@article{DAN_1971_201_4_a3,
     author = {V. Ya. Ivrii},
     title = {The {Cauchy} problem for nonstrictly hyperbolic equations of second order},
     journal = {Doklady Akademii Nauk},
     pages = {778--781},
     publisher = {mathdoc},
     volume = {201},
     number = {4},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1971_201_4_a3/}
}