## Boundary Value Problems

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# Some existence results for a nonlinear fractional differential equation on partially ordered Banach spaces

Boundary Value Problems20132013:112

DOI: 10.1186/1687-2770-2013-112

Accepted: 16 April 2013

Published: 3 May 2013

## Abstract

By using fixed point results on cones, we study the existence and uniqueness of positive solutions for some nonlinear fractional differential equations via given boundary value problems. Examples are presented in order to illustrate the obtained results.

## 1 Introduction

The field of fractional differential equations has been subjected to an intensive development of the theory and the applications (see, for example, [16] and the references therein). 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. There are some papers dealing with the existence of solutions of nonlinear initial value problems of fractional differential equations by using the techniques of nonlinear analysis such as fixed point results, the Leray-Schauder theorem, stability, etc. (see, for example, [719] and the references therein). In fact, fractional differential equations arise in many engineering and scientific disciplines such as physics, chemistry, biology, economics, control theory, signal and image processing, biophysics, blood flow phenomena and aerodynamics (see, for example, [2023] and the references therein). The main advantage of using the fractional nonlinear differential equations is related to the fact that we can describe the dynamics of complex non-local systems with memory. In this line of taught, the equations involving various fractional orders are important from both theoretical and applied view points. We need the following notions.

Definition 1.1 ([1, 4])

For a continuous function , the Caputo derivative of fractional order α is defined by

where , and denotes the integer part of α.

Definition 1.2 ([1, 4])

The Riemann-Liouville fractional derivative of order α for a continuous function f is defined by

where the right-hand side is pointwise defined on .

Definition 1.3 ([1, 4])

Let be an interval in and . The Riemann-Liouville fractional order integral of a function is defined by

whenever the integral exists.

Suppose that E is a Banach space which is partially ordered by a cone , that is, if and only if . We denote the zero element of E by θ. A cone P is called normal if there exists a constant such that implies (see [24]). Also, we define the order interval for all [24]. We say that an operator is increasing whenever implies . Also, means that there exist and such that (see [24]). Finally, put for all . It is easy to see that is convex and for all . We recall the following in our results. Let E be a real Banach space and let P be a cone in E. Let be an interval and let τ and φ be two positive-valued functions such that for all and is a surjection. We say that an operator is τ-φ-concave whenever for all and [13]. We say that A is φ-concave whenever for all t [13]. We recall the following result.

Theorem 1.1 ([13])

Let E be a Banach space, let P be a normal cone in E, and let be an increasing and τ-φ-concave operator. Suppose that there exists such that . Then there are and such that and , the operator A has a unique fixed point , and for and the sequence with , we have .

## 2 Main results

We study the existence and uniqueness of a solution for the fractional differential equation

on partially ordered Banach spaces with two types of boundary conditions and two types of fractional derivatives, Riemann-Liouville and Caputo.

### 2.1 Existence results for the fractional differential equation with the Riemann-Liouville fractional derivative

First, we study the existence and uniqueness of a positive solution for the fractional differential equation
(2.1)
(2.2)

where is the Riemann-Liouville fractional derivative of order α. Let . Consider the Banach space of continuous functions on with the sup norm and set . Then P is a normal cone.

Lemma 2.1 Let , , , and . Then the problem with the boundary value condition has a solution if and only if is a solution of the fractional integral equation
where
Proof From and the boundary condition, it is easy to see that . By the definition of a fractional integral, we get
Thus, and
Since , we obtain
Hence,

This completes the proof. □

Now, we are ready to state and prove our first main result.

Theorem 2.2 Let be given and let τ and φ be two functions on such that for all . Suppose that is a surjection and is increasing in u for each fixed t, and for all and . Assume that there exist , and such that

for all , where is the green function defined in Lemma 2.1. Then the problem (2.1) with the boundary value condition (2.2) has a unique positive solution . Moreover, for the sequence , we have for all .

Proof By using Lemma 2.1, the problem is equivalent to the integral equation
where
Define the operator by . Then u is a solution for the problem if and only if . It is easy to check that the operator A is increasing on P. On the other hand,
for all and . Thus, the operator A is τ-φ-concave. Since

for all , we get . Now, by using Theorem 1.1, the operator A has a unique positive solution . This completes the proof. □

Here, we give the following example to illustrate Theorem 2.2.

Example 2.1 Let be given. Consider the periodic boundary value problem
where , g is continuous on and . Put
Then . Now, define , , , and also for all t. Then is a surjection and for all . For each , we have
Now, put , and . Then we get
and

Thus, by using Theorem 2.2, the problem has a unique solution in .

### 2.2 Existence results for the fractional differential equation with the Caputo fractional derivative

Here, we study the existence and uniqueness of a positive solution for the fractional differential equation
(2.3)
(2.4)
where is the Caputo fractional derivative of order α. Let be the Banach space of continuous functions on with the sup norm and

It is known that P is a normal cone. Similar to the proof of Lemma 2.1, we can prove the following result.

Lemma 2.3 Let , , , and . Then the problem with the boundary value conditions and has a solution if and only if is a solution of the fractional integral equation , where
Theorem 2.4 Let be given and let τ and φ be two positive-valued functions on such that for all . Suppose that is a surjection and is increasing in u for each fixed t, whenever and otherwise, and also for all and . Assume that there exist , and such that

for all , where is the green function defined in Lemma 2.3. Then the problem (2.3) with the boundary value conditions (2.4) has a unique positive solution . Moreover, for the sequence , we have for all .

Proof It is sufficient to define the operator by

Now, by using a similar proof of Theorem 2.2, one can show that for all and , and also the operator A is τ-φ-concave. By using Theorem 1.1, the operator A has a unique positive solution . This completes the proof by using Lemma 2.3. □

Below we present an example to illustrate Theorem 2.4.

Example 2.2 Let . Consider the periodic boundary value problem
where g is a continuous function on with . Put , and
Then . Now, define , , , and . Then it is easy to see that is a surjection map and for . Also, we have
for all . Now, put , and also . Then we have
and

Thus, by using Theorem 2.4, the problem has a unique solution in .

## Declarations

### Acknowledgements

This work is partially supported by the Scientific and Technical Research Council of Turkey. Research of the third and forth authors was supported by Azarbaidjan Shahid Madani University. Also, the authors express their gratitude to the referees for their helpful suggestions which improved final version of this paper.

## Authors’ Affiliations

(1)
Department of Chemical and Materials Engineering, Faculty of Engineering, King Abdulaziz University
(2)
Department of Mathematics, Cankaya University
(3)
Institute of Space Sciences
(4)
Department of Mathematics, Texas A&M University
(5)
Department of Mathematics, Azarbaidjan Shahid Madani University

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