Reduced Residue System under Multiplication forms Abelian Group

Theorem
Let $\Z_m$ be the set of set of all residue classes modulo $m$.

Let $\Z'_m$ be the reduced residue system modulo $m$.

Then the structure $\struct {\Z'_m, \times}$ is an abelian group, precisely equal to the group of units of $\Z_m$.