Research

# Strong convergence theorems for total quasi-ϕ-asymptotically nonexpansive multi-valued mappings in Banach spaces

Jinfang Tang1 and Shih-sen Chang2*

Author Affiliations

1 Department of Mathematics, Yibin University, Yibin, Sichuan 644007, China

2 College of statistics and mathematics, Yunnan University of Finance and Economics, Kunming, Yunnan 650221, China

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Fixed Point Theory and Applications 2012, 2012:63  doi:10.1186/1687-1812-2012-63

 Received: 6 December 2011 Accepted: 19 April 2012 Published: 19 April 2012

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

The main purpose of this article is to introduce the concept of total quasi-ϕ-asymptotically nonexpansive multi-valued mapping and prove the strong convergence theorem in a real uniformly smooth and strictly convex Banach space with Kadec-Klee property. In order to get the theorems, the hybrid algorithms are presented and are used to approximate the fixed point. The results presented in this article improve and extend some recent results announced by some authors.

AMS (MOS) Subject Classification: 47J06, 47J25.

##### Keywords:
total quasi-ϕ-asymptotically nonexpansive multi-valued mappings; total quasi-ϕ-asymptotically nonexpansive mappings; quasi-ϕ-asymptotically non-expansive mappings.

### 1 Introduction

Throughout this article, we always assume that X is a real Banach space with the dual X* and J : X → 2X is the normalized duality mapping defined by

In the sequal, we use F(T) to denote the set of fixed points of a mapping T, and use ℛ and ℛ+ to denote the set of all real numbers and the set of all nonnegative real numbers, respectively. We denote by xn x and xn x the strong convergence and weak convergence of a sequence {xn}, respectively.

A Banach space X is said to be strictly convex if for all x, y U = {z X : ||z|| = 1} with x y. X is said to be uniformly convex if, for each ϵ ∈ (0, 2], there exists δ > 0 such that for all x, y U with ||x - y|| ≥ ϵ. X is said to be smooth if the limit

exists for all x, y U. X is said to be uniformly smooth if the above limit is attained uniformly in x, y U.

Remark 1.1. The following basic properties of a Banach space X can be found in Cioranescu [1].

(i) If X is uniformly smooth, then J is uniformly continuous on each bounded subset of X;

(ii) If X is a reflexive and strictly convex Banach space, then J-1 is norm-weak-continuous;

(iii) If X is a smooth, strictly convex and reflexive Banach space, then J is single-valued, one-to-one and onto;

(iv) A Banach space X is uniformly smooth if and only if X* is uniformly convex;

(v) Each uniformly convex Banach space X has the Kadec-Klee property, i.e., for any sequence {xn} ⊂ X, if xn x X and ||xn|| → ||x||, then xn x.

Let X be a smooth Banach space. We always use ϕ : X × X → ℛ+ to denote the Lyapunov functional defined by

(1.1)

It is obvious from the definition of the function ϕ that

(1.2)

Following Alber [2], the generalized projection ΠC : X C is defined by

Lemma 1.2. [2] Let X be a smooth, strictly convex and reflexive Banach space and C be a nonempty closed convex subset of X. Then the following conclusions hold:

(a) ϕ(x, ΠCy) + ϕCy, y) ≤ ϕ(x, y) for all x C and y X;

(b) If x X and z C, then

(c) For x, y X, ϕ(x, y) = 0 if and only if x = y.

Let X be a smooth, strictly convex and reflexive Banach space and C be a nonempty closed convex subset of X and T : C C be a mapping. A point p C is said to be an asymptotic fixed point of T if there exists a sequence {xn} ⊂ C such that xn p and ||xn - Txn|| → 0. We denoted the set of all asymptotic fixed points of T by .

Definition 1.3. (1) A mapping T : C C is said to be relatively nonexpansive [3] if and

(2) A mapping T : C C is said to be closed if, for any sequence {xn} ⊂ C with xn x and Txn y, then Tx = y.

Definition 1.4. (1) A mapping T : C C is said to be quasi-ϕ-nonexpansive if and

(2) A mapping T : C C is said to be quasi-ϕ-asymptotically nonexpansive if and there exists a real sequence {kn} ⊂ [1, ∞) with kn → 1 such that

(1.3)

(3) A mapping T : C C is said to be total quasi-ϕ-asymptotically nonexpansive if and there exist nonnegative real sequences {νn}, {μn} with νn → 0, μn → 0 (as n → ∞) and a strictly increasing continuous function ζ : ℛ+ → ℛ+ with ζ(0) = 0 such that for all x C, p F(T)

(1.4)

Remark 1.5. From the definitions, it is easy to know that

(1) Taking ζ(t) = t, t ≥ 0, νn = kn - 1 and μn = 0, then νn → 0(as n → ∞) and (1.3) can be rewritten as

(1.5)

This implies that the class of total quasi-ϕ-asymptotically nonexpansive mappings contains properly the class of quasi-ϕ-asymptotically nonexpansive mappings as a subclass, but the converse is not true.

(2) The class of quasi-ϕ-asymptotically nonexpansive mappings contains properly the class of quasi-ϕ-nonexpansive mappings as a subclass, but the converse is not true.

(3) The class of quasi-ϕ-nonexpansive mappings contains properly the class of relatively nonexpansive mappings as a subclass, but the converse is not true.

Let C be a nonempty closed convex subset of a Banach space X. Let N(C) be the family of nonempty subsets of C.

Definition 1.6. (1) A multi-valued mapping T : C N(C) is said to be relatively nonexpansive [3] if and

(2) A multi-valued mapping T : C N(C) is said to be closed if, for any sequence {xn} ⊂ C with xn x and wn T(xn) with wn y, then y Tx.

Definition 1.7. (1) A multi-valued mapping T : C N(C) is said to be quasi-ϕ-nonexpansive if and

(2) A multi-valued mapping T : C N(C) is said to be quasi-ϕ-asymptotically non-expansive if and there exists a real sequence {kn} ⊂ [1, ∞) with kn → 1 such that

(1.6)

(3) A multi-valued mapping T : C N(C) is said to be total quasi-ϕ-asymptotically nonexpansive if and there exist nonnegative real sequences {νn}, {μn} with νn → 0, μn → 0(as n → ∞) and a strictly increasing continuous function ζ : ℛ+ → ℛ+ with ζ(0) = 0 such that for all x C, p F(T)

(1.7)

(4) A total quasi-ϕ-asymptotically nonexpansive multi-valued mapping T : C N(C) is said to be uniformly L-Lipschitz continuous if there exists a constant L > 0 such that

Remark 1.8. From the definitions, it is easy to know that

(1) Taking ζ(t) = t, t ≥ 0, νn = kn - 1 and μn = 0, then νn → 0 (as n → ∞) and (1.6) can be rewritten as

This implies that the class of total quasi-ϕ-asymptotically nonexpansive multi-valued mappings contains properly the class of quasi-ϕ-asymptotically nonexpansive multi-valued mappings as a subclass, but the converse is not true.

(2) The class of quasi-ϕ-asymptotically nonexpansive multi-valued mappings contains properly the class of quasi-ϕ-nonexpansive multi-valued mappings as a subclass, but the converse is not true.

(3) The class of quasi-ϕ-nonexpansive multi-valued mappings contains properly the class of relatively nonexpansive multi-valued mappings as a subclass, but the converse is not true.

In 2005, Matsushita and Takahashi [3] proved weak and strong convergence theorems to approximate a fixed point of a single relatively nonexpansive mapping in a uniformly convex and uniformly smooth Banach space X. In 2008, Plubtieng and Ungchittrakool [4] proved the strong convergence theorems to approximate a fixed point of two relatively nonexpansive mapping in a uniformly convex and uniformly smooth Banach space X. In 2010, Chang et al. [5] obtained the strong convergence theorem for an infinite family of quasi-ϕ-asymptotically nonexpansive mappings in a uniformly smooth and strictly convex Banach space X with Kadec-Klee property. In 2011, Chang et al. [6] proved some approximation theorems of common fixed points for countable families of total quasi-ϕ-asymptotically nonexpansive mappings in a uniformly smooth and strictly convex Banach space X with Kadec-Klee property. In 2011, Homaeipour and Razani [7] proved weak and strong convergence theorems for a single relatively nonexpansive multi-valued mapping in a uniformly convex and uniformly smooth Banach space X.

Motivated and inspired by the researches going on in this direction, the purpose of this article is first to introduce the concept of total quasi-ϕ-asymptotically nonexpansive multivalued mapping which contains many kinds of mappings as its special cases, and then by using the hybrid iterative algorithm to prove some strong convergence theorems in uniformly smooth and strictly convex Banach space with Kadec-Klee property. The results presented in the article improve and extend some recent results announced by some authors.

### 2 Preliminaries

Lemma 2.1. [6] Let X be a real uniformly smooth and strictly convex Banach space with Kadec-Klee property, and C be a nonempty closed convex set of X. Let {xn} and {yn} be two sequences in C such that xn p and ϕ(xn, yn) → 0, where ϕ is the function defined by (1.1), then yn p.

Lemma 2.2. Let X and C be as in Lemma 2.1. Let T : C N(C) be a closed and total quasi-ϕ-asymptotically nonexpansive multi-valued mapping with nonnegative real sequences {νn}, {μn} and a strictly increasing continuous function ζ : ℛ+ → ℛ+ such that νn → 0, μn → 0(as n → ∞) and ζ(0) = 0. If μ1 = 0, then the fixed point set F(T) is a closed and convex subset of C.

Proof. Let {xn} be a sequence in F(T) with xn p (as n → ∞), we prove that p F(T). In fact, by the assumption that T is total quasi-ϕ-asymptotically nonexpansive multi-valued mapping and μ1 = 0, we have

Furthermore, we have

By Lemma 1.2(c), p = u. Hence, p Tp. This implies that p F(T), i.e., F(T) is closed.

Next, we prove that F(T) is convex. For any x, y F(T), t ∈ (0, 1), putting q = tx + (1 - t)y, we prove that q F(T). Indeed, let {un} be a sequence generated by

(2.1)

In view of the definition of ϕ(x, y), for all un Tun-1 Tnq, we have

(2.2)

Since

(2.3)

Substituting (2.2) into (2.1) and simplifying it we have

By Lemma 2.1, we have un q (as n → ∞). This implies that un+1 q (as n → ∞). Since T is closed, we have q Tq, i.e., q F(T).

This completes the proof of Lemma2.2.

Lemma 2.3. [8] Let X be a uniformly convex Banach space, r > 0 be a positive number and Br(0) be a closed ball of X. Then, there exists a continuous, strictly increasing and convex function g : [0, ∞) → [0, ∞) with g(0) = 0 such that

(2.4)

for all x, y Br(0) and all α, β ∈ [0, 1] with α + β = 1.

### 3 Main results

In this section, we shall use the hybrid iterative algorithm to study the iterative solutions of nonlinear operator equations with a closed and uniformly total quasi-ϕ-asymptotically nonexpansive multi-valued mapping in Banach space.

Theorem 3.1. Let X be a real uniformly smooth and strictly convex Banach space with Kadec-Klee property, and C be a nonempty closed and convex subset of X. Let T : C N(C) be a closed and total quasi-ϕ-asymptotically nonexpansive multi-valued mapping with nonnegative real sequences {νn},{μn} and a strictly increasing continuous function ζ : ℛ+ → ℛ+ such that μ1 = 0, νn → 0, μn → 0 (as n → ∞) and ζ(0) = 0. Let x0 C, C0 = C and {xn} be a sequence generated by

(3.1)

where wn Tnxn, ∀n ≥ 1, ξn = νn suppF(T) ζ(ϕ(p, xn)) + μn, is the generalized projection of X onto Cn+1, {αn} and {βn} are sequences in [0,1] satisfies the following conditions:

(a) lim infn→∞ βn(1 - βn) > 0;

(b) 0 ≤ αn α < 1 for some α ∈ (0, 1).

If F(T) is a nonempty and bounded subset of C, then the sequence {xn} converges strongly to ΠF(T)x0.

Proof. We divide the proof of Theorem 3.1 into six steps.

(I) Cn is closed and convex for each n ≥ 0.

In fact, by the assumption, C0 = C is closed and convex. Suppose that Cn is closed and convex for some n ≥ 1. Since the condition ϕ(ν, yn) ≤ ϕ(ν, xn) + ξn is equivalent to

hence the set

is closed and convex. Therefore Cn is closed and convex for each n ≥ 0.

(II) {xn} is bounded and {ϕ(xn, x0)} is a convergent sequence.

Indeed, it follows from (3.1) and Lemma 1.2(a) that for all n ≥ 0, u F(T)

This implies that {ϕ(xn, x0)} is bounded. By virtue of (1.2), we know that {xn} is bounded.

In view of structure of {Cn}, we have Cn+1 Cn, and . This implies that xn+1 Cn and

Therefore {ϕ(xn, x0)} is a convergent sequence.

(III) F(T) ⊂ Cn for all n ≥ 0.

It is obvious that F(T) ⊂ C0 = C. Suppose that F(T) ⊂ Cn for some n ≥ 1. Since X is uniformly smooth, X* is uniformly convex. For any given u F(T) ⊂ Cn and n ≥ 1 we have

(3.2)

Furthermore, it follows from Lemma 2.3 that for any u F(T), wn Tnxn we have

(3.3)

Substituting (3.3) into (3.2) and simplifying it, we have

(3.4)

i.e., u Cn+1 and so F(T) ⊂ Cn+1 for all n ≥ 0.

By the way, in view of the assumption on {νn}, {μn} we have

(IV) {xn} converges strongly to some point p* ∈ C.

In fact, since {xn} is bounded and X is reflexive, there exists a subsequence such that (some point in C). Since Cn is closed and convex and Cn+1 Cn, this implies that Cn is weakly closed and p* ∈ Cn for each n ≥ 0. In view of , we have

Since the norm || · || is weakly lower semi-continuous, we have

and so

This implies that , and so . Since , by virtue of Kadec-Klee property of X, we obtain that

Since {ϕ(xn, x0)} is convergent, this together with , which shows that limn→∞ ϕ(xn, x0) = ϕ(p*, x0). If there exists some sequence such that , then from Lemma 1.2(a) we have that

This implies that p* = q and

(3.5)

(V) Now we prove that p* ∈ F(T).

In fact, since xn+1 Cn+1 Cn, it follows from (3.1) and (3.5) that

Since xn p*, by the virtue of Lemma 2.1

(3.6)

From (3.2) and (3.3), for any u F(T) and wn Tnxn, we have

i.e.,

By conditions (a) and (b) it shows that limn→∞ g(||Jxn - Jwn||) = 0. In view of property of g, we have

Since Jxn Jp*, this implies that Jwn Jp*. From Remark 1.1 (ii) it yields

(3.7)

Again since

this together with (3.7) and the Kadec-Klee property of X shows that

(3.8)

Let {sn} be a sequence generated by

By the assumption that T is uniformly L-Lipschitz continuous, hence for any wn Tnxn and sn+1 Twn Tn+1xn we have

(3.9)

This together with (3.5) and (3.8) shows that limn→∞ ||sn+1 - wn|| = 0 and limn→∞ sn+1 = p*. In view of the closeness of T, it yields that p* ∈ Tp*, i.e.,

(VI) we prove that xn p* = ΠF(T)x0.

Let t = ΠF(T)x0. Since t F(T) ⊂ Cn and , we have

This implies that

(3.10)

In view of the definition of ΠF(T)x0, from (3.10) we have p* = t. Therefore, xn p* = ΠF(T)x0.

This completes the proof of Theorem 3.1.

From Remark 1.8, the following theorems can be obtained from Theorem 3.1 immediately.

Theorem 3.2. Let X be a real uniformly smooth and strictly convex Banach space with Kadec-Klee property, and C be a nonempty closed and convex subset of X. Let T : C N(C) be a closed and quasi-ϕ-asymptotically nonexpansive multi-valued mapping with a real sequences {kn} ⊂ [1, ∞) and kn → 1(n → (∞). Let x0 C, C0 = C and {xn} be a sequence generated by

where ξn = (kn - 1) suppF(T)(ϕ(p, xn)), is the generalized projection of X onto Cn+1, {αn} and {βn} are sequences in [0,1] satisfies the following conditions:

(a) lim infn→∞ βn(1 - βn) > 0;

(b) 0 ≤ αn α < 1 for some α ∈ (0, 1).

If F(T) is a nonempty and bounded subset of C, then the sequence {xn} converges strongly to ΠF(T)x0.

Theorem 3.3. Let X be a real uniformly smooth and strictly convex Banach space with Kadec-Klee property, and C be a nonempty closed and convex subset of X. Let T : C N(C) be a closed and quasi-ϕ nonexpansive multi-valued mapping. Let x0 C, C0 = C and {xn} be a sequence generated by

where is the generalized projection of X onto Cn+1, {αn} and {βn} are sequences in [0,1] satisfies the following conditions:

(a) lim infn→∞ βn(1 - βn) > 0;

(b) 0 ≤ αn α < 1 for some α ∈ (0, 1).

If F(T) is a nonempty and bounded subset of C, then the sequence {xn} converges strongly to ΠF(T)x0.

Theorem 3.4. Let X be a real uniformly smooth and strictly convex Banach space with Kadec-Klee property, and C be a nonempty closed and convex subset of X. Let T : C N(C) be a closed and relatively nonexpansive multi-valued mapping. Let x0 C, C0 = C and {xn} be a sequence generated by

where is the generalized projection of X onto Cn+1, {αn} and {βn} are sequences in [0,1] satisfies the following conditions:

(a) lim infn→∞ βn(1 - βn) > 0;

(b) 0 ≤ αn α < 1 for some α ∈ (0, 1).

If F(T) is a nonempty and bounded subset of C, then the sequence {xn} converges strongly to ΠF(T)x0.

### Competing interests

The authors declare that they have no competing interests.

### Authors' contributions

All the authors contributed equally to the writing of the present article. And they also read and approved the final manuscript.

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