@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/}
}