# Injective module

In

mathematics , especially in the area ofabstract algebra known asmodule theory , an**injective module**is a module "Q" that shares certain desirable properties with the**Z**-module**Q**of allrational number s. Specifically, if "Q" is asubmodule of some other module, then it is already adirect summand of that module; also, given a submodule of a module "Y", then anymodule homomorphism from this submodule to "Q" can be extended to a homomorphism from all of "Y" to "Q". This concept is dual to that ofprojective module s. Injective modules were introduced in harv|Baer|1940 and are discussed in some detail in the textbook harv|Lam|1999|loc=§3.Injective modules have been heavily studied, and a variety of additional notions are defined in terms of them:

Injective cogenerator s are injective modules that faithfully represent the entire category of modules. Injective resolutions measure how far from injective a module is in terms of theinjective dimension and represent modules in thederived category .Injective hull s are maximalessential extension s, and turn out to be minimal injective extensions. Over aNoetherian ring , every injective module is uniquely a direct sum of indecomposable modules, and their structure is well understood. An injective module over one ring, may not be injective over another, but there are well-understood methods of changing rings which handle special cases. Rings which are themselves injective modules have a number of interest properties and include rings such asgroup ring s offinite group s over fields. Injective modules includedivisible group s and are generalized by the notion ofinjective object s incategory theory .**Definition**A left module "Q" over the ring "R" is injective if it satisfies one (and therefore all) of the following equivalent conditions:

* If "Q" is a submodule of some other left "R"-module "M", then there exists another submodule "K" of "M" such that "M" is the internal direct sum of "Q" and "K", i.e. "Q" + "K" = "M" and "Q" ∩ "K" = {0}.

* If "X" is a submodule of the left "R"-module "Y" and "g" : "X" → "Q" is a module homomorphism, then there exists a module homomorphism "h" : "Y" → "Q" such that "h"("x") = "g"("x") for all "x" in "X".

* If "X" and "Y" are left-"R" modules and "f" : "X" → "Y" is aninjective module homomorphism and "g" : "X" → "Q" is an arbitrary module homomorphism, then there exists a module homomorphism "h" : "Y" → "Q" such that "hf" = "g", i.e. such that the following diagram commutes:::

* Anyshort exact sequence 0 →"Q" → "M" → "K" → 0 of left "R"-modules splits.

* Thecontravariant functor Hom(-,"Q") from the category of left "R"-modules to the category ofabelian group s is exact.Injective right "R"-modules are defined in complete analogy.

**Examples****First examples**Trivially, the zero module {0} is injective.

Given a field "k", every "k"-

vector space "Q" is an injective "k"-module. Reason: if "Q" is a subspace of "V", we can find a basis of "Q" and extend it to a basis of "V". The new extending basis vectors span a subspace "K" of "V" and "V" is the internal direct sum of "Q" and "K". Note that the direct complement "K" of "Q" is not uniquely determined by "Q", and likewise the extending map "g" in the above definition is typically not unique.The rationals

**Q**(with addition) form an injective abelian group (i.e. an injective**Z**-module). Thefactor group **Q**/**Z**and thecircle group are also injective**Z**-modules. The factor group**Z**/"n"**Z**for "n" > 1 is injective as a**Z**/"n"**Z**-module, but "not" injective as an abelian group.**Commutative examples**More generally, for any

integral domain "R" with field of fractions "K", the "R"-module "K" is an injective "R"-module, and indeed the smallest injective "R"-module containing "R". For anyDedekind domain , thequotient module "K"/"R" is also injective, and its indecomposable summands are the localizations $R\_\{mathfrak\{p/R$ for the nonzeroprime ideal s $mathfrak\{p\}$. Thezero ideal is also prime and corresponds to the injective "K". In this way there is a 1-1 correspondence between prime ideals and indecomposable injective modules.A particularly rich theory is available for commutative

noetherian ring s due toEben Matlis , harv|Lam|1999|loc=§3I. Every injective module is uniquely a direct sum of indecomposable injective modules, and the indecomposable injective modules are uniquely identified as the injective hulls of the quotients "R"/"P" where "P" varies over theprime spectrum of the ring. The injective hull of "R"/"P" as an "R"-module is canonically an "R"_{"P"}module, and is the "R"_{"P"}-injective hull of "R"/"P". In other words, it suffices to considerlocal ring s. Theendomorphism ring of the injective hull of "R"/"P" is the completion $hat\; R\_P$ of "R" at "P".Two particularly instructive examples are the injective hull of the

**Z**-module**Z**/"p**"Z**(thePrüfer group ), and the injective hull of the "k" ["x"] -module "k" (the ring of inverse polynomials). The latter is easily described as "k"("x")/"k" ["x"] , but the form of the elements is quite transparent: The module has a basis consisting of "inverse monomials", that is "x"^{−"n"}for "n" = 1, 2, …. Multiplication by scalars is as expected, and multiplication by "x" behaves normally except that "x"·"x"^{−1}= 0. The endomorphism ring is simply the ring offormal power series .**Artinian examples**If "G" is a

finite group and "k" a field with characteristic 0, then one shows in the theory ofgroup representation s that any subrepresentation of a given one is already a direct summand of the given one. Translated into module language, this means that all modules over thegroup algebra "kG" are injective. If the characteristic of "k" is not zero, the following example may help.If "A" is a unital

associative algebra over the field "k" with finite dimension over "k", then Hom_{"k"}(−, "k") is a duality between finitely generated left "A"-modules and finitely generated right "A"-modules. Therefore, the finitely generated injective left "A"-modules are precisely the modules of the form Hom_{"k"}("P", "k") where "P" is a finitely generated projective right "A"-module. For symmetric algebras, the duality is particularly well-behaved and projective modules and injective modules coincide.For any

Artinian ring , just as forcommutative ring s, there is a 1-1 correspondence between prime ideals and indecomposable injective modules. The correspondence in this case is perhaps even simpler: a prime ideal is an annihilator of a unique simple module, and the corresponding indecomposable injective module is itsinjective hull . For finite dimensional algebras over fields, these injective hulls arefinitely-generated module s harv|Lam|1999|loc=§3G, §3J.**Theory****Submodules, quotients, products, and sums**Any product of (even infinitely many) injective modules is injective; conversely, if a direct product of modules is injective, then each module is injective harv|Lam|1999|p=61. Every direct sum of finitely many injective modules is injective. In general, submodules, factor modules, or infinite

direct sum s of injective modules need not be injective. Every submodule of every injective module is injective if and only if the ring is Artinian semisimple harv|Golan|Head|1991|p=152; every factor module of every injective module is injective if and only if the ring is hereditary, harv|Lam|1999|loc=Th. 3.22; every infinite direct sum of injective modules is injective if and only if the ring is Noetherian, harv|Lam|1999|loc=Th 3.46 [*This is the Bass-Papp theorem, see harv|Papp|1959 and harv|Chase|1960*] .**Baer's criterion**In Baer's original paper, he proved a useful result, usually known as Baer's Criterion, for checking whether a module is injective: a left "R"-module "Q" is injective if and only if any homomorphism "g" : "I" → "Q" defined on a left ideal "I" of "R" can be extended to all of "R".

Using this criterion, one can show that

**Q**is an injectiveabelian group (i.e. an injective module over**Z**). More generally, an abelian group is injective if and only if it is divisible. More generally still: a module over aprincipal ideal domain is injective if and only if it is divisible (the case of vector spaces is an example of this theorem, as every field is a principal ideal domain and every vector space is divisible). Over a general integral domain, we still have one implication: every injective module over an integral domain is divisible.Baer's criterion has been refined in many ways harv|Golan|Head|1991|p=119, including a result of harv|Smith|1981 and harv|Vamos|1983 that for a commutative Noetherian ring, it suffices to consider only

prime ideal s "I". The dual of Baer's Criterion would give a simple test for projectivity, but even for the ring**Z**of integers, this becomes the unsolvableWhitehead problem .**Injective cogenerators**Maybe the most important injective module is the abelian group

**Q**/**Z**. It is aninjective cogenerator in thecategory of abelian groups , which means that it is injective and any other module is contained in a suitably large product of copies of**Q**/**Z**. So in particular, every abelian group is subgroup of an injective one. It is quite significant that this is also true over any ring: every module is a submodule of an injective one, or "the category of left "R"-modules has enough injectives." To prove this, one uses the peculiar properties of the abelian group**Q**/**Z**to construct an injective cogenerator in the category of left "R"-modules.For a left "R"-module "M", the so called "character module" "M"

^{+}= Hom_{Z}("M",**Q**/**Z**) is a right "R"-module that exhibits an interesting duality, not between injective modules andprojective module s, but between injective modules andflat module s harv|Enochs|Jenda|2001|pp=78–80. For any ring "R", a left "R"-module is flat if and only if its character module is injective. If "R" is left noetherian, then a left "R"-module is injective if and only if its character module is flat.**Injective resolutions**Every module "M" also has an

**injective resolution**: anexact sequence of the form:0 → "M" → "I"^{0}→ "I"^{1}→ "I"^{2}→ ...where the "I"^{ "j"}are injective. These injective resolutions can be used to definederived functor s such as theExt functor .**Injective dimension**Injective resolutions are also used to define the

**injective dimension**of a module, harv|Lam|1999|loc=§5C. The injective dimension is the length of the shortest injective resolution ending in zeros, if such a finite resolution exists, otherwise the injective dimension is ∞. Equivalently, the injective dimension is the minimal integer (if there is such, otherwise ∞) "n" such that $operatorname\{Ext\}^N\_A(-,M)\; =\; 0$ for all "N" > "n".**Injective hulls**The

injective hull of a module is essentially the smallest injective module containing the given one and was described in harv|Eckmann|Shopf|1953.One can use injective hulls to define a minimal injective resolution. If each term of the injective resolution is the injective hull of the cokernel of the previous map, then the injective resolution has minimal length.

**Indecomposables**Every injective submodule of an injective module is a direct summand, so it is important to understand indecomposable injective modules, harv|Lam|1999|loc=§3F.

Every indecomposable injective module has a local

endomorphism ring . A module is called**uniform**if every two nontrivial submodules intersect nontrivially. For an injective module "M" the following are equivalent:

* "M" is indecomposable

* "M" is nonzero and is the injective hull of every nonzero submodule

* "M" is uniform

* "M" is the injective hull of a uniform module

* "M" is the injective hull of a uniformcyclic module

* "M" has a local endomorphism ringOver a Noetherian ring, every injective module is the direct sum of (uniquely determined) indecomposable injective modules. Over a commutative Noetherian ring, this gives a particularly nice understanding of all injective modules, described in harv|Matlis|1958.

**Change of rings**It is important to be able to consider modules over

subring s orquotient ring s, especially for instancepolynomial ring s. In general, this is difficult, but a number of results are known, harv|Lam|1999|p=62.Let "S" and "R" be rings, and "P" be a left-"R", right-"S"

bimodule that is flat as a left-"R" module. For any injective right "S"-module "M", the set ofmodule homomorphism s Hom_{"S"}( "P", "M" ) is an injective right "R"-module. For instance, if "R" is a subring of "S" such that "S" is a flat "R"-module, then every injective "S"-module is an injective "R"-module. In particular, if "R" is an integral domain and "S" itsfield of fractions , then every vector space over "S" is an injective "R"-module. Similarly, every injective "R" ["x"] -module is an injective "R"-module.For quotient rings "R"/"I", the change of rings is also very clear. An "R"-module is an "R"/"I"-module precisely when it is annihilated by "I". The submodule ann

_{"I"}("M") = { "m" in "M" : "im" = 0 for all "i" in "I" } is a a left submodule of the left "R"-module "M", and is the largest submodule of "M" that is an "R"/"I"-module. If "M" is an injective left "R"-module, then ann_{"I"}("M") is an injective left "R"/"I"-module. Applying this to "R"=**Z**, "I"="n**"Z**and "M"=**Q**/**Z**, one gets the familiar fact that**Z**/"n**"Z**is injective as a module over itself. While it is easy to convert injective "R"-modules into injective "R"/"I"-modules, this process does not convert injective "R"-resolutions into injective "R"/"I"-resolutions, and the homology of the resulting complex is one of the early and fundamental areas of study of relative homological algebra.The textbook harv|Rotman|1979|p=103 has an erroneous proof that localization preserves injectives, but a counterexample was given in harv|Dade|1981.

**elf-injective rings**Every ring is projective as a module over itself, but it is rarer for a ring to be injective as a module over itself, harv|Lam|1999|loc=§3B. Such rings are called self-injective rings, or to be more precise, if the ring is injective over itself as a right module, then it is called a right self-injective ring. Every

Frobenius algebra is self-injective, but nointegral domain that is not a field is self-injective. Every proper quotient of aDedekind domain is self-injective.A right Noetherian, right self-injective ring is called a

**quasi-Frobenius ring**, and is two-sided Artinian and two-sided injective, harv|Lam|1999|loc=Th. 15.1.**Ring structure**In many cases, for "R" a subring of a self-injective ring "S", the injective hull of "R" will also have a ring structure, harv|Lam|1999|pp=78–80. For instance, taking "S" to be a full

matrix ring over a field, and taking "R" to be any ring containing every matrix which is zero in all but the last column, the injective hull of the right "R"-module "R" is "S". For instance, one can take "R" to be the ring of all upper triangular matrices. However, it is not always the case that the injective hull of a ring has a ring structure, as an example in harv|Osofsky|1964 shows.A large class of rings which do have ring structures on their injective hulls are the so-called non-singular rings, harv|Lam|1999|p=366, including all reduced

commutative rings , that is commutative rings withoutnilpotent element s, harv|Lam|1999|p=253. In particular, for anintegral domain , one recalls the injective hull of the ring is thefield of fractions . The injective hulls of nonsingular rings provide the more general analogue of fields of fractions for non-commutative rings; compare to the more restrictive localization defined forOre domain s. This type of "ring of quotients" (as these more general "fields of fractions" are called) was pioneered in harv|Utumi|1956, and the connection to injective hulls was recognized in harv|Lambek|1963.**Generalizations and specializations****Injective objects**One also talks about

injective object s in categories more general than module categories, for instance in functor categories or in categories of sheaves of O_{"X"}-modules over someringed space ("X",O_{"X"}). The following general definition is used: an object "Q" of the category "C" is**injective**if for anymonomorphism "f" : "X" → "Y" in "C" and any morphism "g" : "X" → "Q" there exists a morphism "h" : "Y" → "Q" with "hf" = "g".**Divisible groups**The notion of injective object in the category of abelian groups was studied somewhat independently of injective modules under the term

divisible group . Here a**Z**-module "M" is injective if and only if "n"⋅"M" = "M" for every nonzero integer "n". Here the relationships betweenflat module s,pure submodule s, and injective modules is more clear, as it simply refers to certain divisibility properties of module elements by integers.**Pure injectives**In relative homological algebra, the extension property of homomorphisms may be required only for certain submodules, rather than for all. For instance, a

pure injective module is a module in which a homomorphism from apure submodule can extended to the whole module.**References****Notes****Textbooks***

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*Citation | last1=Golan | first1=Jonathan S. | last2=Head | first2=Tom | title=Modules and the structure of rings | publisher=Marcel Dekker | series=Monographs and Textbooks in Pure and Applied Mathematics | isbn=978-0-8247-8555-0 | id=MathSciNet | id = 1201818 | year=1991 | volume=147

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