In this paper, we investigate the existence of solutions for a class of Caputo fractional differential inclusions with two integral boundary conditions. The convex and nonconvex cases are separately considered. For the first case, an existence result is obtained by applying the Bohnenblust–Karlin’s fixed point theorem. For the second case, a fixed point theorem for contraction multi-valued maps due to Covitz and Nadler is used. Further, an illustrating example is presented.
In Section 2, we recall some basic concepts of the fractional calculus and the theory of multi-valued maps. Some well-known existence results are also recalled. In Section 3, we prove the existence result for the problem (1.1)–(1.3) when, in one case, the right-hand side is convex valued, and in the other case, nonconvex valued. The first result relies on the Bohnenblust–Karlin theorem, while the other is based upon a fixed point theorem for contraction multi-valued maps due to Covitz and Nadler [37]. In Section 4, we propose an example to illustrate our results.
This paper is concerned with the existence of solutions for a certain class of fractional differential inclusion with integral boundary conditions. Thanks to the Bohnenblust–Karlin’s fixed point theorem, an existence result is obtained.
Considering the particular case of single-valued second member, this result is then used to derive an existence result for a certain type of singular boundary value problems.
1. Introduction
Differential equations of fractional order have recently proved to be valuable tools in the modeling of many phenomena in various fields of science and engineering. There are applications in biology [1, 2], chaotic systems [3, 4], control [5–7], diffusion process [8–10], economics [11], electrochemistry [12], electromagnetism [13], epidemiology [14], photoelasticity [15], variational problems [16, 17] and other fields.
The theory of fractional calculus has also seen the emergence of new types of fractional differential in the modern era, we refer to the monographs of Hilfer [18], Jarad et al. [19, 20], Kilbas et al. [21], Podlubny [22], Zhou [23].
On the other hand, applied problems require the definitions of fractional derivatives allowing the utilization of physically interpretable initial data, which contain y(0), y′(0), etc…, and the same requirements for boundary conditions. Caputo's fractional derivative satisfies these demands. It should be noted that there are several Caputo derivatives. Each of them is obtained by a modification of a previously defined derivative in order to maintain the so useful and fundamental property of the classical derivative, namely that the derivative of any constant is the null function. Thus, we have the Caputo-Liouvile, the Caputo-Hadamard, the Caputo-Atangana-Baleanu derivatives and others. For more details concerning geometric, physical interpretations of fractional Caputo derivatives and their applications see Refs. [19–22, 24, 25] and references therein.
Realistic problems arising from economics, optimal control and so on, can be modeled as differential inclusions, so differential inclusions are widely investigated by many authors, see ([26–41]) and references therein.
In 2015, Rezapour et al. [42] studied the following singular one integral boundary-value problem:
where has only one singularity at the point 0. The authors used the Banach contraction principle and the compressive form of Krasnosel'skii's cone fixed point theorem to prove successively an uniqueness and an existence results. In 2020, Kheyryan and Rezapour [43] investigated the Ulam - Hyers stability of this problem via its equivalent integral formulation. In this paper, motivated by the work of Rezapour [42], we wish to discuss the existence of solutions for the Boundary value problems (BVP for short), for the fractional differential inclusions of the form.
Where CDα is the Caputo fractional derivative, . is a multi-valued map, j1, j2, q1, q2, p1, p2 are positive integers such that
It should be noted that if F is a single-valued function and
In Section 2, we recall some basic concepts of the fractional calculus and the theory of multi-valued maps. Some well-known existence results are also recalled. In Section 3, we firstly use the Bohnenblust-Karlin's fixed point theorem to prove the existence result for the problem (1.4)-(1.6) in the case of convex valued right hand side. Secondly, we prove the existence result for the nonconvex valued right hand side by applying a fixed point theorem for contraction multi-valued maps due to Covitz and Nadler [44]. In Section 4, we propose an example to illustrate our results.
Of course, approaches adopted in the present work are not the only possible. Other approaches as the end-point method, the nonlinear alternative of Leray-Schauder type and the nonlinear alternative for Kakutani maps, may also be considered under the appropriate conditions.
2. Preliminaries
In this section, we introduce notations, definitions, and preliminary facts that will be used in the remainder of this paper.
Let J = [0, T]. In the sequel, we will use the following real Banach spaces:
C(J) is the Banach space of continuous functions from J into with the norm
L1(J) is the Banach space of real functions x that are integrable on J with the norm
where Γ is the gamma function.
where n: = [α] + 1 and h(n) is the differential order n of h.
If α > 0, ν > −1, then
[21] Let the function h ∈ L1(J). Then, the general solution of the fractional differential equation
is given by
where c0, c1, …, cn−1 are real constants.
We now recall some useful concepts from the theory of multi-valued maps [45, 46]. If X is an infinite dimensional real Banach space with the norm ‖.‖X, then a multivalued map.
is convex (closed) valued if H(x) is convex (closed) for every x ∈ X.
is bounded on bounded subsets if is bounded in X for any bounded subset of X.
is called upper semi-continuous (u.s.c.) on X if for each x0 ∈ X, the set H(x0) is a nonempty, closed in X and for each open set of X containing H(x0), there exists an open neighborhood of x0 such that .
is said to be completely continuous if is relatively compact for every bounded subset of X.
has a fixed point if there exists x ∈ X such that x ∈ H(x).
is said to be Carathéodory if it is measurable with respect to t ∈ J for each fixed x ∈ X and u.s.c. on X for almost all t ∈ J.
Notice that if H is completely continuous with nonempty compact values, then H is u.s.c. if and only if H has a closed graph, that is:
For each , define the selection SF,y of F by
Let (X, d) be a metric space induced from the normed space (X, ‖ ⋅‖). Consider given by:
.
For our purpose, we need to introduce the following subsets of :
Pcl(X) (respectively Pc(X) is the set of all closed (convex respectively) subsets of X.
Pcp(X) is the set of all compact subsets of X.
Pcl,c(X) (respectively Pcp,c(X)) is the set of all nonempty, closed and convex (nonempty, compact and convex respectively) subsets of X.
A multivalued operator N: X → Pcl(X) is called
γ-Lipschitz if and only if there exists γ > 0 such that
Hd(N(x), N(y)) ≤ γd(x, y), for each x, y ∈ X,
a contraction if and only if it is γ-Lipschitz with γ < 1.
(Covitz and Nadler [44]) Let (X, d) be a complete metric space. If N: X → Pcl(X) is a contraction, then Fix(N) ≠ ∅, where Fix(N) is the set of all fixed points for N.
(Bohnenblust-Karlin [47]) Let D ∈ Pcl,c(X) and let H: D → Pcl,c(D) be u.s.c. such that is compact. Then, H has a fixed point.
([48]) Let I be a compact real interval, ϕ a linear continuous map from L1(I) into C(I). Let also be a multi-valued map satisfying the following assumptions:
F is Carathéodory.
For each fixed , is nonempty.
Then, the operator ϕ◦SF defined from C(I) into Pcl,c(C(I)) by (ϕ◦SF) (x) = ϕ(SF,x) is a closed graph operator in C(I) × C(I).
3. Existence of solutions
3.1 Auxiliary lemmas
We will use the following auxiliary scalar functions:
There is no difficulty to check that
and
Let h ∈ L1(J), A ≠ 0 and assume that conditions 1.7 are satisfied. Then, the solution of the equation
subject to the conditions
and
is given by
where
where, c0, c1, …, cn−1 are real constants.
Using initial conditions at t = 0, one obtains that ck = 0 for and then,
Using integral conditions and after some transformations, we get that constants and satisfy the following linear system:
System (S) admits a unique solution given by
Remarking that
and replacing in the expression of x(t), we get
it is not difficult to check that and that initial conditions (3.9) are all satisfied. Let us proof the condition (3.10) for k = 1 (the case k = 2 is demonstrated in a similar way). Direct calculations give us,
Consequently,
Using 3.6, we get finally,
□
We have A ≠ 0 for example if we have simultaneously
Indeed, in this case either
or,
Suppose that A ≠ 0 and let
Then,
Proof. It is a direct calculation that uses the expression of each gi(s), i = 1, 2 and the obvious inequalities
□
3.2 The convex case
In this section, we present an existence result of solutions for the problem (1.4)-(1.7) under the condition that F(t, s) is a convex multi-valued map. Let us start by defining what we mean by a solution of the problem.
Let A ≠ 0 and assume the following hypothesis hold:
is a Carathéodory multi-valued map.
For each r > 0, there exists a positive function ρr ∈ L1(J) such that
If
Proof. We begin by transform this problem into a fixed point one. For this, consider the multi-valued operator,
defined by
where the function G(t, s) is given by (3.12). For r > 0 let
Step 1: We prove that there exists r0 > 0 such that .
If it is not true, we get
This is equivalent to
with
According to (H2), there exists a positive function ρr ∈ L1(J) such that
Thus,
Dividing both sides by r and taking the lower limit as r → ∞, we obtain that
which contradicts 3.16. Hence for a certain r0 > 0.
Let N0 be the restriction of N to closed and convex subset . Clearly, each fixed point for N0 is also a fixed point for N. So, we shall follow the scheme proposed in Ref. [35] to prove that N0 satisfies all assumptions of the theorem 2.7.
Step 2: We prove that N0(x) is convex for each .
Let h1, h2 belong to N0(x). There exist f1, f2 ∈ SF,x such that for each t ∈ J and for each i = 1, 2, we have
Let 0 ≤ λ ≤ 1. Then, for each t ∈ J, we have
Since SF,x is convex (F has convex values), we have
λh1 + (1 − λ)h2 ∈ N0(y).
Step 3: We prove that is a compact subset of .
According to the Step 1, is a subset of and hence is uniformly bounded. Thus, it suffices to prove that is equicontinuous. Let and h ∈ N0(x). There exists f ∈ SF,x such that
Thus, for t1, t2 ∈ J: t1 < t2, and applying (H2), we get
Since and G(t, s) is continuous with respect to t ∈ J, the right hand side of the above inequality tends to zero independently of and h ∈ N(x), whenever t1 → t2. This implies that is equicontinuous and that is a compact subset of .
Step 4: We prove that N0 has a closed graph.
We need to prove that
We have
Consider the linear operator ϕ: L1(J) → C(J) given by
It is easy to see that
This means that ϕ is bounded and continuous. By Lemma 2.8, ϕ◦SF has a closed graph. Since, , we deduce that
This is equivalent to,
In other words, N0 has a closed graph.
Summarizing, we get that the multi-valued N0 has a closed graph, bounded, convex and closed values which are also compact since is so. Hence, by Lemma 2.8, N0 is upper semicontinuous. Applying the theorem 2.7, we deduce that N0 has a fixed point x in Dr which is a solution of the problem (1.4)-(1.6). □
3.3 The nonconvex case
We present now a result for the problem (1.4)-(1.5) with a nonconvex valued right hand side. Our considerations are based on the fixed point theorem for contraction multivalued maps given by Covitz and Nadler ([44]).
Assume that the following hypotheses hold:
has the property that is measurable for each .
For each y ∈ C(J), there exists a constant My > 0 depending only of y such that
|w| ≤ My for each w ∈⋃t ∈ JF(t, y(t)).
There exists , such that
for every ,
If
By (H3), we can see that SF,x is nonempty for each , so F has a measurable selection (see Ref. [51], Theorem III.6)
Proof. We shall show that N satisfies the assumptions of Lemma (2.6). The proof will be given in two steps.
Step1: for each .
Let be such that in . Then and for each natural number n there exists a null-measure subset Jn of J = [0, T] together with a function vn ∈ L1(J) such that
The subset is null-measure and vn(t) ∈ F(t, x(t)) for each natural n and each .
Using the compactness of each F(t, x(t)), t ∈ J, we may pass to a subsequence in necessary to get that converges to a certain function .
Clearly, .
By the hypothesis (H4),
This implies that .
Thus, by the dominated convergence Lebesgue theorem, we have for each t ∈ J
So, and thus, for each .
Step2: There exists γ < 1 such that for each .
Let and h1 ∈ N(x). There exists v1 ∈ F(t, x(t)) such that for each t ∈ J
From (H5) it follows that
Hence, there exists such that
Consider given by
Since the multivalued operator is measurable, there exists a function v2(t) which is a measurable selection for V. So, and for each t ∈ J,
Let us define for each t ∈ J
Then, and for each t ∈ J
Thus
We obtain an analogous relation by interchanging the roles of x and . Consequently, we can affirm that
4. Example
In this section, we give an example illustrating our main results. Consider the boundary fractional differential value problem
where
In this case,
and A: = det(S) = 131, 99. Recalling formula (3.15), we obtain that Δ ≃ 5, 3979.
Clearly, F is Carathéodory with closed and convex values. For each x fixed in function fx defined on [0, 1] by
is an element of SF, x. Moreover,
Finally, it follows from inequality
that

