ZETA _ _ _ _ _ _ _ _ _ _ _ _ operator
The Zeta operator returns Riemann's Zeta function,
Zeta (z) := sum(1/(k**z),k,1,infinity)
Zeta(2); 2 pi / 6 on rounded; Zeta 1.01; 100.577943338
Numerical computation for the Zeta function for arguments close to 1 are tedious, because the series is converging very slowly. In this case a formula (e.g. found in Bender/Orzag: Advanced Mathematical Methods for Scientists and Engineers, McGraw-Hill) is used.
No numerical approximation for complex arguments is done.