1. Geometrical Methods in the Theory of Ordinary Differential Equations;Arnold,1988
2. Asymptotic Methods in the Theory of Non-linear Oscillations;Bogoliubov,1961
3. Ordinary Differential Equations in Banach Spaces;Deimling,1977
4. Nonlinear Oscillations, Dynamical Systems, and Bifurcations of Vector Fields;Guckenheimer,1983
5. Theory of Functional Differential Equations;Hale,1977