Decoupling of modes for low regularity hyperbolic systems

We show that the coupling operator between distinct modes of a second-order hyperbolic system is smoothing of degree one, where we assume that the eigenvalues of the symbol are of constant rank, and that the coefficients of the system have bounded derivatives of second order. An important example is the wave equation for linear isotropic elasticity, where our assumption states that the Lamé parameters and mass density have bounded derivatives of second order. This extends a result for the elastic wave equation established by Brytik, et.al.