How to Compute $\zeta (0)$? Ultimately, I am interested in analytically continuing the function
$$
\eta _a(s):=\sum _{n=1}^\infty \frac{1}{(n^2+a^2)^s},
$$
where $a$ is a non-negative real number, and calculating $\eta _a$ and its derivatives (at least the first derivative) at the origin:  $\eta _a(0),\eta _a'(0),\ldots $.
It is well-known that $\zeta (0)=-\tfrac{1}{2}$ and that $\zeta '(0)=-\tfrac{1}{2}\ln (2\pi)$, but I do not actually know how to obtain these ($\zeta$ is of course the Riemann Zeta function).  I figured that, perhaps if I knew how to calculate these values, I would be able to generalize the technique to be able to calculate the corresponding values of $\eta _a$.
So then, how does one calculate $\zeta (0)$, $\zeta '(0)$, etc.?  If this technique does not obviously generalize to $\eta _a$, any ideas how I might go about calculating these values?
 A: By the functional equation of the zeta function:
$$\zeta(s)=2^s\pi^{s-1}\sin\frac{\pi s}2\Gamma(1-s)\zeta(1-s)$$
We now use the fact that the zeta function has a simple pole at $\,s=1\,$ with residue $\,1\,$ (this is, in my opinion, one of the most beautiful elementary things that can be proved about this wonderful function), and this means that
$$\lim_{s\to 1}(s-1)\zeta(s)=1$$
Now, using the functional equation for the Gamma Function $\,s\Gamma(s)=\Gamma(s+1)\;$, we multiply the functional equation for zeta by $\,(1-s)\;$ and then pass to the limit when $\,s\to 1\;$:
$$(1-s)\zeta(s)=2^s\pi^{s-1}\sin\frac{\pi s}2\left[(1-s)\Gamma(1-s)\right]\zeta(1-s)\implies$$
$$\lim_{s\to 1}(1-s)\zeta(s)=-1=\lim_{s\to 1}\;\Gamma(2-s)2^s\pi^{s-1}\zeta(1-s)=1\cdot 2\zeta(0)\implies$$
$$\zeta(0)=-\frac12$$
A: There is a paper listed online which should answer your questions. If you let $P(x)=1$ and $Q(x)=x^2 + a^2$ then you are looking at $$\eta_a(s)= \sum P(k)/Q(k)^s.$$
This paper suggests that $\eta_a(s)$ continues to the $s$-plane and provides values of $\eta_a(0),$ $\eta_a'(0).$
http://www.math.nagoya-u.ac.jp/~kohjimat/weng.pdf
