Multiplicative function

Definition

Let $f$ be an arithmetic function: in other words, $f$ is a function from the set of natural numbers to a commutative unital ring $R$. We say that $f$ is multiplicative if it satisfies the following two conditions:

• $f(1)=1$.
• $f(mn)=f(m)f(n)$ for all pairs of relatively prime numbers $m,n\in \mathbb {N}$.

Facts

Determined by values at prime powers

A multiplicative function $f$ is determined completely by the values it takes at powers of primes. Further, the values taken by $f$ at prime powers are completely independent. In other words, any function from the set of prime powers to the commutative unital ring extends uniquely to a multiplicative function.

Dirichlet series

There is a nice Dirichlet series expression for multiplicative functions. Specifically, the Dirichlet series for a multiplicative function is a product of series for values at powers of each prime. If $f$ is a multiplicative function and $\mathbb {P}$ denotes the set of primes, we have:

$\sum _{n\in \mathbb {N} }{\frac {f(n)}{n^{s}}}=\prod _{p\in \mathbb {P} }\left(\sum _{k=0}^{\infty }{\frac {f(p^{k})}{p^{ks}}}\right)$.

Relation with other properties

Stronger properties

• Completely multiplicative function

Incomparable properties

• Divisibility-preserving function