1. E. Bernardi and A. Bove, Propagation of Gevrey singularities for a class of operators with triple characteristics. I, II, Duke Math. J. 60 (1990), 187–205, 207–220.
2. J. M. Bony, Equivalence des diverses notions de spectre singulier analytique, Séminaire Goulaouic-Schwartz (1976/1977), Équations aux dérivées partielles et analyse fonctionnelle, Exp. No. 3, École Polytech., Palaiseau, 1977.
3. M. D. Bronshtein, The Cauchy problem for hyperbolic operators with variable multiple characteristics, Trudy Moskov. Mat. Obschch. 41 (1980), 83–99.
4. M. Christ, Intermediate optimal Gevrey exponents occur, Comm. Partial Differential Equations 22 (1997), 359–379.
5. V. Ja. Ivrii, Wave fronts of solutions of certain pseudodifferential equations, Trudy Moskov. Mat. Obshch. 39 (1979), 49–82, 235.