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Coupled common fixed point results involving a -contractive condition for mixed g-monotone operators in partially ordered metric spaces
Journal of Inequalities and Applications volume 2012, Article number: 285 (2012)
Abstract
In the setting of partially ordered metric spaces, using the notion of compatible mappings, we establish the existence and uniqueness of coupled common fixed points involving a -contractive condition for mixed g-monotone operators. Our results extend and generalize the well-known results of Berinde (Nonlinear Anal. TMA 74:7347-7355, 2011; Nonlinear Anal. TMA 75:3218-3228, 2012) and weaken the contractive conditions involved in the results of Alotaibi et al. (Fixed Point Theory Appl. 2011:44, 2011), Bhaskar et al. (Nonlinear Anal. TMA 65:1379-1393, 2006), and Luong et al. (Nonlinear Anal. TMA 74:983-992, 2011). The effectiveness of the presented work is validated with the help of suitable examples.
MSC:54H10, 54H25.
1 Introduction and preliminaries
Bhaskar and Lakshmikantham [1] introduced the notion of coupled fixed points and proved some coupled fixed point theorems for a mapping with the mixed monotone property in the setting of partially ordered metric spaces. These concepts are defined as follows.
Definition 1.1 [1]
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 monotone non-increasing in y; that is, for any ,
and
Definition 1.2 [1]
An element is called a coupled fixed point of the mapping if and .
Bhaskar and Lakshmikantham [1] proved the following results.
Theorem 1.3 [1]
Let be a partially ordered set and suppose 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 a with
for all and .
If there exist two elements with and , then there exist such that and .
Theorem 1.4 [1]
Let be a partially ordered set and suppose there exists a metric d on X such that is a complete metric space. Assume that X has the following property:
-
(i)
if a non-decreasing sequence , then for all n,
-
(ii)
if a non-increasing sequence , then for all n.
Let be a mapping having the mixed monotone property on X. Assume that there exists a with the condition (1.1). If there exist two elements with and , then there exist such that and .
Lakshmikantham and Ćirić [2] extended the notion of mixed monotone property to mixed g-monotone property and generalized the results of Bhaskar and Lakshmikantham [1] by establishing the existence of coupled coincidence point results using a pair of commutative maps.
Definition 1.5 [2]
Let be a partially ordered set and and . We say F has the mixed g-monotone property if is monotone g-nondecreasing in its first argument and is monotone g-nonincreasing in its second argument; that is, for any ,
and
Definition 1.6 [2]
An element is called a coupled coincidence point of the mappings and if and .
Definition 1.7 [2]
An element is called a coupled common fixed point of the mappings and if and .
Definition 1.8 [2]
Let X be a non-empty set and and . We say F and g are commutative if for all .
Later, Choudhury and Kundu [3] introduced the notion of compatibility in the context of coupled coincidence point problems and used the notion to improve the results of Lakshmikantham and Ćirić [2].
Definition 1.9 [3]
The mappings and are said to be compatible if
whenever and are sequences in X such that and for some .
In recent years, following Bhaskar and Lakhsmikantham [1], the existence and uniqueness of coupled fixed points under more general contractive conditions were established by various authors. One can refer to [2, 4–15].
In order to generalize the results of Bhaskar and Lakshmikantham [1], Luong and Thuan [7] considered the following class of control functions.
Definition 1.10 [7]
Let Φ denote the class of functions which satisfy
() φ is continuous and non-decreasing;
() if and only if ;
() , for all .
Definition 1.11 [7]
Let Ψ denote the class of functions which satisfy
(i ψ ) for all and .
The contractive condition considered by Luong and Thuan [7] is given below:
where , and , .
On the other hand, Alotaibi and Alsulami [16] extended the results of Luong and Thuan [7] for a compatible pair , where and are the maps satisfying the following contractive condition:
with , and , .
We consider the class Φ redefined by Berinde [5] as follows.
Definition 1.12 [5]
Let Φ denote the class of functions which satisfy
(i φ ) φ is continuous and (strictly) increasing;
(ii φ ) for all ;
(iii φ ) for all .
Note that by (i φ ) and (ii φ ), we have if and only if .
Berinde [5] weakened the contractive conditions (1.1) and (1.2) by considering the more general one
for a mixed monotone mapping , , , where and .
The present work extends and generalizes several results presented in the literature of fixed point theory. Our theorems directly derive the main results of Berinde [4, 5]. We give suitable examples to show how our results extend the well-known results of Alotaibi et al. [16], Bhaskar et al. [1] and Luong et al. [7] by significantly weakening the involved contractive condition.
2 Main results
Theorem 2.1 Let be a partially ordered set and suppose there exists a metric d on X such that is a complete metric space. Let , be two maps with F having the mixed g-monotone property on X such that there exist two elements with and . Suppose there exist and such that
for all with and .
Suppose that , g is continuous and the pair is compatible.
Also suppose either
-
(a)
F is continuous, or
-
(b)
X has the following property:
-
(i)
if a non-decreasing sequence , then for all n;
-
(ii)
if a non-increasing sequence , then for all n.
Then there exist such that and ; that is, F and g have a coupled coincidence point in X.
Proof Let such that and . Since , we can choose such that , . Again, we can choose such that , .
Continuing this process, we can construct sequences and in X such that
We shall prove, for all , that
Since and , , , we have , ; that is, (2.3) and (2.4) hold for .
Suppose that (2.3) and (2.4) hold for some , i.e., , . As F has the mixed g-monotone property, by (2.2), we have
and
that is,
Then, by mathematical induction, it follows that (2.3) and (2.4) hold for all .
If, for some , we have , then and ; that is, F and g have a coincidence point. So, now onwards, we suppose for all ; that is, we suppose that either or .
Since and , by (2.1) and (2.2), we have, for all , that
Since ψ is non-negative, we have
By the monotonicity of φ, we have
Let , then is a monotone decreasing sequence of non-negative real numbers. Therefore, there exists some such that
We claim that .
On the contrary, suppose that .
Taking limit as on both sides of (2.5) and using the properties of φ and ψ, we have
a contradiction.
Thus, ; that is,
Next, we shall show that and are Cauchy sequences.
If possible, suppose that at least one of and is not a Cauchy sequence. Then there exists an for which we can find subsequences , of and , of with such that
Further, corresponding to , we can choose in such a way that it is the smallest integer with and satisfies (2.8). Then
By (2.8), (2.9) and the triangle inequality, we have
Letting and using (2.7) in the last inequality, we have
Again, by the triangle inequality
By the monotonicity of φ and the property (iii φ ), we have
Since , and .
Then by (2.1) and (2.2), we have
By (2.11) and (2.12), we have
Letting , using (2.7), (2.10) and the properties of φ and ψ in the last inequality, we have
a contradiction.
Therefore, both and are Cauchy sequences in X. By the completeness of X, there exist such that
Since F and g are compatible mappings, we have from (2.13)
Let the condition (a) hold.
For all , we have
Taking in the last inequality, using the inequalities (2.13), (2.14) and the continuities of F and g, we have ; that is, . Again, for all ,
Taking in the last inequality, using the inequalities (2.13), (2.15) and the continuities of F and g, we have ; that is, . Hence, the element is a coupled coincidence point of the mappings and .
Next, we suppose that the condition (b) holds.
By (2.3), (2.4) and (2.13), we have is a non-decreasing sequence, and is a non-increasing sequence, as . Hence, by the assumption (b), we have for all ,
Since F and g are compatible mappings and g is continuous, by inequalities (2.13)-(2.15), we have
and
Now,
that is,
Taking in the last inequality and using (2.17), we have
Similarly,
By (2.19), (2.20) and the property (i φ ), we have
By (2.1) and (2.16), we have
Inserting (2.22) in (2.21), we have
By (2.17), (2.18), the continuity of φ and , we get
Since φ is non-negative and , we have
that is,
Hence, the element is a coupled coincidence point of the mappings and . □
Now, we give an example in support of Theorem 2.1.
Example 2.1 Let . Then is a partially ordered set with the natural ordering of real numbers.
Let for .
Then is a complete metric space.
Let : be defined as
Let be defined as
Let and be two sequences in X such that
Now, for all ,
and
Obviously, and .
Then it follows that
and
Hence, the mappings F and g are compatible in X. Clearly, F obeys the mixed g-monotone property. Also, .
Let φ, be defined as , , for .
Also, and (>0) are two points in X such that and .
Next, we verify inequality (2.1) of Theorem 2.1. We take such that and ; that is, and . We discuss the following cases.
Case 1: , .
Then
Case 2: , .
Then
Case 3: , .
Then
Case 4: , .
Then
Hence, the inequality (2.1) of Theorem 2.1 is satisfied.
Thus, all the conditions of Theorem 2.1 are satisfied, and it can be easily seen that is the required coupled coincidence point of F and g in X.
Remark 2.1 If we choose the functions and , for , then with this choice of functions, we can obtain the already existing contractive condition. Since φ and ψ are actually contractions, this will be cleared in Corollary 2.3. But if we choose and , for , then with this choice of φ and ψ, the contractive condition (2.1) does not turn to the existing contractive condition.
The next example shows that Theorem 2.1 is more general than Theorem 3.1 in [16] since the contractive condition (2.1) is more general than (1.3).
Example 2.2 Let . Then is a partially ordered set with the natural ordering of real numbers. Let be defined by
Then is a complete metric space.
Define by , and by , .
Clearly, , F is continuous and has the mixed g-monotone property, the pair is compatible and satisfies the condition (2.1) but does not satisfy the condition (1.3). Assume, to the contrary, that there exist (in accordance with Definition 1.10) and such that (1.3) holds. Then we must have
for all and . Take , in the last inequality and let , we obtain
But by () we have and hence we deduce that, for all , , that is, , which contradicts (i ψ ). This shows that F does not satisfy (1.3).
Now, we prove that (2.1) holds. Indeed, for and , we have
and
By summing up the last two inequalities, we get exactly (2.1) with , . Also, , are the two points in X such that and . F, g, φ, ψ satisfy all the conditions of Theorem 2.1. So, by Theorem 2.1, we obtain that F and g have a coupled coincidence point , but Theorem 3.1 in [16] cannot be applied to F and g in this example.
The following Corollary 2.1 is Theorem 2 in [5].
Corollary 2.1 [5]
Let be a partially ordered set and suppose 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 such that there exist two elements with and . Suppose there exist and such that
for all with and . Suppose either
-
(a)
F is continuous, or
-
(b)
X has the following property:
-
(i)
if a non-decreasing sequence , then for all n;
-
(ii)
if a non-increasing sequence , then for all n.
Then there exist such that and .
Proof Taking g to be an identity mapping in Theorem 2.1, we obtain Corollary 2.1. □
The following example shows that Corollary 2.1 is more general than Theorem 1.3 (i.e., Theorem 2.1 in [1]) and Theorem 2.1 in [7], since the contractive condition (2.23) is more general than (1.1) and (1.2).
Example 2.3 Let . Then is a partially ordered set with the natural ordering of real numbers. Let be defined by
Then is a complete metric space.
Define by , .
Then F is continuous, has the mixed monotone property and satisfies the condition (2.23) but does not satisfy either the condition (1.1) or the condition (1.2). Indeed, assume there exists such that (1.1) holds. Then we must have
by which, for , we get
which for implies , a contradiction, since . Hence, F does not satisfy (1.1).
Further, (1.2) is also not satisfied. Assume, to the contrary, that there exist (in accordance with Definition 1.10) and such that (1.2) holds. Then we must have
for all and . Take , in the last inequality and let , we obtain
But by (), we have and hence we deduce that, for all , , that is, , which contradicts (i ψ ). This shows that F does not satisfy (1.2).
Now, we prove that (2.23) holds. Indeed, for and , we have
and
By summing up the last two inequalities, we get exactly (2.23) with , . Also, , are the two points in X such that and .
So, by Corollary 2.1, we obtain that F has a coupled fixed point but neither Theorem 2.1 in [1] nor Theorem 2.1 in [7] can be applied to F in this example.
The following Corollary 2.2 is Corollary 1 in [5].
Corollary 2.2 [5]
Let be a partially ordered set and suppose 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 such that there exist two elements with and . Suppose there exists such that
for all with and . Suppose either
-
(a)
F is continuous, or
-
(b)
X has the following property:
-
(i)
if a non-decreasing sequence , then for all n;
-
(ii)
if a non-increasing sequence , then for all n.
Then F has a coupled fixed point in X.
Proof Note that if , then for all , . Now divide (2.24) by 4 and take , , then the condition (2.24) reduces to (2.1) with and ; and hence by Theorem 2.1, we obtain Corollary 2.2. □
Corollary 2.3 Let be a partially ordered set and suppose there exists a metric d on X such that is a complete metric space. Let , be two maps with F having the mixed g-monotone property on X such that there exist two elements with and . Suppose there exists a real number such that
for all with , . Suppose either
-
(a)
F is continuous, or
-
(b)
X has the following property:
-
(i)
if a non-decreasing sequence , then for all n;
-
(ii)
if a non-increasing sequence , then for all n.
Suppose that , g is continuous and the pair is compatible, then there exist X such that and .
Proof Taking and , , in Theorem 2.1, we obtain Corollary 2.3. □
Remark 2.2 (i) Corollary 2.3 is an extension of the recent coupled fixed point result of Berinde (Theorem 3 in [4]) to a coupled coincidence point theorem for a pair of compatible mappings having the mixed g-monotone property.
-
(ii)
Again, the choice of functions F and g in Example 2.2 shows that Corollary 2.3 is more general than Theorem 3.1 in [16], since the contractive condition (2.23) is more general than (1.3). Indeed, the contractive condition (1.3) does not hold for the choice of functions F and g, but (2.25) holds exactly for with and and yields as the coupled coincidence point of F and g.
Corollary 2.4 Let be a partially ordered set and suppose 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 such that there exist two elements with and . Suppose there exists a real number such that
for all with , . Suppose either
-
(a)
F is continuous, or
-
(b)
X has the following property:
-
(i)
if a non-decreasing sequence , then for all n;
-
(ii)
If a non-increasing sequence , then for all n.
Then F has a coupled fixed point in X.
Proof Taking g to be the identity mapping in Corollary 2.3, we obtain Corollary 2.4. □
Remark 2.3 (i) By considering the condition of continuity of F in Corollary 2.4, we obtain Theorem 3 in [4].
-
(ii)
Again, the choice of the function F in Example 2.3 shows that Corollary 2.4 is more general than Theorem 1.3 (i.e., Theorem 2.1 in [1]) and Theorem 2.1 in [7], since the contractive condition (2.26) is more general than (1.1) and (1.2). Indeed, the contractive conditions (1.1) and (1.2) do not hold for the choice of the function F, but (2.26) holds exactly for with and and yields as the coupled fixed point of F.
Now, in order to prove the existence and uniqueness of the coupled common fixed point for our main results, we need the following lemma.
Lemma 2.1 Let and be compatible maps and let an element such that and exist, then and .
Proof Since the pair is compatible, it follows that
whenever and are sequences in X such that , for some . Taking , and using , , it follows that
Hence, and . □
Theorem 2.2 In addition to the hypothesis of Theorem 2.1, suppose that for every , there exists a such that is comparable to and . Then F and g have a unique coupled common fixed point; that is, there exists a unique such that and .
Proof By Theorem 2.1, the set of coupled coincidences is non-empty. In order to prove the theorem, we shall first show that if and are coupled coincidence points, that is, if , and , , then
By assumption, there is such that is comparable with and . Put , and choose so that , .
Then, similarly as in the proof of Theorem 2.1, we can inductively define sequences and such that and .
Further, set , , , and, in the same way, define the sequences , and , . Then it is easy to show that
and
Since and are comparable, then and . It is easy to show that and are comparable, that is, and for all . Thus by (2.1),
Since ψ is non-negative, we have
By the monotonicity of φ, we have
Thus, the sequence defined by , is a monotonically decreasing sequence of non-negative real numbers, so there exists some such that .
We shall show that . Suppose, to the contrary, that . Then taking limit as , in (2.28) and using the continuity of φ, we have
a contradiction. Thus, ; that is, .
Hence, it follows that , .
Similarly, one can show that , .
By the uniqueness of the limit, it follows that and . Thus, we proved (2.27).
Since , and the pair is compatible, then by Lemma 2.1, it follows that
Denote , . Then by (2.30),
Thus, is a coupled coincidence point.
Then by (2.27) with and , it follows that and ; that is,
By (2.31) and (2.32),
Therefore, is the coupled common fixed point of F and g.
To prove the uniqueness, assume that is another coupled common fixed point of F and g. Then by (2.27), we have and . □
Corollary 2.5 In addition to the hypothesis of Corollary 2.3, suppose that for every , there exists a such that is comparable to and . Then F and g have a unique coupled common fixed point; that is, there exists a unique such that and .
Proof Taking and , in Theorem 2.2, we obtain Corollary 2.5. □
Remark 2.4 Indeed, is the unique coupled common fixed point of the maps F and g in Example 2.1 in view of Theorem 2.2 and Corollary 2.5.
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Jain, M., Tas, K., Kumar, S. et al. Coupled common fixed point results involving a -contractive condition for mixed g-monotone operators in partially ordered metric spaces. J Inequal Appl 2012, 285 (2012). https://doi.org/10.1186/1029-242X-2012-285
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DOI: https://doi.org/10.1186/1029-242X-2012-285