1. Finitely generated subgroups of 𝐻𝑁𝑁 groups;Burns, R. G.;Canadian J. Math.,1973
2. A presentation of 𝑆𝐿₂ for Euclidean imaginary quadratic number fields;Cohn, P. M.;Mathematika,1968
3. 3. Benjamin Fine, The structure of PSL2(R), Ann. of Math. Studies, no. 79, Discontinuous groups and Riemann surfaces, May 1973.
4. 4. Benjamin Fine and Marvin Tretkoff, The SQ-universality of certain linear groups (to appear).
5. Subgroups of 𝐻𝑁𝑁 groups and groups with one defining relation;Karrass, A.;Canadian J. Math.,1971