We will obtain a decay estimate of the Fourier image of the solution for problem (1.11)–(1.12) in this section. This approach enables us to provide the decay rate of the solution in the energy space by utilising Plancherel’s theorem along with some integral estimates, such as Lemma (1.1). Using the energy approach in Fourier space, we create the proper Lyapunov functionals for this problem. Lastly, we prove our major finding.
2.1 The energy method in the Fourier space
.
Now, we introduce the new variables to construct the Lyapunov functional in the Fourier space
$$\begin{aligned} \begin{aligned} &r=(\varsigma _{x}-\hbar -l\Im ),\qquad g=\varsigma _{t},\qquad v=a \hbar _{x},\qquad w=\hbar _{t} \\ &\phi =k_{0}(\Im _{x}-l\varsigma ),\qquad \varpi =\Im _{t}, \qquad\vartheta =\upsilon _{t},\qquad \sigma =\upsilon _{x}. \end{aligned} \end{aligned}$$
(2.1)
Then, the system (1.11) takes the following form
$$\begin{aligned} \textstyle\begin{cases} r_{t}-g_{x}+w+l\varpi =0, \\ g_{t}-r_{x}-k_{0}l\phi =0, \\ v_{t}-ay_{x}=0, \\ w_{t}-az_{x}-r+m\vartheta _{x}=0, \\ \phi _{t}-k_{0}\varpi _{x}+k_{0}lu=0, \\ \varpi _{t}-k_{0}\phi _{x}-lv+\aleph _{1}\varpi + \int _{ \wp _{1}}^{\wp _{2}}\aleph _{2}(s)\mathcal{Y} ( x, 1,s,t )\,ds=0, \\ \vartheta _{t}-k_{1}\sigma _{x}+\beta w_{x}-k_{2}\vartheta _{xx}=0, \\ \sigma _{t}-\vartheta _{x}=0, \\ s\mathcal{Y}_{t}+\mathcal{Y}_{\jmath}=0, \end{cases}\displaystyle \end{aligned}$$
(2.2)
with initial conditions
$$\begin{aligned} (r,g,v,w,\phi,\varpi,\vartheta,\sigma,\mathcal{Y}) (x,0)=(r_{0},g_{0},v_{0},w_{0}, \phi _{0},\varpi _{0},\vartheta _{0},\sigma _{0},f_{0}),\quad x\in \mathbb{R}, \end{aligned}$$
(2.3)
where
$$\begin{aligned} &r_{0}=(\varsigma _{0,x}-\hbar _{0}-l\Im _{0}),\qquad g_{0}=\varsigma _{1}, \qquad v_{0}=a \hbar _{0,x}, \qquad w_{0}=\hbar _{1}, \\ &\phi _{0}=k_{0}(\Im _{0,x}-l\varsigma _{0}), \qquad \varpi _{0}=\Im _{1},\qquad \vartheta _{0}=\upsilon _{1},\qquad \sigma _{0}=\upsilon _{0,x}. \end{aligned}$$
Hence, the problem (2.2)–(2.3) is written as
$$\begin{aligned} \textstyle\begin{cases} Z_{t}+\mathcal{A}Z_{x}+\mathcal{L}Z=\mathcal{B}Z_{xx}, \\ Z(x,0)=Z_{0}(x), \end{cases}\displaystyle \end{aligned}$$
(2.4)
with \(Z=(r,g,v,w,\phi,\varpi,\vartheta,\sigma,\mathcal{Y})^{T}, Z_{0}=(r_{0},g_{0},v_{0},w_{0},\phi _{0},\varpi _{0},\vartheta _{0}, \sigma _{0},f_{0})\) and
$$\begin{aligned} \begin{aligned} &\mathcal{A}Z= \begin{pmatrix} -g \\ -r \\ -ay \\ -az+m\vartheta \\ -k_{0}\varpi \\ -k_{0}\phi \\ -k_{1}\sigma +\beta w \\ -\vartheta \\ 0 \end{pmatrix},\qquad \mathcal{L}Z= \begin{pmatrix} w+l\varpi \\ -k_{0}l\phi \\ 0 \\ r \\ k_{0}lu \\ -lv+\aleph _{1}\varpi +\int _{\wp _{1}}^{\wp _{2}}\aleph _{2}(s) \mathcal{Y} ( x, 1,s,t )\,ds \\ 0 \\ 0 \\ \frac{1}{s}\mathcal{Y}_{\jmath} \end{pmatrix},\\ &\mathcal{B}Z= \begin{pmatrix} 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ 0 \\ k_{2}\vartheta \\ 0 \\ 0 \end{pmatrix}. \end{aligned} \end{aligned}$$
(2.5)
Utilizing the Fourier transform to (2.4), we get
$$\begin{aligned} \textstyle\begin{cases} \widehat{Z}_{t}+i\imath \mathcal{A}\widehat{Z}+\mathcal{L}\widehat{Z}=- \imath ^{2}\mathcal{B}\widehat{Z}, \\ \widehat{Z}(\imath,0)=\widehat{Z}_{0}(\imath ), \end{cases}\displaystyle \end{aligned}$$
(2.6)
where \(\widehat{Z}(\imath,t)=(\widehat{r},\widehat{g},\widehat{v}, \widehat{w},\widehat{\phi},\widehat{\varpi},\widehat{\vartheta}, \widehat{\sigma},\widehat{\mathcal{Y}})^{T}(\imath,t)\). The equation (2.6)1 can be stated as
$$\begin{aligned} \textstyle\begin{cases} \widehat{r}_{t}-i\imath \widehat{g}+\widehat{w}+l\widehat{\varpi}=0, \\ \widehat{g}_{t}-i\imath \widehat{r}-k_{0}l\widehat{\phi}=0, \\ \widehat{v}_{t}-ai\imath \widehat{w}=0, \\ \widehat{w}_{t}-ai\imath \widehat{v}-\widehat{r}+mi\imath \widehat{\vartheta}=0, \\ \widehat{\phi}_{t}-k_{0}i\imath \widehat{\varpi}+k_{0}l\widehat{g}=0, \\ \widehat{\varpi}_{t}-k_{0}i\imath \widehat{\phi}-l\widehat{r}+\aleph _{1} \widehat{\varpi}+ \int _{\wp _{1}}^{\wp _{2}}\aleph _{2}(s) \widehat{\mathcal{Y}} (\imath, 1,s,t )\,ds=0, \\ \widehat{\vartheta}_{t}-k_{1}i\imath \widehat{\sigma}+\beta \widehat{w}+\imath ^{2}k_{2}\widehat{\vartheta}=0, \\ \widehat{\sigma}_{t}-i\imath \widehat{\vartheta}=0, \\ s\widehat{\mathcal{Y}}_{t}+\widehat{\mathcal{Y}}_{\jmath}=0. \end{cases}\displaystyle \end{aligned}$$
(2.7)
Lemma 2.1
Suppose that (1.8) holds. Assume that \(\widehat{Z}(\imath,t)\) is the solution of (2.6), then the energy functional \(\widehat{V}(\imath,t)\) is stated as
$$\begin{aligned} \widehat{V}(\imath,t)={}&\frac{\beta}{2} \biggl\{ \vert \widehat{r} \vert ^{2}+ \vert \widehat{g} \vert ^{2}+ \vert \widehat{v} \vert ^{2}+ \vert \widehat{w} \vert ^{2} + \vert \widehat{\phi} \vert ^{2}+ \vert \widehat{\varpi} \vert ^{2}+\frac{m}{\beta} \vert \widehat{\vartheta} \vert ^{2}+\frac{mk_{1}}{\beta} \vert \widehat{\sigma} \vert ^{2} \biggr\} \\ &{}+\frac{\beta}{2} \int _{0}^{1} \int _{ \wp _{1}}^{\wp _{2}}s \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath, \end{aligned}$$
(2.8)
satisfies
$$\begin{aligned} \frac{d\widehat{V}(\imath,t)}{dt}\leq - C_{1} \vert \widehat{\varpi} \vert ^{2}-k_{2}m\imath ^{2} \vert \widehat{\vartheta} \vert ^{2}\leq 0, \end{aligned}$$
(2.9)
where \(C_{1}=\beta (\aleph _{1}-\int _{\wp _{1}}^{\wp _{2}}\vert \aleph _{2}(s)\vert \,ds )>0\).
Proof
First of all, multiplying (2.7)1,2,3,4,5,6 by \(\beta \overline{\widehat{r}},\beta \overline{\widehat{g}},\beta \overline{\widehat{v}},\beta \overline{\widehat{w}},\beta \overline{\widehat{\phi}}\), and \(\beta \overline{\widehat{\varpi}}\), respectively. Further, multiplying (2.7)7,8 by \(m\overline{\widehat{\vartheta}}\) and \(k_{1}m\overline{\widehat{\sigma}}\). Then by adding these equalities and taking the real part, we obtain
$$\begin{aligned} &\frac{\beta}{2}\frac{d}{dt} \biggl[ \vert \widehat{r} \vert ^{2}+ \vert \widehat{g} \vert ^{2}+ \vert \widehat{v} \vert ^{2}+ \vert \widehat{w} \vert ^{2} + \vert \widehat{\phi} \vert ^{2}+ \vert \widehat{\varpi} \vert ^{2}+\frac{m}{\beta} \vert \widehat{\vartheta} \vert ^{2}+\frac{mk_{1}}{\beta} \vert \widehat{\sigma} \vert ^{2} \biggr] \,dx \\ &\quad{}+k_{2}m\imath ^{2} \vert \widehat{\vartheta} \vert ^{2}+\beta \aleph _{1} \vert \widehat{\varpi} \vert ^{2}+\Re e \biggl\{ \beta \int _{\wp _{1}}^{ \wp _{2}} \aleph _{2}(s) \overline{ \widehat{\varpi}} \widehat{\mathcal{Y}} ( \imath, 1, s, t )\,ds \biggr\} =0. \end{aligned}$$
(2.10)
In second step, by multiplying (2.7)9 by \(\overline{\widehat{\mathcal{Y}}}\vert \aleph _{2}(s)\vert \) and integrating the result over \((0, 1)\times (\wp _{1}, \wp _{2})\)
$$\begin{aligned} &\frac{d}{dt }\frac{\beta}{2} \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}}s \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}}(\imath, \jmath, s, t) \bigr\vert ^{2}\,ds \,d\jmath \\ &\quad=-\frac{\beta }{2 } \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \frac{d}{ \,d\jmath} \bigl\vert \widehat{\mathcal{Y}}(\imath, \jmath, s, t) \bigr\vert ^{2}\,ds \,d\jmath \\ &\quad =\frac{\beta }{2 } \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl( \bigl\vert \widehat{\mathcal{Y}}(\imath, 0, s, t) \bigr\vert ^{2} - \bigl\vert \widehat{\mathcal{Y}}(\imath, 1, s, t) \bigr\vert ^{2} \bigr)\,ds \\ &\quad=\frac{\beta}{2 } \biggl( \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \,ds \biggr) \vert \widehat{\varpi} \vert ^{2}- \frac{\beta}{2} \int _{ \wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}}(\imath,1, s, t) \bigr\vert ^{2} \,ds, \end{aligned}$$
(2.11)
utilizing Young’s inequality, we get
$$\begin{aligned} &\Re e \biggl\{ \beta \int _{\wp _{1}}^{\wp _{2}} \aleph _{2}(s) \overline{ \widehat{\varpi}}\widehat{\mathcal{Y}} ( \imath, 1, s, t )\,ds \biggr\} \\ &\quad\leq \frac{\beta}{2 } \biggl( \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \,ds \biggr) \vert \widehat{\varpi} \vert ^{2}+ \frac{\beta}{2} \int _{ \wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}}(\imath,1, s, t) \bigr\vert ^{2} \,ds, \end{aligned}$$
(2.12)
by substituting (2.11) and (2.12) into (2.10), we find
$$\begin{aligned} \frac{d\widehat{V}(\imath,t)}{dt}\leq - \beta \biggl(\aleph _{1}- \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \,ds \biggr) \vert \widehat{\varpi} \vert ^{2}-k_{2}m \imath ^{2} \vert \widehat{\vartheta} \vert ^{2}, \end{aligned}$$
then, by (1.8), \(\exists C_{1}=\beta (\aleph _{1}-\int _{\wp _{1}}^{\wp _{2}}\vert \aleph _{2}(s)\vert \,ds)>0\) such that
$$\begin{aligned} \frac{d\widehat{V}(\imath,t)}{dt}\leq - C_{1} \vert \widehat{\varpi} \vert ^{2}-k_{2}m\imath ^{2} \vert \widehat{\vartheta} \vert ^{2}\leq 0. \end{aligned}$$
(2.13)
Hence, we get the required result. □
The following Lemma is required in order to get the main result.
Lemma 2.2
The functional
$$\begin{aligned} \mathcal{D}_{1}(\imath,t):= \Re e \bigl\{ i\imath ( \widehat{\varpi} \overline{\widehat{\phi}} +l\widehat{\phi} \overline{\widehat{w}} ) \bigr\} , \end{aligned}$$
(2.14)
satisfies the following for any
\(\varepsilon _{1}>0\)
$$\begin{aligned} \frac{d\mathcal{D}_{1}(\imath,t)}{dt}\leq {}& {-}\frac{k_{0}}{2}\imath ^{2} \vert \widehat{\phi} \vert ^{2} +2\varepsilon _{1} \frac{\imath ^{2}}{1+\imath ^{2}} \vert \widehat{g} \vert ^{2}+c( \varepsilon _{1}) \bigl(1+\imath ^{2}\bigr) \vert \widehat{\varpi} \vert ^{2} \\ &{}+c(\varepsilon _{1}) \bigl(1+\imath ^{2}\bigr) \vert \widehat{w} \vert ^{2} +c \vert \widehat{\vartheta} \vert ^{2} \\ &{}+c \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}}(\imath,1, s, t) \bigr\vert ^{2} \,ds. \end{aligned}$$
(2.15)
Proof
By differentiating \(\mathcal{D}_{1}\) and using (2.7), we get
$$\begin{aligned} \frac{d\mathcal{D}_{1}(\imath,t)}{dt}={}&\Re e \{i\imath \widehat{\varpi}_{t}\overline{ \widehat{\phi}} -i\imath \widehat{\phi}_{t} \overline{\widehat{\varpi}} +i\imath l\widehat{\phi}_{t} \overline{\widehat{w}} -i\imath l \widehat{w}_{t} \overline{\widehat{\phi}} \} \\ ={}&{-}k_{0}\imath ^{2} \vert \widehat{\phi} \vert ^{2}+k_{0}\imath ^{2} \vert \widehat{\varpi} \vert ^{2}-\Re e \{i\aleph _{1}\imath \widehat{\varpi} \overline{\widehat{\phi}} \}+\Re e \bigl\{ al \imath ^{2}\widehat{v} \overline{\widehat{\phi}} \bigr\} \\ &{}+\Re e \{ik_{0}l\imath \widehat{g}\overline{\widehat{\varpi}} \}- \Re e \bigl\{ k_{0}l\imath ^{2}\widehat{\varpi} \overline{ \widehat{w}} \bigr\} -\Re e \bigl\{ ik_{0}l^{2}\imath \widehat{g}\overline{\widehat{w}} \bigr\} \\ &{}-\Re e \bigl\{ ml\imath ^{2}\widehat{\vartheta} \overline{\widehat{ \phi}} \bigr\} -\Re e \biggl\{ i\imath \int _{\wp _{1}}^{ \wp _{2}} \aleph _{2}(s) \overline{ \widehat{\phi}} \widehat{\mathcal{Y}}(\imath,1, s, t) \,ds \biggr\} . \end{aligned}$$
(2.16)
The terms in the RHS of (2.16) are obtained by utilizing the Young’s inequality. For any \(\varepsilon _{1},\delta _{1},\delta _{2}>0\), we have
$$\begin{aligned} \begin{aligned} &{-}\Re e \{i\aleph _{1}\imath \widehat{\varpi} \overline{\widehat{ \phi}} \}\leq \delta _{1}\imath ^{2} \vert \widehat{ \phi} \vert ^{2} +c(\delta _{1}) \vert \widehat{\varpi} \vert ^{2}, \\ &\Re e \{ik_{0}l\imath \widehat{g}\overline{\widehat{\varpi}} \}\leq \varepsilon _{1}\frac{\imath ^{2}}{1+\imath ^{2}} \vert \widehat{g} \vert ^{2} +c(\varepsilon _{1}) \bigl(1+\imath ^{2} \bigr) \vert \widehat{\varpi} \vert ^{2}, \\ &{-}\Re e \{lk_{0}\imath \widehat{\varpi}\overline{\widehat{w}} \}\leq c\imath ^{2} \vert \widehat{w} \vert ^{2} +c \vert \widehat{\varpi} \vert ^{2}, \\ &{-}\Re e \bigl\{ ik_{0}l^{2}\imath \widehat{g}\overline{ \widehat{w}} \bigr\} \leq \varepsilon _{1}\frac{\imath ^{2}}{1+\imath ^{2}} \vert \widehat{g} \vert ^{2} +c(\varepsilon _{1}) \bigl(1+\imath ^{2}\bigr) \vert \widehat{w} \vert ^{2}, \\ &\Re e \bigl\{ al\imath ^{2}\widehat{v}\overline{\widehat{\phi}} \bigr\} \leq \delta _{1}\imath ^{2} \vert \widehat{\phi} \vert ^{2} +c( \delta _{1})\imath ^{2} \vert \widehat{v} \vert ^{2}, \\ &{-}\Re e \bigl\{ ml\imath ^{2}\widehat{\vartheta} \overline{\widehat{ \phi}} \bigr\} \leq \delta _{1}\imath ^{2} \vert \widehat{\phi} \vert ^{2} +c(\delta _{1})\imath ^{2} \vert \widehat{\vartheta} \vert ^{2}, \\ &{-}\Re e \biggl\{ i\imath \int _{\wp _{1}}^{\wp _{2}} \aleph _{2}(s) \overline{ \widehat{\phi}}\widehat{\mathcal{Y}}(\imath,1, s, t) \,ds \biggr\} \\ &\quad\leq \delta _{2}\aleph _{1}\imath ^{2} \vert \widehat{\phi} \vert ^{2} +c(\delta _{2}) \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}}(\imath,1, s, t) \bigr\vert ^{2} \,ds. \end{aligned} \end{aligned}$$
(2.17)
Inserting the above estimates (2.17) into (2.16) and by letting \(\delta _{1}=\frac{k_{0}}{12},\delta _{2}=\frac{k_{0}}{4\aleph _{1}}\), we get the required (2.15). □
Lemma 2.3
The functional
$$\begin{aligned} \mathcal{D}_{2}(\imath,t):= \Re e \bigl\{ i\imath (ak_{1} \widehat{\vartheta}\overline{\widehat{\sigma}} +a\beta \widehat{\vartheta} \overline{\widehat{w}} +2k_{1}\widehat{v} \overline{\widehat{\sigma}} ) \bigr\} , \end{aligned}$$
(2.18)
satisfies the following for any
\(\varepsilon _{2},\varepsilon _{3}>0\)
$$\begin{aligned} \frac{d\mathcal{D}_{2}(\imath,t)}{dt}\leq {}& {-}\frac{ak_{1}^{2}}{2} \imath ^{2} \vert \widehat{\sigma} \vert ^{2} -\frac{a\beta ^{2}}{2} \imath ^{2} \vert \widehat{w} \vert ^{2}+\varepsilon _{2}\imath ^{2} \vert \widehat{r} \vert ^{2} +\varepsilon _{3}\imath ^{2} \vert \widehat{v} \vert ^{2} \\ &{} +c(\varepsilon _{2},\varepsilon _{3}) \bigl(1+\imath ^{2}+\imath ^{4}\bigr) \vert \widehat{\vartheta} \vert ^{2}. \end{aligned}$$
(2.19)
Proof
By differentiating \(\mathcal{D}_{2}\) and using (2.7), we get
$$\begin{aligned} \frac{\mathcal{D}_{2}(\imath,t)}{dt}={}&\Re e \{i\imath ak_{1} \widehat{ \vartheta}_{t}\overline{\widehat{\sigma}} -i\imath ak_{1} \widehat{\sigma}_{t}\overline{\widehat{\vartheta}} -i\imath \beta a \widehat{\vartheta}_{t}\overline{\widehat{w}} +i\imath \beta a \widehat{w}_{t}\overline{\widehat{\vartheta}} \} \\ &{}+\Re e \{2i\imath \beta k_{1}\widehat{v}_{t} \overline{ \widehat{\sigma}} -2i\imath \beta k_{1}\widehat{\sigma}_{t} \overline{\widehat{v}} \} \\ ={}&{-}ak_{1}\imath ^{2} \vert \widehat{\sigma} \vert ^{2}-a\beta ^{2} \imath ^{2} \vert \widehat{w} \vert ^{2}+a(k_{1}+m\beta )\imath ^{2} \vert \widehat{\vartheta} \vert ^{2} \\ &{}+\Re e \bigl\{ \beta \imath ^{2}\bigl(2k_{1}-a^{2} \bigr)\widehat{v} \overline{\widehat{\vartheta}} \bigr\} +\Re e \{ia\beta \imath \widehat{r}\overline{\widehat{\vartheta}} \} \\ &{}-\Re e \bigl\{ iak_{1}k_{2}\imath ^{3}\widehat{ \vartheta} \overline{\widehat{\sigma}} \bigr\} +\Re e \bigl\{ ia\beta k_{2}\imath ^{3} \widehat{\vartheta}\overline{\widehat{w}} \bigr\} . \end{aligned}$$
(2.20)
The terms in the RHS of (2.20) are obtained by utilizing Young’s inequality. Next, for any \(\varepsilon _{2},\varepsilon _{3},\delta _{3},\delta _{4}>0\), we can find
$$\begin{aligned} \begin{aligned} &\Re e \{ia\beta \imath \widehat{r}\overline{\widehat{\vartheta}} \}\leq \varepsilon _{2}\imath ^{2} \vert \widehat{r} \vert ^{2} +c( \varepsilon _{2}) \vert \widehat{\vartheta} \vert ^{2}, \\ &\Re e \bigl\{ \beta \imath ^{2}\bigl(2k_{1}-a^{2} \bigr)\widehat{v} \overline{\widehat{\vartheta}} \bigr\} \leq \varepsilon _{3}\imath ^{2} \vert \widehat{v} \vert ^{2} +c(\varepsilon _{3}) \vert \widehat{\vartheta} \vert ^{2}, \\ &{-}\Re e \bigl\{ iak_{1}k_{2}\imath ^{3}\widehat{ \vartheta} \overline{\widehat{\sigma}} \bigr\} \leq \delta _{3}\imath ^{2} \vert \widehat{\sigma} \vert ^{2} +c(\delta _{3})\imath ^{4} \vert \widehat{\vartheta} \vert ^{2}, \\ &\Re e \bigl\{ ia\beta k_{2}\imath ^{3}\widehat{\vartheta} \overline{\widehat{w}} \bigr\} \leq \delta _{4}\imath ^{2} \vert \widehat{w} \vert ^{2} +c(\delta _{4})\imath ^{4} \vert \widehat{\vartheta} \vert ^{2}. \end{aligned} \end{aligned}$$
(2.21)
By substituting (2.21) into (2.20) and letting \(\delta _{3}=\frac{ak_{1}^{2}}{2},\delta _{4}=\frac{a\beta ^{2}}{2}\), we get (2.19). □
Lemma 2.4
The functional
$$\begin{aligned} \mathcal{D}_{3}(\imath,t):= \Re e \{\widehat{\phi} \overline{ \widehat{g}} \}, \end{aligned}$$
(2.22)
satisfies the below for any
\(\varepsilon _{4}>0\)
$$\begin{aligned} \frac{d\mathcal{D}_{3}(\imath,t)}{dt}\leq -\frac{k_{0}l^{2}}{2} \vert \widehat{g} \vert ^{2} +\varepsilon _{4} \vert \widehat{r} \vert ^{2}+c \imath ^{2} \vert \widehat{\varpi} \vert ^{2}+c(\varepsilon _{4}) \bigl(1+ \imath ^{2} \bigr) \vert \widehat{\phi} \vert ^{2}. \end{aligned}$$
(2.23)
Proof
By differentiating \(\mathcal{D}_{3}\) and using (2.7), we have
$$\begin{aligned} \frac{\mathcal{D}_{3}(\imath,t)}{dt}={}&\Re e \{ \widehat{\phi}_{t} \overline{ \widehat{g}} + \widehat{g}_{t}\overline{\widehat{\phi}} \} \\ ={}&-k_{0}l \vert \widehat{g} \vert ^{2}+k_{0}l \vert \widehat{\phi} \vert ^{2} \\ &{}+\Re e \{ik_{0}\imath \widehat{\varpi}\overline{\widehat{g}} \}+\Re e \{i\imath \widehat{r}\overline{\widehat{\phi}} \}. \end{aligned}$$
(2.24)
The last two terms in the RHS of (2.24) are obtained by Young’s inequality, which we solve for any \(\varepsilon _{4},\delta _{5}>0\)
$$\begin{aligned} \begin{aligned} &\Re e \{ik_{0}\imath \widehat{\varpi}\overline{\widehat{g}} \}\leq \delta _{5} \vert \widehat{g} \vert ^{2} +c(\delta _{5}) \imath ^{2} \vert \widehat{\varpi} \vert ^{2}, \\ &\Re e \{i\imath \widehat{r}\overline{\widehat{\phi}} \}\leq \varepsilon _{4} \vert \widehat{r} \vert ^{2} +c(\varepsilon _{4}) \imath ^{2} \vert \widehat{\phi} \vert ^{2}. \end{aligned} \end{aligned}$$
(2.25)
By substituting (2.25) into (2.24) and letting \(\delta _{5}=\frac{k_{0}l^{2}}{2}\), we obtained (2.23). □
Next, we have the following lemma.
Lemma 2.5
The functional
$$\begin{aligned} \mathcal{D}_{4}(\imath,t):=al\mathcal{F}_{1}(\imath,t)- \imath ^{2} \mathcal{F}_{2}(\imath,t), \end{aligned}$$
(2.26)
where
$$\begin{aligned} \mathcal{F}_{1}(\imath,t):= \Re e \bigl\{ i\imath (l\widehat{w} \overline{\widehat{v}} +\widehat{v}\overline{\widehat{\varpi}} ) \bigr\} \quad \textit{and}\quad \mathcal{F}_{2}(\imath,t):= \Re e \bigl\{ (\widehat{w} \overline{\widehat{r}} +a\widehat{g}\overline{\widehat{v}} ) \bigr\} , \end{aligned}$$
(2.27)
satisfies
-
(1)
For \(a=1\). Then,
$$\begin{aligned} \frac{d\mathcal{D}_{4}(\imath,t)}{dt}\leq {}& {-}\frac{a^{2}l^{2}}{2} \imath ^{2} \vert \widehat{v} \vert ^{2} -\frac{1}{2}\imath ^{2} \vert \widehat{r} \vert ^{2}+c \vert \widehat{\varpi} \vert ^{2} +\bigl(1+a^{2}l^{2}\bigr) \imath ^{2} \vert \widehat{w} \vert ^{2} \\ &{} +c\bigl(\imath ^{2}+\imath ^{4}\bigr) \vert \widehat{ \vartheta} \vert ^{2}+c \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}}(\imath,1, s, t) \bigr\vert ^{2} \,ds. \end{aligned}$$
(2.28)
-
(2)
For \(a\neq 1\). Then, for any \(\varepsilon _{5}>0\)
$$\begin{aligned} \frac{d\mathcal{D}_{4}(\imath,t)}{dt}\leq {}& {-}\frac{a^{2}l^{2}}{2} \imath ^{2} \vert \widehat{v} \vert ^{2} -\frac{1}{2}\imath ^{2} \vert \widehat{r} \vert ^{2}+\varepsilon _{5} \frac{\imath ^{4}}{(1+\imath ^{2})^{2}} \vert \widehat{g} \vert ^{2} +c( \varepsilon _{5})\imath ^{2}\bigl(1+\imath ^{2} \bigr)^{2} \vert \widehat{w} \vert ^{2} \\ &{}+c\bigl(1+\imath ^{2}\bigr) \vert \widehat{\varpi} \vert ^{2} +c\bigl(\imath ^{2}+ \imath ^{4}\bigr) \vert \widehat{\vartheta} \vert ^{2} \\ &{}+c \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}}(\imath,1, s, t) \bigr\vert ^{2} \,ds. \end{aligned}$$
(2.29)
Proof
Firstly, by differentiating \(\mathcal{F}_{1}, \mathcal{F}_{2}\) and using (2.7), we get
$$\begin{aligned} \frac{d\mathcal{F}_{1}(\imath,t)}{dt}={}&\Re e \{i\imath l \widehat{w}_{t}\overline{ \widehat{v}} -i\imath l\widehat{v}_{t} \overline{\widehat{w}} +i\imath \widehat{v}_{t} \overline{\widehat{\varpi}} -i\imath \widehat{ \varpi}_{t} \overline{\widehat{v}} \} \\ ={}&{-}al\imath ^{2} \vert \widehat{v} \vert ^{2}+al\imath ^{2} \vert \widehat{w} \vert ^{2}+\Re e \{i\aleph _{1}\imath \widehat{\varpi} \overline{\widehat{v}} \}-\Re e \bigl\{ a \imath ^{2}\widehat{w} \overline{\widehat{\varpi}} \bigr\} \\ &{}+\Re e \bigl\{ k_{0}\imath ^{2}\widehat{\phi}\overline{ \widehat{v}} \bigr\} +\Re e \bigl\{ ml\imath ^{2}\widehat{\vartheta} \overline{\widehat{v}} \bigr\} \\ &{}+\Re e \biggl\{ i\imath \int _{\wp _{1}}^{\wp _{2}} \aleph _{2}(s) \overline{ \widehat{v}}\widehat{\mathcal{Y}}(\imath,1, s, t) \,ds \biggr\} , \end{aligned}$$
(2.30)
and
$$\begin{aligned} \frac{d\mathcal{F}_{2}(\imath,t)}{dt}={}&\Re e \{\widehat{w}_{t} \overline{\widehat{r}} + \widehat{r}_{t}\overline{\widehat{w}} +a \widehat{v}_{t} \overline{\widehat{g}} +a\widehat{g}_{t} \overline{\widehat{v}} \} \\ ={}&{-} \vert \widehat{w} \vert ^{2}+ \vert \widehat{r} \vert ^{2}+\Re e \bigl\{ i\bigl(a^{2}-1\bigr)\imath \widehat{w} \overline{\widehat{g}} \bigr\} -\Re e \{im\imath \widehat{\vartheta}\overline{ \widehat{r}} \} \\ &{}-\Re e \{l\widehat{\varpi}\overline{\widehat{w}} \}+\Re e \{alk_{0} \widehat{\phi}\overline{\widehat{v}} \}. \end{aligned}$$
(2.31)
Now, differentiating \(\mathcal{D}_{4}\) and by (2.30) and (2.31), we have
$$\begin{aligned} \frac{d\mathcal{D}_{4}(\imath,t)}{dt}={}&{-}a^{2}l^{2}\imath ^{2} \vert \widehat{v} \vert ^{2}-\imath ^{2} \vert \widehat{r} \vert ^{2}+\bigl(1+a^{2}l^{2}\bigr) \imath ^{2} \vert \widehat{w} \vert ^{2} +\Re e \{ial \aleph _{1} \imath \widehat{\varpi}\overline{\widehat{v}} \} \\ &{}+\Re e \bigl\{ i\bigl(1-a^{2}\bigr)\imath ^{3}\widehat{w} \overline{\widehat{g}} \bigr\} +\Re e \bigl\{ im\imath ^{3}\widehat{ \vartheta} \overline{\widehat{r}} \bigr\} +\Re e \bigl\{ l\bigl(1-a^{2} \bigr)\imath ^{2} \widehat{\varpi}\overline{\widehat{w}} \bigr\} \\ &{}+\Re e \bigl\{ aml^{2}\imath ^{2}\widehat{\vartheta} \overline{\widehat{v}} \bigr\} +\Re e \biggl\{ ial\imath \int _{\wp _{1}}^{ \wp _{2}} \aleph _{2}(s) \overline{ \widehat{v}}\widehat{\mathcal{Y}}( \imath,1, s, t) \,ds \biggr\} . \end{aligned}$$
(2.32)
At this point, we discuss two cases:
Case 1. \((a=1)\).
In this case, by applying the Young’s inequality to the terms on the RHS of (2.32). Then, for any \(\delta _{6},\delta _{7},\delta _{8}>0\), we get
$$\begin{aligned} \begin{aligned} &\Re e \{ial\aleph _{1}\imath \widehat{\varpi} \overline{\widehat{v}} \}\leq \delta _{6}\imath ^{2} \vert \widehat{v} \vert ^{2} +c(\delta _{6}) \vert \widehat{\varpi} \vert ^{2}, \\ &\Re e \bigl\{ im\imath ^{3}\widehat{\vartheta}\overline{\widehat{r}} \bigr\} \leq \delta _{7}\imath ^{2} \vert \widehat{r} \vert ^{2} +c( \delta _{7})\imath ^{4} \vert \widehat{\vartheta} \vert ^{2}, \\ &\Re e \bigl\{ aml^{2}\imath ^{2}\widehat{\vartheta} \overline{\widehat{v}} \bigr\} \leq \delta _{6}\imath ^{2} \vert \widehat{v} \vert ^{2} +c(\delta _{6})\imath ^{2} \vert \widehat{\vartheta} \vert ^{2}, \\ &\Re e \biggl\{ i\imath \int _{\wp _{1}}^{\wp _{2}} \aleph _{2}(s) \overline{ \widehat{v}}\widehat{\mathcal{Y}}(\imath,1, s, t) \,ds \biggr\} \\ &\quad\leq \delta _{8}\aleph _{1}\imath ^{2} \vert \widehat{v} \vert ^{2} +c(\delta _{8}) \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}}(\imath,1, s, t) \bigr\vert ^{2} \,ds. \end{aligned} \end{aligned}$$
(2.33)
Inserting the above estimates of (2.33) into (2.32).
Finally, by letting \(\delta _{6}=\frac{a^{2}l^{2}}{8},\delta _{7}=\frac{1}{2},\delta _{8}= \frac{a^{2}l^{2}}{4\aleph _{1}}\), we obtained (2.28).
Case 2. \((a\neq 1)\).
In this case, using the Young’s inequality to the terms on the RHS of (2.32) for any \(\varepsilon _{5},\delta _{9},\delta _{10},\delta _{11}>0 \) gives
$$\begin{aligned} \begin{aligned} &\Re e \{ial\aleph _{1}\imath \widehat{\varpi} \overline{\widehat{v}} \}\leq \delta _{9}\imath ^{2} \vert \widehat{v} \vert ^{2} +c(\delta _{9}) \vert \widehat{\varpi} \vert ^{2}, \\ &\Re e \bigl\{ i\bigl(1-a^{2}\bigr)\imath ^{3}\widehat{w} \overline{\widehat{g}} \bigr\} \leq \varepsilon _{5} \frac{\imath ^{4}}{(1+\imath ^{2})^{2}} \vert \widehat{g} \vert ^{2} +c( \varepsilon _{5})\imath ^{2}\bigl(1+\imath ^{2} \bigr)^{2} \vert \widehat{w} \vert ^{2}, \\ &\Re e \bigl\{ im\imath ^{3}\widehat{\vartheta}\overline{\widehat{r}} \bigr\} \leq \delta _{10}\imath ^{2} \vert \widehat{r} \vert ^{2} +c( \delta _{10})\imath ^{4} \vert \widehat{\vartheta} \vert ^{2}, \\ &\Re e \bigl\{ l\bigl(1-a^{2}\bigr)\imath ^{2}\widehat{\varpi} \overline{\widehat{w}} \bigr\} \leq c\imath ^{2} \vert \widehat{ \varpi} \vert ^{2} +c\imath ^{2} \vert \widehat{w} \vert ^{2}, \\ &\Re e \bigl\{ aml^{2}\imath ^{2}\widehat{\vartheta} \overline{\widehat{v}} \bigr\} \leq \delta _{9}\imath ^{2} \vert \widehat{v} \vert ^{2} +c(\delta _{9})\imath ^{2} \vert \widehat{\vartheta} \vert ^{2}, \\ &\Re e \biggl\{ i\imath \int _{\wp _{1}}^{\wp _{2}} \aleph _{2}(s) \overline{ \widehat{v}}\widehat{\mathcal{Y}}(\imath,1, s, t) \,ds \biggr\} \\ &\quad\leq \delta _{11}\aleph _{1}\imath ^{2} \vert \widehat{v} \vert ^{2} +c(\delta _{11}) \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}}(\imath,1, s, t) \bigr\vert ^{2} \,ds. \end{aligned} \end{aligned}$$
(2.34)
Inserting (2.34) into (2.32), and letting \(\delta _{9}=\frac{a^{2}l^{2}}{8},\delta _{10}=\frac{1}{2},\delta _{11}= \frac{a^{2}l^{2}}{4\aleph _{1}}\), we get (2.29. The proof of Lemma 2.5 is completed. □
Now, introducing the following functional.
Lemma 2.6
The functional
$$\begin{aligned} \mathcal{D}_{5} (\imath, t ):= \int _{0}^{1} \int _{\wp _{1}}^{ \wp _{2}} s e^{-s\jmath } \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath, \end{aligned}$$
satisfies
$$\begin{aligned} \frac{d\mathcal{D}_{5} (\imath, t )}{dt} \leq {}&{ -}\zeta _{1} \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} s \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath +\aleph _{1} \vert \widehat{\varpi} \vert ^{2} \\ &{}-\zeta _{1} \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, 1, s, t ) \bigr\vert ^{2} \,ds , \end{aligned}$$
(2.35)
where \(\zeta _{1}>0\).
Proof
By differentiating \(\mathcal{D}_{5}\) with respect to t and utilizing (2.7)9, we have
$$\begin{aligned} \frac{d\mathcal{D}_{5} (\imath, t )}{dt} ={}&{-} \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} s e^{-s\jmath} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath \\ &{}- \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl[e^{-s} \bigl\vert \widehat{\mathcal{Y}} ( \imath, 1, s,t ) \bigr\vert ^{2}- \bigl\vert \widehat{\mathcal{Y}} ( \imath, 0, s, t ) \bigr\vert ^{2}\bigr] \,ds. \end{aligned}$$
Using \(\mathcal{Y}(\imath, 0, s, t)=\Im _{t}(\imath, t)=\varpi \), & \(e^{-s}\leq e^{-s\jmath}\leq 1\), ∀ \(0<\jmath <1\), we have
$$\begin{aligned} \frac{d\mathcal{D}_{5} (\imath, t )}{dt} \leq {}&{-} \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} se^{-s} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath \\ &{}- \int _{\wp _{1}}^{\wp _{2}} e^{-s} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, 1, s, t ) \bigr\vert ^{2} \,ds +\biggl( \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \,ds\biggr) \vert \widehat{\varpi} \vert ^{2}. \end{aligned}$$
Next, we have \(-e^{-s}\leq -e^{-\wp _{2}}\), for all \(s\in [\wp _{1}, \wp _{2}]\), since \(-e^{-s}\) is an increasing function. Assuming that \(\zeta _{1}=e^{-\wp _{2}}\) and remembering (1.8), we obtain (2.35). □
We define the Lyapunov functionals at this point
-
For \(a=1\):
$$\begin{aligned} \mathcal{K}_{1}(\imath,t):={}&N\widehat{V}(\imath,t)+N_{1} \frac{\imath ^{4}}{(1+\imath ^{2})^{3}}\mathcal{D}_{1}(\imath,t)+N_{2} \frac{\imath ^{2}}{(1+\imath ^{2})^{2}}\mathcal{D}_{2}(\imath,t) \\ &{} +N_{3}\frac{\imath ^{6}}{(1+\imath ^{2})^{4}}\mathcal{D}_{3}( \imath,t)+N_{4} \frac{\imath ^{2}}{(1+\imath ^{2})^{2}}\mathcal{D}_{4}( \imath,t)+N_{5} \mathcal{D}_{5}(\imath,t). \end{aligned}$$
(2.36)
-
For \(a\neq 1\):
$$\begin{aligned} \mathcal{K}_{2}(\imath,t):={}&M\widehat{V}(\imath,t)+M_{1} \frac{\imath ^{4}}{(1+\imath ^{2})^{6}}\mathcal{D}_{1}(\imath,t)+M_{2} \frac{\imath ^{2}}{(1+\imath ^{2})^{3}}\mathcal{D}_{2}(\imath,t) \\ &{} +M_{3}\frac{\imath ^{6}}{(1+\imath ^{2})^{7}}\mathcal{D}_{3}( \imath,t)+M_{4} \frac{\imath ^{2}}{(1+\imath ^{2})^{5}}\mathcal{D}_{4}( \imath,t)+M_{5} \mathcal{D}_{5}(\imath,t), \end{aligned}$$
(2.37)
where \(N,M,N_{i},M_{i}, i=1,\ldots,5\) are positive constants and will be selected later.
Lemma 2.7
There exist \(\mu _{i}>0,i=1,\ldots,6\) such that the functionals \(\mathcal{K}_{1}(\imath,t)\) and \(\mathcal{K}_{2}(\imath,t)\) given by (2.36) and (2.37) satisfies
-
For \(a=1\):
$$\begin{aligned} \textstyle\begin{cases} \mu _{1}\widehat{V}(\imath,t)\leq \mathcal{K}_{1}(\imath,t)\leq \mu _{2}\widehat{V}(\imath,t), \\ \mathcal{K}_{1}'(\imath,t)\leq -\mu _{3}\jmath _{1}(\imath ) \mathcal{K}_{1}(\imath,t),\quad \forall t>0. \end{cases}\displaystyle \end{aligned}$$
(2.38)
-
For \(a\neq 1\):
$$\begin{aligned} \textstyle\begin{cases} \mu _{4}\widehat{V}(\imath,t)\leq \mathcal{K}_{2}(\imath,t)\leq \mu _{5}\widehat{V}(\imath,t), \\ \mathcal{K}_{2}'(\imath,t)\leq -\mu _{6}\jmath _{2}(\imath ) \mathcal{K}_{2}(\imath,t),\quad \forall t>0, \end{cases}\displaystyle \end{aligned}$$
(2.39)
where
$$\begin{aligned} \jmath _{1}(\imath )=\frac{\imath ^{6}}{(1+\imath ^{2})^{4}} \quad\textit{and}\quad \jmath _{2}(\imath )=\frac{\imath ^{6}}{(1+\imath ^{2})^{7}}. \end{aligned}$$
(2.40)
Proof
First, by differentiating (2.36) and using (2.9), (2.15), (2.19), (2.23), (2.28) and (2.35) with the fact that \(\frac{\imath ^{2}}{1+\imath ^{2}}\leq \min \{1,\imath ^{2}\}\) and \(\frac{1}{1+\imath ^{2}}\leq 1\), we have
$$\begin{aligned} \mathcal{K}_{1}'(\imath,t)\leq {}&{- }\frac{\imath ^{6}}{(1+\imath ^{2})^{4}} \biggl[\frac{k_{0}l^{2}}{2}N_{3}-2 \varepsilon _{1}N_{1} \biggr] \vert \widehat{g} \vert ^{2} \\ &{}-\frac{\imath ^{4}}{(1+\imath ^{2})^{2}} \biggl[\frac{1}{2}N_{4}- \varepsilon _{2}N_{2}-\varepsilon _{4}N_{3} \biggr] \vert \widehat{r} \vert ^{2} \\ &{}-\frac{\imath ^{4}}{(1+\imath ^{2})^{2}} \biggl[\frac{a\beta ^{2}}{2}N_{2}-c( \varepsilon _{1})N_{1}-cN_{4} \biggr] \vert \widehat{w} \vert ^{2} \\ &{}-\frac{\imath ^{4}}{(1+\imath ^{2})^{2}} \biggl[\frac{a^{2}l^{2}}{2}N_{4}- \varepsilon _{3}N_{2} \biggr] \vert \widehat{v} \vert ^{2} \\ &{}-\frac{\imath ^{6}}{(1+\imath ^{2})^{3}} \biggl[\frac{k_{0}}{2}N_{1}-c( \varepsilon _{4})N_{3} \biggr] \vert \widehat{\phi} \vert ^{2}- \frac{\imath ^{4}}{(1+\imath ^{2})^{2}} \biggl[\frac{ak_{1}^{2}}{2}N_{2} \biggr] \vert \widehat{\sigma} \vert ^{2} \\ &{}-\imath ^{2} \bigl[mk_{2}N-cN_{1}-c(\varepsilon _{2},\varepsilon _{3})N_{2}-cN_{4} \bigr] \vert \widehat{\vartheta} \vert ^{2} \\ &{}- \bigl[C_{1}N-c(\varepsilon _{1}) N_{1}-cN_{3}-cN_{4}- \aleph _{1}N_{5} \bigr] \vert \widehat{\varpi} \vert ^{2} \\ &{}-\zeta _{1}N_{5} \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} s \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath \\ &{}- [\zeta _{1}N_{5}-cN_{1}-cN_{4} ] \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, 1, s, t ) \bigr\vert ^{2} \,ds. \end{aligned}$$
(2.41)
By setting
$$\begin{aligned} \varepsilon _{1}=\frac{k_{0}l^{2} N_{3}}{8 N_{1}},\qquad \varepsilon _{2}= \frac{N_{4}}{8N_{2}},\qquad \varepsilon _{3}=\frac{a^{2}l^{2}N_{4}}{4N_{2}}, \qquad\varepsilon _{4}=\frac{N_{4}}{8N_{3}}. \end{aligned}$$
We obtain the following
$$\begin{aligned} \mathcal{K}_{1}'(\imath,t)\leq {}&{- }\frac{\imath ^{6}}{(1+\imath ^{2})^{4}} \biggl[\frac{k_{0}l^{2}}{4}N_{3} \biggr] \vert \widehat{g} \vert ^{2}- \frac{\imath ^{4}}{(1+\imath ^{2})^{2}} \biggl[\frac{1}{4}N_{4} \biggr] \vert \widehat{r} \vert ^{2} \\ &{}-\frac{\imath ^{4}}{(1+\imath ^{2})^{2}} \biggl[\frac{a\beta ^{2}}{2}N_{2}-c(N_{1},N_{3})N_{1}-cN_{4} \biggr] \vert \widehat{w} \vert ^{2} \\ &{}-\frac{\imath ^{4}}{(1+\imath ^{2})^{2}} \biggl[\frac{a^{2}l^{2}}{4}N_{4} \biggr] \vert \widehat{v} \vert ^{2}- \frac{\imath ^{4}}{(1+\imath ^{2})^{2}} \biggl[ \frac{ak_{1}^{2}}{2}N_{2} \biggr] \vert \widehat{\sigma} \vert ^{2} \\ &{}-\frac{\imath ^{6}}{(1+\imath ^{2})^{3}} \biggl[\frac{k_{0}}{2}N_{1}-c(N_{3},N_{4})N_{3} \biggr] \vert \widehat{\phi} \vert ^{2} \\ &{}-\imath ^{2} \bigl[mk_{2}N-cN_{1}-c(N_{2},N_{4})N_{2}-cN_{4} \bigr] \vert \widehat{\vartheta} \vert ^{2} \\ &{}- \bigl[C_{1}N-c(N_{1},N_{3})N_{1}-cN_{3}-cN_{4}- \aleph _{1}N_{5} \bigr] \vert \widehat{\varpi} \vert ^{2} \\ &{}-\zeta _{1}N_{5} \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} s \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath \\ &{}- [\zeta _{1}N_{5}-cN_{1}-cN_{4} ] \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, 1, s, t ) \bigr\vert ^{2} \,ds . \end{aligned}$$
(2.42)
Next, we fix \(N_{3},N_{4}\) and choose \(N_{1}\) large enough such that
$$\begin{aligned} \frac{k_{0}}{2}N_{1}-c(N_{3},N_{4})N_{3}>0, \end{aligned}$$
then, we pick \(N_{2}\) and \(N_{5}\) large enough in such a way that
$$\begin{aligned} &\frac{a\beta ^{2}}{2}N_{2}-c(N_{1},N_{3})N_{1}-cN_{4}>0, \\ &\zeta _{1}N_{5}-cN_{1}-cN_{4}>0. \end{aligned}$$
Hence, we have
$$\begin{aligned} \mathcal{K}_{1}'(\imath,t)\leq{}& -\alpha _{0} \frac{\imath ^{6}}{(1+\imath ^{2})^{4}} \vert \widehat{g} \vert ^{2}- \alpha _{5}\frac{\imath ^{6}}{(1+\imath ^{2})^{3}} \vert \widehat{\phi} \vert ^{2}-\imath ^{2} [mk_{2}N-c ] \vert \widehat{ \vartheta} \vert ^{2} \\ &{} -\frac{\imath ^{4}}{(1+\imath ^{2})^{2}} \bigl(\alpha _{1} \vert \widehat{r} \vert ^{2}+\alpha _{2} \vert \widehat{w} \vert ^{2} + \alpha _{3} \vert \widehat{v} \vert ^{2}+\alpha _{4} \vert \widehat{\sigma} \vert ^{2} \bigr)- [C_{1}N-c ] \vert \widehat{\varpi} \vert ^{2} \\ &{} -\alpha _{6} \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} s \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath. \end{aligned}$$
(2.43)
Secondly, we have
$$\begin{aligned} \bigl\vert \mathcal{K}_{1}(\imath,t)-N\widehat{V}(\imath,t) \bigr\vert ={}&N_{1}\frac{\imath ^{4}}{(1+\imath ^{2})^{3}} \bigl\vert \mathcal{D}_{1}( \imath,t) \bigr\vert +N_{2} \frac{\imath ^{2}}{(1+\imath ^{2})^{2}} \bigl\vert \mathcal{D}_{2}( \imath,t) \bigr\vert \\ &{} +N_{3}\frac{\imath ^{6}}{(1+\imath ^{2})^{4}} \bigl\vert \mathcal{D}_{3}( \imath,t) \bigr\vert +N_{4}\frac{\imath ^{2}}{(1+\imath ^{2})^{2}} \bigl\vert \mathcal{D}_{4}(\imath,t) \bigr\vert +N_{5} \bigl\vert \mathcal{D}_{5}(\imath,t) \bigr\vert \\ \leq {}&aN_{1}\frac{\imath ^{4}}{(1+\imath ^{2})^{3}} \bigl\vert \Re e \bigl\{ i\imath ( \widehat{\varpi}\overline{\widehat{\phi}} +l \widehat{\phi}\overline{\widehat{w}} ) \bigr\} \bigr\vert \\ &{}+N_{2}\frac{\imath ^{2}}{(1+\imath ^{2})^{2}} \bigl\vert \Re e \bigl\{ i\imath (ak_{1}\widehat{\vartheta} \overline{\widehat{\sigma}} +a\beta \widehat{\vartheta} \overline{\widehat{w}} +2k_{1}\widehat{v}\overline{ \widehat{\sigma}} ) \bigr\} \bigr\vert \\ &{}+N_{3}\frac{\imath ^{6}}{(1+\imath ^{2})^{4}} \bigl\vert \Re e \{\widehat{\phi} \overline{\widehat{g}} \} \bigr\vert \\ &{}+N_{4}\frac{\imath ^{2}}{(1+\imath ^{2})^{2}} \bigl\vert \Re e \bigl\{ i\imath (l \widehat{w}\overline{\widehat{v}} +\widehat{v} \overline{\widehat{\varpi}} ) \bigr\} \bigr\vert \\ &{}+N_{4}\frac{\imath ^{2}}{(1+\imath ^{2})^{2}} \bigl\vert \Re e \bigl\{ (\widehat{w} \overline{\widehat{r}} +a\widehat{g} \overline{\widehat{v}} ) \bigr\} \bigr\vert \\ &{}+N_{5} \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} s e^{-s\jmath } \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath. \end{aligned}$$
By utilizing Young’s inequality, the fact that \(\frac{\imath ^{2}}{1+\imath ^{2}}\leq \min \{1,\imath ^{2}\}\) and \(\frac{1}{1+\imath ^{2}}\leq 1\), we find
$$\begin{aligned} \bigl\vert \mathcal{K}_{1}(\imath,t)-N\widehat{V}(\imath,t) \bigr\vert \leq c\widehat{V}(\imath,t). \end{aligned}$$
Hence, we get
$$\begin{aligned} (N-c)\widehat{V}(\imath,t)\leq \mathcal{K}_{1}(\imath,t)\leq (N+c) \widehat{V}(\imath,t). \end{aligned}$$
(2.44)
Now, we choose N large enough in such a way that
$$\begin{aligned} N-c>0,\qquad C_{1}N-c>0,\qquad mk_{2}N-c>0, \end{aligned}$$
and utilizing (2.8), estimates (2.43) and (2.44), respectively.
One can find a positive constant \(\alpha >0\), then ∀ \(t>0\) & ∀ \(\imath \in \mathbb{R}\), we obtain
$$\begin{aligned} \mu _{1}\widehat{V}(\imath,t)\leq \mathcal{K}_{1}( \imath,t)\leq \mu _{2}\widehat{V}(\imath,t). \end{aligned}$$
(2.45)
and
$$\begin{aligned} \mathcal{K}_{1}'(\imath,t)\leq {}&-\alpha \frac{\imath ^{6}}{(1+\imath ^{2})^{4}} \biggl( \vert \widehat{g} \vert ^{2}+ \vert \widehat{\phi} \vert ^{2}+ \vert \widehat{\vartheta} \vert ^{2} + \vert \widehat{r} \vert ^{2}+ \vert \widehat{w} \vert ^{2} + \vert \widehat{v} \vert ^{2}+ \vert \widehat{\sigma} \vert ^{2}+ \vert \widehat{\varpi} \vert ^{2} \\ &{}+ \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} s \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath \biggr), \end{aligned}$$
(2.46)
then
$$\begin{aligned} \mathcal{K}_{1}'(\imath,t)\leq -\lambda _{1} \jmath _{1}(\imath ) \widehat{V}(\imath,t), \quad\forall t\geq 0. \end{aligned}$$
(2.47)
Therefore, for some positive constant \(\mu _{3}=\frac{\lambda _{1}}{\mu _{2}}>0\), we get
$$\begin{aligned} \mathcal{K}_{1}'(\imath,t)\leq -\mu _{3} \jmath _{1}(\imath ) \mathcal{K}_{1}(\imath,t), \quad\forall t \geq 0, \end{aligned}$$
(2.48)
where \(\jmath _{1}(\imath )=\frac{\imath ^{6}}{(1+\imath ^{2})^{4}}\), for some \(\lambda _{1},\mu _{i}>0, i=1,2,3\). The proof of the first result (2.38) is finished.
Before the proof of the second result (2.39). In the estimates (2.21), we used the inequalities
$$\begin{aligned} &\Re e \{ia\beta \imath \widehat{r}\overline{\widehat{\vartheta}} \}\leq \varepsilon _{2} \frac{\imath ^{2}}{(1+\imath ^{2})^{2}} \vert \widehat{r} \vert ^{2} +c( \varepsilon _{2}) \bigl(1+\imath ^{2} \bigr)^{2} \vert \widehat{\vartheta} \vert ^{2}, \\ &\Re e \bigl\{ \beta \imath ^{2}\bigl(2k_{1}-a^{2} \bigr)\widehat{v} \overline{\widehat{\vartheta}} \bigr\} \leq \varepsilon _{3} \frac{\imath ^{2}}{(1+\imath ^{2})^{2}} \vert \widehat{v} \vert ^{2} +c( \varepsilon _{3}) \bigl(1+\imath ^{2}\bigr)^{2} \vert \widehat{\vartheta} \vert ^{2}. \end{aligned}$$
(2.49)
Hence, the estimate (2.19) can also be written as
$$\begin{aligned} \frac{d\mathcal{D}_{2}(\imath,t)}{dt}\leq {}& -\frac{ak_{1}^{2}}{2} \imath ^{2} \vert \widehat{\sigma} \vert ^{2} -\frac{a\beta ^{2}}{2} \imath ^{2} \vert \widehat{w} \vert ^{2}+\varepsilon _{2} \frac{\imath ^{2}}{(1+\imath ^{2})^{2}} \vert \widehat{r} \vert ^{2} \\ &{}+\varepsilon _{3}\frac{\imath ^{2}}{(1+\imath ^{2})^{2}} \vert \widehat{v} \vert ^{2} +c(\varepsilon _{2},\varepsilon _{3}) \bigl(1+\imath ^{2}\bigr)^{2} \vert \widehat{\vartheta} \vert ^{2}. \end{aligned}$$
(2.50)
Similarly, we can prove the second result.
So, we derive (2.37) and by using (2.9), (2.15), (2.50), (2.23), (2.29) and (2.35) with the fact that \(\frac{\imath ^{2}}{1+\imath ^{2}}\leq \min \{1,\imath ^{2}\}\) and \(\frac{1}{1+\imath ^{2}}\leq 1\), we get
$$\begin{aligned} \mathcal{K}_{2}'(\imath,t)\leq {}&- \frac{\imath ^{6}}{(1+\imath ^{2})^{7}} \biggl[\frac{k_{0}l^{2}}{2}M_{3}-2 \varepsilon _{1}M_{1}- \varepsilon _{5}M_{4} \biggr] \vert \widehat{g} \vert ^{2} \\ &{}-\frac{\imath ^{4}}{(1+\imath ^{2})^{5}} \biggl[\frac{1}{2}M_{4}- \varepsilon _{2}M_{2}-\varepsilon _{4}M_{3} \biggr] \vert \widehat{r} \vert ^{2} \\ &{}-\frac{\imath ^{4}}{(1+\imath ^{2})^{3}} \biggl[\frac{a\beta ^{2}}{2}M_{2}-c( \varepsilon _{1})M_{1}-cM_{4} \biggr] \vert \widehat{w} \vert ^{2} \\ &{}-\frac{\imath ^{4}}{(1+\imath ^{2})^{5}} \biggl[\frac{a^{2}l^{2}}{2}M_{4}- \varepsilon _{3}M_{2} \biggr] \vert \widehat{v} \vert ^{2} \\ &{}-\frac{\imath ^{6}}{(1+\imath ^{2})^{6}} \biggl[\frac{k_{0}}{2}M_{1}-c( \varepsilon _{4})M_{3} \biggr] \vert \widehat{\phi} \vert ^{2}- \frac{\imath ^{4}}{(1+\imath ^{2})^{3}} \biggl[\frac{ak_{1}^{2}}{2}M_{2} \biggr] \vert \widehat{\sigma} \vert ^{2} \\ &{}-\imath ^{2} \bigl[mk_{2}M-cM_{1}-c(\varepsilon _{2},\varepsilon _{3})M_{2}-cM_{4} \bigr] \vert \widehat{\vartheta} \vert ^{2} \\ &{}- \bigl[C_{1}M-c(\varepsilon _{1}) M_{1}-cM_{3}-cM_{4}- \aleph _{1}M_{5} \bigr] \vert \widehat{\varpi} \vert ^{2} \\ &{}-\zeta _{1}M_{5} \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} s \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath \\ &{}- [\zeta _{1}M_{5}-cM_{1}-cM_{4} ] \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, 1, s, t ) \bigr\vert ^{2} \,ds. \end{aligned}$$
(2.51)
By setting
$$\begin{aligned} \varepsilon _{1}=\frac{k_{0}l^{2} M_{3}}{16 M_{1}},\qquad \varepsilon _{2}= \frac{M_{4}}{8M_{2}},\qquad \varepsilon _{3}=\frac{a^{2}l^{2}M_{4}}{4M_{2}},\qquad \varepsilon _{4}=\frac{M_{4}}{8M_{3}}, \qquad\varepsilon _{5}= \frac{k_{0}l^{2} M_{3}}{8 M_{4}}, \end{aligned}$$
we obtain the following
$$\begin{aligned} \mathcal{K}_{2}'(\imath,t)\leq{} &{-} \frac{\imath ^{6}}{(1+\imath ^{2})^{7}} \biggl[\frac{k_{0}l^{2}}{4}M_{3} \biggr] \vert \widehat{g} \vert ^{2}- \frac{\imath ^{4}}{(1+\imath ^{2})^{5}} \biggl[\frac{1}{4}M_{4} \biggr] \vert \widehat{r} \vert ^{2} \\ &{}-\frac{\imath ^{4}}{(1+\imath ^{2})^{3}} \biggl[\frac{a\beta ^{2}}{2}M_{2}-c(M_{1},M_{3})M_{1}-cM_{4} \biggr] \vert \widehat{w} \vert ^{2} \\ &{}-\frac{\imath ^{4}}{(1+\imath ^{2})^{5}} \biggl[\frac{a^{2}l^{2}}{4}M_{4} \biggr] \vert \widehat{v} \vert ^{2}- \frac{\imath ^{4}}{(1+\imath ^{2})^{3}} \biggl[ \frac{ak_{1}^{2}}{2}M_{2} \biggr] \vert \widehat{\sigma} \vert ^{2} \\ &{}-\frac{\imath ^{6}}{(1+\imath ^{2})^{6}} \biggl[\frac{k_{0}}{2}M_{1}-c(M_{3},M_{4})M_{3} \biggr] \vert \widehat{\phi} \vert ^{2} \\ &{}-\imath ^{2} \bigl[mk_{2}M-cM_{1}-c(M_{2},M_{4})M_{2}-cM_{4} \bigr] \vert \widehat{\vartheta} \vert ^{2} \\ &{}- \bigl[C_{1}M-c(M_{1},M_{3}) M_{1}-cM_{3}-cM_{4}-\aleph _{1}M_{5} \bigr] \vert \widehat{\varpi} \vert ^{2} \\ &{}-\zeta _{1}M_{5} \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} s \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath \\ &{}- [\zeta _{1}M_{5}-cM_{1}-cM_{4} ] \int _{\wp _{1}}^{\wp _{2}} \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, 1, s, t ) \bigr\vert ^{2} \,ds. \end{aligned}$$
(2.52)
Next, we fix \(M_{3},M_{4}\) and choose \(M_{1}\) large enough such that
$$\begin{aligned} \frac{k_{0}}{2}M_{1}-c(M_{3},M_{4})M_{3}>0, \end{aligned}$$
then, we select \(M_{2},M_{5}\) large enough such that
$$\begin{aligned} &\frac{a\beta ^{2}}{2}M_{2}-c(M_{1},M_{3})M_{1}-cM_{4}>0, \\ &\zeta _{1}M_{5}-cM_{1}-cM_{4}>0. \end{aligned}$$
Hence, we arrive at
$$\begin{aligned} \mathcal{K}_{2}'(\imath,t)\leq{}&{ -}\kappa _{0} \frac{\imath ^{6}}{(1+\imath ^{2})^{7}} \vert \widehat{g} \vert ^{2}- \kappa _{5}\frac{\imath ^{6}}{(1+\imath ^{2})^{6}} \vert \widehat{\phi} \vert ^{2}- \imath ^{2} [mk_{2}M-c ] \vert \widehat{\vartheta} \vert ^{2} \\ &{} -\frac{\imath ^{4}}{(1+\imath ^{2})^{5}} \bigl(\kappa _{1} \vert \widehat{r} \vert ^{2} +\kappa _{3} \vert \widehat{v} \vert ^{2} \bigr) - \frac{\imath ^{4}}{(1+\imath ^{2})^{3}} \bigl(\kappa _{2} \vert \widehat{w} \vert ^{2} +\kappa _{4} \vert \widehat{\sigma} \vert ^{2} \bigr) \\ &{} - [C_{1}M-c ] \vert \widehat{\varpi} \vert ^{2}- \kappa _{6} \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} s \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath. \end{aligned}$$
(2.53)
On the other hand, we have
$$\begin{aligned} \bigl\vert \mathcal{K}_{2}(\imath,t)-M\widehat{V}(\imath,t) \bigr\vert ={}&M_{1}\frac{\imath ^{4}}{(1+\imath ^{2})^{6}} \bigl\vert \mathcal{D}_{1}( \imath,t) \bigr\vert +M_{2} \frac{\imath ^{2}}{(1+\imath ^{2})^{3}} \bigl\vert \mathcal{D}_{2}( \imath,t) \bigr\vert \\ &{} +M_{3}\frac{\imath ^{6}}{(1+\imath ^{2})^{7}} \bigl\vert \mathcal{D}_{3}( \imath,t) \bigr\vert +M_{4}\frac{\imath ^{2}}{(1+\imath ^{2})^{5}} \bigl\vert \mathcal{D}_{4}(\imath,t) \bigr\vert +M_{5} \bigl\vert \mathcal{D}_{5}(\imath,t) \bigr\vert \\ \leq {}&aM_{1}\frac{\imath ^{4}}{(1+\imath ^{2})^{6}} \bigl\vert \Re e \bigl\{ i\imath ( \widehat{\varpi}\overline{\widehat{\phi}} +l \widehat{\phi}\overline{\widehat{w}} ) \bigr\} \bigr\vert \\ &{}+M_{2}\frac{\imath ^{2}}{(1+\imath ^{2})^{3}} \bigl\vert \Re e \bigl\{ i\imath (ak_{1}\widehat{\vartheta} \overline{\widehat{\sigma}} +a\beta \widehat{\vartheta} \overline{\widehat{w}} +2k_{1}\widehat{v}\overline{ \widehat{\sigma}} ) \bigr\} \bigr\vert \\ &{}+M_{3}\frac{\imath ^{6}}{(1+\imath ^{2})^{7}} \bigl\vert \Re e \{\widehat{\phi} \overline{\widehat{g}} \} \bigr\vert \\ &{}+M_{4}\frac{\imath ^{2}}{(1+\imath ^{2})^{5}} \bigl\vert \Re e \bigl\{ i\imath (l \widehat{w}\overline{\widehat{v}} +\widehat{v} \overline{\widehat{\varpi}} ) \bigr\} \bigr\vert \\ &{}+M_{4}\frac{\imath ^{2}}{(1+\imath ^{2})^{5}} \bigl\vert \Re e \bigl\{ (\widehat{w} \overline{\widehat{r}} +a\widehat{g} \overline{\widehat{v}} ) \bigr\} \bigr\vert \\ &{}+M_{5} \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} s e^{-s\jmath } \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath. \end{aligned}$$
Utilizing Young’s inequality, and the fact that \(\frac{\imath ^{2}}{1+\imath ^{2}}\leq \min \{1,\imath ^{2}\}\) and \(\frac{1}{1+\imath ^{2}}\leq 1\), we find
$$\begin{aligned} \bigl\vert \mathcal{K}_{2}(\imath,t)-M\widehat{V}(\imath,t) \bigr\vert \leq c\widehat{V}(\imath,t). \end{aligned}$$
Hence, we get
$$\begin{aligned} (M-c)\widehat{V}(\imath,t)\leq \mathcal{K}_{2}(\imath,t)\leq (M+c) \widehat{V}(\imath,t). \end{aligned}$$
(2.54)
Now, we choose M large enough in such a way that
$$\begin{aligned} M-c>0,\qquad C_{1}M-c>0,\qquad mk_{2}M-c>0, \end{aligned}$$
using (2.8), we get (2.53) and (2.54), respectively. One can find a positive constant \(\kappa >0\), then ∀ \(t>0\) & ∀ \(\imath \in \mathbb{R}\), we get
$$\begin{aligned} \mu _{4}\widehat{V}(\imath,t)\leq \mathcal{K}_{2}(\imath,t) \leq \mu _{5}\widehat{V}(\imath,t). \end{aligned}$$
(2.55)
and
$$\begin{aligned} \mathcal{K}_{2}'(\imath,t)\leq {}&{-}\kappa \frac{\imath ^{6}}{(1+\imath ^{2})^{7}} \biggl( \vert \widehat{g} \vert ^{2}+ \vert \widehat{\phi} \vert ^{2}+ \vert \widehat{\vartheta} \vert ^{2} + \vert \widehat{r} \vert ^{2}+ \vert \widehat{w} \vert ^{2} + \vert \widehat{v} \vert ^{2}+ \vert \widehat{\sigma} \vert ^{2}+ \vert \widehat{\varpi} \vert ^{2} \\ &{} + \int _{0}^{1} \int _{\wp _{1}}^{\wp _{2}} s \bigl\vert \aleph _{2}(s) \bigr\vert \bigl\vert \widehat{\mathcal{Y}} ( \imath, \jmath, s, t ) \bigr\vert ^{2} \,ds \,d\jmath \biggr). \end{aligned}$$
(2.56)
then
$$\begin{aligned} \mathcal{K}_{2}'(\imath,t)\leq -\lambda _{2} \jmath _{2}(\imath ) \widehat{V}(\imath,t), \quad\forall t\geq 0. \end{aligned}$$
(2.57)
Therefore, for some positive constant \(\mu _{6}=\frac{\lambda _{2}}{\mu _{5}}>0\), we get
$$\begin{aligned} \mathcal{K}_{2}'(\imath,t)\leq -\mu _{6}\jmath _{2}(\imath ) \mathcal{K}_{2}(\imath,t), \quad\forall t\geq 0, \end{aligned}$$
(2.58)
where \(\jmath _{2}(\imath )=\frac{\imath ^{6}}{(1+\imath ^{2})^{7}}\), for some \(\lambda _{2},\mu _{i}>0, i=4,5,6\). The proof of the second result (2.39) is finished. □
The pointwise estimates of the functional \(\widehat{V}(\imath,t)\) are given in the following result.
Proposition 2.8
Suppose (1.8) holds. Then, for any \(t\geq 0\) and \(\imath \in \mathbb{R}\), there exist a positive constants \(d_{1},d_{2}>0\) such that the energy functional stated by (2.8) holds
$$\begin{aligned} \textstyle\begin{cases} \widehat{V}(\imath,t)\leq d_{1}\widehat{V}(\imath,0)e^{-\mu _{3} \jmath _{1}(\imath )t}& \textit{if } a=1, \\ \widehat{V}(\imath,t)\leq d_{2}\widehat{V}(\imath,0)e^{-\mu _{6} \jmath _{2}(\imath )t}& \textit{if } a\neq 1, \end{cases}\displaystyle \end{aligned}$$
(2.59)
where \(\jmath _{1}(\imath )=\frac {\imath ^{6}}{(1+\imath ^{2})^{4}}, \jmath _{2}(\imath )=\frac {\imath ^{6}}{(1+\imath ^{2})^{7}}\).
Proof
From (2.38)2 and (2.39)2, we have
$$\begin{aligned} &\mathcal{K}_{1}(\imath,t)\leq \mathcal{K}_{1}( \imath,0)e^{-\mu _{3} \jmath _{1}(\imath )t}, \quad\forall t\geq 0, \text{ if } a=1 \end{aligned}$$
(2.60)
$$\begin{aligned} &\mathcal{K}_{2}(\imath,t)\leq \mathcal{K}_{2}( \imath,0)e^{-\mu _{6} \jmath _{2}(\imath )t},\quad \forall t\geq 0, \text{ if } a\neq 1. \end{aligned}$$
(2.61)
Hence, according of (2.38)1, (2.39)1 and (2.60), (2.61), we established (2.59). □
2.2 Decay estimates
Now, we will show the following important result.
Theorem 2.9
Let s be a nonnegative integer, and \(Z_{0}\in H^{s}(\mathbb{R})\cap L^{1}(\mathbb{R})\). Then, the solution Z of problem (2.2)–(2.3) holds, ∀ \(t\geq 0\) the following decay estimates
-
For \(a=1\)
$$\begin{aligned} \bigl\Vert \partial _{x}^{k}Z(t) \bigr\Vert _{2}\leq C \Vert Z_{0} \Vert _{1}(1+t)^{- \frac{1}{12}-\frac{k}{6}} +C(1+t)^{-\frac{\ell}{2}} \bigl\Vert \partial _{x}^{k+ \ell}Z_{0} \bigr\Vert _{2} \end{aligned}$$
(2.62)
-
For \(a\neq 1\)
$$\begin{aligned} \bigl\Vert \partial _{x}^{k}Z(t) \bigr\Vert _{2}\leq C \Vert Z_{0} \Vert _{1}(1+t)^{- \frac{1}{12}-\frac{k}{6}} +C(1+t)^{-\frac{\ell}{8}} \bigl\Vert \partial _{x}^{k+ \ell}Z_{0} \bigr\Vert _{2}, \end{aligned}$$
(2.63)
where ℓ and k are nonnegative integers \(k+\ell \leq s\) and \(C>0\) is a positive constant.
Proof
From (2.8), we get \(\vert \widehat{Z}(\imath,t)\vert ^{2}\sim \widehat{V}(\imath,t)\).
-
If \(a=1\), then by using the Plancherel theorem and (2.59)1, we have
$$\begin{aligned} \bigl\Vert \partial _{x}^{k}Z(t) \bigr\Vert _{2}^{2}={}& \int _{\mathbb{R}} \vert \imath \vert ^{2k} \bigl\vert \widehat{Z}(\imath,t) \bigr\vert ^{2}\,d\imath \\ \leq {}&c \int _{\mathbb{R}} \vert \imath \vert ^{2k}e^{-\mu _{3}\jmath _{1}( \imath )t} \bigl\vert \widehat{Z}(\imath,0) \bigr\vert ^{2}\,d\imath \\ \leq {}& \underbrace{c \int _{ \vert \imath \vert \leq 1} \vert \imath \vert ^{2k}e^{-\mu _{3}\jmath _{1}(\imath )t} \bigl\vert \widehat{Z}(\imath,0) \bigr\vert ^{2}\,d \imath}_{R_{1}} \\ &{}+ \underbrace{c \int _{ \vert \imath \vert \geq 1} \vert \imath \vert ^{2k}e^{-\mu _{3}\jmath _{1}(\imath )t} \bigl\vert \widehat{Z}(\imath,0) \bigr\vert ^{2}\,d \imath}_{R_{2}}. \end{aligned}$$
(2.64)
Now, we estimate \(R_{1},R_{2}\), the low-frequency part \(\vert \imath \vert \leq 1\) and the high-frequency part \(\vert \imath \vert \geq 1\), respectively. First, we have \(\jmath _{1}(\imath )\geq \frac{1}{16}\imath ^{6}\), for \(\vert \imath \vert \leq 1\). Then
$$\begin{aligned} R_{1}\leq {}&c \int _{ \vert \imath \vert \leq 1} \vert \imath \vert ^{2k}e^{- \frac{\mu _{3}}{16} \vert \imath \vert ^{6}t} \bigl\vert \widehat{Z}(\imath,0) \bigr\vert ^{2}\,d\imath \\ \leq {}&c\sup_{ \vert \imath \vert \leq 1}\bigl\{ \bigl\vert \widehat{Z}(\imath,0) \bigr\vert ^{2}\bigr\} \int _{ \vert \imath \vert \leq 1} \vert \imath \vert ^{2k}e^{- \frac{\mu _{3}}{16} \vert \imath \vert ^{6}t} \,d\imath, \end{aligned}$$
(2.65)
by utilizing Lemma 1.1, we get
$$\begin{aligned} R_{1}&\leq c\sup_{ \vert \imath \vert \leq 1}\bigl\{ \bigl\vert \widehat{Z}( \imath,0) \bigr\vert ^{2}\bigr\} (1+t)^{-\frac{k}{3}-\frac{1}{6}} \\ &\leq c \Vert Z_{0} \Vert ^{2}_{1}(1+t)^{-\frac{k}{3}-\frac{1}{6}}. \end{aligned}$$
(2.66)
Secondly, we have \(\jmath _{1}(\imath )\geq \frac{1}{16}\imath ^{-2}\), for \(\vert \imath \vert \geq 1\). Then
$$\begin{aligned} R_{2}\leq c \int _{ \vert \imath \vert \geq 1} \vert \imath \vert ^{2k}e^{- \frac{\mu _{3}}{16} \vert \imath \vert ^{-2}t} \bigl\vert \widehat{Z}( \imath,0) \bigr\vert ^{2}\,d\imath, \quad\forall t\geq 0. \end{aligned}$$
(2.67)
Then, through the inequality
$$\begin{aligned} \sup_{ \vert \imath \vert \geq 1} \bigl\{ \vert \imath \vert ^{-2\ell}e^{-c \frac{1}{16} \vert \imath \vert ^{-2}t} \bigr\} \leq C(1+t)^{-\ell}, \end{aligned}$$
(2.68)
we get that
$$\begin{aligned} R_{2}&\leq c\sup_{ \vert \imath \vert \geq 1} \bigl\{ \vert \imath \vert ^{-2\ell}e^{-\frac{\mu _{3}}{16} \vert \imath \vert ^{-2}t} \bigr\} \int _{ \vert \imath \vert \geq 1} \vert \imath \vert ^{2(k+ \ell )} \bigl\vert \widehat{Z}(\imath,0) \bigr\vert ^{2}\,d\imath \\ &\leq c(1+t)^{-\ell} \bigl\Vert \partial ^{k+\ell}_{x}Z(x,0) \bigr\Vert _{2}^{2},\quad \forall t\geq 0. \end{aligned}$$
(2.69)
Substituting (2.66) and (2.69) into (2.64), we find (2.62).
-
If \(a\neq 1\), similar to the first estimate, we apply the Plancherel theorem and using (2.59)2, we get
$$\begin{aligned} \bigl\Vert \partial _{x}^{k}Z(t) \bigr\Vert _{2}^{2}={}& \int _{\mathbb{R}} \vert \imath \vert ^{2k} \bigl\vert \widehat{Z}(\imath,t) \bigr\vert ^{2}\,d\imath \\ \leq {}&c \int _{\mathbb{R}} \vert \imath \vert ^{2k}e^{-\mu _{6}\jmath _{2}( \imath )t} \bigl\vert \widehat{Z}(\imath,0) \bigr\vert ^{2}\,d\imath \\ \leq {}& \underbrace{c \int _{ \vert \imath \vert \leq 1} \vert \imath \vert ^{2k}e^{-\mu _{6}\jmath _{2}(\imath )t} \bigl\vert \widehat{Z}(\imath,0) \bigr\vert ^{2}\,d \imath}_{R_{3}} \\ &{}+ \underbrace{c \int _{ \vert \imath \vert \geq 1} \vert \imath \vert ^{2k}e^{-\mu _{6}\jmath _{2}(\imath )t} \bigl\vert \widehat{Z}(\imath,0) \bigr\vert ^{2}\,d \imath}_{R_{4}}. \end{aligned}$$
(2.70)
Now, we estimate \(R_{3},R_{4}\), the low-frequency part \(\vert \imath \vert \leq 1\) and the high-frequency part \(\vert \imath \vert \geq 1\), respectively. First, we have \(\jmath _{2}(\imath )\geq \frac{1}{64}\imath ^{6}\), for \(\vert \imath \vert \leq 1\). Then
$$\begin{aligned} R_{3}&\leq c \int _{ \vert \imath \vert \leq 1} \vert \imath \vert ^{2k}e^{- \frac{\mu _{6}}{64} \vert \imath \vert ^{6}t} \bigl\vert \widehat{Z}(\imath,0) \bigr\vert ^{2}\,d\imath \\ &\leq c\sup_{ \vert \imath \vert \leq 1}\bigl\{ \bigl\vert \widehat{Z}(\imath,0) \bigr\vert ^{2}\bigr\} \int _{ \vert \imath \vert \leq 1} \vert \imath \vert ^{2k}e^{- \frac{\mu _{6}}{64} \vert \imath \vert ^{6}t} \,d\imath, \end{aligned}$$
(2.71)
by utilizing Lemma 1.1, we get
$$\begin{aligned} R_{3}&\leq c\sup_{ \vert \imath \vert \leq 1}\bigl\{ \bigl\vert \widehat{Z}( \imath,0) \bigr\vert ^{2}\bigr\} (1+t)^{-\frac{k}{3}-\frac{1}{6}} \\ &\leq c \Vert Z_{0} \Vert ^{2}_{1}(1+t)^{-\frac{k}{3}-\frac{1}{6}}. \end{aligned}$$
(2.72)
Secondly, we have \(\jmath _{2}(\imath )\geq \frac{1}{64}\imath ^{-8}\), for \(\vert \imath \vert \geq 1\). Then
$$\begin{aligned} R_{4}\leq c \int _{ \vert \imath \vert \geq 1} \vert \imath \vert ^{2k}e^{- \frac{\mu _{6}}{64} \vert \imath \vert ^{-8}t} \bigl\vert \widehat{Z}( \imath,0) \bigr\vert ^{2}\,d\imath,\quad \forall t\geq 0. \end{aligned}$$
(2.73)
By (2.68), we find
$$\begin{aligned} R_{4}&\leq c\sup_{ \vert \imath \vert \geq 1} \bigl\{ \vert \imath \vert ^{-2\ell}e^{-\frac{\mu _{6}}{64} \vert \imath \vert ^{-8}t} \bigr\} \int _{ \vert \imath \vert \geq 1} \vert \imath \vert ^{2(k+ \ell )} \bigl\vert \widehat{Z}(\imath,0) \bigr\vert ^{2}\,d\imath \\ &\leq c(1+t)^{-\frac{\ell}{4}} \bigl\Vert \partial ^{k+\ell}_{x}Z(x,0) \bigr\Vert _{2}^{2},\quad \forall t\geq 0. \end{aligned}$$
(2.74)
Substituting (2.72) and (2.74) into (2.70), we obtain (2.63).
□