摘要
Many papers have been published on the existence of periodic solutions for the Duffing equation (?)+g(x)=p(t)=p(t+2π).(1) Their main assumptions imposed on g are super-linear, sub-linear and exclude the resonance cases (see Ref. [1] and Prof. T. R. Ding’s, Prof. W.G. Ge’s recent papers). Relatively speaking, there are few papers concerning the
