Approximate solutions in General Relativity via deformation of   embeddings
  A systematic study of deformations of four-dimensional Einsteinian space-times embedded in a pseudo-Euclidean space $E^N$ of higher dimension is presented. Infinitesimal deformations, seen as vector fields in $E^N$, can be divided in two parts, tangent to the embedded hypersurface and orthogonal to it; only the second ones are relevant, the tangent ones being equivalent to coordinate transformations in the embedded manifold.   The geometrical quantities can be then expressed in terms of embedding functions $z^A$ and their infinitesimal deformations $v^A z^A \to {\tilde{z}}^A = z^A + \epsilon v^A$. The deformations are called Einsteinian if they keep Einstein equations satisfied up to a given order in $\epsilon$. The system so obtained is then analyzed in particular in the case of the Schwarzschild metric taken as the starting point, and some solutions of the first-order deformation of Einstein's equations are found.   We discuss also second and third order deformations leading to wave-like solutions and to the departure from spherical symmetry towards an axial one (the approximate Kerr solution) 
Richard Kerner and Salvatore Vitale