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New conditions on fuzzy coupled coincidence fixed point theorem
Fixed Point Theory and Applications volume 2014, Article number: 153 (2014)
Abstract
Recently, Choudhury et al. proved a coupled coincidence point theorem in a partial order fuzzy metric space. In this paper, we give a new version of the result of Choudhury et al. by removing some restrictions. In our result, the mappings are not required to be compatible, continuous or commutable, and the t-norm is not required to be of Hadžić-type. Finally, two examples are presented to illustrate the main result of this paper.
MSC:54E70, 47H25.
1 Introduction
The concept of fuzzy metric spaces was defined in different ways [1–3]. Grabiec [4] presented a fuzzy version of Banach contraction principle in a fuzzy metric space of Kramosi and Michalek’s sense. Fang [5] proved some fixed point theorems in fuzzy metric spaces, which improve, generalize, unify, and extend some main results of Edelstein [6], Istratescu [7], Sehgal and Bharucha-Reid [8].
In order to obtain a Hausdorff topology, George and Veeramani [9, 10] modified the concept of fuzzy metric space due to Kramosil and Michalek [11]. Many fixed point theorems in complete fuzzy metric spaces in the sense of George and Veeramani [9, 10] have been obtained. For example, Singh and Chauhan [12] proved some common fixed point theorems for four mappings in GV fuzzy metric spaces. Gregori and Sapena [13] proved that each fuzzy contractive mapping has a unique fixed point in a complete GV fuzzy metric space in which fuzzy contractive sequences are Cauchy.
The coupled fixed point theorem and its applications in metric spaces are firstly obtained by Bhaskar and Lakshmikantham [14]. Recently, some authors considered coupled fixed point theorems in fuzzy metric spaces; see [15–18].
In [15], the authors gave the following results.
Theorem 1.1 [[15], Theorem 2.5]
Let for all and be a complete fuzzy metric space such that M has n-property. Let and be two functions such that
for all , where , and g is continuous and commutes with F. Then there exists a unique such that .
Let , where and each satisfies the following conditions:
(ϕ-1) ϕ is non-decreasing;
(ϕ-2) ϕ is upper semicontinuous from the right;
(ϕ-3) for all where , .
In [16], Hu proved the following result.
Theorem 1.2 [[16], Theorem 1]
Let be a complete fuzzy metric space, where ∗ is a continuous t-norm of H-type. Let and be two mappings and let there exist such that
for all , . Suppose that , and g is continuous; F and g are compatible. Then there exists such that , that is, F and g have a unique common fixed point in X.
Choudhury et al. [17] gave the following coupled coincidence fixed point result in a partial order fuzzy metric space.
Theorem 1.3 [[17], Theorem 3.1]
Let be a complete fuzzy metric space with a Hadžić type t-norm as for all . Let ⪯ be a partial order defined on X. Let and be two mappings such that F has mixed g-monotone property and satisfies the following conditions:
-
(i)
,
-
(ii)
g is continuous and monotonic increasing,
-
(iii)
is a compatible pair,
-
(iv)
for all , with and , where , is a continuous function such that for each .
Also suppose that X has the following properties:
-
(a)
if we have a non-decreasing sequence , then for all ,
-
(b)
if we have a non-increasing sequence , then for all .
If there exist such that , , and for all , then there exist such that and , that is, g and F have a coupled coincidence point in X.
Wang et al. [18] proved the following coupled fixed point result in a fuzzy metric space.
Theorem 1.4 [[18], Theorem 3.1]
Let be a fuzzy metric space under a continuous t-norm ∗ of H-type. Let be a function satisfying for any . Let and be two mappings with and assume that for any ,
for all . Suppose that is complete and g and F are w-compatible, then g and F have a unique common fixed point , that is, .
In this paper, by modifying the conditions on the result of Choudhury et al. [17], we give a new coupled coincidence fixed point theorem in partial order fuzzy metric spaces. In our result, we do not require that the t-norm is of Hadžić-type [19], the mappings are compatible [16], commutable, continuous or monotonic increasing. Our proof method is different from the one of Choudhury et al. Finally, some examples are presented to illustrate our result.
2 Preliminaries
Definition 2.1 [9]
A binary operation is continuous t-norm if ∗ satisfies the following conditions:
-
(1)
∗ is associative and commutative,
-
(2)
∗ is continuous,
-
(3)
for all ,
-
(4)
whenever and for all .
Typical examples of the continuous t-norm are and for all .
A t-norm ∗ is said to be positive if for all . Obviously, ∗1 and ∗2 are positive t-norms.
Definition 2.2 [9]
The 3-tuple is called a fuzzy metric space if X is an arbitrary non-empty set, ∗ is a continuous t-norm and M is a fuzzy set on satisfying the following conditions for each and :
(GV-1) ,
(GV-2) if and only if ,
(GV-3) ,
(KM-4) is continuous,
(KM-5) .
Lemma 2.1 [4]
Let be a fuzzy metric space. Then is non-decreasing for all .
Lemma 2.2 [20]
Let be a fuzzy metric space. Then M is a continuous function on .
Definition 2.3 [9]
Let be a fuzzy metric space. A sequence in X is called an M-Cauchy sequence, if for each and there is such that for all . The fuzzy metric space is called M-complete if every M-Cauchy sequence is convergent.
Let be a partially ordered set and F be a mapping from X to itself. A sequence in X is said to be non-decreasing if for each , . A mapping is called monotonic increasing if for all with , .
Definition 2.4 [21]
Let be a partially ordered set and and be two mappings. The mapping F is said to have the mixed g-monotone property if for all , implies for all , and for all , implies for all .
Definition 2.5 [14]
An element is called a coupled coincidence point of the mappings and if
Here is called a coupled point of coincidence.
3 Main results
Lemma 3.1 Let be a left continuous function and ∗ be a continuous t-norm. Assume that for all . Then .
Proof Let be a non-decreasing sequence with . By hypothesis we have
Since γ is left continuous and ∗ is continuous, we get
which implies that . Since , one has . This completes the proof. □
Theorem 3.1 Let be a fuzzy metric space with a continuous and positive t-norm. Let ⪯ be a partial order defined on X. Let be a function satisfying for all and let be a left continuous and increasing function satisfying for all . Let and be two mappings such that F has the mixed g-monotone property and assume that is complete. Suppose that the following conditions hold:
-
(i)
,
-
(ii)
we have
(3.1)
for all , with and ,
-
(iii)
if a non-decreasing sequence , then for all ,
-
(iv)
if a non-increasing sequence , then for all .
If there exist such that , and for all , then there exist such that and .
Proof Let such that and . Define the sequences and in X by
Along the lines of the proof of [17], we see that
By (3.1) and (3.2) we have
and
Since ∗ is positive, we have
Repeating the process (3.3) and (3.4), we get
and further we have
Continuing the above process, we get, for each ,
and
Since ∗ is positive, one has
Now we prove by induction that, for each and with , one has
Obviously (3.5) holds for . Assume that (3.5) holds for some with . Then we have
Since , , and ∗ is positive, we have
Similarly, we have
Therefore, (3.5) holds for all with .
Now we use the method of Wang [22] to show that both and are Cauchy sequences. Fix . Let
For , by (3.1) and (3.2) we have
Similarly,
So, by (3.5) and the hypothesis we have
which implies that
Since is bounded, there exists such that . Assume that . Since γ is increasing, we have
and further
From (3.6) and (3.7) it follows that
i.e.,
Since γ is left continuous, by hypothesis we get
This is a contradiction. So .
For any given , there exists such that
Thus for each ,
which implies that
It follows that both and are Cauchy sequences. Since is complete, there exist such that and as .
By hypothesis, we have
Now, for all , by (3.1) and (3.8) we have
Since γ is left continuous and ∗ is continuous, letting in (3.9), we get
It follows that . Similarly, we can prove that . This completes the proof. □
If for all in Theorem 3.1, we get the following corollary.
Corollary 3.1 Let be a fuzzy metric space with a positive t-norm. Let ⪯ be a partial order defined on X. Let be a left continuous and increasing function satisfying for all . Let and be two mappings such that F has mixed g-monotone property and assume that is complete. Suppose that the following conditions hold:
-
(i)
.
-
(ii)
We have
for all , with and .
-
(iii)
If we have a non-decreasing sequence , then for all .
-
(iv)
If we have a non-increasing sequence , then for all .
If there exist such that , and for all , then there exist such that and .
Letting for all in Theorem 3.1 and Corollary 3.1, we get the following corollaries.
Corollary 3.2 Let be a complete fuzzy metric space with a positive t-norm. Let ⪯ be a partial order defined on X. Let be a function satisfying for all and let be a left continuous and increasing function satisfying for all . Let and assume F has mixed monotone property. Suppose that the following conditions hold:
-
(i)
We have
for all , with and .
-
(ii)
If we have a non-decreasing sequence , then for all .
-
(iii)
If we have a non-increasing sequence , then for all .
If there exist such that , and for all , then there exist such that and .
Corollary 3.3 Let be a complete fuzzy metric space with a positive t-norm. Let ⪯ be a partial order defined on X. Let be a left continuous and increasing function satisfying for all . Let and assume F has mixed monotone property. Suppose that the following conditions hold:
-
(i)
We have
for all , with and .
-
(ii)
If we have a non-decreasing sequence , then for all .
-
(iii)
If we have a non-increasing sequence , then for all .
If there exist such that , , and for all , then there exist such that and .
First, we illustrate Theorem 3.1 by modifying [[17], Example 3.4] as follows.
Example 3.1 Let is the partially ordered set with and the natural ordering ≤ of the real numbers as the partial ordering ⪯. Define by
Let for all . Then is a (complete) fuzzy metric space.
Let for all and for all . It is easy to see that for all .
Define the mappings by
and by
Then , F satisfies the mixed g-monotone property; see [[17], Example 3.4]. Obviously is complete.
Let and , then and ; see [[17], Example 3.4]. Moreover, for all .
Next we show that for all and all with and , i.e., and , one has
We prove the above inequality by a contradiction. Assume
Then
i.e.,
This is a contradiction. Thus, (3.10) holds. Therefore, all the conditions of Theorem 3.1 are satisfied. Then by Theorem 3.1 we conclude that there exist , such that and . It is easy to see that , as desired.
Example 3.2 Let is the partially ordered set with and the natural ordering ≤ of the real numbers as the partial ordering ⪯. Define a mapping by for all and . Let for all . Then is a fuzzy metric space but not complete.
Define the mappings and by
and for all . Then , F satisfies the mixed g-monotone property, and is complete. Take . By a simple calculation we see that and . Moreover, for all .
Let for all . Let γ be a function from to defined by
Obviously, γ is left continuous and increasing, and for all .
Let and with and , i.e., and , since
Hence (3.1) is satisfied. Therefore, all the conditions of Theorem 3.1 are satisfied. Then by Theorem 3.1 F and g have a coincidence point. It is easy to check that .
The above two examples cannot be applied to [[17], Theorem 3.1], since ∗ is not of Hadžić-type, or g is not monotonic increasing or continuous, or as for all .
4 Conclusion
In this paper, we prove a new coupled coincidence fixed point result in a partial order fuzzy metric space in which some restrictions required in [[17], Theorem 3.1] are removed, such that the conditions required in our result are fewer than the ones required in [[17], Theorem 3.1]. The purpose of this paper is to give some new conditions on the coupled coincidence fixed point theorem. Our result is not an improvement of [[17], Theorem 3.1], since we add some other restrictions such as requiring that the function γ is increasing and for all . As pointed out in the conclusion part of [17], it still is an interesting open problem to find simpler or fewer conditions on the coupled coincidence fixed point theorem in a fuzzy metric space.
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Acknowledgements
This work is supported by the Fundamental Research Funds for the Central Universities (Grant Numbers: 13MS109, 2014ZD44) and funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah. The authors Alsulami and Ćirić, therefore, acknowledge with thanks the DSR financial support.
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Wang, S., Luo, T., Ćirić, L. et al. New conditions on fuzzy coupled coincidence fixed point theorem. Fixed Point Theory Appl 2014, 153 (2014). https://doi.org/10.1186/1687-1812-2014-153
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DOI: https://doi.org/10.1186/1687-1812-2014-153