In mathematics , a rational zeta series is the representation of an arbitrary real number in terms of a series consisting of rational numbers and the Riemann zeta function or the Hurwitz zeta function . Specifically, given a real number x , the rational zeta series for x is given by
x
=
∑
n
=
2
∞
q
n
ζ
(
n
,
m
)
{\displaystyle x=\sum _{n=2}^{\infty }q_{n}\zeta (n,m)}
where each q n is a rational number, the value m is held fixed, and ζ(s , m ) is the Hurwitz zeta function. It is not hard to show that any real number x can be expanded in this way.
Elementary series
edit
For integer m>1 , one has
x
=
∑
n
=
2
∞
q
n
[
ζ
(
n
)
−
∑
k
=
1
m
−
1
k
−
n
]
{\displaystyle x=\sum _{n=2}^{\infty }q_{n}\left[\zeta (n)-\sum _{k=1}^{m-1}k^{-n}\right]}
For m=2 , a number of interesting numbers have a simple expression as rational zeta series:
1
=
∑
n
=
2
∞
[
ζ
(
n
)
−
1
]
{\displaystyle 1=\sum _{n=2}^{\infty }\left[\zeta (n)-1\right]}
and
1
−
γ
=
∑
n
=
2
∞
1
n
[
ζ
(
n
)
−
1
]
{\displaystyle 1-\gamma =\sum _{n=2}^{\infty }{\frac {1}{n}}\left[\zeta (n)-1\right]}
where γ is the Euler–Mascheroni constant . The series
log
2
=
∑
n
=
1
∞
1
n
[
ζ
(
2
n
)
−
1
]
{\displaystyle \log 2=\sum _{n=1}^{\infty }{\frac {1}{n}}\left[\zeta (2n)-1\right]}
follows by summing the Gauss–Kuzmin distribution . There are also series for π:
log
π
=
∑
n
=
2
∞
2
(
3
/
2
)
n
−
3
n
[
ζ
(
n
)
−
1
]
{\displaystyle \log \pi =\sum _{n=2}^{\infty }{\frac {2(3/2)^{n}-3}{n}}\left[\zeta (n)-1\right]}
and
13
30
−
π
8
=
∑
n
=
1
∞
1
4
2
n
[
ζ
(
2
n
)
−
1
]
{\displaystyle {\frac {13}{30}}-{\frac {\pi }{8}}=\sum _{n=1}^{\infty }{\frac {1}{4^{2n}}}\left[\zeta (2n)-1\right]}
being notable because of its fast convergence. This last series follows from the general identity
∑
n
=
1
∞
(
−
1
)
n
t
2
n
[
ζ
(
2
n
)
−
1
]
=
t
2
1
+
t
2
+
1
−
π
t
2
−
π
t
e
2
π
t
−
1
{\displaystyle \sum _{n=1}^{\infty }(-1)^{n}t^{2n}\left[\zeta (2n)-1\right]={\frac {t^{2}}{1+t^{2}}}+{\frac {1-\pi t}{2}}-{\frac {\pi t}{e^{2\pi t}-1}}}
which in turn follows from the generating function for the Bernoulli numbers
t
e
t
−
1
=
∑
n
=
0
∞
B
n
t
n
n
!
{\displaystyle {\frac {t}{e^{t}-1}}=\sum _{n=0}^{\infty }B_{n}{\frac {t^{n}}{n!}}}
Adamchik and Srivastava give a similar series
∑
n
=
1
∞
t
2
n
n
ζ
(
2
n
)
=
log
(
π
t
sin
(
π
t
)
)
{\displaystyle \sum _{n=1}^{\infty }{\frac {t^{2n}}{n}}\zeta (2n)=\log \left({\frac {\pi t}{\sin(\pi t)}}\right)}
edit
A number of additional relationships can be derived from the Taylor series for the polygamma function at z = 1, which is
ψ
(
m
)
(
z
+
1
)
=
∑
k
=
0
∞
(
−
1
)
m
+
k
+
1
(
m
+
k
)
!
ζ
(
m
+
k
+
1
)
z
k
k
!
{\displaystyle \psi ^{(m)}(z+1)=\sum _{k=0}^{\infty }(-1)^{m+k+1}(m+k)!\;\zeta (m+k+1)\;{\frac {z^{k}}{k!}}}
.
The above converges for |z | < 1. A special case is
∑
n
=
2
∞
t
n
[
ζ
(
n
)
−
1
]
=
−
t
[
γ
+
ψ
(
1
−
t
)
−
t
1
−
t
]
{\displaystyle \sum _{n=2}^{\infty }t^{n}\left[\zeta (n)-1\right]=-t\left[\gamma +\psi (1-t)-{\frac {t}{1-t}}\right]}
which holds for |t | < 2. Here, ψ is the digamma function and ψ(m ) is the polygamma function. Many series involving the binomial coefficient may be derived:
∑
k
=
0
∞
(
k
+
ν
+
1
k
)
[
ζ
(
k
+
ν
+
2
)
−
1
]
=
ζ
(
ν
+
2
)
{\displaystyle \sum _{k=0}^{\infty }{k+\nu +1 \choose k}\left[\zeta (k+\nu +2)-1\right]=\zeta (\nu +2)}
where ν is a complex number. The above follows from the series expansion for the Hurwitz zeta
ζ
(
s
,
x
+
y
)
=
∑
k
=
0
∞
(
s
+
k
−
1
s
−
1
)
(
−
y
)
k
ζ
(
s
+
k
,
x
)
{\displaystyle \zeta (s,x+y)=\sum _{k=0}^{\infty }{s+k-1 \choose s-1}(-y)^{k}\zeta (s+k,x)}
taken at y = −1. Similar series may be obtained by simple algebra:
∑
k
=
0
∞
(
k
+
ν
+
1
k
+
1
)
[
ζ
(
k
+
ν
+
2
)
−
1
]
=
1
{\displaystyle \sum _{k=0}^{\infty }{k+\nu +1 \choose k+1}\left[\zeta (k+\nu +2)-1\right]=1}
and
∑
k
=
0
∞
(
−
1
)
k
(
k
+
ν
+
1
k
+
1
)
[
ζ
(
k
+
ν
+
2
)
−
1
]
=
2
−
(
ν
+
1
)
{\displaystyle \sum _{k=0}^{\infty }(-1)^{k}{k+\nu +1 \choose k+1}\left[\zeta (k+\nu +2)-1\right]=2^{-(\nu +1)}}
and
∑
k
=
0
∞
(
−
1
)
k
(
k
+
ν
+
1
k
+
2
)
[
ζ
(
k
+
ν
+
2
)
−
1
]
=
ν
[
ζ
(
ν
+
1
)
−
1
]
−
2
−
ν
{\displaystyle \sum _{k=0}^{\infty }(-1)^{k}{k+\nu +1 \choose k+2}\left[\zeta (k+\nu +2)-1\right]=\nu \left[\zeta (\nu +1)-1\right]-2^{-\nu }}
and
∑
k
=
0
∞
(
−
1
)
k
(
k
+
ν
+
1
k
)
[
ζ
(
k
+
ν
+
2
)
−
1
]
=
ζ
(
ν
+
2
)
−
1
−
2
−
(
ν
+
2
)
{\displaystyle \sum _{k=0}^{\infty }(-1)^{k}{k+\nu +1 \choose k}\left[\zeta (k+\nu +2)-1\right]=\zeta (\nu +2)-1-2^{-(\nu +2)}}
For integer n ≥ 0, the series
S
n
=
∑
k
=
0
∞
(
k
+
n
k
)
[
ζ
(
k
+
n
+
2
)
−
1
]
{\displaystyle S_{n}=\sum _{k=0}^{\infty }{k+n \choose k}\left[\zeta (k+n+2)-1\right]}
can be written as the finite sum
S
n
=
(
−
1
)
n
[
1
+
∑
k
=
1
n
ζ
(
k
+
1
)
]
{\displaystyle S_{n}=(-1)^{n}\left[1+\sum _{k=1}^{n}\zeta (k+1)\right]}
The above follows from the simple recursion relation S n + S n + 1 = ζ(n + 2). Next, the series
T
n
=
∑
k
=
0
∞
(
k
+
n
−
1
k
)
[
ζ
(
k
+
n
+
2
)
−
1
]
{\displaystyle T_{n}=\sum _{k=0}^{\infty }{k+n-1 \choose k}\left[\zeta (k+n+2)-1\right]}
may be written as
T
n
=
(
−
1
)
n
+
1
[
n
+
1
−
ζ
(
2
)
+
∑
k
=
1
n
−
1
(
−
1
)
k
(
n
−
k
)
ζ
(
k
+
1
)
]
{\displaystyle T_{n}=(-1)^{n+1}\left[n+1-\zeta (2)+\sum _{k=1}^{n-1}(-1)^{k}(n-k)\zeta (k+1)\right]}
for integer n ≥ 1. The above follows from the identity T n + T n + 1 = S n . This process may be applied recursively to obtain finite series for general expressions of the form
∑
k
=
0
∞
(
k
+
n
−
m
k
)
[
ζ
(
k
+
n
+
2
)
−
1
]
{\displaystyle \sum _{k=0}^{\infty }{k+n-m \choose k}\left[\zeta (k+n+2)-1\right]}
for positive integers m .
Half-integer power series
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Similar series may be obtained by exploring the Hurwitz zeta function at half-integer values. Thus, for example, one has
∑
k
=
0
∞
ζ
(
k
+
n
+
2
)
−
1
2
k
(
n
+
k
+
1
n
+
1
)
=
(
2
n
+
2
−
1
)
(
ζ
(
n
+
2
)
−
1
)
−
1
{\displaystyle \sum _{k=0}^{\infty }{\frac {\zeta (k+n+2)-1}{2^{k}}}{{n+k+1} \choose {n+1}}=\left(2^{n+2}-1\right)\left(\zeta (n+2)-1\right)-1}
edit
Adamchik and Srivastava give
∑
n
=
2
∞
n
m
[
ζ
(
n
)
−
1
]
=
1
+
∑
k
=
1
m
k
!
S
(
m
+
1
,
k
+
1
)
ζ
(
k
+
1
)
{\displaystyle \sum _{n=2}^{\infty }n^{m}\left[\zeta (n)-1\right]=1\,+\sum _{k=1}^{m}k!\;S(m+1,k+1)\zeta (k+1)}
and
∑
n
=
2
∞
(
−
1
)
n
n
m
[
ζ
(
n
)
−
1
]
=
−
1
+
1
−
2
m
+
1
m
+
1
B
m
+
1
−
∑
k
=
1
m
(
−
1
)
k
k
!
S
(
m
+
1
,
k
+
1
)
ζ
(
k
+
1
)
{\displaystyle \sum _{n=2}^{\infty }(-1)^{n}n^{m}\left[\zeta (n)-1\right]=-1\,+\,{\frac {1-2^{m+1}}{m+1}}B_{m+1}\,-\sum _{k=1}^{m}(-1)^{k}k!\;S(m+1,k+1)\zeta (k+1)}
where
B
k
{\displaystyle B_{k}}
are the Bernoulli numbers and
S
(
m
,
k
)
{\displaystyle S(m,k)}
are the Stirling numbers of the second kind .
Other series
edit
Other constants that have notable rational zeta series are:
References
edit
Jonathan M. Borwein, David M. Bradley, Richard E. Crandall (2000). "Computational Strategies for the Riemann Zeta Function" (PDF) . J. Comput. Appl. Math . 121 (1–2): 247–296. Bibcode :2000JCoAM.121..247B. doi :10.1016/s0377-0427(00)00336-8 . {{cite journal}}
: CS1 maint: multiple names: authors list (link)
Victor S. Adamchik and H. M. Srivastava (1998). "Some series of the zeta and related functions" (PDF) . Analysis . 18 (2): 131–144. CiteSeerX 10.1.1.127.9800 . doi :10.1524/anly.1998.18.2.131. S2CID 11370668.