In this paper we describe the integral transform that allows to write solutions of one partial differential equation via solution of another one. This transform was suggested by the author in the case when the last equation is a wave equation, and then it was used to investigate several well-known equations such as Tricomi-type equation, the Klein-Gordon equation in the de Sitter and Einstein-de Sitter spacetimes. A generalization given in this paper allows us to consider also the Klein-Gordon equations with coefficients depending on the spatial variables.
Keywords: Klein-Gordon equation; Curved spacetime; Representation of solution
Introduction and Statement of ResultsIn this paper we give some generalization of the approach suggested in [28], which is aimed to reduce equations with variable coefficients to more simple ones. This transform was used in a series of papers [10, 11], [28]-[33] to investigate in a unified way several equations such as the linear and semilinear Tricomi equations, Gellerstedt equation, the wave equation in Einstein-de Sitter spacetime, the wave and the Klein-Gordon equations in the de Sitter and anti-de Sitter spacetimes. The listed equations play an important role in the gas dynamics, elementary particle physics, quantum field theory in curved spaces, and cosmology.Consider for the smooth function f = f (x, t) the solution w = w A,f (x, t; b) to the problemwith the parameter b ∈ I = [t 0 , T ] ⊆ R, t 0 < T ≤ ∞, and with 0 < T 1 ≤ ∞.Here Ω is a domain in R n , while A(x, ∂ x ) is the partial differential operator A(x, ∂ x ) = |α|≤m a α (x)D α x . We are going to present the integral operator In fact, the function u = u(x, t) takes initial values as follows u(x, t 0 ) = 0, u t (x, t 0 ) = 0, x ∈ Ω .