@article{DAN_1963_148_6_a1,
     author = {V. V. Grushin},
     title = {Extension of smoothness to solutions of differential equations of smooth type},
     journal = {Doklady Akademii Nauk},
     pages = {1241--1244},
     publisher = {mathdoc},
     volume = {148},
     number = {6},
     year = {1963},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1963_148_6_a1/}
}