  In this paper we write down the equation for a scalar conformally coupled
field simultaneously for de Sitter (dS), anti-de Sitter (AdS) and Minkowski
spacetime in d-dimensions. The curvature dependence appears in a very simple
way through a conformal factor. As a consequence the process of curvature free
limit, including wave functions limit and two-points functions, turns to be a
straightforward issue. We determine a set of modes, that we call de Sitter
plane waves, which become ordinary plane waves when the curvature vanishes.
