- Research Article
- Open Access
Strong Convergence of a New Iteration for a Finite Family of Accretive Operators
© L.-G. Hu and J.-P.Wang. 2009
- Received: 9 March 2009
- Accepted: 17 May 2009
- Published: 16 June 2009
The viscosity approximation methods are employed to establish strong convergence of the modified Mann iteration scheme to a common zero of a finite family of accretive operators on a strictly convex Banach space with uniformly Gâteaux differentiable norm. Our work improves and extends various results existing in the current literature.
- Banach Space
- Nonexpansive Mapping
- Strong Convergence
- Nonempty Closed Convex Subset
- Convex Banach Space
An operator is said to be accretive if , for all , and . If is accretive and is identity mapping, then we define, for each , a nonexpansive single-valued mapping by , which is called the resolvent of . we also know that for an accretive operator , , where and . An accretive operator is said to be -accretive, if for all . If is a Hilbert space, then accretive operator is monotone operator. There are many papers throughout literature dealing with the solution of ( ) by utilizing certain iterative sequence (see [1–3, 8–10, 13, 16, 20]).
where and , for some , satisfy the following conditions:
(C3) , and
They proved that the iterative sequence converges strongly to a zero of .
where with , , for , , and satisfies the conditions: (C1), (C2), (C3), or ( ). ).
They proved that the sequence converges strongly to a common zero of .
where with , for , and , and . The iterative sequence (1.7) is a natural generalization of all the above mentioned iterative sequences.
(i)In contrast to the iterations (1.3)–(1.5), the convex composition of the iteration (1.7) deals with only instead of and .
converges weakly to a zero of . However, the Mann iteration scheme has only weak convergence for nonexpansive mappings even in a Hilbert space (see ).
Our main purpose is to prove strong convergence theorems for a finite family of accretive operators on a strictly convex Banach space with uniformly G teaux differentiable norm by using viscosity approximation methods. Our theorems extend the comparable results in the following three aspects.
(1)In contrast to weak convergence results on a Hilbert Space in , strong convergence of the iterative sequence is obtained in the general setup of a Banach space.
(3)A single mapping of the results in  is replaced by a finite family of mappings.
exists for each , where . The norm of is uniformly G teaux differentiable if for each , the limit is attained uniformly for . The norm of is uniformly Fréchet differentiable ( is also called uniformly smooth) if the limit is attained uniformly for each . It is well known that if is uniformly G teaux differentiable norm, then the duality mapping is single-valued and uniformly continuous on each bounded subset of .
A Banach space is called strictly convex if for , , and , we have for , and for . In a strictly convex Banach space , we have that if , for , , and , then .
Lemma 2.1 (The Resolvent Identity).
for each . In general, we use instead of . Let with , and let be a Banach limit on . Then . Further, we know the following result.
Let be a nonempty closed convex subset of a Banach space with uniformly G teaux differentiable norm. Assume that is a bounded sequence in . Let , and let a Banach limit. Then if and only if , .
Let be a closed convex and, let a mapping of onto . Then is said to be sunny [12, 13] if for all and . A mapping of onto is said to be retraction if ; If a mapping is a retraction then for any , the range of . A subset of is said to be a sunny nonexpansive retraction of if there exists a sunny nonexpansive retraction of onto , and it is said to be a nonexpansive retraction of if there exists a nonexpansive retraction of onto . In a smooth Banach space , it is known ([5, Page 48]) that is a sunny nonexpansive retraction if and only if the following condition holds: , and .
Lemma 2.3 (see ).
Lemma 2.5 (see ).
where is a sequence in and is a sequence in satisfying the following conditions:
(ii) or .
Lemma 2.6 ().
Let be real numbers in with and , where and . Then is nonexpansive and .
For the sake of convenience, we list the assumptions to be used in this paper as follows.
(i) is a strictly convex Banach space which has uniformly G teaux differentiable norm, and is a nonempty closed convex subset of which has the fixed point property for nonexpansive mappings.
(ii)The real sequence satisfies the conditions: (C1). and (C2). .
Thus the net is well defined.
and there exists a subsequence which is still denoted by such that .
for all .
In addition, if is a uniformly smooth Banach space in Theorem 3.1 and we define , then we obtain from Theorem 3.1 and [19, Theorem 4.1] that the net converges strongly to , as , where and is a sunny nonexpansive retraction of onto .
Then the sequence converges strongly to , where is the unique solution of a variational inequality .
From (3.47), (3.48), (C1), (C2), and , it follows that and . Consequently applying Lemma 2.5 to (3.50), we conclude that .
If we take , for all , in the iteration (1.7), then, from Theorem 3.3, we have what follows
where with , for , and . Then the sequence converges strongly to .
Theorem 3.3 and Corollary 3.4 prove strong convergence results of the new iterative sequences which are different from the iterative sequences (1.4) and (1.5). In contrast to , the restriction: (C3). or is removed.
If we consider the case of an accretive operator , then as a direct consequence of Theorem 3.1 and Theorem 3.3, we have the following corollaries.
Corollary 3.6 ([3, Theorem 3.1]).
where . If , for some , then converges strongly to , as , where is the unique solution of a variational inequality:
where . Then the sequence converges strongly to , where is the unique solution of a variational inequality .
(i)Corollary 3.7 describes strong convergence result in Banach spaces for a modification of Mann iteration scheme in contrast to the weak convergence result on Hilbert spaces given in [9, Theorem 3].
(ii)In contrast to the result [10, Theorem 4.2], the iterative sequence in Corollary 3.7 is different from the iteration (1.3), and the conditions and are not required.
The work was supported partly by NNSF of China (no. 60872095), the NSF of Zhejiang Province (no. Y606093), K. C. Wong Magna Fund of NIngbo University, NIngbo Natural Science Foundation (no. 2008A610018), and Subject Foundation of Ningbo University (no. XK109050).
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