In mathematics a free module is a free object in a category of modules. Given a set S a free module on S is a free module with basis S.

Every vector space is free and the free vector space on a set is a special case of a free module on a set. Contents 1 Definition 2 Construction 3 Universal property 4 See also 5 References 6 External links Definition

A free module is a module with a basis: a linearly independent generating set.

For an R-module M the set E is a basis for M if: E is a generating set for M; that is to say every element of M is a finite sum of elements of E multiplied by coefficients in R; E is linearly independent that is if for distinct elements of E then (where 0M is the zero element of M and 0R is the zero element of R).

If R has invariant basis number then by definition any two bases have the same cardinality. The cardinality of any (and therefore every) basis is called the rank of the free module M and M is said to be free of rank n or simply free of finite rank if the cardinality is finite.

Note that an immediate corollary of (2) is that the coefficients in (1) are unique for each x.

The definition of an infinite free basis is similar except that E will have infinitely many elements. However the sum must still be finite and thus for any particular x only finitely many of the elements of E are involved.

In the case of an infinite basis the rank of M is the cardinality of E. Construction

Given a set E we can construct a free R-module over E. The module is simply the direct sum of E copies of R often denoted R(E). We give a concrete realization of this direct sum denoted by C(E) as follows: Carrier: C(E) contains the functions such that f(x) 0 for cofinitely many (all but finitely many) . Addition: for two elements we define by . Inverse: for we define by . Scalar multiplication: for we define by . A basis for C(E) is given by the set where (a variant of the Kronecker delta and a particular case of the indicator function for the set a). Define the mapping by (a) a. This mapping gives a bijection between E and the basis vectors . We can thus identify these sets. Thus E may be considered as a linearly independent basis for C(E). Universal property The mapping defined above is universal in the following sense. If there is an arbitrary R-module M and an arbitrary mapping then there exists a unique module homomorphism such that . See also Free object References Iain T. Adamson (1972). Elementary rings and modules. University Mathematical Texts. Oliver and Boyd. pp. 6566. ISBN 0-05-002192-3.  External links This article incorporates material from free vector space over a set on PlanetMath which is licensed under the Creative Commons Attribution/Share-Alike License.