Research

# Positive periodic solutions for a second-order functional differential equation

Yongxiang Li* and Qiang Li

### Author affiliations

Department of Mathematics, Northwest Normal University, Lanzhou, 730070, People’s Republic of China

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Boundary Value Problems 2012, 2012:140  doi:10.1186/1687-2770-2012-140

 Received: 12 June 2012 Accepted: 12 November 2012 Published: 27 November 2012

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 cited.

### Abstract

In this paper, the existence results of positive ω-periodic solutions are obtained for the second-order functional differential equation

where is a continuous function which is ω-periodic in t, is a ω-periodic function, . Our discussion is based on the fixed point index theory in cones.

MSC: 34C25, 47H10.

##### Keywords:
functional differential equation; positive periodic solution; cone; fixed point index

### 1 Introduction

In this paper, we discuss the existence of positive ω-periodic solutions of the second-order functional differential equation with the delay terms of first-order derivative in nonlinearity,

(1)

where is a continuous function which is ω-periodic in t and is a ω-periodic delay function, .

For the second-order differential equation without delay and the first-order derivative term in nonlinearity,

(2)

the existence problems of periodic solutions have attracted many authors’ attention and concern. Many theorems and methods of nonlinear functional analysis have been applied to research the periodic problems of Equation (2), such as the upper and lower solutions method and monotone iterative technique [1-4], the continuation method of topological degree [5-7], variational method and critical point theory [8-10], the theory of the fixed point index in cones [11-16], etc.

In recent years, the existence of periodic solutions for the second-order delayed differential equations have also been researched by many authors; see [17-24] and the references therein. In some practice models, only positive periodic solutions are significant. In [20,21,23], the authors obtained the existence of positive periodic solutions for some delayed second-order differential equations as a special form of the following equation:

(3)

by using Krasnoselskii’s fixed point theorem of cone mapping or the theory of the fixed point index in cones. In these works, the positivity of Green’s function of the corresponding linear second-order periodic problems plays an important role. The positivity guarantees that the integral operators of the second-order periodic problems are cone-preserving in the cone

(4)

in the Banach space , where is a constant. Hence, the fixed point theorems of cone mapping can be applied to periodic problems of the second-order delay equation (3) as well as Equation (2) (for Equation (2), see [11-16]). However, few people consider the existence of positive periodic solutions of Equation (1). Since the nonlinearity of Equation (1) explicitly contains the delayed first-order derivative term, the corresponding integral operator has no definition on the cone P. Thus, the argument methods used in [20,21,23] are not applicable to Equation (1).

The purpose of this paper is to discuss the existence of positive periodic solutions of Equation (1). We will use a different method to treat Equation (1). Our main results will be given in Section 3. Some preliminaries to discuss Equation (1) are presented in Section 2.

### 2 Preliminaries

Let denote the Banach space of all continuous ω-periodic function with the norm . Let be the Banach space of all continuous differentiable ω-periodic function with the norm

Generally, denotes the nth-order continuous differentiable ω-periodic function space for . Let be the cone of all nonnegative functions in .

Let be a constant. For , we consider the linear second-order differential equation

(5)

The ω-periodic solutions of Equation (5) are closely related to the linear second-order boundary value problem

(6)

see [14]. It is easy to see that problem (6) has a unique solution which is explicitly given by

(7)

where . By [[14], Lemma 1], we have

Lemma 2.1Let. Then, for every, the linear equation (5) has a uniqueω-periodic solutionwhich is given by

(8)

Moreover, is a completely continuous linear operator.

Since , for every , by (8), if and , then the ω-periodic solution of Equation (5) for every , and we term it the positive ω-periodic solution. Let

(9)

Define a set K in by

(10)

It is easy to verify that K is a closed convex cone in .

Lemma 2.2Let. Then, for every, the positiveω-periodic solution of Equation (5) . Namely, .

Proof Let , . For every , from (8) it follows that

and therefore,

Using (8), we obtain that

For every , since

we have

Hence, . □

Now we consider the nonlinear delay equation (1). Hereafter, we assume that the nonlinearity f satisfies the condition

(F0) There exists such that

Let , then for , , , and Equation (1) is rewritten to

(11)

For every , set

(12)

Then is continuous. We define an integral operator by

(13)

By the definition of the operator S, the positive ω-periodic solution of Equation (1) is equivalent to the nontrivial fixed point of A. From assumption (F0), Lemma 2.1 and Lemma 2.2, we easily see that

Lemma 2.3andis completely continuous.

We will find the non-zero fixed point of A by using the fixed point index theory in cones. We recall some concepts and conclusions on the fixed point index in [25,26]. Let E be a Banach space and be a closed convex cone in E. Assume Ω is a bounded open subset of E with the boundary Ω, and . Let be a completely continuous mapping. If for any , then the fixed point index has a definition. One important fact is that if , then A has a fixed point in . The following two lemmas are needed in our argument.

Lemma 2.4 ([26])

Let Ω be a bounded open subset ofEwithandbe a completely continuous mapping. Iffor everyand, then.

Lemma 2.5 ([26])

Let Ω be a bounded open subset ofEandbe a completely continuous mapping. If there exists ansuch thatfor everyand, then.

In the next section, we will use Lemma 2.4 and Lemma 2.5 to discuss the existence of positive ω-periodic solutions of Equation (1).

### 3 Main results

We consider the existence of positive ω-periodic solutions of the functional differential equation (1). Let satisfy assumption (F0) and be ω-periodic in t. Let be the constant defined by (9) and . For convenience, we introduce the notations

Our main results are as follows.

Theorem 3.1Letandbeω-periodic int, . Iffsatisfies assumption (F0) and the condition

(F1) , ,

then Equation (1) has at least one positiveω-periodic solution.

Theorem 3.2Letandbeω-periodic int, . Iffsatisfies assumption (F0) and the conditions

(F2) ,

then Equation (1) has at least one positiveω-periodic solution.

In Theorem 3.1, the condition (F1) allows to be superlinear growth on x and . For example,

satisfies (F0) with and (F1) with and .

In Theorem 3.2, the condition (F2) allows to be sublinear growth on x and . For example,

satisfies (F0) with and (F2) with and .

Proof of Theorem 3.1 Choose the working space . Let be the closed convex cone in defined by (10) and be the operator defined by (13). Then the positive ω-periodic solution of Equation (1) is equivalent to the nontrivial fixed point of A. Let and set

(14)

We show that the operator A has a fixed point in when r is small enough and R is large enough.

By and the definition of , there exist and such that

(15)

Let . We now prove that A satisfies the condition of Lemma 2.4 in , namely for every and . In fact, if there exist and such that , then by the definition of A and Lemma 2.1, satisfies the delay differential equation

(16)

Since , by the definitions of K and , we have

(17)

Hence, from (15) it follows that

By this, (16) and the definition of , we have

Integrating both sides of this inequality from 0 to ω and using the periodicity of , we obtain that

Since , it follows that , which is a contradiction. Hence, A satisfies the condition of Lemma 2.4 in . By Lemma 2.4, we have

(18)

On the other hand, since , by the definition of , there exist and such that

(19)

Choose and let . Clearly, . We show that A satisfies the condition of Lemma 2.5 in , namely for every and . In fact, if there exist and such that , since , by the definition of A and Lemma 2.1, satisfies the differential equation

(20)

Since , by the definition of K, we have

(21)

By the latter inequality of (21), we have that . This implies that . Consequently,

(22)

By (22) and the former inequality of (21), we have

From this, the latter inequality of (21) and (19), it follows that

By this inequality, (20) and the definition of , we have

Integrating this inequality on I and using the periodicity of , we get that

Since , from this inequality it follows that , which is a contradiction. This means that A satisfies the condition of Lemma 2.5 in . By Lemma 2.5,

(23)

Now, by the additivity of fixed point index, (18) and (23), we have

Hence, A has a fixed-point in , which is a positive ω-periodic solution of Equation (1). □

Proof of Theorem 3.2 Let be defined by (14). We prove that the operator A defined by (13) has a fixed point in if r is small enough and R is large enough.

By and the definition of , there exist and such that

(24)

Let and . We prove that A satisfies the condition of Lemma 2.5 in , namely for every and . In fact, if there exist and such that , since , by the definition of A and Lemma 2.1, satisfies the delay differential equation

(25)

Since , by the definitions of K and , satisfies (17). From (17) and (24) it follows that

By this, (25) and the definition of , we have

Integrating this inequality on and using the periodicity of , we obtain that

Since , from this inequality it follows that , which is a contradiction. Hence, A satisfies the condition of Lemma 2.5 in . By Lemma 2.5, we have

(26)

Since , by the definition of , there exist and such that

(27)

Choosing , we show that A satisfies the condition of Lemma 2.4 in , namely for every and . In fact, if there exist and such that , then by the definition of A and Lemma 2.1, satisfies the differential equation

(28)

Since , by the definition of K, satisfies (21). From the second inequality of (21), it follows that (22) holds. By (22) and the first inequality of (21), we have

From this, the second inequality of (21) and (27), it follows that

By this and (28), we have

Integrating this inequality on and using the periodicity of , we obtain that

Since , from this inequality it follows that , which is a contradiction. This means that A satisfies the condition of Lemma 2.4 in . By Lemma 2.4,

(29)

Now, from (26) and (29), it follows that

Hence, A has a fixed-point in , which is a positive ω-periodic solution of Equation (1). □

Example 1 Consider the following second-order differential equation with delay:

(30)

where , . If and for , we can verify that

satisfies the conditions (F0) and (F1) for . By Theorem 3.1, the delay equation (30) has at least one positive ω-periodic solution.

Example 2 Consider the functional differential equation

(31)

where , , and . If and for . We easily see that

satisfies the conditions (F0) and (F2) for . By Theorem 3.2, the functional differential equation (31) has a positive ω-periodic solution.

### Competing interests

The authors declare that they have no competing interests.

### Authors’ contributions

YL carried out the main part of this article. All authors read and approved the final manuscript.

### Acknowledgements

The research was supported by the NNSFs of China (11261053, 11061031).

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