Abstract
We prove some results on the existence of fixed points for multivalued generalized -contractive maps not involving the extended Hausdorff metric. Consequently, several known fixed point results are either generalized or improved.
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1. Introduction
Throughout this paper, unless otherwise specified, is a metric space with metric
. Let
, and
denote the collection of nonempty subsets of
, nonempty closed subsets of
, and nonempty closed bounded subsets of
, respectively. Let
be the Hausdorff metric on
, that is,
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ1_HTML.gif)
A multivalued map is called
(i)contraction [1] if for a fixed constant and for each
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ2_HTML.gif)
(ii)generalized contraction [2] if for any
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ3_HTML.gif)
where is a function from
to
with
for every
;
(iii)contractive [3] if there exist constants such that for any
there is
satisfying
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ4_HTML.gif)
where ;
(iv)generalized contractive [4] if there exist such that for any
there is
satisfying
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ5_HTML.gif)
where is a function from
to
with
for every
An element is called a fixed point of a multivalued map
if
. We denote
A sequence in
is called an
of
at
if
for all
. A map
is called lower semicontinuous if for any sequence
with
imply that
.
Using the concept of Hausdorff metric, Nadler Jr. [1] established the following fixed point result for multivalued contraction maps which in turn is a generalization of the well-known Banach contraction principle.
Theorem 1.1 (see [1]).
Let be a complete space and let
be a contraction map. Then
This result has been generalized in many directions. For instance, Mizoguchi and Takahashi [2] have obtained the following general form of the Nadler's theorem.
Theorem 1.2 (see [2]).
Let be a complete space and let
be a generalized contraction map. Then
Another extension of Nadler's result obtained recently by Feng and Liu [3]. Without using the concept of the Hausdorff metric, they proved the following result.
Theorem 1.3 (see [3]).
Let X be a complete space and let be a multivalued contractive map. Suppose that a real-valued function
on
,
, is lower semicontinuous. Then
Most recently, Klim and Wardowski [4] generalized Theorem 1.3 as follows:
Theorem 1.4 (see [4]).
Let X be a complete metric space and let be a multivalued generalized contractive map such that a real-valued function
on
,
is lower semicontinuous. Then
Recently, Kada et al. [5] introduced the concept of -distance on a metric space as follows.
A function is called
-
on
if it satisfies the following for any
:
()
() a map is lower semicontinuous;
() for any there exists
such that
and
imply
Using the concept of -distance, they improved Caristi's fixed point theorem, Ekland's variational principle, and Takahashi's existence theorem. In [6], Susuki and Takahashi proved a fixed point theorem for contractive type multivalued maps with respect to
-distance. See also [7–12].
Let us give some examples of -distance [5].
-
(a)
The metric
is a
-distance on
.
-
(b)
Let
be normed space with norm
Then the functions
defined by
and
for every
, are
-distance.
The following lemmas concerning -distance are crucial for the proofs of our results.
Lemma 1.5 (see [5]).
Let
and
be sequences in
and let
and
be sequences in
converging to
Then, for the w-distance
on
the following hold for every
:
(a)if and
for any
then
in particular, if
and
then
;
-
(b)
if
and
for any
then
converges to
;
-
(c)
if
for any
with
then
is a Cauchy sequence;
-
(d)
if
for any
then
is a Cauchy sequence.
Lemma 1.6 (see [9]).
Let be a closed subset of
and let
be a w-distance on
Suppose that there exists
such that
. Then
(where
)
We say a multivalued map is generalized
-contractive if there exist a
-distance
on
and a constant
such that for any
there is
satisfying
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ6_HTML.gif)
where and
is a function from
to
with
for every
Note that if we take then the definition of generalized
-contractive map reduces to the definition of generalized contractive map due to Klim and Wardowski [4]. In particular, if we take a constant map
then the map
is weakly contractive (in short,
-contractive) [8], and further if we take
then we obtain
and
is contractive [3].
In this paper, using the concept of -distance, we first establish key lemma and then obtain fixed point results for multivalued generalized
-contractive maps not involving the extended Hausdorff metric. Our results either generalize or improve a number of fixed point results including the corresponding results of Feng and Liu [3], Latif and Albar [8], and Klim and Wardowski [4].
2. Results
First, we prove key lemma in the setting of metric spaces.
Lemma 2.1.
Let
be a generalized
-contractive map. Then, there exists an orbit
of
in
such that the sequence of nonnegative real numbers
is decreasing to zero and the sequence
is Cauchy.
Proof.
Since for each ,
is closed, the set
is nonempty for any
Let
be an arbitrary but fixed element of
. Since
is generalized
-contractive, there is
such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ7_HTML.gif)
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ8_HTML.gif)
Using (2.1) and (2.2), we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ9_HTML.gif)
Similarly, there is such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ10_HTML.gif)
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ11_HTML.gif)
Using (2.4) and (2.5), we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ12_HTML.gif)
From (2.5) and (2.1), it follows that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ13_HTML.gif)
Continuing this process, we get an orbit of
in
such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ14_HTML.gif)
Using (2.8), we get
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ15_HTML.gif)
and thus for all
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ16_HTML.gif)
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ17_HTML.gif)
Note that the sequences and
are decreasing, and thus convergent. Now, by the definition of the function
there exists
such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ18_HTML.gif)
Thus, for any there exists
such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ19_HTML.gif)
and thus for all we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ20_HTML.gif)
Also, it follows from (2.9) that for all ,
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ21_HTML.gif)
where Note that for all
we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ22_HTML.gif)
and thus
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ23_HTML.gif)
Now, since we have
and hence the decreasing sequence
converges to
. Now, we show that
is a Cauchy sequence. Note that for all
,
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ24_HTML.gif)
where Now, for any
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ25_HTML.gif)
and thus by Lemma 1.5, is a Cauchy sequence.
Using Lemma 2.1, we obtain the following fixed point result which is an improved version of Theorem 1.4 and contains Theorem 1.3 as a special case.
Theorem 2.2.
Let be a complete space and let
be a generalized
-contractive map. Suppose that a real-valued function
on
defined by
is lower semicontinous. Then there exists
such that
Further, if
then
.
Proof.
Since is a generalized
-contractive map, it follows from Lemma 2.1 that there exists a Cauchy sequence
in
such that the decreasing sequence
converges to 0. Due to the completeness of
, there exists some
such that
Since
is lower semicontinuous, we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ26_HTML.gif)
and thus, Since
and
is closed, it follows from Lemma 1.6 that
As a consequence, we also obtain the following fixed point result.
Corollary 2.3 (see [8]).
Let be a complete space and let
be a
-contractive map. If the real-valued function
on
defined by
is lower semicontinous, then there exists
such that
Further, if
then
Applying Lemma 2.1, we also obtain a fixed point result for multivalued generalized -contractive map satisfying another suitable condition.
Theorem 2.4.
Let
be a complete space and let
be a generalized
-contractive map. Assume that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ27_HTML.gif)
for every with
Then
Proof.
By Lemma 2.1, there exists an orbit of
, which is a Cauchy sequence in
. Due to the completeness of
, there exists
such that
Since
is lower semicontinuous and
it follows from the proof of Lemma 2.1 that for all
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ28_HTML.gif)
where Also, we get
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ29_HTML.gif)
Assume that Then, we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ30_HTML.gif)
which is impossible and hence .
Corollary 2.5 (see [8]).
Let
be a complete space and let
be
-contractive map. Assume that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1155%2F2009%2F487161/MediaObjects/13663_2008_Article_1146_Equ31_HTML.gif)
for every with
Then
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Latif, A., Abdou, A.A.N. Fixed Points of Generalized Contractive Maps. Fixed Point Theory Appl 2009, 487161 (2009). https://doi.org/10.1155/2009/487161
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DOI: https://doi.org/10.1155/2009/487161