Solvability in Gevrey Classes of Some Nonlinear Fractional Functional Differential Equations
Abstract
Our purpose in this paper is to prove, under some regularity conditions on the data, the solvability in a Gevrey class of bound −1 on the interval [−1,1] of a class of nonlinear fractional functional differential equations.
1. Introduction
Fractional calculus has evolved from the speculations of early mathematicians of the 17th and 18th centuries like G. W. Leibnitz, I. Newton, L. Euler, G. F. de L’Hospital, and J. L. Lagrange [1]. In the 19th century, other eminent mathematicians like P. S. Laplace, J. Liouville, B. Riemann, E. A. Holmgren, O. Heaviside, A. Grunwald, A. Letnikov, J. B. J. Fourier, and N. H. Abel have used the ideas of fractional calculus to solve some physical or mathematical problems [1]. In the 20th century, several mathematicians (S. Pincherle, O. Heaviside, G. H. Hardy, H. Weyl, E. Post, T. J. Fa Bromwich, A. Zygmund, A. Erdelyi, R. G. Buschman, M. Caputo, etc.) have made considerable progress in their quest for rigor and generality to build fractional calculus and its applications on rigorous and solid mathematical foundations [1]. Actually, fractional calculus allows mathematical modeling of social and natural phenomena in a more powerful way than the classical calculus. Indeed fractional calculus has a lot of applications in different areas of pure and applied sciences like mathematics, physics, engineering, fractal phenomena, biology, social sciences, finance, economy, chemistry, anomalous diffusion, and rheology [1–22]. It is then of capital importance to develop for fractional calculus the mathematical tools analogous to those of classical calculus [1, 3, 4, 19, 23]. The fractional differential equations [23–28] are a particularly important case of such fundamental tools. An important type of fractional differential equations is that of fractional functional differential equations (FFDEs) [10, 29–31] which are the fractional analogues to functional differential equations [17, 32–34], enable the study of some physical, biological, social, and economical processes (automatic control, financial dynamics, economical planning, population dynamics, blood cell dynamics, infectious disease dynamics, etc.) with fractal memory and nonlocality effects, where the rate of change of the state of the systems depends not only on the present time but on other different times which are functions of the present time [11, 35, 36]. The question then arises of the choice of a suitable framework for the study of the solvability of these equations. But, since the functional Gevrey spaces play an important role in various branches of partial and ordinary differential equations [37– 40], we think that these functional spaces can play the role of such convenient framework. However, let us point out that in order to make these spaces adequate to our specific setting, it is necessary to make a modification to their definition. This leads us to the definition of new Gevrey classes, namely, the Gevrey classes of bound q1 and index l > 0 on an interval [q1, q2]. Our purpose in this paper is to prove, under some regularity conditions on the data, the solvability in a Gevrey class of the form Gk,−1([−1,1]) of a class of nonlinear FFDE. Our approach is mainly based on a theorem that we have proved in [41]. The notion of fractional calculus we are interested in is the Caputo fractional calculus. Some examples are given to illustrate our main results.
2. Preliminary Notes and Statement of the Main Result
2.1. Basic Notations
Let F : E⟶E be a mapping from a nonempty set E into itself. F〈n〉(n ∈ ℕ) denotes the iterate of F of order n for the composition of mappings.
For z ∈ ℂ and h > 0, B(z, h) is the open ball in ℂ≃ℝ2 with the center z and radius h.
Let S1 and S2 be two nonempty subsets of ℂ such that S1 ⊂ S2 and f : S2⟶ℂ a mapping. We denote by the restriction of the mapping f to the set S1.
Remark 1. The following inclusions hold for every d ∈ ]q1, q2], r ∈ ]0, d − q1[ and n ∈ ℕ∗:
Let f : S⟶ℂ be a bounded function. ‖f‖∞,S denotes the quantity:
By C0([q1, q2]) (resp. C1([q1, q2])), we denote the complex vector space of all complex valued functions defined and continuous (resp. defined and of class C1) on the interval [q1, q2]. C0([q1, q2]) is a Banach space when it is endowed with the uniform norm:
Let ξ1, ξ2 ∈ ℂ. We denote by the linear path joining ξ1 to ξ2:
In this paper, k > 0 and α ∈ ]0,1[ are fixed numbers.
2.2. Fractional Derivatives and Integrals
Definition 1. Let δ ∈ ]0,1[ and f be a Lebesgue-integrable function on the nontrivial compact interval [q1, q2]. The Caputo fractional integral of order δ and lower bound q1 of the function f [19, 23, 25, 26, 28] is the function denoted by and defined by
Remark 2. If the function f is continuous on the interval [q1, q2], then the function is well defined and continuous on the entire interval [q1, q2], and we have
Definition 2. Let f : [q1, q2]⟶ℂ be an absolutely continuous function on [q1, q2]; then, the Caputo fractional derivative of f of order δ and lower bound q1 [19, 23, 25, 26, 28] is the function denoted by and defined by
2.3. Gevrey Classes
Definition 3. Let l > 0. The Gevrey class of index l on [q1, q2], denoted by Gl([q1, q2]), is the set of all functions f of class C∞ on [q1, q2] such that
Definition 4. The Gevrey class of bound q1 and index l on the interval [q1, q2], denoted by is the set of all functions f of class C1 on [q1, q2] and of class C∞ on ]q1, q2] such that the restriction of f belongs to the Gevrey class Gl([q, q2]), for every q ∈ ]q1, q2[ .
2.4. The Property S(l)
Definition 5. A function φ defined on the set is said to satisfy the property S(l) on the interval [q1, q2] if is holomorphic on is a function of class C1 on [q1, q2], and there exists a constant τφ ∈ ]0, π[ such that for all D ∈ ]0, τφ] there exist Nl,φ(D) ∈ ℕ∗ depending only on D, l, and φ such that the inclusion
Remark 4. Let φ be a function verifying the property S(l). Then,
On the other hand, it follows from (14) that we have for every D ∈ ]0, τφ[
Thence, we have
It follows that for every D ∈ ]0, τφ[ there exists E ∈ ]0, D[ such that
2.5. Statement of the Main Result
Our main result in this paper is the following.
Theorem 1. Let λ ∈ ℂ and σ > 0. Let a, b, and ψ be holomorphic functions on [−1,1]σ and Φ be an entire function. We assume that the function a is not identically vanishing and that there exist constants α0, β0 > 0 such that
Then, the FFDE
3. Proof of the Main Result
The proof of the theorem is subdivided in three steps.
Step 1. The localisation of the solutions of the equation:
The study of the variations of the function
Therefore, the equation (ℑ) has on ℝ+ exactly two solutions R0 < R1 and the following inequalities hold:
Step 2. Proof of the existence of a solution u of the FFDE (E) in C1([−1,1]) such that the initial condition (E1) holds.
Consider the operator T : C0([−1,1])⟶C0([−1,1]) defined by the following formula:
We have for all
Thence, the closed ball is stable by the operator T. On the other hand, we have for all f,
Since 0 < R0 < (ln (αΓ(α)/α0β02α‖a‖∞,[−1,1])/β0), it follows from condition (23) that
Thence, T has, in a unique fixed point u.
Consider the sequence of functions defined on [−1,1] by the following formula:
Let us set for each n ∈ ℕ, Fn = fn+1 − fn. Since Q ∈ [0,1[, it follows that the function series ∑Fn is uniformly convergent on [−1,1] to a function which is a fixed point of the operator T. It follows that v = u. Consequently, the function series ∑Fn is uniformly convergent on [−1,1] to the function u ∈C0([−1,1]).
On the other hand, we have for all x ∈ ]−1,1] and n ∈ ℕ∗:
Since a(−1) = 0, it follows that
To achieve the proof of this step we need the following result.
Proposition 1. The sequence is bounded.
Proof. We have for all x ∈ ]−1,1] and n ∈ ℕ∗
It follows from assumption (20) that
But, according to assumption (24) and (35) we have
Consequently, the following inequality holds for each n ∈ ℕ∗
Since Q ∈ [0,1[, it follows that the sequence is bounded.
The proof of the proposition is complete.
Now, we set
Then, we can write
Direct computations show then that
Since Q ∈ [0,1[, it follows that the function series is uniformly convergent on [−1,1]. Thence, the function u is of class C1 on [−1,1] and satisfies the following relation:
Consequently, according to assumption (20), we can write for all t ∈ [−1,1]
So, u is a solution of the FFDE (E) which belongs to C1([−1,1]) and fulfills the relation u(−1) = λ.
Step 3. Proof that u belongs to the Gevrey class Gk,−1([−1,1]).
Since the function Λ defined on [0, min(1, σ)[ by
Consider the sequence of functions , where
Proposition 2. The inclusion holds for every n ∈ ℕ∗.
Proof. We denote the last inclusion by P(n). We denote for every z ∈ ℂ by the closest point of [−1,1] to z. It is obvious that P(1) is true. Assume for a certain n ∈ ℕ∗ that P(p) is true for every p ∈ {1 … n}. Since the function ωn+1 is holomorphic on we have then for each
Thence, the assertion P(n + 1) is true. Consequently, P(n) is true for all n ∈ ℕ∗.
The proof of the proposition is then complete.
By virtue of the Proposition 2., we have for all n ∈ ℕ∗\{1} and
It follows that
Let us set Ω1 = ω1 and denote, for all n ∈ ℕ∗\{1}, by Ωn the function
Then, the function Ωn is holomorphic on for each n ∈ ℕ∗. Furthermore, the following relations hold for every n ∈ ℕ∗\{1}:
Since Λ(s2) ∈ [0,1[, it follows then from (59) that the function series ∑Ωn|[−1,1] is uniformly convergent on [−1,1] to the function u. However, we know, according to relation (4) of Remark 1, that the following inclusion hold:
It follows then that
The relations (61) entail, thanks to the main result of [41], that u|[d, 1] belongs to Gk([d, 1]), for each d ϵ ] − 1,1[ . Thence, since u is of class C1 on [−1,1], it follows that u belongs to the Gevrey class Gk,−1([−1,1]).
The proof of the main result is then complete.
4. Examples
To obtain examples illustrating our main result, we need first to prove the following proposition.
Proposition 3. The function
Proof. Let l ∈ ]0,1], ε ∈ ]0,1] and z ∈ [−1,1]ε. We have
It follows that
We consider then the principal argument arg(ℒ(z) + 1) of ℒ(z) + 1 which satisfies the following estimates:
But, direct computations prove that
Thence, we have
It follows that
On the other hand, we have
But, we know that
It follows that
We derive, from the estimates (68) and (71), the following inclusion:
Let n ∈ ℕ∗ and A ∈ ]0, 1/μl[ . We have
But, we have
It follows that there exists an integer NA,l ≥ 1 such that the following inequality holds for every integer n ≥ NA,l:
Consequently, we have
It follows that the function satisfies the property S (l).
The proof of the proposition is then complete.
Example 1. Let C ∈ ℂ and γ ∈ ] − 1,1[ . We assume that
Consider the FFDE
Consider then the following entire functions:
It is clear that a1 is not identically vanishing and that a1(−1) = b1(−1) = 0. Furthermore, we have
We also have
Consequently, it follows from the main result that the problem has a solution which belongs to the Gevrey class G1,−1([−1,1]).
Example 2. Let η > 0 and λ ∈ ℂ. We assume that
Consider the FFDE
Consider then the following functions:
It is clear that a2 is not identically vanishing and that a2(−1) = b2(−1) = 0. Furthermore, we have the following inequalities:
Consequently, it follows from the main result that the problem has a solution which belongs to the Gevrey class G1,−1([−1,1]).
Disclosure
This modest work is dedicated to the memories of two great men: our beloved master Ahmed Intissar (1951–2017), a brilliant mathematician (PhD at M.I.T, Cambridge), a distinguished professor, a man with a golden heart; our brother and indeed friend Mohamed Saber Bensaid (1965–2019), the man who belongs to the time of jasmine and sincere love, the comrade who devoted his whole life to the fight for socialism, democracy, and human rights.
Conflicts of Interest
The author declares that there are no conflicts of interest.
Open Research
Data Availability
No data were used to support this study.