A remark concerning universal curvature identities on 4-dimensional   Riemannian manifolds
  We shall prove the universality of the curvature identity for the 4-dimensional Riemannian manifold using a different method than that used by Gilkey, Park, and Sekigawa \cite{GPS}. 
Yunhee Euh, Chohee Jeong, and JeongHyeong Park
math.DG