Multiplicative arithmetic function

An arithmetic function of one argument, $f(m)$, satisfying the condition

$$ f(mn) = f(m) f(n) $$

for any pair of coprime integers $m,n$. It is usually assumed that $f$ is not identically zero (which is equivalent to the condition $f(1)=1$). A multiplicative arithmetic function is called strongly multiplicative if $f(p^a) = f(p)$ for all prime numbers $p$ and all natural numbers $a$. If (*) holds for any two numbers $m,n$, and not just for coprime numbers, then $f$ is called totally multiplicative; in this case $f(p^a) = f(p)^a$.

Examples of multiplicative arithmetic functions. The function $\tau(m)$, the number of natural divisors of a natural number $m$; the function $\sigma(m)$, the sum of the natural divisors of the natural number $m$; the Euler function $\phi(m)$; and the Möbius function $\mu(m)$. The function $\phi(m)/m$ is a strongly multiplicative arithmetic function, a power function $m^s$ is a totally multiplicative arithmetic function.

Comments

The Dirichlet convolution product

$$ (f*g)(n) = \sum_{d\vert n} f(d) g(n/d) $$

yields a group structure on the multiplicative functions. The unit element is given by the function $e$, where $e(1)=1$ and $e(m) = 0$ for all $m > 1$. Another standard multiplicative function is the constant function $\zeta(n)$ with $\zeta(m) = 1$ for all $m$ and its inverse $\mu$, the Möbius function. Note that $\phi = \mu * N_1$, where $N_1(n) = n$ for all $n$, and that $\tau = \zeta * \zeta$, $\sigma = \zeta * N_1$.

Formally, the Dirichlet series of a multiplicative function $f$ has an Euler product:

$$ \sum_{n=1}^\infty f(n) n^{-s} = \prod_p \left({1 + f(p) p^{-s} + f(p^2) p^{-2s} + \cdots }\right) \ , $$

whose form simplifies considerably if $f$ is strongly or totally multiplicative.

References

[a1]	G.H. Hardy, E.M. Wright, "An introduction to the theory of numbers" , Clarendon Press (1960) pp. Chapts. XVI-XVII