@article{DE_1970_6_4_a7,
     author = {V. F. Butuzov and A. B. Vasil'eva},
     title = {Differential and difference systems of equations with a small parameter in the case when the unperturbed (degenerate) system is situated on the spectrum},
     journal = {Differencialʹnye uravneni\^a},
     pages = {650--664},
     publisher = {mathdoc},
     volume = {6},
     number = {4},
     year = {1970},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1970_6_4_a7/}
}