[1] A. H. Clifford, G. B. Preston: The Algebraic Theory of Semigroups, Vol. 1. Math. Surveys No. 7, Amer. Math. Soc, Providence, R. I., 1962.
[2] Š. Schwarz:
On the semigroup of binary relations on a finite set. Czech. math. J. 20 (1970), 632-679.
MR 0296190 |
Zbl 0228.20034