Hindawi Publishing Corporation Advances in Difference Equations Volume 2011, Article ID 296353, 13 pages doi:10.1155/2011/296353 Research Article Employing of Some Basic Theory for Class of Fractional Differential Equations Azizollah Babakhani1 and Dumitru Baleanu2, 3 1 Faculty of Basic Science, Babol University of Technology, Babol 47148-71167, Iran Department of Mathematics and Computer Science, Cankaya University, 06530 Ankara, Turkey 3 Institute of Space Sciences, P.O. Box MG-23, Magurele, 077125 Bucharest, Romania 2 Correspondence should be addressed to Azizollah Babakhani, babakhani@nit.ac.ir and Dumitru Baleanu, dumitru@cankaya.edu.tr Received 19 September 2010; Accepted 15 November 2010 Academic Editor: J. J. Trujillo Copyright q 2011 A. Babakhani and D. Baleanu. 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. Basic theory on a class of initial value problem of some fractional differential equation involving Riemann-Liouville differential operators is discussed by employing the classical approach from the work of Lakshmikantham and A. S. Vatsala 2008. The theory of inequalities, local existence, extremal solutions, comparison result and global existence of solutions are considered. Our work employed recent literature from the work of Lakshmikantham and A. S. Vatsala, 2008. 1. Introduction Differential equations of fractional order have recently proven to be valuable tools in the modeling of many phenomena in various fields of science and engineering. Indeed, we can find numerous applications in viscoelasticity, electrochemistry, control, porous media, electromagnetism, and so forth 1–5. There has been a significant development in the study of fractional differential equations and inclusions in recent years; see the monographs of Kilbas et al. 6, Lakshmikantham et al. 7, Podlubny 4, and the survey by Agarwal et al. 8. For some recent contributions on fractional differential equations, see 9–20 and the references therein. Very recently in 10, 11, 21, 22, the author and other researchers studied the existence and uniqueness of solutions of some classes of fractional differential equations with delay. For more details on the geometric and physical interpretation for fractional derivatives of both the Caputo types see 5, 23. Băleanu and Mustafa 16 allowed for immediate applications of a general comparison-type result from the recent literature 24. 2 Advances in Difference Equations Lakshmikantham and Vatsal have discussed basic theory of fractional differential equations for initial value problem fractional differential equations type 24 Dα xt ft, xt, x0 x0 , t ∈ 0, T . 1.1 This paper deals with the basic theory of initial value problem IVP for a generalized class of fractional order differential equation of the form LDxt − x0 ft, xt, x0 x0 , t ∈ 0, T , 1.2 β where LD D0α − tn D0 , f ∈ C0, T , R and 0 < β ≤ α < 1. Recall that C0, T , R is the Banach space of continuous functions from the interval 0, T into R endowed with uniform norm. We begin in this section with the recall of some definitions and results for fractional calculus which are used throughout this paper 4, 19, 20. The left-sided Riemann-Liouville fractional integral of a function x of order α > 0 is defined as I0α xt 1 Γα t t − sα−1 xsds, t > 0, 1.3 0 and the left sided Riemann-Liouville fractional derivative operator of order 0 < α < 1 is defined by D0α xt d 1−α I0 xt , dt 1.4 ∞ where Γα 0 e−t tα−1 dt. We denote D0α xt by D0α xt and I0α xt by I0α xt. Also Dα xt and I α xt refer to D0α xt and I0α xt, respectively. β Assume that 0 < β < 1, β ≤ α, if the fractional derivative D0 xt is integrable, then 4, page 72 β 1−β α−β I0α D0 xt I0 xt − I0 xt tα−1 . t0 Γα 1.5 β If xt is continuous on 0, T then D0 xt is integrable, I 1−β xt|t0 0 25, and 1.5 reduces to β α−β I0α D0 xt I0 xt, 0 < β < 1, β ≤ α. Proposition 1.1. Let x ∈ C0, T , R and 0 < β < 1, β ≤ α. Then nk0 k n αk n−k i I α tn xt nk0 −α xt nk0 −α /n − k!I αk xt, k D t I k n!t n−k /n − k!I α−βk xt, ii I α tn Dβ xt nk0 −α k n!t 1.6 Advances in Difference Equations 3 where n is a nonnegative integer, and −α k −1k Γα k . k! Γα 1.7 Proof. i can be found in 19, page 53 and ii is an immediate consequence of 1.6 and i. 2. Strict and Nonstrict Inequalities Since f is assumed to be continuous, the initial value problem 1.2 is equivalent to the following Volterra fractional integral 10, 25: n −α n!tn−k α−βk xt x0 I {xt − x0} I α ft, xt − k! n k k0 x0 n −α k k0 1 Γα t n!tn−k n − k! Γ α − β k t − sα−1 fs, xsds, t t − sα−βk−1 xsds 2.1 0 0 ≤ t ≤ T. 0 Note. Let us consider the notation of Ixt for second term in 2.1 which is used in throughout of text and so that n −α n!tn−k α−βk I Ixt {xt − x0} k n − k! k0 n −α k0 k n!tn−k n − k! Γ α − β k t 2.2 t − sα−βk−1 xsds. 0 We now first discuss a fundamental result relative to the fractional inequalities. Theorem 2.1. Let v, w ∈ C0, T , R, f ∈ C0, T × R, R, and t i vt ≤ v0 Ivt 1/Γα 0 t − sα−1 fs, vsds, t ii wt ≥ w0 Iwt 1/Γα 0 t − sα−1 fs, wsds, one of the foregoing inequalities being strict. Moreover, if ft, x is nondecreasing in x for each t and v0 < w0, then one has vt < wt, t ∈ 0, T . Proof. Suppose that for each t ∈ 0, T , the conclusion vt < wt is not true. Then, because of the continuity of the functions involved and v0 < w0 it follows that there exists a t1 such that 0 < t1 ≤ T and vt1 wt1 , vt < wt, 0 < t ≤ t1 . 2.3 4 Advances in Difference Equations Let us suppose that the inequality ii is strict. Then using the nondecreasing nature of f and 2.3 we get t1 1 t − sα−1 fs, wsds Γα 0 t1 1 ≥ v0 Ivt1 t − sα−1 fs, vsds ≥ vt1 , Γα 0 wt1 > w0 Iwt1 2.4 which is a contradiction in view of 2.3. Hence the conclusion of this theorem holds and the proof is complete. The next result is for nonstrict inequalities, which require a one-sided Lipschitz type condition. Theorem 2.2. Assume that the conditions of Theorem 2.1 hold with nonstrict inequalities (i) and (ii). Suppose further that ft, x − f t, y ≤ L x−y , 2 1t 2.5 whenever x ≥ y and L > 0. Then, v0 ≤ w0, and L < Γα 1 implies vt ≤ wt, 0 ≤ t ≤ T. 2.6 Proof. Set w t wt 1 tα , for small > 0 so that we have, w 0 w0 > w0, w t > wt, 0 ≤ t ≤ T. 2.7 Now, n −α n!tn−k α−βk I w t ≥ w0 {w t − ω0} k n − k! k0 1 Γα t 2.8 t − sα−1 fs, wsds 1 tα . 0 In view of 2.7, using one-sided Lipschitz condition 2.5 we see that t L1 sα ds tα t − sα−1 fs, w s − 1 sα 0 2.9 t 1 L t α−1 α−1 w 0 Iw t t − s fs, w sds − t − s ds tα . Γα 0 Γα 0 w t ≥ w 0 Iw t 1 Γα Advances in Difference Equations Now, since t 0 t − sα−1 ds tα 1 0 5 1 − τα−1 dτ Γα/Γα 1tα , we arrive at w t > w 0 Iw t 1 Γα t t − sα−1 fs, w sds, 2.10 0 in view of the condition L < Γα 1. We now apply Theorem 2.1 to the inequalities i, 2.9, and 2.10 to get vt < w t, t ∈ 0, T . Since > 0 is arbitrary, we conclude that 2.6 is true, and we are done. 3. Local Existence and Extremal Conditions In this section, we will consider the local existence and the existence of extremal solutions for the IVP 1.2. We now first discuss Peano’s type existence result. Theorem 3.1. Assume that f ∈ Ω0 , R, where Ω0 {t, x : 0 ≤ t ≤ r and |x − x0 | ≤ b}, and let |ft, x| ≤ M on Ω. Then the IVP 1.2 has at least one solution xt on 0, λ, where 1/α−βk b Γα 1 1/α b Γα − β k 1 λ min r, , , k 0, n , n 2 M n 2 ρ b 3.1 so that ρ and x0 will be observing in the proof of this theorem. Proof. Let x0 t be a continuous function on −δ, 0, δ > 0 such that x0 0 x0 , |x0 t − x0 | ≤ b. For ≤ δ, we define a function x t x0 t on −δ, 0 and x t x0 Ix t − I α ft, x t − 3.2 on 0, λ1 , where λ1 min{λ, }. We observe that t n −α n−k 1 n!t α−βk I − x − − x ≤ t t t − sα−1 fs, x s − ds, |x |x 0| 0| Γα − k! n k 0 k0 t n 1 ≤ ρ I α−βk |x t − − x0 | t − sα−1 fs, x s − ds, Γα 0 k0 3.3 where −α n!μn−k −α n! 1 ρ max , : k 0, n . k n − k! k n − k! 3.4 6 Advances in Difference Equations Note that, 0 < s ≤ t ≤ λ1 ≤ , as λ1 min{λ, }. Then −δ ≤ − < s − ≤ 0, and hence we can consider x s − t x0 s − in the last above inequality. Thus |x s − − x0 | ≤ b. n ρb |x t − x0 | ≤ k0 Γ α − β k t t − s α−βk−1 0 M Γα t t − sα−1 ds 0 n ρb M tα tα−βk Γα 1 k0 Γ α − β k 1 3.5 n ρb M λα ≤ b, λα−βk Γα 1 Γ α − β k 1 k0 because of the choice of λ1 . If λ1 < λ, we can employ 3.2 to extend as a continuous function on −δ, λ2 , λ2 min{λ, 2}, such that |x − x0 | ≤ b holds. Continuing this process, we can define x t over −δ, λ so that |x t − x0 | ≤ b is satisfied on −δ, λ. Furthermore, letting 0 ≤ t1 ≤ t2 ≤ λ we see that |x t1 − x t2 | |Ix t1 − Ix t2 | t t2 1 fs, x s − fs, x s − 1 ds − ds Γα 0 t1 − s1−α t2 − s1−α 0 t1 1 |Ix t1 − Ix t2 | t1 − s1−α − t2 − s1−α fs, x s − ds Γα 0 1 t2 1−α t2 − s fs, x s − ds Γα t1 ≤ |Ix t1 − Ix t2 | M t1 M t2 1−α 1−α 1−α ds − t2 − s ds. t1 − s − t2 − s Γα 0 Γα t1 3.6 Notice that −α 2k > 0 and −α 2k1 < 0, when 0 < α < 1. Let, n n!λn−2k −α 2k >0 Λ1 : max , k 0, 1, 2, . . . , 2 n − 2k! Γ α − β 2k Λ2 : min n!λ n−2k−1 −α 2k1 n , k 0, 1, . . . , 1 2 n − 2k − 1! Γ α − β 2k 1 3.7 < 0, Advances in Difference Equations 7 firstly we have Ix t1 − Ix t2 n −α k k0 − n!tn−k 1 n − k! Γ α − β k n −α k k0 Λ1 n/2 t Λ2 t1 − sβ−α−2k1 0 k0 t n/21 1 ≤ bΛ1 n/2 t 1 k0 0 bΛ1 t1 − s n/2 t2 k0 bΛ2 bΛ2 ≤ bΛ1 k0 t2 − sβ−α−2k1 t2 0 − t2 − s ds x s − − x0 t2 − sβ−α−2k α−β2k−1 ds 3.8 ds t1 − s α−β2k − t2 − s α−β2k ds 0 n/21 t2 n/2 ds − x s − − x0 t2 − sα−β2k−1 ds t 1 k0 t2 t2 − sα−βk−1 {x s − − x0 }ds 0 t1 n/21 k0 α−β2k−1 t2 0 x s − − x0 t1 − sβ−α−2k 0 k0 ds − t1 − sα−βk−1 {x s − − x0 }ds 0 n!t2 n−k n − k! Γ α − β k x s − − x0 1 t1 t2 − sα−β2k ds t1 n/21 t2 − t1 α−β2k1 t2 − t1 α−β2k b|Λ2 | . α − β 2k α − β 2k 1 k0 We, therefore, set Λ max{b|Λ1 |, b |Λ2 |}, and get n/2 t2 − t1 α−β2k n/21 t2 − t1 α−β2k1 . Ix t1 − Ix t2 ≤ 2Λ α − β 2k α − β 2k 1 k0 k0 3.9 8 Advances in Difference Equations Finally, inequality 3.6 from inequality 3.9 becomes |x t1 − x t2 | ≤ M 2t2 − t1 α tα1 − tα2 Γα 1 n/2 t2 − t1 α−β2k n/21 t2 − t1 α−β2k1 2Λ α − β 2k α − β 2k 1 k0 k0 n/2 t2 − t1 α−β2k n/21 t2 − t1 α−β2k1 ≤ 2Λ α − β 2k α − β 2k 1 k0 k0 3.10 2M t2 − t1 α < , Γα 1 provided that |t2 − t1 | < δ0 min Γα 1 2n 1M n 1/α α − β 2k 1/α−β2k α − β 2k 1 1/α−β2k1 , , . 2n 1Λ 2n 1Λ k0 3.11 It then follows from 3.4 and 3.6 that the family {x t} forms an equicontinuous and uniformly bounded functions. Ascoli-Arzela theorem implies that the existence of a sequence {n } such that 1 > 2 > · · · > m → 0 as m → ∞, and xt limm → ∞ xm t exists uniformly on −δ, λ. Since f is uniformly continuous, we obtain that ft, xn t − m tends uniformly to ft, xt as m → ∞, and, hence, term by term integration of 3.2 with m , λ1 λ yields xt x0 Ixt I α ft, xt. 3.12 This proves that xt is a solution of the IVP 1.2, and the proof is complete. Theorem 3.2. Under the hypothesis of Theorem 3.1, there exists extremal solution for the IVP 1.2 on the interval 0 ≤ t ≤ λ0 , provided ft, x is nondecreasing in x for each t, where λ0 will be observed in the proof of this theorem. Proof. We will prove the existence of maximal solution only, since the case of minimal solution is very similar. Let 0 < ≤ b/2 and consider the fractional differential equation with an initial condition LDxt − x0 ft, xt , x0 x0 . 3.13 We observe that f t, x ft, x is defined and continuous on Ω b , t, x : 0 ≤ t ≤ λ, |x − x0 | ≤ 2 3.14 Advances in Difference Equations 9 Ω ⊂ Ω0 and |f t, x ≤ M b/2 on Ω . We then deduce from Theorem 3.1 that IVP 3.6 has a solution xt, on the interval o ≤ t ≤ λ0 . Now for 0 < 2 < 1 ≤ , we have x0, 2 x0 2 < x0 1 x0, 1 , 3.15 xt, 2 ≤ x0, 2 Ixt, 2 I α f2 t, xt, 2 xt, 1 ≥ x0, 1 Ixt, 1 I α f1 t, xt, 1 , where Ix was introduced by 2.2. In view of Theorem 2.1 to get xt, 2 < xt, 1 , t ∈ 0, λ0 . Consider the family of functions {xt, } on 0 ≤ t ≤ λ0 . We have |xt, − x0, | Ixt, I α f t, xt, Ixt, 2 I α ft, xt, n ρ1 x t ≤ s − tα−βk−1 Γ α − β k 0 k0 ≤ 1 Γα t t − sα−1 ft, xt, ds 3.16 0 n ρ1 x M α t tα−βk Γα 1 Γ α − β k 1 k0 n ρ1 x 2M b α λ0 ≤ b, λ0 α−βk 2Γα 1 Γ α − β k 1 k0 where ρ was denoted by 3.4, x max{|xt, | : t ∈ 0, λ0 , ∈ 0, b/2} and λ0 min r, 2bΓα 1 n 22M b 1/α bΓα − β k 1 1/α−βk , , k 0, n . ρ x 1n 2 3.17 Showing that the family {xt, } is uniformly bounded. Also, if 0 ≤ t1 ≤ t2 ≤ λ0 then there exists a constant Λ∗ such that ∗ |xt1 , − xt2 , | ≤ 2Λ n/2 t2 − t1 α−β2k k0 α − β 2k n/21 k0 t2 − t1 α−β2k1 α − β 2k 1 2M b t2 − t1 α . 2Γα 1 3.18 Following the computation similar to 3.10 with suitable changes. This proves that the family {xt, } is equicontinuous. Hence there exists a sequence {m } with m → 0 as m → ∞ and the uniform limit μt lim xt, m m→∞ 3.19 10 Advances in Difference Equations exists on 0, λ0 . Clearly μ0 x0 . The uniform continuity of f gives argument as before as in Theorem 3.1, that μt is a solution of IVP 1.2. Next, we show that μt is required maximal solution of 1.2, on 0 ≤ t ≤ λ0 . Let xt be a any solution of 1.2 on 0 ≤ t ≤ λ0 . Then we have x0 < x0 x0, t 1 xt < x0 Ixt t − sα−1 f s, xsds, Γα 0 t 1 xt, ≥ x0, Ixt, t − sα−1 f s, xs, , Γα 0 3.20 where x0, x0 , f t, xt ft, xt , and f t, xt, ft, xt, . Using Theorem 2.1, we get xt < xt, on 0, λ0 as → 0. Therefore, the proof is complete. 4. Global Existence We need the following comparison result before we proceed further. Theorem 4.1. Assume that h : 0, T → R , g : 0, T × R → R are continuous, and gt, u is nondecreasing with respect to the second argument such that ht ≤ h0 Iht 1 Γα t t − sα−1 gs, hsds, t ∈ 0, T , 4.1 0 where I is defined by 3.5. Let mt be the maximal solution of LDut − u0 gt, u, u0 u0 ≥ 0, 4.2 existing on 0, T such that h0 ≤ u0. Then we have ht ≤ mt, t ∈ 0, T . 4.3 Proof. In view of the definition of the maximal solution mt, it is enough to prove, to conclude 4.3, that ht < ut, , t ∈ 0, T , 4.4 where ut, is any solution of LDut − u0 gt, u , u0 u0 , > 0. 4.5 Advances in Difference Equations 11 Now, it follows from 4.4 that 1 ut, > u0, Iut, Γα t t − sα−1 f s, us, ds, 4.6 0 where u0, u0 . Then applying Theorem 2.1, we get immediately 4.1 and since lim → 0 ut, mt uniformly on each t ∈ 0, T0 , T0 < T , the proof is complete. We are now in position to prove global existence result. Theorem 4.2. Assume that f : 0, ∞ × R → R continuous. Let there exists function g : 0, ∞ × R → R continuous and nondecreasing with respect to the second argument such that ft, x ≤ gt, |x| ∀t ≥ 0, x ∈ R. 4.7 If the maximal solution of the initial value problem LDut − u0 gt, ut, u0 u0 , t > 0 4.8 exists in 0, ∞ then all solutions of the initial value problem LDxt − x0 ft, xt, x0 x0 4.9 with |x0 | ≤ u0 exist in 0, ∞. Proof. Let xt, x0 be any solution of IVP 4.8 such that |x0 | ≤ u0 , which exists on 0, γ for γ ∈ 0, ∞, and the value of γ cannot be increased further. Set ht |xt, x0 | for 0 ≤ t < γ. Then using the assumption 4.4, we get ht ≤ |x0 | Iht 1 Γα t t − sα−1 gs, hsds. 4.10 0 Applying the comparison Theorem 4.1, we obtain ht |xt, x0 | ≤ mt, 0 ≤ t < γ. 4.11 Since mt is assumed to exist on 0, ∞, it follows that gt, mt ≤ M, 0 ≤ t ≤ γ. 4.12 12 Advances in Difference Equations Now, let 0 ≤ t1 ≤ t2 < γ. Then employing the arguments similar to estimating 3.19 and using 4.7 and the bounded M of g we arrive at |xt2 , x0 − xt1 , x0 | ≤ 2M t2 − t1 α Γα 1 n/2 t2 − t1 α−β2k n/21 t2 − t1 α−β2k1 . 2Λ α − β 2k α − β 2k 1 k0 k0 4.13 Letting t1 , t2 → γ − and using Cauchy criterion, it follows that limt → γ − xt, x0 exists. 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