I want to know about reference of formulas for 
$$
L(s,D)=\sum\_{n=1}^\infty \left(\frac{D}{n}\right)\,n^{-s}
$$
for $s$ a positive integer number and $D$ a fundamental discriminant. For $s=1$ we have the *Dirichlet class number formula*. 


I would like to have some reference for $s\geq2$. Thanks.-.



Values of Dirichlet L-funcions at natural numbers
Let $\chi$ be any Dirichlet character modulo $q$, and let $m$ be a positive integer. Then Theorem 4.2 of Washington's *Introduction to Cyclotomic Fields* states that


$$ L(1-m,\chi) = - \frac{q^{m-1}}{m} \sum\_{a=1}^q \chi(a)B\_{m}(\tfrac{a}{q}). $$


Here $B\_m(x)$ is the usual Bernoulli polynomial, defined by


$$ \frac{t e^{Xt}}{e^t-1} = \sum\_{n=0}^\infty B\_n(X) \frac{t^n}{n!}.$$


As Stopple pointed out, you can use the functional equation for $L(s,\chi)$ to evaluate $L(m,\chi)$, for some values of $m$, if you know the value of $ L(1-m,\chi).$


