# Coupled fixed point theorems on partially ordered *G*-metric spaces

- Erdal Karapınar
^{1}, - Poom Kumam
^{2, 3}and - Inci M Erhan
^{1}Email author

**2012**:174

**DOI: **10.1186/1687-1812-2012-174

© Karapınar et al.; licensee Springer 2012

**Received: **7 July 2012

**Accepted: **26 September 2012

**Published: **11 October 2012

## Abstract

The purpose of this paper is to extend some recent coupled fixed point theorems in the context of partially ordered *G*-metric spaces in a virtually different and more natural way.

**MSC:**46N40, 47H10, 54H25, 46T99.

### Keywords

coupled fixed point coupled coincidence point mixed*g*-monotone property ordered set

*G*-metric space

## 1 Introduction and preliminaries

The notion of metric space was introduced by Fréchet [1] in 1906. In almost all fields of quantitative sciences which require the use of analysis, metric spaces play a major role. Internet search engines, image classification, protein classification (see, *e.g.*, [2]) can be listed as examples in which metric spaces have been extensively used to solve major problems. It is conceivable that metric spaces will be needed to explore new problems that will arise in quantitative sciences in the future. Therefore, it is necessary to consider various generalizations of metrics and metric spaces to broaden the scope of applied sciences. In this respect, cone metric spaces, fuzzy metric spaces, partial metric spaces, quasi-metric spaces and *b*-metric spaces can be given as the main examples. Applications of these different approaches to metrics and metric spaces make it evident that fixed point theorems are important not only for the branches of mainstream mathematics, but also for many divisions of applied sciences.

Inspired by this motivation Mustafa and Sims [3] introduced the notion of a *G*-metric space in 2004 (see also [4–7]). In their introductory paper, the authors investigated versions of the celebrated theorems of the fixed point theory such as the Banach contraction mapping principle [8] from the point of view of *G*-metrics. Another fundamental aspect in the theory of existence and uniqueness of fixed points was considered by Ran and Reurings [9] in partially ordered metric spaces. After Ran and Reurings’ pioneering work, several authors have focused on the fixed points in ordered metric spaces and have used the obtained results to discuss the existence and uniqueness of solutions of differential equations, more precisely, of boundary value problems (see, *e.g.*, [10–20]). Upon the introduction of the notion of coupled fixed points by Guo and Laksmikantham [14], Gnana-Bhaskar and Lakshmikantham [15] obtained interesting results related to differential equations with periodic boundary conditions by developing the mixed monotone property in the context of partially ordered metric spaces. As a continuation of this trend, many authors conducted research on the coupled fixed point theory and many results in this direction were published (see, for example, [21–35]).

In this paper, we prove the theorem that amalgamates these three seminal approaches in the study of fixed point theory, the so called *G*-metrics, coupled fixed points and partially ordered spaces.

We shall start with some necessary definitions and a detailed overview of the fundamental results developed in the remarkable works mentioned above. Throughout this paper, ℕ and ${\mathbb{N}}^{\ast}$ denote the set of non-negative integers and the set of positive integers respectively.

**Definition 1** (See [3])

Let *X* be a non-empty set, $G:X\times X\times X\to {\mathbb{R}}^{+}$ be a function satisfying the following properties:

(G1) $G(x,y,z)=0$ if $x=y=z$,

(G2) $G(x,x,y)>0$ for all $x,y\in X$ with $x\ne y$,

(G3) $G(x,x,y)\le G(x,y,z)$ for all $x,y,z\in X$ with $y\ne z$,

(G4) $G(x,y,z)=G(x,z,y)=G(y,z,x)=\cdots $ (symmetry in all three variables),

(G5) $G(x,y,z)\le G(x,a,a)+G(a,y,z)$ for all $x,y,z,a\in X$ (rectangle inequality).

Then the function *G* is called a generalized metric or, more specially, a *G*-metric on *X*, and the pair $(X,G)$ is called a *G*-metric space.

*G*-metric on

*X*induces a metric ${d}_{G}$ on

*X*given by

Trivial examples of *G*-metric are as follows.

**Example 2**Let $(X,d)$ be a metric space. The function $G:X\times X\times X\to [0,+\mathrm{\infty})$, defined by

for all $x,y,z\in X$, is a *G*-metric on *X*.

The concepts of convergence, continuity, completeness and Cauchy sequence have also been defined in [3].

**Definition 3** (See [3])

Let $(X,G)$ be a *G*-metric space, and let $\{{x}_{n}\}$ be a sequence of points of *X*. We say that $\{{x}_{n}\}$ is *G*-convergent to $x\in X$ if ${lim}_{n,m\to +\mathrm{\infty}}G(x,{x}_{n},{x}_{m})=0$, that is, if for any $\epsilon >0$, there exists $N\in \mathbb{N}$ such that $G(x,{x}_{n},{x}_{m})<\epsilon $ for all $n,m\ge N$. We call *x* the limit of the sequence and write ${x}_{n}\to x$ or ${lim}_{n\to +\mathrm{\infty}}{x}_{n}=x$.

**Proposition 4** (See [3])

*Let*$(X,G)$

*be a*

*G*-

*metric space*.

*The following statements are equivalent*:

- (1)
$\{{x}_{n}\}$

*is**G*-*convergent to**x*, - (2)
$G({x}_{n},{x}_{n},x)\to 0$

*as*$n\to +\mathrm{\infty}$, - (3)
$G({x}_{n},x,x)\to 0$

*as*$n\to +\mathrm{\infty}$, - (4)
$G({x}_{n},{x}_{m},x)\to 0$

*as*$n,m\to +\mathrm{\infty}$.

**Definition 5** (See [3])

Let $(X,G)$ be a *G*-metric space. A sequence $\{{x}_{n}\}$ is called *G*-Cauchy sequence if for any $\epsilon >0$, there is $N\in \mathbb{N}$ such that $G({x}_{n},{x}_{m},{x}_{l})<\epsilon $ for all $m,n,l\ge N$, that is, $G({x}_{n},{x}_{m},{x}_{l})\to 0$ as $n,m,l\to +\mathrm{\infty}$.

**Proposition 6** (See [3])

*Let*$(X,G)$

*be a*

*G*-

*metric space*.

*The following statements are equivalent*:

- (1)
*The sequence*$\{{x}_{n}\}$*is**G*-*Cauchy*. - (2)
*For any*$\epsilon >0$,*there exists*$N\in \mathbb{N}$*such that*$G({x}_{n},{x}_{m},{x}_{m})<\epsilon $,*for all*$m,n\ge N$.

**Definition 7** (See [3])

A *G*-metric space $(X,G)$ is called *G*-complete if every *G*-Cauchy sequence is *G*-convergent in $(X,G)$.

**Definition 8** Let $(X,G)$ be a *G*-metric space. A mapping $F:X\times X\times X\to X$ is said to be continuous if for any three *G*-convergent sequences $\{{x}_{n}\}$, $\{{y}_{n}\}$ and $\{{z}_{n}\}$ converging to *x*, *y* and *z* respectively, $\{F({x}_{n},{y}_{n},{z}_{n})\}$ is *G*-convergent to $F(x,y,z)$.

We define below *g*-ordered complete *G*-metric spaces.

**Definition 9** Let $(X,\u2aaf)$ be a partially ordered set, $(X,G)$ be a *G*-metric space and $g:X\to X$ be a mapping. A partially ordered *G*-metric space, $(X,G,\u2aaf)$, is called *g*-ordered complete if for each *G*-convergent sequence ${\{{x}_{n}\}}_{n=0}^{\mathrm{\infty}}\subset X$, the following conditions hold:

($O{C}_{1}$) If $\{{x}_{n}\}$ is a non-increasing sequence in *X* such that ${x}_{n}\to {x}^{\ast}$, then $g{x}^{\ast}\u2aafg{x}_{n}$ $\mathrm{\forall}n\in \mathbb{N}$.

($O{C}_{2}$) If $\{{x}_{n}\}$ is a non-decreasing sequence in *X* such that ${x}_{n}\to {x}^{\ast}$, then $g{x}^{\ast}\u2ab0g{x}_{n}$ $\mathrm{\forall}n\in \mathbb{N}$.

In particular, if *g* is the identity mapping in ($O{C}_{1}$) and ($O{C}_{2}$), the partially ordered *G*-metric space, $(X,G,\u2aaf)$, is called ordered complete.

We next recall some basic notions from the coupled fixed point theory. In 1987 Guo and Lakshmikantham [14] defined the concept of a coupled fixed point. In 2006, in order to prove the existence and uniqueness of the coupled fixed point of an operator $F:X\times X\to X$ on a partially ordered metric space, Gnana-Bhaskar and Lakshmikantham [15] reconsidered the notion of a coupled fixed point via the mixed monotone property.

**Definition 10** ([15])

*F*is said to have the mixed monotone property if $F(x,y)$ is monotone non-decreasing in

*x*and is monotone non-increasing in

*y*, that is, for any $x,y\in X$,

**Definition 11** ([15])

The results in [15] were extended by Lakshmikantham and Ćirić in [16] by defining the mixed *g*-monotone property.

**Definition 12**Let $(X,\u2aaf)$ be a partially ordered set, $F:X\times X\to X$ and $g:X\to X$. The function

*F*is said to have mixed

*g*-monotone property if $F(x,y)$ is monotone

*g*-non-decreasing in

*x*and is monotone

*g*-non-increasing in

*y*, that is, for any $x,y\in X$,

It is clear that Definition 12 reduces to Definition 10 when *g* is the identity mapping.

**Definition 13**An element $(x,y)\in X\times X$ is called a coupled coincidence point of the mappings $F:X\times X\to X$ and $g:X\to X$ if

*F*and

*g*if

**Definition 14**The mappings $F:X\times X\to X$ and $g:X\to X$ are said to commute if

Throughout the rest of the paper, we shall use the notation *gx* instead of $g(x)$, where $g:X\to X$ and $x\in X$, for brevity. In [35], Nashine proved the following theorems.

**Theorem 15**

*Let*$(X,G,\u2aaf)$

*be a partially ordered*

*G*-

*metric space*.

*Let*$F:X\times X\to X$

*and*$g:X\to X$

*be mappings such that*

*F*

*has the mixed*

*g*-

*monotone property*,

*and let there exist*${x}_{0},{y}_{0}\in X$

*such that*$g{x}_{0}\u2aafF({x}_{0},{y}_{0})$

*and*$F({y}_{0},{x}_{0})\u2aafg{y}_{0}$.

*Suppose that there exists*$k\in [0,\frac{1}{2})$

*such that for all*$x,y,u,v,w,z\in X$

*the following holds*:

*for all*$gw\u2aafgu\u2aafgx$

*and*$gy\u2aafgv\u2aafgz$,

*where either*$gu\ne gz$

*or*$gv\ne gw$.

*Assume the following hypotheses*:

- (i)
$F(X\times X)\subseteq g(X)$,

- (ii)
$g(X)$

*is**G*-*complete*, - (iii)
*g**is**G*-*continuous and commutes with**F*.

*Then* *F* *and* *g* *have a coupled coincidence point*, *that is*, *there exists* $(x,y)\in X\times X$ *such that* $gx=F(x,y)$ *and* $gy=F(y,x)$. *If* $gu=gz$ *and* $gv=gw$, *then* *F* *and* *g* *have a common fixed point*, *that is*, *there exists* $x\in X$ *such that* $gx=F(x,x)=x$.

**Theorem 16** *If in the above theorem*, *we replace the condition* (ii) *by the assumption that* *X* *is* *g*-*ordered complete*, *then we have the conclusions of Theorem * 15.

We next give the definition of *G*-compatible mappings inspired by the definition of compatible mappings in [13].

**Definition 17**Let $(X,G)$ be a

*G*-metric space. The mappings $F:X\times X\to X$, $g:X\to X$ are said to be

*G*-compatible if

where $\{{x}_{n}\}$ and $\{{y}_{n}\}$ are sequences in *X* such that ${lim}_{n\to \mathrm{\infty}}F({x}_{n},{y}_{n})={lim}_{n\to \mathrm{\infty}}g{x}_{n}=x$ and ${lim}_{n\to \mathrm{\infty}}F({y}_{n},{x}_{n})={lim}_{n\to \mathrm{\infty}}g{y}_{n}=y$ for all $x,y\in X$ are satisfied.

In this paper, we aim to extend the results on coupled fixed points mentioned above. Our results improve, enrich and extend some existing theorems in the literature. We also give examples to illustrate our results. This paper can also be considered as a continuation of the works of Berinde [11, 12].

## 2 Main results

We start with an example which shows the weakness of Theorem 15.

**Example 18**Let $X=\mathbb{R}$. Define $G:X\times X\times X\to [0,\mathrm{\infty})$ by

*G*-metric space. Define a map $F:X\times X\to X$ by $F(x,y)=\frac{1}{8}x+\frac{5}{8}y$ and $g:X\to X$ by $g(x)=\frac{7x}{8}$ for all $x,y\in X$. Let $x=u=z$. Then we have

It is clear that there is no $k\in [0,\frac{1}{2})$ for which the statement (1.4) of Theorem 15 holds. Notice, however, that $(0,0)$ is the unique coupled coincidence point of *F* and *g*. In fact, it is a common fixed point of *F* and *g*, that is, $F(0,0)=g0=0$.

We now state our first result which successively guarantees the existence of a coupled coincidence point.

**Theorem 19**

*Let*$(X,\u2aaf)$

*be a partially ordered set and*$(X,G)$

*be a*

*G*-

*complete*

*G*-

*metric space*.

*Let*$F:X\times X\to X$

*and*$g:X\to X$

*be two mappings such that*

*F*

*has the mixed*

*g*-

*monotone property on*

*X*

*and*

*for all*$x,y,u,v,z,w\in X$

*with*$gx\u2ab0gu\u2ab0gw$, $gy\u2aafgv\u2aafgz$.

*Assume that*$F(X\times X)\subset g(X)$,

*g*

*is*

*G*-

*continuous and that*

*F*

*and*

*g*

*are*

*G*-

*compatible mappings*.

*Suppose further that either*

- (a)
*F**is continuous or* - (b)
$(X,G,\u2aaf)$

*is**g*-*ordered complete*.

*Suppose also that there exist* ${x}_{0},{y}_{0}\in X$ *such that* $g{x}_{0}\u2aafF({x}_{0},{y}_{0})$ *and* $F({y}_{0},{x}_{0})\u2aafg{y}_{0}$. *If* $k\in [0,1)$, *then* *F* *and* *g* *have a coupled coincidence point*, *that is*, *there exists* $(x,y)\in (X\times X)$ *such that* $g(x)=F(x,y)$ *and* $g(y)=F(y,x)$.

*Proof*Let ${x}_{0},{y}_{0}\in X$ be such that $g{x}_{0}\u2aafF({x}_{0},{y}_{0})$ and $F({y}_{0},{x}_{0})\u2aafg{y}_{0}$. Using the fact that $F(X\times X)\subset g(X)$, we can construct two sequences $\{{x}_{n}\}$ and $\{{y}_{n}\}$ in

*X*in the following way:

*F*has the mixed

*g*-monotone property, from (2.4), we have

*F*and

*g*have a coupled coincidence point. Indeed, in that case we would have

We suppose that $(g{x}_{n+1},g{y}_{n+1})\ne (g{x}_{n},g{y}_{n})$ for all $n\in \mathbb{N}$. More precisely, we assume that either $g{x}_{n+1}=F({x}_{n},{y}_{n})\ne g{x}_{n}$ or $g{y}_{n+1}=F({y}_{n},{x}_{n})\ne g{y}_{n}$.

*G*-metric and (2.10), we get

Then by Proposition 6, we conclude that the sequences $\{g{x}_{n}\}$ and $\{g{y}_{n}\}$ are *G*-Cauchy.

*G*-complete, there exist $x,y\in g(X)$ such that $\{g{x}_{n}\}$ and $\{g{y}_{n}\}$ are

*G*-convergent to

*x*and

*y*respectively,

*i.e.*,

*F*and

*g*are

*G*-compatible mappings, by (2.11), we have

*F*and

*g*, we have

*F*and

*g*. Suppose that the condition (b) holds. By (2.8), (2.9) and (2.11), we have

*F*and

*g*are

*G*-compatible mappings and

*g*is continuous, by (2.11) and (2.12), we have

*g*, we conclude that

By (G1), we have $gx=F(x,y)$ and $gy=F(y,x)$. Consequently, the element $(x,y)\in X\times X$ is a coupled coincidence point of the mappings *F* and *g*. □

**Corollary 20**

*Let*$(X,\u2aaf)$

*be a partially ordered set and*$(X,G)$

*be a*

*G*-

*metric space such that*$(X,G)$

*is*

*G*-

*complete*.

*Let*$F:X\times X\to X$

*and*$g:X\to X$

*be two mappings such that*

*F*

*has the mixed*

*g*-

*monotone property on*

*X*

*and*

*for all*$x,y,u,v\in X$

*with*$gx\u2ab0gu$, $gy\u2aafgv$.

*Assume that*$F(X\times X)\subset g(X)$,

*the self*-

*mapping*

*g*

*is*

*G*-

*continuous and*

*F*

*and*

*g*

*are*

*G*-

*compatible mappings*.

*Suppose that either*

- (a)
*F**is continuous or* - (b)
$(X,G,\u2aaf)$

*is**g*-*ordered complete*.

*Suppose also that there exist* ${x}_{0},{y}_{0}\in X$ *such that* $g{x}_{0}\u2aafF({x}_{0},{y}_{0})$ *and* $g{y}_{0}\u2ab0F({y}_{0},{x}_{0})$. *If* $k\in [0,1)$, *then* *F* *and* *g* *have a coupled coincidence point*.

*Proof* It is sufficient to take $z=u$ and $w=v$ in Theorem 19. □

**Corollary 21**

*Let*$(X,\u2aaf)$

*be a partially ordered set and*$(X,G)$

*be a*

*G*-

*metric space such that*$(X,G)$

*is*

*G*-

*complete*.

*Let*$F:X\times X\to X$

*and*$g:X\to X$

*be two mappings such that*

*F*

*has the mixed*

*g*-

*monotone property on*

*X*

*and*

*for all*$x,y,u,v\in X$

*with*$gx\u2ab0gu\u2ab0gw$, $gy\u2aafgv\u2aafgz$.

*Assume that*$F(X\times X)\subset g(X)$

*and that the self*-

*mapping*

*g*

*is*

*G*-

*continuous and commutes with*

*F*.

*Suppose that either*

- (a)
*F**is continuous or* - (b)
$(X,G,\u2aaf)$

*is**g*-*ordered complete*.

*Suppose further that there exist* ${x}_{0},{y}_{0}\in X$ *such that* $g{x}_{0}\u2aafF({x}_{0},{y}_{0})$ *and* $g{y}_{0}\u2ab0F({y}_{0},{x}_{0})$. *If* $k\in [0,1)$, *then* *F* *and* *g* *have a coupled coincidence point*.

*Proof* Since *g* commutes with *F*, then *F* and *g* are *G*-compatible mappings. Thus, the result follows from Theorem 19. □

**Corollary 22**

*Let*$(X,\u2aaf)$

*be a partially ordered set and*$(X,G)$

*be a*

*G*-

*metric space such that*$(X,G)$

*is*

*G*-

*complete*.

*Let*$F:X\times X\to X$

*and*$g:X\to X$

*be two mappings such that*

*F*

*has the mixed*

*g*-

*monotone property on*

*X*

*and*

*for all*$x,y,u,v\in X$

*with*$gx\u2ab0gu$, $gy\u2aafgv$.

*Assume that*$F(X\times X)\subset g(X)$

*and that*

*g*

*is*

*G*-

*continuous and commutes with*

*F*.

*Suppose that either*

- (a)
*F**is continuous or* - (b)
$(X,G,\u2aaf)$

*is**g*-*ordered complete*.

*Assume also that there exist* ${x}_{0},{y}_{0}\in X$ *such that* $g{x}_{0}\u2aafF({x}_{0},{y}_{0})$ *and* $g{y}_{0}\u2ab0F({y}_{0},{x}_{0})$. *If* $k\in [0,1)$, *then* *F* *and* *g* *have a coupled coincidence point*.

*Proof* Since *g* commutes with *F*, then *F* and *g* are *G*-compatible mappings. Thus, the result follows from Corollary 20. □

Letting $g=I$ in Theorem 19 and in Corollary 20, we get the following results.

**Corollary 23**

*Let*$(X,\u2aaf)$

*be a partially ordered set and*$(X,G)$

*be a*

*G*-

*metric space such that*$(X,G)$

*is*

*G*-

*complete*.

*Let*$F:X\times X\to X$

*be a mapping having the mixed monotone property on*

*X*

*and*

*for all*$x,y,u,v,z,w\in X$

*with*$x\u2ab0u\u2ab0w$, $y\u2aafv\u2aafz$.

*Suppose that either*

- (a)
*F**is continuous or* - (b)
$(X,G,\u2aaf)$

*is ordered complete*.

*Suppose also that there exist* ${x}_{0},{y}_{0}\in X$ *such that* ${x}_{0}\u2aafF({x}_{0},{y}_{0})$ *and* ${y}_{0}\u2ab0F({y}_{0},{x}_{0})$. *If* $k\in [0,1)$, *then* *F* *has a coupled fixed point*.

**Corollary 24**

*Let*$(X,\u2aaf)$

*be a partially ordered set and*$(X,G)$

*be a*

*G*-

*metric space such that*$(X,G)$

*is*

*G*-

*complete*.

*Let*$F:X\times X\to X$

*be a mapping having the mixed monotone property on*

*X*

*and*

*for all*$x,y,u,v\in X$

*with*$x\u2ab0u$, $y\u2aafv$.

*Suppose that either*

- (a)
*F**is continuous or* - (b)
$(X,G,\u2aaf)$

*is ordered complete*.

*Suppose further that there exist* ${x}_{0},{y}_{0}\in X$ *such that* ${x}_{0}\u2aafF({x}_{0},{y}_{0})$ *and* ${y}_{0}\u2ab0F({y}_{0},{x}_{0})$. *If* $k\in [0,1)$, *then* *F* *has a coupled fixed point*.

**Example 25**Let us recall Example 18. We have

It is clear that there any $k\in [\frac{6}{7},1)$ provides the statement (2.3) of Theorem 19.

Notice that $(0,0)$ is the unique coupled coincidence point of *F* and *g* which is also common coupled fixed point, that is, $F(0,0)=g0=0$.

**Example 26**Let $X=\mathbb{R}$. Define $G:X\times X\times X\to [0,\mathrm{\infty})$ by

for all $x,y,z\in X$. Let ⪯ be usual order. Then $(X,G)$ is a *G*-metric space.

then the statement (2.3) of Theorem 19 is satisfied for any $k\in (\frac{3}{4},1)$ and $(0,0)$.

where $k\in [0,\frac{1}{2})$, then the coupled coincidence point exists even though the contractive condition is not satisfied.

It is clear that the condition (2.27) holds for $k>\frac{5}{8}$.

## Declarations

### Acknowledgements

The second author gratefully acknowledges the support provided by the Department of Mathematics and Faculty of Science, King Mongkut’s University of Technology Thonburi (KMUTT) during his stay at the Department of Mathematical and Statistical Sciences, University of Alberta as a visitor for the short term research.

## Authors’ Affiliations

## References

- Fréchet F: Sur quelques points du calcul fonctionnel.
*Rend. Circ. Mat. Palermo*1906, 22: 1–74. 10.1007/BF03018603View ArticleGoogle Scholar - Fenwick DH, Batycky RP: Using metric space methods to analyse reservoir uncertainty.
*Proceedings of the 2011 Gussow Conference*2011.Google Scholar - Mustafa Z, Sims B: A new approach to generalized metric spaces.
*J. Nonlinear Convex Anal.*2006, 7(2):289–297.MathSciNetGoogle Scholar - Mustafa Z, Obiedat H, Awawdeh F: Some fixed point theorem for mapping on complete
*G*-metric spaces.*Fixed Point Theory Appl.*2008., 2008: Article ID 189870. doi:10.1155/2008/189870Google Scholar - Mustafa Z, Khandaqji M, Shatanawi W: Fixed point results on complete
*G*-metric spaces.*Studia Sci. Math. Hung.*2011, 48: 304–319.Google Scholar - Mustafa Z, Sims B: Fixed point theorems for contractive mappings in complete
*G*-metric spaces.*Fixed Point Theory Appl.*2009., 2009: Article ID 917175. doi:10.1155/2009/917175Google Scholar - Mustafa Z, Shatanawi W, Bataineh M: Existence of fixed point results in
*G*-metric spaces.*Int. J. Math. Math. Sci.*2009., 2009: Article ID 283028. doi:10.1155/2009/283028Google Scholar - Banach S: Sur les opérations dans les ensembles abstraits et leur application aux équations intégrales.
*Fundam. Math.*1922, 3: 133–181.Google Scholar - Ran ACM, Reurings MCB: A fixed point theorem in partially ordered sets and some application to matrix equations.
*Proc. Am. Math. Soc.*2004, 132: 1435–1443. 10.1090/S0002-9939-03-07220-4MathSciNetView ArticleGoogle Scholar - Aydi H, Damjanović B, Samet B, Shatanawi W: Coupled fixed point theorems for nonlinear contractions in partially ordered
*G*-metric spaces.*Math. Comput. Model.*2011, 54: 2443–2450. 10.1016/j.mcm.2011.05.059View ArticleGoogle Scholar - Berinde V: Generalized coupled fixed point theorems for mixed monotone mappings in partially ordered metric spaces.
*Nonlinear Anal.*2011, 74: 7347–7355. 10.1016/j.na.2011.07.053MathSciNetView ArticleGoogle Scholar - Berinde V: Coupled coincidence point theorems for mixed monotone nonlinear operators.
*Comput. Math. Appl.*2012, 64: 1770–1777. 10.1016/j.camwa.2012.02.012MathSciNetView ArticleGoogle Scholar - Choudhury BS, Kundu A: A coupled coincidence point result in partially ordered metric spaces for compatible mappings.
*Nonlinear Anal.*2010, 73: 2524–2531. 10.1016/j.na.2010.06.025MathSciNetView ArticleGoogle Scholar - Guo D, Lakshmikantham V: Coupled fixed points of nonlinear operators with applications.
*Nonlinear Anal., Theory Methods Appl.*1987, 11: 623–632. 10.1016/0362-546X(87)90077-0MathSciNetView ArticleGoogle Scholar - Gnana-Bhaskar T, Lakshmikantham V: Fixed point theorems in partially ordered metric spaces and applications.
*Nonlinear Anal.*2006, 65: 1379–1393. 10.1016/j.na.2005.10.017MathSciNetView ArticleGoogle Scholar - Ćirić LJ, Lakshmikantham V: Coupled fixed point theorems for nonlinear contractions in partially ordered metric spaces.
*Nonlinear Anal.*2009, 70: 4341–4349. 10.1016/j.na.2008.09.020MathSciNetView ArticleGoogle Scholar - Nieto JJ, Lopez RR: Contractive mapping theorems in partially ordered sets and applications to ordinary differential equations.
*Order*2005, 22: 223–239. 10.1007/s11083-005-9018-5MathSciNetView ArticleGoogle Scholar - Luong NV, Thuan NX: Coupled fixed point theorems in partially ordered
*G*-metric spaces.*Math. Comput. Model.*2012, 55: 1601–1609. 10.1016/j.mcm.2011.10.058MathSciNetView ArticleGoogle Scholar - Shatanawi W: Fixed point theory for contractive mappings satisfying Φ-maps in
*G*-metric spaces.*Fixed Point Theory Appl.*2010., 2010: Article ID 181650. doi:10.1155/2010/181650Google Scholar - Shatanawi W: Some fixed point theorems in ordered
*G*-metric spaces and applications.*Abstr. Appl. Anal.*2011., 2011: Article ID 126205. doi:10.1155/2011/126205Google Scholar - Shatanawi W: Coupled fixed point theorems in generalized metric spaces.
*Hacet. J. Math. Stat.*2011, 40: 441–447.MathSciNetGoogle Scholar - Shatanawi W, Abbas M, Nazir T: Common coupled coincidence and coupled fixed point results in two generalized metric spaces.
*Fixed Point Theory Appl.*2011., 2011: Article ID 80. doi:10.1186/1687–1812–2011–80Google Scholar - Tahat N, Aydi H, Karapınar E, Shatanawi W: Common fixed points for single-valued and multi-valued maps satisfying a generalized contraction in
*G*-metric spaces.*Fixed Point Theory Appl.*2012., 2012: Article ID 48Google Scholar - Abbas M, Cho YJ, Nazir T: Common fixed points of Ciric-type contractive mappings in two ordered generalized metric spaces.
*Fixed Point Theory Appl.*2012., 2012: Article ID 139Google Scholar - Cho YJ, Rhoades BE, Saadati R, Samet B, Shatanawi W: Nonlinear coupled fixed point theorems in ordered generalized metric spaces with integral type.
*Fixed Point Theory Appl.*2012., 2012: Article ID 8Google Scholar - Gordji ME, Akbartabar E, Cho YJ, Ramezani M: Coupled common fixed point theorems for mixed weakly monotone mappings in partially ordered metric spaces.
*Fixed Point Theory Appl.*2012., 2012: Article ID 95Google Scholar - Huang NJ, Fang YP, Cho YJ: Fixed point and coupled fixed point theorems for multi-valued increasing operators in ordered metric spaces. 3. In
*Fixed Point Theory and Applications*. Edited by: Cho YJ, Kim JK, Kang SM. Nova Science Publishers, New York; 2002:91–98.Google Scholar - Karapınar E, Luong NV, Thuan NX, Hai TT: Coupled coincidence points for mixed monotone operators in partially ordered metric spaces.
*Arab. J. Math.*2012, 1(3):329–339. 10.1007/s40065-012-0027-0MathSciNetView ArticleGoogle Scholar - Karapınar E: Coupled fixed point theorems for nonlinear contractions in cone metric spaces.
*Comput. Math. Appl.*2010, 59: 3656–3668. 10.1016/j.camwa.2010.03.062MathSciNetView ArticleGoogle Scholar - Karapınar E: Coupled fixed points on cone metric spaces.
*Gazi Univ. J. Sci.*2011, 24: 51–58.Google Scholar - Ding HS, Li L: Coupled fixed point theorems in partially ordered cone metric spaces.
*Filomat*2011, 25: 137–149.MathSciNetView ArticleGoogle Scholar - Abbas M, Sintunavarat W, Kumam P: Coupled fixed point of generalized contractive mappings on partially ordered
*G*-metric spaces.*Fixed Point Theory Appl.*2012., 2012: Article ID 31Google Scholar - Aydi H, Postolache M, Shatanawi W: Coupled fixed point results for $(\psi ,\varphi )$-weakly contractive mappings in ordered
*G*-metric spaces.*Comput. Math. Appl.*2012, 63: 298–309. 10.1016/j.camwa.2011.11.022MathSciNetView ArticleGoogle Scholar - Aydi H, Karapınar E, Shatnawi W: Tripled fixed point results in generalized metric spaces.
*J. Appl. Math.*2012., 2012: Article ID 314279Google Scholar - Nashine HK: Coupled common fixed point results in ordered
*G*-metric spaces.*J. Nonlinear Sci. Appl.*2012, 1: 1–13.MathSciNetView ArticleGoogle Scholar

## Copyright

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.