Livia Corsi, Roberto Feola, Guido Gentile
Convergent series for quasi-periodically forced strongly dissipative systems
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ABSTRACT. We study the ordinary differential equation $\epsilon x''+x'+\epsilon g(x)=\epsilon f(\omega t)$, with $f$ and $g$ analytic and $f$ quasi-periodic in $t$ with frequency vector $\omega \in
R^d$. We show that if there exists $c_0\in R$ such that $g(c_0)$ equals the average of $f$ and the first non-zero derivative of $g$ at $c_0$ is of odd order $n$, then, for ε small enough and under very mild Diophantine conditions on $\omega$, there exists a quasi-periodic solution close to $c_0$ with the same frequency vector as $f$. In particular if $f$ is a trigonometric polynomial the Diophantine condition on $\omega$ can be completely removed. This extends results previously available in the literature for $n=1$. We also point out that, if $n=1$ and the first derivative of $g$ at $c_0$ is positive, then the quasi-periodic solution is locally unique and attractive.