Necessary and sufficient conditions for the well posedness of the Cauchy problem for a class of hyperbolic operators with high variable multiplicity V. Sordoni (1992) Rendiconti del Seminario Matematico della Università di Padova
Nonlinear effectively hyperbolic equations N. Iwasaki (1985/1986) Séminaire Équations aux dérivées partielles (Polytechnique)