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1 Homework Assignment 5 Homework 5. Due day: 11/6/06 (5A) Do each of the following. (i) Compute the multiplication: (12)(16) in Z 24. (ii) Determine the set of units in Z 5. Can we extend our conclusion on Z 5 to Z p, for an arbitrary prime integer p? (iii) Determine the set of units in Z 6. Can we extend our conclusion on Z 6 to Z n, for an arbitrary integer n? (iv) Determine if 2x 10 is a prime element in R[x], the ring of all real coefficient polynomials. (v) Determine if 2x 10 is a prime element in Z[x], the ring of all integral coefficient polynomials. Solution: (i) (12)(16) = (12)(2)(8) 0 (mod 24). Some students misunderstood is as the product of two principal ideals. In that case, = (12)(16) = 0. (ii) If a U(Z 5 ), then for some u Z, ua 1 (mod 5), and so for some v Z, ua + 5v = 1. Therefore, a and 5 are relatively prime. On the other hand, if a and 5 are relatively prime, then for some integers u and v, au + 5v = 1, and so au 1 (mod 5). Thus a U(Z 5 ). Hence U(Z 5 ) = {a : a and 5 are relatvely prime} = Z 5 {0}. (iii) The same argument in (ii) indicates that In general, we have U(Z 6 ) = {a : a and 6 are relatvely prime} = {1, 5} Z 6. U(Z n ) = {a : a and n are relatvely prime}. (iv) Let f(x) = 2x 10. Since R[x] is a PID, an element is prime if and only if it is irreducible. If f(x) = a(x)b(x), then compare the degree both sides we may assume that a(x) has degree 1 and b(x) has degree zero. Thus b(x) = b R {0}. Since R is a field, every non zero element in a field is a unit of R, which is also is a unit of R[x]. Hence f(x) is irreducible, and so f(x) is prime in R[x]. (iv) Let f(x) = 2x 10. Then f(x) = 2(x 5), where 2 and x 5 are nonzero nonunit elements in Z[x]. It follows that f(x) is not an irreducible element in Z[x]. But Z[x] is a commutative ring with identity, an element in Z[x] is prime only if it is irreducible. Since f(x) is not irreducible in Z[x], f(x) is not a prime in Z[x]. (5B) = (3.3) Let R = {a + b 10 : a, b Z} be a subring of the field of the reals. (a) The map N : R Z given by N(a + b 10) = a 2 10b 2 is multiplicative (that is, u, v R, N(uv) = N(u)N(v)) and N(u) = 0 if and only if u = 0. 1

2 (b) u is a unit if and only if N(u) = ±1. (c) 2, 3, and 4 10 are irreducibles in R. (d) 2, 3, and 4 10 are not primes of R. (e) Explain why this ring is an integral domain. Obtain two different factorizations of 6, and conclude that the factorization in this integral domain is not unique. Proof: (a) Let u = a + b 10 and v = c + d 10. Then N(uv) = N((a + b 10)(c + d 10) = N(ac + 10bd + (ad + bc) 10) = (ac + 10bd) 2 10(ad + bc) 2 = a 2 c abcd + 100b 2 d 2 (10a 2 d abcd + 10b 2 c 2 ) = a 2 c b 2 d 2 10a 2 d 2 10b 2 c 2 = a 2 (c 2 10d 2 ) 10b 2 (c 2 10d 2 ) = (a 2 10b 2 )(c 2 10d 2 ) = N(u)N(v). Note that if for some positive d, a = da 1 and b = db 1, then N(u) = N(a+b 10) = a 2 10b 2 = d 2 (N(a 1 + b 1 10). Therefore, if 0 = N(u) = a 2 10b 2, for some non zero a and b, then we may assume that (a, b) = 1, (that is, a and b are relatively prime). Suppose that for some non zero a and b such that N(a + b 10) = 0, and such that (a, b) = 1. Now a 2 = 10b 2. Then 2 a 2 implies that 2 a. Therefore, we can write a = 2a 1. Thus 4a 2 1 = 10b2, and so 2a 2 1 = 5b2. Then 2 b and so 2 is a common factor of a and b, contrary to the assumption that (a, b) = 1. Therefore, we must have a = b = 0. (b) Let u = a + b 10. Suppose that u is a unit. Then there must be a v R, such that uv = 1. Since N is multiplicative, N(u)N(v) = N(1) = 1. Since N(u) and N(v) are both integers, we must have N(u) = ±1. Conversely, we assume that N(u) = ±1. Then u(a b 10) = a 2 10b 2 = ±1. Therefore, ±(a b 10) is the inverse of u. (c) To prove the statements in (c), we first note that the following fact holds in number theory. (3c-1) For any n Z, both n 2 2 (mod 5) and n 2 2 (mod 5). In fact, we can write n = 5k + r with 0 r 4. Then n 2 r 2 (mod 5). But 0 2 = 0, (mod 5) and (mod 5). This proves (3c-1). Suppose that u 0 is not an irreducible element. The for some non unit nonzero x and y R, u = xy. It follows that N(u) = N(x)N(y), where both N(x) and N(y) are integers other than ±1. Now consider u = 2. Note that N(2) = 4, and that 4 = (2)(2) = ( 2)( 2) are the only factorizations. If u = xy, for some non units x and y, then we must have N(x) = N(y) = ±2. Let u = a + b 10. Then 2 = N(u) = a 2 10b 2. Then we have 2 a 2 10b 2 = a 2 (mod 5), contrary to (3c-1). Therefore, 2 must be irreducible. Similarly, 3 must be irreducible. 2

3 Now suppose that u = = xy is a product of two non unit nonzero elements. Then 6 = N(u) = N(x)N(y). Therefore, N(x) {±2, ±3}. This contradicts to (3c-1), as shown above. Therefore, must be irreducible. Similarly, 4 10 is also irreducible. (d) As (2)(3) = 6 = (4 + 10)(4 10), we have 2 (4 + 10)(4 10). As N(2) = 4 and N(4 ± 10) = 6, if for some x R, 2x = , then 4N(x) = N(2)N(x) = N(4 ± 10) = 6, forcing N(x) = 2/3 which is not an integer. Therefore, 2 4 ± 10, and so 2 is not a prime in R. Similarly, the other three elements are not primes in R. (e) Since R is a subring of the real number field, and since a field does not have zero divisors, R has 1, is commutative, and has no zero divisors. Therefore, R is an integral domain. Note that 6 = (2)(3) = (4 10)(4 + 10). To see that 4 10 is not associate with either 2 or 3, we use the function N. Suppose that 4 10 = 2u for some unit u R. Then by (a) and (b) of this problem, 6 = N(4 10) = N(2)N(u) = 4 1 = 4, a contradiction. Similarly, 4 10 is not associate with 3, and so the factorization of 6 in R is not unique. (5C) Consider this solution of the equation X 2 = I in M 3 (R), the ring of all 3 3 real matrices with matrix addition and multiplication: As X 2 = I implies X 2 I = 0, the zero matrix, so factorization shows that (X I)(X + I) = 0. It follows that either X = I or X = I. Is this reasoning correct? If your answer is not, point out the error and present an counterexample. Solution: The reasoning is incorrect. The error is that it ignores the fact that zero divisors may exist and so AB = 0 does not always imply A = 0 or B = 0. In fact, if X = 0 1 0, then X 2 = I but X I and X I. (5D) Mark each of the following true or false. True a. Every field is also a ring. (Definition) False b. Every ring with identity has at least two elements. (The subring {0} of Z has multiplicative identity 0). True c. Addition in every ring is commutative. (Definition) False d. As a ring, nz is isomorphic to Z. (When n 2, nz has no multiplicative identity). False e. Z is a subfield of R, the field of the real numbers. (Lack of multiplicative inverses for nonzero nonunit elements). True f. A ring homomorphism f is 1-1 if and only if the kernel of f is {0}. (A ring homomorphism 3

4 is also a group homomorphism for the additive groups. This holds for group homomorphisms). False g. Every subring of a ring R is also an ideal of R. (The center of M 2, the ring of all 2 by 2 real matrices, is not an ideal). True h. Every ideal of a ring R is also a subring of R. (Definition) True i. A quotient ring of an integral domain can be a field. (Z/ 5 = Z 5 is a field.) True h. A quotient ring of an integral domain can have zero divisor. (Z/ 6 = Z 6 has zero divisors.) (5E) = (2.12) Let R be a ring without identity and with no zero divisors. Let S be the ring whose additive group is R Z (with multiplication defined by (r 1, k 1 )(r 2, k 2 ) = (r 1 r 2 +k 2 r 1 +k 1 r 2, k 1 k 2 ), for any r 1, r 2 R and k 1, k 2 Z). Let A = {(r, n) : rx + nx = 0, x R}. (a) A is an idea in S. (b) S/A has an identity and contains a subring isomorphic to R. (c) If R is commutative, then S/A has no zero divisor. (d) What is A if R = 2Z? Proof: (a) Firstly, (0, 0) A and so A. Let (r, n ) S and (r, n) A. Then (r, n ) (r, n) = (r r, n n). If both (r, n ) A and (r, n) A, then x R, r x + n x = 0 and rx + nx = 0. It follows that (r r)x + (n n)x = 0 and so (r, n ) (r, n) = (r r, n n) A. By definition of multiplication, (r, n )(r, n) = (r r + n r + nr, n n). For any x R, (r r + n r + nr )x + n nx = r (rx + nx) + n (rx + nx) = 0, and so (r, n )(r, n) A. Similarly, (r, n)(r, n ) = (rr + n r + nr, n n) A. Thus A is an ideal of S. (b) Note that for any (r, n) S, (0, 1)(r, n) = (r, n) = (r, n)(0, 1) and so (0, 1) is the identity of S. It follows that the element (0, 1) + A is the identity of S/A. and Define f : R S by f(r) = (r, 0) + A. Then f(r 1 + r 2 ) = (r 1 + r 2, 0) + A = (r 1, 0) + A + (r 2, 0) + A = f(r 1 ) + f(r 2 ), f(r 1 r 2 ) = (r 1 r 2, 0) + A = (r 1, 0)(r 2, 0) + A = f(r 1 )f(r 2 ). Thus f is a ring homomorphism. Suppose that for some r, r R, f(r) = f(r ). Then f(r r ) A, and so x R, (r r )x = 0. Since R does not have zero divisors, we must have r r = 0 and so r = r. This implies that f is a monomorphism. By the First Isomorphism Theorem, f(r) is isomorphic to R. (c) Since in S, (a, m)(x, 0) = (ax + mx, 0) and since (0, 0) is the additive identity of S, the subset A of S can be defined as A = {(r, n) : rx + nx = 0, x R} = {(r, n) : (r, n)(x, 0) = (0, 0), x R}. Let (a, m)+a and (b, n)+a be two non zero elements in S/A. We assume that (a, m)(b, n)+a = ((a, m) + B)((b, n) + A) = A (or (a, m)(b, n) A) to derive a contradiction. Since (a, m) + A and 4

5 (b, n) + A are nonzero elements in S/A, (a, m), (b, n) A. Now suppose (ab + mb + na, mn) = (a, m)(b, n) A. Since (a, m) A, y R, such that (ay + my 0 in R. Note that (y, 0)(a, m)(b, n) A since A is an ideal. By the definition of A, x R, ((ay + my)(bx + nx), 0) = (y, 0)(a, m)(b, n)(x, 0) = (0, 0), where the first equality holds as R is commutative. It follows by the assumption that R has no zero divisor that bx + nx = 0 in R, x R, and so (b, n) A, contrary to the assumption that (b, n) + A A. 5

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