Research Article | Open Access

Changyou Wang, Haiqiang Zhang, Shu Wang, "Positive Solution of a Nonlinear Fractional Differential Equation Involving Caputo Derivative", *Discrete Dynamics in Nature and Society*, vol. 2012, Article ID 425408, 16 pages, 2012. https://doi.org/10.1155/2012/425408

# Positive Solution of a Nonlinear Fractional Differential Equation Involving Caputo Derivative

**Academic Editor:**Seenith Sivasundaram

#### Abstract

This paper is concerned with a nonlinear fractional differential equation involving Caputo derivative. By constructing the upper and lower control functions of the nonlinear term without any monotone requirement and applying the method of upper and lower solutions and the Schauder fixed point theorem, the existence and uniqueness of positive solution for the initial value problem are investigated. Moreover, the existence of maximal and minimal solutions is also obtained.

#### 1. Introduction

Fractional differential equation can be extensively applied to various disciplines such as physics, mechanics, chemistry, and engineering, see [1–3]. Hence, in recent years, fractional differential equations have been of great interest and there have been many results on existence and uniqueness of the solution of FDE, see [4–8]. Especially, Diethelm and Ford [9] have gained existence, uniqueness, and structural stability of solution of the type of fractional differential equation

where is a real number, denotes the Riemann-Liouville differential operator of order , and is the Taylor polynomial of order for the function at . Recently, Daftardar-Gejji and Jafari [10] have discussed the existence, uniqueness, and stability of solution of the system of nonlinear fractional differential equation where and denotes Caputo fractional derivative (see Definition 2.3). Delbosco and Rodino [11] have proved existence and uniqueness theorems for the nonlinear fractional equation where , is the Riemann-Liouville fractional derivative. Zhang [12] used the method of the upper and lower solution and cone fixed point theorem to obtain the existence and uniqueness of positive solution to (1.3). Yao [13] considered the existence of positive solution to (1.3) controlled by the power function employing Krasnosel’skii fixed point theorem of cone expansion-compression type. The existence of the local and global solution for (1.3) was obtained by Lakshmikantham and Vatsala [14] utilizing classical differential equation theorem.

More recently, Zhang [15] shows the existence of positive solutions to the singular boundary value problem for fractional differential equation where is the Riemann-Liouville fractional derivative of order , .

However, in the previous works, the nonlinear term has to satisfy the monotone or others control conditions. In fact, the fractional differential equations with nonmonotone function can respond better to impersonal law, so it is very important to weaken monotone condition. Considering this, in this paper, we mainly investigate the fractional differential Equation (1.2) without any monotone requirement on nonlinear term by constructing upper and lower control function and exploiting the method of upper and lower solutions and the Schauder fixed point theorem. The existence and uniqueness of positive solution for (1.2) are obtained. Some properties concerning the maximal and minimal solutions are also given. This work is motivated by the above references and my previous work [16, 17]. Other related results on the fractional differential equations can be found in [18–24].

This paper is organized as follow. In Section 2, we recall briefly some notions of the fractional calculus and the theory of the operators for integration and differentiation of fractional order. Section 3 is devoted to the study of the existence and uniqueness of positive solution for (1.2) utilizing the upper and lower solution method and the Schauder fixed point theorem. The existence of maximal and minimal solutions for (1.2) is given in Section 4.

#### 2. Preliminaries and Notations

First, we give some basic definitions and theorems which are basically used throughout this paper. denotes the space of continuous functions defined on and denotes the class of all real valued functions defined on which have continuous th order derivative.

*Definition 2.1. *Let and , then the expression
is called the (left-sided) Riemann-Liouville integral of order .

*Definition 2.2. *Let , then the expression
is called the (left-sided) Riemann-Liouville derivative of of order whenever the expression on the right-hand side is defined.

*Definition 2.3. *Let and , then the expression
is called the (left-sided) Caputo derivative of of order .

In further discussion we will denote , , and as , , and , respectively.

Lemma 2.4 (see [25, 26]). *Let and , then we one has
*

Lemma 2.5 (see, [10]). *If the function is , then the initial value problem (1.2) is equivalent to the Volterra integral equations
*

*Proof. *Suppose satisfies the initial value problem (1.2), then applying to both sides of (1.2) and using Lemma 2.4 (2.7) follows. Conversely, suppose satisfies (2.7). Then observe that exists and is integrable, because
which exists and is integrable as is . Thus exists.

Applying on both sides of (2.7), one has
as is continuous and . Hence satisfies (1.2). Moreover, from (2.4), hold.

Let be the Banach space endowed with the infinity norm and a nonempty closed subset of defined as . The positive solution which we consider in this paper is a function such that .

According to Lemma 2.5, (1.2) is equivalent to the fractional integral Equation (2.7). The integral equation (2.7) is also equivalent to fixed point equation , where operator is defined as then we have the following lemma.

Lemma 2.6. *Let a given continuous function. Then the operator is completely continuous.*

*Proof. *Let be bounded, that is, there exists a positive constant such that for any . Since is a given continuous function, we have
where .

Let , then for any , we have
Thus,
Hence is uniformly bounded.

Now, we prove that is continuous. Since is continuous function in a compact set , then it is uniformly continuous there. Thus given , we can find such that whenever , where . Then
proving the continuity of the operators .

Now, we will prove that the operator is equicontinuous. For each , any and . Let , then when , we have
The Arzela-Ascoli Theorem implies that is completely continuous. The proof is therefore completed.

Lemma 2.7. *If the operator is the contraction mapping, where is the Banach space, then has a unique fixed point in .**Let be a given function. Take , and . For any one defines the upper-control function , and lower-control function , obviously is monotonous nondecreasing on and .*

*Definition 2.8. *Let , and satisfy
then the functions are called a pair of order upper and lower solutions for (1.2).

#### 3. Existence and Uniqueness of Positive Solution

Now, we give and prove the main results of this paper.

Theorem 3.1. *Assume is continuous, and are a pair of order upper and lower solutions of (1.2), then the boundary value problem (1.2) exists one solution ; moreover,
*

*Proof. *Let
endowed with the norm , then we have . Hence is a convex, bounded, and closed subset of the Banach space . According to Lemma 2.6, the operator is completely continuous. Then we need only to prove .

For any , we have , then
Hence , that is, . According to Schauder fixed point theorem, the operator exists at least one fixed point . Therefore the boundary value problem (1.2) exists at least one solution , and .

Corollary 3.2. *Assume is continuous, and there exist , such that
**
then the boundary value problem (1.2) exists at least one positive solution , moreover
*

*Proof. *By assumption (3.4) and the definition of control function, we have
Now, we consider the equation
Obviously, (3.7) has a positive solution
namely, is a upper solution of (1.2). In the similar way, we obtain is the lower solution of (1.2). An application of Theorem 3.1 now yields that the boundary value problem (1.2) exists at least one positive solution , moreover

Corollary 3.3. *Assume is continuous, where , moreover
**
then the boundary value problem (1.2) has at least one positive solution .*

*Proof. *By assumption (3.9), there are positive constants , such that whenever . Let , then , . By the definition of control function, one has .

Now, we consider the equation
Obviously, (3.11) has a positive solution
namely, is the upper solution of (1.2). In the similar way, we obtain is the lower solution of (1.2). Therefore, the boundary value problem of (1.2) has at least one positive solution , what is more, we have

Corollary 3.4. *Assume is continuous, where , moreover
**
then the boundary value problem (1.2) exists at least at one positive solution .*

*Proof. *According to , there exists , such that for any , we have
By the definition of control function, we have
We now consider the equation
According to Lemma 2.5, (3.17) is equivalent to the integral equation
Let be an operator as follows:
by Lemma 2.6, the operator is completely continuous.

Let
where and satisfies that , then is convex, bounded, and closed subset of the Banach space . For any , we have
then
thus
Hence, the Schauder fixed theorem assures that the operator has at least one fixed point and then (3.17) has at least one positive solution , therefore we have
Combining condition (3.16), we have
Obviously, is the upper solution of initial value problem (1.2), and is the lower solution. By Theorem 3.1, system (1.2) has at least one positive solution .

Corollary 3.5. *Assume is continuous and there exists , such that
**
then the boundary value problem (1.2) has at least one positive solution , moreover
*

*Proof. *By the definition of control function, we have
By Corollary 3.2, the boundary value problem (1.2) has at least one positive solution , moreover

Theorem 3.6. *Let the conditions in Theorem 3.1 hold. Moreover for any , there exists , such that
**
then when , the boundary value problem (1.2) has a unique positive solution .*

*Proof. *According to Theorem 3.1, if the conditions in Theorem 3.1 hold, then the boundary value problem (1.2) has at least one positive solution in . Hence we need only to prove that the operator defined in (2.10) is the contraction mapping in . In fact, for any , by assumption (3.30), we have
Thus, when , the operator is the contraction mapping. Then by Lemma 2.7, the boundary value problem (1.2) has a unique positive solution .

#### 4. Maximal and Minimal Solutions Theorem

In this section, we consider the existence of maximal and minimal solutions for (1.2).

*Definition 4.1. *Let be a solution of (1.2) in , then is said to be a maximal solution of (1.2), if for every solution of (1.2) existing on the inequality holds. A minimal solution may be defined similarly by reversing the last inequality.

Theorem 4.2. *Let be a given continuous and monotone nondecreasing with respect to the second variable. Assume that there exist two positive constants such that
**
Then there exist maximal solution and minimal solution of (1.2) on , moreover
*

*Proof. *It is easy to know that and are the upper and lower solutions of (1.2), respectively. Then by using as a pair of coupled initial iterations we construct two sequences from the following linear iteration process:
It is easy to show from the monotone property of and condition (4.1) that the sequences possess the following monotone property:
The above property implies that
exist and satisfy the relation
Letting in (4.3) shows that and satisfy the equations
It is easy to verify that the limits and are maximal and minimal solutions of (1.2) in , respectively, furthermore, if then is the unique solution in , and hence the proof is completed.

#### Acknowledgments

The authors are grateful to the referee for the comments. This work is supported by the Science and Technology Project of Chongqing Municipal Education Committee (Grants nos. KJ110501 and KJ120520) of China, Natural Science Foundation Project of CQ CSTC (Grant no. cstc2012jjA20016) of China, and the NSFC (Grant no. 11101298) of China.

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#### Copyright

Copyright © 2012 Changyou Wang et al. 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.