Hindawi Publishing Corporation Journal of Applied Mathematics Volume 2012, Article ID 879657, 14 pages doi:10.1155/2012/879657 Research Article Asymptotic Stability Results for Nonlinear Fractional Difference Equations Fulai Chen and Zhigang Liu Department of Mathematics, Xiangnan University, Chenzhou 423000, China Correspondence should be addressed to Fulai Chen, cflmath@163.com Received 1 August 2011; Revised 27 December 2011; Accepted 2 January 2012 Academic Editor: Michela Redivo-Zaglia Copyright q 2012 F. Chen and Z. Liu. 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 present some results for the asymptotic stability of solutions for nonlinear fractional difference equations involvingRiemann-Liouville-like difference operator. The results are obtained by using Krasnoselskii’s fixed point theorem and discrete Arzela-Ascoli’s theorem. Three examples are also provided to illustrate our main results. 1. Introduction In this paper we consider the asymptotic stability of solutions for nonlinear fractional difference equations: Δα xt ft α, xt α, α−1 Δ t ∈ N0 , 0 < α ≤ 1, xt|t0 x0 , 1.1 where Δα is aRiemann-Liouville-like discrete fractional difference, f : 0, ∞ × R → R is continuous with respect to t and x, Na {a, a 1, a 2, . . .}. Fractional differential equations have received increasing attention during recent years since these equations have been proved to be valuable tools in the modeling of many phenomena in various fields of science and engineering. Most of the present works were focused on fractional differential equations, see 1–12 and the references therein. However, very little progress has been made to develop the theory of the analogous fractional finite difference equation 13–19. Due to the lack of geometry interpretation of the fractional derivatives, it is difficult to find a valid tool to analyze the stability of fractional difference equations. In the case that it 2 Journal of Applied Mathematics is difficult to employ Liapunov’s direct method, fixed point theorems are usually considered in stability 20–25. Motivated by this idea, in this paper, we discuss asymptotic stability of nonlinear fractional difference equations by using Krasnoselskii’s fixed point theorem and discrete Arzela-Ascoli’s theorem. Different from our previous work 18, in this paper, the sufficient conditions of attractivity are irrelevant to the initial value x0 . 2. Preliminaries In this section, we introduce preliminary facts of discrete fractional calculus. For more details, see 14. Definition 2.1 see 14. Let ν > 0. The ν-th fractional sum x is defined by Δ−ν ft t−ν 1 t − s − 1ν−1 fs, Γν sa 2.1 where f is defined for s a mod 1 and Δ−ν f is defined for t a ν mod 1, and tν Γt1/Γt−ν 1. The fractional sum Δ−ν maps functions defined on Na to functions defined on Naν . Definition 2.2 see 14. Let μ > 0 and m − 1 < μ < m, where m denotes a positive integer, m μ, · ceiling of number. Set ν m − μ. The μ-th fractional difference is defined as Δμ ft Δm−ν ft Δm Δ−ν ft . 2.2 Theorem 2.3 see 15. Let f be a real-value function defined on Na and μ, ν > 0, then the following equalities hold: i Δ−ν Δ−μ ft Δ−μν ft Δ−μ Δ−ν ft; ii Δ−ν Δft ΔΔ−ν ft − t − aν−1 fa. Γν Lemma 2.4 see 15. Let μ / 1 and assume μ ν 1 is not a nonpositive integer, then −ν μ Δ t Γ μ1 tμν . Γ μν1 2.3 Lemma 2.5 see 15. Assume that the following factorial functions are well defined: i If 0 < α < 1, then tαγ ≥ tγ α ; ii tβγ t − γβ tγ . Lemma 2.6 see 13. Let μ > 0 be noninteger, m μ, ·, ν m − μ, thus one has t−μ saν t − s − 1μ−1 t − a − νμ . μ 2.4 Journal of Applied Mathematics 3 Lemma 2.7. The equivalent fractional Taylor’s difference formula of 1.1 is xt t−α x0 α−1 1 t t − s − 1α−1 fs α, xs α, Γα Γα s0 t ∈ Nα . 2.5 Proof. Apply the Δ−α operator to each side of the first formula of 1.1 to obtain Δ−α Δα xt Δ−α ft α, xt α, t ∈ Nα . 2.6 Apply Theorem 2.3 to the left-hand side of 2.6 to obtain Δ−α Δα xt Δ−α ΔΔ−1−α xt ΔΔ−α Δ−1−α xt − x0 α−1 xt − t . Γα tα−1 xα − 1 Γα 2.7 So, applying Definition 2.1 to the right-hand side of 2.6, for t ∈ Nα we obtain 2.5. The recursive iteration to this Taylor’s difference formula implies that 2.5 represents the unique solution of the IVP 1.1. This completes the proof. Lemma 2.8 see 4, 1.5.15. The quotient expansion of two gamma functions at infinityis Γz a 1 za−b 1 O , Γz b z argz a < π, |z| −→ ∞ . 2.8 Corollary 2.9. One has t−β > t α−β for α, β, t > 0. 2.9 Proof. According to Lemma 2.8, t−β t α−β Γ tαβ1 Γt 1 · Γt α 1 Γ tβ1 Γ tαβ1 Γt 1 · Γt α 1 Γ tβ1 α 1 1 −α · tβ 1O t 1O t tβ β α 1 1 1O 1 1O t t tβ > 1. Then, t−β > t α−β for α, β, t > 0. This completes the proof. 2.10 4 Journal of Applied Mathematics Definition 2.10. The solution x ϕt of the IVP 1.1 is said to be i stable if for any ε > 0 and t0 ∈ R , there exists a δ δt0 , ε > 0 such that xt, x0 , t0 − ϕt < ε 2.11 for |x0 − ϕt0 | ≤ δt0 , ε and all t ≥ t0 ; ii attractive if there exists ηt0 > 0 such that x0 ≤ η implies lim xt, x0 , t0 0; t→∞ 2.12 iii asymptotically stable if it is stable and attractive. The space ln∞0 is the set of real sequences defined on the set of positive integers where any individual sequence is bounded with respect to the usual supremum norm. It is well known that under the supremum norm ln∞0 is a Banach space 26. Definition 2.11 see 27. A set Ω of sequences in ln∞0 is uniformly Cauchy or equi-Cauchy, if for every ε > 0, there exists an integer N such that |xi − xj| < ε, whenever i, j > N for any x {xn} in Ω. Theorem 2.12 see 27, discrete Arzela-Ascoli’s theorem. A bounded, uniformly Cauchy subset Ω of ln∞0 is relatively compact. Theorem 2.13 see 20, Krasnoselskii’s fixed point theorem. Let S be a nonempty, closed, convex, and bounded subset of the Banach space X and let A : X → X and B : S → X be two operators such that a A is a contraction with constant L < 1, b B is continuous, BS resides in a compact subset of X, c x Ax By, y ∈ S ⇒ x ∈ S. Then the operator equation Ax Bx x has a solution in S. 3. Main Results ∞ Let lα∞ be the set ofall real sequences x {xt}∞ tα with norm x supt∈Nα |xt|, then lα is a Banach space. Define the operator t−α 1 x0 α−1 t t − s − 1α−1 fs α, xs α, Γα Γα s0 x0 α−1 t , Axt Γα t−α 1 Bxt t − s − 1α−1 fs α, xs α, t ∈ Nα . Γα s0 P xt 3.1 Journal of Applied Mathematics 5 Obviously, P x Ax Bx, the operator A is a contraction with the constant 0, which implies that condition a of Theorem 2.13 holds, and xt is a solution of 1.1 if it is a fixed point of the operator P . Lemma 3.1. Assume that the following condition is satisfied: H1 there exist constants β1 ∈ α, 1 and L1 ≥ 0 such that ft, xt ≤ L1 t−β1 for t ∈ Nα . 3.2 Then the operator B is continuous and BS1 is a compact subset of R for t ∈ Nαn1 , where S1 xt : |xt| ≤ t−γ1 for t ∈ Nαn1 , 3.3 γ1 −1/2α − β1 , and n1 ∈ N satisfies that L1 Γ 1 − β1 1/2αβ1 −1 −γ1 |x0 | ≤ 1. α n1 γ1 α n1 γ1 Γα Γ 1 α − β1 3.4 Proof. For t ∈ Nα , apply Lemma 2.8 and γ1 > 0, Γt 1 1 , t−γ1 t−γ1 1 O t Γ t γ1 1 3.5 and we have that t−γ1 → 0 as t → ∞, then there exists a n1 ∈ N such that inequality 3.4 holds, which implies that the set S1 exists. We firstly show that B maps S1 in S1 . It is easy to know that S1 is a closed, bounded, and convex subset of R. Apply condition H1 , Lemma 2.5, Corollary 2.9 and 3.4, for t ∈ Nαn1 , we have |Bxt| ≤ ≤ t−α 1 t − s − 1α−1 fs α, xs α Γα s0 t−α 1 t − s − 1α−1 L1 s α−β1 Γα s0 L1 Δ−α t α−β1 L1 Γ 1 − β1 t αα−β1 Γ 1 α − β1 L1 Γ 1 − β1 α−β1 < t Γ 1 α − β1 L1 Γ 1 − β1 −γ1 −γ1 t t γ1 Γ 1 α − β1 6 Journal of Applied Mathematics L1 Γ 1 − β1 −γ1 −γ1 t ≤ α n1 γ1 Γ 1 α − β1 ≤ t−γ1 , 3.6 which implies that BS1 ⊂ S1 for t ∈ Nαn1 . Nextly, we show that B is continuous on S1 . Let ε > 0 be given then there exist T1 ∈ N and T1 ≥ n1 such that t ∈ NαT1 implies that L1 Γ 1 − β1 α−β1 ε < . t 2 Γ 1 α − β1 3.7 Let {xn } be a sequence such that xn → x. For t ∈ {α n1 , α n1 1, . . . , α T1 − 1}, applying the continuity of f and Lemma 2.6, we have t−α 1 t − s − 1α−1 fs α, xn s α − fs α, xs α Γα s0 ≤ max fs α, xn s α − fs α, xs α |Bxn t − Bxt| ≤ s∈{0,1,...,T1 −1} × t−α 1 t − s − 1α−1 Γα s0 tα max fs α, xn s α − fs α, xs α Γα 1 s∈{0,1,...,T1 −1} α T1 − 1α ≤ max fs α, xn s α − fs α, xs α Γα 1 s∈{0,1,...,T1 −1} −→ 0 3.8 as n −→ ∞. For t ∈ NαT1 , |Bxn t − Bxt| ≤ ≤ t−α 1 t − s − 1α−1 fs α, xn s α fs α, xs α Γα s0 t−α 2L1 t − s − 1α−1 s α−β1 Γα s0 2L1 Δ−α t α−β1 2L1 Γ 1 − β1 t αα−β1 Γ 1 α − β1 2L1 Γ 1 − β1 α−β1 < t Γ 1 α − β1 < ε. 3.9 Journal of Applied Mathematics 7 Thus, for all t ∈ Nαn1 , we have |Bxn t − Bxt| −→ 0 as n → ∞. 3.10 which implies that B is continuous. Lastly, we show that BS1 is relatively compact. Let t1 , t2 ∈ NαT1 and t2 > t1 , thus we have 1 t 2 −α t − s − 1α−1 fs α, xs α |Bxt2 − Bxt1 | Γα s0 2 1 −α 1 t α−1 − fs α, xs α t1 − s − 1 Γα s0 ≤ 2 −α 1 t t2 − s − 1α−1 fs α, xs α Γα s0 1 −α 1 t t1 − s − 1α−1 fs α, xs α Γα s0 L1 Γ 1 − β1 L1 Γ 1 − β1 α−β1 ≤ t2 α t1 αα−β1 Γ 1 α − β1 Γ 1 α − β1 L1 Γ 1 − β1 L1 Γ 1 − β1 α−β1 < t2 t1 α−β1 Γ 1 α − β1 Γ 1 α − β1 3.11 < ε. Thus, {Bx : x ∈ S1 } is a bounded and uniformly Cauchy subset by Definition 2.11, and BS1 is relatively compact by means of Theorem 2.12. This completes the proof. Lemma 3.2. Assume that condition H1 holds, then a solution of 1.1 is in S1 for t ∈ Nαn1 . Proof. Notice if that xt is a fixed point of P , then it is a solution of 1.1. To prove this, it remains to show that, for fixed y ∈ S1 , x Ax By ⇒ x ∈ S1 holds. If x Ax By, applying condition H1 and 3.4, for t ∈ Nαn1 , we have |xt| ≤ |Axt| Byt t−α 1 |x0 | α−1 t t − s − 1α−1 f s α, ys α Γα Γα s0 L1 Γ 1 − β1 |x0 | α−1 ≤ t t αα−β1 Γα Γ 1 α − β1 L1 Γ 1 − β1 α−β1 |x0 | α−1 t < t Γα Γ 1 α − β1 ≤ 8 Journal of Applied Mathematics L1 Γ 1 − β1 1/2αβ1 −1 −γ1 −γ1 |x0 | t t γ1 t γ1 Γα Γ 1 α − β1 L1 Γ 1 − β1 1/2αβ1 −1 −γ1 −γ1 |x0 | t ≤ α n1 γ1 α n1 γ1 Γα Γ 1 α − β1 ≤ t−γ1 . 3.12 Thus, xt ∈ S1 for t ∈ Nαn1 . According to Theorem 2.13 and Lemma 3.1, there exists a x ∈ S1 such that x AxBx, that is, P has a fixed point in S1 which is a solution of 1.1 for t ∈ Nαn1 . This completes the proof. Theorem 3.3. Assume that condition H1 holds, then the solutions of 1.1 is attractive. Proof. By Lemma 3.2, the solutions of 1.1 exist and are in S1 . All functions xt in S1 tend to 0 as t → ∞. Then the solutions of 1.1 tend to zero as t → ∞. This completes the proof. Theorem 3.4. Assume that the following condition is satisfied: H2 there exist constants β2 ∈ α, 1 and L2 ≥ 0 such that ft, xt − f t, yt ≤ L2 t−β2 x − y for t ∈ Nα . 3.13 Then the solutions of 1.1 are stable provided that L2 Γ1 αΓ 1 − β2 c : < 1. Γ 1 α − β2 Γ 1 β2 3.14 Proof. Let xt be a solution of 1.1, and let xt be a solution of 1.1 satisfying the initial value condition x0 x0 . For t ∈ Nα , applying condition H2 , we have ≤ |xt − xt| ≤ t−α tα−1 1 t − s − 1α−1 |x0 − x0 | Γα Γα s0 α × fs α, xs α − fs α, xs t−α tα−1 L2 t − s − 1α−1 s α−β2 x − x |x0 − x0 | Γα Γα s0 tα−1 |x0 − x0 | L2 Δ−α t α−β2 x − x Γα L2 Γ 1 − β2 tα−1 t αα−β2 x − x |x0 − x0 | Γα Γ 1 α − β2 L2 Γ 1 − β2 αα−1 ≤ αα−β2 x − x |x0 − x0 | Γα Γ 1 α − β2 Journal of Applied Mathematics 9 L2 Γ1 αΓ 1 − β2 α|x0 − x0 | x − x Γ 1 α − β2 Γ 1 β2 α|x0 − x0 | cx − x, 3.15 which implies that ≤ x − x α |x0 − x0 |. 1−c 3.16 For any given ε > 0, let δ 1 − c/αε, |x0 − x0 | < δ follows that x − x < ε, which yields that the solutions of 1.1 are stable. This completes the proof. Theorem 3.5. Assume that conditions H1 and H2 hold, then the solutions of 1.1 are asymptotically stable provided that 3.14 holds. Theorem 3.5 is the simple consequence of Theorems 3.3 and 3.4. Theorem 3.6. Assume that the following condition is satisfied: H3 there exist constants β3 ∈ α, 1/21 α, γ2 1/21 − α, and L3 ≥ 0 such that ft, xt ≤ L3 t γ2 −β3 |xt| for t ∈ Nα . 3.17 Then the solutions of 1.1 is attractive. Proof. Set S2 xt : |xt| ≤ t−γ2 for t ∈ Nαn2 , 3.18 where n2 ∈ N satisfies that L3 Γ 1 − β3 − γ2 −γ2 α−β3 |x0 | ≤ 1. α n2 γ2 α n2 γ2 Γα Γ 1 α − β3 − γ2 3.19 We first prove condition c of Theorem 2.13, that is, for fixed y ∈ S2 and for all x ∈ R, x Ax By ⇒ x ∈ S2 holds. 10 Journal of Applied Mathematics If x Ax By, applying condition H3 and 3.19, for t ∈ Nαn2 , we have |xt| ≤ |Axt| Byt ≤ t−α 1 |x0 | α−1 t t − s − 1α−1 f s α, ys α Γα Γα s0 ≤ t−α −β3 1 |x0 | α−1 ys α t t − s − 1α−1 L3 s α γ2 Γα Γα s0 ≤ t−α −β3 L3 |x0 | α−1 t t − s − 1α−1 s α γ2 s α−γ2 Γα Γα s0 t−α L3 |x0 | α−1 t t − s − 1α−1 s α−β3 −γ2 Γα Γα s0 L3 Γ 1 − β3 − γ2 |x0 | α−1 t ≤ t αα−β3 −γ2 Γα Γ 1 α − β3 − γ2 L3 Γ 1 − β3 − γ2 α−β3 −γ2 |x0 | α−1 t < t Γα Γ 1 α − β3 − γ2 L3 Γ 1 − β3 − γ2 −γ2 α−β3 −γ2 |x0 | t t γ2 t γ2 Γα Γ 1 α − β3 − γ2 L3 Γ 1 − β3 − γ2 −γ2 α−β3 −γ2 |x0 | t ≤ α n2 γ2 α n2 γ2 Γα Γ 1 α − β3 − γ2 ≤ 3.20 ≤ t−γ2 . Thus, condition c of Theorem 2.13 holds. The proof of condition b of Theorem 2.13 is similar to that of Lemma 3.1, and we omit it. Therefore, P has a fixed point in S2 by using Theorem 2.13, that is, the IVP 1.1 has a solution in S2 . Moreover, all functions in S2 tend to 0 as t → ∞, then the solution of 1.1 tends to zero as t → ∞, which shows that the zero solution of 1.1 is attractive. This completes the proof. Theorem 3.7. Assume that conditions H2 and H3 hold, then the solutions of 1.1 are asymptotically stable provided that 3.14 holds. Theorem 3.8. Assume that the following condition is satisfied: H4 there exist constants η ∈ 0, 1, β4 ∈ α, 2 αη/2 η, and L4 ≥ 0 such that ft, xt ≤ L4 t 1−β4 |xt|η for t ∈ Nα . 3.21 Then the solutions of 1.1 is attractive. Proof. Set S3 xt : |xt| ≤ t−γ3 for t ∈ Nαn3 , 3.22 Journal of Applied Mathematics 11 where γ3 1/2β4 − α, and n3 ∈ N satisfies that L4 Γ 1 − β4 − γ3 η α−1γ3 −γ |x0 | α n3 γ3 α n3 γ3 3 ≤ 1. Γα Γ 1 α − β4 − γ3 η 3.23 Here we only prove that condition c of Theorem 2.13 holds, and the remaining part of the proof is similar to that of Theorem 3.6. Since η ∈ 0, 1, β4 ∈ α, 2 αη/2 η, and γ3 1/2β4 − α, then γ3 , γ3 η, α γ3 ∈ 0, 1, β4 γ3 η ∈ α, 1. If x Ax By, applying condition H4 , Lemma 2.5 and 3.23, for t ∈ Nαn3 , we have t−α 1 |x0 | α−1 t t − s − 1α−1 f s α, ys α |xt| ≤ |Axt| Byt ≤ Γα Γα s0 ≤ t−α η 1 |x0 | α−1 t t − s − 1α−1 L4 s α 1−β4 ys α Γα Γα s0 ≤ t−α η −β4 L4 |x0 | α−1 t t − s − 1α−1 s α γ3 η s α−γ3 Γα Γα s0 ≤ t−α −β4 L4 |x0 | α−1 t t − s − 1α−1 s α γ3 η s α−γ3 η Γα Γα s0 t−α L4 |x0 | α−1 t t − s − 1α−1 s α−β4 −γ3 η Γα Γα s0 L4 Γ 1 − β4 − γ3 η |x0 | α−1 t ≤ t αα−β4 −γ3 η Γα Γ 1 α − β4 − γ3 η L4 Γ 1 − β4 − γ3 η α−β4 −γ3 η |x0 | α−1 t < t Γα Γ 1 α − β4 − γ3 η L4 Γ 1 − β4 − γ3 η α−β4 |x0 | α−1 t ≤ t Γα Γ 1 α − β4 − γ3 η L4 Γ 1 − β4 − γ3 η α−1γ3 −γ3 −γ3 |x0 | t t γ3 t γ3 Γα Γ 1 α − β4 − γ3 η α−1γ3 |x0 | ≤ α n3 γ3 Γα L4 Γ 1 − β4 − γ3 η −γ3 −γ3 t α n3 γ3 Γ 1 α − β4 − γ3 η ≤ t−γ3 . 3.24 Thus, condition c of Theorem 2.13 holds. This completes the proof. 12 Journal of Applied Mathematics 4. Examples Example 4.1. Consider Δ0.5 xt 0.2t 0.5−0.75 sinxt 0.5, t ∈ N0 , Δ−0.5 xt|t0 x0 , 4.1 where ft, xt 0.2t−0.75 sinxt, t ∈ N0.5 . Since ft, xt 0.2t−0.75 sinxt ≤ 0.2t−0.75 , 4.2 this implies that condition H1 holds. In addition, ft, xt − f t, yt ≤ 0.2t−0.75 x − y. 4.3 Thus, condition H2 is satisfied. Moreover, from L2 0.2, α 0.5, and β2 0.75, we have L2 Γ1 αΓ 1 − β2 0.2Γ1.5Γ0.25 ≈ 0.7716 < 1, c Γ1.25Γ1.75 Γ 1 α − β2 Γ 1 β2 4.4 which implies that inequality 3.14 holds. Thus the solutions of 4.1 are asymptotically stable by Theorem 3.5. Example 4.2. Consider Δ0.5 xt 0.2t 1.5−0.6 xt 0.5, t ∈ N0 , Δ−0.5 xt|t0 x0 , 4.5 where ft, xt 0.2t 1−0.6 xt, t ∈ N0.5 . Since β3 0.6, α 0.5, we have that β3 ∈ α, 1/21 α, γ2 0.25 and ft, xt 0.2t 1−0.6 xt ≤ 0.2t 0.25−0.6 |xt|, 4.6 which implies that condition H3 is satisfied. Meanwhile, ft, xt − f t, yt ≤ 0.2t 1−0.6 x − y ≤ 0.2t−0.6 x − y, which implies that condition H2 is satisfied. 4.7 Journal of Applied Mathematics 13 From L2 0.2, α 0.5, and β2 0.6, we have L2 Γ1 αΓ 1 − β2 0.2Γ1.5Γ0.4 c ≈ 0.4120 < 1, Γ0.9Γ1.6 Γ 1 α − β2 Γ 1 β2 4.8 which implies that inequality 3.14 holds. Thus the solutions of 4.5 are asymptotically stable by Theorem 3.7. Example 4.3. Consider Δ0.5 xt t 1.5−0.6 x1/3 t 0.5, t ∈ N0 , Δ−0.5 xt|t0 x0 , 4.9 where ft, xt t 1−0.6 x1/3 t, t ∈ N0.5 . Since α 0.5, β4 0.6, η 1/3, we have that η ∈ 0, 1, β4 ∈ α, 2 αη/2 η and ft, xt ≤ t 1−0.6 |xt|1/3 , 4.10 then condition H4 is satisfied. The solutions of 4.9 are attractive by Theorem 3.8. Acknowledgments This research was supported by the NSF of Hunan Province 10JJ6007, 2011FJ3013, the Scientific Research Foundation of Hunan Provincial Education Department, and the Construct Program of the Key Discipline in Hunan Province. References 1 R. P. Agarwal, V. Lakshmikantham, and J. J. Nieto, “On the concept of solution for fractional differential equations with uncertainty,” Nonlinear Analysis. Theory, Methods & Applications, vol. 72, no. 6, pp. 2859–2862, 2010. 2 M. Benchohra, J. Henderson, S. K. Ntouyas, and A. Ouahab, “Existence results for fractional order functional differential equations with infinite delay,” Journal of Mathematical Analysis and Applications, vol. 338, no. 2, pp. 1340–1350, 2008. 3 F. Chen, A. Chen, and X. Wang, “On the solutions for impulsive fractional functional differential equations,” Differential Equations and Dynamical Systems, vol. 17, no. 4, pp. 379–391, 2009. 4 A. A. Kilbas, H. M. Srivastava, and J. J. Trujillo, Theory and Applications of Fractional Differential Equations, North-Holland Mathematics Studies, Elsevier Science B.V., Amsterdam, the Netherlands, 2006. 5 V. Lakshmikantham, “Theory of fractional functional differential equations,” Nonlinear Analysis. Theory, Methods & Applications, vol. 69, no. 10, pp. 3337–3343, 2008. 6 V. Lakshmikantham, S. Leela, and J. V. Devi, Theory of Fractional Dynamic Systems, Cambridge Scientific Publishers, Cambridge, UK, 2009. 7 K. S. Miller and B. Ross, An Introduction to the Fractional Calculus and Fractional Differential Equations, John Wiley & Sons, New York, NY, USA, 1993. 8 J. J. Nieto, “Maximum principles for fractional differential equations derived from Mittag-Leffler functions,” Applied Mathematics Letters, vol. 23, no. 10, pp. 1248–1251, 2010. 14 Journal of Applied Mathematics 9 I. Podlubny, Fractional Differential Equations, Academic Press, San Diego, Calif, USA, 1999. 10 J. Wang and Y. Zhou, “A class of fractional evolution equations and optimal controls,” Nonlinear Analysis. Real World Applications, vol. 12, no. 1, pp. 262–272, 2011. 11 Y. Zhou, F. Jiao, and J. Li, “Existence and uniqueness for P-type fractional neutral differential equations,” Nonlinear Analysis. Theory, Methods & Applications, vol. 71, no. 7-8, pp. 2724–2733, 2009. 12 Y. Zhou, F. Jiao, and J. Li, “Existence and uniqueness for fractional neutral differential equations with infinite delay,” Nonlinear Analysis. Theory, Methods & Applications, vol. 71, no. 7-8, pp. 3249–3256, 2009. 13 G. A. Anastassiou, “Discrete fractional calculus andinequalities,” Classical Analysis and ODEs. In press. 14 F. M. Atici and P. W. Eloe, “Initial value problems in discrete fractional calculus,” Proceedings of the American Mathematical Society, vol. 137, no. 3, pp. 981–989, 2009. 15 F. M. Atici and P. W. Eloe, “A transform method in discrete fractional calculus,” International Journal of Difference Equations, vol. 2, no. 2, pp. 165–176, 2007. 16 F. M. Atıcı and P. W. Eloe, “Discrete fractional calculus with the nabla operator,” Electronic Journal of Qualitative Theory of Differential Equations I, no. 3, pp. 1–12, 2009. 17 F. M. Atici and S. Sengül, “Modeling with fractional difference equations,” Journal of Mathematical Analysis and Applications, vol. 369, no. 1, pp. 1–9, 2010. 18 F. Chen, “Fixed points and asymptotic stability of nonlinear fractional difference equations,” Electronic Journal of Qualitative Theory of Differential Equations, vol. 39, pp. 1–18, 2011. 19 F. Chen and Y. Zhou, “Attractivity of fractional functional differential equations,” Computers & Mathematics with Applications, vol. 62, no. 3, pp. 1359–1369, 2011. 20 T. A. Burton and T. Furumochi, “Krasnoselskii’s fixed point theorem and stability,” Nonlinear Analysis. Theory, Methods & Applications, vol. 49, no. 4, pp. 445–454, 2002. 21 T. A. Burton, “Fixed points, stability, and exact linearization,” Nonlinear Analysis. Theory, Methods & Applications, vol. 61, no. 5, pp. 857–870, 2005. 22 T. A. Burton, Stability by Fixed Point Theory for Functional Differential Equations, Dover Publications, Mineola, NY, USA, 2006. 23 B. C. Dhage, “Global attractivity results for nonlinear functional integral equations via a Krasnoselskii type fixed point theorem,” Nonlinear Analysis. Theory, Methods & Applications, vol. 70, no. 7, pp. 2485– 2493, 2009. 24 C. Jin and J. Luo, “Stability in functional differential equations established using fixed point theory,” Nonlinear Analysis. Theory, Methods & Applications, vol. 68, no. 11, pp. 3307–3315, 2008. 25 Y. N. Raffoul, “Stability in neutral nonlinear differential equations with functional delays using fixedpoint theory,” Mathematical and Computer Modelling, vol. 40, no. 7-8, pp. 691–700, 2004. 26 Y. Zhou, Oscillatory Behavior of Delay Differential Equations, Science Press, Beijing, China, 2007. 27 S. S. Cheng and W. T. Patula, “An existence theorem for a nonlinear difference equation,” Nonlinear Analysis. Theory, Methods & Applications, vol. 20, no. 3, pp. 193–203, 1993. 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