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1 Pacific Journal of Mathematics INJECTIVE MODULES OVER NOETHERIAN RINGS EBEN MATLIS Vol 8, No 3 May 1958

2 INJECTIVE MODULES OVER NOETHERIAN RINGS EBEN MATLIS Introduction In this discussion every module over a ring R will be understood to be a left i2-module R will always have a unit, and every module will be unitary The aim of this paper is to study the structure and properties of injective modules, particularly over Noetherian rings B Eckmann and A Schopf have shown that if M is a module over any ring, then there exists a unique, minimal, injective module E(M) containing it The module E(M) will be a major tool in our investigations, and we shall systematically exploit its properties In 1 we show that if a module M has a maximal, injective submodule C (as is the case for left-noetherian rings), then C contains a carbon-copy of every injective submodule of M, and MjC has no injective submodules different from 0 Although C is unique up to an automorphism of M y C does not in general contain every injective submodule of M, In fact, the sum of two injective submodules of a module is always injective if and only if the ring is left-hereditary In 2 we show that for any ring R a module E is an indecomposable, injective module if and only if E = E(R\J)y where J is an irreducible, left ideal of R We prove that if R is a left-noetherian ring, then every injective Jϋ-module has a decomposition as a direct sum of indecomposable, injective submodules Strong uniqueness assertions can be made concerning such decompositions over any ring In 3 we take R to be a commutative, Noetherian ring, and P to be a prime ideal of R We prove there is a one-to-one correspondence between the prime ideals of R and the indecomposable, injective R- modules given by P**E(RjP) We examine the structure of the module E = E{RjP)y and show that if A t is the annihilator in E of P\ then E = U A t and -4 ί+1 /A t is a finite dimensional vector space over the quotient field of R/P The ring of iϋ-endomorphisms of E is isomorphic in a natural way to R p, the completion of the ring of quotients of R with respect to R-P As an ^-module E is an injective envelope of RpjP, where P is the maximal ideal of R p If P is a maximal ideal of Ry then E is a countably generated β-module Every indecomposable, injective i2-module is finitely generated if and only if R has the minimum condition on ideals In 4 we take R to be a commutative, Noetherian, complete, local ring, P the maximal ideal of R and E = E{RjP) Then the eontravariant, Received April 10, 1958 In revised form June 9,

3 512 EBEN MATLIS exact functor Ήom E (A, E) establishes a one-to-one correspondence between the category of modules with ACC and the category of modules with DCC such that Hom i2 (Hom β (A, E),E) = A, for A in either category In particular, there is a lattice anti-isomorphism of the ideals of R and the submodules of E given by annihilation This paper is essentially the first half of a doctoral dissertation submitted to the University of Chicago The author would like to express his deep appreciation for the assistance and inspiration given to to him by Professor I Kaplansky 1 Maximal Injective Submodules* DEFINITION Let R be a ring and S an β-module Then an iϋ-module M is said to be an essential extension of S, if S c M, and if the only submodule T of M such that S Π T = 0 is the submodule T = 0 B Eckmann and A Schopf have shown [4] that if R is a ring and M an ΛJ-module, then there exists an injective ^-module E(M) which is an essential extension of M, If C is any other injective extension of M y then there exists in C a maximal, essential extension A of M, and there is an isomorphism of E(M) onto A which is the identity on M We shall call E(M) the injective envelope of M A Rosenberg and D Zelinsky have called this module the injective hull of M [9] LEMMA 11 Let R be a ring, S and T R-modules, and D an injective submodule of Sξ )T Let E be an injective envelope o/dpis in D, and let F be a complementary summand of E in D Thus D = EQ)F; and E and F project monomorphically into S and T, respectively Proof It is clear that F projects monomorphically into T Let / be the projection of E into S Since Ker /c Γ, Ker /Π φ Π S) = 0 However, E is an essential extension of D Π S, and so Ker / = 0 The following proposition is well known [3, Ch 1, Ex 8], but we state it for the sake of reference PROPOSITION 12 Let R be a left-noetherian ring Then: (1) A direct limit of injective R-modules is injective (2) A direct sum of R-modules is injective if and only if each summand is injective It is an immediate consequence of Proposition 12 that if R is a left-noetherian ring and M an iϋ-module, then M possesses a maximal, injective submodule C Concerning this situation we have the following : THEOREM 13 Let R be any ring and M an R-module such that M has a maximal, injective submodule C Then:

4 INJECTIVE MODULES OVER NOETHERIAN RINGS 513 (1) If N is a complementary summand of C in M, then M C φ N, and MjC = N has no injective submodules different from 0 (2 ) If E is any injective submodule of M, then the projection of E into C maps E isomorphically onto an injective envelope ofeγ\c in C (3) If D is any other maximal, injective submodule of M, then there is an automorphism of M which carries C onto D and is the identity on N Proof (1) is obvious, and (2) follows immediately from (1) and Lemma 11 We will prove (3) Let / be the projection of D into C By (2) f(d) is an injective envelope of D Π C in C and thus C = f(d) φ C l9 where C λ is an injective submodule of C However, since (DίlQc f(d), D Π CΊ = 0; and thus by the maximality of D, CΊ = 0 Therefore, since / is one-to-one by (2),/ is an isomorphism of D onto C, and so D Π N = 0 Thus M = D φ iv, and this completes the proof of the theorem If the sum of the injective submodules of a module M is always injective, then M has a unique, maximal, injective submodule which contains every injective submodule of M However, in general, this is not the case In fact, we have the following THEOREM 14 Let R be any ring The sum of tiυo injective submodules of an R-module is always injective if and only if R is lefthereditary Proof If R is left-hereditary and N lf N z are injective submodules of an i2-module N, then N L + N 2 is a homomorphic image of the injective i2-module Λ^φΛ^, and hence is injective Conversely, assume that the sum of two injective submodules of any i2-module is injective Let M be any injective iϋ-module and H a submodule of M We will show that MjH is injective, and this will prove that R is left-hereditary Let M v, M 2 be two copies of M, N = M D the submodule of N consisting of the elements (A, h), where h e H The canonical homomorphism : N -> N/D maps M 19 M 2 isomorphically onto submodules M lf M 2 of N/D, respectively and, therefore, (NΊD)ίM ι is injective Since NjD = M L + M 2, N/D is injective The composite mapping: Λf-> M 2 -> M 2 -> (NID)!M 1 defines a homomorphism of M onto (N/D)IM 1 with kernel H Therefore, MjH is injective and the proof is complete It follows easily from Proposition 12 and Theorem 14 that if M is any module over a left-noetherian, left-hereditary ring, then M has a unique, maximal, injective submodule C which contains every injective submodule of M

5 514 EBEN MATLIS 2 Indecomposable Injective Modules PROPOSITION 21 Let R be a ring and {M t } a finite family of R- modules Then the natural imbedding of Σ M* into Σ W ) can be extended to an isomorphism of (Σθ^ί) cmd Σ Φ W ) If R is left-noetherian, the finiteness restriction can be omitted Proof We identify ΣφM 4 with its image in Σ Θ W If the family is finite, or if R is left-noetherian, the latter module is injective Hence it suffices to show that Σ 2?(Λf 4 ) is an essential extension of Σφikf έ, regardless of whether the family is finite or not Let χ = 0 e Σ Φ E(M t ); then since x has only a finite number of non-zero coordinates, we can by working successively on each coordinate find an element r e R such that rx Φ 0 and π e Σ Θ ^ This proves the assertion DEFINITION Let A be a module over a ring R We say that A is indecomposable, if its only direct summands are 0 and A The following proposition follows readily from the definitions : PROPOSITION 22 Let M be a module over a ring R Then the following statements are equivalent: (1) E(M) is an injective envelope of every one of its non-zero submodules ( 2 ) M contains no non-zero submodules S and T such that S Π T = 0 ( 3 ) E(M) is indecomposable In general it is not true that if M is indecomposable, then E(M) is also indecomposable For let M be an indecomposable, torsion-free module of rank two over an integral domain R Let Q be the quotient field of R Then E(M) ~Q^)Q [5, Th 19] DEFINITION Let ί be a ring and / a left ideal of R such that / = J λ Π Π J n, where J i is a left ideal of R We call this a decomposition of I, and we say the decomposition is irredundant, if no J % contains the intersection of the others THEOREM 23 Let R be a ring and I J τ Π Π J n &n irredundant decomposition of the left ideal I by left ideals Ji Assume that each E{RlJi) is indecomposable Then the natural imbedding of R\l into C E(RIJ τ ) φ φ E(R/J n ) can be extended to an isomorphism of E(R/I) onto C Proof We identify R/I with the submodule of C consisting of all elements of the form (r + J lf, r + J n ), where r e R Since C is

6 INJECTIVE MODULES OVER NOETHERIAN RINGS 515 injective, it is sufficient to show that it is an essential extension of Rjl We first show that Rjl (Ί R\Jι φ 0 for all i for simplicity we will prove only the case i 1 By the irredundancy of the decomposition of / we can find r e R such that r e J 2 Π Π J nf but r \$ J x Then (r + J 19 r + </ 2,, r + J w ) = (r + Λ, 0,, 0) is a non-zero element of RII Π R\J λ By Proposition 22 EiR/J^ is an injective envelope of Rjl Π RfJi Let x φ 0 6 C By working successively in each component we can find s e R such that sx φ 0, and the ith component of sx is in R\l Π RjJ t Thus s# is a sum of elements of Rjl, and hence sx e R/L Thus C is an essential extension of RII, DEFINITION Let J be a left ideal of a ring B We say that J is irreducible, if there do not exist left ideals K and L of R, properly containing J, such that K Π L J NOTATION Let M be a module over a ring i?, and let S be a subset of M Then we define O(S) = {r e i2 rs = 0} Clearly O(S) is a left ideal of R THEOREM 24 ^4 module E over α ring R is an indecomposable, injective module if and only if E = E(R/J), where J is an irreducible left ideal of R In this case, for every x φ 0 e E, O(x) is an irreducible left ideal and E = E{RIO{x)) Proof Let J be an irreducible left ideal of R, and K, L left ideals of R such that KIJ Π L/J = 0 Then K Π L = J, and thus either K= J or L = J Hence E(R/J) is indecomposable by Proposition 22 Conversely, let Z? be an indecomposable, injective module, # = 0 6 #, and J = O(x) By Proposition 22 E = E(Rx); and since ifa; ^ u?/j, we have # = E(R/J) Suppose that J = if Π L is an irredundant decomposition of J by the left ideals K, L We imbed R\J in C = E(R\K)< )E(R\L), and let D be an injective envelope of R\J in C Due to the irredundancy of the decomposition of J, R\J Π R\K Φ 0 Therefore, by Lemma 11 and the indecomposability of D, D projects monomorphically into E{RjK) The image of D is an injective module containing R\K, and hence is equal to E(R/K) Thus E(RIK) is indecomposable similarly, E(R/L) is indecomposable Thus by Theorem 23 E{RIJ)^E{RIK) E{RIL) This contradicts the indecomposability of E(RjJ), and thus Jis irreducible REMARKS (1) Every ring R possesses indecomposable, injective modules For if J is a maximal, left ideal of R (such exist by Zorn's lemma and the fact that R has an identity element), then J is an irreducible ideal,

7 516 EBEN MATLIS and so E(RjJ) is indecomposable by Theorem 24 (2) It can be shown that every injective module over a ring R has an indecomposable, direct summand if and only if every left ideal of R has an irredundant decomposition by left ideals, at least one of which is irreducible THEOREM 25 Let R be a left-noetherian ring Then every injective R-module has a decomposition as a direct sum of indecomposable, injective submodules Proof Let M be an injective J?-module Then by Zorn's lemma we can find a submodule C of M which is maximal with respect to the property of being a direct sum of indecomposable, injective submodules Suppose that C Φ M By Proposition 12 C is injective, and hence there is a non-zero submodule D of M such that M = Cφ D Let x Φ 0 e D; since R is left-noetherian, O(x) is an intersection of a finite number of irreducible, left ideals [8, Lemma 182] Therefore, by Theorems 23 and 24, E(R\O(x)) is a direct sum of a finite number of indecomposable, injective iί-modules Now Rx = RjO(x) 9 and so we can consider that E(R/O(x)) is imbedded in D But then C E(RIO(x)) contradicts the maximality of C, and thus C = M This concludes the proof of the theorem Theorem 25 is a generalization of a theorem of Y Kawada, K Morita, and H Tachikawa [6, Th 32] who considered the case of a ring with the minimum condition on left ideals Their theorem, in turn, is a generalization of a theorem of H Nagao and T Nakayama [7] concerning finite dimensional algebras over a field DEFINITION Following the most general definition [3, p 147] we will call a ring H a local ring if the set of non-units of H forms a twosided ideal PROPOSITION 26 Let E be an injective module over a ring R and H = Ήom R (E, E) Then H is a local ring if and only if E is indecomposable In this case f e H is a unit if and only if Ker / = 0 Proof If E is not indecomposable, then H has a non-zero idempotent different from the identity, which can't happen in a local ring Conversely, assume that E is indecomposable If / e H is a unit, then of course Ker /=0 while if Ker /= 0, then f(e) is injective, f(e)=e f and so / is a unit of H Let g, h be non-units of H Then Ker g Φ 0, Ker h Φ 0, and by Proposition 22 Ker g Π Ker h Φ 0 Since Ker g Π Ker h c Ker (g + h), it follows that g + h is a non-unit It is well-

8 INJECTIVE MODULES OVER NOETHERIAN RINGS 517 known that if the sum of two non-units of a ring is always a non-unit, then the ring is a local ring The preceding proposition shows that G Azumaya's generalization of the Krull-Remak-Schmidt theorem [1, Th 1] applies to direct sums of indecomposable, injective modules We state this theorem without proof in the form in which it applies to our considerations PROPOSITION 27 Let R be a ring and M = Σφ α (a e A) an R- module which is a direct sum of indecomposable, injective submodules E a Then: (1) Given a second decomposition of M into indecomposable submodules F b (b 6 B): M = Σ φ F b, then there exists a one-to-one mapping a -» b(a) of A onto B such that E a is isomorphic to F b ( a ), for each a e A In other words the decomposition of M into indecomposable submodules is unique up to an automorphism of M (2) For any non-zero idempotent element f in Hom^M, M) there exists at least one E a such that f is an isomorphism on E a In particular every indecomposable, direct summand of M is isomorphic to one of the E a 's It is an open question whether every direct summand of M is also a direct sum of indecomposable, injective modules Of course if R is left-noetherian, then Theorem 25 provides an affirmative answer The following proposition will be needed in 3 PROPOSITION 28 Let R be a ring, B an R-module, and C an injective R-module let y e C and x lf, x n e B Then Π 0{x i )(Z O(y) if and only if there exist f 19,/ w e Hom B (B, C) such that y f 1 x 1 + Proof If y = fx τ f n χ nt then clearly Π O(x t ) c O(y) Now assume that Π O(# 4 ) c O(y) Rj Π Ofe) is a cyclic i2-module generated by an element z with O(z) = Π O(# 4 ) There exists an TMiomomorphism : Rz ~> Ry such that z-> y and there exists a natural imbedding of Rz into Σ θ ^ ( such that z -» (x l9, x n ) This yields the diagram : I Ry I c Since C is injective, there is an i2-homomorphism /: Σ Φ- β ^ -* C such that f(x 19, x n ) = y Now / = Σ/t, where f : Rx h ~> C is the restriction of / to Rx t We have the diagram :

9 518 EBEN MATLIS 0 -> Rx t ^ B A i c Therefore, we have imiomomorphisms fi'b-^c such that f t extends A for each i Thus f λ x λ H + f n x n = A%i + / 3 Injective Modules over a Commutative, Noetherian Ring Throughout this section R will be a fixed commutative, Noetherian ring PROPOSITION 31 There is a one-to-one correspondence between the prime ideals of R and the indecomposable, injective R-modules given by P <-> E{R\P), for P a prime ideal of R If Q is an irreducible P-primary ideal, then E(R/Q) = Proof It is obvious that a prime ideal is irreducible and thus E(RIP) is an indecomposable, injective module by Theorem 24 Let P τ,pz be two prime ideals of R such that E(RIP Ύ ) = E(RIP Z ) We consider that RjP Ύ and RjP 2 are imbedded in E(R\P Ύ ) Hence by Proposition 22, R/P ι Π RIP* Φ 0 However, every nonzero element of R/P t (resp RlPi) has order ideal P 1 (resp P 2 ) Thus P Ύ P 2, and the mapping P ~> E{R(P) is one-to-one Let E be any indecomposable, injective iϋ-module Then by Theorem 24 there is an irreducible ideal Q of R such that E ~ E{RjQ) Now Q is a primary ideal with a unique associated prime ideal P [8, Lemma 183] If Q P, we are finished hence assume that Q Φ P Then there is a smallest integer n > 1 such that P n c Q Take b e P n ~ λ such that b \$ Q, and denote the image of b in R/Q by δ Clearly 0(5) D P on the other hand if a e 0(5), then ab e Q, and so a e P, showing that 0(5) = p Therefore, there is an element of E(RjQ) with order ideal P, and thus E{RjQ) ^ #(i?/p) by Theorem 24 This completes the proof of the proposition LEMMA 32 Let P be a prime ideal of R and E Έ(R\P) Then : (1) Q is an irreducible, P-primary ideal if and only if there is an x Φ 0 e E such that 0(x) = Q (2) Ifre R-P, then 0{rx) = 0(x) for all x e E, and the homomorphism : E -» E defined by x -* rx is an automorphism of E Proof (1) This is an immediate consequence of Theorem 24 and Proposition 31

10 INJECTIVE MODULES OVER NOETHERIAN RINGS 519 ( 2 ) Let r e R P the map : E -» E defined by x -* rx, for x e E, has kernel O by (1); therefore, it is an automorphism by Proposition 26 It follows that O(x) = O(rx) for every x e E DEFINITION of R Define and Let M be an injective i?-module, and P a prime ideal M p = {x e MI Pc Rad O(α)} ivp = {a? e Jlί, I P ^ Rad O(α)} Let M 2 Φ ^α (a e A) be a direct decomposition of Λf by indecomposable, injective submodules E a By Proposition 31 there is associated with each E a a prime ideal which we shall designate by P a Define H p to be the direct sum of those E a '& such that P a = P We shall call H p the P-component of the decomposition of M and we define the P-index of M to be the cardinal number of summands E a in the decomposition of H p THEOREM 33 (1) M p is the direct sum of those E a 's such that P a Z) P, and N p is the direct sum of those E y a s such that P a \$ P Thus M p and N p are submodules of M; and, in fact, are direct summands of M In addition, the direct sums of E a 's just defined are independent of the decomposition of M (2) M is a direct sum of the P a -components H p We have H p = Mp/Np, and H p is unique up to an automorphism of M In other words M is determined up to isomorphism by a collection of cardinal numbers, namely its P-indices If P is a maximal ideal of R, then H p M p, and thus H p is independent of the decomposition Proof (1) Let x Φ 0 e M; then x = x x + + x n, where x % Φ 0 e E a Thus O(x) = Π Ofa); and by Lemma 32 (1) 0{x t ) is P α -primary From this it follows that Rad O(x) = Π P α Hence it is clear that x e M p if and only if x is an element of the direct sum of those E a '& such that P a z) P It also follows that x e N p if and only if x is an element of those E a '8 such that P a J P (2) This follows immediately from (1) and Proposition 27 We now proceed to examine the structure of a typical indecomposable, injective i2-module

11 520 EBEN MATLIS THEOREM 34 Let P be a prime ideal of R, E = E(RIP), and A t = {xe E\I*x = 0} Then: (1) A t is a submodule of E, A t c A i+l9 and E U A % (2) Π 0(x) = P (ί), w/^rβ P (ΐ) is the ith symbolic prime power of P xea (3) T%# non-zero elements of A ί+ι IAi form the set of elements of E\A % having order ideal P (4) Let K be the quotient field of RjP Then A i+ί IAi is a vector space over K, and A λ ~ K Proof (1) Clearly A t is a submodule of E, and A ( c4 i+1 Let x Φ 0 e E; then by Lemma 32 (1) O(x) is a P-primary ideal Thus there exists a positive integer i such that PcO(4 and so x e A t Therefore, E = U A, (2) By Lemma 32 (1) Π O(#) is the intersection of all irreducible, P-primary ideals containing F\ It is easily seen that this intersection is equal to P (i) (3) Since PA i+ι cz A if it follows from Lemma 32 (2) that every non-zero element of A ί+i /A has order ideal P Conversely, if x e E is an element such that Pxc:A ίf then x e A ί+ί Therefore, every element of ElAi having order ideal P is an element of A i+1 IAi ( 4 ) If r e R, denote its image in RjP by r Similarly, if x 6 A i+1, denote its image in A i+1 /Ai by x If s e R P, then by Lemma 32 (2) there exists a unique y e A ί+1 such that x = sy Define an operation of K on A ί+ll IAi by (r/s) x = ry It is easily verified that with this definition A i+1 jai becomes a vector space over K Take x Φ 0 e A τ Since A o = 0, A x is a vector space over K and so we can define a iί-monomorphism g : K-> A τ by #(r/s) = (r/s) a?, for r/s e JL Let z Φ 0 e Ai Since E 1 is an essential extension of A 19 there exist tywer P such that to = wz Thus <7( /w) = z 9 and # is an isomorphism REMARKS (1) Let 7c P be an ideal of R, and A(l) = {# e E 1 1 /^ = 0} Then it follows from [2, Th 17] that A(/), considered as an i2//-module, is an injective envelope of (JB/I)/(P//) Therefore, A(I) is an indecomposable, injective Rj/-module We obtain from [8, Lemma 1] that Hom Λ (B/I, E) ~ A(/); and if J is an ideal of R containing 7, then Hom Λ (J/I, #) ^ A(I)!A(J) (2) If P 0, R is an integral domain, and E A x ~ K, which is then the quotient field of 72 If P Φ 0, the module E is the natural generalization of the typical divisible, torsion, Abelian group Z p We

12 INJECTIVE MODULES OVER NOETHERIAN RINGS 521 will show in a later theorem that A i+1!a h is a finite dimensional vector space over K, although in general it is not one dimensional as is the case for Z Poo The ring of endomorphisms of Z p^ is the p-adic integers This will generalize to the theorem that the ring of iϋ-endomorphisms of E is R p, the completion of the generalized ring of quotients of R with respect to R P DEFINITION Let P be b prime ideal of R, S = R P, and N the ^-component of the 0-ideal of R; ie N= {a e R there exists an element ce S with ca = 0} Let R* = RjN, P* = P/JV, and let S* be the image of S in F No element of S* is a zero divisor in 22*, and thus we can form the ordinary ring of quotients of R* with respect to S* We denote this ring by i? p ; it is a commutative, Noetherian, local ring with maximal ideal P = 22 P P We will denote the completion of R v by R p, and its maximal ideal by P may be found in [8, Ch 2] Further details of the construction LEMMA 35 Let K be the quotient field of RIP Then K and R P \P are isomorphic as fields and P 1^1^ (x) K R/P and Pn /p fi+1 are isomorphic as vector spaces over K Proof Let rer, ser P and denote the images of r, s in RjN by r*, s*, and their images in R\P by r, s It is easily verified that the mapping: K -> R p jp r defined by rjs ~> (r*/s*) + P r is a field isomorphism We make every KJP'-module into a iγ-module by means of this isomorphism Let q e P ι and denote its image in RIN by g* We define a mapping : pηp^ <g) Λ/F If -* P's/P'*- 1 by [g + P ί+1 ] (g) r/s -> (r/β)[?* + P /ί+1 ] We define a mapping in the reverse direction by g*/s*+p'* +1 -»[g+p ί+1 ](x)r/s These mappings are if-homomorphisms, and their product in either order is the identity mapping Thus P i jp i+l (g) RjPK is isomorphic as a vector space over K to pηp*+\ It is easily seen that if Q is a P-primary ideal, then NaQ Hence by Lemma 32 (1) E(RjP) is an i?*-module We will henceforth assume (without loss of generality) that N = 0 The simplification amounts to this : R p is an ordinary ring of quotients of R THEOREM 36 With the notation as already given let E = E{RIP) Then E is in a natural way an R p -module, and as such it is an injective envelope of R P \P The R-submodules A t {x e E \ P ι x 0} ofe are equal to the corresponding submodules defined for R p,

13 522 EBEN MATLIS Proof We define on operation of R p on E as follows : let x e E, r e R, and s e R P then by Lemma 32 (2) there exists a unique y e E such that x sy Define (rjs)x = r?/ It is easily verified that with this definition E becomes an iϋ^-module If G is an i? p -module containing E, then E is an iϋ-direct summand of G But then E is an indirect summand of G also, and thus E is iϋp-injective Since E is indecomposable as an β-module, it is a fortiori indecomposable as an If x e E has order ideal O(x) in R, then a straightforward calculation shows that the order ideal of x in R p is R p O(x) From this it follows readily that P ι x = 0 if and only if P ri x 0 These remarks show that we may assume that R is a local ring with maximal ideal P We denote its completion by R, and the maximal ideal of R by P = RP Let a e R; then there exists a Cauchy sequence {α w } in R such that α w -* a Let x e E then x e A t for some i There exists an integer iv such that a n a m e P* whenever n > m^ N Thus α w # = α m #, and if we define ax = α^, it is easily verified that this definition makes E into an ΐ?-module Let E be the iϋ-injectίve envelope of E Since A τ = iϋ/p = JB/P over -ff, it follows from Proposition 22 that E is the ϋϊ-injective envelope of R/P Therefore, by Theorem 34 E = U A 4, where A, = {x e E \ P ι x =0} As an i2-module E splits into a direct sum of E and an ϋj-module C If x e C, then x e A % for some i and iΐ r e R, there exists r 6 R such that r = r (mod P*) Hence rx = rx e C, and thus C is an R- module However, E is indecomposable as an iu-module and thus C = 0 and E = E A simple calculation shows that if x e E has order ideal O(x) in iϋ, then it has order ideal RO(x) in R From this it follows readily that A t = A t THEOREM 37 With the notation of Theorem 36 let H = Rom R (E, E) Then H is R-isomorphic to R p ; more precisely, every R-homomorphism of E into itself can be realized by multiplication by exactly one element of R p Proof It is easily seen that H = Hom^ (E, E) Therefore, by Theorem 36 we can assume without loss of generality that R is a complete, local ring with maximal ideal P Since Π P ι 0, if r Φ 0 e R, there exists an integer i such that r \$ P ι Hence ra% Φ 0 by Theorem 34 (2); and E is a faithful iϋ-module Consequently, we can identify R with the subring of H consisting of multiplications by elements of R Define iϊ 4 = {he H\ h{a % ) = 0} Then Π H, = 0, and since g(a t ) c A t

14 INJECTIVE MODULES OVER NOETHERIAN RINGS 523 for every g e H, Hi is a two-sided ideal of H We will prove by induction on i that if / e H, then for each integer i there exists an element p t e R such that /= p t (mod H t ) Since A λ = R/P, there exists y e A ± such that A λ Ry Let f e H; then there exists p τ e R such that fy PiV- Thus f= p τ (mod HJ We assume the assertion has been proved for i ^ 1 Let P ί+1 Q 1 f) ΓiQ n be an irredundant decomposition of P ί+ι by irreducible, P-primary ideals Qj By Lemma 32 (1) we can find elements Xj e A i+ι such that O(xj) = Qj forj> i,, n By the irredundancy of the decomposition we can find q 5 e Q : Π ΓΊ Qj Π Q n (where Qj means Q j is left out) such that q 5 e Q jy Thus q j x j Φ 0 and q 5 x h 0 for j Φ k By induction there exists p t e R such that g = / p L e H^ Since Pxj c A i9 P(gXj) g(pxj) = 0 and thus gxj e A τ Therefore, by Proposition 22 there exists r 3 e R such that r^qpcj) = flra? Je Let <7 = r λ q L + + r n q n Then ^^ = qx ό for i = 1,, n Let OJ 6 A j+1 ; then O(x) z) P* +1 = 0{x λ ) Π Π Ofe) Hence by Proposition 28 there exist f u f e H such that x f x + + f x 9 n 1 ί n n By the induction hypothesis f ό s ό + h j9 where Sj e R and h 3 e H t Since Pxj c A u P(hjXj) = hj(pxj) = 0 hence hjxj e A L and g{h 3 x ό ) 0 Since g^ e A if q(hjxj) h ό {qx ό ) 0 Thus g(x) = SxtoαO + + s n (gx n ) = s^qxj + + s n (qx n ) = g(a ) Therefore, g q e H i+1 We let p i+1 = p t + q then /= p i+ι (mod H ί+ι ), and we have verified the induction Thus we have associated with / a sequence {p t } of elements of R If n^m, then p n - p m = (p w -/) + (/- p m ) e H n + H m = H n Therefore, (p» - Pm)A w = 0, and so by Theorem 34 (2) p n - p m e P\ Thus {p 4 } is a Cauchy sequence in it! and since R is complete, Pι-> p e R Since P 1 c H if pi-* p in fί But p t -*f in iί, and so / = p e R Thus H R and the theorem is proved COROLLARY 38 (1) Let y, x l9, x n e E Then Π O(x ό ) c O(i/) i/ α^d only if there exist r 19, r n e R p such that y r x x x + + r n x n (2) Every R-homomorphism from one submodule of E into another can be realized by multiplication by an element of R p Proof (1) This is an immediate consequence of Proposition 28, Theorem

15 524 EBEN MATLIS 37, and the definition of the operation of R p on E (2) Since E is injective, every ϋj-homomorphism from one submodule of E to another can be extended to an endomorphism of E DEFINITION Let P be a prime ideal of R, and let P (ί) = Qι Π Π Q t Π Qt+i (Ί Q n be an irredundant decomposition of the symbolic prime power P (ί) by irreducible ideals Q m such that Q m φ p( έ ~ υ for m 1,, t and Q m ID P^-^ for m = ί + 1,, n We say that this is a minimal decomposition of P (i) and that P (i) belongs to t, if is the smallest integer for which we can obtain such a decomposition Clearly for i > 0 P (ί) has a minimal decomposition and t > 0 if and only if P^ Φ P*'- 1 ) THEOREM 39 Let P be a prime ideal of R, E = E(RIP), and assume that P( i+1 > belongs to t Then the dimension of A i+1 jai as a vector space over the quotient field of RjP is equal to t Proof We can assume without loss of generality that R is a local ring with maximal ideal P Then P (ί) = P ι for every i Let P ί+1 Qι Π Π Q t Π Q t+ ι Π c Q w be a minimal decomposition of P ί+ \ where Q m φ P ι for m = 1,, t and Q m z) P* for m = + 1,, n By Lemma 32 (1) we can find x m e A i+1 such that O(x m ) = Q m for m = 1,, n Let x m = ίr m + A t ; we will show that x lf, form a basis for A ^t t+ i/ai over RIP (1) δ lf, x t are linearly independent over R/P Suppose that r x x λ + + r s x s 0 where s S t and r 5 r 5 + P for some r 5 e i2 We can assume that r t Φ 0 for i = 1,, s Now r x x x + + r s x s y e A % \ and so r λ x x = r 2 α; 2 r s α; s + y Applying Lemma 32 (2) we have 0{x x ) ID 0{x 2 ) n Π O(x s ) n O(ι/) Hence P i+1 = O(α,) n Π O(^s) n O(?/) We can refine this irreducible decomposition of P i+ι to an irredundant one but since O(y) D P\ we will then have t 1, or fewer, components not containing P\ This contradicts the minimal nature of t and therefore, x ly <-,x t are linearly independent over R/P (2) x u,x t span A ί+ i/a 4 over K/P For let a;6i w ; then O(x) ID P* +1 = n O(x m ) Therefore, by Corollary 37 (1) there exist n,, r n 6 R such that x = r^ + + r n α; M Since Q m ID P 4 for m = t +1,, n, it follows that x m e A t for m = έ + 1,, n Thus x + A* is a linear combination of x lf, x % with coefficients in i2/p The main part of the following theorem was communicated to me by A Rosenberg and D Zelinsky THEOREM 310 With the notation of Theorem 39 let K be the quotient field of R/P Then A i+1 IAi is isomorphic as a vector space over K to the dual space of P i jp i^1 (g) Λjp K

16 INJECTIVE MODULES OVER NOETHER1AN RINGS 525 Proof By Lemma 35 and Theorem 36 we can assume that R is a local ring with maximal ideal P; and then we must prove that A i+ι ja is isomorphic as a vector space over R\P to the dual space of P i lp i+1 By [9, Lemma 1] we know that A i+1 IA t is isomorphic over R/P to Hom^PVP^1, E) It is clear that any i2-homomorphism of pηp i+1 into E has its range in A τ (which is isomorphic to RjP), and is actually an 22/P-homomorphism Thus we can identify Hom Λ (P*/P* +1, E) with Hom^pίP^P^1, RjP) Since the latter is the dual space of pηp ί+1 as a vector space over R/P, this concludes the proof Theorem 310 provides a second proof that A i+1 IAi is a finite dimensional vector space over K; and in addition shows that this dimension is equal to the dimension over K of F^P 1 * 1 RIP K- Combining this result with Theorem 39 we see that P< ΐ+J ) belongs to t if and only if piipi+i g) jζ k dimension t over K Rip as THEOREM 311 (1) If P is a maximal ideal of R, then A % c E(R\P) is a finitely generated R-module for every integer i and thus E(R/P) is a countably generated R-module (2) R has the minimum condition on ideals if and only if every indecomposable, injective R-module is finitely generated Proof (1) The proof is by induction on i Since the case i 0 is trivial, assume that A 4 -χ is finitely generated By Theorem 39 we can find x u, x n e Ai such that if x 6 A i9 then there exists b e R P such that bx is in the module generated by x lf *,x n and A^ Since P is maximal and Px c A t - l9 x is in the module generated by A t - τ and x lf, x n Thus Ai is finitely generated (2) Suppose that R has the minimum condition on ideals Let P be a prime ideal of R then P is a maximal ideal, and there exists an integer i such that P* - P ί+ι - P* +2 = Therefore, E(RIP) = A t and by (1) E{RjP) is finitely generated over R Since E(RjP) is typical, all indecomposable, injective ^-modules are finitely generated Conversely, assume that every indecomposable, injective i2-module is finitely generated Let P be a prime ideal of R Then A ι c E(RjP) is finitely generated over R/P But A ι is isomorphic to the quotient field of RjP, and hence R/P is a field Thus prime ideals of R are maximal and since R is a Noetherian ring, this implies that R has the minimum condition on ideals 4 Duality DEFINITION Let R be any ring and Man i?-module Then we say

17 526 EBEN MATLIS that M has ACC {Ascending Chain Condition) if every ascending chain of submodules of M terminates, and we say that M has DCC {Descending Chain Condition) if every descending chain of submodules of M terminates PROPOSITION 41 Let R be a left-noetherian ring, and M an R- module having DCC Then E{M) is a direct sum of a finite number of indecomposable injective R-modules Proof By Theorem 25 E(M) = Σφ α (α e A), where the E a '\$ are indecomposable, injective i2-modules Assume that the index set A is infinite Then A has a properly descending chain of subsets: idapip Define the submodule C t of E(M) by C, = Σ E b {be At) Then M ZD (d Π M) =) (C 2 Π M) z> is a descending chain of submodules of M Since M Π E a Φ 0 for all a e A, the chain is properly descending This contradiction shows that A is finite DEFINITION Let R be any ring and M an iϋ-module For any positive integer n let M n denote a direct sum of n copies of M If x = (#!,, χ n ) e Λf\ where x 1 e M, we will denote x by fe) For a fixed pair i?*, Λf w we make the following definitions : let S be a subset of R n and B a subset of M n ; then define S' = {(»<) e Λf w Σ^Λ = 0 for all (r t ) e S} and B r = {(r f ) e iϊ w Σ^Λ = 0 for all (a? 4 ) e } If i? is commutative, then S r is a submodule of M n and β r is a submodule of iϋ\ THEOREM 42 Lei R be a commutative, Noetherian, complete, local ring, P its maximal ideal, and E E{R[P) Take a fixed pair R n, E n let S be a submodule of R n and B a submodule of E n Then : (1) S' = Rom R {R n IS, E), and E n /S r ~ Rom R (S, E), and S" = S ( 2 ) B f = Hom^^/5, E), and R n \B r ^ Hom Λ (5, E), and B" = S i/ A is α submodule or factor module of either R n or Έ n, then m^a, E), E) = A The operation ' is a lattice anti-isomorphism of the submodules of R n and E n Therefore, E n has DCC Proof {1) R n /S is generated by elements u i9 i = 1,, n, such that (r i )6 5φφΣ^w* = 0 LetfeΉom R (R n /S,E) and define φ: Ήom E (B n /S,E)->S' by Φ{f) = (fui) Clearly φ is a well-defined ϋmiomomorphism If Φ(f) = 0, then /% Λ = 0 for all i, and so / = 0 Thus φ is one-to-one If (x t ) e S', define /: R n /S^E by /%< = α? 4 ; and then φ(f) = (x t ) Thus φ is onto, and so Rom R {R n IS, E) = is' From the exact sequence :

18 INJECTIVE MODULES OVER NOETHERIAN RINGS 527 we derive the commutative diagram : 0 -> Hom Λ (i?7s, E) -* Rom R (R n, E) -> Hom Λ (S, E)-* 0 I 0 -» S' -> # w -> JS*/S' -> 0 Since E 1 is injective, the top row is exact Of course the bottom row is exact, and the vertical maps are isomorphisms Thus Hom jβ (*S, E) ~ Clearly ScS^c R n Suppose that (s 4 ) e S", but (s { ) \$ S Then V = Σ Si%«^Oe iϊ w /S Now 0(2/) c P, and so by Proposition 28 there exists an ίί-homomorphism g : R n js-> E such that g(y) Φ 0 However, S((y) = Σ s i9( u d 0, since (gui) e S r This is a contradiction, and so ( 2 ) Let (r 4 ) 6 B ; and (j/') e lw Define Φ \ B -* Rom B (E n IB, E) by ^(^*)[(2/0 + -δ] Σ r ivi- Then (P is a well defined β-homomorphism If φ(r % ) = 0, then Σ ^Vt = 0 for all (&) 6 ^w Thus r t E = 0 for all i Since iί 1 is a faithful i2-module by Theorem 37 r t = 0 for all i Thus Φ is one-to-one Let f e Hom R (E n IB, E), and for each i 1, -, w let ^ be the ίth component of E n Then the composite mapping : E -> ^ -> " w -» E n (B f -> " induces an iϋ-endomorphism of *, and so by Theorem 37 can be realized by multiplication by an element r t e R Thus if (Vi) e E n, fl(y t ) + B] = Σ r t y t If fe) e S, then Σ W =/[(a?,) + B] -0 and thus (r 4 ) e ί', Clearly φ(r t ) = f, and thus 0 is onto Therefore, B - Hom Λ (E"IB, E) From the exact sequence : 0^B->E n -> E n IB -* 0 we derive the commutative diagram : I 0 -> Hom β (ΐ; w /S, E) -+ Kom R (E n, E) -» Hom /e (5, E) ~> 0 The top row is exact, the vertical maps are isomorphisms and since E is injective, the bottom row is exact Thus R n \B' = Hom β (5, E) Clearly B c B" c E n Suppose that (z t ) e B'\ but (z t ) < B Since O((Zi) + B) c P, we can find by Proposition 28 an i2-homomorphism /: E n IB~+E such that f((z) + B) Φ 0 However, f=φ(r i ), where (n) e B'\ and thus f((z t ) + B)= Φir^z) + B) = Σ Λ^i = 0, since (s 4 ) 6 S r/ This contradiction shows that B B", REMARKS (1) It follows easily from Proposition 28 that if R is any ring, C an injective β-module which contains a copy of every simple

19 528 EBEN MATLIS i2-module, and M any R-moάvλe, then M is imbedded in a natural way in Hom^Hom^M, C), C) G Azumaya has also obtained this result [2, Prop 6] However, his other duality statements concern non-commutative rings with minimum conditions on both left and right ideals (2) It is easily seen that if R is a commutative, Noetherian ring, and C an injective iϋ-module such Hoiϊi Λ (Hom Λ (A, C), C) = A for every cyclic JS-module A, then R is a finite direct sum of complete, local rings R t And if C 4 is the ^-component of C, and P* the maximal ideal of B i9 then C 4 = E(B l jp i ) COROLLARY 43 Lβί R be a commutative, Noetherian, complete, local ring P its maximal ideal and E = E(RjP) Then : (1) Aw R-module M has ACC i/ αrcd owίi/ i/ it is a homomorphic image of R n for some n (2) An R-module M has DCC if and only if it is a submodule of E n for some n (3) If X, Y are the categories of R-modules with ACC and DCC, respectively, then the contravariant, exact functor Hom^, E) establishes a one-to-one correspondence Xt-> Y In particular, Hom i2 (Hom jr (lί, '), E=M for M in either category Proof (1) This is true for any left-noetherian ring (2) By Theorem 42 every submodule of E n has DCC On the other hand let M be a module with DCC By Proposition 41 E(M) = Ciφ φc n, where the C/s are indecomposable, injective iϋ-modules Let x Φ 0 6 M Then there exists an integer j such that P 3 x = P J+1 x, and so P j c O(x) + P j+ \ This implies that P j c O(x) [8, Prop 421] and thus O(x) is a P-primary ideal It follows from Lemma 32 (1) that Ci~E for i = 1,, n (3) This follows immediately from (1), (2), and Theorem 42 REFERENCES 1 G Azumaya, Corrections and supplementaries to my paper concerning Krull-Remak- Schmidfs theorem, Nagoya Math 1 (1950), , Λ duality theory for injective modules, (forthcoming) 3 H Cartan and S Eilenberg, Homological algebra, Princeton University Press, B Eckmann and A Schopf, Uber injective Moduln, Arch Math, 4 (1953), I Kaplansky, Infinite Abelian groups, University of Michigan Press, Y Kawada, K Morita, and H Tachikawa, On injective modules, Math Z 68, (1957), H Nagao and T Nakayama, On the structure of (MQ) and (M u ) modules, Math Z 59 (1953), D Northcott, Ideal theory, Cambridge University Press, A Rosenberg and D Zelinsky, Finiteness of the injective hull, (forthcoming) UNIVERSITY OF CHICAGO

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