Hindawi Publishing Corporation Abstract and Applied Analysis Volume 2012, Article ID 387629, 15 pages doi:10.1155/2012/387629 Research Article Some Existence Results for Impulsive Nonlinear Fractional Differential Equations with Closed Boundary Conditions Hilmi Ergören1 and Adem Kiliçman2 1 2 Department of Mathematics, Faculty of Sciences, Yuzuncu Yil University, 65080 Van, Turkey Department of Mathematics and Institute for Mathematical Research, University Putra Malaysia, 43400 Serdang, Malaysia Correspondence should be addressed to Adem Kiliçman, akilicman@putra.upm.edu.my Received 30 September 2012; Accepted 22 October 2012 Academic Editor: Beata Rzepka Copyright q 2012 H. Ergören and A. Kiliçman. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We investigate some existence results for the solutions to impulsive fractional differential equations having closed boundary conditions. Our results are based on contracting mapping principle and Burton-Kirk fixed point theorem. 1. Introduction This paper considers the existence and uniqueness of the solutions to the closed boundary value problem BVP, for the following impulsive fractional differential equation: Dα xt ft, xt, t ∈ J : 0, T , t / tk , 1 < α ≤ 2, − Δxtk Ik x tk , Δx tk Ik∗ x t−k , k 1, 2, . . . , p, C xT ax0 bT x 0, 1.1 T x T cx0 dT x 0, α where C D is Caputo fractional derivative, f ∈ CJ × R, R, Ik , Ik∗ ∈ CR, R, Δxtk x tk − x t−k , 1.2 2 Abstract and Applied Analysis with x tk lim xtk h, h→0 x t−k lim− xtk h, h→0 1.3 and Δx tk has a similar meaning for x t, where 0 t0 < t1 < t2 < · · · < tp < tp1 T, 1.4 a, b, c, and d are real constants with Δ : c1 − b 1 − a1 − d / 0. The boundary value problems for nonlinear fractional differential equations have been addressed by several researchers during last decades. That is why, the fractional derivatives serve an excellent tool for the description of hereditary properties of various materials and processes. Actually, fractional differential equations arise in many engineering and scientific disciplines such as, physics, chemistry, biology, electrochemistry, electromagnetic, control theory, economics, signal and image processing, aerodynamics, and porous media see 1– 7. For some recent development, see, for example, 8–14. On the other hand, theory of impulsive differential equations for integer order has become important and found its extensive applications in mathematical modeling of phenomena and practical situations in both physical and social sciences in recent years. One can see a noticeable development in impulsive theory. For instance, for the general theory and applications of impulsive differential equations we refer the readers to 15–17. Moreover, boundary value problems for impulsive fractional differential equations have been studied by some authors see 18–20 and references therein. However, to the best of our knowledge, there is no study considering closed boundary value problems for impulsive fractional differential equations. Here, we notice that the closed boundary conditions in 1.1 include quasi-periodic boundary conditions b c 0 and interpolate between periodic a d 1, b c 0 and antiperiodic a d −1, b c 0 boundary conditions. Motivated by the mentioned recent work above, in this study, we investigate the existence and uniqueness of solutions to the closed boundary value problem for impulsive fractional differential equation 1.1. Throughout this paper, in Section 2, we present some notations and preliminary results about fractional calculus and differential equations to be used in the following sections. In Section 3, we discuss some existence and uniqueness results for solutions of BVP 1.1, that is, the first one is based on Banach’s fixed point theorem, the second one is based on the Burton-Kirk fixed point theorem. At the end, we give an illustrative example for our results. 2. Preliminaries Let us set J0 0, t1 , J1 t1 , t2 , . . . , Jk−1 tk−1 , tk , Jk tk , tk1 , J : 0, T \ {t1 , t2 , . . . , tp } and introduce the set of functions: P CJ, R {x : J → R : x ∈ Ctk , tk1 , R, k 0, 1, 2, . . . , p and there exist xtk and − xtk , k 1, 2, . . . , p with xt−k xtk } and P C1 J, R {x ∈ P CJ, R, x ∈ Ctk , tk1 , R, k 0, 1, 2, . . . , p and there exist x tk − and x tk , k 1, 2, . . . , p with x t−k x tk } which is a Banach space with the norm x supt∈J {xP C , x P C } where xP C : sup{|xt| : t ∈ J}. The following definitions and lemmas were given in 4. Abstract and Applied Analysis 3 Definition 2.1. The fractional arbitrary order integral of the function h ∈ L1 J, R of order α ∈ R is defined by I0α ht 1 Γα t 2.1 t − sα−1 hsds, 0 where Γ· is the Euler gamma function. Definition 2.2. For a function h given on the interval J, Caputo fractional derivative of order α > 0 is defined by C α D0 ht 1 Γn − α t t − sn−α−1 hn sds, n α 1, 2.2 0 where the function ht has absolutely continuous derivatives up to order n − 1. Lemma 2.3. Let α > 0, then the differential equation C Dα ht 0 2.3 has solutions ht c0 c1 t c2 t2 · · · cn−1 tn−1 , ci ∈ R, i 0, 1, 2, . . . , n − 1, n α 1. 2.4 The following lemma was given in 4, 10. Lemma 2.4. Let α > 0, then α I αC D ht ht c0 c1 t c2 t2 · · · cn−1 tn−1 , 2.5 for some ci ∈ R, i 0, 1, 2, . . . , n − 1, n α 1. The following theorem is known as Burton-Kirk fixed point theorem and proved in 21. Theorem 2.5. Let X be a Banach space and A, D : X → X two operators satisfying: a A is a contraction, and b D is completely continuous. Then either i the operator equation x Ax Dx has a solution, or ii the set ε {x ∈ X : x λAx/λ λDx} is unbounded for λ ∈ 0, 1. Theorem 2.6 see 22, Banach’s fixed point theorem. Let S be a nonempty closed subset of a Banach space X, then any contraction mapping T of S into itself has a unique fixed point. 4 Abstract and Applied Analysis Next we prove the following lemma. Lemma 2.7. Let 1 < α ≤ 2 and let h : J → R be continuous. A function xt is a solution of the fractional integral equation: ⎧ t t − sα−1 ⎪ ⎪ hsds Ω1 tΛ1 Ω2 tΛ2 , t ∈ J0 ⎪ ⎪ ⎪ Γα ⎪ 0 ⎪ ⎪ ⎪ t ⎪ α−1 k ti ⎪ ti − sα−1 ⎪ ⎪ t − s hsds 1 Ω1 t hsds ⎪ ⎪ ⎪ Γα Γα ⎪ tk i1 ti−1 ⎪ ⎪ ⎪ ⎪ k ti ⎪ ti − sα−2 ⎪ ⎪ ⎪ hsds T − t Ω − t Ω t t t k 1 2 k ⎪ ⎪ ⎪ ti−1 Γα − 1 i1 ⎪ ⎪ ⎪ ti T ⎨ k−1 ti − sα−2 T − sα−1 xt hsds Ω1 t hsds 1 Ω1 t t k − ti ⎪ Γα ⎪ tk ti−1 Γα − 1 i1 ⎪ ⎪ ⎪ T ⎪ k ⎪ − − ⎪ T − sα−2 ∗ ⎪ ⎪ hsds 1 Ω1 t Ω2 t I i x ti I k x tk ⎪ ⎪ ⎪ tk Γα − 1 ⎪ i1 ⎪ ⎪ ⎪ k−1 k−1 ⎪ − ⎪ ⎪ ∗ ⎪ Ω1 t Ii∗ x t−i − t x t Ω T I t ⎪ i 2 i i ⎪ ⎪ ⎪ i1 i1 ⎪ ⎪ k−1 ⎪ ⎪ − ⎪ ∗ ⎪ t − ti Ii x ti , t ∈ Jk , k 1, 2, . . . , p ⎩ i1 2.6 if and only if xt is a solution of the fractional BVP Dα xt ht, t ∈ J , Δxtk Ik x t−k , Δx tk Ik∗ x t−k , C xT ax0 bT x 0, T x T cx0 dT x 0, where Λ1 : T tk k T − sα−1 hsds Γα i1 k i1 T − tk ti ti−1 ti ti−1 ti − sα−1 hsds Γα k−1 ti − sα−2 hsds tk − ti Γα − 1 i1 ti ti−1 k k−1 Ii x t−i T − ti Ii∗ x t−i Ik∗ x t−k , i1 i1 ti − sα−2 hsds Γα − 1 2.7 Abstract and Applied Analysis Λ2 : T tk 5 k T − sα−2 hsds Γα − 1 i1 ti Ω1 t : − Ω2 t : ti−1 k−1 ti − sα−2 hsds Ii∗ x t−i , Γα − 1 i1 ct 1 − d − , Δ TΔ 1 − bT 1 − at − . Δ Δ 2.8 Proof. Let x be the solution of 2.7. If t ∈ J0 , then Lemma 2.4 implies that xt I ht − c0 − c1 t α x t t 0 t 0 t − sα−1 hsds − c0 − c1 t, Γα t − sα−2 hsds − c1 , Γα − 1 2.9 for some c0 , c1 ∈ R. If t ∈ J1 , then Lemma 2.4 implies that t t − sα−1 hsds − d0 − d1 t − t1 , Γα t1 t t − sα−2 hsds − d1 , x t t1 Γα − 1 xt 2.10 for some d0 , d1 ∈ R. Thus we have t1 t1 − sα−1 hsds − c0 − c1 t1 , x t1 −d0 , Γα t0 t1 t1 − sα−2 hsds − c1 , x t1 −d1 . x t−1 Γα − 1 t0 x t−1 2.11 Observing that Δxt1 x t1 − x t−1 I1 x t−1 , Δx t1 x t1 − x t−1 I1∗ x t−1 , 2.12 6 Abstract and Applied Analysis then we have t1 t1 − sα−1 hsds − c0 − c1 t1 I1 x t−1 , Γα t0 t1 t1 − sα−2 −d1 hsds − c1 I1∗ x t−1 , Γα − 1 t0 −d0 2.13 hence, for t ∈ t1 , t2 , t1 t − sα−1 t1 − sα−1 hsds hsds Γα Γα t1 t0 t1 t1 − sα−2 hsds I1 x t−1 t − t1 I1∗ x t−1 − c0 − c1 t, t − t1 t0 Γα − 1 t1 t t − sα−2 t1 − sα−2 hsds hsds I1∗ x t−1 − c1 . x t t1 Γα − 1 t0 Γα − 1 xt t 2.14 If t ∈ J2 , then Lemma 2.4 implies that t t − sα−1 hsds − e0 − e1 t − t2 , Γα t2 t t − sα−2 hsds − e1 , x t t2 Γα − 1 xt 2.15 for some e0 , e1 ∈ R. Thus we have x t−2 t2 t1 t2 − sα−1 t1 − sα−1 hsds hsds Γα Γα t1 t0 t1 t1 − sα−2 hsds I1 x t−1 t2 − t1 I1∗ x t−1 − c0 − c1 t2 , t2 − t1 t0 Γα − 1 x t2 − e0 , t1 t2 t2 − sα−2 t1 − sα−2 − hsds hsds I1∗ x t−1 − c1 , x t2 Γα − 1 Γα − 1 t1 t0 x t2 − e 1 . 2.16 Similarly we observe that Δxt2 x t2 − x t−2 I2 x t−2 , Δx t2 x t2 − x t−2 I2∗ x t−2 , 2.17 Abstract and Applied Analysis 7 thus we have −e0 x t−2 I2 x t−2 , −e1 x t−2 I2∗ x t−2 . 2.18 Hence, for t ∈ t2 , t3 , xt t1 t2 t − sα−1 t1 − sα−1 t2 − sα−1 hsds hsds hsds Γα Γα Γα t2 t0 t1 t1 t1 − sα−2 hsds t2 − t1 t0 Γα − 1 t t2 1 t1 − sα−1 t2 − sα−1 t − t2 hsds hsds t0 Γα − 1 t1 Γα − 1 t 2.19 I1 x t−1 I2 x t−2 t − t1 I1∗ x t−1 I2∗ x t−2 − c0 − c1 t. By a similar process, if t ∈ Jk , then again from Lemma 2.4 we get ⎧ t k ti ⎪ t − sα−1 ti − sα−1 ⎪ ⎪ hsds hsds ⎪ ⎪ ⎪ Γα Γα ⎪ tk i1 ti−1 ⎪ ⎪ ⎪ ti ti ⎨ k k−1 ti − sα−2 ti − sα−2 xt hsds hsds t − tk tk − ti ⎪ ⎪ ti−1 Γα − 1 ti−1 Γα − 1 i1 i1 ⎪ ⎪ ⎪ ⎪ k k−1 ⎪ ⎪ ⎪ ⎪ Ii x t−i t − ti Ii∗ x t−i Ik∗ x t−k − c0 − c1 t, ⎩ x t i1 t tk 2.20 i1 k t − sα−2 hsds Γα − 1 i1 ti ti−1 k−1 ti − sα−2 hsds Ii∗ x t−i − c1 . Γα − 1 i1 Now if we apply the conditions: xT ax0 bT x 0, T x T cx0 dT x 0, 2.21 we have Λ1 1 − ac0 T 1 − bc1 , c Λ2 − c0 1 − dc1 , T −c0 − 1 − dΛ1 1 − bT Λ2 , Δ Δ −c1 − cΛ1 1 − aΛ2 − . TΔ Δ 2.22 8 Abstract and Applied Analysis In view of the relations 2.8, when the values of −c0 and −c1 are replaced in 2.9 and 2.20, the integral equation 2.7 is obtained. Conversely, assume that x satisfies the impulsive fractional integral equation 2.6, then by direct computation, it can be seen that the solution given by 2.6 satisfies 2.7. The proof is complete. 3. Main Results Definition 3.1. A function x ∈ P C1 J, R with its α-derivative existing on J is said to be α a solution of 1.1, if x satisfies the equation C D xt ft, xt on J and satisfies the conditions: Δxtk Ik x t−k , Δx tk Ik∗ x t−k , xT ax0 bT x 0, 3.1 T x T cx0 dT x 0. For the sake of convenience, we define Ω∗1 sup|Ω1 t|, Ω∗2 sup|Ω2 t|, t∈J t∈J Ω∗∗ 1 sup Ω1 t , t∈J Ω∗∗ 2 sup Ω2 t . t∈J 3.2 The followings are main results of this paper. Theorem 3.2. Assume that A1 the function f : J × R → R is continuous and there exists a constant L1 > 0 such that ft, u − ft, v ≤ L1 u − v, for all t ∈ J, and u, v ∈ R, A2 Ik , Ik∗ : R → R are continuous, and there exist constants L2 > 0 and L3 > 0 such that Ik u − Ik v ≤ L2 u − v, Ik∗ u − Ik∗ v ≤ L3 u − v for each u, v ∈ R and k 1, 2, . . . , p. Moreover, consider the following: L1 T α−1 ∗ L1 T α Ω 1p 1 Ω∗1 1 p 2pα Γα 1 Γα 2 ∗ ∗ ∗ 1 Ω1 pL2 L3 Ω1 T Ω2 T pL3 < 1. Then, BVP 1.1 has a unique solution on J. Proof. Define an operator F : P C1 J, R → P C1 J, R by Fxt t tk k t − sα−1 fs, xsds 1 Ω1 t Γα i1 T − tk Ω1 t Ω2 t t − tk ti ti−1 ti − sα−1 fs, xsds Γα k ti ti − sα−2 fs, xsds Γα − 1 i1 ti−1 3.3 Abstract and Applied Analysis 1 Ω1 t k−1 9 tk − ti ti ti−1 i1 Ω1 t T tk Ω2 t T tk Ω1 t k−1 ti − sα−2 fs, xsds Γα − 1 T − sα−1 fs, xsds Γα k − − T − sα−2 ∗ fs, xsds 1 Ω1 t I i x ti I k x tk Γα − 1 i1 k−1 k−1 Ii∗ x t−i T − ti Ii∗ x t−i Ω2 t t − ti Ii∗ x t−i . i1 i1 i1 3.4 Now, for x, y ∈ P CJ, R and for each t ∈ J, we obtain Fxt − Fy t ≤ t tk t − sα−1 fs, xs − f s, ys ds Γα |1 Ω1 t| k ti ti − sα−1 fs, xs − f s, ys ds Γα i1 ti−1 |T − tk Ω1 t Ω2 t t − tk | k ti ti − sα−2 fs, xs − f s, ys ds × Γα − 1 i1 ti−1 |1 Ω1 t| k−1 tk − ti ti ti−1 i1 |Ω1 t| T tk ti − sα−2 fs, xs − f s, ys ds Γα − 1 T − sα−1 fs, xs − f s, ys ds Γα T T − sα−2 fs, xs − f s, ys ds tk Γα − 1 k − − ∗ − − ∗ Ii x t |1 Ω1 t| − Ii y t I x t −I y t |Ω2 t| i i i1 |Ω1 t| k−1 T − ti Ii∗ x t−i − Ii∗ y t−i i1 |Ω2 t| k−1 I ∗ x t− − I ∗ y t− i i i i i1 k−1 i1 |t − ti |Ii∗ x t−i − Ii∗ y t−i , k k k k 10 Abstract and Applied Analysis Fxt − Fy t ≤ L1 T α−1 ∗ L1 T α 1 Ω∗1 1 p 2pα Ω 1p Γα 1 Γα 2 ∗ ∗ ∗ 1 Ω pL2 L3 Ω T Ω T pL3 xs − ys. 1 2 1 3.5 Therefore, by 3.3, the operator F is a contraction mapping. In a consequence of Banach’s fixed theorem, the BVP 1.1 has a unique solution. Now, our second result relies on the Burton-Kirk fixed point theorem. Theorem 3.3. Assume that (A1)-(A2) hold, and A3 there exist constants M1 > 0, M2 > 0, M3 > 0 such that ft, u ≤ M1 , Ik u ≤ M2 , Ik∗ u ≤ M3 for each u, v ∈ R and k 1, 2, . . . , p. Then the BVP 1.1 has at least one solution on J. Proof. We define the operators A, D : P C1 J, R → P C1 J, R by k − − ∗ I i x ti I k x tk Axt 1 Ω1 t i1 Ω1 t k−1 k−1 k−1 Ii∗ x t−i T − ti Ii∗ x t−i Ω2 t t − ti Ii∗ x t−i , i1 Dxt t tk i1 t − sα−1 fs, xsds 1 Ω1 t Γα T − tk Ω1 t Ω2 t t − tk k−1 tk − ti ti−1 i1 Ω1 t T tk ti ti−1 i1 k ti i1 1 Ω1 t i1 k ti ti−1 ti − sα−1 fs, xsds Γα 3.6 ti − sα−2 fs, xsds Γα − 1 ti − sα−2 fs, xsds Γα − 1 T − sα−1 fs, xsds Ω2 t Γα T tk T − sα−2 fs, xsds. Γα − 1 It is obvious that A is contraction mapping for 1 Ω∗1 pL2 L3 Ω∗1 T Ω∗2 T pL3 < 1. 3.7 Now, in order to check that D is completely continuous, let us follow the sequence of the following steps. Abstract and Applied Analysis 11 Step 1 D is continuous. Let {xn } be a sequence such that xn → x in P CJ, R. Then for t ∈ J, we have |Dxn t − Dxt| ≤ t tk t − sα−1 fs, xn s − fs, xsds Γα k ti ti − sα−1 |1 Ω1 t| fs, xn s − fs, xsds Γα i1 ti−1 |T − tk Ω1 t Ω2 t t − tk | k ti ti − sα−2 fs, xn s − fs, xsds × Γα − 1 i1 ti−1 k−1 |1 Ω1 t| tk − ti ti−1 i1 |Ω1 t| T tk |Ω2 t| T tk ti ti − sα−2 fs, xn s − fs, xsds Γα − 1 T − sα−1 fs, xn s − fs, xsds Γα T − sα−2 fs, xn s − fs, xsds. Γα − 1 3.8 Since f is continuous function, we get Dxn t − Dxt −→ 0 as n −→ ∞. 3.9 Step 2 D maps bounded sets into bounded sets in P CJ, R. Indeed, it is enough to show that for any r > 0, there exists a positive constant l such that for each x ∈ Br {x ∈ P CJ, R : x ≤ r}, we have Dx ≤ l. By A3, we have for each t ∈ J, |Dxt| ≤ t tk t − sα−1 fs, xsds Γα |1 Ω1 t| k ti ti − sα−1 fs, xsds Γα i1 ti−1 |T − tk Ω1 t Ω2 t t − tk | |1 Ω1 t| k−1 i1 tk − ti ti ti−1 k ti ti − sα−2 fs, xsds Γα − 1 i1 ti−1 ti − sα−2 fs, xsds Γα − 1 12 Abstract and Applied Analysis |Ω1 t| T tk Dx ≤ T − sα−1 fs, xsds |Ω2 t| Γα T tk T − sα−2 fs, xsds, Γα − 1 M1 T α−1 ∗ M1 T α Ω2 1 p : l. 1 Ω∗1 1 p 2pα Γα 1 Γα 3.10 Step 3 D maps bounded sets into equicontinuous sets in P C1 J, R. Let τ1 , τ2 ∈ Jk , 0 ≤ k ≤ p with τ1 < τ2 and let Br be a bounded set of P C1 J, R as in Step 2, and let x ∈ Br . Then Dy τ2 − Dτ1 ≤ τ2 Dy sds ≤ Lτ2 − τ1 , 3.11 τ1 where Dx t ≤ t tk k t − sα−2 fs, xsds Ω1 t Γα − 1 i1 k T − tk Ω1 t Ω2 t 1 i1 k−1 Ω1 t tk − ti i1 Ω1 t ≤ T tk ti ti−1 ti ti−1 ti ti−1 ti − sα−1 fs, xsds Γα ti − sα−2 fs, xsds Γα − 1 ti − sα−2 fs, xsds Γα − 1 T − sα−1 fs, xsds Ω2 t Γα T tk T − sα−2 fs, xsds, Γα − 1 M1 T α−1 M1 T α ∗ Ω1 1 p 2pα 1 Ω∗2 1 p : L. Γα 1 Γα 3.12 This implies that A is equicontinuous on all the subintervals Jk , k 0, 1, 2, . . . , p. Therefore, by the Arzela-Ascoli Theorem, the operator D : P C1 J, R → P C1 J, R is completely continuous. To conclude the existence of a fixed point of the operator A D, it remains to show that the set x λDx for some λ ∈ 0, 1 ε x ∈ X : x λA λ 3.13 is bounded. Let x ∈ ε, for each t ∈ J, x xt λDxt λA λ t. 3.14 Abstract and Applied Analysis 13 Hence, from A3, we have |xt| ≤ λ t tk k t − sα−1 fs, xsds λ|1 Ω1 t| Γα i1 λ|T − tk Ω1 t Ω2 t t − tk | k ti i1 λ|1 Ω1 t| k−1 tk − ti ti ti−1 i1 ti−1 ti ti−1 ti − sα−1 fs, xsds Γα ti − sα−2 fs, xsds Γα − 1 ti − sα−2 fs, xsds Γα − 1 T T T − sα−1 T − sα−2 λ|Ω1 t| fs, xs ds λ|Ω2 t| fs, xsds Γα Γα − 1 tk tk − − k x ti ∗ x tk λ|1 Ω1 t| Ii Ik 3.15 λ λ i1 λ|Ω1 t| k−1 i1 − x ti T − ti Ii∗ λ − − k−1 k−1 x ti ∗ x ti λ|Ω2 t| t − ti Ii∗ Ii λ , λ λ i1 i1 xt ≤ M1 T α−1 ∗ M1 T α Ω2 1 p 1 Ω∗1 1 p 2pα Γα 1 Γα ∗ ∗ ∗ 1 Ω1 pM2 M3 Ω1 T Ω2 T pM3 . Consequently, we conclude the result of our theorem based on the Burton-Kirk fixed point theorem. 4. An Example Consider the following impulsive fractional boundary value problem: sin 2t|xt| 1 t ∈ 0, 1, t / , 3 t 5 1 |xt| 1 |x1− /3| |x 1− /3| 1 Δx , Δx , − 3 15 |x1 /3| 3 10 |x 1− /3| C D 3/2 xt 2 x1 5x0 − 2x 0, , x 1 x0 4x 0. 4.1 14 Abstract and Applied Analysis Here, a 5, b −2, c 1, d 4, α 3/2, T 1, p 1. Obviously, L1 1/25, L2 1/15, L3 1/10, Ω∗1 4/15, Ω∗2 7/15. Further, L1 T α−1 ∗ L1 T α Ω2 1 p Ω∗1 T Ω∗2 T pL3 1 Ω∗1 1 p1 L2 2pα L3 Γα 1 Γα 4.2 464 173 < 1. √ 1125 π 450 Since the assumptions of Theorem 3.2 are satisfied, the closed boundary value problem 4.1 has a unique solution on 0, 1. Moreover, it is easy to check the conclusion of Theorem 3.3. Acknowledgments The authors would like to express their sincere thanks and gratitude to the reviewers for their valuable comments and suggestions for the improvement of this paper. 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