# Congruence (Number Theory)/Examples

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## Examples of Congruence

### Congruence Modulo $1$

Let $x \equiv y \pmod 1$ be defined as congruence on the real numbers modulo $1$:

- $\forall x, y \in \R: x \equiv y \pmod 1 \iff \exists k \in \Z: x - y = k$

That is, if their difference $x - y$ is an integer.

The equivalence classes of this equivalence relation are of the form:

- $\eqclass x 1 = \set {\dotsc, x - 2, x - 1, x, x + 1, x + 2, \dotsc}$

Each equivalence class has exactly one representative in the half-open real interval:

- $\hointr 0 1 = \set {x \in \R: 0 \le x < 1}$

### Congruence Modulo $2 \pi$ as Angular Measurement

Let $\RR$ denote the relation on the real numbers $\R$ defined as:

- $\forall x, y \in \R: \tuple {x, y} \in \RR \iff \text {$x$ and $y$}$ measure the same angle in radians

Then $\RR$ is the congruence relation modulo $2 \pi$.

The equivalence classes of this equivalence relation are of the form:

- $\eqclass \theta {2 \pi} = \set {\theta + 2 k \pi: k \in \Z}$

Hence for example:

- $\eqclass 0 {2 \pi} = \set {2 k \pi: k \in \Z}$

and:

- $\eqclass {\dfrac \pi 2} {2 \pi} = \set {\dfrac {\paren {4 k + 1} \pi} 2: k \in \Z}$

Each equivalence class has exactly one representative in the half-open real interval:

- $\hointr 0 {2 \pi} = \set {x \in \R: 0 \le x < 2 \pi}$

and have a one-to-one correspondence with the points on the circumference of a circle.