1. [1] E. Bernardi and A. Bove, Necessary and sufficient conditions for the well-posedness of the Cauchy problem for a class of hyperbolic operators with triple characteristics, preprint, 1988.
2. [2] C. Parenti, E. Bernardi, and A. Bove, Propagation of $C\sp \infty$-singularities for a class of hyperbolic operators with double characteristics, Recent developments in hyperbolic equations (Pisa, 1987), Pitman Res. Notes Math. Ser., vol. 183, Longman Sci. Tech., Harlow, 1988, pp. 255–264.
3. [3] L. Hörmander, The Cauchy problem for differential equations with double characteristics, J. Analyse Math. 32 (1977), 118–196.
4. [4] L. Hörmander, The Analysis of Linear Partial Differential Operators, III, Grundlehren der Mathematischen Wissenschaften [Fundamental Principles of Mathematical Sciences], vol. 274, Springer-Verlag, Berlin, 1985.
5. [5] V. Ya. Ivrii, Correctness of the Cauchy problem in Gevrey classes for non-strictly hyperbolic operators, Mat. U.S.S.R Sbornik 25 (1975), 365–387.