# Iterative algorithms with errors for zero points of *m*-accretive operators

- Xiaolong Qin
^{1}, - Sun Young Cho
^{2}and - Lin Wang
^{3}Email author

**2013**:148

https://doi.org/10.1186/1687-1812-2013-148

© Qin et al.; licensee Springer. 2013

**Received: **20 March 2013

**Accepted: **21 May 2013

**Published: **6 June 2013

## Abstract

In this paper, we study the convergence of paths for continuous pseudocontractions in a real Banach space. As an application, we consider the problem of finding zeros of *m*-accretive operators based on an iterative algorithm with errors. Strong convergence theorems for zeros of *m*-accretive operators are established in a real Banach space.

**MSC:**47H05, 47H09, 47J25, 65J15.

### Keywords

accretive operator fixed point nonexpansive mapping iterative algorithm zero point## 1 Introduction and preliminaries

*C*be a nonempty closed convex subset of a real Banach space

*E*and let ${E}^{\ast}$ be the dual space of

*E*. Let $\u3008\cdot ,\cdot \u3009$ denote the pairing between

*E*and ${E}^{\ast}$. The normalized duality mapping $J:E\to {2}^{{E}^{\ast}}$ is defined by

*j*to denote the single-valued normalized duality mapping. Let ${U}_{E}=\{x\in E:\parallel x\parallel =1\}$.

*E*is said to be

*smooth*or is said to have a

*Gâteaux differentiable norm*if the limit

exists for each $x,y\in {U}_{E}$. *E* is said to have a *uniformly Gâteaux differentiable norm* if for each $y\in {U}_{E}$, the limit is attained uniformly for all $x\in {U}_{E}$. *E* is said to be *uniformly smooth* or is said to have a *uniformly Fréchet differentiable norm* if the limit is attained uniformly for $x,y\in {U}_{E}$. It is known that if the norm of *E* is uniformly Gâteaux differentiable, then the duality mapping *J* is single-valued and uniformly norm to weak^{∗} continuous on each bounded subset of *E*.

*C*of a Banach space

*E*is said to have a

*normal structure*if for each bounded closed convex subset

*K*of

*C*which contains at least two points, there exists an element

*x*of

*K*which is not a diametral point of

*K*,

*i.e.*,

where $d(K)$ is the diameter of *K*. It is well known that a closed convex subset of a uniformly convex Banach space has the normal structure and a compact convex subset of a Banach space has the normal structure; see [1] for more details.

*T*. Recall that

*T*is said to be

*contractive*if there exits a constant $\alpha \in (0,1)$ such that

*T*an

*α*-

*contraction*.

*T*is said to be a

*strong contraction*if there exits a constant $\alpha \in (0,1)$ and some $j(x-y)\in J(x-y)$ such that

*T*an

*α*-

*strong pseudocontraction.*

*T*is said to be

*nonexpansive*if

*T*is said to be pseudocontractive if there exists some $j(x-y)\in J(x-y)$ such that

*C*. That is,

In the case that *T* enjoys a fixed point, Browder [2] proved that if *E* is a Hilbert space, then ${x}_{t}$ converges strongly to a fixed point of *T*. Reich [3] extended Browder’s result to the setting of Banach spaces and proved that if *E* is a uniformly smooth Banach space, then ${x}_{t}$ converges strongly to a fixed point of *T*.

Let *D* be a nonempty subset of *C*. Let $Q:C\to D$. *Q* is said to be *contraction* if ${Q}^{2}=Q$; *sunny* if for each $x\in C$ and $t\in (0,1)$, we have $Q(tx+(1-t)Qx)=Qx$; *sunny nonexpansive retraction* if *Q* is sunny, nonexpansive, and contraction. *K* is said to be a *nonexpansive retract* of *C* if there exists a nonexpansive retraction from *C* onto *D*.

The following result, which was established in [4, 5], describes a characterization of sunny nonexpansive retractions on a smooth Banach space.

*E*be a smooth Banach space and let

*C*be a nonempty subset of

*E*. Let $Q:E\to C$ 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)
${\parallel Qx-Qy\parallel}^{2}\le \u3008x-y,j(Qx-Qy)\u3009$, $\mathrm{\forall}x,y\in E$;

- (3)
$\u3008x-Qx,j(y-Qx)\u3009\le 0$, $\mathrm{\forall}x\in E$, $y\in C$.

*f*is an

*α*-contraction and

*T*is a nonexpansive mapping. He showed that the sequence $\{{x}_{t}\}$ converges strongly to a point in $F(T)$ in a uniformly smooth Banach space. If one defines $Q:{\mathrm{\Pi}}_{C}\to F(T)$, where ${\mathrm{\Pi}}_{C}$ denotes the set of

*α*-contractions, by

- (1)
Can one extend the framework of the space from uniformly smooth Banach spaces to a more general Banach space?

- (2)
Can one extend the viscosity approximation method by considering strong pseudocontractions instead of contractions?

- (3)
Do Xu’s results still hold for a larger class of nonlinear mappings?

In Section 2, we give an affirmative answer to the above questions.

*I*denote the identity operator on

*E*. An operator $A\subset E\times E$ with domain $D(A)=\{z\in E:Az\ne \mathrm{\varnothing}\}$ and range $R(A)=\bigcup \{Az:z\in D(A)\}$ is said to be

*accretive*if for each ${x}_{i}\in D(A)$ and ${y}_{i}\in A{x}_{i}$, $i=1,2$, there exists $j({x}_{1}-{x}_{2})\in J({x}_{1}-{x}_{2})$ such that

*A*is said to be

*m*-

*accretive*if $R(I+rA)=E$ for all $r>0$. In a real Hilbert space, an operator

*A*is

*m*-accretive if and only if

*A*is maximal monotone. In this paper, we use ${A}^{-1}(0)$ to denote the set of zeros of

*A*. Interest in accretive operators, which is an important class of nonlinear operators, stems mainly from their firm connection with equations of evolution. It is known that many physically significant problems can be modeled by initial value problems of the form

*A*is an accretive operator in an appropriate Banach space. Typical examples where such evolution equations occur can be found in the heat, wave or Schrödinger equations. If $x(t)$ is dependent on

*t*, then (1.1) is reduced to

whose solutions correspond to the equilibrium points of the system (1.1). Consequently, considerable research efforts have been devoted, especially within the past 40 years or so, to methods for finding approximate solutions (when they exist) of equation (1.2). An early fundamental result in the theory of accretive operators, due to Browder [8], states that the initial value problem (1.1) is solvable if *A* is locally Lipschitz and accretive on *E*.

For an accretive operator *A*, we can define a nonexpansive single-valued mapping ${J}_{r}:R(I+rA)\to D(A)$ by ${J}_{r}={(I+rA)}^{-1}$ for each $r>0$, which is called the *resolvent* of *A*.

where ${J}_{{r}_{n}}={(I+{r}_{n}A)}^{-1}$ and $\{{r}_{n}\}$ is a sequence of positive real numbers.

As we know, many well-known problems arising in various branches of science can be studied by using algorithms which are iterative in their nature. As an example, in computer tomography with limited data, each piece of information implies the existence of a convex set in which the required solution lies. The problem of finding a point in the intersection of the sets is then of crucial interest, and it cannot be usually solved directly. Therefore, an iterative algorithm must be used to approximate such a point.

where $\{{\alpha}_{n}\}$ is a sequence in the interval $(0,1)$. If *T* is a nonexpansive mapping with a fixed point and the control sequence $\{{\alpha}_{n}\}$ is chosen so that ${\sum}_{n=1}^{\mathrm{\infty}}{\alpha}_{n}(1-{\alpha}_{n})=\mathrm{\infty}$, then the sequence $\{{x}_{n}\}$ generated in the normal Mann iterative process converges weakly to a fixed point of *T*. In an infinite-dimensional Hilbert space, the normal Mann iteration process has only weak convergence, in general, even for nonexpansive mappings. Therefore, many authors try to modify the normal Mann iterative process to have strong convergence for nonexpansive mappings; see, *e.g.*, [10–24] and the references therein.

*m*-accretive operators. To be more precise, they considered the following iterative process:

where *u* is a fixed element in *C* and ${J}_{{r}_{n}}={(I+{r}_{n}A)}^{-1}$. They proved that the sequence $\{{x}_{n}\}$ generated in the above iterative process converges strongly to a zero of *A*.

In this paper, we study the convergence of paths for continuous pseudocontractions in a real Banach space by viscosity approximation methods. As applications, we consider the problem of finding zeros of *m*-accretive operators based on an iterative process with errors. Strong convergence theorems of zeros are established in a real Banach space.

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

**Lemma 1.1** [25]

*Let*$\{{a}_{n}\}$, $\{{b}_{n}\}$,

*and*$\{{c}_{n}\}$

*be three nonnegative real sequences satisfying*

*where*$\{{t}_{n}\}$

*is a sequence in*$(0,1)$.

*Assume that the following conditions are satisfied*:

- (a)
${\sum}_{n=0}^{\mathrm{\infty}}{t}_{n}=\mathrm{\infty}$

*and*${b}_{n}=o({t}_{n})$; - (b)
${\sum}_{n=0}^{\mathrm{\infty}}{c}_{n}<\mathrm{\infty}$.

*Then* ${lim}_{n\to \mathrm{\infty}}{a}_{n}=0$.

**Lemma 1.2** [26]

*Let* *C* *be a nonempty*, *bounded*, *closed*, *and convex subset of a reflexive Banach space* *E* *which also has the normal structure*. *Let* *T* *be a nonexpansive mapping of* *C* *into itself*. *Then* $F(T)$ *is nonempty*.

**Lemma 1.3** [27]

*Let* *C* *be a nonempty*, *closed*, *and convex subset of a Banach space* *E*, *and let* $T:C\to C$ *be a continuous and strong pseudocontraction*. *Then* *T* *has a unique fixed point in* *C*.

**Lemma 1.4** [28]

*In a Banach space*

*E*,

*there holds the inequality*

*where* $j(x+y)\in J(x+y)$.

Set $A=(0,1)$, let $B(A)$ denote the Banach space of all bounded real-valued functions on *A* with a supremum norm and let *X* be a subspace of $B(A)$.

**Lemma 1.5** [1]

*Let* *C* *be a nonempty*, *closed*, *and convex subset of a Banach space* *E*. *Suppose that the norm of* *E* *is uniformly Gâteaux differentiable*. *Let* $\{{x}_{t}\}$ *be a bounded set in* *E*, *and* $z\in C$. *Let* ${\mu}_{t}$ *be a mean on* *X*. *Then* ${\mu}_{t}{\parallel {x}_{t}-z\parallel}^{2}={min}_{y\in C}\parallel {x}_{t}-y\parallel $ *if and only if* ${\mu}_{t}\u3008y-z,{x}_{t}-z\u3009\le 0$ *for all* $y\in C$.

**Lemma 1.6** [29]

*Let*

*E*

*be a Banach space and let*

*A*

*be an*

*m*-

*accretive operator*.

*For*$\lambda >0$, $\mu >0$,

*and*$x\in E$,

*we have*

*where* ${J}_{\lambda}={(I+\lambda A)}^{-1}$ *and* ${J}_{\mu}={(I+\mu A)}^{-1}$.

**Lemma 1.7** [30]

*Let*$\{{x}_{n}\}$

*and*$\{{y}_{n}\}$

*be bounded sequences in a Banach space*

*E*,

*and let*$\{{\beta}_{n}\}$

*be a sequence in*$(0,1)$

*with*

*Suppose that*${x}_{n+1}=(1-{\beta}_{n}){y}_{n}+{\beta}_{n}{x}_{n}$, $\mathrm{\forall}n\ge 1$

*and*

*Then* ${lim}_{n\to \mathrm{\infty}}\parallel {y}_{n}-{x}_{n}\parallel =0$.

## 2 Main results

Now, we are in a position to prove the strong convergence of paths for continuous pseudocontractions.

**Theorem 2.1**

*Let*

*E*

*be a real reflexive Banach space with the uniformly Gâteaux differentiable norm and the normal structure*,

*and let*

*C*

*be a nonempty closed convex subset of E*.

*Let*$T:C\to C$

*be a continuous pseudocontraction with a fixed point*,

*and let*$f:C\to C$

*be a fixed continuous bounded and strong pseudocontraction with the coefficient*$\alpha \in (0,1)$.

*Let*$\{{x}_{t}\}$

*be a sequence generated by the following*:

*where*$t\in (0,1)$.

*Then*$\{{x}_{t}\}$

*converges strongly as*$t\to 0$

*to a fixed point*${x}^{\ast}$

*of*

*T*,

*which is the unique solution in*$F(T)$

*to the following variational inequality*:

*Proof*For $t\in (0,1)$, define a mapping ${T}_{t}^{f}:C\to C$ by

*C*for each $t\in (0,1)$. Hence (2.1) is well defined. Next, we show that $\{{x}_{t}\}$ is bounded. For any $p\in F(T)$ and $t\in (0,1)$, we see that

*f*and $\{{x}_{t}\}$ are bounded, we obtain that $f({x}_{t})$ is bounded. It follows that

*g*attains its infinimum over

*C*(see,

*e.g.*, [1]). Set

Then *D* is a nonempty bounded closed convex subset of *C*. Next, we show that there exits a point ${x}^{\ast}\in D$ such that $T{x}^{\ast}={x}^{\ast}$.

*S*is a continuous strong pseudocontraction. From Lemma 1.3, we see that

*S*has a unique fixed point

*x*in

*C*, that is, $x=\frac{1}{2}y+\frac{1}{2}Tx$. This implies that

*h*is a nonexpansive mapping. Indeed, we have

*C*is a nonempty closed convex subset of

*E*which has the normal structure, we see that

*h*has a fixed point. We denote it by ${x}^{\ast}$. It follows from $F(h)=F(T)$ that $T{x}^{\ast}={x}^{\ast}$. From Lemma 1.5, we see that

This implies that there exists a subnet $\{{x}_{{t}_{\alpha}}\}$ of $\{{x}_{t}\}$ such that ${x}_{{t}_{\alpha}}\to {x}^{\ast}$.

This completes the proof. □

**Remark 2.2**Note that uniformly smooth Banach spaces enjoy the uniformly Gâteaux differentiable norm and the uniform normal structure. Theorem 2.1, which gives an affirmative answer to the questions presented in Section 1, improves Theorem 4.1 of Xu [6] in the following aspects:

- (1)
improves the framework of spaces from uniformly smooth Banach spaces to the Banach space with the uniformly Gâteaux differentiable norm;

- (2)
improves the mapping

*f*from the class of contractions to the class of strongly pseudocontractions; - (3)
improves the mapping

*T*from the class of nonexpansive mappings to the class of pseudocontractions.

**Remark 2.3**Under the conditions of Theorem 2.1, we can define a mapping $Q:{\mathrm{\Pi}}_{C}\to F(T)$, where ${\mathrm{\Pi}}_{C}$ denotes the set of continuous bounded and strong pseudocontractions, by

It is not hard to see that *Q* is a sunny nonexpansive retraction from ${\mathrm{\Pi}}_{C}$ onto $F(T)$.

**Remark 2.4**If

*T*is a nonexpansive mapping and $f(x)=u$, a fixed element for every $x\in C$ in Theorem 2.1, then $\{{x}_{t}\}$ defined by

*T*, where

*Q*is the sunny nonexpansive retraction from

*C*onto $F(T)$. The point $Q(u)$ is the unique solution in $F(T)$ to the following variational inequality:

Next, we prove strong convergence of iterative processes with errors for *m*-accretive operators.

**Theorem 2.5**

*Let*

*E*

*be a real reflexive Banach space with the uniformly Gâteaux differentiable norm and let*

*A*

*be an*

*m*-

*accretive operator in*

*E*.

*Assume that*$C:=\overline{D(A)}$

*is convex and has the normal structure*.

*Let*$\{{\alpha}_{n}\}$, $\{{\beta}_{n}\}$, $\{{\gamma}_{n}\}$,

*and*$\{{\delta}_{n}\}$

*be real number sequences in*$(0,1)$.

*Let*${Q}_{C}$

*be a sunny nonexpansive retraction from*

*E*

*onto*

*C*

*and let*$\{{x}_{n}\}$

*be a sequence generated in the following manner*:

*where*$\{{e}_{n}\}$

*is a sequence in*

*E*, $\{{f}_{n}\}$

*is a bounded sequence in*

*E*, $\{{r}_{n}\}$

*is a positive real numbers sequence*,

*u*

*is a fixed element in*

*C*

*and*${J}_{{r}_{n}}={(I+{r}_{n}A)}^{-1}$.

*Assume that*${A}^{-1}(0)\ne \mathrm{\varnothing}$

*and the above control sequences satisfy the following restrictions*:

- (a)
${\beta}_{n}+{\gamma}_{n}+{\delta}_{n}=1$

*for each*$n\ge 0$; - (b)
${lim}_{n\to \mathrm{\infty}}{\alpha}_{n}=0$

*and*${\sum}_{n=1}^{\mathrm{\infty}}{\alpha}_{n}=\mathrm{\infty}$; - (c)
$0<{lim\hspace{0.17em}inf}_{n\to \mathrm{\infty}}{\beta}_{n}\le {lim\hspace{0.17em}sup}_{n\to \mathrm{\infty}}{\beta}_{n}<1$;

- (d)
${\sum}_{n=1}^{\mathrm{\infty}}\parallel {e}_{n}\parallel <\mathrm{\infty}$

*and*${\sum}_{n=0}^{\mathrm{\infty}}{\delta}_{n}<\mathrm{\infty}$; - (e)
${r}_{n}\ge \u03f5$

*for each*$n\ge 1$*and*${lim}_{n\to \mathrm{\infty}}|{r}_{n}-{r}_{n+1}|=0$.

*Then the sequence*$\{{x}_{n}\}$

*generated by*(Δ)

*converges strongly to a zero*$Q(u)$

*of*

*A*,

*which is the unique solution to the following variational inequality*:

*Proof*First, we prove that $\{{x}_{n}\}$ is bounded. Fixing $p\in {A}^{-1}(0)$, we see that

*n*. It follows that

*r*such that $\u03f5>r>0$. From Lemma 1.6, we obtain that

*j*is strong to weak

^{∗}uniformly continuous on bounded subsets of

*E*, we see that

*ϵ*is arbitrary and (2.25), we see that ${lim\hspace{0.17em}sup}_{n\to \mathrm{\infty}}\u3008u-Q(u),j({x}_{n}-Q(u))\u3009\le 0$. That is,

Put ${a}_{n}={\parallel {x}_{n}-Q(u)\parallel}^{2}$, ${t}_{n}={\alpha}_{n}$, ${b}_{n}=2{\alpha}_{n}{\lambda}_{n+1}$ and ${c}_{n}={\eta}_{n}{M}_{3}$ for every $n\ge 0$. In view of Lemma 1.1, we can obtain the desired conclusion easily. □

As an application of Theorem 2.5, we have the following results.

**Corollary 2.6**

*Let*

*E*

*be a real reflexive Banach space with the uniformly Gâteaux differentiable norm and let*

*A*

*be an*

*m*-

*accretive operator in*

*E*.

*Assume that*$C:=\overline{D(A)}$

*is convex and has the normal structure*.

*Let*$\{{\alpha}_{n}\}$

*and*$\{{\beta}_{n}\}$

*be real number sequences in*$(0,1)$.

*Let*$\{{x}_{n}\}$

*be a sequence generated by*(

*QS*) ,

*where*$\{{r}_{n}\}$

*is a positive real numbers sequence*,

*u*

*is a fixed element in*

*C*

*and*${J}_{{r}_{n}}={(I+{r}_{n}A)}^{-1}$.

*Assume that*${A}^{-1}(0)\ne \mathrm{\varnothing}$

*and the above control sequences satisfy the following restrictions*:

- (a)
${lim}_{n\to \mathrm{\infty}}{\alpha}_{n}=0$

*and*${\sum}_{n=1}^{\mathrm{\infty}}{\alpha}_{n}=\mathrm{\infty}$; - (b)
$0<{lim\hspace{0.17em}inf}_{n\to \mathrm{\infty}}{\beta}_{n}\le {lim\hspace{0.17em}sup}_{n\to \mathrm{\infty}}{\beta}_{n}<1$;

- (c)
${r}_{n}\ge \u03f5$

*for each*$n\ge 1$*and*${lim}_{n\to \mathrm{\infty}}|{r}_{n}-{r}_{n+1}|=0$.

*Then the sequence* $\{{x}_{n}\}$ *converges strongly to a zero of* *A*.

**Remark 2.7** Corollary 2.6 improves Theorem 2.1 of Qin and Su [18]. To be more precise, we partially relax the restrictions on the parameters and extend the framework of the space; see [18] for more details.

**Remark 2.8** It is of interest to design an explicit iterative process to approximate zeros of accretive operators by Moudafi’s viscosity approximation method with continuous strong pseudocontractions.

## Declarations

## Authors’ Affiliations

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