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数学物理学报, 2019, 39(3): 441-450 doi:

论文

一类次临界Bose-Einstein凝聚型方程组的渐近收敛行为和相位分离

张晶,

The Asymptotic Behaviors and Phase Separation for a Class of Subcritical Bose-Einstein Condensation System

Zhang Jing,

收稿日期: 2018-04-28  

基金资助: 国家自然科学基金(11326098)、哈尔滨师范大学博士科研启动基金(XKB201311)和2018年黑龙江省普通本科高等学校青年创新人才培养计划(UNPYSCT-2018177)

Received: 2018-04-28  

Fund supported: 国家自然科学基金(11326098)、哈尔滨师范大学博士科研启动基金(XKB201311)和2018年黑龙江省普通本科高等学校青年创新人才培养计划(UNPYSCT-2018177)

作者简介 About authors

张晶,zhjmath11@163.com , E-mail:zhjmath11@163.com

摘要

该文利用变分法和椭圆方程理论研究有界光滑区域上次临界Bose-Einstein凝聚型方程组耦合系数趋于负无穷时解的极限产生的相位分离现象.

关键词: Bose-Einstein凝聚型方程组 ; 次临界指数 ; 变分法 ; 相位分离

Abstract

In this paper, we study the phase separation phenomena of the limit profile as the coupling constant tending to minus infinity for some Bose-Einstein condensation system with subcritical exponent in a general smooth bounded domain via variational methods and elliptic equations theories.

Keywords: Bose-Einstein condensation system ; Subcritical exponent ; Variational methods ; Phase separation phenomena

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本文引用格式

张晶. 一类次临界Bose-Einstein凝聚型方程组的渐近收敛行为和相位分离. 数学物理学报[J], 2019, 39(3): 441-450 doi:

Zhang Jing. The Asymptotic Behaviors and Phase Separation for a Class of Subcritical Bose-Einstein Condensation System. Acta Mathematica Scientia[J], 2019, 39(3): 441-450 doi:

1 引言

薛定谔方程是量子力学中最重要的方程,最近十几年来,对于与物理学密切相关的非线性薛定谔方程解的存在性和解的各种性质的研究,一直是全世界数学和物理工作者关注的焦点和研究热点.

考虑下面次临界薛定谔方程组

{Δu1+α1u1=μ1|u1|p1u1+β|u1|p32u1|u2|p+12,xΩ,Δu2+α2u2=μ2|u2|p1u2+β|u1|p+12|u2|p32u2,xΩ,u1=u2=0,xΩ,
(1.1)

其中ΩRN是有界光滑区域, N3, αi,μi>0, i=1,2, β<0, 1<p<21, 2=2NN2是临界Sobolev指数.

近年来,此类方程组由于在非线性力学和Bose-Einstein凝聚等物理中有着非常重要的作用,引起了广大数学和物理工作者的极大关注.大量杰出的研究成果不断涌现.下面简单介绍一下已有的重要结果,然后给出该文的研究问题.

大家关心的是方程组(1.1)的非平凡解的存在性及其相关性质,参见文献[1-6].下面简单回顾几个重要的研究成果.当Ω=RN, N=2,3, αi>0, i=1,2,数学上关于方程组(1.1)最早结果由Lin和Wei[5]2005年给出了基态解存在性.除了基态解,在过去十多年中,许多知名数学家对于多解性也做了深入细致的研究.文献[2-4, 6-8]中得到了方程(1.1)无穷多个正解、径向对称解等存在性.另外,解的各种性质也是研究的焦点.文献[3, 6, 9]研究了方程组(1.1)的正解的唯一性,正解的先验估计,在β时解的极限的光滑性,正解的渐近收敛行为及其相应正解产生的相位分离现象等.

前面已经指出,关于方程组(1.1)还有很多大家非常关心的重要问题没有解决.该文主要研究一些与方程组(1.1)密切相关的重要问题.从上面问题介绍中可知,方程组(1.1)正解研究方面已经取得了丰富成果.当ΩRN是有界光滑区域, N3, αi,μi>0, i=1,2, β<0, 1<p<21时,参考文献[11]证明了无穷个变号解和半变号解的存在性,但是此时解的极限行为及其相位分离现象,却没有相应的研究结果.

该文中,定义Lp(Ω)的范数为|u|p=(Ω|u|pdx)1p, H10(Ω)的范数为

C表示常数(不同地方取值可以不一样).记H:= H_0^1(\Omega)\times H_0^1(\Omega),其上的范数为

\|(u_1, \, u_2)\|_{H}^2:=\|u_1\|_{\alpha_1}^2+\|u_2\|_{\alpha_2}^2,

其中

\|u_i\|_{\alpha_i}^2:=\int_{\Omega}(|\nabla u_i|^2+\alpha_i |u_i|^2)\, {\rm d}x, \quad i=1, \, 2.

2 非平凡解的渐近收敛行为

方程组(1.1)的解对应于C^2泛函I_\beta:\, H\rightarrow \mathbb{R} ,

\begin{eqnarray} \label{21} I_\beta(u_1, \, u_2) &:=& \frac{1}{2} (\| u_1\|_{\alpha_1}^2+\| u_2\|_{\alpha_2}^2)-\frac{1}{p+1}(\mu_1\, |u_1|_{p+1}^{p+1}+\mu_2\, |u_2|_{p+1}^{p+1})\\ &&+\frac{2|\beta|}{p+1} \int_{\Omega} |u_1|^{\frac{p+1}{2}}|u_2|^{\frac{p+1}{2}}\, {\rm d}x \end{eqnarray}
(2.1)

的临界点.由于该文只关心非平凡解,记\widetilde{H}:=\{(u_1, \, u_2)\in H:\, u_i\neq 0, \, i=1, \, 2 \}.

定义2.1  若u_1\not\equiv 0, \, u_2\not\equiv 0,则称解(u_1, \, u_2)是非平凡解;若(u_1, \, u_2)形如(u_1, \, 0)(0, \, u_2),则称解(u_1, \, u_2)是半平凡解;若在\Omegau_i>0对于i=1, \, 2都成立,则称解(u_1, \, u_2)是正解;若u_1u_2都是变号的,则称解(u_1, \, u_2)是变号解;若(u_1, \, u_2)的一个分量是正的而另一个分量变号,则称解(u_1, \, u_2)是半变号解;若变号解(u_1, \, u_2)的能量泛函在所有变号解的泛函能量中是最小的,则称变号解(u_1, \, u_2)是最小能量变号解.

由文献[11]可知,对任意\beta < 0,存在(u_{1, \beta}, \, u_{2, \beta})\in H使得(u_{1, \beta}, \, u_{2, \beta})或是方程组(1.1)变号解或是方程组(1.1)半变号解.这一节我们研究当\beta\rightarrow -\infty时,方程组(1.1)解(u_{1, \beta}, \, u_{2, \beta})的渐近收敛行为.这节的主要结果如下:

定理2.1  存在\textrm{Lipschitz}连续的(u_1, \, u_2)\in \widetilde{H},使得在子列的意义下满足:

(1)对任意的0 < r < 1,当\beta\rightarrow -\infty时,有u_{1, \beta}\rightarrow u_1, u_{2, \beta}\rightarrow u_2H_0^1(\Omega)\cap C^{0, r}(\overline{\Omega})中强收敛;

(2)在开集\{u_i\neq 0\}, i=1, \, 2上,

\begin{eqnarray}\label{22}-\Delta u_i+\alpha_i \, u_i=\mu_i\, u_i^p, \qquad i=1, \, 2;\end{eqnarray}
(2.2)

(3) u_1\cdot u_2\equiv 0,当\beta\rightarrow -\infty时,有

|\beta|\, \int_{\Omega} u_{1, \beta}^{\frac{p+1}{2}}\, u_{2, \beta}^{\frac{p+1}{2}}\, {\rm d}x\rightarrow 0 .

为了证明定理2.1,该文需要用文献[10]中介绍的向量指标定义恰当的极大极小值.考虑集合

{\cal F}=\{A\subset {\cal A}:\, A \hbox{是闭集}, \, \sigma_i(u_1, \, u_2)\in A, \, \forall (u_1, \, u_2)\in A, \, i=1, \, 2\} ,

其中{\cal A}:=\{(u_1, \, u_2)\in H:\, |u_1|_{p+1}=|u_2|_{p+1}=1 \},变换\sigma_i:\, {\cal A}\rightarrow {\cal A}, i=1, \, 2\sigma_1(u_1, \, u_2)=(-u_1, \, u_2), \sigma_2(u_1, \, u_2)=(u_1, \, -u_2).对任意的A\in {\cal F}k_1, \, k_2\in {\mathbb N},考虑集合

\begin{eqnarray*} F_{(k_1, \, k_2)}(A)&=&\Big\{f=(f_1, \, f_2):\, A\rightarrow \prod\limits_{i=1}^2 \mathbb{R} ^{k_i-1}:\, f_i:\, A\rightarrow \mathbb{R} ^{k_i-1}\hbox{连续}, \\&&\ \ f_i(\sigma_i(u_1, \, u_2))=-f_i(u_1, \, u_2), \, f_i(\sigma_j(u_1, \, u_2))=f_i(u_1, \, u_2), \, i\neq j, \, i, j=1, 2 \Big\}, \end{eqnarray*}

这里记\mathbb{R} ^0:=\{0\}.下面给出文献[10]中定义的向量指标.

定义2.2 (向量指标[10])  令A\in {\cal F}并任取k_1, \, k_2\in {\mathbb N}.若对任意的f\in F_{(k_1, \, k_2)}(A)都存在(u_1, \, u_2)\in A满足f(u_1, \, u_2)=(0, \, 0).则称A的向量指标\gamma(A)\geq (k_1, \, k_2).

\Gamma^{(k_1, \, k_2)}:=\{A\in {\cal F}:\, \gamma(A)\geq (k_1, \, k_2)\}.

引理2.1[10]  (1)设A_1\times A_2\subset {\cal A}, \eta_i:\, S^{k_i-1}\rightarrow A_i是同胚,对任意x\in S^{k_i-1}, \eta_i(-x)=-\eta_i(x), i=1, \, 2,则A_1\times A_2 \in \Gamma^{(k_1, \, k_2)},其中S^{k_i-1}=\{x\in \mathbb{R} ^{k_i}:\, |x|=1\}.

(2)若A \in\Gamma^{(k_1, \, k_2)},连续映射\eta:\, A\rightarrow {\cal A}满足\eta \circ \sigma_i=\sigma_i\circ \eta, \, i=1, \, 2.则有\overline{\eta(A)}\in \Gamma^{(k_1, \, k_2)}.

引理2.2  存在A\in \Gamma^{(k_1, \, k_2)}和正常数 d^{k_1, k_2}, k_1, \, k_2\geq 2,使得

\sup\limits_{(u_1, u_1)\in A} \sup\limits_{t, s\geq 0} I_\beta(tu_1, \, su_2)\leq d^{k_1, k_2},

其中d^{k_1, k_2}不依赖于\beta < 0的选取.

  任取非空开集\Omega_1, \, \Omega_2\subset \Omega满足\Omega_1\cap\Omega_2=\emptyset.定义

A_i:= \Big\{u_i\in \hbox{span} \{U^i_1, \, \cdots, U^i_{k_i}\}:\, U^i_j\in H_0^1(\Omega_i), \, U^i_j \, \hbox{线性无关, }\, 1\leq j\leq k_i, \, |u_i|_{p+1}=1 \Big\},

其中i=1, \, 2.显然存在从S^{k_i-1}A_i的奇同胚, i=1, \, 2.则由引理2.1(1)可知, A :=A_1\times A_2\in \Gamma^{(k_1, \, k_2)}.对于任意的(u_1, \, u_2)\in A,由于u_i\in H_0^1(\Omega_i),所以u_1\cdot u_2\equiv 0,则由于有限维线性空间的范数都是等价的,所以存在常数d_{k_i}>0使得\| u_i\|_{\alpha_i}^2\leq d_{k_i}\, |u_i|_{p+1}^2.由文献[11]可知存在常数T_1\leq T_2满足0 < T_1\leq t_{u_1, u_2, \beta}, \, s_{u_1, u_2, \beta}\leq T_2 < +\infty满足

\sup\limits_{t, \, s\geq 0}\, I_\beta (tu_1, \, su_2)=I_\beta (t_{u_1, u_2, \beta}u_1, \, s_{u_1, u_2, \beta}u_2).

\widehat{t}:=t_{u_1, u_2, \beta}, \, \widehat{s}:=s_{u_1, u_2, \beta}.因此,

\begin{eqnarray*} \sup\limits_{(u_1, u_1)\in A} \sup\limits_{t, s\geq 0} I_\beta(tu_1, \, su_2)&= &\Big(\frac{1}{2}-\frac{1}{p+1}\Big)\sup\limits_{(u_1, u_1)\in A}\, \Big(\widehat{t}^2\| u_1\|_{\alpha_1}^2+\widehat{s}^2\|u_2\|_{\alpha_2}^2\Big)\\ &\leq&\Big(\frac{1}{2}-\frac{1}{p+1}\Big) \sup\limits_{(u_1, u_1)\in A}\, \Big(\widehat{t}^2\, d_{k_1}\, |u_1|_{p+1}^2+\widehat{s}^2\, d_{k_2}\, |u_2|_{p+1}^2\Big) \\&\leq& d^{k_1, k_2} . \end{eqnarray*}

故存在与\beta < 0无关的常数d^{k_1, k_2}>0使得\sup\limits_{(u_1, u_1)\in A} \sup\limits_{t, s\geq 0} I_\beta(tu_1, \, su_2)\leq d^{k_1, k_2}对任意\beta < 0都成立.

对任意k_1, \, k_2\geq 10 < \delta < 2^{-\frac{1}{p+1}},定义

d_{\beta, \delta}^{k_1, k_2}:=\inf\limits_{A\in \Gamma_\beta^{(k_1, k_2)}}\sup\limits_{A\backslash {\cal P}_\delta}\sup _{t, s\geq 0}\, I_\beta(tu_1, \, su_2),

其中

\Gamma_\beta^{(k_1, \, k_2)}:=\{A\in \Gamma^{(k_1, \, k_2)} :\, \sup\limits_{(u_1, u_1)\in A} \sup\limits_{t, s\geq 0} I_\beta(tu_1, \, su_2) < d^{k_1, k_2}+1\},

正锥{\cal P}_i:=\{(u_1, \, u_2)\in H:\, u_i\geq 0\}, \, i=1, \, 2, {\cal P}:=\bigcup\limits_{i=1}^2\big({\cal P}_i\cup -{\cal P}_i \big),

{\cal P}_\delta:=\{(u_1, \, u_2)\in H:\, \mbox{dist}_{p+1}((u_1, \, u_2), \, {\cal P}) <\delta\},

\mbox{dist}_{p+1} ((u_1, \, u_2), \, {\cal P}):=\min \{\mbox{dist}_{p+1} (u_i, \, \pm{\cal P}_i) , \, \mbox{dist}_{p+1} (u_i, \, \pm{\cal P}_i)\}, \quad i=1, \, 2,

由于\mbox{dist}_{p+1} (u_i, \, \pm{\cal P}_i) :=\inf\limits_{\omega\in \pm{\cal P}_i} |u_i-\omega |_{p+1},且u_i=u_i^+-u_i^-,可知\mbox{dist}_{p+1} (u_i, \, \pm{\cal P}_i)=|u_i^\mp|_{p+1},其中u^\pm:=\max \{0, \, \pm u\}.引理2.2表明, \Gamma_\beta^{(k_1, \, k_2)}\neq \emptyset,所以\Gamma_\beta^{(k_1, \, k_2)}定义合理,并且d_{\beta, \delta}^{k_1, k_2}\leq d^{k_1, k_2}.

任意固定的k_1, \, k_2\in {\mathbb N}, k_1\geq 2, k_2\geq 1.由参考文献[11]可知,对任意\beta < 0,存在0 < \delta_\beta < 2^{-\frac{1}{p+1}}(u_{1, \beta}, \, u_{2, \beta})\in H使得(u_{1, \beta}, \, u_{2, \beta})或是方程组(1.1)变号解或是方程组(1.1)半变号解且满足

\begin{eqnarray}\label{23} I_\beta(u_{1, \beta}, \, u_{2, \beta})= d_{\beta, \delta_\beta}^{k_1, k_2}\leq d^{k_1, k_2} <+\infty. \end{eqnarray}
(2.3)

引理2.3  假设k_1, \, k_2\geq 2.则对于任意的0 < \delta < 2^{-\frac{1}{p+1}}和任意的A \in\Gamma^{(k_1, \, k_2)},有A\backslash{\cal P}_\delta \neq \emptyset.

  任取A \in\Gamma^{(k_1, \, k_2)},定义f=(f_1, \, f_2),

f_1(u_1, \, u_2)=\bigg(\int_{\Omega} |u_1|^pu_1\, {\rm d}x, \, 0, \, \cdots, \, 0\bigg), \quad f_2(u_1, \, u_2)=\bigg(\int_{\Omega} |u_2|^p u_2\, {\rm d}x, \, 0, \, \cdots, \, 0\bigg),

则有f_i:\, A\rightarrow \mathbb{R} ^{k_i-1}连续,

f_1(\sigma_1(u_1, \, u_2))=f_1(-u_1, \, u_2)=-\bigg(\int_{\Omega} |u_1|^pu_1\, {\rm d}x, \, 0, \, \cdots, \, 0\bigg)=-f_1(u_1, \, u_2),

f_1(\sigma_2(u_1, \, u_2))=f_1(u_1, \, -u_2)=\bigg(\int_{\Omega} |u_1|^pu_1\, {\rm d}x, \, 0, \, \cdots, \, 0\bigg)=f_1(u_1, \, u_2),

同理,

f_2(\sigma_1(u_1, \, u_2))=f_2(u_1, \, u_2), f_2(\sigma_2(u_1, \, u_2))=-f_2(u_1, \, u_2).

则由F_{(k_1, \, k_2)}(A)定义可知f\in F_{(k_1, \, k_2)}(A),所以存在(\widetilde{u}_1, \, \widetilde{u}_2)\in A使得f(\widetilde{u}_1, \, \widetilde{u}_2)=(0, \, 0).由于A\in {\cal A},则可推出

\int_{\Omega} (\widetilde{u}_1^+)^{p+1}\, {\rm d}x=\int_{\Omega} (\widetilde{u}_1^-)^{p+1}\, {\rm d}x=\frac{1}{2},

\int_{\Omega} (\widetilde{u}_2^+)^{p+1}\, {\rm d}x=\int_{\Omega} (\widetilde{u}_2^-)^{p+1}\, {\rm d}x=\frac{1}{2},

\mbox{dist} ((\widetilde{u}_1, \, \widetilde{u}_2), \, {\cal P})=2^{-\frac{1}{p+1}}.所以(\widetilde{u}_1, \, \widetilde{u}_2)\in A \backslash{\cal P}_\delta对任意的0 < \delta < 2^{-\frac{1}{p+1}}都成立.

定理2.1的证明  由于

I_\beta(u_{1, \beta}, \, u_{2, \beta})= \Big(\frac{1}{2}-\frac{1}{p+1}\Big)\, \Big(\| u_{1, \beta}\|_{\alpha_1}^2+\|u_{2, \beta}\|_{\alpha_2}^2\Big),

则由(2.3)式可知(u_{1, \beta}, \, u_{2, \beta})H中一致有界.因为H_0^1(\Omega)\hookrightarrow L^2(\Omega),则存在常数C>0使得

| u_{i, \beta}|_2\leq C, \qquad i=1, \, 2.

假设存在\delta>0,满足u_{i, \beta}\in L^{2+\delta}(\Omega),方程(1.1)两端乘以u_{i, \beta}\, |u_{i, \beta}|^\delta,然后积分,可知存在常数C>0满足

\begin{eqnarray*}&&\frac{1+\delta}{(1+\frac{\delta}{2})^2}\, \int_\Omega |\nabla |u_{i, \beta}|^{1+\frac{\delta}{2}} |^2\, {\rm d}x\\&\leq& \frac{1+\delta}{(1+\frac{\delta}{2})^2}\, \int_\Omega |\nabla |u_{i, \beta}|^{1+\frac{\delta}{2}} |^2\, {\rm d}x+|\beta|\, \int_\Omega|u_{i, \beta}|^{\frac{p-3}{2}}|u_{i, \beta}|^{2+\delta}|u_{j, \beta}|^{\frac{p+1}{2}}\, {\rm d}x\\&\leq &C\, \int_\Omega|u_{i, \beta}|^{2+\delta}\, {\rm d}x.\end{eqnarray*}

因此

\begin{eqnarray*}|u_{i, \beta}|_{2^*(1+\frac{\delta}{2})}&\leq&\Big(S^2\, \frac{(1+\frac{\delta}{2})^2}{1+\delta} \Big)^{\frac{1}{2+\delta}}\Big(C\, \int_\Omega|u_{i, \beta}|^{2+\delta}\, {\rm d}x\Big)^{\frac{1}{2+\delta}}\\&\leq& \Big(C\, \frac{(1+\frac{\delta}{2})^2}{1+\delta} \Big)^{\frac{1}{2+\delta}}|u_{i, \beta}|_{2+\delta}, \end{eqnarray*}

其中SH_0^1(\Omega)\hookrightarrow L^{2^*}(\Omega)的嵌入常数, i\neq j, \, i, \, j=1, \, 2.

\delta(1)=0, 2+\delta(m+1)=2^*\frac{2+\delta(m)}{2}.由于\delta(m)\geq (\frac{2^*}{2})^{m-1},则有\delta(m)\rightarrow +\infty.则通过标准的\textrm{Moser}迭代可知

\begin{eqnarray*}|u_{i, \beta}|_{2^*(1+\frac{\delta(m)}{2})}&\leq& \prod\limits_{m=1}^k\, \Big(C\, \frac{(1+\frac{\delta(m)}{2})^2}{1+\delta(m)} \Big)^{\frac{1}{2+\delta(m)}}|u_{i, \beta}|_{2}\\&\leq& {\rm e}^{\Big(\sum\limits_{j=1}^\infty \frac{1}{2+\delta(m)} \, \textrm{log}\, \Big(C\, \frac{(1+\frac{\delta(m)}{2})^2}{1+\delta(m)} \Big)\Big)}|u_{i, \beta}|_{2}, \end{eqnarray*}

由于\delta(m)\geq (\frac{2^*}{2})^{m-1},则有

\sum\limits_{j=1}^\infty \frac{1}{2+\delta(m)} \log \Big(C\, \frac{(1+\frac{\delta(m)}{2})^2}{1+\delta(m)} \Big) <+\infty.

由此可知u_{i, \beta}L^\infty(\Omega)中对任意\beta < 0i=1, \, 2都是一致有界的.则由椭圆正则性理论知道u_{i, \beta}\in C(\overline{\Omega})\cap C^2(\Omega),所以定理2.1可由文献[12]的定理1.1和定理1.2推出.虽然文献[12]的结果是关于正解的,通过分别处理非平凡解的正部和负部,文献[12]所用讨论都能用于没有符号限制的解成立.

3 相位分离

在这一节,我们研究当\beta\rightarrow -\infty时非平凡解的相位分离现象.这节的主要结果如下:

定理3.1  对于定理2.1中存在的(u_1, \, u_2)\in \widetilde{H},使得在子列的意义下满足:

(1)若k_1, \, k_2\geq 2,则有u_1u_2都是变号;

(2)若(k_1, \, k_2)=(2, \, 2),则有\{u_i\neq 0\}恰有两个连通分支,且u_i是方程

\begin{eqnarray}\label{31} -\Delta u+\alpha_i \, u=\mu_i\, u^p, \qquad u\in H_0^1(\{u_i\neq 0\}) \end{eqnarray}
(3.1)

的最小能量变号解, i=1, \, 2;

(3)若k_1\geq2k_2=1,则有u_1变号, \{u_1\neq 0\}至多有k_1个连通分支, u_2\{u_2\neq 0\}上是正的;

(4)若(k_1, \, k_2)=(2, \, 1),则有\{u_1\neq 0\}恰有两个连通分支, \{u_2\neq 0\}连通且u_i是方程(3.1)的最小能量变号解, i=1, \, 2.

   (1)首先考虑k_1, \, k_2\geq 2的情形.由于u_{i, \beta}\in C(\overline{\Omega})\cap C^2(\Omega)u_{i, \beta}是变号解,所以存在x^\pm_{i, \beta}\in \Omega满足

u_{i, \beta}(x^+_{i, \beta})=\max\limits_{x\in \Omega}u_{i, \beta}(x)>0\quad \mbox{且}\quad u_{i, \beta}(x^-_{i, \beta})=\min\limits_{x\in \Omega}u_{i, \beta}(x) <0, \quad i=1, \, 2.

则有\Delta u_{i, \beta}(x^+_{i, \beta})\leq 0\Delta u_{i, \beta}(x^-_{i, \beta})\geq 0.由于(u_{1, \beta}, \, u_{2, \beta})满足方程(1.1),对于i\neq j, i, j=1, \, 2,可得

\begin{eqnarray*}&& -\Delta u_{i, \beta}(x^+_{i, \beta})+\alpha_i\, u_{i, \beta}(x^+_{i, \beta})\\ &=&\mu_i\, |u_{i, \beta}(x^+_{i, \beta})|^{p-1}\, u_{i, \beta}(x^+_{i, \beta})+ \beta \, |u_{i, \beta}(x^+_{i, \beta})|^{\frac{p-3}{2}}u_{i, \beta}(x^+_{i, \beta}) |u_{j, \beta}(x^+_{i, \beta})|^{\frac{p+1}{2}}, \end{eqnarray*}

所以\alpha_i\, u_{i, \beta}(x^+_{i, \beta})\leq \mu_i\, u_{i, \beta}(x^+_{i, \beta})^{p},即

u_{i, \beta}(x^+_{i, \beta})\geq \Big(\frac{\alpha_i}{\mu_i}\Big)^{\frac{1}{p-1}}, \qquad \forall\quad \beta <0.

同理

\begin{eqnarray*}&& -\Delta u_{i, \beta}(x^-_{i, \beta})+\alpha_i\, u_{i, \beta}(x^-_{i, \beta})\\ &=&\mu_i\, |u_{i, \beta}(x^-_{i, \beta})|^{p-1}\, u_{i, \beta}(x^-_{i, \beta})+ \beta \, |u_{i, \beta}(x^-_{i, \beta})|^{\frac{p-3}{2}}u_{i, \beta}(x^-_{i, \beta}) |u_{j, \beta}(x^-_{i, \beta})|^{\frac{p+1}{2}}, \end{eqnarray*}

所以\alpha_i\, u_{i, \beta}(x^-_{i, \beta})\geq - \mu_i\, |u_{i, \beta}(x^-_{i, \beta})|^{p-1}u_{i, \beta}(x^-_{i, \beta}),即

u_{i, \beta}(x^-_{i, \beta})\leq -\Big(\frac{\alpha_i}{\mu_i}\Big)^{\frac{1}{p-1}}, \qquad \forall\quad \beta <0.

则由定理2.1(1)可知u_1u_2变号,从而\{u_i\neq 0\}至少有两个连通分支, i=1, \, 2.

(2)若(k_1, \, k_2)=(2, \, 2).假设\{u_1\neq 0\}至少有三个连通分支{\cal O}_1, {\cal O}_2{\cal O}_3.假设在{\cal O}_1\cup{\cal O}_2上有u_1>0,在{\cal O}_3上有u_1 < 0.定义

\omega^+:=u_1\, \chi_{{\cal O}_1}, \qquad \omega^-:=u_1\, \chi_{{\cal O}_2}, \qquad v:=u_1\, \chi_{{\cal O}_3},

其中

\chi_{\Omega}(x) := \left\{\begin{array}{ll} 1, \quad& x\in \Omega, \\ 0, &x\in \mathbb{R} ^N\backslash \Omega, \end{array}\right.

则有\omega^\pm, \, v\in H^1_0(\Omega)\backslash \{0\},且有

\begin{eqnarray}\label{32} \|u_1 \|_{\alpha_1}^2>\|\omega^+ \|_{\alpha_1}^2+\|\omega^-\|_{\alpha_1}^2. \end{eqnarray}
(3.2)

由于I_\beta'(u_1, \, u_2)(\omega^\pm, \, 0)=0I_\beta'(u_1, \, u_2)(0, \, u_{2}^\pm)=0,可得

\begin{equation}\label{33} \|\omega^\pm\|_{\alpha_1}^2=\mu_1\, |\omega^\pm|_{p+1}^{p+1}-|\beta|\, \int_{\Omega} |\omega^\pm|^{\frac{p+1}{2}}|u_2|^{\frac{p+1}{2}}\, {\rm d}x, \end{equation}
(3.3)

\begin{equation}\label{34} \|u_2^\pm\|_{\alpha_2}^2=\mu_2\, |u_2^\pm|_{p+1}^{p+1}-|\beta|\, \int_{\Omega} |u_1|^{\frac{p+1}{2}}|u_2^\pm|^{\frac{p+1}{2}}\, {\rm d}x. \end{equation}
(3.4)

定义集合

{\cal N}_\beta:=\{(u_1, u_2)\in H:\, u_1, \, u_2\mbox{都变号}, \, I_\beta'(u_1, \, u_2)(u_1^\pm, \, 0)= I_\beta'(u_1, \, u_2)(0, \, u_2^\pm)=0\},

定义d_\beta:=\inf\limits_{(u_1, u_2)\in {\cal N}_\beta}\, I_\beta(u_1, \, u_2),则d_\beta\leq d_{\beta, \delta_\beta}^{2, 2},若证 d_{\beta, \delta_\beta}^{2, 2}\leq d_\beta,可得d_\beta=d_{\beta, \delta_\beta}^{2, 2}.由(3.3)和(3.4)式可得(\omega^+-\omega^-, \, u_2)\in {\cal N}_\beta对任意\beta < 0都成立,则d_{\beta, \delta_\beta}^{2, 2}\leq I_\beta(\omega^+-\omega^-, \, u_2).下证 d_{\beta, \delta_\beta}^{2, 2}\leq d_\beta,事实上,选取任意的(u_0, v_0)\in {\cal N}_\beta满足I_\beta(u_0, \, v_0) < d^{2, 2}+1,定义

A_1:=\{u\in \mbox{span}\{u_0^+, \, u_0^- \}:\, |u|_{p+1}=1\},

A_2:=\{v\in \mbox{span}\{v_0^+, \, v_0^- \}:\, |v|_{p+1}=1\},

A:=A_1\times A_2\in \Gamma ^{(2, 2)}.对于任意的(u, \, v)\in A,存在l_i\in{\mathbb R}, i=1, 2, 3, 4,满足

u=l_1\, u_0^++l_2\, u_0^-, \qquad v=l_3\, v_0^++l_4\, v_0^-.

所以

\begin{eqnarray*}\sup\limits_{t, \, s\geq 0}\, I_\beta(tu, \, sv)&=& \sup\limits_{t, \, s\geq0}\, I_\beta\big(t(l_1\, u_0^++l_2\, u_0^-), \, s(l_3\, v_0^++l_4\, v_0^-)\big)\\&=&\sup\limits_{t, \, s\geq0}\, I_\beta\big(t\, |l_1|\, u_0^+-t\, |l_2|\, u_0^-, \, s\, |l_3|\, v_0^+-s\, |l_4|\, v_0^-\big)\\&\leq& I_\beta(u_0, \, v_0), \end{eqnarray*}

\sup\limits_{(u_1, u_2)\in A }\sup\limits_{t, \, s\geq 0}\, I_\beta(tu, \, sv)\leq I_\beta(u_0, \, v_0) <d^{2, 2}+1,

A\in \Gamma ^{(2, 2)}_\lambda

d^{2, 2}_{\beta, \delta_\beta}\leq \sup\limits_{A \setminus P_{\delta_\beta}} \sup\limits_{t, \, s\geq 0}\, I_\beta(tu, \, sv)\leq I_\beta(u_0, \, v_0),

因此d^{2, 2}_{\beta, \delta_\beta}\leq d_\beta.所以由(3.2)式可得

\begin{eqnarray}\label{35}\Big(\frac{1}{2}-\frac{1}{p+1}\Big)\, (\|u_1 \|_{\alpha_1}^2+\|u_2\|_{\alpha_2}^2)&=&\lim\limits_{\beta\rightarrow -\infty}\Big(\frac{1}{2}-\frac{1}{p+1}\Big)\, (\|u_{1, \beta} \|_{\alpha_1}^2+\|u_{2, \beta}\|_{\alpha_2}^2)\\&=&\lim\limits_{\beta\rightarrow -\infty}I_\beta(u_{1, \beta}, \, u_{2, \beta}) =\lim\limits_{\beta\rightarrow -\infty}\, d_{\beta, \delta_\beta}^{2, 2}\\&\leq& \lim\limits_{\beta\rightarrow -\infty}I_\beta(\omega^+-\omega^-, \, u_2)\\&=&\Big(\frac{1}{2}-\frac{1}{p+1}\Big)\, (\|\omega^+ \|_{\alpha_1}^2+\|\omega^- \|_{\alpha_1}^2+\|u_2\|_{\alpha_2}^2)\\&<&\Big(\frac{1}{2}-\frac{1}{p+1}\Big)\, (\|u_1\|_{\alpha_1}^2+\|u_2\|_{\alpha_2}^2).\end{eqnarray}
(3.5)

得到矛盾.所以\{u_i\neq 0\}恰有两个连通分支.

假设v_i\in H_0^1(\{u_i\neq 0\})是方程(3.1)的任意一个变号解, i=1, \, 2.则对任意的\beta < 0,有(v_1, \, u_2), \, (u_1, \, v_2)\in {\cal N}_\beta.于是,有

\begin{eqnarray*}\Big (\frac{1}{2}-\frac{1}{p+1}\Big)\, (\|u_1 \|_{\alpha_1}^2+\|u_2\|_{\alpha_2}^2)&=&\lim\limits_{\beta\rightarrow -\infty}I_\beta(u_{1, \beta}, \, u_{2, \beta})\\&\leq& \lim\limits_{\beta\rightarrow -\infty}I_\beta(v_1, \, u_2)\\&=&\Big(\frac{1}{2}-\frac{1}{p+1}\Big)\, (\|v_1 \|_{\alpha_1}^2+\|u_2\|_{\alpha_2}^2)\end{eqnarray*}

\begin{eqnarray*}\Big (\frac{1}{2}-\frac{1}{p+1}\Big)\, (\|u_1 \|_{\alpha_1}^2+\|u_2\|_{\alpha_2}^2)&=&\lim\limits_{\beta\rightarrow -\infty}I_\beta(u_{1, \beta}, \, u_{2, \beta})\\ &\leq &\lim\limits_{\beta\rightarrow -\infty}I_\beta(u_1, \, v_2)\\ &=&\Big(\frac{1}{2}-\frac{1}{p+1}\Big)\, (\|u_1 \|_{\alpha_1}^2+\|v_2\|_{\alpha_2}^2), \end{eqnarray*}

\|u_i\|_{\alpha_i}^2\leq \|v_i\|_{\alpha_i}^2, i=1, \, 2,所以(u_1, \, u_2)是方程(3.1)的最小能量变号解.

(3)若k_1\geq 2k_2=1.由参考文献[11]可知, u_{1, \beta}变号, u_{2, \beta}是正的,所以在\{u_2\neq 0\}u_2>0.假设\{u_1\neq 0\}至少有k_1+1个连通分支.定义

\begin{eqnarray*} {\cal M}_\beta:&=&\Big\{(u_1, u_2)\in H:\, u_1\mbox{变号}, \, \{u_1\neq 0\} \mbox{至少有} k_1+1 \mbox{个连通分支} {\cal O}_k, \, u_2\neq0, \, u_2\geq 0, \\&&\ \ I_\beta'(u_1, \, u_2)(u_1\, \chi_{{\cal O}_k}, \, 0)=I_\beta'(u_1, \, u_2)(0, \, u_2)=0, \, \forall 1\leq k\leq k_1 \Big\}.\end{eqnarray*}

假设(u_1, u_2)\in {\cal M}_\beta,则\{u_1\neq 0\}至少有k_1+1个连通分支{\cal O}_k\, (1\leq k\leq k_1+1).对于 1\leq k\leq k_1, u_1\, \chi_{{\cal O}_k}\in H^1_0(\Omega),由I_\beta'(u_1, \, u_2)(u_1\, \chi_{{\cal O}_k}, \, 0)=0, \, I_\beta'(u_1, \, u_2)(0, \, u_2)=0可知,有

\|u_1\, \chi_{{\cal O}_k}\|_{\alpha_1}^2=\mu_1\, |u_1\, \chi_{{\cal O}_k}|_{p+1}^{p+1}-|\beta|\, \int_{\Omega} |u_1\, \chi_{{\cal O}_k}|^{\frac{p+1}{2}}|u_2|^{\frac{p+1}{2}}\, {\rm d}x, \quad 1\leq k\leq k_1

\|u_2\|_{\alpha_2}^2=\mu_2\, |u_2|_{p+1}^{p+1}-|\beta|\, \int_{\Omega} |u_1|^{\frac{p+1}{2}}|u_2|^{\frac{p+1}{2}}\, {\rm d}x.

则有

\begin{eqnarray*}&&\int_{\Omega}\bigg|\sum\limits_{k=1}^{k_1} t_k \, u_1\, \chi_{{\cal O}_k}\bigg|^{\frac{p+1}{2}}|s\, u_2 |^{\frac{p+1}{2}}\, {\rm d}x\\&\leq& \frac{1}{2}\sum\limits_{k=1}^{k_1} t_k^{p+1}\int_{\Omega} |u_1\, \chi_{{\cal O}_k}|^{\frac{p+1}{2}}|u_2|^{\frac{p+1}{2}}\, {\rm d}x+ \frac{1}{2} s^{p+1}\int_{\Omega} |u_1|^{\frac{p+1}{2}}|u_2|^{\frac{p+1}{2}}\, {\rm d}x\\&=&\frac{1}{2|\beta|}\bigg(\mu_1\, \sum\limits_{k=1}^{k_1} t_k^{p+1}|u_1\, \chi_{{\cal O}_k}|_{p+1}^{p+1}+\mu_2\, s^{p+1} |u_2|_{p+1}^{p+1}\bigg)\\&& -\frac{1}{2|\beta|}\bigg( \sum\limits_{k=1}^{k_1} t_k^{p+1}\|v_n\, \chi_{{\cal O}_k} \|_{\alpha_1}^2+s^{p+1} \|u_2\|_{\alpha_2}^2\bigg).\end{eqnarray*}

由于u_2\geq 0,所以对于t_1, \, \cdots, t_{k_1}, \, s\geq 0,我们有

\begin{eqnarray}\label{36} I_\beta\bigg(\sum\limits_{k=1}^{k_1} t_k \, u_1\, \chi_{{\cal O}_k}, \, s\, u_2\bigg)&=&\frac{1}{2}\sum\limits_{k=1}^{k_1} t_k^2\|u_1\, \chi_{{\cal O}_k}\|_{\alpha_1}^2+ \frac{1}{2}s^{2}\|u_2\|_{\alpha_2}^2-\frac{\mu_1}{p+1}\sum\limits_{k=1}^{k_1} t_k^{p+1}|u_1\, \chi_{{\cal O}_k}|^{p+1}_{p+1}\\&& -\frac{\mu_2}{p+1} s ^{p+1}|u_2|^{p+1}_{p+1} + \frac{2|\beta|}{p+1} \int_{\Omega} \bigg|\sum\limits_{k=1}^{k_1} t_k \, u_1\, \chi_{{\cal O}_k}\bigg|^{\frac{p+1}{2}}|s\, u_2|^{\frac{p+1}{2}}\, {\rm d}x\\&\leq& \sum\limits_{k=1}^{k_1} \bigg(\frac{1}{2}t_k^2-\frac{1}{p+1}t_k^{p+1}\bigg) \|u_1\, \chi_{{\cal O}_k}\|_{\alpha_1}^2+ \bigg(\frac{1}{2}s^2-\frac{1}{p+1}s^{p+1}\bigg) \|u_2\|_{\alpha_2}^2\\&\leq&\bigg(\frac{1}{2}-\frac{1}{p+1}\bigg)\bigg(\sum\limits_{k=1}^{k_1}\|u_1\, \chi_{{\cal O}_k}\|_{\alpha_1}^2+ \|u_2\|_{\alpha_2}^2\bigg).\end{eqnarray}
(3.6)

再定义集合

A_1:=\Big\{u\in \mbox{span}\{u_1\, \chi_{{\cal O}_1}, \, \cdots, u_1\, \chi_{{\cal O}_{k_1}}\}:\, |u|_{p+1}=1\Big\},

A_2:=\bigg\{\frac{u_2}{|u_2|_{p+1}}\bigg\},

则由引理2.1(1)可知A:=A_1\times A_2\in \Gamma^{(k_1, \, 1)}.于是由(3.6)式可知

\begin{eqnarray*} \sup\limits_{A}\sup _{t, s\geq 0}\, I_\beta(tu_1, \, su_2)&\leq& \bigg(\frac{1}{2}-\frac{1}{p+1}\bigg)\bigg( \sum\limits_{k=1}^{k_1}\|u_1\, \chi_{{\cal O}_k}\|_{\alpha_1}^2+ \|u_2\|_{\alpha_2}^2\bigg)\\&< &\bigg(\frac{1}{2}-\frac{1}{p+1}\bigg)(\|u_1\|_{\alpha_1}^2+\|u_2\|_{\alpha_2}^2)\\& =&I_\beta(u_1, \, u_2)=d_{\beta, \delta_n}^{k_1, 1}\leq d^{k_1, 1}, \end{eqnarray*}

所以A\in \Gamma_\beta^{(k_1, 1)}.于是

\begin{eqnarray*} d_{\beta, \delta_n}^{k_1, 1}&\leq& \sup\limits_{A\setminus P_{\delta_n}}\sup _{t, s\geq 0}\, I_\beta(tu_1, \, su_2)\\&\leq& \bigg(\frac{1}{2}-\frac{1}{p+1}\bigg)\bigg(\sum\limits_{k=1}^{k_1}\|u_1\, \chi_{{\cal O}_k}\|_{\alpha_1}^2+\|u_2\|_{\alpha_2}^2\bigg) <I_\beta(u_1, \, u_2).\end{eqnarray*}

矛盾.所以, \{u_1\neq 0\}至多有k_1个连通分支.

(4)若(k_1, \, k_2)=(2, \, 1).\{u_1\neq 0\}恰有两个连通分支.假设\{u_2\neq 0\}至少有两个连通分支{\cal O}_1{\cal O}_2,则对任意的\beta < 0, (u_1, \, u_2\chi_{{\cal O}_1})\in {\cal M}_\beta.于是和(3.5)式类似讨论得到矛盾,所以\{u_2\neq 0\}连通.最后,和上面类似的讨论我们可证u_1是方程(3.1)当i=1的最小能量变号解,而u_2是方程(3.1)当i=2的最小能量变号解.

参考文献

Ambrosetti A , Colorado E .

Bound and ground states of coupled nonlinear Schrödinger equations

C R Math Acad Sci Paris, 2006, 342 (7): 453- 458

DOI:10.1016/j.crma.2006.01.024      [本文引用: 1]

Bartsch T , Dancer N , Wang Z .

A Liouville theorem, a priori bounds, and bifurcating branches of positive solutions for a nonlinear elliptic system

Calc Var PDE, 2010, 37 (3/4): 345- 361

URL     [本文引用: 1]

Dancer N , Wei J , Weth T .

A priori bounds versus multiple existence of positive solutions for a nonlinear Schrödinger system

Ann Inst H Poincaré Anal Non Linéaire, 2010, 27 (3): 953- 969

DOI:10.1016/j.anihpc.2010.01.009      [本文引用: 1]

Liu Z , Wang Z .

Multiple bound states of nonlinear Schrödinger systems

Comm Math Phys, 2008, 282 (3): 721- 731

DOI:10.1007/s00220-008-0546-x      [本文引用: 1]

Lin T , Wei J .

Ground state of N coupled nonlinear Schrödinger equations in \mathbb{R} ^n, n \leq 3

Comm Math Phys, 2005, 255 (3): 629- 653

DOI:10.1007/s00220-005-1313-x      [本文引用: 1]

Wei J , Weth T .

Radial solutions and phase separation in a system of two coupled Schrödinger equations

Arch Ration Mech Anal, 2008, 190 (1): 83- 106

DOI:10.1007/s00205-008-0121-9      [本文引用: 3]

Sato Y , Wang Z .

On the multiple existence of semi-positive solutions for a nonlinear Schrödinger system

Ann Inst H Poincaré Anal Non Linéaire, 2013, 30 (1): 1- 22

DOI:10.1016/j.anihpc.2012.05.002     

Maia L , Montefusco E , Pellacci B .

Infinitely many nodal solutions for a weakly coupled nonlinear Schrödinger system

Comm Comtemp Math, 2008, 10 (5): 651- 669

DOI:10.1142/S0219199708002934      [本文引用: 1]

Wei J , Yao W .

Uniqueness of positive solutions to some coupled nonlinear Schrödinger equations

Comm Pure Appl Anal, 2012, 11 (3): 1003- 1011

[本文引用: 1]

Tavares H , Terracini S .

Sign-changing solutions of competition diffusion elliptic systems and optimal partition problems

Ann I H Poincaré AN, 2012, 29: 279- 300

DOI:10.1016/j.anihpc.2011.10.006      [本文引用: 4]

Zhang J, Zou W M. Infinitely many sign-changing solutions for a coupled Schrödinger system with subcritical exponent. Submitted

[本文引用: 5]

Noris B , Tavares H , Terracini S , et al.

Uniform Hölder bounds for nonlinear Schrödinger systems with strong competition

Comm Pure Appl Math, 2010, 63 (3): 267- 302

DOI:10.1002/cpa.v63:3      [本文引用: 3]

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