ABSTRACT
Extending Fermat’s principle, we derived in section 3.2.7 the nonlinear ray equations and then using the conservation of energy along a ray tube, we derived the transport equation for the amplitude along the nonlinear rays for an isotropic wave propagation. This lead to a set of coupled equations of a weakly nonlinear ray theory for such a system. Extension of the theory to a more general system is quite difficult.
