@article{SF_1970__1_133800,
     author = {R.J. Plemmons and B.M. Schein},
     title = {Groups of binary relations.},
     journal = {Semigroup forum},
     pages = {267--271},
     publisher = {mathdoc},
     volume = {1},
     year = {1970},
     zbl = {0211.03901},
     url = {http://geodesic.mathdoc.fr/item/SF_1970__1_133800/}
}