Group of Rationals Modulo One is Group

Theorem
The set of equivalence classes $\Q/\Z$ with respect to the relation
 * $a \sim b :\Longleftrightarrow a-b \mathop\in\Z$

with the binary operation
 * $\Q/\Z \times \Q/\Z \to \Q/\Z, \quad \struct{[a],[b]} \mapsto [a+b]$

is an infinite abelian group.