For a new class of g(t, x), the existence, uniqueness and stability of 2π\documentclass[12pt]{minimal}
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\begin{document}$${2\pi}$$\end{document}-periodic solution of Duffing equation x′′+cx′+g(t,x)=h(t)\documentclass[12pt]{minimal}
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\begin{document}$${x'' + cx' + g(t, x) = h(t)}$$\end{document} are presented. Moreover, the unique 2π\documentclass[12pt]{minimal}
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\begin{document}$${2\pi}$$\end{document}-periodic solution is (exponentially asymptotically stable) and its rate of exponential decay c/2 is sharp. The new criterion characterizes gx′(t,x)-c2/4\documentclass[12pt]{minimal}
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\begin{document}$${g_{x}^{\prime}(t, x) - c^2/4}$$\end{document} with Lp-norms (p∈[1,∞])\documentclass[12pt]{minimal}
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\begin{document}$${(p \in [1, \infty])}$$\end{document}, and the classical criterion employs the L∞\documentclass[12pt]{minimal}
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\begin{document}$${L^{\infty}}$$\end{document}-norm. The advantage is that we can deal with the case that gx′(t,x)-c2/4\documentclass[12pt]{minimal}
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\begin{document}$${g_{x}^{\prime}(t, x) - c^2/4}$$\end{document} is beyond the optimal bounds of the L∞\documentclass[12pt]{minimal}
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\begin{document}$${L^{\infty}}$$\end{document}-norm, because of the difference between the Lp-norm and the L∞\documentclass[12pt]{minimal}
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\begin{document}$${L^{\infty}}$$\end{document}-norm.
