1. P. R. Sethna, On Averaged and Normal Form Equations,Nonlinear Dynamics,7, 1–10 (1995).
2. S. N. Chow & J. R. Hale,Methods of Bifurcation Theory, Springer-Verlag, New York (1982).
3. R. Courant & D. Hilbert,Methods of Mathematical Physics, Vol. II: Partial Differential Equations, Wiley-Interscience Publishers, New York (1962).
4. W. H. Warner.An Extension of Normal Form Methods for Calculating Approximate Solutions, applied Mathematics: Methods and Applications (G. Oyibo, ed.), Proceedings of a conference in honor of Dr. George H. Handelman, Rensselaer Polytechnic Institute, Troy, NY, March 1995, Nova Science Publishers, Inc. (to appear).
5. Advances in Soviet Mathematics;Yu. S. Il'yashenko,1991