Dirichlet convolution
In mathematics, the Dirichlet convolution is a binary operation defined for arithmetic functions; it is important in number theory. It was developed by Peter Gustav Lejeune Dirichlet.
Contents
Definition
If f and g are two arithmetic functions (i.e. functions from the positive integers to the complex numbers), one defines a new arithmetic function f ∗ g, the Dirichlet convolution of f and g, by
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.
Properties
The set of arithmetic functions forms a commutative ring, the Dirichlet ring, under pointwise addition (i.e. 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 (i.e. invertible elements) of this ring are the arithmetic functions f with f(1) ≠ 0.
Specifically, Dirichlet convolution is^{[1]} associative,
 (f ∗ g) ∗ h = f ∗ (g ∗ h),
distributes over addition
 f ∗ (g + h) = f ∗ g + f ∗ h = (g + h) ∗ f,
is commutative,
 f ∗ g = g ∗ f,
and has an identity element,
 f ∗ ε = ε ∗ f = f.
Furthermore, for each f for which f(1) ≠ 0 there exists a g such that f ∗ g = ε, called the Dirichlet inverse of f.
The Dirichlet convolution of two multiplicative functions is again multiplicative, and every multiplicative function has a Dirichlet inverse that is also multiplicative. The article on multiplicative functions lists several convolution relations among important multiplicative functions.
Given a completely multiplicative function f, then f(g ∗ h) = (fg) ∗ (fh), where juxtaposition represents pointwise multiplication.^{[2]} The convolution of two completely multiplicative functions is multiplicative, but not necessarily completely multiplicative.
Examples
In these formulas
 ε is the multiplicative identity. (I.e. ε(1) = 1, all other values 0.)
 1 is the constant function whose value is 1 for all n. (I.e. 1(n) = 1.) Keep in mind that 1 is not the identity.
 1_{C}, where is a set is the indicator function. (I.e. 1_{C}(n) = 1 if n ∈ C, 0 otherwise.)
 Id is the identity function whose value is n. (I.e. Id(n) = n.)
 Id_{k} is the kth power function. (I.e. Id_{k}(n) = n^{k}.)
 The other functions are defined in the article arithmetical function.
 1 ∗ μ = ε (the Dirichlet inverse of the constant function 1 is the Möbius function.) This implies
 g = f ∗ 1 if and only if f = g ∗ μ (the Möbius inversion formula).
 λ ∗ μ = ε where λ is Liouville's function.
 λ ∗ 1 = 1_{Sq} where Sq = {1, 4, 9, ...} is the set of squares
 Id_{k} ∗ (Id_{k} μ) = ε
 σ_{k} = Id_{k} ∗ 1 definition of the divisor function σ_{k}
 σ = Id ∗ 1 definition of the function σ = σ_{1}
 d = 1 ∗ 1 definition of the function d(n) = σ_{0}
 Id_{k} = σ_{k} ∗ μ Möbius inversion of the formulas for σ_{k}, σ, and d.
 Id = σ ∗ μ
 1 = d ∗ μ
 d^{3} ∗ 1 = (d ∗ 1)^{2}
 φ ∗ 1 = Id This formula is proved in the article Euler's totient function.
 J_{k} ∗ 1 = Id_{k} The Jordan's totient function.
 (Id_{s}J_{r}) ∗ J_{s} = J_{s + r}
 σ = φ ∗ d Proof: convolve 1 to both sides of Id = φ ∗ 1.
 Λ ∗ 1 = log where Λ is von Mangoldt function.
Dirichlet inverse
Given an arithmetic function f its Dirichlet inverse g = f^{−1} may be calculated recursively (i.e. the value of g(n) is in terms of g(m) for m < n) from the definition of Dirichlet inverse.
For n = 1:
 (f ∗ g) (1) = f(1) g(1) = ε(1) = 1, so
 g(1) = 1/f(1). This implies that f does not have a Dirichlet inverse if f(1) = 0.
For n = 2
 (f ∗ g) (2) = f(1) g(2) + f(2) g(1) = ε(2) = 0,
 g(2) = −1/f(1) (f(2) g(1)),
For n = 3
 (f ∗ g) (3) = f(1) g(3) + f(3) g(1) = ε(3) = 0,
 g(3) = −1/f(1) (f(3) g(1)),
For n = 4
 (f ∗ g) (4) = f(1) g(4) + f(2) g(2) + f(4) g(1) = ε(4) = 0,
 g(4) = −1/f(1) (f(4) g(1) + f(2) g(2)),
and in general for n > 1,
Since the only division is by f(1) this shows that f has a Dirichlet inverse if and only if f(1) ≠ 0.
Here is a useful table of Dirichlet inverses of common arithmetic functions:
Arithmetic function  Dirichlet inverse 

Constant function equal to 1  Möbius function μ 
Liouville's function λ  Absolute value of Möbius function μ 
Dirichlet series
If f is an arithmetic function, one defines its Dirichlet series generating function by
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:
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.
Related Concepts
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The restriction of the divisors in the convolution to unitary, biunitary 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, Eulertype product formulas over associated primes, etc.).
Dirichlet convolution is the convolution of the incidence algebra for the positive integers ordered by divisibility.
References
 ^ Proofs of all these facts are in Chan, ch. 2
 ^ A proof is in the article Completely multiplicative function#Proof of pseudoassociative property.
 Apostol, Tom M. (1976), Introduction to analytic number theory, Undergraduate Texts in Mathematics, New YorkHeidelberg: SpringerVerlag, ISBN 9780387901633, MR 0434929, Zbl 0335.10001
 Chan Heng Huat (2009). Analytic Number Theory for Undergraduates. Monographs in Number Theory. World Scientific Publishing Company. ISBN 9814271365.
 Hugh L. Montgomery; Robert C. Vaughan (2007). Multiplicative number theory I. Classical theory. Cambridge tracts in advanced mathematics. 97. Cambridge: Cambridge Univ. Press. p. 38. ISBN 0521849039.
 Cohen, Eckford (1959). "A class of residue systems (mod r) and related arithmetical functions. I. A generalization of Möbius inversion". Pacific J. Math. 9 (1). pp. 13–23. MR 0109806.
 Cohen, Eckford (1960). "Arithmetical functions associated with the unitary divisors of an integer". Mathematische Zeitschrift. 74. pp. 66–80. MR 0112861. doi:10.1007/BF01180473.
 Cohen, Eckford (1960). "The number of unitary divisors of an integer". American Mathematical Monthly. 67 (9). pp. 879–880. MR 0122790.
 Cohen, Graeme L. (1990). "On an integers' infinitary divisors". Math. Comp. 54 (189). pp. 395–411. MR 0993927. doi:10.1090/S00255718199009939275.
 Cohen, Graeme L. (1993). "Arithmetic functions associated with infinitary divisors of an integer". Intl. J. Math. Math. Sci. 16 (2). pp. 373–383. doi:10.1155/S0161171293000456.
 Sandor, Jozsef; Berge, Antal (2003). "The Möbius function: generalizations and extensions". Adv. Stud. Contemp. Math. (Kyungshang). 6 (2): 77–128. MR 1962765.
 Finch, Steven (2004). "Unitarism and Infinitarism" (PDF). Archived from the original (PDF) on 20150222.
External links
 Hazewinkel, Michiel, ed. (2001) [1994], "Dirichlet convolution", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 9781556080104