@article{RM_1975_30_6_a12,
     author = {V. Ya. Ivrii},
     title = {The energy integral for nonstrictly hyperbolic operators},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     publisher = {mathdoc},
     volume = {30},
     number = {6},
     year = {1975},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/RM_1975_30_6_a12/}
}