# Projective and Injective Modules

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1 Projective and Injective Modules Push-outs and Pull-backs. Proposition. Let P be an R-module. The following conditions are equivalent: (1) P is projective. (2) Hom R (P, ) is an exact functor. (3) Every epimorphism β : M P splits. (4) P is isomorphic to a summand of a free R-module. Definition. A ring is called left hereditary if all left ideals are projective. A hereditary integral domain is called a Dedekind domain. Theorem. (1) If R is a left hereditary ring then every submodule of a projective left R-module is projective. (2) If R is a Dedekind domain then every finitely generated torsion free R-module is isomorphic to a direct sum of ideals. Furthermore, if a 1,...,a m and b 1,...,b n are ideals in a Dedekind domain R then a 1 a m b 1 b n if and only if m = n and the ideals a 1 a m and b 1 b n are isomorphic. Proposition. If P is a projective R-module and I a (two-sided) ideal, then P/IP is a projective R/I-module. Proposition. Let P 1 and P 2 be projective R-modules and let J be the Jacobson radical of R. IfP 1 /JP 1 P 2 /JP 2 then P 1 P 2. Proposition. If R is a local ring or a PID then all projective modules are free. Corollary. A finitely generated module M over a commutative noetherian ring R is projective if and only if M p is a free R p -module for every prime ideal p. Proposition. If an indecomposable projective module M has finite length then there exists a unique maximal proper submodule M 0 of M. Furthermore, M/M 0 is a simple module and M is determined up to isomorphism by M/M 0.

2 Proposition. Let R be an integral domain and K its quotient field. Let M be an R-submodule of K. Then M is projective if and only if there is an R-submodule P of K such that MP = R. 2 Proposition. Let E be an R-module. The following conditions are equivalent: (1) E is injective. (2) Hom R (,E) is an exact functor. (3) Every monomorphism α: E M splits. Definition. Divisible module. Proposition An injective module over an integral domain is divisible. Corollary. An injective module over an integral domain is faithful. Proposition. If R is an integral domain with quotient field K,then K is an injective R-module. Baer s Theorem. AleftR-module E is injective if and only if every homomorphism ϕ from a left ideal L of R into E extends to a homomorphism from R into E. Proposition. (1) A direct product of injective modules is injective. (2) If R is noetherian, then a direct sum of injective modules is injective. Proposition. An injective module over a noetherian (not necessarily commutative) ring R is a (possibly infinite) direct sum of indecomposable injective modules. Corollary. If R is a PID then a module M is injective if and only if it is divisible. Theorem. A ring Ris semi-simple if and only if every R-module is projective [injective].

3 3 Essential Extensions and Injective Envelopes Propostion. Let N be a submodule of M. The following are equivalent: (1) Every m 0 M has a non-trivial multiple which belongs to N. (2) Every non-trivial submodule of M has a non-trivial intersection with N. (3) Whenever ϕ: M X is a homomorphism and the restriction of ϕ to N is monic then ϕ is monic. Definition. In this case we say that N is an essential submodule of M or that M is an essential extension of N. Example. If S is a multiplicative set in the center of R and M an R-module and θ : M S 1 M is the canonical map, then S 1 M is an essential extension of θ(m). Proposition. If N is an injective module and M is an essential extension of N,then M=N. Corollary. If M is an injective module over a commutative noetherian ring R and S = R Ass M,thenS 1 M M. Theorem. Let E be an injective module and M E E. The following conditions are equivalent: (1) E is a maximal essential extension of M. (2) E is a minimal injective submodule of E containing M. (3) E is an essential extension of M and is injective. (4) E is injective and every monomorphism from M into an injective module Q extends to a monomorphism from E into Q. Definition. Injective envelope. Proposition. If N is a submodule of an injective module E then there exists an injective envelope of N contained in E. Theorem. Existence of injective envelopes.

4 4 Proposition. Let E be an injective module. The following conditions are equivalent: (1) E is indecomposable. (2) E is the injective envelope of each of its non-trivial submodules. (3) Every two non-trivial submodules of E intersect non-trivially. Corollary. If E is an indecomposable injective module over a commutative noetherian ring R, thenasse={p}for some prime ideal p, E is isomorphic to the injective envelope of R/p, ande E p. Example. The indecomposable injective Z-modules. Proposition. Let E be an indecomposable injective module. (1) Every monic endomorphism of E is in fact an automorphism. (2) End R E is a local ring. Theorem. If R is a (not necessarily commutative) artinian ring, then every injective R-module is a direct sum of indecomposable injective R-modules. Furthermore every indecomposable injective module is the injective envelope of a unique simple module. In most situations, at least, an indecomposable module over an artinian ring has finite length. Lemma. Let R be a commutative local noetherian ring with maximal ideal m and let E be the injective envelope of R/m. IfMis any module with finite length, then Hom R (M, E) also has finite length, in fact length Hom R (M, E) = length M. Proposition. A finitely generated injective module M over a commutative noetherian ring has finite length. proof: It suffices to show that every associated prime for M is maximal. Now if p Ass M then M contains a summand isomorphic to E(R/p) and thus E = E(R/p) is finitely generated. Now if p m then me = E. Furthermore, E E m,sowemay replace R by R m. Then by NAK we conclude that E =0,acontradiction. Thus p must be maximal.

5 Proposition. If R is a commutative local ring with maximal ideal m then R is artinian if and only if there exists a non-trivial finitely generated injective R-module. In this case, the injective envelope of R/m is finitely generated. proof: Let E betheinjectiveenvelopeofr/m. SinceE Hom R (R, E), it follows from the lemma above that E has finite length if and only if R does (which is equivalent to R being artinian.) On the other hand, if there exists any finitely generated injective module M then we have seen that M has finite length. Hence Ass M = {m} and so M contains a copy of R/m. ThenMcontains a copy of the injective envelope E of R/m, soe has finite length. 5

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