Dirichlet convolution

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Johann Peter Gustav Lejeune Dirichlet Dirichlet.jpg
Johann Peter Gustav Lejeune Dirichlet

In mathematics, the Dirichlet convolution (or divisor 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 are two arithmetic functions from the positive integers to the complex numbers, the Dirichlet convolution fg is a new arithmetic function defined 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.

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:

Properties

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,

distributive over addition

,

commutative,

,

and has an identity element,

= .

Furthermore, for each having , there exists an arithmetic function with , called the Dirichlet inverse of .

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 ), 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 , pointwise multiplication by distributes over Dirichlet convolution: . [2] The convolution of two completely multiplicative functions is multiplicative, but not necessarily completely multiplicative.

Properties and examples

In these formulas, we use the following arithmetical functions:

The following relations hold:

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

where is the Mertens function and 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]

Dirichlet inverse

Examples

Given an arithmetic function its Dirichlet inverse may be calculated recursively: the value of is in terms of for .

For :

, so
. This implies that does not have a Dirichlet inverse if .

For :

,
,

For :

,
,

For :

,
,

and in general for ,

Properties

The following properties of the Dirichlet inverse hold: [4]

Other formulas

Arithmetic functionDirichlet inverse: [5]
Constant function with value 1 Möbius function μ
Liouville's function λAbsolute value of Möbius function |μ|
Euler's totient function
The generalized sum-of-divisors function

An exact, non-recursive formula for the Dirichlet inverse of any arithmetic function f is given in Divisor sum identities. A more partition theoretic expression for the Dirichlet inverse of f is given by

The following formula provides a compact way of expressing the Dirichlet inverse of an invertible arithmetic function f :

where the expression stands for the arithmetic function convoluted with itself k times. Notice that, for a fixed positive integer , if then , this is because and every way of expressing n as a product of k positive integers must include a 1, so the series on the right hand side converges for every fixed positive integer n.

Dirichlet series

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

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 a special case of the convolution multiplication for the incidence algebra of a poset, in this case the poset of positive integers ordered by divisibility.

The Dirichlet hyperbola method computes the summation of a convolution in terms of its functions and their summation functions.

See also

Related Research Articles

In number theory, an arithmetic, arithmetical, or number-theoretic function is generally any function f(n) whose domain is the positive integers and whose range is a subset of the complex numbers. Hardy & Wright include in their definition the requirement that an arithmetical function "expresses some arithmetical property of n". There is a larger class of number-theoretic functions that do not fit this definition, for example, the prime-counting functions. This article provides links to functions of both classes.

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

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In mathematics, the classic Möbius inversion formula is a relation between pairs of arithmetic functions, each defined from the other by sums over divisors. It was introduced into number theory in 1832 by August Ferdinand Möbius.

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<span class="mw-page-title-main">Lambert series</span> Mathematical term

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The purpose of this page is to catalog new, interesting, and useful identities related to number-theoretic divisor sums, i.e., sums of an arithmetic function over the divisors of a natural number , or equivalently the Dirichlet convolution of an arithmetic function with one:

In mathematics, a greatest common divisor matrix is a matrix that may also be referred to as Smith's matrix. The study was initiated by H.J.S. Smith (1875). A new inspiration was begun from the paper of Bourque & Ligh (1992). This led to intensive investigations on singularity and divisibility of GCD type matrices. A brief review of papers on GCD type matrices before that time is presented in Haukkanen, Wang & Sillanpää (1997).

References

  1. Proofs are in Chan, ch. 2
  2. A proof is in the article Completely multiplicative function#Proof of distributive property.
  3. Schmidt, Maxie. Apostol's Introduction to Analytic Number Theory. This identity is a little special something I call "croutons". It follows from several chapters worth of exercises in Apostol's classic book.
  4. Again see Apostol Chapter 2 and the exercises at the end of the chapter.
  5. See Apostol Chapter 2.