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Persistence and almost periodic solutions for a discrete ratio-dependent Leslie system with feedback control
Advances in Difference Equations volume 2014, Article number: 214 (2014)
Abstract
In this paper, by utilizing the comparison theorem of the differential equation and constructing a suitable Lyapunov functional, we consider the existence of almost periodic solutions to a discrete time ratio-dependent Leslie system with feedback control. Some sufficient conditions for the existence of positive almost periodic solutions for the model are obtained. An example is given to illustrate the effectiveness of the main results.
MSC:34K14, 92D25.
1 Introduction
In 1948, Leslie considered the following differential equation (see [1]):
where and stand for the population (the density) of the prey and the predator at time t, respectively, and is the so-called predator functional response to prey.
Recently, more and more obvious evidences of biology and physiology show that in many conditions, especially when the predators have to search for food (consequently, have to share or compete for food), a more realistic and general predator-prey system should rely on the theory of ratio-dependence, this theory is confirmed by lots of experimental results (see [2, 3]). A ratio-dependent Leslie system with the functional response of Holling-Tanner type is as follows:
where has the same means as before. In particular, Wang et al. [3] considered a ratio-dependent Leslie predator-prey model with feedback controls as follows:
where and stand for the population (the density) of the prey and the predator at time t, respectively, () are control variables, the prey grows logistically with growth rate and carries capacity in the absence of predation. The parameter is a measure of food quality that the prey provides, which is converted to the predator birth. Under the assumption that the coefficients of the above system are all T-periodic functions, by applying Mawhin’s continuation theorem and constructing a suitable Lyapunov function, they obtained sufficient conditions which guarantee the existence of a unique globally attractive positive T-periodic solution to system (1.1).
Feedback control is the basic mechanism by which systems, whether mechanical, electrical, or biological, maintain their equilibrium or homeostasis. During the last decade, a series of mathematical systems have been established to describe the dynamics of feedback control systems, we refer to [4–9]. Furthermore, in recent research on species, dynamics of the Leslie system has important significance, see [1–3, 5, 6, 10–16] and the references therein for details. Moreover, since the discrete time models can also provide efficient computational models of continuous models for numerical simulations, it is reasonable to study discrete time models governed by difference equations. Motivated by the above idea, we consider a discrete ratio-dependent Leslie system with feedback control:
where () denote the density of the prey and the predator at time n, respectively. , are control variables, , , , , , , , , , , () are all almost ω-periodic functions of n; denotes the constant of capturing half-saturation. For more biological background of system (1.2), one could refer to [3] and the references cited therein.
To the best of our knowledge, though many works have been done for population dynamic systems with feedback controls, most of the works deal with continuous time models. For more results about the existence of almost periodic solutions of a continuous time system, we can refer to [5] and the references cited therein. There are few works that consider the existence of almost periodic solutions for a discrete time population dynamic model with feedback controls. On the other hand, in fact, it is more realistic to consider almost periodic systems than periodic systems. On the existence and stability of almost periodic sequence solutions for the discrete biological models, some results are found in the literature, we refer to [8, 9, 17, 18]. Therefore, our main purpose of this paper is to study the existence and uniqueness of almost periodic solutions for model (1.2).
Throughout this paper, we assume that
(H1) , , , , , , , , and for are bounded nonnegative almost periodic sequences such that
, , ,
, , ,
, , (),
().
Here, for any bounded sequence , and . Furthermore, we need the following assumptions:
(H2) ;
(H3) .
By the biological meaning, we focus our discussion on the positive solution of model (1.2). So it is assumed that the initial conditions of model (1.2) are of the form
One can easily show that all the solutions of model (1.2) with the initial condition (1.3) are defined and remain positive for all .
The organization of this paper is as follows. In Section 2, we give some basic definitions and necessary lemmas which will be used in later sections. In Section 3, the persistence of model (1.2) is established. In Section 4, based on the persistence result, we show the existence and uniform asymptotic stability of an almost periodic solution to model (1.2). An example is given in Section 5.
2 Definitions and lemmas
Now let us state several definitions and lemmas which will be useful in proving the main result of this section.
Definition 2.1 [17]
A sequence is called an almost periodic sequence if the ϵ-translation number set of x,
is a relatively dense set in ℤ for all ; that is, for any given , there exists an integer such that each interval of length contains an integer such that
τ is called the ϵ-translation number of .
Definition 2.2 [17]
Let , where is an open set in , is said to be almost periodic in n uniformly for , or uniformly almost periodic for short, if for any and any compact set in , there exists a positive integer such that any interval of length contains an integer τ for which
τ is called the ϵ-translation number of .
Lemma 2.1 [18]
is an almost periodic sequence if and only if for any sequence there exists a subsequence such that converges uniformly on as . Furthermore, the limit sequence is also an almost periodic sequence.
In [17], Zhang and Zheng consider the following almost periodic delay difference system
where , , with , is almost periodic in n uniformly for and is continuous in ϕ, while is defined as for all .
The product system of (1.2) is in the form of
A discrete Lyapunov functional of (1.2) is a functional which is continuous in its second and third variables. Define the difference of V along the solution of system (1.2) by
where is a solution of system (1.2) through , .
Lemma 2.2 [17]
Suppose that there exists a Lyapunov functional satisfying the following conditions:
-
(1)
, where with .
-
(2)
, where is a constant.
-
(3)
, where is a constant.
Moreover, if there exists a solution of (1.2) such that for all , then there exists a unique uniformly asymptotically stable almost periodic solution of (1.2) which satisfies for all . In particular, if is periodic of period ω, then (1.2) has a unique uniformly asymptotically stable periodic solution of period ω.
3 Persistence
In this section, we establish a persistence result for system (1.2).
Proposition 3.1 Assume that (H1) holds. For every solution of system (1.2),
where , , ().
Proof We first present two cases to prove that
Case 1. By the first equation of system (1.2), from (H1) and (1.3), we have
Then there exists such that . So, . Hence, , and
Here we used for . We claim that for .
In fact, if there exists an integer such that , and letting be the least integer between and m such that , then and , which implies . This is impossible. The claim is proved.
Case 2. for . In particular, exists, denoted by . We claim that . By way of contradiction, assume that . Taking . Noting that , therefore
for , which is a contradiction. This proves the claim.
Similarly to the above analysis, next we prove .
Case 1. By the second equation of system (1.2), from (H1) and (1.3), we can obtain
Then there exists such that . So, . Hence, , and
In fact, if there exists an integer such that , and letting be the least integer between and m such that , then and , which implies . This is impossible. The claim is proved.
Case 2. for . In particular, exists, denoted by . We claim that . By way of contradiction, assume that . Taking . Noting that , therefore
for , which is a contradiction. This proves the claim.
Similarly, by the third and fourth equations of system (1.2), for all , we can get
Since , we can find a positive number such that . Using Stolz’s theorem, we have
Hence
By the arbitrariness of ϵ, is valid. So the proof of Proposition 3.1 is complete. □
Proposition 3.2 Assume that (H1)-(H3) hold, where and () are the same in Proposition 3.1. Then
where
Proof Firstly, we also present two cases to prove that
For any which satisfies and , according to Proposition 3.1, there exists such that
for .
Case 1. There exists a positive integer such that . Note that for , we have
In particular, with , we obtain
which implies that . Then
We claim that for .
By way of contradiction, assume that there exists such that . Then , let be the smallest integer such that . Then . The above argument produces that , a contradiction. This proves the claim.
Case 2. We assume that for all large . Then exists, denoted by . We claim that . By way of contradiction, assume that . Take
which is a contradiction since
Noting that , we see that , and . We can easily see that holds.
The same as in the above equality analysis, we will obtain the result from the second equation of system (1.2).
Case 1. By the second equation of system (1.2), (H1)-(H3) and (1.3), we can obtain
In particular with , we have
which implies that
Then
Let . We claim that for .
By way of contradiction, assume that there exists such that . Then , let , let be the smallest integer such that . Then . The above argument produces that , a contradiction. This proves the claim.
Case 2. We assume that for . Then exists, denoted by . We claim that
By way of contradiction, assume that . Take , which is a contradiction, since
Noting that , we see that , and . We can easily see that holds. Thus, for any small enough, there exists a positive integer , such that for .
The proof of , , is very similar to that of Proposition 2 in [19]. Here we omit the details. □
Now the main result of this section is obtained as follows.
Theorem 3.1 Suppose that assumptions (H1)-(H3) hold. Then system (1.2) is persistent.
4 Existence of a unique almost periodic solution
According to Lemma 2.2, we first prove that there exists a bounded solution of system (1.2) and then construct an adaptive Lyapunov functional for system (1.2).
The next results tells that there exists a bounded solution of system (1.2).
Proposition 4.1 Assume that (H1)-(H3) hold, then .
Proof It is now possible to show by an inductive argument that system (1.2) leads to
From Proposition 3.1 and Proposition 3.2, any solution of system (1.2) with initial condition (1.3) satisfies system (4.1). Hence, for any , there exists . If is sufficiently large, we have
Let be any integer-valued sequence such that as . We claim that there exists a subsequence of , we still denote it by , such that
uniformly in n on any finite subset B of Z as , where , () and m is a finite number.
In fact, for any finite subset , when α is large enough, , . So
That is, , are uniformly bounded for large enough n.
Similarly, for , we can choose a subsequence of such that , uniformly converges on for n large enough.
Repeating this procedure, for , we obtain a subsequence of such that , uniformly converges on for n large enough.
Now pick the sequence which is a subsequence of , we still denote it by , then for all , we have , uniformly in as .
By the arbitrariness of , the conclusion is valid.
Since , , , , , , , , and are almost periodic sequences, for the above sequence , as , there exists a subsequence still denoted by (if necessary, we take a subsequence) such that
as uniformly on . For any , we can assume that for p large enough. Let and , an inductive argument of system (1.2) from to leads to
Then, for , we have
Let , for any ,
By the arbitrariness of σ, is a solution of system (1.2) on . It is clear that , , for all , . So . Proposition 4.1 is valid. □
The main results of the following theorem concern the existence of a uniformly asymptotically stable almost periodic sequence solution of system (1.2).
Theorem 4.1 Assume that (H1)-(H3) hold. Suppose further that (H4): , here , where
and
then there exists a unique uniformly asymptotically stable almost periodic solution of system (1.2) which is bounded by Ω for all .
Proof Let . From (1.2), we have
where . From Proposition 4.1, we know that system (4.8) has a bounded solution satisfying
Hence, , , where , , .
For , we define the norm .
Consider the product system of system (4.8)
Suppose that , are any two solutions of system (4.10) defined on , then , , where
Consider the Lyapunov function defined on as follows:
It is easy to see that the norm and the norm are equivalent, that is, there exist two constants , such that
then
Let , , , , thus condition (1) in Lemma 2.2 is satisfied.
In addition,
where (). Hence condition (2) of Lemma 2.2 is satisfied.
Finally, calculating ΔV of along the solutions of (4.10), we can obtain
In view of system (4.1) and using the mean value theorem, we get
where lies between and , ,
where
and
Similarly, we also have
where
and
From system (4.10), we also obtain
where
From (4.16), (4.17), (4.18) and (4.19), we have
where . That is, there exists a positive constant such that . From , condition (3) of Lemma 2.2 is satisfied. So, from Lemma 2.2, there exists a unique uniformly asymptotically stable almost periodic solution of system (4.10) which is bounded by for all , which means that there exists a unique uniformly asymptotically stable almost periodic solution of system (1.2) which is bounded by Ω for all . This completes the proof. □
5 An example
In this section, we present an example to illustrate the feasibility of our results.
Example 5.1 Consider the following discrete ratio-dependent Leslie model:
where , , , , , , , , , , , . Then system (5.1) is persistent and has a unique uniformly asymptotically stable almost periodic sequence solution.
Proof It is easy to see that , , , , , , , , and for are bounded nonnegative almost periodic sequences. By calculation of Mathematica software, we get
Then , and . So we can see that all the conditions of Theorem 4.1 hold. According to Theorem 4.1, system (5.1) has a unique uniformly asymptotically stable almost periodic solution which is bounded by Ω for all . □
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Acknowledgements
The author thanks the anonymous referees for their careful reading of this manuscript, and for their valuable comments and suggestions. This work is supported by Yunnan Province Education Department Scientific Research Fund Project (No. 2012Z065) and the Young Teachers Program of Yuxi Normal University.
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Li, Z. Persistence and almost periodic solutions for a discrete ratio-dependent Leslie system with feedback control. Adv Differ Equ 2014, 214 (2014). https://doi.org/10.1186/1687-1847-2014-214
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DOI: https://doi.org/10.1186/1687-1847-2014-214