@article{DE_1999_35_7_a4,
     author = {E. N. Krupchik},
     title = {Lower characteristic exponents of nontrivial solutions of {Pfaffian} systems},
     journal = {Differencialʹnye uravneni\^a},
     pages = {899--908},
     publisher = {mathdoc},
     volume = {35},
     number = {7},
     year = {1999},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1999_35_7_a4/}
}