cpp/experimental/special functions/riemann zeta

@1@ Computes the Riemann zeta function of. @4@ A set of overloads or a function template accepting an argument of any integral type. Equivalent to after casting the argument to.

Return value
If no errors occur, value of the Riemann zeta function of, $ζ(arg)$, defined for the entire real axis:
 * For $arg>1$, $Σ∞ n=1n-arg$
 * For $0≤arg≤1$, $Σ∞ n=1(-1)n-1 n-arg$
 * For $arg<0$, $2arg πarg-1 sinΓ(1−arg)ζ(1−arg)$

Error handling
Errors may be reported as specified in math_errhandling


 * If the argument is NaN, NaN is returned and domain error is not reported