# MATH 420 FINAL EXAM J. Beachy, 5/7/97

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1 MATH 420 FINAL EXAM J. Beachy, 5/7/97 1. (a) For positive integers a and b, define gcd(a, b). (b) Compute gcd(1776, 1492). (c) Show that if a, b, c are positive integers, then gcd(a, bc) = 1 if and only if gcd(a, b) = 1 and gcd(a, c) = (a) Find ϕ(32). (b) Use the Euclidean algorithm to find [5] 1 32 in Z 32. (c) Find all powers of [5] 32 in Z Define σ = ( (a) Compute στ and τσ. ) and τ = ( (b) Write each of σ, τ, στ, and τσ as a product of disjoint cycles (c) Compute the order (in S 7 ) of each of the elements σ, τ, στ, and τσ. 4. Let S be the set of all ordered pairs (m, n) of positive integers m, n. On S, define (m 1, n 1 ) (m 2, n 2 ) if m 1 + n 2 = m 2 + n 1. (a) Show that defines an equivalence relation on S. (b) On the equivalence classes S/, define an addition as follows: [(m 1, n 1 )] + [(m 2, n 2 )] = [(m 1 + m 2, n 1 + n 2 )]. Show that there is an identity element for this addition. Then find a formula for the additive inverse of [(m, n)]. (You may assume that the formula for addition gives a well-defined and associative binary operation.) 5. (a) State these definitions: group; subgroup. (b) State Lagrange s theorem. (c) Let G be a group, and let H be a nonempty subset of G. Suppose that if x and y are any elements of H, then xy 1 H. Show that H must be a subgroup of G. 6. Let m and n be positive integers with gcd(m, n) = 1. Define φ : Z mn Z m Z n by φ([x] mn ) = ([x] m, [x] n ), for all [x] mn Z mn. (a) Show that φ is a well-defined function. (b) State the definition of an isomorphism of groups. (c) Show that φ is an isomorphism. 7. For each of the following, either indicate that the statement is true, or give a counterexample if the statement is false. (a) If G is a finite group of order n, then every element x of G satisfies the equation x n = e. (b) If G is a finite group of order n, then every element (except the identity e) has order n. (c) If G is a finite group of order n, then there is at least one element of G that has order n. (d) If G is a finite group of order n, and n is prime, then there is at least one element of G that has order n. (e) If a and b are group elements of order m and n, respectively, then the element ab has order lcm[m, n]. 8. Prove ONE of the following theorems from the text. I. Every subgroup of a cyclic group is cyclic. II. If G is a cyclic group of order n, then G is isomorphic to Z n. III. Every group is isomorphic to a group of permutations. ).

2 MATH 420 FINAL EXAM J. Beachy, 12/15/95 1. Solve the following system of congruences: 2. Find [91] (in Z 501 ). 3. Let σ = (2, 4, 9, 7, )(6, 4, 2, 5, 9)(1, 6)(3, 8, 6) S 9. (i) Write σ as a product of disjoint cycles. (ii) What is the order of σ? (iii) Compute σ Let G be a group. (a) State the definition of a subgroup of G. 2x 9 (mod 15) x 8 (mod 11) (b) State a result that tells you which conditions to check when determining whether or not a subset of G is a subgroup of G. Use this result in proving part (c). (c) Let H and K be subgroups of G. Prove that H K = {g G g H and g K} is a subgroup of G. 5. (a) State the definition of a cyclic group. (b) Write out ONE of the following proofs from the text: I. Any subgroup of a cyclic group is cyclic. II. If G is a cyclic group of order n, then G is isomorphic to Z n. 6. Do ONE of the following problems. I. Find all subgroups of Z 11, and give the lattice diagram which shows the inclusions between them. II. Show that the three groups Z 6, Z 9, and Z 18 are isomorphic to each other. 7. Let G be the subgroup of GL 3 (R) consisting of all matrices of the form 1 a b such that a, b R Show that G is a subgroup of GL 3 (R). 8. Show that the group G in problem 7 is isomorphic to the direct product R R.

3 MATH 420 FINAL EXAM J. Beachy, 12/10/93 1. (20 pts) Find gcd(980, 189) and express it as a linear combination of 980 and (20 pts) (a) Is divisible by 3? (b) What is the last digit in the decimal expansion of 4 123? ( ) ( (20 pts) Let σ = and τ = (a) Write σ, τ, στ, and τ σ as products of disjoint cycles. (b) Find the order of each of σ, τ, στ, and τσ. 4. (20 pts) Define f : Z n Z m by f([x] n ) = [kx] m. Show that the formula f defines a function if and only if m kn. Find conditions on n, m, k that determine when f is a one-to-one correspondence. 5. (20 pts) Let G be a group and let H be a subgroup of G. For elements x, y G, define x y if y 1 x H. Check that defines an equivalence relation on G. 6. (25 pts) (a) Define the following terms: group; cyclic group; order of an element of a group. (b) State the following theorems: Lagrange s theorem (about the order of a subgroup); Cayley s theorem (about groups of permutations). 7. (25 pts) Let G be any cyclic group. Prove that G is isomorphic to either Z or Z n, for some positive integer n. 8. (20 pts) (a) Show that Z 5 is isomorphic to Z 10. (b) Show that Z 30 is not isomorphic to Z (30 pts) Let G and G be groups, and let φ : G G be a function (not required to be either one-to-one or onto) such that φ(ab) = φ(a)φ(b) for all a, b G. (a) Let e and e denote the identity element of G and G. Show that φ(e) = e, and that φ(g 1 ) = (φ(g)) 1 for all g G. (b) Show that the subset {g G φ(g) = e } is a subgroup of G. (c) Show that the subset {y G y = φ(x) for some x G} is a subgroup of G. ).

4 MATH 420 FINAL EXAM, J. Beachy, 12/12/90 Answer any eight questions. 1. Let a and b be nonzero integers. Prove that (a, b) = 1 if and only if gcd(a + b, ab) = Solve the following system of congruences: 2x 7 (mod 15) 3x 5 (mod 14) 3. Let G be any cyclic group. Prove that G is isomorphic to either Z or Z n, for some positive integer n. 4. Let G be a group and let H be a subgroup of G. For any element a G, define Ha = {x G x = ha for some h H}. Prove that the collection of all such subsets partitions G. 5. Let σ = (2, 4, 9, 7, )(6, 4, 2, 5, 9)(1, 6)(3, 8, 6) S 9. (i) Write σ as a product of disjoint cycles. (ii) What is the order of σ? (iii) Compute σ Let G be a group. Show that G is abelian if and only if (ab) 1 = a 1 b 1 for all a, b G. 7. If a nontrivial group G has no proper nontrivial subgroups, prove that G is cyclic and that the order of G is a prime number. 8. Let G be any group. In the proof of Cayley s theorem, for each a G a function λ a : G G is defined by λ a (x) = ax, for all x G. (a) Prove that λ a is a permutation of G, for any a G. (b) Prove that {λ a a G} is a subgroup of Sym(G). 9. Let G be a group and let H and K be subgroups of G. Prove that H K is a subgroup of G. 10. Define the following terms: one-to-one function; onto function; group; cyclic group.

5 MATH 420 FINAL EXAM J. Beachy, 5/12/88 1. (30) (a) State the Division Algorithm. (b) State the definition of one-to-one function; onto function. (c) State the definition of a group. 2. (20) Let G, G be groups, and let φ : G G be a function such that φ(ab) = φ(a)φ(b) for all a, b G. Prove that K = {x G φ(x) = e} is a subgroup of G. 3. (15) Define a function φ from the multiplicative group C of complex numbers into itself by φ(a+bi) = a bi. Prove that φ is an isomorphism. 4. (35) (a) State the proposition which gives the solution to all linear congruences of the form as b (mod n). (b) State the proposition which tells how to compute the order of any element in a cyclic group of order n. (c) For the special case of the cyclic group Z n, show that the result in (a) can be used to prove (b). 5. (30) Let N be a subgroup of the group G. (a) For a, b G define a b if ab 1 N. Show that defines an equivalence relation. (b) Assume that gxg 1 N for all x N and g G. Prove that if a b and b d, then ab cd. Hint: Show that if ac 1 N and bd 1 N, then ac 1 cbd 1 c 1 N. 6. (25) State and prove Lagrange s Theorem. OR State and prove Cayley s Theorem. 1 a b 7. (25) Let G be the set of matrices of the form 0 1 c, where a, b, c Z 2. Prove that G is a group Is it abelian? Is it cyclic? Compute the order of each of its elements. 8. (20) Prove that if gcd(m, n) = 1, then n ϕ(m) + m ϕ(n) 1 (mod mn).

6 MATH 420 FINAL EXAM J. Beachy, 12/18/87 Each problem is worth 25 points. 1. Write out the definitions of the following concepts: (a) group (b) cyclic group (c) one-to-one function; onto function (d) greatest common divisor of two integers. 2. Write out the statements of the following theorems: (a) The Division Algorithm (b) Lagrange s Theorem (on the order of a subgroup of a finite group) (c) The theorem which gives ϕ(n) in terms of the prime factorization of n. 3. Write out the proof of the theorem which states that every subgroup of a cyclic group is cyclic. 4. Write out a proof of Cayley s Theorem, which states that every group is isomorphic to a group of permutations. 5. Let a, b, d, m, n be integers. If gcd(a, b) = d and an + bm = d, then prove that gcd(n, m) = Let σ = (2, 8, 5)(5, 7, 8)(3, 8)(8, 6) in S 9. Write σ as a product of disjoint cycles. What is the order of σ? Compute σ For real numbers a, b we define a b if a b is an integer. Show that is an equivalence relation of the set of real numbers. {[ ] 1 x 8. Show that the set G = x R} is a group under matrix multiplication, and show that G 0 1 is isomorphic to R, the group of all real numbers under addition.

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