@article{SF_1992__44_1_135137,
     author = {M. Breen},
     title = {Principal ideals in the semigroup of binary relations  on a finite set: {What} happens when one element is added to the set.},
     journal = {Semigroup forum},
     pages = {129--132},
     publisher = {mathdoc},
     volume = {44},
     number = {1},
     year = {1992},
     zbl = {0763.20023},
     url = {http://geodesic.mathdoc.fr/item/SF_1992__44_1_135137/}
}