@article{SF_1982__25_134568,
     author = {G. Baumslag and Y. Roitberg},
     title = {Groupes with free 2-generator subsemigroups.},
     journal = {Semigroup forum},
     pages = {135--144},
     publisher = {mathdoc},
     volume = {25},
     year = {1982},
     zbl = {0494.20014},
     url = {http://geodesic.mathdoc.fr/item/SF_1982__25_134568/}
}