MathSciNet bibliographic data MR342774 34C25 Butler, G. J.; Freedman, H. I. Periodic solutions of $x\sp{\prime\prime}+g(x)+\mu h(x)=0$$x\sp{\prime\prime}+g(x)+\mu h(x)=0$. Trans. Amer. Math. Soc. 197 (1974), 59–74. Article

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