Volume 2012, Issue 1 185104
Research Article
Open Access

Convergence of an Iterative Algorithm for Common Solutions for Zeros of Maximal Accretive Operator with Applications

Uamporn Witthayarat

Uamporn Witthayarat

Department of Mathematics, Faculty of Science, King Mongkut’s University of Technology Thonburi (KMUTT), Bangmod, Bangkok 10140, Thailand kmutt.ac.th

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Yeol Je Cho

Corresponding Author

Yeol Je Cho

Department of Mathematics Education and RINS, Gyeongsang National University, Chinju 660-701, Republic of Korea gnu.ac.kr

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Poom Kumam

Corresponding Author

Poom Kumam

Department of Mathematics, Faculty of Science, King Mongkut’s University of Technology Thonburi (KMUTT), Bangmod, Bangkok 10140, Thailand kmutt.ac.th

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First published: 10 April 2012
Citations: 3
Academic Editor: Yeong-Cheng Liou

Abstract

The aim of this paper is to introduce an iterative algorithm for finding a common solution of the sets (0) and (0), where M is a maximal accretive operator in a Banach space and, by using the proposed algorithm, to establish some strong convergence theorems for common solutions of the two sets above in a uniformly convex and 2-uniformly smooth Banach space. The results obtained in this paper extend and improve the corresponding results of Qin et al. 2011 from Hilbert spaces to Banach spaces and Petrot et al. 2011. Moreover, we also apply our results to some applications for solving convex feasibility problems.

1. Introduction

Let E be a real Banach space with norm ∥·∥ with the dual space E* and let 〈·, ·〉 denote the pairing between E and E*. Let C be a nonempty closed convex subset of E. We define the generalized duality mapping by
(1.1)
for all q > 1. In the special case, for q = 2, we called the mapping J2 as the normalized duality mapping and as usual we write J2 = J. The following is the well-known properties of the generalized duality mapping Jq:
  • (1)

    Jq(x) = ∥xq−2J2(x) for all xE with x ≠ 0;

  • (2)

    Jq(tx) = tq−1Jq(x) for all xE and t ∈ [0, );

  • (3)

    Jq(−x) = −Jq(x) for all xE.

It is well known that if X is smooth, then J is single valued, which is denoted by j. Recall that the duality mapping j is said to be weakly sequentially continuous if, for each sequence {xn} with xnx weakly, we have j(xn) → j(x) weakly*. We know that, if X admits a weakly sequentially continuous duality mapping, then X is smooth. For the details, see [13].

Let U = {xE : ∥x∥ = 1}. A Banach space E is said to be
  • (1)

    uniformly convex if there exists δ > 0 such that, for any x, yU and, for any ϵ ∈ (0,2], ∥xy∥≥ϵ implies ∥(x + y)/2∥≤1 − δ.

We can see that every uniformly convex Banach space is also reflexive and strictly convex.
  • (2)

    Smooth if lim t→0(∥x + ty∥−∥x∥)/t exists for all x, yU.

  • (3)

    Uniformly smooth if the limit is attained uniformly for x, yU. The modulus of smoothness of E is defined by

    (1.2)
    where ρ : [0, )→[0, ) is a function. In the other way, E is uniformly smooth if and only if lim τ→0ρ(τ)/τ = 0.

  • (4)

    q-uniformly smooth if there exists a constant c > 0 such that ρ(τ) ≤ cτq for all τ > 0 where q is a fixed real number with 1 < q ≤ 2. (see, for instance, [1, 4]).

We note that E is a uniformly smooth Banach space if and only if Jq is single valued and uniformly continuous on any bounded subset of E. Examples of both uniformly convex and uniformly smooth Banach spaces are Lp, where p > 1. More precisely, Lp is min{p, 2}-uniformly smooth for any p > 1. Note also that no Banach space is q-uniformly smooth for q > 2 (see [1, 5] for more details).

Let A : CE be a nonlinear mapping. The mapping A is said to be
  • (1)

    accretive if

    (1.3)

  • (2)

    λ-strongly accretive if there exists a constant α > 0 such that

    (1.4)

  • (3)

    λ-inverse-strongly accretive if there exists a constant α > 0 such that

(1.5)

Definition 1.1. Let M : E → 2E be a multivalued maximal accretive mapping. The single-valued mapping JM,ρ : EE defined by

(1.6)
is called the resolvent operator associated with M, where ρ is any positive number and I is the identity mapping.

Let T be a mapping from E into itself. We use F(T) to denote the set of fixed points of the mapping T. Recall that the mapping T is said to be nonexpansive if
(1.7)
A mapping f : CC is said to be contractive if there exists a constant α ∈ (0,1) such that
(1.8)
Recently, Aoyama et al. [4] considered the following generalized variational inequality problem in a smooth Banach space: Find a point xC such that
(1.9)
where A is an accretive operator of C into E. This problem is related to the fixed point problem for nonlinear mappings, the problem of finding a zero point of an accretive operator, and so on. For the problem of finding a zero point of an accretive operator by the proximal point algorithm, see Agarwal et al. [6], Cho et al. [7, 8], Kamimura and Takahashi [9, 10], Qin et al. [11], Song et al. [12], and Wei and Cho [13]. In order to find a solution of the variational inequality (1.9), Aoyama et al. [4] studied the weak convergence theorem for accretive operators in Banach spaces, which is a generalization of the result by Iiduka et al. [14] from the class of Hilbert spaces.

Theorem AIT (see [4], Aoyama et al. Theorem  3.1.)Let E be a uniformly convex and 2-uniformly smooth Banach space and let C be a nonempty closed convex subset of E. Let QC be a sunny nonexpansive retraction from E onto C, α > 0 and A be an α-inverse strongly accretive operator of C into E with S(C, A) ≠ , where

(1.10)
If {λn} and {αn} are chosen such that λn ∈ [a, α/K2] for some a > 0 and αn ∈ [b, c] for some b, c with 0 < b < c < 1, then the sequence {xn} defined by the following manners: x1 = xC and
(1.11)
converges weakly to an element z of S(C, A), where K is the 2-uniformly smoothness constant of E and QC is a sunny nonexpansive retraction.

In 2011, Katchang and Kumam [15] presented an iterative algorithm for finding a common solution of fixed point problems and a general system of variational inequality problems for two accretive operators as shown in the following: for all n ≥ 0,
(1.12)
They proved that the sequence {xn} generated by the above algorithm converges strongly to a point . Moreover, they apply their theorem to find zeros of accretive operators and the class of k-strictly pseudocontractive mappings.
Recently, Petrot et al. [16] considered the problem so-called quasivariational inclusion problem, that is, determine an element uH such that
(1.13)
where A : HH is a single-valued nonlinear mapping and M : H → 2H is a multivalued mapping. The set of solutions of the above problem is denoted by VI (H, A, M). Therefore, they presented a new iterative scheme for finding a common element of the set of fixed points of a nonexpansive mapping and the set of solutions of variational inclusion problem with a multivalued maximal monotone mapping and an α-inverse-strongly monotone mapping by using the iterative sequence {xn} defined as follows:
(1.14)
and, under appropriated conditions, they proved the the sequence {xn} generated by (1.14) converges strongly to a point z0H, which is the unique solution in F(S)∩VI (H, A, M) to the following variational inequality:
(1.15)
Very recently, Qin et al. [17] introduced an iterative scheme for a general variational inequality (VI ) and proved the strong convergence theorems of common solutions of two variational inequalities in a uniformly convex and 2-uniformly smooth Banach space by using the following iterative sequence {xn}:
(1.16)
They proved that the sequence {xn} generated by the above algorithm converges strongly to a point q = QVIu, where QVI is the unique sunny nonexpansive retraction from C onto VI.

Motivated and inspired by the above recent works, in this paper, we introduce an iterative scheme for finding zeros of maximal accretive operators. Furthermore, we prove some strong convergence theorems and also propose applications for solving the convex feasibility problems. Our results improve and extend the corresponding results of Qin et al. [17] and Katchang and Kumam [15], Petrot et al. [16], and many others.

2. Preliminaries

Note that, if C and D are nonempty subsets of a Banach space E such that D is a subset of a closed convex subset C and Q : CD. Then Q is said to be sunny if
(2.1)
whenever Qx + t(xQx) ∈ C for any xC and t ≥ 0. A subset D of C is said to be a sunny nonexpansive retract of C if there exists a sunny nonexpansive retraction Q of C onto D. A mapping Q : CC is called a retraction if Q2 = Q. If a mapping Q : CC is a retraction, then Qz = z for all z is in the range of Q (see [4, 18] for more details).

The following result describes a characterization of sunny nonexpansive retractions on a smooth Banach space.

Proposition 2.1 (see [19].)Let E be a smooth Banach space and let C be a nonempty subset of E. Let Q : EC 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)

    QxQy2 ≤ 〈xy, J(QxQy)〉 for all x, yE;

  • (3)

    xQx, J(yQx)〉≤0 for all xE and yC.

Proposition 2.2 (see [20].)Let C be a nonempty closed convex subset of a uniformly convex and let uniformly smooth Banach space E and T be a nonexpansive mapping of C into itself with F(T) ≠ . Then the set F(T) is a sunny nonexpansive retract of C.

We need the following lemmas in order to prove our main results.

Lemma 2.3 (see [5].)Let E be a real 2-uniformly smooth Banach space with the best smooth constant K. Then the following inequality holds:

(2.2)

Lemma 2.4 (see [21].)Let {xn} and {yn} be bounded sequences in a Banach space E and {βn} be a sequence in [0,1] with

(2.3)
Suppose that xn+1 = (1 − βn)yn + βnxn for all n ≥ 0 and
(2.4)
Then lim nynxn∥ = 0.

Lemma 2.5 (see [22].)Assume that {an} is a sequence of nonnegative real numbers such that

(2.5)
where {αn} is a sequence in (0,1) and {δn} is a sequence in such that
  • (1)

    ;

  • (2)

    limsup nδn/αn ≤ 0 or .

Then lim nan = 0.

Lemma 2.6 (see [23].)Let C be a closed convex subset of a strictly convex Banach space E. Let Tm : CC be a nonexpansive mappings for each 1 ≤ mr, where r is some integer. Suppose that is nonempty. Let {λn} be a sequence of positive numbers with . Then the mapping S : CC defined by

(2.6)
is well defined, nonexpansive, and holds.

Lemma 2.7 (see [24].)Let C be a nonempty bounded closed convex subset of a uniformly convex Banach space E and T be nonexpansive mapping of C into itself. If {xn} is a sequence in C such that xnx weakly and xnTxn → 0 strongly, then x is a fixed point of T.

Lemma 2.8 (see [3], [4].)Let C be a nonempty closed convex subset of a real 2-uniformly smooth Banach space E. Let a mapping A : CE be λ-inverse-strongly accretive. Then one has

(2.7)

If λρ2K2, then Iρ2A is nonexpansive.

Proof. For any x, yC, it follows from Lemma 2.3 that

(2.8)
If λρ2K2, then Iρ2A is nonexpansive. This completes the proof.

Lemma 2.9. Let C be a nonempty subset of a Banach space E. Let A be a mapping of C into E, M be a maximal accretive operator on E and JM,ρ = (I + ρM) −1 be the resolvent of M for any ρ > 0. Then F(JM,ρ(IρA)) = (A + M) −1(0) for all ρ > 0.

Proof. Let ρ > 0 be fixed. Then we have

(2.9)
This completes the proof.

Lemma 2.10. Let E be a Banach space. Then for all x, yE,

(2.10)

3. Main Results

In this section, we prove strong convergence theorems for a λ-inverse-strongly accretive mapping A : CE and a β-inverse-strongly accretive B : CE in a real 2-uniformly smooth Banach space E.

In order to prove our main results, we need the following lemma.

Lemma 3.1. Let C be a nonempty closed convex subset of a real 2-uniformly smooth Banach space E with the best smooth constant K. Let be a resolvent operator associated with M1, M2, where M1, M2 : E → 2E is a multivalued maximal accretive mapping. Let the mappings A, B : CE be λ-inverse-strongly accretive and β-inverse-strongly accretive, respectively. Let G : CC be a mapping defined by

(3.1)
If λρ2K2 and βρ1K2, then G is nonexpansive.

Proof. Since and are nonexpansive, for any x, yC, it follows from Lemma 2.8 that

(3.2)
Therefore, G is nonexpansive. This completes the proof.

Next, we state the main result of this work.

Theorem 3.2. Let E be a uniformly convex and 2-uniformly smooth Banach space which admits a weakly sequentially continuous duality mapping and C be a nonempty closed convex subset of E. Let A, B : CE be λ-inverse-strongly accretive and β-inverse-strongly accretive, respectively, and K be the best smooth constant. Let f be a contraction of E into itself with coefficient α ∈ [0,1). Suppose that Ω : = (A + M2) −1(0)∩(B + M1) −1(0) ≠ and G is a mapping defined by Lemma 3.1. Let ρ1, ρ2 be any positive real numbers such that ρ1β/K2 and ρ2λ/K2. For arbitrary x0 = xC, define the iterative sequence {xn} as follows:

(3.3)
where the sequences {αn}, {βn}, and {γn} in (0,1) satisfy the following conditions:
  • (C1)

    αn + βn + γn = 1;

  • (C2)

    lim nαn = 0 and ;

  • (C3)

    0 < liminf nβn ≤ limsup nβn < 1;

  • (C4)

    lim nδn = δ ∈ (0,1).

Then the sequence {xn} generated by (3.3) converges strongly to a point q = QΩf(q), where QΩ is a sunny nonexpansive retraction on Ω.

Proof. First, we prove that and are nonexpansive mappings. Consider the following:

(3.4)
Thus, it follows that is nonexpansive and so is .

Step 1. We show that {xn} is bounded. For any p ∈ Ω, we have

(3.5)
It follows by induction that
(3.6)
Thus the sequence {xn} is bounded and so is {yn}.

Step 2. We show that lim nxn+1xn∥ = 0. Let and for each n ≥ 0. Then we have

(3.7)
and so
(3.8)
where M1 ia an appropriate constant such that M1 ≥ sup n≥0{∥unvn∥}.

Next, let zn = (xn+1βnxn)/(1 − βn) for all n ≥ 0. Then we have xn+1 = (1 − βn)zn + βnxn for all n ≥ 0. Now, we compute

(3.9)
and so
(3.10)
Substituting (3.8) into (3.10), we get
(3.11)
that is,
(3.12)
From the conditions (C2) and (C3), it follows that
(3.13)
Thus, from Lemma 2.4, it follows that
(3.14)
From the definition of xn+1 in this step, we observe that xn+1xn = (1 − βn)(znxn). Then we have
(3.15)

Step 3. We show that limsup n〈(fI)q, J(xnq)〉≤0, where q = QΩf(q). Define a mapping G : CC by Lemma 3.1 Then, it follows that G is a nonexpansive mapping such that

(3.16)
Consider the following:
(3.17)
From the condition (C4), we have
(3.18)
Next, we consider
(3.19)
Therefore, we have
(3.20)
From the conditions (C2), (C3), (3.15), (3.18), and the inequality above, we obtain
(3.21)
Thus, since QΩf(q) is a contraction, there exists a unique fixed point. We denote that q is the unique fixed point to the mapping QΩf(q) which means that q = QΩf(q).

Since {xn} is bounded, there exists a subsequence of {xn} such that , it follows from (3.21) that

(3.22)
Since G is nonexpansive, it follows from Lemma  2.7 that p = Gp we obtain that pF(G). By (3.22), we have p ∈ Ω.

Furthermore, with the reason that {xn} is bounded, we can choose the sequence of {xn} which such that

(3.23)
Now, from (3.23) and Proposition 2.1(3) and the weakly sequential continuity of the duality mapping J, we have
(3.24)
From (3.15), it follows that
(3.25)

Step 4. We show that {xn} converges strongly to a point q = QΩf(q). In fact, observe that

(3.26)
Thus it follows that
(3.27)
Therefore, from Condition (C2), (3.25), and Lemma 2.5, we get ∥xnq∥→0 as n. This completes the proof.

Corollary 3.3. Let E be a uniformly convex and 2-uniformly smooth Banach space which admits a weakly sequentially continuous duality mapping and let C be a nonempty closed convex subset of E. Let A, B : CE be λ-inverse-strongly accretive and β-inverse-strongly accretive, respectively, and K be the best smooth constant. Suppose that Ω : = (A + M2) −1(0)∩(B + M1) −1(0) ≠ , where G is a mapping defined by Lemma 3.1. Let ρ1, ρ2 be any positive real numbers such that ρ1β/K2 and ρ2λ/K2. For arbitrary x0 = xC, define the iterative sequence {xn} by

(3.28)
where the sequences {αn}, {βn}, and {γn} in (0,1) satisfy the following conditions:
  • (C1)

    αn + βn + γn = 1;

  • (C2)

    lim nαn = 0 and ;

  • (C3)

    0 < liminf nβn ≤ limsup nβn < 1;

  • (C4)

    lim nδn = δ ∈ (0,1).

Then the sequence {xn} generated by (3.28) converges strongly to a point q = QΩf(q), where QΩ is a sunny nonexpansive retraction on Ω.

Proof. Take f(xn) = u for all n ≥ 1 for any fixed uC in (3.3). Then, by Theorem 3.2, we can conclude the desired conclusion easily.

4. Applications

4.1. Application to Convex Feasibility Problems

In this part, we consider the following convex feasibility problem (CFP): find , where j ∈ {1,2, …, N} and Cj denotes the set of zeros of a maximal accretive operator.

The following result can be obtained from Theorem 3.2.

Theorem 4.1. Let E be a uniformly convex and 2-uniformly smooth Banach space which admits a weakly sequentially continuous duality mapping and let C be a nonempty closed convex subset of E. Let be an λi-inverse-strongly accretive and K be the best smooth constant. Let f be a contraction of E into itself with coefficient α ∈ [0,1). Suppose that , where G is a mapping defined by Lemma 3.1. Let ρi be any positive real numbers such that ρiλi/K2, i = 1,2, 3, …, N. For arbitrary x0 = xC, define the iterative sequence {xn} by

(4.1)
where the sequences {αn}, {βn}, and {γn} in (0,1) satisfy the following conditions:
  • (C1)

    αn + βn + γn = 1 and ;

  • (C2)

    lim nαn = 0 and ;

  • (C3)

    0 < liminf nβn ≤ limsup nβn < 1;

  • (C4)

    lim nδi,n = δ ∈ (0,1).

Then the sequence {xn} generated by (4.1) converges strongly to a point q = QΩ, where QΩ is a sunny nonexpansive retraction on Ω.

4.2. Application to Hilbert Spaces

Assume that H is a real Hilbert space with inner product 〈·, ·〉 and norm ∥·∥. Let A : HH be a single-valued nonlinear mapping and M : H → 2H be a multivalued mapping. The problem of finding uH such that
(4.2)
is called the quasivariational inclusion problem, and we denote the set of solutions of the above variational inclusion by VI (H, A, M).
If M = δC, where C is a nonempty closed convex subset of H and δC : H → [0, ] is the indicator function of C, that is,
(4.3)
Then the variational inclusion problem (4.2) is equivalent to the problem of finding uC such that
(4.4)
which is the well-known Hartman-Stampacchia variational inequality problem [25].

Theorem 4.2. Let C be a closed convex subset of a real Hilbert space H. Let A, B : CH be λ-inverse-strongly monotone and β-inverse-strongly monotone, respectively. Let f be a contraction of E into itself with coefficient α ∈ [0,1). Suppose that Ω : = VI (C, A)∩VI (C, B) ≠ , where VI (C, A) and VI (C, B) are the sets of solutions of variational inequality (4.4). For arbitrary x0 = xC, define the iterative sequence {xn} by

(4.5)
where the sequences {αn}, {βn}, and {γn} in (0,1) satisfy the following conditions:
  • (C1)

    αn + βn + γn = 1;

  • (C2)

    lim nαn = 0 and ;

  • (C3)

    0 < liminf nβn ≤ limsup nβn < 1;

  • (C4)

    lim nδn = δ ∈ (0,1).

Then the sequence {xn} generated by (4.5) converges strongly to a point PΩx0.

Proof. Take M = δC : H → 2H, where δC : H → [0, ] is the indicator function of C. Let J(M, ρ) = I. Then we get

(4.6)
Thus the conclusion can be obtained from Theorem 3.2 immediately.

Acknowledgments

This paper was supported by the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission (under the project NRU-CSEC no. 54000267) for financial support during the preparation of this paper. Furthermore, the first author would like to thank the Office of the Higher Education Commission, Thailand, for the financial support of the Ph.D. program at KMUTT. This research was partially finished at Department of Mathematics Education, Gyeongsang National University, Republic of Korea. Also, the second author was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (Grant no. 2011-0021821).

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