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@article{DE_1974_10_2_a0, author = {A. F. Andreev}, title = {The exceptional direction of an $n$th order system at a stationary point}, journal = {Differencialʹnye uravneni\^a}, pages = {187--194}, publisher = {mathdoc}, volume = {10}, number = {2}, year = {1974}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/DE_1974_10_2_a0/} }
A. F. Andreev. The exceptional direction of an $n$th order system at a stationary point. Differencialʹnye uravneniâ, Tome 10 (1974) no. 2, pp. 187-194. http://geodesic.mathdoc.fr/item/DE_1974_10_2_a0/