# Mobius function

From Number

This article defines an arithmetic function or number-theoretic function: a function from the natural numbers to a ring (usually, the ring of integers, rational numbers, real numbers, or complex numbers).

View a complete list of arithmetic functions

## Contents

## Definition

The **Mobius function** is an integer-valued function defined on the natural numbers as follows. The Mobius function at , denoted , is defined as:

- .
- if are pairwise distinct primes.
- if is divisible by the square of a prime.

### Definition in terms of Dirichlet product

The Mobius function is defined as the inverse, with respect to the Dirichlet product, of the all ones function , which is defined as the function sending every natural number to . In other words:

.

Here, is the identity element for the Dirichlet product, and is the function that is at and elsewhere.

## Facts

### Mobius inversion formula

`Further information: Mobius inversion formula`

In terms of Dirichlet products, the Mobius inversion formula states that:

.

The group-theoretic proof of this involves taking the Dirichlet product of both sides with .

In more explicit terms, it states that:

.