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@article{SF_1971__3_133864, author = {A.A. Markov}, title = {On finitely generated subsemigroups of a free semigroup.}, journal = {Semigroup forum}, pages = {251--258}, publisher = {mathdoc}, volume = {3}, year = {1971}, zbl = {0225.20034}, url = {http://geodesic.mathdoc.fr/item/SF_1971__3_133864/} }
A.A. Markov. On finitely generated subsemigroups of a free semigroup.. Semigroup forum, Tome 3 (1971), pp. 251-258. http://geodesic.mathdoc.fr/item/SF_1971__3_133864/