Form class groups and class fields of CM-fields
Let $F$ be a totally real number field of class number one, and let $K$ be a CM-field with $F$ as its maximal real subfield. For each positive integer $N$, we construct a class group of certain binary quadratic forms over $F$ which is isomorphic to the ray class group of $K$ modulo $N$. Assuming further that the narrow class number of $F$ is one, we construct a class field of the reflex field of $K$ in terms of the singular values of Hilbert modular functions.