@article{DE_1973_9_3_a25,
     author = {A. I. Kalinin},
     title = {Necessary optimality conditions for singular controls},
     journal = {Differencialʹnye uravneni\^a},
     pages = {568--573},
     publisher = {mathdoc},
     volume = {9},
     number = {3},
     year = {1973},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1973_9_3_a25/}
}