Free Modules

Definition:

An \(R\)-module \(M\) is a free \(R\)-module if \(M\) has a basis. A subset \(A\) of an \(R\)-module \(M\) is a basis of \(M\) if \(A\) is linearly independent and generates \(M\).

Examples:

1. \(R=R\{1_R\}\) is a free \(R\)-module
2. \(R^2=R\oplus R\) has basis \(\{(1_R,0_R),(0_R,1_R)\}\)
3. \(R[x]\) is a free \(R\)-module with (infinite) basis \(\{1,x,x^2,...,x^i,...\}\)
4. \(R[x,y]\) has basis \(\{x^n,y^m\ |\ n,m\geq 0 \}\) as free \(R\)-module
5. \(R[x,y]\) has basis \(\{1,y,y^2, ...,y^i,...\}\) as free \(R[x]\)-module

All free \(R\)-modules \(M \) have some basis \(B=\{b_1,...,b_n\} \) so have rank \(n \), and can be written as \(R^n\cong M \). Additionally every element \(m\in M \) can be uniquely written \( m=\sum_{i=1}^n r_ib_i\).

Theorems:

Theorem: (UMP for free modules) Let \(R\) be a ring, \(M\) a free \(R\)-module with basis \(B\), \(N\) an \(R\)-module, \(j:B\to N\) a function. Then there exists a unique \(R\)-module homomorphism \(h:M\to N\) such that \(h(b)=j(b)\ \forall b\in B\).

Corollary: If A and B are sets of the same cardinality, and fix a bijection j : A → B. If M and N are free R-modules with bases A and B respectively, then there is an isomorphism of R-modules M ∼= N.

Theorem:(Uniqueness of rank over commutative rings). Let R be a commutative ring with 1 6= 0 and let M be a free R-module. If A and B are both bases for M, then A and B have the same cardinality, meaning that there exists a bijection A → B.

Theorem: Let R be a commutative ring with 1 6= 0. Let V and W be finitely generated free R-modules of ranks n and m respectively. Fixing ordered bases B for V and C for W gives an isomorphism of R-modules

\(\text{Hom}_R(V,W)\cong \text{M}_{m\times n}(R) \quad f \mapsto [f]_B^C \)

If V = W, so that in particular m = n, and B = C, then the above map is an R-algebra isomorphism EndR(V ) ∼= Mn(R).

Lemma. Given any ring \(R\) with \(1 \ne 0\), any direct sum of copies of R is always a free R-module.

Theorem. Every R-module is a quotient of a free R-module