@article{SM_1987_56_2_a3,
     author = {G. G. Kazaryan},
     title = {On a~functional index of hypoellipticity},
     journal = {Sbornik. Mathematics},
     pages = {333--347},
     publisher = {mathdoc},
     volume = {56},
     number = {2},
     year = {1987},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1987_56_2_a3/}
}