[1] R. H. Brück: A survey of binary systems. Springer, Berhn, 1958. MR 0093552

[2] A. H. Clifford, G. B. Preston: The algebraic theory of semigroups, Vol. I. Math. Surveys No. 7, Amer. Math. Soc, Providence, R. I., 1961. MR 0132791 | Zbl 0111.03403

[3] P. H. H. Fantham: On the classification of a certain type of semigroup. Proc. London Math. Soc. (3) 10 (1960), 409-427. MR 0121411 | Zbl 0228.20035

[4] B. Kolibiarová: On semigroups, every subsemigroup of which has a left identify. Mat.-Fyz. Časopis Slovensk. Akad. Vied 7 (1957), 177-182 (Slovak; Russian and English summaries). MR 0100643

[5] B. Kolibiarová: On semigroups, every left ideal of which has a one-sided identity. Mat.-Fyz. Časopis Slovensk. Akad. Vied 10 (1960), 9-17 (Slovak; Russian and German summaries). MR 0132115

[6] B. Kolibiarová: On semigroups, every principal left ideal of which has an identity. Mat.-Fyz. Časopis Slovensk. Akad. Vied 11 (1961), 275-281 (Slovak; Russian and German summaries).

[7] M. Petrich: Sur certaines classes de demi-groupes, I. Acad. Roy. Belg. Bull. CI. Sci. 49 (1963), 785-798. MR 0166283 | Zbl 0124.25704

[8] M. Petrich: The maximal semilattice decomposition of a semigroup. Math. Zeitschrift 85 (1964), 68-82. DOI 10.1007/BF01114879 | MR 0167552 | Zbl 0124.25801

[9] Š. Schwarz: Contribution to the theory of periodic semigroups. Czechoslovak Math. J. 3 (1953), 7-21 (Russian; English summary). MR 0061593

[10] N. N. Vorob'ev: Associative systems of which every subsystem has an identity. Doklady Akad. Nauk SSSR 88 (1953), 393-396 (Russian). MR 0053077

[11] N. N. Vorob'ev: On associative systems of which every left ideal has an identity. Leningrad. Gos. Ped. Inst. Uc. Zap. 103 (1955), 83-90 (Russian).