@article{SM_2000_191_4_a1,
     author = {A. V. Deryabina},
     title = {Second-order hyperbolic equations with strong characteristic degeneracy at the initial hypersurface},
     journal = {Sbornik. Mathematics},
     pages = {503--527},
     publisher = {mathdoc},
     volume = {191},
     number = {4},
     year = {2000},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_2000_191_4_a1/}
}