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We study the dynamics of solutions of nonlinear Schrödinger equation near unstable ground states. The existence of the local center stable manifold around ground states and the asymptotic stability for the solutions on the manifold is proved. The novelty of our result is that we allow the existence of internal modes.

In this paper, we construct center stable manifolds of unstable line solitary waves for the Zakharov--Kuznetsov equation on $\mathbb{R}\times \mathbb{T}_L$ and show the orbital stability of the unstable line solitary waves on the center stable manifolds, which yields the asymptotic stability of unstable solitary waves on the center stable manifolds near by stable line solitary waves. The construction is based on the graph transform approach by Nakanishi and Schlag. Applying the bilinear estimate on Fourier restriction spaces by Molinet and Pilod and modifying the mobile distance by Nakanishi and Schlag, we construct a contraction map on the graph space.

In this paper, we consider the stability for line solitary waves of the two dimensional Zakharov-Kuznetsov equation on $\mathbb{R}\times\mathbb{T}_L$ which is one of a high dimensional generalization of Korteweg-de Vries equation , where $\mathbb{T}_L$ is the torus with the period $2\pi L$. The orbital and asymptotic stability of the one soliton of Korteweg-de Vries equation on the energy space has been proved by Benjamin, Pego and Weinstein and Martel and Merle. We regard the one soliton of Korteweg-de Vries equation as a line solitary wave of Zakharov-Kuznetsov equation on $\mathbb{R}\times\mathbb{T}_L$. We prove the stability and the transverse instability of the line solitary waves of Zakharov-Kuznetsov equation by applying Evans' function method and the argument of Rousset and Tzvetkov. Moreover, we prove the asymptotic stability for the orbitally stable line solitary wave of Zakharov-Kuznetsov equation by using the argument of Martel and Merle, a Liouville type theorem and a corrected virial type estimate.

In this paper we consider the transverse instability for a nonlinear Schr��dinger equation with a linear potential on $\mathbb{R} \times \mathbb{T}_L$, where $2��L$ is the period of the torus $\mathbb{T}_L$. Rose and Weinstein showed the existence of a stable standing wave for a nonlinear Schr��dinger equation with a linear potential. We regard the standing wave of nonlinear Schr��dinger equation on $\mathbb{R}$ as a line standing wave of nonlinear Schr��dinger equation on $\mathbb{R} \times \mathbb{T}_L$. We show the stability of line standing waves for all $L>0$.