# Multiplicative function

In number theory, a multiplicative function is an arithmetic function f(n) of the positive integer n with the property that f(1) = 1 and whenever a and b are coprime, then

f(ab) = f(a) f(b).

An arithmetic function f(n) is said to be completely (totally) multiplicative if f(1) = 1 and f(ab) = f(a) f(b) holds for all positive integers a and b, even when they are not coprime.

Outside number theory, the term multiplicative is usually used for functions with the property f(ab) = f(a) f(b) for all arguments a and b; this requires either f(1) = 1, or f(a) = 0 for all a except a = 1. This article discusses number theoretic multiplicative functions.

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## Examples

Examples of multiplicative functions include many functions of importance in number theory, such as:

• [itex]\phi[itex](n): Euler's Totient function [itex]\phi[itex], counting the positive integers coprime to (but not bigger than) n
• [itex]\mu[itex](n): the Mbius function, related to the number of prime factors of square-free numbers
• gcd(n,k): the greatest common divisor of n and k, where k is a fixed integer.
• d(n): the number of positive divisors of n,
• [itex]\sigma[itex](n): the sum of all the positive divisors of n,
• [itex]\sigma[itex]k(n): the divisor function, which is the sum of the k-th powers of all the positive divisors of n (where k may be any complex number). In special cases we have
• [itex]\sigma[itex]0(n) = d(n) and
• [itex]\sigma[itex]1(n) = [itex]\sigma[itex](n),
• 1(n): the constant function, defined by 1(n) = 1 (completely multiplicative)
• Id(n): identity function, defined by Id(n) = n (completely multiplicative)
• Idk(n): the power functions, defined by Idk(n) = nk for any natural (or even complex) number k (completely multiplicative). As special cases we have
• Id0(n) = 1(n) and
• Id1(n) = Id(n),
• [itex]\epsilon[itex](n): the function defined by [itex]\epsilon[itex](n) = 1 if n = 1 and = 0 if n > 1, sometimes called multiplication unit for Dirichlet convolution or simply the unit function; sometimes written as u(n), not to be confused with [itex]\mu[itex](n) (completely multiplicative).
• (n/p), the Legendre symbol, where p is a fixed prime number (completely multiplicative).
• [itex]\lambda[itex](n): the Liouville function, related to the number of prime factors dividing n (completely multiplicative).
• [itex]\gamma[itex](n), defined by [itex]\gamma[itex](n)=(-1)[itex]\omega[itex](n), where the additive function [itex]\omega[itex](n) is the number of distinct primes dividing n.
• All Dirichlet characters are completely multiplicative functions.

An example of a non-multiplicative function is the arithmetic function r2(n) - the number of representations of n as a sum of squares of two integers, positive, negative, or zero, where in counting the number of ways, reversal of order is allowed. For example:

1 = 12 + 02 = (-1)2 + 02 = 02 + 12 = 02 + (-1)2

and therefore r2(1) = 4 ≠ 1. This shows that the function is not multiplicative. However, r2(n)/4 is multiplicative.

See arithmetic function for some other examples of non-multiplicative functions.

## Properties

A multiplicative function is completely determined by its values at the powers of prime numbers, a consequence of the fundamental theorem of arithmetic. Thus, if n is a product of powers of distinct primes, say n = pa qb ..., then f(n) = f(pa) f(qb) ...

This property of multiplicative functions significantly reduces the need for computation, as in the following examples for n = 144 = 24 32:

d(144) = [itex]\sigma[itex]0(144) = [itex]\sigma[itex]0(24)[itex]\sigma[itex]0(32) = (10 + 20 + 40 + 80 + 160)(10 + 30 + 90) = 5 3 = 15,
[itex]\sigma[itex](144) = [itex]\sigma[itex]1(144) = [itex]\sigma[itex]1(24)[itex]\sigma[itex]1(32) = (11 + 21 + 41 + 81 + 161)(11 + 31 + 91) = 31 13 = 403,
[itex]\sigma[itex]*(144) = [itex]\sigma[itex]*(24)[itex]\sigma[itex]*(32) = (11 + 161)(11 + 91) = 17 10 = 170.

Similarly, we have:

[itex]\phi[itex](144)=[itex]\phi[itex](24)[itex]\phi[itex](32) = 8 · 6 = 48

In general, if f(n) is a multiplicative function and a, b are any two positive integers, then

f(a) f(b) = f(gcd(a,b)) f(lcm(a,b)).

Every completely multiplicative function is a homomorphism of monoids and is completely determined by its restriction to the prime numbers.

## Convolution

If f and g are two multiplicative functions, one defines a new multiplicative function f * g, the Dirichlet convolution of f and g, by

(f * g)(n) = ∑d|n f(d)g(n/d)

where the sum extends over all positive divisors d of n. With this operation, the set of all multiplicative functions turns into an abelian group; the identity element is [itex]\epsilon[itex].

Relations among the multiplicative functions discussed above include:

• [itex]\epsilon[itex] = [itex]\mu[itex] * 1 (the Mbius inversion formula)
• [itex]\phi[itex] = [itex]\mu[itex] * Id
• d = 1 * 1
• [itex]\sigma[itex] = Id * 1 = [itex]\phi[itex] * d
• [itex]\sigma[itex]k = Idk * 1
• Id = [itex]\phi[itex] * 1 = [itex]\sigma[itex] * [itex]\mu[itex]
• Idk = [itex]\sigma[itex]k * [itex]\mu[itex]

The Dirichlet convolution can be defined for general arithmetic functions, and yields a ring structure, the Dirichlet ring.

## References

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