1. Clifford, A. H. and G. B. Preston,The algebraic theory of semigroups, Amer. Math. Soc., Providence, R. I., Vol. 1, 1961; Vol. 2, 1967.
2. Imaoka, T.,On fundamental regular *semigroups, Mem. Fac. Sci., Shimane Univ., 14 (1980), 19–23.
3. Imaoka, T.,Some remarks on fundamental regular *-semigroups, to appear.
4. Meakin, J.,Congruences on orthodox semigroups, J. Austral. Math. Soc. 12 (1971), 323–341.
5. Preston, G. B.,Inverse semigroups, J. London Math. Soc. 29 (1954), 396–403.