$\forall$$a$, ${\it a'}$, $b$, ${\it b'}$:$\mathbb{Z}$. ($a$ $\sim$ ${\it a'}$) $\Rightarrow$ ($b$ $\sim$ ${\it b'}$) $\Rightarrow$ (($a$$\cdot$$b$) $\sim$ (${\it a'}$$\cdot$${\it b'}$))