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In mathematics, and more particularly in analytic number theory, Perron's formula is a formula due to Oskar Perron to calculate the sum of an arithmetical function, by means of an inverse Mellin transform.

Statement

Let $\{a(n)\}$ be an arithmetic function, and let

$g(s)=\sum _{n=1}^{\infty }{\frac {a(n)}{n^{s}}}$

be the corresponding Dirichlet series. Presume the Dirichlet series to be absolutely convergent for $\Re (s)>\sigma _{a}$. Then Perron's formula is

$A(x)={\sum _{n\leq x}}^{\star }a(n)={\frac {1}{2\pi i}}\int _{c-i\infty }^{c+i\infty }g(z){\frac {x^{z}}{z}}dz.\;$

Here, the star on the summation indicates that the last term of the sum must be multiplied by 1/2 when x is an integer. The formula requires $c>\sigma _{a}$ and $x>0$ real, but otherwise arbitrary.

Proof

An easy sketch of the proof comes from taking Abel's sum formula

$g(s)=\sum _{n=1}^{\infty }{\frac {a(n)}{n^{s}}}=s\int _{0}^{\infty }A(x)x^{-(s+1)}dx.$

This is nothing but a Laplace transform under the variable change $x=e^{t}.$ Inverting it one gets Perron's formula.

Examples

Because of its general relationship to Dirichlet series, the formula is commonly applied to many number-theoretic sums. Thus, for example, one has the famous integral representation for the Riemann zeta function:

$\zeta (s)=s\int _{1}^{\infty }{\frac {\lfloor x\rfloor }{x^{s+1}}}\,dx$

and a similar formula for Dirichlet L-functions:

$L(s,\chi )=s\int _{1}^{\infty }{\frac {A(x)}{x^{s+1}}}\,dx$

where

$A(x)=\sum _{n\leq x}\chi (n)$

and $\chi (n)$ is a Dirichlet character. Other examples appear in the articles on the Mertens function and the von Mangoldt function.

References

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