A priori estimates and existence for a class of fully nonlinear elliptic   equations in conformal geometry
  In this paper we prove the interior gradient and second derivative estimates for a class of fully nonlinear elliptic equations determined by symmetric functions of eigenvalues of the Ricci or Schouten tensors. As an application we prove the existence of solutions to the equations when the manifold is locally conformally flat or the Ricci curvature is positive. 
Xu-Jia Wang