# $L$-functions for quadratic orders and Siegel's solution of the class number problem

Let $$K$$ be an imaginary quadratic field and $$D_K$$ its discriminant. Further let $$\mathcal O$$ be an order in $$K$$ with conductor $$f$$ and

$$L(\chi,s)=\sum_{\mathfrak a}\chi(\mathfrak a)N(\mathfrak a)^{-s}$$

where the sum is over all integral ideals of $$\mathcal O$$ relatively prime to $$f$$, and $$\chi$$ is a character of the ideal group of $$\mathcal O$$ (ideals taken prime to $$f$$).

In his paper Zum Beweise des Starkschen Satzes, Siegel deduces the formula

$$\frac {1}{2\pi}f |D_K|^{1/2}L(\chi, 1)=-\sum_{\mathfrak C}\bar\chi(\mathfrak c)\log \left( \sqrt{\Im(\omega_{\mathfrak c})\eta(\omega_{\mathfrak c})^2}\right).$$ Here the sum on the right is over ideal classes in $$\mathcal O$$ (again prime to $$f$$), $$\eta$$ is the dedekind eta function, and $$\omega_\mathfrak c=-\beta/\alpha$$ if $$\mathfrak c= [\alpha,\beta]$$. This should follow from the Kronecker's first limit formula. Is there some English reference for the Siegel's formula, and the mentioned $$L$$-functions for non-maximal orders?

• A representative of $\mathfrak{c}$ is a lattice $u \mathbb{Z}+v \mathbb{Z}, u/v = a+ib$ and you can look at $h(x) = \sum_{n \in \mathbb{Z}^2} \exp(-\pi x \|\begin{pmatrix} 1 & a \\ 0 & b \end{pmatrix} n\|^2)$. The Poisson summation formula tells you $h(1/x) = |b|^{-1/2} x^{-1} \sum_{n \in \mathbb{Z}^2} \exp(-\pi x \| \begin{pmatrix} 1 & a \\ 0 & b \end{pmatrix}^{-\top} n\|^2)$ (which is related to the sum for a representative of $\mathfrak{c}^{-1}$). From this you know the asymptotic of $h$ as $x \to 0$ and the pole of its Dirichlet series, that you can relate to $\log \eta(u/v)$ – reuns Nov 9 '18 at 0:11
• @reuns Could you please elaborate on your comment? – Shimrod Nov 9 '18 at 16:58