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:<math>\lim_{s\to 1} \left[ \zeta(s,a) - \frac{1}{s-1}\right] =
:<math>\lim_{s\to 1} \left[ \zeta(s,a) - \frac{1}{s-1}\right] =
\frac{-\Gamma'(a)}{\Gamma(a)} = -\psi(a)</math>
\frac{-\Gamma'(a)}{\Gamma(a)} = -\psi(a)</math>
where <math>\Gamma</math> is the [[gamma function]] and <math>\psi = \Gamma / \Gamma'</math> is the [[digamma function]]. As a special case, <math>\gamma_0(1) = -\psi(1) = \gamma_0 = \gamma</math>.
where <math>\Gamma</math> is the [[gamma function]] and <math>\psi = \Gamma' / \Gamma</math> is the [[digamma function]]. As a special case, <math>\gamma_0(1) = -\psi(1) = \gamma_0 = \gamma</math>.


==Fourier transform==
==Fourier transform==

Revision as of 08:40, 1 July 2021

In mathematics, the Hurwitz zeta function is one of the many zeta functions. It is formally defined for complex arguments s with Re(s) > 1 and a with Re(a) > 0 by

This series is absolutely convergent for the given values of s and a and can be extended to a meromorphic function defined for all s≠1. The Riemann zeta function is ζ(s,1). The Hurwitz zeta function is named after Adolf Hurwitz, who introduced it in 1882.[1]

Hurwitz zeta function corresponding to a = 1/3. It is generated as a Matplotlib plot using a version of the Domain coloring method.[2]
Hurwitz zeta function corresponding to a = 24/25.
Hurwitz zeta function as a function of a with s = 3+4i.

Integral representation

The Hurwitz zeta function has an integral representation

for and (This integral can be viewed as a Mellin transform.) The formula can be obtained, roughly, by writing

and then interchanging the sum and integral.[3]

The integral representation above can be converted to a contour integral representation

where is a Hankel contour counterclockwise around the positive real axis, and the principal branch is used for the complex exponentiation . Unlike the previous integral, this integral is valid for all s, and indeed is an entire function of s.[4]

The contour integral representation provides an analytic continuation of to all . At , it has a simple pole with residue .[5]

Hurwitz's formula

The Hurwitz zeta function satisfies an identity which generalizes the functional equation of the Riemann zeta function:[6]

valid for Re(s) > 1 and 0 < a ≤ 1. Apostol calls this Hurwitz's formula.[6] The Riemann zeta functional equation is the special case a = 1:[7]

Hurwitz's formula can also be expressed as[8]

(for Re(s) < 0 and 0 < a ≤ 1).

Functional equation

The functional equation relates values of the zeta on the left- and right-hand sides of the complex plane. For integers ,

holds for all values of s.

Some finite sums

Closely related to the functional equation are the following finite sums, some of which may be evaluated in a closed form

where m is positive integer greater than 2 and s is complex, see e.g. Appendix B in.[9]

Series representation

A convergent Newton series representation defined for (real) a > 0 and any complex s ≠ 1 was given by Helmut Hasse in 1930:[10]

This series converges uniformly on compact subsets of the s-plane to an entire function. The inner sum may be understood to be the nth forward difference of ; that is,

where Δ is the forward difference operator. Thus, one may write

Other series converging globally include these examples

where Hn are the Harmonic numbers, are the Stirling numbers of the first kind, is the Pochhammer symbol, Gn are the Gregory coefficients, G(k)
n
are the Gregory coefficients of higher order and Cn are the Cauchy numbers of the second kind (C1 = 1/2, C2 = 5/12, C3 = 3/8,...), see Blagouchine's paper.[11]

Taylor series

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

Thus, the Taylor series can be written as:

Alternatively,

with .[12]

Closely related is the Stark–Keiper formula:

which holds for integer N and arbitrary s. See also Faulhaber's formula for a similar relation on finite sums of powers of integers.

Laurent series

The Laurent series expansion can be used to define generalized Stieltjes constants that occur in the series

In particular, the constant term is given by

where is the gamma function and is the digamma function. As a special case, .

Fourier transform

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

Relation to Bernoulli polynomials

The values of ζ(s, a) at s = 0, −1, −2, ... are related to the Bernoulli polynomials:[13]

For example, the case gives[14]

Relation to Jacobi theta function

If is the Jacobi theta function, then

holds for and z complex, but not an integer. For z=n an integer, this simplifies to

where ζ here is the Riemann zeta function. Note that this latter form is the functional equation for the Riemann zeta function, as originally given by Riemann. The distinction based on z being an integer or not accounts for the fact that the Jacobi theta function converges to the periodic delta function, or Dirac comb in z as .

Relation to Dirichlet L-functions

At rational arguments the Hurwitz zeta function may be expressed as a linear combination of Dirichlet L-functions and vice versa: The Hurwitz zeta function coincides with Riemann's zeta function ζ(s) when a = 1, when a = 1/2 it is equal to (2s−1)ζ(s),[15] and if a = n/k with k > 2, (n,k) > 1 and 0 < n < k, then[16]

the sum running over all Dirichlet characters mod k. In the opposite direction we have the linear combination[15]

There is also the multiplication theorem

of which a useful generalization is the distribution relation[17]

(This last form is valid whenever q a natural number and 1 − qa is not.)

Zeros

If a=1 the Hurwitz zeta function reduces to the Riemann zeta function itself; if a=1/2 it reduces to the Riemann zeta function multiplied by a simple function of the complex argument s (vide supra), leading in each case to the difficult study of the zeros of Riemann's zeta function. In particular, there will be no zeros with real part greater than or equal to 1. However, if 0<a<1 and a≠1/2, then there are zeros of Hurwitz's zeta function in the strip 1<Re(s)<1+ε for any positive real number ε. This was proved by Davenport and Heilbronn for rational or transcendental irrational a,[18] and by Cassels for algebraic irrational a.[15][19]

Rational values

The Hurwitz zeta function occurs in a number of striking identities at rational values.[20] In particular, values in terms of the Euler polynomials :

and

One also has

which holds for . Here, the and are defined by means of the Legendre chi function as

and

For integer values of ν, these may be expressed in terms of the Euler polynomials. These relations may be derived by employing the functional equation together with Hurwitz's formula, given above.

Applications

Hurwitz's zeta function occurs in a variety of disciplines. Most commonly, it occurs in number theory, where its theory is the deepest and most developed. However, it also occurs in the study of fractals and dynamical systems. In applied statistics, it occurs in Zipf's law and the Zipf–Mandelbrot law. In particle physics, it occurs in a formula by Julian Schwinger,[21] giving an exact result for the pair production rate of a Dirac electron in a uniform electric field.

Special cases and generalizations

The Hurwitz zeta function with a positive integer m is related to the polygamma function:

The Barnes zeta function generalizes the Hurwitz zeta function.

The Lerch transcendent generalizes the Hurwitz zeta:

and thus

Hypergeometric function

where

Meijer G-function

Notes

  1. ^ Hurwitz, Adolf (1882). "Einige Eigenschaften der Dirichlet'schen Functionen , die bei der Bestimmung der Classenanzahlen binärer quadratischer Formen auftreten". Zeitschrift für Mathematik und Physik (in German). 27: 86–101.
  2. ^ http://nbviewer.ipython.org/github/empet/Math/blob/master/DomainColoring.ipynb
  3. ^ Apostol 1976, p. 251, Theorem 12.2
  4. ^ Whittaker & Watson 1927, p. 266, Section 13.13
  5. ^ Apostol 1976, p. 255, Theorem 12.4
  6. ^ a b Apostol 1976, p. 257, Theorem 12.6
  7. ^ Apostol 1976, p. 259, Theorem 12.7
  8. ^ Whittaker & Watson 1927, pp. 268–269, Section 13.15
  9. ^ Blagouchine, I.V. (2014). "A theorem for the closed-form evaluation of the first generalized Stieltjes constant at rational arguments and some related summations". Journal of Number Theory. 148. Elsevier: 537–592. arXiv:1401.3724. doi:10.1016/j.jnt.2014.08.009.
  10. ^ Hasse, Helmut (1930), "Ein Summierungsverfahren für die Riemannsche ζ-Reihe", Mathematische Zeitschrift, 32 (1): 458–464, doi:10.1007/BF01194645, JFM 56.0894.03
  11. ^ Blagouchine, Iaroslav V. (2018). "Three Notes on Ser's and Hasse's Representations for the Zeta-functions". INTEGERS: The Electronic Journal of Combinatorial Number Theory. 18A: 1–45. arXiv:1606.02044. Bibcode:2016arXiv160602044B.
  12. ^ Vepstas, Linas (2007). "An efficient algorithm for accelerating the convergence of oscillatory series, useful for computing the polylogarithm and Hurwitz zeta functions". Numerical Algorithms. 47 (3): 211–252. arXiv:math/0702243. Bibcode:2008NuAlg..47..211V. doi:10.1007/s11075-007-9153-8.
  13. ^ Apostol 1976, p. 264, Theorem 12.13
  14. ^ Apostol 1976, p. 268
  15. ^ a b c Davenport (1967) p.73
  16. ^ Lowry, David. "Hurwitz Zeta is a sum of Dirichlet L functions, and vice-versa". mixedmath. Retrieved 8 February 2013.
  17. ^ Kubert, Daniel S.; Lang, Serge (1981). Modular Units. Grundlehren der Mathematischen Wissenschaften. Vol. 244. Springer-Verlag. p. 13. ISBN 0-387-90517-0. Zbl 0492.12002.
  18. ^ Davenport, H. & Heilbronn, H. (1936), "On the zeros of certain Dirichlet series", Journal of the London Mathematical Society, 11 (3): 181–185, doi:10.1112/jlms/s1-11.3.181, Zbl 0014.21601
  19. ^ Cassels, J. W. S. (1961), "Footnote to a note of Davenport and Heilbronn", Journal of the London Mathematical Society, 36 (1): 177–184, doi:10.1112/jlms/s1-36.1.177, Zbl 0097.03403
  20. ^ Given by Cvijović, Djurdje & Klinowski, Jacek (1999), "Values of the Legendre chi and Hurwitz zeta functions at rational arguments", Mathematics of Computation, 68 (228): 1623–1630, Bibcode:1999MaCom..68.1623C, doi:10.1090/S0025-5718-99-01091-1
  21. ^ Schwinger, J. (1951), "On gauge invariance and vacuum polarization", Physical Review, 82 (5): 664–679, Bibcode:1951PhRv...82..664S, doi:10.1103/PhysRev.82.664

References