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# The existence of solutions for nonlinear fractional multipoint boundary value problems at resonance

Na Xu, Wenbin Liu* and Lishun Xiao

### Author affiliations

College of Sciences, China University of Mining and Technology, Xuzhou, Jiangsu, 221116, P.R. China

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Boundary Value Problems 2012, 2012:65  doi:10.1186/1687-2770-2012-65

 Received: 17 January 2012 Accepted: 18 May 2012 Published: 28 June 2012

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

A class of nonlinear fractional multipoint boundary value problems at resonance is considered in this article. The existence results are obtained by the method of the coincidence degree theory of Mawhin. An example is given to illustrate the results.

MSC: 34A08.

##### Keywords:
coincidence degree; fractional differential equation; resonance; multipoint boundary conditions

### 1 Introduction

The subject of fractional calculus has gained considerable popularity during the past decades, due mainly to its frequent appearance in a variety of different areas such as physics, aerodynamics, polymer rheology, etc. (see [1-3]). Many methods have been introduced for solving fractional differential equations (FDEs for short in the remaining), such as the Laplace transform method, the iteration method, the Fourier transform method, etc. (see [4]).

Recently, there have been many works related to the existence of solutions for multipoint boundary value problems (BVPs for short in the remaining) at nonresonance of FDEs (see [5-11]). Motivated by the above articles and recent studies on FDEs (see [12-19]), we consider the existence of solutions for a nonlinear fractional multipoint BVPs at resonance in this article.

In [16], Zhang and Bai considered the following fractional three-point boundary value problems at resonance:

where is a natural number; is a real number; and are the standard Riemann-Liouville derivative and integral respectively; is continuous; ; are given constants such that . In their article, they made the operator and got . In [17], Bai discussed fractional m-point boundary value problems at resonance with the case of .

In 2010, Bai and Jiang studied the fractional differential equation of boundary value problems at resonance with the case of respectively (see [18,19]), and we can see that they obtained the results by the assumption that a specific algebraic expression is not equal to zero; for example,

is referred to as a condition in [18]. We will show that the assumption like above is not necessary.

In this article, we will use the coincidence degree theory to study the existence of solutions for a nonlinear FDEs at resonance which is given by

(1.1)

with boundary conditions

(1.2)

where ; ; ; ; with satisfying Carathéodory conditions; and are the standard Riemann-Liouville fractional derivative and fractional integral respectively.

BVPs (1.1)-(1.2) being at resonance means that the associated linear homogeneous equation with boundary conditions (1.2) has as a nontrivial solution, where , .

We will always suppose that the following conditions hold:

(C)

The rest of this article is organized as follows: In Section 2, we give some definitions, lemmas and notations. In Section 3, we establish theorems of existence result for BVPs (1.1)-(1.2). In Section 4, we give an example to illustrate our result.

### 2 Preliminaries

We present here some necessary basic knowledge and definitions of the fractional calculus theory, which can be found in [1-3].

Definition 2.1 The Riemann-Liouville fractional integral of order of a function is given by

where is the Gamma function, provided the right side is pointwise defined on .

Definition 2.2 The Riemann-Liouville fractional derivative of order of a function is given by

where , provided the right side is pointwise defined on .

Definition 2.3 ([18])

We say that the map satisfies Carathéodory conditions with respect to if the following conditions are satisfied:

(i) for each , the mapping is Lebesgue measurable;

(ii) for almost every , the mapping is continuous on ;

(iii) for each , there exists a such that, for a.e. and every , we have .

Lemma 2.4 ([2])

Assume, , then. And, for all, , we have that

Lemma 2.5 ([2])

Let, and assume that, then the following equality holds almost everywhere on,

Now, we briefly recall some notations and an abstract existence result, which can be found in [20]. Let YZ be real Banach spaces, be a Fredholm map of index zero, and be continuous projectors such that

It follows that is invertible. We denote the inverse by . If Ω is an open bounded subset of Y such that , the map will be called L-compact on Ω if is bounded and is compact.

Lemma 2.6 ([20])

LetLbe a Fredholm operator of index zero andNbeL-compact on. The equationhas at least one solution inif the following conditions are satisfied:

(i) for each;

(ii) for each;

(iii) ,

whereis a projection such thatandis a any isomorphism.

In this article, we use the Banach space with the norm .

Lemma 2.7 ([16])

Givenand, for any, (), we can define a Banach space

with the norm defined by.

Lemma 2.8 ([16])

is a sequentially compact set if and only ifEis uniformly bounded and equicontinuous. Here, a uniform bound means that there exists a constantwith each, such that

and equicontinuation means that there exists awithfor any, and, such that

In this article, let with the norm and with the norm . Define the operator by

(2.1)

where . Define the operator by

(2.2)

Thus, BVP (1.1) can be written as for each .

### 3 Main results

First, let us introduce the following notations for convenience, with setting and with ,

Then, let us make some assumptions which will be used throughout the article.

(H1) There exist functions and a constant such that for all , ,

(H2) For any , , there exists a constant such that if , then either

(H3) For any , , there exists a constant such that if , then either

Theorem 3.1If conditions (C), (H1)-(H3) hold, then BVPs (1.1)-(1.2) have at least one solution provided that.

In order to obtain our main result, we first present and prove Lemmas 3.2-3.8. Now, let us define operators () as follows:

Lemma 3.2If condition (C) holds andLis defined by (2.1), then

Proof By (2.1) and Lemma 2.5, has a solution

Combining with the condition (1.2), we get .

Suppose , then there exists such that , i.e., , . By Lemma 2.5, we have

Then in view of condition (C), (1.2) and Lemma 2.4, x satisfies

(3.1)

On the other hand, suppose and it satisfies (3.1), let , then , , i.e., . Therefore, we obtain that

□

Lemma 3.3If condition (C) holds, then there exist two constantsandwithsuch that.

Proof From , we obtain that for any nonnegative integer l, there exists such that . If else, we obtain that , , .

If , we have

It is equal to

Since the determinant of coefficients is not equal to zero, we have that (), which is a contradiction to condition (C).

If , we get

Similarly, we can deduce that the determinant of coefficients is not equal to zero, so we have that (), which is a contradiction to condition (C). Thus, there exists such that .

Similarly, from , we have that there exists a constant such that

(3.2)

Let

we shall prove that S is a finite set. If else, there exists a strict increasing sequence such that

Since , we have . Thus,

which is a contradiction to (3.2). Therefore, there exists two constants and with such that . □

Lemma 3.4If the condition (C) holds andLis defined by (2.1), thenLis a Fredholm operator of index zero. Define the linear operatorwith, then it is the inverse ofL. Furthermore, we have

Proof For each and with , define operator by

(3.3)

where

(3.4)

It is clear that . It follows from (3.4), the definition of and that

(3.5)

similarly, we can derive that

(3.6)

Hence, for each and , it follows from the (3.3)-(3.6) that

Furthermore, Q is a continuous linear projector.

For each , we have , i.e., . On the other hand, for each , we have that

However, the determinant of coefficients is as follows

then we have (), i.e., . Thus, .

Take any in the type , obviously, and , so . For any with , by Lemma 3.2, we have

That is,

but the determinant of coefficients is as follows

we can deduce that . Hence, . Furthermore, we get . Therefore, , which means that L is a Fredholm operator of index zero.

Let operator and

(3.7)

It is easy to calculate that ; furthermore, P is a continuous linear projector. Obviously

It is clear that .

For any , in view of the definition of operators Kp and L, we have . On the other hand, if , we have , . Therefore, by Lemma 2.5 and definitions of operators and L, we know that , which implies that . By the definition of , we have

It follows from Lemma 2.4 that

Then, we have

By the definition of the norm in space Y, we get . □

Lemma 3.5Assumeis an open bounded subset such that, andNis defined by (2.2), thenNisL-compact on.

Proof In order to prove N is L-compact, we only need to prove that is bounded and is compact. Since the function f satisfies Carathéodory conditions and , for each , there exists a such that, for a.e. and every , we have . By the definition of operators Q and on the interval , it is easy to get that and are bounded. Thus, there exists a constant with each , such that .

For all , , , we have

and

Since is uniformly continuous on and , so and are equicontinuous. By Lemma 2.8, we get that is completely continuous. □

Lemma 3.6Suppose (H1)-(H3) hold, then the setis bounded.

Proof Taking any , then we have , which yields and , i.e., for all . It follows from (H2) and (H3) that there exists such that . Then we can get that

Furthermore, we have that, with setting ,

(3.8)

(3.9)

By (3.7)-(3.9) and Lemma 2.4, we have that

As before, for any , we have and . From Lemma 3.4 and for each , we can get

Furthermore, we have

By (H1) and the definition of N, we have

where . Since and hold true, we can get that

which yield that

Furthermore, from the previous inequalities, we know that

Since , there exist constants such that

Therefore, is bounded. □

Lemma 3.7Suppose (H2) and (H3) hold, then the setis bounded.

Proof For any and , then and . By (H2), we get that , then we have . By (H3), we have that , thus . Therefore, is bounded. □

Lemma 3.8If the first parts of (H2) and (H3) hold, then the setis bounded.

Proof Taking any and , we have . For all , we define the isomorphism by

By the definition of the set , we can get that

(3.10)

If , we have

By the first parts of (H2) and (H3), similar to the proof of Lemma 3.7, then

Therefore, is bounded.

If , we have .

If , we get that and , similar to the proof of Lemma 3.7, is bounded. If else, we have that and . It contradicts (3.10), thus is bounded. □

Remark 3.9 If the other parts of (H2) and (H3) hold, then the set is bounded.

Now with Lemmas 3.2-3.8 in hands, we can begin to prove our main result - Theorem 3.1.

Proof of Theorem 3.1 Assume that Ω is a bounded open set of Y with . By Lemma 3.5, N is L-compact on . Then by Lemmas 3.6 and 3.7, we have

(i) for every ;

(ii) for every .

Finally, we will prove that (iii) of Lemma 2.6 is satisfied. We let I as the identity operator in the Banach space Y and , according to Lemma 3.8 (or Remark 3.9) we know that for all , . By the homotopic property of degree, we have

so (iii) of Lemma 2.6 is satisfied.

Consequently, by Lemma 2.6, the equation has at least one solution in . Namely, BVPs (1.1)-(1.2) have at least one solution in the space Y. □

According to Theorem 3.1, we have the following corollary.

Corollary 3.10Suppose that (H1) is replaced by the following condition,

(H4) there exist functionsand a constantsuch that for all, ,

or

and the others in Theorem 3.1 are not changed, then BVPs (1.1)-(1.2) have at least one solution.

### 4 An example

Example Consider the following boundary value problem for all :

(4.1)

Let , and , , , , , , , . We can get that the condition (C) holds, i.e., , , . Moreover,

Thus, we have

Taking , , , , , , , , , , , , we can calculate that (H1)-(H3) hold. Furthermore, we can get

By Corollary 3.10, the BVP (4.1) has at least one solution in .

### Competing interests

The authors declare that they have no competing interests.

### Author’s contributions

NX designed all the steps of proof in this research and also wrote the article. WBL suggested many good ideas in this article. LSX helped to draft the first manuscript and gave an example to illustrate our result. All authors read and approved the final manuscript.

### Acknowledgement

The authors would like to acknowledge the anonymous referee for many helpful comments and valuable suggestions on this article. This work is sponsored by Fundamental Research Funds for the Central Universities (2012LWB44).

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