# Solutions to Homework 1. All rings are commutative with identity!

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1 Solutions to Homework 1. All rings are commutative with identity! (1) [4pts] Let R be a finite ring. Show that R = NZD(R). Proof. Let a NZD(R) and t a : R R the map defined by t a (r) = ar for all r R. Since a is a nonzero divisor on R, t a is injective. An injective map from a finite set into itself is surjective. Thus there is an element b R with ab = 1. (2) [8pts] Suppose that R is a subring of a ring S and that R is a direct summand of S as an R-module (i.e. S = R N as an R-module). (a) Prove that each ideal is the contraction of an ideal of S. (b) Conclude that R is Noetherian (Artinian) if S is Noetherian (Artinian). Proof. Suppose that S = R N as an R-module. (a) Let I R be an ideal. Then IS = IR IN and IS R = IR R = I since N R = 0. (b) Let M = {I λ λ Λ} be a set of ideals of R. Then M S = {I λ S λ Λ} is a set of ideals in S. If S is Noetherian (Artinian) then M S has a maximal (minimal) element. Since every ideal I λ is the contraction of I λ S, the set M has a maximal (minimal) element. (3) [16pts] Let R be a ring. (a) Suppose that I R is an ideal which is not principal, but every ideal which properly contains I is principal. Show that I is a prime ideal. (b) If every prime ideal of R is principal then every ideal of R is principal. Proof. Assume that a, b R with ab I and a / I, b / I. By assumption: In particular, a 1 b 1 I. Continue: I : (a) = (b 1 ) with I + (b) (b 1 ) I : (b 1 ) = (a 1 ) with I + (a) (a 1 ) I : (a 1 ) = (b 2 ) with I + (b 1 ) (b 2 ) I : (b 2 ) = (a 2 ) with I + (a 1 ) (a 2 ) This yields increasing sequences of ideals in R: Their unions are principal ideals: I (a 1 ) (a 2 ) (a 3 )... I (b 1 ) (b 2 ) (b 3 )... (a 0 ) = i N(a i ) (b 0 ) = i N(b i ) 1

2 2 with a 0 b 0 I, I : (a 0 ) = (b 0 ), and I : (b 0 ) = (a 0 ). We claim that I = (a 0 b 0 ). If x I, then x (a 0 ) and x = a 0 s for some s R. Then s I : (a 0 ) = (b 0 ) and s = b 0 t for some t R. Thus x = a 0 b 0 t and x (a 0 b 0 ). Contradiction. (b) Let M = {I R I a non-principal ideal} be the set of non-principal ideals of R. M is partially ordered by inclusion. If M is not empty then M is inductively ordered. If K = {I λ } λ Λ is a chain in M, then J = λ Λ I λ is not a principal ideal in R, since otherwise J = (a) = I λ for some λ Λ. Thus J M and by Zorn s Lemma, M has a maximal element I. By part (a), the ideal I is prime and thus principal by assumption, contradiction. (4) [8pts] Let R be a ring and S R a nonempty subset. Show that the following are equivalent: (a) R S is the union of prime ideals. (b) 1 S and (ab S a S and b S). Proof. (a) (b) Suppose that Γ is a set of prime ideals of R and that R S = Then 1 S and if ab S then ab / P for all P Γ. Thus a / P and b / P. Therefore a S and b S. (b) (a) If a, b S then ab S and S is multiplicatively closed. If 0 S then S = R. The empty set R S is the union of an empty set of prime ideals. We may suppose that 0 / S. Theorem (1.12) applied to S and the ideal I = (0) yields that there is a prime ideal P R with P S =. Let Γ be the set of prime ideals Q with Q S =. Then R S = Q Γ Q. P Γ P. Obviously, Q R S. In order to show the other inclusion, let a R S. By assumption (b): (a) S = and by Theorem (1.12) there is a prime ideal P R with a P and P S =. (5) [10pts] Let R be an integral domain, P = {p λ } λ Λ the set of prime elements of R, and S the multiplicative set generated by P and R. (a) Show that S is saturated. (b) Show that the following are equivalent: (i) R is a UFD (factorial). (ii) S 1 R is the quotient field of R. (iii) Each proper prime ideal of R contains some p λ. Proof. (a) Let a, b R with ab S. Then ab = ɛp 1... p r where ɛ R and p i prime elements. Since p 1,..., p r are prime elements, we may assume by renumbering and counting multiplicities correctly that a = αp 1... p s and b = βp s+1... p r where α, β R. Then αβ = ɛ and a, b S. (b) (i) (ii) clear

3 (ii) (iii) Use that the prime ideals Q of S 1 R are in 1 1 correspondence to the prime ideals P of R which satisfy P S =. (ii) (i) Let x R (R {0}). Then there is a γ S 1 R with (x/1)γ = 1/1. Write γ = a/b with a R and b S. Then xa = b S. By (a) the multiplicative set S is saturated, hence x S and x is product of prime elements in R. R is a UFD. (6) [12pts] Assumptions as in problem (5). Suppose in addition that every nonzero nonunit of R is a finite product of irreducible elements of R. Show that if S 1 R is a UFD, then R is a UFD. Proof. Suppose that {p λ } λ Λ is a set of prime elements of R and that S is the multiplicative set generated by {p λ } λ Λ and R. By the same proof as in (5)(a) the multiplicative set S is saturated. Assume that S 1 R is a UFD and that every element x R (R {0}) is a finite product of irreducible elements. In order to show that R is a UFD it suffices to show that every irreducible element of R is a prime element of R. Let x R be irreducible. (a) x/1 S 1 R is a unit or an irreducible element of R. If x/1 is not a unit, suppose that x/1 = (a/s)(b/t) in S 1 R where a, b R and s, t S. Then xst = ab in R. By assumption s = αp 1... p n and t = βq 1... q m where α, β R and p i, q j {p λ } λ Λ. Then for all 1 i n, p i a or p i b and for all 1 j m, q j a or q j b. Thus we can cancel s and t and write x = a 1 b 1 where a 1 a and b 1 b. Since x is irreducible in R, a 1 or b 1 is a unit in R. Hence a/s or b/t is a unit in S 1 R and x/1 is irreducible in S 1 R. (b) This shows that if x is irreducible in R then x/1 is a unit or a prime element in S 1 R. If x/1 is a unit, then (x) S and x S, since S is saturated. In this case x is a prime element of R. Suppose that x/1 is a prime element of R and let a, b, c R with ab = xc. We have to show that x a or x b. Since x/1 is prime in S 1 R, (x/1) (a/1) or (x/1) (b/1). Suppose that (x/1) (a/1). Then ( ) a/1 = (x/1)(d/e) where d R and s S. In particular, e = ɛp 1... p r where ɛ R and p i R prime elements. Since e is a product of prime elements we can cancel those with p i d and may assume that (d, e) = 1. ( ) yields that ae = xd in R. Hence for all 1 i r, p i x, since p i R prime and p i d. By assumption x R is irreducible, hence x = αp i with α R. But then x S and x/1 a unit in S 1 R, a contradiction. This shows that e = ɛ R and x a. (7) [8pts] Let M be an R-module and I R and ideal. Suppose that M m = 0 for every maximal ideal m R with I m. Show that M = IM. Proof. Consider the R-module M = M/IM and let m R be a maximal ideal of R. If I m then I m = R m and M m = 0. If I m then by assumption M m = 0 and M m = 0. By the local-global principle M = 0. (8) [8pts] Suppose that R is an integral domain. An R-module M is called torsion free if for all a R (0) and all m M (0) it holds that am 0. Show that M is a torsion free R-module if and only if M m is a torsion free R m -module for all maximal ideals m R. Proof. Suppose that M m is torsion free for all maximal ideals m R. Let n M and a R (0) with an = 0. If n 0 the annihilator ann(n) is a proper ideal of R and there is a maximal ideal m of R with ann(n) m. This implies that n/1 0 in 3

4 4 M m. Since R is an integral domain the canonical map R R m is injective. Let t ann(n) (0), then t/1 0 in R m and (t/1)(n/1) = 0 in M m, a contradiction. The converse also holds true: Suppose that M is torsion free. Let m R be a maximal ideal of R. Suppose that r, t R m, s R and n M with (s/t)(n/r) = 0 in M m. Then there is an element u R m so that (us)n = 0 in M. Since M is torsion free either n = 0 or us = 0. Thus n/r = 0 or s/t = 0. (9) [16pts] Let R be a ring and f = n i=0 a ix i R[x] be an element in the polynomial ring over R. Show: (a) f is invertible in R[x] if and only if a 0 R and for all i 1: a i is nilpotent. (b) f is a zerodivisor in R[x] if and only if there is an element b R (0) so that bf = 0. Proof. (a) : Let f = n i=0 a ix i R[x] with a 0 R and a i nilpotent for all 1 i n. Then there is an N N so that ( n a i x i ) N = 0. i=1 With g = (1/a 0 ) n i=1 a ix i we have that (1 + g)(1 g + g 2... ± g N 1 ) = 0 and f is invertible. Let g = m i=0 b ix i R[x] with fg = 1. Then a 0 b 0 = 1 and a 0 R. The case where n = 0 or m = 0 is trivial. Assume n, m 1. Claim: For all 0 r m it holds that a r+1 n b m r = 0. The proof of the claim is by induction on r. If r = 0 then a n b m = 0. Suppose that a k+1 n b m k = 0 for all 0 k < r m. Then a r n = a r nfg = Consider the coefficient of x n+m r : n+m l=0 a r n( i+j=l a i b j )x l = 0. a r n(a n b m r + a n 1 b m r a n r b m ) = 0. Since a r nb l = 0 for all l > m r it follows that a r+1 n b m r = 0. Thus a m+1 n b 0 = 0. Since b 0 is invertible we obtain that a m+1 n = 0. Since a n is nilpotent the element a n x n is contained in every maximal ideal of R[x]. Thus h = f a n x n is invertible and we can apply the same argument in order to obtain that a n 1 is nilpotent, etc. (b) The backward direction is trivial. In order to prove the forward direction let f = n i=0 a ix i, g = m i=0 b ix i R[x] with fg = 0. We may assume that f 0 and that g is a polynomial of minimal degree with fg = 0. Since a n b m = 0 the polynomial a n g is either zero or has a smaller degree than g. Since f(a n g m ) = 0 it follows that a n g = 0. Claim: a i g = 0 for all 0 i n.

5 We show by induction on r that a n r g = 0 for all 0 r n. We already know that a n g = 0. Suppose the statement is shown for all 0 k < r. If a n r g 0 then deg(g) = deg(a n r g) since f(a n r g) = 0 and g of minimal degree. This implies that a n r b m, the leading coefficient of a n r g, is nonzero. The coefficient of x n+m r of fg(= 0) is: i+j=m+n r a i b j = a n r b m + i+j=m+n r;i>n r a i b j = 0. By induction hypothesis the right hand sum is 0. Thus a n r b m = 0. The claim implies that b m f = 0. (10) [10pts] For any ring R show that Jrad(R[x]) = nil(r[x]). Proof. Obviously, nil(r[x]) Jrad(R[x]). Let f = n i=0 a ix i Jrad(R[x]). Then for all g R[x]: 1 fg R[x]. Thus, for g = x the polynomial 1 xf = 1 n i=0 a ix i+1 R[x]. By Problem 9: a i nil(r) for all 0 i n and f nil(r[x]). 5

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