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数学物理学报, 2019, 39(6): 1314-1322 doi:

论文

R2上对偶Minkowski问题的可解性

魏娜,

The Solvability of Dual Minkowski Problem in R2

Wei Na,

收稿日期: 2019-05-24  

基金资助: 中央高校基本科研业务费专项资金.  2722019PY053
湖北省自然科学基金.  2019CFB570

Received: 2019-05-24  

Fund supported: the Fundamental Research Funds for the Central Universities.  2722019PY053
the Natural Science Foundation of Hubei Province.  2019CFB570

作者简介 About authors

魏娜,E-mail:weina@zuel.edu.cn , E-mail:weina@zuel.edu.cn

摘要

该文研究Sobolev空间W1,4 (S)中一类约束变分问题存在极小可达元.在Sg(θ)dθ>0条件下,该极小可达元是相应Euler-Lagrange方程

u+u=g(θ)u(u2+u2),θS

的严格正解.基于此,该文在R2上证明了文献[Huang-Lutwak-Yang-Zhang.Acta Math,2016,216(2):325-338]提出的对偶Minkowski问题的可解性.

关键词: 对偶Minkowski问题 ; 非线性方程 ; 变分方法

Abstract

In this paper, we study the existence of minimum of a constrained variational problem in the Sobolev space W1, 4(S). If ∫Sg(θ)dθ>0, the minimum is a positive solution to the related Euler-Lagrange equation

u+u=g(θ)u(u2+u2),θS

Based on this, we prove the solvability of the dual Minkowski problem in R2 posed by Huang-Lutwak-Yang-Zhang[Acta Math, 2016, 216(2):325-338].

Keywords: Dual Minkowski problem ; Nonlinear equation ; Variational method

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本文引用格式

魏娜. R2上对偶Minkowski问题的可解性. 数学物理学报[J], 2019, 39(6): 1314-1322 doi:

Wei Na. The Solvability of Dual Minkowski Problem in R2. Acta Mathematica Scientia[J], 2019, 39(6): 1314-1322 doi:

1 引言及主要结果

kR和球面SN1上的有限Borel测度μ.文献[12]等提出的对偶Minkowski问题是研究使μ能够生成RN上某一凸体Kk -阶对偶曲率测度˜Ck(K,)的充分必要条件.与Lp-Minkowski问题[10-11, 16]类似,对偶Minkowski问题被认为是经典Minkowski问题[17-18]的重要推广.在分析上, Minkowski类问题的研究与非线性微分方程联系紧密,相关重要工作见文献[5-6, 8-9, 12, 19-20, 22].实际上, Minkowski问题的可解性等价于求解非线性微分方程

det(uij+ei,ju)=g(x)f(u),xSN1,
(1.1)

其中u:SN1(0,+)表示凸体K的支撑函数, ei,jSN1上的标准黎曼度量, gSN1上给定的有限Borel测度μ的密度函数.当f(u)1时方程(1.1)的可解性等价于经典的Minkowski问题[1-2, 17-18];当f(u)=u1p时方程(1.1)的可解性等价于Lp-Minkowski问题[10].对于

f(u)=u1(u2+|u|2)nk2,
(1.2)

其中kR, 表示SN1标准坐标架下的梯度算子,方程(1.1)的可解性等价于RN上对偶Minkowski问题[12].特别地,当k=N时,对偶Minkowski问题就是log-Minkowski问题(参见文献[3-4, 24]).

k<0时, RN上对偶Minkowski问题已被完全解决,见文献[15, 23].当k>0时,对偶Minkowski问题的可解性相对较困难,需要新的估计和技巧.对于1kN和关于原点对称的Borel正测度μ,黄勇等在文献[12]中研究了对偶Minkowski问题的存在性.对于R2上的对偶Minkowski问题

u
(1.3)

k=2时,问题(1.3)的可解性等价于\mathbb{R} ^2上的Log-Minkowski问题.文献[8, 13]等给出了解的存在性方面的研究成果.当k>1时,有关问题(1.3)解的存在性研究也有一些新的工作.文献[14]利用不动点定理结合椭圆方程估计得到了k>1情形对偶Minkowski问题可解性的充分条件;针对k>0和严格正的连续函数g,文献[7]利用连续性方法证明了对偶Minkowski问题解的存在性.这些参考文献在g为严格正的连续函数条件下研究对偶Minkowski问题C^2光滑正解的存在性.

本文在g为非负可积函数的条件下研究k=4\mathbb{R} ^2上对偶Minkowski问题(1.3),即非线性方程

\begin{eqnarray}\label{eq:2}u''(\theta)+u(\theta)=\frac{g(\theta)}{u(u^2+u'^2)}, \ \theta\in{\Bbb S}\end{eqnarray}
(1.4)

的可解性.我们在条件

(g_1): g(\theta)\in L^1(S)

下应用约束变分方法来研究方程(1.4).为此,假设g满足如下条件

{\rm(}g_2{\rm)}: g(\theta)=g(\theta+{\pi}/{m}), m\in{\Bbb N};\ \ {\rm(}g_3{\rm)}: \int_0^{\pi/m}g(\theta){\rm d}\theta>0.

m\in{\Bbb N},考虑Banach空间

W^{1, 4}({\Bbb S})=\bigg\{u\in L^4({\Bbb S}):\int_{{\Bbb S}}u'^4{\rm d}\theta<+\infty\bigg\}

W^{1, 4}_m=\{u\in W^{1, 4}({\Bbb S}):u(\theta)=u(\theta+2\pi/m), \ \mbox{对所有的}\ \theta\in{\Bbb S}\},

其范数为

\|u\|=\left\{\int_0^{\frac{\pi}{m}}u^2+u'^2{\rm d}\theta\right\}^{1/2}.

定义W^{1, 4}_m上的正锥

\begin{eqnarray}\label{eq:G} G_m=\{u\in W^{1, 4}_m:u(\theta)>0, \theta\in{\Bbb S}\}. \end{eqnarray}
(1.5)

G_m上赋予由W^{1, 4}_m范数诱导的距离.假设(g_1)-(g_3),在G_m上定义与问题(1.3)相关的泛函

\begin{eqnarray}\label{eq:f2} J(u)=\int_0^{\frac{\pi}{m}}u^4-\frac{1}{3}u'^4-2u'^2u^2{\rm d}\theta \end{eqnarray}
(1.6)

\begin{eqnarray}\label{eq:f1} I(u)=\int_0^{\frac{\pi}{m}}g(\theta)\ln (u){\rm d}\theta. \end{eqnarray}
(1.7)

应用文献[21]中的标准化过程可证明泛函IJG_m上为C^1泛函.本文通过研究约束变分问题

\begin{eqnarray}\label{eq:min}I_0=\inf\limits_{u\in G_m, J(u)=I(e)}I(u)\end{eqnarray}
(1.8)

极小可达元的存在性来证明\mathbb{R} ^{2}上对偶Minkowski问题(1.4)的可解性.

本文主要结论如下.

定理1.1  假设(g_1)-(g_3)成立.如果m\geq 2,则变分问题(1.8)存在极小可达元w\in G_m,且w是问题(1.4)的一个严格正的弱解,即

\begin{eqnarray}\label{eq:df}\int_0^{\frac{\pi}{m}}w^3\varphi -\frac{1}{3} w'^3\varphi'-w^2w'\varphi'-w'^2w\varphi {\rm d}\theta= \int_0^{\frac{\pi}{m} }\frac{g(\theta)}{w}\varphi {\rm d}\theta, \ \varphi\in W^{1, 4}_m. \end{eqnarray}
(1.9)

如果w二阶可导,则有

\begin{eqnarray}\label{eq:ws}\int_0^{\frac{\pi}{m}}(w'^2+w^2)(w''+w)\varphi {\rm d}\theta=\int_0^{\frac{\pi}{m}}\frac{g(\theta)}{w}\varphi {\rm d}\theta, \varphi\in W^{1, 4}_m .\end{eqnarray}
(1.10)

等式(1.10)表明了w{\Bbb S}上几乎处处满足方程(1.4),由(1.9)式定义问题(1.4)的弱解是合理的.这里我们假设测度\mu的密度函数g为非负可积函数,没有假设g为严格正的连续函数,适用范围更广.针对\mathbb{R} ^{2}上具有非负可积密度函数的测度,定理1.1给出了对偶Minkowski问题(1.4)可解性的一个充分条件.

2 主要结果的证明

对任意的m\in{\Bbb N},考虑约束变分问题(1.8).我们有序列\{u_n\}\in G_m使得

\lim\limits_{n\rightarrow+\infty}I(u_n)=I_0=\inf\limits_{u\in G_m, J(u)=I(e)}I(u) \ \mbox{以及}\ J(u_n)\equiv\int_0^{\frac{\pi}{m}}g(\theta){\rm d}\theta=I(e).

u逼近0时,泛函I(u)趋于负无穷且具有奇性.证明I_0>-\infty以及\{u_n\}W^{1, 4}_m({\Bbb S})中有界并不显然.这是本文研究问题(1.8)存在极小可达元的关键.为研究极小可达元的逼近序列\{u_n\}的紧性,本文先给出周期函数震荡幅度的估计.

引理2.1  取m\in{\Bbb N}, \{w_n\}\subset C^1({\Bbb S})\cap W^{1, 4}_m.w_n(\beta_n)=\sup\limits_{\theta\in {\Bbb S}}w_n(\theta)w_n(\alpha_n)=\inf\limits_{\theta\in {\Bbb S}}w_n(\theta),则有

\begin{eqnarray}\label{eq:lemestim.1}\frac{16[w_n(\beta_n)-w_n(\alpha_n)]^4}{3(\pi/m)^3}+\frac{2[w_n^2(\beta_n)-w_n^2(\alpha_n)]^2}{\pi/m}\leq\int_{0}^{\frac{\pi}{m}}\frac{1}{3}w_n'^4(\theta){\rm d}\theta+2w_n'^2(\theta)w_n^2(\theta){\rm d}\theta.\end{eqnarray}

  利用Hölder不等式和Newton-Libniz公式可得

w_n(\beta_n)-w_n(\alpha_n)=\int_{\alpha_n}^{\beta_n}w_n'(\theta){\rm d}\theta\leq|\beta_n-\alpha_n|^{\frac{3}{4}}\left\{\int_{\alpha_n}^{\beta_n}w_n'^4{\rm d}\theta\right\}^{\frac{1}{4}},

以及

\begin{eqnarray*}w_n^2(\beta_n)-w_n^2(\alpha_n)&=&\int_{\alpha_n}^{\beta_n}\frac{{\rm d} w_n^2(\theta)}{{\rm d}\theta}{\rm d}\theta=2\int_{\alpha_n}^{\beta_n}w_n(\theta)w_n'(\theta){\rm d}\theta\\&\leq&2|\beta_n-\alpha_n|^{\frac{1}{2}}\left\{\int_{\alpha_n}^{\beta_n}w_n^2(\theta)w_n'^2(\theta){\rm d}\theta\right\}^{\frac{1}{2}}.\end{eqnarray*}

|\beta_n-\alpha_n|=\gamma_n\pi/m,其中\gamma_n\in(0, 1/2],于是可得

\begin{equation}\label{eq:lem1.6}[w_n(\beta_n)-w_n(\alpha_n)]^4/[\gamma_n^3(\pi/m)^3]\leq \int_{\alpha_n}^{\beta_n}w_n'^4(\theta){\rm d}\theta, \end{equation}
(2.2)

\begin{equation}\label{eq:lem1.7}[w_n^2(\beta_n)-w_n^2(\alpha_n)]^2/[2\gamma_n\pi/m]\leq 2\int_{\alpha_n}^{\beta_n}w_n'^2(\theta)w_n^2(\theta){\rm d}\theta.\end{equation}
(2.3)

结合(2.2)和(2.3)式推知

\begin{eqnarray}\label{eq:lem1.8}&&[w_n(\beta_n)-w_n(\alpha_n)]^4/[3\gamma_n^3(\pi/m)^3]+[w_n^2(\beta_n)-w_n^2(\alpha_n)]^2/[2\gamma_n\pi/m]\\&\leq& \int_{\alpha_n}^{\beta_n}\frac{1}{3}w_n'^4(\theta)+2w_n'^2(\theta)w_n^2(\theta){\rm d}\theta.\end{eqnarray}
(2.4)

类似地有

\begin{eqnarray}\label{eq:lem1.9}&&[w_n(\beta_n)-w_n(\alpha_n)]^4/[3(1-\gamma_n)^3(\pi/m)^3]+[w_n^2(\beta_n)-w_n^2(\alpha_n)]^2/[2(1-\gamma_n)\pi/m]\\&\leq& \int_{\beta_n}^{\alpha_n+\frac{\pi}{m}}\frac{1}{3}w_n'^4(\theta)+2w_n'^2(\theta)w_n^2(\theta){\rm d}\theta.\end{eqnarray}
(2.5)

合并(2.4)和(2.5)式可得

\begin{eqnarray}\label{eq:lem1.10}&&\frac{(1-\gamma_n)^3+\gamma_n^3}{\gamma_n^3(1-\gamma_n)^3}\frac{[w_n(\beta_n)-w_n(\alpha_n)]^4}{3(\pi/m)^3}+\frac{1}{\gamma_n(1-\gamma_n)}\frac{[w_n^2(\beta_n)-w_n^2(\alpha_n)]^2}{2\pi/m}\\&\leq&\int_{\alpha_n}^{\alpha_n+\frac{\pi}{m}}\frac{1}{3}w_n'^4(\theta){\rm d}\theta+2w_n'^2(\theta)w_n^2(\theta){\rm d}\theta\\&=&\int_{0}^{\frac{\pi}{m}}\frac{1}{3}w_n'^4(\theta){\rm d}\theta+2w_n'^2(\theta)w_n^2(\theta){\rm d}\theta.\end{eqnarray}
(2.6)

对(2.6)式左端由\gamma_n组成的系数估计下界即得(2.1)式.

利用上述有关周期函数震荡幅度的估计,我们得到针对逼近序列的二择一性质估计.

引理2.2  设m\geq2, \{u_n\}\subset G_m\liminf\limits_{n\rightarrow+\infty}J(u_n)\geq 0.如果存在常数c_0>0使得对所有的n\in {\Bbb N}, \min\limits_{\theta \in {\Bbb S}}u_n(\theta)\leq c_0,则存在\{u_n\}的子序列(仍记为\{u_n\},下文简称\{u_n\}的子序列)使得

\begin{equation}\label{eq:lem1.2}\lim\limits_{n\rightarrow+\infty}\sup\limits_{\theta\in{\Bbb S}}u_n(\theta)= 0;\end{equation}
(2.7)

或者存在常数c>0(依赖于c_0)使得

\begin{equation}\label{eq:lem1.1} c<u_n(\theta)<1/c, \ \mbox{对所有的$ n\in {\Bbb N}, \theta\in {\Bbb S}$成立.} \end{equation}
(2.8)

  用逼近方法证明该引理.因为C^1({\Bbb S})W^{1, 4}_m内稠密,则有\pi/m -周期函数列\{w_n\}\subset C^1({\Bbb S})使得

\begin{equation}\label{eq:lem1.3} \|u_n-w_n\|<\min\left\{1/n, \inf\limits_{\theta\in {\Bbb S}}u_n(\theta)/2\right\}. \end{equation}
(2.9)

简单计算可得

\begin{eqnarray}\label{eq:lem1.4} \inf\limits_{\theta\in {\Bbb S}}w_n(\theta)&=&\inf\limits_{\theta\in {\Bbb S}}\left(w_n(\theta)-u_n(\theta)+u_n(\theta)\right) \\&\geq &\inf\limits_{\theta\in {\Bbb S}}u_n(\theta)-\sup\limits_{\theta\in {\Bbb S}}|u_n(\theta)-w_n(\theta)|\nonumber\\ &\geq& \inf\limits_{\theta\in {\Bbb S}}u_n(\theta)-\|u_n-w_n\| \\&\geq & \frac{1}{2}\inf\limits_{\theta\in {\Bbb S}}u_n(\theta)>0. \end{eqnarray}
(2.10)

既然有上述逼近估计(2.9)和(2.10),如果证明\{w_n\}具有二择一性质(2.7)或(2.8),则\{u_n\}也具有二择一性质(2.7)或(2.8).引理得证.

下面证明\{w_n\}具有二择一性质(2.7)或(2.8).由(2.9)式和J的连续性可知

\begin{equation}\label{eq:lem1.5}\liminf\limits_{n\rightarrow+\infty}\int_0^{\frac{\pi}{m}}w_n^4-\frac{1}{3}w_n'^4-2w_n'^2w_n^2{\rm d}\theta=\liminf\limits_{n\rightarrow+\infty}J(w_n)\geq 0.\end{equation}
(2.11)

对任意n\in{\Bbb N},取\alpha_n, \beta_n\in {\Bbb S}使得w_n(\beta_n)=\sup\limits_{\theta\in {\Bbb S}}w_n(\theta), w_n(\alpha_n)=\inf\limits_{\theta\in {\Bbb S}}w_n(\theta).由假设和(2.9)式知:存在常数c_1>0使得w_n(\alpha_n)<c_1.对于\{w_n(\beta_n)\},则有如下三种情形

\begin{eqnarray*} {\rm (I)} &&\liminf\limits_{n\rightarrow \infty}w_n(\beta_n)=0;\\ {\rm (Ⅱ)} &&\limsup\limits_{n\rightarrow \infty}w_n(\beta_n)=+\infty; \\{\rm (Ⅲ)} && 0<\liminf\limits_{n\rightarrow \infty}w_n(\beta_n)\leq\limsup\limits_{n\rightarrow \infty}w_n(\beta_n)<+\infty.\end{eqnarray*}

如果(Ⅰ) \liminf\limits_{n\rightarrow \infty}w_n(\beta_n)=0发生,则存在\{w_n\}的子列使得\liminf\limits_{n\rightarrow+\infty}\int_0^{\frac{\pi}{m}} w_n^4{\rm d}\theta=0.由Hölder不等式和(2.11)式知

\liminf\limits_{n\rightarrow \infty} \int_0^{\frac{\pi}{m}}w_n'^4{\rm d}\theta=0.

所以\liminf\limits_{n\rightarrow+\infty}\|w_n\|= 0.结合嵌入定理知, \{w_n\}存在子列满足性质(2.7).

由引理2.1可知\{w_n\}满足不等式(2.1).令n\rightarrow +\infty,由(2.11)和(2.1)式可得

\begin{eqnarray}\label{eq:est11}\frac{16[w_n(\beta_n)-w_n(\alpha_n)]^4}{3(\pi/m)^4}+\frac{2[w_n^2(\beta_n)-w_n^2(\alpha_n)]^2}{(\pi/m)^2}\leq w^4_n(\beta_n)+o(1).\end{eqnarray}
(2.12)

如果(Ⅱ) \limsup\limits_{n\rightarrow \infty}w_n(\beta_n)=+\infty发生.令n\rightarrow +\infty,对(2.12)式两边分别乘以1/w_n^4(\beta_n)得到如下矛盾

\begin{eqnarray}\label{eq:est12}\frac{3}{2}<\frac{16}{3(\pi/m)^4}+\frac{2}{(\pi/m)^2}\leq 1+o(1).\end{eqnarray}
(2.13)

如果(Ⅲ) 0<\liminf\limits_{n\rightarrow \infty}w_n(\beta_n)\leq\limsup\limits_{n\rightarrow \infty}w_n(\beta_n)<+\infty发生,则存在常数c_2\geq c_1使得对所有的 n\in{\Bbb N},有

\begin{eqnarray}\label{eq:est13}\frac{1}{c_2}<w_n(\beta_n)<c_2.\end{eqnarray}
(2.14)

如果\liminf\limits_{n\rightarrow+\infty}w_n(\alpha_n)=0,令n\rightarrow +\infty,并对(2.12)式两边乘以1/w_n^4(\beta_n)得到矛盾不等式(2.13).所以存在常数c_3>0使得

\begin{eqnarray}\label{eq:est14}w_n(\alpha_n)>c_3\ \mbox{对$ n\in {\Bbb N} $一致成立}.\end{eqnarray}
(2.15)

结合(2.14)和(2.15)式,可得\{w_n\}满足性质(2.8).

应用上述二择一引理,我们证明约束变分问题(1.8)存在极小可达元.

定理2.1  假设m\geq2(g_1)-(g_3)成立,则存在u\in G_m使得

I(u)=\inf\limits_{v\in G_m, J(v)=I(e)}I(v), J(u)=I(e).

  取\{u_n\}\subset G_m为极小化序列,即

\begin{eqnarray}\label{eq:th1}J(u_n)=I(e)=\int_0^{\frac{\pi}{m}}g(\theta){\rm d}\theta>0 \mbox{且}\lim\limits_{n\rightarrow+\infty}I(u_n)=\inf\limits_{v\in G_m, J(v)=I(e)}I(v).\end{eqnarray}
(2.16)

我们通过下面几步证明该定理.

第1步 存在常数c>0, u_n(\alpha_n)=\inf\limits_{\theta\in {\Bbb S}}v_n(\theta)<cn\in {\Bbb N}一致成立.

v_n(\theta)=u_n(\theta)/u_n(\alpha_n),则\{v_n(\theta)\}\subset G_m,并且

\begin{eqnarray}\label{eq:th2}J(v_n)\geq0, \ \inf\limits_{\theta\in{\Bbb S}}v_n(\theta)\equiv 1.\end{eqnarray}
(2.17)

因此\{v_n\}满足引理2.2的假设,应用引理2.2可得

\begin{eqnarray}\label{eq:th3} c<v_n(\theta)<1/c, \ \mbox{对所有的$ n\in {\Bbb N}, \theta\in {\Bbb S} $成立}; \end{eqnarray}
(2.18)

或者存在\{v_n\}的子序列使得

\begin{eqnarray}\label{eq:th4}\lim\limits_{n\rightarrow+\infty}\sup\limits_{\theta\in{\Bbb S}}v_n(\theta)= 0.\end{eqnarray}
(2.19)

显然(2.19)与(2.17)式矛盾.由此排除估计式(2.19),即\{v_n\}满足估计式(2.18).所以

\begin{eqnarray}\label{eq:th5}|I(v_n)|=\left|\int_0^{\frac{\pi}{m}} g(\theta)\ln v_n(\theta){\rm d}\theta\right|<+\infty.\end{eqnarray}
(2.20)

另一方面,有

\begin{eqnarray*}\nonumber\ln (u_n(\alpha_n))\int_0^{\frac{\pi}{m}} g(\theta){\rm d}\theta&=&\int_0^{\frac{\pi}{m}} g(\theta)\ln u_n(\theta){\rm d}\theta-\int_0^{\frac{\pi}{m}} g(\theta)\ln v_n(\theta){\rm d}\theta\\&\leq& I(e)-I(v_n).\end{eqnarray*}

结合(2.20)式得到第1步结论.

第2步 存在u\in G_m\{u_n\}的子列使得u_n\mathop{\rightharpoonup}\limits^{n\rightarrow+\infty} uW^{1, 4}_m中弱收敛且I(u)=\inf\limits_{v\in G_m, J(v)=I(e)}I(v).

由第1步结论知\{u_n(\theta)\}满足引理2.2的假设,因此\{u_n\}满足

\begin{eqnarray}\label{eq:th6}c<u_n(\theta)<\frac{1}{c}, \ \mbox{对所有的$ n\in {\Bbb N}, \theta\in{\Bbb S} $成立} ;\end{eqnarray}
(2.21)

或者存在\{u_n\}的子列使得

\begin{eqnarray}\label{eq:th7}\lim\limits_{n\rightarrow+\infty}\sup\limits_{\theta\in{\Bbb S}}u_n(\theta)= 0.\end{eqnarray}
(2.22)

如果(2.22)式发生,则有\liminf\limits_{n\rightarrow+\infty}\int_0^{\frac{\pi}{m}} u_n^4 {\rm d}\theta=0 .由此可得矛盾: 0<I(e)=\liminf\limits_{n\rightarrow+\infty}J(u_n)\leq 0.因此排除二择一估计中的(2.22)式,我们有(2.21)式成立.由(2.21)和(2.16)式可推知\{u_n\}W^{1, 4}_m中有界.所以存在u\in W^{1, 4}_m\{u_n\}的子列使得u_n\rightharpoonup u_0W^{1, 4}_m中弱收敛.由0<\alpha<{1/2}时嵌入H^1({\Bbb S})\hookrightarrow C^\alpha({\Bbb S})的紧性,我们有

\lim\limits_{n\rightarrow+\infty}u_n(\theta)= u(\theta)\ \mbox{对$ \theta\in{\Bbb S}$一致成立}.

结合(2.21)式知,对所有的\theta\in{\Bbb S}有估计

\begin{eqnarray}\nonumberu(\theta)\geq c>0.\end{eqnarray}

因此u\in G_mI(u)=\lim\limits_{n\rightarrow+\infty}I(u_n)=\inf\limits_{v\in G_m, J(v)=I(e)}I(v).

第3步J(u)=I(e).

由积分的凸性质可得

\begin{eqnarray}\nonumber\int_0^{\frac{\pi}{m}} u'^4{\rm d}\theta \leq \liminf\limits_{n\rightarrow \infty}\int_0^{\frac{\pi}{m}} u_n'^4{\rm d}\theta, \mbox{且}\int_0^{\frac{\pi}{m}} u'^2u_0^2{\rm d}\theta \leq \liminf\limits_{n\rightarrow \infty}\int_0^{\frac{\pi}{m}} u_n'^2u_n^2{\rm d}\theta.\end{eqnarray}

因此J(u)\geq \lim\limits_{n\rightarrow \infty} J(u_n)=I(e)>0.如果J(u)>I(e),则有常数\lambda>1使得J(u/\lambda)=I(e)u/\lambda\in G_m.另一方面

\begin{eqnarray*}I(u/\lambda)&=&\int_0^{\frac{\pi}{m}} g(\theta)\ln u{\rm d}\theta-\int_0^{\frac{\pi}{m}} g(\theta)\ln \lambda {\rm d}\theta<\int_0^{\frac{\pi}{m}} g(\theta)\ln u{\rm d}\theta\\&=&\lim\limits_{n\rightarrow\infty}I(u_n)=\inf\limits_{v\in G_m, J(v)=I(e)}I(v), \end{eqnarray*}

这与约束极小的定义矛盾.因此J(u)=I(e).

定理1.1的证明  由定理2.1可知存在集合G_m的内点w使得J(w)=I(e)

\begin{eqnarray*}I(w)=\inf\limits_{v\in G_m, J(v)=I(e)}I(v).\end{eqnarray*}

应用约束变分原理知,存在参数\omega \in \mathbb{R} 使得在W^{1, 4}_m的对偶空间中有

\begin{eqnarray*}J'(w)+\omega I'(w)=0.\end{eqnarray*}

因此,对所有的\varphi\in W^{1, 4}_m我们有

\begin{eqnarray}\label{eq:th2.1}\omega \int_0^{\frac{\pi}{m} }\frac{g(\theta)}{w}\varphi {\rm d}\theta+4\int_0^{\frac{\pi}{m}}\frac{1}{3} w'^3\varphi'+w^2w'\varphi'+w'^2w\varphi-w^3\varphi {\rm d}\theta=0.\end{eqnarray}
(2.23)

在(2.23)式中令\varphi=w,则有

\begin{eqnarray}\label{eq:th2.2}\omega\int_0^{\frac{\pi}{m}}g(\theta){\rm d}\theta=4\int_0^{\frac{\pi}{m}} w^4-\frac{1}{3}w'^4-2w^2w'^2{\rm d}\theta=4J(w).\end{eqnarray}
(2.24)

因此,我们有\omega=4J(w)/I(e)=4.结合(2.23)式可得w满足积分等式(1.9).即w是问题(1.3)的弱解.如果w二阶可导,对任意的\varphi\in W^{1, 4}_m通过分部积分得

\begin{eqnarray}\label{eq:th2.3}J'(w)\varphi&=&\int_0^{\frac{\pi}{m}} \frac{1}{3}w'^3\varphi'+w^2w'\varphi'+w'^2w\varphi-w^3\varphi {\rm d}\theta\nonumber\\&=&-\int_0^{\frac{\pi}{m}} w'^2w''\varphi {\rm d}\theta-\int_0^{\frac{\pi}{m}} w^2w''\varphi {\rm d}\theta-2\int_0^{\frac{\pi}{m}} ww'^2\varphi {\rm d}\theta\\&&+\int_0^{\frac{\pi}{m}} w'^2w\varphi {\rm d}\theta-\int_0^{\frac{\pi}{m}} w^3\varphi {\rm d}\theta\nonumber\\&=&-\int_0^{\frac{\pi}{m}} w'^2w''\varphi {\rm d}\theta-\int_0^{\frac{\pi}{m}} w^2w''\varphi {\rm d}\theta-\int_0^{\frac{\pi}{m}} w w'^2\varphi {\rm d}\theta-\int_0^{\frac{\pi}{m}} w^3\varphi {\rm d}\theta\nonumber\\&=&-\int_0^{\frac{\pi}{m}} [w'^2w''+w^2w''+ww'^2+w^3]\varphi {\rm d}\theta\nonumber\\&=&-\int_0^{\frac{\pi}{m}} (w'^2+w^2)(w''+w)\varphi {\rm d}\theta.\end{eqnarray}

由(1.9)和(2.25)式知w满足积分等式(1.10).即w{\Bbb S}上几乎处处满足(1.4)式.

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