@article{DAN_1960_133_3_a9,
     author = {L. N. Shevrin},
     title = {On subsemigroups of free semigroups},
     journal = {Doklady Akademii Nauk},
     pages = {537--539},
     publisher = {mathdoc},
     volume = {133},
     number = {3},
     year = {1960},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1960_133_3_a9/}
}