# Dynamics of a fourth-order system of rational difference equations

- Q Din
^{1}Email author, - MN Qureshi
^{1}and - A Qadeer Khan
^{1}

**2012**:215

https://doi.org/10.1186/1687-1847-2012-215

© Din et al.; licensee Springer 2012

**Received: **3 October 2012

**Accepted: **3 December 2012

**Published: **17 December 2012

## Abstract

In this paper, we study the equilibrium points, local asymptotic stability of an equilibrium point, instability of equilibrium points, periodicity behavior of positive solutions, and global character of an equilibrium point of a fourth-order system of rational difference equations of the form

$n=0,1,\dots $ , where the parameters *α*, *β*, *γ*, ${\alpha}_{1}$, ${\beta}_{1}$, ${\gamma}_{1}$ and initial conditions ${x}_{0}$, ${x}_{-1}$, ${x}_{-2}$, ${x}_{-3}$, ${y}_{0}$, ${y}_{-1}$, ${y}_{-2}$, ${y}_{-3}$ are positive real numbers. Some numerical examples are given to verify our theoretical results.

**MSC:**39A10, 40A05.

## Keywords

## 1 Introduction and preliminaries

The theory of discrete dynamical systems and difference equations developed greatly during the last twenty-five years of the twentieth century. Applications of difference equations also experienced enormous growth in many areas. Many applications of discrete dynamical systems and difference equations have appeared recently in the areas of biology, economics, physics, resource management, and others. The theory of difference equations occupies a central position in applicable analysis. There is no doubt that the theory of difference equations will continue to play an important role in mathematics as a whole. Nonlinear difference equations of order greater than one are of paramount importance in applications. Such equations also appear naturally as discrete analogues and as numerical solutions of differential and delay differential equations which model various diverse phenomena in biology, ecology, physiology, physics, engineering and economics. It is very interesting to investigate the behavior of solutions of a system of higher-order rational difference equations and to discuss the local asymptotic stability of their equilibrium points.

where ${u}_{n}$, ${v}_{n}$, ${w}_{n}$, ${s}_{n}$ are some sequences ${x}_{n}$ or ${y}_{n}$.

for all values of real parameters *a*, *b*.

$n=0,1,\dots $ , where the parameters *α*, *β*, *γ*, ${\alpha}_{1}$, ${\beta}_{1}$, ${\gamma}_{1}$ and initial conditions ${x}_{0}$, ${x}_{-1}$, ${x}_{-2}$, ${x}_{-3}$, ${y}_{0}$, ${y}_{-1}$, ${y}_{-2}$, ${y}_{-3}$ are positive real numbers. This paper is a natural extension of [9, 10].

*I*,

*J*are some intervals of real numbers. Furthermore, a solution ${\{({x}_{n},{y}_{n})\}}_{n=-3}^{\mathrm{\infty}}$ of the system (1.2) is uniquely determined by initial conditions $({x}_{i},{y}_{i})\in I\times J$ for $i\in \{-3,-2,-1,0\}$. Along with the system (1.2), we consider the corresponding vector map $F=(f,{x}_{n},{x}_{n-1},{x}_{n-2},{x}_{n-3},g,{y}_{n},{y}_{n-1},{y}_{n-2},{y}_{n-3})$. An equilibrium point of (1.2) is a point $(\overline{x},\overline{y})$ that satisfies

The point $(\overline{x},\overline{y})$ is also called a fixed point of the vector map *F*.

**Definition 1.1**Let $(\overline{x},\overline{y})$ be an equilibrium point of the system (1.2).

- (i)
An equilibrium point $(\overline{x},\overline{y})$ is said to be stable if for every $\epsilon >0$, there exists $\delta >0$ such that for every initial condition $({x}_{i},{y}_{i})$, $i\in \{-3,-2,-1,0\}$ if $\parallel {\sum}_{i=-3}^{0}({x}_{i},{y}_{i})-(\overline{x},\overline{y})\parallel <\delta $ implies $\parallel ({x}_{n},{y}_{n})-(\overline{x},\overline{y})\parallel <\epsilon $ for all $n>0$, where $\parallel \cdot \parallel $ is the usual Euclidean norm in ${\mathbb{R}}^{2}$.

- (ii)
An equilibrium point $(\overline{x},\overline{y})$ is said to be unstable if it is not stable.

- (iii)
An equilibrium point $(\overline{x},\overline{y})$ is said to be asymptotically stable if there exists $\eta >0$ such that $\parallel {\sum}_{i=-3}^{0}({x}_{i},{y}_{i})-(\overline{x},\overline{y})\parallel <\eta $ and $({x}_{n},{y}_{n})\to (\overline{x},\overline{y})$ as $n\to \mathrm{\infty}$.

- (iv)
An equilibrium point $(\overline{x},\overline{y})$ is called a global attractor if $({x}_{n},{y}_{n})\to (\overline{x},\overline{y})$ as $n\to \mathrm{\infty}$.

- (v)
An equilibrium point $(\overline{x},\overline{y})$ is called an asymptotic global attractor if it is a global attractor and stable.

**Definition 1.2**Let $(\overline{x},\overline{y})$ be an equilibrium point of the map

*f*and

*g*are continuously differentiable functions at $(\overline{x},\overline{y})$. The linearized system of (1.2) about the equilibrium point $(\overline{x},\overline{y})$ is

and ${F}_{J}$ is a Jacobian matrix of the system (1.2) about the equilibrium point $(\overline{x},\overline{y})$.

where $A=\frac{\alpha}{\beta +\gamma {\overline{y}}^{4}}$, $B=-\frac{\alpha \gamma \overline{x}{\overline{y}}^{3}}{{(\beta +\gamma {\overline{y}}^{4})}^{2}}$, $C=-\frac{{\alpha}_{1}{\gamma}_{1}\overline{y}{\overline{x}}^{3}}{{({\beta}_{1}+{\gamma}_{1}{\overline{x}}^{4})}^{2}}$ and $D=\frac{{\alpha}_{1}}{{\beta}_{1}+{\gamma}_{1}{\overline{x}}^{4}}$.

**Theorem 1.3** *For the system* ${X}_{n+1}=F({X}_{n})$, $n=0,1,\dots $ , *of difference equations such that* $\overline{X}$ *is a fixed point of* *F*. *If all eigenvalues of the Jacobian matrix* ${J}_{F}$ *about* $\overline{X}$ *lie inside the open unit disk* $|\lambda |<1$, *then* $\overline{X}$ *is locally asymptotically stable*. *If one of them has a modulus greater than one*, *then* $\overline{X}$ *is unstable*.

**Theorem 1.4** (Routh-Hurwitz criterion)

*For real numbers*${a}_{1},{a}_{2},\dots ,{a}_{n}$,

*let*

*Consider the polynomial equation*

*The following statements are true*:

- (i)
*A necessary and sufficient condition for all of the roots of*(1.5)*to have a negative real part is*$det({H}_{j})>0$*for*$j=1,2,\dots ,n$. - (ii)
*A necessary and sufficient condition for the existence of a root of*(1.5)*with a positive real part is*$det({H}_{j})<0$*for some*$j\in \{1,2,\dots ,n\}$.

## 2 Main results

where $A={(\frac{{\alpha}_{1}-{\beta}_{1}}{{\gamma}_{1}})}^{\frac{1}{4}}$ and $B={(\frac{\alpha -\beta}{\gamma})}^{\frac{1}{4}}$.

**Theorem 2.1**

*Let*$({x}_{n},{y}_{n})$

*be a positive solution of the system*(1.1),

*then for every*$m\ge 0$,

*the following results hold*:

□

**Theorem 2.2**

*For the equilibrium point*${P}_{0}=(0,0)$

*of Equation*(1.1),

*the following results hold*:

- (i)
*Let*$\alpha <\beta $*and*${\alpha}_{1}<{\beta}_{1}$,*then the equilibrium point*${P}_{0}=(0,0)$*of the system*(1.1)*is locally asymptotically stable*. - (ii)
*If*$\alpha >\beta $*or*${\alpha}_{1}>{\beta}_{1}$,*then the equilibrium point*${P}_{0}=(0,0)$*of the system*(1.1)*is unstable*.

*Proof*(i) The linearized system of (1.1) about the equilibrium point $(0,0)$ is given by

- (ii)
It is easy to see that if $\alpha >\beta $ or ${\alpha}_{1}>{\beta}_{1}$, then there exists at least one root

*λ*of Equation (2.1) such that $|\lambda |>1$. Hence, by Theorem 1.3 if $\alpha >\beta $ or ${\alpha}_{1}>{\beta}_{1}$, then $(0,0)$ is unstable. □

**Theorem 2.3** *If* $\alpha >\beta $ *and* ${\alpha}_{1}>{\beta}_{1}$, *then a positive equilibrium point* ${P}_{1}=({(\frac{{\alpha}_{1}-{\beta}_{1}}{{\gamma}_{1}})}^{\frac{1}{4}},{(\frac{\alpha -\beta}{\gamma})}^{\frac{1}{4}})$ *of Equation* (1.1) *is unstable*.

*Proof*The linearized system of (1.1) about the equilibrium point ${P}_{1}$ is given by

Hence, by Theorem 1.3 if $\alpha >\beta $ and ${\alpha}_{1}>{\beta}_{1}$, then ${P}_{1}$ is unstable. □

**Theorem 2.4** *If* $\alpha >\beta $ *and* ${\alpha}_{1}>{\beta}_{1}$, *then the equilibrium points* ${P}_{2}$, ${P}_{3}$, ${P}_{4}$ *of Equation* (1.1) *are unstable*.

*Proof* The proof is similar to Theorem 2.3, so it is omitted. □

The following theorem is similar to Theorem 3.4 of [9].

**Theorem 2.5**

*Let*$\alpha >\beta $

*and*${\alpha}_{1}>{\beta}_{1}$,

*and let*$({x}_{n},{y}_{n})$

*be a solution of the system*(1.1).

*Then*,

*for*$k=-3,-2,-1,0$,

*the following statements are true*:

- (i)
*If*$({x}_{k},{y}_{k})\in (0,{(\frac{{\alpha}_{1}-{\beta}_{1}}{{\gamma}_{1}})}^{\frac{1}{4}})\times ({(\frac{\alpha -\beta}{\gamma})}^{\frac{1}{4}},\mathrm{\infty})$,*then*$({x}_{n},{y}_{n})\in (0,{(\frac{{\alpha}_{1}-{\beta}_{1}}{{\gamma}_{1}})}^{\frac{1}{4}})\times ({(\frac{\alpha -\beta}{\gamma})}^{\frac{1}{4}},\mathrm{\infty})$. - (ii)
*If*$({x}_{k},{y}_{k})\in ({(\frac{{\alpha}_{1}-{\beta}_{1}}{{\gamma}_{1}})}^{\frac{1}{4}},\mathrm{\infty})\times (0,{(\frac{\alpha -\beta}{\gamma})}^{\frac{1}{4}})$,*then*$({x}_{n},{y}_{n})\in ({(\frac{{\alpha}_{1}-{\beta}_{1}}{{\gamma}_{1}})}^{\frac{1}{4}},\mathrm{\infty})\times (0,{(\frac{\alpha -\beta}{\gamma})}^{\frac{1}{4}})$.

**Theorem 2.6** *The system* (1.1) *has no prime period*-*two solutions*.

*Proof*Assume that $({p}_{1},{q}_{1}),({p}_{2},{q}_{2}),({p}_{1},{q}_{1}),\dots $ is a prime period-two solution of Equation (1.1) such that ${p}_{i},{q}_{i}\ne 0$ and ${p}_{i}\ne {q}_{i}$ for $i=1,2$. Then, from the system (1.1), one has

From (2.3) and (2.4), one has ${p}_{i},{q}_{i}=0$ for $i=1,2$. Which is a contradiction. Hence, the system (1.1) has no prime period-two solutions. □

**Theorem 2.7** *Let* $\alpha <\beta $ *and* ${\alpha}_{1}<{\beta}_{1}$, *then the equilibrium point* ${P}_{0}=(0,0)$ *of Equation* (1.1) *is globally asymptotically stable*.

*Proof*For $\alpha <\beta $ and ${\alpha}_{1}<{\beta}_{1}$, from Theorem 2.2, $(0,0)$ is locally asymptotically stable. From Theorem 2.1, it is easy to see that every positive solution $({x}_{n},{y}_{n})$ is bounded,

*i.e.*, $0\le {x}_{n}\le \mu $ and $0\le {y}_{n}\le \nu $ for all $n=0,1,2,\dots $ , where $\mu =max\{{x}_{-3},{x}_{-2},{x}_{-1},{x}_{0}\}$ and $\nu =max\{{y}_{-3},{y}_{-2},{y}_{-1},{y}_{0}\}$. Now, it is sufficient to prove that $({x}_{n},{y}_{n})$ is decreasing. From the system (1.1), one has

*i.e.*, the sequence $\{{x}_{n}\}$ is decreasing. Similarly, one has

This implies that ${y}_{4n+1}<{y}_{4n-3}$ and ${y}_{4n+5}<{y}_{4n+1}$. Hence, the subsequences $\{{y}_{4n+1}\}$, $\{{y}_{4n+2}\}$, $\{{y}_{4n+3}\}$, $\{{y}_{4n+4}\}$ are decreasing, *i.e.*, the sequence $\{{y}_{n}\}$ is decreasing. Hence, ${lim}_{n\to \mathrm{\infty}}{x}_{n}={lim}_{n\to \mathrm{\infty}}{y}_{n}=0$. □

**Theorem 2.8**

*Let*$\alpha >\beta $

*and*${\alpha}_{1}>{\beta}_{1}$.

*Then*,

*for a solution*$({x}_{n},{y}_{n})$

*of the system*(1.1),

*the following statements are true*:

- (i)
*If*${x}_{n}\to 0$,*then*${y}_{n}\to \mathrm{\infty}$. - (ii)
*If*${y}_{n}\to 0$,*then*${x}_{n}\to \mathrm{\infty}$.

## 3 Examples

In order to verify our theoretical results and to support our theoretical discussions, we consider several interesting numerical examples in this section. These examples represent different types of qualitative behavior of solutions to the system of nonlinear difference equations (1.1). All plots in this section are drawn with mathematica.

**Example**Consider the system (1.1) with initial conditions ${x}_{-3}=1.1$, ${x}_{-2}=2.6$, ${x}_{-1}=1.6$, ${x}_{0}=1.7$, ${y}_{-3}=1.5$, ${y}_{-2}=1.3$, ${y}_{-1}=2.5$, ${y}_{0}=0.5$. Moreover, choosing the parameters $\alpha =0.01$, $\beta =0.011$, $\gamma =50$, ${\alpha}_{1}=0.03$, ${\beta}_{1}=0.031$, ${\gamma}_{1}=70$, the system (1.1) can be written as follows:

**Example**Consider the system (1.1) with initial conditions ${x}_{-3}=2.1$, ${x}_{-2}=2.6$, ${x}_{-1}=0.6$, ${x}_{0}=1.4$, ${y}_{-3}=1.5$, ${y}_{-2}=1.6$, ${y}_{-1}=2.7$, ${y}_{0}=0.45$. Moreover, choosing the parameters $\alpha =12$, $\beta =12.5$, $\gamma =90$, ${\alpha}_{1}=15$, ${\beta}_{1}=15.5$, ${\gamma}_{1}=75$, the system (1.1) can be written as follows:

**Example**Consider the system (1.1) with initial conditions ${x}_{-3}=9.2$, ${x}_{-2}=1.8$, ${x}_{-1}=0.76$, ${x}_{0}=1.1$, ${y}_{-3}=1.1$, ${y}_{-2}=1.2$, ${y}_{-1}=8.1$, ${y}_{0}=0.52$. Moreover, choosing the parameters $\alpha =200$, $\beta =225$, $\gamma =1000$, ${\alpha}_{1}=150$, ${\beta}_{1}=160$, ${\gamma}_{1}=700$, the system (1.1) can be written as follows:

**Example**Consider the system (1.1) with initial conditions ${x}_{-3}=1.3$, ${x}_{-2}=1.8$, ${x}_{-1}=2.6$, ${x}_{0}=2.1$, ${y}_{-3}=0.01$, ${y}_{-2}=1.2$, ${y}_{-1}=2.8$, ${y}_{0}=1.5$. Moreover, choosing the parameters $\alpha =12$, $\beta =10.5$, $\gamma =15$, ${\alpha}_{1}=14$, ${\beta}_{1}=13$, ${\gamma}_{1}=0.2$, the system (1.1) can be written as follows:

## 4 Conclusion

This work is a natural extension of [9, 10]. In the paper, we investigated some dynamics of an eight-dimensional discrete system. The system has five equilibrium points all of which except $(0,0)$ are unstable. The linearization method is used to show that the equilibrium point $(0,0)$ is locally asymptotically stable. We prove that the system has no prime period-two solutions. The main objective of dynamical systems theory is to predict the global behavior of a system based on the knowledge of its present state. An approach to this problem consists of determining the possible global behaviors of the system and determining which initial conditions lead to these long-term behaviors. In case of higher-order dynamical systems, it is crucial to discuss global behavior of the system. Some powerful tools such as semiconjugacy and weak contraction cannot be used to analyze global behavior of the system (1.1). In the paper, we prove the global asymptotic stability of the equilibrium point $(0,0)$ by using simple techniques. Some numerical examples are provided to support our theoretical results. These examples are experimental verifications of theoretical discussions.

## Declarations

### Acknowledgements

Authors would like to thank the referees for their comments and suggestions on the manuscript. This work was supported by the Higher Education Commission of Pakistan.

## Authors’ Affiliations

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