@article{SM_1970_12_3_a7,
     author = {V. V. Grushin},
     title = {On~a~class of hypoelliptic operators},
     journal = {Sbornik. Mathematics},
     pages = {458--476},
     publisher = {mathdoc},
     volume = {12},
     number = {3},
     year = {1970},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1970_12_3_a7/}
}