# free module

Let $R$ be a commutative ring with unity. A over $R$ is a (unital) module isomorphic to a direct sum of copies of $R$. In particular, as every abelian group is a $\mathbb{Z}$-module, a free abelian group is a direct sum of copies of $\mathbb{Z}$. This is equivalent to saying that the module has a free basis, i.e. a set of elements with the that every element of the module can be uniquely expressed as an linear combination over $R$ of elements of the free basis. In the case that a free module over $R$ is a sum of finitely many copies of $R$, then the number of copies is called the rank of the free module.

An alternative definition of a free module is via its universal property: Given a set $X$, the free $R$-module $F(X)$ on the set $X$ is equipped with a function $i:X\rightarrow F(X)$ satisfying the property that for any other $R$-module $A$ and any function $f:X\rightarrow A$, there exists a unique $R$-module map $h:F(X)\rightarrow A$ such that $(h\circ i)=f$.

Title free module FreeModule 2013-03-22 12:10:10 2013-03-22 12:10:10 mathcam (2727) mathcam (2727) 9 mathcam (2727) Definition msc 16D40 FreeGroup free module free abelian group free basis rank of a free module