# Definition:Congruence (Number Theory)/Integers/Remainder after Division

## Definition

Let $m \in \Z_{> 0}$ be an integer.

Congruence modulo $m$ is defined as the relation $\equiv \pmod m$ on the set of all $a, b \in \Z$:

$a \equiv b \pmod m := \set {\tuple {a, b} \in \Z \times \Z: \exists k \in \Z: a = b + k m}$

That is, such that $a$ and $b$ have the same remainder when divided by $m$.

## Notation

The relation $x$ is congruent to $y$ modulo $z$, usually denoted:

$x \equiv y \pmod z$

is also frequently seen denoted as:

$x \equiv y \ \paren {\mathop {\operatorname{modulo} } z}$

Some (usually older) sources render it as:

$x \equiv y \ \paren {\mathop {\operatorname{mod.} } z}$

## Historical Note

The concept of congruence modulo an integer was first explored by Carl Friedrich Gauss.

He originated the notation $a \equiv b \pmod m$ in his work Disquisitiones Arithmeticae, published in $1801$.

## Linguistic Note

The word modulo comes from the Latin for with modulus, that is, with measure.