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@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/} }
P. I. Petrov. Second order invariants of the quaternary differential quadratic form. Doklady Akademii Nauk, Tome 113 (1957) no. 6, pp. 1214-1217. http://geodesic.mathdoc.fr/item/DAN_1957_113_6_a6/