@article{SM_1973_19_2_a3,
     author = {M. A. Frumkin},
     title = {A~filtration in the three-dimensional {Cremona} group},
     journal = {Sbornik. Mathematics},
     pages = {191--208},
     publisher = {mathdoc},
     volume = {19},
     number = {2},
     year = {1973},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1973_19_2_a3/}
}