@article{RM_1971_26_2_a8,
     author = {O. A. Oleinik and E. V. Radkevich},
     title = {On local smoothness of generalized and hypoellipticity of second order differential equations},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {139--156},
     publisher = {mathdoc},
     volume = {26},
     number = {2},
     year = {1971},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/RM_1971_26_2_a8/}
}