Symmetry, singularities and integrability in complex dynamics III:   approximate symmetries and invariants
  The different natures of approximate symmetries and their corresponding first integrals/invariants are delineated in the contexts of both Lie symmetries of ordinary differential equations and Noether symmetries of the Action Integral. Particular note is taken of the effect of taking higher orders of the perturbation parameter. Approximate symmetries of approximate first integrals/invariants and the problems of calculating them using the Lie method are considered. 
Peter Leach, Sibusio Moyo, Spiros Cotsakis, R. L. Lemmer