@article{DE_1972_8_9_a2,
     author = {V. G. Boltyanskii},
     title = {Necessary conditions for the extremum of a function},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1553--1559},
     publisher = {mathdoc},
     volume = {8},
     number = {9},
     year = {1972},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1972_8_9_a2/}
}