no. 3
On a weakly hyperbolic equation with a term of order zero
Annales de la Faculté des sciences de Toulouse : Mathématiques, Série 6, Tome 6 (1997) no. 3, pp. 525-534 
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     author = {Orr\`u, Nicola},
     title = {On a weakly hyperbolic equation with a term of order zero},
     journal = {Annales de la Facult\'e des sciences de Toulouse : Math\'ematiques},
     pages = {525--534},
     year = {1997},
     publisher = {Universit\'e Paul Sabatier. Facult\'e des sciences},
     address = {Toulouse},
     volume = {Ser. 6, 6},
     number = {3},
     mrnumber = {1610911},
     zbl = {0895.35057},
     language = {en},
     url = {https://www.numdam.org/item/AFST_1997_6_6_3_525_0/}
}
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Orrù, Nicola. On a weakly hyperbolic equation with a term of order zero. Annales de la Faculté des sciences de Toulouse : Mathématiques, Série 6, Tome 6 (1997) no. 3, pp. 525-534. https://www.numdam.org/item/AFST_1997_6_6_3_525_0/

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[CJS] Colombini (F.), Jannelli (E.) and Spagnolo (S.) .- Well-posedness in the Gevrey classes of the Cauchy problem for a non-strictly hyperbolic equation with coefficients depending on time, Ann. Scuola Norm. Sup. Pisa, 10 (1983), pp. 291-312. | Zbl | MR | Numdam

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[Ka] Kajitani (K.) .- The well posed Cauchy problem for hyperbolic operators, Exposé au Séminaire Vaillant (1989).

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