[1] Hewitt E., H. S. Zuckerman: Ternary operations and semigroups. Semigroups: Proceedings 1968 Wayne State U. Symposium on Semigroups, K. W. Folley, ed., Academic Press (New York, 1969), 55-83. MR 0260918
[2] Petrich M.: Introduction to Semigroups. Charles E. Merril Pub. Co. (Columbus, 1973). MR 0393206 | Zbl 0321.20037