1. The algebraic theory of semigroups;Clifford,1961
2. An extension of Tietze's theorem;Dugundji;Pacific J. Math.,1951
3. Semigroups;Ljapin,1963
4. Some homomorphism theorems for a class of semigroups;Magill,1965
5. Subsemigroups of S(X);Magill;Math. Japonicae,1966