Recently, Anders Petersson, Omar Ortiz and myself have developed a rather general theory for second order hyperbolic systems based on Laplace and Fourier transforms, with particular emphasis on boundary processes corresponding to generalized eigenvalues. Our theory uses mode analysis and builds upon the theory for first order systems, developed a long time ago. This approach has many desirable properties: 1) Once a second order system has been Laplace and Fourier transformed it can always be written as a system of 2n first order pseudo-differential equations. Therefore the above theory also applies here. 2) We can localize the problem, i.e., it is only necessary to study the Cauchy problem and halfplane problems with constant coefficients. 3) The class of problems we can treat is much larger than previous approaches based on “integration by parts”. 4) The relation between boundary conditions and boundary phenomena becomes transparent.
I will not bore you with complicated proofs. Instead, I will discuss a number of examples.