First we give the Klein-Gordon equation with mass parameter that is considered in this paper
$$ h^{2}\frac{\partial^{2}\Psi(t,x)}{\partial t^{2}}-h^{2}c^{2} \frac {\partial ^{2}\Psi(t,x)}{\partial x^{2}}+c^{4}m^{2}\Psi=0. $$
(4)
The Klein-Gordon equation with mass parameter m has solutions with complex-valued functions of the time variable t and space variables of x. Theorems of the derivation of the numerical approximation of three Riemann-Liouville types of fractional derivatives can be found in paper [12].
3.1 Second approximation of Riemann Liouville approach and a stability analysis of the numerical scheme
Theorem 1
Let
f
be a function not necessary differentiable within an interval
\([a,T]\), then the fractional derivative of
f
of order
\(1<\alpha\leq2\)
in Riemann-Liouville sense is given as follows:
$$ \begin{aligned}[b] &{}_{0}^{\mathrm{RL}}D_{t}^{\alpha} \bigl[ f(x) \bigr] \\ &\quad=\frac{ ( \Delta x ) ^{-1-\alpha}}{2\Gamma(3-\alpha )} \Biggl[ \sum _{k=0}^{j}f(x_{k+1})g_{j,k}^{\alpha} -2\sum_{k=0}^{j-1}f(x_{k+1})g_{j,k}^{\alpha ,1}+ \sum_{k=0}^{j-1}f(x_{k+1})g_{j,k}^{\alpha,2} \Biggr] +R_{\alpha,j,k}, \end{aligned}$$
(5)
where
$$ \begin{aligned}[b] R_{\alpha,j,k}={}&\frac{ ( \Delta x ) ^{-2}}{2\Gamma (2-\alpha)} \Biggl[ \sum_{k=0}^{j} \int_{x_{k}}^{x_{k+1}}\frac{f(y)-f(x_{k+1})}{ ( x_{j+1}-y ) ^{\alpha-1}}\,dy \\ &-2\sum_{k=0}^{j-1} \int_{x_{k}}^{x_{k+1}}\frac{f(y)-f(x_{k+1})}{ ( x_{j}-y ) ^{\alpha-1}}\,dy +\sum _{k=0}^{j-1}\int _{x_{k}}^{x_{k+1}} \frac{f(y)-f(x_{k+1})}{ ( x_{j-1}-y ) ^{\alpha-1}}\,dy \Biggr] , \end{aligned} $$
(6)
and
$$ \begin{gathered} g_{j,k}^{\alpha} =(j-k)^{1-\alpha}-(j-k+1)^{1-\alpha}, \\ g_{j,k}^{\alpha,1} =(j-k-1)^{1-\alpha}-(j-k)^{1-\alpha}, \\ g_{j,k}^{\alpha,2} =(j-k-1)^{1-\alpha}-(j-k-1)^{1-\alpha}. \end{gathered} $$
(7)
Now, we can consider the equation with the second order Riemann-Liouville derivative.
$$\begin{aligned}& h^{2}{}_{0}^{\mathrm{RL}}D_{t}^{\alpha} \bigl[ \Psi(t,x) \bigr] = {}_{0}^{\mathrm{RL}}D_{x}^{\alpha} \bigl[ \Psi(t,x) \bigr] h^{2}c-c^{4}m^{2}\Psi(t,x), \end{aligned}$$
(8)
$$\begin{aligned}& \begin{aligned}[b] &h^{2} \frac{ ( \Delta t ) ^{-1-\alpha}}{2\Gamma(3-\alpha )} \Biggl[ \sum _{k=0}^{j}\Psi(t_{k+1})g_{j,k}^{\alpha} -2\sum_{k=0}^{j-1}\Psi(t_{k+1})g_{j,k}^{\alpha ,1}+ \sum_{k=0}^{j-1}\Psi(t_{k+1})g_{j,k}^{\alpha,2} \Biggr] \\ &\quad=h^{2}c^{2} \biggl[ \frac{ ( \Psi_{i+1}^{j+1}-2\Psi _{i}^{j+1}+\Psi _{i-1}^{j+1} ) - ( \Psi_{i+1}^{j}-2\Psi_{i}^{j}+\Psi _{i-1}^{j} ) }{2 ( \triangle x ) ^{2}} \biggr] \\ &\qquad{}-c^{4}m^{2} \biggl[ \frac{\Psi_{i}^{j+1}+\Psi_{i}^{j}}{2} \biggr] . \end{aligned} \end{aligned}$$
(9)
To make further work clearer, let us do regulation in the equation with sufficient parameters as follows:
$$ c_{i}^{j}=\frac{h^{2} ( \Delta t ) ^{-1-\alpha}}{2\Gamma (3-\alpha )},\qquad d_{i}^{j}= \frac{h^{2}c^{2}}{2 ( \triangle x ) ^{2}},\qquad e_{i}^{j}= \frac{c^{4}m^{2}}{2}. $$
(10)
Then we rewrite the equation with parameters
$$ \begin{aligned}[b] &c_{i}^{j} \bigl( \Psi_{i}^{j+1}g_{j,k}^{\alpha}-2 \Psi _{i}^{j}g_{j,k}^{\alpha,1}+ \Psi_{i}^{j}g_{j,k}^{\alpha,2} \bigr) \\ &\quad\quad{}+c_{i}^{j} \Biggl[ \sum _{k=0}^{j-1} \Psi_{i}^{j+1}g_{j,k}^{\alpha }-2 \sum_{k=0}^{j-2}\Psi_{i}^{j+1}g_{j,k}^{\alpha ,1}+ \sum_{k=0}^{j-2}\Psi_{i}^{j+1}g_{j,k}^{\alpha,2} \Biggr] \\ &\quad=d_{i}^{j} \bigl[ \bigl( \Psi_{i+1}^{j+1}-2 \Psi_{i}^{j+1}+\Psi _{i-1}^{j+1} \bigr) - \bigl( \Psi_{i+1}^{j}-2\Psi_{i}^{j}+\Psi _{i-1}^{j} \bigr) \bigr] \\ &\qquad{}-e_{i}^{j} \bigl( \Psi_{i}^{j+1}+ \Psi_{i}^{j} \bigr) . \end{aligned} $$
(11)
Finally, we have the following equation for the numerical scheme:
$$ \begin{aligned}[b] &\Psi_{i}^{j+1} \bigl( c_{i}^{j}g_{j,k}^{\alpha }+2d_{i}^{j}+e_{i}^{j} \bigr) \\ &\quad=\Psi_{i}^{j} \bigl( 2c_{i}^{j}g_{j,k}^{\alpha ,1}-c_{i}^{j}g_{j,k}^{\alpha,2}+2d_{i}^{j}-e_{i}^{j} \bigr) \\ &\quad\quad{}-c_{i}^{j} \Biggl[ \sum _{k=0}^{j-1} \Psi_{i}^{j+1}g_{j,k}^{\alpha }-2 \sum_{k=0}^{j-2}\Psi_{i}^{j+1}g_{j,k}^{\alpha ,1}+ \sum_{k=0}^{j-2}\Psi_{i}^{j+1}g_{j,k}^{\alpha,2} \Biggr] \\ &\qquad{}+d_{i}^{j} \bigl[ \Psi_{i+1}^{j+1}+ \Psi_{i-1}^{j+1}-\Psi _{i+1}^{j}-\Psi _{i-1}^{j} \bigr]. \end{aligned} $$
(12)
3.1.1 Stability analysis of the numerical scheme
Let us represent a stability analysis of the numerical scheme by supposing
$$ v_{i}^{j}=\Psi_{i}^{j}-w_{i}^{j}, $$
(13)
where \(w_{i}^{j}\) is the approximate solution of the equation in time and space \((x_{i},t_{j})\) (\({i=1,2,\ldots,N}\), \(j=1,2,\ldots,M \)) [4].
Also the error for approximation is given as
$$ v_{i}^{j}= \bigl[ v_{1}^{j},v_{2}^{j}, \ldots,v_{N}^{j} \bigr] {.} $$
(14)
So we have the following error expression for the Klein-Gordon equation with mass parameter:
$$ \begin{aligned}[b] &v_{i}^{j+1} \bigl( c_{i}^{j}g_{j,k}^{\alpha }+2d_{i}^{j}+e_{i}^{j} \bigr) \\ &\quad=v_{i}^{j} \bigl( 2c_{i}^{j}g_{j,k}^{\alpha ,1}-c_{i}^{j}g_{j,k}^{\alpha ,2}+2d_{i}^{j}-e_{i}^{j} \bigr) \\ &\qquad{}-c_{i}^{j} \Biggl[ \sum _{k=0}^{j-1}v_{i}^{j+1}g_{j,k}^{\alpha }-2 \sum_{k=0}^{j-2}v_{i}^{j+1}g_{j,k}^{\alpha ,1}+ \sum_{k=0}^{j-2}v_{i}^{j+1}g_{j,k}^{\alpha,2} \Biggr] \\ &\quad\quad{}+d_{i}^{j} \bigl[ v_{i+1}^{j+1}+v_{i-1}^{j+1}-v_{i+1}^{j}-v_{i-1}^{j} \bigr]. \end{aligned} $$
(15)
Then let us take the following equality for the stability analysis.
$$ \begin{gathered} v_{m}(x,t)=\exp[at] \exp[ik_{m}x], \\ v_{m}^{j} =\exp[at] \exp[ik_{m}x], \\ v_{m}^{j+1} =\exp \bigl[a ( t+\Delta t ) \bigr] \exp[ik_{m}x], \\ v_{m+1}^{j} =\exp[at]\exp \bigl[ik_{m} ( x+ \Delta x ) \bigr], \\ v_{m-1}^{j} =\exp[at]\exp \bigl[ik_{m} ( x- \Delta x ) \bigr], \\ v_{m+1}^{j+1} =\exp \bigl[a ( t+\Delta t ) \bigr]\exp \bigl[ik_{m} ( x+\Delta x ) \bigr], \\ v_{m-1}^{j+1} =\exp \bigl[a ( t+\Delta t ) \bigr]\exp \bigl[ik_{m} ( x-\Delta x ) \bigr], \\ v_{m-1}^{j-1} =\exp \bigl[a ( t-\Delta t ) \bigr]\exp \bigl[ik_{m} ( x-\Delta x ) \bigr],\end{gathered} $$
(16)
where \(k_{m}=\frac{\pi m}{L}\), \(m=1,2,\ldots,M=\frac{L}{\Delta x}\). If we use the equalities above, equation can be reconsidered as follows:
$$\begin{aligned} \begin{aligned}[b] &\exp \bigl[a ( t+\Delta t ) \bigr] \exp[ik_{m}x] \bigl( c_{i}^{j}g_{j,k}^{\alpha}+2d_{i}^{j}+e_{i}^{j} \bigr) \\ &\quad=\exp[at]\exp[ik_{m}x] \bigl( 2c_{i}^{j}g_{j,k}^{\alpha ,1}-c_{i}^{j}g_{j,k}^{\alpha,2}+2d_{i}^{j}-e_{i}^{j} \bigr) \\ &\qquad{}-c_{i}^{j} \Biggl[ \sum _{k=0}^{j-1}\exp \bigl[a ( t+\Delta t ) \bigr] \exp [ik_{m}x]g_{j,k}^{\alpha} \\ &\qquad{} -2\sum_{k=0}^{j-2}\exp \bigl[a ( t+\Delta t ) \bigr]\exp [ik_{m}x]g_{j,k}^{\alpha,1} +\sum _{k=0}^{j-2}\exp \bigl[a ( t+ \Delta t ) \bigr]\exp [ik_{m}x]g_{j,k}^{\alpha,2} \Biggr] \\ &\quad\quad{}+d_{i}^{j} \bigl[ \exp \bigl[a ( t+\Delta t ) \bigr]\exp \bigl[ik_{m} ( x+\Delta x ) \bigr] + \exp \bigl[a ( t+\Delta t ) \bigr]\exp \bigl[ik_{m} ( x-\Delta x ) \bigr] \\ &\quad\quad{}-\exp[at]\exp \bigl[ik_{m} ( x+\Delta x ) \bigr] - \exp[at]\exp \bigl[ik_{m} ( x-\Delta x ) \bigr] \bigr]. \end{aligned} \end{aligned}$$
(17)
If we do simplification with \(\exp[at]\exp[ik_{m}x]\), we will obtain the following:
$$\begin{aligned}& \begin{aligned}[b] &\exp \bigl[a ( \Delta t ) \bigr] \bigl( c_{i}^{j}g_{j,k}^{\alpha }+2d_{i}^{j}+e_{i}^{j} \bigr) \\ &\quad= \bigl( 2c_{i}^{j}g_{j,k}^{\alpha,1}-c_{i}^{j}g_{j,k}^{\alpha ,2}+2d_{i}^{j}-e_{i}^{j} \bigr) \\ &\qquad{}-c_{i}^{j} \Biggl[ \sum _{k=0}^{j-1}\exp \bigl[a ( \Delta t ) \bigr]g_{j,k}^{\alpha} \\ &\qquad{}-2\sum_{k=0}^{j-2} \exp \bigl[a ( \Delta t ) \bigr]g_{j,k}^{\alpha,1} +\sum _{k=0}^{j-2}\exp \bigl[a ( \Delta t ) \bigr]g_{j,k}^{\alpha,2} \Biggr] \\ &\qquad{}+d_{i}^{j} \bigl[ \exp \bigl[a ( \Delta t ) \bigr]\exp \bigl[ik_{m} ( \Delta x ) \bigr] +\exp \bigl[a ( \Delta t ) \bigr]\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \\ &\qquad{} -\exp \bigl[ik_{m} ( \Delta x ) \bigr] -\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr] \end{aligned} \end{aligned}$$
(18)
$$\begin{aligned}& \begin{aligned}[b] &\exp \bigl[a ( \Delta t ) \bigr] \bigl[ c_{i}^{j}g_{j,k}^{\alpha}+2d_{i}^{j}+e_{i}^{j}+c_{i}^{j}.J \bigl( g_{j,k}^{\alpha}-2g_{j,k}^{\alpha,1}+g_{j,k}^{\alpha,2} \bigr) \\ &\qquad{}-d_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr] \\ &\quad= \bigl( 2c_{i}^{j}g_{j,k}^{\alpha,1}-c_{i}^{j}g_{j,k}^{\alpha ,2}+2d_{i}^{j}-e_{i}^{j} \bigr) -d_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \end{aligned} \end{aligned}$$
(19)
$$\begin{aligned}& \begin{aligned}[b] \exp \bigl[a ( \Delta t ) \bigr]={}&{ \bigl\{ \bigl( 2c_{i}^{j}g_{j,k}^{\alpha,1}-c_{i}^{j}g_{j,k}^{\alpha ,2}+2d_{i}^{j}-e_{i}^{j} \bigr) -d_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr\} } \\ &/ \bigl\{ c_{i}^{j}g_{j,k}^{\alpha}+2d_{i}^{j}+e_{i}^{j}+c_{i}^{j}.J \bigl( g_{j,k}^{\alpha}-2g_{j,k}^{\alpha,1}+g_{j,k}^{\alpha,2} \bigr) \\ &-d_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr\} . \end{aligned} \end{aligned}$$
(20)
With the help of the following inequality step by step, we have the condition for the stability analysis.
$$ \begin{gathered} \frac{v_{i}^{j+1}}{v_{i}^{j}}=\exp \bigl[a(\Delta t) \bigr], \\ \biggl\vert \frac{v_{i}^{j+1}}{v_{i}^{j}} \biggr\vert = \bigl\vert \exp \bigl[a( \Delta t) \bigr] \bigr\vert \leq1.\end{gathered} $$
(21)
Then the stability condition is given as
$$ \begin{aligned}[b] & \bigl\vert { \bigl\{ \bigl( 2c_{i}^{j}g_{j,k}^{\alpha,1}-c_{i}^{j}g_{j,k}^{\alpha ,2}+2d_{i}^{j}-e_{i}^{j} \bigr) -d_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr\} } \\ &\qquad/ \bigl\{ c_{i}^{j}g_{j,k}^{\alpha}+2d_{i}^{j}+e_{i}^{j}+c_{i}^{j}.J \bigl( g_{j,k}^{\alpha}-2g_{j,k}^{\alpha,1}+g_{j,k}^{\alpha,2} \bigr) \\ &\qquad{}-d_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr\} \bigr\vert \\ &\quad \leq1.\end{aligned} $$
(22)
Theorem 2
The Crank-Nicholson scheme for solving the Klein-Gordon equation with second order Riemann-Liouville is stable if inequality (22) is satisfied.
3.2 Second approximation of the Caputo-Fabrizio derivative in Riemann-Liouville sense and a stability analysis of the numerical scheme
Theorem 3
Let
f
be a function not necessary differentiable within an interval
\([a,T]\), then the fractional derivative of
f
of order
\(1<\alpha\leq2\)
in the Caputo-Fabrizio derivative in Riemann-Liouville sense is given as follows:
$$ {}_{0}^{\mathrm{CFR}}D_{t}^{\alpha} \bigl[ f(x) \bigr] =\frac{1}{2 ( \Delta x ) ^{2}} \Biggl[ \sum _{k=0}^{j}\frac{f(x_{k+1})}{2}d_{j,k}^{\alpha,1} -2\sum_{k=0}^{j-1}\frac{f(x_{k+1})}{2}d_{j,k}^{\alpha ,2}+ \sum_{k=0}^{j-1}\frac{f(x_{k+1})}{2}d_{j,k}^{\alpha,3} \Biggr] +F, $$
(23)
where
$$ \begin{aligned} F={}&\frac{\alpha}{(1-\alpha)\sqrt{\pi}} \Biggl[ \sum _{k=0}^{j} \int_{x_{k}}^{x_{k+1}} \bigl( f(\tau )-f(x_{j+1}) \bigr) \exp \biggl( - \biggl( \frac{\alpha}{1-\alpha } \biggr) ^{2} ( x_{j+1}-\tau ) ^{2} \biggr) \,d\tau \\ &-2\sum_{k=0}^{j-1} \int_{x_{k}}^{x_{k+1}} \bigl( f(\tau )-f(x_{j+1}) \bigr) \exp \biggl( - \biggl( \frac{\alpha}{1-\alpha } \biggr) ^{2} ( x_{j}-\tau ) ^{2} \biggr) \,d\tau \\ &+\sum_{k=0}^{j-1} \int_{x_{k}}^{x_{k+1}} \bigl( f(\tau )-f(x_{j+1}) \bigr) \exp \biggl( - \biggl( \frac{\alpha}{1-\alpha } \biggr) ^{2} ( x_{j+1}-\tau ) ^{2} \biggr) \,d\tau \Biggr] ,\end{aligned} $$
and
$$ \begin{gathered} d_{j,k}^{\alpha,1} = \operatorname{{erf}c} \biggl\{ - \alpha\frac{x_{j+1}-x_{k+1}}{1-\alpha} \biggr\} - \operatorname{{erf}c} \biggl\{ -\alpha\frac {x_{j+1}-x_{k}}{1-\alpha } \biggr\} , \\ d_{j,k}^{\alpha,2} =\operatorname{{erf}c} \biggl\{ -\alpha \frac{x_{j}-x_{k+1}}{1-\alpha} \biggr\} -\operatorname{{erf}c} \biggl\{ -\alpha \frac {x_{j}-x_{k}}{1-\alpha} \biggr\} , \\ g_{j,k}^{\alpha,3} =\operatorname{{erf}c} \biggl\{ -\alpha \frac{x_{j-1}-x_{k+1}}{1-\alpha} \biggr\} -\operatorname{{erf}c} \biggl\{ -\alpha \frac {x_{j-1}-x_{k}}{1-\alpha } \biggr\} . \end{gathered} $$
(24)
Also we will consider the following equation with the Caputo-Fabrizio derivative in Riemann-Liouville sense of order
\(1<\alpha\leq2\):
$$ \begin{aligned}[b] &\frac{1}{2 ( \Delta x ) ^{2}} \Biggl[ \sum_{k=0}^{j} \frac{\Psi(x_{k+1})}{2}d_{j,k}^{\alpha,1} -2\sum _{k=0}^{j-1}\frac{\Psi(x_{k+1})}{2}d_{j,k}^{\alpha ,2}+ \sum_{k=0}^{j-1}\frac{\Psi(x_{k+1})}{2}d_{j,k}^{\alpha,3} \Biggr] \\ &\quad=h^{2}c^{2} \biggl[ \frac{ ( \Psi_{i+1}^{j+1}-2\Psi _{i}^{j+1}+\Psi _{i-1}^{j+1} ) - ( \Psi_{i+1}^{j}-2\Psi_{i}^{j}+\Psi _{i-1}^{j} ) }{2 ( \triangle x ) ^{2}} \biggr] -c^{4}m^{2} \biggl[ \frac{\Psi_{i}^{j+1}+\Psi_{i}^{j}}{2} \biggr] . \end{aligned} $$
(25)
To continue easier, let us do simplification in the equation with sufficient parameters as follows:
$$ f_{i}^{j}=\frac{1}{4 ( \Delta x ) ^{2}},\qquad g_{i}^{j}= \frac{h^{2}c^{2}}{2 ( \triangle x ) ^{2}},\qquad h_{i}^{j}=\frac {c^{4}m^{2}}{2}. $$
(26)
Then we rewrite the equation with parameters
$$ \begin{aligned}[b] &f_{i}^{j} \bigl( \Psi_{i}^{j+1}d_{j,k}^{\alpha,1}-2 \Psi _{i}^{j}d_{j,k}^{\alpha,2}+ \Psi_{i}^{j}d_{j,k}^{\alpha,3} \bigr) \\ &\qquad{}+f_{i}^{j} \Biggl[ \sum _{k=0}^{j-1} \Psi_{i}^{j+1}d_{j,k}^{\alpha ,1}-2 \sum_{k=0}^{j-2}\Psi_{i}^{j+1}d_{j,k}^{\alpha ,2}+ \sum_{k=0}^{j-2}\Psi_{i}^{j+1}d_{j,k}^{\alpha,3} \Biggr] \\ &\quad=g_{i}^{j} \bigl[ \bigl( \Psi_{i+1}^{j+1}-2 \Psi_{i}^{j+1}+\Psi _{i-1}^{j+1} \bigr) - \bigl( \Psi_{i+1}^{j}-2\Psi_{i}^{j}+\Psi _{i-1}^{j} \bigr) \bigr] \\ &\qquad{}-h_{i}^{j} \bigl( \Psi_{i}^{j+1}+ \Psi_{i}^{j} \bigr) . \end{aligned} $$
(27)
Finally, we have the following equation for the numerical scheme:
$$\begin{aligned} \begin{aligned}[b] &\Psi_{i}^{j+1} \bigl( f_{i}^{j}d_{j,k}^{\alpha ,1}+2g_{i}^{j}+h_{i}^{j} \bigr) \\ &\quad=\Psi_{i}^{j} \bigl( 2f_{i}^{j}d_{j,k}^{\alpha ,2}-f_{i}^{j}d_{j,k}^{\alpha,3}+2g_{i}^{j}-h_{i}^{j} \bigr) \\ &\qquad{}-f_{i}^{j} \Biggl[ \sum _{k=0}^{j-1} \Psi_{i}^{j+1}d_{j,k}^{\alpha ,1}-2 \sum_{k=0}^{j-2}\Psi_{i}^{j+1}d_{j,k}^{\alpha ,2}+ \sum_{k=0}^{j-2}\Psi_{i}^{j+1}d_{j,k}^{\alpha,3} \Biggr] \\ &\quad\quad{}+g_{i}^{j} \bigl[ \Psi_{i+1}^{j+1}+ \Psi_{i-1}^{j+1}-\Psi _{i+1}^{j}-\Psi _{i-1}^{j} \bigr] . \end{aligned} \end{aligned}$$
(28)
3.2.1 Stability analysis of the numerical scheme for the Caputo-Fabrizio derivative in Riemann-Liouville sense
Let us represent a stability analysis of the numerical scheme by supposing
$$ s_{i}^{j}=\Psi_{i}^{j}-l_{i}^{j}, $$
(29)
where \(l_{i}^{j}\) is the approximate solution of the equation in time and space \((x_{i},t_{j})\) (\(i=1,2,\ldots,N\), \(j=1,2,\ldots,M \)).
Also the error for approximation is given as
$$ s_{i}^{j}= \bigl[ s_{1}^{j},s_{2}^{j}, \ldots,s_{N}^{j} \bigr] {.} $$
(30)
So we have the following error expression for the Klein-Gordon equation with mass parameter:
$$ \begin{aligned}[b] &s_{i}^{j+1} \bigl( f_{i}^{j}d_{j,k}^{\alpha ,1}+2g_{i}^{j}+h_{i}^{j} \bigr) \\ &\quad=s_{i}^{j} \bigl( 2f_{i}^{j}d_{j,k}^{\alpha ,2}-f_{i}^{j}d_{j,k}^{\alpha ,3}+2g_{i}^{j}-h_{i}^{j} \bigr) \\ &\qquad{}-f_{i}^{j} \Biggl[ \sum _{k=0}^{j-1}s_{i}^{j+1}d_{j,k}^{\alpha ,1}-2 \sum_{k=0}^{j-2}s_{i}^{j+1}d_{j,k}^{\alpha ,2}+ \sum_{k=0}^{j-2}s_{i}^{j+1}d_{j,k}^{\alpha,3} \Biggr] \\ &\quad\quad{}+g_{i}^{j} \bigl[ s_{i+1}^{j+1}+s_{i-1}^{j+1}-s_{i+1}^{j}-s_{i-1}^{j} \bigr] .\end{aligned} $$
(31)
Then let us take the following equality for doing the stability analysis.
$$ \begin{gathered} s_{m}(x,t)=\exp[at] \exp[ik_{m}x], \\ s_{m}^{j} =\exp[at] \exp[ik_{m}x], \\ s_{m}^{j+1} =\exp \bigl[a ( t+\Delta t ) \bigr] \exp[ik_{m}x], \\ s_{m+1}^{j} =\exp[at]\exp \bigl[ik_{m} ( x+ \Delta x ) \bigr], \\ s_{m-1}^{j} =\exp[at]\exp \bigl[ik_{m} ( x- \Delta x ) \bigr], \\ s_{m+1}^{j+1} =\exp \bigl[a ( t+\Delta t ) \bigr]\exp \bigl[ik_{m} ( x+\Delta x ) \bigr], \\ s_{m-1}^{j+1} =\exp \bigl[a ( t+\Delta t ) \bigr]\exp \bigl[ik_{m} ( x-\Delta x ) \bigr], \\ s_{m-1}^{j-1} =\exp \bigl[a ( t-\Delta t ) \bigr]\exp \bigl[ik_{m} ( x-\Delta x ) \bigr], \end{gathered} $$
(32)
where \(k_{m}=\frac{\pi m}{L}\), \(m=1,2,\ldots,M=\frac{L}{\Delta x}\). If we use the equalities above, the equation can be reconsidered as follows:
$$ \begin{aligned}[b] &\exp \bigl[a ( t+\Delta t ) \bigr] \exp[ik_{m}x] \bigl( f_{i}^{j}d_{j,k}^{\alpha,1}+2g_{i}^{j}+h_{i}^{j} \bigr) \\ &\quad=\exp[at]\exp[ik_{m}x] \bigl( 2f_{i}^{j}d_{j,k}^{\alpha ,2}-f_{i}^{j}d_{j,k}^{\alpha,3}+2g_{i}^{j}-h_{i}^{j} \bigr) \\ &\qquad{}-f_{i}^{j} \Biggl[ \sum _{k=0}^{j-1}\exp \bigl[a ( t+\Delta t ) \bigr] \exp [ik_{m}x]d_{j,k}^{\alpha,1} \\ &\qquad{} -2\sum_{k=0}^{j-2}\exp \bigl[a ( t+\Delta t ) \bigr]\exp [ik_{m}x]d_{j,k}^{\alpha,2} +\sum _{k=0}^{j-2}\exp \bigl[a ( t+\Delta t ) \bigr] \exp [ik_{m}x]d_{j,k}^{\alpha,3} \Biggr] \\ &\quad\quad{} +g_{i}^{j} \bigl[ \exp \bigl[a ( t+\Delta t ) \bigr]\exp \bigl[ik_{m} ( x+\Delta x ) \bigr] + \exp \bigl[a ( t+\Delta t ) \bigr]\exp \bigl[ik_{m} ( x-\Delta x ) \bigr] \\ &\qquad{}-\exp[at]\exp \bigl[ik_{m} ( x+\Delta x ) \bigr] -\exp[at] \exp \bigl[ik_{m} ( x-\Delta x ) \bigr] \bigr]. \end{aligned} $$
(33)
If we do simplification with \(\exp[at]\exp[ik_{m}x]\), we will obtain the following:
$$\begin{aligned}& \begin{aligned}[b] &\exp \bigl[a ( \Delta t ) \bigr] \bigl( f_{i}^{j}d_{j,k}^{\alpha ,1}+2g_{i}^{j}+h_{i}^{j} \bigr) \\ &\quad= \bigl( 2f_{i}^{j}d_{j,k}^{\alpha,2}-f_{i}^{j}d_{j,k}^{\alpha ,3}+2g_{i}^{j}-h_{i}^{j} \bigr) \\ &\qquad{}-f_{i}^{j} \Biggl[ \sum _{k=0}^{j-1}\exp \bigl[a ( \Delta t ) \bigr]d_{j,k}^{\alpha,1} \\ &\qquad{}-2\sum_{k=0}^{j-2} \exp \bigl[a ( \Delta t ) \bigr]d_{j,k}^{\alpha,2} +\sum _{k=0}^{j-2}\exp \bigl[a ( \Delta t ) \bigr]d_{j,k}^{\alpha,3} \Biggr] \\ &\quad\quad{}+g_{i}^{j} \bigl[ \exp \bigl[a ( \Delta t ) \bigr]\exp \bigl[ik_{m} ( \Delta x ) \bigr] +\exp \bigl[a ( \Delta t ) \bigr]\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \\ &\qquad{}-\exp \bigl[ik_{m} ( \Delta x ) \bigr] -\exp \bigl[ik_{m} ( - \Delta x ) \bigr] \bigr] \end{aligned} \end{aligned}$$
(34)
$$\begin{aligned}& \begin{aligned}[b] &\exp \bigl[a ( \Delta t ) \bigr] \bigl[ f_{i}^{j}d_{j,k}^{\alpha,1}+2g_{i}^{j}+h_{i}^{j}+f_{i}^{j}.J \bigl( d_{j,k}^{\alpha,1}-2d_{j,k}^{\alpha,2}+d_{j,k}^{\alpha,3} \bigr) \\ &\qquad{}-g_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr] \\ &\quad= \bigl( 2f_{i}^{j}d_{j,k}^{\alpha,2}-f_{i}^{j}d_{j,k}^{\alpha ,3}+2g_{i}^{j}-h_{i}^{j} \bigr) -g_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \end{aligned} \end{aligned}$$
(35)
$$\begin{aligned}& \begin{aligned}[b] \exp \bigl[a ( \Delta t ) \bigr]={}&{ \bigl\{ \bigl( 2f_{i}^{j}d_{j,k}^{\alpha,2}-f_{i}^{j}d_{j,k}^{\alpha ,3}+2g_{i}^{j}-h_{i}^{j} \bigr) -g_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr\} } \\ &/ \bigl\{ f_{i}^{j}d_{j,k}^{\alpha,1}+2g_{i}^{j}+h_{i}^{j}+f_{i}^{j}.J \bigl( d_{j,k}^{\alpha,1}-2d_{j,k}^{\alpha,2}+d_{j,k}^{\alpha,3} \bigr) \\ & -g_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr\} . \end{aligned} \end{aligned}$$
(36)
With the help of the following inequality step by step, we have the condition for the stability analysis.
$$ \begin{gathered} \frac{s_{i}^{j+1}}{s_{i}^{j}}=\exp \bigl[a(\Delta t) \bigr], \\ \biggl\vert \frac{s_{i}^{j+1}}{s_{i}^{j}} \biggr\vert = \bigl\vert \exp \bigl[a( \Delta t) \bigr] \bigr\vert \leq1.\end{gathered} $$
(37)
Then the stability condition is given as
$$ \begin{aligned}[b] & \bigl\vert { \bigl\{ \bigl( 2f_{i}^{j}d_{j,k}^{\alpha,2}-f_{i}^{j}d_{j,k}^{\alpha ,3}+2g_{i}^{j}-h_{i}^{j} \bigr) -g_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr\} } \\ &\qquad/ \bigl\{ f_{i}^{j}d_{j,k}^{\alpha,1}+2g_{i}^{j}+h_{i}^{j}+f_{i}^{j}.J \bigl( d_{j,k}^{\alpha,1}-2d_{j,k}^{\alpha,2}+d_{j,k}^{\alpha,3} \bigr) \\ &\qquad{} -g_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr\} \bigr\vert \\ &\quad\leq1. \end{aligned} $$
(38)
Theorem 4
The Crank-Nicholson scheme for solving the Klein-Gordon equation with the second order Caputo-Fabrizio derivative in Riemann-Liouville sense is stable if inequality (38) is satisfied.
3.3 Second approximation of the Atangana-Baleanu derivative in Riemann-Liouville sense and a stability analysis of the numerical scheme
Theorem 5
Let
f
be a function not necessary differentiable within an interval
\([a,T]\), then the fractional derivative of
f
of order
\(1<\alpha\leq2\)
in the Atangana-Baleanu derivative in Riemann-Liouville sense is given as follows:
$$ \begin{aligned}[b] &{}_{0}^{\mathrm{ABR}}D_{t}^{\alpha} \bigl[ f(x) \bigr] \\ &\quad=\frac{1}{2 ( \Delta x ) ^{2}} \Biggl[ \sum _{k=0}^{j}\frac{f(x_{k+1})}{2}a_{j,k}^{\gamma,1} -2\sum_{k=0}^{j-1}\frac{f(x_{k+1})}{2}a_{j,k}^{\gamma ,2}+ \sum_{k=0}^{j-1}\frac{f(x_{k+1})}{2}a_{j,k}^{\gamma,3} \Biggr] +G,\end{aligned} $$
(39)
where
$$ \begin{aligned} G={}&\frac{\gamma}{(1-\gamma)\sqrt{\pi}} \Biggl[ \sum _{k=0}^{j} \int_{x_{k}}^{x_{k+1}} \bigl( f(\varepsilon )-f(x_{j+1}) \bigr) E_{\gamma,2} \biggl( - \biggl( \frac{\gamma }{1-\gamma} \biggr) ^{2} ( x_{j+1}- \varepsilon ) ^{2} \biggr) \,d\varepsilon \\ &-2\sum_{k=0}^{j-1} \int_{x_{k}}^{x_{k+1}} \bigl( f(\varepsilon )-f(x_{j+1}) \bigr) E_{\gamma,2} \biggl( - \biggl( \frac{\gamma }{1-\gamma} \biggr) ^{2} ( x_{j}- \varepsilon ) ^{2} \biggr) \,d\varepsilon \\ &+\sum_{k=0}^{j-1} \int_{x_{k}}^{x_{k+1}} \bigl( f(\varepsilon )-f(x_{j+1}) \bigr) E_{\gamma,2} \biggl( - \biggl( \frac{\gamma }{1-\gamma} \biggr) ^{2} ( x_{j+1}- \varepsilon ) ^{2} \biggr) \,d\varepsilon \Biggr] ,\end{aligned} $$
and
$$ \begin{gathered} a_{j,k}^{\gamma,1} =E_{\gamma,2} \biggl\{ -\gamma \frac {x_{j+1}-x_{k+1}}{1-\gamma} \biggr\} -E_{\gamma,2} \biggl\{ -\gamma \frac {x_{j+1}-x_{k}}{1-\gamma} \biggr\} , \\ a_{j,k}^{\gamma,2} =E_{\gamma,2} \biggl\{ -\gamma \frac {x_{j}-x_{k+1}}{1-\gamma} \biggr\} -E_{\gamma,2} \biggl\{ -\gamma\frac {x_{j}-x_{k}}{1-\gamma } \biggr\} , \\ a_{j,k}^{\gamma,3} =E_{\gamma,2} \biggl\{ -\gamma \frac {x_{j-1}-x_{k+1}}{1-\gamma} \biggr\} -E_{\gamma,2} \biggl\{ -\gamma\frac {x_{j-1}-x_{k}}{1-\gamma} \biggr\} .\end{gathered} $$
(40)
Now we can consider the equation again as follows:
$$ \begin{aligned}[b] &\frac{1}{2 ( \Delta x ) ^{2}} \Biggl[ \sum_{k=0}^{j} \frac{\Psi(x_{k+1})}{2}a_{j,k}^{\gamma,1} -2\sum _{k=0}^{j-1}\frac{\Psi(x_{k+1})}{2}a_{j,k}^{\gamma ,2}+ \sum_{k=0}^{j-1}\frac{\Psi(x_{k+1})}{2}a_{j,k}^{\gamma,3} \Biggr] \\ &\quad=h^{2}c^{2} \biggl[ \frac{ ( \Psi_{i+1}^{j+1}-2\Psi _{i}^{j+1}+\Psi _{i-1}^{j+1} ) - ( \Psi_{i+1}^{j}-2\Psi_{i}^{j}+\Psi _{i-1}^{j} ) }{2 ( \triangle x ) ^{2}} \biggr] -c^{4}m^{2} \biggl[ \frac{\Psi_{i}^{j+1}+\Psi_{i}^{j}}{2} \biggr] .\end{aligned} $$
(41)
To continue easier, let us do simplification in the equation with sufficient parameters as follows:
$$ m_{i}^{j}=\frac{1}{4 ( \Delta x ) ^{2}},\quad\quad n_{i}^{j}= \frac{h^{2}c^{2}}{2 ( \triangle x ) ^{2}},\quad\quad r_{i}^{j}=\frac {c^{4}m^{2}}{2}. $$
(42)
Then we rewrite the equation with parameters
$$ \begin{aligned}[b] &m_{i}^{j} \bigl( \Psi_{i}^{j+1}a_{j,k}^{\gamma,1}-2 \Psi _{i}^{j}a_{j,k}^{\gamma,2}+ \Psi_{i}^{j}a_{j,k}^{\gamma,3} \bigr) \\ &\qquad{}+m_{i}^{j} \Biggl[ \sum _{k=0}^{j-1} \Psi_{i}^{j+1}a_{j,k}^{\gamma ,1}-2 \sum_{k=0}^{j-2}\Psi_{i}^{j+1}a_{j,k}^{\gamma ,2}+ \sum_{k=0}^{j-2}\Psi_{i}^{j+1}a_{j,k}^{\gamma,3} \Biggr] \\ &\quad=n_{i}^{j} \bigl[ \bigl( \Psi_{i+1}^{j+1}-2 \Psi_{i}^{j+1}+\Psi _{i-1}^{j+1} \bigr) - \bigl( \Psi_{i+1}^{j}-2\Psi_{i}^{j}+\Psi _{i-1}^{j} \bigr) \bigr] \\ &\qquad{}-r_{i}^{j} \bigl( \Psi_{i}^{j+1}+ \Psi_{i}^{j} \bigr) .\end{aligned} $$
(43)
Finally, we have the following equation for the numerical scheme:
$$\begin{aligned} \begin{aligned}[b] &\Psi_{i}^{j+1} \bigl( m_{i}^{j}a_{j,k}^{\gamma ,1}+2n_{i}^{j}+r_{i}^{j} \bigr) \\ &\quad=\Psi_{i}^{j} \bigl( 2m_{i}^{j}a_{j,k}^{\gamma ,2}-m_{i}^{j}a_{j,k}^{\gamma,3}+2n_{i}^{j}-r_{i}^{j} \bigr) \\ &\qquad{}-m_{i}^{j} \Biggl[ \sum _{k=0}^{j-1} \Psi_{i}^{j+1}a_{j,k}^{\gamma ,1}-2 \sum_{k=0}^{j-2}\Psi_{i}^{j+1}a_{j,k}^{\gamma ,2}+ \sum_{k=0}^{j-2}\Psi_{i}^{j+1}a_{j,k}^{\gamma,3} \Biggr] \\ &\qquad{}+n_{i}^{j} \bigl[ \Psi_{i+1}^{j+1}+ \Psi_{i-1}^{j+1}-\Psi _{i+1}^{j}-\Psi _{i-1}^{j} \bigr] . \end{aligned} \end{aligned}$$
(44)
3.3.1 Stability analysis of the numerical scheme for the Atangana-Baleanu derivative in Riemann-Liouville sense
Let us represent a stability analysis of the numerical scheme by supposing
$$ u_{i}^{j}=\Psi_{i}^{j}-y_{i}^{j}, $$
(45)
where \(l_{i}^{j}\) is the approximate solution of the equation in time and space \((x_{i}, t_{j})\) (\(i=1,2,\ldots,N\), \(j=1,2,\ldots,M \)).
Also the error for approximation is given as
$$ u_{i}^{j}= \bigl[ u_{1}^{j},u_{2}^{j}, \ldots,u_{N}^{j} \bigr] {.} $$
(46)
So we have the following error expression for the Klein-Gordon equation with mass parameter:
$$ \begin{aligned}[b] &u_{i}^{j+1} \bigl( m_{i}^{j}a_{j,k}^{\gamma ,1}+2n_{i}^{j}+r_{i}^{j} \bigr) \\ &\quad=u_{i}^{j} \bigl( 2m_{i}^{j}a_{j,k}^{\gamma ,2}-m_{i}^{j}a_{j,k}^{\gamma ,3}+2n_{i}^{j}-r_{i}^{j} \bigr) \\ &\quad\quad{}-m_{i}^{j} \Biggl[ \sum _{k=0}^{j-1}u_{i}^{j+1}a_{j,k}^{\gamma ,1}-2 \sum_{k=0}^{j-2}u_{i}^{j+1}a_{j,k}^{\gamma ,2}+ \sum_{k=0}^{j-2}u_{i}^{j+1}a_{j,k}^{\gamma,3} \Biggr] \\ &\quad\quad{}+n_{i}^{j} \bigl[ u_{i+1}^{j+1}+u_{i-1}^{j+1}-u_{i+1}^{j}-u_{i-1}^{j} \bigr] . \end{aligned} $$
(47)
Then let us take the following equality for doing the stability analysis.
$$ \begin{gathered} u_{m}(x,t)=\exp[at] \exp[ik_{m}x], \\ u_{m}^{j} =\exp[at] \exp[ik_{m}x], \\ u_{m}^{j+1} =\exp \bigl[a ( t+\Delta t ) \bigr] \exp[ik_{m}x], \\ u_{m+1}^{j} =\exp[at]\exp \bigl[ik_{m} ( x+ \Delta x ) \bigr], \\ u_{m-1}^{j} =\exp[at]\exp \bigl[ik_{m} ( x- \Delta x ) \bigr], \\ u_{m+1}^{j+1} =\exp \bigl[a ( t+\Delta t ) \bigr]\exp \bigl[ik_{m} ( x+\Delta x ) \bigr], \\ u_{m-1}^{j+1} =\exp \bigl[a ( t+\Delta t ) \bigr]\exp \bigl[ik_{m} ( x-\Delta x ) \bigr], \\ u_{m-1}^{j-1} =\exp \bigl[a ( t-\Delta t ) \bigr]\exp \bigl[ik_{m} ( x-\Delta x ) \bigr], \end{gathered} $$
(48)
where \(k_{m}=\frac{\pi m}{L}\), \(m=1,2,\ldots,M=\frac{L}{\Delta x}\). If we use the equalities above, the equation can be reconsidered as follows:
$$ \begin{aligned}[b] &\exp \bigl[a ( t+\Delta t ) \bigr] \exp[ik_{m}x] \bigl( m_{i}^{j}a_{j,k}^{\gamma,1}+2n_{i}^{j}+r_{i}^{j} \bigr) \\ &\quad=\exp[at]\exp[ik_{m}x] \bigl( 2m_{i}^{j}a_{j,k}^{\gamma ,2}-m_{i}^{j}a_{j,k}^{\gamma,3}+2n_{i}^{j}-r_{i}^{j} \bigr) \\ &\quad\quad{}-m_{i}^{j} \Biggl[ \sum _{k=0}^{j-1}\exp \bigl[a ( t+\Delta t ) \bigr] \exp [ik_{m}x]a_{j,k}^{\gamma,1} \\ &\qquad{}-2\sum_{k=0}^{j-2}\exp \bigl[a ( t+ \Delta t ) \bigr]\exp [ik_{m}x]a_{j,k}^{\gamma,2} +\sum _{k=0}^{j-2}\exp \bigl[a ( t+\Delta t ) \bigr] \exp [ik_{m}x]a_{j,k}^{\gamma,3} \Biggr] \\ &\qquad{}+n_{i}^{j} \bigl[ \exp \bigl[a ( t+\Delta t ) \bigr]\exp \bigl[ik_{m} ( x+\Delta x ) \bigr] + \exp \bigl[a ( t+\Delta t ) \bigr]\exp \bigl[ik_{m} ( x-\Delta x ) \bigr] \\ &\qquad{}-\exp[at]\exp \bigl[ik_{m} ( x+\Delta x ) \bigr] -\exp[at] \exp \bigl[ik_{m} ( x-\Delta x ) \bigr] \bigr]. \end{aligned} $$
(49)
If we do simplification with \(\exp[at]\exp[ik_{m}x]\), we will obtain the following:
$$\begin{aligned}& \begin{aligned}[b] &\exp \bigl[a ( \Delta t ) \bigr] \bigl( m_{i}^{j}a_{j,k}^{\gamma ,1}+2n_{i}^{j}+r_{i}^{j} \bigr) \\ &\quad= \bigl( 2m_{i}^{j}a_{j,k}^{\gamma,2}-m_{i}^{j}a_{j,k}^{\gamma ,3}+2n_{i}^{j}-r_{i}^{j} \bigr) \\ &\qquad{}-m_{i}^{j} \Biggl[ \sum _{k=0}^{j-1}\exp \bigl[a ( \Delta t ) \bigr]a_{j,k}^{\gamma,1} -2\sum_{k=0}^{j-2} \exp \bigl[a ( \Delta t ) \bigr]a_{j,k}^{\gamma,2} +\sum _{k=0}^{j-2}\exp \bigl[a ( \Delta t ) \bigr]a_{j,k}^{\gamma,3} \Biggr] \\ &\qquad{}+n_{i}^{j} \bigl[ \exp \bigl[a ( \Delta t ) \bigr]\exp \bigl[ik_{m} ( \Delta x ) \bigr] +\exp \bigl[a ( \Delta t ) \bigr]\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \\ &\qquad{} -\exp \bigl[ik_{m} ( \Delta x ) \bigr] -\exp \bigl[ik_{m} ( - \Delta x ) \bigr] \bigr] \end{aligned} \end{aligned}$$
(50)
$$\begin{aligned}& \begin{aligned}[b] &\exp \bigl[a ( \Delta t ) \bigr] \bigl[ m_{i}^{j}a_{j,k}^{\gamma,1}+2n_{i}^{j}+r_{i}^{j}+m_{i}^{j}.J \bigl( a_{j,k}^{\gamma,1}-2a_{j,k}^{\gamma,2}+a_{j,k}^{\gamma,3} \bigr) \\ &\qquad{} -n_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr] \\ &\quad= \bigl( 2m_{i}^{j}a_{j,k}^{\gamma,2}-m_{i}^{j}a_{j,k}^{\gamma ,3}+2n_{i}^{j}-r_{i}^{j} \bigr) -n_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \end{aligned} \end{aligned}$$
(51)
$$\begin{aligned}& \begin{aligned}[b] \exp \bigl[a ( \Delta t ) \bigr]={}&{ \bigl\{ \bigl( 2m_{i}^{j}a_{j,k}^{\gamma,2}-m_{i}^{j}a_{j,k}^{\gamma ,3}+2n_{i}^{j}-r_{i}^{j} \bigr) -n_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr\} } \\ &/ \bigl\{ m_{i}^{j}a_{j,k}^{\gamma,1}+2n_{i}^{j}+r_{i}^{j}+m_{i}^{j}.J \bigl( a_{j,k}^{\gamma,1}-2a_{j,k}^{\gamma,2}+a_{j,k}^{\gamma,3} \bigr) \\ &-n_{i}^{j} \bigl( \exp \bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr\} . \end{aligned} \end{aligned}$$
(52)
With the help of the following inequality step by step, we have the condition for the stability analysis:
$$ \biggl\vert \frac{u_{i}^{j+1}}{u_{i}^{j}} \biggr\vert = \bigl\vert \exp \bigl[a( \Delta t) \bigr] \bigr\vert \leq1. $$
Then the stability condition is given as
$$ \begin{aligned}[b] & \bigl\vert {\bigl\{ \bigl( 2m_{i}^{j}a_{j,k}^{\gamma,2}-m_{i}^{j}a_{j,k}^{\gamma ,3}+2n_{i}^{j}-r_{i}^{j} \bigr) -n_{i}^{j} \bigl( \exp\bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr\} } \\ &\qquad/\bigl\{ m_{i}^{j}a_{j,k}^{\gamma,1}+2n_{i}^{j}+r_{i}^{j}+m_{i}^{j}.J \bigl( a_{j,k}^{\gamma,1}-2a_{j,k}^{\gamma,2}+a_{j,k}^{\gamma,3} \bigr) \\ &\qquad{}-n_{i}^{j} \bigl( \exp\bigl[ik_{m} ( \Delta x ) \bigr]+\exp \bigl[ik_{m} ( -\Delta x ) \bigr] \bigr) \bigr\} \bigr\vert \\ &\quad\leq1. \end{aligned} $$
(53)
Theorem 6
The Crank-Nicholson scheme for solving the Klein-Gordon equation with the second order Atangana-Baleanu derivative in Riemann-Liouville sense is stable if inequality (50) is satisfied.