@article{DAN_1990_314_1_a18,
     author = {S. G. Krein and V. I. Fomin},
     title = {Small perturbations of singular differential equations with},
     journal = {Doklady Akademii Nauk},
     pages = {77--79},
     publisher = {mathdoc},
     volume = {314},
     number = {1},
     year = {1990},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1990_314_1_a18/}
}