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Abstract and Applied Analysis

Volume 2013 (2013), Article ID 321930, 15 pages

http://dx.doi.org/10.1155/2013/321930

## Stability and Global Hopf Bifurcation Analysis on a Ratio-Dependent Predator-Prey Model with Two Time Delays

^{1}Department of Applied Mathematics, Kunming University of Science and Technology, Kunming, Yunnan 650093, China^{2}Department of Mathematics and Information Science, Zhoukou Normal University, Zhoukou, Henan 466001, China

Received 4 September 2013; Revised 3 November 2013; Accepted 5 November 2013

Academic Editor: István Györi

Copyright © 2013 Huitao Zhao et al. 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

A ratio-dependent predator-prey model with two time delays is studied. By means of an iteration technique, sufficient conditions are obtained for the global attractiveness of the positive equilibrium. By comparison arguments, the global stability of the semitrivial equilibrium is addressed. By using the theory of functional equation and Hopf bifurcation, the conditions on which positive equilibrium exists and the quality of Hopf bifurcation are given. Using a global Hopf bifurcation result of Wu (1998) for functional differential equations, the global existence of the periodic solutions is obtained. Finally, an example for numerical simulations is also included.

#### 1. Introduction

The main purpose of this paper is to investigate the bifurcation phenomena from the delays for the following predator-prey system: where and stand for the population (or density) of the prey and the predator at time , respectively. From the biological sense, we assume that . , and are positive constants, in which denotes the intrinsic growth rate of the prey, is the intraspecific competition rate of the prey, is the capturing rate of the predator, describes the efficiency of the predator in converting consumed prey into predator offspring, is the interference coefficient of the predators, and is the predator mortality rate. The delay denotes the gestation period of the predator; is the hunting delay of the predator to prey.

This model is labeled “ratio-dependent,” which means that the functional and numerical responses depend on the densities of both prey and predators, especially when predator has to search for food. Such a functional response is called a ratio-dependent response function (see [1] for more details). In system (1), the ratio-dependent response function is of the form .

The ratio-dependent predator-prey model has been studied by several researchers recently and very rich dynamics have been observed [2–5]. For example, Xu et al. [4] studied a delayed ratio-dependent predator-prey model with the same ratio-dependent response function of system (1). By means of an iteration technique, they obtained the sufficient conditions for the global attractiveness of the positive equilibrium. By comparison arguments, they proved the global stability of the semitrivial equilibrium. Finally using the theory of functional equation and Hopf bifurcation, they gave the condition on which positive equilibrium exists and the formulae to determine the quality of Hopf bifurcation. But in their work, the global continuation of local Hopf bifurcation was not mentioned.

In general, periodic solutions through the Hopf bifurcation in delay differential equations are local for the values of parameters which are only in a small neighborhood of the critical values (see, e.g., [6, 7]). Therefore we would like to know if these nonconstant periodic solutions obtained through local bifurcation can continue for a large range of parameter values. Recently, a great deal of research has been devoted to the topics [8–12]. One of the methods used in them is the global Hopf bifurcation theorem by Wu [13]. For example, Song et al. [12] studied a predator-prey system with two delays, and using the methods in [13], they get the global existence of periodic solutions.

Motivated by [12], we will study the system (1); special attention is paid to the global continuation of local Hopf bifurcation. We suppose that the initial condition for system (1) takes the form where , which is the Banach space of continuous functions mapping the interval into , where .

By the fundamental theory of functional differential equations [14], system (1) has a unique solution satisfying initial condition (2).

The rest of the paper is organized as follows. In Section 2, we show the positivity and the boundedness of solutions of system (1) with initial condition (2). In Section 3, we study the existence of Hopf bifurcation for system (1) at the positive equilibrium. In Section 4, using the normal form theory and the center manifold reduction, explicit formulae are derived to determine the direction of bifurcation and the stability and other properties of bifurcating periodic solutions. In Section 5, by means of an iteration technique, sufficient conditions are obtained for the global attractiveness of the positive equilibrium. In Section 6, we consider the global existence of bifurcating periodic solutions and give some numerical simulations. In Section 7, a brief discussion is given.

#### 2. Positivity and Boundedness

In this section, we study the positivity and boundedness of solutions of system (1) with initial conditions (2).

Theorem 1. *Solutions of system (1) with initial condition (2) are positive for all .*

*Proof. *Assume to be a solution of system (1) with initial condition (2). Let us consider for . It follows from the second equation of system (1) that
then, from initial condition (2), we have , for . We derive from the first equation of system (1) that
that is, for . This ends the proof.

For the following discussion of boundedness, we first consider the following ordinary differential equation: where , and are positive constants. From Lemma 2.1 in , it is easy to verify the following result.

Lemma 2. *If , the trivial equilibrium of (5) is globally stable. If , then (5) admits a unique positive equilibrium which is globally asymptotically stable in .*

Theorem 3. *Positive solutions of system (1) with initial condition (2) are ultimately bounded.*

*Proof. *Let be a positive solution of system (1) with initial condition (2). From the first equation of system (1), we have
which yields
hence, for sufficiently small, there is a such that if , .

We now consider the boundedness of . If , we derive from the second equation of system (1) that
from monotone bounded theorem, it is easy to show that .

Therefore, we assume below that . We derive from the second equation of system (1) that, for ,
noting that , by Lemma 2, a comparison argument shows that
This completes the proof.

#### 3. Local Stability and Hopf Bifurcation

In this section, we discuss the local stability of the positive equilibrium and the semitrivial equilibrium of system (1) and establish the existence of Hopf bifurcation at the positive equilibrium.

It is easy to show that system (1) always has a semitrivial equilibrium . Further, if the following condition holds: (H1) ,then system (1) has a unique positive equilibrium , where where

For convenience, let us introduce new variables , rewriting as , so that system (1) can be written as the following system with a single delay: Clearly, system (13) has the same equilibrium as system (1).

The characteristic equation of system (13) at the semitrivial equilibrium is of the form Clearly, (14) always has a root , and if , the other root of (14) is negative; if , the other root of (14) is positive. Hence the semitrivial equilibrium is locally asymptotically stable (unstable) if ().

The characteristic equation of system (13) at the positive equilibrium is of the form where where is defined as (12).

When , (15) becomes It is easy to show that Obviously, if (H1) holds, then . Hence, the positive equilibrium of system (13) is locally stable when if and it is unstable when if

We assume that is a root of (15); this is the case if and only if satisfies the following equation: Separating the real and imaginary parts, we obtain the following system for : It follows that

Letting , (42) becomes By a direct calculation, it follows that Note that if (H1) holds, then . Hence if (H1) and hold, (24) has no positive roots. Accordingly, if (H1) and hold, the positive equilibrium of system (13) exists and is locally asymptotically stable for all . If (H1) and hold, then (24) has a unique positive root , where Then, we can get at which (15) admits a pair of purely imaginary roots of the form .

Let and be defined above. Denote the root of (15) satisfying

It is not difficult to verify that the following result holds.

Lemma 4. *If (H1) and hold, the transversal condition holds.*

*Proof. *Differentiating (15) with respect , we obtain that
it follows that
from (15) and (31), we have

We therefore derive that
Noting that , hence, if (H1) and hold, we have . Accordingly, the transversal condition holds and a Hopf bifurcation occurs at .

By Lemma B in [5], we have the following results.

Theorem 5. *Suppose (H1) holds and let be defined in (12), for system (13), one has the following.*(i)*If and , then the positive equilibrium is locally asymptotically stable for all .*(ii)*If and , then there exists a positive number such that the positive equilibrium is locally asymptotically stable if and is unstable if . Further, system (13) undergoes a Hopf bifurcation at when .*

#### 4. Direction and Stability of Hopf Bifurcations

In Section 3, we have shown that system (13) admits a periodic solution bifurcated from the positive equilibrium at the critical value . In this section, we derive explicit formulae to determine the direction of Hopf bifurcations and stability of periodic solutions bifurcated from the positive equilibrium at critical value by using the normal form theory and the center manifold reduction (see, e.g., [15, 16]).

Set ; then is a Hopf bifurcation value of system (13). Thus we can consider the problem above in the phase space .

Let . System (13) is transformed into where

For the simplicity of notations, we rewrite (34) as where , is defined by , and , are given, respectively, by By the Riesz representation theorem, there exists a function of bounded variation for such that In fact, we can choose where is the Dirac delta function. For , define

Then when , system (36) is equivalent to where for .

For , define and a bilinear inner product, where and denotes the conjugate complex of . Then and are adjoint operators. By the discussion in Section 3, we know that are eigenvalues of . Thus, they are also eigenvalues of . We first need to compute the eigenvector of and corresponding to and , respectively.

Suppose that is the eigenvector of corresponding to . Then . From the definition of , it is easy to get .

Similarly, let be the eigenvector of corresponding to . By the definition of , we can compute .

In order to assure , we need to determine the value of . From (44) and the definitions of and , we have such that and .

In the following, we first compute the coordinates to describe the center manifold at . Define On the center manifold , we have where and are local coordinates for center manifold in the directions of and . Note that is real if is real. We consider only real solutions. For the solution , since , we have where By (45), we have It follows from (38) and (48) that

In order to assure the value of , we need to compute and . By (42) and (45), we have where Notice that near the origin on the center manifold , we have thus, we have By (51), for , we have Comparing the coefficients with (51) gives that From (56), (54), and the definition of , we can get

Notice that ; we have where is a constant vector. In the same way, we can also obtain where is also a constant vector. In what follows, we will compute and . From the definition of and (54), we have where .

From (51), (58), and (60) and noting that we have where

From (52), (59), and (61) and noting that we have where

Thus, we can determine and from (58) and (59). Furthermore, we can determine each . Therefore, each is determined by the parameters and delay in (13). Thus, we can compute the following values [15]: which determine the quantities of bifurcating periodic solutions in the center manifold at the critical value ; that is, determines the directions of the Hopf bifurcation: if , then the Hopf bifurcation is supercritical (subcritical) and the bifurcation exists for ; determines the stability of the bifurcation periodic solutions: the bifurcating periodic solutions are stable (unstable) if ; and determines the period of the bifurcating periodic solutions: the period increases (decreases) if .

#### 5. Global Attractiveness

In this section, following Chaplygin [17], taking into account the upper and lower solution technique and using monotone iterative methods [18, 19], we discuss the global attractiveness of the positive equilibrium and the global stability of the semitrivial equilibrium of system (1), respectively.

Theorem 6. *Suppose (H1) holds and let be defined above, then the positive equilibrium of system (1) is globally attractive provided that the following holds:* *(H2) ,*

*Proof. *Let be any positive solution of system (1) with initial conditions (2).

Let
Using iteration method, we will proof that .

From the first equation of system (1), we have
by comparison, it follows that
hence, for sufficiently small, there exists a such that if .

From the second equation of system (1), we have, for ,
Consider the following auxiliary equation:
Since (H1) holds, by Lemma 2, it follows from (73) that
where is defined in (12). By comparison, we obtain that
since this inequality holds true for arbitrary sufficiently small, it follows that , where
Hence, for sufficiently small, there is a such that if .

For sufficiently small, noting that , we derive from the first equation of system (1) that, for ,
by comparison, it follows that
hence, for sufficiently small, there is a , such that if , .

For sufficiently small, we derive from the second equation of system (1) that, for ,
Consider the following auxiliary equation:
Since (H1) holds, by Lemma (5), it follows from (80) that
by comparison we derive that
Since this inequality holds true for arbitrary sufficiently small, we conclude that , where
Therefore, for sufficiently small, there is a such that if , .

Again, for sufficiently small, it follows from the first equation of system (1) that, for ,
by comparison we derive that
Since the above inequality holds true for arbitrary sufficiently small, it follows that , where
hence, for sufficiently small, there is a such that if , .

It follows from the second equation of system (1) that, for ,
By Lemma 2 and a comparison argument we derive from (87) that
since this inequality holds true for sufficiently small, we get , where
hence, for sufficiently small, there is a such that if , .

For sufficiently small, it follows from the first equation of system (1) that, for ,
by comparison, we can obtain that
Since the above inequality holds true for arbitrary sufficiently small, it follows that , where
therefore, for sufficiently small, there is a such that if , .

For sufficiently small, we derive from the second equation of system (1) that, for ,
Since (H1) holds, by Lemma 2 and a comparison argument, it follows (93) that
since, for arbitrary sufficiently small, this inequality holds true, we conclude that , where
Continuing this process, we obtain four sequences such that, for ,
where is defined in (12). It is readily seen that
It is easy to know that the sequences are not increasing and the sequences are not decreasing; from accumulation point theorem, the limit of each sequence in , and exists, Denote
We therefore obtain from (96) and (98) that
To complete the proof, it is sufficient to prove that . It follows from (99) that
Letting (100) minus (101), we have
If , we derive from (102) that
Letting , we derive from (103) that
It follows from (104) that
noting that , we derive from (105) that
Substituting (78) into (106), it follows that
Hence, if (H2) holds, we have ; this is a contradiction. Accordingly, we have . Therefore, from (99), we have . Hence, the positive equilibrium is globally attractive. The proof is complete.

Using the same methods in [4, 20], we can also get a similar result.

Theorem 7. *If and , the semitrivial equilibrium of system (1) is globally asymptotically stable.*

#### 6. Global Continuation of Local Hopf Bifurcations

In this section, we study the global continuation of periodic solutions bifurcating from the positive equilibrium of system (13). Throughout this section, we follow closely the notations in [13]. For simplification of notations, setting , we may rewrite system (13) as the following functional differential equation: where . It is obvious that if (H1) holds, then system (13) has a semitrivial equilibrium and a positive equilibrium . Following the work of [13], we need to define Let denote the connected component passing through in , where is defined by (26). From Theorem 5, we know that is nonempty.

We first state the global Hopf bifurcation theory due to Wu [13] for functional differential equations.

Lemma 8. *Assume that is an isolated center satisfying the hypotheses ()–() in [13]. Denote by the connected component of in . Then either*(i)* is unbounded or*(ii)* is bounded; is finite and
**for all , where is the crossing number of if or it is zero if otherwise.*

Clearly, if (ii) in Lemma 8 is not true, then is unbounded. Thus, if the projections of onto -space and onto -space are bounded, then the projection onto -space is unbounded. Further, if we can show that the projection of onto -space is away from zero, then the projection of onto -space must include interval . Following this ideal, we can prove our results on the global continuation of local Hopf bifurcation.

Lemma 9. *If condition (H1) holds, then all nonconstant periodic solutions of (13) with initial conditions,
*