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// Derived from SciPy's special/cephes/zeta.c
// https://github.com/scipy/scipy/blob/master/scipy/special/cephes/zeta.c
// Made freely available by Stephen L. Moshier without support or guarantee.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Copyright ©1984, ©1987 by Stephen L. Moshier
// Portions Copyright ©2016 The Gonum Authors. All rights reserved.
package cephes
import "math"
// zetaCoegs are the expansion coefficients for Euler-Maclaurin summation
// formula:
//
// \frac{(2k)!}{B_{2k}}
//
// where
//
// B_{2k}
//
// are Bernoulli numbers.
var zetaCoefs = [...]float64{
12.0,
-720.0,
30240.0,
-1209600.0,
47900160.0,
-1.307674368e12 / 691,
7.47242496e10,
-1.067062284288e16 / 3617,
5.109094217170944e18 / 43867,
-8.028576626982912e20 / 174611,
1.5511210043330985984e23 / 854513,
-1.6938241367317436694528e27 / 236364091,
}
// Zeta computes the Riemann zeta function of two arguments.
//
// Zeta(x,q) = \sum_{k=0}^{\infty} (k+q)^{-x}
//
// Note that Zeta returns +Inf if x is 1 and will panic if x is less than 1,
// q is either zero or a negative integer, or q is negative and x is not an
// integer.
//
// Note that:
//
// zeta(x,1) = zetac(x) + 1
func Zeta(x, q float64) float64 {
// REFERENCE: Gradshteyn, I. S., and I. M. Ryzhik, Tables of Integrals, Series,
// and Products, p. 1073; Academic Press, 1980.
if x == 1 {
return math.Inf(1)
}
if x < 1 {
panic(paramOutOfBounds)
}
if q <= 0 {
if q == math.Floor(q) {
panic(errParamFunctionSingularity)
}
if x != math.Floor(x) {
panic(paramOutOfBounds) // Because q^-x not defined
}
}
// Asymptotic expansion: http://dlmf.nist.gov/25.11#E43
if q > 1e8 {
return (1/(x-1) + 1/(2*q)) * math.Pow(q, 1-x)
}
// The Euler-Maclaurin summation formula is used to obtain the expansion:
// Zeta(x,q) = \sum_{k=1}^n (k+q)^{-x} + \frac{(n+q)^{1-x}}{x-1} - \frac{1}{2(n+q)^x} + \sum_{j=1}^{\infty} \frac{B_{2j}x(x+1)...(x+2j)}{(2j)! (n+q)^{x+2j+1}}
// where
// B_{2j}
// are Bernoulli numbers.
// Permit negative q but continue sum until n+q > 9. This case should be
// handled by a reflection formula. If q<0 and x is an integer, there is a
// relation to the polyGamma function.
s := math.Pow(q, -x)
a := q
i := 0
b := 0.0
for i < 9 || a <= 9 {
i++
a += 1.0
b = math.Pow(a, -x)
s += b
if math.Abs(b/s) < machEp {
return s
}
}
w := a
s += b * w / (x - 1)
s -= 0.5 * b
a = 1.0
k := 0.0
for _, coef := range zetaCoefs {
a *= x + k
b /= w
t := a * b / coef
s = s + t
t = math.Abs(t / s)
if t < machEp {
return s
}
k += 1.0
a *= x + k
b /= w
k += 1.0
}
return s
}
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