Some non-congruence subgroups and the associated modular curves ☆

Abstract In this paper, we study two families of normal subgroups of Γ 0 ( 2 ) with abelian quotients and their associated modular curves. They are similar to Fermat groups and Fermat curves in some aspects but very different in other aspects. Almost all of them are non-congruence subgroups. These modular curves are either projective lines or hyperelliptic curves. There are few modular forms of weight 1 for these groups. We also determine their cuspidal divisor class groups and show that these groups are finite.