The geometric optics for a class of hyperbolic second order operators with Hölder continuous coefficients with respect to time
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Volume 18 (1991) no. 1, p. 39-66 
@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},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 18},
     number = {1},
     year = {1991},
     pages = {39-66},
     zbl = {0761.35055},
     mrnumber = {1118220},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1991_4_18_1_39_0}
}

Cicognani, Massimo. The geometric optics for a class of hyperbolic second order operators with Hölder continuous coefficients with respect to time. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Volume 18 (1991) no. 1, pp. 39-66. http://www.numdam.org/item/ASNSP_1991_4_18_1_39_0/

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