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: Let R be a ring, M be a free R-module with basis B, N be any R-module, and let j : B → N be any function. Then there is a unique R-module homomorphism h : M → N such that h(b) = j(b) for all b ∈ 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.