1. Defining relations and algorithmic problems for groups and semigroups;Adjan,1966
2. The word problem for a class of semigroups with a finite number of defining relations;Grindlinger;Siberian Math. J.,1964
3. On Dehn's algorithm;Lyndon;Math. Ann.,1966
4. Combinatorial Group Theory;Lyndon,1977
5. On the immersion of an algebraic ring into a field;Malcev;Math. Ann.,1937