Special values of generalized $\mathbf{\lambda}$ functions at imaginary quadratic points

Abstract

We study a modular function $\Lambda_{k,\ell}$ which is one of generalized $\lambda$ functions. We show $\Lambda_{k,\ell}$ and the modular invariant function $j$ generate the modular function field with respect to the modular subgroup $\Gamma_1(N)$. Further we prove that $\Lambda_{k,\ell}$ is integral over $\mathbf Z[j]$. From these results, we obtain that the value of $\Lambda_{k,\ell}$ at an imaginary quadratic point is an algebraic integer and generates a ray class field over the Hilbert class field.