On some extension of Gauss' work and applications (II)

Abstract: Let $K$ be an imaginary quadratic field of discriminant $d_K$, and let $\mathfrak{n}$ be a nontrivial integral ideal of $K$ in which $N$ is the smallest positive integer. Let $\mathcal{Q}N(d_K)$ be the set of primitive positive definite binary quadratic forms of discriminant $d_K$ whose leading coefficients are relatively prime to $N$. We adopt an equivalence relation $\sim\mathfrak{n}$ on $\mathcal{Q}N(d_K)$ so that the set of equivalence classes $\mathcal{Q}_N(d_K)/\sim\mathfrak{n}$ can be regarded as a group isomorphic to the ray class group of $K$ modulo $\mathfrak{n}$. We further present an explicit isomorphism of $\mathcal{Q}N(d_K)/\sim\mathfrak{n}$ onto $\mathrm{Gal}(K_\mathfrak{n}/K)$ in terms of Fricke invariants, where $K_\mathfrak{n}$ is the ray class field of $K$ modulo $\mathfrak{n}$. This would be certain extension of the classical composition theory of binary quadratic forms, originated and developed by Gauss and Dirichlet.