@article{DE_1975_11_4_a18,
     author = {E. I. Zheleznov},
     title = {Necessary optimality conditions for singular extremals},
     journal = {Differencialʹnye uravneni\^a},
     pages = {749--752},
     publisher = {mathdoc},
     volume = {11},
     number = {4},
     year = {1975},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1975_11_4_a18/}
}