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Strong convergence theorems for two total asymptotically nonexpansive nonself mappings in Banach spaces
Fixed Point Theory and Applications volume 2013, Article number: 90 (2013)
Abstract
In this paper, we define and study the convergence theorems of a new two-steps iterative scheme for two total asymptotically nonexpansive nonself-mappings in Banach spaces. The results of this paper can be viewed as an improvement and extension of the corresponding results of (Shahzad in Nonlinear Anal. 61:1031-1039, 2005; Thianwan in Thai J. Math. 6:27-38, 2008; Ozdemir et al. in Discrete Dyn. Nat. Soc. 2010:307245, 2010) and all the others.
MSC:47H09, 47H10, 46B20.
1 Introduction
Let E be a real normed space and K be a nonempty subset of E. A mapping is called nonexpansive if for all . A mapping is called asymptotically nonexpansive if there exists a sequence with such that
for all and . Goebel and Kirk [1] proved that if K is a nonempty closed and bounded subset of a uniformly convex Banach space, then every asymptotically nonexpansive self-mapping has a fixed point.
A mapping T is said to be asymptotically nonexpansive in the intermediate sense (see, e.g., [2]) if it is continuous and the following inequality holds:
If and (1.2) holds for all , , then T is called asymptotically quasi-nonexpansive in the intermediate sense. Observe that if we define
then as and (1.2) is reduced to
The class of mappings which are asymptotically nonexpansive in the intermediate sense was introduced by Bruck et al. [2]. It is known in [3] that if K is a nonempty closed convex bounded subset of a uniformly convex Banach space E and T is a self-mapping of K which is asymptotically nonexpansive in the intermediate sense, then T has a fixed point. It is worth mentioning that the class of mappings which are asymptotically nonexpansive in the intermediate sense contains, properly, the class of asymptotically nonexpansive mappings.
Albert et al. [4] introduced a more general class of asymptotically nonexpansive mappings called total asymptotically nonexpansive mappings and studied methods of approximation of fixed points of mappings belonging to this class.
Definition 1 A mapping is said to be total asymptotically nonexpansive if there exist nonnegative real sequences and , with as and strictly increasing continuous function with such that for all ,
Remark 1 If , then (1.5) is reduced to
In addition, if for all , then total asymptotically nonexpansive mappings coincide with asymptotically nonexpansive mappings. If and for all , we obtain from (1.5) the class of mappings that includes the class of nonexpansive mappings. If and , where for all , then (1.5) is reduced to (1.4) which has been studied as mappings which are asymptotically nonexpansive in the intermediate sense.
Iterative techniques for nonexpansive and asymptotically nonexpansive mappings in Banach space including Mann type and Ishikawa type iteration processes have been studied extensively by various authors; see [1, 5–11]. However, if the domain of T, , is a proper subset of E (and this is the case in several applications) and T maps into E, then the iteration processes of Mann type and Ishikawa type have been studied by the authors mentioned above, their modifications introduced may fail to be well defined.
A subset K of E is said to be a retract of E if there exists a continuous map such that , for all . Every closed convex subset of a uniformly convex Banach space is a retract. A map is said to be a retraction if . It follows that if a map P is a retraction, then for all , the range of P.
The concept of asymptotically nonexpansive nonself-mappings was firstly introduced by Chidume et al. [7] as the generalization of asymptotically nonexpansive self-mappings. The asymptotically nonexpansive nonself-mapping is defined as follows:
Let K be a nonempty subset of real normed linear space E. Let be the nonexpansive retraction of E onto K. A nonself mapping is called asymptotically nonexpansive if there exists sequence , () such that
Chidume et al. [12] introduce a more general class of total asymptotically nonexpansive mappings as the generalization of asymptotically nonexpansive nonself-mappings.
Definition 2 Let K be a nonempty closed and convex subset of E. Let be the nonexpansive retraction of E onto K. A nonself map is said to be total asymptotically nonexpansive if there exist sequences , in with as and a strictly increasing continuous function with such that for all ,
Proposition 1 Let K be a nonempty closed and convex subset of E which is also a nonexpansive retraction of E and be two total nonself asymptotically nonexpansive mappings. Then there exist nonnegative real sequences , in with as and a strictly increasing continuous function with such that for all ,
for .
Proof Since is a total nonself asymptotically nonexpansive mappings for , there exist nonnegative real sequences , , with as and strictly increasing continuous function with such that for all ,
Setting
then we get nonnegative real sequences , , with as and strictly increasing continuous function with such that
for all and each . □
In [7], Chidume et al. study the following iterative sequence:
to approximate some fixed point of T under suitable conditions. In [13], Wang generalized the iteration process (1.10) as follows:
where are asymptotically nonexpansive nonself-mappings and , are sequences in . They studied the strong and weak convergence of the iterative scheme (1.11) under proper conditions. Meanwhile, the results of [13] generalized the results of [7].
In [14], Shahzad studied the following iterative sequence:
where is a nonexpansive nonself-mapping and K is a nonempty closed convex nonexpansive retract of a real uniformly convex Banach space E with P nonexpansive retraction.
Recently, Thianwan [15] generalized the iteration process (1.12) as follows:
where , , , , , are appropriate sequences in and , are bounded sequences in K. He proved weak and strong convergence theorems for nonexpansive nonself-mappings in uniformly convex Banach spaces.
Inspired and motivated by this facts, we define and study the convergence theorems of two steps iterative sequences for total asymptotically nonexpansive nonself-mappings in Banach spaces. The results of this paper can be viewed as an improvement and extension of the corresponding results of [14–16] and others. The scheme (1.14) is defined as follows.
Let E be a normed space, K a nonempty convex subset of the nonexpansive retraction of E onto K and be two total asymptotically nonexpansive nonself-mappings. Then, for given and , we define the sequence by the iterative scheme:
where , , , are appropriate sequences in . Clearly, the iterative scheme (1.14) is the generalization of the iterative schemes (1.11), (1.12) and (1.13).
Under suitable conditions, the sequence defined by (1.14) can also be generalized to iterative sequence with errors. Thus, all the results proved in this paper can also be proved for the iterative process with errors. In this case, our main iterative process (1.14) looks like
where , , , , , are appropriate sequences in satisfying and , are bounded sequences in K. Observe that the iterative process (1.15) with errors is reduced to the iterative process (1.14) when .
The purpose of this paper is to define and study the strong convergence theorems of the new iterations for two total asymptotically nonexpansive nonself-mappings in Banach spaces.
2 Preliminaries
Now, we recall the well-known concepts and results.
Let E be a Banach space with dimension . The modulus of E is the function defined by
A Banach space E is uniformly convex if and only if for all .
The mapping with is said to satisfy condition (A) [17] if there is a nondecreasing function with , for all such that
for all , where .
Two mappings are said to satisfy condition (A′) [18] if there is a nondecreasing function with , for all such that
for all where .
Note that condition (A′) reduces to condition (A) when and hence is more general than the demicompactness of and [17]. A mapping is called: (1) demicompact if any bounded sequence in K such that converges has a convergent subsequence; (2) semicompact (or hemicompact) if any bounded sequence in K such that as has a convergent subsequence. Every demicompact mapping is semicompact but the converse is not true in general.
Senter and Dotson [17] have approximated fixed points of a nonexpansive mapping T by Mann iterates, whereas Maiti and Ghosh [18] and Tan and Xu [8] have approximated the fixed points using Ishikawa iterates under the condition (A) of Senter and Dotson [17]. Tan and Xu [8] pointed out that condition (A) is weaker than the compactness of K. We shall use condition (A′) instead of compactness of K to study the strong convergence of defined in (1.14).
In the sequel, we need the following useful known lemmas to prove our main results.
Lemma 1 [8]
Let , and be sequences of nonnegative real numbers satisfying the inequality
If and , then
-
(i)
exists;
-
(ii)
In particular, if has a subsequence which converges strongly to zero, then .
Lemma 2 [19]
Let and be two fixed numbers and E a Banach space. Then E is uniformly convex if and only if there exists a continuous, strictly increasing and convex function with such that
for all and , where .
3 Main results
We shall make use of the following lemmas.
Lemma 3 Let E be a real Banach space, let K be a nonempty closed convex subset of E which is also a nonexpansive retract of E and be two total asymptotically nonexpansive nonself-mappings with sequences , defined by (1.9) such that , and . Assume that there exist such that for all , . Starting from an arbitrary , define the sequence by recursion (1.14). Then, the sequence is bounded and exists, .
Proof Let . Set and .
Firstly, we note that
Note that Ï• is an increasing function, it follows that whenever and (by hypothesis) if . In either case, we have
for some , . Hence, from (3.1) and (3.2), we have
for some constant . From (1.14) and (3.3), we have
for some constant . Similarly, we have
for some constant . Substituting (3.4) into (3.5)
for some constant . It follows from (1.14) and (3.6) that
for some constant . Since , , by Lemma 1, we get exists. This completes the proof. □
Theorem 1 Let K be a nonempty convex subset of a real Banach space E which is also a nonexpansive retract of E and be two continuous total asymptotically nonexpansive nonself-mappings with sequences , defined by (1.9) such that , and . Assume that there exist such that for all , . Starting from an arbitrary , define the sequence by recursion (1.14). Then, the sequence converges strongly to a common fixed point of , if and only if , where , .
Proof The necessity is obvious. Indeed, if (), then
Now we prove sufficiency. It follows from (3.7) that for , we have
where . Since is bounded and , , we have . Hence, (3.8) implies
that is
by Lemma 1(i), it follows from (3.9) that we get exists. Noticing , it follows from (3.9) and Lemma 1(ii) that we have .
Now, we prove that is a Cauchy sequence in E. In fact, from (3.8) that for any , any and any , we have that
So by (3.10), we have that
By the arbitrariness of and from (3.11), we have
For any given , there exists a positive integer , such that for any , and , we have and so for any
This show that is a Cauchy sequence in K. Since K is a closed subset of E and so it is complete. Hence, there exists a such that as .
Finally, we have to prove that . By contradiction, we assume that p is not in . Since ℱ is a closed set, . Thus for all , we have that
This implies that
From (3.14) and (3.15) (), we have that . This is a contradiction. Thus . This completes the proof. □
On the lines similar to this theorem, we can also prove the following theorem which addresses the error terms.
Theorem 2 Let K be a nonempty convex subset of a real Banach space E which is also a nonexpansive retract of E and be two continuous total asymptotically nonexpansive nonself-mappings with sequences , defined by (1.9) such that , and . Assume that there exist such that for all , . Starting from an arbitrary , define the sequence by recursion (1.15). Suppose that , are bounded sequences in K such that , . Then, the sequence converges strongly to a common fixed point of , if and only if , where , .
Lemma 4 Let K be a nonempty convex subset of a uniformly convex Banach space E which is also a nonexpansive retract of E and be two total asymptotically nonexpansive nonself-mappings with sequences , defined by (1.9) such that , and . Assume that there exist such that for all , . Starting from an arbitrary , define the sequence by recursion (1.14). Suppose that
-
(i)
and , and
-
(ii)
and .
Then for .
Proof Let . Then by Lemma 3, exists. Let . If , then by the continuity of and the conclusion follows. Now suppose . Set and . Since is bounded, there exists an such that for all . Using Lemma 2, we have, for some constant , that
It follows from (1.14), Lemma 2, (3.2) and (3.16) that for some constant ,
Using Lemma 2 and (3.17), we have, for some constant , that
Similarly, it follows from (1.14), Lemma 2, (3.2) and (3.18) that for some constant ,
It follows from (3.19) that
and
Since , and , there exists and such that , and for all . This implies by (3.20) that
for all . It follows from (3.24) that , we have
Then and therefore . Since g is strictly increasing and continuous with , we have
By a similar method, together with (3.21), (3.22) and (3.23), it can be show that
It follows from (1.14) that
This together with (3.26) implies that
It follows from (3.26) and (3.27) that
That is . The proof is completed. □
Theorem 3 Let K be a nonempty convex subset of a real Banach space E which is also a nonexpansive retract of E and be two continuous total asymptotically nonexpansive nonself-mappings with sequences , defined by (1.9) such that , and . Assume that there exist such that for all , ; and that one of , is demicompact (without loss of generality, we assume is demicompact). Starting from an arbitrary , define the sequence by recursion (1.14). Suppose that
-
(i)
and , and
-
(ii)
and .
Then the sequence converges strongly to some common fixed points of and .
Proof It follows from (1.14) and (3.26) that
It follows Lemma 4 and (3.29) that
Since is continuous and P is nonexpansive retraction, it follows from (3.30) that for
Hence, by Lemma 4 and (3.31), we have
Since is demicompact, from the fact that and is bounded, there exists a subsequence of that converges strongly to some as . Hence, it follows from (3.32) that , as and it follows from (3.31) and is continuous that
Observe that
Taking limit as and using the fact that Lemma 4 and (3.33) we have that and so . Also we get
Taking limit as and using the fact that Lemma 4 and (3.33) we have that and so . Therefore, we obtain that . It follows from (3.7), Lemma 1 and that converges strongly to . This completes the proof. □
Theorem 4 Let K be a nonempty convex subset of a real Banach space E which is also a nonexpansive retract of E and be two continuous total asymptotically nonexpansive nonself-mappings with sequences , defined by (1.9) such that , and satisfying the condition (A′). Assume that there exist such that for all , and . Starting from an arbitrary , define the sequence by recursion (1.14). Suppose that
-
(i)
and , and
-
(ii)
and .
Then the sequence converges strongly to some common fixed points of and .
Proof By Lemma 3, we see that and so, exists for all . Also, by (3.32), for . It follows from condition (A′) that
That is,
Since is a nondecreasing function satisfying , for all , therefore, we have
Now we can take a subsequence of and sequence such that for all integers . Using the proof method of Tan and Xu [8], we have
and hence
We get that is a Cauchy sequence in ℱ and so it converges. Let . Since ℱ is closed, therefore, and then . As exists, . This completes the proof. □
In a way similar to the above, we can also prove the results involving error terms as follows.
Theorem 5 Let K be a nonempty convex subset of a real Banach space E which is also a nonexpansive retract of E and be two continuous total asymptotically nonexpansive nonself-mappings with sequences , defined by (1.9) such that , and . Assume that there exist such that for all , ; and that one of , is demicompact (without loss of generality, we assume is demicompact). Starting from an arbitrary , define the sequence by recursion (1.15). Suppose that , are bounded sequences in K such that , . Suppose that
-
(i)
and , and
-
(ii)
and .
Then the sequence converges strongly to some common fixed points of and .
Theorem 6 Let K be a nonempty convex subset of a real Banach space E which is also a nonexpansive retract of E and be two continuous total asymptotically nonexpansive nonself-mappings with sequences , defined by (1.9) such that , and satisfying the condition (A′). Assume that there exist such that for all , and . Starting from an arbitrary , define the sequence by recursion (1.15). Suppose that , are bounded sequences in K such that , . Suppose that
-
(i)
and , and
-
(ii)
and .
Then the sequence converges strongly to some common fixed points of and .
Remark 2 If and are asymptotically nonexpansive mappings, then and so that the assumption that there exist such that for all , in the above theorems is no longer needed. Hence, the results in the above theorems also hold for asymptotically nonexpansive mappings. Thus, the results in this paper improvement and extension the corresponding results of [14, 15] and [16] from asymptotically nonexpansive (or nonexpansive) mappings to total asymptotically nonexpansive nonself-mappings under general conditions.
Example 1 Let E is the real line with the usual norm , and P be the identity mapping. Assume that and for . Let Ï• be a strictly increasing continuous function such that with . Let and be two nonnegative real sequences defined by and , for all ( and ). Since for , we have
For all , we obtain
for all  , and with as and so is a total asymptotically nonexpansive mapping. Also, for , we have
For all , we obtain
for all  , and with as and so is a total asymptotically nonexpansive mapping. Clearly, . Set
for . Thus, the conditions of Theorem 2 are fulfilled. Therefore, we can invoke Theorem 2 to demonstrate that the iterative sequence defined by (1.15) converges strongly to 0.
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Acknowledgements
Dedicated to Professor Hari M Srivastava.
The authors are very grateful to the referees for their careful reading of manuscript, valuable comments and suggestions.
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Kiziltunc, H., Yolacan, E. Strong convergence theorems for two total asymptotically nonexpansive nonself mappings in Banach spaces. Fixed Point Theory Appl 2013, 90 (2013). https://doi.org/10.1186/1687-1812-2013-90
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DOI: https://doi.org/10.1186/1687-1812-2013-90