Contents

# Contents

## Definition

For $n \in \mathbb{N}$, the group of units of the ring of integers modulo n

$\left( \mathbb{Z}/n \right)^\times$

is typically called the multiplicative group of integers modulo $n$.

This consists of all those elements $k \in \mathbb{Z}/n$ which are represented by coprime integers to $n$. It is typically denoted $\mathbb{Z}/n ^\ast$.

## Euler totient function

The cardinality ${|(\mathbb{Z}/n)^\times|}$ is often denoted $\phi(n)$ (after Leonhard Euler), and $\phi$ is known as the Euler totient function.

The function $\phi$ is multiplicative in the standard number theory sense: $\phi(m n) = \phi(m)\phi(n)$ is $m$ and $n$ are coprime. This is a corollary of the Chinese remainder theorem?, which asserts that the canonical ring map

$\mathbb{Z}/m n \to \mathbb{Z}/m \times \mathbb{Z}/n$

is an isomorphism if $m, n$ are coprime. Thus, if $n = p_1^{r_1} \ldots p_k^{r_k}$ is the prime factorization of $n$, we have

$\phi(n) = \phi(p_1^{r_1}) \ldots \phi(p_k^{r_k}).$

Furthermore, as $\mathbb{Z}/p^r$ is a local ring with maximal ideal $p(\mathbb{Z}/p^r)$, the cardinality of the group of units is

$\phi(p^r) = p^r - p^{r-1} = p^{r-1}(p-1).$