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Coupled fixed point theorems for mixed g-monotone mappings in partially ordered metric spaces
Fixed Point Theory and Applications volume 2013, Article number: 348 (2013)
Abstract
In this paper, we prove some coupled coincidence point results for mixed g-monotone mappings in partially ordered metric spaces. The main results of this paper are generalizations of the main results of Luong and Thuan (Nonlinear Anal. 74:983-992, 2011).
MSC:54H25, 47H10.
1 Introduction and preliminaries
Fixed point theory plays a major role in mathematics. The Banach contraction principle [1] is the simplest one corresponding to fixed point theory. So a large number of mathematicians have extended it and have been interested in fixed point theory in some metric spaces. One of these spaces is a partially ordered metric space, that is, metric spaces endowed with a partial ordering. The first result in this direction was given by Ran and Reurings [2] who presented their applications to a matrix equation. Subsequently, the existence of solutions for matrix equations or ordinary differential equations by applying fixed point theorems were presented in [2–7].
The existence of a fixed point for contraction type mappings in partially ordered metric spaces has been considered by Ran and Reurings [2], Bhaskar and Lakshmikantham [4], Nieto and Rodriquez-Lopez [6, 7], Lakshmikantham and Ćirić [8], Agarwal et al. [9] and Samet [10]. Bhaskar and Lakshmikantham [4] introduced the notion of coupled fixed point and proved some coupled fixed point theorems for mappings satisfying the mixed monotone property and discussed the existence and uniqueness of a solution for a periodic boundary value problem. Lakshmikantham and Ćirić [8] introduced the concept of a mixed g-monotone mapping and proved coupled coincidence and common fixed point theorems that extend theorems from [4]. Subsequently, many authors obtained several coupled coincidence and coupled fixed point theorems in some ordered metric spaces [11–27].
Definition 1 ([4])
Let be a partially ordered set and . The mapping F is said to have the mixed monotone property if is monotone non-decreasing in x and is monotone non-decreasing in y, that is, for any ,
and
Definition 2 ([4])
An element is called a coupled fixed point of the mapping if , .
Definition 3 ([8])
An element is called a coupled coincidence point of mappings and if , .
Definition 4 ([8])
Let X be non-empty set and and . We say F and g are commutative if for all .
Definition 5 ([8])
Let be a partially ordered set and , be mappings. The mapping F is said to have the mixed g-monotone property if F is monotone g-non-decreasing in its first argument and is monotone g-non-increasing in the second argument, that is, for any ,
and
Lemma 1 ([28])
Let X be a non-empty set and and be mappings. Then there exists a subset such that and is one-to-one.
Theorem 1 ([4])
Let be a partially ordered set and suppose that there exists a metric d on X such that is a complete metric space. Let be a continuous mapping having the mixed monotone property on X. Assume that there exists with
If there exist two elements with
then there exist such that
Theorem 2 ([4])
Let be a partially ordered set and suppose that there exists a metric d on X such that is a complete metric space. Assume that X has the following property:
-
(1)
if a non-decreasing sequence , then for all ,
-
(2)
if a non-increasing sequence , then for all .
Let be a mapping having the mixed monotone property on X. Assume that there exists with
If there exist two elements with
then there exist such that
Theorem 3 ([29])
Let be a partially ordered set and suppose that there exists a metric d on X such that is a complete metric space. Let be a partially ordered set and suppose that there exists a metric d on X such that is a complete metric space. Let be a mapping having the mixed monotone property on X and there exist two elements with and . Suppose that F, g satisfy
for all with and . Suppose that either
-
(1)
F is continuous or
-
(2)
X has the following property:
-
(a)
if a non-decreasing sequence , then for all ,
-
(b)
if a non-increasing sequence , then for all .
Then there exist such that
that is, F has a coupled fixed point in X.
2 The main results
In this paper, we prove coupled coincidence and common fixed point theorems for mixed g-monotone mappings satisfying more general contractive conditions in partially ordered metric spaces. We also present results on existence and uniqueness of coupled common fixed points. Our results improve those of Luong and Thuan [29]. Our work generalizes, extends and unifies several well known comparable results in the literature.
Let Φ denote all functions which satisfy
-
(1)
φ is continuous and non-decreasing,
-
(2)
and only if ,
-
(3)
,
and Ψ denote all functions which satisfy for all and .
Theorem 4 Let be a partially ordered set and suppose that there exists a metric d on X such that is a complete metric space. Let be a partially ordered set and suppose that there exists a metric d on X such that is a complete metric space. Let be a mapping having the mixed monotone property on X and there exist two elements with and . Suppose that F, g satisfy
for all with and , , is complete and g is continuous.
Suppose that either
-
(1)
F is continuous or
-
(2)
X has the following property:
-
(a)
if a non-decreasing sequence , then for all ,
-
(b)
if a non-increasing sequence , then for all .
Then there exist such that
that is, F and g have a coupled coincidence point in .
Proof Using Lemma 1, there exists such that and is one-to-one. We define a mapping by
As g is one-to-one on , so A is well defined. Thus, it follows from (2.1) and (2.2) that
for all with and . Since F has the mixed g-monotone property, for all , we have
and
Thus, it follows from (2.2), (2.4) and (2.5) that, for all ,
and
which implies that A has the mixed monotone property.
Suppose that assumption (1) holds. Since F is continuous, A is also continuous. Using Theorem 3 with the mapping A, it follows that A has a coupled fixed point .
Suppose that assumption (2) holds. We can conclude similarly in the proof of Theorem 3 that the mapping A has a coupled fixed point .
Finally, we prove that F and g have a coupled fixed point in X. Since is a coupled fixed point of A, we get
Since , there exists a point such that
Thus, it follows from (2.6) and (2.7) that
Also, from (2.2) and (2.8), we get
Therefore, is a coupled coincidence point of F and g. This completes the proof. □
Corollary 1 Let be a partially ordered set and suppose that there exists a metric d on X such that is a complete metric space. Let be a partially ordered set and suppose that there exists a metric d on X such that is a complete metric space. Let be a mapping having the mixed monotone property on X and there exist two elements with and . Suppose that F, g satisfy
for all with and , , is complete and g is continuous.
Suppose that either
-
(1)
F is continuous or
-
(2)
X has the following property:
-
(a)
if a non-decreasing sequence , then for all ,
-
(b)
if a non-increasing sequence , then for all .
Then there exist such that
that is, F and g have a coupled coincidence point in .
Proof In Theorem 4, taking , we get Corollary 1. □
Corollary 2 Let be a partially ordered set and suppose that there exists a metric d on X such that is a complete metric space. Let be a partially ordered set and suppose that there exists a metric d on X such that is a complete metric space. Let be a mapping having the mixed monotone property on X, and there exist two elements with and . Suppose that F, g satisfy
for all with and , , is complete and g is continuous.
Suppose that either
-
(1)
F is continuous or
-
(2)
X has the following property:
-
(a)
if a non-decreasing sequence , then for all ,
-
(b)
if a non-increasing sequence , then for all .
Then there exist such that
that is, F and g have a coupled coincidence point in .
Proof In Corollary 1, taking , we get Corollary 2. □
Theorem 5 Let be a partially ordered set and suppose that there exists a metric d on X such that is a complete metric space. Let be a partially ordered set and suppose that there exists a metric d on X such that is a complete metric space. Let be a mapping having the mixed monotone property on X and there exist two elements with and . Suppose that F, g satisfy
for all with and , , is complete and g is continuous.
Suppose that either
-
(1)
F is continuous or
-
(2)
X has the following property:
-
(a)
if a non-decreasing sequence , then for all ,
-
(b)
if a non-increasing sequence , then for all .
Then there exist such that
and
that is, F and g have a coupled common fixed point .
Proof Following the proof of Theorem 4, F and g have a coupled coincidence point. We only have to show that and .
Now, and are two points in the statement of Theorem 4. Since , we can choose such that and . In the same way, we construct and . Continuing in this way, we can construct two sequences and in X such that
Since and , from (2.1) and (2.9), we have
Similarly, since and , from (2.1) and (2.9), we have
From (2.10) and (2.11), we have
By property (3) of φ, we have
From (2.12) and (2.13), we have
which implies
Using the fact that φ is non-decreasing, we get
Set , then sequence is decreasing. Therefore, there is some such that
We shall show that . Suppose, to the contrary, that . Then taking the limit as (equivalently, ) of both sides of (2.13) and having in mind that we suppose that for all and φ is continuous, we have
a contradiction. Thus , that is,
Hence and , that is, and . □
Theorem 6 In addition to the hypotheses of Theorem 4, suppose that for every , in , there exists in that is comparable to and , then F and g have a unique coupled fixed point.
Proof From Theorem 4, the set of coupled fixed points of F is non-empty. Suppose that and are coupled coincidence points of F, that is, , , and . We will prove that
By assumption, there exists in such that is comparable with and . Put and and choose so that and . Then, similarly as in the proof of Theorem 3, we can inductively define sequences , with
Further set , , and , in a similar way, define the sequences , and , . Then it is easy to show that
as . Since
are comparable, then and , or vice versa. It is easy to show that, similarly, and are comparable for all , that is, and , or vice versa. Thus from (2.1), we have
Similarly,
From (2.16), (2.17) and the property of φ, we have
which implies
Thus,
That is, the sequence is decreasing. Therefore, there exists such that
We shall show that . Suppose, to the contrary, that . Taking the limit as in (2.18), we have
a contradiction. Thus, , that is,
It implies
Similarly, we show that
From (2.19), (2.20) and by the uniqueness of the limit, it follows that we have and . Hence is the unique coupled point of coincidence of F and g. □
Example 1 Let endowed with the standard metric for all . Then is a complete metric space. Define the mapping by
Suppose that is such that for all and is such that . Assume that .
It is easy to show that for all with and , we have
Thus, it satisfies all the conditions of Theorem 4. So we deduce that F and g have a coupled coincidence point . Here, is a coupled coincidence point of F and g.
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Turkoglu, D., Sangurlu, M. Coupled fixed point theorems for mixed g-monotone mappings in partially ordered metric spaces. Fixed Point Theory Appl 2013, 348 (2013). https://doi.org/10.1186/1687-1812-2013-348
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DOI: https://doi.org/10.1186/1687-1812-2013-348