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Implicit nonlinear fractional differential equations of variable order
Boundary Value Problems volume 2021, Article number: 64 (2021)
Abstract
In this manuscript, we examine both the existence and the stability of solutions to the implicit boundary value problem of Caputo fractional differential equations of variable order. We construct an example to illustrate the validity of the observed results.
1 Introduction
The idea of fractional calculus is to replace the natural numbers in the derivative’s order with rational ones. Although it seems an elementary consideration, it has an exciting relevance explaining some physical phenomena. Especially in the last two decades, significant numbers of papers appeared on this topic, some papers deal with the existence of solutions to problems of variable order; see e.g. [3, 4, 9, 10, 12].
In particular, [2] Benchohra et al. studied the existence and uniqueness results for the following nonlinear implicit fractional differential equations:
where f is a given function, \(x_{0}, x_{1}\in \Re \), and \({}^{c}D^{u}_{0^{+}}\) is the Caputo fractional derivative of order u.
Inspired by [2] and [3, 4, 9, 10, 12], we deal with the boundary value problem (BVP)
where \(u: J \rightarrow (1, 2]\), \(f_{1}:J\times \Re \times \Re \rightarrow \Re \) is a continuous function and \({}^{c}D^{u(t)}_{0^{+}}\) is the Caputo fractional derivative of variable-order \(u(t)\).
In this paper, we shall look for a solution of (1). Further, we study the stability of the obtained solution of (1) in the sense of Ulam–Hyers (UH).
2 Preliminaries
This section introduces some important fundamental definitions that will be needed for obtaining our results in the next sections.
The symbol \(C(J, \Re )\) represents the Banach space of continuous functions \(x:J \to \Re \) with the norm
For \(- \infty < a_{1} < a_{2} < + \infty \), we consider the mappings \(u(t): [a_{1}, a_{2}]\rightarrow (0, +\infty ) \) and \(v(t): [a_{1}, a_{2}]\rightarrow (n-1, n)\), \(n \in N \). Then the left Caputo fractional integral (CFI) of variable-order \(u(t)\) for the function \(f_{2}(t)\) [7, 8, 11] is
and the left Caputo fractional derivative (CFD) of variable-order \(v(t)\) for the function \(f_{2}(t)\) [7, 8, 11] is
As anticipated, in the case of \(u(t)\) and \(v(t)\) being constant, then CFI and CFD coincide with the standard Caputo fractional derivative and integral, respectively; see e.g. [6–8].
Recall the following pivotal observation.
Lemma 2.1
([6])
Let \(\alpha _{1}, \alpha _{2} >0\), \(a_{1} >0\), \(f_{2} \in L(a_{1}, a_{2})\), \({}^{c}D_{a_{1}^{+}}^{\alpha _{1}}f_{2}\in L(a_{1}, a_{2})\). Then the differential equation
has the unique solution
and
with \(n-1 < \alpha _{1} \leq n\), \(\omega _{\ell }\in \Re \), \(\ell =0,1,\ldots,n-1\).
Furthermore,
and
Remark 2.1
Note that the semigroup property is not fulfilled for general functions \(u(t)\), \(v(t)\), i.e.,
Example 2.1
Let
and
So, we get
Therefore, we obtain
Lemma 2.2
([18])
Let \(u: J \rightarrow (1, 2]\) be a continuous function, then, for \(f_{2} \in C_{\delta }(J, {\Re })=\{ f_{2}(t)\in C(J, {\Re }), t^{ \delta }f_{2}(t) \in C(J, {\Re }), 0 \leq \delta \leq 1 \}\), the variable order fractional integral \(I^{u(t)}_{0^{+}}f_{2}(t)\) exists for any points on J.
Lemma 2.3
([18])
Let \(u: J \rightarrow (1, 2]\) be a continuous function, then \(I^{u(t)}_{0^{+}} f_{2}(t)\in C(J, \Re )\) for \(f_{2} \in C(J, \Re )\).
Definition 2.1
Let \(I \subset \Re \), I is called a generalized interval if it is either an interval, or \(\{a_{1}\}\) or ∅.
A finite set \({\mathcal{P}}\) is called a partition of I if each x in I lies in exactly one of the generalized intervals E in \({\mathcal{P}}\).
A function \(g: I \rightarrow \Re \) is called piecewise constant with respect to partition \({\mathcal{P}}\) of I if for any \(E \in {\mathcal{P}}\), g is constant on E.
Theorem 2.1
(Krasnoselskii fixed point theorem [6])
Let S be a closed, bounded and convex subset of a real Banach space E and let \(W_{1}\) and \(W_{2}\) be operators on S satisfying the following conditions:
(i) \(W_{1}(S)+W_{2}(S)\subset S\),
(ii) \(W_{1}\) is continuous on S and \(W_{1}(S)\) is a relatively compact subset of E,
(iii) \(W_{2}\) is a strict contraction on S, i.e., there exists \(k\in [0,1)\), such that
for every \(x, y \in S\).
Then there exists \(x \in S\) such that \(W_{1}(x)+W_{2}(x)=x\).
Definition 2.2
([1])
Equation (1) is (UH) stable if there exists \(c_{f_{1}}>0\), such that, for any \(\epsilon >0\) and for every solution \(z \in C(J, \Re )\) of the following inequality:
there exists a solution \(x \in C(J, \Re )\) of Eq. (1) with
3 Existence of solutions
Let us introduce the following assumption:
- (H1):
-
Let \(n\in N\) be an integer, \({\mathcal{P}} =\{J_{1}:=[0,T_{1}], J_{2}:=(T_{1},T_{2}], J_{3}:=(T_{2},T_{3}],\ldots,J_{n}:=(T_{n-1},T] \}\) be a partition of the interval J, and let \(u(t): J \rightarrow (1,2]\) be a piecewise constant function with respect to \({\mathcal{P}}\), i.e.,
where \(1< u_{\ell } \leq 2 \) are constants, and \(I_{\ell }\) is the indicator of the interval \(J_{\ell }:=(T_{\ell -1},T_{\ell }], \ell =1,2,\ldots,n\), (with \(T_{0}=0, T_{n}=T\)) such that
For each \(\ell \in \{1, 2,\ldots,n \}\), the symbol \(E_{\ell }= C(J_{\ell },\Re )\), indicates the Banach space of continuous functions \(x:J_{\ell } \to \Re \) equipped with the norm
Then, for any \(t \in J_{\ell }, \ell = 1, 2, \ldots, n\), the left Caputo fractional derivative of variable order \(u(t)\) for the function \(x(t) \in C(J,\Re )\), defined by (3), could be presented as a sum of left Caputo fractional derivatives of constant-orders \(u_{\ell }, \ell = 1, 2, \ldots, n\)
Thus, according to (5), the BVP (1) can be written for any \(t \in J_{\ell }, \ell = 1, 2, \ldots, n\) in the form
In what follows we shall introduce the solution to the BVP (1).
Definition 3.1
The BVP (1) has a solution, if there are functions \(x_{\ell }, \ell =1, 2,\ldots, n\), so that \(x_{\ell } \in C([0, T_{\ell }], \Re )\), fulfilling Eq. (6), and \(x_{\ell }(0) = 0 = x_{\ell }(T_{\ell })\).
Let the function \(x \in C(J, \Re )\) be such that \(x(t) \equiv 0\) on \(t \in [0, T_{\ell -1}]\) and such that it solves the integral equation (6). Then (6) is reduced to
We shall deal with the following BVP:
For our purpose, the upcoming lemma will be a corner stone of the solution of the BVP (7).
Lemma 3.1
Let \(\ell \in \{1,2,\ldots,n\}\) be a natural number, \(f_{1}\in C(J_{\ell } \times \Re \times \Re, \Re )\) and there exists a number \(\delta \in (0, 1)\) such that \(t^{\delta } f_{1}\in C(J_{\ell } \times \Re \times \Re, \Re )\).
Then the function \(x \in E_{\ell }\) is a solution of the BVP (7) if and only if x solves the integral equation
where
Proof
We presume that \(x \in E_{\ell }\) is solution of the BVP (7) and we take \({}^{c}D^{u_{\ell }}_{T_{\ell -1}^{+}} x(t)= y(t)\). Employing the operator \(I^{u_{\ell }}_{T_{\ell -1}^{+}}\) to both sides of (7) and regarding Lemma 2.1, we find
By \(x(T_{\ell -1}) = 0\), we get \(\omega _{1}=0\).
Let \(x(t)\) satisfy \(x(T_{\ell })=0\). So, we observe that
Then we find
where
Conversely, let \(x \in E_{\ell }\) be a solution of the integral equation (8). Regarding the continuity of the function \(t^{\delta } f_{1}\) and Lemma 2.1, we deduce that x is the solution of the BVP (7).
We will prove the existence result for the BVP (7). This result is based on Theorem 2.1. □
Theorem 3.1
Let the conditions of Lemma 3.1be satisfied and there exist constants \(K, L >0\), such that \(t^{\delta }|f_{1}(t,y_{1}, z_{1})- f_{1}(t,y_{2}, z_{2})|\leq K|y_{1}-y_{2}|+ L|z_{1}-z_{2}|\), for any \(y_{i}, z_{i} \in \Re \), \(i = 1, 2\), \(t\in J_{\ell }\). and the inequality
holds.
Then the BVP (7) possesses at least one solution in \(E_{\ell }\).
Proof
We construct the operators
as follows:
where
It follows from the properties of fractional integrals and from the continuity of the function \(t^{\delta }f_{1}\) that the operators \(W_{1}, W_{2}: E_{\ell }\) → \(E_{\ell }\) defined in (10) are well defined.
Let
where
We consider the set
Clearly \(B_{R_{\ell }}\) is nonempty, closed, convex and bounded.
Now, we demonstrate that \(W_{1}, W_{2}\) satisfy the assumption of Theorem 2.1. We shall prove it in four phases.
STEP 1: Claim: \(W_{1}(B_{R_{\ell }})+ W_{2}(B_{R_{\ell }})\subseteq (B_{R_{\ell }})\).
For \(y \in B_{R_{\ell }}\), we have
which means that \(W_{1}(B_{R_{\ell }})+ W_{2}(B_{R_{\ell }}) \subseteq B_{R_{\ell }} \).
STEP 2: Claim: \(W_{1}\) is continuous.
We presume that the sequence \((y_{n})\) converges to y in \(E_{\ell }\) and \(t \in J_{\ell }\). Then
i.e., we obtain
Ergo, the operator \(W_{1}\) is a continuous on \(E_{\ell }\).
STEP 3: \(W_{1}\) is compact
Now, we will show that \(W_{1}(B_{R_{\ell }})\) is relatively compact, meaning that \(W_{1}\) is compact. Clearly \(W_{1}(B_{R_{\ell }})\) is uniformly bounded because by Step 1, we have \(W_{1}(B_{R_{\ell }})= \{W_{1}(y): y \in B_{R_{\ell }} \}\subset W_{1}(B_{R_{ \ell }})+ W_{2}(B_{R_{\ell }})\subseteq (B_{R_{\ell }})\) thus for each \(y \in B_{R_{\ell }}\) we have \(\|W_{1}(y)\|_{E_{\ell }} \leq R_{\ell }\), which means that \(W_{1}(B_{R_{\ell }})\) is bounded. It remains to show that \(W_{1}(B_{R_{\ell }})\) is equicontinuous.
For \(t_{1},t_{2}\in J_{\ell }, t_{1} < t_{2}\) and \(y \in B_{R_{\ell }}\), we have
Hence \(\|(W_{1}y)(t_{2})-(W_{1}y)(t_{1})\|_{E_{\ell }}\rightarrow 0\) as \(|t_{2}-t_{1}|\rightarrow 0\). It implies that \(W_{1}(B_{R_{\ell }})\) is equicontinuous.
STEP 4: \(W_{2}\) is a strict contraction
For \(x(t), y(t) \in E_{\ell }\), we obtain
Consequently by (9), the operator \(W_{2}\) is a strict contraction.
Therefore, all conditions of Theorem 2.1 are fulfilled and thus there exists \(\widetilde{x_{\ell }}\in B_{R_{\ell }}\), such that \(W_{1}\widetilde{x_{\ell }}+W_{2}\widetilde{x_{\ell }}=\widetilde{x_{ \ell }}\), which is a solution of the BVP (7). Since \(B_{R_{\ell }} \subset E_{\ell }\), the claim of Theorem 3.1 is proved.
Now, we will prove the existence result for the BVP (1).
Introduce the following assumption:
- (H2):
-
Let \(f_{1}\in C(J \times \Re \times \Re, \Re )\) and there exists a number \(\delta \in (0, 1)\) such that \(t^{\delta } f_{1}\in C(J \times \Re \times \Re, \Re )\) and there exist constants \(K, L >0\), such that \(t^{\delta }|f_{1}(t,y_{1}, z_{1})- f_{1}(t,y_{2}, z_{2})|\leq K|y_{1}-y_{2}|+ L|z_{1}-z_{2}|\), for any \(y_{1}, y_{2}, z_{1}, z_{2} \in \Re \) and \(t\in J\). □
Theorem 3.2
Let the conditions (H1), (H2) and inequality (9) be satisfied for all \(\ell \in \{1,2,\ldots,n\}\).
Then the problem (1) possesses at least one solution in \(C(J, \Re )\).
Proof
For any \(\ell \in \{1,2,\ldots,n\}\) according to Theorem 3.1 the BVP (7) possesses at least one solution \(\widetilde{x_{\ell }}\in E_{\ell }\).
For any \(\ell \in \{1,2,\ldots,n\}\) we define the function
Thus, the function \(x_{\ell } \in C([0, T_{\ell }], \Re )\) solves the integral equation (6) for \(t \in J_{\ell }\) with \(x_{\ell }(0) =0, x_{\ell }(T_{\ell }) = \widetilde{x}_{\ell }(T_{\ell }) = 0\).
Then the function
is a solution of the BVP (1) in \(C(J, \Re )\). □
4 Ulam–Hyers stability
Theorem 4.1
Let the conditions (H1), (H2) and inequality (9) be satisfied. Then BVP (1) is (UH) stable.
Proof
Let \(\epsilon >0\) an arbitrary number and the function \(z(t)\) from \(z \in C(J_{\ell }, \Re )\) satisfy inequality (4).
For any \(\ell \in \{1,2,\ldots,n\}\) we define the functions \(z_{1}(t)\equiv z(t), t \in [0, T_{1}]\) and for \(\ell =2,3,\ldots,n\):
For any \(\ell \in \{1,2,\ldots,n\}\) according to equality (5) for \(t \in J\) we get
Taking the (CFI) \(I^{u_{\ell }}_{T_{\ell -1}^{+}}\) of both sides of the inequality (4), we obtain
According to Theorem 3.2, BVP (1) has a solution \(x \in C(J, \Re )\) defined by \(x(t) = x_{\ell }(t)\) for \(t \in J_{\ell }, \ell = 1, 2,\ldots, n\), where
and \(\widetilde{x}_{\ell } \in E_{\ell }\) is a solution of (7). According to Lemma 3.1 the integral equation
holds.
Let \(t \in J_{\ell }, \ell = 1, 2,\ldots, n\). Then by Eqs. (12) and (13) we get
where
Then
We obtain, for each \(t \in J_{\ell }\),
Therefore, the BVP (1) is (UH) stable. □
5 Example
Let us consider the following fractional boundary value problem:
Let
Then we have
Hence the condition (H2) holds with \(\delta =\frac{1}{3}\) and \(K =L = \frac{1}{e+5}\).
By (15), according to (7) we consider two auxiliary BVPs for Caputo fractional differential equations of constant order,
and
Next, we prove that the condition (9) is fulfilled for \(\ell = 1\). Indeed,
Accordingly the condition (9) is achieved. By Theorem 3.1, the problem (16) has a solution \(\widetilde{x}_{1} \in E_{1}\).
We prove that the condition (9) is fulfilled for \(\ell = 2\). Indeed,
Thus, the condition (9) is satisfied.
According to Theorem 3.1, the BVP (17) possesses a solution \(\widetilde{x}_{2} \in E_{2}\).
Then, by Theorem 3.2, the BVP (14) has a solution
where
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Acknowledgements
The third author would like to thank the Department of Mathematics, Faculty of Applied Science, King Mongkut’s University of Technology North Bangkok.
Funding
This research was funded by Thailand Science Research and Innovation Fund, and King Mongkut’s University of Technology North Bangkok with Contract no. KMUTNB-BasicR-64-33-1.
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Benkerrouche, A., Souid, M.S., Sitthithakerngkiet, K. et al. Implicit nonlinear fractional differential equations of variable order. Bound Value Probl 2021, 64 (2021). https://doi.org/10.1186/s13661-021-01540-7
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DOI: https://doi.org/10.1186/s13661-021-01540-7
MSC
- 26A33
- 34K37
Keywords
- Fractional differential equations of variable order
- Boundary value problem
- Fixed point theorem
- Ulam–Hyers stability