Hyperbolic operators with non Lipschitz coefficients F. Colombini; N. Lerner Séminaire Équations aux dérivées partielles (Polytechnique)
Some examples of hyperbolic equations without local solvability F. Colombini; S. Spagnolo — 1989 Annales scientifiques de l'École Normale Supérieure
Well-posedness in the Gevrey classes of the Cauchy problem for a non-strictly hyperbolic equation with coefficients depending on time F. Colombini; E. Jannelli; S. Spagnolo — 1983 Annali della Scuola Normale Superiore di Pisa - Classe di Scienze