KAM Tori for a Two Dimensional Beam Equation with a Quintic Nonlinear Term and Quasi-periodic Forcing

Springer Science and Business Media LLC - Tập 21 - Trang 1-43 - 2022
Min Zhang1, Jie Rui1, Yan Li1, Jian Zhang1
1College of Science, China University of Petroleum (East China), Qingdao, People’s Republic of China

Tóm tắt

This work studies a two-dimensional beam equation with a quintic nonlinear term and quasi-periodic forcing $$\begin{aligned} u_{tt}+\Delta ^2 u+ \varepsilon \phi (t)h(u)=0,\quad x\in {\mathbb {T}}^2,\quad t\in {\mathbb {R}} \end{aligned}$$ with periodic boundary conditions, where $$\varepsilon $$ is a small positive parameter; $$\phi (t)$$ is a real analytic quasi-periodic function in t with frequency vector $$\eta =(\eta _1 ,\eta _2 \ldots ,\eta _{n^*})\subset [\varrho , 2\varrho ]^{n^*}$$ for a given positive integer $$n^*$$ and some constant $$\varrho >0$$ ; and h is a real analytic function of the form $$\begin{aligned} h(u)=c_1u+c_5u^5+\sum _{{\hat{i}}\ge 6}c_{{\hat{i}}}u^{{\hat{i}}},\quad c_1,c_5\ne 0. \end{aligned}$$ Firstly, the linear part of Hamiltonian system corresponding to the equation is transformed to constant coefficients by a linear quasi-periodic change of variables. Then, a symplectic transformation is used to convert the Hamiltonian system into an angle-dependent block-diagonal normal form, which can be achieved by selecting the appropriate tangential sites. Finally, it is obtained that a Whitney smooth family of small-amplitude quasi-periodic solutions for the equation by developing an abstract KAM (Kolmogorov–Arnold–Moser) theorem for infinite dimensional Hamiltonian systems.

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