Rational zeta series

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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=\sum_{n=2}^\infty q_n \zeta (n,m)

where qn is a rational number, the value m is held fixed, and ζ(sm) 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=\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=\sum_{n=2}^\infty \left[\zeta(n)-1\right]

and

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 =\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 \pi =\sum_{n=2}^\infty \frac{2(3/2)^n-3}{n}\left[\zeta(n)-1\right]

and

\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

\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

\frac{x}{e^x-1} = \sum_{n=0}^\infty B_n \frac{t^n}{n!}

Adamchik and Srivastava give a similar series

\sum_{n=1}^\infty \frac{t^{2n}}{n} \zeta(2n) = 
\log \left(\frac{\pi t} {\sin (\pi t)}\right)

Polygamma-related series[edit]

A number of additional relationships can be derived from the Taylor series for the polygamma function at z = 1, which is

\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

\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:

\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

\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:

\sum_{k=0}^\infty {k+\nu+1 \choose k+1} \left[\zeta(k+\nu+2)-1\right] 
= 1

and

\sum_{k=0}^\infty (-1)^k {k+\nu+1 \choose k+1} \left[\zeta(k+\nu+2)-1\right] 
= 2^{-(\nu+1)}

and

\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

\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 = \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\left[1+\sum_{k=1}^n \zeta(k+1) \right]

The above follows from the simple recursion relation Sn + Sn + 1 = ζ(n + 2). Next, the series

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}\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 Tn + Tn + 1 = Sn. This process may be applied recursively to obtain finite series for general expressions of the form

\sum_{k=0}^\infty {k+n-m \choose k} \left[\zeta(k+n+2)-1\right]

for positive integers m.

Half-integer power series[edit]

Similar series may be obtained by exploring the Hurwitz zeta function at half-integer values. Thus, for example, one has

\sum_{k=0}^\infty \frac {\zeta(k+n+2)-1}{2^k} 
{{n+k+1} \choose {n+1}}=\left(2^{n+2}-1\right)\zeta(n+2)-1

Expressions in the form of p-series[edit]

Adamchik and Srivastava give

\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

\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 are the Bernoulli numbers and 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]