On globally smooth oscillating solutions of non-strictly hyperbolic systems

A class of non-strictly hyperbolic systems of quasilinear equations with oscillatory solutions of the Cauchy problem, globally smooth in time in some open neighborhood of the zero stationary state, is found. For such systems, the period of oscillation of solutions does not depend on the initial point of the Lagrangian trajectory. The question of the possibility of constructing these systems in a physical context is also discussed, and non-relativistic and relativistic equations of cold plasma are studied from this point of view.