@article{DE_1975_11_1_a10,
     author = {Yu. P. Krivenkov},
     title = {Sufficient optimality conditions for problems with second order differential equations of elliptic type in the presence of phase constraints},
     journal = {Differencialʹnye uravneni\^a},
     pages = {85--99},
     publisher = {mathdoc},
     volume = {11},
     number = {1},
     year = {1975},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1975_11_1_a10/}
}