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Fixed point theorems for some generalized nonexpansive mappings in Ptolemy spaces
Fixed Point Theory and Applications volume 2014, Article number: 76 (2014)
Abstract
In this paper, some existence fixed point theorems for some classes of mappings such as , , , and the class of fundamentally nonexpanisve mappings are obtained in Ptolemy spaces.
MSC:47H10.
1 Introduction
Let be a metric space; the inequality
is called the Ptolemy inequality, where . A Ptolemy metric space is a metric space where the Ptolemy inequality holds. It was shown in [1] that a normed space is an inner product space if and only if it is a Ptolemy space.
Remark 1.1 [2]
-
(I)
spaces are Ptolemy spaces.
-
(II)
A geodesic Ptolemy space is not necessarily a space (see [3] for more details).
For more details on nonexpansive mappings and related topics in fixed point theory see [4–7]. Espinola and Nicolae [8] study some properties of uniformly convex geodesic spaces in geodesic Ptolemy spaces. They prove that a geodesic Ptolemy space with a uniformly continuous midpoint map is reflexive. As a result, every bounded sequence has a unique asymptotic center. This result is used to prove some fixed point theorems for geodesic Ptolemy spaces with a uniformly continuous midpoint map. Karapınar and Tas [9] introduce some generalization of condition C such as conditions , , , and , which are strong enough to generate a fixed point for discontinuous mappings. In this paper, the existence of fixed point for some class of mappings such as , , , , and the class of fundamentally nonexpansive mapping in Ptolemy spaces is studied.
2 Preliminaries
In this section, we introduce some notations (see [8, 10], for more details).
Let be a metric space; a geodesic path joining to is a map c from a closed interval to X such that , and for all . In particular, c is an isometry and . The image α of c is called a geodesic (or metric) segment joining x and y. When it is unique, this geodesic is denoted by . The space is said to be a geodesic space if every two points of X are joined by a geodesic, and X is said to be uniquely geodesic if there is exactly one geodesic joining x to y for each .
Definition 2.1 A subset Y of a geodesic space is called convex if the geodesic segment joining any two points of Y is entirely contained in Y.
Definition 2.2 [8]
Let X be a geodesic space. We say that X admits a continuous midpoint map if there exists a map such that
and for , such that and , we have .
A geodesic triangle consist of three points , and in X (the vertex of the triangle) and three geodesic segment corresponding to each pair of points (the edges of the triangle). For the geodesic triangle , a comparison triangle is a triangle in the Euclidean plan such that for .
A geodesic triangle Δ satisfies the inequality if for every comparison triangle of Δ and for every we have
where are the comparison triangle points of x and y.
A geodesic space is space if every geodesic triangle satisfies the inequality.
Let X be a metric space and be a bounded sequence in X. For let
The asymptotic radius of in K is given by
and the asymptotic center of in K is the set:
Nonexpansive mappings and its generalization are important form applications of view. Here, we present some new generalization of this kind of mapping in Ptolemy spaces. But at first, we recall the definition of nonexpansive and quasi-nonexpansive mappings as follows.
Definition 2.3 A map T on a subset C of a metric space E is called nonexpansive if
for all .
Definition 2.4 A map T on a subset C of a metric space E is called quasi-nonexpansive if
for all and , where is the set of all fixed points of T.
In 2008, Suzuki [11] introduced condition C as follows.
Definition 2.5 Let T be a mapping on a subset C of a metric space E. Then T is said to satisfy condition C if
for all .
It is obvious that every nonexpansive mapping satisfies condition C, but the converse is not true. The next simple example can show this fact.
Example 2.6 [11]
Define a mapping T on by
Then T satisfies condition C, but T is not nonexpansive.
Here, we introduce the following definition and recall some other conditions which generalize the Suzuki one:
Definition 2.7 Let X be a metric space and K be a subset of X. A mapping is said to be fundamentally nonexpansive if
for all .
Proposition 2.8 Every mapping which satisfies condition C is fundamentally nonexpansive, but the converse is not true.
Proof By taking and , we see that every nonexpansive mapping is fundamentally nonexpansive. So by [[11], Lemma 3.4 part (iii)] the result is obtained. □
Example 2.9 Suppose . Define
Define T on X by
T does not satisfy condition C, i.e.
Let , , then
and
Thus condition C does not hold. Now, T is fundamentally nonexpansive. To show this, we show (2.1) holds. Let and , then
Thus (2.1) is satisfied. Also, (2.1) holds for the other points. Moreover, in this example is not the zero constant function.
Example 2.10 Define a mapping T on by
It is easy to show T is quasi-nonexpansive but it is not fundamentally nonexpansive mapping.
Karapınar and Tas [9] state some new definitions which are modifications of Suzuki’s condition C, as follows.
Definition 2.11 Let T be a mapping on a subset K of a metric space E.
(i) T is said to satisfy condition if
where
(ii) T is said to satisfy condition if
where
(iii) T is said to satisfy condition if
(iv) T is said to satisfy condition if
It is clear, every nonexpansive mapping satisfies condition ([[9], Proposition 9]).
Example 2.12 Define a mapping T on with the usual metric by
It is obvious, and T is quasi-nonexpansive. However, since
and
T does not satisfy condition C, but T satisfies condition . Consider the following cases:
-
(1)
If then .
-
(2)
If then .
-
(3)
If , then
Karapınar and Tas [9] proved the following useful propositions.
Theorem 2.13 Let T be a mapping on a closed subset K of a metric space E. Suppose T satisfies condition , then is closed. Moreover, if E is strictly convex and K is convex, then is convex.
Remark 2.14 Theorem 2.13 holds if one replaces condition by one of the conditions , , and .
Theorem 2.15 Let T be a mapping on a closed subset K of a metric space E and T satisfy condition , then holds for .
Remark 2.16 Theorem 2.15 holds if one replaces condition by one of the conditions , , and .
See [12] for the next definition.
Definition 2.17 Let T be a mapping on a subset K of a metric space X and . T is said to satisfy condition if
Moreover, T is said to satisfy condition E, whenever T satisfies the condition for some .
Therefore, if T satisfies one of the conditions , , , and , then T satisfies condition for .
Definition 2.18 Let X be a metric space, , and . T is said to satisfy condition if
Note that, if , then the condition implies the condition . The class of mappings satisfying the condition is broader than the class of mappings satisfying the condition C. The next lemma and theorem play important roles to obtain fixed point in the Ptolemy spaces.
Lemma 2.19 [13]
Let and be bounded sequences in K, where K is nonempty, bounded, closed, and convex subset of a metric space X and . Suppose that and for all . Then .
Theorem 2.20 [8]
Let X be a complete geodesic Ptolemy space with a uniformly continuous midpoint map, a bounded sequence and is nonempty, closed, and convex. Then has a unique asymptotic center in K.
The following theorem can be useful for finding fixed points for nonexpansive mappings in Ptolemy spaces [8].
Theorem 2.21 Let X be a complete geodesic Ptolemy space with a uniformly continuous midpoint map, and nonempty, bounded, closed, and convex. Suppose that is a nonexpansive mapping. Then is nonempty, closed, and convex.
3 Main results
Let X be complete geodesic Ptolemy space with a uniformly continuous midpoint map and T be a mapping. In this section, we apply some different conditions for T and give some new results for .
Theorem 3.1 Let X be a complete geodesic Ptolemy space with a uniformly continuous midpoint map, and suppose that is nonempty, bounded, closed, and convex. If satisfies condition C, then is nonempty, closed, and convex.
Proof First, there exists an approximate fixed point sequence for T. To show this, define a sequence in K by and
for , where α is a real number in . Then, by the assumption,
for , hence
So, by Lemma 2.19
Now, by Theorem 2.20, the asymptotic center of any bounded sequence is in K, particularly, the asymptotic center of approximate fixed point sequence for T is in K. Let , we show that y is a fixed point of T. In order to prove this, one writes
therefore
Uniqueness of the asymptotic center implies . □
Theorem 3.2 Let K be a nonempty closed, convex, and bounded subset of a complete geodesic Ptolemy space with a uniformly continuous midpoint map X. Suppose that satisfies the conditions E and for some . Then T has a fixed point in K.
Proof Define a sequence by and for all . Then we have
By the condition , we have
Apply Lemma 2.19 to conclude .
Let , by Theorem 2.20, we have . Since T satisfies the condition E, we have
Taking the limit superior on both side in the above inequality, we obtain
Uniqueness of the asymptotic center implies . □
Theorem 3.3 Let X be a complete geodesic Ptolemy space with a uniformly continuous midpoint map. Suppose that is nonempty, bounded, closed, and convex. If satisfies condition and , then is closed and convex.
Proof Assume is a sequence in which converges to some . We show . In order to prove this, one can write
therefore
Uniqueness of asymptotic center implies . is convex, let , then
and
For , we have
Therefore and , because if or , then which is contradiction, therefore and , which means . □
There is an example [[2], p.6] to guarantee that the Ptolemy space differs from the space. Thanks to this example, we construct a function , where X is Ptolemy, but it is not a space.
Example 3.4 Consider the space
with metric,
X is geodesic Ptolemy space, but it is not a space (see [2]).
Define a mapping T on X by
T has condition . Suppose that and
and
thus
One can check that condition holds for the other points of the space X.
Note that , and is closed and convex.
Corollary 3.5 Let X be a complete geodesic Ptolemy space with a uniformly continuous midpoint map. Suppose that is nonempty, bounded, closed, and convex. If satisfies condition , then is closed and convex.
Remark 3.6 Corollary 3.5 holds if we replace condition by one of the conditions and .
Theorem 3.7 Let K be a bounded, closed, and convex subset of a complete geodesic Ptolemy space X with a uniformly continues midpoint map. Suppose satisfies condition . Suppose that is a sequence in K with . If , then y is a fixed point of T.
Proof Assume that there exists some approximate fixed point sequence . By Theorem 2.20, the asymptotic center of any bounded sequence is in K, particularly, the asymptotic center of approximate fixed point sequence for T is in K. Let . y is a fixed point of T. In order to prove this, one can write
therefore
Uniqueness of the asymptotic center shows . □
Corollary 3.8 Let K be a bounded, closed, and convex subset of a complete geodesic Ptolemy space X with a uniformly continues midpoint map. Suppose satisfies condition . Suppose that is a sequence in K with . If , then y is a fixed point of T.
Remark 3.9 Corollary 3.8 holds if we replace condition by one of the conditions and .
Corollary 3.10 Let K be a bounded, closed, and convex subset of a complete geodesic Ptolemy space X with a uniformly continues midpoint map. Suppose satisfies condition . Suppose that is a sequence in K with . If , then y is a fixed point of T.
Corollary 3.11 Let K be a bounded, closed, and convex subset of a complete geodesic Ptolemy space X with a uniformly continues midpoint map. Suppose satisfies condition . Suppose that is a sequence in K with . If , then y is a fixed point of T.
Theorem 3.12 Let X be a complete geodesic Ptolemy space with a uniformly continuous midpoint map, and nonempty, bounded, closed, and convex. Suppose that is fundamentally nonexpansive mapping. Then is nonempty, closed, and convex.
Proof The proof is similar to the one in Theorem 3.3. □
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Hosseini Ghoncheh, S., Razani, A. Fixed point theorems for some generalized nonexpansive mappings in Ptolemy spaces. Fixed Point Theory Appl 2014, 76 (2014). https://doi.org/10.1186/1687-1812-2014-76
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DOI: https://doi.org/10.1186/1687-1812-2014-76