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Quasi-periodic solutions for the fourth order nonlinear Schr$\dd...
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四阶非线性薛定谔方程的拟周期解,高美娜,刘建军,本文研究周期边条件的立方非线性项的四阶薛定谔方程:$$\mathbf{i}u_t=\partial_x^4u\pm|u|^2u,\hspace{12pt}x\in\mathbb{T}:=\mathbb{R}/2\pi\mathbb{Z}.$$得到小
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˖ڍመڙጲ
http://www.paper.edu.cn
四阶非线性薛定谔方程的拟周期解
高美娜
1
,刘建军
2
1
上海第二工业大学文理学部,上海 201209
2
四川大学数学学院,成都 610065
摘要:本文研究周期边条件的立方非线性项的四阶薛定谔方程:
iu
t
= ∂
4
x
u ± |u|
2
u, x ∈ T := R/2πZ.
得到小振幅拟周期解构成的不变康托流形。
关键词:基础数学,薛定谔方程,拟周期解
中图分类号: O175.14, O175.29
Quasi-periodic solutions for the fourth order
nonlinear Schr
¨
odinger equation
GAO Meina
1
, LIU Jianjun
2
1
College of Arts and Sciences, Shanghai Polytechnic University, Shanghai 201209
2
School of Mathematics, Sichuan University, Chengdu 610065
Abstract: This paper is concerned with the cubic fourth order nonlinear Schr
¨
odinger
equation with periodic boundary conditions
iu
t
= ∂
4
x
u ± |u|
2
u, x ∈ T := R/2πZ.
We show the above equation possesses Cantor families of quasi-periodic solutions of small
amplitude.
Key words: Fundamental Mathematics, Schr
¨
odinger Equation, Quasi-periodic Solution
0 Introduction
This paper is concerned with the cubic fourth order nonlinear Schr
¨
odinger equation with
periodic boundary conditions
iu
t
= ∂
4
x
u ± |u|
2
u, (t, x) ∈ T × R, (1)
where u is a complex-valued function on T × R. The equation is also called the biharmonic
NLS and it was studied in [1, 2] in the context of stability of solitons in magnetic materials.
Foundations: National Research Foundation for the Doctoral Program of Higher Education of China (20130181120104)
Author Introduction: GAO Meina(1983-),female,associate professor,major research direction:dynamical system,
email:mngao@sspu.edu.cn. Correspondence author:LIU Jianjun (1983-),male,professor,major research direction:
dynamical system,email:071018018@fudan.edu.cn.
- 1 -
˖ڍመڙጲ
http://www.paper.edu.cn
Also see [3, 4, 5, 6, 7, 8] for the references therein. This equation is a Hamiltonian PDE with
the following Hamiltonian:
H(u) =
1
2
T
|∂
2
x
u|
2
dx ±
1
4
T
|u|
4
dx. (2)
Our aim is to construct many small amplitude solutions of (1) that are quasi-periodic in time.
For simplicity, the solutions are restricted to be odd in x, i.e.,
u(t, −x) = −u(t, x). (3)
1 Main result
In first approximation the high order term ±|u|
2
u may be considered as a small pertur-
bation of the linear equation iu
t
− ∂
4
x
u = 0. The latter is of course well understood and has a
plenty of quasi-periodic solutions. To be more precise, let
ϕ
j
(x) =
1
π
sin jx, λ
j
= j
4
, j = 1, 2, · · ·
be the basic modes and frequencies. Then every solution is the superposition of their harmonic
oscillations and of the form
u(t, x) =
j≥1
q
j
(t)ϕ
j
(x), q
j
(t) = I
j
e
iλ
j
t
.
Their combined motions are periodic, quasi-periodic or almost-periodic, respectively, depending
on whether one, finitely many or infinitely many modes are excited. In particular, for every
choice
J = {j
1
< · · · < j
n
} ⊂ N
of n ≥ 1 basic modes there is an invariant 2n-dimensional linear subspace E
J
that is foliated
into rotational tori with frequencies λ
j
1
, · · · , λ
j
n
:
E
J
= {u = q
1
ϕ
j
1
+ · · · + q
n
ϕ
j
n
: q ∈ C
n
} =
I∈P
n
T
J
(I),
where P
n
= {I : I
j
> 0, 1 ≤ j ≤ n} is the positive quadrant in R
n
and
T
J
(I) = {u = q
1
ϕ
j
1
+ · · · + q
n
ϕ
j
n
: |q
j
|
2
= I
j
, 1 ≤ j ≤ n}.
Upon restoring the nonlinearity ±|u|
2
u, E
J
with their quasi-periodic solutions will not persist
in their entirety. However, there does persist a Cantor subfamily of rotational n-torus which
are only slightly deformed. More exactly, we have the following theorem:
- 2 -
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