Abstract
We prove that under minimal conditions the modified Mann and Ishikawa iterations converge when dealing with an asymptotically pseudocontractive map. We give an affirmative answer to the open question from C.E. Chidume and H. Zegeye, Approximate fixed point sequences and convergence theorems for asymptotically pseudocontractive mappings, J. Math. Anal. Appl., 278 (2003), 354–366.
Authors
Stefan M. Soltuz
Tiberiu Popoviciu Institute of Numerical Analysis, Romanian Academy
Keywords
Asymptotically hemicontractive map; Modified Mann; Modified Ishikawa iteration
Paper coordinates
Stefan M. Soltuz, The convergence of modified Mann-Ishikawa iterations when applied to an asymptotically pseudocontractive map, Austral. J. Math Anal. Appl., Volume 4, Issue 2, Article 16, pp. 1-8, 2007.
About this paper
Journal
The Australian Journal of Mathematical
Analysis and Applications
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Print ISSN
1449-5910
Online ISSN
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[1] S.S. CHANG, Some results for asymptotically pseudo-contractive mappings and asymptotically nonexpansive mappings, Proc. Amer. Math. Soc., 129 (2000), 845–853.
[2] S.S. CHANG, J.Y. PARK and Y.J. CHO, Iterative approximations of fixed points for asymptotically nonexpansive mappings in Banach spaces, Bull. Korean Math. Soc., 37 (2000), 109–119.
[3] C.E. CHIDUME, Convergence theorems for asymptotically pseudocontractive mappings, Nonlinear Analysis, 49 (2002), 1–11.
[4] C.E. CHIDUME and H. ZEGEYE, Approximate fixed point sequences and convergence theorems for asymptotically pseudocontractive mappings, J. Math. Anal. Appl., 278 (2003), 354–366.
[5] D.I. IGBOKWE, Iterative construction of fixed points of asymptotically pseudocontractive maps, Panamer. Math. J., 13 (2003), 83–97.
[6] S. ISHIKAWA, Fixed points by a new iteration method, Proc. Amer. Math. Soc., 44 (1974), 147-150.
[7] T. KATO, Nonlinear semigroup and evolution equations, J. Math. Soc. Japan, 19(1967), 508–520.
[8] W.R. MANN, Mean value in iteration, Proc. Amer. Math. Soc., 4 (1953), 506–510.
[9] M.O. OSILIKE, Iterative approximation of fiexd points of asymptotically demicontractive mappings, Indian J. Pure Appl. Math., 29 (1998), 1291–1300.
[10] M.O. OSILIKE and D.I. IGBBOKWE, Convergence theorems for asymptotically pseudocontractive maps, Bull. Korean Math. Soc., 39 (2002), 389–399.
[11] B.E. RHOADES and ¸STEFAN M. ¸SOLTUZ, The equivalence between the convergences of Ishikawa and Mann iterations for asymptotically pseudocontractive map, J. Math. Anal. Appl., 283 (2003), 681–688.
[12] B.K. SHARMA and D.R. SAHU, Existence and approximation results for asymptotically pseudocontractive mappings, Indian J. Pure Appl. Math., 31 (2000), 185–196.
[13] J. SCHU, Iterative construction of fixed points of asymptotically nonexpansive mappings, J. Math. Anal. Appl., 158 (1991), 407–413.
[14] H. ZHOU and J. YUTING, Approximation of fixed points of strongly pseudocontractive maps without Lipschitz assumption, Proc. Amer. Math. Soc., 125 (1997), 1705–1709.
Paper (preprint) in HTML form
THE CONVERGENCE OF MODIFIED MANN-ISHIKAWA ITERATIONS WHEN APPLIED TO AN ASYMPTOTICALLY PSEUDOCONTRACTIVE MAP
Abstract
We prove that under minimal conditions the modified Mann and Ishikawa iterations converge when dealing with an asymptotically pseudocontractive map. We give an affirmative answer to the open question from C.E. Chidume and H. Zegeye, Approximate fixed point sequences and convergence theorems for asymptotically pseudocontractive mappings, J. Math. Anal. Appl., 278 (2003), 354-366.
Received 26 January, 2007; accepted 5 September, 2007; published 30 November, 2007. Communicated by: M. Mariani
1. Introduction
Let be an arbitrary real Banach space and the normalized duality mapping given by
| (1.1) |
In [13] the following class of maps was introduced:
Definition 1.1. Let be a normed space and a subset of . A map is said to be asymptotically pseudocontractive if there exists a sequence 1 , and there exists such that
| (1.2) |
If there exists such that , by setting in (1.2) we get
| (1.3) |
such a map is called asymptotically hemicontractive.
The modified Mann iteration, (see [8]), is defined by
| (1.4) |
The modified Ishikawa iteration is defined, (see [6]), by
| (1.5) | ||||
The sequences satisfy
| (1.6) |
We shall give here the most general result concerning the convergence of Mann and Ishikawa iterations dealing with a uniformly Lipschitzian and asymptotically pseudocontractive map. Our result generalizes the main results from [3], [4], [10] and [12]. We also give an affirmative answer to the open question from [4] if the Mann or Ishikawa iteration converges when applied to an asymptotically pseudocontractive (respectively an asymptotically hemicontractive map), in more general spaces than Hilbert spaces.
2. Preliminaries
We recall the following auxiliary results.
Lemma 2.1. [7] Let be a Banach space and . Then
| (2.1) |
for all if and only if there exists such that .
Lemma 2.2 [11] Let be a nonempty subset of a Banach space and let be a map. Then the following conditions are equivalent:
(i) is an asymptotically pseudocontractive map,
(ii) for , we have
| (2.2) |
Definition 2.1. Let be a normed space and a subset of , then the map is a uniformly Lipschitzian map if for some , we have .
Lemma 2.3. [14] Let be a nonnegative sequence satisfying
| (2.3) |
where and . Then .
3. Main Result
Theorem 3.1. Let be a closed convex subset of an arbitrary Banach space and defined by (1.4) with and satisfying (1.6). Let be an asymptotically pseudocontractive (or asymptotically hemicontractive) and uniformly Lipschitzian map with self-map of . If , then the modified Mann iteration (1.4) strongly converges to the nearest fixed point of .
Proof. From (1.4) we obtain
| (3.1) | ||||
By using we observe that
| (3.2) |
From (3.1) and (3.2) we get
| (3.3) | |||
The norm of the sum of the first two terms on the right-hand side of 3.3) is equal to
| (3.4) |
Using (2.1) with
| (3.5) | ||||
we obtain
| (3.6) | |||
From (3.3) and (3.6) it follows that
| (3.7) | |||
We shall prove later the first inequality from 3.7). Supposing that 3.7) holds, we obtain
| (3.8) | |||
Also, we know that
| (3.9) | ||||
Using (1.4), 3.9 and the fact that is a uniformly Lipschitzian map, we obtain
| (3.10) | ||||
From (3.8), (3.9) and (3.10), by using , we get
| (3.11) | |||
The condition implies the existence of , such that
| (3.12) |
Condition (3.12) assures the following inequalities, ,
| (3.13) | ||||
Using (3.11) and (3.13) we observe that
| (3.14) | |||
Relations (3.11), (3.14), lead us to
| (3.15) |
Setting in (2.3) from lemma 2.3
| (3.16) | ||||
we get
| (3.17) |
We prove now the first inequality from 3.7 . Set in 3.7
| (3.18) | ||||
to obtain
| (3.19) | |||
We shall now prove (3.19) using the following relations
| (3.20) | |||
The last inequality from (3.20) is given by (2.1), that is . We further prove that
| (3.21) |
By using
| (3.22) |
we obtain
| (3.23) | ||||
The last equality is true because . Finally, we have
| (3.24) | ||||
The last equality is true because we already know that
| (3.25) |
Remark 3.1. If , then our Theorem 3.1 holds supposing condition (3.12) is satisfied.
The modified Ishikawa iteration also converges, being equivalent to the modified Mann iteration.
Theorem 3.2. [11] Let be a closed convex subset of an arbitrary Banach space and defined by (1.5) and (1.4) with and satisfying (1.6). Let be an asymptotically pseudocontractive and uniformly Lipschitzian with self-map of . Let be a fixed point of . If , then the following two assertions are equivalent:
(i) the modified Mann iteration (1.4) strongly converges to ,
(ii) the modified Ishikawa iteration (1.5) strongly converges to .
Remark 3.2. Each fixed point has its own basin of attraction. The map has no unique fixed point. The starting point is crucial for the convergence of Mann or Ishikawa iteration. For example, take , the identity map on , with . Each point of becomes a fixed point and the starting point is directly a fixed point.
Theorem 3.1 generalizes the Theorem from [3] because in [3] the set is bounded, the space is uniformly convex and and satisfy some additional conditions. We also generalize Theorem 1 from [12], because the space is smooth and the following conditions are required: and . Our Theorem 3.1 generalizes the main results from [2] and [10] because the map satisfies the following restrictive condition:
| (3.26) |
where is the modified Mann (respectively modified Ishikawa) iterations, is a fixed point and is a strictly increasing function with .
In [1] and [5] the convergence of (1.4) and (1.5) is shown, dealing with an asymptotically pseudocontractive map without being uniformly Lipschitzian. However, in [1] and [5] the assumptions are more restrictive than those from our Theorem 3.1; the Banach space is uniformly smooth, the set is bounded, respectively is bounded and the map satisfies condition (3.26).
References
[1] S.S. CHANG, Some results for asymptotically pseudo-contractive mappings and asymptotically nonexpansive mappings, Proc. Amer. Math. Soc., 129 (2000), 845-853.
[2] S.S. CHANG, J.Y. PARK and Y.J. CHO, Iterative approximations of fixed points for asymptotically nonexpansive mappings in Banach spaces, Bull. Korean Math. Soc., 37 (2000), 109-119.
[3] C.E. CHIDUME, Convergence theorems for asymptotically pseudocontractive mappings, Nonlinear Analysis, 49 (2002), 1-11.
[4] C.E. CHIDUME and H. ZEGEYE, Approximate fixed point sequences and convergence theorems for asymptotically pseudocontractive mappings, J. Math. Anal. Appl., 278 (2003), 354-366.
[5] D.I. IGBOKWE, Iterative construction of fixed points of asymptotically pseudocontractive maps, Panamer. Math. J., 13 (2003), 83-97.
[6] S. ISHIKAWA, Fixed points by a new iteration method, Proc. Amer. Math. Soc., 44 (1974), 147150.
[7] T. KATO, Nonlinear semigroup and evolution equations, J. Math. Soc. Japan, 19(1967), 508-520.
[8] W.R. MANN, Mean value in iteration, Proc. Amer. Math. Soc., 4 (1953), 506-510.
[9] M.O. OSILIKE, Iterative approximation of fiexd points of asymptotically demicontractive mappings, Indian J. Pure Appl. Math., 29 (1998), 1291-1300.
[10] M.O. OSILIKE and D.I. IGBBOKWE, Convergence theorems for asymptotically pseudocontractive maps, Bull. Korean Math. Soc., 39 (2002), 389-399.
[11] B.E. RHOADES and ŞTEFAN M. ŞOLTUZ, The equivalence between the convergences of Ishikawa and Mann iterations for asymptotically pseudocontractive map, J. Math. Anal. Appl., 283 (2003), 681-688.
[12] B.K. SHARMA and D.R. SAHU, Existence and approximation results for asymptotically pseudocontractive mappings, Indian J. Pure Appl. Math., 31 (2000), 185-196.
[13] J. SCHU, Iterative construction of fixed points of asymptotically nonexpansive mappings, J. Math. Anal. Appl., 158 (1991), 407-413.
[14] H. ZHOU and J. YUTING, Approximation of fixed points of strongly pseudocontractive maps without Lipschitz assumption, Proc. Amer. Math. Soc., 125 (1997), 1705-1709.
