1. J. Chazarain, Opérateurs hyperboliques à caractéristiques de multiplicité constant. Ann. Inst. Fourier (Grenoble) 24, 173–202 (1974)
2. F. Colombini, E. De Giorgi, S. Spagnolo, Sur les équations hyperboliques avec des coefficients qui ne dépendent que du temps. Ann. Sc. Norm. Super. Pisa 4, 511–559 (1979)
3. F. Colombini, E. Jannelli, S. Spagnolo, Well posedness in the Gevrey class of the Cauchy problem for a non strictly hyperbolic equation with coefficients depending on time. Ann. Sc. Norm. Super. Pisa 10, 291–312 (1983)
4. F. Colombini, T. Nishitani, N. Orrù, Some well-posed Cauchy problem for second order hyperbolic equations with two independent variables. Osaka J. Math. 48, 647–673 (2011)
5. P. D’Ancona, Well posedness in C ∞ for a weakly hyperbolic second order equation. Rend. Sem. Mat. Univ. Padova 91, 65–83 (1994)