[1] Harbans Lal: Commutative semiprimary semigroups. Czech Math. Journal, 25 (100), (1975), 1-3.

[2] M. Petrich: Introduction to semigroups. Charles E. Merril Publishing company, Columbus, Ohio, 1973. MR 0393206 | Zbl 0321.20037

[3] B. Pondělíček: On a certain relation for a closure operation on a semigroup. Czech. Math. Journal, 20 (1970), 220-231. MR 0260905

[4] B. Pondělíček: On weakly commutative semigroups. Czech. Math. Journal, 25 (100), (1975), 20-23. MR 0387460

[5] M. Satyanarayana: Commutative primary semigroups. Czech. Math. Journal, 22 (97), (1972), 509-516. MR 0308304 | Zbl 0248.20072