# (a) Write each of p and q as a polynomial in x with coefficients in Z[y, z]. deg(p) = 7 deg(q) = 9

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1 Homework #01, due 1/20/10 = 9.1.2, 9.1.4, 9.1.6, 9.1.8, Additional problems for study: 9.1.1, 9.1.3, 9.1.5, , 9.2.1, 9.2.2, 9.2.4, 9.2.5, 9.2.6, 9.3.2, (This problem was not assigned except for study, but it s useful for the next problem.) Let p and q be polynomials in Z[x, y, z], where p = p(x, y, z) = 2x 2 y 3xy 3 z + 4y 2 z 5 q = q(x, y, z) = 7x 2 + 5x 2 y 3 z 4 3x 2 z 3 (a) Write each of p and q as a polynomial in x with coefficients in Z[y, z]. p = (2y)x 2 (3y 3 z)x + 4y 2 z 5 q = (7 + 5y 3 z 4 3z 3 )x 2 (b) Find the degree of each of p and q. deg(p) = 7 deg(q) = 9 (c) Find the degree of p and q in each of the three variables x, y, and z. deg x (p) = 2 deg x (q) = 2 deg y (p) = 3 deg y (q) = 3 deg z (p) = 5 deg z (q) = 4

2 2 (d) Compute pq and find the degree of pq in each of the three variables x, y, and z. pq = (2x 2 y 3xy 3 z + 4y 2 z 5 )(7x 2 + 5x 2 y 3 z 4 3x 2 z 3 ) = 2x 2 y(7x 2 + 5x 2 y 3 z 4 3x 2 z 3 ) 3xy 3 z(7x 2 + 5x 2 y 3 z 4 3x 2 z 3 ) + 4y 2 z 5 (7x 2 + 5x 2 y 3 z 4 3x 2 z 3 ) = 2x 2 y7x 2 + 2x 2 y5x 2 y 3 z 4 2x 2 y3x 2 z 3 3xy 3 z7x 2 3xy 3 z5x 2 y 3 z 4 + 3xy 3 z3x 2 z 3 + 4y 2 z 5 7x 2 + 4y 2 z 5 5x 2 y 3 z 4 4y 2 z 5 3x 2 z 3 = 14x 4 y + 10x 4 y 4 z 4 6x 4 yz 3 21x 3 y 3 z 15x 3 y 6 z 5 + 9x 3 y 3 z x 2 y 2 z x 2 y 5 z 9 12x 2 y 2 z 8 deg x (pq) = = 4 deg y (pq) = = 6 deg z (pq) = = 9 (e) Write pq as a polynomial in the variable z with coefficients in Z[x, y]. pq = 14x 4 y (21x 3 y 3 )z (6x 4 y)z 3 + (10x 4 y 4 + 9x 3 y 3 )z 4 + (28x 2 y 2 15x 3 y 6 )z 5 (12x 2 y 2 )z 8 + (20x 2 y 5 )z Repeat the preceding exercise under the assumption that the coefficients of p and q are in Z/3Z. Let p and q be polynomials in Z/3Z[x, y, z], where p = p(x, y, z) = 2x 2 y + y 2 z 5 q = q(x, y, z) = x 2 + 2x 2 y 3 z 4 (a) Write each of p and q as a polynomial in x with coefficients in Z/3Z[y, z].

3 We need only reduce the coefficients modulo 3, which gives us p = (2y)x 2 + y 2 z 5 q = (1 + 2y 3 z 4 )x 2 (b) Find the degree of each of p and q. deg(p) = 7 deg(q) = 9 (c) Find the degree of p and q in each of the three variables x, y, and z. deg x (p) = 2 deg x (q) = 2 deg y (p) = 2 deg y (q) = 3 deg z (p) = 5 deg z (q) = 4 (d) Compute pq and find the degree of pq in each of the three variables x, y, and z. pq = (2x 2 y + y 2 z 5 )(x 2 + 2x 2 y 3 z 4 ) = 2x 2 y(x 2 + 2x 2 y 3 z 4 ) + y 2 z 5 (x 2 + 2x 2 y 3 z 4 ) = 2x 2 y(x 2 ) + 2x 2 y(2x 2 y 3 z 4 ) + y 2 z 5 x 2 + y 2 z 5 (2x 2 y 3 z 4 ) = 2x 4 y + x 4 y 4 z 4 + x 2 y 2 z 5 + 2x 2 y 5 z 9 deg x (pq) = = 4 deg y (pq) = = 6 deg z (pq) = = 9 (e) Write pq as a polynomial in the variable z with coefficients in Z/3Z[x, y]. pq = 2x 4 y + x 4 y 4 z 4 + x 2 y 2 z 5 + 2x 2 y 5 z Prove that the ideals (x) and (x, y) are prime ideals in Q[x, y] but only the latter ideal is a maximal ideal. To show (x) is a prime ideal of Q[x, y], we must assume pq (x), where p, q Q[x, y], and show that either p (x) or q (x). Suppose p = a n x n + + a 1 x + a 0 and p = b m x m + + b 1 x + b 0, where a n,..., a 0, b m,..., b 0 Q[y]. Then pq has exactly one term with x- degree 0, namely the polynomial a 0 b 0 Q[y]. 3

4 4 From the assumption pq (x) we know there is some polynomial s Q[x, y] such that pq = sx. Note that every (non-zero) term in sx has x as a factor, hence its x-degree is at least 1. Hence every nonzero term in pq has degree at least 1. It therefore follows from pq = sx that a 0 b 0 = 0. Now Q is an integral domain, hence Q[y] is also an integral domain by Prop. 9.2, so from a 0 b 0 = 0 we conclude that either a 0 = 0 or b 0 = 0. If a 0 = 0 then every term of p has x as a factor, hence p is equal to x multiplied by some polynomial, so p (x). Similarly, if b 0 = 0, then q (x). This shows that p or q is in (x), so (x) is prime. To show (x, y) is a prime ideal of Q[x, y], we must assume pq (x, y), where p, q Q[x, y], and show that either p (x, y) or q (x, y). From pq (x, y) we get pq = sx + ty for some polynomials s, t Q[x, y]. This means that the constant term of pq is 0, because sx + ty is a polynomial in which every term either has x as a factor or has y as a factor. Let a 0 Q be the constant term of p and let b 0 Q be the constant term of q. Then a 0 b 0 = 0, so either a 0 = 0 or b 0 = 0. But if a 0 = 0 then every term of p has either x or y (or both) as a factor, and this implies that p (x, y). Similarly, if b 0 = 0 then q (x, y). This shows that (x, y) is prime. Define a function ϕ : Q Q[x, y]/(x, y) by ϕ(q) = q +(x, y) for every q Q. It is easy to show that ϕ preserves + and and is therefore a ring homomorphism. Note that ϕ sends each rational q to the set of polynomials in Q[x, y] which have constant term equal to q. Now every polynomial in Q[x, y] has a constant term, so ϕ is onto. Furthermore, polynomials with distinct constant terms are distinct, so ϕ is injective. These considerations show that Q[x, y]/(x, y) is isomorphic to Q. But Q is a field, so the ideal (x, y) is a maximal ideal by Prop (which says, if R a commutative ring then an ideal M is maximal iff R/M is a field). One the other hand, (x) is not a maximal ideal because (as we will show) it is a proper subset of the maximal ideal (x, y). We have (x) (x, y) since {x} {x, y}. To show the inclusion is proper it is enough

5 to note that y (x, y) but y / (x) because the y-degree of y is 1 but the y-degree of every polynomial in (x) is Prove that (x, y) is not a principal ideal in Q[x, y]. Let us suppose, to the contrary, that (x, y) = (p) for some polynomial p Q[x, y]. Then, since x, y (x, y) = (p), there are s, t Q[x, y] such that x = sp and y = tp. Then 0 = deg y (x) = deg y (s) + deg y (p), so 0 = deg y (p), and 0 = deg x (y) = deg x (t)+deg x (p), so 0 = deg x (p). From 0 = deg y (p) = deg x (p we get deg(p) = 0 and p Q. But p (p) = (x, y), so p = fx+gy for some f, g Q[x, y], so deg(p) = deg(fx+gy) = min(deg(f)+deg(x), deg(g)+deg(y)) = min(deg(f)+1, deg(g)+1) 1, contradicting deg(p) = Let F be a field and let R = F [x, x 2 y, x 3 y 2,..., x n y n 1,... ] be a subring of the polynomial ring F [x, y]. (a) Prove that the field of fractions of R and F [x, y] are the same. Just as a review, we note that Th says, if R is a commutative ring, D R, 0 / D, D is closed under, and D contains no 0-divisors, then there is a ring of quotients Q = D 1 R R such that (1) Q is a commutative ring with 1, (2) D Q, (3) Q = {d 1 r d D, r R}, (4) if D is a set of units in S R, a commutative ring with 1 extending R, then Q is isomorphic to a subring Q of S extending R, i.e., Q = Q and R Q S. If R is an integral domain, then D = R {0} has the needed properties and D 1 R is a field, called the field of quotients of R. If F is a field, then the polynomial ring F [x, y] is an integral domain, so F [x, y] has a field of fractions obtained by choosing D = F [x, y] {0}. Let Q 1 be the field of fractions of F [x, y]. Then 5 R = F [x, x 2 y, x 3 y 2,..., x n y n 1,... ] F [x, y] Q 1 = D 1 F [x, y]

6 6 By Theorem 7.15, there is a subring Q 2 of Q 1 which is isomorphic to the field of quotients of R, so R = F [x, x 2 y, x 3 y 2,..., x n y n 1,... ] Q 2 Q 1 and we need only show Q 1 Q 2. Note that x R so x Q 2. Also, 0 x, x 2 y R so x, x 2 y Q 2, but x is a unit of Q 2, so y = (x 1 ) 2 x 2 y Q. From x, y Q 2 we get F [x, y] Q 2. Consider an arbitrary element d 1 p Q 1, where 0 d F [x, y] and p F [x, y]. Then d, p Q 2 and d 0 and Q 2 is a field, so d 1 = p d Q 2. This shows Q 1 Q 2, completing the proof that Q 1 = Q 2. (b) Prove that R contains an ideal that is not finitely generated. For every n Z + let G n := {x,, x n y n 1 }, and let G n be the closure of G n under multiplication. Then a polynomial f F [x, y] belongs to F [x, x 2 y, x 3 y 2,..., x n y n 1 ] if and only if f is a linear combination of a finite subset of G n, that is, there is a finite set of monomials M G n and a finite set of coefficients α m F, m M, such that f = Σ m M α m m. For every f F [x, y] let r(f) = deg x (f) deg y (f). m G n then r(m) = 1. If f, g F [x, y] then r(fg) = deg x (fg) deg y (fg) = deg x (f) + deg x (g) (deg y (f) + deg y (g)) = deg x (f) deg y (f) + deg x (g) deg y (g) = r(f) + r(g) Note that if Therefore, for every m G n, either m G n and r(m) = 1, or else r(m) > 1 (because m is a nontrivial product of two or more monomials from G n ). Claim x n+1 y n / F [x,, x n y n 1 ]. Proof Suppose, to the contrary, that x n+1 y n F [x,, x n y n 1 ]. Then there is a finite set of monomials M G n and a finite set of

7 coefficients α m F, m M, such that x n+1 y n = Σ m M α m m. Now two polynomials in F [x, y] are equal iff they have the same coefficients. In this case, this means that x n+1 y n is one of the monomials in M, say x n+1 y n = m M, and its coefficient is α m = 1, and all the other coefficients are 0. Now m M G n, but r(m) = r(x n+1 y n ) = 1, so in fact m G n. However, this is a contradiction because x n+1 y n / G n. R is an ideal of itself, so by the following claim R contains an ideal (itself) that is not finitely generated. Claim R is not finitely generated. Proof Suppose, to the contrary, that G R is a finite set of generators of R. Now R is the union of a chain of subrings since R = F [x, x 2 y,, x n y n 1, ] = n Z + F [x,, x n y n 1 ]. Each element of G is in one of the subrings F [x,, x n y n 1 ] R, n = 1, 2, 3,. For each element of G, choose such a subring containing that element. There are only finitely many such subrings since G is finite, and one of them, say F [x,, x N y N 1 ] for some N Z +, contains all the others (because every finite subset of a linearly ordered set has a maximum element). Thus G F [x,, x N y N 1 ], so, since G generates R, we have R F [x,, x N y N 1 ], but F [x,, x N y N 1 ] R, so F [x,, x N y N 1 ] = R. We now have a contradiction since, by the claim above, x N+1 y N / F [x,, x N y N 1 ] Let f(x) be a polynomial in F [x]. Prove that F [x]/(f(x)) is a field if and only if f(x) is irreducible. [Use Proposition 7, Section 8.2] We note that Prop. 8.7 says, Every nonzero prime ideal in a PID is maximal. The proof actually shows that, in an integral domain, prime ideals are maximal among principal ideals. In a PID, all ideals are principal, so prime ideals are maximal. We assume (although it is not explicitly mentioned in the text of the problem) that F is a field. Then F [x] is a Euclidean domain, hence F [x] is a PID, so by Prop. 8.7, prime ideals in F [x] are maximal. 7

8 8 Prop says that, in a commutative ring, every maximal ideal is prime. Therefore, (1) in F [x] an ideal is prime iff it is maximal. Now complete the proof as follows F [x]/(f(x)) is a field (f(x)) is a maximal ideal of F [x] (f(x)) is a prime ideal of F [x] Prop. 7.12, below see (1) above f(x) is prime in F [x] Def. (2), p. 284 f(x) is irreducible in F [x] Prop. 8.11, below Prop says, If R a commutative ring then an ideal M is maximal iff R/M is a field. Prop says, In a PID a nonzero element is a prime iff it is irreducible.

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