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104 Algebra Qualifying Exam August 2001 Do all 5 problems. 1. Let G be afinite group of order 504 = a. Show that G cannot be isomorphic to a subgroup of the alternating group Alt 7. (5 points) b. If G is simple, determine the number of Sylow 3-subgroups of G. (5points) 2. Let R be acommutative ring with 1 and let M be a maximal ideal of R. a. Show that the ring R/M 2 has no idempotents other than 0 and 1. (4 points) b. We know that M/M 2 is naturally an R/M-module. If R is Noetherian, prove that this module is finitely generated. (2 points) c. Finally, assume that R = K[x 1,x 2,...,x t ]isapolynomial ring in finitely many variables over the field K. Prove that dim K (R/M 2 ) <. (4points) 3. Let F E be fields and suppose 0 α E with E = F [α]. Assume that some power of α lies in F and let n be the smallest positive integer such that α n F. a. If α m F with m>0, show that m is a multiple of n. (2points) b. If E is a separable extension of F, prove that the characteristic of F does not divide n. (4 points) c. If every root of unity in E lies in F, show that E : F = n. (4points) 4. Let A be a real n n matrix. We say that A is a difference of two squares if there exist real n n matrices B and C with BC = CB =0and A = B 2 C 2. a. If A is a diagonal matrix, show that it is a difference of two squares. (3 points) b. If A is a symmetric matrix that is not necessarily diagonal, again show that it is a difference of two squares. (3 points) c. Suppose A is a difference of two squares, with corresponding matrices B and C as above. If B has a nonzero real eigenvalue, prove that A has a positive real eigenvalue. (4 points) 5. Let K be afield of characteristic 0 and view the polynomial ring V = K[x] asa K-vector space. Let M: V V be the linear operator given by multiplication by x, sothat M(x n )=x n+1 for all integers n 0. In addition, let D: V V be the linear operator given by differentiation with respect to x, sothat D(x n )=nx n 1 for all n 0. Let L denote the set of all linear operators of the form M i D j with i, j 0, where M 0 = D 0 = I is the identity operator on V. a. Prove that DM MD = I. (3points) b. Show that L is a K-linearly independent set. (4 points) c. For all nonnegative integers t, prove that DM t is in the K-linear span of the set L. (3 points)

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112 Algebra Qualifying Exam January 2004 Do all 5 problems. 1. Let G be a finite group and let H G be a subgroup of index G : H = n. a. Show that H : (H H g ) n for all g G. (2 points) b. If H is a maximal subgroup of G and H is abelian, show that (H H g ) G for all g / H. (3 points) c. Now suppose that G is simple. If H is abelian and n is a prime, prove that H = 1. (5 points) 2. Let K be a field and let R be the subring of the polynomial ring K[X] given by all polynomials with X-coefficient equal to 0. a. Prove that the elements X 2 and X 3 are irreducible but not prime in the ring R. (5 points) b. Show that R is a Noetherian ring, and that the ideal I of R consisting of all polynomials in R with constant term 0 is not principal. (5 points) 3. Recall that a field K is algebraically closed if every polynomial f K[X] splits over K (is a product of linear factors in K[X]). Now let F E be an algebraic field extension. a. If every polynomial f(x) F [X] splits over E, prove that E is algebraically closed. (4 points) b. If every polynomial f(x) F [X] has a root in E and if F has characteristic 0, prove that E is algebraically closed. (6 points) 4. Let V be a finite dimensional vector space over the field F. Suppose T : V V is a linear operator and let f(x) F [X] be its minimal polynomial. a. If f(x) has a nonconstant polynomial factor of degree m, show that V has a nonzero subspace W of dimension m with T (W ) W. (5 points) b. Conversely, if V has a nonzero subspace W of dimension n with T (W ) W, show that f(x) has a nonconstant polynomial factor of degree n. (5 points) 5. Let R be a ring with 1 and let V be a right R-module. Suppose that V = X +Y is the internal direct sum of the two nonzero submodules X and Y. a. Show that 0, X, Y and V are the only R-submodules of V if and only if X and Y are nonisomorphic simple R-modules. (6 points) b. If X and Y are nonisomorphic simple R-modules, prove that End R (V ), the ring of R-endomorphisms of V, is isomorphic to the direct sum of two division rings. (4 points)

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114 Algebra Qualifying Exam - January 2005 Do all 5 problems. Show all work. 1. Let G be a finite group with G = 660 = and suppose that E G is a subgroup of order 11. Assume that C G (E) = E. (a) Prove that N G (E) = 55. (3 points) (b) If M G, show that either E M or M 1 mod 11. (3 points) (c) Show that every minimal normal subgroup of G contains E. (4 points) 2. All rings in this problem are commutative with 1. A ring S is said to be finitely generated if there exist finitely many elements s 1, s 2,..., s n S such that every element of S can be written as a sum of products of these generators. Now let R be a ring, let G be a finite group of automorphisms of R, and let R G = {r R r g = r for all g G} be the fixed subring. (a) If r R, prove that R G contains a finitely generated subring T such that r is integral over T. (4 points) (b) If R is finitely generated, show that R G contains a finitely generated subring S such that R is integral over S. (2 points) (c) Let R and S be as in (b). Deduce that R is a finitely generated S-module and hence that R G is a finitely generated S-module. Conclude that R G is a finitely generated ring. (Hint. You can use the fact that any finitely generated ring is a homomorphic image of a polynomial ring in finitely many variables over the integers and hence is a Noetherian ring.) (4 points) 3. Let F be a field and let f(x) F [X] be an irreducible polynomial. Suppose E F is an extension field of F containing a root α of f(x) satisfying f(α 2 ) = 0. Show that f(x) splits over E. (10 points) 4. Let F be an algebraically closed field and let M n (F ) be the ring of n n matrices over F. Describe those matrices X M n (F ) with the property that all matrices that commute with X are diagonalizable. (10 points) 5. An additive abelian group U is said to be uniform if, for every two nonzero subgroups X and Y, we have X Y 0. Let us also say that U is max-uniform if U is uniform and if U is not contained in any properly larger uniform group. (a) If U is uniform and has a nonzero element of finite order, show that there exists a prime p such that every element of U has order a power of p. (3 points) (b) Let A be an abelian group and let U be a uniform subgroup. Suppose M is a subgroup of A maximal with the property that M U = 0. Show that A/M is a uniform group. (3 points) (c) Let A be an abelian group and let U be a max-uniform subgroup. Prove that there exists a subgroup M of A with A = U +M, the internal direct sum of U and M. Include details of the Zorn s Lemma argument. (4 points)

115 Algebra Qualifying Exam - January 2006 Do all 5 problems. Show all work. 1. Let A, B and K be minimal normal subgroups of the group G with K A, K B and K AB. (a) Show that KA = AB = KB. (4 points) (b) Prove that A = K = B. (3 points) (c) Show that AB is abelian. (3 points) 2. Let Z[x] be the polynomial ring over the integers Z in the indeterminant x. Let R be the subring of Z[x] consisting of all polynomials having their coefficients of x and x 2 equal to 0. (a) Prove that Q(x) is the field of fractions of R, where Q is the field of rational numbers. (2 points) (b) Find the integral closure of R in Q(x). (4 points) (c) Does there exist a polynomial g(x) R such that R is generated as a ring by 1 and g(x)? (4 points) 3. Let n be a positive integer and let F be a field of characteristic not dividing n. Let f(x) F [x] be the polynomial x n a for some 0 a F and let E be a splitting field for f(x) over F. (a) Show that E contains a primitive nth root of unity ε. (3 points) (b) If ε F, show that all irreducible factors of f(x) in F [x] have the same degree and that E : F divides n. (3 points) (c) Now assume that n is a power of 2, but do not assume that ε F. Prove that E : F is a power of 2. (4 points) 4. Let V be a finite-dimensional vector space over the real numbers R. (a) If dim R V is odd, prove that every linear operator A: V V has at least one real eigenvalue. (3 points) (b) Suppose A 1, A 2,..., A n are finitely many pairwise commuting linear operators on V. Assume that none of the operators A i has a negative real eigenvalue. If the sum A 1 + A A n is equal to the negative of the identity operator on V, show that dim R V is even. (Hint. Use induction on the dimension of V.) (7 points) 5. Let R be a ring with 1 and let M be a right R-module. We say that the module M has property ( ) if every nonzero homomorphic image of M has a simple submodule. (a) If M is generated by its artinian submodules, show that M has property ( ). (5 points) (b) If M has property ( ) and is noetherian, show that it is artinian. (5 points)

116 Algebra Qualifying Exam August 2006 Do all 5 problems. 1. Let M be a minimal normal subgroup of the finite group G and let N/M be a nontrivial normal subgroup of G/M. Assume that M is a p-group and that N/M is a q-group for some primes p and q, not necessarily distinct. a. Show that G = M H where H is a subgroup of G having a nontrivial normal q-subgroup. (4 points) b. If M is self-centralizing in G, prove that p # q. (3 points) c. If M is self-centralizing and if H is as in part a, prove that M n H = 1. (3 points) 2. Let R be a ring with 1, not necessarily commutative. Recall that an element e of R is an idempotent if e2 = e, and an element 0 # r E R is a zero divisor if there exists 0 # s E R with rs = 0 or sr = 0. Now assume that R has a nil ideal N such that R/N has no zero divisors. a. Show that the only idempotents of R are the elements 0 and 1. (5 points) b. If R/N is a division ring, prove that every zero divisor in R is nilpotent. (5 points) 3. Let C > E > K > Q be a chain of fields, where C is the field of complex numbers, Q is the field of rational numbers, E = Q[a] with an E Q, and K is generated by all roots of unity in E. Assume that E is a Galois extension of Q. a. Show that the Galois group Gal(E/K) is cyclic. (5 points) b. If the restriction T of complex conjugation to E is in the center of Gal(E/Q), prove that lai2 E Q, where I ( denotes complex absolute value. (5 points) 4. Let V # 0 be a finite dimensional vector space over a field F and let T: V + V be a linear transformation. We say that T is regular if its characteristic polynomial and minimal polynomial are equal. a. If there exists a vector v E V such that V is spanned by v, T(v), T2(v),..., prove that T is regular. (5 points) b. Assume that T is regular and let W be a subspace of V with T(W) C W. Show that Tw, the restriction of T to W, and Tvlw, the induced action of T on V/W, are both regular. (5 points) 5. Let F = GF(q) be the finite field with q elements and let M2(F) be the ring of 2 x 2 matrices over F. a. If A E M2(F) has equal eigenvalues in the algebraic closure of F, show that the eigenvalues of A actually belong to F. (4 points) b. Determine the number of nonzero nilpotent matrices in M2(F) as a function of q. (Hint. Use Jordan canonical form and note that the group G of invertible 2 x 2 matrices over F has order (q2-1) (q2 - q).) (6 points)

117 Algebra Qualifying Exam January 2007 Do all 5 problems. 1. Let G be a finite group and let Syl p (G) denote its set of Sylow p-subgroups. a. Suppose that S and T are distinct members of Syl p (G) chosen so that S T is maximal among all such intersections. Prove that the normalizer N G (S T ) has more than one Sylow p-subgroup. (5 points) b. Show that S T = 1 for all S, T Syl p (G), with T S, if and only if N G (P ) has exactly one Sylow p-subgroup for every nonidentity p-subgroup P of G. (5 points) 2. Let R be a commutative, Noetherian integral domain. a. If P is a prime ideal of R, show that the radical of P n is P. (2 points) b. If R has a unique nonzero prime ideal P, prove that all ideals of R are primary. (3 points) c. Conversely, let us now assume that all ideals of R are primary, and let P and Q be distinct prime ideals of R with Q P. Since P n Q is primary, deduce first that P n Q and then that Q = 0. (Hint. Consider whether the intersection P n Q can be irredundant.) (5 points) 3. Let F be a field of characteristic 0 and let f F [X] be an irreducible polynomial of degree > 1 with splitting field E F. Define Ω = {α E f(α) = 0}. a. Let α Ω and let m be a positive integer. If g F [X] is the minimal polynomial of α m over F, show that {β m β Ω} is the set of roots of g. (3 points) b. Now fix α Ω and suppose that αr Ω for some r F. Show that, for all β Ω and integers i 0, we have βr i Ω. Conclude that r is a root of unity. (3 points) c. If α and r are as in (b) and if m is the multiplicative order of the root of unity r, show that f(x) = g(x m ), where g is the minimal polynomial of α m over F. (4 points) 4. Let V be a finite dimensional vector space over a field K and assume that V is endowed with a not necessarily symmetric bilinear form, : V V K. We Let R and L denote the right and left radicals of, given by R = {x V V, x = 0} and L = {x V x, V = 0}, so that these are both subspaces of V. a. Use the bilinear form to construct a linear transformation T from V to the dual space (V/R) of V/R such that ker(t ) = L. (6 points) b. Show that dim K L = dim K R, and deduce that the map T is surjective. (4 points) 5. Let A be an additive abelian group and let B be a subgroup. We say that B is essential in A, and write B ess A, if and only if B X 0 for all nonzero subgroups X of A. a. If B 1 ess A 1 and B 2 ess A 2, prove that (B 1 B 2 ) ess (A 1 A 2 ). (5 points) b. If B ess A, and B has no nonzero elements of finite order, prove that A has no nonzero elements of finite order. (2 points) c. Let Q denote the additive group of rational numbers and suppose that Q ess A, for some abelian group A. Prove that Q = A. (3 points)

118 Algebra Qualifying Exam August 2007 Do all 5 problems. 1. Let G be a finite group of order G = 504 = a. If G has a normal subgroup N of order 8, show that G has at most 8 Sylow 7-subgroups, that is Syl 7 (G) 8. (5 points) b. If Syl 7 (G) 8, prove that G has an element of order 21. (4 points) c. If G is isomorphic to a subgroup of Sym 9, the symmetric group of degree 9, show that G cannot have a normal subgroup of order 8. (1 point) 2. Let R be a commutative integral domain with field of fractions F, and assume that R is integrally closed. a. Suppose K is a field containing F and let α K be integral over R. Show that the minimal monic polynomial of α over F is contained in R[x]. (5 points) b. Let f(x) R[x] be a monic polynomial. Show that f(x) is irreducible in R[x] if and only if it is irreducible in F [x]. (5 points) 3. Let F be a field of characteristic 0 and let E be a finite Galois extension of F. a. If 0 α E with E = F [α], show that F [α 2 ] E if and only if there exists an automorphism σ Gal(E/F ) with α σ = α. (6 points) b. Prove that there exists an element α E with E = F [α 2 ]. (4 points) 4. Let V be a finite-dimensional vector space over the field F with dim F V = n, and let (, ): V V F be a symmetric bilinear form. If X is a subset of V, write X = {v V (X, v) = 0} for the subspace of V perpendicular to X. a. If W is a subspace of V, show that dim F W + dim F W dim F V. (Hint. If w W, note that {w} has codimension 1 in V.) (2 points) b. Now suppose (, ) is nonsingular, so that V = 0. If A = {a 1, a 2,..., a n } is a basis for V, prove that there exists a unique dual basis A = {a 1, a 2,..., a n}. That is, A is a basis with (a i, a j ) = 0 if i j and (a i, a i ) = 1. (4 points) c. Again suppose (, ) is nonsingular, and let B = {b 1, b 2,..., b n } be a second basis for V with dual basis B = {b 1, b 2,..., b n}. Compare the change of basis matrix from A to B with the change of basis matrix from B to A. (4 points) 5. Let R be a not necessarily commutative ring with 1. a. If V 1, V 2,..., V n are n nonisomorphic irreducible right R-modules, show that there exists an R-module epimorphism from R, viewed as a right R-module, to the external direct sum V 1 V 2 V n. (5 points) b. Suppose R, viewed as a right R-module, has a finite composition series with nonisomorphic composition factors. Prove that the Jacobson radical of R is equal to 0. (5 points)

119 Algebra Qualifying Exam January 2008 Do all 5 problems. 1. Let G be a finite nonabelian group with center Z. a. If G/Z is a p-group, for some prime p, show that G has a normal Sylow p- subgroup and that p divides Z. (5 points) b. If G/Z is solvable, show that G has a nonidentity normal p-subgroup for some prime dividing G : Z. (5 points) 2. Let R S be commutative rings with the same 1, and suppose that S is finitely generated as an R-module. a. If an element r R is not invertible in R, prove that it is not invertible in S. HINT. If r is invertible in S, consider a polynomial in R[X] having 1/r as a root. (5 points) b. If the ideals of R satisfy the ascending chain condition, show that the ideals of S satisfy the ascending chain condition. (5 points) 3. Working in the field of complex numbers, let ε be a primitive 16 th root of unity, and let α = ε 2. Set E = Q[ε], where Q is the field of rational numbers, let f(x) = X Q[X], and note that α is a root of f(x). a. Show that 2 Q[ε 2 ]. (3 points) b. Conclude that f(x) splits in E[X]. (2 points) c. If G = Gal(E/Q), prove that no nonidentity element of G fixes α. Conclude that f(x) is irreducible in Q[X]. (5 points) 4. Let V be a finite-dimensional vector space over the field F of characteristic p > 0, let T : V V be a linear operator on V, and set W = {v V vt = v}. Suppose that T p = I, the identity, and that dim F W = 1. a. Show that (T I) p = 0 and conclude that dim F V p. (4 points) b. If dim F V < p, prove that (T I) p 1 = 0. (3 points) c. If, for some vector v V, we have v + vt + vt vt p 1 0, prove that dim F V = p. (3 points) 5. Let R be a ring with 1. A (right) R-module V is said to be strongly n-generated, for some integer n, if every submodule of V is generated as an R-module by some set of n elements. a. If V is strongly n-generated and if W is a submodule of V, prove that both W and V/W are strongly n-generated. (3 points) b. Let W be a submodule of V. If W is strongly n-generated and if V/W is strongly m-generated, prove that V is strongly (n + m)-generated. (5 points) c. If V has composition length n, prove that V is strongly n-generated. (2 points)

120 Algebra Qualifying Exam August 2008 Do all 5 problems. 1. In this problem we prove that a Sylow 2-subgroup of a simple group of order 168 is its own normalizer. a. If G is a group of order 24 and G has a normal Sylow 2-subgroup, show that G contains an element of order 6. (4 points) b. If G is a simple group and H is a subgroup of G with G : H = 7, show that H contains no element of order 6. (3 points) c. Let G be a simple group with G = 168 and let P be a Sylow 2-subgroup of G. Prove that N G (P ) = P. (3 points) 2. Let Z be the ring of integers and let S = Z Z be the ring external direct sum of two copies of Z. Now let R be the subring of S given by R = { (a, b) Z Z a b mod 6 }. a. Show that R is a finitely generated Z-module and conclude that R is a Noetherian ring. (3 points) b. Prove that the ideal P of R given by P = { (a, 0) Z Z a 0 mod 6 } is prime. (2 points) c. If Q is a primary ideal of R with P = Q, the radical of Q, show that Q = P. (5 points) 3. Let C denote the complex number field and let E C be the splitting field over the rational numbers Q of the polynomial x 3 2. a. Show that E : Q = 6. (2 points) b. If α E and α 5 Q, prove that α Q. (5 points) c. Show that there exists β E with β 2 Q, but β / Q. (3 points) (over)

121 4. Let S, T and M be n n matrices over the complex numbers C and suppose that SM = MT. a. If f(x) C[x] is the minimal polynomial of T, show that f(s)m = 0. (4 points) b. If M 0, deduce that S and T have a common eigenvalue. (3 points) c. Now suppose n = 2, S = ( ) and T = ( ) Find a nonzero matrix M with SM = MT and show that it is impossible to find an invertible matrix M with this property. (3 points) 5. Let R be a subring of the ring M n (C) of all complex n n matrices, and suppose that R is finitely generated as module over the integers Z. Let M R. a. Show that M is contained in a commutative subring S of M n (C) that is finitely generated as a Z-module. (3 points) b. Deduce that there is a monic polynomial f(x) Z[x] such that f(m) = 0. (2 points) c. Prove that tr(m), the matrix trace of M, is an algebraic integer. (5 points)

122 Algebra Qualifying Exam January 2009 Do all 5 problems. 1. Let G be a finite group of order p(p + 1), where p is an odd prime, and assume that G does not have a normal Sylow p-subgroup. (a) Find (with proof) the number of elements of G with order different from p. (3 points) (b) Show that each nonidentity conjugacy class of elements with order different from p has size at least p, and conclude that there is precisely one such conjugacy class. (5 points) (c) Prove that p + 1 is a power of 2. (2 points) 2. Let R be the field of real numbers and let C R be the complex field. Define S to be the subring of the polynomial ring C[X] consisting of all polynomials with real constant term so that S = R + CX + CX 2 + CX 3 +. (a) Show that the ideal of S consisting of all polynomials with 0 constant term is not principal. (4 points) (b) Let I be a nonzero ideal of S and choose 0 f I to have minimal possible degree n. If g I, show that there exists s S with g sf either equal to 0 or to a polynomial of degree n. Conclude that I is generated by f and perhaps one additional polynomial of degree n. (6 points) 3. Let F be the field GF(p) of prime order p > 2 and suppose that the polynomial f(x) = X m + 1 F [X] is irreducible. (a) Show that every root of f in a splitting field of the polynomial has multiplicative order 2m. (4 points) (b) Prove that 2m divides p m 1, but that 2m does not divide p n 1 for any integer n with 0 < n < m. (3 points) (c) Show that m 4. (3 points) (over)

123 4. Let V be a finite-dimensional vector space over the complex numbers C and let T : V V be a linear operator on V. (a) If T is diagonalizable on V and if W is a subspace of V with T (W ) W, prove that T is diagonalizable on W. (6 points) (b) If T has the matrix with respect to some basis of V, decide (with proof) whether T is diagonalizable on V. (4 points) 5. In the following, all groups are additive abelian groups, and recall that an abelian group is said to be noetherian if its set of subgroups satisfies the ascending chain condition or equivalently the maximal condition. Furthermore, a nonzero group is said to be uniform if it contains no direct sum of nonzero subgroups. (a) Show that every nonzero noetherian group contains a nonzero uniform subgroup. (4 points) (b) Suppose G = U + V is the internal direct sum of the two subgroups U and V with U uniform. If G contains the direct sum A + B with A and B both nonzero, prove that (A + B) V 0. (3 points) (c) Let G = U + V be as above with U uniform. If G contains the direct sum A + B + C with A, B and C all nonzero, prove that V is not uniform. (3 points)

124 Algebra Qualifying Exam August 2009 Do all 5 problems. 1. Let H be a maximal subgroup of the finite group G and let X be the set of normal subgroups X of G such that X 1 and X H = 1. a. Show that all members of X are minimal normal subgroups of G of the same order. (3 points) b. If some member of X is abelian, show that all members of X are abelian p-groups for some prime p. (3 points) c. Let U, V X be distinct and assume that X contains at least one additional member different from U and V. Show that (UV H) G and conclude that (UV H) Z(UV ). (4 points) 2. Let R S be commutative rings with the same 1, and assume that every element of S is integral over R. a. If r R has an inverse in S, prove that this inverse is contained in R. (3 points) b. Suppose R is a field and let s S be a regular element (that is, if sx = 0 for some x S, then x = 0). Show that s is invertible in S. (3 points) c. If P is a prime ideal of S, prove that P is a maximal ideal of S if and only if R P is a maximal ideal of R. (4 points) 3. Let F be a field and let f(x) F [x] be an irreducible polynomial with splitting field E over F. Choose α E with f(α) = 0. Furthermore, for some fixed integer n 1, let g(x) be an irreducible polynomial in F [x] with g(α n ) = 0. a. Show that deg(g) divides deg(f) and that deg(f)/ deg(g) n. (5 points) b. If deg(f)/ deg(g) = n and if the characteristic of F does not divide n, prove that E contains a primitive nth root of unity. (5 points) 4. Let V be a vector space over a field F and let (, ): V V F be a bilinear form. For each x V define A(x) = { y V (x, y) = (y, x) }. Now suppose v is a fixed element of V with (v, v) 0. a. For all x V, show that A(x) is a subspace of V of codimension at most 1. (4 points) b. If the characteristic of F is different from 2, prove that A(v) is a subspace of V of codimension precisely 1. (1 point) c. If F is algebraically closed and has characteristic different from 2, show that either (a, a) = 0 for every element a A(v), or there exists y V \ A(v) with (y, y) = 0. (5 points) 5. A multiplicative abelian group A is said to be divisible if, for all a A and positive integers n, there exists b A with b n = a. a. If A is divisible and A is a homomorphic image of A, prove that A is divisible. (2 points) b. If A is a finite divisible group, prove that A = 1. (3 points) c. Suppose A is divisible and that A is a subgroup of the abelian group B. If A X > 1 for all nonidentity subgroups X of B, prove that A = B. (5 points)

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