# On the existence of solutions for some infinite coefficient-symmetric Caputo-Fabrizio fractional integro-differential equations

## Abstract

By mixing the idea of 2-arrays, continued fractions, and Caputo-Fabrizio fractional derivative, we introduce a new operator entitled the infinite coefficient-symmetric Caputo-Fabrizio fractional derivative. We investigate the approximate solutions for two infinite coefficient-symmetric Caputo-Fabrizio fractional integro-differential problems. Finally, we analyze two examples to confirm our main results.

## Introduction

Fractional calculus has many real-world applications in various fields of science and engineering . During the recent years, the researchers started to think how to enlarge the range of fractional calculus by constructing operators with different nonlocal kernels. For example, a new nonlocal derivative without singular kernel was introduced in . After that, this new fractional operator was utilized to get more information from solving different fractional differential equations corresponding to complex phenomena (the reader can see, for example, , and the references therein). Let use consider $$b>0$$ and $$x\in H^{1}(0,b)$$ together with $$\alpha\in(0,1)$$. For a function x, Caputo and Fabrizio defined its fractional derivative (CF) of order α as $${}^{\mathrm{CF}}C^{\alpha}x(p)=\frac{(2-\alpha)M(\alpha)}{2(1-\alpha)}\int _{0}^{p}\exp (\frac{-\alpha}{1-\alpha}(p-w))x^{\prime}(w)\,dw$$, where $$t\geq0$$, and $$M(\alpha)$$ is such that $$M(0)=M(1)=1$$ . The corresponding fractional integral of order α for the function x is $${}^{\mathrm{CF}}I^{\alpha} x(p)=\frac{2(1-\alpha)}{(2-\alpha)M(\alpha)}x(p) +\frac{2\alpha}{(2-\alpha)M(\alpha)}\int_{0}^{p} x(w) \,dw$$ whenever $$0<\alpha<1$$ . Also, the values of the function M were found as $$M(\alpha)=\frac{2}{2-\alpha}$$ for all $$0\leq\alpha\leq1$$ . Taking into account the results mentioned, for a given function x, its fractional CF of order α becomes $${}^{\mathrm{CF}}C^{\alpha}x(p)=\frac{1}{1-\alpha}\int_{0}^{p}\exp(-\frac {\alpha}{1-\alpha}(p-w))x^{\prime}(w)\,dw$$ for $$t\geq0$$ and $$0<\alpha<1$$ . In this way a new type of fractional calculus was established. The aim of the manuscript is to propose a new operator named the infinite coefficient-symmetric Caputo-Fabrizio fractional derivative and to study some its properties.

## Basic tools and new fractional operators

We further introduce some basic notation.

### Lemma 2.1



Let us consider the equation $${}^{\mathrm{CF}}C^{\alpha}x(t)=y(t)$$ such that $$x(0)=c$$ and $$0<\alpha<1$$. The solutions of this equation has the form $$x(p)=c+a_{\alpha}(y(p)-y(0))+b_{\alpha}\int_{0}^{p} y(z)\,dz$$, where $$a_{\alpha}=\frac{2(1-\alpha)}{(2-\alpha)M(\alpha)}=1-\alpha$$ and $$b_{\alpha}=\frac{2\alpha}{(2-\alpha)M(\alpha)}=\alpha$$.

Let $$\varepsilon> 0$$. We consider a metric space $$(Z,d_{1})$$, a selfmap G on Z, and a mapping $$\alpha : Z\times Z \to[ 0 , \infty)$$. As a result, we say that G is α-admissible whenever $$\alpha(t,s) \geq1$$ implies $$\alpha (Gt , Gs )\geq1$$ . An element $$z_{0}\in Z$$ is called an ε-fixed point of G if $$d(G z_{0},z_{0}) \leq\varepsilon$$. We say that G possess the approximate fixed point property if G possesses an ε-fixed point for all $$\varepsilon> 0$$ . Denote by $$\mathcal{R}$$ the set of all continuous mappings $$j : [0,\infty)^{5} \to[0,\infty)$$ satisfying $$j(1,1,1,2,0)= j(1,1,1,0,2):=l \in(0,1)$$, $$j(\mu t_{1},\mu t_{2},\mu t_{3},\mu t_{4},\mu t_{5}) \leq\mu j(t_{1}, t_{2},t_{3},t_{4},t_{5})$$ for all $$(t_{1},t_{2},t_{3},t_{4},t_{5}) \in[0,\infty)^{5}$$ and $$\mu\geq0$$ and also $$j( t_{1},t_{2},t_{3},0,t_{4}) \leq j( s_{1},s_{2},s_{3},0,s_{4})$$ and $$j(t_{1},t_{2},t_{3},t_{4},0)\leq j(s_{1},s_{2},s_{3},s_{4},0)$$ whenever $$t_{1},\dots,t_{4},s_{1},\dots,s_{4} \in[0,\infty)$$ with $$t_{k}< s_{k}$$ for $$k=1,2,3,4$$ . Next, we recall that G is called a generalized α-contractive mapping if there exists $$j \in\mathcal{R}$$ such that $$\alpha(t,s)d_{1}(Gt,Gs)\leq j(d_{1}(t_{1},s_{1}),d_{1}(t_{1},Gt_{1}),d_{1}(s_{1},Gs_{1}),d_{1}(t_{1},Gs_{1}), d_{1}(s_{1},Gt_{1}))$$ for all $$t_{1},s_{1} \in Z$$ . We need the following key result.

### Theorem 2.2



Suppose that there exists $$t_{0}\in Z$$ such that $$\alpha(t_{0},Gt_{0}) \geq1$$. Then G possesses an approximate fixed point, where $$(Z,d)$$ is a metric space, $$\alpha: Z\times Z \to [0,\infty)$$ denotes a mapping, and G represents a generalized α-contractive and α-admissible selfmap on Z.

Let $$\{L_{i,2^{i}}\}_{i\geq1}$$ be a sequence of operators on a set. For reduction and approximation in large and infinite potential-driven flow networks, there is a method of using 2-arrays and continued fractions (see  and ). In fact, it is sufficient to arrange the operators $$\{L_{i,2^{i}}\}_{i\geq1}$$ symmetrically on a 2-array, and by using a continued fraction we make a new operator $$L_{N}$$ from the operators $$L_{i,2^{i}}$$, where N is a natural number (see  and ). First, we arrange the operators $$L_{i,2^{i}}$$ on a 2-array (tree) as in Figure 1 (see ).

Now, using a finite continued fraction, consider the new operator $$L_{N}$$ defined by

$$L_{N}=\frac{1}{\frac{1}{L_{11}+\frac{1}{\frac{1}{L_{21}+\cdots+\frac {1}{\frac{1}{L_{N1}}+\frac{1}{L_{N2}}}}+\frac{1}{L_{22}+\cdots+\frac {1}{\frac{1}{L_{N3}}+\frac{1}{L_{N4}}}}}}+\frac{1}{L_{12}+\frac{1}{\frac {1}{L_{23}+\cdots+\frac{1}{\frac{1}{L_{N2^{N-3}}}+\frac {1}{L_{N2^{N-2}}}}}+\frac{1}{L_{24}+\cdots+\frac{1}{\frac {1}{L_{N2^{N-1}}}+\frac{1}{L_{N2^{N}}}}}}}}.$$

Here, we replace symmetrically the operators $$L_{ij}$$ with $${}^{\mathrm{CF}}C^{\alpha}$$ for j odd (the upper branch) and $${}^{\mathrm{CF}}C^{\beta}$$ for j even (the lower branch) as in Figure 2.

Put

$${}^{\mathrm{CF}}\mathbb{C}^{(\alpha,\beta)}_{1}=\frac{1}{\frac {1}{{}^{\mathrm{CF}}C^{\alpha} }+\frac{1}{{}^{\mathrm{CF}}C^{\beta} }},\quad \quad {}^{\mathrm{CF}}\mathbb{C}^{(\alpha,\beta)}_{2}=\frac{1}{\frac {1}{{}^{\mathrm{CF}}C^{\alpha}+\frac{1}{\frac{1}{{}^{\mathrm{CF}}C^{\alpha}}+\frac {1}{{}^{\mathrm{CF}}C^{\beta}}}}+\frac{1}{{}^{\mathrm{CF}}C^{\beta}+\frac{1}{\frac {1}{{}^{\mathrm{CF}}C^{\alpha}}+\frac{1}{{}^{\mathrm{CF}}C^{\beta}}}}}$$

and

\begin{aligned}& {}^{\mathrm{CF}}\mathbb{C}^{(\alpha,\beta)}_{3} \\& \quad= \frac{1}{\frac{1}{{}^{\mathrm{CF}}C^{\alpha}+\frac{1}{\frac {1}{{}^{\mathrm{CF}}C^{\alpha}+\frac{1}{\frac{1}{{}^{\mathrm{CF}}C^{\alpha} }+\frac{1}{{}^{\mathrm{CF}}C^{\beta} }}}+\frac{1}{{}^{\mathrm{CF}}C^{\beta}+\frac{1}{\frac{1}{{}^{\mathrm{CF}}C^{\alpha} }+\frac{1}{{}^{\mathrm{CF}}C^{\beta} }}}}}+\frac{1}{{}^{\mathrm{CF}}C^{\beta}+\frac{1}{\frac{1}{{}^{\mathrm{CF}}C^{\alpha }+\frac{1}{\frac{1}{{}^{\mathrm{CF}}C^{\alpha} }+\frac{1}{{}^{\mathrm{CF}}C^{\beta} }}}+\frac{1}{{}^{\mathrm{CF}}C^{\beta}+\frac{1}{\frac{1}{{}^{\mathrm{CF}}C^{\alpha} }+\frac{1}{{}^{\mathrm{CF}}C^{\beta} }}}}}}. \end{aligned}

Continuing this process, we can define the new operator $${}^{\mathrm{CF}}\mathbb{C}^{(\alpha,\beta)}_{N}$$. Now, we define the infinite symmetric CF fractional derivative by $${}^{\mathrm{CF}}\mathbb{C}^{(\alpha,\beta)}_{\infty}=\lim_{N \to \infty} {}^{\mathrm{CF}}\mathbb{C}^{(\alpha,\beta)}_{N}$$. A simple calculation shows that $${}^{\mathrm{CF}}\mathbb{C}^{(\alpha,\beta)}_{\infty} =({}^{\mathrm{CF}}C^{\alpha} {}^{\mathrm{CF}}C^{\beta})^{\frac{1}{2}}$$. Similarly, we can define the infinite symmetric CF fractional integral $${}^{\mathrm{CF}}\mathbb{I}^{(\alpha,\beta)}_{\infty}$$ by

$${}^{\mathrm{CF}}\mathbb{I}^{(\alpha,\beta)}_{\infty}= \frac{1}{\frac {1}{{}^{\mathrm{CF}}I^{\alpha}+\frac{1}{\frac{1}{{}^{\mathrm{CF}}I^{\alpha}+\cdots }+\frac{1}{{}^{\mathrm{CF}}I^{\beta}+\cdots}}}+\frac{1}{{}^{\mathrm{CF}}I^{\beta}+\frac {1}{\frac{1}{{}^{\mathrm{CF}}I^{\alpha}+\cdots}+\frac{1}{{}^{\mathrm{CF}}I^{\beta }+\cdots}}}}.$$

Let $$\mu\geq0$$, $$\mu\neq2$$. Putting $$\mu^{i-1} {}^{\mathrm{CF}}C^{\alpha}$$ on the upper branch and $$\mu^{i-1} {}^{\mathrm{CF}}C^{\beta}$$ on the lower branch in the ith stage as in Figure 3, we can make the infinite coefficient-symmetric CF fractional derivative as a generalization for last case.

In fact, we define

$${}^{\mathrm{CF}}\mathbb{C}^{(\alpha,\beta)}_{(\mu,\infty)}=\frac{1}{\frac {1}{{}^{\mathrm{CF}}C^{\alpha}+\frac{1}{\frac{1}{\mu {}^{\mathrm{CF}}C^{\alpha}+\cdots}+\frac{1}{\mu {}^{\mathrm{CF}}C^{\beta}+\cdots}}}+\frac{1}{{}^{\mathrm{CF}}C^{\beta}+\frac{1}{\frac {1}{\mu {}^{\mathrm{CF}}C^{\alpha}+\cdots}+\frac{1}{\mu{}^{\mathrm{CF}}C^{\beta}+\cdots}}}},$$

and so

$${}^{\mathrm{CF}}\mathbb{C}^{(\alpha,\alpha)}_{(\mu,\infty)}=\frac{1}{\frac {1}{{}^{\mathrm{CF}}C^{\alpha}+ \mu{}^{\mathrm{CF}}\mathbb{C}^{(\alpha,\alpha)}_{(\mu,\infty)}}+\frac {1}{{}^{\mathrm{CF}}C^{\alpha}+ \mu{}^{\mathrm{CF}}\mathbb{C}^{(\alpha,\alpha)}_{(\mu,\infty)}}}.$$

This implies that

$$(*)\quad\quad {}^{\mathrm{CF}}\mathbb{C}^{(\alpha,\beta)}_{(\mu,\infty)} = \frac{1}{2-\mu} {}^{\mathrm{CF}}C^{\alpha}.$$

## Results

To show our results, we recall below two lemmas  under the assumption that $$x,y\in H^{1}(0,1)$$.

### Lemma 3.1



If there exists a real number $$K_{1}$$ such that $$\vert x(p)-y(p) \vert \leq K_{1}$$ for all $$p\in[0,1]$$, then $$\vert {}^{\mathrm{CF}}C^{\alpha}x(p)-{{}^{\mathrm{CF}}}C^{\alpha}y(p) \vert \leq \frac{2-\alpha}{(1-\alpha)^{2}}K_{1}$$ for all $$p \in[0,1]$$.

### Lemma 3.2



Assume that $$x(0)=y(0)$$ and there exists a real number $$K_{1}$$ such that $$\vert x(p)-y(p) \vert \leq K_{1}$$ for $$p\in[0,1]$$. Then $$\vert {}^{\mathrm{CF}}C^{\alpha}x(p)-{{}^{\mathrm{CF}}}C^{\alpha}y(p) \vert \leq\frac{1}{(1-\alpha)^{2}}K_{1}$$ for all $$p\in[0,1]$$.

Let $$x,y\in C_{\mathbb{R}}[0,1]$$.

### Lemma 3.3



If there is $$K_{1}\geq0$$ such that $$\vert x(p)-y(p) \vert \leq K_{1}$$ for all $$p\in[0,1]$$, then $$\vert {}^{\mathrm{CF}}I^{\alpha}x(p)-{}^{\mathrm{CF}}I^{\alpha}y(p) \vert \leq K_{1}$$ for $$p\in[0,1]$$.

Now we are ready to show our main results. Using Lemmas 3.1 and 3.2, we obtain the next key results.

### Lemma 3.4

Let $$x,y\in H^{1}$$. If there exists a real number $$K_{1}$$ such that $$\vert x(p)-y(p) \vert \leq K_{1}$$ for all $$p\in[0,1]$$, then $$\vert {}^{\mathrm{CF}}\mathbb{C}_{\infty}^{(\alpha,\alpha )}x(p)-{{}^{\mathrm{CF}}}\mathbb{C}_{\infty}^{(\alpha,\alpha)}y(p) \vert \leq\frac{2-\alpha}{(1-\alpha)^{2}}K_{1}$$ for all $$p\in[0,1]$$.

### Lemma 3.5

Let $$x,y\in H^{1}$$ with $$x(0)=y(0)$$ and $$K_{1}\in\mathbb{R}$$. If $$\vert x(p)-y(p) \vert \leq K_{1}$$ for $$p\in[0,1]$$, then $$\vert {}^{\mathrm{CF}}\mathbb{C}_{\infty}^{(\alpha,\alpha )}x(p)-{{}^{\mathrm{CF}}}\mathbb{C}_{\infty}^{(\alpha,\alpha)}y(p) \vert \leq\frac{1}{(1-\alpha)^{2}}K_{1}$$ for all $$p\in[0,1]$$.

Using Lemmas 3.4 and 3.5 and (*), we get the following results.

### Lemma 3.6

Let $$x,y\in H^{1}$$. If there exists a real number $$K_{1}$$ such that $$\vert x(p)-y(p) \vert \leq K_{1}$$ for all $$p\in[0,1]$$, then $$\vert {}^{\mathrm{CF}}\mathbb{C}_{(\mu,\infty)}^{(\alpha,\alpha )}x(p)-{{}^{\mathrm{CF}}}\mathbb{C}_{(\mu,\infty)}^{(\alpha,\alpha)}y(p) \vert \leq\frac{(2-\alpha)}{(2-\mu)(1-\alpha)^{2}} K_{1}$$ for all $$p\in [0,1]$$.

### Lemma 3.7

Let $$x,y\in H^{1}$$ with $$x(0)=y(0)$$ and $$K_{1}\in\mathbb{R}$$. If $$\vert x(p)-y(p) \vert \leq K_{1}$$ for all $$p\in[0,1]$$, then $$\vert {}^{\mathrm{CF}}\mathbb{C}_{(\mu,\infty)}^{(\alpha,\alpha )}x(p)-{{}^{\mathrm{CF}}}\mathbb{C}_{(\mu,\infty)}^{(\alpha,\alpha)}y(p) \vert \leq\frac{1}{(2-\mu)(1-\alpha)^{2}} K_{1}$$ for all $$p\in[0,1]$$.

### Lemma 3.8

Let $$x,y\in C_{\mathbb{R}}[0,1]$$. Let $$K_{1}$$ be a real number such that $$\vert x(p)-y(p) \vert \leq K_{1}$$ for all $$p\in[0,1]$$, then $$\vert {}^{\mathrm{CF}}\mathbb{I}_{\infty}^{(\alpha,\alpha )}x(p)-{{}^{\mathrm{CF}}}\mathbb{I}_{\infty}^{(\alpha,\alpha)}y(p) \vert \leq K_{1}$$ for all $$p\in[0,1]$$.

Using Lemma 2.1, we can prove the next key result.

### Lemma 3.9

Let $$\alpha\in(0,1)$$ and $$c\in\mathbb{R}$$. The unique solution of the problem

$${}^{\mathrm{CF}}\mathbb{C}_{\infty}^{(\alpha,\alpha)}x(p)=y(p)$$

with boundary condition $$x(0)=c$$ is given by $$x(p)=c+a_{\alpha}(y(p)-y(0))+b_{\alpha}\int_{0}^{t} y (s)\,ds$$.

Also, using Lemma 2.1 and (*), we can prove the next key result.

### Lemma 3.10

Let $$\alpha\in(0,1)$$ and $$c\in\mathbb{R}$$. The unique solution of the problem

$${}^{\mathrm{CF}}\mathbb{C}_{(\mu,\infty)}^{(\alpha,\alpha)}x(p)=y(p)$$

with boundary condition $$x(0)=c$$ is given by

$$x(p)=c+a_{\alpha}(2-\mu) \bigl(y(p)-y(0) \bigr)+b_{\alpha}(2-\mu) \int_{0}^{p} y (s)\,ds.$$

Let $$I=[0,1]$$, and let $$\gamma,\lambda:[0,1] \times[0,1]\to [0,\infty)$$ be two continuous maps such that $$\sup_{p\in I} \vert \int_{0}^{p} \lambda(p,r) \,dr \vert <\infty$$ and $$\sup_{p\in I} \vert \int_{0}^{p} \gamma(p,r) \,dr \vert <\infty$$. We introduce the following maps ϕ and φ defined by $$(\phi u)(p)= \int_{0}^{p} \gamma(p,r)u(r)\,dr$$ and $$(\varphi u)(p)= \int_{0}^{p} \lambda(p,r)u(r)\,dr$$, respectively. Let us consider $$\gamma_{0}=\sup \vert \int_{0}^{p} \gamma(p,r) \,dr \vert$$ and $$\lambda_{0}=\sup \vert \int_{0}^{p} \lambda(p,r) \,dr \vert$$, respectively. Let $$\eta(p)\in L^{\infty}(I)$$ with $$\eta^{\ast}=\sup_{p\in I} \vert \eta(p) \vert$$. We further are going to investigate the infinite CF fractional integro-differential problem, namely

\begin{aligned}[b] {}^{\mathrm{CF}}\mathbb{C}^{(\alpha,\alpha )}_{\infty}u_{1}^{\prime}(r)&= \mu{ \bigl(} ^{\mathrm{CF}}\mathbb{C}^{(\beta,\beta )}_{\infty}u_{1}^{\prime}(r)+^{\mathrm{CF}} \mathbb{C}^{(\gamma,\gamma)}_{\infty}u_{1}^{\prime}(r) { \bigr)} \\ &\quad{}+f^{\prime} \bigl(r,u_{1}^{\prime}(r), \bigl(\phi u_{1}^{\prime} \bigr) (r), \bigl(\varphi u_{1}^{\prime} \bigr) (r), ^{\mathrm{CF}}\mathbb{I}^{(\theta,\theta )}_{\infty} u_{1}^{\prime}(r),^{\mathrm{CF}}\mathbb{C}^{ (\delta,\delta)}_{\infty}u_{1}^{\prime}(r) \bigr) \end{aligned}
(1)

with $$u_{1}^{\prime}(0)=0$$. Here $$\alpha,\beta,\gamma,\theta ,\delta\in (0,1)$$, and $$\mu\geq0$$.

### Theorem 3.11

Let $$f^{\prime}:[0,1]\times\mathbb{R}^{5}\rightarrow\mathbb{R}$$ be a continuous function satisfying

\begin{aligned}& \bigl\vert f^{\prime}(r,x_{1},y_{1},w_{1},u_{1},u_{2})-f^{\prime} \bigl(r,x_{1}^{\prime},y_{1}^{\prime},w_{1}^{\prime},v_{1},v_{2} \bigr) \bigr\vert \\& \quad \leq\eta(r) \bigl( \bigl\vert x_{1}-x_{1}^{\prime} \bigr\vert + \bigl\vert y_{1}-y_{1}^{\prime} \bigr\vert + \bigl\vert w_{1}-w_{1}^{\prime} \bigr\vert + \vert u_{1}-v_{1} \vert + \vert u_{2}-v_{2} \vert \bigr) \end{aligned}

for all $$r\in I$$ and $$x_{1},y_{1},w_{1},x_{1}^{\prime},y_{1}^{\prime },w_{1}^{\prime},u_{1}, u_{2},v_{1},v_{2} \in\mathbb{R}$$. If $$\Delta= [\eta^{*}(2+\gamma _{0} + \lambda_{0} + \frac{1}{(1-{\delta})^{2}})+{ \mu}( \frac{1}{(1-{\gamma})^{2} }+\frac{1}{(1-{\beta})^{2}})]<1$$, then problem (1) possesses an approximate solution.

### Proof

Let $$H^{1}$$ be equipped with $$d(u_{1}^{\prime},v_{1}^{\prime })= \Vert u_{1}^{\prime}-v_{1}^{\prime} \Vert$$, where $$\Vert u_{1}^{\prime} \Vert =\sup_{t\in I} \vert u_{1}^{\prime}(t) \vert$$. Now, consider the selfmap $$F:H^{1}\to H^{1}$$ defined by

\begin{aligned} \bigl(Fu_{1}^{\prime} \bigr) (r) &=a_{ \alpha} \bigl[ \mu{ \bigl(} ^{\mathrm{CF}}\mathbb{C}^{(\beta,\beta)}_{\infty }u_{1}^{\prime}(r)+^{\mathrm{CF}} \mathbb{C}^{(\gamma,\gamma)}_{\infty}u_{1}^{\prime}(r) { \bigr)} \\ &\quad{} {}+f^{\prime} \bigl(r,u_{1}^{\prime}(r), \bigl(\phi u_{1}^{\prime} \bigr) (r), \bigl(\varphi u_{1}^{\prime} \bigr) (r), ^{\mathrm{CF}}\mathbb{I}^{(\theta,\theta)}_{\infty} u_{1}^{\prime}(r),^{\mathrm{CF}}\mathbb{C}^{(\delta,\delta )}_{\infty} u_{1}^{\prime}(r) \bigr) \bigr] \\ &\quad {}+b_{\alpha} \int_{0}^{r} \bigl[ \mu{ \bigl(} ^{\mathrm{CF}} \mathbb{C}^{(\beta,\beta)}_{\infty} u_{1}^{\prime}(s)+^{\mathrm{CF}} \mathbb{C}^{(\gamma,\gamma)}_{\infty}u_{1}^{\prime}(s) { \bigr)} \\ &\quad {}+ f^{\prime} \bigl(s,u_{1}^{\prime}(s), \bigl(\phi u_{1}^{\prime} \bigr) (s), \bigl(\varphi u_{1}^{\prime} \bigr) (s), ^{\mathrm{CF}}\mathbb{I}^{(\theta,\theta )}_{\infty} u_{1}^{\prime}(r),^{\mathrm{CF}}\mathbb{C}^{(\delta ,\delta)}_{\infty} u_{1}^{\prime}(s) \bigr) \bigr]\,ds \end{aligned}

for all $$r\in I$$ and $$u_{1}^{\prime},v_{1}^{\prime}\in H^{1}$$, where $$a_{\alpha}$$ and $$b_{\alpha}$$ have the meaning given in Lemma 3.9. Now, utilizing Lemmas 3.5 and 3.8, we get

\begin{aligned} & \bigl\vert \bigl(Fu_{1}^{\prime} \bigr) (r) - \bigl(F v_{1}^{\prime} \bigr) (r) \bigr\vert \\ &\quad\leq a_{\alpha}\bigl( \mu \bigl\vert \bigl( ^{\mathrm{CF}} \mathbb{C}^{(\beta,\beta )}_{\infty}u_{1}^{\prime}(r)+ ^{\mathrm{CF}}\mathbb{C}^{(\gamma,\gamma)}_{\infty}u_{1}^{\prime}(r) \bigr)- \bigl( ^{\mathrm{CF}}\mathbb{C}^{(\beta,\beta )}_{\infty}v_{1}^{\prime}(r)+^{\mathrm{CF}} \mathbb{C}^{(\gamma,\gamma)}_{\infty}v_{1}^{\prime}(r) \bigr) \bigr\vert \\ &\quad\quad {}+ \bigl\vert f^{\prime} \bigl(r,u_{1}^{\prime}(r), \bigl(\phi u_{1}^{\prime} \bigr) (r), \bigl(\varphi u_{1}^{\prime} \bigr) (r), ^{\mathrm{CF}}\mathbb{I}^{(\theta,\theta)}_{\infty} u_{1}^{\prime }(r),^{\mathrm{CF}}\mathbb{C}^{(\delta,\delta)}_{\infty} u_{1}^{\prime}(r) \bigr) \\ &\quad\quad {}-f^{\prime} \bigl(r,v_{1}^{\prime}(t), \bigl( \phi v_{1}^{\prime} \bigr) (r), \bigl(\varphi v_{1}^{\prime} \bigr) (r), ^{\mathrm{CF}}\mathbb{I}^{(\theta,\theta)}_{\infty} v_{1}^{\prime}(r), ^{\mathrm{CF}}\mathbb{C}^{(\delta,\delta )}_{\infty} v_{1}^{\prime}(r) \bigr) \bigr\vert \bigr) \\ &\quad\quad {}+b_{\alpha} \int_{0}^{r} \bigl[ \mu \bigl\vert \bigl( ^{\mathrm{CF}}\mathbb{C}^{(\beta,\beta)}_{\infty}u_{1}^{\prime }(s)+^{\mathrm{CF}} \mathbb{C}^{(\gamma,\gamma)}_{\infty}u_{1}^{\prime}(s) \bigr)- \bigl( ^{\mathrm{CF}}\mathbb{C}^{(\beta,\beta)}_{\infty }v_{1}^{\prime}(s)+^{\mathrm{CF}} \mathbb{C}^{(\gamma,\gamma)}_{\infty}v_{1}^{\prime}(s) \bigr) \bigr\vert \\ &\quad\quad {}+ \bigl\vert f^{\prime} \bigl(s,u_{1}^{\prime}(r), \bigl( \phi u_{1}^{\prime} \bigr) (s), \bigl(\varphi u_{1}^{\prime} \bigr) (s), ^{\mathrm{CF}}\mathbb{I}^{(\theta,\theta)}_{\infty} u_{1}^{\prime}(s),^{\mathrm{CF}}\mathbb{C}^{(\delta,\delta )}_{\infty} u_{1}^{\prime}(s) \bigr) \\ &\quad\quad {}-f^{\prime} \bigl(s,v_{1}^{\prime}(s), \bigl( \phi v_{1}^{\prime} \bigr) (s), \bigl(\varphi v_{1}^{\prime} \bigr) (s), ^{\mathrm{CF}}\mathbb{I}^{(\theta,\theta)}_{\infty} v_{1}^{\prime}(s), ^{\mathrm{CF}}\mathbb{C}^{(\delta,\delta )}_{\infty} v_{1}^{\prime}(s) \bigr) \bigr\vert \bigr] \,ds \\ &\quad \leq a_{ \alpha} \mu \bigl[ \bigl\vert ^{\mathrm{CF}} \mathbb{C}^{(\beta,\beta)}_{\infty} \bigl(u_{1}^{\prime}(r)-v_{1}^{\prime}(r) \bigr) \bigr\vert \\ &\quad\quad {}+ \bigl\vert ^{\mathrm{CF}}\mathbb{C}^{(\gamma,\gamma)}_{\infty} \bigl(u_{1}^{\prime}(r)-v_{1}^{\prime}(r) \bigr) \bigr\vert \bigr]+a_{ \alpha} \bigl\vert \eta(r) \bigr\vert \bigl[ \bigl\vert u_{1}^{\prime}(r)-v_{1}^{\prime}(r) \bigr\vert + \bigl\vert \bigl(\phi u_{1}^{\prime} \bigr) (r)- \bigl( \phi v_{1}^{\prime} \bigr) (r) \bigr\vert \\ &\quad\quad {}+ \bigl\vert \bigl(\varphi u_{1}^{\prime} \bigr) (r)- \bigl( \varphi v_{1}^{\prime} \bigr) (r) \bigr\vert + \bigl\vert ^{\mathrm{CF}}\mathbb{I}^{(\theta,\theta)}_{\infty} u_{1}^{\prime}(r)- ^{\mathrm{CF}}\mathbb{I}^{(\theta,\theta)}_{\infty} v_{1}^{\prime}(r) \bigr\vert \\ &\quad\quad{} + \bigl\vert ^{\mathrm{CF}}\mathbb{C}^{(\delta,\delta)} _{\infty} u_{1}^{\prime}(r)-^{\mathrm{CF}} \mathbb{C}^{(\delta,\delta)}_{\infty} v_{1}^{\prime}(r) \bigr\vert \bigr] \\ &\quad\quad {}+b_{\alpha} \int_{0}^{r} \bigl[ \mu \bigl( \bigl\vert ^{\mathrm{CF}}\mathbb{C}^{(\beta,\beta)}_{\infty} \bigl(u_{1}^{\prime}(s)-v_{1}^{\prime}(s) \bigr) \bigr\vert + \bigl\vert ^{\mathrm{CF}}\mathbb{C}^{(\gamma ,\gamma)}_{\infty} \bigl(u_{1}^{\prime}(s)-v_{1}^{\prime}(s) \bigr) \bigr\vert \bigr) \\ &\quad\quad{} + \bigl\vert \eta(s) \bigr\vert \bigl( \bigl\vert u_{1}^{\prime}(s)-v_{1}^{\prime}(s) \bigr\vert \\ &\quad\quad {}+ \bigl\vert \bigl(\phi u_{1}^{\prime} \bigr) (s)- \bigl( \phi v_{1}^{\prime} \bigr) (s) \bigr\vert + \bigl\vert \bigl( \varphi u_{1}^{\prime} \bigr) (s)- \bigl(\varphi v_{1}^{\prime} \bigr) (s) \bigr\vert + \bigl\vert ^{\mathrm{CF}}\mathbb{I}^{(\theta,\theta)}_{\infty} u_{1}^{\prime}(s) - ^{\mathrm{CF}}I^{(\theta,\theta)}_{\infty} v_{1}^{\prime}(s) \bigr\vert \\ &\quad\quad {}+ \bigl\vert ^{\mathrm{CF}}\mathbb{C}^{(\delta,\delta)}_{\infty} u_{1}^{\prime}(s)-^{\mathrm{CF}}\mathbb{C}^{(\delta,\delta )}_{\infty} v_{1}^{\prime}(s) \bigr\vert \bigr) \bigr] \,ds \\ &\quad \leq \biggl[\eta^{*} \biggl(2+\gamma_{0} + \lambda _{0} + \frac{1}{(1-{\delta})^{2}} \biggr)+{ \mu}\biggl( \frac{1}{(1-{\gamma})^{2}}+ \frac{1}{(1-{\beta})^{2}}\biggr) \biggr] [a_{\alpha}+b_{\alpha}] \bigl\Vert u_{1}^{\prime}-v_{1}^{\prime} \bigr\Vert \end{aligned}

for all $$r\in I$$ and $$u_{1}^{\prime},v_{1}^{\prime}\in H^{1}$$. Hence,

$$\bigl\Vert Fu_{1}^{\prime}-Fv_{1}^{\prime} \bigr\Vert \leq \biggl[\eta^{*} \biggl(2+\gamma_{0} + \lambda_{0} + \frac{1}{(1-{\delta})^{2}} \biggr)+{ \mu} \biggl( \frac{1}{(1-{\gamma})^{2} }+\frac{1}{(1-{\beta})^{2}} \biggr) \biggr] \bigl\Vert u_{1}^{\prime}-v_{1}^{\prime} \bigr\Vert$$

for all $$u_{1}^{\prime},v_{1}^{\prime}\in H^{1}$$. Consider the mappings $$j:[0,\infty)^{5} \to[0,\infty)$$ and $$\alpha :H^{1}\times H^{1}\to[0,\infty)$$ defined by $$j(t_{1},t_{2},t_{3},t_{4},t_{5})= \Delta t_{1}$$ and $$\alpha(t,s) =1$$ for all $$t,s\in H^{1}$$. We can check that $$j \in\mathcal{R}$$ and F is a generalized α-contraction. From Theorem 2.2 we conclude that F possesses an approximate fixed point, which is an approximate solution for problem (1). □

Let c be a real number, and k, s, and q bounded functions on $$I=[0,1]$$ with $$M_{1}=\sup_{p\in I} \vert k(p) \vert <\infty$$, $$M_{2}=\sup_{p\in I} \vert s(p) \vert <\infty$$, and $$M_{3}=\sup_{t\in I} \vert q(p) \vert <\infty$$. We investigate the infinite coefficient-symmetric CF fractional integro-differential problem

\begin{aligned}[b] ^{\mathrm{CF}}\mathbb{C}_{(\mu,\infty)}^{(\alpha ,\alpha)}x(p)&= \lambda k(p)^{\mathrm{CF}}\mathbb{C}_{\infty}^{(\delta,\delta )}x(p) +{\rho }s(p)^{\mathrm{CF}}\mathbb{I}_{\infty}^{(\theta,\theta)}x(p) \\ &\quad{}+ \int_{0}^{p} f \bigl(w,x(w),(\varphi x) (w),q(w)^{\mathrm{CF}}\mathbb{C}_{(m,\infty)}^{(\gamma, \gamma )}x(w) \bigr)\,dw \end{aligned}
(2)

with $$x(0)=c$$, where $$\lambda,\rho\geq0$$ and $$\alpha, \gamma ,\delta,\theta\in(0,1)$$.

### Theorem 3.12

Let $$\xi_{1},\xi_{2},\xi_{3}\geq0$$, and let $$f:[0,1]\times\mathbb {R}^{3}\rightarrow\mathbb{R}$$ be a bounded integrable function satisfying $$\vert f(p,x_{1},y_{1},w_{1})-f(p,x_{1}^{\prime },y_{1}^{\prime},w_{1}^{\prime}) \vert \leq \xi_{1} \vert x_{1} -x_{1}^{\prime} \vert +\xi_{2} \vert y_{1} -y_{1}^{\prime} \vert +\xi_{3} \vert w_{1}-w_{1}^{\prime} \vert$$ for all $$p \in I$$ and $$x_{1},y_{1},w_{1},v_{1},x_{1}^{\prime },y_{1}^{\prime},w_{1}^{\prime}\in\mathbb{R}$$. If $$\Delta= \vert 2-\mu \vert [\lambda\frac{M_{1}}{(1-\delta )^{2}}+\rho M_{2}+ \xi_{1} + \xi_{2} \gamma_{0} +\xi_{3} \frac {M_{3}}{(1-\gamma)^{2} \vert 2-m \vert }]<1$$, then problem (2) admits an approximate solution.

### Proof

Let $$H^{1}$$ be equipped with $$d(x ,y )= \Vert x -y \Vert$$, where $$\Vert x \Vert =\sup_{t\in I} \vert x(t) \vert$$. Now, consider the selfmap $$\mathcal{F}:H^{1}\to H^{1}$$ defined by

\begin{aligned} (\mathcal{F}x) (p)&=x(0)+(2-\mu)a_{\alpha} \biggl[ (\lambda k(p)^{\mathrm{CF}} \mathbb{C}_{\infty}^{(\delta,\delta)}x(p)+{ \rho}s(p)^{\mathrm{CF}} \mathbb{I}_{\infty}^{(\theta,\theta)}x(p) \\ &\quad {}+ \int_{0}^{p} f \bigl(w,x(w),(\varphi x) (w),q(w)^{\mathrm{CF}}\mathbb{C}_{(m,\infty)}^{ (\gamma, \gamma )}x(w) \bigr)\,dw \biggr] \\ &\quad{}+ b_{\alpha}(2-\mu) \int_{0}^{p} \biggl[ \lambda k(w)^{\mathrm{CF}} \mathbb{C}_{\infty }^{(\delta,\delta)}x(w)+{\rho}s(w)^{\mathrm{CF}} \mathbb{I}_{\infty }^{(\theta,\theta)}x(w)\,dw \\ &\quad {}+ \int_{0}^{w}f \bigl(r,x(r),(\varphi x) (r),q(r)^{\mathrm{CF}}\mathbb{C}_{(m,\infty)}^{ (\gamma, \gamma )}x(r) \bigr)\,dr \biggr] \,dw \end{aligned}

for all $$p\in I$$ and $$x,y\in H^{1}$$, where $$a_{\alpha}$$ and $$b_{\alpha}$$ are given in Lemma 3.10. As a result, utilizing Lemmas 3.5, 3.7, and 3.8, we get

\begin{aligned} & \biggl\vert \biggl[\lambda k(p)^{\mathrm{CF}}\mathbb{C}_{\infty }^{(\delta,\delta)}x(p)+{ \rho}s(p)^{\mathrm{CF}}\mathbb{I}_{\infty}^{(\theta,\theta)}x(p)+ \int_{0}^{p} f \bigl(w,x(w),(\varphi x) (w), q(w)^{\mathrm{CF}}\mathbb{C}_{(m,\infty)}^{(\gamma, \gamma)}x(w) \bigr)\,dw \biggr] \\ &\quad\quad{} - \biggl[\lambda k(p)^{\mathrm{CF}}\mathbb{C}_{\infty }^{(\delta,\delta)}y(p)+{ \rho} s(w)^{\mathrm{CF}}\mathbb{I}_{\infty}^{(\theta,\theta)}y(p) \\ &\quad\quad{} + \int _{0}^{p} f \bigl(w,y(w),(\varphi y) (w),q(w)^{\mathrm{CF}}\mathbb {C}_{(m,\infty)}^{(\gamma, \gamma)}y(w) \bigr)\,dw \biggr] \biggr\vert \\ &\quad \leq \biggl[\lambda\frac{M_{1}}{(1-\delta)^{2}}+\rho M_{2} \biggr] \Vert x-y \Vert + \xi_{1} \Vert x-y \Vert + \xi_{2} \gamma _{0} \Vert x-y \Vert +\xi_{3} \frac{M_{3}}{(1-\gamma)^{2} \vert 2-m \vert } \Vert x-y \Vert \\ &\quad\leq \biggl[\lambda\frac{M_{1}}{(1-\delta)^{2}}+\rho M_{2}+ \xi _{1} + \xi_{2} \gamma_{0} +\xi_{3} \frac{M_{3}}{(1-\gamma)^{2} \vert 2-m \vert } \biggr] \Vert x-y \Vert \end{aligned}

for all $$p\in I$$ and $$x,y\in H^{1}$$. As a result, we get

\begin{aligned} & \bigl\vert (\mathcal{F}x) (p)-(\mathcal{F}x) (p) \bigr\vert \\ &\quad \leq a_{\alpha} \vert 2-\mu \vert \biggl[\lambda\frac{M_{1}}{(1-\delta )^{2}}+\rho M_{2}+ \xi_{1} + \xi_{2} \gamma_{0} + \xi_{3} \frac{M_{3}}{(1-\gamma)^{2} \vert 2-m \vert } \biggr] \Vert x-y \Vert \\ &\quad \quad{}+b_{\alpha} \vert 2-\mu \vert \int _{0}^{p} \biggl[\lambda\frac{M_{1}}{(1-\delta)^{2}}+\rho M_{2}+ \xi_{1} + \xi_{2} \gamma_{0} + \xi_{3} \frac{M_{3}}{(1-\gamma)^{2} \vert 2-m \vert } \biggr] \Vert x-y \Vert \,ds \\ &\quad \leq \vert 2-\mu \vert \biggl[\lambda\frac{M_{1}}{(1-\delta )^{2}}+\rho M_{2}+ \xi_{1} + \xi_{2} \gamma_{0} +\xi_{3} \frac{M_{3}}{(1-\gamma)^{2} \vert 2-m \vert } \biggr] \Vert x-y \Vert \end{aligned}

for all $$p \in I$$ and $$x,y \in H^{1}$$. Now we consider the mappings $$j:[0,\infty)^{5} \to[0,\infty)$$ and $$\alpha:H^{1}\times H^{1}\to[0,\infty)$$ defined by $$\alpha(t,s) = 1$$ and $$j(t_{1}, t_{2},t_{3},t_{4},t_{5})=\frac{\Delta}{3}(t_{1} +2t_{2})$$. We can check that $$j \in\mathcal{R}$$ and $$\mathcal{F}$$ is a generalized α-contraction. With the help of Theorem 2.2, we conclude that $$\mathcal{F}$$ possesses an approximate fixed point, which represents an approximate solution for the investigated problem (2). □

The next step is to study two applications to describe the reported results.

### Example 1

Let us define $$\eta\in L^{\infty}([0,1])$$ and $$\gamma ,\lambda:[0,1]\times[0,1]\to[0,\infty)$$ by $$\eta(p)=\frac{\pi}{e^{(p+12)}}$$, $$\gamma(p,s)=e^{ p-s}$$ and $$\lambda(p,s)= \ln(5^{\sin(\pi p -s)})$$. Then, we have $$\eta^{*}=\frac{\pi}{e^{12}}$$, $$\gamma_{0}\leq e$$, and $$\lambda_{0}\leq\ln5$$. Let us consider $$\alpha=\frac{1}{5}$$, $$\mu= \frac{1}{20}$$, $$\beta =\frac{1}{4}$$, $$\gamma=\frac{1}{2}$$, $$\theta=\frac{3}{4}$$, and $$\delta=\frac{3}{5}$$. Consider the problem

\begin{aligned}[b] ^{\mathrm{CF}}\mathbb{C}^{(\frac{1}{5},\frac {1}{5})}_{\infty}u_{1}^{\prime}(p)&= \frac{1}{20} { \bigl(}^{\mathrm{CF}}\mathbb{C}_{\infty} ^{(\frac{1}{4},\frac{1}{4})} u_{1}^{\prime}(p)+ ^{\mathrm{CF}} \mathbb{C}_{\infty} ^{(\frac{1}{2},\frac{1}{2})}u_{1}^{\prime}(p) { \bigr)} \\ &\quad{} +e^{-\pi(t+12)} \biggl[p+u_{1}^{\prime}(p)+ \int_{0}^{p} e^{ p-s}u_{1}^{\prime}(s) \,ds \\ &\quad{}+ \int_{0}^{p} \ln \bigl(5^{\sin(\pi p -s)} \bigr)u_{1}^{\prime}(s)\,ds+^{\mathrm{CF}}\mathbb{I}_{\infty}^{(\frac {3}{4},\frac{3}{4})} u_{1}^{\prime}(p)+^{\mathrm{CF}}\mathbb{C}_{\infty}^{(\frac{3}{5}, \frac{3}{5})}u_{1}^{\prime}(p) \biggr] \end{aligned}
(3)

with $$u_{1}^{\prime}(0)=0$$. Considering $$f(p,x,y,w,u_{1},u_{2})= e^{-\pi(p+12)}(p+x+y+w+u_{1}+u_{2})$$, we note that $$\Delta= [\eta^{*}(2+\gamma_{0} + \lambda_{0} + \frac{1}{(1-{\delta })^{2}})+{ \mu}( \frac{1}{(1-{\gamma})^{2}(1-{\beta})^{2}} )]<0/4447<1$$. Now, by Theorem 3.11 problem (3) admits an approximate solution.

### Example 2

Consider the function $$\lambda:[0,1] \times[0,1]\to[0,\infty)$$ by $$\lambda(p,s )=\frac{e^{2p-s}}{e}$$. Thus, $${\lambda_{0}\leq e}$$. Let us consider $$\mu=3$$, $$m=\frac{1}{2}$$, $$\alpha=\frac{1}{4}$$, $$\delta=\frac{1}{4}$$, $$\theta=\frac{1}{2}$$, $$\gamma=\frac{1}{2}$$, $$\lambda= \frac{1}{200}$$, $$\rho=\frac{1}{122}$$, $$\xi_{1}=\frac{1}{320}$$, $$\xi_{2}=\frac{1}{40}$$, and $$\xi_{3}=\frac {1}{119}$$. Let $$k(t)=\frac{2-p}{p+1}$$, $$s(p)=\sin p$$ and $$q(p)=\tan^{-1}(p)$$. Then, $$M_{1}=\sup_{p\in[0,1]} \vert k(p) \vert =2$$, $$M_{2}=\sup_{t\in[0,1]} \vert s(p) \vert =1$$, and $$M_{3}=\sup_{t\in[0,1]} \vert q(p) \vert =\frac{\pi}{2}$$. As a next step, we consider the problem

\begin{aligned}[b] ^{\mathrm{CF}}\mathbb{C}_{(\mu,\infty)}^{(\frac {1}{4},\frac{1}{4})}x(p) &= \frac{1}{200}k(p)^{\mathrm{CF}}\mathbb{C}_{\infty}^{(\frac{1}{4}, \frac{1}{4})}x(p)+ \frac{1}{122}s(p)^{\mathrm{CF}}\mathbb{I}_{\infty}^{(\frac {1}{2},\frac{1}{2})}x(p) \\ &\quad{}+ \int_{0}^{p} \biggl[\frac{2}{56}s+ \frac{1}{320}x(s)+ \frac{1}{40} \int_{0}^{s}\frac{e^{2s-r}}{e}x(r) \,dr \\ &\quad{} + \frac{1}{119}\tan^{-1}(s) ^{\mathrm{CF}}\mathbb{C}_{(m,\infty )}^{(\frac{1}{2},\frac{1}{2})}x(s) \biggr]\,ds \end{aligned}
(4)

with $$x(0)=0$$. Considering $$f(p,x_{1},y_{1},w_{1})= \frac{2}{56}p+\xi _{1}x_{1}+\xi_{2}y_{1}+\xi_{3}w_{1}$$ for all $$p\in I$$ and $$x_{1},y_{1},w_{1},v\in\mathbb{R}$$, we note that

$$\Delta= \vert 2-\mu \vert \biggl[\lambda\frac{M_{1}}{(1-\delta )^{2}}+\rho M_{2}+ \xi_{1} + \xi_{2} \gamma_{0} + \xi_{3} \frac{M_{3}}{(1-\gamma)^{2} \vert 2-m \vert } \biggr]< 0.111< 1.$$

Now, by Theorem 3.12, problem (4) admits an approximate solution.

## Conclusion

Fractional derivatives with nonsingular kernels started to be utilized from both theoretical and applied viewpoints. Particularly, the fractional Caputo-Fabrizio derivative was applied to models possessing memory effect of exponential type. Therefore, new generalizations of this operator should be investigated and applied to the dynamics of real-world problems. In this manuscript, we suggested a new operator called the infinite coefficient-symmetric CF fractional derivative. Besides, its properties were investigated, and two examples clearly show the advantages of the newly introduced concept.

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## Acknowledgements

The second and third authors were supported by Azarbaijan Shahid Madani University.

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Correspondence to Dumitru Baleanu.

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All authors contributed equally and significantly in writing this article. All authors read and approved the final manuscript.

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