# Definition:Reduced Residue System

## Definition

Let $m \in \Z_{> 0}$ be a (strictly) positive integer.

The reduced residue system modulo $m$, denoted $\Z'_m$, is the set of all residue classes of $k$ (modulo $m$) which are prime to $m$:

$\Z'_m = \set {\eqclass k m \in \Z_m: k \perp m}$

Thus $\Z'_m$ is the set of all coprime residue classes modulo $m$:

$\Z'_m = \set {\eqclass {a_1} m, \eqclass {a_2} m, \ldots, \eqclass {a_{\map \phi m} } m}$

where:

$\forall k: a_k \perp m$
$\map \phi m$ denotes the Euler phi function of $m$.

## Least Positive Coprime Residues

The least positive reduced residue system modulo $m$ is the set of integers:

$\set {a_1, a_2, \ldots, a_{\map \phi m} }$

with the following properties:

$\map \phi m$ is the Euler $\phi$ function
$\forall i: 0 < a_i < m$
each of which is prime to $m$
no two of which are congruent modulo $m$.

## Also known as

A reduced residue system modulo $m$ is also known as a reduced set of residues modulo $m$.

Some authors refer to this as the set of relatively prime residue classes modulo $m$.

Some sources denote it $\Z^*_m$ or $Z^*_m$.

## Examples

The reduced sets of residues modulo $n$ for the first few (strictly) positive integers are:

 $\ds 1$ $:$ $\ds \set 1$ $\ds 2$ $:$ $\ds \set 1$ $\ds 3$ $:$ $\ds \set {1, 2}$ $\ds 4$ $:$ $\ds \set {1, 3}$ $\ds 5$ $:$ $\ds \set {1, 2, 3, 4}$ $\ds 6$ $:$ $\ds \set {1, 5}$ $\ds 7$ $:$ $\ds \set {1, 2, 3, 4, 5, 6}$ $\ds 8$ $:$ $\ds \set {1, 3, 5, 7}$ $\ds 9$ $:$ $\ds \set {1, 2, 4, 5, 7, 8}$ $\ds 10$ $:$ $\ds \set {1, 3, 7, 9}$

## Also see

• Results about reduced residue systems can be found here.