@article{ASENS_1989_4_22_1_109_0,
     author = {Colombini, F. and Spagnolo, S.},
     title = {Some examples of hyperbolic equations without local solvability},
     journal = {Annales scientifiques de l'\'Ecole Normale Sup\'erieure},
     pages = {109--125},
     publisher = {Elsevier},
     volume = {Ser. 4, 22},
     number = {1},
     year = {1989},
     doi = {10.24033/asens.1578},
     mrnumber = {90d:35164},
     zbl = {0702.35146},
     language = {en},
     url = {http://www.numdam.org/articles/10.24033/asens.1578/}
}