# Existence Results of Three-Point Boundary Value Problems for Second-order Ordinary Differential Equations

- Sheng-Ping Wang
^{1}Email author and - Long-Yi Tsai
^{2}

**2011**:901796

**DOI: **10.1155/2011/901796

© S.-P.Wang and L.-Y. Tsai. 2011

**Received: **19 May 2010

**Accepted: **24 September 2010

**Published: **28 September 2010

## Abstract

We establish existence results of the following three-point boundary value problems: , , and , where and . The approach applied in this paper is upper and lower solution method associated with basic degree theory or Schauder's fixed point theorem. We deal with this problem with the function which is Carathéodory or singular on its domain.

## 1. Introduction

where and .

In the mathematical literature, a number of works have appeared on nonlocal boundary value problems, and one of the first of these was [1]. Il'in and Moiseev initiated the research of multipoint boundary value problems for second-order linear ordinary differential equations, see [2, 3], motivated by the study [4–6] of Bitsadze and Samarskii.

Recently, nonlinear multipoint boundary value problems have been receiving considerable attention, and have been studied extensively by using iteration scheme (e.g., [7]), fixed point theorems in cones (e.g., [8]), and the Leray-Schauder continuation theorem (e.g., [9]). We refer more detailed treatment to more interesting research [10, 11] and the references therein.

The theory of upper and lower solutions is also a powerful tool in studying boundary value problems. For the existence results of two-point boundary value problem, there already are lots of interesting works by applying this essential technique (see [12, 13]). Recently, it is shown that this method plays an important role in proving the existence of solutions for three-point boundary value problems (see [14–16]).

Last but not least, as the singular source term appearing in two-point problems, singular three-point boundary value problems also attract more attention (e.g., [17]).

In this paper, we will discuss the existence of solutions of some general types on three-point boundary value problems by using upper and lower solution method associated with basic degree theory or Schauder's fixed point theorem.

and prove the existence of -solution in Theorems 4.1 and 4.4. Some sufficient conditions for constructing upper and lower solutions are given in each section for applications.

## 2. Preliminaries

By direct computations, we get the following results.

- (i)The function defined by (2.1), is the Green function corresponding for the problem(2.3)

- (iii)In the case , we have(2.4)

Lemma 2.2.

where is defined by (2.1).

## 3. Carathéodory Case

In this section we first introduce the Carathéodory function as follows.

Definition 3.1.

A function defined on is called a Carathéodory function on if

(i)for almost every is continuous on ;

(ii)for any the function is measurable on ;

(iii)for any , there exists such that for any and for almost every with , we have .

We in this section assume that is a Carathéodory function and discuss the existence of -solution by assuming the existence of upper and lower solutions.

### 3.1. Existence of -Solutions

We first introduce the definitions of -upper and lower solutions as below.

Definition 3.2.

A function is called a -lower solution of problem (1.1) and (1.2) if it satisfies

(i) , , and

Definition 3.3.

A function is called a -upper solution of problem (1.1) and (1.2) if it satisfies

(i) , , and

Proposition 3.4.

Let and be respective -lower and upper solutions of problem (1.1) and (1.2) with on . If is a solution of problem (3.3) and (1.2), then , for any .

Proof.

Case 1.

This implies that the minimum of cannot occur at , a contradiction.

Case 2.

And we get a contradiction.

Case 3.

which is impossible.

Theorem 3.5.

Proof.

It is clear that is a closed, bounded and convex set in and one can show that is a completely continuous mapping by Arzelà-Ascoli theorem and Lebesgue dominated convergence theorem. By applying Schauder's fixed point theorem, we obtain that has a fixed point in which is a solution of problem (3.3) and (1.2). From Proposition 3.4, this fixed point is also a solution of problem (1.1) and (1.2). Hence, we complete the proof.

with the boundary condition (1.2).

Corollary 3.6.

Then, problem (3.18) and (1.2) has at least one solution.

Proof.

Hence, if is large enough, we can show that and , where , which implies that is a positive -upper solution. In the same way we construct a -lower solution on .

### 3.2. Nontangency Solution

In this subsection, we afford another stronger lower and upper solutions to get a strict inequality of the solution between them.

Definition 3.7.

A function is a strict -lower solution of problem (1.1) and (1.2), if it is not a solution of problem (1.1) and (1.2), , and for any , one of the following is satisfied:

(i) ;

Definition 3.8.

A function is a strict -upper solution of problem (1.1) and (1.2), if it is not a solution of problem (1.1) and (1.2), , and for any , one of the following is satisfied:

(i) ,

Remark 3.9.

Every strict -lower(upper) solution of problem (1.1) and (1.2) is a -lower(upper) solution.

Now we are going to show that the solution curve of problem (1.1) and (1.2) cannot be tangent to upper or lower solutions from below or above.

Proposition 3.10.

Let and be respective strict -lower and upper solutions of problem (1.1) and (1.2) with on . If is a solution of problem (1.1) and (1.2) with on , then , for any .

Proof.

exists. Hence, has minimum at , that is, .

Case 1.

Case 2.

And we get a contradiction.

Case 3.

If , repeat the same arguments in Case 3 of the proof of Proposition 3.4. Therefore, we obtain on . The inequality on can be proved by the similar arguments as above.

Theorem 3.11.

Proof.

This is a consequence of Theorem 3.5 and Proposition 3.10 and hence, we omits this proof.

## 4. Singular Case

In this section we give a more general existence result than Theorem 3.11 by assuming the existence of -lower and upper solutions. This makes us to deal with problem (1.1) and (1.2), where the function is singular at the end point and .

Theorem 4.1.

Let and be -lower and upper solutions of problem (1.1) and (1.2) such that on and let satisfy the following conditions:

(i)for almost every is continuous on ;

(ii)for any the function is measurable on ;

Proof.

and is defined by (3.17). The rest arguments are similar to the proof of Theorem 3.5.

Remark 4.2.

where is a constant and , are given as (1.2).

Example 4.3.

Notice that in Theorem 4.1, one can only deal with the case that is singular at end points , . However, when is singular at , there is no hope to obtain the solutions directly from Theorem 4.1. We will establish the following theorem to deal with this case by constructing upper and lower solutions to solve this problem.

Theorem 4.4.

Assume

the function is continuous;

where is defined as in Lemma 2.1.

Remark 4.5 (see [12, Remark ]).

Assumption is equivalent to the assumption that there exists and a function such that:

(i) for all ,

(ii) , for all , ,

(iii) , for all ,

Proof.

Step 1.

Step 2.

where .

Step 3.

It follows from (4.25) and (4.27) that is a lower solution of ( ).

Step 4.

Step 5.

Step 6.

it remains only to check the continuity of at . This can be deduced from the continuity of and the fact that as .

Example 4.6.

## Authors’ Affiliations

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