1. G.Birkhoff,Lattice Theory (3rd Edition), Amer. Math. Soc. Colloq. Publ. #25, Providence, 1967.
2. I. E. Burmistrovi?,Commutative Bands of Caneelative Semigroups (Russian), Sibirsk. Mat. Z.,6 (1965), 284?299.
3. A. H.Clifford and G. B.Preston,The Algebraic Theory of Semigroups, Vol. I, Amer. Math. Soc. Math. Surveys #, Providence, 1961.
4. P. M. Cohn,Universal algebra, Harper & Row, New York, 1965.
5. B. CsÁkány,Congruences and subalgebras, Ann. Univ. Sci. Budapest. Eotros Sect. Math.18 (1975), 37?44.