组合KdV-Burgers方程
$\;\;\quad {u_t} + g{(u)_x} + r{u_{xx}} + \beta {u_{xxx}} = 0,\;\;\;b > 0,r \leqslant 0,\beta > 0\!\!\!\!\!\!\!\!\!\!$ | (1) |
是非线性研究领域重要的模型方程,在等离子体物理、量子场理论以及固态物理中有着广泛的应用[1-6]。其中,
${u_t} + au{u_x} + r{u_{xx}} + \beta {u_{xxx}} = 0$ | (2) |
方程(2)可作为许多具有某种耗散作用的实际问题的控制方程,如粘性液体中的浅水波、弹性管内液体的流动和波动、等离子体中的磁声波等。当方程(2)中
${u_t} + au{u_x} + \beta {u_{xxx}} = 0$ |
文献[7-10]研究了方程(1)孤波解的求解问题,在文献[7]中求出了方程(1)的扭状孤波解;文献[8-9]分别运用齐次平衡法和直接法与假设法的一种结合得到了方程(1)的精确解;随后,文献[10]应用Liapunov稳定性分析法证明了广义组合KdV-Burgers方程的扭状孤波解是线性稳定的,并得到了孤波解线性稳定的条件。文献[11]研究了组合KdV-Burgers方程(1)行波解与耗散系数
引理1 假设波速
a. 当
b. 当
c. 当
文献[11]利用平面动力系统的理论和方法研究了方程(1)的行波解。对于引理1中涉及的方程(1)的衰减振荡解,文献[11]利用解轨线在相图中的演化关系、假设待定法,求出了方程(1)衰减震荡解的近似解,进一步得到了近似解与真解间的误差估计,证明了误差是以指数形式速降的无穷小量。对于引理1中方程(1)所具有的波形函数为单调的行波解(也可称为扭状孤波解),目前尚未发现有对它的稳定性研究的文献发表。本文研究当
假定
$U(\xi ) \to {u_ \pm },\quad\xi \to \pm \infty $ |
则
将
$ \qquad\qquad\quad- c{U_\xi } - \alpha {U_{\xi \xi }} + \beta {U_{\xi \xi \xi }} + g{(U)_\xi } = 0$ | (3) |
将式(3)对
$\begin{split} - \alpha {U_\xi } + \beta {U_{\xi \xi }} = cU - g(U) + a \\ a = - c{u_ \pm } + g({u_ \pm }) \qquad\!\! \end{split} $ |
性质1 假设
$c({u_ + } - {u_ - }) = g({u_ + }) - g({u_ - })$ |
性质2 假设
$g'({u_ + }) < c < g'({u_ - })$ |
性质3 假设
$\left| {{U_\xi }} \right|,\left| {{U_{\xi \xi }}} \right| \leqslant C\left| {{u_ - } - {u_ + }} \right|$ |
证明 因为,
$\qquad 0 - \alpha {U_\xi } + \beta {U_{\xi \xi }} = g(U) - g({u_ + }) - c(U - {u_ + })$ | (4) |
利用积分中值定理,将式(4)化为
$\begin{gathered} - \alpha {U_\xi } + \beta {U_{\xi \xi }} = g'(\theta (U - {u_ + }))(U - {u_ + }) - c(U - {u_ + })= \\ \;\;\;\;\; (g'(\theta (U - {u_ + })) - c)(U - {u_ + }),\begin{array}{*{20}{c}} {}&{0 < \theta < 1} \end{array} \\ \end{gathered} $ |
a. 当
$ - \alpha {U_\xi } + \beta {U_{\xi \xi }} > 0$ |
$ - c(U - {u_ + }) \leqslant - \alpha {U_\xi } + \beta {U_{\xi \xi }} \leqslant g(U) - g({u_ + })$ |
故存在常数
$\qquad\quad - C\left| {{u_ - } - {u_ + }} \right| \leqslant - \alpha {U_\xi } + \beta {U_{\xi \xi }} \leqslant C\left| {{u_ - } - {u_ + }} \right|$ | (5) |
将式(5)两边同乘以
$ \begin{split} - C{{\rm e}^{ - \frac{\alpha }{\beta }\xi }}\left| {{u_ - } - {u_ + }} \right| \leqslant \beta \frac{\rm d}{{{\rm d}\xi }}({{\rm e}^{ - \frac{\alpha }{\beta }\xi }}{U_\xi }) \leqslant \\ C{{\rm e}^{ - \frac{\alpha }{\beta }\xi }}\left| {{u_ - } - {u_ + }} \right|\qquad\qquad\end{split}$ | (6) |
将式(6)对
$\left| {{U_\xi }} \right| \leqslant C\left| {{u_ - } - {u_ + }} \right|$ |
b. 当
$\begin{array}{c} - \alpha {U_\xi } + \beta {U_{\xi \xi }} < 0\\ - c(U - {u_ + }) \leqslant - \alpha {U_\xi } + \beta {U_{\xi \xi }} \leqslant U - {u_ + }\end{array} $ |
故存在常数
$\qquad\quad - C\left| {{u_ - } - {u_ + }} \right| \leqslant - \alpha {U_\xi } + \beta {U_{\xi \xi }} \leqslant C\left| {{u_ - } - {u_ + }} \right|$ | (7) |
将式(7)两边同乘以
$\begin{split} - C{{\rm e}^{ - \frac{r}{\beta }\xi }}\left| {{u_ - } - {u_ + }} \right| \leqslant \beta \frac{{\rm d}}{{{\rm d}\xi }}({{\rm e}^{ - \frac{r}{\beta }\xi }}{U_\xi }) \leqslant \\ C{{\rm e}^{ - \frac{r}{\beta }\xi }}\left| {{u_ - } - {u_ + }} \right|\qquad\quad\end{split}$ | (8) |
将式(8)对
$\left| {{U_\xi }} \right| \leqslant C\left| {{u_ - } - {u_ + }} \right|$ | (9) |
同理,利用微分中值定理以及式(9),可得
$\left| {{U_{\xi \xi }}} \right| \leqslant C\left| {{u_ - } - {u_ + }} \right|$ |
故性质3得证。
3 单调递减扭状孤波解的渐近稳定性定理考虑方程(1)的初值问题,初值条件为
$u(0,x) = {u_0}(x)$ | (10) |
这里
${u_0}(x) \to {u_ \pm },\;\;x \to \pm \infty $ |
设
${u_0} - U \in {H^1}$ | (11) |
并且对任意
$\qquad\qquad\quad\left\{ \begin{array}{l} {\varPhi _0}(x) = \displaystyle\int_{ - \infty }^x {\left( {{u_0} - U} \right)} (y){\rm d}y \\ {\varPhi _0} \in {L^2} \\ \end{array} \right.$ | (12) |
由条件式(11)和式(12)可知,
${\varPhi _0}(x) = \int_{ - \infty }^{ + \infty } {\left( {{u_0} - U} \right)} (y){\rm d}y = 0$ |
定理1(渐近稳定性定理) 设
${N_0} = {\left\| {{u_0} - U} \right\|_{{H^1}}} + {\left\| {{\varPhi _0}} \right\|_{{H^2}}}$ |
则存在与
$\qquad\qquad u - U \in {C^0}(0,\infty ;{H^1}) \cap {L^2}(0,\infty ;{H^2})$ | (13) |
进一步,该解以最大范数的形式趋近于行波解
$\qquad\qquad\underset{x\in \mathbb{R}}{\mathop{\text{sup}}}\,\left| u(t,x)-U(x-ct) \right|\to 0,\;\; t\to \infty $ | (14) |
定理1的证明可以分为两部分:第一部分是证明解的整体存在性;第二部分则是证明解的渐近稳定性。
将
${\psi _t} - c{\psi _\xi } - \alpha {\psi _{\xi \xi }} + \beta {\psi _{\xi \xi \xi }} + {({{g}}(U + \psi ) - g(U))_\xi } = 0$ | (15) |
$\psi (0,\xi ) = {\psi _0}(\xi ) = ({u_0} - U)(\xi )$ | (16) |
于是,问题(14)就化为证明
令
$\begin{split}&{\varPhi _{t\xi }} - c{\varPhi _{\xi \xi }} - \alpha {\varPhi _{\xi \xi \xi }} + \\ &\qquad\beta {\varPhi _{\xi \xi \xi \xi }} + {({{g}}(U + {\varPhi _\xi }) - g(U))_\xi } = 0\end{split}$ | (17) |
将式(17)对
$\begin{split}&{\varPhi _t} - c{\varPhi _\xi } - \alpha {\varPhi _{\xi \xi }} + \beta {\varPhi _{\xi \xi \xi }} +\\ &\qquad{({{g}}(U + {\varPhi _\xi }) - g(U))} = 0\end{split}$ | (18) |
将式(18)线性化,则式(18)化为
${\varPhi _t} - c{\varPhi _\xi } - \alpha {\varPhi _{\xi \xi }} + \beta {\varPhi _{\xi \xi \xi }} + g'(U){\varPhi _\xi } = F(U,{\varPhi _\xi })$ | (19) |
其中,
$F(U,{\varPhi _\xi }) = - ({{g}}(U + {\varPhi _\xi }) - g(U)) + g'(U){\varPhi _\xi}$ |
而原初值
$\varPhi(0,\xi ) = {\varPhi _0}(\xi )$ | (20) |
定义初值问题式(18)和式(20)的解空间为
$X(0,T) = \left\{ {\varPhi \in {L^\infty }(0,T;{H^2}),{\varPhi _\xi } \in {L^2}(0,T;{H^2})} \right\}$ |
于是,有定理2。
定理2 假设
$\begin{split}&\left\| \varPhi \right\|_{{H^2}}^2 + \left\| \psi \right\|_{{H^1}}^2 +\\ &\quad\int_{0}^{t}{\left( {{\left\| \sqrt{\left| {{U}_{\xi }} \right|}\varPhi \right\|}^{2}}+{{\left\| {{\varPhi}_{\xi }} \right\|}^{2}}+{{\left\| {{\psi }_{\xi }} \right\|}^{2}}+{{\left\| {{\psi }_{\xi \xi }} \right\|}^{2}} \right)}{\rm d}\tau \leqslant \!\!\!\!\!\!\!\!\!\!\\ &\quad\quad{C_1}N_0^2\end{split}$ | (21) |
其中,
实际上,定理2中的
对于定理2的证明也可以分为两个部分:第一部分证明初值问题式(18)和式(20)解的局部存在性;第二部分证明解的全局存在性。对于初值问题式(18)和式(20)解的局部存在性的证明,可运用Galerkin方法按标准方式进行证明,可参考文献[12-13]等。本文省略证明而给出定理3。
定理3(局部存在性) 假设
$\left\| \varPhi \right\|_{{H^2}}^2 \leqslant K\left\| {{\varPhi _0}} \right\|_{{H^2}}^2$ | (22) |
对于定理2中的初值问题式(18)和式(20)的全局存在性及不等式(21),需要在局部解存在的基础上给出一致先验估计。
引理2(Young不等式[14]) a. 令
$ab \leqslant \theta {a^p} + C(\theta ){b^q}$ |
其中,
b. 如果
$\int_\varOmega {\left| {u(x)v(x)} \right|{\rm d}x} \leqslant \frac{1}{p}\left\| {u(x)} \right\|_p^p + \frac{1}{q}\left\| {v(x)} \right\|_q^q$ |
命题1(先验估计) 假设
$N(t) = \mathop {\sup }\limits_{0 \leqslant \tau \leqslant t} \{ {\left\| {\varPhi(\tau )} \right\|_{{H^2}}} + {\left\| {\psi (\tau )} \right\|_{{H^1}}}\} $ |
其中,
$\begin{split} &{N^2}(t) + \int_{0}^{t}{\left( {{\left\| \sqrt{\left| {{U}_{\xi }} \right|}\varPhi \right\|}^{2}}+{{\left\| {{\varPhi}_{\xi }} \right\|}^{2}}+{{\left\| {{\psi }_{\xi }} \right\|}^{2}}+{{\left\| {{\psi }_{\xi \xi }} \right\|}^{2}} \right)}{\rm d}\tau \leqslant\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\! \\ &\qquad{C_2}N_0^2\end{split}$ | (23) |
证明 通过对
a. 低阶先验估计。
首先,用
$\begin{split}&\varPhi{\varPhi _t} - c\varPhi{\varPhi _\xi } - \alpha \varPhi{\varPhi _{\xi \xi }} + \\ & \qquad\beta \varPhi{\varPhi _{\xi \xi \xi }} +g'(U)\varPhi{\varPhi _\xi } = \varPhi F(U,{\varPhi _\xi })\end{split}$ | (24) |
由于
$\begin{split} & \qquad \varPhi{\varPhi _t} = \frac{1}{2}{\left( {{\varPhi^2}} \right)_t}\\ & \qquad - c\varPhi{\varPhi _\xi } = - \frac{c}{2}{\left( {{\varPhi^2}} \right)_\xi }\\ & - \alpha \varPhi{\varPhi _{\xi \xi }} + \beta \varPhi{\varPhi _{\xi \xi \xi }} = \varPhi{( - \alpha {\varPhi _\xi } + \beta {\varPhi _{\xi \xi }})_\xi } =\\ &\qquad {\left[ {\varPhi( - \alpha {\varPhi _\xi } + \beta {\varPhi _{\xi \xi }})} \right]_\xi } - {\varPhi _\xi }( - \alpha {\varPhi _\xi } + \beta {\varPhi _{\xi \xi }})=\\ & \qquad {\left[ { - \alpha \varPhi{\varPhi _\xi } + \beta \varPhi{\varPhi _{\xi \xi }} - \frac{\beta }{2}\varPhi _\xi ^2} \right]_\xi } + \alpha {({\varPhi _\xi })^2}\\ & \qquad g'(U)\varPhi{\varPhi _\xi } = \frac{1}{2}{({{g}}'(U){\varPhi^2})_\xi } - \frac{1}{2}{{g}}''(U){U_\xi }{\varPhi^2} \end{split} $ |
故式(24)可写为
$\begin{split} {\left(\frac{1}{2}{\varPhi^2}\right)_t} +& \alpha \varPhi _\xi ^2 - \frac{1}{2}g''(U){U_\xi }{\varPhi^2} +\\ & {\left\{ {{Q_1}} \right\}_\xi } = \varPhi F(U,{\varPhi _\xi }) \end{split}$ | (25) |
其中,
${Q_1} = - \alpha \varPhi{\varPhi _\xi } + \beta \varPhi{\varPhi _{\xi \xi }} - \frac{\beta }{2}\varPhi _\xi ^2 - \frac{c}{2}{\varPhi^2} + \frac{1}{2}g'(U){\varPhi^2}$ |
又
$\begin{split}&\frac{1}{2}{({\varPhi^2})_t} - {\lambda _0}\varPhi _\xi ^2 - \frac{1}{2}{{g}}''(U){U_\xi }{\varPhi^2} +\\ & \qquad\quad{\left\{ {{Q_1}} \right\}_\xi } \leqslant \varPhi F(U,{\varPhi _\xi })\end{split}$ | (26) |
将式(26)分别对
$\begin{gathered} \frac{1}{2}{\left\| \varPhi \right\|^2} + \int_0^t {\int_\mathbb{R} {\biggr( - {\lambda _0}\varPhi _\xi ^2} } - \frac{1}{2}{{g}}''(U){U_\xi }{\varPhi^2}\biggr){\rm d}\xi {\rm d}\tau \leqslant \\ \frac{1}{2}{\left\| {{\varPhi _0}} \right\|^2} + \int_0^t {\int_\mathbb{R} {\varPhi F(U,{\varPhi _\xi }){\rm d}\xi {\rm d}\tau } } \\ \end{gathered} $ |
即
$\begin{split} &\frac{1}{2}{\left\| \varPhi \right\|^2} + \int_{0}^{t}{\left( \left| {{\lambda }_{0}} \right|{{\left\| {{\varPhi}_{\xi }} \right\|}^{2}}+\left| \frac{1}{2}{g}''(U) \right|{{\left\| \sqrt{\left| {{U}_{\xi }} \right|}\varPhi \right\|}^{2}} \right)}{\rm d}\tau\leqslant \!\!\!\!\!\!\!\!\!\!\! \!\!\!\!\!\!\!\!\!\!\! \\ &\qquad \frac{1}{2}{\left\| {{\varPhi _0}} \right\|^2} + \int_0^t {\int_\mathbb{R} {\left| {\varPhi F(U,{\varPhi _\xi })} \right|{\rm d}\xi {\rm d}\tau}} \end{split}$ | (27) |
由此推知,存在
$\begin{split}&{\left\| \varPhi \right\|^2} + \int_{0}^{t}{\left( {{\left\| \sqrt{\left| {{U}_{\xi }} \right|}\varPhi \right\|}^{2}}+{{\left\| {{\varPhi}_{\xi }} \right\|}^{2}} \right)}{\rm d}\tau \leqslant\\ &\qquad{C_1}\left\{ {{{\left\| {{\varPhi _0}} \right\|}^2} + \int_0^t {\int_\mathbb{R} {\left| {\varPhi F(U,{\varPhi _\xi })} \right|{\rm d}\xi {\rm d}\tau } } } \right\}\end{split}$ |
b. 高阶先验估计。
将式(15)线性化,有
$\begin{split}{\psi _t} - c{\psi _\xi } &- \alpha {\psi _{\xi \xi }} + \beta {\psi _{\xi \xi \xi }} +\\ &\;g''(U){U_\xi }\psi + f'(U){\psi _\xi } = {F_\xi }\end{split}$ | (28) |
令
$\begin{split}&G = [{{g}}'(U) - {{g}}'(U + \psi )]{U_\xi } - [{{g}}'(U + \psi ) -\\ &\qquad {{g}}'(U)]{\psi _\xi } + {{g}}''(U){U_\xi }\psi \end{split}$ |
(a) 用
$\begin{split}&\psi {\psi _t} - c\psi {\psi _\xi } - \alpha \psi {\psi _{\xi \xi }} + \beta \psi {\psi _{\xi \xi \xi }} + \\ &\qquad g''(U){U_\xi }{\psi ^2} +g'(U)\psi {\psi _\xi } = \psi G\end{split}$ | (29) |
类似于对式(24)的分析,则式(29)可改写为
$\qquad\frac{1}{2}{({\psi ^2})_t} + \alpha \psi _\xi ^2 + \frac{1}{2}g''(U){U_\xi }{\psi ^2} + {\left\{ {{Q_2}} \right\}_\xi } = \psi G$ | (30) |
其中,
${Q_2} = - \alpha \psi {\psi _\xi } + \beta \psi {\psi _{\xi \xi }} - \frac{1}{2}\beta \psi _\xi ^2 - \frac{c}{2}{\psi ^2} + \frac{1}{2}{\rm{g}}'(U){\psi ^2}$ |
又由
$\qquad\frac{1}{2}{({\psi ^2})_t} - {\lambda _0}{\psi _\xi }^2 + {\left\{ {{Q_2}} \right\}_\xi } \leqslant \psi G - \frac{1}{2}g''(U){U_\xi }{\psi ^2}\!\!\!\!\!\!$ | (31) |
将式(31)分别对t和
$\begin{split}& \frac{1}{2}{\left\| \psi \right\|^2} + \int_0^t {\left| {{\lambda _0}} \right|\left\| {{\psi _\xi }} \right\|^2} {\rm d}\tau \leqslant \frac{1}{2}{\left\| {{\psi _0}} \right\|^2} +\\ &\qquad\quad \int_0^t {\int_\mathbb{R} {\left| {(\psi G - \frac{1}{2}g''(U){U_\xi }{\psi ^2})} \right|} } {\rm d}\xi {\rm d}\tau \end{split}$ | (32) |
(b) 将式(28)对
$\begin{split}&{\psi _{t\xi }} - c{\psi _{\xi \xi }} - \alpha {\psi _{\xi \xi \xi }} + \beta {\psi _{\xi \xi \xi \xi }} + \\ &\qquad g'{(U)_{\xi \xi }}\psi +2g'{(U)_\xi }{\psi _\xi } + g'(U){\psi _{\xi \xi }} = {G_\xi }\end{split}$ | (33) |
用
$\begin{split} &{\psi _\xi }{\psi _{t\xi }} - c{\psi _\xi }{\psi _{\xi \xi }} - \alpha {\psi _\xi }{\psi _{\xi \xi \xi }} + \beta {\psi _\xi }{\psi _{\xi \xi \xi \xi }} + \\ & \qquad g'{(U)_{\xi \xi }}\psi {\psi _\xi } +2g'{(U)_\xi }{\psi _\xi }^2 + \\ & \qquad g'(U){\psi _\xi }{\psi _{\xi \xi }} = {\psi _\xi }{G_\xi } \end{split} $ | (34) |
同理,类似于对式(24)的分析,则式(34)可改写为
$\begin{split}&\frac{1}{2}{(\psi {_\xi ^2})_t} + \alpha \psi {_{\xi \xi }^2} + {\left\{ {{Q_3}} \right\}_\xi } = \\ &\qquad - \frac{3}{2}g''(U){U_\xi }\psi {_\xi ^2} -g'{(U)_{\xi \xi }}\psi {\psi _\xi } + {\psi _\xi }{G_\xi }\end{split}$ | (35) |
其中,
${Q_3} = - \frac{c}{2}{\psi _\xi }^2 - \alpha {\psi _\xi }{\psi _{\xi \xi }} + \beta {\psi _\xi }{\psi _{\xi \xi \xi }} - \frac{\beta }{2}{\psi _{\xi \xi }}^2 + \frac{1}{2}{\rm{g}}'(U){\psi _\xi }^2$ |
又式(35)左边第2项有
$\begin{split}&\frac{1}{2}{(\psi _\xi ^2)_t} - {\lambda _0}\psi _{\xi \xi }^2 + {\left\{ {{Q_3}} \right\}_\xi } \leqslant \\ &\quad\quad - \frac{3}{2}{\rm{g}}''(U){U_\xi }\psi {_\xi ^2} -g'{(U)_{\xi \xi }}\psi {\psi _\xi } + {\psi _\xi }{G_\xi }\end{split}$ | (36) |
将式(36)分别对
$\begin{split} &\frac{1}{2}{\left\| {\psi _\xi } \right\|^2} + \int_0^t {\left| {{\lambda _0}} \right|{{\left\| {\psi _{\xi \xi }} \right\|}^2}} {\rm d}\tau \leqslant \\ & \qquad \frac{1}{2}{\left\| {\psi _{0\xi }} \right\|^2} + \int _0 ^t {\int _\mathbb{R} {\Biggr( - \frac{3}{2}g''(U){U_\xi }\psi {{_\xi }^2}} } -\\ &\qquad g'{(U)_{\xi \xi }}\psi {\psi _\xi } + {\psi _\xi }{G_\xi }\Biggr)\,{\rm d}\xi {\rm d}\tau \end{split} $ | (37) |
再将式(27),式(32),式(37)这3式相加,有
$\begin{gathered} \frac{1}{2}{\left\| \varPhi \right\|^2} + \frac{1}{2}{\left\| \psi \right\|^2} + \frac{1}{2}{\left\| {\psi _\xi } \right\|^2} + \int_{0}^{t}{\left( \left| \frac{1}{2}{g}''(U) \right|{{\left\| \sqrt{\left| {{U}_{\xi }} \right|}\varPhi \right\|}^{2}}+\left| {{\lambda }_{0}} \right|{{\left\| {{\varPhi}_{\xi }} \right\|}^{2}}+\left| {{\lambda }_{0}} \right|{{\left\| {{\psi }_{\xi }} \right\|}^{2}}+\left| {{\lambda }_{0}} \right|{{\left\| {{\psi }_{\xi \xi }} \right\|}^{2}} \right)}\text{d}\tau \leqslant \frac{1}{2}{\left\| {{\varPhi _0}} \right\|^2} + \frac{1}{2}{\left\| {{\psi _0}} \right\|^2} +\\ \frac{1}{2}{\left\| {\psi _{0\xi }} \right\|^2} + \int_0^t {\int_\mathbb{R} {\Bigg(\left| {\frac{1}{2}g''(U){U_\xi }{\psi ^2}} \right| + \left| {\frac{3}{2}g''(U){U_\xi }\psi {{_\xi }^2}} \right| + \left| {g'{{(U)}_{\xi \xi }}\psi {\psi _\xi }} \right| + \left| {\varPhi F(U,{\varPhi _\xi })} \right| + \left| {\psi G} \right| + \left| {{\psi _\xi }{G_\xi }} \right|} } \Bigg){\rm d}\xi {\rm d}\tau \\ \end{gathered} $ |
进而存在常数
$ \begin{split} &{\left\| \varPhi \right\|^2} + {\left\| \psi \right\|^2} + {\left\| {\psi _\xi } \right\|^2} + \int_0^t {\Bigg({{\left\| {\sqrt {\left| {{U_\xi }} \right|} \varPhi} \right\|}^2} + {{\left\| {{\varPhi _\xi }} \right\|}^2} + \left\| {{\psi _\xi }} \right\|^2} + {\left\| {\psi _{\xi \xi }} \right\|^2}\Bigg){\rm d}\tau \leqslant {C_2}\Biggr\{ {\left\| {{\varPhi _0}} \right\|^2} + {\left\| {{\psi _0}} \right\|^2} +{\left\| {\psi _{0\xi }} \right\|^2} +\\ &\qquad \int_0^t {\int_\mathbb{R} {\Bigg(\left| {g''(U){U_\xi }{\psi ^2}} \right| + \left| {g''(U){U_\xi }\psi {{_\xi }^2}} \right| + \left| {g'{{(U)}_{\xi \xi }}\psi {\psi _\xi }} \right| + \left| {\varPhi F(U,{\varPhi _\xi })} \right| + \left| {\psi G} \right| + \left| {{\psi _\xi }{G_\xi }} \right|} } \Bigg){\rm d}\xi {\rm d}\tau\Biggr\} \end{split} $ | (38) |
注意到式(38)右端第4项,利用Young不等式,有
$\int_\mathbb{R} \!\!{g''\!(U){U_\xi }{\psi ^2}{\rm d}\xi } \!\leqslant\! \eta {(g''(U))^2}{\left\| \psi \right\|^2} \!+\! C(\eta ){U_\xi }^2{\left\| \psi \right\|^2}\!\!\!\!\!\!\!\!\!\!$ | (39) |
其中,
同理,对式 (38) 中的
$\begin{split} & {{\left\| \varPhi \right\|}^{2}}+{{\left\| \psi \right\|}^{2}}+{{\left\| {{\psi }_{\xi }} \right\|}^{2}}+\int_{0}^{t}\left({{{\left\| \sqrt{\left| {{U}_{\xi }} \right|}\varPhi \right\|}^{2}}+}{{\left\| {{\varPhi}_{\xi }} \right\|}^{2}}+ \right.\\ &\qquad\left. {{\left\| {{\psi }_{\xi }} \right\|}^{2}}+{{\left\| {{\psi }_{\xi \xi }} \right\|}^{2}}\right)\text{d}\tau \leqslant{{C}_{3}}\{{{\left\| {{\varPhi}_{0}} \right\|}^{2}}+{{\left\| {{\psi }_{0}} \right\|}^{2}}+ {{\left\| {{\psi }_{0\xi }} \right\|}^{2}}+\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\\ &\qquad\int_{0}^{t}{\int_\mathbb{R}{\left( \left| \varPhi F(U,{{\varPhi}_{\xi }}) \right|+\left| \psi G \right|+\left| {{\psi }_{\xi }}{{G}_{\xi }} \right| \right)}}\text{d}\xi \text{d}\tau \end{split} $ | (40) |
又因为,
$\begin{split} &F(U,{\varPhi _\xi }) = - \{ ({{g}}(U + {\varPhi _\xi }) - g(U)) - g'(U){\varPhi _\xi }\} =\\ &\quad- \{ g'(U + \theta {\varPhi _\xi }){\varPhi _\xi } - g'(U){\varPhi _\xi }\} \end{split}$ |
$\begin{split} &G = {F_\xi } = - \{ g'(U + {\varPhi _\xi })({U_\xi } + {\varPhi _{\xi \xi }}) - \\ &\quad g'(U){U_\xi } - g'(U){\varPhi _{\xi \xi }} - g''(U){U_\xi }{\varPhi _\xi }\}=\\ &\quad - \{ g''(U + \theta {\varPhi _\xi }){U_\xi }{\varPhi _\xi } + g''(U + \theta {\varPhi _\xi }){\varPhi _\xi }{\varPhi _{\xi \xi }} -\\ &\quad g''(U){U_\xi }{\varPhi _\xi }\} \end{split} $ |
所以,
$\begin{split} &\int_\mathbb{R} {\varPhi F(U,{\varPhi _\xi })} {\rm d}\xi = \int_\mathbb{R} {({{g}}'(U + \theta {\varPhi _\xi }) - {{g}}'(U))\varPhi{\varPhi _\xi }} {\rm d}\xi = \quad\quad\\ &\;\;\;\int_\mathbb{R} \left({{{ (({{g}}'(U + \theta {\varPhi _\xi }) - {{g}}'(U)){\varPhi^2})}_\xi } - ({{g}}'(U + \theta {\varPhi _\xi }) - } \right. \\ &\;\;\;\left.{{g}}'(U))'{\varPhi^2}\right){\rm d}\xi = - \int_\mathbb{R} {({{g}}''(U + \theta {\varPhi _\xi }) - {{g}}''(U)){U_\xi }{\varPhi^2}{\rm d}\xi } \end{split} $ |
利用Young不等式,可得
$\begin{split} &\int_\mathbb{R} {\left| {(g''(U + \theta {\varPhi _\xi }) - g''(U)){U_\xi }{\varPhi^2}} \right|{\rm d}\xi } \leqslant \\ &\qquad{\eta _0}{(g''(U + \theta {\varPhi _\xi }) - g''(U))^2}{\left\| \varPhi \right\|^2} + C({\eta _0}){U_\xi }^2{\left\| \varPhi \right\|^2}\end{split}\quad\quad\quad $ |
即
$\begin{gathered} \int_\mathbb{R} {\varPhi F(U,{\varPhi _\xi })} {\rm d}\xi \leqslant {\eta _0}{(g''(U + \theta {\varPhi _\xi }) - g''(U))^2}{\left\| \varPhi \right\|^2} + \\ C({\eta _0}){U_\xi }^2{\left\| \varPhi \right\|^2} { \leqslant {C_4}{{\left| {{u_-} - {u_ + }} \right|}^2}{{\left\| \varPhi \right\|}^2}} \end{gathered} $ |
同样,利用Young不等式,可得
$ \begin{split} &\qquad\int_\mathbb{R} {\left| {g''(U + \theta {\varPhi _\xi }){U_\xi }\psi {\varPhi _\xi }} \right|{\rm d}\xi } \leqslant \\ & \qquad\qquad {\eta _1}{(g''(U + \theta {\varPhi _\xi }))^2}{\left\| \psi \right\|^2} +C({\eta _1}){U_\xi }^2{\left\| {{\varPhi _\xi }} \right\|^2}\\ &\int_\mathbb{R} {\left| {g''(U + \theta {\varPhi _\xi })\psi {\varPhi _\xi }{\varPhi _{\xi \xi }}} \right|{\rm d}\xi } \leqslant \\ & \qquad{C_5}N(t)\int_\mathbb{R} {g''(U + \theta {\varPhi _\xi }){\varPhi _\xi }{\varPhi _{\xi \xi }}{\rm d}\xi } \leqslant \\ &\qquad{C_5}N(t)({\eta _2}{\left\| {{\varPhi _\xi }} \right\|^2} + C({\eta _2}){(g''(U + \theta {\varPhi _\xi }))^2}{\left\| {{\varPhi _{\xi \xi }}} \right\|^2}) \\ &\int_\mathbb{R} {\left| { - g''(U){U_\xi }\psi {\varPhi _\xi }} \right|{\rm d}\xi }\! \leqslant\! {\eta _3}{(g''(U))^2}{\left\| \psi \right\|^2} \!\!+\! C({\eta _3}){U_\xi }^2{\left\| {{\varPhi _\xi }} \right\|^2} \end{split} $ |
所以,
$\int_\mathbb{R} {\left| {\psi G} \right|{\rm d}\xi \leqslant {C_6}N(t)({{\left\| \psi \right\|}^2} + {{\left\| {{\psi _\xi }} \right\|}^2})}\!\! + {\left| {{u_-}\! - {u_ + }} \right|^2}{\left\| \psi \right\|^2}\!\!\!\!\!\!\!$ | (41) |
同理,类似于对
$\quad \int_\mathbb{R} {\left| {{\psi _\xi }{G_\xi }} \right|{\rm d}\xi } \leqslant {C_7}N(t){\left\| {{\psi _\xi }} \right\|^2} + {\left| {{u_-} - {u_ + }} \right|^2}{\left\| {{\psi _{\xi \xi }}} \right\|^2}\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!$ | (42) |
从而
$\begin{split} &\int_0^t {\int_\mathbb{R} {(\left| {\varPhi F(U,{\varPhi _\xi })} \right| + \left| {\psi G} \right| + \left| {{\psi _\xi }{G_\xi }} \right|} } ){\rm d}\xi {\rm d}\tau \leqslant \\ &\quad\qquad{C_8}N(t)\int_0^t {({{\left\| \psi \right\|}^2} + {{|| {{\psi _\xi }} ||}^2})} {\rm d}\tau + \\ &\quad\qquad{C_9}{\left| {{u_-} - {u_ + }} \right|^2}\int_0^t {({{\left\| \varPhi \right\|}^2} + {{\left\| \psi \right\|}^2} + {{|| {{\psi _{\xi \xi }}} ||}^2}){\rm d}\tau } \end{split} \!\!\!\!\!\!\!\!\!$ | (43) |
由于在局部存在的时间区间内,式(22)成立,可得
$\begin{gathered} N(t) \leqslant \mathop {\sup }\limits_{0 \leqslant \tau \leqslant T} \{ K\left\| {{\varPhi _0}(\tau )} \right\|_{{H^2}}^2 + K|| {{\varPhi _{0\xi }}(\tau )} ||_{{H^1}}^2\} = \\ \qquad\quad K\mathop {\sup }\limits_{0 \leqslant \tau \leqslant T} \{ \left\| {{\varPhi _0}(\tau )} \right\|_{{H^2}}^2 + \left\| {{\psi _0}(\tau )} \right\|_{{H^1}}^2\} = KN_0^2(t) \end{gathered} \qquad\quad$ |
故存在
现证明定理1
证明 a. 因为,命题1成立,由定理3可知,
$\varPhi \in X(0,\infty ) = \left\{ {\varPhi \in {C^0}(0,\infty ;{H^2}),{\varPhi _\xi } \in {L^2}(0,\infty ;{H^2})} \right\}$ |
由此可知:
(a)
据(a)和(b)可以推知
$\psi \in {C^0}(0,\infty ;{H^1}) \cap {L^2}(0,\infty ;{H^2})$ |
即
$u - U \in {C^0}(0,\infty ;{H^1}) \cap {L^2}(0,\infty ;{H^2})$ |
故式(13)得证。
b. 由式(23)可知,
$\mathop {\lim }\limits_{t \to \infty } \;\left\| {\psi (t)} \right\|_{{H^2}}^2 = 0$ | (44) |
式(44)意味着对任意的
$\int_0^t {\left\| \psi \right\|_{{H^2}}^2{\rm d}\tau } \leqslant {C_2}N_0^2$ |
由于被积函数非负,可知
$\mathop {\lim }\limits_{t \to \infty }\; \left\| {\psi (t)} \right\|_{{H^2}}^2 = 0$ | (45) |
根据不等式[15]
${\left\| f \right\|_{{L^\infty }({R^n})}} \leqslant C{\left\| f \right\|_{{H^{\frac{n}{2}}}({R^n})}}$ |
可得
$\mathop {\sup }\limits_{x \in \mathbb{R}} \left| \psi \right| \leqslant C{\left\| \psi \right\|_{{H^2}}}$ |
由此,根据式(45)即可推得
$ \mathop {\sup }\limits_{x \in \mathbb{R}} \left| \psi \right| \to 0,\text{当}\;t \to \infty $ |
故定理1得证。
4 方程(1)单调递减扭状孤波解扰动的衰减估计前面已经证明了方程(1)单调递减行波解
引理3(Gagliardo-Nirenberg不等式[16]) 假设
${\left\| {{D^j}u} \right\|_r} \leqslant C\left\| u \right\|_p^{1 - \lambda }\left\| {{D^m}u} \right\|_q^\lambda $ |
其中,
$\frac{1}{r} - \frac{j}{n} = \lambda \left(\frac{1}{q} - \frac{m}{n}\right) + (1 - \lambda )\frac{1}{p},\;\frac{j}{m} \leqslant \lambda \leqslant 1$ |
当
定理4 假设
$\begin{split} (1 + t){\left\| {{\varPhi _\xi }} \right\|^2} &+ \int_0^t \left({(1 + \tau ){{\left\| {{\varPhi _\xi }} \right\|}^2} + (1 + \tau ){{\left\| {{\varPhi _{\xi \xi }}} \right\|}^2}}\right) {\rm d}\tau \leqslant\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\\ &{C_{11}}{\sigma _0}^2 \end{split}\qquad$ | (46) |
成立。其中,
证明 将式(19)对
$ \begin{split} &{\varPhi _{t\xi }} - c{\varPhi _{\xi \xi }} - \alpha {\varPhi _{\xi \xi \xi }} + \beta {\varPhi _{\xi \xi \xi \xi }} + \\ &\qquad{(g'(U){\varPhi _\xi })_\xi } = {F_\xi }(U,{\varPhi _\xi }) \end{split} $ | (47) |
在式(47)两边同乘以
$\begin{split} &(1 + t){\varPhi _\xi }{\varPhi _{t\xi }} - c(1 + t){\varPhi _\xi }{\varPhi _{\xi \xi }} - \alpha (1 + t){\varPhi _\xi }{\varPhi _{\xi \xi \xi }} +\\ &\qquad\quad\beta (1 + t){\varPhi _\xi }{\varPhi _{\xi \xi \xi \xi }} + (1 + t){\varPhi _\xi }{(g'(U){\varPhi _\xi })_\xi } = \\ & \quad\qquad(1 + t){\varPhi _\xi }{F_\xi }(U,{\varPhi _\xi }) \\ \end{split} $ | (48) |
由于
$\begin{split}&(1 + t){\varPhi _\xi }{\varPhi _{t\xi }} = {\left(\frac{1}{2}(1 + t){\varPhi _\xi }^2\right)_t} - \frac{1}{2}{\varPhi _\xi }^2- c(1 + t){\varPhi _\xi }{\varPhi _{\xi \xi }} = \\ &\quad{\left(\frac{c}{2}(1 + t){\varPhi _\xi }^2\right)_\xi} - \alpha (1 + t){\varPhi _\xi }{\varPhi _{\xi \xi \xi }} + \beta (1 + t){\varPhi _\xi }{\varPhi _{\xi \xi \xi \xi }} =\\ &\quad (1 + t){\varPhi _\xi }{( - \alpha {\varPhi _{\xi \xi }} + \beta {\varPhi _{\xi \xi \xi }})_\xi }={[(1 + t){\varPhi _\xi }( - \alpha {\varPhi _{\xi \xi }} +}\\ &\quad{ \beta {\varPhi _{\xi \xi \xi }})]_\xi } -(1 + t){\varPhi _\xi }{( - \alpha {\varPhi _{\xi \xi }} + \beta {\varPhi _{\xi \xi \xi }})}=\\ &\quad{[(1 + t){\varPhi _\xi }( - \alpha {\varPhi _{\xi \xi }} + \beta {\varPhi _{\xi \xi \xi }})]_\xi }-\alpha (1 + t){\varPhi _{\xi \xi }}^2 - \\ & \quad \frac{\beta }{2}(1 + t){({\varPhi _{\xi \xi }}^2)_\xi }= \Biggr\{ (1 + t){\varPhi _\xi }( - \alpha {\varPhi _{\xi \xi }} + \beta {\varPhi _{\xi \xi \xi }}) -\\ &\quad { \frac{\beta }{2}(1 + t){\varPhi _{\xi \xi }}^2\Biggr\} } \,_{_\xi} + \alpha (1 + t){\varPhi _{\xi \xi }}^2 \end{split}$ |
$\begin{split} &(1 + t){\varPhi _\xi }{(g'(U){\varPhi _\xi })_\xi } = {[(1 + t){\varPhi _\xi }g'(U){\varPhi _\xi }]_\xi } -\qquad\\ &\quad(1 + t){\varPhi _{\xi \xi }}{\varPhi _\xi }g'(U) (1 + t)g''(U){U_\xi }{\varPhi _\xi }^2 +\\ &\quad{\{ (1 + t){\varPhi _\xi }^2g'(U)\} _\xi } \end{split} $ |
故式(48)可写为
$\begin{split} &{\left(\frac{1}{2}(1 + t){\varPhi _\xi }^2\right)_t} + \alpha (1 + t){\varPhi _{\xi \xi }}^2 + \frac{1}{2}(1 + t)g''(U){U_\xi }{\varPhi _\xi }^2 + \!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\\ &\qquad {\{ {Q_4}\} _\xi } = \frac{1}{2}{\varPhi _\xi }^2 + (1 + t){\varPhi _\xi }{F_\xi }(U,{\varPhi _\xi }) \end{split} $ | (49) |
其中,
$\begin{split}&{Q_4} = \frac{c}{2}(1 + t){\varPhi _\xi }^2 + (1 + t){\varPhi _\xi }( - \alpha {\varPhi _{\xi \xi }} + \beta {\varPhi _{\xi \xi \xi }}) -\\ &\qquad\frac{\beta }{2}(1 + t){\varPhi _{\xi \xi }}^2 + (1 + t){\varPhi _\xi }^2{{g}}'(U)\end{split}$ |
又由式(49)左边第2项,有
$\alpha (1 + t){\varPhi _{\xi \xi }}^2 \geqslant - {\lambda _0}(1 + t){\varPhi _{\xi \xi }}^2$ | (50) |
将式(50)代入式(49)中,有
$ \begin{split}&{\left(\frac{1}{2}(1 + t){\varPhi _\xi }^2\right)_t} - {\lambda _0}(1 + t){\varPhi _{\xi \xi }}^2{\rm{ + }}\frac{1}{2}(1 + t)g''(U){U_\xi }{\varPhi _\xi }^2 + \!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\\ &\qquad{\{ {Q_4}\} _\xi } \leqslant \frac{1}{2}{\varPhi _\xi }^2 + (1 + t){\varPhi _\xi }{F_\xi }(U,{\varPhi _\xi }) \end{split} $ | (51) |
将式(51)分别对
$\begin{split} & \frac{1}{2}(1+t){{\left\| {{\varPhi}_{\xi }} \right\|}^{2}}+\int_{0}^{t}\int_\mathbb{R}\Biggr(\left| {{\lambda }_{0}}(1+\tau ){{\varPhi}_{\xi \xi }}^{2} \right|+ \\ &\quad \left| \frac{1}{2}(1+\tau ){g}''(U){{U}_{\xi }}{{\varPhi}_{\xi }}^{2} \right|\Biggr){\rm d}\xi {\rm d}\tau \frac{1}{2}{{\left\| {{\varPhi}_{0\xi }} \right\|}^{2}}+ \\ &\quad \int_{0}^{t}{\int_\mathbb{R}{\left(\left| \frac{1}{2}{{\varPhi}_{\xi }}^{2} \right|+\left| (1+\tau ){{\varPhi}_{\xi }}{{F}_{\xi }}(U,{{\varPhi}_{\xi }}) \right|\right){\rm d}\xi {\rm d}\tau }} \end{split} \qquad\;\;$ |
故存在常数
$\begin{split} & \!\!\!\!\!\!(1 \!\!+\!\! t){\left\| {{\varPhi _\xi }} \right\|^2} \!\!+\!\! \int_0^t {\int_\mathbb{R} {((1 \!\!+\!\! \tau ){\varPhi _{\xi \xi }}^2 \!\!+\!\! (1 \!\!+\!\! \tau ){\varPhi _\xi }^2){\rm d}\xi {\rm d}\tau } } \leqslant \!\!\!\!\!\!\!\!\!\!\!\!\!\!\\ &\;\;\;\!\!{C_{10}}\biggr\{ {\left\| {{\varPhi _{0\xi }}} \right\|^2} \!\!+\!\! \int_0^t {\int_\mathbb{R} \!\!{({\varPhi _\xi }^2 \!\!\!+\!\!\! (1 \!\!+\!\! \tau ){\varPhi _\xi }{F_\xi }(U,{\varPhi _\xi })){\rm d}\xi {\rm d}\tau \biggr\} } } \!\!\!\!\!\!\!\!\! \end{split}$ | (52) |
又由式(23)可知
$\int_0^t {{{\left\| {{\varPhi _\xi }} \right\|}^2}{\rm d}\tau \leqslant {C_2}N_0^2 \leqslant {C_2}{{\left| {{u_-} - {u_ + }} \right|}^2}} $ |
注意到式(52)中,
$\begin{split}&\int_0^t {\int_\mathbb{R} {(1 + \tau ){\varPhi _\xi }{F_\xi }(U,{\varPhi _\xi }){\rm d}\xi {\rm d}\tau } } =\qquad\qquad\\ &\quad\int_0^t {\int_\mathbb{R} {(1 + \tau )\psi G(U,{\varPhi _\xi }){\rm d}\xi {\rm d}\tau } } \end{split}$ |
类似于对式(41)与式(43)的分析,有
$\begin{split} &\int_0^t {\int_\mathbb{R} {(1 + \tau )\psi G(U,{\varPhi _\xi }){\rm d}\xi {\rm d}\tau } } \leqslant \\ &\quad\int_0^t {(1 + \tau )({C_6}N(t)({{\left\| \psi \right\|}^2} + {{\left\| {{\psi _\xi }} \right\|}^2}) + {{\left| {{u_-} - {u_ + }} \right|}^2}{{\left\| \psi \right\|}^2})} {\rm d}\tau \leqslant \\ &\quad\int_0^t {(1 + \tau )({C_6}N_0^2({{\left\| \psi \right\|}^2} + {{\left\| {{\psi _\xi }} \right\|}^2}) + {{\left| {{u_-} - {u_ + }} \right|}^2}{{\left\| \psi \right\|}^2})}{\rm d}\tau \leqslant \\ &\quad {\sigma _1}^2\int_0^t {(1 + \tau )({{\left\| \psi \right\|}^2} + {{\left\| {{\psi _\xi }} \right\|}^2})} {\rm d}\tau \\ \end{split} $ |
其中,
$\begin{split} (1 + t){\left\| {{\varPhi _\xi }} \right\|^2} &+ \int_0^t {\biggr((1 + \tau ){{\left\| {{\varPhi _\xi }} \right\|}^2} + (1 + \tau ){{\left\| {{\varPhi _{\xi \xi }}} \right\|}^2}\biggr)} {\rm d}\tau \leqslant \!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\\ &{C_{11}}\sigma _0^2 \end{split}$ | (53) |
故定理4得证。
定理5 假设
$\begin{split} & {{(1+t)}^{2}}{{\left\| {{\varPhi}_{\xi \xi }} \right\|}^{2}}+\int_{0}^{t}{\int_\mathbb{R}{\biggr({{(1+\tau )}^{2}}{{\varPhi}_{\xi \xi }}^{2}+}} \\ &\qquad {{(1+\tau )}^{2}}{{\varPhi}_{\xi \xi \xi }}^{2}\biggr)\text{d}\xi \text{d}\tau \leqslant{{C}_{12}}\sigma _{0}^{2} \end{split}$ |
成立,其中,
证明 将式(19)对
$\begin{split}&{\varPhi _{t\xi \xi }} - c{\varPhi _{\xi \xi \xi }} - \alpha {\varPhi _{\xi \xi \xi \xi }} + \beta {\varPhi _{\xi \xi \xi \xi \xi }} + \\ &\qquad{(g'(U){\varPhi _\xi })_{\xi \xi }} = {F_{\xi \xi }}(U,{\varPhi _\xi })\end{split}$ | (54) |
在式(54)两边同时乘以
$\begin{split} &{(1 \!+\! t)^2}{\varPhi _{\xi \xi }}{\varPhi _{t\xi \xi }} \!-\! c{(1 \!+\! t)^2}{\varPhi _{\xi \xi }}{\varPhi _{\xi \xi \xi }} -\\ &\quad \alpha {(1 \!+\! t)^2}{\varPhi _{\xi \xi }}{\varPhi _{\xi \xi \xi \xi }} \!+\! \beta {(1 \!+\! t)^2}{\varPhi _{\xi \xi }}{\varPhi _{\xi \xi \xi \xi \xi }} \!+ \\ &\quad {(1 \!+\! t)^2}{\varPhi _{\xi \xi }}{(g'(U){\varPhi _\xi })_{\xi \xi }} \!=\!{(1 \!+ t\!)^2}{\varPhi _{\xi \xi }}{F_{\xi \xi }}(U,{\varPhi _\xi }) \end{split} \!\!\!\!\!\!\!\!$ | (55) |
类似于对式(48)的分析,式(55)可改写为
$\begin{split} &\frac{1}{2}{({(1 + t)^2}{\varPhi _{\xi \xi }}^2)_t} + \alpha {(1 + t)^2}{\varPhi _{\xi \xi \xi }}^2 + \\ &\qquad\frac{1}{2}{(1 + t)^2}g''(U){U_\xi }{\varPhi _{\xi \xi }}^2 + {\{ {Q_5}\} _\xi } =\\ &\qquad (1 + t){\varPhi _{\xi \xi }}^2 + {(1 + t)^2}g''(U){U_\xi }{\varPhi _\xi }{\varPhi _{\xi \xi \xi }} + \\ &\qquad{(1 + t)^2}{\varPhi _{\xi \xi }}{F_{\xi \xi }}(U,{\varPhi _\xi }) \end{split} $ | (56) |
其中,
$\begin{split} &{Q_5} = \frac{c}{2}{(1 + t)^2}{\varPhi _{\xi \xi }}^2 + {(1 + t)^2}{\varPhi _{\xi \xi }}( - \alpha {\varPhi _{\xi \xi \xi }} + \beta {\varPhi _{\xi \xi \xi \xi }}) - \\ &\quad\frac{\beta }{2}{(1 + t)^2}{\varPhi _{\xi \xi \xi }}^2 { + {{(1 + t)}^2}{\varPhi _{\xi \xi }}(g''(U){U_\xi }{\varPhi _\xi } + g'(U){\varPhi _{\xi \xi }})} \end{split} $ |
又由式(56)左边第2项,有
$\alpha {(1 + t)^2}{\varPhi _{\xi \xi \xi }}^2 \geqslant - {\lambda _0}{(1 + t)^2}{\varPhi _{\xi \xi \xi }}^2$ | (57) |
将式(57)代入式(56)中,有
$\begin{split} &\frac{1}{2}{({(1 + t)^2}{\varPhi _{\xi \xi }}^2)_t} - {\lambda _0}{(1 + t)^2}{\varPhi _{\xi \xi \xi }}^2 +\\ &\qquad\frac{1}{2}{(1 + t)^2}g''(U){U_\xi }{\varPhi _{\xi \xi }}^2 + {\{ {Q_5}\} _\xi } \leqslant \\ &\qquad(1 + t){\varPhi _{\xi \xi }}^2 + {(1 + t)^2}g''(U){U_\xi }{\varPhi _\xi }{\varPhi _{\xi \xi \xi }} +\\ &\qquad{(1 + t)^2}{\varPhi _{\xi \xi }}{F_{\xi \xi }}(U,{\varPhi _\xi }) \end{split} $ | (58) |
将式(58)分别对
$ \begin{split} &\frac{1}{2}{(1 + t)^2}{\left\| {{\varPhi _{\xi \xi }}} \right\|^2} + \int_0^t {\int_\mathbb{R} { \biggr(\left|{{\lambda _0}{{(1 + \tau )}^2}{\varPhi _{\xi \xi \xi }}^2} \right| + } } \\ &\quad\left| {\frac{1}{2}{{(1 + \tau )}^2}g''(U){U_\xi }{\varPhi _{\xi \xi }}^2} \right|\biggr){\rm{d}}\xi {\rm{d}}\tau \frac{1}{2}{\left\| {{\varPhi _{0\xi \xi }}} \right\|^2} + \\ &\quad\int_0^t {\int_\mathbb{R} {\biggr(\left| {(1 + \tau ){\varPhi _{\xi \xi }}^2} \right| + \left| {{{(1 + \tau )}^2}g''(U){U_\xi }{\varPhi _\xi }{\varPhi _{\xi \xi \xi }}} \right| + } } \!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\\ &\quad\left| {{{(1 + \tau )}^2}{\varPhi _{\xi \xi }}{F_{\xi \xi }}(U,{\varPhi _\xi })} \right|\biggr){\rm{d}}\xi {\rm{d}}\tau \end{split} $ | (59) |
由式(53)可知
$\qquad\qquad {\int_0^t {(1 + \tau )\left\| {{\varPhi _{\xi \xi }}} \right\|} ^2}{\rm d}\tau \leqslant {C_{11}}{\sigma _0}^2$ |
注意到不等式(59)右端第3项,运用Young不等式,有
$\begin{split}&\int_0^t {\int_\mathbb{R} {g''(U){U_\xi }{\varPhi _\xi }{\varPhi _{\xi \xi \xi }}} {\rm d}\xi {\rm d}\tau } \leqslant \qquad\qquad\qquad\\ &\quad\int_0^t {\biggr(\eta {{(g''(U))}^2}{{\left\| {{\varPhi _\xi }} \right\|}^2} + C(\eta ){U_\xi }^2{{\left\| {{\varPhi _{\xi \xi \xi }}} \right\|}^2}\biggr){\rm d}\tau }\end{split} $ |
其中,
$\int_0^t \biggr( {\left\| {{\varPhi _\xi }} \right\|^2} + {\left\| {{\psi _{\xi \xi }}} \right\|^2}\biggr){\rm d}\tau \leqslant {C_2}N_0^2$ |
从而,
$\begin{split}&\int_0^t {\int_\mathbb{R} {g''(U){U_\xi }{\varPhi _\xi }{\varPhi _{\xi \xi \xi }}} {\rm d}\xi {\rm d}\tau } \leqslant\\ &\quad\int_0^t \biggr({\eta {{(g''(U))}^2}{{\left\| {{\varPhi _\xi }} \right\|}^2} + C(\eta ){U_\xi }^2{{\left\| {{\varPhi _{\xi \xi \xi }}} \right\|}^2}\biggr){\rm d}\tau \leqslant } {C_2}N_0^2\end{split}$ |
又
$\begin{split}&\int_0^t {\int_\mathbb{R} {{{(1 + \tau )}^2}{\varPhi _{\xi \xi }}{F_{\xi \xi }}(U,{\varPhi _\xi })} {\rm d}\xi {\rm d}\tau } =\\ &\quad\int_0^t {\int_\mathbb{R} {{{(1 + \tau )}^2}{\psi _\xi }{G_\xi }(U,{\varPhi _\xi })} {\rm d}\xi {\rm d}\tau }\end{split} $ |
根据式(42),有
$\begin{split} & \int_0^t {\int_\mathbb{R} {{{(1 + \tau )}^2}{\psi _\xi }{G_\xi }(U,{\varPhi _\xi })}{\rm d}\xi {\rm d}\tau } \leqslant \\ &\quad \int_0^t {{{(1 + \tau )}^2}({C_7}N(t){{\left\| {{\psi _\xi }} \right\|}^2} + {{\left| {{u_-} - {u_ + }} \right|}^2}{{\left\| {{\psi _{\xi \xi }}} \right\|}^2}){\rm d}\tau } \leqslant\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\! \\ &\quad { \sigma _2^2} \int_0^t {{{(1 + \tau )}^2}({C_7}N_0^2{{\left\| {{\psi _\xi }} \right\|}^2} + {{\left| {{u_-} - {u_ + }} \right|}^2}{{\left\| {{\psi _{\xi \xi }}} \right\|}^2}){\rm d}\tau } \leqslant \!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\\ &\quad \sigma _2^2\int_0^t {{{(1 + \tau )}^2}({{\left\| {{\psi _\xi }} \right\|}^2} + {{\left\| {{\psi _{\xi \xi }}} \right\|}^2}){\rm d}\tau } \end{split} $ | (60) |
其中,
$\begin{split} {(1 + t)^2}&{\left\| {{\varPhi _{\xi \xi }}} \right\|^2} + \int_0^t {\int_\mathbb{R} {({{(1 + \tau )}^2}{\varPhi _{\xi \xi }}^2 + } } {C_{12}}\sigma _0^2\\ &{(1 + \tau )^2}{\varPhi _{\xi \xi \xi }}^2){\rm d}\xi {\rm d}\tau \leqslant{C_{12}}\sigma _0^2 \end{split}$ | (61) |
故定理5得证。
由式(53)可知
$(1 + t){\left\| {{\varPhi _\xi }} \right\|^2} \leqslant {C_{11}}{\sigma _0}^2$ | (62) |
又由式(61),有
${(1 + t)^2}{\left\| {{\varPhi _{\xi \xi }}} \right\|^2} \leqslant {C_{12}}{\sigma _0}^2$ | (63) |
由式(62)和式(63),有
$\qquad\quad{(1 + t)^k}{\left\| {{D^k}\varPhi( \cdot ,t)} \right\|^2} \leqslant {C_{1k}}{\sigma _0}^2,k = 1,2$ |
因此,可得
$\qquad\quad {\left\| {{D^k}\varPhi( \cdot ,t)} \right\|_{{L^2}}} \leqslant {C_{1k}}{\sigma _0}{(1 + t)^{ - \frac{k}{2}}},\;k = 1,2$ |
再由Gargliado-Nirenberg不等式,得到
$\begin{split}&{\left\| {{\varPhi _\xi }( \cdot ,t)} \right\|_{{L^\infty }}} \leqslant {C_{13}}\left\| {{\varPhi _\xi }( \cdot ,t)} \right\|_{{L^2}}^{1 - \frac{1}{4}} \left\| {{D^2}{\varPhi _\xi }( \cdot ,t)} \right\|_{{L^2}}^{\frac{1}{4}} \leqslant \!\!\!\!\!\!\!\!\!\!\!\!\!\\ &\quad\qquad C{\sigma _0}{(1 + t)^{ - \frac{1}{4}}}\end{split}$ |
又
a. 组合KdV-Burgers方程(1)在耗散
b. 组合KdV-Burgers方程(1)单调递减扭状孤波解的扰动在
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