1. The existence of periodic solutions of systems of differential equations;Massera;Duke Math. J.,1950
2. Note on the existence theorem of a periodic solution of the nonlinear differential equation;Yoshizawa,1954
3. Asymptotic Behavior and Stability Problems in Ordinary Differential Equations;Cesari,1963
4. Differential Equations: Geometric Theory;Lefschetz,1963
5. Nonlinear Differential Equations;Sansone,1964