Let $\Gamma_n(\mathcal{\scriptstyle{O}}_\mathbb{K})$ denote the Hermitian modular group of degree $n$ over an imaginary-quadratic number field $\mathbb{K}$. In this paper we determine its maximal discrete extension in $SU(n,n;\mathbb{C})$, which coincides with normalizer $\Gamma_n(\mathcal{\scriptstyle{O}}_{\mathbb{K}})$. The description involves $n$-torsion subgroup ideal class This is defined...

Abstract We introduce and study families of finite index subgroups the modular group that generalize congruence subgroups. Such groups, termed ϕ‐congruence subgroups, are obtained by reducing homomorphisms ϕ from into a linear algebraic modulo integers. In particular, we examine two examples, arising on one hand map quasi‐unipotent group, other maps symplectic groups degree four. case, also pro...