@article{SF_1984__29_134688,
     author = {L. Makar-Limanov},
     title = {On free subsemigroups.},
     journal = {Semigroup forum},
     pages = {253--254},
     publisher = {mathdoc},
     volume = {29},
     year = {1984},
     zbl = {0552.20043},
     url = {http://geodesic.mathdoc.fr/item/SF_1984__29_134688/}
}