[1] A. H. Clifford, G. B. Preston:
The algebraic theory of semigroups. Amer. Math. Soc. Surveys, No. 7, Vol. 1, Providence, R.I., 1961.
MR 0132791 |
Zbl 0111.03403
[4] C. S. Johnson, Jr., F. R. McMorris:
Injective hulls of certain $S$-systems over a semilattice. Proc. Amer. Math. Soc. 32 (1972), 371-375.
MR 0289687 |
Zbl 0226.06003