# Existence and Uniqueness of Periodic Solution for Nonlinear Second-Order Ordinary Differential Equations

Jian Zu

### Author affiliations

College of Mathematics, Jilin University, Changchun 130012, China

Boundary Value Problems 2011, 2011:192156  doi:10.1155/2011/192156

 Received: 22 May 2010 Accepted: 6 March 2011 Published: 15 March 2011

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.

We study periodic solutions for nonlinear second-order ordinary differential problem . By constructing upper and lower boundaries and using Leray-Schauder degree theory, we present a result about the existence and uniqueness of a periodic solution for second-order ordinary differential equations with some assumption.

### 1. Introduction

The study on periodic solutions for ordinary differential equations is a very important branch in the differential equation theory. Many results about the existence of periodic solutions for second-order differential equations have been obtained by combining the classical method of lower and upper solutions and the method of alternative problems (The Lyapunov-Schmidt method) as discussed by many authors [110]. In [11], the author gives a simple method to discuss the existence and uniqueness of nonlinear two-point boundary value problems. In this paper, we will extend this method to the periodic problem.

We consider the second-order ordinary differential equation

(11)

Throughout this paper, we will study the existence of periodic solutions of (1.1) with the following assumptions:

are continuous in , and

(12)

(13)

where is some positive integer,

(14)

The following is our main result.

Theorem 1.1.

Assume that and hold, then (1.1) has a unique periodic solution.

### 2. Basic Lemmas

The following results will be used later.

Lemma 2.1 (see [12]).

Let with

(21)

then

(22)

and the constant is optimal.

Lemma 2.2 (see [12]).

Let with the boundary value conditions , then

(23)

Consider the periodic boundary value problem

(24)

Lemma 2.3.

Suppose that are -integrable periodic function, where satisfy the condition (H2), with

(25)

then (2.4) has only the trivial -periodic solution .

Proof.

If on the contrary, (2.4) has a nonzero -periodic solution , then using (2.4), we have

(26)

where is undetermined.

Firstly, we prove that has at least one zero in . If , we may assume . Since is a -periodic solution, there exists a with . Then,

(27)

Without loss of generality, we may assume that ; then there exists a sufficiently small such that . Since is a continuous function, there must exist a with .

Secondly, we prove that has at least zeros on . Considering the initial value problem

(28)

Obviously,

(29)

is the solution of (2.8) and

(210)

where with . Since

(211)

holds under the assumptions of , there is a , such that

(212)

Now, let . By the conditions (H2), (2.11), and (2.12), we have

(213)

(214)

Since is decreasing in , we have . Therefore,

(215)

We also consider the initial value problem

(216)

Clearly,

(217)

is the solution of (2.16), where is the same as the previous one, and

(218)

Hence, there exists a with , such that

(219)

Then,

(220)

From (2.12) and (2.19), it follows that

(221)

By and (2.21), we have

(222)

Since is decreasing on , we have , and

(223)

We now prove that has a zero point in . If on the contrary for , then we would have the following inequalities:

(224)

(225)

In fact, from(2.4), (2.8), and (2.15), we have

(226)

with . Setting , and since

(227)

we obtain

(228)

Notice that , which implies

(229)

So, we have

(230)

Integrating from 0 to , we obtain

(231)

Therefore,

(232)

which implies (2.24). By a similar argument, we have (2.25). Therefore, , a contradiction, which shows that has at least one zero in , with .

We let . If , then from a similar argument, there is a , such that and so on. So, we obtain that has at least zeros on .

Thirdly, we prove that has at least zeros on . If, on the contrary, we assume that only has zeros on , we write them as

(233)

Obviously,

(234)

Without loss of generality, we may assume that . Since

(235)

we obtain , which contradicts . Therefore, has at least zeros on .

Finally, we prove Lemma 2.3. Since has at least zeros on , there are two zeros and with . By Lemmas 2.1 and 2.2, we have

(236)

From , it follows that

(237)

Hence,

(238)

which implies for . Also . Therefore, for, a contradiction. The proof is complete.

### 3. Proof of Theorem 1.1

Firstly, we prove the existence of the solution. Consider the homotopy equation

(31)

where and . When , it holds (1.1). We assume that is the fundamental solution matrix of with . Equation (3.1) can be transformed into the integral equation

(32)

From , is a periodic solution of (3.2), then

(33)

For is invertible,

(34)

We substitute (3.4) into (3.2),

(35)

Define an operator

(36)

such that

(37)

Clearly, is a completely continuous operator in .

There exists , such that every possible periodic solution satisfies ( denote the usual normal in . If not, there exists and the solution with .

We can rewrite (3.1) in the following form:

(38)

Let , obviously . It satisfies the following problem:

(39)

in which we have

(310)

Since , are uniformly bounded and equicontinuous, there exists continuous function , and a subsequence of (denote it again by ), such that ,  uniformly in . Using and , and are uniformly bounded. By the Hahn-Banach theorem, there exists integrable function ,, and a subsequence of (denote it again by ), such that

(311)

where denotes "weakly converges to" in . As a consequence, we have

(312)

that is,

(313)

Denote that , , then we get

(314)

which also satisfy the condition . Notice that and are integrable on , so satisfies Lemma 2.3. Hence, we have , which contradicts . Therefore, is bounded.

Denote

(315)

Because for , by Leray-Schauder degree theory, we have

(316)

So, we conclude that has at least one fixed point in , that is, (1.1) has at least one solution.

Finally, we prove the uniqueness of the equation when the condition and holds. Let and be two -periodic solutions of the problem. Denote , then is a solution of the following problem:

(317)

By Lemma 2.3, we have for .

Let . We have

(318)

with . Denote by . So, is the solution of the problem (1.1). The proof is complete.

### 4. An Example

Consider the system

(41)

where is a continuous function. Obviously,

(42)

satisfy Theorem 1.1, then there is a unique -periodic solution in this system.

### Acknowledgments

The author expresses sincere thanks to Professor Yong Li for useful discussion. He would like to thank the reviewers for helpful comments on an earlier draft of this paper.

### References

1. Bereanu, C, Mawhin, J: Existence and multiplicity results for some nonlinear problems with singular -Laplacian. Journal of Differential Equations. 243(2), 536–557 (2007). Publisher Full Text

2. Ehme, J, Eloe, PW, Henderson, J: Upper and lower solution methods for fully nonlinear boundary value problems. Journal of Differential Equations. 180(1), 51–64 (2002). Publisher Full Text

3. Kannan, R, Lakshmikantham, V: Existence of periodic solutions of nonlinear boundary value problems and the method of upper and lower solutions. Applicable Analysis. 17(2), 103–113 (1983)

4. Knobloch, H-W: On the existence of periodic solutions for second order vector differential equations. Journal of Differential Equations. 9, 67–85 (1971)

5. Knobloch, HW, Schmitt, K: Non-linear boundary value problems for systems of differential equations. Proceedings of the Royal Society of Edinburgh. Section A. 78(1-2), 139–159 (1977)

6. Liu, Y, Ge, W: Positive periodic solutions of nonlinear Duffing equations with delay and variable coefficients. Tamsui Oxford Journal of Mathematical Sciences. 20(2), 235–255 (2004)

7. Ortega, R, Tarallo, M: Almost periodic upper and lower solutions. Journal of Differential Equations. 193(2), 343–358 (2003). Publisher Full Text

8. Rachůnková, I, Tvrdý, M: Existence results for impulsive second-order periodic problems. Nonlinear Analysis. Theory, Methods & Applications. 59(1-2), 133–146 (2004). PubMed Abstract | Publisher Full Text

9. Schmitt, K: Periodic solutions of linear second order differential equations with deviating argument. Proceedings of the American Mathematical Society. 26, 282–285 (1970). Publisher Full Text

10. Sędziwy, S: Nonlinear periodic boundary value problem for a second order ordinary differential equation. Nonlinear Analysis. Theory, Methods & Applications. 32(7), 881–890 (1998). PubMed Abstract | Publisher Full Text

11. Li, Y: Boundary value problems for nonlinear ordinary differential equations. Northeastern Mathematical Journal. 6(3), 297–302 (1990)

12. Mitrinović, DS: Analytic Inequalities,p. xii+400. Springer, New York, NY, USA (1970)