no. 2
Well-posedness in the Gevrey classes of the Cauchy problem for a non-strictly hyperbolic equation with coefficients depending on time
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 10 (1983) no. 2, pp. 291-312 
@article{ASNSP_1983_4_10_2_291_0,
     author = {Colombini, F. and Jannelli, E. and Spagnolo, S.},
     title = {Well-posedness in the {Gevrey} classes of the {Cauchy} problem for a non-strictly hyperbolic equation with coefficients depending on time},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {291--312},
     year = {1983},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 10},
     number = {2},
     mrnumber = {728438},
     zbl = {0543.35056},
     language = {en},
     url = {https://www.numdam.org/item/ASNSP_1983_4_10_2_291_0/}
}
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Colombini, F.; Jannelli, E.; Spagnolo, S. Well-posedness in the Gevrey classes of the Cauchy problem for a non-strictly hyperbolic equation with coefficients depending on time. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 10 (1983) no. 2, pp. 291-312. https://www.numdam.org/item/ASNSP_1983_4_10_2_291_0/

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