众所周知, 调和函数定理在研究流形的拓扑结构和非紧致黎曼流形的曲率中扮演着很重要的角色.另一方面, Hodge定理表明调和微分形式在刻画紧致黎曼流形的结构中是很重要的研究工具, 而在非紧致流形中, Hodge定理依然有效.同时, 调和
do Carmo, Peng[1]和Fischer-Colbrie, Schoen[2]证明了3维欧氏空间
$ {\left\| A \right\|_{{L^n}(M)}}: = {\left( {\int_M {{{\left| A \right|}^n}{\rm{d}}v} } \right)^{\frac{1}{n}}} \le \frac{{n - 2}}{{2(n - 1)C(n)}}, $ |
则
$ \|\Phi\|_{L^n(M^n)}<\frac{(n-2)(1-\alpha)}{(n-1)\sqrt{n-1}C(n)}, $ |
$ \|{H}\|_{L^n(M^n)}\leqslant\frac{\alpha}{nC(n)}, $ |
则
对于调和
$ \begin{align*} D= \begin{cases} \sqrt2, & n=3, \\ 2, & n\geqslant4. \end{cases} \end{align*} $ |
同时还证明了若
$ \begin{align*} \delta(n)= \begin{cases} \dfrac{\sqrt[3]{{{\omega _3}}}}{3\cdot\sqrt{2}\cdot4^4}, & n=3, \\ \dfrac{(n-2)\sqrt[n]{\omega_n}}{2n(n-1)\cdot4^{n+1}}, & n\geqslant4. \end{cases} \end{align*} $ |
由于文献[5]、[6]做了4维及以下欧氏空间(或球空间)中超曲面上非平凡
定理0.1 设
定理0.2 设
$ \begin{align*} \mu(n)= \begin{cases} \dfrac{1-\alpha}{4C(3)}, & n=3, \\ \sqrt{\dfrac{n-2}{2n(n-1)}}\cdot\dfrac{1-\alpha}{C(n)}, & n\geqslant4. \end{cases} \end{align*} $ |
注 定理0.1中是关于无迹张量和平均曲率向量的一个整体关系式, 是一个整体限制, 而定理0.2中是对无迹张量和平均曲率向量分别进行限制, 二者不再相互制约.
1 预备知识及引理设
$ \omega=\sum{a_{i_1\cdots{i_p}}}e^{i_p}\wedge\cdots\wedge{e^{i_1}}=a_I\omega_I, \\ \theta=\sum{b_{i_1\cdots{i_p}}}e^{i_p}\wedge\cdots\wedge{e^{i_1}}=b_I\omega_I. $ |
则有
$ |\omega|^2=\sum\limits_I{a_I}^2, ~~ |\nabla\omega|^2=\sum\limits_i|\nabla_{e_i}\omega|^2, ~~ \langle\omega, \theta\rangle=\sum\limits_I{a_Ib_I}. $ |
记
$\Phi(X, Y)=A(X, Y)-H\langle{X}, Y\rangle, $ |
其中
记
$*e^{i_1}\wedge\cdots\wedge{e^{i_p}}=\text{sgn}~\delta(i_1, i_2, \cdots, i_p, i_{p+1}, \cdots, i_n)e^{i_{p+1}}\wedge\cdots\wedge{e^{i_n}}, $ |
其中
Hodge余星算子
$\text{d}^*\omega=(-1)^{np+n+1}*\text{d}*\omega.$ |
进而, 作用在
$\Delta\omega=-(\text{dd}^*+\text{d}^*\text{d})\omega.$ |
对于
引理1.1[9] 设
$\frac{1}{2}\Delta|\omega|^2=\langle\Delta\omega, \omega\rangle+|\nabla{\omega}|^2+ \langle{E}(\omega), \omega\rangle, $ |
其中
引理1.2[10] 设
$ {\left( {\int_M {{{\left| f \right|}^{\frac{n}{{n - 1}}}}{\rm{d}}v} } \right)^{\frac{{n - 1}}{n}}} \le C(n)\left( {\int_M {\left| {\nabla f} \right|{\rm{d}}v + n} \int_M {\left| {H||f} \right|{\rm{d}}v} } \right), $ |
其中
引理1.3[11-12] 令
$ (1+K_p)\big|\nabla|\omega|\big|^2\leqslant|\nabla{\omega}|^2, $ |
其中
$ \begin{equation*} {K_p}= \begin{cases} \dfrac{1}{n-p}, & 1\leqslant{p}\leqslant\dfrac{n}{2}, \\ \dfrac{1}{p}, & \dfrac{n}{2}<p\leqslant{n-1}. \end{cases} \end{equation*} $ |
命题1.1 设
$ \langle{E}(\omega), \omega\rangle\geqslant \frac{n}{2}\big(|H|^2-|\Phi|^2\big)|\omega|^2. $ |
证明 由引理1.1得
$ \begin{align*} E(\omega) &=R_{k_1i_1j_1i_1}a_{k_1i_2}e^{i_2}\wedge{e^{j_1}}+ R_{k_2i_2j_2i_2}a_{i_1k_2}e^{j_2}\wedge{e^{i_1}}\\ &~~~+R_{k_2i_2j_1i_1}a_{i_1k_2}e^{i_2}\wedge{e^{j_1}}+ R_{k_1i_1j_2i_2}a_{k_1i_2}e^{j_2}\wedge{e^{i_1}}\\ &=\text{Ric}_{k_1j_1}a_{k_1i_2}e^{i_2}\wedge{e^{j_1}}+ \text{Ric}_{k_2j_2}a_{i_1k_2}e^{j_2}\wedge{e^{i_1}}\\ &~~~+R_{k_2i_2j_1i_1}a_{i_1k_2}e^{i_2}\wedge{e^{j_1}}+ R_{k_1i_1j_2i_2}a_{k_1i_2}e^{j_2}\wedge{e^{i_1}}. \end{align*} $ |
则
$ \begin{align*} \langle{E}(\omega), \omega\rangle &=\text{Ric}_{k_1j_1}a_{k_1i_2}a_{j_1i_2}+ \text{Ric}_{k_2j_2}a_{i_2k_2}a_{i_1j_2}\\ &~~~+R_{k_2i_2j_1i_1}a_{i_1k_2}a_{j_1i_2}+R_{k_1i_1j_2i_2}a_{k_1i_2}a_{i_1j_2}. \end{align*} $ |
由Gauss方程
$ \begin{align*} \text{Ric}_{k_1j_1}=\, &nHh_{k_1j_1}-h_{k_1i}h_{ij_1}, \quad \text{Ric}_{k_2j_2}=nHh_{k_2j_2}-h_{k_2i}h_{ij_2}, \\ R_{k_2i_2j_1i_1}=\, &h_{k_2j_1}h_{i_2i_1}-h_{k_2i_1}h_{i_2j_1}, \quad R_{k_1i_1j_2i_2}=h_{k_1j_2}h_{i_1i_2}-h_{k_1i_2}h_{i_1j_2}. \end{align*} $ |
在任意点
$n|H|=\lambda_1+\cdots+\lambda_n.$ |
故
$ \begin{align*} \langle{E}(\omega), \omega\rangle &=\text{Ric}_{k_1j_1}a_{k_1i_2}a_{j_1i_2}+\text{Ric}_{k_2j_2}a_{i_2k_2}a_{i_1j_2} +R_{k_2i_2j_1i_1}a_{i_1k_2}a_{j_1i_2}+R_{k_1i_1j_2i_2}a_{k_1i_2}a_{i_1j_2}\\ &=\sum{n|H|\lambda_{k_1}(a_{k_1i_2})^2}-\sum{{\lambda_{k_1}}^2(a_{k_1i_2})^2}+\sum{n|H|\lambda_{k_2}(a_{i_1k_2})^2}-\sum{{\lambda_{k_2}}^2(a_{i_1k_2})^2}\\ &~~~-\sum{\lambda_{k_2}\lambda_{i_2}(a_{k_2i_2})^2}-\sum{\lambda_{j_2}\lambda_{i_2}(a_{j_2i_2})^2}\\ &=2n\sum{|H|\lambda_i}(a_{ij})^2-2\sum\lambda_i^2(a_{ij})^2-2\sum\lambda_i\lambda_j(a_{ij})^2\\ &=2\sum\limits_{i\neq{j}} \big((\lambda_1+\cdots+\lambda_n)\lambda_i-\lambda_i^2 -\lambda_i\lambda_j\big)(a_{ij})^2. \end{align*} $ |
当
$ \begin{align*} \langle{E}(\omega), \omega\rangle&=2\sum\limits_{i\neq{j}} \big((\lambda_1+\lambda_2+\lambda_3)\lambda_i-\lambda_i^2- \lambda_i\lambda_j\big)(a_{ij})^2\\ &=\sum\limits_{i\neq{j}} (\lambda_1+\cdots+\hat{\lambda}_i+\cdots+\hat{\lambda}_j+\cdots+\lambda_3) (\lambda_i+\lambda_j)(a_{ij})^2\\ &\geqslant\frac{1}{2}\sum\limits_{i\neq{j}} \big((3|H|)^2-(\lambda_1+\cdots+\hat{\lambda}_i+\cdots+\hat{\lambda}_j+ \cdots+\lambda_3)^2-2({\lambda_i}^2+{\lambda_j}^2)\big)(a_{ij})^2\\ &\geqslant\frac{1}{2}\big(|A|^2-3|\Phi|^2\big)|\omega|^2. \end{align*} $ |
又
$ \begin{align} \langle{E}(\omega), \omega\rangle &\geqslant\frac{1}{2}\big(|A|^2-3|\Phi|^2\big)|\omega|^2 \geqslant\frac{1}{2}\big(3|H|^2-3|\Phi|^2\big)|\omega|^2\notag\\ &=\frac{3}{2}\big(|H|^2-|\Phi|^2\big)|\omega|^2. \end{align} $ | (1) |
当
$ \begin{align*} \langle{E}(\omega), \omega\rangle &=2\sum\limits_{i\neq{j}} \big((\lambda_1+\cdots+\lambda_n)\lambda_i-\lambda_i^2 -\lambda_i\lambda_j\big)(a_{ij})^2\\ &=\sum\limits_{i\neq{j}} (\lambda_1+\cdots+\hat{\lambda}_i+\cdots+\hat{\lambda}_j+\cdots+\lambda_n) (\lambda_i+\lambda_j)(a_{ij})^2\\ &\geqslant\frac{1}{2}\sum\limits_{i\neq{j}} \bigg((n|H|)^2-(n-2)\bigg(\sum\limits_{k\neq{i, j}}^{n}(\lambda_k)^2+ ({\lambda_i}^2+{\lambda_j}^2)\bigg)\bigg)(a_{ij})^2\\ &\geqslant\bigg(|A|^2-\frac{n}{2}|\Phi|^2\bigg)|\omega|^2. \end{align*} $ |
又
$ \langle{E}(\omega), \omega\rangle \geqslant\big(|A|^2-\frac{n}{2}|\Phi|^2\big)|\omega|^2 \geqslant\frac{n}{2}\big(|H|^2-|\Phi|^2\big)|\omega|^2. $ | (2) |
综合式(1)、(2), 对于
定理0.1的证明 设
$ \frac{1}{2}\Delta|\omega|^2=|\nabla{\omega}|^2+\langle{E}(\omega), \omega\rangle. $ | (3) |
记
$ \begin{align} \eta= \begin{cases} 1, & \rho<r, \\ a\in[0, 1], & |\nabla\eta|<\dfrac{2}{r}, r\leqslant\rho\leqslant2r, ~~~~~~~~~~~~~~~~~\\ 0, & \rho>2r. \end{cases} \end{align} $ | (4) |
在式(3)两边同时乘以
$ \begin{align} \int_M {{\eta ^2}{{\left| {\nabla \omega } \right|}^2}{\rm{d}}v} + \int_M {{\eta ^2}\langle E(\omega ), \omega \rangle {\rm{d}}v} &=\frac{1}{2}\int_M {{\eta ^2}\Delta {{\left| \omega \right|}^2}{\rm{d}}v} \\ & = - \frac{1}{2}\int_M {\langle \nabla {\eta ^2}, \nabla {{\left| \omega \right|}^2}\rangle {\rm{d}}v} \\ & = - 2\int_M {\eta \left| \omega \right|\langle \nabla \eta, \nabla \left| \omega \right|\rangle {\rm{d}}v} . \end{align} $ | (5) |
又对于任意的正常数
$ \pm 2\int_M {\eta |\omega |\langle \nabla \eta, \nabla |\omega |\rangle {\rm{d}}v} \le \varepsilon \int_M {{\eta ^2}{{\left| {\nabla |\omega |} \right|}^2}{\rm{d}}v} + \frac{1}{\varepsilon }\int_M {|\omega {|^2}|\nabla \eta {|^2}{\rm{d}}v} $ |
以及引理1.3, 式(5)可改写为
$ (1 - \frac{\varepsilon }{{1 + {K_p}}})\int_M {{\eta ^2}|\nabla \omega {|^2}{\rm{d}}v} + \int_M {{\eta ^2}\langle E(\omega ), \omega \rangle {\rm{d}}v} \le \frac{1}{\varepsilon }\int_M {|\omega {|^2}|\nabla \eta {|^2}{\rm{d}}v} . $ | (6) |
又
$ (1 - \frac{\varepsilon }{{1 + {K_p}}})\int_{{B_{{x_0}}}(r)} {|\nabla \omega {|^2}{\rm{d}}v} + \int_{{B_{{x_0}}}(r)} {\langle E(\omega ), \omega \rangle {\rm{d}}v} \le \frac{4}{{\varepsilon {r^2}}}\int_{{B_{{x_0}}}(2r)} {|\omega {|^2}{\rm{d}}v} . $ | (7) |
根据命题1.1,
$|\nabla\omega|=\langle{E}(\omega), \omega\rangle=0.$ |
所以
定理0.2的证明 设
$ (1 - \frac{\varepsilon }{{1 + {K_p}}})\int_M {{\eta ^2}|\nabla \omega {|^2}{\rm{d}}v + \frac{n}{2}} \int_M {{\eta ^2}(|H{|^2} - |\Phi {|^2})|\omega {|^2}{\rm{d}}v} \le \frac{1}{\varepsilon }\int_M {|\omega {|^2}|\nabla \eta {|^2}{\rm{d}}v} . $ | (8) |
依条件
$ \begin{align*} nC(n)\int_M {|H||f|{\rm{d}}v} & \le nC(n){\left( {\int_M {|H{|^n}{\rm{d}}v} } \right)^{\frac{1}{n}}}{\left( {\int_M {|f{|^{\frac{n}{{n - 1}}}}{\rm{d}}v} } \right)^{\frac{{n - 1}}{n}}}\\ & \le \alpha {\left( {\int_M {|f{|^{\frac{n}{{n - 1}}}}{\rm{d}}v} } \right)^{\frac{{n - 1}}{n}}}. \end{align*} $ |
结合引理1.2得
${\left( {\int_M {|f{|^{\frac{n}{{n - 1}}}}{\rm{d}}v} } \right)^{\frac{{n - 1}}{n}}} \le \frac{{C(n)}}{{1 - \alpha }}\int_M {|\nabla f|{\rm{d}}v}, $ |
上式中令
${\left( {\int_M {|g{|^{\frac{{2n}}{{n - 2}}}}{\rm{d}}v} } \right)^{\frac{{n - 2}}{n}}} \le \frac{{4{{(n - 1)}^2}C{{(n)}^2}}}{{{{(1 - \alpha )}^2}{{(n - 2)}^2}}}\int_M {|\nabla g{|^2}{\rm{d}}v} .$ |
利用上式和Hölder不等式得
$ \begin{align*} \int_M {{\eta ^2}|\Phi {|^2}|\omega {|^2}{\rm{d}}v} & \le {\left( {\int_M {|\Phi {|^n}{\rm{d}}v} } \right)^{\frac{2}{n}}}{\left( {\int_M {{{(\eta |\omega |)}^{\frac{{2n}}{{n - 2}}}}{\rm{d}}v} } \right)^{\frac{{n - 2}}{n}}}\\ & \le \frac{{4{{(n - 1)}^2}{\phi _0}C{{(n)}^2}}}{{{{(1 - \alpha )}^2}{{(n - 2)}^2}}}\int_M {|\nabla (\eta |\omega |){|^2}{\rm{d}}v} \\ & \le \frac{{4{{(n - 1)}^2}{\phi _0}C{{(n)}^2}}}{{{{(1 - \alpha )}^2}{{(n - 2)}^2}}}\int_M {((1 + \varepsilon ){\eta ^2}|\nabla |\omega |{|^2} + (1 + \frac{1}{\varepsilon })|\omega {|^2}|\nabla \eta {|^2}){\rm{d}}v} \\ & \le \frac{{4{{(n - 1)}^2}{\phi _0}C{{(n)}^2}}}{{{{(1 - \alpha )}^2}{{(n - 2)}^2}}}\int_M {(\frac{{1 + \varepsilon }}{{1 + {K_p}}}{\eta ^2}|\nabla \omega {|^2} + (1 + \frac{1}{\varepsilon })|\omega {|^2}|\nabla \eta {|^2}){\rm{d}}v.} \end{align*} $ |
其中
$ {l_1}\int_M {{\eta ^2}|\nabla \omega {|^2}{\rm{d}}v} + \frac{n}{2}\int_M {{\eta ^2}|H{|^2}|\omega {|^2}{\rm{d}}v} \le {l_2}\int_M {|\omega {|^2}|\nabla \eta {|^2}{\rm{d}}v}, $ | (9) |
其中
$l_1=1-\frac{2n(n-1)^2\phi_0{C(n)}^2}{(1+K_p)(1-\alpha)^2(n-2)^2}-\frac{\varepsilon}{1+K_p}\bigg(1+\frac{2n(n-1)^2\phi_0{C(n)}^2}{(1-\alpha)^2(n-2)^2}\bigg), $ |
$l_2=\frac{1}{\varepsilon}+\frac{2n(n-1)^2\phi_0{C(n)}^2}{(1-\alpha)^2(n-2)^2} \bigg(1+\frac{1}{\varepsilon}\bigg).$ |
又
$ {l_1}\int_{{B_{{x_0}}}(r)} {|\nabla \omega {|^2}{\rm{d}}v} + \frac{n}{2}\int_{{B_{{x_0}}}(r)} {|H{|^2}|\omega {|^2}{\rm{d}}v} \le \frac{{4{l_2}}}{{{r^2}}}\int_{{B_{{x_0}}}(2r)} {|\omega {|^2}{\rm{d}}v} . $ | (10) |
显然
当
$|\nabla\omega|=|H||\omega|=0.$ |
所以
当
$|\nabla\omega|=|H||\omega|=0.$ |
所以
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