[1] Clifford A.H., Preston G.B.: The Algebraic Theory of Semigroups. Amer. Math. Soc., Providence I (1961). MR 0132791

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

[3] Fabrici I.: $\mathcal J$-classes in the direct product of two semigroups. Czechoslovak Math. J. 44 (1994), 325–335. MR 1281027

[4] Fabrici I.: One-sided principal ideals in the direct product of two semigroups. Math. Bohem. 4 (1993), 337–342. MR 1251880 | Zbl 0810.20043

[5] Tamura T.: Note on finite semigroups and determination of semigroups of order 4. J.  Gakugei, Tokushima Univ. 5 (1954), 17–28. MR 0069813