  Let $(M,g)$ be a compact conformally flat manifold of dimension $n\geq4$ with
positive scalar curvature. According to a positive mass theorem by Schoen and
Yau, the constant term in the development of the Green function of the
conformal Laplacian is positive if $(M,g)$ is not conformally equivalent to the
sphere. On spin manifolds, there is an elementary proof of this fact by Ammann
and Humbert, based on a proof of Witten. Using differential forms instead of
spinors, we give an elementary proof on even dimensional manifolds, without any
other topological assumption.
