Geodesic
The energy integral for nonstrictly hyperbolic operators
Trudy Matematicheskogo Instituta imeni V.A. Steklova, Tome 30 (1975) no. 6 
V. Ya. Ivrii. The energy integral for nonstrictly hyperbolic operators. Trudy Matematicheskogo Instituta imeni V.A. Steklova, Tome 30 (1975) no. 6. http://geodesic.mathdoc.fr/item/RM_1975_30_6_a12/
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     author = {V. Ya. Ivrii},
     title = {The energy integral for nonstrictly hyperbolic operators},
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     number = {6},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/RM_1975_30_6_a12/}
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Voir la notice de l'article provenant de la source Math-Net.Ru

[1] L. Gording, Zadacha Koshi dlya giperbolicheskikh uravnenii, IL, M., 1961

[2] V. Ya. Ivrii, V. M. Petkov, “Neobkhodimye usloviya korrektnosti zadachi Koshi dlya nestrogo giperbolicheskikh uravnenii”, UMN, 29:5 (1974), 3–70 | MR | Zbl