Periodic solutions of certain second order nonlinear differential equations
Tài liệu tham khảo
Cronin, 1964, Fixed Points and Topological Degree in Nonlinear Analysis
Lazer, 1968, On Schauder's fixed point theorem and forced second-order nonlinear oscillations, J. Math. Anal. Appl., 21, 421, 10.1016/0022-247X(68)90225-4