Bulletin of the Brazilian Mathematical Society. 1991;22(1):81-92. doi: 10.1007/bf01244899 Q20.92025

On the existence of periodic solutions for the equation(ddot x + f(x)dot x + g(x) = 0)

关于方程（$\ddot{x} + f(x)\dot{x} + g(x) = 0$）周期解的存在性

DOI: 10.1007/bf01244899

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