1. Andersen, G. M. and Geer, J. F., Power series expansions for the frequency and period of the limit cycle of the van der Pol equation. SIAM J. Appl. Math. 42 (1983), 678–693.
2. Arnold, V. I. and Anosov, D. V. (eds.), Dynamical Systems I. Encyclopaedia of Mathematical Sciences. Springer-Verlag, New York, 1988.
3. Bibikov, Yu. N., Local Theory of Nonlinear Analytic Ordinary Differential Equations. LNM 702, Springer-Verlag, New York, 1979.
4. Boege, W., Gebauer, R. and Kredel, H., Some examples for solving systems of algebraic equations by calculating Groebner bases. J. Symb. Comput. 1 (1986), 83–98.
5. Bruno (Brjuno), A. D., Analytical form of differential equations I. Trans. Mosc. Mat. Soc. 25 (1971), 131–288.