@article{IM2_1988_30_3_a8,
     author = {A. N. Varchenko},
     title = {On~normal forms of nonsmoothness of solutions of hyperbolic equations},
     journal = {Izvestiya. Mathematics },
     pages = {615--628},
     publisher = {mathdoc},
     volume = {30},
     number = {3},
     year = {1988},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1988_30_3_a8/}
}