[1] CLIFFORD A. H., PRESTON G. B.: The algebгaic theory of semigroups, I. Providence 1961.

[2] LAJOS S.: Generalized ideals in semigroups. Acta Sci. Math. Szeged, 22, 1961, 217-222. MR 0136671 | Zbl 0108.25904

[3] LJAPIN E. S.: Polugruppy. Moskva 1960. MR 0120299

[4] SCHWARZ Š.: K teóгii pologrúp. Sborník pгác Príгodovedeckej fakulty Slovenskej univerzity v Bratislave, 6, 1943, 1-64.

[5] STEINFELD O.: Über die Quasiideale von Halbgruppen. Publ. Math. Debrecen, 4, 1956, 262-275. MR 0083998 | Zbl 0070.25301

[6] ŠULKA R.: On the nilpotency in semigroups. Mat. časop. 18, 1968, 148-157. MR 0242979 | Zbl 0159.02701

[7] ŠULKA R.: The maximal semilattice decomposition of a semigroup, radicals and nilpotency. Mat. časop. 20, 1970, 172-180. MR 0311821 | Zbl 0249.20031