[1] Croisot R.: Demi-groupes inversifs et demi-groupes reunions des demi-groupes simples. Ann. scient. École norm. supér. 70 (1953), 361-379. MR 0060504

[2] Fabrici I.: Classes of regularity in semigroups. Mat. časop. 19 (1969), 299-304. MR 0306365 | Zbl 0186.31202

[3] Ляпин E. C.: Полугрупп. Mocквa 1960. Zbl 1004.90500

[4] Clifford A. H., Preston G. B.: The algebraic theory of semigroups I. Amer. Math. Soc. Translat. Providence 1961. MR 0132791

[5] Green J. A.: On the structure of semigroups. Annals of Math., 54 (1951), 163-172. MR 0042380 | Zbl 0043.25601

[6] Petrich M.: Semigroups certain of whose subsemigroups have identities. Czechoslovak Math. J. 16 (91) (1966), 186-198. MR 0200370 | Zbl 0143.03203

[7] Pondělíček B.: On a certain relation for closure operations on a semigroup. Czechoslovak Math. J. 20 (1970), 220-231. MR 0260905

[8] Schwarz Št.: A theorem on normal semigroups. Czechoslovak Math. J. 10 (85) (1960), 197-200. MR 0116075 | Zbl 0098.01704