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@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/} }
L. Makar-Limanov. On free subsemigroups.. Semigroup forum, Tome 29 (1984), pp. 253-254. http://geodesic.mathdoc.fr/item/SF_1984__29_134688/