@article{SM_1970_12_4_a7,
     author = {Z. D. Usmanov},
     title = {On~infinitesimal deformations of surfaces of positive curvature with an isolated flat point},
     journal = {Sbornik. Mathematics},
     pages = {595--614},
     publisher = {mathdoc},
     volume = {12},
     number = {4},
     year = {1970},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1970_12_4_a7/}
}