@article{SF_1972__5_133922,
     author = {D.W. Hardy and G. Markowsky and R.L. Brandon},
     title = {The {Sch\"utzenberger} group of an {H-class} in the semigroup of binary relations.},
     journal = {Semigroup forum},
     pages = {45--53},
     publisher = {mathdoc},
     volume = {5},
     year = {1972},
     zbl = {0259.20056},
     url = {http://geodesic.mathdoc.fr/item/SF_1972__5_133922/}
}