Convergence of an Iterative Algorithm for Common Solutions for Zeros of Maximal Accretive Operator with Applications
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
- (1)
Jq(x) = ∥x∥q−2J2(x) for all x ∈ E with x ≠ 0;
- (2)
Jq(tx) = tq−1Jq(x) for all x ∈ E and t ∈ [0, ∞);
- (3)
Jq(−x) = −Jq(x) for all x ∈ E.
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 xn → x 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 [1–3].
- (1)
uniformly convex if there exists δ > 0 such that, for any x, y ∈ U and, for any ϵ ∈ (0,2], ∥x − y∥≥ϵ implies ∥(x + y)/2∥≤1 − δ.
- (2)
Smooth if lim t→0(∥x + ty∥−∥x∥)/t exists for all x, y ∈ U.
- (3)
Uniformly smooth if the limit is attained uniformly for x, y ∈ U. 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→0 ρ(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).
- (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
Definition 1.1. Let M : E → 2E be a multivalued maximal accretive mapping. The single-valued mapping JM,ρ : E → E defined by
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
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
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 : E → C 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)
∥Qx − Qy∥2 ≤ 〈x − y, J(Qx − Qy)〉 for all x, y ∈ E;
- (3)
〈x − Qx, J(y − Qx)〉≤0 for all x ∈ E and y ∈ C.
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:
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
Lemma 2.5 (see [22].)Assume that {an} is a sequence of nonnegative real numbers such that
- (1)
;
- (2)
limsup n→∞δn/αn ≤ 0 or .
Lemma 2.6 (see [23].)Let C be a closed convex subset of a strictly convex Banach space E. Let Tm : C → C be a nonexpansive mappings for each 1 ≤ m ≤ r, where r is some integer. Suppose that is nonempty. Let {λn} be a sequence of positive numbers with . Then the mapping S : C → C defined by
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 xn → x weakly and xn − Txn → 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 : C → E be λ-inverse-strongly accretive. Then one has
If λ ≥ ρ2K2, then I − ρ2A is nonexpansive.
Proof. For any x, y ∈ C, it follows from Lemma 2.3 that
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(I − ρA)) = (A + M) −1(0) for all ρ > 0.
Proof. Let ρ > 0 be fixed. Then we have
Lemma 2.10. Let E be a Banach space. Then for all x, y ∈ E,
3. Main Results
In this section, we prove strong convergence theorems for a λ-inverse-strongly accretive mapping A : C → E and a β-inverse-strongly accretive B : C → E 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 : C → E be λ-inverse-strongly accretive and β-inverse-strongly accretive, respectively. Let G : C → C be a mapping defined by
Proof. Since and are nonexpansive, for any x, y ∈ C, it follows from Lemma 2.8 that
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 : C → E 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 ρ1 ≤ β/K2 and ρ2 ≤ λ/K2. For arbitrary x0 = x ∈ C, define the iterative sequence {xn} as follows:
- (C1)
αn + βn + γn = 1;
- (C2)
lim n→∞αn = 0 and ;
- (C3)
0 < liminf n→∞βn ≤ limsup n→∞βn < 1;
- (C4)
lim n→∞δn = δ ∈ (0,1).
Proof. First, we prove that and are nonexpansive mappings. Consider the following:
Step 1. We show that {xn} is bounded. For any p ∈ Ω, we have
Step 2. We show that lim n→∞∥xn+1 − xn∥ = 0. Let and for each n ≥ 0. Then we have
Next, let zn = (xn+1 − βnxn)/(1 − βn) for all n ≥ 0. Then we have xn+1 = (1 − βn)zn + βnxn for all n ≥ 0. Now, we compute
Step 3. We show that limsup n→∞〈(f − I)q, J(xn − q)〉≤0, where q = QΩf(q). Define a mapping G : C → C by Lemma 3.1 Then, it follows that G is a nonexpansive mapping such that
Since {xn} is bounded, there exists a subsequence of {xn} such that , it follows from (3.21) that
Furthermore, with the reason that {xn} is bounded, we can choose the sequence of {xn} which such that
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 : C → E 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 ρ1 ≤ β/K2 and ρ2 ≤ λ/K2. For arbitrary x0 = x ∈ C, define the iterative sequence {xn} by
- (C1)
αn + βn + γn = 1;
- (C2)
lim n→∞αn = 0 and ;
- (C3)
0 < liminf n→∞βn ≤ limsup n→∞βn < 1;
- (C4)
lim n→∞δn = δ ∈ (0,1).
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 ρi ≤ λi/K2, i = 1,2, 3, …, N. For arbitrary x0 = x ∈ C, define the iterative sequence {xn} by
- (C1)
αn + βn + γn = 1 and ;
- (C2)
lim n→∞αn = 0 and ;
- (C3)
0 < liminf n→∞βn ≤ limsup n→∞βn < 1;
- (C4)
lim n→∞δi,n = δ ∈ (0,1).
4.2. Application to Hilbert Spaces
Theorem 4.2. Let C be a closed convex subset of a real Hilbert space H. Let A, B : C → H 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 = x ∈ C, define the iterative sequence {xn} by
- (C1)
αn + βn + γn = 1;
- (C2)
lim n→∞αn = 0 and ;
- (C3)
0 < liminf n→∞βn ≤ limsup n→∞βn < 1;
- (C4)
lim n→∞δn = δ ∈ (0,1).
Proof. Take M = ∂δC : H → 2H, where δC : H → [0, ∞] is the indicator function of C. Let J(M, ρ) = I. Then we get
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).