at most 2 such that β = q(α). 3. Consider p(x) = x Q[x]. Let S be the splitting field of p(x) Q[x] and assume S is contained in C, the field of complex numbers. You will be graded on the correctness and thoroughness of your explanations: provide relevant theorems, facts etc. to support each of your assertions. (a) Determine [S : Q], showing all details. (b) Determine Gal(S/Q), up to isomorphism. Provide thorough explanation, citing relevant theorems, facts, etc. to support your conclusions. (c) Explain why S contains exactly one subfield J, with Q J S, such that [J : Q] = 2. 3

32 Ph.D. Qualifying Examination in Algebra 1 Groups Department of Mathematics University of Louisville Spring 2005 Do any two problems from this section. 1. Prove that a group that has only a finite number of subgroups must be finite. [ ] a b 2. Prove that G = { : a, b real and a > 0} is a group under matrix 0 1 multiplication. Show that G contains a subgroup H such that H is isomorphic to the group R + of positive real numbers under multiplication. 3. Classify up to isomorphism all groups with 225 elements. 4. Do all four parts to receive full credit. Let G be a finite group. (a) What is a subnormal series for G? (b) What is a composition series for G? (c) What condition or conditions must G satisfy to be a solvable group? (d) Show that S 3 is solvable. 2 Rings Do any two problems from this section. 1. Let x 4 16 be an element of the polynomial ring E = Z[x] and use the bar notation to denote passage to the quotient ring Z[x]/(x 4 16). 1

33 (a) Find a polynomial of degree 3 that is congruent to 7x 13 11x 9 + 5x 5 2x modulo (x 4 16). (b) Prove that x 2 and x + 2 are zero divisors in E. 2. Prove that every Euclidean domain is a principal ideal domain (PID). 3. An integral domain D is said to be Artinian if for any descending chain of ideals I 1 I 2 I 3... of D, there is an integer n such that I i = I n for all i n. Prove that an integral domain D is Artinian if and only if it is a field. (Artinian rings are also referred to as rings satisfying the Descending Chain Condition.) 4. Prove the Third Isomorphism Theorem for Rings. That is prove that if R is a ring and I and J are ideals of R with I J, then R/J (R/I)/(J/I). 3 Fields Do two problems from this section with one of the problems being either 1 or Let p(x) Q[x] be a polynomial of degree 3. (a) Prove that if p(x) is irreducible, then its Galois group is isomorphic to either A 3 or S 3. (b) Provide an example of a degree three polynomial p(x) Q[x] whose Galois group is not isomorphic to A 3 or S Let q(x) = x 7 5 Q[x] and let S be a splitting field for q(x). (a) Determine [S : Q], the dimension of S over Q. (b) Determine two distinct subfields K and L of S such that K and L are properly contained in S and both properly contain Q. Demonstrate that K and L are not equal and that they are indeed proper and properly contain Q. 2

34 3. Let F be a finite field of characteristic p. Consider the Frobenius map g : F F, given by g(x) = x p for x F. (a) Prove that g is an automorphism of F. (b) Determine the set {α : g(α) = α}, providing complete justification for your solution. 4. Let E be a finite extension of F. Prove that E is algebraic over F. 3

35 Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville Spring Groups Do any two questions from this section. 1. Let G be a finite group of order 4q 2 where q 5 is a prime number. Prove that there is a normal subgroup of order q Prove that an abelian group has a composition series if and only if it is finite. 3. Determine up to isomorphism all the abelian groups of order Rings Do any two questions from this section. 4. Let Z[ 3] = {m + n 3 : m, n Z}. Prove that Z[ 3] is not a unique factorization domain. (Hint: You may want to use the function N(a) = m 2 + 3n 2 for all a = m + n 3 Z[ 3].) 5. Recall that an element a from a ring R is nilpotent if a n = 0 for some positive integer n. Let R be a ring with no nonzero nilpotent elements. Prove that if f(x) R[x] is a zero divisor, then there exists b R such that b f(x) = Prove the following result. In a commutative ring R with identity 1 R 0 an ideal P is prime if and only if R/P is an integral domain. 3 Fields Do any two questions from this section. 7. Let Q be the field of rational numbers. In the field of complex numbers C, prove that the subfields Q(i) and Q( 2) are isomorphic as vector spaces over Q, but not as fields. 8. Let f(x) = x 5 6x + 2 (a) Show that f(x) is irreducible over Q, and that in C, it has exactly three real roots. (For the last part you need calculus.) (ii) Deduce that if L is the splitting field of f over Q, G = Gal(L/Q), when identified with a subgroup of S 5, contains a 5-cycle and a 2-cycle. (This implies that G = S 5, but you don t need to prove this.) 9. Prove or disprove the following: (i) There are precisely p automorphisms of the field of order p. (ii) All fields with 49 elements are isomorphic. (iii) Q( 2, 3) is a simple extension of Q.

36 Ph.D. Qualifying Examination in Algebra 1 Groups Department of Mathematics University of Louisville Spring 2003 Do any three problems out of the set below. Specify clearly which problems are you attempting. 1. Let G be a simple group of order 168. How many elements are there in G of order 7? What can you say about the number of elements of order 7 in G if G is not simple? 2. Show that a group with precisely two conjugacy classes is simple. Find an example of such a group. 3. Determine up to isomorphism all the abelian groups of order Prove that if G is a finite group and H is a maximal normal subgroup then the quotient group G/H is simple. Prove that every finite group has a composition series. 5. Let G be a group of odd order, and let p be an odd prime that divides G. Prove that there are even number of elements of G of order p. Do any four problems out of the sets Rings and Fields below; do at least one problem in each set. Specify clearly which problems are you attempting. 2 Rings 1. Consider the ideals (3), (1 + i), (2) in the ring Z[i] of the Gaussian integers. (a) Are these ideals maximal? (b) Are these ideals prime? 1

37 (c) Which of the quotient rings are fields? (d) What are the characteristics of the fields above in 1c? 2. Let F be a field, and let S be the ring of all 2 2 matrices over F. Prove or disprove each statement: (a) The center Z(S) of S is {λi : λ F }, where I is the identity matrix. (b) Z(S) is an ideal of S. (c) Let A, B S, then (AB BA) 2 is in Z(S). 3. (a) Prove that if D is a unique factorization domain, then so is the polynomial ring D[x]. (b) Prove or disprove that Z[x] is a unique factorization domain. (c) Decide if the following equations contradict the above statements: x 1 = ( x 1)( x + 1) 12x 3 + 4x 2 = 4x 2 (3x + 1) = 2x(6x 2 + 2x) 3 Fields 1. Let K be a field and f K[x] be an irreducible separable polynomial of degree 2. Determine the Galois group G of f. 2. Find the splitting field F of f = x 4 + x over Q and calculate [F : Q]. 3. Given g = x 2 + x + 1 Z 2 [x], construct a field of four elements. Construct the addition and multiplication tables for this field. 2

38 Ph.D. Qualifying Examination in Algebra Department of Mathematics University of Louisville 17 th October 2003 The questions on this examination are presented in three sections - one section on Groups, one on Rings and one on Fields. You should attempt at least two questions from each section. If you attempt three questions from one section you will be awarded the scores on your best two questions from that section. 1 Groups Do any two questions from this section. 1. Let G be an abelian group. For any integer n > 0 show that the map φ : x x n is a homomorphism of G into G. Characterize the kernel of φ. Show that if G is finite and n is relatively prime to the order of G then φ is an isomorphism. Deduce that in this case, for each element g G there is a unique element x G such that g = x n. 2. For any group G the center Z(G) of G is the set of elements of G that commute with all elements of G, Z(G) = {x G xg = gx g G}. (a) Prove that if G/Z(G) is cyclic then G is abelian. (b) Suppose that G/Z(G) is abelian. Prove or disprove that G must also be abelian. 1

39 3. The Sylow theorems may be summarized as follows: If p n is the largest power of a prime p dividing the order of a finite group G then G has a subgroup of order p n. Any two such subgroups are conjugate in G and the number of such subgroups is conjugate to 1 (mod p) and divides their common index. (a) Let p and q be primes with p > q and let G be a group of order pq. Then G has a p Sylow subgroup P of order p and a q Sylow subgroup Q of order q. Prove that G = P Q. (b) Show that every group of order 56 has a proper normal subgroup. 2 Rings Do any two questions from this section. 1. (a) Let R be the ring given by defining new operations on the integers Z by letting m n = m + n 1 and m n = m + n m.n where + and. are the usual operations of addition and multiplication on Z. Define φ : Z R by φ(n) = 1 n. Prove that φ is an isomorphism. (b) Note that φ maps the identity element 1 of Z(+,.) to the identity element 0 of R(, ). Prove that if θ is a homomorphism of a commutative ring with identity S to an integral doman D then θ maps the identity element of S to the identity element of D. (c) Show that a homomorphism of a ring V with identity into a ring W with identity does not necessarily map the identity element of V to the identity element of W. 2. Let R be a commutative ring with ideals I, J such that I J R. It is well known that J/I is an ideal of R/I. (a) Show that the factor ring (R/I)/(J/I) is isomorphic to R/J. Hint: Define a homomorphism from R/I onto R/J and apply the fundamental homomorphism theorem for rings. (b) Using the result of Part (a), or otherwise, determine all prime ideals and all maximal ideals of Z n. 2

40 3. Let R be a ring with identity and let x R have a right inverse y and a left inverse z. Prove that y = z. Suppose that K is a ring with identity 1 and let x be an element of K that has a right inverse y but has no left inverse. Prove that the function φ : z y + zx 1 maps the set of right inverses of x into itself. Show that φ is one-to-one but not onto and deduce that if an element of a ring has more than one right inverse then it has an infinite number of right inverses. 3 Fields Do any two questions from this section. 1. Let K E F where E is a finite extension field of the field K and F is a finite extension field of the field E. It is well known that [F : K] = [F : E][E : K]. Using this result, or otherwise, prove that [Q( 2 + 3) :Q] = Construct a splitting field for the polynomial x 3 2 over Q. 3. Define φ : GF (2 2 ) GF (2 2 ) by φ(x) = x 2, for all x GF (2 2 ). (a) Show that φ is an isomorphism. (b) Choose an irreducible polynomial p(x) to represent GF (2 2 ) as Z 2 [x]/ < p(x) >. For your choice of p(x) give an explicit computation of φ(β) for each element β of Z 2 [x]/ < p(x) >. 3

### Algebra Ph.D. Entrance Exam Fall 2009 September 3, 2009 Algebra Ph.D. Entrance Exam Fall 2009 September 3, 2009 Directions: Solve 10 of the following problems. Mark which of the problems are to be graded. Without clear indication which problems are to be graded

### Algebra Exam, Spring 2017 Algebra Exam, Spring 2017 There are 5 problems, some with several parts. Easier parts count for less than harder ones, but each part counts. Each part may be assumed in later parts and problems. Unjustified

### 1. Group Theory Permutations. 1.1. Permutations. 1. Group Theory Problem 1.1. Let G be a subgroup of S n of index 2. Show that G = A n. Problem 1.2. Find two elements of S 7 that have the same order but are not conjugate. Let π S 7

### Algebra Qualifying Exam, Fall 2018 Algebra Qualifying Exam, Fall 2018 Name: Student ID: Instructions: Show all work clearly and in order. Use full sentences in your proofs and solutions. All answers count. In this exam, you may use the

### φ(xy) = (xy) n = x n y n = φ(x)φ(y) Groups 1. (Algebra Comp S03) Let A, B and C be normal subgroups of a group G with A B. If A C = B C and AC = BC then prove that A = B. Let b B. Since b = b1 BC = AC, there are a A and c C such that b =

### IUPUI Qualifying Exam Abstract Algebra IUPUI Qualifying Exam Abstract Algebra January 2017 Daniel Ramras (1) a) Prove that if G is a group of order 2 2 5 2 11, then G contains either a normal subgroup of order 11, or a normal subgroup of order

### ALGEBRA QUALIFYING EXAM SPRING 2012 ALGEBRA QUALIFYING EXAM SPRING 2012 Work all of the problems. Justify the statements in your solutions by reference to specific results, as appropriate. Partial credit is awarded for partial solutions.

### CSIR - Algebra Problems CSIR - Algebra Problems N. Annamalai DST - INSPIRE Fellow (SRF) Department of Mathematics Bharathidasan University Tiruchirappalli -620024 E-mail: algebra.annamalai@gmail.com Website: https://annamalaimaths.wordpress.com

### Math 553 Qualifying Exam. In this test, you may assume all theorems proved in the lectures. All other claims must be proved. Math 553 Qualifying Exam January, 2019 Ron Ji In this test, you may assume all theorems proved in the lectures. All other claims must be proved. 1. Let G be a group of order 3825 = 5 2 3 2 17. Show that

### ALGEBRA PH.D. QUALIFYING EXAM September 27, 2008 ALGEBRA PH.D. QUALIFYING EXAM September 27, 2008 A passing paper consists of four problems solved completely plus significant progress on two other problems; moreover, the set of problems solved completely

### SUMMARY ALGEBRA I LOUIS-PHILIPPE THIBAULT SUMMARY ALGEBRA I LOUIS-PHILIPPE THIBAULT Contents 1. Group Theory 1 1.1. Basic Notions 1 1.2. Isomorphism Theorems 2 1.3. Jordan- Holder Theorem 2 1.4. Symmetric Group 3 1.5. Group action on Sets 3 1.6. Graduate Preliminary Examination Algebra II 18.2.2005: 3 hours Problem 1. Prove or give a counter-example to the following statement: If M/L and L/K are algebraic extensions of fields, then M/K is algebraic. Algebra Qualifying Exam August 2001 Do all 5 problems. 1. Let G be afinite group of order 504 = 23 32 7. a. Show that G cannot be isomorphic to a subgroup of the alternating group Alt 7. (5 points) b.

### Exercises on chapter 1 Exercises on chapter 1 1. Let G be a group and H and K be subgroups. Let HK = {hk h H, k K}. (i) Prove that HK is a subgroup of G if and only if HK = KH. (ii) If either H or K is a normal subgroup of G

### Algebra Exam Syllabus Algebra Exam Syllabus The Algebra comprehensive exam covers four broad areas of algebra: (1) Groups; (2) Rings; (3) Modules; and (4) Linear Algebra. These topics are all covered in the first semester graduate

### RINGS: SUMMARY OF MATERIAL RINGS: SUMMARY OF MATERIAL BRIAN OSSERMAN This is a summary of terms used and main results proved in the subject of rings, from Chapters 11-13 of Artin. Definitions not included here may be considered

### Algebra Qualifying Exam Solutions. Thomas Goller Algebra Qualifying Exam Solutions Thomas Goller September 4, 2 Contents Spring 2 2 2 Fall 2 8 3 Spring 2 3 4 Fall 29 7 5 Spring 29 2 6 Fall 28 25 Chapter Spring 2. The claim as stated is false. The identity

### CHAPTER I. Rings. Definition A ring R is a set with two binary operations, addition + and CHAPTER I Rings 1.1 Definitions and Examples Definition 1.1.1. A ring R is a set with two binary operations, addition + and multiplication satisfying the following conditions for all a, b, c in R : (i)

### Ohio State University Department of Mathematics Algebra Qualifier Exam Solutions. Timothy All Michael Belfanti Ohio State University Department of Mathematics Algebra Qualifier Exam Solutions Timothy All Michael Belfanti July 22, 2013 Contents Spring 2012 1 1. Let G be a finite group and H a non-normal subgroup

### Math 429/581 (Advanced) Group Theory. Summary of Definitions, Examples, and Theorems by Stefan Gille Math 429/581 (Advanced) Group Theory Summary of Definitions, Examples, and Theorems by Stefan Gille 1 2 0. Group Operations 0.1. Definition. Let G be a group and X a set. A (left) operation of G on X is

### QUALIFYING EXAM IN ALGEBRA August 2011 QUALIFYING EXAM IN ALGEBRA August 2011 1. There are 18 problems on the exam. Work and turn in 10 problems, in the following categories. I. Linear Algebra 1 problem II. Group Theory 3 problems III. Ring

### 1 Rings 1 RINGS 1. Theorem 1.1 (Substitution Principle). Let ϕ : R R be a ring homomorphism 1 RINGS 1 1 Rings Theorem 1.1 (Substitution Principle). Let ϕ : R R be a ring homomorphism (a) Given an element α R there is a unique homomorphism Φ : R[x] R which agrees with the map ϕ on constant polynomials

### Algebra Prelim Notes Algebra Prelim Notes Eric Staron Summer 2007 1 Groups Define C G (A) = {g G gag 1 = a for all a A} to be the centralizer of A in G. In particular, this is the subset of G which commuted with every element

### ALGEBRA QUALIFYING EXAM PROBLEMS ALGEBRA QUALIFYING EXAM PROBLEMS Kent State University Department of Mathematical Sciences Compiled and Maintained by Donald L. White Version: August 29, 2017 CONTENTS LINEAR ALGEBRA AND MODULES General

### Algebraic structures I MTH5100 Assignment 1-10 Algebraic structures I For handing in on various dates January March 2011 1 FUNCTIONS. Say which of the following rules successfully define functions, giving reasons. For each one

### Modern Computer Algebra Modern Computer Algebra Exercises to Chapter 25: Fundamental concepts 11 May 1999 JOACHIM VON ZUR GATHEN and JÜRGEN GERHARD Universität Paderborn 25.1 Show that any subgroup of a group G contains the neutral

### Supplement. Dr. Bob s Modern Algebra Glossary Based on Fraleigh s A First Course on Abstract Algebra, 7th Edition, Sections 0 through IV. Glossary 1 Supplement. Dr. Bob s Modern Algebra Glossary Based on Fraleigh s A First Course on Abstract Algebra, 7th Edition, Sections 0 through IV.23 Abelian Group. A group G, (or just G for short) is

### SUMMARY OF GROUPS AND RINGS GROUPS AND RINGS III Week 1 Lecture 1 Tuesday 3 March. SUMMARY OF GROUPS AND RINGS GROUPS AND RINGS III 2009 Week 1 Lecture 1 Tuesday 3 March. 1. Introduction (Background from Algebra II) 1.1. Groups and Subgroups. Definition 1.1. A binary operation on a set

### School of Mathematics and Statistics. MT5836 Galois Theory. Handout 0: Course Information MRQ 2017 School of Mathematics and Statistics MT5836 Galois Theory Handout 0: Course Information Lecturer: Martyn Quick, Room 326. Prerequisite: MT3505 (or MT4517) Rings & Fields Lectures: Tutorials: Mon

### Outline of the course School of Mathematical Sciences PURE MTH 3007 Groups and Rings III, Semester 1, 2010 Outline of the course Week 1 Lecture 1 Monday 1 March 2010. 1. Introduction (Background from Algebra II) 1.1. Groups

### PROBLEMS FROM GROUP THEORY PROBLEMS FROM GROUP THEORY Page 1 of 12 In the problems below, G, H, K, and N generally denote groups. We use p to stand for a positive prime integer. Aut( G ) denotes the group of automorphisms of G.

### Sample algebra qualifying exam Sample algebra qualifying exam University of Hawai i at Mānoa Spring 2016 2 Part I 1. Group theory In this section, D n and C n denote, respectively, the symmetry group of the regular n-gon (of order 2n)

### Math 4400, Spring 08, Sample problems Final Exam. Math 4400, Spring 08, Sample problems Final Exam. 1. Groups (1) (a) Let a be an element of a group G. Define the notions of exponent of a and period of a. (b) Suppose a has a finite period. Prove that

### Group Theory. Ring and Module Theory Department of Mathematics University of South Florida QUALIFYING EXAM ON ALGEBRA Saturday, May 14, 016 from 9:00 am to 1:00 noon Examiners: Brian Curtin and Dmytro Savchuk This is a three hour examination.

### Chapter 3. Rings. The basic commutative rings in mathematics are the integers Z, the. Examples Chapter 3 Rings Rings are additive abelian groups with a second operation called multiplication. The connection between the two operations is provided by the distributive law. Assuming the results of Chapter

### 6]. (10) (i) Determine the units in the rings Z[i] and Z[ 10]. If n is a squarefree Quadratic extensions Definition: Let R, S be commutative rings, R S. An extension of rings R S is said to be quadratic there is α S \R and monic polynomial f(x) R[x] of degree such that f(α) = 0 and S

### ALGEBRA PH.D. QUALIFYING EXAM SOLUTIONS October 20, 2011 ALGEBRA PH.D. QUALIFYING EXAM SOLUTIONS October 20, 2011 A passing paper consists of four problems solved completely plus significant progress on two other problems; moreover, the set of problems solved

### Algebra Homework, Edition 2 9 September 2010 Algebra Homework, Edition 2 9 September 2010 Problem 6. (1) Let I and J be ideals of a commutative ring R with I + J = R. Prove that IJ = I J. (2) Let I, J, and K be ideals of a principal ideal domain.

### Rings. Chapter 1. Definition 1.2. A commutative ring R is a ring in which multiplication is commutative. That is, ab = ba for all a, b R. Chapter 1 Rings We have spent the term studying groups. A group is a set with a binary operation that satisfies certain properties. But many algebraic structures such as R, Z, and Z n come with two binary

### Page Points Possible Points. Total 200 Instructions: 1. The point value of each exercise occurs adjacent to the problem. 2. No books or notes or calculators are allowed. Page Points Possible Points 2 20 3 20 4 18 5 18 6 24 7 18 8 24 9 20 10

### Finite Fields. Sophie Huczynska. Semester 2, Academic Year Finite Fields Sophie Huczynska Semester 2, Academic Year 2005-06 2 Chapter 1. Introduction Finite fields is a branch of mathematics which has come to the fore in the last 50 years due to its numerous applications,

### ALGEBRA EXERCISES, PhD EXAMINATION LEVEL ALGEBRA EXERCISES, PhD EXAMINATION LEVEL 1. Suppose that G is a finite group. (a) Prove that if G is nilpotent, and H is any proper subgroup, then H is a proper subgroup of its normalizer. (b) Use (a)

### (Rgs) Rings Math 683L (Summer 2003) (Rgs) Rings Math 683L (Summer 2003) We will first summarise the general results that we will need from the theory of rings. A unital ring, R, is a set equipped with two binary operations + and such that

### The Galois group of a polynomial f(x) K[x] is the Galois group of E over K where E is a splitting field for f(x) over K. The third exam will be on Monday, April 9, 013. The syllabus for Exam III is sections 1 3 of Chapter 10. Some of the main examples and facts from this material are listed below. If F is an extension field

### MATH 3030, Abstract Algebra Winter 2012 Toby Kenney Sample Midterm Examination Model Solutions MATH 3030, Abstract Algebra Winter 2012 Toby Kenney Sample Midterm Examination Model Solutions Basic Questions 1. Give an example of a prime ideal which is not maximal. In the ring Z Z, the ideal {(0,

### Math 120 HW 9 Solutions Math 120 HW 9 Solutions June 8, 2018 Question 1 Write down a ring homomorphism (no proof required) f from R = Z[ 11] = {a + b 11 a, b Z} to S = Z/35Z. The main difficulty is to find an element x Z/35Z

### Algebra Review. Instructor: Laszlo Babai Notes by Vincent Lucarelli and the instructor. June 15, 2001 Algebra Review Instructor: Laszlo Babai Notes by Vincent Lucarelli and the instructor June 15, 2001 1 Groups Definition 1.1 A semigroup (G, ) is a set G with a binary operation such that: Axiom 1 ( a,

### List of topics for the preliminary exam in algebra List of topics for the preliminary exam in algebra 1 Basic concepts 1. Binary relations. Reflexive, symmetric/antisymmetryc, and transitive relations. Order and equivalence relations. Equivalence classes.

### THE JOHNS HOPKINS UNIVERSITY Faculty of Arts and Sciences FINAL EXAM - FALL SESSION ADVANCED ALGEBRA I. THE JOHNS HOPKINS UNIVERSITY Faculty of Arts and Sciences FINAL EXAM - FALL SESSION 2006 110.401 - ADVANCED ALGEBRA I. Examiner: Professor C. Consani Duration: take home final. No calculators allowed.

### Module MA3411: Abstract Algebra Galois Theory Michaelmas Term 2013 Module MA3411: Abstract Algebra Galois Theory Michaelmas Term 2013 D. R. Wilkins Copyright c David R. Wilkins 1997 2013 Contents 1 Basic Principles of Group Theory 1 1.1 Groups...............................

### Name: Solutions Final Exam Instructions. Answer each of the questions on your own paper. Be sure to show your work so that partial credit can be adequately assessed. Put your name on each page of your paper. 1. [10 Points] All of

### Quizzes for Math 401 Quizzes for Math 401 QUIZ 1. a) Let a,b be integers such that λa+µb = 1 for some inetegrs λ,µ. Prove that gcd(a,b) = 1. b) Use Euclid s algorithm to compute gcd(803, 154) and find integers λ,µ such that

### Problem 1. Let I and J be ideals in a ring commutative ring R with 1 R. Recall I. Take-Home Portion: Math 350 Final Exam Due by 5:00pm on Tues. 5/12/15 No resources/devices other than our class textbook and class notes/handouts may be used. You must work alone. Choose any 5 problems

### Math 430 Final Exam, Fall 2008 IIT Dept. Applied Mathematics, December 9, 2008 1 PRINT Last name: Signature: First name: Student ID: Math 430 Final Exam, Fall 2008 Grades should be posted Friday 12/12. Have a good break, and don t forget

### MATH 1530 ABSTRACT ALGEBRA Selected solutions to problems. a + b = a + b, MATH 1530 ABSTRACT ALGEBRA Selected solutions to problems Problem Set 2 2. Define a relation on R given by a b if a b Z. (a) Prove that is an equivalence relation. (b) Let R/Z denote the set of equivalence

### Algebra. Travis Dirle. December 4, 2016 Abstract Algebra 2 Algebra Travis Dirle December 4, 2016 2 Contents 1 Groups 1 1.1 Semigroups, Monoids and Groups................ 1 1.2 Homomorphisms and Subgroups................. 2 1.3 Cyclic Groups...........................

### NOTES ON FINITE FIELDS NOTES ON FINITE FIELDS AARON LANDESMAN CONTENTS 1. Introduction to finite fields 2 2. Definition and constructions of fields 3 2.1. The definition of a field 3 2.2. Constructing field extensions by adjoining

### 2 (17) Find non-trivial left and right ideals of the ring of 22 matrices over R. Show that there are no nontrivial two sided ideals. (18) State and pr MATHEMATICS Introduction to Modern Algebra II Review. (1) Give an example of a non-commutative ring; a ring without unit; a division ring which is not a eld and a ring which is not a domain. (2) Show that

### PRACTICE FINAL MATH , MIT, SPRING 13. You have three hours. This test is closed book, closed notes, no calculators. PRACTICE FINAL MATH 18.703, MIT, SPRING 13 You have three hours. This test is closed book, closed notes, no calculators. There are 11 problems, and the total number of points is 180. Show all your work.

### Field Theory Qual Review Field Theory Qual Review Robert Won Prof. Rogalski 1 (Some) qual problems ˆ (Fall 2007, 5) Let F be a field of characteristic p and f F [x] a polynomial f(x) = i f ix i. Give necessary and sufficient conditions

### Algebra Exam Fall Alexander J. Wertheim Last Updated: October 26, Groups Problem Problem Problem 3... Algebra Exam Fall 2006 Alexander J. Wertheim Last Updated: October 26, 2017 Contents 1 Groups 2 1.1 Problem 1..................................... 2 1.2 Problem 2..................................... 2

### COURSE SUMMARY FOR MATH 504, FALL QUARTER : MODERN ALGEBRA COURSE SUMMARY FOR MATH 504, FALL QUARTER 2017-8: MODERN ALGEBRA JAROD ALPER Week 1, Sept 27, 29: Introduction to Groups Lecture 1: Introduction to groups. Defined a group and discussed basic properties

### TEST CODE: PMB SYLLABUS TEST CODE: PMB SYLLABUS Convergence and divergence of sequence and series; Cauchy sequence and completeness; Bolzano-Weierstrass theorem; continuity, uniform continuity, differentiability; directional

### Modern Algebra (MA 521) Synopsis of lectures July-Nov 2015 semester, IIT Guwahati Modern Algebra (MA 521) Synopsis of lectures July-Nov 2015 semester, IIT Guwahati Shyamashree Upadhyay Contents 1 Lecture 1 4 1.1 Properties of Integers....................... 4 1.2 Sets, relations and

### CHAPTER 14. Ideals and Factor Rings CHAPTER 14 Ideals and Factor Rings Ideals Definition (Ideal). A subring A of a ring R is called a (two-sided) ideal of R if for every r 2 R and every a 2 A, ra 2 A and ar 2 A. Note. (1) A absorbs elements

### 1 Finite abelian groups Last revised: May 16, 2014 A.Miller M542 www.math.wisc.edu/ miller/ Each Problem is due one week from the date it is assigned. Do not hand them in early. Please put them on the desk in front of the room

### Honors Algebra 4, MATH 371 Winter 2010 Assignment 4 Due Wednesday, February 17 at 08:35 Honors Algebra 4, MATH 371 Winter 2010 Assignment 4 Due Wednesday, February 17 at 08:35 1. Let R be a commutative ring with 1 0. (a) Prove that the nilradical of R is equal to the intersection of the prime

### Finite Fields. Sophie Huczynska (with changes by Max Neunhöffer) Semester 2, Academic Year 2012/13 Finite Fields Sophie Huczynska (with changes by Max Neunhöffer) Semester 2, Academic Year 2012/13 Contents 1 Introduction 3 1 Group theory: a brief summary............................ 3 2 Rings and fields....................................

### School of Mathematics and Statistics. MT5824 Topics in Groups. Problem Sheet I: Revision and Re-Activation MRQ 2009 School of Mathematics and Statistics MT5824 Topics in Groups Problem Sheet I: Revision and Re-Activation 1. Let H and K be subgroups of a group G. Define HK = {hk h H, k K }. (a) Show that HK

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

### Algebraic Structures Exam File Fall 2013 Exam #1 Algebraic Structures Exam File Fall 2013 Exam #1 1.) Find all four solutions to the equation x 4 + 16 = 0. Give your answers as complex numbers in standard form, a + bi. 2.) Do the following. a.) Write

### Rings. Chapter Definitions and Examples Chapter 5 Rings Nothing proves more clearly that the mind seeks truth, and nothing reflects more glory upon it, than the delight it takes, sometimes in spite of itself, in the driest and thorniest researches

### RUDIMENTARY GALOIS THEORY RUDIMENTARY GALOIS THEORY JACK LIANG Abstract. This paper introduces basic Galois Theory, primarily over fields with characteristic 0, beginning with polynomials and fields and ultimately relating the

### Some practice problems for midterm 2 Some practice problems for midterm 2 Kiumars Kaveh November 14, 2011 Problem: Let Z = {a G ax = xa, x G} be the center of a group G. Prove that Z is a normal subgroup of G. Solution: First we prove Z is

### University of Ottawa University of Ottawa Department of Mathematics and Statistics MAT3143: Ring Theory Professor: Hadi Salmasian Final Exam April 21, 2015 Surname First Name Instructions: (a) You have 3 hours to complete

### THE JOHNS HOPKINS UNIVERSITY Faculty of Arts and Sciences FINAL EXAM - SPRING SESSION ADVANCED ALGEBRA II. THE JOHNS HOPKINS UNIVERSITY Faculty of Arts and Sciences FINAL EXAM - SPRING SESSION 2006 110.402 - ADVANCED ALGEBRA II. Examiner: Professor C. Consani Duration: 3 HOURS (9am-12:00pm), May 15, 2006. No

### Math 2070BC Term 2 Weeks 1 13 Lecture Notes Math 2070BC 2017 18 Term 2 Weeks 1 13 Lecture Notes Keywords: group operation multiplication associative identity element inverse commutative abelian group Special Linear Group order infinite order cyclic

### Fields and Galois Theory. Below are some results dealing with fields, up to and including the fundamental theorem of Galois theory. Fields and Galois Theory Below are some results dealing with fields, up to and including the fundamental theorem of Galois theory. This should be a reasonably logical ordering, so that a result here should

### Homework problems from Chapters IV-VI: answers and solutions Homework problems from Chapters IV-VI: answers and solutions IV.21.1. In this problem we have to describe the field F of quotients of the domain D. Note that by definition, F is the set of equivalence

### MATH 431 PART 2: POLYNOMIAL RINGS AND FACTORIZATION MATH 431 PART 2: POLYNOMIAL RINGS AND FACTORIZATION 1. Polynomial rings (review) Definition 1. A polynomial f(x) with coefficients in a ring R is n f(x) = a i x i = a 0 + a 1 x + a 2 x 2 + + a n x n i=0

### ABSTRACT ALGEBRA MODULUS SPRING 2006 by Jutta Hausen, University of Houston ABSTRACT ALGEBRA MODULUS SPRING 2006 by Jutta Hausen, University of Houston Undergraduate abstract algebra is usually focused on three topics: Group Theory, Ring Theory, and Field Theory. Of the myriad

### MATH 113 FINAL EXAM December 14, 2012 p.1 MATH 113 FINAL EXAM December 14, 2012 This exam has 9 problems on 18 pages, including this cover sheet. The only thing you may have out during the exam is one or more writing utensils. You have 180

### FIELD THEORY. Contents FIELD THEORY MATH 552 Contents 1. Algebraic Extensions 1 1.1. Finite and Algebraic Extensions 1 1.2. Algebraic Closure 5 1.3. Splitting Fields 7 1.4. Separable Extensions 8 1.5. Inseparable Extensions

### AN INTRODUCTION TO GALOIS THEORY AN INTRODUCTION TO GALOIS THEORY STEVEN DALE CUTKOSKY In these notes we consider the problem of constructing the roots of a polynomial. Suppose that F is a subfield of the complex numbers, and f(x) is

### INTRODUCTION TO THE GROUP THEORY Lecture Notes on Structure of Algebra INTRODUCTION TO THE GROUP THEORY By : Drs. Antonius Cahya Prihandoko, M.App.Sc e-mail: antoniuscp.fkip@unej.ac.id Mathematics Education Study Program Faculty of Teacher

### 22M: 121 Final Exam. Answer any three in this section. Each question is worth 10 points. 22M: 121 Final Exam This is 2 hour exam. Begin each question on a new sheet of paper. All notations are standard and the ones used in class. Please write clearly and provide all details of your work. Good

### Algebra SEP Solutions Algebra SEP Solutions 17 July 2017 1. (January 2017 problem 1) For example: (a) G = Z/4Z, N = Z/2Z. More generally, G = Z/p n Z, N = Z/pZ, p any prime number, n 2. Also G = Z, N = nz for any n 2, since

### Galois Theory, summary Galois Theory, summary Chapter 11 11.1. UFD, definition. Any two elements have gcd 11.2 PID. Every PID is a UFD. There are UFD s which are not PID s (example F [x, y]). 11.3 ED. Every ED is a PID (and

### August 2015 Qualifying Examination Solutions August 2015 Qualifying Examination Solutions If you have any difficulty with the wording of the following problems please contact the supervisor immediately. All persons responsible for these problems,

### Algebra Exercises in group theory Algebra 3 2010 Exercises in group theory February 2010 Exercise 1*: Discuss the Exercises in the sections 1.1-1.3 in Chapter I of the notes. Exercise 2: Show that an infinite group G has to contain a non-trivial

### GALOIS THEORY. Contents GALOIS THEORY MARIUS VAN DER PUT & JAAP TOP Contents 1. Basic definitions 1 1.1. Exercises 2 2. Solving polynomial equations 2 2.1. Exercises 4 3. Galois extensions and examples 4 3.1. Exercises. 6 4.

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

### Math 120. Groups and Rings Midterm Exam (November 8, 2017) 2 Hours Math 120. Groups and Rings Midterm Exam (November 8, 2017) 2 Hours Name: Please read the questions carefully. You will not be given partial credit on the basis of having misunderstood a question, and please

### Practice problems for first midterm, Spring 98 Practice problems for first midterm, Spring 98 midterm to be held Wednesday, February 25, 1998, in class Dave Bayer, Modern Algebra All rings are assumed to be commutative with identity, as in our text.

### Math 581 Problem Set 3 Solutions Math 581 Problem Set 3 Solutions 1. Prove that complex conjugation is a isomorphism from C to C. Proof: First we prove that it is a homomorphism. Define : C C by (z) = z. Note that (1) = 1. The other properties

### 1 First Theme: Sums of Squares I will try to organize the work of this semester around several classical questions. The first is, When is a prime p the sum of two squares? The question was raised by Fermat who gave the correct answer Rings and Fields Theorems Rajesh Kumar PMATH 334 Intro to Rings and Fields Fall 2009 October 25, 2009 12 Rings and Fields 12.1 Definition Groups and Abelian Groups Let R be a non-empty set. Let + and (multiplication) Page 1 Definitions Tuesday, May 8, 2018 12:23 AM Notations " " means "equals, by definition" the set of all real numbers the set of integers Denote a function from a set to a set by Denote the image of