International Journal of Differential Equations

Volume 2013, Article ID 890281, 13 pages

http://dx.doi.org/10.1155/2013/890281

## Existence of Positive Periodic Solutions for Periodic Neutral Lotka-Volterra System with Distributed Delays and Impulses

^{1}Department of Mathematics, National University of Defense Technology, Changsha 410073, China^{2}Department of Mathematics, Hengyang Normal University, Hengyang, Hunan 421008, China

Received 20 April 2013; Accepted 13 May 2013

Academic Editor: Norio Yoshida

Copyright © 2013 Zhenguo Luo and Liping Luo. 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

By using a fixed-point theorem of strict-set-contraction, we investigate the existence of positive periodic solutions for a class of the following impulsive neutral Lotka-Volterra system with distributed delays: Some verifiable criteria are established easily.

#### 1. Introduction

It is well known that natural environments are physically highly variable, and in response, birth rates, death rates, and other vital rates of populations vary greatly in time. Theoretical evidence suggests that many population and community patterns represent intricate interactions between biology and variation in the physical environment (see [1–4]). Thus, the focus in theoretical models of population and community dynamics must be not only on how populations depend on their own population densities or the population densities of other organisms, but also on how populations change in response to the physical environment. It is reasonable to study the models of population with periodic coefficients. In addition to the theoretical and practical significance, the Lotka-Volterra model is one of the famous models for dynamics of population; therefore it has been studied extensively [5–9]. In view of the above effects, by applying a fixed-point theorem of strict-set-contraction, Li [10] established criteria to guarantee the existence of positive periodic solutions of the following neutral Lotka-Volterra system with distributed delays: where are -periodic functions and and satisfying , , . On the other side, birth of many species is an annual birth pulse or harvesting. To have a more accurate description of many mathematical ecology systems, we need to consider the use of impulsive differential equations [11–13]. Some qualitative properties such as oscillation, periodicity, asymptotic behavior, and stability properties have been investigated extensively by many authors over the past few years [14–18]. However, to our knowledge, there are few published papers discussing the existence of periodic solutions for neutral Lotka-Volterra system with distributed delays and impulses. In this paper, we are concerned with the following neutral Lotka-Volterra system with distributed delays and impulses: where are -periodic functions and and satisfying , . Moreover, (here represents the right limit of at the point ), ; that is, changes decreasingly suddenly at times ; is a constant, , . We assume that there exists an integer such that , where .

The main purpose of this paper is by using a fixed-point theorem of strict-set-contraction [19, 20] to establish new criteria to guarantee the existence of positive periodic solutions of the system (2).

For convenience, we introduce the notation where is a continuous -periodic function.

Throughout this paper, we assume that; ; ; ; .

The paper is organized as follows. In Section 2, we give some definitions and lemmas to prove the main results of this paper. In Section 3, by using a fixed-point theorem of strict-set-contraction, we established some criteria to guarantee the existence of at least one positive periodic solution of system (2). Finally, in Section 4, we give an example to show the validity of our result.

#### 2. Preliminaries

In order to obtain the existence of a periodic solution of system (2), we first introduce some definitions and lemmas.

*Definition 1 (see [13]). *A function is said to be a positive solution of (2), if the following conditions are satisfied:(a) is absolutely continuous on each ;(b)for each , and exist and ;(c) satisfies the first equation of (2) for almost everywhere in and satisfies the second equation of (2) at impulsive point .

*Definition 2 (see [21]). *Let be a real Banach space and a closed, nonempty subset of . is a cone provided(i) for all and all ,(ii) imply .

*Definition 3 (see [21]). *Let be a bounded subset in . Define
where denotes the diameter of the set , obviously, . So is called the Kuratowski measure of noncompactness of .

*Definition 4 (see [21]). *Let be two Banach spaces and , a continuous and bounded map , is called *k*-set contractive if, for any bounded set , we have
is called strict-set-contractive if it is -set-contractive for some .

*Definition 5 (see [22]). *The set is said to be quasi-equicontinuous in if for any there exists such that if , , , and , then .

Lemma 6 (see [22]). *The set is relatively compact if and only if*(1)* is bounded; that is, , for each , and some ;*(2)* is quasi-equicontinuous in .*

Lemma 7. * is an-periodic solution of (2) is equivalent to is an-periodic solution of the following equation:
**
where
*

*Proof. *Assume that , is a periodic solution of (2). Then, we have
Integrating the previous equation with , we can have
where , , , . Therefore,
which can be transformed into

Thus, is a periodic solution of (6). If , is a periodic solution of (6), for any , from (6) we have
For any , , we have from (6) that

Hence is a positive -periodic solution of (2). Thus we complete the proof of Lemma 7.

Lemma 8 (see [20, 23]). *Let be a cone of the real Banach space and with . Assume that is strict-set-contractive such that one of the following two conditions is satisfied:*(a)*, for all and , for all ;*(b)*, for all and , for all .**Then has at least one fixed point in .**In order to apply Lemma 7 to system (1), one sets
**
Define
**
with the norm defined by , where , and
**
with the norm defined by , where . Then and are both Banach spaces. Define the cone in by
**
Let the map be defined by
**
where , ,
**
where
**
It is obvious to see that , and
*

In what follows, we will give some lemmas concerning and defined by (17) and (18), respectively.

Lemma 9. *Assume that hold.*(i)*If , then is well defined.*(ii)*If holds and , then is well defined.*

* Proof. *For any , it is clear that . From (18), for , we have

That is, . So . In view of , for , we have

Therefore, for , we find
Now, we show that , , . From (18), we obtain
It follows from (23) and (25) that if , then
On the other hand, from (25) and , if , then

It follows from (27) and (28) that . So . By (25), we have . Hence, . This completes the proof of (i). In view of the proof of (ii), we only need to prove that , implies
From (23), (26), , and , we have
The proof of (ii) is complete. Thus we complete the proof of Lemma 9.

Lemma 10. *Assume that hold and .*(i)*If , then is strict-set-contractive.*(ii)*If holds and , then is strict-set-contractive, where .*

* Proof. *We only need to prove (i), since the proof of (ii) is similar. It is easy to see that is continuous and bounded. Now we prove that a for any bounded set . Let . Then, for any positive number , there is a finite family of subsets satisfying with . Therefore
As and are precompact in , it follows that there is a finite family of subsets of such that and
In addition, for any and , we have
Hence,
Applying the Arzela-Ascoli theorem, we know that is precompact in . Then, there is a finite family of subsets of such that and
From (23), (26), (31)–(34), and , for any , we have
where
From (36) and (37) we obtain
As is arbitrarily small, it follows that
Therefore, is strict-set-contractive. The proof of Lemma 10 is complete.

#### 3. Main Results

Our main result of this paper is as follows.

Theorem 11. *Assume that , hold.*(i)*If , then system (2) has at least one positive -periodic solution.*(ii)*If holds and , then system (2) has at least one positive -periodic solution.*

* Proof. *We only need to prove (i), since the proof of (ii) is similar. Let
Then it is easy to see that . From Lemmas 9 and 10, we know that is strict-set-contractive on . In view of (26), we see that if there exists such that , then is one positive -periodic solution of system (2). Now, we will prove that condition (ii) of Lemma 7 holds. First, we prove that , for all . Otherwise, there exists , such that . So and , which implies that
Moreover, for , we have
In view of (42) and (43), we obtain
which is a contradiction. Finally, we prove that , for all , also hold. For this case, suppose, for the sake of contradiction, that there exist such that . Furthermore, for any , we have
In addition, for any , we find
Thus, we have
From (45) and (47), we obtain
which is a contradiction. Therefore, condition (ii) of Lemma 7 holds. By Lemma 7, we see that has at least one nonzero fixed point in . Thus, the system (6) has at least one positive -periodic solution. Therefore, it follows from Lemma 6 that system (2) has a positive -periodic solution. The proof of Theorem 11 is complete.

*Remark 12. *If , we can easily derive the corresponding results in [10]. So we extend the corresponding results in [10].

#### 4. Example

In this section, we give an example to show the effectiveness of our result.

*Example 1. *Consider the following nonimpulsive system: