@article{MZ_1994__215_1_174626,
     author = {Mitsuru Sugimoto},
     title = {A priori estimates for higher order hyperbolic equations.},
     journal = {Mathematische Zeitschrift},
     pages = {519--532},
     year = {1994},
     volume = {215},
     number = {1},
     zbl = {0790.35063},
     url = {http://geodesic.mathdoc.fr/item/MZ_1994__215_1_174626/}
}