Friedrichs systems for the three-dimensional wave equation

Abstract

The three-dimensional wave equation is reduced to a Friedrichs symmetric hyperbolic system. We describe all these reductions and find those of them that preserve the velocity of propagation of perturbations. We also exhibit transformations of a Friedrichs system under the Lorentz transformation of coordinates. The construction of the reduction of the wave equation and justification of the properties of this reduction are based on the use of quaternions.