# Multiple monotone positive solutions for higher order differential equations with integral boundary conditions

Xinan Hao* and Lishan Liu

Author Affiliations

School of Mathematical Sciences, Qufu Normal University, Qufu, Shandong, 273165, P.R. China

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Boundary Value Problems 2014, 2014:74  doi:10.1186/1687-2770-2014-74

 Received: 17 January 2014 Accepted: 19 March 2014 Published: 28 March 2014

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 credited.

### Abstract

This paper investigates the higher order differential equations with nonlocal boundary conditions

The existence results of multiple monotone positive solutions are obtained by means of fixed point index theory for operators in a cone.

MSC: 34B10, 34B18.

##### Keywords:
monotone positive solutions; multiplicity; higher order differential equations; nonlocal boundary conditions

### 1 Introduction

In this paper, we are concerned with the existence of multiple monotone positive solutions for the higher order differential equation

(1.1)

subject to the following integral boundary conditions:

(1.2)

where , is continuous in which , A and B are right continuous on , left continuous at , and nondecreasing on , with ; and denote the Riemann-Stieltjes integrals of v with respect to A and B, respectively.

Boundary value problems (BVPs for short) for nonlinear differential equations arise in many areas of applied mathematics and physics. Many authors have discussed the existence of positive solutions for second order or higher order differential equations with boundary conditions defined at a finite number of points, for instance, [1-12]. In [2], Graef and Yang considered the following nth-order multi-point BVP:

where , is a parameter, g and f are continuous functions, , for and . The authors obtained the existence and nonexistence results of positive solutions by using Krasnosel’skii’s fixed point theorem in cones. In [5], we studied the following second order m-point nonhomogeneous BVP:

where , (), , . The authors obtained the existence, nonexistence and multiplicity of positive solutions by using the Krasnosel’skii-Guo fixed point theorem, the upper-lower solutions method and topological degree theory.

Boundary value problems with integral boundary conditions for ordinary differential equations represent a very interesting and important class of problems and arise in the study of various physical, biological and chemical processes [13-15], such as heat conduction, chemical engineering, underground water flow, thermo-elasticity, and plasma physics. They include two, three, multi-point and nonlocal BVPs as special cases. The existence and multiplicity of positive solutions for such problems have received a great deal of attention, see [16-32] and the references therein. In [17], Feng, Ji and Ge considered the existence and multiplicity of positive solutions for a class of nonlinear boundary value problems of second order differential equations with integral boundary conditions in ordered Banach spaces

The arguments are based upon a specially constructed cone and fixed point theory in a cone for strict set contraction operators.

Motivated by the works mentioned above, in this paper, we consider the existence of multiple monotone positive solutions for BVP (1.1) and (1.2). In comparison with previous works, our paper has several new features. Firstly, we consider higher order boundary value problems, and we allow the nonlinearity f to contain derivatives of the unknown function up to order. Secondly, we discuss the boundary value problem with integral boundary conditions, i.e., BVP (1.1) and (1.2), which includes two-point, three-point, multi-point and nonlocal boundary value problems as special cases. Thirdly, we consider the existence of multiple monotone positive solutions. To our knowledge, few papers have considered the monotone positive solutions for a higher order differential equation with integral boundary conditions. We shall emphasize here that with these new features our work improves and generalizes the results of [2] and some other known results to some degree. In this work we shall also utilize the following fixed point theorem in cones.

Lemma 1.1 ([33,34])

LetKbe a cone in a Banach spaceE. LetDbe an open bounded subset ofEwithand. Assume thatis a compact operator such thatfor. Then the following results hold.

(1) If, , then.

(2) If there existssuch thatfor alland, then.

(3) LetUbe open inEsuch that. Ifand, thenAhas a fixed point in. The same result holds ifand.

### 2 Preliminary lemmas

Let , then E is a Banach space with the norm for each .

We make the following assumptions:

() is continuous.

() , , , where ,

Lemma 2.1Assume that () holds. Then, for any, the BVP

(2.1)

has a unique solutionuthat can be expressed in the form

(2.2)

where

(2.3)

Proof Firstly, we prove that if u is a solution of BVP (2.1), then it will take the form of (2.2). Now, integrating differential equation (2.1) from 0 to t twice, we have

(2.4)

Letting in (2.4), we get

(2.5)

Substituting the boundary conditions of (2.1) and (2.5) into (2.4) yields

(2.6)

and, consequently,

Solving the above two equations for and , we have

and so

(2.7)

Hence, (2.2) follows from (2.6) and (2.7).

Next we prove that the u given by (2.2) satisfies the differential equation and boundary conditions of (2.1). From (2.2), we have

(2.8)

Direct differentiation of (2.8) gives . Also, from (2.2) we have

and, similarly,

Therefore, by solving the above two equations with the double integrals as unknowns, we have

(2.9)

and

(2.10)

Hence (2.6) follows from (2.2), (2.9) and (2.10), and thus , . This completes the proof. □

Defining

then is the Green function of BVP (1.1) and (1.2). Moreover, solving BVP (1.1) and (1.2) is equivalent to finding a solution of the following integral equation:

Remark 2.1 If () holds, then for any , it is easy to testify that

(2.11)

Lemma 2.2Let, then for any, , we have

Proof It is easy to show that , , . For , , we have

For any , we define . From Lemma 2.2, we know that

(2.12)

□

Lemma 2.3Assume that () holds. Ifsatisfies the boundary conditions (1.2) and

then

(2.13)

Proof Let , , then we have

For , implies that

Thus

i.e.,

On the other hand,

and so

i.e., , therefore , and so .

Now, , and is concave downward, so we have

(2.14)

From (2.14) and , we obtain (2.13). This completes the proof of Lemma 2.3. □

Remark 2.2 From Lemma 2.3, if u is a positive solution of BVP (1.1) and (1.2), then u is nondecreasing on , i.e., u is a monotone positive solution of BVP (1.1) and (1.2).

Let

Obviously, K is a cone in E. For any , let , and . Define an operator as follows:

(2.15)

Then u is a solution of BVP (1.1) and (1.2) if and only if u solves the operator equation .

Lemma 2.4Suppose that () and () hold, thenis completely continuous.

Proof For all , , by (), (2.11), (2.12) and (2.15), we have

and

Thus, further from the first inequality of (2.12), we have

Hence, and .

Next by standard methods and the Ascoli-Arzela theorem, one can prove that is completely continuous. So this is omitted. □

Let

Proceeding as for the proof of Lemma 2.5 in [33], we have the following.

Lemma 2.5has the following properties:

(a) is open relative toK;

(b) ;

(c) if and only if;

(d) if, thenfor.

Now for convenience we introduce the following notations:

To prove our main results, we need the following lemmas.

Lemma 2.6Assume that (), () hold andfsatisfies

(2.16)

then.

Proof For , we have and , , . Then by (2.16) we have, for ,

This implies that for . By the point (1) in Lemma 1.1, we have . □

Lemma 2.7Assume that (), () hold andfsatisfies

(2.17)

then.

Proof Let , , then with . Next we prove that

In fact, if not, then there exist and such that . By (2.17) and the point (d) in Lemma 2.5, we have, for ,

This implies that , and so by the point (c) in Lemma 2.5, this is a contradiction. It follows from the point (2) of Lemma 1.1 that . □

### 3 Main results

In the following, we shall give the main results on the existence of multiple positive solutions of BVP (1.1) and (1.2).

Theorem 3.1Suppose that () and () are satisfied. In addition, assume that one of the following conditions holds.

() There existwithandsuch that

() There existwithsuch that

Then BVP (1.1) and (1.2) has two nondecreasing positive solutions, inK. Moreover, if in () is replaced by, then BVP (1.1) and (1.2) has a third nondecreasing positive solution.

Proof Assume that () holds. We show that either T has a fixed point or in . If for , by Lemmas 2.6 and 2.7, we have , , and . By Lemma 2.5(b), we have since . It follows from Lemma 1.1(3) that T has a fixed point . Similarly, T has a fixed point . The proof is similar when () holds. □

Corollary 3.1If there existssuch that one of the following conditions holds:

() , , for, ,

() , , for, ,

then BVP (1.1) and (1.2) has at least two nondecreasing positive solutions inK.

Proof We show that () implies (). It is easy to verify that implies that there exists such that . Let , by , there exists such that for , . Let

then for , we have

This implies that and () holds. Similarly, () implies (). This completes the proof. □

Remark 3.1 We establish the multiplicity of monotone positive solutions for a higher order differential equation with integral boundary conditions, and we allow the nonlinearity f to contain derivatives of the unknown function up to order, so our work improves and generalizes the results of [2] to some degree.

### Competing interests

The authors declare that they have no competing interests.

### Authors’ contributions

XH wrote the first manuscript and LL corrected and improved the final version. Both authors read and approved the final manuscript.

### Acknowledgements

Research supported by the National Natural Science Foundation of China (11371221, 11201260), the Specialized Research Fund for the Doctoral Program of Higher Education of China (20123705120004, 20123705110001), a Project of Shandong Province Higher Educational Science and Technology Program (J11LA06) and Foundation of Qufu Normal University (BSQD20100103).

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