[1] FRASER G. A., HORN A.: Congruence relations in direct products. Proc. Amer. Math. Soc., 26, 1970, 390-394. MR 0265258 | Zbl 0241.08004

[2] SZÁSZ G.: Introduction to lattice theory. Akad. Kiadó, Budapest 1963. MR 0110652