@article{SM_1992_72_1_a7,
     author = {P. P. Zabreiko and V. I. Nazarov},
     title = {Smoothness properties of solutions of nonlinear differential equations},
     journal = {Sbornik. Mathematics},
     pages = {135--150},
     publisher = {mathdoc},
     volume = {72},
     number = {1},
     year = {1992},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1992_72_1_a7/}
}