@article{SM_1992_73_2_a7,
     author = {V. A. Roman'kov},
     title = {Primitive elements of free groups of rank~3},
     journal = {Sbornik. Mathematics},
     pages = {445--454},
     publisher = {mathdoc},
     volume = {73},
     number = {2},
     year = {1992},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1992_73_2_a7/}
}