Riemann's function for a Klein-Gordon equation with a non-constant coefficient
C Wahlberg · Journal of Physics A Mathematical and General · 1977
The Riemann function (or the Riemann-Green function) for the equation U tt -U xx +(1+ lambda x)U=0 is derived by means of a Laplace transform technique. It is shown that the solution given by Whitham's (1974) variational method can be identified as the first term in an asymptotic expansion, in lambda and 1/t, of the exact solution.