You should study  two seminal monographs:

1. Cubic and quadratic nonlinearities:

M.I. Rabinovich, D.I. Trubetskov. Oscillations and waves: in linear and nonlinear systems. Published by Kluwer Academic Publishers in 1989. Look the beginning of the part 2.

2. Perturbation methods:

A.J. Lichtenberg, M.A. Lieberman. Regular and stochastic motion. Published by Springer-Verlag in 1983.

Thank you very much Mr. Denis