@article{TM_1976_140_a7,
     author = {G. G. Kazaryan},
     title = {A~characterization of hypoellipticity by estimates from below},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {162--168},
     publisher = {mathdoc},
     volume = {140},
     year = {1976},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/TM_1976_140_a7/}
}