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Viscosity approximation methods for two nonexpansive semigroups in CAT(0) spaces
Journal of Inequalities and Applications volume 2014, Article number: 283 (2014)
Abstract
The purpose of this paper is by using the viscosity approximation method to study the strong convergence problem for two one-parameter continuous semigroups of nonexpansive mappings in CAT(0) spaces. Under suitable conditions, some strong convergence theorems for the proposed implicit and explicit iterative schemes to converge to a common fixed point of two one-parameter continuous semigroups of nonexpansive mappings are proved, which is also a unique solution of some kind of variational inequalities. The results presented in this paper extend and improve the corresponding results of some others.
MSC:47J05, 47H09, 49J25.
1 Introduction
Throughout this paper, we assume that X is a CAT(0) space, ℕ is the set of positive integers, ℝ is the set of real numbers, is the set of nonnegative real numbers and C is a nonempty closed and convex subset of a complete CAT(0) space X.
A family of mappings is called a one-parameter continuous semigroup of nonexpansive mappings if the following conditions are satisfied:
-
(i)
for each , is a nonexpansive mapping on C, i.e.,
-
(ii)
for all ;
-
(iii)
for each , the mapping from into C is continuous.
A family of mappings is called a one-parameter strongly continuous semigroup of nonexpansive mappings if conditions (i), (ii), (iii) and the following condition are satisfied:
-
(iv)
for all .
In the sequel, we shall denote by ℱ the common fixed point set of , that is,
It is well known that one classical way to study nonexpansive mappings is to use the contractions to approximate nonexpansive mappings. More precisely, take and define a contraction by
where is an arbitrary fixed element. In the case of T having a fixed point, Browder [1] proved that converged strongly to a fixed point of T that is nearest to u in the framework of Hilbert spaces. Reich [2] extended Browder’s result to the setting of a uniformly smooth Banach space and proved that converged strongly to a fixed point of T.
Halpern [3] introduced the following explicit iterative scheme (1.2) for a nonexpansive mapping T on a subset C of a Hilbert space:
He proved that the sequence converged to a fixed point of T. In [4], Shioji and Takahashi introduced the following implicit iteration in a Hilbert space:
Under suitable conditions, they proved strong convergence of to a member of ℱ.
Later, Suzuki [5] introduced in a Hilbert space the following iteration process:
where is a strongly continuous semigroup of nonexpansive mappings on C such that . Under suitable conditions he proved that converged strongly to the element of ℱ nearest to u. Using Moudafi’s viscosity approximation methods, Song and Xu [6], Cho and Kang [7] introduced the following iteration process:
and
They proved that defined by (1.5) and (1.6) both converged to the same point of ℱ in a reflexive strictly convex Banach space with a uniformly Gâteaux differentiable norm.
In a similar way, Dhompongsa et al. [8] extended Browder’s implicit iteration to a strongly continuous semigroup of nonexpansive mappings in a complete CAT(0) space X. Under suitable conditions he proved that the sequence converged strongly to the element of ℱ nearest to u. Using Moudafi’s viscosity approximation methods, Shi and Chen [9] studied the convergence theorems of the following Moudafi’s viscosity iterations for a nonexpansive mapping T:
and
They proved that defined by (1.7) and defined by (1.8) converged strongly to a fixed point of T in the framework of CAT(0) spaces.
Very recently, Wangkeeree and Preechasilp [10] extended the results of [9] to a one-parameter continuous semigroup of nonexpansive mappings in CAT(0) spaces. Under suitable conditions they proved that the iterative schemes both converged strongly to the same point such that , which is the unique solution of the variational inequality
Motivated and inspired by the research going on in this direction, especially inspired by Wangkeeree and Preechasilp [10], in this paper we study the strong convergence theorems of Moudafi’s viscosity approximation methods for two one-parameter continuous semigroups of nonexpansive mappings in CAT(0) spaces. We prove that the implicit and explicit iteration algorithms both converge strongly to the same point such that , which is the unique solution of the variational inequality (1.9) where ℱ is the set of common fixed points of the two semigroups of nonexpansive mappings.
2 Preliminaries and lemmas
In this paper, we write for the unique point z in the geodesic segment joining from x to y such that
Lemma 2.1 [11]
A geodesic space X is a CAT(0) space if and only if the following inequality
is satisfied for all and . In particular, if x, y, z are points in a CAT(0) space and , then
Lemma 2.2 [12]
Let X be a CAT(0) space, and . Then
By induction, we write
Lemma 2.3 Let X be a CAT(0) space, then, for any sequence in satisfying and for any , the following conclusions hold:
and
Proof It is obvious that (2.5) holds for . Suppose that (2.5) holds for some . From (2.3) and (2.4) we have
This implies that (2.5) holds.
Next, we prove that (2.6) holds.
Indeed, it is obvious that (2.6) holds for . Suppose that (2.6) holds for some . Next we prove that (2.6) is also true for .
In fact, we have
From (2.2) and (2.4) and the assumption of induction, we have
This completes the proof of (2.6). □
The concept of Δ-convergence introduced by Lim [13] in 1976 was shown by Kirk and Panyanak [14] in CAT(0) spaces to be very similar to the weak convergence in the Banach space setting (see also [15]). Now, we give the concept of Δ-convergence.
Let be a bounded sequence in a CAT(0) space X. For , we set
The asymptotic radius of is given by
and the asymptotic center of is the set
It is known from Proposition 7 of [16] that in a complete CAT(0) space, consists of exactly one point. A sequence is said to Δ-converge to if for every subsequence of .
The uniqueness of an asymptotic center implies that a CAT(0) space X satisfies Opial’s property, i.e., for given such that Δ-converges to x and given with ,
Lemma 2.4 [14]
Every bounded sequence in a complete CAT(0) space always has a Δ-convergent subsequence.
Berg and Nikolaev [17]introduced the concept of quasilinearization as follows. Let us denote a pair by and call it a vector. Then quasilinearization is defined as a map defined by
It is easily seen that , and for all . We say that X satisfies the Cauchy-Schwarz inequality if
for all .
Recently, Dehghan and Rooin [18] presented a characterization of metric projection in CAT(0) spaces as follows.
Lemma 2.5 Let C be a nonempty convex subset of a complete CAT(0) space X, and . Then if and only if
Lemma 2.6 [19]
Let X be a complete CAT(0) space, be a sequence in X and . Then Δ-converges to x if and only if for all .
Lemma 2.7 [20]
Let be a sequence of nonnegative real numbers satisfying the property , , where and such that
-
(i)
;
-
(ii)
or .
Then converges to zero as .
3 Viscosity approximation iteration algorithms
In this section, we present the strong convergence theorems of Moudafi’s viscosity approximation implicit and explicit iteration algorithms for two one-parameter continuous semigroups of nonexpansive mappings and in CAT(0) spaces.
Before proving main results, we need the following two vital lemmas.
Let X be a complete CAT(0) space. Then, for all , the following inequality holds:
Lemma 3.2 Let X be a complete CAT(0) space. For any and , , let . Then, for any , the following inequality holds:
Proof It follows from (2.1) and (2.6) that
Similarly, we can obtain and . Therefore, we have
From (2.6) and (3.1), we have that
which is the desired result. □
Now we are in a position to state and prove our main results.
Theorem 3.3 Let C be a closed convex subset of a complete CAT(0) space X, and let and be two one-parameter continuous semigroups of nonexpansive mappings on C satisfying and both uniformly asymptotically regular (in short, u.a.r.) on C, that is, for all and any bounded subset B of C,
Let f be a contraction on C with coefficient . Suppose that the sequence is given by
for all , where and satisfy the following conditions:
-
(i)
;
-
(ii)
, ;
-
(iii)
, ;
-
(iv)
for any bounded subset B of C, .
Then converges strongly to such that , which is equivalent to the following variational inequality:
Proof We shall divide the proof of Theorem 3.3 into five steps.
Step 1. The sequence defined by (3.2) is well defined for all .
In fact, let us define mappings by
and
respectively. For any , from Lemma 2.2, we have
Therefore we have that
This implies that is a contraction mapping. Hence, the sequence is well defined for all .
Step 2. The sequence is bounded.
For any , from Lemma 2.3, we have that
Then
This implies that
Hence is bounded, so are , and .
Step 3. For any , and .
From Lemma 2.3 and condition (ii), we have
and
Since and is u.a.r., we obtain that for all ,
and
where B is any bounded subset of C containing . Hence, we have
and
Step 4. The sequence contains a subsequence converging strongly to such that , which is equivalent to (3.3).
Since is bounded, by Lemma 2.4, there exists a subsequence of (without loss of generality, we denote it by ) which Δ-converges to a point .
First we claim that . Since every CAT(0) space has Opial’s property, for any , if , we have
This is a contraction, and hence . Similarly, we can obtain that . So we have .
Next we prove that converges strongly to . Indeed, it follows from Lemma 3.2 that
where . It follows that
and thus
Since Δ-converges to , by Lemma 2.6 we have
It follows from (3.5) and that converges strongly to .
Next we show that solves the variational inequality (3.3). Applying Lemma 2.3, for any , we have
This implies that
Taking the limit through , we can obtain
On the other hand, from (2.7) we have
From (3.6) and (3.7) we have
That is, solves inequality (3.3).
Step 5. The sequence converges strongly to .
Assume that as . By the same argument, we get that and solves the variational inequality (3.3), i.e.,
and
Adding up (3.8) and (3.9), we get that
Since , we have that , and so . Hence the sequence converges strongly to , which is the unique solution to the variational inequality (3.3).
This completes the proof. □
Theorem 3.4 Let C be a closed convex subset of a complete CAT(0) space X, and let and be two one-parameter continuous semigroups of nonexpansive mappings on C satisfying and both uniformly asymptotically regular on C. Let f be a contraction on C with coefficient . Suppose that is given by
for all , where and satisfy the following conditions:
-
(i)
;
-
(ii)
, and ;
-
(iii)
for all , ;
-
(iv)
and ;
-
(iv)
for any bounded subset B of C, .
Then converges strongly to such that , which is equivalent to the variational inequality (3.3).
Proof We first show that the sequence is bounded. For any , we have that
By induction, we have
for all . Hence is bounded, so are , and .
In view of condition (ii), we have
Since is u.a.r and , then for all , we obtain that
where B is any bounded subset of C containing . Hence
Similarly, for all , we have
Let be a sequence in C such that
It follows from Theorem 3.3 that converges strongly to a fixed point , which solves the variational inequality (3.3).
Now we claim that
Indeed, it follows from Lemma 3.2 that
where
and
This implies that
Taking the upper limit as first, and then , from (3.11), (3.12) and , we get
Since
Thus, by taking the upper limit as first, and then , it follows from and (3.14) that
Finally, we prove that as . In fact, for any , letting
from Lemma 3.1 and Lemma 3.2, we have that
where . This implies that
Then it follows that
where
Applying Lemma 2.7 and , we can conclude that as . This completes the proof. □
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Acknowledgements
The authors would like to express their thanks to the editors and referees for their helpful comments and advice. This work was supported by the Scientific Research Fund of Sichuan Provincial Education Department (13ZA0199) and the Scientific Research Fund of Sichuan Provincial Department of Science and Technology (2012JYZ011) and the National Natural Science Foundation of China (Grant No. 11361070).
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Tang, J., Chang, Ss. Viscosity approximation methods for two nonexpansive semigroups in CAT(0) spaces. J Inequal Appl 2014, 283 (2014). https://doi.org/10.1186/1029-242X-2014-283
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DOI: https://doi.org/10.1186/1029-242X-2014-283