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Abstract and Applied Analysis

Volume 2011 (2011), Article ID 390543, 16 pages

http://dx.doi.org/10.1155/2011/390543

## Positive Solutions to Boundary Value Problems of Nonlinear Fractional Differential Equations

^{1}School of Science, University of Jinan, Jinan, Shandong 250022, China^{2}Department of Mathematics and Statistics, Missouri University of Science and Technology Rolla, MO 65409-0020, USA

Received 23 September 2010; Revised 5 November 2010; Accepted 6 December 2010

Academic Editor: Josef Diblík

Copyright © 2011 Yige Zhao 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.

#### Abstract

We study the existence of positive solutions for the boundary value problem of nonlinear fractional differential equations , , , where is a real number, is the Riemann-Liouville fractional derivative, is a positive parameter, and is continuous. By the properties of the Green function and Guo-Krasnosel'skii fixed point theorem on cones, the eigenvalue intervals of the nonlinear fractional differential equation boundary value problem are considered, some sufficient conditions for the nonexistence and existence of at least one or two positive solutions for the boundary value problem are established. As an application, some examples are presented to illustrate the main results.

#### 1. Introduction

Fractional differential equations have been of great interest recently. It is caused both by the intensive development of the theory of fractional calculus itself and by the applications; see [1–4]. It should be noted that most of papers and books on fractional calculus are devoted to the solvability of linear initial fractional differential equations on terms of special functions.

Recently, there are some papers dealing with the existence of solutions (or positive solutions) of nonlinear initial fractional differential equations by the use of techniques of nonlinear analysis (fixed-point theorems, Leray-Schauder theory, Adomian decomposition method, etc.); see [5–11]. In fact, there has the same requirements for boundary conditions. However, there exist some papers considered the boundary value problems of fractional differential equations; see [12–19].

Yu and Jiang [19] examined the existence of positive solutions for the following problem: where is a real number, , and is the Riemann-Liouville fractional differentiation. By using the properties of the Green function, they obtained some existence criteria for one or two positive solutions for singular and nonsingular boundary value problems by means of the Krasnosel'skii fixed point theorem and a mixed monotone method.

To the best of our knowledge, there is very little known about the existence of positive solutions for the following problem: where is a real number, is the Riemann-Liouville fractional derivative, is a positive parameter and is continuous.

On one hand, the boundary value problem in [19] is the particular case of problem (1.2) as the case of . On the other hand, as Yu and Jiang discussed in [19], we also give some existence results by the fixed point theorem on a cone in this paper. Moreover, the purpose of this paper is to derive a -interval such that, for any lying in this interval, the problem (1.2) has existence and multiplicity on positive solutions.

In this paper, by analogy with boundary value problems for differential equations of integer order, we firstly give the corresponding Green function named by fractional Green's function and some properties of the Green function. Consequently, the problem (1.2) is reduced to an equivalent Fredholm integral equation. Finally, by the properties of the Green function and Guo-Krasnosel'skii fixed point theorem on cones, the eigenvalue intervals of the nonlinear fractional differential equation boundary value problem are considered, some sufficient conditions for the nonexistence and existence of at least one or two positive solutions for the boundary value problem are established. As an application, some examples are presented to illustrate the main results.

#### 2. Preliminaries

For the convenience of the reader, we give some background materials from fractional calculus theory to facilitate analysis of problem (1.2). These materials can be found in the recent literature; see [19–21].

*Definition 2.1 (see [20]). *The Riemann-Liouville fractional derivative of order of a continuous function is given by
where , denotes the integer part of number *α*, provided that the right side is pointwise defined on .

*Definition 2.2 (see [20]). *The Riemann-Liouville fractional integral of order of a function is given by
provided that the right side is pointwise defined on .

From the definition of the Riemann-Liouville derivative, we can obtain the following statement.

Lemma 2.3 (see [20]). *Let . If we assume , then the fractional differential equation
**
has , , , as unique solutions, where is the smallest integer greater than or equal to α.*

Lemma 2.4 (see [20]). *Assume that with a fractional derivative of order that belongs to . Then
**
for some , , where is the smallest integer greater than or equal to α.*

In the following, we present the Green function of fractional differential equation boundary value problem.

Lemma 2.5 (see [19]). *Let and . The unique solution of problem
**
is
**
where
**
Here is called the Green function of boundary value problem (2.5).*

The following properties of the Green function play important roles in this paper.

Lemma 2.6 (see [19]). *The function defined by (2.7) satisfies the following conditions: *(1)*, for ; *(2)*, for ; *(3)*, for ; *(4)*, for . *

The following lemma is fundamental in the proofs of our main results.

Lemma 2.7 (see [21]). *Let be a Banach space, and let be a cone in . Assume are open subsets of with , and let be a completely continuous operator such that, either *(1)*, , , or *(2)*, , , . **Then has a fixed point in .*

For convenience, we set ; then

#### 3. Main Results

In this section, we establish the existence of positive solutions for boundary value problem (1.2).

Let Banach space be endowed with the norm . Define the cone by

Suppose that is a solution of boundary value problem (1.2). Then

We define an operator as follows:

By Lemma 2.6, we have Thus, .

Then we have the following lemma.

Lemma 3.1. * is completely continuous.*

*Proof. * The operator is continuous in view of continuity of and . By means of the Arzela-Ascoli theorem, is completely continuous.

For convenience, we denote

Theorem 3.2. *If there exists such that holds, then for each
**
the boundary value problem (1.2) has at least one positive solution. Here we impose if and if .*

*Proof. * Let satisfy (3.6) and be such that
By the definition of , we see that there exists such that
So if with , then by (3.7) and (3.8), we have
Hence, if we choose , then

Let be such that
If with , then by (3.7) and (3.11), we have
Thus, if we set , then
Now, from (3.10), (3.13), and Lemma 2.7, we guarantee that has a fixed-point with , and clearly is a positive solution of (1.2). The proof is complete.

Theorem 3.3. *If there exists such that holds, then for each
**
the boundary value problem (1.2) has at least one positive solution. Here we impose if and if .*

*Proof. * Let satisfy (3.14) and be such that
From the definition of , we see that there exists such that
Further, if with , then similar to the second part of Theorem 3.2, we can obtain that . Thus, if we choose , then

Next, we may choose such that
We consider two cases.*Case 1. * Suppose is bounded. Then there exists some , such that
We define , and with , then
Hence,
*Case 2. * Suppose is unbounded. Then there exists some , such that
Let with . Then by (3.15) and (3.18), we have
Thus, (3.21) is also true.In both Cases 1 and 2, if we set , then
Now that we obtain (3.17) and (3.24), it follows from Lemma 2.7 that has a fixed-point with . It is clear is a positive solution of (1.2). The proof is complete.

Theorem 3.4. *Suppose there exist , such that , and satisfy
**
Then the boundary value problem (1.2) has a positive solution with .*

*Proof. * Choose ; then for , we have

On the other hand, choose , then for , we have
Thus, by Lemma 2.7, the boundary value problem (1.2) has a positive solution with . The proof is complete.

For the reminder of the paper, we will need the following condition., where .

Denote

In view of the continuity of and , we have and .

Theorem 3.5. *Assume holds. If and , then the boundary value problem (1.2) has at least two positive solutions for each .*

*Proof. * Define
By the continuity of , and , we have that is continuous and
By (3.28), there exists , such that
then for , there exist constants with
Thus,

On the other hand, applying the conditions and , there exist constants , with
Then
By (3.34) and (3.37), (3.35) and (3.38), combining with Theorem 3.4 and Lemma 2.7, we can complete the proof.

Corollary 3.6. *Assume holds. If or , then the boundary value problem (1.2) has at least one positive solution for each .*

Theorem 3.7. *Assume holds. If and , then for each , the boundary value problem (1.2) has at least two positive solutions.*

*Proof. * Define
By the continuity of , and , we easily see that is continuous and
By (3.29), there exists , such that
For , there exist constants , with
Therefore,

On the other hand, using , we know that there exists a constant with
In view of , there exists a constant such that

Let
It is easily seen that
By (3.45) and (3.48), combining with Theorem 3.4 and Lemma 2.7, the proof is complete.

Corollary 3.8. *Assume holds. If or , then for each , the boundary value problem (1.2) has at least one positive solution.*

By the above theorems, we can obtain the following results.

Corollary 3.9. *Assume holds. If , , or , , then for any , the boundary value problem (1.2) has at least one positive solution.*

Corollary 3.10. *Assume holds. If ,, or if , , then for any , the boundary value problem (1.2) has at least one positive solution.*

*Remark 3.11. *For the integer derivative case , Theorems 3.2–3.7 also hold; we can find the corresponding existence results in [22].

#### 4. Nonexistence

In this section, we give some sufficient conditions for the nonexistence of positive solution to the problem (1.2).

Theorem 4.1. *Assume holds. If and , then there exists a such that for all , the boundary value problem (1.2) has no positive solution.*

*Proof. * Since and , there exist positive numbers , and , such that and
Let . Then we have
Assume is a positive solution of (1.2). We will show that this leads to a contradiction for . Since for ,
which is a contradiction. Therefore, (1.2) has no positive solution. The proof is complete.

Theorem 4.2. *Assume holds. If and , then there exists a such that for all , the boundary value problem (1.2) has no positive solution.*

*Proof. * By and , we know that there exist positive numbers , and , such that and
Let . Then we get
Assume is a positive solution of (1.2). We will show that this leads to a contradiction for . Since for ,
which is a contradiction. Thus, (1.2) has no positive solution. The proof is complete.

#### 5. Examples

In this section, we will present some examples to illustrate the main results.

*Example 5.1. *Consider the boundary value problem

Since , we have
Let . Then we have , . Choose . Then . So holds. Thus, by Theorem 3.2, the boundary value problem (5.1) has a positive solution for each .

*Example 5.2. *Discuss the boundary value problem

Since , we have and . Let . Then we have . Choose . Then . So holds. Thus, by Theorem 3.3, the boundary value problem (5.3) has a positive solution for each .

*Example 5.3. *Consider the boundary value problem

Since , we have and . Let . Then we have , , and .

(i)Choose . Then . So holds. Thus, by Theorem 3.2, the boundary value problem (5.4) has a positive solution for each .(ii)By Theorem 4.1, the boundary value problem (5.4) has no positive solution for all .(iii)By Theorem 4.2, the boundary value problem (5.4) has no positive solution for all .

*Example 5.4. *Consider the boundary value problem

Since , we have and . Let . Then we have , , , and .

(i)Choose . Then . So holds. Thus, by Theorem 3.3, the boundary value problem (5.5) has a positive solution for each .(ii)By Theorem 4.1, the boundary value problem (5.5) has no positive solution for all .(iii)By Theorem 4.2, the boundary value problem (5.5) has no positive solution for all .

#### Acknowledgments

The authors sincerely thank the reviewers for their valuable suggestions and useful comments that have led to the present improved version of the original manuscript. This research is supported by the Natural Science Foundation of China (11071143, 11026112, 60904024), the Natural Science Foundation of Shandong (Y2008A28, ZR2009AL003), University of Jinan Research Funds for Doctors (XBS0843) and University of Jinan Innovation Funds for Graduate Students (YCX09014).

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