@article{DE_1990_26_4_a16,
     author = {Nguyẽn Minh Tr{\'\i}},
     title = {On the global hypoellipticity property of a higher-order differential operator},
     journal = {Differencialʹnye uravneni\^a},
     pages = {687--692},
     publisher = {mathdoc},
     volume = {26},
     number = {4},
     year = {1990},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1990_26_4_a16/}
}