Transversally strictly hyperbolic systems

Abstract

We consider the Cauchy problem for first order systems. Assuming that the set of the singular points of the characteristic variety is a smooth manifold and the characteristic values are real and semi-simple we introduce a new class which is strictly hyperbolic in the transverse direction to the characteristic manifold. We prove that if the characteristic manifold is either involutive or symplectic then these transversally strictly hyperbolic systems are strongly hyperbolic. On the other hand if the characteristic manifold is neither involutive nor symplectic transversally strictly hyperbolic systems are much more involved which is discussed taking an interesting example.