1. Clifford, A. H., and G. B. Preston:The Algebraic Theory of Semigroups, 2nd Edition, Vol. 1, A.M.S., Providence, 1964.
2. Clifford, A. H., and G. B. Preston:The Algebraic Theory of Semigroups, Vol. 2. A. M. S., Providence, 1967.
3. Feller, E. H., and R. L. Gantos:Indecomposable and Injective S-systems with Zero, Math. Nachr. 41 (1969), 37–48.
4. Johnson, R. E.:The Extended Centralizer of a Ring Over a Module, Proc. Amer. Math. Soc. 2(1951), 891–895.
5. McMorris, F. R.:On
Quotient
Semigroups, submitted.