In
mathematics
Mathematics is a field of study that discovers and organizes methods, Mathematical theory, theories and theorems that are developed and Mathematical proof, proved for the needs of empirical sciences and mathematics itself. There are many ar ...
, a free module is a
module that has a ''basis'', that is, a
generating set that is
linearly independent. Every
vector space
In mathematics and physics, a vector space (also called a linear space) is a set (mathematics), set whose elements, often called vector (mathematics and physics), ''vectors'', can be added together and multiplied ("scaled") by numbers called sc ...
is a free module, but, if the
ring of the coefficients is not a
division ring (not a
field in the
commutative
In mathematics, a binary operation is commutative if changing the order of the operands does not change the result. It is a fundamental property of many binary operations, and many mathematical proofs depend on it. Perhaps most familiar as a pr ...
case), then there exist non-free modules.
Given any
set and ring , there is a free -module with basis , which is called the ''free module on'' or ''module of formal'' -''linear combinations'' of the elements of .
A
free abelian group is precisely a free module over the ring
of
integer
An integer is the number zero (0), a positive natural number (1, 2, 3, ...), or the negation of a positive natural number (−1, −2, −3, ...). The negations or additive inverses of the positive natural numbers are referred to as negative in ...
s.
Definition
For a
ring and an
-
module , the set
is a basis for
if:
*
is a
generating set for
; that is to say, every element of
is a finite sum of elements of
multiplied by coefficients in
; and
*
is
linearly independent: for every set
of distinct elements,
implies that
(where
is the zero element of
and
is the zero element of
).
A free module is a module with a basis.
An immediate consequence of the second half of the definition is that the coefficients in the first half are unique for each element of ''M''.
If
has
invariant basis number, then by definition any two bases have the same cardinality. For example, nonzero commutative rings have invariant basis number. The cardinality of any (and therefore every) basis is called the rank of the free module
. If this cardinality is finite, the free module is said to be ''free of finite rank'', or ''free of rank'' if the rank is known to be .
Examples
Let ''R'' be a ring.
* ''R'' is a free module of rank one over itself (either as a left or right module); any unit element is a basis.
* More generally, If ''R'' is commutative, a nonzero ideal ''I'' of ''R'' is free if and only if it is a
principal ideal generated by a
nonzerodivisor, with a generator being a basis.
* Over a
principal ideal domain (e.g.,
), a submodule of a free module is free.
* If ''R'' is commutative, the polynomial ring
2, ....
* Let
cartesian product
In mathematics, specifically set theory, the Cartesian product of two sets and , denoted , is the set of all ordered pairs where is an element of and is an element of . In terms of set-builder notation, that is
A\times B = \.
A table c ...
of ''n'' copies of ''R'' as a left ''R''-module, is free. If ''R'' has
invariant basis number, then its
rank is ''n''.
* A
direct sum of free modules is free, while an infinite cartesian product of free modules is generally ''not'' free (cf. the
Baer–Specker group).
* A finitely generated module over a commutative
local ring
In mathematics, more specifically in ring theory, local rings are certain rings that are comparatively simple, and serve to describe what is called "local behaviour", in the sense of functions defined on algebraic varieties or manifolds, or of ...
is free if and only if it is
faithfully flat.
Also,
Kaplansky's theorem states a projective module over a (possibly non-commutative) local ring is free.
* Sometimes, whether a module is free or not is
undecidable in the set-theoretic sense. A famous example is the
Whitehead problem, which asks whether a Whitehead group is free or not. As it turns out, the problem is independent of ZFC.
Formal linear combinations
Given a set and ring , there is a free -module that has as a basis: namely, the
direct sum of copies of ''R'' indexed by ''E''
:
R^ = \bigoplus_ R.
Explicitly, it is the submodule of the
Cartesian product
In mathematics, specifically set theory, the Cartesian product of two sets and , denoted , is the set of all ordered pairs where is an element of and is an element of . In terms of set-builder notation, that is
A\times B = \.
A table c ...
\prod_E R (''R'' is viewed as say a left module) that consists of the elements that have only finitely many nonzero components. One can
embed ''E'' into as a subset by identifying an element ''e'' with that of whose ''e''-th component is 1 (the unity of ''R'') and all the other components are zero. Then each element of can be written uniquely as
:
\sum_ c_e e ,
where only finitely many
c_e are nonzero. It is called a ''
formal linear combination'' of elements of .
A similar argument shows that every free left (resp. right) ''R''-module is isomorphic to a direct sum of copies of ''R'' as left (resp. right) module.
Another construction
The free module may also be constructed in the following equivalent way.
Given a ring ''R'' and a set ''E'', first as a set we let
:
R^ = \.
We equip it with a structure of a left module such that the addition is defined by: for ''x'' in ''E'',
:
(f+g)(x) = f(x) + g(x)
and the scalar multiplication by: for ''r'' in ''R'' and ''x'' in ''E'',
:
(r f)(x) = r f(x)
Now, as an ''R''-valued
function on ''E'', each ''f'' in
R^ can be written uniquely as
:
f = \sum_ c_e \delta_e
where
c_e are in ''R'' and only finitely many of them are nonzero and
\delta_e is given as
:
\delta_e(x) = \begin 1_R \quad\mbox x=e \\ 0_R \quad\mbox x\neq e \end
(this is a variant of the
Kronecker delta). The above means that the subset
\ of
R^ is a basis of
R^. The mapping
e \mapsto \delta_e is a
bijection
In mathematics, a bijection, bijective function, or one-to-one correspondence is a function between two sets such that each element of the second set (the codomain) is the image of exactly one element of the first set (the domain). Equival ...
between and this basis. Through this bijection,
R^ is a free module with the basis ''E''.
Universal property
The inclusion mapping
\iota : E\to R^ defined above is
universal in the following sense. Given an arbitrary function
f : E\to N from a set to a left -module , there exists a unique
module homomorphism \overline: R^\to N such that
f = \overline \circ\iota; namely,
\overline is defined by the formula:
:
\overline\left (\sum_ r_e e \right) = \sum_ r_e f(e)
and
\overline is said to be obtained by ''extending
f by linearity.'' The uniqueness means that each ''R''-linear map
R^ \to N is uniquely determined by its
restriction to ''E''.
As usual for universal properties, this defines
up to a
canonical isomorphism. Also the formation of
\iota : E\to R^ for each set ''E'' determines a
functor
In mathematics, specifically category theory, a functor is a Map (mathematics), mapping between Category (mathematics), categories. Functors were first considered in algebraic topology, where algebraic objects (such as the fundamental group) ar ...
:
R^: \textbf \to R\text\mathsf, \, E \mapsto R^,
from the
category of sets to the category of left -modules. It is called the
free functor and satisfies a natural relation: for each set ''E'' and a left module ''N'',
:
\operatorname_(E, U(N)) \simeq \operatorname_R(R^, N), \, f \mapsto \overline
where
U: R\text\mathsf \to \textbf is the
forgetful functor
In mathematics, more specifically in the area of category theory, a forgetful functor (also known as a stripping functor) "forgets" or drops some or all of the input's structure or properties mapping to the output. For an algebraic structure of ...
, meaning
R^ is a
left adjoint of the forgetful functor.
Generalizations
Many statements true for free modules extend to certain larger classes of modules.
Projective modules are direct summands of free modules.
Flat modules are defined by the property that tensoring with them preserves exact sequences.
Torsion-free modules form an even broader class. For a finitely generated module over a PID (such as Z), the properties free, projective, flat, and torsion-free are equivalent.
:

See
local ring
In mathematics, more specifically in ring theory, local rings are certain rings that are comparatively simple, and serve to describe what is called "local behaviour", in the sense of functions defined on algebraic varieties or manifolds, or of ...
,
perfect ring and
Dedekind ring.
See also
*
Free object
*
free presentation
*
free resolution
*
Quillen–Suslin theorem
*
stably free module
*
generic freeness
Notes
References
*
*
* .
*
{{Dimension topics
Module theory
Free algebraic structures