1. An associative calculus with an insoluble problem of equivalence (Russian);Céjtin;Trudy. Mat. Inst. Steklov,1958
2. D. J. Collins, A universal semigroup, to appear in Algebra i Logika.
3. The word problem for semigroups with two generators;Hall;J. Symb. Logic,1949
4. Simple examples of undecidable associative calculi;Matijasevid;Soviet Math.,1967
5. Isomorphic embedding of a semigroup with an enumerable set of defining relations in a finitely presented semigroup (Russian);Murskii;Matem. Sametki,1967