@article{DE_1980_16_4_a9,
     author = {V. P. Fedotov},
     title = {The extremum principle for a system of equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {679--683},
     publisher = {mathdoc},
     volume = {16},
     number = {4},
     year = {1980},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1980_16_4_a9/}
}