[2] Schwarz Š.: 
On the semigroup of binary relations on a finite set. Czech. Math. J. 20 (1970), 632-679. 
MR 0296190 | 
Zbl 0228.20034[3] Schwarz Š.: 
On a sharp estimation in the theory of binary relations on a finite set. Czech. Math. J. 20 (1970), 703-714. 
MR 0282862 | 
Zbl 0226.20061