@article{MZM_1991_49_6_a0,
     author = {V. N. Aref'ev},
     title = {Smoothness of solutions of second-order hyperbolic equations with discontinuous coefficients},
     journal = {Matemati\v{c}eskie zametki},
     pages = {3--11},
     publisher = {mathdoc},
     volume = {49},
     number = {6},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1991_49_6_a0/}
}