Open Access

# Suzuki-type fixed point theorem for fuzzy mappings in ordered metric spaces

Fixed Point Theory and Applications20132013:9

DOI: 10.1186/1687-1812-2013-9

Accepted: 18 December 2012

Published: 10 January 2013

## Abstract

In this paper, a Suzuki-type fixed fuzzy point result for fuzzy mappings in complete ordered metric spaces is obtained. As an application, we establish the existence of coincidence fuzzy points and common fixed fuzzy points for a hybrid pair of a single-valued self-mapping and a fuzzy mapping. An example is also provided to support the main result presented herein.

MSC:47H10, 47H04, 47H07.

### Keywords

fixed fuzzy point fuzzy mapping fuzzy set approximate quantity

## 1 Introduction and preliminaries

Let X be a space of points with generic elements of X denoted by x and . A fuzzy subset of X is characterized by a membership function such that each element in X is associated with a real number in the interval I. Let be a metric space and a fuzzy set A in X is characterized by a membership function A. Then α-level set of A, denoted by , is defined as
for and for , we have
where denotes the closure of the non-fuzzy set B. A fuzzy set A in X is said to be an approximate quantity if and only if for , is a compact, convex subset of X and

Let be a family of all approximate quantities in X. A fuzzy set A is said to be more accurate than a fuzzy set B denoted by (that is, B includes A) if and only if for each x in X, where and denote the membership function of A and B, respectively. It is easy to see that if , then .

Corresponding to each and , the fuzzy point of X is the fuzzy set given by
For , we have
Let be a collection of all fuzzy subsets of X and be a subcollection of all approximate quantities. For and , define
and

Note that is a nondecreasing function of α and D is a metric on . Let . Define . Let be a metric space and Y be an arbitrary set. A mapping is called a fuzzy mapping, that is, for each y in Y. Thus, if we characterize a fuzzy set Fy in a metric space X by a membership function Fy, then is the grade of membership of x in Fy. Therefore, a fuzzy mapping F is a fuzzy subset of with a membership function .

In a more general sense than that given in [1], a mapping is a fuzzy mapping over X[2] and is the fixed degree of x in .

Definition 1 ([3])

A fuzzy point in X is called a fixed fuzzy point of the fuzzy mapping F if , that is, or . That is, the fixed degree of x in Fx is at least α. If , then x is a fixed point of a fuzzy mapping F.

Let and .

A fuzzy point in X is called a coincidence fuzzy point of the hybrid pair if , that is, or . That is, the fixed degree of gx in Fx is at least α. A fuzzy point in X is called a common fixed fuzzy point of the hybrid pair if , that is, (the fixed degree of x and gx in Fx is the same and is at least α).

We denote by and the set of all coincidence fuzzy points and the set of all common fixed fuzzy points of the hybrid pair , respectively.

A hybrid pair is called w-fuzzy compatible if whenever .

A mapping g is called F-fuzzy weakly commuting at some point if .

Lemma 1 ([4])

Let X be a nonempty set and. Then there exists a subsetsuch thatandis one-to-one.

Definition 2 Let X be a nonempty set. Then is called an ordered metric space if is a metric space and is partially ordered.

Let be a partially ordered set. Then are said to be comparable if or holds.

Define

An ordered metric space is said to satisfy the order sequential limit property if for all n, whenever a sequence and for all n.

A mapping is said to be an ordered fuzzy mapping if the following conditions are satisfied:
1. (a)

implies that .

2. (b)

implies that whenever and .

The following lemmas are needed in the sequel.

Lemma 2 (Heilpern [1])

Letbe a metric space, and:
1. 1.

if , then ;

2. 2.

;

3. 3.

if , then .

Lemma 3 (Lee and Cho [5])

Letbe a complete metric space and F be a fuzzy mapping from X intoand. Then there exists ansuch that.

Zadeh [6] introduced the concept of a fuzzy set. Heilpern [1] introduced the concept of fuzzy mappings in a metric space and proved a fixed point theorem for fuzzy contraction mappings as a generalization of the fixed point theorem for multivalued mappings given by Nadler [7]. Estruch and Vidal [3] proved a fixed point theorem for fuzzy contraction mappings in complete metric spaces which in turn generalizes the Heilpern fixed point theorem. Further generalizations of the result given in [3] were proved in [8, 9]. Recently, Suzuki [10] generalized the Banach contraction principle and characterized the metric completeness property of an underlying space. Among many generalizations (see [1113]) of the results given in [10], Dorić and Lazović [14] obtained Suzuki-type fixed point results for a generalized multivalued contraction in complete metric spaces.

On the other hand, the existence of fixed points in ordered metric spaces has been introduced and applied by Ran and Reurings [15]. Fixed point theorems in partially ordered metric spaces are hybrid of two fundamental principles: Banach contraction theorem with a contractive condition for comparable elements and a selection of an initial point to generate a monotone sequence. For results concerning fixed points and common fixed points in partially ordered metrics spaces, we refer to [1622].

The aim of this paper is to investigate Suzuki-type fixed point results for fuzzy mappings in complete ordered metric spaces. As an application, a coincidence fuzzy point and a common fixed fuzzy point of the hybrid pair of a single-valued self-mapping and a fuzzy mapping are obtained. We provide an example to support the result.

Throughout this paper, let be the nonincreasing function defined by
(1)

## 2 Main results

The following theorem is the main result of the paper and is a generalization of [[14], Theorem 2.1] for fuzzy mappings in ordered metric spaces.

Theorem 4 Letbe a complete ordered metric space. If an ordered fuzzy mappingsatisfies
(2)
for all, where

Then there exists a pointsuch thatprovided that X satisfies the order sequential limit property.

Proof Let be a real number such that and . Since is nonempty and compact, there exists such that
By the given assumption, we have . Since is nonempty and compact, there exists such that
Also, . Since , we obtain
That is,
So, we have
Note that . If not, then the above inequality gives
a contradiction. Hence, . Continuing this process, we construct a sequence in X such that and with
By the given assumption, we have and . As , so
Therefore,
We claim that . If not, then by the above inequality, we obtain
a contradiction as . So, we have
and
(3)
Hence, is a Cauchy sequence in X. Since X is complete, there is some point such that . As for all n, then by the assumption, . Now, we show that for every pair with , the following inequality holds:
As , there exists a positive integer such that for all , we have
(4)
Now, for all ,
implies that
which on taking limit as gives
If
then
Hence,
(5)
Now, we show that for each . First, consider the case . Assume on the contrary that , that is, . Let , as is nonempty and compact, so for each , we have
(6)
Now, implies and . From (5) we have
(7)
Now,
implies that
Hence,
which further implies that
We claim that . If not, then the above inequality becomes
a contradiction, so we deduce that . From inequality (7), we have
Therefore,

Now, when , we first prove that
(8)
for all . If , then (8) holds trivially. So, assume that . For every , one may find a sequence such that
As , this implies . Using (7) we have
for all . If , then
This implies that
Hence, for , we obtain
On taking the limit as , we have
If , then
On taking the limit as , we have
By the given assumption, we have
Thus, for any , (8) holds true. Put in the above inequality to obtain

as , we get . Hence by Lemma 2, . □

Corollary 5 Letbe a complete ordered metric space. If an ordered fuzzy mappingsatisfies
for all, where

Then there exists a pointsuch thatprovided that X satisfies the order sequential limit property.

Corollary 6 Letbe a complete ordered metric space. If an ordered fuzzy mappingsatisfies
for all, where

and, . Then there exists a pointsuch thatprovided that X satisfies the order sequential limit property.

## 3 An application

Let and . A pair is said to be an ordered fuzzy hybrid pair if the following conditions are satisfied:
1. (c)

implies that .

2. (d)

gives whenever and .

3. (e)

whenever for all .

Theorem 7 Letbe a complete ordered metric space. If an ordered fuzzy hybrid pairsatisfies
(9)
for all, where
Thenprovided that X satisfies the order sequential limit property andfor each α. Moreover, F and g have a common fixed fuzzy point if any of the following conditions holds:
1. (f)

F and g are w-fuzzy compatible, and for some , and g is continuous at u.

2. (g)

g is F-fuzzy weakly commuting for some and is a fixed point of g, that is, .

3. (h)

g is continuous at x for some and for some such that .

Proof By Lemma 1, there exists such that is one-to-one and . Define a mapping by
(10)
As g is one-to-one on E, is well defined. Also,
(11)
for all . Therefore,
for all . Hence, satisfies (2) and all the conditions of Theorem 4. Using Theorem 4 with a mapping , it follows that has a fixed fuzzy point . Now, it is left to prove that F and g have a coincidence fuzzy point. Since has a fixed fuzzy point , we get . As , so there exists such that , thus it follows that . This implies that is a coincidence fuzzy point of F and g. Hence, . Suppose now that (f) holds. Then for some , we have , where . Thus . Since g is continuous at u, we have that u is a fixed point of g. As F and g are w-fuzzy compatible, and for all . That is, for all . Now,
implies that

On taking limit as , we get and therefore . By Lemma 2 we obtain . Consequently, . Hence, is a common fixed fuzzy point of F and g. Suppose now that (g) holds. If for some , g is F-fuzzy weakly commuting and , then . Hence, is a common fixed fuzzy point of F and g. Suppose now that (h) holds and assume that for some and for some , and . By the continuity of g at x and y, we get . The result follows. □

Example 1 Let be endowed with the usual metric. Let and , then . Define a fuzzy mapping F from X into as
and for ,
Define a self-map by . Then
Note that for all , we have
Also, for all , we have
And
If and , then . So, for all , with , we have . Hence, for all ,
hold true, where
and

Hence, all the conditions of Theorem 7 are satisfied. Moreover, for each , we have and . For , we have .

## 4 Conclusion

The Banach contraction principle has become a classical tool to show the existence of solutions of functional equations in nonlinear analysis (see for details [2326]). Suzuki-type fixed point theorems [10, 14] are the generalizations of the Banach contraction principle that characterize metric completeness of underlying spaces. Fuzzy sets and mappings play important roles in the process of fuzzification of systems. Suzuki-type fixed point theorems for fuzzy mappings obtained in this article can further be used in the process of finding the solutions of functional equations involving fuzzy mappings in fuzzy systems. In the main result, we not only extended the mapping to a fuzzy mapping, but also the underlying metric space has been replaced with ordered metric spaces. In this article, we defined coincidence fuzzy points and common fixed fuzzy points of the hybrid pair of a single-valued self-mapping and a fuzzy mapping and applied our main result to obtain the existence of coincidence fuzzy points and common fixed fuzzy points of the hybrid pair.

## Declarations

### Acknowledgements

The authors are thankful to the referees for their critical remarks which helped to improve the presentation of this paper.

## Authors’ Affiliations

(1)
Department of Mathematics, School of Science and Engineering, Lahore University of Management Sciences
(2)
Department of Mathematics and Applied Mathematics, University of Pretoria

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