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Discrete Dynamics in Nature and Society
Volume 2013 (2013), Article ID 459161, 7 pages
Existence and Ulam Stability of Solutions for Discrete Fractional Boundary Value Problem
1Department of Mathematics, Xiangnan University, Chenzhou 423000, China
2School of Mathematics and Computational Science, Xiangtan University, Xiangtan 411005, China
Received 28 May 2013; Accepted 9 July 2013
Academic Editor: Shurong Sun
Copyright © 2013 Fulai Chen and Yong Zhou. 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 discuss the existence of solutions for antiperiodic boundary value problem and the Ulam stability for nonlinear fractional difference equations. Two examples are also provided to illustrate our main results.
This paper investigates the existence of solutions for antiperiodic boundary value problem and the Ulam stability for nonlinear fractional difference equations: where is a Caputo fractional difference operator, and for any number and each interval of , and is a continuous function with respect to the second variable.
Accompanied with the development of the theory on fractional differential equations, fractional difference equations have also been studied more intensively of late. In particular, some properties and inequalities of the fractional difference calculus are discussed in [1–7], the existence and asymptotic stability of the solutions for fractional difference equations are investigated in [8–10], and the boundary value problems of fractional difference equations are considered in [11–13]. But there are a lot of works to do in the future, and to the best of my knowledge, there is no work on the existence of solutions for antiperiodic boundary value problem and the Ulam stability for nonlinear fractional difference equations.
To research the boundary value problem of fractional difference equations, we need to select a suitable fixed-point theorem because of the discrete property of the difference operator; here, we choose the Banach contraction mapping principle and the Brower fixed-point theorem. Motivated by the work of the Ulam stability for fractional differential equations , in this paper, we also introduce four types of the Ulam stability definitions for fractional difference equations and study the Ulam-Hyers stable and the Ulam-Hyers-Rassias stable.
The rest of the paper is organized as follows. In Section 2, we introduce some useful preliminaries. In Section 3, we consider the existence of solutions for antiperiodic boundary value problem of fractional difference equations. In Section 4, we discuss the Ulam stability for fractional difference equations. Finally, two examples are given to illustrate our main results.
In this section, we introduce preliminary facts which are used throughout this paper.
In (2), the fractional sum maps functions defined on to functions defined on . Atici and Eloe  pointed out that this definition is the development of the theory of the fractional calculus on time scales.
Definition 2 (see ). Let and , where denotes a positive integer and , ceiling of number. Set . The th fractional Caputo difference operator is defined as where is the th order forward difference operator; the fractional Caputo like difference maps functions defined on to functions defined on .
Lemma 4. One has
Corollary 5. One has (i), (ii).
3. Antiperiodic Boundary Value Problem
In this section, we consider the following antiperiodic boundary value problem: where is a forward difference operator.
Let be the set of all real sequences with norm . Then, is a Banach space.
Lemma 6. A solution is a solution for antiperiodic boundary value problem if and only if is a solution of the the following fractional Taylor’s difference formula:
Proof. Suppose that defined on is a solution of (10). Using Lemma 3, for some constants , we have
Then, we obtain 
In view of and , we have Then, Substituting the values of and into (12), we obtain (11).
Conversely, if is a solution of (11), by a direct computation, it follows that the solution given by (11) satisfies (10). The proof is completed.
The following fixed-point theorems are needed to prove the existence and uniqueness of solutions for the BVP (9).
Lemma 7 (see  (Banach contraction mapping principle)). A contraction mapping on a complete metric space has exactly one fixed point.
Lemma 8 (see  (Brower fixed-point theorem)). Let be a continuous mapping, where is a nonempty, bounded, close, and convex set. Then, has a fixed point.
Define the operator Obviously, is a solution of (9) if it is a fixed point of the operator .
Theorem 9. Assume that.
There exists a constant such that for each and all .
Then, the BVP (9) has a unique solution on provided that
Proof. Let ; then for each , we have
According to (17), we obtain Then, which implies that is a contraction. Therefore, the Banach fixed-point theorem (Lemma 7) guarantees that has a unique fixed point which is a unique solution of the BVP (9). This completes the proof.
Theorem 10. Assume that.
There exists a bounded function such that for all .
Then, the BVP (9) has at least a solution on provided that where .
Proof. Let ; define the set
To prove this theorem, we only need to show that maps in .
For , we have From (21), we have ; then, which implies that maps in . has at least a fixed point which is a solution of the BVP (9) according to Brower fixed-point theorem (Lemma 8). This completes the proof.
4. The Ulam Stability
Similar to the definitions of the Ulam stability for fractional differential equation , we introduce four types of the Ulam stability definitions for fractional difference equation.
Consider (1) and the following inequalities:
Definition 11. Equation (1) is the Ulam-Hyers stable if there exists a real number such that for each and for each solution of inequality (24), there exists a solution of (1) with Equation (1) is the generalized Ulam-Hyers stable if we substitute the function for the constant on inequality (26), where and .
Definition 12. Equation (1) is the Ulam-Hyers-Rassias stable with respect to if there exists a real number such that for each and for each solution of inequality (25), there exists a solution of (1) with Equation (1) is the generalized Ulam-Hyers-Rassias stable if we substitute the function for the function on inequalities (25) and (27).
Proof. By Lemma 3, the solution of the BVP (28) is given by From inequality (24), for , it follows that Combining (30) and (31), for , we have Then, Applying (29) to the previous inequality yields that where , thus; (1) is the Ulam-Hyers stable.
Theorem 16. Assume that and the following condition hold.
Let be an increasing function. There exists a constant such that
Let be a solution of inequality (25) and let be a solution of the following boundary value problem (28). Then, (1) is the Ulam-Hyers-Rassias stable provided that (29) holds.
As the applications of our main results, we consider the following examples.
Example 1. Consider the fractional difference BVP where for and conditions and are satisfied. Since inequalities (17) and (21) are satisfied if . According to Theorem 9, the BVP (36) has a unique solution. At the same time, the BVP (36) has at least a solution by Theorem 10.
This work was supported in part by the Natural Science Foundation of China (10971173), the National Natural Science Foundation of Hunan Province under Grant 13JJ3120, and the Construct Program of the Key Discipline in Hunan Province.
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