Mobius function

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

Definition

The Mobius function is an integer-valued function defined on the natural numbers as follows. The Mobius function at $n$, denoted $\mu (n)$, is defined as:

• $\mu (1)=1$.
• $\mu (p_1{p}_2\dots p_r)=(-1{)}^r$ if $p_i$ are pairwise distinct primes.
• $\mu (n)=0$ if $n$ is divisible by the square of a prime.

Definition in terms of Dirichlet product

The Mobius function $\mu$ is defined as the inverse, with respect to the Dirichlet product, of the all ones function $U$, which is defined as the function sending every natural number to $1$. In other words:

$\mu *U=U*\mu =I$.

Here, $I$ is the identity element for the Dirichlet product, and is the function that is $1$ at $1$ and $0$ elsewhere.

Facts

Mobius inversion formula

Further information: Mobius inversion formula

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

$f*U=g⟺f=g*\mu$.

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

In more explicit terms, it states that:

$g(n)={\sum }_{d|n}f(d)\mathrm{\forall }n\in \mathbb{N}⟺f(n)={\sum }_{d|n}g(d)\mu (n/d)\mathrm{\forall }n\in \mathbb{N}$.