1. Ault, J.E.: Group congruences on a bisimple $$\omega $$ ω -semigroup. Semigroup Forum 10, 351–366 (1975)
2. Clifford, A.H., Preston, G.B.: The Algebraic Theory of Semigroups. Mathematical Surveys No. 7, vol. I. American Mathematical Society, Providence (1961)
3. Clifford, A.H., Preston, G.B.: The Algebraic Theory of Semigroups. Mathematical Surveys No. 7, vol. II. American Mathematical Society, Providence (1967)
4. Fortunatov, V.A.: Perfect semigroups. Izv. Vyssh. Uchebn. Zaved. Mat. 3, 80–90 (1972) (in Russian)
5. Fortunatov, V.A.: Perfect congruences on monogenic inverse semigroups. In: Algebra Number Theory, Nalchik, vol. 4, pp. 93–98 (1979) (in Russian)