@article{DAN_1957_113_6_a6,
     author = {P. I. Petrov},
     title = {Second order invariants of the quaternary differential quadratic form},
     journal = {Doklady Akademii Nauk},
     pages = {1214--1217},
     publisher = {mathdoc},
     volume = {113},
     number = {6},
     year = {1957},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1957_113_6_a6/}
}