 Research
 Open Access
Necessary and sufficient condition for existence of periodic solutions of predatorprey dynamic systems with BeddingtonDeAngelistype functional response
 Neslihan Nesliye Pelen^{1}Email author,
 A. Feza Güvenilir^{2} and
 Billur Kaymakçalan^{3}
https://doi.org/10.1186/s1366201607470
© Pelen et al. 2016
 Received: 30 July 2015
 Accepted: 10 January 2016
 Published: 22 January 2016
Abstract
We consider twodimensional predatorprey systems with BeddingtonDeAngelistype functional response on periodic time scales. For this special case, we try to find the necessary and sufficient conditions for the considered system when it has at least one wperiodic solution. This study is mainly based on continuation theorem in coincidence degree theory and will also give beneficial results for continuous and discrete cases. Especially, for the continuous case, by using the study of Cui and Takeuchi (J. Math. Anal. Appl. 317:464474, 2006), to obtain the globally attractive wperiodic solution of the given system, an inequality is given as a necessary and sufficient condition. Additionally, for the continuous case in this study, the open problem given in the discussion part of the study of Fan and Kuang (J. Math. Anal. Appl. 295:1539, 2004) is solved.
Keywords
 predatorprey dynamic systems
 BeddingtonDeAngelistype functional response
 continuation theorem
 globally attractive solution
 periodic solution
 timescale calculus
1 Introduction
The relationships between species and the outer environment and the connections between different species are the description of the predatorprey dynamic systems, which are the subject of mathematical ecology in biomathematics. Various types of functional responses in a predatorprey dynamic system such as Monodtype, semiratiodependent, and Hollingtype have been studied in [3–5].
The key concepts in this study are the functional response in the periodic environment and the timescale calculus.
First of all, we investigate the predatorprey system with BeddingtonDeAngelistype functional response for a general time scale and its continuous case. This type of functional response first appeared in [6] and [7]. At low densities, with this type of functional response, some of the singular behaviors of ratiodependent models are avoided. Also, predator feeding can be described much better over a range of predatorprey abundances by using BeddingtonDeAngelistype functional response.
Secondly, being in a periodic environment is important because, in such an environment, the global existence and stability of a positive periodic solution is a significant problem in population growth model. This plays a similar role as a globally stable equilibrium in an autonomous model. Therefore, it is important to consider under which conditions the resulting periodic nonautonomous system would have a positive periodic solution that is globally asymptotically stable, and the globally asymptotically stable periodic solution of the given system in the continuous case is investigated in this study as an application. For nonautonomous case, there are many studies on the existence of periodic solutions of predatorprey systems in continuous and discrete models based on the coincidence theory such as [5, 8–16].
Here x̃ and ỹ represent the densities of the populations of the prey and predator. In other words, they represent the numbers of individuals in the prey and predator population per unit area, respectively. For general nonautonomous case, Fan and Kuang studied the permanence, extinction, and global asymptotic stability of the given system. For the periodic case, Fan and Kuang established two sufficient criteria for the existence of a positive periodic solution by using Brouwer fixed point theorem and continuation theorem in coincidence degree theory, respectively. These criteria are easy to be verified for the given system in the form of (1). At the same time, authors pointed that these criteria have room for further improvement. They presented numerical simulation to indicate that (1) may admit positive periodic solutions when the conditions in the theorems fail.
On the basis of these obtained results for system (1) with periodic coefficients, Cui and Takeuchi continue the study on the periodic solution and permanence of that system. Cui and Takeuchi obtained some new conditions for the permanence and existence of a positive periodic solution of system (1). These results improve those obtained by Fan and Kuang [2]. In addition to this improvement, in their paper [1], they also give the equivalence between the permanence and satisfaction of the inequality in Theorem 2.1 in [1]. However, they could not show whether there is the equivalence between the existence of at least one wperiodic solution and satisfaction of the inequality in Theorem 2.1 in [1].
In this study, for the continuous case, this equivalence is shown by using coincidence degree theory as an application of general timescale case. In addition to that, we also show the global attractivity or global asymptotic stability of this wperiodic solution. In other words, the contribution of this paper is that we prove that any predatorprey dynamic systems with BeddingtonDeAngelistype functional response satisfying the inequality in Theorem 2.1 in paper [1] have the same meaning with having globally attractive wperiodic solution. Therefore, we are able to show that to obtain a periodic solution for the periodic case of system (1), the improvement of this inequality becomes impossible.
Thirdly, the unification of continuous and discrete analysis is also significant for this study. To unify the study of differential and difference equations, the theory of timescale calculus is initiated by Stephan Hilger [17]. In [4, 18] unification of the existence of periodic solutions of population models modeled by ordinary differential equations and their discrete analogues in the form of difference equations and extension of these results to more general time scales is studied. The aim of this study is to find a necessary and sufficient condition for the periodic solution of the given system with BeddingtonDeAngelistype functional response for a general time scale and apply this result to the continuous case.
2 Preliminaries
The necessary information is taken from [21]. Let X, Z be normed vector spaces, \(L : \operatorname{Dom}L \subset X \rightarrow Z\) be a linear mapping, and \(N : X \rightarrow Z\) be a continuous mapping. The mapping L will is called a Fredholm mapping of index zero if \(\operatorname{dim} \operatorname{Ker}L=\operatorname {codim}\operatorname{Im}L<+\infty\) and ImL is closed in Z. If L is a Fredholm mapping of index zero and there exist continuous projections \(P : X \rightarrow X\) and \(Q : Z \rightarrow Z\) such that \(\operatorname{Im} P = \operatorname{Ker}L\) and \(\operatorname {Im}L = \operatorname{Ker}Q = \operatorname{Im} (I  Q)\), then it follows that \(L_{\operatorname{Dom}L \cap\operatorname{Ker} P} : (I  P)X \rightarrow\operatorname{Im}L\) is invertible. We denote the inverse of that map by \(K_{P}\). If Ω is an open bounded subset of X, then the mapping N is called Lcompact on Ω if \(QN(\Omega)\) is bounded and \(K_{P} (I  Q)N : \Omega\rightarrow X\) is compact. Since ImQ is isomorphic to KerL, there exists an isomorphism \(J : \operatorname{Im}Q \rightarrow\operatorname{Ker}L\).
Definition 1
([22])
The codimension (or quotient or factor dimension) of a subspace L of a vector space V is the dimension of the quotient space \(V/L\); it is denoted by \(\operatorname{codim}_{V}L\) or simply by codimL and is equal to the dimension of the orthogonal complement of L in V, and we have \(\operatorname{dim} L+\operatorname{codim} L=\operatorname{dim} V\).
The given information is necessary for the following continuation theorem.
Theorem 1
([23], continuation theorem)
 (a)
For each \(\lambda\in(0, 1)\), every solution z of \(Lz = \lambda Nz\) is such that \(z \notin\delta\Omega\);
 (b)
\(QNz \neq0\) for each \(z \in\delta\Omega\cap \operatorname{Ker}L\), and the Brouwer degree \(\operatorname{deg} \{JQN,\delta\Omega\cap\operatorname{Ker} L, 0\} \neq0\).
Then the operator equation \(Lz = Nz\) has at least one solution in \(\operatorname{Dom}L \cap\delta\Omega\).
Definition 2
([24])

For all right dense points of \(t_{0}\in X\) and for each \(\epsilon >0\), there exists \(\delta>0\) such that for all \(t_{0}\in\mathbb{T}\), we have$$\bigl\vert f(t)f(t_{0}) \bigr\vert < \epsilon \quad \text{for all }f\in A, \vert tt_{0}\vert < \delta. $$

For all left dense points of \(t_{0}\in X\), there exists a δneighborhood \(U_{ld}\) of \(t_{0}\) such that$$\bigl\vert f \bigl(t' \bigr)f \bigl(t'' \bigr) \bigr\vert < \epsilon \quad \text{for all }f\in A, \bigl\vert t't'' \bigr\vert < \delta. $$
The above definition is significant for the explanation of the following ArzelaAscoli theorem for time scales.
Theorem 2
([24], ArzelaAscoli theorem for time scales)

A is uniformly bounded subset of \(C_{rd}(X,\mathbb{R})\).

A is rdequicontinuous in X.
We also give the following lemma, which is essential for the proof of the consequent theorems.
Lemma 1
([4])
Remark 1
In [25] predatorprey dynamic models with several types of functional responses with impulses on time scales are studied and a general result is obtained. On the other hand, in their study, only the effect of functional response is seen on the prey, but the effect of the given functional response cannot be seen on predator. Therefore, our results are also important since the impact of BeddingtonDeAngelistype functional response is taken into account for both prey and predator.
3 Main result
3.1 General case
Remark 2
([4])
Let \(\mathbb{T}=\mathbb{R}\). In (2), by taking \(\exp (x(t))=\tilde{x}(t)\) and \(\exp(y(t))=\tilde{y}(t)\), we obtain equality (1), which is the standard predatorprey system with BeddingtonDeAngelis functional response governed by ordinary differential equations. Many studies have been done on this system, and [1, 2, 14] are their examples.
For equation (2), \(\exp(x(t))\) and \(\exp(y(t))\) denote the densities of the prey and predator. Therefore, \(x(t)\) and \(y(t)\) may be negative. By taking the exponentials of \(x(t)\) and \(y(t)\) we obtain the numbers of preys and predators that are living per unit of an area. In other words, for the general timescale case, our equation is based on the natural logarithm of the densities of the predator and prey. Hence, \(x(t)\) and \(y(t)\) may be negative.
For equations (1) and (3), since \(\exp(x(t))=\tilde {x}(t)\) and \(\exp(y(t))=\tilde{y}(t)\), the given dynamic systems directly depend on the densities of the prey and predator.
Definition 3
In system (2), if for all solutions of \(x(t)\) (\(y(t)\)), \(\exp (x(t))\) (\(\exp(y(t))\)) tends to 0 as t tends to infinity, then we say that the prey (predator) goes to extinction.
Lemma 2
Proof
Since, \(\int_{0}^{w} (d(t)+\frac{f(t)}{\beta(t)})\Delta t<0\), \(\lim_{t\to \infty}\exp(y(t))=0\). □
Lemma 3
If the predator does not go to extinction, then neither prey does. In other words, if for all solutions of \(y(t)\), \(\exp(y(t))\) does not tend to zero as t tends to infinity, then for all solutions of \(x(t)\), \(\exp (x(t))\) does not tend to zero as t tends to infinity.
Proof
Theorem 3
Assume that all the coefficient functions in system (2) are bounded, positive, and wperiodic, and from \(C_{rd}(\mathbb{T},\mathbb {R}^{2})\). Then at least one wperiodic solution exists if and only if the predator does not go to extinction.
Proof
Let \(X:=\{\bigl [ {\scriptsize\begin{matrix}{}u \cr v\end{matrix}} \bigr ]\in C_{rd}(\mathbb{T},\mathbb{R}^{2}): u(t+w)=u(t),v(t+w)=v(t) \}\) with the norm \(\Vert \bigl [ {\scriptsize\begin{matrix}{}u \cr v\end{matrix}} \bigr ] \Vert =\sup_{t\in[0,w]_{\mathbb{T}}} (\vert u(t)\vert ,\vert v(t)\vert )\) and \(Y:=\{\bigl [ {\scriptsize\begin{matrix}{}u \cr v\end{matrix}} \bigr ]\in C_{rd}(\mathbb{T},\mathbb{R}^{2}), u(t+w)=u(t),v(t+w)=v(t) \}\) with the norm \(\Vert \bigl [ {\scriptsize\begin{matrix}{}u \cr v\end{matrix}} \bigr ] \Vert =\sup_{t\in[0,w]_{\mathbb{T}}}(\vert u(t)\vert ,\vert v(t)\vert )\).
In this subsection, from now on, instead of \([0,w]_{\mathbb{T}}\), we use \([0,w]\).
By the first equation of (6) and by (7) we get \(x(\xi_{1})< l_{1}\), where \(l_{1} :=\ln (\frac{\int_{0}^{w} a(t) \Delta t}{\int_{0}^{w} b(t) \Delta t} )\). Since \(x(\xi_{1})\) is the infimum of \(x(t)\) for \(t\in[0,w]\), there exists \(t_{1}\in[0,w]\) such that \(x(\xi _{1})\leq x(t_{1})< l_{1}\).
Thus, all the conditions of Theorem 1 are satisfied. Therefore, system (2) has at least one positive wperiodic solution.
If the given system (2) has at least one periodic solution, then for all the solutions of \(y(t)\), \(\exp (y(t))\) does not go to zero as t goes to infinity, which means that the predator does not go to extinction. Hence, we are done. □
Remark 3
It is obvious that if system (2) has at least one periodic solution, then the inequality \(\int_{0}^{w} d(t) \Delta t<\int_{0}^{w}\frac {f(t)}{\beta(t)}\Delta t\) must be satisfied. For the continuous case, this was done in [2]. But although this inequality is satisfied, system (2) does not have any periodic solution, which means that the predator can go to extinction by Theorem 3. Therefore, if we are able to extend the conditions that make the predator go to extinction, then we have more information about the systems that have at least one periodic solution.
Example 1
Example 2
3.2 Continuous case
3.2.1 Preliminaries for continuous case
Definition 4
([28])
Solutions of a wperiodic system generate a wperiodic semiflow \(T(t):X\rightarrow X\) (X is the initial value space) in the sense that \(T(t)x\) is continuous in \((t,x)\in[0,+\infty)\times X\), \(T(0)=I\), and \(T(t+w)=T(t)T(w)\) for all \(t>0\).
Definition 5
([28])
The periodic semiflow \(T(t)\) is said to be uniformly persistent with respect to \((X_{0},\partial X_{0})\) if there exists \(\eta>0\) such that for any \(x\in X_{0}\), \(\liminf_{t\rightarrow\infty}d(T(t)x, \partial X_{0})\geq \eta\).
Definition 6
([29])
Let \(T:\mathbb{R}^{n}\rightarrow\mathbb{R}^{n}\). The map T is point dissipative if there exists a bounded set B such that, for each \(x\in\mathbb{R}^{n}\), there is an integer \(n_{0}=n_{0}(x,B)\) such that \(T^{n}(x)\in B\) for each \(n\geq n_{0}\).
Lemma 4
([28])
Let \(S:X\rightarrow X\) be a continuous map with \(S(X_{0})\subset X_{0}\). Assume that S is point dissipative, compact, and uniformly persistent with respect to \((X_{0},\delta X_{0})\). Then there exists a global attractor \(A_{0}\) for S in \(X_{0}\) relative to strongly bounded sets in \(X_{0}\), and S has a coexistence state \(x_{0}\in A_{0}\).
Definition 7
([12])
Theorem 4
([1])
In [1], the following corollary of Theorem 4 is given.
Corollary 1
([1])
Theorem 5
([1])
Assume that all the coefficient functions in system (1) are positive. Then system (1) is permanent if and only if inequality (15) holds.
Definition 8
In system (17), for all solutions of \(x(t)\) (\(y(t)\)), if \(\exp (x(t))\) (\(\exp (y(t))\)) tends to 0 as t tends to infinity, then we say that the prey (predator) goes to extinction. In other words, in system (1), if \(\tilde {x}(t)\) (\(\tilde{y}(t)\)) tends to 0 as t tends to infinity, then we say that the prey (predator) goes to extinction.
In [1], a sufficient and necessary condition for the permanence of system (1) is established by the theorem, which Theorem 5 in this study. Additionally, in the discussion part of that paper, the following corollary is stated.
3.2.2 Application of the main result to the continuous case
Theorem 6
Assume that all the coefficient functions in system (1) are bounded, positive, wperiodic, and from \(C(\mathbb{T},\mathbb{R}^{2})\). Then, there exists at least one wperiodic solution of system (1) if and only if inequality (15) is satisfied.
Proof
First, let us assume that inequality (15) is satisfied. Then system (1) becomes permanent by Theorem 5, and the predator does not go to extinction. Since system (1) and system (17) are equivalent, the predator does not go to extinction in system (17). Then, by Theorem 3 we obtain that system (17) has at least one wperiodic solution. Therefore, system (1) also has at least one wperiodic solution.
For the other part, let us assume that our system (1) has at least one wperiodic solution. Then system (17) has at least one wperiodic solution. By Theorem 3 the predator does not go to extinction. By Lemma 3 the prey also does not go to extinction. Then \(\tilde{x}(t)\) and \(\tilde{y}(t)\) do not go to 0 as t tends to infinity. Then by Corollary 2 we obtain that if system (1) does not go to extinction, then inequality (15) is satisfied. Hence, we are done. □
The following lemma is similar to Lemma 4.4 in [3], but with zero impulses.
Lemma 5
Suppose that inequality (15) holds. Then, the wperiodic solution of system (1) is globally asymptotically stable or globally attractive.
Proof
3.2.3 Examples for continuous case
Example 3
For Example 3, \((f^{L}d^{M} {\beta}^{M})(\frac {a}{b})^{L}=(3.65(2)\cdot(1.5))\cdot(5)=3.25\) and \(d^{M} {\alpha }^{M}=2\cdot(2.2)=4.4\), but \(4.4>3.25\). Therefore, this example does not satisfy inequality (16) in Corollary 1, but since it satisfies inequality (15), we can say that this system has a 1periodic globally attractive solution.
Example 4
All of the figures in this study are obtained by Matlab program.
4 Discussion
In [2], Figure 1, for the continuous case, there was a discussion about why Figure (1a) does not satisfy the conditions of Theorem 3.2 in that paper, but the solutions are still periodic. We can answer this question by using our Theorem 3. When the coefficient functions in Figure (1a) are bounded, positive, 1periodic, continuous, and the predator does not go to extinction, then we have at least one 1periodic solution, which is globally asymptotically stable. Inequality (15) was first found by Cui and Takeuchi [1]. However, what they have found was the equivalence between satisfaction of inequality (15) and the permanence of the predatorprey dynamic systems with BeddingtonDeAngelistype function response. Although they have found that if system (1) satisfies inequality (15), then it has at least one wperiodic solution, they could not say anything about when system (1) has at least one wperiodic solution, whether it satisfies inequality (15) or not. In that paper, by using Theorem 3 and Theorem 6, we can say that for system (1), having at least one wperiodic solution of is equivalent to satisfaction of inequality (15), which means that a much better development of the inequality for system (1) to investigate the periodic solution is impossible. In addition, by using Corollary 3 we are able to say that satisfaction of inequality (15) is equivalent to the existence of a globally attractive wperiodic solution.
Hence, for any continuous predatorprey dynamic system with BeddingtonDeAngelistype functional response, there is a globally attractive wperiodic solution if and only if inequality (15) is satisfied, and the predator of this system goes to extinction if and only if inequality (15) is not satisfied.
As a result, the importance of this paper is that we can enlarge conditions for the existence of the positive periodic solutions of predatorprey dynamic systems with BeddingtonDeAngelis functional response for continuous case and for a general timescale case.
Declarations
Acknowledgements
We thank the reviewers and the academic editor of this article for all their contributions in the review process.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Authors’ Affiliations
References
 Cui, J, Takeuchi, Y: Permanence, extinction and periodic solution of predatorprey system with BeddingtonDeAngelis functional response. J. Math. Anal. Appl. 317, 464474 (2006) View ArticleMathSciNetMATHGoogle Scholar
 Fan, M, Kuang, Y: Dynamics of a nonautonomous predatorprey system with the BeddingtonDeAngelis functional response. J. Math. Anal. Appl. 295, 1539 (2004) View ArticleMathSciNetMATHGoogle Scholar
 Wang, W, Shen, J, Nieto, J: Permanence and periodic solution of predatorprey system with Holling type functional response and impulses. Discrete Dyn. Nat. Soc. 2007, Article ID 81756 (2007) View ArticleMathSciNetGoogle Scholar
 Bohner, M, Fan, M, Zhang, J: Existence of periodic solutions in predatorprey and competition dynamic systems. Nonlinear Anal., Real World Appl. 7, 11931204 (2006) View ArticleMathSciNetMATHGoogle Scholar
 Fan, M, Wang, Q: Periodic solutions of a class of nonautonomous discrete time semiratiodependent predatorprey systems. Discrete Contin. Dyn. Syst., Ser. B 4(3), 563574 (2004) View ArticleMathSciNetMATHGoogle Scholar
 Beddington, JR: Mutual interference between parasites or predators and its effect on searching efficiency. J. Anim. Ecol. 44, 331340 (1975) View ArticleGoogle Scholar
 DeAngelis, DL, Goldstein, RA, O’Neill, RV: A model for trophic interaction. Ecology 56, 881892 (1975) View ArticleGoogle Scholar
 Chen, F: Permanence and global stability of nonautonomous LotkaVolterra system with predatorprey and deviating arguments. Appl. Math. Comput. 173, 10821100 (2006) View ArticleMathSciNetMATHGoogle Scholar
 Fan, M, Agarwal, S: Periodic solutions for a class of discrete time competition systems. Nonlinear Stud. 9(3), 249261 (2002) MathSciNetMATHGoogle Scholar
 Fan, M, Wang, K: Global periodic solutions of a generalized nspecies GilpinAyala competition model. Comput. Math. Appl. 40(1011), 11411151 (2000) View ArticleMathSciNetMATHGoogle Scholar
 Fan, M, Wang, K: Periodicity in a delayed ratiodependent predatorprey system. J. Math. Anal. Appl. 262(1), 179190 (2001) View ArticleMathSciNetMATHGoogle Scholar
 Fang, Q, Li, X, Cao, M: Dynamics of a discrete predatorprey system with BeddingtonDeAngelis function response. Appl. Math. 3, 389394 (2012) View ArticleMathSciNetGoogle Scholar
 Huo, HF: Periodic solutions for a semiratiodependent predatorprey system with functional responses. Appl. Math. Lett. 18, 313320 (2005) View ArticleMathSciNetMATHGoogle Scholar
 Li, H, Takeuchi, Y: Dynamics of density dependent predatorprey system with BeddingtonDeAngelis functional response. J. Math. Anal. Appl. 374, 644654 (2011) View ArticleMathSciNetMATHGoogle Scholar
 Wang, Q, Fan, M, Wang, K: Dynamics of a class of nonautonomous semiratiodependent predatorprey systems with functional responses. J. Math. Anal. Appl. 278(2), 443471 (2003) View ArticleMathSciNetMATHGoogle Scholar
 Xu, R, Chaplain, MAJ, Davidson, FA: Periodic solutions for a predatorprey model with Hollingtype functional response and time delays. Appl. Math. Comput. 161(2), 637654 (2005) View ArticleMathSciNetMATHGoogle Scholar
 Hilger, S: Analysis on measure chains  a unified approach to continuous and discrete calculus. Results Math. 18, 1856 (1990) View ArticleMathSciNetMATHGoogle Scholar
 Fazly, M, Hesaaraki, M: Periodic solutions for predatorprey systems with BeddingtonDeAngelis functional response on time scales. Nonlinear Anal., Real World Appl. 9, 12241235 (2008) View ArticleMathSciNetMATHGoogle Scholar
 Güvenilir, AF, Kaymakçalan, B, Pelen, NN: Impulsive predatorprey dynamic systems with BeddingtonDeAngelis type functional response on the unification of discrete and continuous systems. Appl. Math. 6, 1649 (2015) View ArticleGoogle Scholar
 Yang, L, Yang, J, Zhog, Q: Periodic solutions for a predatorprey model with BeddingtonDeAngelis type functional response on time scales. Gen. Math. Notes 3(1), 4654 (2011) Google Scholar
 Bohner, M, Peterson, A: Dynamic Equations on Times Scales: An Introduction with Applications. Birkhäuser, Basel (2001) View ArticleGoogle Scholar
 Bourbaki, N: Elements of Mathematics. Algebra: Algebraic Structures. Linear Algebra, 1. AddisonWesley, Reading (1974) (Chaps. 1, 2) Google Scholar
 Gaines, RE, Mawhin, JL: Coincidence Degree and NonLinear Differential Equations. Springer, Berlin (1977) Google Scholar
 Gong, Y, Xiang, X: A class of optimal control problems of systems governed by the first order linear dynamic equations on time scales. J. Ind. Manag. Optim. 5(1), 110 (2009) MathSciNetMATHGoogle Scholar
 Liu, X, Liu, X: Necessary and sufficient conditions for the existence of periodic solutions in a predatorprey model on time scales. Electron. J. Differ. Equ. 2012 199 (2012) View ArticleGoogle Scholar
 Zhang, J, Wang, J: Periodic solutions for discrete predatorprey systems with the BeddingtonDeAngelis functional response. Appl. Math. Lett. 19, 13611366 (2006) View ArticleMathSciNetMATHGoogle Scholar
 Xu, C, Liao, M: Existence of periodic solutions in a discrete predatorprey system with BeddingtonDeAngelis functional responses. Int. J. Math. Math. Sci. 2011, Article ID 970763 (2011) View ArticleMathSciNetGoogle Scholar
 Zhao, XQ: Uniform persistence and periodic coexistence states in infinitedimensional periodic semiflows with applications. Can. Appl. Math. Q. 3(4), 473495 (1995) MATHGoogle Scholar
 Hale, JK: Asymptotic Behavior of Dissipative Systems. Math. Surveys and Monographs, vol. 25. Am. Math. Soc., Providence (1988) MATHGoogle Scholar
 Bainov, D, Simeonov, P: Impulsive Differential Equations: Periodic Solutions and Applications. Pitman Monogr. Surveys Pure Appl. Math., vol. 66. Longman, Harlow (1993) MATHGoogle Scholar