@article{SF_1976__13_134200,
     author = {B.M. Schein},
     title = {Regular elements of the semigroup of all binary relations.},
     journal = {Semigroup forum},
     pages = {95--102},
     publisher = {mathdoc},
     volume = {13},
     year = {1976},
     zbl = {0355.20058},
     url = {http://geodesic.mathdoc.fr/item/SF_1976__13_134200/}
}