1. Longtime existence of classical solutions to nonlinear wave equations in exterior domains;Datti,1985
2. Hyperbolic differential equations and waves;Duff,1977
3. On Sobolev spaces associated with some Lie algebras;Hörmander;Mittag-Leffler Inst. Reports,1985
4. Remarks on the Klein-Gordon equation;Hörmander,1987
5. Mixed problems for hyperbolic equations of second order;Ikawa;J. math. Soc. Japan,1968