# Existence and uniqueness of solutions for periodic-integrable boundary value problem of second order differential equation

Boundary Value Problems20122012:89

DOI: 10.1186/1687-2770-2012-89

Accepted: 16 July 2012

Published: 7 August 2012

## Abstract

In this paper we deal with one kind of second order periodic-integrable boundary value problem. Using the lemma on bilinear form and Schauder’s fixed point theorem, we give the existence and uniqueness of solutions for the problem under Lazer type nonresonant condition.

MSC:34B15, 34B16, 37J40.

### Keywords

lemma on bilinear forms Schauder’s fixed point theorem existence and uniqueness periodic-integrable boundary value problems

## 1 Introduction and main results

In this paper, we consider the solutions to the following periodic-integrable boundary value problem (for short, PIBVP):
(1.1)

where is a given T-periodic function in , and ; is T-periodic in t.

Throughout this paper, we assume

(A1) there exist two constants m and M such that

for all and ;

(A2) there exists such that
Recently, boundary value problems with integral conditions have been studied extensively [610]. As we all know Lazer type conditions are essential for the existence and uniqueness of periodic solutions of equations [14]. In [5] the existence of periodic solutions has been considered for the following second order equation:

Motivated by the above works, we will consider periodic-integrable boundary value problem (1.1). The main result obtained by us is the following theorem.

Theorem 1 Assume that (A 1) and (A 2) are satisfied. Then PIBVP (1.1) has a unique solution.

This paper is organized as follows. Section 2 deals with a linear problem. There, using the bilinear lemma developed by Lazer, one proves the uniqueness of solutions for linear equations. In Section 3, applying the result in Section 2 and Schauder’s fixed point theorem, we complete the proof of Theorem 1.

## 2 Linear equation

Consider the following linear PIBVP:
(2.1)

here is a given T-periodic function in , and ; is a T-periodic function. Assume that

(L1) there exist two constants m and M such that

for all . Moreover, m and M suit (A2).

Theorem 2 Assume that (L 1) and (A 2) are satisfied, then PIBVP (2.1) has only a trivial solution.

In order to prove Theorem 2, let us give some following concepts.

First, for any interval , define
It is clear that is a linear space with the norm as follows:
Define a bilinear form on as follows:
for any and . Let

where N suits assumption (L1), and , , , and are some constants. Then .

From and , we can obtain that there exist two constants and such that
for all . Then from assumptions (L1) and (A2), we have
for all , and

for all . Thus, is positive definite on and negative definite on . By the lemma in [1], we assert that if for all , then .

For every x on with , we introduce an auxiliary function

The following lemma is very useful in our proofs.

Lemma 1 If , are continuous and satisfy (L 1) and (A 2), then the following two points boundary value problem
(2.2)

has only a trivial solution.

Proof It is clear that 0 is a solution of two points boundary value problem (2.2). If is a solution of problem (2.2), then . For any , we have
by using (2.2). Integrating the first terms by parts, we derive

By assumption (L1), is positive definite on and negative definite on . These show for , that is, for . The proof of Lemma 1 is ended. □

Proof of Theorem 2 It is clear that PIBVP (2.1) has at least one solution, for example, . Assume that PIBVP (2.1) possesses a nontrivial solution . The proof is divided into three parts.

Case 1: . By Lemma 1 ( and ), PIBVP (2.1) has only a trivial solution. This contradicts .

Case 2: . Denote
Take
From , there are at least two points in the set S, which implies that and . By Lemma 1 ( and ) the two points boundary value problem
(2.3)
only has a trivial solution. Hence we obtain , . By the definitions of a and b, one has
From , we get

Case 3: . This case is similar to Case 2.

Thus, we complete the proof of Theorem 2. □

Theorem 3 If , are continuous and satisfy (L 1) and (A 2), then the following PIBVP
(2.4)

has a unique solution.

Proof Let and be two linear independent solutions of the following linear equation:
Assume that is a solution of PIBVP (2.1), where and are constants. Then by the boundary value conditions of (2.1),
By Theorem 3, PIBVP (2.1) has only a trivial solution, which shows
(2.5)
Let be a solution of PIBVP (2.4), where is a solution of the equation
From the boundary value conditions, we have

From (2.5) constants , are unique. Thus, PIBVP (2.4) has only one solution. □

## 3 Nonlinear equations

Let us prove Theorem 1. Rewrite (1.1) as follows:
(3.1)
where
Define
Fix , introduce an auxiliary PIBVP
(3.2)

To prove the main result, we need the following Lemma 2.

Lemma 2 If f satisfies (A 1) and (A 2), then for any given , PIBVP (3.2) has only one solution, denoted as and .

Proof From condition (A2), it follows that

By Theorem 3, PIBVP (3.2) has only one solution . If does not hold, there would exist a sequence such that , . Choose a subsequence of , without loss of generality, express as itself, such that the sequences are weakly convergent in . Denote the limit as . It is obvious that .

Because the set
is bounded convex in , by the Mazur theorem, we have . Hence,
By the Arzela-Ascoli theorem, passing to a subsequence, we may assume that

and in . Thus, and .

By
one has

which implies , for any . Hence, .

From PIBVP (3.2), we obtain
(3.3)
This shows that is a nontrivial solution of the following PIBVP:
(3.4)

On the other hand, by Theorem 2, PIBVP (3.4) has only zero, which leads to a contradiction. The proof of Lemma 2 is completed. □

Set

Define an operator by . Applying Lemma 2, .

Lemma 3 Operator F is completely continuous on .

Proof We first prove that F is continuous. Given any such that . Put . From the definition
(3.5)
We would prove that in . If not, then there would be a such that
Utilizing Lemma 2 and Arzela-Ascoli theorem, passing to a subsequence, we may assume that . Similar to the proof of Lemma 2, we have . Then
(3.6)
Moreover,

Hence, from Theorem 2, . This implies F is continuous. By Lemma 2, for any bounded subset , is also bounded. Hence, applying the continuity of F and Arzela-Ascoli theorem, is relatively compact. This shows F is completely continuous on . The proof of Lemma 3 is completed. □

Proof of Theorem 1 By Lemma 2, Lemma 3 and Schauder’s fixed point theorem, F has a fixed point in , that is, PIBVP (1.1) has a solution .

The following is to prove uniqueness. Let and be any two solutions of equation (1.1). Then is a solution of the equation
Employing (A2), we have

Hence by Theorem 3, . The uniqueness is proved. □

## Declarations

### Acknowledgements

The authors are grateful to the referees for their useful comments. The research of F. Cong was partially supported by NSFC Grant (11171350) and Natural Science Foundation of Jilin Province of China (201115133).

## Authors’ Affiliations

(1)
Fundamental Department, Aviation University of Air Force
(2)
Institute of Mathematics, Jilin University

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