We presented proofs that place the Fréchet-derivative approach as a ray-theory concept.  

 

Our proofs are based on properties of the Cauchy problem of partial differential equations.

 

Presently, as a part of ray theory, the so-called finite-frequency approach benefits from the wealth of theoretical methods.  

 

Prior to this work, it was commonly stated that the recently developed finite-frequency approach was a fundamental generalization of ray theory.

 

 This approach, however, neither generalizes nor supplants ray theory; it is precisely a consequence of that theory.