Nonlinear Perturbation Theory
  An explicit perturbative solution to all orders is given for a general class of nonlinear differential equations. This solution is written as a sum indexed by rooted trees and uses the Green function of a linearization of the equations. The modifications due to the presence of zero-modes is considered. Possible divergence of the integrals can be avoided by using approximate Green functions. 
Stephanie Rossano and Christian Brouder