Isomorphism between free abelian groups Let $A_1$ and $A_2$ be two free abelian group generated on the same finite set $X$. I have to demonstrate that are isomorphic if and only if have the same characteristic $rank(A_1)=rank(A_2)$. 
The "only if" is easy. But I don't know how to proceed to deduce the isomorphims of the two free abelian groups if $rank(A_1)=rank(A_2)$.
Note: if $A$ is a finetely generated abelian group it can be written as 
$$A=\mathbb{Z_{n_1}} \otimes...\otimes\mathbb{Z_{n_1}}\otimes\mathbb{Z^r}$$ 
in this contest by definition the rank of the abelian group is $rank(A)=r$.
Can anybody give me an hint?

A finitely generated free Abelian group is isomorphic to $\Bbb Z^r$
where the rank $r$ is the size of the generating set. It is clear that $r$
determines the structure of $\Bbb Z^r$.
If $\Bbb Z^r\cong\Bbb Z^s$ then $(\Bbb Z/2\Bbb Z)^r\cong(\Bbb Z/2\Bbb Z)^s$
so $2^r=2^s$ and $r=s$. (For Abelian $G$ and $H$, $G\cong H$ implies $G/2G\cong H/2H$.)
