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Some generalizations of Mizoguchi-Takahashi’s fixed point theorem with new local constraints
Fixed Point Theory and Applications volume 2014, Article number: 31 (2014)
Abstract
In this paper, motivated by Kikkawa-Suzuki’s fixed point theorem, we establish some new generalizations of Mizoguchi-Takahashi’s fixed point theorem with new local constraints on discussion maps.
MSC:47H10, 54C60, 54H25, 55M20.
1 Introduction and preliminaries
Let be a metric space. Denote by the family of all nonempty subsets of X, the class of all nonempty closed subsets of X and the family of all nonempty closed and bounded subsets of X. For each and , let . A function defined by
is said to be the Hausdorff metric on induced by the metric d on X. Let be a multivalued map. A point v in X is said to be a fixed point of T if . The set of fixed points of T is denoted by . The map T is said to have the approximate fixed point property [1–3] on X provided . It is obvious that implies that T has the approximate fixed point property. The symbols ℕ and ℝ are used to denote the sets of positive integers and real numbers, respectively.
A function is said to be an -function (or ℛ-function) [2–5] if for all . It is evident that if is a nondecreasing function or a nonincreasing function, then φ is a -function. So the set of -functions is a rich class.
Recently, Du [5] first proved the following characterizations of -functions.
Theorem 1.1 ([5])
Let be a function. Then the following statements are equivalent.
-
(a)
φ is an -function.
-
(b)
For each , there exist and such that for all .
-
(c)
For each , there exist and such that for all .
-
(d)
For each , there exist and such that for all .
-
(e)
For each , there exist and such that for all .
-
(f)
For any nonincreasing sequence in , we have .
-
(g)
φ is a function of contractive factor; that is, for any strictly decreasing sequence in , we have .
In 1989, Mizoguchi and Takahashi [6] proved a famous generalization of Nadler’s fixed point theorem, which gives a partial answer of Problem 9 in Reich [7].
Theorem 1.2 (Mizoguchi and Takahashi [6])
Let be a complete metric space, be a -function and be a multivalued map. Assume that
for all . Then .
A number of generalizations in various different directions of research of Mizoguchi-Takahashi’s fixed point theorem were investigated by several authors; see, e.g., [2–5, 8–12] and references therein.
In 2008, Suzuki [13] presented a new type of generalization of the celebrated Banach contraction principle [14] which characterized the metric completeness.
Theorem 1.3 (Suzuki [13])
Define a nonincreasing function θ from onto by
Then for a metric space , the following are equivalent:
-
(1)
X is complete.
-
(2)
Every mapping T on X satisfying the following has a fixed point:
-
There exists such that implies for all .
-
(3)
There exists such that every mapping T on X satisfying the following has a fixed point:
-
implies for all .
Remark 1.1 ([13])
For every , is the best constant.
Later, Kikkawa and Suzuki [15] proved an interesting generalization of both Theorem 1.1 and Nadler’s fixed point theorem. In fact, Kikkawa-Suzuki’s fixed point theorem can be regarded as a generalization of Nadler fixed point theorem with a local constraint on the discussion map.
Theorem 1.4 (Kikkawa and Suzuki [15])
Define a strictly decreasing function η from onto by
Let be a complete metric space and let T be a map from X into . Assume that there exists such that
for all . Then .
In this paper, motivated by Kikkawa-Suzuki’s fixed point theorem, we establish some new generalizations of Mizoguchi-Takahashi’s fixed point theorem with new local constraints on discussion maps. Our new results generalize and improve Mizoguchi-Takahashi’s fixed point theorem, Nadler’s fixed point theorem and Banach contraction principle.
2 Main results
Very recently, Du and Khojasteh [12] first introduced the concept of manageable functions.
Definition 2.1 ([12])
A function is called manageable if the following conditions hold:
(η 1) for all .
(η 2) For any bounded sequence and any nonincreasing sequence , we have
We denote the sets of all manageable functions by .
Remark 2.1 If , then for all .
Example 2.1 Let and . Then the function defined by is manageable.
Example 2.2 ([12])
Let be any function and be an -function. Define by
Then η is a manageable function. Indeed, one can verify easily that (η 1) holds. Next, we verify that η satisfies (η 2). Let be a bounded sequence and be a nonincreasing sequence. Then for some . Since φ is an -function, by Theorem 1.1, there exist and such that for all . Since , there exists , such that
Hence we have
which means that (η 2) holds. Thus we prove .
In this paper, we first introduce the concepts of weakly transmitted functions and -strongly transmitted functions.
Definition 2.2 A function is called
-
(i)
weakly transmitted if for all ;
-
(ii)
-strongly transmitted if there exists , such that for all .
We denote by and , the sets of all weakly transmitted functions and -strongly transmitted functions, respectively. It is quite obvious that for all .
Example 2.3 Let , be functions and . Define by
Then .
The following simple example shows that there exists a weakly transmitted function which is not -strongly transmitted for all . In other words, for all .
Example 2.4 Let be defined by
Then ξ is a weakly transmitted function which is not -strongly transmitted for all .
The following result is simple, but it is very crucial in our proofs.
Lemma 2.1 Let be a metric space, be a nonempty subset of and be a multivalued map. Suppose that there exists such that
If with , then .
Proof Since , . If , then, by Remark 2.2, we have , a contradiction. If , then, by (η 1), we have
which also leads a contradiction. Therefore . □
Now, we establish an existence theorem for approximate fixed point property and fixed points by using manageable functions and transmitted functions which is one of the main results of this paper.
Theorem 2.1 Let be a metric space and be a multivalued map. Assume that there exist and such that
where
Then T has the approximate fixed property on X.
Moreover, if is complete and , then .
Proof Let . If , then is a fixed point of T and we are done. Suppose that . Then . Since , we can find with . Thus
Since , we get
which means that . Therefore, by (2.1), we obtain
By Lemma 2.1, we have
If , then we have nothing to prove. So we assume that . Hence we have
Define by
By (η 1), we know that
Since and , by the definition of h and (2.2), we have
Take
Then . Since
there exists such that and
If , then the proof is finished. Otherwise, we have
By (2.2) and , we get
which implies . By (2.1), we obtain
By Lemma 2.1 and (2.5), we have
Taking into account (2.5), (2.6) and the definition of h conclude that
By taking
there exists with such that
Hence, by induction, we can establish a sequences in X satisfying for each ,
and
We claim that is a Cauchy sequence in X. For each , let
By (2.3), we know that
so, from (2.8) and (2.9), we obtain and
Hence the sequence is strictly decreasing in . Thus
By (2.7), we get
which means that is a bounded sequence. By (η 2) and the definition of h, we have
which implies . So, there exists and , such that
For any , since for all and , taking into account (2.10) and (2.12), we conclude
Put , . For with , from the last inequality, we have
Since , and hence
So is a Cauchy sequence in X. Combining (2.11) and (2.13), we get
Since for each , we have
Combining (2.14) and (2.15) yields
which means that T has the approximate fixed property on X.
Now, we assume that is complete and . Since is a Cauchy sequence in X, by the completeness of X, there exists such that as . We will proceed with the following claims to prove .
Claim 1. for all .
Given with . Let
Suppose that , where is the cardinal number of S. Then there exists such that for all . So as . By the uniqueness of the limit, we get , a contradiction. Hence which deduces that there exists such that for all with . For any , put
Thus we have
-
for all ;
-
for all ;
-
as .
Since , there exists such that
For with , from (2.16), we have
which implies that . Applying Lemma 2.1,
Since as and
by taking the limit from both sides of (2.17), we get
Claim 2. .
We first prove for all . Suppose that there exists with such that . So
Note that for any , there exists such that
Thus, for any , by Claim 1, we have
Hence we get . Since , there exists , such that for all . So, we obtain
a contradiction. Therefore for all . By Lemma 2.1, we have
Therefore,
From (2.18), we obtain
By taking limit from both side of (2.19), we get . By the closedness of Tv, we have . The proof is completed. □
Theorem 2.2 Let be a complete metric space, be a multivalued map and . Assume that there exist an -function and a function such that, for ,
Then .
Proof Define and , respectively, by
and
Thus . By Example 2.2, we know . By (2.20), we obtain
where
Therefore the desired conclusion follows from Theorem 2.1 immediately. □
In Theorem 2.2, if we take for all , then we obtain the following new generalization of Mizoguchi-Takahashi’s fixed point theorem.
Theorem 2.3 Let be a complete metric space, be a multivalued map and . Assume that there exists an -function such that for ,
Then .
Remark 2.2 Theorems 2.1, 2.2 and 2.3 generalize and improve Mizoguchi-Takahashi’s fixed point theorem, Nadler’s fixed point theorem, and Banach contraction principle.
Finally, a question arises naturally.
Question Can we give new generalizations of Mizoguchi-Takahashi’s fixed point theorem with other new local constraints which also extend Kikkawa-Suzuki’s fixed point theorem?
References
Hussain N, Amini-Harandi A, Cho YJ: Approximate endpoints for set-valued contractions in metric spaces. Fixed Point Theory Appl. 2010., 2010: Article ID 614867 10.1155/2010/614867
Du W-S: On generalized weakly directional contractions and approximate fixed point property with applications. Fixed Point Theory Appl. 2012., 2012: Article ID 6 10.1186/1687-1812-2012-6
Du W-S: New existence results and generalizations for coincidence points and fixed points without global completeness. Abstr. Appl. Anal. 2013., 2013: Article ID 214230 10.1155/2013/214230
Du W-S: Some new results and generalizations in metric fixed point theory. Nonlinear Anal. 2010, 73: 1439–1446. 10.1016/j.na.2010.05.007
Du W-S: On coincidence point and fixed point theorems for nonlinear multivalued maps. Topol. Appl. 2012, 159: 49–56. 10.1016/j.topol.2011.07.021
Mizoguchi N, Takahashi W: Fixed point theorems for multivalued mappings on complete metric spaces. J. Math. Anal. Appl. 1989, 141: 177–188. 10.1016/0022-247X(89)90214-X
Reich S: Some problems and results in fixed point theory. Contemp. Math. 1983, 21: 179–187.
Daffer PZ, Kaneko H: Fixed points of generalized contractive multi-valued mappings. J. Math. Anal. Appl. 1995, 192: 655–666. 10.1006/jmaa.1995.1194
Berinde M, Berinde V: On a general class of multi-valued weakly Picard mappings. J. Math. Anal. Appl. 2007, 326: 772–782. 10.1016/j.jmaa.2006.03.016
Gordji ME, Ramezani M: A generalization of Mizoguchi and Takahashi’s theorem for single-valued mappings in partially ordered metric spaces. Nonlinear Anal. 2011, 74: 4544–4549. 10.1016/j.na.2011.04.020
Ćirić L, Damjanović B, Jleli M, Samet B: Coupled fixed point theorems for generalized Mizoguchi-Takahashi contractions with applications. Fixed Point Theory Appl. 2012., 2012: Article ID 51
Du, W-S, Khojasteh, F: New results and generalizations for approximate fixed point property and their applications. Abstr. Appl. Anal. 2014 (2014, in press)
Suzuki T: A generalized Banach contraction principle that characterizes metric completeness. Proc. Am. Math. Soc. 2008, 136: 1861–1869.
Banach S: Sur les opérations dans les ensembles abstraits et leurs applications aux équations intégrales. Fundam. Math. 1922, 3: 133–181.
Kikkawa M, Suzuki T: Three fixed point theorems for generalized contractions with constants in complete metric spaces. Nonlinear Anal. 2008, 69: 2942–2949. 10.1016/j.na.2007.08.064
Acknowledgements
The first author was supported by grant no. NSC 102-2115-M-017-001 of the National Science Council of the Republic of China.
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Du, WS., Khojasteh, F. & Chiu, YN. Some generalizations of Mizoguchi-Takahashi’s fixed point theorem with new local constraints. Fixed Point Theory Appl 2014, 31 (2014). https://doi.org/10.1186/1687-1812-2014-31
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DOI: https://doi.org/10.1186/1687-1812-2014-31