`International Journal of AnalysisVolume 2013 (2013), Article ID 486357, 12 pageshttp://dx.doi.org/10.1155/2013/486357`
Research Article

## Global Attractivity Results on Complete Ordered Metric Spaces for Third-Order Difference Equations

1Department of Mathematics and Applied Mathematics, University of Pretoria, Lynnwood road, Pretoria 0002, South Africa
2University of Tunisia, Tunis College of Sciences and Techniques, 5 Avenue Taha Hussein, BP 56, Bab Manara, Tunis, Tunisia

Received 24 October 2012; Accepted 31 January 2013

Copyright © 2013 Mujahid Abbas and Maher Berzig. 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

We establish fixed-point theorems for mixed monotone mappings in the setting of ordered metric spaces which satisfy a contractive condition for all points that are related by a given ordering. We also give a global attractivity result for all solutions of the difference equation where satisfies certain monotonicity conditions with respect to the given ordering. As an application of our obtained results, we present some iterative algorithms to solve a class of matrix equations. A numerical example is also presented to test the validity of the algorithms.

#### 1. Introduction

The following global attractivity result from [1] (see also [2]) is very useful in establishing convergence results in many situations.

Theorem 1 (see [1]). Let be a closed and bounded interval of real numbers and let satisfy the following conditions.(i)The function is monotonic in each of its arguments.(ii)For each and for each , one defines
and assume that if is a solution of the system
then .
Then there exists exactly one equilibrium of the equation and every solution of the above equation converges to .

The above result in Theorem 1 attracted considerable attention of the leading specialists in difference equations and discrete dynamic systems and has been generalized and extended to the case of maps in , see [3], and maps in Banach space with cone, see [4####^~^~^~^~^~^####x2013;6].

Moreover, there has been recent interest in establishing fixed-point theorems in partially ordered complete metric spaces with a contractivity condition which holds for all points that are related by partial ordering, (see, e.g., [7####^~^~^~^~^~^####x2013;19]). These fixed-point results have been applied mainly to the existence of solutions of boundary value problems for differential equations, namely [18], has been applied for solving a class of matrix equations.

In [20], Burgi####^~^~^~^~^~^####x107; et al. obtained the following global attractivity result for mixed monotone mappings in partially ordered complete metric spaces (see also Gnana Bhaskar and Lakshmikantham [10]).

Theorem 2 (see Burgi####^~^~^~^~^~^####x107; et al. [20]). Let be a partially ordered set and suppose there is a metric on such that is a complete metric space. Let be a map such that is nonincreasing in for all and nondecreasing in for all . Suppose that the following conditions hold.(i)There exists with (ii)There exists such that the following condition holds: (iii)If is a nondecreasing convergent sequence such that , then , for all and if is a nonincreasing convergent sequence such that , then , for all ; if for every , then .
Then one has the following.(a)For every initial point such that condition (ii) holds, , , , where satisfy (b)If in condition (ii), then . If in addition , then , converge to the equilibrium of the equation (c)In particular, every solution of
such that , converges to the equilibrium of (7).

In this paper, motivated by the results and ideas in a recent work of Berinde and Borcut [9], we extend Theorem 2 to mappings . Such extension allows us to study the third-order difference equation The presented theorems also extend and generalize the work in [9]. We use our obtained results to build some iterative algorithms to solve a class of matrix equations. A numerical example is also presented to test the validity of the algorithms.

Now we introduce the following concepts.

Definition 3. Let be a nonempty set and a given mapping. One says that is a fixed-point of the third order if

Definition 4. Let be a partially ordered set and a given mapping. one says that has the mixed monotone property if is monotonously increasing in and and is monotonously decreasing in ; that is, for any ,(i),(ii), and(iii).

Through this paper we will use the following notations.

Let be a partially ordered set. (i)For the notation means that and .(ii)We endow with the partial order that we denote also by , defined by (iii)Let be a given mapping. For all , we denote

#### 2. Main Result

Our first result is the following.

Theorem 5. Let be a partially ordered set and suppose there is a metric on such that is a complete metric space. Let be a mapping having the mixed monotone property on . Suppose that the following conditions hold.(i) There exists with ####^~^~^~^~^~^####x2009;for all . (ii) There exist such that (iii) If is a nondecreasing convergent sequence such that , then for all , if is a nonincreasing convergent sequence such that , then for every , and if for every , then .
Then one has the following.(a) For every initial point such that condition (14) holds: ####^~^~^~^~^~^####x2009;where satisfy ####^~^~^~^~^~^####x2009;If and in condition (14), then and , , and converge to the equilibrium of the equation (b) In particular, every solution of ####^~^~^~^~^~^####x2009;such that (or ) converges to the equilibrium of (18).(c) The following estimates hold:

Proof. Let such that condition (14) is satisfied. Denote , , and . Since , , and , from the mixed monotone property of , we get that Consider the sequences , , and defined by (17). By induction and using the mixed monotone property of , we obtain easily that For the sake of clarity, for all , denote We claim that, for all , we have By (13) and (21), we obtain Thus we get that Then our claim holds for . Suppose now that (23) holds for some fixed . Similarly, by (13) and (21), we obtain Similarly, one can show that Then by the induction principle, (23) holds for all .
Now we will prove that , , and are Cauchy sequences in the metric space . Using (23) and the triangular inequality, for , we have This implies that is a Cauchy sequence. Similarly, one can prove that and are Cauchy sequences.
Since is complete, there exist such that From condition (iii) and (21), we get that We claim that . Indeed, we have
Then, our claim holds; that is, . Similarly, one can show that and . Thus we proved (16).
On the other hand, from (28), for with being fixed, we have Letting in the above inequality, we obtain that is, Similarly, one can show that Thus we proved (19).
Now, if and , we claim that, for all and . Indeed, by the mixed monotone property of , Assume that and for some . Then, and similarly for and . Thus we proved that
Next, from (38), we have which implies that . Similarly, we obtain that , that is, . Then .
Now, assume that . Then, in view of the monotonicity of , we have Continuing this process, one can show that If we assume that , in view of the monotonicity of , we have Continuing in a similar way, we can prove that Letting and using (iii) and (29), we get that as , where is the equilibrium of (17). Similarly, if and , we obtain also the same result.

Remark 6. If we replace the condition, if is a nondecreasing convergent sequence such that , then for all and if is a nonincreasing convergent sequence such that , then for every , by the continuity of , we can check easily that the result of Theorem 5 holds also in this case.

Theorem 7. In addition to the hypotheses of Theorem 5, suppose that for every , , there exists such that and . Then one obtains the uniqueness of the fixed point of the third order.

Proof. From (a) of Theorem 5, we know that admits a fixed point of the third order ; that is, where Suppose that is another fixed point of the third order of ; that is, We will prove that where From the hypothesis of Theorem 7, there exists such that Since is a mixed monotone operator, we have We have which implies that that is,
Example 8. Let be an ordered set with the natural ordering of real numbers and a usual metric on . Let be defined by It is easy to check that has the mixed monotone property on . For with , we have Thus (13) is satisfied for . Thus all the conditions of Theorems 5 and 7 are satisfied. Moreover, there exists a unique in such that

Corollary 9. Let be a partially ordered set and suppose there is a metric on such that is a complete metric space. Let be a mapping having the mixed monotone property on . Suppose that the following conditions hold. (1) There exists for and with ####^~^~^~^~^~^####x2009;for all . (2) There exist such that (3) If is a nondecreasing convergent sequence such that , then for all , if is a nonincreasing convergent sequence such that , then for every , and if for every , then .
Then one has the following.(a) For every initial point such that condition (14) holds, ####^~^~^~^~^~^####x2009;where satisfy ####^~^~^~^~^~^####x2009;If and in condition (14), then and , , and converge to the equilibrium of the equation (b) In particular, every solution of ####^~^~^~^~^~^####x2009;such that (or ) converges to the equilibrium of (18).

Proof. Note that (14) implies that for all , where and the result follows from Theorem 5.

Corollary 10. In addition to the hypotheses of Corollary 9, suppose that for every , , there exists such that and . Then one obtains the uniqueness of the fixed point of the third order.

Corollary 11. Let be a partially ordered set and suppose there is a metric on such that is a complete metric space. Let be a mapping having the mixed monotone property on . Suppose that the following conditions hold. (1)There exists with (2)There exist such that (3)If is a nondecreasing convergent sequence such that , then for all ; if is a nonincreasing convergent sequence such that , then for every , and if for every , then .
Then one has the following.(a) For every initial point such that condition (14) holds, ####^~^~^~^~^~^####x2009;where satisfy ####^~^~^~^~^~^####x2009;If and in condition (14), then and , , and converge to the equilibrium of the equation (b) In particular, every solution of ####^~^~^~^~^~^####x2009;such that (or ) converges to the equilibrium of (18).(c) The following estimates hold:

Corollary 12. In addition to the hypotheses of Corollary 11, suppose that for every , , there exists such that and . Then one obtains the uniqueness of the fixed point of the third order.

#### 3. Application: Solving a Class of Third-Order Difference Matrix Equations

In this section, we apply our main results to the study of a class of third-order difference matrix equations. At first, we start by fixing some notations and recalling some preliminaries.

We will use the symbol for the set of all Hermitian matrices. We denote by the set of all Hermitian positive definite matrices. Instead of we will also write . Furthermore, means that is positive semidefinite. As a different notation for and we will use, respectively, , and . The symbol denotes the spectral norm, that is, , the largest eigenvalue of . We denote by the Ky Fan norm defined by , where are the singular values of . For a given , we define the modified norm given by . The set endowed with this norm is a complete metric space for any positive definite matrix . For any matrix , we denote by tr the trace of the matrix .

The following lemmas will be useful later.

Lemma 13 (see [18]). Let and be matrices, then

Lemma 14 (See [21]). Let satisfy , then .

Finally, we recall the well-known Schauder fixed-point theorem.

Theorem 15 (The Schauder fixed-point theorem). Let be a nonempty, compact, and convex subset of a normed vector space. Every continuous function mapping into itself has a fixed point.

Now, we consider the class of third-order difference matrix equations: for given , where and , , and are Hermitian matrices. This type of difference equations often arises from many areas such as ladder networks [22, 23], dynamic programming [24, 25], and control theory [26, 27].

##### 3.1. A Convergence Result

Theorem 16. Suppose that Then, one has the following.(i) Equation (72) has one and only one equilibrium point .(ii), where (iii) The sequences and defined by , , and ####^~^~^~^~^~^####x2009;converge to , and the error estimation is given by ####^~^~^~^~^~^####x2009;for all , where is a certain constant in .(iv) For every , every solution of (72) converges to .

Proof. In order to make the proof easy, we divide it into several steps.
Step 1. We claim that there exists a unique solution to the system Consider the mapping defined by for all . It is clear that is a mapping having the mixed monotone property with respect to the partial order . Let such that , , and . Using Lemma 13, we have where From (73) and Lemma 14, we have . Thus, the contractive condition of Theorem 5 is satisfied for all with , , and . Moreover, from (73), we have and .
Now, all the hypotheses of Theorem 5 are satisfied. Consequently, there exists solution to (77). Since for every there is a greatest lower bound and a least upper bound with respect to the partial order , we deduce from Theorem 7 the uniqueness of the solution to (77). Then, our claim holds.
Step 2. We claim that .
Since (note that is a positive definite matrix), applying Theorem 5, we obtain the equality . This proves our claim.
From Steps 1 and 2, we know that (72) has a unique equilibrium point . Now, we need to prove that . This is the goal of the next step.
Step 3 (proof of (i)####^~^~^~^~^~^####x2013;(iii)). Define the mapping by for all .
From (73), one can show that . Now, applying the Schauder fixed-point theorem (see Theorem 15), we deduce the existence of a fixed point of in . But a fixed point of is an equilibrium point of (72), and from Step 2, we know that (72) has a unique equilibrium point in . Consequently, . This proves (i) and (ii). For the proof of (iii), we have only to apply the inequalities (19).
Step 4. Let , , and . Then, we have , thus by (b) Theorem 5, every solution to (72) converges to .

##### 3.2. Numerical Example

Consider a third-order difference matrix equation where , , , , , , and are given by

We are interested to approximate the unique positive equilibrium to (82).

We use Algorithm (iii) of Theorem 16 with and .

After iterations, we get the unique solution given by