@article{TRSPY_1976_140_a7, author = {G. G. Kazaryan}, title = {A~characterization of hypoellipticity by estimates from below}, journal = {Informatics and Automation}, pages = {162--168}, publisher = {mathdoc}, volume = {140}, year = {1976}, language = {ru}, url = {http://geodesic.mathdoc.fr/item/TRSPY_1976_140_a7/} }