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Common fixed points of a family of strictly pseudocontractive mappings
Fixed Point Theory and Applications volume 2013, Article number: 298 (2013)
Abstract
In this article, fixed point problems of a family of strictly pseudocontractive mappings are investigated based on a viscosity iterative process. Strong convergence theorems are established in a real q-uniformly Banach space.
MSC:47H09, 47J05, 47J25.
1 Introduction
Fixed point problems of nonlinear mappings as an important branch of nonlinear analysis theory have been applied in many disciplines, including economics, optimization, image recovery, mechanics, quantum physics, transportation and control theory; for more details, see [1–31] and the references therein.
Strictly pseudocontractive mappings, which act as a link between nonexpansive mappings and pseudocontractive mappings, have been extensively studied by many authors; see [20–31] and the references therein. The computation of fixed points is important in the study of many real world problems, including inverse problems; for instance, it is not hard to show that the split feasibility problem and the convex feasibility problem in signal processing and image reconstruction can both be formulated as a problem of finding fixed points of certain operators, respectively.
Recently, many authors studied the following convex feasibility problem (CFP): , where is an integer, and each is assumed to be the fixed point set of a nonlinear mapping , . There is a considerable investigation on CFP in the setting of Hilbert spaces which captures applications in various disciplines such as image restoration [32], computer tomography [33] and radiation therapy treatment planning [34].
In this paper, we investigate the problem of finding a common fixed point of a finite family of strictly pseudocontractive mappings based on a viscosity approximation iterative process. Strong convergence theorems of common fixed points are established in a real q-uniformly Banach space.
2 Preliminaries
Throughout this paper, we always assume that E is a real Banach space. Let be the dual space of E. Let () denote the generalized duality mapping from E into given by
where denotes the generalized duality pairing. In particular, is called the normalized duality mapping, which is usually denoted by J. In this paper, we use j to denote the single-valued normalized duality mapping. It is known that if . If E is a Hilbert space, then , the identity mapping. Further, we have the following properties of the generalized duality mapping :
-
(1)
for all and ;
-
(2)
for all .
A Banach space E is said to be smooth if the limit
exists for all . It is also said to be uniformly smooth if the limit is attained uniformly for all . The norm of E is said to be Fréchet differentiable if, for any , the above limit is attained uniformly for all . The modulus of smoothness of E is the function defined by
The Banach space E is uniformly smooth if and only if . Let . The Banach space E is said to be q-uniformly smooth if there exists a constant such that . Indeed, there is no Banach space which is q-uniformly smooth with . Hilbert spaces, (or ) spaces and Sobolev spaces , where , are 2-uniformly smooth.
Let C be a nonempty closed convex subset of E and be a mapping. In this paper, we use to denote the fixed point set of T. A mapping T is said to be κ-contractive iff there exists a constant such that
A mapping T is said to be nonexpansive iff
A mapping T is said to be κ-strictly pseudocontractive iff there exist a constant and such that
It is clear that (2.1) is equivalent to the following:
The class of κ-strictly pseudocontractive mappings was first introduced by Browder and Petryshyn [35] in Hilbert spaces.
One classical way to study nonexpansive mappings is to use contractions to approximate a nonexpansive mapping. More precisely, take and define a contraction by
where is a fixed point. Banach’s contraction mapping principle guarantees that has a unique fixed point in C. In the case of T having a fixed point, Browder [36] proved that converges strongly to a fixed point of T in the framework of Hilbert spaces. Reich [37] extended Browder’s result to the setting of Banach spaces and proved that if E is a uniformly smooth Banach space, then converges strongly to a fixed point of T, and the limit defines the (unique) sunny nonexpansive retraction from C onto ; for more details, see [37] and the reference therein.
Recently, Xu [38] investigated the viscosity approximation process in a smooth Banach space. Let be a contraction. Take and define a mapping by
It is not hard to see that also enjoys a unique fixed point. Xu proved that converges to a fixed point of T as , and defines the unique sunny nonexpansive retraction from C onto .
Recently, construction of fixed points for nonexpansive mappings via the normal Mann iterative process has been extensively investigated by many authors. The normal Mann iterative process generates a sequence in the following manner:
where the sequence is in the interval .
In an infinite-dimensional Hilbert space, the normal Mann iteration algorithm has only weak convergence. In many disciplines, including economics, image recovery and control theory, problems arise in infinite dimension spaces. In such problems, strong convergence is often much more desirable than weak convergence, for it translates the physically tangible property that the energy of the error between the iterate and the solution x eventually becomes arbitrarily small. We also remark here that many authors have been instigating the problem of modifying the normal Mann iteration process to have strong convergence for κ-strictly pseudocontractive mappings; see [24–27] and the references therein.
Let D be a nonempty subset of C. Let . Q is said to be a contraction iff ; sunny iff for each and , we have ; sunny nonexpansive retraction iff Q is sunny, nonexpansive and a contraction. K is said to be a nonexpansive retract of C if there exists a nonexpansive retraction from C onto D. The following result, which was established in [39], describes a characterization of sunny nonexpansive retractions on a smooth Banach space.
Let be a retraction, and let j be the normalized duality mapping on E. Then the following are equivalent:
-
(1)
Q is sunny and nonexpansive;
-
(2)
, ;
-
(3)
, , .
In this paper, we investigate the problem of modifying the normal Mann iteration process for a family of κ-strictly pseudocontractive mappings. Strong convergence of the purposed iterative process is obtained in a real q-uniformly Banach space. In order to prove our main results, we need the following tools.
Lemma 2.1 [27]
Let C be a nonempty subset of a real q-uniformly smooth Banach space E, and let be a κ-strict pseudocontraction. For , we define for every . Then, as , where , is nonexpansive such that .
Lemma 2.2 [40]
Let E be a real q-uniformly smooth Banach space. Then the following inequality holds:
where D is some fixed positive constant.
Lemma 2.3 [41]
Assume that is a sequence of nonnegative real numbers such that
where and satisfy the following restrictions:
-
(i)
, ;
-
(ii)
.
Then .
Lemma 2.4 [42]
Let and be bounded sequences in a Banach space X, and let be a sequence in with
Suppose for all integers and
Then .
Lemma 2.5 [25]
Let E be a smooth Banach space, and let C be a nonempty convex subset of E. Given an integer , assume that is a finite family of -strict pseudocontractions such that . Assume that is a positive sequence such that . Then .
Lemma 2.6 [43]
Let . Then the following inequality holds:
for arbitrary positive real numbers a and b.
3 Main results
Theorem 3.1 Let C be a nonempty closed convex subset of a real q-uniformly smooth Banach space E, and let N be some positive integer. Let be a -strictly pseudocontractive mapping for each . Assume that . Let f be an α-contractive mapping. Let be a sequence generated in the following process:
where , and are real number sequences in satisfying the following restrictions:
-
(a)
;
-
(b)
, ;
-
(c)
;
-
(d)
, ;
-
(e)
,
where is some real number, and . Then converges strongly as to some point in , which is the unique solution in to the following variational inequality:
Proof First, we show that and are bounded. Putting , we see that T is a κ-strictly pseudocontractive mapping. Indeed, we have the following:
This proves that T is a κ-strictly pseudocontractive mapping. Fix and put , . It follows from Lemma 2.1 that is nonexpansive. This in turn implies that
It follows that
This in turn implies that
which gives that the sequence is bounded, so is . Notice that
Putting , we see that
Now, we compute . Noticing that
we have
Substituting (3.2) into (3.4), we arrive at
It follows from the restrictions (b) and (c) that
In view of Lemma 2.4, we obtain that . This implies from the restriction (c) that
Notice that
It follows that . On the other hand, we have . It follows that . This in turn implies that
where . Next, we show that
where , where Q is a sunny nonexpansive retraction from C onto , is the strong limit of the sequence defined by
It follows that
For any , we see that
where . It follows that
Fixing t and letting yields that
Since E is q-uniformly smooth, is uniformly continuous on any bounded sets of E, which ensures that and are interchangeable, hence
Finally, we show that as . In view of Lemma 2.6, we see that
where . This implies that
In view of Lemma 2.3, we find the desired conclusion immediately. This completes the proof. □
Remark 3.2 Theorem 3.1 mainly improves the corresponding results in Yuan et al. [22] from 2-uniformly smooth Banach spaces to q-uniformly smooth Banach spaces. Theorem 3.1 is applicable to the spaces and for all .
From Theorem 3.1, we have the following result immediately.
Corollary 3.3 Let C be a nonempty closed convex subset of a real q-uniformly smooth Banach space E, and let N be some positive integer. Let be a -strictly pseudocontractive mapping for each . Assume that . Let be a sequence generated in the following process:
where u is a fixed element in C, , and are real number sequences in satisfying the following restrictions:
-
(a)
;
-
(b)
, ;
-
(c)
;
-
(d)
, ;
-
(e)
,
where is some real number, and . Then converges strongly as to some point in , which is the unique solution in to the following variational inequality:
Remark 3.4 Corollary 3.3 improves the corresponding results in Zhou [26] from 2-uniformly smooth Banach spaces to q-uniformly smooth Banach spaces and relaxes the restrictions imposed on the parameter in Zhang and Su [27].
References
Park S:A review of the KKM theory on -space or GFC-spaces. Adv. Fixed Point Theory 2013, 3: 355–382.
Cho SY, Li W, Kang SM: Convergence analysis of an iterative algorithm for monotone operators. J. Inequal. Appl. 2013., 2013: Article ID 199
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.
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
Dhage BC, Jadhav NS: Differential inequalities and comparison theorems for first order hybrid integro-differential equations. Adv. Inequal. Appl. 2013, 2: 61–80.
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
Chen JH: Fixed point iterations of semigroups of nonexpansive mappings. J. Semigroup Theory Appl. 2013., 2013: Article ID 9
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.
Lv S, Wu C: Convergence of iterative algorithms for a generalized variational inequality and a nonexpansive mapping. Eng. Math. Lett. 2012, 1: 44–57.
Noor MA, Noor KI, Waseem M: Decomposition method for solving system of linear equations. Eng. Math. Lett. 2013, 2: 34–41.
Cho SY, Kang SM: Approximation of common solutions of variational inequalities via strict pseudocontractions. Acta Math. Sci. 2012, 32: 1607–1618.
Shen J, Pang LP: An approximate bundle method for solving variational inequalities. Commun. Optim. Theory 2012, 1: 1–18.
Fattorini HO: Infinite-Dimensional Optimization and Control Theory. Cambridge University Press, Cambridge; 1999.
Dhage BC, Kamble GP, Metkar RG: On generalized Mellin-Hardy integral transformations. Eng. Math. Lett. 2013, 2: 67–80.
Lions PL, Mercier B: Splitting algorithms for the sum of two nonlinear operators. SIAM J. Numer. Anal. 1979, 16: 964–979. 10.1137/0716071
Al-Bayati AY, Al-Kawaz RZ: A new hybrid WC-FR conjugate gradient-algorithm with modified secant condition for unconstrained optimization. J. Math. Comput. Sci. 2012, 2: 937–966.
Cho SY, Qin X, Kang SM: Iterative processes for common fixed points of two different families of mappings with applications. J. Glob. Optim. 2013. 10.1007/s10898-012-0017-y
Chen JH: Iterations for equilibrium and fixed point problems. J. Nonlinear Funct. Anal. 2013., 2013: Article ID 4
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.
Qin X, Cho SY, Kang SM: Iterative algorithms for variational inequality and equilibrium problems with applications. J. Glob. Optim. 2010, 48: 423–445. 10.1007/s10898-009-9498-8
Kangtunyakarn A, Suantai S: Strong convergence of a new iterative scheme for a finite family of strict pseudo-contractions. Comput. Math. Appl. 2010, 60: 680–694. 10.1016/j.camwa.2010.05.016
Yuan Q, Cho SY, Shang M: Strong convergence of an iterative process for a family of strictly pseudocontractive mappings. Fixed Point Theory Appl. 2013., 2013: Article ID 117
Qin X, Cho YJ, Kang SM, Shang MJ: A hybrid iterative scheme for asymptotically κ -strict pseudo-contractions in Hilbert spaces. Nonlinear Anal. 2009, 70: 1902–1911. 10.1016/j.na.2008.02.090
Qin X, Shang M, Kang SM: Strong convergence theorems of modified Mann iterative process for strict pseudo-contractions in Hilbert spaces. Nonlinear Anal. 2009, 70: 1257–1264. 10.1016/j.na.2008.02.009
Zhou H: Strong convergence of an explicit iterative algorithm for continuous pseudo-contractions in Banach spaces. Nonlinear Anal. 2009, 70: 4039–4046. 10.1016/j.na.2008.08.012
Zhou H: Convergence theorems for λ -strict pseudo-contractions in 2-uniformly smooth Banach spaces. Nonlinear Anal. 2008, 69: 3160–3173. 10.1016/j.na.2007.09.009
Zhang H, Su Y: Strong convergence theorems for strict pseudo-contractions in q -uniformly smooth Banach spaces. Nonlinear Anal. 2009, 70: 3236–3242. 10.1016/j.na.2008.04.030
Wang ZM: Convergence theorem on total asymptotically pseudocontractive mapping. J. Math. Comput. Sci. 2013, 3: 788–798.
Ceng LC, Yao JC: Strong convergence theorems for variational inequalities and fixed point problems of asymptotically strict pseudocontractive mappings in the intermediate sense. Acta Appl. Math. 2011, 115: 167–191. 10.1007/s10440-011-9614-x
Takahshi W, Wong NC, Yao JC: Iterative common solutions for monotone inclusion problems, fixed point problems and equilibrium problems. Fixed Point Theory Appl. 2012., 2012: Article ID 181
Ceng LC, Ansari QH, Yao JC: Strong and weak convergence theorems for asymptotically strict pseudocontractive mappings in intermediate sense. J. Nonlinear Convex Anal. 2010, 11: 283–308.
Kotzer T, Cohen N, Shamir J: Image restoration by a novel method of parallel projection onto constraint sets. Opt. Lett. 1995, 20: 1172–1174. 10.1364/OL.20.001172
Sezan MI, Stark H: Application of convex projection theory to image recovery in tomograph and related areas. In Image Recovery: Theory and Application. Edited by: Stark H. Academic Press, Orlando; 1987:155–270.
Censor Y, Zenios SA Numerical Mathematics and Scientific Computation. In Parallel Optimization: Theory, Algorithms, and Applications. Oxford University Press, New York; 1997.
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
Browder FE: Convergence of approximants to fixed points of nonexpansive nonlinear mappings in Banach space. Arch. Ration. Mech. Anal. 1967, 24: 82–90.
Reich S: Weak convergence theorems for nonexpansive mappings in Banach spaces. J. Math. Anal. Appl. 1979, 67: 274–276. 10.1016/0022-247X(79)90024-6
Xu HK: Viscosity approximation methods for nonexpansive mappings. J. Math. Anal. Appl. 2004, 298: 279–291. 10.1016/j.jmaa.2004.04.059
Bruck RE: Nonexpansive projections on subsets of Banach spaces. Pac. J. Math. 1973, 47: 341–355. 10.2140/pjm.1973.47.341
Xu HK: Inequalities in Banach spaces with applications. Nonlinear Anal. 1991, 16: 1127–1138. 10.1016/0362-546X(91)90200-K
Liu LS: Ishikawa and Mann iteration process with errors for nonlinear strongly accretive mappings in Banach spaces. J. Math. Anal. Appl. 1995, 194: 114–125. 10.1006/jmaa.1995.1289
Suzuki T: Strong convergence of Krasnoselskii and Mann’s type sequences for one-parameter nonexpansive semigroups without Bochne integrals. J. Math. Anal. Appl. 2005, 305: 227–239. 10.1016/j.jmaa.2004.11.017
Mitrinovic DS: Analytic Inequalities. Springer, New York; 1970.
Acknowledgements
The first author was supported by the Natural Science Foundation of Zhejiang Province (Q12A010097) and the National Natural Science Foundation of China (11126334). The second author was supported by the Fundamental Research Funds for the Central Universities of China (2011YJS075) and the Scientific Research Fund of Hebei Provincial Education Department (QN20132030). The authors are grateful to the editor and two anonymous reviewers’ suggestions which improved the contents of the article.
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Qin, X., Shang, M. & Qing, Y. Common fixed points of a family of strictly pseudocontractive mappings. Fixed Point Theory Appl 2013, 298 (2013). https://doi.org/10.1186/1687-1812-2013-298
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DOI: https://doi.org/10.1186/1687-1812-2013-298