# Math 312/ AMS 351 (Fall 17) Sample Questions for Final

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1 Math 312/ AMS 351 (Fall 17) Sample Questions for Final 1. Solve the system of equations 2x 1 mod 3 x 2 mod 7 x 7 mod 8 First note that the inverse of 2 is 2 mod 3. Thus, the first equation becomes (multiply both sides be 2 1 = 2) x 2 mod 3 We also have x 2 mod 7. Thus, obviously x 2 mod 21 is a solution for the first two equations. We are now reduced to x 2 mod 21 x 7 mod 8 We need to write 1 = gcd(21, 8) as linear combination of 21 and 8. Apply the Euclid algorithm and get Going in reverse, we get 1 = = = = = = 3 (5 3) = = 2 (8 5) 5 = = 2 8 3(21 2 8) =

2 Indeed, 1 = (= 64 63). Thus, the solution to the equation is x = mod 168(= 8 21) Get x = 313 = 23 mod 168. (indeed 23 2 mod 21, 23 7 mod 8). 2. Can we write 12 as a linear combination of 24 and 114. If yes, find a and b such that 12 = 24a + 114b. Answer: A necessary and sufficient condition to write x = a n + b m is gcd(n, m) x. Here, gcd(24, 114) = 6, and Thus, we can write 12 as a linear combination. By Euclid, or just by inspection, we get Multiply this by 2 and get 6 = = Compute 6 76 mod 13 By Euler, we know mod 13. Thus 6 76 = = 6 4 mod 13. Then 6 2 = 36 = 10 mod 13. Then 6 4 = (6 2 ) 2 = 100 = 9 mod 13. Suppose a 4 mod 10. What are the possible last 2 digits of a n. We have a 0 mod 2 (i.e. a is even). Thus a n 0 mod 4 for n 2. On the other hand a 4 mod 5. This gives a can be 5k + 4 mod 24, i.e. 4, 9, 14, 19, or 24. a {4, 9, 14, 19, 24} mod 25 For concreteness, let s take n = 102. We know a n 0 mod 4, we need to compute a 102 mod 25. Since, we know by Euler a φ(25) = a 20 = 1. We get a 102 = a 2 {4 2, 9 2, 14 2, 19 2, 24 2 } mod 25 which gives a 102 = {16, 6, 21, 11, 1} mod 25 2

3 And a 102 = 0 mod 4 Now we need to apply the Chinese reminder theorem. Note first 1 = So, the answer is if a 102 = 16, then a 102 = = 16 mod 100 Similarly, a 102 = 6 mod 25 gives a 102 = = 56 mod 100. The other 3 cases are similar. In conclusion, starting with a 4 mod 10, we get that the last 2 digits of a 102 are: 16, 56, 96, 36, 76 (depending on a mod 25). 4. We define the quaternion group Q to be the group with 8 elements {±1, ±i, ±j, ±k} such that i 2 = j 2 = k 2 = 1, and ij = k, jk = i, and ki = j. Show that Q is not isomorphic to Z 8 Z 4 Z 2 Σ 4 D(4) Q is not abelian, while Z 8 and Z 4 Z 2 are abelian. Thus, they cannot be isomorphic. Q has order 8, while Σ 4 has order 24, again non-isomorphic. Finally, to distinguish Q and D(4) we need to count the elements of order 4: there are 6 such elements in Q (±i, ±j, ±k), while there are only 2 in D(4) (ρ and ρ 3, where ρ is a rotation of order 4). 5. Give an example of a field with finitely many elements: Z p (p prime) two different examples of integral domains, which are not fields: Z, Z 2 [X]. a ring (commutative and with unit) which is not an integral domain: Z n a ring which doesn t have a unit: 2Z 3

4 a ring which is not commutative: M n,n (R) (n n matrices, with real coefficients) 6. Find the decomposition into irreducible factors for i) x 3 3x 2 + 3x 2 over Z 7 Let f = x 3 3x 2 + 3x 2. We compute f(0) = 2, f(1) = 1, f(2) = = 0, f(3) = = 7 = 0, f(4) = = 26, f(5) = = 63 = 0. Thus, we get 3 roots, 2,3, 5. We conclude f = (x 2)(x 3)(x 5) ii) x 4 x 2 6 over R x 4 x 2 6 = (x 2 3)(x 2 + 2) = (x 3)(x + 3)(x 2 + 2) iii) same as (ii), but over C x 4 x 2 6 = (x 3)(x+ 3)(x 2 +2) = (x 3)(x+ 3)(x+i 2)(x i 2) 7. Find the gcd and lcm of the following polynomials x 4 +x+1 and x 3 +x+1 over Z 3. Use both methods: factorization and Euclid s Algorithm. Euclid Algorithm: (Step 1) divide x 4 + x + 1 by x 3 + x + 1. We get x 4 + x + 1 = x (x 3 + x + 1) + 2x (Step 2) divide (x 3 + x + 1) by the reminder 2x x 3 + x + 1 = 2x (2x 2 + 1) (x 1) (Step 3) Repeat: divide 2x by x 1. We get (2x 2 + 1) = 2(x 1)(x + 1) thus remainder 0. Euclid tells us that the last non-zero remainder (i.e. (x 1) is the gcd). 4

5 In general, we have gcd(f, g) lcm(f, g) = f g Thus lcm = (x4 + x + 1)(x 3 + x + 1) x 1 = (x 4 + x + 1)(x 2 + x + 2) 8. Find all irreducible cubic polynomials over Z 2. List all polynomials of degree 3 over Z 2, then eliminate those that have 0 or 1 as root. Note that 0 is a root iff the coefficient of the constant is 0, and 1 is a root iff the sum of the coefficients is even. Thus, we get two possibilities: x 3 +x 2 +1 and x 3 +x+1 (the coefficient of x 3 and of the constant have to be 1, and then we need an odd number of non-zero coefficients). 9. Let f = x 2 + x + 2 over Z 3 i) Show that f is irreducible. f(0) = 2, f(1) = 4 = 1, f(2) = 2. Thus, degree 2 and no root; it implies irreducible. ii) Write down the 9 representatives for the congruence classes mod f. Just the polynomials of degree less than 1. Thus, answer: x, x+1, x + 2, 2x, 2x + 1, 2x + 2, 0, 1, 2. iii) Compute (x + 1) 3 mod f. We know x 2 + x + 2 = 0 (because we work modulo x 2 + x + 2). Thus x 2 = 2x + 1 Also note x 3 = x x 2 = 2x 2 + x = 2(2x + 1) + x = 2x + 2 Back to the question (x + 1) 3 = x 3 + 3x 2 + 3x + 1 = x = 2x + 3 = 2x 5

6 iv) Find the inverse of [x + 1] f. We look for ax + b such that Expanding we get Use x 2 = 2x + 1 and get (x + 1)(ax + b) = 1 ax 2 + (a + b)x + b = 1 1 = ax 2 + (a + b)x + b = 2ax + a + (a + b)x + b = bx + (a + b) giving b = 0, a + b = 1 Thus, b = 0, a = 1. Thus the inverse of x + 1 is just x. Let s check: x(x + 1) = x 2 + x = 2x x = 3x + 1 = Give example of a field with 9 elements. In general, the answer to such a question (a field with p n elements, here p = 3, n = 2) is to say polynomials over Z p modulo an irreducible polynomial of degree n. In this case, you are given a degree 2 polynomial over Z 3 in the example above... (f = x 2 + x + 2). In general, you have to find such an irreducible polynomial. Typically, you can find irreducible polynomials of type x n + a (exception over Z 2 ). 6

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