The Hurwitz-Kronecker Class Number is given by the formula $H(d)=\sum_{Q\in Q_d/(\Gamma=PSL_2(\mathbb{Z}))}\frac{1}{w_Q}$ where $w_Q=card(stab(\alpha_Q))$ with $\alpha_Q$ being the unique zero attached to the quadratic form $Q(X,1)$ of determinant $-d$. I would like to know if there is a specific reason to why we should divide by the size of the stab. Thank you ! This is from Zagier's paper Traces of Singular Moduli : https://pdfs.semanticscholar.org/ad32/ffe3debb0169cea3d102151a74b1edb76fed.pdf
2026-03-30 21:09:33.1774904973
Understanding the Hurwitz-Kronecker Class Number Formula
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