Fundamental Solutions for the Klein-Gordon Equation in de Sitter   Spacetime
  In this article we construct the fundamental solutions for the Klein-Gordon equation in de Sitter spacetime. We use these fundamental solutions to represent solutions of the Cauchy problem and to prove $L^p-L^q$ estimates for the solutions of the equation with and without a source term. 
Karen Yagdjian and Anahit Galstian