Mappings Between Residue Classes

Theorem
Let $\eqclass a m$ be the residue class of $a$ (modulo $m$).

Let $\phi: \Z_m \to \Z_n$ be a mapping given by:
 * $\map \phi {\eqclass x m} = \eqclass x n$

Then $\phi$ is well defined $m$ is a divisor of $n$.

Proof
For $\phi$ to be well defined, we require that:


 * $\forall x, y \in \Z_m: \eqclass x m = \eqclass y m \implies \map \phi {\eqclass x m} = \map \phi {\eqclass y m}$

Now:
 * $\eqclass x m = \eqclass y m \implies x - y \divides m$

For $\map \phi {\eqclass x m} = \map \phi {\eqclass y m}$ we require that:
 * $\eqclass x n = \eqclass y n \implies x - y \mathrel \backslash n$

Thus $\phi$ is well defined :
 * $x - y \divides m \implies x - y \divides n$

That is, $m \divides n$.