Published online by Cambridge University Press: 09 March 2020
In this paper we consider the following nonlinear quasi-periodic system:$$\begin{eqnarray}{\dot{x}}=(A+\unicode[STIX]{x1D716}P(t,\unicode[STIX]{x1D716}))x+\unicode[STIX]{x1D716}g(t,\unicode[STIX]{x1D716})+h(x,t,\unicode[STIX]{x1D716}),\quad x\in \mathbb{R}^{d},\end{eqnarray}$$
$A$ is a
$d\times d$ constant matrix of elliptic type,
$\unicode[STIX]{x1D716}g(t,\unicode[STIX]{x1D716})$ is a small perturbation with
$\unicode[STIX]{x1D716}$ as a small parameter,
$h(x,t,\unicode[STIX]{x1D716})=O(x^{2})$ as
$x\rightarrow 0$, and
$P,g$ and
$h$ are all analytic quasi-periodic in
$t$ with basic frequencies
$\unicode[STIX]{x1D714}=(1,\unicode[STIX]{x1D6FC})$, where
$\unicode[STIX]{x1D6FC}$ is irrational. It is proved that for most sufficiently small
$\unicode[STIX]{x1D716}$, the system is reducible to the following form:
$$\begin{eqnarray}{\dot{x}}=(A+B_{\ast }(t))x+h_{\ast }(x,t,\unicode[STIX]{x1D716}),\quad x\in \mathbb{R}^{d},\end{eqnarray}$$
$h_{\ast }(x,t,\unicode[STIX]{x1D716})=O(x^{2})~(x\rightarrow 0)$ is a high-order term. Therefore, the system has a quasi-periodic solution with basic frequencies
$\unicode[STIX]{x1D714}=(1,\unicode[STIX]{x1D6FC})$, such that it goes to zero when
$\unicode[STIX]{x1D716}$ does.