1. Embedding theorems using amalgamation bases;Ash,1980
2. Absolutely flat semigroups;Bulman-Fleming;Pacific J. Math.,1983
3. Flatness and amalgamation in semigroups;Bulman-Fleming,1984
4. The Algebraic Theory of Semigroups;Clifford,1961
5. The Algebraic Theory of Semigroups;Clifford,1967