Journal of Applied Mathematics

Volume 2014, Article ID 972135, 5 pages

http://dx.doi.org/10.1155/2014/972135

## Existence of Positive Solutions for an Elastic Beam Equation with Nonlinear Boundary Conditions

Department of Mathematics, Northwest Normal University, Lanzhou 730070, China

Received 9 December 2013; Accepted 18 February 2014; Published 20 March 2014

Academic Editor: Ch. Tsitouras

Copyright © 2014 Ruikuan Liu and Ruyun Ma. 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 and nonexistence of positive solutions for the following fourth-order two-point boundary value problem subject to nonlinear boundary conditions , where are parameters, and , and are continuous. By using the fixed-point index theory, we prove that the problem has at least one positive solution for sufficiently small and has no positive solution for large enough.

#### 1. Introduction

In this paper, we study the existence and nonexistence of positive solutions for the following fourth-order boundary value problem with nonlinear boundary conditions where , are parameters and and are real functions. If , , in mechanics, problem (1) is called cantilever beam equation [1, 2]. The equation describes the deflection of an elastic beam fixed at the left and freed at the right. There are some papers discussing the existence of solutions of the equation by using various methods, such as the lower and upper solution method, the Leray-Schauder continuation method, fixed-point theory, and the monotone iterative method; see [3–11]. If , problem (1) has also been studied; see [12–17].

In the case , , Yang et al. [12] obtained sufficient conditions of the existence of two solutions of problem (1) by using variational technique and a three-critical-point theorem. Recently, Li and Zhang [13] are concerned with the existence and uniqueness of monotone positive solution of problem (1) with , by using a new fixed-point theorem of generalized concave operators, but some monotone assumptions on and are needed.

In 2013, by using a three-critical-point theorem, Cabada and Tersian [14] studied the existence and multiplicity of solutions of problem (1) with , .

A natural question is what would happen if and ?

Motivated by the above papers, we will prove the existence and nonexistence of positive solution for problem (1) by using the fixed-point index theory with the nonlinearity satisfying superlinear growth condition at infinity.

We make the following assumptions. (A1) is continuous. (A2) is continuous and is continuous. (A3) For any , .

The main results of the present paper are summarized as follows.

Theorem 1. *Assume that (A1)–(A3) hold. Then problem (1) has at least one positive solution for , sufficiently small.*

Theorem 2. *Assume that (A1)–(A3) hold. Then problem (1) has no positive solution for large enough.*

*Remark 3. *The results obtained in this paper are not a consequence of the previous theorem in the previous literature. Clearly, the boundary condition of (1) is more general than the above pieces of literature and problem (1) considers two different parameters which is more extensive.

*Remark 4. * It is pointed out that we do not need any monotone assumption on , , and , which is weaker than the corresponding assumptions on , in [13]. References [12, 14] are only considered with the existence of solution for problem (1) with nonlinear boundary condition; however, we study the existence and nonexistence of positive solution of (1).

The remainder of this paper is arranged as follows. Section 2 presents some preliminaries. The proofs of Theorems 1 and 2 are given in Section 3. Finally, we give a simple example to illustrate our main results.

#### 2. Preliminaries

In this section we collect some preliminary results that will be used in subsequent sections. Let ; then is a Banach space under the norm . Define ; then is a nonnegative cone.

Lemma 5 (see [11, Lemma 2.2]). *Let ; then the solution of the problem
**
is
**
where
*

*Now, let us set
It follows from (3) and (5) with simple computation that problem (1) is equivalent to integral equation
*

*Note that for any the function satisfies the boundary conditions in (1) by the definition of Green’s function . In view of Lemma 5, it is easy to see that is a fixed point of the operator if and only if is a solution of problem (1).*

*Lemma 6 (see [6, Lemma 2.2], [13, Lemma 2.1]). , have the following properties:(i), , ;(ii), ;(iii), ;(iv), .*

*Lemma 7. has the following properties:(i), ;(ii), .*

*Proof. *First, we will prove (i). For any , it follows that
From (A2), . So is nondecreasing for . Moreover,
so (i) holds.

Now, we prove (ii). For any , obviously, .

From (i), is nondecreasing for ; then .

*For any fixed , define the cone
Then is a positive cone; letting , with is a constant.*

*Lemma 8. Assume that (A1) and (A2) hold; then is completely continuous.*

*Proof. *Obviously, for any , , it follows that .

For any , from (6) and Lemma 7(i), we have
Hence

For any fixed , (11) together with Lemma 7(ii), we get
So, .

According to (A1), (A2), and Arzela-Ascoli theorem, it is not difficult to verify that is completely continuous.

*The following well-known fixed-point index theorem in cones is crucial to our arguments.*

*Lemma 9 (see [18]). Let be a Banach space and let be a cone. For , define . Assume that is compact map such that for .(i)If for , then .(ii)If for , then .*

*3. Proof of Main Results*

*3. Proof of Main Results**In this section, we will prove our main results.*

*Proof of Theorem 1. *Let
Let ; set
For any number , let . Choose so that
Then, for , and , combining (6) with Lemma 7, we have
which implies that
Thus, Lemma 9 implies that

According to (A3), for any ], there exists a such that
where is chosen so that
For any , choose , and set . If , then
It follows from (A2) and Lemma 6 that
which implies that
Hence, from Lemma 9, we get
From (18) and (24), it follows that
Therefore, has a fixed point in ; that is, problem (1) has at least one positive solution.

*Proof of Theorem 2. *From (A1) and (A3), for any , there is a constant such that
Let be a positive solution of (1); then satisfies (6). Choose large enough such that
For any , we have
which is a contradiction. Therefore, problem (1) has no positive solution for large enough.

*Finally, we give an example to illustrate our main result.*

*Example 10. *Consider the following fourth-order two-point problem with nonlinear boundary conditions:
Clearly, the nonlinearity

*It is easy to check that (A1)–(A3) are satisfied. By simple computation, we have . Set ; then .*

*If (where can be any real number greater than 0), , then , , and take ; then obviously holds for .*

*Choosing , it is easy to verify that
*

*From Theorem 1, has a fixed point in ; that is, problem (29) has at least one positive solution.*

*For any , there is a constant , so that
If , then problem (29) has no positive solution for .*

*Conflict of Interests*

*Conflict of Interests**The authors declare that there is no conflict of interests regarding the publication of this paper.*

*Acknowledgments*

*Acknowledgments**The authors are very grateful to the anonymous referees for their valuable suggestions. This work was supported by NSFC (nos. 11361054 and 11201378), SRFDP (no. 2012 6203110004), and Gansu Provincial National Science Foundation of China (no. 1208RJZA258).*

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