〔1〕Wu Xian,Chen Shaoxiong,Teng Kaimin.On variational
methods for a class of damped vibration problems〔J〕.
Nonlinear Anal.,2008,68(6):1432-1441.
〔2〕Berger M,Schechter M.On the solvability of semi-linear
gradient operator equations〔J〕.Adv. Math.,1977,25(2):
97-132.
〔3〕Mawhin J,Willem M.Critical Point Theory and Hamiltonian
Systems〔M〕.Springer-Verlag:Berlin/ New York,1989.
〔4〕Mawhin J.Semi-coercive monotone variational problems〔J〕.
Acad. Roy. Belg. Bull. Cl. Sci.,1987,73(5):118-130.
〔5〕Ma Jian,Tang Chunlei.Periodic solutions for some non -
autonomous second order systems〔J〕. J. Math. Anal. Appl.,
2002,275(2),482-492.
〔6〕Tang Chunlei.Eriodic solutions of non-autonomous second
order systems with -quasisub-additive potential〔J〕. J. Math.
Anal. Appl.,1995(189):671-675.
〔7〕Tang Chunlei.Periodic solutions of non-autonomous second
order systems〔J〕. J. Math. Anal. Appl.,1996(202):465-469.
〔8〕Tang Chunlei.Periodic solutions for non-autonomous second
order systems with sublinear nonlinearity〔J〕. Proc. Amer.
Math. Soc.,1998(126):3263-3270.
〔9〕Tang Chunlei,Wu Xingping.Periodic solutions for second
order systems with not uniformly coercive potential〔J〕. J.
Math. Anal. Appl.,2001(259):386-397.
〔10〕Wu Xingping,Tang Chunlei.Periodic solutions of a class
of non-autonomous second order systems〔J〕. J. Math. Anal.
Appl.,1999(236):227-235.
〔11〕王少敏,熊明,茶国智.一类非自治二阶系统的周期解
〔J〕.大理学院学报,2012,11(4):11-13.
〔12〕张绍康.一阶时滞微分方程的周期解〔J〕.云南师范大学
学报:自然科学版,2010,30(4):18-21.