[1] G. G. Hamedani: Periodic Boundary Value Problems for Nonlinear Second Order Vector Differential Equations. To appear in Revue Roum. De Math. Pures Et App.
[2] B. Mehri, G. G. Hamedani: 
On the Existence of Periodic Solutions of Nonlinear Second Order Differential Equations. SIAM J. Appî. Math. Vol. 29, No. 1, July 1975. 
MR 0369824 | 
Zbl 0322.34027