@article{DAN_1970_194_1_a15,
     author = {R. F. Polishchuk},
     title = {On the classification of the curvature tensor by means of principal congruences},
     journal = {Doklady Akademii Nauk},
     pages = {62--65},
     year = {1970},
     volume = {194},
     number = {1},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1970_194_1_a15/}
}