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2. Ordinary Differential Equations;Hale,1969
3. Alternative problems for nonlinear functional equations;Bancroft;J. Differential Equations,1968
4. Degré topologique et solutions périodiques des systèmes différentiels non-linéaires;Mawhin;Bull. Soc. Roy. Sci. Liège,1969
5. Autosynartetic solutions of differential equations;Lewis;Amer. J. Math.,1961