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HIGHER DERIVATIVES OF THE HURWITZ ZETA FUNCTION
A Thesis Presented to The Faculty of the Department of Mathematics Western Kentucky University Bowling Green, Kentucky
In Partial Fulfillment Of the Requirements for the Degree Master of Science
By Jason Musser
August 2011
ACKNOWLEDGMENTS
I would like to thank Dr. Dominic Lanphier for his insightful guidance in this endeavor.
iii TABLE OF CONTENTS
Abstract v
Chapter 1 The Riemann Zeta and Hurwitz Zeta Functions
1.1 Introduction 1
1.2 The Gamma Function 2
1.3 The Riemann Zeta Function 3
1.4 The Hurwitz Zeta Function 5
Chapter 2 The Stieltjes Constants and Euler’s Constant
2.1 The Stieltjes Constants 8
2.2 Euler’s Constant 9
Chapter 3 Higher Derivatives of the Riemann Zeta Function
3.1 Series Expansion of the Functional Equation 11
Chapter 4 Higher Derivatives of the Hurwitz Zeta Function
4.1 Series Expansion of the Functional Equation 17
4.2 Higher Derivatives of the Hurwitz Zeta Function 21
4.3 Main Results 26
4.4 Applications to the Stieltjes Constants 28
4.5 Examples 29
iv Chapter 5 Numerical Results
5.1 Numerical Results 32
Appendix 39
References 47
v HIGHER DERIVATIVES OF THE HURWITZ ZETA FUNCTION
Jason Musser August 2011 46 Pages
Directed by: Dominic Lanphier, Tilak Bhattacharya, Claus Ernst
Department of Mathematics Western Kentucky University
The Riemann zeta function ζ(s) is one of the most fundamental functions in number theory. Euler demonstrated that ζ(s) is closely connected to the prime numbers and Riemann gave proofs of the basic analytic properties of the zeta function. Values of the zeta function and its derivatives have been studied by several mathematicians. Apostol in particular gave a computable formula for the values of the derivatives of ζ(s) at s = 0. The Hurwitz zeta function ζ(s, q) is a generalization of ζ(s). We modify Apostol’s methods to find values of the derivatives of ζ(s, q) with respect to s at s = 0. As a consequence, we obtain relations among certain important constants, the generalized Stieltjes constants. We also give numerical estimates of several values of the derivatives of ζ(s, q).
vi Chapter 1
The Riemann Zeta and Hurwitz Zeta Functions
In this chapter an introduction, definitions of the gamma function, the Riemann zeta function, and the Hurwitz zeta function are given. Various properties of each function including integral representations that give their meromorphic continuation to C which can be found in [1] and [6] are also shown. This is done in preparation for ultimately discussing the derivatives of the Hurwitz zeta function. Many of these properties can be found in any introductory analytic number theory text. 1.1 Introduction In 1859 Bernhard Riemann discussed the significance of the Riemann zeta function ζ(s) in his paper Uber¨ die Anzahl der Primzahlen unter einer gegebenen Gr¨osse (On the Number of Primes Less Than a Given Magnitude). He outlined a method whereby one can study prime numbers from the analytic properties of ζ(s). These ideas culminated in a proof of the celebrated prime number theorem in 1900 by Hadamard and de la Vall´eePoussin, which gives an asymptotic formula for the number of primes less than a given value. In Riemann’s original paper he stated a conjecture that is now famously known as the Riemann hypothesis. The critical strip of the Riemann zeta function is the set s ∈ C so that 0 < Re(s) < 1. From properties of ζ(s) we know that ζ(s) 6= 0 for Re(s) > 1. The analytic continuation of ζ(s) allows us to define ζ(s) for any s ∈ C, s 6= 1. It can be shown that ζ(s) 6= 0 for Re(s) ≥ 1 and this result implies the Prime 1 Number Theorem. It is known that there are values σ0 = 2 + it so that ζ(σ0) = 0. The Riemann hypothesis asserts that all nontrivial zeros of ζ(s) lie on the vertical 1 line at Re(s) = 2. Therefore, the Riemann hypothesis asserts that ζ(s) 6= 0 for 1 Re(s) > 2. Although it has not been shown to be true or false, many zeros lie on this line. One of the reasons the Riemann hypothesis is important is because given
1 that the Riemann hypothesis is true, further very powerful deductions can be made about the distribution of prime numbers. To further study the Riemann zeta function, Adolf Hurwitz defined a more generalized form of the Riemann zeta function refered to as the Hurwitz zeta function ζ(s, q). Although the relationship to the prime numbers is lost in the generalization, insight can be gained on the Riemann zeta function by studying the Hurwitz zeta function due to relationships between the two. One of the properties that has been studied is the derivatives of the Riemann zeta function at s = 0 [2]. As with ζ(s), the Hurwitz zeta function has an analytic continuation to all s ∈ C, s 6= 1. Thus ζ(s, q) is analytic at s = 0. The Taylor series at s = 0 has a radius of convergence 1. In this thesis we find the derivatives at s = 0 of the Hurwitz zeta function by modifying methods used to study ζ(s). Some numerical results are also found. For all instances that refer to Mathematica the ` software Mathematica 7.0 was used. Note that in all chapters f(`)(s) = d f is ds` defined for any meromorphic f. 1.2 The Gamma Function The gamma function Γ(n) is defined as follows from Chapter one of [6] : For
n ∈ Z, n > 0, Γ(n) = (n − 1)!, n = 1, 2,... and 0! = 1.
The gamma function has an integral representation from chapter one of [6] given by
Z ∞ Γ(s) = ts−1e−tdt 0
that converges absolutely for Re(s) > 0 and allows a definition of Γ(s) for all s ∈ C. The integral converges for all complex s with s 6= 0, −1, −2, ··· . The gamma function has simple poles at these points.
2 Some selected properties of the gamma function follow. Three formulas that are functional equations of the gamma function are given. From integration by parts the recursion formula can be obtained ((1.1.6) in [6])
Γ(s + 1) = sΓ(s) and Γ(n) = (n − 1)!, n = 1, 2,...
Euler’s reflection formula ((1.2.1) in [6]) states
π Γ(1 − n)Γ(n) = . sin(πn)
The duplication formula ((1.5.1) in [6]) states
1 1 Γ(z)Γ z + = 21−2z Γ Γ(2z) 2 2
A well known value of the gamma function ((2.14) in [3]) is
1 √ (1) Γ = π. 2
1.3 The Riemann Zeta Function The Riemann zeta function ζ(s) as found in Chapter twelve of [1] is defined by the series ∞ X 1 ζ(s) = ns n=1 for Re(s) > 1. It has an analytic continuation to all s = σ + it except for a pole at s = 1 with residue 1 as found in the introduction of [10], and it can be obtained from the integral representation
Γ(1 − s) Z (−z)s−1 ζ(s) = − z dz. 2πi C e − 1
3 The contour C starts at infinity on the positive real axis, encircles the origin once, excluding the points ±2πi, ±4πi, . . . and returns to where it begins. Γ(s) is the gamma function. Also, if Re(s) > 1 then we can express the product of the gamma function and Riemann zeta function as found in the introduction of [10] by
Z ∞ xs−1 Γ(s)ζ(s) = x dx 0 e − 1
In the introduction of [10], Euler found that for Re(s) > 1,
∞ X 1 Y 1 = . ns 1 − p−s n=1 ∀ primes p
This shows an interesting relationship between the Riemann zeta function and the prime numbers. Like the gamma function, the Riemann zeta function obeys certain formulas. For example, the Riemann zeta function as found in Chapter twelve of [1] has a functional equation
πs (2) ζ(s) = 2(2π)s−1sin Γ(1 − s)ζ(1 − s), ∀s ∈ . 2 C
Many values of the Riemann zeta function are particularly interesting. Values of the Riemann zeta function have a connection with the Bernoulli numbers. The Bernoulli
numbers Bm are the coefficients of the following series expansion ((1.2.10)in [6])
∞ m t X Bmt (3) = et − 1 m! m=0
1 1 1 1 for t ∈ R and t 6= 0. For example B0 = 1, B1 = 2, B2 = 6, B4 = −30, B6 = 42,
4 1 5 B8 = −30, B10 = 66. If k is an even and positive integer we have ((1.2.11)in [6])
k B (2π)k ζ(k) = (−1) 2 +1 k (2k)!
As examples, this equation gives
∞ X 1 π2 ζ(2) = = n2 6 n=1
and ∞ X 1 π4 ζ(4) = = . n4 90 n=1 These results were first discovered by Euler. 1.4 The Hurwitz Zeta Function The Hurwitz zeta function ζ(s, q) is a generalized form of the Riemann zeta function and as found in Chapter twelve of [1] and is defined as
∞ X 1 ζ(s, q) = (n + q)s n=0
where Re(s) > 1 and 0 < q ≤ 1. Note that ζ(s, 1) = ζ(s). The Hurwitz zeta function also has an analytic continuation for all s = σ + it (except for the pole at s = 1 with residue 1) as found in chapter twelve of [1] and is given by 1 Z zs−1eqz ζ(s, q) = Γ(1 − s) z dz. 2πi C 1 − e
The contour C loops around the negative real axis by starting at negative infinity on the real axis, encircles the origin once and returns to where it begins. Similar to the Riemann zeta function, the Hurwitz Zeta function also has a
5 functional equation as found in Chapter twelve of [1] and is given by
k h 2Γ(s) X πs 2πrh r (4) ζ 1 − s, = cos − ζ s, , ∀s ∈ k (2πk)s 2 k k C r=1 where h and k are integers and 1≤ h ≤ k. Similar to the Riemann zeta function and the gamma function, the Hurwitz zeta function obeys several identities. The Hurwitz zeta function also has the following integral representation as found in chapter twelve of [1] 1 Z ∞ xs−1e−qx ζ(s, q) = −x dx Γ(s) 0 1 − e
where Re(s) > 1 and 0 < q ≤ 1. Note that, however, the Hurwitz zeta function does not have an Euler product expansion over the prime numbers. Hurwitz discovered a formula which as found in Chapter twelve of [1] is refered to as Hurwitz’s formula.
Given that Re(s) > 1 and t ∈ R we have
∞ X e2πint F (t, s) = ns n=1
where F (t, s) is periodic with period 1 and is called the periodic zeta function. If Re(s) > 1 then F (t, s) converges absolutely. Note that F (1, s) = ζ(s). Hurwitz’s formula is Γ(s) −iπs iπs ζ(1 − s, q) = e 2 F (q, s) + e 2 F (−q, s) (2π)s
where Re(s) > 1 and 0 < q ≤ 1. Hurwitz’s formula is also valid if q 6= 1 and Re(s) > 0. As with the Riemann zeta function, some values of the Hurwitz zeta function can be obtained explicitly. We have a formula for the values ζ(−n, q) for
6 n = 0, 1, 2,... as found in Chapter twelve of [1]. Given
n X n B (x) = B xn−k, ∀x ∈ , n = 0, 1, 2,..., n k k C k=0
where Bn(0) = Bn and Bk are the Bernoulli numbers previously defined we have
B (q) ζ(−n, q) = − n+1 , n = 0, 1, 2,... n + 1
with 0 < q ≤ 1. The formula for the derivative of ζ(s, q) with respect to the second arguement q is ∂ ζ(s, q) = −sζ(s + 1, q), ∂q where 0 < q ≤ 1 and s 6= 0, 1. A multiplication theorem for the second variable of the Hurwitz zeta function can be found in [11]:
∞ X s + n − 1 ζ(s, kz) = (1 − k)nznζ(s + n, z), n n=0
where Re(s) > 1 and 0 < kz ≤ 1.
7 Chapter 2
The Stieltjes Constants and Euler’s Constant
2.1 The Stieltjes Constants
The generalized Stieltjes constants are denoted as γn(q) and arise when
considering the Laurent expansion of the Hurwitz zeta function. Note that γn(q) is
a function with arguement q not to be confused with Stieltjes constants γn = γn(1). From Berndt [4] for 0 < q ≤ 1 and for all complex s 6= 1,
∞ 1 X (−1)n (5) ζ (s, q) = + γ (q)(s − 1)n . s − 1 n! n n=0
Berndt [4] shows the following. Theorem (Theorem 1 [4]) For 0 < q ≤ 1 and n = 0, 1, 2, ···
m ! X logn(k + q) logn+1(m + q) (6) γn(q) = lim − m→∞ k + q n + 1 k=0
It is unknown whether γn(q) is rational or irrational for any q.
The Stieltjes constants denoted γn arise when considering the Laurent expansion of the Riemann zeta function which can be found by taking q = 1 in (5), [2].
∞ 1 X (−1)n ζ(s) = + γ (s − 1)n s − 1 n n! (7) n=0 1 (s − 1)2 = + γ − γ (s − 1) + γ + ··· s − 1 0 1 2 2!
which has one pole at s = 1 with residue 1, where γn = γn(1). We have that
8 γn(1) = γn, and from [2] the nth Stieltjes constant is expressed as
" m ! # X (log(k))n (log(m))n+1 (8) γn = lim − . m→∞ k n + 1 k=1
2.2 Euler’s Constant
Euler’s constant γ0 as defined in the introduction of [10] is defined by
n ! X 1 γ = lim − log(n) 0 n→∞ k (9) k=1 ≈ 0.57721 ...
As with γn(q) and γn very little is known about the arithmetic properties of γ0.
Euler’s constant is the first in the sequence of the Stieltjes constants γn. Some
properties of Euler’s constant follow. We first give some relationships of γ0 to the derivatives of the gamma function discussed previously. From [2] we have
0 Γ (1) = −γ0 00 π2 Γ (1) = γ2 + ζ(2) = γ2 + 0 0 6 000 π2 Γ (1) = −γ3 − 3γ ζ(2) − 2ζ(3) = −γ3 − γ − 2ζ(3) 0 0 0 0 2
From Chapter six of [7],
0 ∞ Γ (x) 1 X 1 1 = − − γ + − for x > 0 and x ∈ . Γ(x) x 0 r r + x R r=1
For example at x = 1 we have 0 Γ (1) = −γ . Γ(1) 0
9 γ0 also has an integral representation from chapter twelve of [7] we have
1 Z 1 1 − e−u − e−u γ0 = du 0 u
The Laurent expansion of Γ(s)ζ(s) ((7) in [2]) involves Euler’s constant and the other Stieltjes constants γn. It is given by
∞ 1 X (10) Γ(s)ζ(s) = + c (s − 1)n, s − 1 n n=0 where
n Γ(n+1)(1) X Γ(n−k)(1) (11) c = γ . n (n + 1)! k (n − k)! k=0
In particular for n = 0, 1 and 2 (11) gives
c0 = γ0, γ4 γ2γ c = − 0 + 0 1 − γ2ζ(2) + γ ζ(2), 1 2 2 0 1 ! ! γ3 2ζ(2)γ γ3 c = − 0 + 0 + ζ(3) 0 + γ ζ(2) − γ γ + γ . 2 6 3 2 0 1 0 2
10 Chapter 3
Higher Derivatives of the Riemann Zeta Function
We review a method to find the values of ζ(n)(0) for arbitrary n. The value 0 00 ζ(0) was found by Riemann, as was ζ (0). Ramanujan found ζ (0). Apostol [2] found a formula for ζ(n)(0). 3.1 Series Expansion of the Functional Equation Using (2), the functional equation of ζ(s), derivatives of the Riemann zeta function at s = 0 were found by Apostol [2]. His method from [2] is as follows. Start with (2) and some manipulation gives an alternate form of the functional equation
πs ζ(1 − s) = 2(2π)−scos Γ(s)ζ(s). 2
Rewriting in yet another form, by substituting
πs iπs iπs 2(2π)−scos = e 2 −s log(2π) + e− 2 −s log(2π), 2
this gives
" # −s log(2π)−πi −s log(2π)+πi (12) ζ(1 − s) = Γ(s)ζ(s) e 2 + e 2 .
Taking the kth derivative of both sides by using Leibniz’s rule found in Chapter five of [5] k X k [f(z)g(z)](k) = f(m)(z)g(k−m)(z) m m=0
11 Apostol then shows that for any k ≥ 1 and for all s ∈ C we have
(−1)kζ(k)(1 − s)
k " k−m X k −s log(2π)+πi πi = e 2 log(2π) + (−1)k−m m 2 m=0 k−m# s − log(2π)+πi πi +e 2 − log(2π) + [Γ(s)ζ(s)](m) . (13) 2 k " k−m X k sπi πi = e−s log(2π) e− 2 log(2π) + (−1)k−m m 2 m=0 # sπi πik−m +e 2 − log(2π) + [Γ(s)ζ(s)](m) . 2
Apostol then rewrites (13) in terms of the sum of its real and imaginary parts.
(−1)kζ(k)(1 − s)
k " " k−m# X k πi πs = Re − log(2π) − cos (14) m 2 2 m=0 " # # πik−m πs +Im − log(2π) − sin [Γ(s)ζ(s)](m) . 2 2
Where Re[z] is the real part of z and Im[z] is the imaginary part of z. Leibniz’s rule is then applied to [Γ(s)ζ(s)](m) in (14) to give
(−1)kζ(k)(1 − s)
k m " " k−m# X X k m πi πs = Re − log(2π) − cos m r 2 2 (15) m=0 r=0 " # # πik−m πs +Im − log(2π) − sin Γ(r)(s)ζ(m−r)(s). 2 2
12 As with (13), for certain sets of integer values of s, (15) can be simplified. Some examples follow. Taking s = 2n + 1, n ∈ N and k ≥ 1 in (15) then
(−1)kζ(k)(−2n) k m 2(−1)n X X k m = (2π)2n+1 m r m=0 r=0 " # # πik−m Im − log(2π) − Γ(r)(2n + 1)ζ(m−r)(2n + 1) . 2
Taking s = 2n, n ∈ N and k ≥ 1 in (15) then
(−1)kζ(k)(1 − 2n) k m 2(−1)n X X k m = (2π)2n m r m=0 r=0 " # # πik−m Re − log(2π) − Γ(r)(2n)ζ(m−r)(2n) . 2
Now series expansions of the members of (12) can be found. Since ζ(1 − s) is analytic at s = 1 the expansion at s = 1 follows
∞ X (−1)nζ(n)(0) (16) ζ(1 − s) = (s − 1)n. n! n=0
Looking at the exponential factor in (12), we can expand it in a Taylor series about s = 1,
−s log(2π)+πi − log(2π)+πi (s−1) − log(2π)−πi e 2 = e 2 e 2 (17) ∞ n X − log(2π)+πi πi (−1)n = e 2 log(2π) + (s − 1)n 2 n! n=0
13 Previously in (10) we have the Laurent expansion of the product Γ(s)ζ(s),
∞ 1 X Γ(s)ζ(s) = + c (s − 1)n, s − 1 n n=0 where as before n Γ(n+1)(1) X Γ(n−k)(1) c = γ , n (n + 1)! k (n − k)! k=0
and γn denote the Stieltjes constants as in (8). Using the expansions of Γ(s)ζ(s) −s log(2π)+πi from (10) and e 2 from (17), we take the product and have the Laurent expansion
(18) −s log(2π)+πi Γ(s)ζ(s)e 2 ∞ ! 1 X = + c (s − 1)n s − 1 n n=0 ∞ n ! X − log(2π)+πi πi (−1)n e 2 log(2π) + (s − 1)n 2 n! n=0 − log(2π)+πi 1 = e 2 s − 1 ∞ " n+1 X − log(2π)+πi πi (−1)n+1 + e 2 log(2π) + 2 (n + 1)! n=0 ∞ n−k # X − log(2π)+πi πi (−1)n−k + c e 2 log(2π) + (s − 1)n. k 2 (n − k)! k=n
The numbers ck are defined as in (11). Starting from the functional equation (12) −s log(2π)+πi and applying the product of Γ(s)ζ(s) with e 2 from ( 18) above, and
14 equating the coefficients of the (s − 1)n terms for n ≥ 0, Apostol then shows
ζ(n)(0) (−1)n n! n+1 − log(2π)+πi πi (−1)n+1 = e 2 log(2π) + 2 (n + 1)! n+1 − log(2π)−πi πi (−1)n+1 + e 2 log(2π) − 2 (n + 1)! n " n−k X − log(2π)+πi πi (−1)n−k + c e 2 log(2π) + k 2 (n − k)! k=0 n−k # − log(2π)−πi πi (−1)n−k +e 2 log(2π) − . 2 (n − k)!
After simplifying the above formula Apostol obtains the following Theorem. Theorem 1 [2] For n ≥ 0
ζ(n)(0) (−1)n n! " # πin+1 1 = Im − log(2π) − (19) 2 π(n + 1)! n−1 " n−k# 1 X πi 1 + c Im − log(2π) − , π k 2 (n − k)! k=1
where the numbers ck are defined as in (11). This formula can be used to find exact values of ζ(n)(0) for small n. It can also be used to find good numerical approximations to ζ(n)(0) for large n. Using (19), ζ(n)(0) is given below for 0 ≤ n ≤ 4 . The first two results were obtained by Riemann, the third was obtained first by Ramanujan, and the subsequent formulas
15 were obtained by Apostol.
1 ζ(0) = − 2
0 1 ζ (0) = − log(2π) 2
00 1 π2 ζ (0) = − log2(2π) + − c 2 24 1 1 π2 γ4 γ2γ π2 π2 (20) = − log2(2π) + + 0 − 0 1 + γ2 − γ 2 24 2 2 0 6 1 6
000 1 π2 ζ (0) = − log3(2π) + log(2π) − 3c log(2π) + 3c 2 8 1 2
1 π2 π4 π2 ζ(4)(0) = − log4(2π) + log2(2π) − − 6c log2(2π) + c 2 4 160 1 2 1
+ 12c2 log(2π) + 12c3
16 Chapter 4
Higher Derivatives of the Hurwitz Zeta Function
As with finding the derivatives of the Riemann zeta function, to find ζ(n)(0, q) we start with (4), the functional equation for the Hurwitz zeta function. We then expand its members in Taylor and Laurent series about s = 1. Then we equate coefficients of (s − 1)n, resulting in a formula for ζ(n)(0, q). 4.1 Series Expansion of the Functional Equation From (5) we have the Laurent expansion of the Hurwitz zeta function about s = 1, ∞ r 1 X (−1)n r ζ s, = + γ (s − 1)n . k s − 1 n! n k n=0
γn(q) are the generalized Stieltjes constants from (6). Taking the Taylor expansion h of ζ(1 − s, k ) about s = 1 we have
∞ ζ(n) 0, h (−1)n(s − 1)n h X k (21) ζ 1 − s, = . k n! n=0
The following Lemma will be used to find the Taylor expansion of Γ(s) about s = 1. This result shows how to obtain the Taylor expansion of f(x) about x = a from the Taylor expansion of log(f(x)) about x = a. Lemma 1[8] Let
a1 a2 a3 ··· an (n − 1) a1 a2 ··· an − 1 [a1, a2, . . . , an] = det 0 (n − 2) a ··· a − 2 1 n ...... . . . . . 0 ··· 0 1 a1
17 and let f(x) be a positive function differentiable at x = a so that
∞ X an log (f(x)) = a + (x − a)n. 0 n n=1
Then ∞ ! X h i (x − a)n f(x) = ea0 1 + a , −a , ..., (−1)n+1a . 1 2 n n! n=0 Proof : Let
j−1 j n−1 (−1) aj (−1) aj+1 ··· (−1) an n−j−1 (n − j) a1 ··· (−1) an−j . . . D = det 0 .. .. . . j,n ...... . . . . 0 ··· 0 1 a1
Then
h n−1 i D1,n = a1, −a2,..., (−1) an
h n−2 i = a1 a1,..., (−1) an−1 − (n − 1)D2,n and for j < n,
j−1 h n−j−2 i Dj,n = (−1) aj a1,..., (−1) an−j − (n − j)Dj+1,n.
18 Applying these equations repeatedly,
h n−1 i D1,n = a1, −a2,..., (−1) an
h n−2 i = a1 a1,..., (−1) an−1 − (n − 1)D2,n
h n−2 i h n−3 i = a1 a1,..., (−1) an−1 + (n − 1)a2 a1,..., (−1) an−2
− (n − 1)(n − 2)D3,n.
Iterating, we obtain
n X (n − 1)! h i (22) D = a a ,..., (−1)n−j−1a . 1,n (n − j)! j 1 n−j j=1
P∞ n a a Setting f(x) = n=0 bn(x − a) , it can then be seen that b0 = e 0 and b1 = e 0a1, so assume that ea0 h i b = a , −a ,..., (−1)m−1a m m! 1 2 m
0 P∞ n P∞ n−1 for each m < n. Since f (x) = ( n=0 bn(x − a) ) n=1 an(x − a) then equating terms gives us n X bnn = ajbn−j. j=1 From the induction hypothesis we can rewrite this as
n X aj h i b n = ea0 a ,..., (−1)n−j−1a . n (n − j)! 1 n−j j=1
From (22),
n X (n − 1)! h i b n! = ea0 a a ,..., (−1)n−j−1a n j (n − j)! 1 n−j j=1 a h n−1 i = e 0 a1,..., (−1) an
19 which is the result.
From [1], ∞ X n n log(Γ(s)) = −γ0(s − 1) + (−1) ζ(n)(s − 1) . n=2 The hypothesis of Lemma 1 gives
a0 = 0,
a1 = −γ0, . .
n an = n(−1) ζ(n) for n>1. Thus Lemma 1 gives
∞ X (s − 1)n (23) Γ(s) = 1 + [−γ , a , ..., (−1)2n+1nζ(n)] 0 2 n! n=1
2n+1 where [−γ0, a2, ..., (−1) nζ(n)] is the determinant as defined in Lemma 1.
Let C0,C1,C2,...,Cn,... be the coefficients of the Taylor expansion of πs 2πrh 2 cos 2 − k (2πk)s about s = 1. Thus we have
∞ 2 πs 2πrh X (24) cos − = C (s − 1)n. (2πk)s 2 k n n=0
20 (n) πs 2πrh 2 f (1) Letting f(s) = cos 2 − k (2πk)s then Cn = n! and
1 2πhr C = sin 0 πk k
−1 2hπr log(k) 2hπr log(2π) 2hπr C = cos − sin − sin 1 2k k kπ k kπ k
log2(k) 2πhr log2(2π) 2πhr π 2πhr (25) C = sin + sin − sin 2 2πk k 2πk k 8k k
log(2π) log(k) 2πhr log(k) 2πhr + sin + cos πk k 2k k
log(2π) 2πhr + cos . 2k k
Note that for every n = 0, 1,..., the number Cn depends on h, k and r. 4.2 Higher Derivatives of the Hurwitz Zeta Function (N) h To find ζ 0, k we apply the series expansions (21),(23), (24) and (5) to the functional equation (4). Another form of the functional equation (4) is then given by
∞ ζ(n) 0, h (−1)n(s − 1)n X k n! n=0 ∞ X h i (s − 1)j (26) = 1 + −γ , a , ..., (−1)2j+1jζ(j) 0 2 j! j=1 k " ∞ ! ∞ !# X X 1 X (−1)m r C (s − 1)` + γ (s − 1)m ` s − 1 m! m k r=1 `=0 m=0
2j+1 where C` (defined in (25)) is in terms of h, k and r and −γ0, a2, ..., (−1) jζ(j)
21 is the determinant found in Lemma 1. Because we are looking for the Nth derivative of ζ(s, q) at s = 0 we take only the Nth term of (21) which is
(N) h N ζ 0, k (−1) (27) (s − 1)N . N!
In the effort to equate (27) with the sum of all coefficients of (s − 1)N from the right hand side of (26), an expression for the sum of these coefficients will now be found. To find such an expression how the coefficients of (s − 1)N arise from the product of the three expansions on the right hand side of (26) must be considered. First the following sum in (26) is considered.
k " ∞ ! ∞ !# X X 1 X (−1)m r (28) C (s − 1)` + γ (s − 1)m ` s − 1 m! m k r=1 `=0 m=0
It is important to now note that for h, k ∈ N and 1 ≤ h < k,
k X 2πhr sin = 0, and k (29) r=1 k X 2πhr cos = 0. k r=1
From (29) for all ` ∈ N0, h, k ∈ N and 1 ≤ h < k,
k X (30) C` = 0. r=1
Now (30) implies that for h, k ∈ N and 1 ≤ h < k,
k " ∞ ! # X X 1 (31) C (s − 1)` = 0. ` s − 1 r=1 `=0
22 Applying (31) it is seen that for h, k ∈ N and 1 ≤ h < k (28) can be expressed as
k " ∞ ! ∞ !# X X X (−1)m r (32) C (s − 1)` γ (s − 1)m ` m! m k r=1 `=0 m=0
Also for h, k ∈ N and 1 ≤ h < k (26) becomes
∞ ζ(n) 0, h (−1)n(s − 1)n X k n! n=0 ∞ X h i (s − 1)j (33) = 1 + −γ , a , ..., (−1)2j+1jζ(j) 0 2 j! j=1 k " ∞ ! ∞ !# X X X (−1)m r C (s − 1)` γ (s − 1)m . ` m! m k r=1 `=0 m=0
Now to ensure that all coefficients of (s − 1)N are included the first N + 1 terms of both factors found within the sum of (33) must be considered and are given by
N X (−1)m r (34) γ (s − 1)m m! m k m=0
and from (24) the first N + 1 terms are
N X ` (35) C`(s − 1) . `=0
We also must consider the first N + 1 terms of (23)
N X (s − 1)j (36) 1 + [−γ , a , ..., (−1)2j+1jζ(j)] . 0 2 j! j=1
To help prevent the expressions from becoming cumbersome the sequence Hm for
23 m 0 ≤ m ≤ N where Hm is the coefficient of (s − 1) from (34) is defined as
(−1)m r (37) H = γ . m m! m k
n So all coefficients of (s − 1) from the product of (34) and (35) for h, k ∈ N, 1 ≤ h < k and 0 ≤ n ≤ N summed from r = 1 to k as is necessary can be expressed as
k N N−m X X X `+m (38) Hm C`(s − 1) . r=1 m=0 `=0
As before, to help prevent the expressions from becoming cumbersome the sequence j Gj for j ≤ N where Gj is the coefficient of (s − 1) from (36) is defined as
Gj = 0, for j < 0,
(39) G0 = 1, 1 G = [−γ , a , ..., (−1)2j+1jζ(j)] , for 1 ≤ j ≤ N. j 0 2 j!
So (36) can be expressed as
N X j (40) Gj(s − 1) . j=0
Taking the product of (40) and (38) for h, k ∈ N and 1 ≤ h < k gives
N k N N−m X j X X X `+m (41) Gj(s − 1) Hm C`(s − 1) . j=0 r=1 m=0 `=0
Now notice that in (41) for the product of any selected terms to result in (s − 1)N then h + ` + m = N. So given some ` and m then h = N − ` − m. So the product of
24 N (40) and (38) for h, k ∈ N and 1 ≤ h < k with only coefficients of (s − 1) can be expressed as
k N N−m X X X N (42) Hm C` GN−`−m (s − 1) . r=1 m=0 `=0
Equating (27) with (42) for h, k ∈ N and 1 ≤ h < k gives
(N) h N ζ 0, k (−1) (s − 1)N N! (43) k N N−m X X X N = Hm C` GN−`−m (s − 1) . r=1 m=0 `=0
(N) h Solving (43) for ζ 0, k for 1 ≤ h < k finally results in
k N N−m h X X X (44) ζ(N) 0, = (−1)N N! H C G . k m ` N−`−m r=1 m=0 `=0
Substituting in (37) and (39) into (44) and keeping in mind that Gj = 0 when j < 0 the following theorem is obtained.
Theorem 2 Let h, k ∈ N and 1 ≤ h < k. Then
(45) h ζ(N) 0, k k N X X (−1)m r = (−1)N N! γ m! m k r=1 m=0 N−m X 2(N−`−m)+1 C` [−γ0, a2, ..., (−1) (N − ` − m)ζ((N − ` − m))] `=0 ## (s − 1)(N−`−m) χ (N − ` − m) (N − ` − m)! N0
25 h where γn and γn k are the Stieltjes constants and generalized Stieltjes constants defined in (8) and (6) respectively, and χ (x) is the characteristic function defined N0 by
1 : x ∈ N ∪ {0} (46) χ (x) = . N0 0 : x∈ / N ∪ {0}
Note that the inclusion of χ (x) is necessary due to the condition G = 0 for N0 j j < 0. Theorem 2 can give exact results for small N and good estimates for large N. 4.3 Main Results (N) h Another representation of ζ 0, k for N = 0 is now found by using the discussion from section 4.2. Starting with (38) for h, k ∈ N and 1 ≤ h < k,
k N N−m X X X `+m Hm C`(s − 1) r=1 m=0 (47) `=0 k X h r i = C γ . 0 0 k r=1
From (36),
N X (s − 1)m (48) 1 + [−γ , a , ..., (−1)2m+1mζ(m)] = 1. 0 2 m! m=1
This gives
k h X h r i (49) ζ 0, = C γ k 0 0 k r=1
26 and so the following Theorem is obtained.
Theorem 3 If h, k ∈ N and 1 ≤ h < k then
k h 1 X 2hπr r (50) ζ 0, = sin γ . k kπ k 0 k r=1