@article{ASNSP_1991_4_18_1_39_0,
     author = {Cicognani, Massimo},
     title = {The geometric optics for a class of hyperbolic second order operators with {H\"older} continuous coefficients with respect to time},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {39--66},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 18},
     number = {1},
     year = {1991},
     mrnumber = {1118220},
     zbl = {0761.35055},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1991_4_18_1_39_0/}
}