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In mathematics, a module is a generalization of the notion of vector space in which the field of scalars is replaced by a ring. The concept of ''module'' generalizes also the notion of
abelian group In mathematics, an abelian group, also called a commutative group, is a group in which the result of applying the group operation to two group elements does not depend on the order in which they are written. That is, the group operation is com ...
, since the abelian groups are exactly the modules over the ring of
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...
s. Like a vector space, a module is an additive abelian group, and scalar multiplication is distributive over the operation of addition between elements of the ring or module and is compatible with the ring multiplication. Modules are very closely related to the representation theory of groups. They are also one of the central notions of commutative algebra and homological algebra, and are used widely in algebraic geometry and algebraic topology.


Introduction and definition


Motivation

In a vector space, the set of scalars is a field and acts on the vectors by scalar multiplication, subject to certain axioms such as the distributive law. In a module, the scalars need only be a ring, so the module concept represents a significant generalization. In commutative algebra, both
ideals Ideal may refer to: Philosophy * Ideal (ethics), values that one actively pursues as goals * Platonic ideal, a philosophical idea of trueness of form, associated with Plato Mathematics * Ideal (ring theory), special subsets of a ring considered ...
and quotient rings are modules, so that many arguments about ideals or quotient rings can be combined into a single argument about modules. In non-commutative algebra, the distinction between left ideals, ideals, and modules becomes more pronounced, though some ring-theoretic conditions can be expressed either about left ideals or left modules. Much of the theory of modules consists of extending as many of the desirable properties of vector spaces as possible to the realm of modules over a " well-behaved" ring, such as a principal ideal domain. However, modules can be quite a bit more complicated than vector spaces; for instance, not all modules have a basis, and even those that do, free modules, need not have a unique rank if the underlying ring does not satisfy the invariant basis number condition, unlike vector spaces, which always have a (possibly infinite) basis whose cardinality is then unique. (These last two assertions require the
axiom of choice In mathematics, the axiom of choice, or AC, is an axiom of set theory equivalent to the statement that ''a Cartesian product of a collection of non-empty sets is non-empty''. Informally put, the axiom of choice says that given any collection ...
in general, but not in the case of finite-dimensional spaces, or certain well-behaved infinite-dimensional spaces such as L''p'' spaces.)


Formal definition

Suppose that ''R'' is a ring, and 1 is its multiplicative identity. A left ''R''-module ''M'' consists of an
abelian group In mathematics, an abelian group, also called a commutative group, is a group in which the result of applying the group operation to two group elements does not depend on the order in which they are written. That is, the group operation is com ...
and an operation such that for all ''r'', ''s'' in ''R'' and ''x'', ''y'' in ''M'', we have # r \cdot ( x + y ) = r \cdot x + r \cdot y # ( r + s ) \cdot x = r \cdot x + s \cdot x # ( r s ) \cdot x = r \cdot ( s \cdot x ) # 1 \cdot x = x . The operation · is called ''scalar multiplication''. Often the symbol · is omitted, but in this article we use it and reserve juxtaposition for multiplication in ''R''. One may write ''R''''M'' to emphasize that ''M'' is a left ''R''-module. A right ''R''-module ''M''''R'' is defined similarly in terms of an operation . Authors who do not require rings to be unital omit condition 4 in the definition above; they would call the structures defined above "unital left ''R''-modules". In this article, consistent with the glossary of ring theory, all rings and modules are assumed to be unital. An (''R'',''S'')- bimodule is an abelian group together with both a left scalar multiplication · by elements of ''R'' and a right scalar multiplication ∗ by elements of ''S'', making it simultaneously a left ''R''-module and a right ''S''-module, satisfying the additional condition for all ''r'' in ''R'', ''x'' in ''M'', and ''s'' in ''S''. If ''R'' is commutative, then left ''R''-modules are the same as right ''R''-modules and are simply called ''R''-modules.


Examples

*If ''K'' is a field, then ''K''- vector spaces (vector spaces over ''K'') and ''K''-modules are identical. *If ''K'' is a field, and ''K'' 'x''a univariate polynomial ring, then a ''K'' 'x''module ''M'' is a ''K''-module with an additional action of ''x'' on ''M'' that commutes with the action of ''K'' on ''M''. In other words, a ''K'' 'x''module is a ''K''-vector space ''M'' combined with a linear map from ''M'' to ''M''. Applying the structure theorem for finitely generated modules over a principal ideal domain to this example shows the existence of the rational and Jordan canonical forms. *The concept of a Z-module agrees with the notion of an abelian group. That is, every
abelian group In mathematics, an abelian group, also called a commutative group, is a group in which the result of applying the group operation to two group elements does not depend on the order in which they are written. That is, the group operation is com ...
is a module over the ring of
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...
s Z in a unique way. For , let (''n'' summands), , and . Such a module need not have a basis—groups containing torsion elements do not. (For example, in the group of integers
modulo In computing, the modulo operation returns the remainder or signed remainder of a division, after one number is divided by another (called the '' modulus'' of the operation). Given two positive numbers and , modulo (often abbreviated as ) is t ...
3, one cannot find even one element which satisfies the definition of a linearly independent set since when an integer such as 3 or 6 multiplies an element, the result is 0. However, if a finite field is considered as a module over the same finite field taken as a ring, it is a vector space and does have a basis.) *The decimal fractions (including negative ones) form a module over the integers. Only singletons are linearly independent sets, but there is no singleton that can serve as a basis, so the module has no basis and no rank. *If ''R'' is any ring and ''n'' a
natural number In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country"). Numbers used for counting are called '' cardinal ...
, then the cartesian product ''R''''n'' is both a left and right ''R''-module over ''R'' if we use the component-wise operations. Hence when , ''R'' is an ''R''-module, where the scalar multiplication is just ring multiplication. The case yields the trivial ''R''-module consisting only of its identity element. Modules of this type are called
free Free may refer to: Concept * Freedom, having the ability to do something, without having to obey anyone/anything * Freethought, a position that beliefs should be formed only on the basis of logic, reason, and empiricism * Emancipate, to procur ...
and if ''R'' has invariant basis number (e.g. any commutative ring or field) the number ''n'' is then the rank of the free module. *If M''n''(''R'') is the ring of matrices over a ring ''R'', ''M'' is an M''n''(''R'')-module, and ''e''''i'' is the matrix with 1 in the -entry (and zeros elsewhere), then ''e''''i''''M'' is an ''R''-module, since . So ''M'' breaks up as the direct sum of ''R''-modules, . Conversely, given an ''R''-module ''M''0, then ''M''0⊕''n'' is an M''n''(''R'')-module. In fact, the category of ''R''-modules and the
category Category, plural categories, may refer to: Philosophy and general uses *Categorization, categories in cognitive science, information science and generally * Category of being * ''Categories'' (Aristotle) * Category (Kant) * Categories (Peirce) ...
of M''n''(''R'')-modules are equivalent. The special case is that the module ''M'' is just ''R'' as a module over itself, then ''R''''n'' is an M''n''(''R'')-module. *If ''S'' is a nonempty set, ''M'' is a left ''R''-module, and ''M''''S'' is the collection of all functions , then with addition and scalar multiplication in ''M''''S'' defined pointwise by and , ''M''''S'' is a left ''R''-module. The right ''R''-module case is analogous. In particular, if ''R'' is commutative then the collection of ''R-module homomorphisms'' (see below) is an ''R''-module (and in fact a ''submodule'' of ''N''''M''). *If ''X'' is a smooth manifold, then the smooth functions from ''X'' to the
real number In mathematics, a real number is a number that can be used to measurement, measure a ''continuous'' one-dimensional quantity such as a distance, time, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small var ...
s form a ring ''C''(''X''). The set of all smooth vector fields defined on ''X'' form a module over ''C''(''X''), and so do the tensor fields and the differential forms on ''X''. More generally, the sections of any vector bundle form a projective module over ''C''(''X''), and by Swan's theorem, every projective module is isomorphic to the module of sections of some bundle; the
category Category, plural categories, may refer to: Philosophy and general uses *Categorization, categories in cognitive science, information science and generally * Category of being * ''Categories'' (Aristotle) * Category (Kant) * Categories (Peirce) ...
of ''C''(''X'')-modules and the category of vector bundles over ''X'' are equivalent. *If ''R'' is any ring and ''I'' is any left ideal in ''R'', then ''I'' is a left ''R''-module, and analogously right ideals in ''R'' are right ''R''-modules. *If ''R'' is a ring, we can define the opposite ring ''R''op which has the same
underlying set In mathematics, an algebraic structure consists of a nonempty set ''A'' (called the underlying set, carrier set or domain), a collection of operations on ''A'' (typically binary operations such as addition and multiplication), and a finite set of ...
and the same addition operation, but the opposite multiplication: if in ''R'', then in ''R''op. Any ''left'' ''R''-module ''M'' can then be seen to be a ''right'' module over ''R''op, and any right module over ''R'' can be considered a left module over ''R''op. * Modules over a Lie algebra are (associative algebra) modules over its universal enveloping algebra. *If ''R'' and ''S'' are rings with a ring homomorphism , then every ''S''-module ''M'' is an ''R''-module by defining . In particular, ''S'' itself is such an ''R''-module.


Submodules and homomorphisms

Suppose ''M'' is a left ''R''-module and ''N'' is a subgroup of ''M''. Then ''N'' is a submodule (or more explicitly an ''R''-submodule) if for any ''n'' in ''N'' and any ''r'' in ''R'', the product (or for a right ''R''-module) is in ''N''. If ''X'' is any
subset In mathematics, set ''A'' is a subset of a set ''B'' if all elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they are unequal, then ''A'' is a proper subset o ...
of an ''R''-module, then the submodule spanned by ''X'' is defined to be \langle X \rangle = \,\bigcap_ N where ''N'' runs over the submodules of ''M'' which contain ''X'', or explicitly \left\, which is important in the definition of tensor products. The set of submodules of a given module ''M'', together with the two binary operations + and ∩, forms a lattice which satisfies the modular law: Given submodules ''U'', ''N''1, ''N''2 of ''M'' such that , then the following two submodules are equal: . If ''M'' and ''N'' are left ''R''-modules, then a map is a homomorphism of ''R''-modules if for any ''m'', ''n'' in ''M'' and ''r'', ''s'' in ''R'', :f(r \cdot m + s \cdot n) = r \cdot f(m) + s \cdot f(n). This, like any homomorphism of mathematical objects, is just a mapping which preserves the structure of the objects. Another name for a homomorphism of ''R''-modules is an ''R''- linear map. A bijective module homomorphism is called a module isomorphism, and the two modules ''M'' and ''N'' are called isomorphic. Two isomorphic modules are identical for all practical purposes, differing solely in the notation for their elements. The kernel of a module homomorphism is the submodule of ''M'' consisting of all elements that are sent to zero by ''f'', and the image of ''f'' is the submodule of ''N'' consisting of values ''f''(''m'') for all elements ''m'' of ''M''. The isomorphism theorems familiar from groups and vector spaces are also valid for ''R''-modules. Given a ring ''R'', the set of all left ''R''-modules together with their module homomorphisms forms an
abelian category In mathematics, an abelian category is a category in which morphisms and objects can be added and in which kernels and cokernels exist and have desirable properties. The motivating prototypical example of an abelian category is the category of ...
, denoted by ''R''-Mod (see category of modules).


Types of modules

; Finitely generated: An ''R''-module ''M'' is finitely generated if there exist finitely many elements ''x''1, ..., ''x''''n'' in ''M'' such that every element of ''M'' is a linear combination of those elements with coefficients from the ring ''R''. ; Cyclic: A module is called a
cyclic module In mathematics, more specifically in ring theory, a cyclic module or monogenous module is a module over a ring that is generated by one element. The concept is a generalization of the notion of a cyclic group, that is, an Abelian group (i.e. Z ...
if it is generated by one element. ; Free: A free ''R''-module is a module that has a basis, or equivalently, one that is isomorphic to a direct sum of copies of the ring ''R''. These are the modules that behave very much like vector spaces. ; Projective: Projective modules are direct summands of free modules and share many of their desirable properties. ; Injective: Injective modules are defined dually to projective modules. ; Flat: A module is called flat if taking the
tensor product In mathematics, the tensor product V \otimes W of two vector spaces and (over the same Field (mathematics), field) is a vector space to which is associated a bilinear map V\times W \to V\otimes W that maps a pair (v,w),\ v\in V, w\in W to an e ...
of it with any exact sequence of ''R''-modules preserves exactness. ; Torsionless: A module is called torsionless if it embeds into its algebraic dual. ; Simple: A simple module ''S'' is a module that is not and whose only submodules are and ''S''. Simple modules are sometimes called ''irreducible''.Jacobson (1964)
p. 4
Def. 1;
; Semisimple: A semisimple module is a direct sum (finite or not) of simple modules. Historically these modules are also called ''completely reducible''. ; Indecomposable: An indecomposable module is a non-zero module that cannot be written as a direct sum of two non-zero submodules. Every simple module is indecomposable, but there are indecomposable modules which are not simple (e.g. uniform modules). ; Faithful: A faithful module ''M'' is one where the action of each in ''R'' on ''M'' is nontrivial (i.e. for some ''x'' in ''M''). Equivalently, the annihilator of ''M'' is the zero ideal. ; Torsion-free: A torsion-free module is a module over a ring such that 0 is the only element annihilated by a regular element (non zero-divisor) of the ring, equivalently implies or . ; Noetherian: A Noetherian module is a module which satisfies the ascending chain condition on submodules, that is, every increasing chain of submodules becomes stationary after finitely many steps. Equivalently, every submodule is finitely generated. ; Artinian: An Artinian module is a module which satisfies the descending chain condition on submodules, that is, every decreasing chain of submodules becomes stationary after finitely many steps. ; Graded: A graded module is a module with a decomposition as a direct sum over a graded ring such that for all ''x'' and ''y''. ; Uniform: A uniform module is a module in which all pairs of nonzero submodules have nonzero intersection.


Further notions


Relation to representation theory

A representation of a group ''G'' over a field ''k'' is a module over the group ring ''k'' 'G'' If ''M'' is a left ''R''-module, then the ''action'' of an element ''r'' in ''R'' is defined to be the map that sends each ''x'' to ''rx'' (or ''xr'' in the case of a right module), and is necessarily a group endomorphism of the abelian group . The set of all group endomorphisms of ''M'' is denoted EndZ(''M'') and forms a ring under addition and composition, and sending a ring element ''r'' of ''R'' to its action actually defines a ring homomorphism from ''R'' to EndZ(''M''). Such a ring homomorphism is called a ''representation'' of ''R'' over the abelian group ''M''; an alternative and equivalent way of defining left ''R''-modules is to say that a left ''R''-module is an abelian group ''M'' together with a representation of ''R'' over it. Such a representation may also be called a ''ring action'' of ''R'' on ''M''. A representation is called ''faithful'' if and only if the map is injective. In terms of modules, this means that if ''r'' is an element of ''R'' such that for all ''x'' in ''M'', then . Every abelian group is a faithful module over the
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...
s or over some ring of integers modulo ''n'', Z/''n''Z.


Generalizations

A ring ''R'' corresponds to a preadditive category R with a single object. With this understanding, a left ''R''-module is just a covariant additive functor from R to the category Ab of abelian groups, and right ''R''-modules are contravariant additive functors. This suggests that, if C is any preadditive category, a covariant additive functor from C to Ab should be considered a generalized left module over C. These functors form a functor category C-Mod which is the natural generalization of the module category ''R''-Mod. Modules over ''commutative'' rings can be generalized in a different direction: take a
ringed space In mathematics, a ringed space is a family of ( commutative) rings parametrized by open subsets of a topological space together with ring homomorphisms that play roles of restrictions. Precisely, it is a topological space equipped with a sheaf ...
(''X'', O''X'') and consider the sheaves of O''X''-modules (see sheaf of modules). These form a category O''X''-Mod, and play an important role in modern algebraic geometry. If ''X'' has only a single point, then this is a module category in the old sense over the commutative ring O''X''(''X''). One can also consider modules over a semiring. Modules over rings are abelian groups, but modules over semirings are only commutative
monoid In abstract algebra, a branch of mathematics, a monoid is a set equipped with an associative binary operation and an identity element. For example, the nonnegative integers with addition form a monoid, the identity element being 0. Monoids ...
s. Most applications of modules are still possible. In particular, for any semiring ''S'', the matrices over ''S'' form a semiring over which the tuples of elements from ''S'' are a module (in this generalized sense only). This allows a further generalization of the concept of vector space incorporating the semirings from theoretical computer science. Over near-rings, one can consider near-ring modules, a nonabelian generalization of modules.


See also

* Group ring * Algebra (ring theory) * Module (model theory) * Module spectrum * Annihilator


Notes


References

* F.W. Anderson and K.R. Fuller: ''Rings and Categories of Modules'', Graduate Texts in Mathematics, Vol. 13, 2nd Ed., Springer-Verlag, New York, 1992, , * Nathan Jacobson. ''Structure of rings''. Colloquium publications, Vol. 37, 2nd Ed., AMS Bookstore, 1964,


External links

* * {{Authority control Algebraic structures * Module