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Strong convergence theorems for uniformly L-Lipschitzian asymptotically pseudocontractive mappings in Banach spaces
Journal of Inequalities and Applications volume 2013, Article number: 79 (2013)
Abstract
In this paper, we establish some strong convergence theorems of the modified Ishikawa and Mann iterations with errors of uniformly L-Lipschitzian asymptotically pseudocontractive mappings in real Banach spaces. Our results not only provide the new proof method, but also extend the known corresponding results given in (Chang in Proc. Am. Math. Soc. 129:845-853, 2001; Chang et al. in Appl. Math. Lett. 22:121-125, 2009; Goebel and Kirk in Proc. Am. Math. Soc. 35:171-174, 1972; Ofoedu in J. Math. Anal. Appl. 321:722-728, 2006; Schu in J. Math. Anal. Appl. 158:407-413, 1991). In order to get some applications of our results, we also provide specific examples.
MSC:47H09, 47H10.
1 Introduction and preliminaries
Let E be a real Banach space and let J denote the normalized duality mapping from E into defined by
for all , where denotes the dual space of E and denotes the generalized duality pairing, respectively. The normalized duality mapping J has the following properties:
-
(1)
J is an odd mapping, i.e., .
-
(2)
J is positive homogeneous, i.e., for any , .
-
(3)
J is bounded, i.e., for any bounded subset A of E, is a bounded subset of .
-
(4)
If E is smooth (or is strictly convex), then J is single-valued.
In the sequel, we denote the single-valued normalized duality mapping by j. In a Hilbert space H, j is the identity mapping.
Let D be a nonempty closed convex subset of E. A mapping is said to be asymptotically nonexpansive with a sequence and if, for all ,
for all . The mapping T is said to be asymptotically pseudocontractive with a sequence and if, for any , there exists such that
for all . Furthermore, the mapping T is said to be uniformly L-Lipschitzian if, for any , there exists a constant such that
for all .
It is easy to see that if T is an asymptotically nonexpansive mapping, then it is both asymptotically pseudocontractive and uniformly L-Lipschitzian. The converse is not true in general. Therefore, it is interesting to study these mappings in fixed point theory and its applications. In fact, the asymptotically nonexpansive and asymptotically pseudocontractive mappings were first introduced by Goebel-Kirk [1] and Schu [2], respectively. Since then, some authors have studied several iterative sequences for asymptotically nonexpansive and asymptotically pseudocontractive mappings in Hilbert spaces and Banach spaces (see [3–11]).
In [2], Schu proved the following theorem.
Theorem 1.1 [2]
Let H be a Hilbert space, K be a nonempty bounded closed convex subset of H and be a completely continuous, uniformly L-Lipschitzian and asymptotically pseudocontractive mapping with a sequence satisfying the following conditions:
(a-1) as ;
(a-2) , where .
Suppose further that and are two sequences in such that for all , where and . For any , let be an iterative sequence defined by
for all . Then converges strongly to a fixed point of T in K.
In [12], Chang extended above Theorem 1.1 to the setting of real uniformly smooth Banach spaces and proved the following.
Theorem 1.2 [12]
Let E be a uniformly smooth Banach space, K be a nonempty bounded closed convex subset of E and be an asymptotically pseudocontractive mapping with a sequence , and , where is the set of fixed points of T in K. Let be a sequence in satisfying the following conditions:
(a-1) as ;
(a-2) .
For any , let be an iterative sequence defined by
for all . If there exists a strictly increasing  function with such that
for all and , where , then as .
In [13], Ofoedu extended Theorem 1.2 in a uniformly smooth Banach space to the setting of arbitrary real Banach spaces and dropped the boundedness assumption in Theorem 1.2.
Theorem 1.3 [13]
Let E be a real Banach space, K be a nonempty closed convex subset of E and be a uniformly L-Lipschitzian asymptotically pseudocontractive mapping with a sequence , and . Let be a sequence in satisfying the following conditions:
(a-1) ;
(a-2) ;
(a-3) .
For any , let be an iterative sequence defined by
for all . If there exists a strictly increasing  function with such that
for all and . Then
-
(1)
is bounded;
-
(2)
converges strongly to .
Theorem 1.4 [13]
Let E be a real Banach space. Let K be a nonempty closed and convex subset of E, be a uniformly L-Lipschitzian asymptotically pseudocontractive mapping with a sequence with and . Let , and be real sequences in satisfying the following conditions:
(a-1) ;
(a-2) ;
(a-3) ;
(a-4) ;
(a-5) .
For arbitrary , let be a sequence in K iteratively defined by
for all , where is a bounded sequence in K. Suppose that there exists a strictly increasing continuous function with such that
for all . Then converges strongly to .
Very recently, in [14], Chang et al. proved the following theorem.
Theorem 1.5 [14]
Let E be a real Banach space. Let K be a nonempty closed convex subset of E, be two uniformly -Lipschitzian mappings with and . Let be a sequence with . Let and be two sequences in satisfying the following conditions:
(a-1) ;
(a-2) ;
(a-3) ;
(a-4) .
For any , let be an iterative sequence in K defined by
for all . If there exists a strictly increasing function with such that
for all and , , then converges strongly to .
Also, some authors have studied the modified Halpern, Mann and Ishikawa iterative sequences for nonlinear mappings in Hilbert spaces and Banach spaces (see [15, 16]).
The aim of this paper is to give some strong convergence theorems for uniformly L-Lipschitzian and asymptotically pseudo contractive mappings in Banach spaces. Our results not only include the past ones known in [3–11], but also provide quite a different proof method.
For our main purpose, we recall some concepts and lemmas.
Definition 1.6 [17]
For arbitrary , the sequence in D defined by
for all is called the modified Ishikawa iteration with errors, where , , , are four real sequences in satisfying , and , are any bounded sequences in D.
In particular, if in (1.4), then the sequence defined by
for all is called the modified Mann iteration with errors.
If in (1.4) and (1.5), then the sequence defined by
and
for all is called the modified Ishikawa iteration and the modified Mann iteration, respectively.
Lemma 1.7 [18]
Let E be a real Banach space and be a normalized duality mapping. Then
for all and .
Lemma 1.8 [19]
Let , and be three nonnegative real sequences and be a strictly increasing continuous function with satisfying the following inequality:
for all , where with and . Then as .
2 Main results
Now, we give our main results in this paper.
Theorem 2.1 Let E be a real Banach space, D be a nonempty closed convex subset of E and be a uniformly L-Lipschitzian asymptotically pseudocontractive mapping with a sequence , and . Let , , and be four real sequences in satisfying the following conditions:
(A-1) as ;
(A-2) ;
(A-3) .
For some , let be a modified Ishikawa iterative sequence with errors defined by (1.4). Suppose that there exists a strictly increasing continuous function with such that
for all , where . Then converges strongly to the fixed point q of T.
Proof Step 1. For any , is a bounded sequence.
Set . Then there exists with such that . Indeed, for any taking and , we denote . If as , then . If with , then there exists a sequence such that as with , thus there exists a positive integer such that for all . We redefine and .
Set . Then we obtain . Denote
Next, we prove that for any . If , then . Now, we assume that it holds for some n, i.e., . We prove that . Suppose that this does not hold. Then . Now, we denote
Since as , without loss of generality, let for any . Thus, we have
and
Applying Lemma 1.7 and the formulas above, we obtain
Since and as , we have as . Thus, without loss of generality, let for any . Then (2.6) implies that
which is a contradiction. Hence, , i.e., is a bounded sequence.
Step 2. We prove that as .
By Step 1, we obtain is a bounded sequence and so is . Let
Observe that
Using Lemma 1.7, (2.6) and (2.8), we have
where
Let , and . Then (2.9) leads to
Therefore, by Lemma 1.8, we obtain , i.e., as . This completes the proof. □
From Theorem 2.1, we have the following corollary.
Corollary 2.2 Let E be a real Banach space. Let D be a nonempty closed convex subset of E, be a uniformly L-Lipschitzian asymptotically pseudocontractive mapping with a sequence , and . Let and be two real sequences in satisfying the following conditions:
(A-1) as ;
(A-2) ;
(A-3) .
For some , let be a modified Mann iterative sequence with errors defined by (1.5). Suppose that there exists a strictly increasing  function with such that
for all , where . Then converges strongly to the fixed point q of T.
Proof In Theorem 2.1, letting , , we can get the convergence of the modified Mann iteration (1.5). □
Theorem 2.3 Let E be a real Banach space. Let D be a nonempty closed convex subset of E and be two uniformly -Lipschitzian mappings with . Let be a sequence with as . Let , , and be four real sequences in satisfying the following conditions:
(A-1) as ;
(A-2) ;
(A-3) , as .
For some , let be an iterative sequence with errors defined by
for all . Suppose that there exists a strictly increasing  function with such that
for all and , where . Then converges strongly to the fixed point q of .
Proof Similarly, we can obtain the result of Theorem 2.3 by using the proof method of Theorem 2.1. □
Remark 2.4 Theorem 2.1 extends, improves and unifies Theorems 3.1, 3.2, 3.3 of [13] and Theorem 3.5 of [14] in the following sense:
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(1)
The modified Mann iteration and modified Ishikawa iteration are replaced by the modified Ishikawa iteration with errors introduced by Xu [17].
-
(2)
The proof method of Theorem 2.1 is quite different from the method of [13, 14].
-
(3)
In [13], the author did not require the function Φ to be surjective. Since is an arbitrary point chosen in D, it is possible that is not well defined.
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(4)
The conditions , , in [[13], Theorem 3.1, Theorem 3.2] and [[14], Theorem 2.1] are replaced by the more general conditions as . Also, the conditions , , in [[13], Theorem 3.3] are replaced by as and of Corollary 2.2.
Remark 2.5 A mapping T is said to be weak uniformly Lipschitzian if there exists a constant such that
for all , and . Then, using the same methods, we can also prove that Theorem 2.1 holds for the more general class of weak uniformly Lipschitzian asymptotically pseudocontractive mappings. In practical application, it can be seen from the following example.
Example 2.6 Let be the set of real numbers with the usual norm and . Define a mapping by
for all . Then T has a fixed point and T is a strictly monotone increasing mapping. Thus, for any , which implies that . Define a function by . Then Φ is a strictly increasing continuous function with . For all and , if and , then we obtain
and
Therefore, T is weakly uniform Lipschitzian and satisfies (2.1) of Theorem 2.1.
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Acknowledgements
The authors are grateful to Professor Yeol-Je Cho for valuable suggestions which helped to improve the manuscript. The first author was supported by Hebei Provincial Natural Science Foundation (Grant No. A2011210033).
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Xue, Z., Lv, G. Strong convergence theorems for uniformly L-Lipschitzian asymptotically pseudocontractive mappings in Banach spaces. J Inequal Appl 2013, 79 (2013). https://doi.org/10.1186/1029-242X-2013-79
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DOI: https://doi.org/10.1186/1029-242X-2013-79