- Research
- Open access
- Published:
A strong convergence theorem for fixed points of generalized asymptotically quasi-ϕ-nonexpansive mappings
Fixed Point Theory and Applications volume 2013, Article number: 279 (2013)
Abstract
The purpose of this paper is to investigate a hybrid projection algorithm for a pair of generalized asymptotically quasi-ϕ-nonexpansive mappings. Strong convergence of the purposed algorithm is obtained in a uniformly smooth and uniformly convex Banach space.
MSC:47H09, 47J25.
1 Introduction
The theory of iterative algorithms is a popular research topic of common interest in two areas of nonlinear analysis and optimization. Applications of iterative algorithms are found in a wide range of areas, including economics, image recovery, optimization, signal processing and a lot of real world applications; see [1–22] and the references therein. Many well-known problems can be studied by using algorithms which are iterative in their nature. As an example, in computer tomography with limited data, each piece of information implies the existence of a convex set in which the required solution lies. The problem of finding a point in the intersection , where is some positive integer, is then of crucial interest, and it cannot be usually solved directly. Therefore, an iterative algorithm must be used to approximate such a point.
The purpose of this paper is to investigate a hybrid projection algorithm for a pair of generalized asymptotically quasi-ϕ-nonexpansive mappings. The organization of this paper is as follows. In Section 2, we provide some necessary preliminaries. In Section 3, a modified Halpern iterative algorithm is investigated. Strong convergence of the purposed algorithm is obtained in a uniformly convex and uniformly smooth Banach space. Some subresults are also deduced.
2 Preliminaries
Let E be a real Banach space, C be a nonempty subset of E and be a nonlinear mapping. The mapping T is said to be asymptotically regular on C if for any bounded subset K of C,
The mapping T is said to be closed if for any sequence such that and , then . A point is a fixed point of T provided . In this paper, we use to denote the fixed point set of T and use → and ⇀ to denote the strong convergence and weak convergence, respectively.
Recall that the mapping T is said to be nonexpansive if
T is said to be asymptotically nonexpansive if there exists a sequence with as such that
The class of asymptotically nonexpansive mappings was introduced by Goebel and Kirk [23] in 1972. In uniformly convex Banach spaces, they proved that if C is nonempty bounded closed and convex, then every asymptotically nonexpansive self-mapping T on C has a fixed point. Further, the fixed point set of T is closed and convex. Since 1972, a host of authors have studied the weak and strong convergence of iterative algorithms for such a class of mappings.
One of classical iterations is the Halpern iteration [24] which generates a sequence in the following manner:
where is a sequence in the interval and is a fixed element.
Since 1967, the Halpern iteration has been studied extensively by many authors; see, for example, [25–31]. It is well known that the following two restrictions
-
(C1)
;
-
(C2)
are necessary if the Halpern iterative sequence is strongly convergent for all nonexpansive self-mappings defined on C. To improve the rate of convergence of the Halpern iterative sequence, we cannot rely only on the iteration itself. Hybrid projection methods recently have been applied to solve the problem.
Martinez-Yanes and Xu [27] considered the hybrid projection algorithm for a single nonexpansive mapping in a Hilbert space. Strong convergence theorems are established under condition (C1) only imposed on the control sequence. To be more precise, they proved the following theorem.
Theorem 2.1 Let H be a real Hilbert space, C be a closed convex subset of H and be a nonexpansive mapping such that . Assume that is such that . Then the sequence defined by
converges strongly to .
Recently, some authors considered the problem of extending Theorem MYX to a Banach space. In this paper, we consider, in the framework of Banach spaces, the problem of modifying the Halpern iteration by hybrid projection algorithms such that strong convergence is available under assumption (C1) only. Before proceeding further, we give some definitions and propositions in Banach spaces first.
Let E be a Banach space with the dual . We denote by J the normalized duality mapping from E to defined by
where denotes the generalized duality pairing.
A Banach space E is said to be strictly convex if for all with and . It is said to be uniformly convex if for any two sequences and in E such that and . Let be the unit sphere of E. Then the Banach space E is said to be smooth provided
exists for each . It is also said to be uniformly smooth if the limit (2.3) is attained uniformly for . It is well known that if E is uniformly smooth, then J is uniformly norm-to-norm continuous on each bounded subset of E. It is also well known that E is uniformly smooth if and only if is uniformly convex.
Recall that a Banach space E enjoys the Kadec-Klee property if for any sequence , and with , and , then as . For more details on the Kadec-Klee property, the readers can refer to [32] and the references therein. It is well known that if E is a uniformly convex Banach space, then E enjoys the Kadec-Klee property.
As we all know, if C is a nonempty closed convex subset of a Hilbert space H and is the metric projection of H onto C, then is nonexpansive. This fact actually characterizes Hilbert spaces and, consequently, it is not available in more general Banach spaces. In this connection, Alber [33] recently introduced a generalized projection operator in a Banach space E which is an analogue of the metric projection in Hilbert spaces.
Next, we assume that E is a smooth Banach space. Consider the functional defined by
Observe that, in a Hilbert space H, (2.4) is reduced to , . The generalized projection is a map that assigns to an arbitrary point the minimum point of the functional , that is, , where is the solution to the minimization problem
Existence and uniqueness of the operator follows from the properties of the functional and strict monotonicity of the mapping J; see, for example, [32]. In Hilbert spaces, . It is obvious from the definition of a function ϕ that
Remark 2.2 If E is a reflexive, strictly convex and smooth Banach space, then for , if and only if . It is sufficient to show that if , then . From (2.5), we have . This implies that . From the definition of J, we have . Therefore, we have ; for more details, see [32] and the references therein.
Let C be a nonempty closed convex subset of E and T be a mapping from C into itself. A point p in C is said to be an asymptotic fixed point of T if C contains a sequence which converges weakly to p such that . The set of asymptotic fixed points of T will be denoted by . A mapping T from C into itself is said to be relatively nonexpansive if and for all and . The mapping T is said to be relatively asymptotically nonexpansive [34] if and there exists a sequence with as such that for all , and .
The mapping T is said to be quasi-ϕ-nonexpansive [35] if and for all and . T is said to be asymptotically quasi-ϕ-nonexpansive [36] and [37] if and there exists a sequence with as such that for all , and .
Remark 2.3 The class of asymptotically quasi-ϕ-nonexpansive mappings is more general than the class of relatively asymptotically nonexpansive mappings which requires the restriction .
Recently, Qin et al. [29] further improved the above results by considering the so-called shrinking projection method for a quasi-ϕ-nonexpansive mapping. To be more precise, they proved the following theorem.
Theorem 2.4 Let C be a nonempty closed and convex subset of a uniformly convex and uniformly smooth Banach space E, and let be a closed and quasi-ϕ-nonexpansive mapping such that . Let be a sequence generated in the following manner:
Assume that the control sequence satisfies the restriction . Then converges strongly to .
Recently, Qin et al. [38] introduced a class of generalized asymptotically quasi-ϕ-nonexpansive mappings. Recall that a mapping T is said to be generalized asymptotically quasi-ϕ-nonexpansive if and there exist a sequence with as and a sequence with as such that for all , and .
In E is a Hilbert space, the mapping T is reduced to a generalized asymptotically quasi-nonexpansive mapping, which was considered by Agarwal et al. [39], Shahzad and Zegeye [40] and Lan [41]. Next, we give examples of the mapping.
Let and . Define the following mapping by
Then T is a generalized asymptotically ϕ-nonexpansive mapping with the fixed point set . We also have the following
and
where . Hence, we have
This shows that T a generalized asymptotically ϕ-nonexpansive mapping instead of an asymptotically ϕ-nonexpansive mapping.
Let with the norm defined by and
Define by
Then T is generalized asymptotically quasi-ϕ-nonexpansive but not asymptotically quasi-ϕ-nonexpansive; for more details, see Lan [41] and the references therein.
In this paper, motivated by the above results, we investigate a hybrid projection algorithm for a pair of generalized asymptotically quasi-ϕ-nonexpansive mappings. Strong convergence of the purposed algorithm is obtained in a uniformly convex and smooth Banach space. The results presented in this paper mainly improve the corresponding results in Wu and Hao [25], Cho et al. [26], Martinez-Yanes and Xu [27], Plubtieng and Ungchittrakool [28], Qin et al. [29] and Qin and Su [31].
In order to give our main results, we need the following lemmas.
Lemma 2.5 [33]
Let C be a nonempty closed convex subset of a smooth Banach space E and . Then if and only if
Lemma 2.6 [33]
Let E be a reflexive, strictly convex and smooth Banach space, C be a nonempty closed convex subset of E and . Then
Lemma 2.7 [42]
Let E be a uniformly convex Banach space and be a closed ball of X. Then there exists a continuous strictly increasing convex function with such that
for all and with .
Lemma 2.8 [43]
Let E be a uniformly convex and smooth Banach space, and let and be two sequences of E. If and either or is bounded, then .
3 Main results
Theorem 3.1 Let E be a uniformly convex and uniformly smooth Banach space. Let C be a nonempty closed and convex subset of E. Let be a closed and generalized asymptotically quasi-ϕ-nonexpansive mapping with a sequence such that as and a sequence , where as . Let be a closed and generalized asymptotically quasi-ϕ-nonexpansive mapping with a sequence such that as and a sequence , where as . Assume that T and S are asymptotically regular on C and is nonempty and bounded. Let be a sequence generated in the following manner:
where , , for each and , , and are real sequences in such that
-
(a)
;
-
(b)
;
-
(c)
.
Then the sequence converges strongly to , where is the generalized projection from C onto ℱ.
Proof First, we show that ℱ is closed and convex. Since T and S are closed, we can easily conclude that and are also closed. This proves that ℱ is closed. Next, we prove the convexity of ℱ. Let , and , where . We see that . Indeed, we see from the definition of T that
and
In view of (2.6), we obtain that
and
Combining (3.1), (3.2), (3.3) with (3.4) yields that
and
Multiplying t and on the both sides of (3.5) and (3.6), respectively, yields that
By Lemma 2.8, we see that as . Hence as . In view of the closedness of T, we can obtain that . This shows that is convex. In the way, we can obtain that is also convex. This completes the proof that ℱ is closed and convex.
Now, we show that is closed and convex for each . It is obvious that is closed and convex. Suppose that is closed and convex for some . For , we see that
is equivalent to
It is not hard to see that is closed and convex. Then, for each , is closed and convex. This shows that is well defined.
Next, we prove that for each . is obvious. Suppose that for some . Then, , we find from Lemma 2.7 that
It follows that
This shows that . This implies that . In view of , we see that
By , we find that
From Lemma 2.6, we see that
for each . Therefore, the sequence is bounded. This implies that is bounded. On the other hand, in view of and , we have
Therefore, is nondecreasing. It follows that the limit of exists. By the construction of , we have that and for any positive integer . It follows that
Letting in (3.8), we see that . It follows from Lemma 2.8 that as . Hence, is a Cauchy sequence. Since E is a Banach space and C is closed and convex, we can assume that
Now, we are in a position to show . By taking , we obtain that
In view of Lemma 2.8, we see that
Since , we obtain that
In view of condition (b), we find from (3.10) that
This in turn implies from Lemma 2.8 that
Note that
Combining (3.11) with (3.13) yields that
Since J is uniformly norm-to-norm continuous on bounded sets, we have
On the other hand, we have . In view of condition (a), we see that
Note that
Combining (3.15) with (3.16), we arrive at
Since is also uniformly norm-to-norm continuous on bounded sets, we obtain that
Since E is a uniformly smooth Banach space, we know that is a uniformly convex Banach space. Let . From Lemma 2.8, we have
It follows that
On the other hand, we have
It follows from (3.17) and (3.18) that
In view of the assumption , we find from (3.19) that
It follows from the property of g that
Since is also uniformly norm-to-norm continuous on bounded sets, we arrive at
On the other hand, we have
From (3.21) and (3.22), we arrive at
On the other hand, we have
In view of restriction (a), we find (3.23) that
It follows from Lemma 2.8 that
Note that
In view of (3.9), (3.18) and (3.25), we find that
On the other hand, we have
Since T is asymptotically regular, we obtain that
That is, as . From the closedness of T, we see that . In the same way, we can also obtain that . This shows that .
Finally, we show that . Taking the limit as in (3.7), we obtain that
and hence by Lemma 2.5. This completes the proof. □
Remark 3.2 Theorem 3.1 includes Theorem 2.4 in Section 2 as a special case. The framework of the space can be applicable to , where . More precisely, is -uniformly smooth and uniformly convex for every .
In the framework of Hilbert spaces, we find the following.
Corollary 3.3 Let E be a Hilbert space. Let C be a nonempty closed and convex subset of E. Let be a closed and generalized asymptotically quasi-nonexpansive mapping with a sequence such that as and a sequence , where as . Let be a closed and generalized asymptotically quasi-nonexpansive mapping with a sequence such that as and a sequence , where as . Assume that T and S are asymptotically regular on C and is nonempty and bounded. Let be a sequence generated in the following manner:
where , , for each and , , and are real sequences in such that
-
(a)
;
-
(b)
;
-
(c)
.
Then the sequence converges strongly to , where is the metric projection from C onto ℱ.
For the class of asymptotically quasi-ϕ-nonexpansive mappings, we find from Theorem 3.1 the following.
Corollary 3.4 Let E be a uniformly convex and uniformly smooth Banach space. Let C be a nonempty closed and convex subset of E. Let be a closed and asymptotically quasi-ϕ-nonexpansive mapping with a sequence such that as . Let be a closed and asymptotically quasi-ϕ-nonexpansive mapping with a sequence such that as . Assume that T and S are asymptotically regular on C and is nonempty and bounded. Let be a sequence generated in the following manner:
where , for each and , , and are real sequences in such that
-
(a)
;
-
(b)
;
-
(c)
.
Then the sequence converges strongly to , where is the generalized projection from C onto ℱ.
If both T and S are quasi-ϕ-nonexpansive, we find from Theorem 3.1 the following.
Corollary 3.5 Let E be a uniformly convex and uniformly smooth Banach space. Let C be a nonempty closed and convex subset of E. Let be a closed quasi-ϕ-nonexpansive mapping, and be a closed quasi-ϕ-nonexpansive mapping with a nonempty common fixed point set. Let be a sequence generated in the following manner:
where , , and are real sequences in such that
-
(a)
;
-
(b)
;
-
(c)
.
Then the sequence converges strongly to , where is the generalized projection from C onto ℱ.
Putting and , we find from Corollary 3.5 the following.
Corollary 3.6 Let E be a uniformly convex and uniformly smooth Banach space. Let C be a nonempty closed and convex subset of E. Let be a closed quasi-ϕ-nonexpansive mapping with a nonempty fixed point set. Let be a sequence generated in the following manner:
where is a real sequence in such that . Then the sequence converges strongly to , where is the generalized projection from C onto .
Remark 3.7 Corollary 3.6 is a Banach version of Theorem 2.1 in Section 2. The sets of are also relaxed.
References
Byrne C: A unified treatment of some iterative algorithms in signal processing and image reconstruction. Inverse Probl. 2008, 20: 103–120.
Fattorini HO: Infinite-Dimensional Optimization and Control Theory. Cambridge University Press, Cambridge; 1999.
Shen J, Pang LP: An approximate bundle method for solving variational inequalities. Commun. Optim. Theory 2012, 1: 1–18.
Abdel-Salam HS, Al-Khaled K: Variational iteration method for solving optimization problems. J. Math. Comput. Sci. 2012, 2: 1475–1497.
Cho SY, Kang SM: Approximation of common solutions of variational inequalities via strict pseudocontractions. Acta Math. Sci. 2012, 32: 1607–1618.
Noor MA, Noor KI, Waseem M: Decomposition method for solving system of linear equations. Eng. Math. Lett. 2013, 2: 31–41.
Dhage BC, Jadhav NS: Differential inequalities and comparison theorems for first order hybrid integro-differential equations. Adv. Inequal. Appl. 2013, 2: 61–80.
Aamir KM, Abbas M, Radenovic S: A logarithmic time complexity algorithm for pattern searching using product-sum property. Comput. Math. Appl. 2011, 62: 2162–2168. 10.1016/j.camwa.2011.07.001
Osu BO, Solomon OU: A stochastic algorithm for the valuation of financial derivatives using the hyperbolic distributional variates. Math. Finance Lett. 2012, 1: 43–56.
Cho SY, Kang SM: Approximation of fixed points of pseudocontraction semigroups based on a viscosity iterative process. Appl. Math. Lett. 2011, 24: 224–228. 10.1016/j.aml.2010.09.008
Luo H, Wang Y: Iterative approximation for the common solutions of a infinite variational inequality system for inverse-strongly accretive mappings. J. Math. Comput. Sci. 2012, 2: 1660–1670.
Cho SY, Qin X, Kang SM: Iterative processes for common fixed points of two different families of mappings with applications. J. Glob. Optim. 2012. 10.1007/s10898-012-0017-y
Pineda MD, Galperin EA: MAPLE code for the gamma algorithm for global optimization of uncertain functions in economy and finance. Comput. Math. Appl. 2010, 59: 2951–2963. 10.1016/j.camwa.2010.02.013
Dhage BC, Nashine HK, Patil VS: Common fixed points for some variants of weakly contraction mappings in partially ordered metric spaces. Adv. Fixed Point Theory 2013, 3: 29–48.
Qin X, Cho SY, Kang SM: An extragradient-type method for generalized equilibrium problems involving strictly pseudocontractive mappings. J. Glob. Optim. 2011, 49: 679–693. 10.1007/s10898-010-9556-2
Wang ZM, Lou W: A new iterative algorithm of common solutions to quasi-variational inclusion and fixed point problems. J. Math. Comput. Sci. 2013, 3: 57–72.
Saluja GS: Strong convergence theorems for two finite families of asymptotically quasi-nonexpansive type mappings in Banach spaces. Adv. Fixed Point Theory 2013, 3: 213–225.
Takahashi W, Tsukiyama N: Approximating fixed points of nonexpansive mappings with compact domains. Commun. Appl. Nonlinear Anal. 2000, 7: 39–47.
Kohsaka F, Takahashi W: Strongly convergent net given by a fixed point theorem for firmly nonexpansive type mappings. Appl. Math. Comput. 2008, 202: 760–765. 10.1016/j.amc.2008.03.019
Cho SY, Li W, Kang SM: Convergence analysis of an iterative algorithm for monotone operators. J. Inequal. Appl. 2013., 2013: Article ID 199
Ye J, Huang J: Strong convergence theorems for fixed point problems and generalized equilibrium problems of three relatively quasi-nonexpansive mappings in Banach spaces. J. Math. Comput. Sci. 2011, 1: 1–18.
Qin X, Chang SS, Cho YJ: Iterative methods for generalized equilibrium problems and fixed point problems with applications. Nonlinear Anal. 2010, 11: 2963–2972. 10.1016/j.nonrwa.2009.10.017
Goebel K, Kirk WA: A fixed point theorem for asymptotically nonexpansive mappings. Proc. Am. Math. Soc. 1972, 35: 171–174. 10.1090/S0002-9939-1972-0298500-3
Halpern B: Fixed points of nonexpanding maps. Bull. Am. Math. Soc. 1967, 73: 957–961. 10.1090/S0002-9904-1967-11864-0
Wu CQ, Hao Y: Strong convergence of a modified Halpern-iteration for asymptotically quasi- ϕ -nonexpansive mappings. An. St. Univ. Ovidius Constanta 2013, 21: 261–276.
Cho YJ, Qin X, Kang SM: Strong convergence of the modified Halpern-type iterative algorithms in Banach spaces. An. St. Univ. Ovidius Constanta 2009, 17: 51–68.
Martinez-Yanes C, Xu HK: Strong convergence of the CQ method for fixed point iteration processes. Nonlinear Anal. 2006, 64: 2400–2411. 10.1016/j.na.2005.08.018
Plubtieng S, Ungchittrakool K: Strong convergence theorems for a common fixed point of two relatively nonexpansive mappings in a Banach space. J. Approx. Theory 2007, 149: 103–115. 10.1016/j.jat.2007.04.014
Qin X, Cho YJ, Kang SM, Zhou H: Convergence of a modified Halpern-type iteration algorithm for quasi- ϕ -nonexpansive mappings. Appl. Math. Lett. 2009, 22: 1051–1055. 10.1016/j.aml.2009.01.015
Su Y, Wang Z, Xu H: Strong convergence theorems for a common fixed point of two hemi-relatively nonexpansive mappings. Nonlinear Anal. 2009, 71: 5616–5628. 10.1016/j.na.2009.04.053
Qin X, Su Y: Strong convergence theorem for relatively nonexpansive mappings in a Banach space. Nonlinear Anal. 2007, 67: 1958–1965. 10.1016/j.na.2006.08.021
Cioranescu I: Geometry of Banach Spaces, Duality Mappings and Nonlinear Problems. Kluwer Academic, Dordrecht; 1990.
Alber YI: Metric and generalized projection operators in Banach spaces: properties and applications. In Theory and Applications of Nonlinear Operators of Accretive and Monotone Type. Edited by: Kartsatos AG. Dekker, New York; 1996.
Agarwal RP, Cho YJ, Qin X: Generalized projection algorithms for nonlinear operators. Numer. Funct. Anal. Optim. 2007, 28: 1197–1215. 10.1080/01630560701766627
Qin X, Cho YJ, Kang SM: Convergence theorems of common elements for equilibrium problems and fixed point problems in Banach spaces. J. Comput. Appl. Math. 2009, 225: 20–30. 10.1016/j.cam.2008.06.011
Zhou H, Gao G, Tan B: Convergence theorems of a modified hybrid algorithm for a family of quasi- ϕ -asymptotically nonexpansive mappings. J. Appl. Math. Comput. 2010, 32: 453–464. 10.1007/s12190-009-0263-4
Qin X, Cho SY, Kang SM: On hybrid projection methods for asymptotically quasi- ϕ -nonexpansive mappings. Appl. Math. Comput. 2010, 215: 3874–3883. 10.1016/j.amc.2009.11.031
Qin X, Agarwal RP, Cho SY, Kang SM: Convergence of algorithms for fixed points of generalized asymptotically quasi- ϕ -nonexpansive mappings with applications. Fixed Point Theory Appl. 2012., 2012: Article ID 58
Agarwal RP, Qin X, Kang SM: An implicit iterative algorithm with errors for two families of generalized asymptotically nonexpansive mappings. Fixed Point Theory Appl. 2011., 2011: Article ID 58
Shahzad N, Zegeye H: Strong convergence of an implicit iteration process for a finite family of generalized asymptotically quasi-nonexpansive maps. Appl. Math. Comput. 2007, 189: 1058–1065. 10.1016/j.amc.2006.11.152
Lan HY: Common fixed point iterative process with errors for generalized asymptotically quasi-nonexpansive mappings. Comput. Math. Appl. 2006, 52: 1403–1412. 10.1016/j.camwa.2006.09.001
Cho YJ, Zhou H, Guo G: Weak and strong convergence theorems for three-step iterations with errors for asymptotically nonexpansive mappings. Comput. Math. Appl. 2004, 47: 707–717. 10.1016/S0898-1221(04)90058-2
Kamimura S, Takahashi W: Strong convergence of a proximal-type algorithm in a Banach space. SIAM J. Optim. 2002, 13: 938–945. 10.1137/S105262340139611X
Acknowledgements
The authors are grateful to the editor and the three anonymous reviewers’ suggestions which improved the contents of the article.
Author information
Authors and Affiliations
Corresponding author
Additional information
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
Both authors contributed equally to this work. Both 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
Zhao, J., Ji, T. A strong convergence theorem for fixed points of generalized asymptotically quasi-ϕ-nonexpansive mappings. Fixed Point Theory Appl 2013, 279 (2013). https://doi.org/10.1186/1687-1812-2013-279
Received:
Accepted:
Published:
DOI: https://doi.org/10.1186/1687-1812-2013-279