@article{SM_1971_14_1_a1,
     author = {V. S. Fedii},
     title = {On a~criterion for hypoellipticity},
     journal = {Sbornik. Mathematics},
     pages = {15--45},
     publisher = {mathdoc},
     volume = {14},
     number = {1},
     year = {1971},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1971_14_1_a1/}
}