@article{SM_1972_18_2_a11,
     author = {S. A. Popov},
     title = {Knotting of contractible two-dimensional polyhedra in~$\mathbf R^4$},
     journal = {Sbornik. Mathematics},
     pages = {333--341},
     publisher = {mathdoc},
     volume = {18},
     number = {2},
     year = {1972},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1972_18_2_a11/}
}