# Hurwitz zeta function

In mathematics, the Hurwitz zeta function is one of the many zeta functions. It defined as

[itex]\zeta(s,q) = \sum_{k=0}^\infty (k+q)^{-s}.[itex]

When q = 1, this coincides with Riemann's zeta function.

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### Relation to Dirichlet L-functions

Fixing an integer Q ≥ 1, the Dirichlet L-functions for characters modulo Q are linear combinations, with constant coefficients, of the ζ(s,q) where q = k/Q and k = 1, 2, ..., Q. This means that the Hurwitz zeta-functions for rational q have analytic properties that are closely related to that class of L-functions.

Specifically, let [itex]\chi[itex] be a character mod Q. Then we can write the Dirichlet L-function as

[itex]L(s,\chi) = \sum_{n=1}^\infty \frac {\chi(n)}{n^s} =

\frac {1}{Q^s} \sum_{k=1}^Q \chi(k)\; \zeta (s,\frac{k}{Q}) [itex].

### Hurwitz's formula

Hurwitz's formula is the theorem that

[itex]\zeta(1-s,x)=\frac{1}{2s}\left[e^{-i\pi s/2}\beta(x;s) + e^{i\pi s/2} \beta(1-x;s) \right][itex]

where

[itex]\beta(x;s)=

2\Gamma(s+1)\sum_{n=1}^\infty \frac {\exp(2\pi inx) } {(2\pi n)^s}= \frac{2\Gamma(s+1)}{(2\pi)^s} \mbox{Li}_s (e^{2\pi ix}) [itex] is a representation of the zeta that is valid for [itex]0\le x\le 1[itex] and [itex]s>1[itex]. Here, [itex]\mbox{Li}_s (z)[itex] is the polylogarithm.

### Relation to Bernoulli polynomials

The function [itex]\beta[itex] defined above generalizes the Bernoulli polynomials:

[itex]B_n(x) = -\Re \left[ (-i)^n \beta(x;n) \right] [itex]

where [itex]\Re z[itex] denotes the real part of z. Alternately,

[itex]\zeta(-n,x)=-{B_{n+1}(x) \over n+1}[itex]

### Relation to the polygamma function

The Hurwitz zeta is generalizes the polygamma function:

[itex]\psi^{(m)}(z)= (-1)^{m+1} m! \zeta (m+1,z)\,[itex]

### Relation to the Lerch transcendant

The Lerch transcendant generalizes the Hurwitz zeta:

[itex]\Phi(z, s, q) = \sum_{k=0}^\infty

\frac { z^k} {(k+q)^s}[itex] and thus

[itex]\zeta (s,q)=\Phi(1, s, q)\,[itex]

### Functional equation

The functional equation relates values of the zeta on the left- and right-hand sides of the complex plane. For integers [itex]1\leq m \leq n [itex],

[itex]\zeta \left(1-s,\frac{m}{n} \right) =

\frac{2\Gamma(s)}{ (2\pi n)^s } \sum_{k=1}^n \cos \left( \frac {\pi s} {2} -\frac {2\pi k m} {n} \right)\; \zeta \left( s,\frac {k}{n} \right) [itex] holds for all values of s.

### Taylor series

The derivative of the zeta in the second argument is a shift:

[itex]\frac {\partial} {\partial q} \zeta (s,q) = -s\zeta(s+1,q)[itex]

Thus, the Taylor series can be written as

[itex]\zeta(s,x+y) = \sum_{k=0}^\infty \frac {y^k} {k!}

\frac {\partial^k} {\partial x^k} \zeta (s,x) = \sum_{k=0}^\infty {s+k-1 \choose s-1} (-y)^k \zeta (s+k,x) [itex]

### Fourier transform

The discrete Fourier transform of the Hurwitz zeta function with respect to the order s is the Legendre chi function.

### Relation to Jacobi theta function

If [itex]\vartheta (z,\tau)[itex] is the Jacobi theta function, then

[itex]\int_0^\infty \left[\vartheta (z,it) -1 \right] t^{s/2} \frac{dt}{t}=

\pi^{-(1-s)/2} \Gamma \left( \frac {1-s}{2} \right) \left[ \zeta(1-s,z) + \zeta(1-s,1-z) \right][itex] holds for [itex]\Re s > 0[itex] and z complex, but not an integer. For z=n an integer, this simplifies to

[itex]\int_0^\infty \left[\vartheta (n,it) -1 \right] t^{s/2} \frac{dt}{t}=

2\ \pi^{-(1-s)/2} \ \Gamma \left( \frac {1-s}{2} \right) \zeta(1-s) =2\ \pi^{-s/2} \ \Gamma \left( \frac {s}{2} \right) \zeta(s)[itex] where ζ here is the Riemann zeta function. This distinction based on z accounts for the fact that the Jacobi theta function converges to the Dirac delta function in z as [itex]t\rightarrow 0[itex].

### Applications

Although Hurwitz's zeta function is thought of by mathematicians as being relevant to the "purest" of mathematical disciplines − number theory, it also occurs in the study of fractals and dynamical systems and in applied statistics; see Zipf's law and Zipf-Mandelbrot law.

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