1. Theory of Ordinary Differential Equations;Coddington,1955
2. Fixed Points and Topological Degree in Nonlinear Analysis;Cronin,1964
3. Periodic solutions of the differential equation ẍ + g(x) = p(t);Harvey;Contris. Diff. Eqs.,1963
4. Periodic solutions of nonlinear differential equations of Duffing type;Loud,1967
5. Sur les solutions périodiques de l'équation différentielle ẍ + g(x) = p(t);Opial;Bull. Acad. Polon. Sci., Sér. Sci. Math. Astron. Phys.,1960