\input amstex
\def\cdeo#1{\lfloor{#1}\rfloor}
\def\akko{\;\Longleftrightarrow\;}
$$\aligned
\cdeo{x}&=m\cdot\Bigl(nx-\cdeo{nx}\Bigr)=mn\cdot\Bigl(x-\cdeo{x}\Bigr)\akko\cr
&\akko\frac{x}{\cdeo{x}}=1+\frac{1}{mn}
\endaligned$$