#### Abstract

We study the predator-prey model proposed by Aziz-Alaoui and Okiye (Appl. Math. Lett. 16 (2003) 1069–1075) First, the structure of equilibria and their linearized stability is investigated. Then, we provide two sufficient conditions on the global asymptotic stability of a positive equilibrium by employing the Fluctuation Lemma and Lyapunov direct method, respectively. The obtained results not only improve but also supplement existing ones.

#### 1. Introduction

One of the important interactions among species is the predator-prey relationship and it has been extensively studied because of its universal existence. There are many factors affecting the dynamics of predator-prey models. One of the familiar factors is the functional response, referring to the change in the density of prey attached per unit time per predator as the prey density changes. In the classical Lotka-Volterra model, the functional response is linear, which is valid first-order approximations of more general interaction. To build more realistic models, Holling [1] suggested three different kinds of functional responses, and Leslie and Gower [2] introduced the so-called Leslie-Gower functional response.

Recently, Aziz-Alaoui and Daher Okiye [3] proposed and studied the following predator-prey model with modified Leslie-Gower and Holling-type II schemes, Here, all the parameters are positive, and we refer to Aziz-Alaoui and Daher Okiye [3] for their biological meanings. System (1.1) can be considered as a representation of an insect pest-spider food chain, nature abounds in systems which exemplify this model; see [3].

Since then, system (1.1) and its nonautonomous versions have been studied by incorporating delay, impulses, harvesting, and so on (see, e.g., [4–11]). In spite of this extensive study, the dynamics of (1.1) is not fully understood and some existing results are not true. For example, the main result (Theorem 6 on global stability of a positive equilibrium) of Aziz-Alaoui and Daher Okiye [3] is not true as the condition (i) and condition (iii) cannot hold simultaneously. In fact, it follows from condition (i), , that . On the other hand, condition (iii), , implies that . Then, one can have , which is impossible. One purpose of this paper is to establish several sufficient conditions on the global asymptotic stability of a positive equilibrium.

Let . As a result of biological meaning, we only consider solutions of (1.1) with . Moreover, solutions of (1.1) with are called positive solutions. An equilibrium of (1.1) is called *globally asymptotically stable* if and as for any positive solution of (1.1). System (1.1) is *permanent* if there exists such that, for any positive solution of (1.1),
The remaining part of this paper is organized as follows. In Section 2, we discuss the structure of nonnegative equilibria to (1.1) and their linearized stability. This has not been done yet, and the results will motivate us to study global asymptotic stability of (1.1) in Section 3. The obtained results not only improve but also supplement existing ones.

#### 2. Nonnegative Equilibria and Their Linearized Stability

The Jacobian matrix of (1.1) is An equilibrium of (1.1) is (linearly) stable if the real parts of both eigenvalues of are negative and therefore a sufficient condition for stability is

Obviously, (1.1) has three boundary equilibria, , , and , whose Jacobian matrices are respectively. As a direct consequence of (2.2), we have the following result.

Proposition 2.1. *
(i) Both and are unstable.**
(ii) is stable if , while it is unstable if . *

Besides the three boundary equilibria, (1.1) may have (componentwise) positive equilibria. Suppose that is such an equilibrium. Then, One can easily see that satisfies where . Moreover, for convenience, we denote . Equation (2.5) can have at most two positive solutions, and hence (1.1) can have at most two positive equilibria. Precisely, we have the following three cases.

*Case 1. *Suppose one of the following conditions holds. (i).(ii) and .(iii), , and .

Then, (1.1) has a unique positive equilibrium with and .

*Case 2. * If , , and , then (1.1) has two positive equilibria , where and .

*Case 3. *If no condition in Case 1 or Case 2 holds, then (1.1) has no positive equilibrium.

For a positive equilibrium , can be simplified to by using (2.4). By simple computation, , .

Then, one can easily see that for Case 1(i)-(ii), for Case 1(iii), , and . Therefore, we obtain the following.

Proposition 2.2. *
(i) The positive equilibrium in Case 1(i)(ii) is stable if .**
(ii) The positive equilibrium is unstable, while the positive equilibrium is stable if . *

*Remark 2.3. *In [3, 7, 8], only existence of the positive equilibrium of (1.1) for Case 1(i) was considered, which is stable if either (a) and [3] or (b) and [7, 8]. Obviously, Proposition 2.2 greatly improves these results.

Propositions 2.1 and 2.2 naturally motivate us to seek sufficient conditions on global asymptotic stability of equilibrium to (1.1) and permanence of (1.1).

Nindjin et al. [5] showed that if then for a positive solution of (1.1). Therefore, system (1.1) is permanent if holds. With the help of these bounds, it was shown that is globally asymptotically stable if holds (see [5]).

In the coming section, we present two results on the global asymptotic stability of a positive equilibrium, which not only supplement Theorem 7 of Nindjin et al. [5] but also improve it by including more situations.

#### 3. Global Asymptotic Stability of a Positive Equilibrium

The first result is established by employing the Fluctuation Lemma, and we refer to [12–16] for details.

Theorem 3.1. * In addition to , further suppose that
**
where is defined in (2.8). Then, system (1.1) has a unique positive equilibrium which is globally asymptotically stable. *

*Proof. *Obviously, implies , that is, condition (i) of Case 1 holds. Thus, (1.1) has a unique positive equilibrium. Let be any positive solution of (1.1). By the results at the end of Section 2, , .

We claim . Otherwise, . According to the Fluctuation lemma, there exist sequences , , , and as such that , , , , , , , and as . First, from the second equation of (1.1),
Letting , we obtain that and . Hence,
Similar arguments as above also produce
Second, from the first equation of (1.1),
Equation (3.4) implies .

Taking limit as , one obtains . This, combined with (3.3), gives us . It follows that
Similarly, one can show that
Multiplying (3.5) by and adding it to (3.6), we have
Due to , one gets which contradicts . Therefore, , and the claim is proved.

The claim implies that exists and we denote it by . Then, it follows from (3.2) and (3.3) that exists and . Letting in (3.4) gives us . Then, one can see that satisfies (2.4), that is, is a positive equilibrium of (1.1). This completes the proof as the positive equilibrium is unique.

Theorem 3.2. * Suppose that (1.1) has a unique positive equilibrium . Further assume that
**
where is defined in (2.7). Then, is globally asymptotically stable. *

* Proof. *Let be any positive solution of (1.1). From , we can choose an such that
Moreover, it follows from (2.7) that there exists such that
According to the proof of Theorem 6 in [3], let
Then, by the positivity of , (3.8), and (3.9),
Therefore, is globally asymptotically stable, and this completes the proof.