Groups commensurable with the modular group.
The author studies the groups commensurable with the well known modular group. It is known that the only Hecke groups commensurable with the modular group are those for \(q=3,4\) and 6. Here the author takes two maximal subgroups \(A\) and \(B\) of the group of isomorphisms of the upper half plane which are commensurable with the modular group, and then determines the index and also the level of the intersection of \(A\) and \(B\) in \(A\). If the index is finite, then the group is classified as commensurable with the modular group.
scientific article; zbMATH DE number 2105375