[1] Clifford A. H., Preston G. B.: The algebraic theory of semigroup, Vol. I. American Math. Society, Providence, 1961. MR 0132791

[2] Fabrici I.: One-sided principal ideals in the direct product of two semigroup. (submitted to Math. Bohemica).

[3] Fabrici I.: Principal two-sided ideals in the direct product of two semigroups. Czechoslovak Math. Journal 41 (1991), 411–421. MR 1117794 | Zbl 0751.20043

[4] Fabrici I., Macko T.: On bases and maximal ideals in semigroups. Math. Slovaca 31 (1981), 115–120. MR 0611622

[5] Howie J. M.: An introduction to semigroup theory. Academic Press, 1976. MR 0466355 | Zbl 0355.20056

[6] Schwarz Š.: On maximal ideals in the theory of semigroups I. Czechoslovak Math. Journal 78 (1953), 139–153. MR 0061591

[7] Schwarz Š.: On maximal ideals in the theory of semigroups II. Czechoslovak Math. Journal 78 (1953), 365–383. MR 0061592