1. Die untergruppen der freien gruppen;Schreier;Abh. Math. Sem. Univ. Hamburg,1927
2. Embedding theorems for groups;Higman;J. Lond. Math. Soc.,1949
3. An essay on free products of groups with amalgamation;Neumann;Philos. Trans. Roy. Soc. London Ser. A,1954
4. Homogeneous universal relational systems;Jónsson;Math. Scand.,1960
5. Universal Algebra;Grätzer,1968