The $\zeta$ function : Zeta

Zeta takes as argument a real x.
Zeta returns for x > 1 :

$$\zeta$$(x) = $$\sum_{{n=1}}^{{+\infty}}$$$${\frac{{1}}{{n^x}}}$$

and for x < 1 it's meromorphic continuation.
Input :

Zeta(2)

Output :

pi^2/6

Input :

Zeta(4)

Output :

pi^4/90