Gravitational radiation from Schwarzschild black holes: the second order   perturbation formalism
  The perturbation theory of black holes has been useful recently for providing estimates of gravitational radiation from black hole collisions. Second order perturbation theory, relatively undeveloped until recently, has proved to be important both for providing refined estimates and for indicating the range of validity of perturbation theory. Here we present the second order formalism for perturbations of Schwarzschild spacetimes. The emphasis is on practical methods for carrying out second order computations of outgoing radiation. General issues are illustrated throughout with examples from ``close-limit'' results, perturbation calculations in which black holes start from small separation. 
Reinaldo Gleiser, Carlos Nicasio, Richard Price and Jorge Pullin