Dirichlet convolution

If ${\displaystyle f,g:\mathbb {N} \to \mathbb {C} }$ are two arithmetic functions from the positive integers to the complex numbers, the Dirichlet convolution f ∗ g is a new arithmetic function defined by:

${\displaystyle (f*g)(n)\ =\ \sum _{d\,\mid \,n}f(d)\,g\!\left({\frac {n}{d}}\right)\ =\ \sum _{ab\,=\,n}\!f(a)\,g(b)}$

where the sum extends over all positive divisors d of n, or equivalently over all distinct pairs (a, b) of positive integers whose product is n.

This product occurs naturally in the study of Dirichlet series such as the Riemann zeta function. It describes the multiplication of two Dirichlet series in terms of their coefficients:

${\displaystyle \left(\sum _{n\geq 1}{\frac {f(n)}{n^{s}}}\right)\left(\sum _{n\geq 1}{\frac {g(n)}{n^{s}}}\right)\ =\ \left(\sum _{n\geq 1}{\frac {(f*g)(n)}{n^{s}}}\right).}$

The set of arithmetic functions forms a commutative ring, the Dirichlet ring, under pointwise addition, where f + g is defined by (f + g)(n) = f(n) + g(n), and Dirichlet convolution. The multiplicative identity is the unit function ε defined by ε(n) = 1 if n = 1 and ε(n) = 0 if n > 1. The units (invertible elements) of this ring are the arithmetic functions f with f(1) ≠ 0.

Specifically,[1] Dirichlet convolution is associative,

${\displaystyle (f*g)*h=f*(g*h),}$

distributive over addition

${\displaystyle f*(g+h)=f*g+f*h}$,

commutative,

${\displaystyle f*g=g*f}$,

and has an identity element,

${\displaystyle f*\varepsilon }$ = ${\displaystyle \varepsilon *f=f}$.

Furthermore, for each ${\displaystyle f}$ having ${\displaystyle f(1)\neq 0}$, there exists an arithmetic function ${\displaystyle f^{-1}}$ with ${\displaystyle f*f^{-1}=\varepsilon }$, called the Dirichlet inverse of ${\displaystyle f}$.

The Dirichlet convolution of two multiplicative functions is again multiplicative, and every not constantly zero multiplicative function has a Dirichlet inverse which is also multiplicative. In other words, multiplicative functions form a subgroup of the group of invertible elements of the Dirichlet ring. Beware however that the sum of two multiplicative functions is not multiplicative (since ${\displaystyle (f+g)(1)=f(1)+g(1)=2\neq 1}$), so the subset of multiplicative functions is not a subring of the Dirichlet ring. The article on multiplicative functions lists several convolution relations among important multiplicative functions.

Another operation on arithmetic functions is pointwise multiplication: fg is defined by (fg)(n) = f(n) g(n). Given a completely multiplicative function ${\displaystyle h}$, pointwise multiplication by ${\displaystyle h}$ distributes over Dirichlet convolution: ${\displaystyle (f*g)h=(fh)*(gh)}$.[2] The convolution of two completely multiplicative functions is multiplicative, but not necessarily completely multiplicative.

In these formulas, we use the following arithmetical functions:

• ${\displaystyle \varepsilon }$ is the multiplicative identity: ${\displaystyle \varepsilon (1)=1}$, otherwise 0 (${\displaystyle \varepsilon (n)=\lfloor {\tfrac {1}{n}}\rfloor }$).
• ${\displaystyle 1}$ is the constant function with value 1: ${\displaystyle 1(n)=1}$ for all ${\displaystyle n}$. Keep in mind that ${\displaystyle 1}$ is not the identity. (Some authors denote this as ${\displaystyle \zeta }$ because the associated Dirichlet series is the Riemann zeta function.)
• ${\displaystyle 1_{C}}$ for ${\displaystyle C\subset \mathbb {N} }$ is a set indicator function: ${\displaystyle 1_{C}(n)=1}$ iff ${\displaystyle n\in C}$, otherwise 0.
• ${\displaystyle {\text{Id}}}$ is the identity function with value n: ${\displaystyle {\text{Id}}(n)=n}$.
• ${\displaystyle {\text{Id}}_{k}}$is the kth power function: ${\displaystyle {\text{Id}}_{k}(n)=n^{k}}$.

The following relations hold:

• ${\displaystyle 1*\mu =\varepsilon }$, the Dirichlet inverse of the constant function ${\displaystyle 1}$ is the Möbius function. Hence:
• ${\displaystyle g=f*1}$ if and only if ${\displaystyle f=g*\mu }$, the Möbius inversion formula
• ${\displaystyle \sigma _{k}={\text{Id}}_{k}*1}$, the kth-power-of-divisors sum function σ_k
• ${\displaystyle \sigma ={\text{Id}}*1}$, the sum-of-divisors function σ = σ₁
• ${\displaystyle d=1*1}$ , the number-of-divisors function d(n) = σ₀
• ${\displaystyle {\text{Id}}_{k}=\sigma _{k}*\mu }$,  by Möbius inversion of the formulas for σ_k, σ, and d
• ${\displaystyle {\text{Id}}=\sigma *\mu }$
• ${\displaystyle 1=d*\mu }$
• ${\displaystyle \phi *1={\text{Id}}}$ , proved under Euler's totient function
• ${\displaystyle \phi ={\text{Id}}*\mu }$ , by Möbius inversion
• ${\displaystyle \sigma =\phi *d}$  , from convolving 1 on both sides of ${\displaystyle \phi *1={\text{Id}}}$
• ${\displaystyle \lambda *|\mu |=\varepsilon }$  where λ is Liouville's function
• ${\displaystyle \lambda *1=1_{\text{Sq}}}$ where Sq = {1, 4, 9, ...} is the set of squares
• ${\displaystyle {\text{Id}}_{k}*({\text{Id}}_{k}\mu )=\varepsilon }$
• ${\displaystyle d^{3}*1=(d*1)^{2}}$
• ${\displaystyle J_{k}*1={\text{Id}}_{k}}$, Jordan's totient function
• ${\displaystyle ({\text{Id}}_{s}J_{r})*J_{s}=J_{s+r}}$
• ${\displaystyle \Lambda *1=\log }$, where ${\displaystyle \Lambda }$ is von Mangoldt's function
• ${\displaystyle |\mu |\ast 1=2^{\omega },}$ where ${\displaystyle \omega (n)}$ is the prime omega function counting distinct prime factors of n
• ${\displaystyle \Omega \ast \mu =1_{\mathcal {P}}}$, the characteristic function of the prime powers.
• ${\displaystyle \omega \ast \mu =1_{\mathbb {P} }}$ where ${\displaystyle 1_{\mathbb {P} }(n)\mapsto \{0,1\}}$ is the characteristic function of the primes.

This last identity shows that the prime-counting function is given by the summatory function

${\displaystyle \pi (x)=\sum _{n\leq x}(\omega \ast \mu )(n)=\sum _{d=1}^{x}\omega (d)M\left(\left\lfloor {\frac {x}{d}}\right\rfloor \right)}$

where ${\displaystyle M(x)}$ is the Mertens function and ${\displaystyle \omega }$ is the distinct prime factor counting function from above. This expansion follows from the identity for the sums over Dirichlet convolutions given on the divisor sum identities page (a standard trick for these sums).[3]

If f is an arithmetic function, the Dirichlet series generating function is defined by

${\displaystyle DG(f;s)=\sum _{n=1}^{\infty }{\frac {f(n)}{n^{s}}}}$

for those complex arguments s for which the series converges (if there are any). The multiplication of Dirichlet series is compatible with Dirichlet convolution in the following sense:

${\displaystyle DG(f;s)DG(g;s)=DG(f*g;s)\,}$

for all s for which both series of the left hand side converge, one of them at least converging absolutely (note that simple convergence of both series of the left hand side does not imply convergence of the right hand side!). This is akin to the convolution theorem if one thinks of Dirichlet series as a Fourier transform.

The restriction of the divisors in the convolution to unitary, bi-unitary or infinitary divisors defines similar commutative operations which share many features with the Dirichlet convolution (existence of a Möbius inversion, persistence of multiplicativity, definitions of totients, Euler-type product formulas over associated primes, etc.).

Dirichlet convolution is the convolution of the incidence algebra for the positive integers ordered by divisibility.

• Arithmetic function
• Divisor sum identities
• Möbius inversion formula

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