The characteristic equation describes the propagation of a discontinuity in space (x,y,z). Let us assume that equation can be solved with respect to t: and . Then we get from equation (20):
That is exactly the Eikonal equation for a medium with elliptic anisotropy. In the case of isotropy, it follows that and equation (21) is reduced to
Let us consider, for example, the linear acoustic equation
In this case and we again get equation (22) at .It is very important that the propagation of a discontinuity depends only on senior terms of the equation (18).