Analysis of Caputo Impulsive Fractional Order Differential Equations with Applications
Abstract
We use Sadovskii′s fixed point method to investigate the existence and uniqueness of solutions of Caputo impulsive fractional differential equations of order α ∈ (0,1) with one example of impulsive logistic model and few other examples as well. We also discuss Caputo impulsive fractional differential equations with finite delay. The results proven are new and compliment the existing one.
1. Introduction
Dynamics of many evolutionary processes from various fields such as population dynamics, control theory, physics, biology, and medicine. undergo abrupt changes at certain moments of time like earthquake, harvesting, shock, and so forth. These perturbations can be well approximated as instantaneous change of states or impulses. These processes are modeled by impulsive differential equations. In 1960, Milman and Myšhkis introduced impulsive differential equations in their paper [1]. Based on their work, several monographs have been published by many authors like Samoilenko and Perestyuk [2], Lakshmikantham et al. [3], Bainov and Simeonov [4, 5], Bainov and Covachev [6], and Benchohra et al. [7]. All the authors mentioned perviously have considered impulsive differential equations as ordinary differential equations coupled with impulsive effects. They considered the impulsive effects as difference equations being satisfied at impulses time. So, the solutions are piecewise continuous with discontinuities at impulses time. In the fields like biology, population dynamics, and so forth, problems with hereditary are best modeled by delay differential equations [8]. Problems associated with impulsive effects and hereditary property are modeled by impulsive delay differential equations.
The origin of fractional calculus (derivatives (dα/dtα)f and integrals Iαf of arbitrary order α > 0) goes back to Newton and Leibniz in the 17th century. In a letter correspondence with Leibniz, L’Hospital asked “What if the order of the derivative is 1/2”? Leibniz replied, “Thus it follows that will be equal to , an apparent paradox, from which one day useful consequences will be drawn.” This letter of Leibniz was in September 1695. So, 1695 is considered as the birthday of fractional calculus. Fractional order differential equations are generalizations of classical integer order differential equations and are increasingly used to model problems in fluid dynamics, finance, and other areas of application. Recent investigations have shown that sometimes physical systems can be modeled more accurately using fractional derivative formulations [9]. There are several excellent monographs available on this field [10–15]. In [11], the authors give a recent and up-to-date description of the developments of fractional differential and fractional integro-differential equations including applications. The existence and uniqueness of solutions to fractional differential equations has been considered by many authors [16–21]. Impulsive fractional differential equations represent a real framework for mathematical modeling to real world problems. Significant progress has been made in the theory of impulsive fractional differential equations [7, 22–24]. Xu et al. in their paper [25] have described an impulsive delay fishing model.
- (a)
the Caputo fractional derivative
(1) - (b)
the Riemann-Liouville fractional derivative
(2)
Definition 1. A solution of fractional differential equation (4) is a piecewise continuous function x ∈ PC([0, T], X) which satisfied (4).
Definition 2 ([29], Definition 11.1). Kuratowskii noncompactness measure: let M be a bounded set in metric space (X, d), then Kuratowskii noncompactness measure, μ(M) is defined as inf {ϵ : M covered by a finite many sets such that the diameter of each set ≤ϵ}.
Definition 3 ([29], Definition 11.6). Condensing map: Let Φ : X → X be a bounded and continuous operator on Banach space X such that μ(Φ(B)) < μ(B) for all bounded set B ⊂ D(Φ), where μ is the Kuratowskii noncompactness measure, then Φ is called condensing map.
Definition 4. Compact map: a map f : X → X is said to be compact if the image of every bounded subset of X under f is precompact (closure is compact).
Theorem 5 (see [30].)Let B be a convex, bounded, and closed subset of a Banach space X and let Φ : B → B be a condensing map. Then, Φ has a fixed point in B.
Lemma 6 ([29], Example 11.7). A map Φ = Φ1 + Φ2 : X → X is k-contraction with 0 ≤ k < 1 if
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(a) Φ1 is k-contraction, that is, ∥Φ1(x) − Φ1(y)∥X ≤ k∥x − y∥X;
-
(b) Φ2 is compact,
and hence Φ is a condensing map.
The structure of the paper is as follows. In Section 2, we prove the existence (Theorem 8) and uniqueness of solutions to (4). We show in Section 3 the existence and uniqueness of solutions for a general class of impulsive functional differential equations of fractional order α ∈ (0,1). In Section 4, we give some examples in favor of our sufficient conditions.
2. Impulsive Fractional Differential Equation
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A.1 f1 is bounded and Lipschitz, in particular, ∥f1(t, x)∥X ≤ M1(t) and ∥f1(t, x) − f1(t, y)∥X ≤ L1(t)∥x − y∥X for all (t, x), (t, y) ∈ R,
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A.2 f2 is compact and bounded, in particular, ∥f2(t, x)∥X ≤ M2(t) for all (t, x) ∈ R,
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A.3 Ik ∈ C(X, X) such that ∥Ik(x)∥X ≤ l1 and ∥Ik(x) − Ik(y)∥X ≤ l2∥x − y∥X,
Lemma 7 (Fečken et al. [24], Lemma 2). The initial value problem (4) is equivalent to the nonlinear integral equation
Theorem 8 (existence of solutions). Under the assumptions (A.1)–(A.3), the problem (4) has at least one solution in [0, T], provided that
Proof. Let Bλ be the closed bounded and convex subset of PC([0, T], X), where Bλ is defined as Bλ = {x : ∥x∥≤λ}, λ = max {λ0, λ1, …, λm}, and
Step 2 (F1 is continuous and γ-contraction). To prove the continuity of F1 for t ∈ [0, T], let us consider a sequence xn converging to x. Taking the norm of F1xn(t) − F1x(t), we have
Step 3 (F2 is compact). For 0 ≤ τ1 ≤ τ2 ≤ T, we have
Step 4 (F is condensing). As F = F1 + F2, F1 is continuous contraction, F2 is compact, so by using Lemma 6, F is condensing map on Br.
And hence by using the Theorem 5, we conclude that (4) has a solution in Br.
Theorem 9. If f is bounded and Lipschitz, in particular, for all (t, x), (t, y) ∈ R and , then the problem (4) has a unique solution in Bλ, provided that
3. Impulsive Fractional Differential Equations with Finite Delay
with sup-norm ∥·∥, defined by ∥x∥ = sup {∥x(t)∥X : t ∈ [−r, T]}.
Definition 10. A solution of fractional differential equation (21) is a piecewise continuous function x ∈ PC([−r, T], X) which satisfies (21).
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A.4 f ∈ C(I × 𝒞, X),
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A.5 f bounded, in particular, ∥f(t, ϕ)∥X ≤ M3(t) for all (t, ϕ) ∈ I × 𝒞,
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A.6 f is Lipschitz, in particular, ∥f(t, ϕ) − f(t, ψ)∥X ≤ L2(t) ∥ϕ − ψ∥r for all (t, ϕ), (t, ψ) ∈ I × 𝒞.
Lemma 11 (Fečken et al. [24], Lemma 2). The initial value problem (21) is equivalent to the nonlinear integral equation
Remark 12. Since history part/initial condition x(t) = ϕ(t), t ∈ [−r, 0], is known, so we will investigate the existence and uniqueness of solution in I = [0, T].
Theorem 13 (existence and uniqueness of solution). Under the assumptions (A.3)–(A.6), the problem (21) has a unique solution in [0, T], provided that
Proof. In this case, we define the operator F : PC(I, X) → PC(I, X) by
The proof is similar to the proof of continuity of F1 in Step 2 of Theorem 8, and hence we omit it.
Step 2 (F is continuous and γ2-contraction). The proof of this step is also similar to the proof of continuous and γ1-contraction of F1 in Step 2 of Theorem 8.
Now by applying Banach’s fixed point theorem, we get that the operator F has an unique fixed point in PC(I, X), and hence the problem (21) has a unique solution in PC([−r, T], X).
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A.3′ Ik bounded, in particular, ,
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A.5′ f bounded, in particular, ∥f(t, ϕ)∥X ≤ M4(t)(1 + ∥ϕ∥r) for all (t, ϕ) ∈ I × 𝒞.
Theorem 14. Under the assumptions (A.3′) and (A.5′), problem (21) has at least one solution.
Proof. We transform the problem into a fixed point problem. For this purpose, consider the operator F : PC(I, X) → PC(I, X) defined by
Step 2 (F maps bounded sets into bounded sets in PC(I, X)). It is enough to show that for any δ > 0, there exists an such that x ∈ Bδ = {x ∈ PC(I, X)∣∥x∥≤δ}, and we have ∥Fx∥≤l.
For t ∈ [0, T], we have
As a consequence of Steps 1–3 together with PC-type Arzela-Ascoli theorem (Fečken et al. [24], Theorem 2.11), the map F : PC(I, X) → PC(I, X) is completely continuous.
Step 4 (A priori bounds). Now, we prove that the set
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E(F) = {x ∈ PC(I, X)∣x = λFx, for some λ ∈ (0,1)} is bounded.
We observe that for t ∈ [0, T] and x ∈ E(F), x(t) = λFx(t) and
As a consequence of Schaefer’s fixed point theorem, the problem (21) has at least one solution in PC([−r, T], X).
Theorem 15. If f is bounded and Lipschitz, in particular, , for all (t, ϕ), (t, ψ) ∈ I × 𝒞 and , then problem (21) has a unique solution in PC([−r, T], X), provided that
Definition 16. A solution of fractional differential equation (32) is a piecewise continuous function x ∈ PC([−r, T], X) which satisfies (32).
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A.7 f ∈ C(I × X × 𝒞, X),
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A.8 f bounded, in particular, ∥f(t, x, ϕ)∥X ≤ M5(t) for all (t, x, ϕ) ∈ I × X × 𝒞,
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A.9 f is Lipschitz, in particular, ∥f(t, x, ϕ) − f(t, y, ψ)∥X ≤ L3(t)∥x − y∥X + L4(t)∥ϕ − ψ∥r for all (t, x, ϕ), (t, y, ψ) ∈ I × X × 𝒞,
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A.8′ ∥f(t, x, ϕ)∥X ≤ M6(t)(1 + ∥x∥X + ∥ϕ∥r).
Lemma 17 (Fečken et al. [24], Lemma 2). The initial value problem (32) is equivalent to the following nonlinear integral equation
Theorem 18. Under the assumptions (A.3), (A.7), (A.8), and (A.9), the problem (32) has a unique solution, provided that
Proof. The proof is similar to the proof of Theorem 13.
Theorem 19. Under the assumptions (A.3′), and (A.8′), the problem (32) has at least one solution in PC([−r, T], X).
Proof. The proof is similar to the proof of Theorem 14.
Theorem 20. If f is bounded and Lipschitz, in particular, , for all (t, x, ϕ), (t, y, ψ) ∈ I × X × 𝒞 and , then the problem (32) has a unique solution in PC([−r, T], X), provided that
4. Examples
Example 21 (fractional impulsive logistic equation). Consider the following class of fractional logistic equations in banach space X with norm ∥·∥X:
Lemma 22 (Fečken et al. [24], Lemma 2). The initial value problem (36) is equivalent to the nonlinear integral equation
We give some more examples which are inspired by [24].
Example 23. Consider the following Caputo impulsive delay fractional differential equations
Set C1 = C([−r, 0], ℝ+), f(t, ϕ) = e−νtϕ/((1 + et)(1 + ϕ)), (t, ϕ)∈[0,1] × C1.
For t ∈ [0,1] and some p ∈ (0, α), L*(t) = m1(t) = e−νt/2 ∈ L1/p([0,1], ℝ+) with , we can arrive at the inequality . We can see that all the assumptions of Theorem 13 are satisfied, and hence the problem (41) has a unique solution in [0,1].
Example 24. Consider the following Caputo impulsive delay fractional differential equation
Set C2 = C([0,1], ℝ+), f(t, ϕ) = ϕ/((1 + et)(1 + ϕ)), (t, ϕ)∈[−r, 0] × C2.
For t ∈ [0,1] and some p ∈ (0, α), m2(t) = e−t/4 ∈ L1/p([0,1], ℝ+) with , we can arrive at the inequality 1/4 + cM/Γ(α) < 1. We can see that all the assumptions of Theorem 14 are satisfied, and hence the problem (44) has a solution in [0,1].
Acknowledgments
The authors are thankful to the anonymous reviewer for his/her valuable comments and suggestions. S. Abbas acknowledge “Erasmus Mundus Lot13, India 4EU” for providing him fellowship to visit University of Bologna, Italy. He also acknowledges Professor Stefan Siegmund, Center for Dynamics of TU Dresden, Germany, for his fruitful discussion on this topic and for hosting his short visit.