@article{IM2_1990_34_2_a5,
     author = {V. A. Kozlov and V. A. Kondrat'ev and V. G. Maz'ya},
     title = {On sign variation and the absence of {\textquotedblleft}strong{\textquotedblright} zeros of solutions of elliptic equations},
     journal = {Izvestiya. Mathematics},
     pages = {337--353},
     year = {1990},
     volume = {34},
     number = {2},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1990_34_2_a5/}
}