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On an open question of Takahashi for nonspreading mappings in Banach spaces
Fixed Point Theory and Applications volume 2013, Article number: 228 (2013)
Abstract
In this paper, we first introduce a new class of mappings called asymptotically nonspreading mappings and establish weak and strong convergence theorems of the iterative sequences generated by these mappings in a real Banach space. We modify Halpern’s iterations for finding a fixed point of an asymptotically nonspreading mapping and provide an affirmative answer to an open problem posed by Kurokawa and Takahashi in their final remark of (Kurokawa and Takahashi in Nonlinear Anal. 73:1562-1568, 2010) for nonspreading mappings. Furthermore, we investigate the approximation of common fixed points of asymptotically nonspreading mappings and nonexpansive mappings and derive a strong convergence theorem by a new hybrid method for these mappings. Our results improve and generalize many known results in the current literature.
MSC:47H10, 37C25.
Dedication
Dedicated to Professor Wataru Takahashi on the occasion of his seventieth birthday
1 Introduction
Throughout this paper, we denote the set of real numbers and the set of positive integers by ℝ and ℕ, respectively. Let E be a Banach space with the norm and the dual space . The modulus δ of convexity of E is denoted by
for every ϵ with . A Banach space E is said to be uniformly convex if for every . Let . The norm of E is said to be Gâteaux differentiable if for each , the limit
exists. In this case, E is called smooth. If the limit (1.1) is attained uniformly in , then E is called uniformly smooth. The Banach space E is said to be strictly convex if whenever and . It is well known that E is uniformly convex if and only if is uniformly smooth. It is also known that if E is reflexive, then E is strictly convex if and only if is smooth; for more details, see [1]. When is a sequence in the Banach space E, we denote the strong convergence of to by and the weak convergence by . For any sequence in , we denote the strong convergence of to by , the weak convergence by and the weak-star convergence by . The normalized duality mapping is defined by
Now, we define a mapping , the modulus of smoothness of E, as follows:
It is well known that E is uniformly smooth if and only if . Let be such that . Then a Banach space E is said to be q-uniformly smooth if there exists a constant such that for all . If a Banach space E admits a sequentially continuous duality mapping J from weak topology to weak-star topology, then J is single-valued and also E is smooth; see [2] for more details. In this case, the normalized duality mapping J is said to be weakly sequentially continuous, i.e., if is a sequence with , then [2]. A Banach space E is said to satisfy the Opial property [3] if for any weakly convergent sequence in E with weak limit x,
for all with . It is well known that all Hilbert spaces, all finite dimensional Banach spaces and the Banach spaces () satisfy the Opial property; see, for example, [2, 3]. It is also known that if E admits a weakly sequentially continuous duality mapping, then E is smooth and enjoys the Opial property; see [2] for more details.
Let C be a nonempty subset of a real Banach space E, and let be a mapping. We denote by the set of fixed points of T, i.e., . A mapping is said to be nonexpansive if for all . A mapping is said to be quasi-nonexpansive if and for all and . Let C be a nonempty, closed and convex subset of a Hilbert space H and . Then there exists a unique nearest point such that . We denote such a correspondence by . The mapping is called metric projection of H onto C.
The concept of nonexpansivity plays an important role in the study of Mann-type iteration for finding fixed points of a mapping , where C is a closed and convex subset of a Banach space E. Recall that the Mann-type iteration [4] is given by the following formula
Here, is a sequence of real numbers in satisfying some appropriate conditions. A more general iteration scheme is the Halpern iteration, given by
where the sequences and satisfy some appropriate conditions. In particular, when all , the Halpern iteration (1.3) becomes the standard Mann iteration (1.2). The construction of fixed points of nonexpansive mappings via Halpern’s algorithm [5] has been extensively investigated recently in the current literature (see, for example, [6] and the references therein). Numerous results have been proved on Mann and Halpern’s iterations for nonexpansive mappings in Hilbert and Banach spaces (see, e.g., [6–14]).
Let E be a smooth, strictly convex and reflexive Banach space, and let J be the normalized duality mapping of E. Let C be a nonempty, closed and convex subset of E. The generalized projection from E onto C is denoted by
for all , where for all .
Following Kohsaka and Takahashi [15, 16] (see also [16–21]), a mapping is said to be nonspreading if
for all , where , . Observe that if E is a real Hilbert space, then J is the identity mapping and .
Recently, Kurakawa and Takahashi [17] proved the following fixed point theorem for nonspreading mappings in a Hilbert space.
Theorem 1.1 [17]
Let C be a nonempty, closed and convex subset of a real Hilbert space H. Let be a nonspreading mapping with . Suppose that is a sequence generated by , and
where , and . Then converges strongly to , where is the metric projection of H onto .
Kurokawa and Takahashi studied strong convergence theorems for nonspreading mappings and posed the following open problem in their final remark of [17].
Question 1.1 Is there any strong convergence theorem of Halpern type for nonspreading mappings in a Hilbert space H?
By using the iterative schemes proposed by Moudafi [8], Iemoto and Takahashi [18] studied the approximation of common fixed points of nonexpansive mappings and nonspreading mappings in a Hilbert space and proved the following strong convergence theorem.
Theorem 1.2 Let C be a nonempty, closed and convex subset of a Hilbert space H. Let be a nonspreading mapping, and let be a nonexpansive mapping such that . Define a sequence as follows:
for all , where . Then the following hold:
-
(i)
If and , then converges weakly to ;
-
(ii)
If and , then converges weakly to ;
-
(iii)
If and , then converges weakly to .
Now, we are in a position to introduce the following new class of nonspreading-type mappings in a Banach space.
Definition 1.1 Let E be a real Banach space. A mapping is said to be asymptotically nonspreading (for short ANS) if
for all and . The mapping T is called nonspreading if
for all , where is the domain of T and J is the normalized duality mapping of E.
Example 1.1 Let be defined by
Then T is an asymptotically nonspreading mapping with . Indeed, for any and , we have , , for all . We define the function by
Then we have
where is the derivative of f at x. This implies that
Observe now that
On the other hand, for any , we have
The other cases can be verified similarly. It is worth mentioning that T is neither nonexpansive nor continuous.
In this paper, we first introduce a new class of asymptotically nonspreading mappings and establish weak and strong convergence theorems of the iterative sequences generated by these mappings in a real Banach space. We modify Mann and Halpern’s iterations for finding a fixed point of an asymptotically nonspreading mapping and provide an affirmative answer to Question 1.1. Furthermore, we study the approximation of common fixed points of asymptotically nonspreading mappings and nonexpansive mappings and derive a strong convergence theorem by a new hybrid method for these mappings. Our results improve and generalize many known results in the current literature; see, for example, [17].
2 Preliminaries
In this section, we collect some lemmas which will be used in the proofs for the main results in the next sections.
Let C and D be nonempty subsets of a real Banach space E with . A mapping is said to be sunny if
for each and . A mapping is said to be a retraction if for each .
Lemma 2.1 [22]
Let C and D be nonempty subsets of a real Banach space E with , and let be a retraction from C into D. Then is sunny and nonexpansive if and only if
for all and , where J is the normalized duality mapping of E.
Lemma 2.2 [22]
Let E be a real Banach space and J be the normalized duality mapping of E. Then
for all .
Proposition 2.1 [19]
Let C be a nonempty, closed and convex subset of a real Hilbert space H, and let be a nonspreading mapping. If , then it is closed and convex.
Let C be a nonempty, closed and convex subset of a Banach space E, and let be a bounded sequence in E. For any , we set
The asymptotic radius of relative to C is defined by
The asymptotic center of relative to C is the set
It is well known that, in a uniformly convex Banach space E, consists of exactly one point; see [3, 22].
Lemma 2.3 [23]
Let be a sequence of nonnegative real numbers satisfying the inequality
where and satisfy the conditions:
-
(i)
and or, equivalently, ;
-
(ii)
, or
(ii′) .
Then .
Lemma 2.4 [24]
Let be a sequence of real numbers such that there exists a subsequence of such that for all . Then there exists a subsequence such that and the following properties are satisfied by all (sufficiently large) numbers :
In fact, .
Let E be a uniformly convex Banach space and , . Then there exists a continuous, strictly increasing and convex function with such that
for all and all with .
3 Fixed point theorems
In the following, we present the existence theorems of fixed points of asymptotically nonspreading mappings in a Banach space.
Theorem 3.1 Let C be a nonempty, closed and convex subset of a uniformly convex Banach space E. Let be an asymptotically nonspreading mapping. Then the following assertions are equivalent.
-
(1)
The fixed point set .
-
(2)
There exists a bounded sequence in C such that .
Proof The implication (1) ⟹ (2) is obvious. For the converse implication, suppose that there exists a bounded sequence in C such that . Consequently, there is a bounded subsequence of such that . Suppose . Let . Since T is an asymptotically nonspreading mapping, we obtain
This implies that
Thus we have
This means that . By the uniform convexity of E, we conclude that , which completes the proof. □
The following result is an immediate consequence of Theorem 3.1.
Proposition 3.1 (Demiclosedness principle)
Let C be a nonempty, closed and convex subset of a real uniformly convex Banach space E. Suppose that is an asymptotically nonspreading mapping with . If is a sequence in C that converges weakly to x and if converges strongly to 0, then .
Theorem 3.2 Let C be a nonempty, closed and convex subset of a uniformly convex Banach space E. Let be an asymptotically nonspreading mapping which is uniformly asymptotically regular, i.e., for all . Then the following assertions are equivalent.
-
(1)
The fixed point set .
-
(2)
There exists such that the sequence is bounded.
Proof The implication (1) ⟹ (2) is obvious. For the converse implication, suppose that there exists such that the sequence is bounded. Setting for all , the uniformly asymptotical regularity of T assures that
Since is bounded, in view of Theorem 3.1, we conclude that , which completes the proof. □
Theorem 3.3 Let C be a nonempty, closed and convex subset of a real Hilbert space H. Let be a nonspreading mapping. Then the following assertions are equivalent.
-
(1)
The fixed point set .
-
(2)
There exists such that the sequence is bounded.
Proof It is obvious that (1) implies (2). Now, suppose that there exists such that the sequence is bounded. Put and for all . Continuing the same process as in the proof of Theorem 3.1 in [17], we conclude that as , which completes the proof. □
4 Weak and strong convergence theorems
In this section, we prove weak and strong convergence theorems for asymptotically nonspreading mappings in a Banach space.
Lemma 4.1 Let C be a nonempty, closed and convex subset of a real Banach space E. Let be an asymptotically nonspreading mapping. Let be a sequence in C such that and as . Then for all .
Proof We divide the proof into several steps.
Step 1. We claim that the following statements hold:
-
(a)
;
-
(b)
;
-
(c)
.
Since T is an asymptotically nonspreading mapping, we obtain
Due to the boundedness of , we deduce that
Observe now that
Thus we have
This implies that
as .
Step 2. We prove the following assertions:
-
(d)
;
-
(e)
.
Since T is an asymptotically nonspreading mapping, we get
Due to the boundedness of and in view of Step 1(c), we deduce that
Observe now that
as .
Step 3. We show that for all .
To this end, we apply the principle of mathematical induction. In view of Step 2(e), for , we deduce that . Now, suppose that for ,
We prove that
Since T is an asymptotically nonspreading mapping, we have
Thus we have for all .
By the triangle inequality, we see that for any ,
In view of Steps 2 and 3, we conclude that for all . This completes the proof. □
Theorem 4.1 Let C be a nonempty, closed and convex subset of a uniformly convex Banach space E with Opial property, and let be an asymptotically nonspreading mapping such that . Assume that is a sequence in such that . Let be a sequence in C generated by the modified Mann iteration process
Then the sequence generated by algorithm (4.1) converges weakly to an element of .
Proof Take any arbitrarily chosen. In view of Lemma 2.5, there exists a continuous, strictly increasing and convex function with such that
Since , we have from (4.2) that
This implies that exists and hence is bounded. Setting
it follows from (4.2) that
which yields that . In view of (4.1), we see that
Thus we have . Employing Proposition 3.1 and Lemma 4.1, we conclude that there exists such that as , which completes the proof. □
Theorem 4.2 Let E be a real uniformly convex Banach space which admits the weakly sequentially continuous duality mapping J, and let C be a nonempty, closed and convex subset of E. Let be an asymptotically nonspreading mapping such that . Let and be two sequences in satisfying the following control conditions:
-
(a)
;
-
(b)
;
-
(c)
.
Let be a sequence generated by
Then the sequence defined in (4.3) converges strongly to , where is the sunny nonexpansive retraction from E onto F.
Proof We divide the proof into several steps.
Since T is a quasi-nonexpansive mapping, so we have F is closed and convex. Set
Step 1. We prove that the sequences , and are bounded.
We first show that is bounded.
Let be fixed. In view of Lemma 2.5, there exists a continuous, strictly increasing and convex function with such that
This implies that
By induction, we obtain
for all . This implies that the sequence is bounded and hence the sequence is bounded. This, together with (4.3), implies that the sequences and are bounded too.
Step 2. We prove that for any ,
Let us show (4.5). For each , in view of (4.4), we obtain
This implies that
Let . It follows from (4.6) that
In view of Lemma 2.2 and (4.4), we obtain
Step 3. We prove that as .
We discuss the following two possible cases.
Case 1. If is eventually decreasing, then there exists such that the sequence is decreasing. Thus, the sequence is convergent and hence as . This, together with condition (c) and (4.7), implies that
From the properties of g, it follows that
On the other hand, we have
This implies that
By the triangle inequality, we conclude that
It follows from (4.9) that
Exploiting Lemma 4.1, (4.8) and (4.10), we obtain
Since is bounded, there exists a subsequence of such that as . In view of Proposition 3.1 and (4.11), we conclude that . This, together with Lemma 2.1, implies that
Thus we have the desired result by Lemma 2.3.
Case 2. If is not eventually decreasing, then there exists a subsequence of such that
for all . In view of Lemma 2.4, there exists a nondecreasing sequence such that
for all . This, together with (4.7), implies that
for all . Then, by conditions (a) and (c) and the properties of g, we get
By the same argument, as in Case 1, we arrive at
Next, it follows from (4.5) that
Since , we conclude that
In particular, since , we obtain
and hence
This, together with (4.13), implies that
On the other hand, we have for all , which implies that as . Thus, we have as , which completes the proof. □
Let C be a nonempty, closed and convex subset of a Hilbert space H, and let be a nonspreading mapping such that . For any real number , we define a mapping by
where I is the identity mapping on H. It is easy to verify that is a nonspreading mapping and . Therefore, in view of Proposition 2.1, is closed and convex. The following strong convergence result provides an affirmative answer to open Question 1.1 in the case where the mapping T is nonspreading. It is worth mentioning that our method of proof is different from that in [19] and can be applied in uniformly convex Banach spaces. In fact, an answer will be given for more general spaces than Hilbert spaces.
Corollary 4.1 Let E be a real uniformly convex Banach space which admits the weakly sequentially continuous duality mapping J, and let C be a nonempty, closed and convex subset of E. Let be a nonspreading mapping such that . Let be a sequence in satisfying the following control conditions:
-
(a)
;
-
(b)
.
For any real number , let be a sequence generated by
Then the sequence converges strongly to , where is the sunny nonexpansive retraction from E onto F.
Corollary 4.2 Let C be a nonempty, closed and convex subset of a Hilbert space H, and let be a nonspreading mapping such that . Let be a sequence in satisfying the following control conditions:
-
(a)
;
-
(b)
.
For any real number , let be defined by (4.15). Let be a sequence generated by
Then the sequence converges strongly to , where is the metric projection from H onto F.
Theorem 4.3 Let E be a uniformly convex Banach space which admits the weakly sequentially continuous duality mapping J, and let C be a nonempty, closed and convex subset of E. Let be an asymptotically nonspreading mapping, and let be a nonexpansive mapping such that . Let , , , be sequences in satisfying the following control conditions:
-
(a)
;
-
(b)
;
-
(c)
, ;
-
(d)
, .
Let be a sequence generated by
Then the sequence defined in (4.16) converges strongly to , where is a sunny nonexpansive retraction from E onto F.
Proof We divide the proof into several steps.
Since T is a quasi-nonexpansive mapping, so we have F is closed and convex. Set
Step 1. We prove that the sequences , , and are bounded.
We first show that is bounded.
Let be fixed. In view of Lemma 2.5, there exists a continuous, strictly increasing and convex function with such that
This implies that
By induction, we obtain
for all . This implies that the sequence is bounded and hence the sequence is bounded. This, together with (4.16), implies that the sequences , and are bounded too.
Step 2. We prove that for any ,
Let us show (4.18). For each and , in view of (4.17), we obtain
This implies that
Let . It follows from (4.19) that
In view of Lemma 2.2 and (4.17), we obtain
Step 3. We prove that as .
We discuss the following two possible cases.
Case 1. If is eventually decreasing, then there exists such that the sequence is decreasing. Thus, the sequence is convergent and hence as . This, together with condition (d) and (4.20), implies that
From the properties of g, it follows that
On the other hand, we have
This implies that
By the triangle inequality, we conclude that
It follows from (4.22) that
Since is bounded, there exists a subsequence of such that as . In view of Proposition 3.1 and (4.21), we conclude that . This, together with Lemma 2.1, implies that
Thus we have the desired result by Lemma 2.3.
Case 2. By the same method, as in the proof of Theorem 4.2, we can prove that as . This completes the proof. □
Remark 4.1 (1) Note that [[18], Theorem 4.1] is a weak convergence result and that our Theorem 4.3 is a strong convergence result. However, it is worth pointing out that the method of proving Theorem 4.3 is very different from the method of proving Theorem 4.1 of [18].
(2) In most cases, strong convergence is more desirable than weak convergence.
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Acknowledgements
The author would like to thank the editor and the referees for sincere evaluation and constructive comments which improved the paper considerably. This work was conducted with a postdoctoral fellowship at the National Sun Yat-sen University of Kaohsiung, Taiwan.
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Naraghirad, E. On an open question of Takahashi for nonspreading mappings in Banach spaces. Fixed Point Theory Appl 2013, 228 (2013). https://doi.org/10.1186/1687-1812-2013-228
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DOI: https://doi.org/10.1186/1687-1812-2013-228