@article{MAN_1987__277_164247,
     author = {Walter Craig},
     title = {Nonstrictly {Hyperbolic} {Nonlinear} {Systems.}},
     journal = {Mathematische Annalen},
     pages = {213--232},
     year = {1987},
     volume = {277},
     zbl = {0614.35060},
     url = {http://geodesic.mathdoc.fr/item/MAN_1987__277_164247/}
}