Iterative Algorithms with Variable Coefficients for Asymptotically Strict Pseudocontractions
© The Author(s). 2010
Received: 8 October 2009
Accepted: 22 January 2010
Published: 28 January 2010
We introduce and study some new CQ-type iterative algorithms with variable coefficients for asymptotically strict pseudocontractions in real Hilbert spaces. General results for asymptotically strict pseudocontractions are established. The main result extends the previous results.
Let be a real Hilbert space, a nonempty closed convex subset of , a self-mapping of and
Recall that a mapping is called to be nonexpansive if
is called to be asymptotically nonexpansive  if there exists a sequence with and such that
is called to be an asymptotically -strict pseudocontraction, if there exist such that
for all and all integers
As , asymptotically -strict pseudocontraction is asymptotically nonexpansive.
In , Nakajo and Takahashi studied the iterative approximation of fixed points of nonexpansive mappings and proved the following strong convergence theorem.
Theorem 1 A.
where is the metric projection from C onto and is chosen so that Then, converges strongly to , where is the metric projection from C onto
Such algorithm in (1.4) is referred to be the (CQ) algorithm in , due to the fact that each iterate is obtained by projecting onto the intersection of the suitably constructed closed convex sets and It is known that the (CQ) algorithm in (1.4) is of independent interest, and the (CQ) algorithm has been extended to various mappings by many authors (cf., e.g., [3–11]).
Very recently, by extending the (CQ) algorithm, Takahashi et al.  studied a family of nonexpansive mappings and gave some good strong convergence theorems. Kim and Xu  extended the (CQ) algorithm to study asymptotically -strict pseudocontractions and established the following interesting result with the help of some boundedness conditions.
Theorem 1 B.
Assume that control sequence is chosen so that Then converges strongly to .
It is our purpose in this paper to try to obtain some new fixed point theorems for asymptotically strict pseudocontractions without the boundedness conditions as in Theorem B. Motivated by Nakajo and Takahashi , Takahashi et al. , and Kim and Xu , we introduce and study certain new CQ-type iterative algorithms with variable coefficients for asymptotically strict pseudocontractions in real Hilbert spaces. Our results improve essentially the corresponding results of .
2. Results and Proofs
Throughout this paper,
(i) means that converges weakly to
(ii) means that converges strongly to
(iii) , that is, the weak -limit set of
(v) is the set of nonnegative integers.
and . Then
Let be a closed convex subset of a Hilbert space and an asymptotically -strict pseudocontraction. Then
is closed and convex so that the projection is well defined.
the sequence is chosen so that , the positive real number is chosen so that , and is as in (1.3). Then converges strongly to .
We divide the proof into five steps.
We prove that is nonempty, convex and closed.
for all , , and all integers
Obviously, , that is, (2.14) holds for . Assume that for some Then, (2.13) implies that and is well defined.
By Lemma 2.1, we get In particular, for each we have This together with the definition of , the inequality (2.14) holds for . So (2.14) is true.
We prove that
We prove that
By (2.18), (2.24), and (2.26), we know that (2.25) holds.
Finally, by Lemma 2.3 and (2.25), we have . Furthermore, it follows from (2.16) and Lemma 2.2 that the sequence converges strongly to
Theorem 2.4 improves [5, Theorem ] since the condition that is satisfied and the boundedness of is dropped off.
the sequence is chosen so that and is as in (1.3). Then converges strongly to .
It is easy to see that in Theorem 2.6. Following the reasoning in the proof of Theorem 2.4 and using instead of , we deduce the conclusion of Theorem 2.6.
The authors are very grateful to the referee for his/her valuable suggestions and comments. The work was supported partly by the NSF of China (10771202), the Research Fund for Shanghai Key Laboratory for Contemporary Applied Mathematics (08DZ2271900), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (2007035805). This work is dedicated to W. Takahashi.
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