# Number is Divisor iff Modulo is Zero

## Theorem

Let $x, y \in \R$ be real numbers.

Let $x \bmod y$ denote the modulo operation:

$x \bmod y := \begin {cases} x - y \floor {\dfrac x y} & : y \ne 0 \\ x & : y = 0 \end {cases}$

where $\floor {\dfrac x y}$ denotes the floor of $\dfrac x y$.

Then $x \bmod y = 0$ if and only if $x$ is an integer multiple of $y$.

## Proof

### Sufficient Condition

Let $x \bmod y = 0$.

$x - \paren {x \bmod y}$

is an integer multiple of $y$.

As $x \bmod y = 0$ it follows that $x - 0 = x$ is an integer multiple of $y$.

$\Box$

### Necessary Condition

If $y = 0$ it follows from Zero Divides Zero that $x = 0$.

Hence the result by definition of the modulo operation:

Otherwise, let $y \ne 0$.

Let $x$ be an integer multiple of $y$.

That is:

$\exists n \in \Z: x = n y$

Hence:

$\dfrac x y = n$
$\dfrac x y - \floor {\dfrac x y} = \dfrac {x \bmod y} y$

From Real Number is Integer iff equals Floor it follows that:

$\dfrac x y = \floor {\dfrac x y}$

Thus:

$\dfrac {x \bmod y} y = 0$

and so multiplying both sides by $y$ the result follows.

$\blacksquare$