Analytical continuation of a Dirichlet series with periodic coefficients  Fix a complex number s and a real number x, does there exist an analytic continuation of the Dirichlet series 
$L(s,x):=\sum_{k=1}^{\infty}\frac{\sin^2(2\pi k x)}{k^s}$
to the whole complex plane except 1?
If yes, is there some functional equation verified which makes it possible to calculate $L(0,x)$?
If yes, what about the modulus of continuity of $x\mapsto L(0,x)$? ($L(\frac{3}{2},x)$ seems to be a nice case.)
Thanks for any comments
Chri
 A: Any Dirichlet series with periodic coefficients is
analytically continuable to the whole plane (maybe with a pole at $1$).
It's a finite linear combination of series like
$$\sum_{m=1}^\infty\frac1{(km+r)^s}$$
where $1\le r\le k$ which equals
$$k^{-s}\sum_{m=1}^\infty\frac1{(m+r/k)^s}.$$
This latter sum is an example of a Hurwitz zeta function
well-known to have an analytic continuation.
http://en.wikipedia.org/wiki/Hurwitz_zeta_function
Added Looking carefully at your question, I note that despite
your title, your series does not actually have periodic coefficients
unless $x$ is rational. In general your $k$-th coefficient is
$$a_k=\sin^2 2\pi kx=\frac{2-\exp(4\pi i x)-\exp(-4\pi i x)}4.$$
Thus your series can be expressed in terms of the Riemann zeta function
and functions of the form
$$f_y(s)=\sum_{n=1}^\infty\frac{\exp(2\pi iky)}{n^s}.$$
In effect this sort of function is dual to the Hurwitz zeta function,
and it has an analytic continuation to the complex plane
with a pole at $1$ proved in the same manner as the Hurwitz zeta function.
In the Wikipedia page it has a brief appearance as essentially $\beta(x;s)$.
One can express $f_y(1-s)$ in terms of the Hurwitz zeta function.
