@article{RM_1972_27_4_a22,
     author = {V. M. Petkov},
     title = {Necessary conditions for the correctness of the {Cauchy} problem for hyperbolic systems with multiple characteristics},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     publisher = {mathdoc},
     volume = {27},
     number = {4},
     year = {1972},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/RM_1972_27_4_a22/}
}