[1] Schmitt K.:
A note on periodic solutions of second order ordinary differential equations. SIAM J. Appl. Math. Vol. 21 (1971), pp. 491-494.
MR 0304781 |
Zbl 0238.34073
[2] Mehri B., Hamedani G. G.:
On the existence of periodic solutions of nonlinear second order differential equations. SIAM J. Appl. Math. Vol. 29, No. 1, July 1975, pp. 72-76.
MR 0369824 |
Zbl 0322.34027
[3] Kiguradze I. T.:
On periodic solutions of non-linear second order differential equations. Prague conference on nonlinear Vibrations 1967 pp. 175-180.
MR 0340726
[4] Krasnoseľskij M. A.: Položiteľnyje rešenija operatornych uravnenij. Moskva 1962.