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2. Theory of Ordinary Differential Equations;Coddington,1955
3. Repeated resonance and homoclinic bifurcation in a periodically forced family of oscillators;Greenspan;SIAM J. Math. Anal.,1984
4. Nonlinear Oscillations, Dynamical Systems, and Bifurcations of Vector Fields;Guckenheimer,1983
5. Invariant Manifolds;Hirsch,1977