- Research
- Open access
- Published:
Convergence theorems for fixed points of uniformly continuous Φ-pseudo-contractive-type operator
Fixed Point Theory and Applications volume 2013, Article number: 321 (2013)
Abstract
Let E be a real normed linear space, let K be a nonempty convex subset of E, and let be a uniformly continuous Φ-pseudo-contractive-type mapping. It is proved that both the Mann-type and Ishikawa-type iteration schemes converge strongly to the unique fixed point of T, without requiring that the sequences associated with the schemes be bounded. Our theorems are significant improvements on the results of Chang et al. (Iterative methods for nonlinear operators in Banach spaces, 2002), those of Gu (Northeast Math. J. 17(3):340-346, 2001), and those of a host of other authors.
MSC:47H06, 47H09, 47J05, 47J25.
1 Introduction
Let E be a real normed linear space. A mapping defined by
where denotes the duality pairing between the elements of E and those of , is called the normalized duality map on E. If is strictly convex, then J is single-valued (see, e.g., [1] or [2]). An operator is called strongly accretive if there exists some such that for each , there exists such that
The mapping A is said to be strongly ϕ-accretive if there exists a strictly increasing function with such that for any , there exists satisfying
The mapping A is called generalized Φ-accretive if there exists a strictly increasing function with such that for any , there exists satisfying
It is well known that the class of generalized Φ-accretive mappings includes the class of strongly ϕ-accretive operators as a special case (one sets for all ).
Let . The mapping A is called strongly quasi-accretive if there exists such that for all , , there exists satisfying
A is called strongly ϕ-quasi-accretive if there exists a strictly increasing function with such that for all , , there exists satisfying
Finally, A is called generalized Φ-quasi-accretive if there exists a strictly increasing function with such that for all , , there exists satisfying
A mapping is called strongly pseudo-contractive if for all , the following inequality holds:
for all and some . If in inequality (1.7), then T is called pseudo-contractive. The relation between the class of accretive-type mappings and those of pseudo-contractive type is contained in the following proposition.
Proposition 1.1 A mapping is strongly pseudo-contractive if and only if is strongly accretive, and is strongly ϕ-pseudo-contractive if and only if is strongly ϕ-accretive. The mapping T is generalized Φ-pseudo-contractive if and only if is generalized Φ-accretive.
Proposition 1.2 If , the mapping T is strongly hemi-contractive if and only if is strongly quasi-accretive; it is ϕ-hemi-contractive if and only if is strongly ϕ-quasi-accretive; and T is generalized Φ-hemi-contractive if and only if is generalized Φ-quasi-accretive.
The class of generalized Φ-hemi-contractive mappings is the most general (among those defined above) for which T has a unique fixed point.
Numerous convergence results have been proved on iterative methods for approximating zeros of Lipschitz Φ-strongly accretive type (or fixed points of Φ-strongly pseudo-contractive type) nonlinear mappings and their stability (see, e.g., Chang et al. [3], Chidume [4, 5], Chidume and Zegeye [6], Deng and Ding [7], Shahzad and Zegeye [8] and the references contained therein). Also, many authors have proved convergence theorems under the assumption that these operators have bounded range (see, e.g., Browder and Petryshyn [9], Hirano and Huang [10] and the references contained therein).
Some of these results have been extended to uniformly continuous mappings. The most general results for uniformly continuous Φ-pseudo-contractive-type and ϕ-hemi-contractive-type mappings seem to be the following theorems (see also [2], Chapter 9).
Theorem G1 ([11], Theorem 2.1)
Let E be a real normed linear space, let K be a nonempty subset of E, and let be a uniformly continuous Φ-pseudo-contractive-type operator, i.e., there exist and a strictly increasing function , such that for all , there exists satisfying
(a) If is a fixed point of T, then , and so T has at most one fixed point in K; (b) Suppose that there exists such that both the Ishikawa iterative sequence with error and the auxiliary sequence defined by
are contained in K, where , are two sequences in E and , are two sequences in satisfying the following conditions: (i) () and ; (ii) and (). If is a bounded sequence in K, then converges strongly to . In particular, if is a fixed point of T in K, then converges strongly to .
Theorem G2 ([11], Theorem 2.2)
Let E be a real normed linear space, let K be a nonempty subset of E such that . Let be a uniformly continuous Φ-pseudo-contractive-type operator.
Let , , , be as in Theorem G1. For any given , the Ishikawa iterative sequence with errors is defined as in Theorem G1. (a) If is a fixed point of T, then , and so T has at most one fixed point in K; (b) If is a bounded sequence, then converges strongly to . In particular, if is a fixed point of T in K, then converges strongly to .
Theorem CCZ1 ([3], Theorem 7.2.1)
Let E be a real normed linear space, let K be a nonempty convex subset of E such that , and let be a uniformly continuous and ϕ-hemi-contractive mapping. Let , be two real sequences in satisfying the following conditions: (i) (); (ii) . Assume that , are two sequences in K satisfying the following conditions: for any sequences , in K with ; and as . Define the Ishikawa iterative sequence with mixed errors in K by
If is bounded, then the sequence converges strongly to the unique fixed point of T.
Theorem CCZ2 ([3], Theorem 7.2.2)
Let E be a real normed linear space, and let be a uniformly continuous and strongly ϕ-quasi-accretive mapping. Let , be two real sequences in satisfying the following conditions:
Let , be as in Theorem CCZ1. Define a mapping by for each . For an arbitrary , define the Ishikawa iterative sequence with mixed errors by
If is bounded, then the sequence converges strongly to the unique fixed point of T.
Remark 1.3 Theorems G1, G2, CCZ1 and CCZ2 are important generalizations of several results. We observe that the class of mappings considered in Theorems CCZ1 and CCZ2 is a proper subclass of the class of mappings studied in Theorems G1 and G2 in which . However, the requirement that be bounded imposed in Theorems G1 and G2 is stronger than the requirement that or be bounded imposed in Theorems CCZ1 and CCZ2, respectively.
In Theorems G1 and G2 of [11], convergence of under other conditions of the theorems is guaranteed if the sequence is bounded.
Remark 1.4 Similarly, the sequence defined in Theorem CCZ1 is guaranteed to converge if is bounded. Finally, the sequence defined in CCZ2 is guaranteed to converge if is bounded. The requirements that , and be bounded before convergence in these theorems is guaranteed is a huge constraint in any possible application of these theorems. The verifications that these sequences are bounded are, in general, very difficult.
It is our purpose in this paper to prove that, under the hypotheses of these theorems, the requirements that , and be bounded can be dispensed with. In fact, we prove that these sequences are necessarily bounded. We achieve this by using the lemma recently proved by Chidume and Chidume [12].
2 Preliminaries
In the sequel, we shall make use of the following results.
Lemma 2.1 Let E be a real normed linear space, and let be the normalized duality map. Then, for any , there exists such that the following inequality holds:
Lemma 2.2 (Chidume and Chidume, [12])
Let X and Y be real normed linear spaces, and let be uniformly continuous. For arbitrary and fixed , let
Then is bounded.
Since this lemma is new and is yet to be published, we reproduce its short proof here.
Proof By uniform continuity of T and by taking , there exists such that for all ,
For , let be arbitrary. Choose fixed such that .
Set , , , … , , , … , , . Then
By uniform continuity of T, . Furthermore,
Hence, is bounded. □
3 Main result
Theorem 3.1 Let E be a real normed linear space, let K be a nonempty subset of E, and let be a uniformly continuous Φ-pseudo-contractive-type operator. Suppose that is as defined in Theorem G1, then there exists such that if , then is bounded.
Proof We have
Let be as defined in Theorem G1 with . Define
Then from (1.8) we obtain that .
Observe that since and are bounded, for some . Let
where , . Set
and
By uniform continuity of T, there exists such that .
Define
Claim .
The proof of this claim is by induction. Clearly it holds for . Assume it holds for some , i.e., . We prove that . Suppose this is not the case. Then . This implies that .
We compute as follows:
Hence, . Now,
Hence, , a contradiction. So, is bounded. □
Corollary 3.2 (Theorem G1 ([11], Theorem 2.1))
Let E be a real normed linear space, let K be a nonempty subset of E, and let be a uniformly continuous Φ-pseudo-contractive-type operator, i.e., there exist and a strictly increasing function , such that for all , there exists satisfying
(a) If is a fixed point of T, then , and so T has at most one fixed point in K; (b) Suppose that there exists such that both the Ishikawa iterative sequence with error and the auxiliary sequence , defined by
are contained in K, where , are two sequences in E and , are two sequences in satisfying the following conditions: (i) () and ; (ii) and (). Then there exists such that if , converges strongly to . In particular, if is a fixed point of T in K, then converges strongly to .
Proof Observe that Corollary 3.2 is the same as Theorem G1 without boundedness assumption on the sequence but with the assumption that there exists such that made in Corollary 3.2. The proof that is bounded follows from Theorem 3.1. The rest of the proof of convergence is as in Theorem G1. This completes the proof. □
Theorem 3.3 Let E be a real normed linear space, let K be a nonempty convex subset of E such that , and let be a uniformly continuous and ϕ-hemi-contractive mapping. Suppose that is as defined in Theorem CCZ1 [3], then there exists such that if , is bounded.
Proof The boundedness of in Theorem 3.3 follows as in Theorem 3.1. Since is bounded, by Lemma 2.2 and the uniform continuity of T, is bounded. This with the boundedness of implies, from the relation , that is bounded. Again, by the uniform continuity of T and Lemma 2.2, is bounded. □
Corollary 3.4 (Theorem CCZ1, [3])
Let E be a real normed linear space, let K be a nonempty convex subset of E such that , and let be a uniformly continuous and ϕ-hemi-contractive mapping. Let , be two real sequences in satisfying the following conditions: (i) (); (ii) . Assume that , are two sequences in K satisfying the following conditions: for any sequences , in K with ; and as . Define the Ishikawa iterative sequence with mixed errors in K by
If is bounded, then the sequence converges strongly to the unique fixed point of T.
Proof Corollary 3.3 is the same as Theorem CCZ1 but without boundedness assumption on the sequence but with the assumption that there exists such that made in Corollary 3.3. The proof that is bounded follows from Theorem 3.3. The remaining proof of convergence is as in Theorem CCZ1. This completes the proof. □
Theorem 3.5 Let E be a real normed linear space, and be a uniformly continuous and strongly ϕ-quasi-accretive mapping. Define a mapping by for each . Suppose that is as defined in Theorem CCZ2 [3], then there exists such that if , is bounded.
Proof . Using the boundedness of and , we have that is bounded. □
Corollary 3.6 (Theorem CCZ2, [3])
Let E be a real normed linear space, and let be a uniformly continuous and strongly ϕ-quasi-accretive mapping. Let , be two real sequences in satisfying the following conditions:
Let , be as in Theorem CCZ1. Define a mapping by for each . For an arbitrary , define the Ishikawa iterative sequence with mixed errors by
If is bounded, then the sequence converges strongly to the unique fixed point of T.
Proof Corollary 3.5 is the same as Theorem CCZ2 without boundedness assumption on the sequence but with the assumption that there exists such that made in Corollary 3.5. The proof that is bounded follows from Theorem 3.5. The rest of the proof of convergence is as in Theorem CCZ2. This completes the proof. □
Remark 3.7 Theorem 3.4 shows that Theorems G1 and G2 remain valid without the requirement that be bounded. Also, it shows that Theorems CCZ1 and CCZ2 remain valid without the requirement that and be bounded, respectively.
Remark 3.8 Part of the aim of including bounded error terms in recursion formulas of Theorem 3.1 and Corollaries 3.4 and 3.6 is to illustrate the fact that if the theorems are proved without these error terms, then addition of bounded error terms in the recursion formulas leads to no generalization. The proof in the case with error terms is in general an unnecessary repetition of the proof using recursion formulas without error terms (see Chapter 9 of [2]).
Prototype An example of iteration parameters satisfying the conditions of our theorems is as follows:
References
Berinde V Lecture Notes in Mathematics 1912. In Iterative Approximation of Fixed Points. Springer, Berlin; 2007.
Chidume C Lecture Notes in Mathematics 1965. Geometric Properties of Banach Spaces and Nonlinear Iterations 2009.
Chang SS, Cho YJ, Zhou H: Iterative Methods for Nonlinear Operators in Banach Spaces. Nova Science Publishers, New York; 2002.
Chidume CE: Iterative methods for nonlinear Lipschitz pseudo-contractive operators. J. Math. Anal. Appl. 2000, 251(1):84–92. 10.1006/jmaa.2000.7021
Chidume CE: Iterative approximation of fixed point of Lipschitz pseudo-contractive operators. Proc. Am. Math. Soc. 2001, 129(8):2245–2251. 10.1090/S0002-9939-01-06078-6
Chidume CE, Zegeye H: Approximate fixed point sequences and convergence theorems for Lipschitz pseudo-contractive maps. Proc. Am. Math. Soc. 2004, 132(3):831–840. 10.1090/S0002-9939-03-07101-6
Ding XP: Iterative approximation of Lipschitz strictly pseudocontractive mappings in uniformly smooth Banach spaces. Nonlinear Anal. 1995, 24(7):981–987. 10.1016/0362-546X(94)00115-X
Shazad N, Zegeye H: On stability for ϕ -strongly pseudocontractive mappings. Nonlinear Anal. 2006, 64(2):2619–2630.
Browder FE, Petryshyn WV: Construction of fixed points of nonlinear mappings in Hilbert spaces. J. Math. Anal. Appl. 1967, 20: 197–228. 10.1016/0022-247X(67)90085-6
Hirano H, Huang Z: Convergence theorems for multi-valued Φ-hemicontractive operators and Φ-strongly accretive operators. Comput. Math. Appl. 2003, 46(10–11):1461–1471. 10.1016/S0898-1221(03)90183-0
Gu F: Convergence theorems for Φ-pseudo-contractive-type mappings in normed linear spaces. Northeast. Math. J. 2001, 17(3):340–346.
Chidume, CE, Chidume, CO: A convergence theorem for zeros of generalized Φ-quasi-accretive mappings (to appear)
Acknowledgements
The third author’s research is supported by the Ebonyi State University, Abakaliki, Nigeria under the ETF PhD Scholarship.
Author information
Authors and Affiliations
Corresponding author
Additional information
Competing interests
The authors declare that there are no competing interests.
Authors’ contributions
All authors contributed equally. All authors read and approved the final manuscript.
Rights and permissions
Open Access This article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
About this article
Cite this article
Chidume, C.E., Djitté, N. & Ezeora, J.N. Convergence theorems for fixed points of uniformly continuous Φ-pseudo-contractive-type operator. Fixed Point Theory Appl 2013, 321 (2013). https://doi.org/10.1186/1687-1812-2013-321
Received:
Accepted:
Published:
DOI: https://doi.org/10.1186/1687-1812-2013-321