1. Note on normal subgroups of the modular group;Greenberg, L.;Proc. Amer. Math. Soc.,1966
2. Congruence subgroups of positive genus of the modular group;Knopp, M. I.;Illinois J. Math.,1965
3. A. M. Macbeath, Fuchsian groups, Lectures at Queen’s College, Dundee, Scotland, 1961.
4. Lattice subgroups of free congruence groups;Mason, A. W.;Glasgow Math. J.,1969
5. Free subgroups and normal subgroups of the modular group;Newman, Morris;Illinois J. Math.,1964