@article{SF_1974__6_133982,
     author = {G.B. Preston},
     title = {Sch\"utzenberger groups of ...-classes in the semigroup of all binary relations on a set.},
     journal = {Semigroup forum},
     pages = {260--262},
     publisher = {mathdoc},
     volume = {6},
     year = {1974},
     zbl = {0267.20063},
     url = {http://geodesic.mathdoc.fr/item/SF_1974__6_133982/}
}