A maximal chain of principal ideals in the semigroup of binary relations on a finite set. M. Breen — 1991 Semigroup forum
Covering in the set of principal ideals in the semigroup of binary relations. M. Breen — 1994 Semigroup forum
Principal ideals in the semigroup of binary relations on a finite set: What happens when one element is added to the set. M. Breen — 1992 Semigroup forum