Maximal normal subgroups of the modular group
Morris Newman · Proceedings of the American Mathematical Society · 1968
Introduction.The purpose of this paper is to initiate the study of the maximal normal subgroups G of the modular group T; i.e. those normal subgroups G such that T/G is simple.The principal congruence subgroups T(p) of prime level p>3 are such groups, since