1. Bonzini, C. and A. Cherubini,On the Putcha Δ-semigroups, Inst. Lombardo Acad. Sci. Lett. Rend. A114 (1980), 179–194.
2. Clifford, A. H. and G. B. Preston,The algebraic theory of semigroups, Amer. Math. Soc., Providence,I (1961),II (1967).
3. Etterbeek, W. A.,Dissertation, University of California, Davis (1980).
4. Nagy, A.,Weakly exponential Δ-semigroups, Semigroup Forum40 (1990), 297–313.
5. Petrich, M.,Lectures in Semigroups Akademie-Verlag Berlin (1977).