1. Geometrical Methods in the Theory of Ordinary Differential Equations;Arnold,1983
2. On the periodic wind-induced vibrations of an oscillator with two degrees of freedom;van der Beek;Nieuw Arch. Wisk.,1987
3. Dynamical Systems;Birkhoff,1927
4. Asymptotic Methods in the Theory of Nonlinear Oscillations;Bogolyubov,1961