@article{RM_1961_16_5_a4,
     author = {E. A. Gorin and V. V. Grushin},
     title = {On the definition of hypo-elliptic equations},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     publisher = {mathdoc},
     volume = {16},
     number = {5},
     year = {1961},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/RM_1961_16_5_a4/}
}