Abstract
We introduce an iterative scheme for finding a common element of the set of common fixed points of a family of infinitely nonexpansive mappings, and the set of solutions of the variational inequality for an inverse-strongly monotone mapping in a Hilbert space. Under suitable conditions, some strong convergence theorems for approximating a common element of the above two sets are obtained. As applications, at the end of the paper we utilize our results to study the problem of finding a common element of the set of fixed points of a family of infinitely nonexpansive mappings and the set of fixed points of a finite family of -strictly pseudocontractive mappings. The results presented in the paper improve some recent results of Qin and Cho (2008).
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1. Introduction
Throughout this paper, we always assume that is a real Hilbert space with inner product and norm , respectively, is a nonempty closed convex subset of , and is the metric projection of onto . In the following, we denote by strong convergence and by weak convergence. Recall that a mapping is called nonexpansive if
We denote by the set of fixed points of . Recall that a mapping is said to be
(i)monotone if , for all ;
(ii)-Lipschitz if there exists a constant such that , for all ;
(iii)-inverse-strongly monotone [1, 2] if there exists a positive real number such that
Remark 1.1.
It is obvious that any -inverse-strongly monotone mapping is monotone and -Lipschitz continuous.
Let be a mapping. The classical variational inequality problem is to find a such that
The set of solutions of variational inequality (1.3) is denoted by . The variational inequality has been extensively studied in the literature; see, for example, [3, 4] and the references therein.
A self-mapping is a contraction if there exists a constant such that
Iterative methods for nonexpansive mappings have recently been applied to solve convex minimization problems; see, for example, [5–8] and the references therein. Convex minimization problems have a great impact and influence in the development of almost all branches of pure and applied sciences. A typical problem is to minimize a quadratic function over the set of the fixed points a nonexpansive mapping on a real Hilbert space:
where is a linear bounded operator, is the fixed point set of a nonexpansive mapping , and is a given point in . Let be a real Hilbert space. Recall that a linear bounded operator is strongly positive if there is a constant with property
Recently, Marino and Xu [9] introduced the following general iterative scheme based on the viscosity approximation method introduced by Moudafi [10]:
where is a strongly positive bounded linear operator on . They proved that if the sequence of parameters satisfies appropriate conditions, then the sequence generated by (1.7) converges strongly to the unique solution of the variational inequality
which is the optimality condition for the minimization problem
where is a potential function for (i.e., for ).
On the other hand, two classical iteration processes are often used to approximate a fixed point of a nonexpansive mapping. The first one is introduced by Mann [11] and is defined as follows:
where the sequence is in the interval .
The second iteration process is referred to as Ishikawa's iteration process [12] which is defined recursively by
where and are sequences in the interval . However, both (1.16) and (1.11) have only weak convergence in general (see [13], e.g.). Very recently, Qin and Cho [14] introduced a composite iterative algorithm defined as follows:
where is a contraction, is a nonexpansive mapping, and is a strongly positive linear bounded self-adjoint operator, proved that, under certain appropriate assumptions on the parameters, defined by (1.12) converges strongly to a fixed point of , which solves some variational inequality and is also the optimality condition for the minimization problem (1.9).
On the other hand, for finding an element of , under the assumption that a set is nonempty, closed, and convex, a mapping is nonexpansive and a mapping is -inverse-strongly monotone, Takahashi and Toyoda [15] introduced the following iterative scheme:
where is a sequence in , and is a sequence in . They proved that if , then the sequence generated by (1.13) converges weakly to some . Recently, Iiduka and Takahashi [16] proposed another iterative scheme as follows
where is an -inverse strongly monotone mapping, and satisfy some parameters controlling conditions. They showed that if is nonempty, then the sequence generated by (1.14) converges strongly to some .
The existence of common fixed points for a finite family of nonexpansive mappings has been considered by many authors (see [17–20] and the references therein). The well-known convex feasibility problem reduces to finding a point in the intersection of the fixed point sets of a family of nonexpansive mappings (see [21, 22]). The problem of finding an optimal point that minimizes a given cost function over the common set of fixed points of a family of nonexpansive mappings is of wide interdisciplinary interest and practical importance (see [18, 23]). A simple algorithmic solution to the problem of minimizing a quadratic function over the common set of fixed points of a family of nonexpansive mappings is of extreme value in many applications including set theoretic signal estimation (see [23, 24]).
In this paper, we study the mapping defined by
where is a nonnegative real sequence with , for all , , form a family of infinitely nonexpansive mappings of into itself. Nonexpansivity of each ensures the nonexpansivity of . Such a is nonexpansive from to and it is called a -mapping generated by and .
In this paper, motivated and inspired by Su et al. [25], Marino and Xu [9], Takahashi and Toyoda [15], and Iiduka and Takahashi [16], we will introduce a new iterative scheme:
where is a mapping defined by (1.15), is a contraction, is strongly positive linear bounded self-adjoint operator, is a -inverse strongly monotone, and we prove that under certain appropriate assumptions on the sequences , , , and , the sequences defined by (1.16) converge strongly to a common element of the set of common fixed points of a family of and the set of solutions of the variational inequality for an inverse-strongly monotone mapping, which solves some variational inequality and is also the optimality condition for the minimization problem (1.9).
2. Preliminaries
Let be a real Hilbert space. It is well known that for any
Let be a nonempty closed convex subset of . For every point , there exists a unique nearest point in , denoted by , such that
is called the metric projection of onto . It is well known that is a nonexpansive mapping of onto and satisfies
for every Moreover, is characterized by the following properties: and
for all . It is easy to see that the following is true:
A Banach space is said to satisfy the Opial's condition if for each sequence in which converges weakly to a point we have
It is well known that each Hilbert space satisfies the Opial's condition.
A set-valued mapping is called monotone if for all , and imply . A monotone mapping is maximal if the graph of of is not properly contained in the graph of any other monotone mapping. It is known that a monotone mapping is maximal if and only if for , for every implies . Let be a monotone map of into and let be the normal cone to at , that is, and define
Then is the maximal monotone and if and only if ; see [26].
Now we collect some useful lemmas for proving the convergence result of this paper.
Lemma 2.1.
In a Hilbert space . Then the following inequality holds
Lemma 2.2 (see [27]).
Let and be bounded sequences in a Banach space and let be a sequence in with Suppose for all integers and Then,
Lemma 2.3 (see [28]).
Assume is a sequence of nonnegative real numbers such that
where is a sequence in and is a sequence in such that
(1)
(2) or
Then
Lemma 2.4 (see [9]).
Assume that is a strongly positive linear bounded self-adjoint operator on a Hilbert space with coefficient and . Then .
Throughout this paper, we will assume that , for all . Concerning defined by (1.15), we have the following lemmas which are important to prove our main result.
Lemma 2.5 (see [29]).
Let be a nonempty closed convex subset of a Hilbert space , let be a family of infinitely nonexpansive mapping with , and let be a real sequence such that , for all . Then
(1) is nonexpansive and for each ;
(2)for each and for each positive integer , the limit exists;
(3)the mapping define by
is a nonexpansive mapping satisfying and it is called the -mapping generated by and
Lemma 2.6 (see [30]).
Let be a nonempty closed convex subset of a Hilbert space , let be a family of infinitely nonexpansive mappings with , and let be a real sequence such that , for all . If is any bounded subset of , then
3. Main Results
Now we are in a position to state and prove the main result in this paper.
Theorem 3.1.
Let be a closed convex subset of a real Hilbert space , let be a contraction of into itself, let be an -inverse strongly monotone mapping of into , and let be a family of infinitely nonexpansive mappings with . Let be a strongly positive linear bounded self-adjoint operator with the coefficient such that . Assume that . Let , , , and be sequences in satisfying the following conditions:
(C1)
(C2)
(C3)
(C4)
(C5).
Then the sequence defined by (1.16) converges strongly to , where which solves the following variational inequality:
Proof.
Since as by the condition (C1), we may assume, without loss of generality that for all . First, we will show that is nonexpansive. Indeed, for all and ,
which implies that is nonexpansive. Noticing that is a linear bounded self-adjoint operator, one has
Observing that
we obtain is positive. It follows that
Next, we observe that is bounded. Indeed, pick and notice that
It follows that
By simple induction, we have
which gives that the sequence is bounded, and so are and .
Next, we claim that
Since and are nonexpansive, we have
where is a constant such that . Similarly, there exists such that .
Observing that
we obtain that
It follows that
Noticing that
we obtain
It follows that
Substituting (3.11) into (3.14), we get
where is an appropriate constant such that
Putting , we get, .
Now, we compute . Observing that
It follows from (3.15) that
It follows that
Observing the conditions (C1) and (C4) and taking the superior limit as , we get
We can obtain easily by Lemma 2.2 since
one obtains that (3.7) holds. Setting , we have
Observing that
we arrive at
This implies
From (3.7) and (C1) we obtain that
Next, we will show that as for any Observe that
where
This impies that
Since and from (3.7), we obtain
From (2.3), we have
so, we obtain
It follows that
which implies that
Applying (3.7), (3.30), and to the last inequality, we obtain that
It follows from (3.26) and (3.35) that
On the other hand, one has
which implies
From the conditions (C3), it follows that
Applying Lemma 2.6 and (3.39), we obtain that
It follows from (3.26) and (3.40) that
We observe that is a contraction. Indeed, for all , we have
Banach's Contraction Mapping Principle guarantees that has a unique fixed point, say . That is, .
Next, we claim that
Indeed, we choose a subsequence of such that
Since is bounded, there exists a subsequence of which converges weakly to . Without loss of generality, we can assume that . From we obtain Therefore, we have
Next we prove that .
First, we prove that .
Suppose the contrary, , that is, . Since , by the Opial's condition and (3.41), we have
This is a contradiction, which shows that .
Next, we prove . For this purpose, let be the maximal monotone mapping defined by (2.7):
For any given , hence . Since we have
On the other hand, from , we have
that is,
Therefore, we obtian
Noting that as and is Lipschitz continuous, hence from (3.18), we obtain
Since is maximal monotone, we have , and hence .
The conclusion is proved.
Hence by (3.45), we obtain
Since , it follows from (3.39), (3.41), and (3.53) that
Hence (3.43) holds. Using (3.26) and (3.54), we have
Now, from Lemma 2.1, it follows that
Since , , and are bounded, we can take a constant such that
for all . It then follows that
where
Using (C1), (3.54), and (3.55), we get . Now applying Lemma 2.3 to (3.58), we conclude that . This completes the proof.
Remark 3.2.
Theorem 3.1 mainly improve the results of Qin and Cho [14] from a single nonexpansive mapping to an infinite family of nonexpansive mappings.
4. Applications
In this section, we obtain two results by using a special case of the proposed method.
Theorem 4.1.
Let be a real Hilbert space, let be an -inverse strongly monotone mapping on , let be a family of infinitely nonexpansive mappings with . Let a contraction with coefficient , and let be a strongly positive bounded linear operator on with coefficient and . Suppose the sequences , , and be generated by
where , , , and are sequences in satisfying the following conditions:
(C1)
(C2)
(C3)
(C4)
(C5).
Then , , and converge strongly to which solves the variational inequality:
Proof.
We have and . Applying Theorem 3.1, we obtain the desired result.
Next, we will apply the main results to the problem for finding a common element of the set of fixed points of a family of infinitely nonexpansive mappings and the set of fixed points of a finite family of -strictly pseudocontractive mappings.
Definition 4.2.
A mappings is said to be a -strictly pseudocontractive mapping if there exists such that
The following lemmas can be obtained from [31, Proposition 2.6] by Acedo and Xu, easily.
Lemma 4.3.
Let be a Hilbert space, let be a closed convex subset of . For any integer , assume that, for each is a -strictly pseudocontractive mapping for some . Assume that is a positive sequence such that . Then is a -strictly pseudocontractive mapping with .
Lemma 4.4.
Let and be as in Lemma 4.3. Suppose that has a common fixed point in . Then .
Let be a -strictly pseudocontractive mapping for some . We define a mapping , where is a positive sequence such that , then is a -inverse-strongly monotone mapping with . In fact, from Lemma 4.3, we have
That is,
On the other hand
Hence, we have
This shows that is -inverse-strongly monotone.
Theorem 4.5.
Let be a closed convex subset of a real Hilbert space . For any integer , assume that, for each is a -strictly pseudocontractive mapping for some . Let be a family of infinitely nonexpansive mappings with . Let a contraction with coefficient and let be a strongly positive bounded linear operator with coefficient and . Let the sequences , , and be generated by
where , , , and are the sequences in satisfying the following conditions:
(C1)
(C2)
(C3)
(C4)
(C5)
Then , , and converge strongly to which solves the variational inequality:
Proof.
Taking , we know that is -inverse strongly monotone with . Hence, is a monotone -Lipschitz continuous mapping with . From Lemma 4.4, we know that is a -strictly pseudocontractive mapping with and then by Chang [30, Proposition 1.3.5]. Observe that
The conclusion of Theorem 4.5 can be obtained from Theorem 3.1.
Remark 4.6.
Theorem 4.5 is a generalization and improvement of the theorems by Qin and Cho [14], Iiduka and Takahashi [16, Thorem 3.1], and Takahashi and Toyoda [15].
References
Browder FE, Petryshyn WV: Construction of fixed points of nonlinear mappings in Hilbert space. Journal of Mathematical Analysis and Applications 1967, 20: 197–228. 10.1016/0022-247X(67)90085-6
Liu F, Nashed MZ: Regularization of nonlinear ill-posed variational inequalities and convergence rates. Set-Valued Analysis 1998,6(4):313–344. 10.1023/A:1008643727926
Yao J-C, Chadli O: Pseudomonotone complementarity problems and variational inequalities. In Handbook of Generalized Convexity and Generalized Monotonicity, Nonconvex Optimization and Its Applications. Volume 76. Springer, New York, NY, USA; 2005:501–558.
Zeng LC, Schaible S, Yao JC: Iterative algorithm for generalized set-valued strongly nonlinear mixed variational-like inequalities. Journal of Optimization Theory and Applications 2005,124(3):725–738. 10.1007/s10957-004-1182-z
Deutsch F, Yamada I: Minimizing certain convex functions over the intersection of the fixed point sets of nonexpansive mappings. Numerical Functional Analysis and Optimization 1998,19(1–2):33–56.
Xu H-K: Iterative algorithms for nonlinear operators. Journal of the London Mathematical Society. Second Series 2002,66(1):240–256. 10.1112/S0024610702003332
Xu HK: An iterative approach to quadratic optimization. Journal of Optimization Theory and Applications 2003,116(3):659–678. 10.1023/A:1023073621589
Yamada I: The hybrid steepest descent method for the variational inequality problem over the intersection of fixed point sets of nonexpansive mappings. In Inherently Parallel Algorithm for Feasibility and Optimization and Their Applications, Studies in Computational Mathematics. Volume 8. Edited by: Butnariu D, Censor Y, Reich S. North-Holland, Amsterdam, The Netherlands; 2001:473–504.
Marino G, Xu H-K: A general iterative method for nonexpansive mappings in Hilbert spaces. Journal of Mathematical Analysis and Applications 2006,318(1):43–52. 10.1016/j.jmaa.2005.05.028
Moudafi A: Viscosity approximation methods for fixed-points problems. Journal of Mathematical Analysis and Applications 2000,241(1):46–55. 10.1006/jmaa.1999.6615
Mann WR: Mean value methods in iteration. Proceedings of the American Mathematical Society 1953, 4: 506–510. 10.1090/S0002-9939-1953-0054846-3
Ishikawa S: Fixed points by a new iteration method. Proceedings of the American Mathematical Society 1974, 44: 147–150. 10.1090/S0002-9939-1974-0336469-5
Genel A, Lindenstrauss J: An example concerning fixed points. Israel Journal of Mathematics 1975,22(1):81–86. 10.1007/BF02757276
Qin X, Cho Y: Convergence of a general iterative method for nonexpansive mappings in Hilbert spaces. Journal of Computational and Applied Mathematics. In press
Takahashi W, Toyoda M: Weak convergence theorems for nonexpansive mappings and monotone mappings. Journal of Optimization Theory and Applications 2003,118(2):417–428. 10.1023/A:1025407607560
Iiduka H, Takahashi W: Strong convergence theorems for nonexpansive mappings and inverse-strongly monotone mappings. Nonlinear Analysis: Theory, Methods & Applications 2005,61(3):341–350. 10.1016/j.na.2003.07.023
Aoyama K, Kimura Y, Takahashi W, Toyoda M: Approximation of common fixed points of a countable family of nonexpansive mappings in a Banach space. Nonlinear Analysis: Theory, Methods & Applications 2007,67(8):2350–2360. 10.1016/j.na.2006.08.032
Bauschke HH: The approximation of fixed points of compositions of nonexpansive mappings in Hilbert space. Journal of Mathematical Analysis and Applications 1996,202(1):150–159. 10.1006/jmaa.1996.0308
Shang M, Su Y, Qin X: Strong convergence theorems for a finite family of nonexpansive mappings. Fixed Point Theory and Applications 2007, Article ID 76971 2007:-9.
Shimoji K, Takahashi W: Strong convergence to common fixed points of infinite nonexpansive mappings and applications. Taiwanese Journal of Mathematics 2001,5(2):387–404.
Bauschke HH, Borwein JM: On projection algorithms for solving convex feasibility problems. SIAM Review 1996,38(3):367–426. 10.1137/S0036144593251710
Combettes PL: The foundations of set theoretic estimation. Proceedings of the IEEE 1993,81(2):182–208.
Youla DC: Mathematical theory of image restoration by the method of convex projections. In Image Recovery: Theory and Application. Edited by: Star H. Academic Press, Orlando, Fla, USA; 1987:29–77.
Iusem AN, De Pierro AR: On the convergence of Han's method for convex programming with quadratic objective. Mathematical Programming Series B 1991,52(1–3):265–284.
Su Y, Shang M, Qin X: A general iterative scheme for nonexpansive mappings and inverse-strongly monotone mappings. Journal of Applied Mathematics and Computing 2008,28(1–2):283–294. 10.1007/s12190-008-0103-y
Rockafellar RT: On the maximality of sums of nonlinear monotone operators. Transactions of the American Mathematical Society 1970,149(1):75–88. 10.1090/S0002-9947-1970-0282272-5
Suzuki T: Strong convergence of Krasnoselskii and Mann's type sequences for one-parameter nonexpansive semigroups without Bochner integrals. Journal of Mathematical Analysis and Applications 2005,305(1):227–239. 10.1016/j.jmaa.2004.11.017
Xu H-K: Viscosity approximation methods for nonexpansive mappings. Journal of Mathematical Analysis and Applications 2004,298(1):279–291. 10.1016/j.jmaa.2004.04.059
Takahashi S, Takahashi W: Viscosity approximation methods for equilibrium problems and fixed point problems in Hilbert spaces. Journal of Mathematical Analysis and Applications 2007,331(1):506–515. 10.1016/j.jmaa.2006.08.036
Chang SS: Variational Inequalities and Related Problems. Chongqing Publishing House, Chongqing, China; 2007.
Acedo GL, Xu H-K: Iterative methods for strict pseudo-contractions in Hilbert spaces. Nonlinear Analysis: Theory, Methods & Applications 2007,67(7):2258–2271. 10.1016/j.na.2006.08.036
Acknowledgments
The authors would like to thank the referee for the comments which improve the manuscript. R. Wangkeeree was supported for CHE-PhD-THA-SUP/2551 from the Commission on Higher Education and the Thailand Research Fund under Grant TRG5280011.
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Wangkeeree, R., Kamraksa, U. A General Iterative Method for Solving the Variational Inequality Problem and Fixed Point Problem of an Infinite Family of Nonexpansive Mappings in Hilbert Spaces. Fixed Point Theory Appl 2009, 369215 (2009). https://doi.org/10.1155/2009/369215
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DOI: https://doi.org/10.1155/2009/369215