@article{DAN_1961_136_6_a9,
     author = {B. V. Shabat},
     title = {On the notion of derivative system according to {M.} {A.~Lavrentʹev}},
     journal = {Doklady Akademii Nauk},
     pages = {1298--1301},
     year = {1961},
     volume = {136},
     number = {6},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1961_136_6_a9/}
}