@article{SM_1972_17_4_a2,
     author = {V. V. Grushin},
     title = {Hypoelliptic differential equations and pseudodifferential operators with operator-valued symbols},
     journal = {Sbornik. Mathematics},
     pages = {497--514},
     publisher = {mathdoc},
     volume = {17},
     number = {4},
     year = {1972},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1972_17_4_a2/}
}