Conformal Einstein spaces and conformally covariant operators

In this article we give general neccessary and sufficient conditions to ensure that a pseudo-Riemannian manifold is conformal to an Einstein space. These conditions are algorithmic in \emph{the metric tensor} whenever the Weyl endomorphism is invertible. Our conditions depend in an essential manner on the $\mathcal{C}$-connection. We also show how to construct \emph{conformally covariant, pseudo-differential} operators which has an independent interest.