@article{SF_1979__17_134325,
     author = {Stefan Schwarz},
     title = {A theorem on binary relations and infinite languages.},
     journal = {Semigroup forum},
     pages = {307--316},
     publisher = {mathdoc},
     volume = {17},
     year = {1979},
     zbl = {0425.20053},
     url = {http://geodesic.mathdoc.fr/item/SF_1979__17_134325/}
}