- Research
- Open Access

# Common solutions of equilibrium and fixed point problems

- Qing-Nian Zhang
^{1}Email author

**2013**:425

https://doi.org/10.1186/1029-242X-2013-425

© Zhang; licensee Springer. 2013

**Received:**22 July 2013**Accepted:**22 August 2013**Published:**8 September 2013

## Abstract

In this paper, common solutions of equilibrium and fixed point problems are investigated. Convergence theorems of common solutions are established in a uniformly smooth and strictly convex Banach space.

**MSC:**47H09, 47H10, 47J25.

## Keywords

- asymptotically quasi-
*ϕ*-nonexpansive mapping - generalized asymptotically quasi-
*ϕ*-nonexpansive mapping - generalized projection
- equilibrium problem
- fixed point

## 1 Introduction and preliminaries

Let *E* be a real Banach space. Let ${U}_{E}=\{x\in E:\parallel x\parallel =1\}$ be the unit sphere of *E*. *E* is said to be smooth iff ${lim}_{t\to 0}\frac{\parallel x+ty\parallel -\parallel x\parallel}{t}$ exists for each $x,y\in {U}_{E}$. It is also said to be uniformly smooth iff the above limit is attained uniformly for $x,y\in {U}_{E}$. *E* is said to be strictly convex iff $\parallel \frac{x+y}{2}\parallel <1$ for all $x,y\in E$ with $\parallel x\parallel =\parallel y\parallel =1$ and $x\ne y$. It is said to be uniformly convex iff ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-{y}_{n}\parallel =0$ for any two sequences $\{{x}_{n}\}$ and $\{{y}_{n}\}$ in *E* such that $\parallel {x}_{n}\parallel =\parallel {y}_{n}\parallel =1$ and ${lim}_{n\to \mathrm{\infty}}\parallel \frac{{x}_{n}+{y}_{n}}{2}\parallel =1$.

*J*from

*E*to ${2}^{{E}^{\ast}}$ is defined by

where $\u3008\cdot ,\cdot \u3009$ denotes the generalized duality pairing. It is well known that if *E* is uniformly smooth, then *J* is uniformly norm-to-norm continuous on each bounded subset of *E*. It is also well known that *E* is (uniformly) smooth if and only if ${E}^{\ast}$ is (uniformly) convex.

In what follows, we use ⇀ and → to stand for the weak and strong convergence, respectively. Recall that *E* enjoys the Kadec-Klee property iff for any sequence $\{{x}_{n}\}\subset E$, and $x\in E$ with ${x}_{n}\rightharpoonup x$, and $\parallel {x}_{n}\parallel \to \parallel x\parallel $, then $\parallel {x}_{n}-x\parallel \to 0$ as $n\to \mathrm{\infty}$. It is well known that if *E* is a uniformly convex Banach space, then *E* enjoys the Kadec-Klee property.

*E*be a smooth Banach space. Consider the functional defined by

*H*, the equality is reduced to $\varphi (x,y)={\parallel x-y\parallel}^{2}$, $x,y\in H$. As we all know, if

*C*is a nonempty closed convex subset of a Hilbert space

*H*and ${P}_{C}:H\to C$ is the metric projection of

*H*onto

*C*, then ${P}_{C}$ is nonexpansive. This fact actually characterizes Hilbert spaces and, consequently, it is not available in more general Banach spaces. In this connection, Alber [1] recently introduced a generalized projection operator ${\mathrm{\Pi}}_{C}$ in a Banach space

*E*, which is an analogue of the metric projection ${P}_{C}$ in Hilbert spaces. Recall that the generalized projection ${\mathrm{\Pi}}_{C}:E\to C$ is a map that assigns to an arbitrary point $x\in E$ the minimum point of the functional $\varphi (x,y)$, that is, ${\mathrm{\Pi}}_{C}x=\overline{x}$, where $\overline{x}$ is the solution to the minimization problem $\varphi (\overline{x},x)={min}_{y\in C}\varphi (y,x)$. Existence and uniqueness of the operator ${\mathrm{\Pi}}_{C}$ follows from the properties of the functional $\varphi (x,y)$ and strict monotonicity of the mapping

*J*. If

*E*is a reflexive, strictly convex and smooth Banach space, then $\varphi (x,y)=0$ if and only if $x=y$; for more details, see [1] and the references therein. In Hilbert spaces, ${\mathrm{\Pi}}_{C}={P}_{C}$. It is obvious from the definition of a function

*ϕ*that

*C*be a nonempty subset of

*E*, and let $T:C\to C$ be a mapping. In this paper, we use $F(T)$ to stand for the fixed point set of

*T*.

*T*is said to be closed iff for any sequence $\{{x}_{n}\}\subset C$ such that ${lim}_{n\to \mathrm{\infty}}{x}_{n}={x}_{0}$ and ${lim}_{n\to \mathrm{\infty}}T{x}_{n}={y}_{0}$, then $T{x}_{0}={y}_{0}$.

*T*is said to be asymptotically regular on

*C*iff for any bounded subset

*K*of

*C*,

*p*in

*C*is said to be an asymptotic fixed point of

*T*iff

*C*contains a sequence $\{{x}_{n}\}$ which converges weakly to

*p*such that ${lim}_{n\to \mathrm{\infty}}\parallel {x}_{n}-T{x}_{n}\parallel =0$. The set of asymptotic fixed points of

*T*will be denoted by $\tilde{F}(T)$.

*T*is said to be relatively nonexpansive iff

*T*is said to be relatively asymptotically nonexpansive iff

where $\{{\mu}_{n}\}\subset [0,\mathrm{\infty})$ is a sequence such that ${\mu}_{n}\to 0$ as $n\to \mathrm{\infty}$.

**Remark 1.1** The class of relatively asymptotically nonexpansive mappings which is an extension of the class of relatively nonexpansive mappings was first considered in [2] and [3].

*T*is said to be quasi-

*ϕ*-nonexpansive iff

*T*is said to be asymptotically quasi-

*ϕ*-nonexpansive iff there exists a sequence $\{{\mu}_{n}\}\subset [0,\mathrm{\infty})$ with ${\mu}_{n}\to 0$ as $n\to \mathrm{\infty}$ such that

**Remark 1.2** The class of asymptotically quasi-*ϕ*-nonexpansive mappings, which is an extension of the class of quasi-*ϕ*-nonexpansive mappings, was considered in [4, 5]; see also [6].

**Remark 1.3** The class of quasi-*ϕ*-nonexpansive mappings and the class of asymptotically quasi-*ϕ*-nonexpansive mappings are more general than the class of relatively nonexpansive mappings and the class of relatively asymptotically nonexpansive mappings. Quasi-*ϕ*-nonexpansive mappings and asymptotically quasi-*ϕ*-nonexpansive mappings do not require the restriction $F(T)=\tilde{F}(T)$.

**Remark 1.4** The class of quasi-*ϕ*-nonexpansive mappings and the class of asymptotically quasi-*ϕ*-nonexpansive mappings are generalizations of the class of quasi-nonexpansive mappings and the class of asymptotically quasi-nonexpansive mappings in Banach spaces.

*T*is said to be generalized asymptotically quasi-

*ϕ*-nonexpansive iff $F(T)\ne \mathrm{\varnothing}$, and there exist two nonnegative sequences $\{{\mu}_{n}\}\subset [0,\mathrm{\infty})$ with ${\mu}_{n}\to 0$, and $\{{\xi}_{n}\}\subset [0,\mathrm{\infty})$ with ${\xi}_{n}\to 0$ as $n\to \mathrm{\infty}$ such that

**Remark 1.5** The class of generalized asymptotically quasi-*ϕ*-nonexpansive mappings [7] is a generalization of the class of generalized asymptotically quasi-nonexpansive mappings in the framework of Banach spaces which was introduced by Agarwal *et al.* [8].

*F*be a bifunction from $C\times C$ to ℝ, where ℝ denotes the set of real numbers. Recall the following equilibrium problem. Find $p\in C$ such that $F(p,y)\ge 0$, $\mathrm{\forall}y\in C$. We use $\mathit{EP}(F)$ to denote the solution set of the equilibrium problem. Given a mapping $Q:C\to {E}^{\ast}$, let

*p*is a solution of the following variational inequality. Find

*p*such that

where ${M}_{n}=sup\{\varphi (z,{x}_{n}):z\in \mathcal{F}\}$ for each $n\ge 1$, $\{{\alpha}_{n}\}$ is a real sequence in $[0,1]$, $\{{r}_{n}\}$ is a real sequence in $[a,\mathrm{\infty})$, where *a* is some positive real number. In a uniformly smooth and strictly convex Banach space, which also enjoys the Kadec-Klee property, the author obtained a strong convergence theorem; for more details, see [25] and the references therein.

In this paper, motivated by the above result, we consider the projection algorithm for treating solutions of the equilibrium problem and fixed points of generalized asymptotically quasi-*ϕ*-nonexpansive mappings. A strong convergence theorem is established in a Banach space. The results presented this paper mainly improve the corresponding results announced in Qin Cho and Kang [5] and Kim [25].

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

**Lemma 1.6** [36]

*Let*

*E*

*be a smooth and uniformly convex Banach space*,

*and let*$r>0$.

*Then there exists a strictly increasing*,

*continuous and convex function*$g:[0,2r]\to R$

*such that*$g(0)=0$

*and*

*for all* $x,y\in {B}_{r}=\{x\in E:\parallel x\parallel \le r\}$ *and* $t\in [0,1]$.

**Lemma 1.7** [1]

*Let*

*C*

*be a nonempty closed convex subset of a smooth Banach space*

*E*

*and*$x\in E$.

*Then*${x}_{0}={\mathrm{\Pi}}_{C}x$

*if and only if*

**Lemma 1.8** [1]

*Let*

*E*

*be a reflexive*,

*strictly convex and smooth Banach space*,

*let*

*C*

*be a nonempty closed convex subset of*

*E*

*and*$x\in E$.

*Then*

*Let*

*C*

*be a closed convex subset of a smooth*,

*strictly convex and reflexive Banach space*

*E*.

*Let*

*F*

*be a bifunction from*$C\times C$

*to*ℝ

*satisfying*(A1)-(A4).

*Let*$r>0$

*and*$x\in E$.

*Then there exists*$z\in C$

*such that*$F(z,y)+\frac{1}{r}\u3008y-z,Jz-Jx\u3009\ge 0$, $\mathrm{\forall}y\in C$.

*Define a mapping*${T}_{r}:E\to C$

*by*

*Then the following conclusions hold*:

- (1)${S}_{r}$
*is a single*-*valued firmly nonexpansive*-*type mapping*,*i*.*e*.,*for all*$x,y\in E$,$\u3008{S}_{r}x-{S}_{r}y,J{S}_{r}x-J{S}_{r}y\u3009\le \u3008{S}_{r}x-{S}_{r}y,Jx-Jy\u3009;$ - (2)
$F({S}_{r})=\mathit{EP}(F)$

*is closed and convex*; - (3)
${S}_{r}$

*is quasi*-*ϕ*-*nonexpansive*; - (4)
$\varphi (q,{S}_{r}x)+\varphi ({S}_{r}x,x)\le \varphi (q,x)$, $\mathrm{\forall}q\in F({S}_{r})$.

**Lemma 1.10** [7]

*Let* *E* *be a uniformly smooth and strictly convex Banach space which also enjoys the Kadec*-*Klee property*, *and let* *C* *be a nonempty closed and convex subset of* *E*. *Let* $T:C\to C$ *be a generalized asymptotically quasi*-*ϕ*-*nonexpansive mapping*. *Then* $F(T)$ *is closed and convex*.

## 2 Main results

**Theorem 2.1**

*Let*

*E*

*be a uniformly smooth and strictly convex Banach space which also enjoys the Kadec*-

*Klee property*,

*and let*

*C*

*be a nonempty closed and convex subset of*

*E*.

*Let*Δ

*be an index set*.

*Let*${F}_{i}$

*be a bifunction from*$C\times C$

*to*ℝ

*satisfying*(A1)-(A4)

*for every*$i\in \mathrm{\Delta}$.

*Let*$T:C\to C$

*be a generalized asymptotically quasi*-

*ϕ*-

*nonexpansive mapping*.

*Assume that*

*T*

*is closed asymptotically regular on*

*C*

*and*$\mathrm{\Omega}:=F(T)\cap {\bigcap}_{i\in \mathrm{\Delta}}EF({F}_{i})$

*is nonempty and bounded*.

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

*be a sequence generated in the following manner*:

*where* ${M}_{n}=sup\{\varphi (z,{x}_{n}):z\in \mathrm{\Omega}\}$, $\{{\alpha}_{n}\}$ *is a real number sequence in* $(0,1)$ *such that* ${lim\hspace{0.17em}inf}_{n\to \mathrm{\infty}}{\alpha}_{n}(1-{\alpha}_{n})>0$, $\{{r}_{n,i}\}$ *is a real number sequence in* $[{a}_{i},\mathrm{\infty})$, *where* $\{{a}_{i}\}$ *is a positive real number sequence*. *Then the sequence* $\{{x}_{n}\}$ *converges strongly to* ${\mathrm{\Pi}}_{\mathrm{\Omega}}{x}_{0}$, *where* ${\mathrm{\Pi}}_{\mathrm{\Omega}}$ *is the generalized projection from* *E* *onto* Ω.

*Proof*In view of Lemmas 1.9 and 1.10, we find that the common solution set Ω is closed and convex. Next, we show that ${C}_{n}$ is closed and convex. It suffices to show, for any fixed but arbitrary $i\in \mathrm{\Delta}$, that ${C}_{n,i}$ is closed and convex. This can be proved by induction on

*n*. It is obvious that ${C}_{1,i}=C$ is closed and convex. Assume that ${C}_{j,i}$ is closed and convex for some $j\ge 1$. We next prove that ${C}_{j+1,i}$ is closed and convex for the same

*j*. This completes the proof that ${C}_{n}$ is closed and convex. It is clear that ${C}_{j+1,i}$ is closed. We only prove the convexity. Indeed, $\mathrm{\forall}a,b\in {C}_{j+1,i}$, we see that $a,b\in {C}_{j,i}$, and

This proves that ${C}_{j+1,i}$ is convex. This completes that ${C}_{n}$ is closed and convex.

*j*and for every $i\in \mathrm{\Delta}$. Then, for $\mathrm{\forall}w\in \mathrm{\Omega}\subset {C}_{j,i}$, we have

This shows that $w\in {C}_{j+1,i}$. This implies that $\mathrm{\Omega}\subset {C}_{n}$ for every $n\ge 1$.

Hence, we have $\parallel {x}_{n}\parallel \to \parallel p\parallel $ as $n\to \mathrm{\infty}$. In view of the Kadec-Klee property of *E*, we obtain that ${x}_{n}\to p$ as $n\to \mathrm{\infty}$.

*E*is reflexive and ${E}^{\ast}$ is also reflexive. We may assume that $J{u}_{n,i}\rightharpoonup {x}^{\ast ,i}\in {E}^{\ast}$. In view of the reflexivity of

*E*, we see that $J(E)={E}^{\ast}$. This shows that there exists an ${x}^{i}\in E$ such that $J{x}^{i}={x}^{\ast ,i}$. It follows that

*E*enjoys the Kadec-Klee property, we obtain that ${u}_{n,i}\to p$ as $n\to \mathrm{\infty}$. Note that

*J*is uniformly norm-to-norm continuous on any bounded sets, we have

*E*is uniformly smooth, we know that ${E}^{\ast}$ is uniformly convex. In view of Lemma 1.6, we see that

*g*that

*E*enjoys the Kadec-Klee property, we obtain that

*T*and (2.6) that

That is, $T{T}^{n}{x}_{n}-p\to 0$ as $n\to \mathrm{\infty}$. It follows from the closedness of *T* that $Tp=p$.

*E*enjoys the Kadec-Klee property, we obtain that ${y}_{n}\to p$ as $n\to \mathrm{\infty}$. Notice that $\parallel {u}_{n,i}-{y}_{n}\parallel \le \parallel {u}_{n,i}-p\parallel +\parallel p-{y}_{n}\parallel $. It follows that

*J*is uniformly norm-to-norm continuous on any bounded sets, we have

Letting ${t}_{i}\downarrow 0$, we find from condition (A3) that ${F}_{i}(p,y)\ge 0$, $\mathrm{\forall}y\in C$. This implies that $p\in \mathit{EP}({F}_{i})$. This completes the proof that $p\in \mathrm{\Omega}$.

In view of Lemma 1.7, we can obtain that $p={\mathrm{\Pi}}_{\mathrm{\Omega}}{x}_{0}$. This completes the proof. □

If *T* is asymptotically quasi-*ϕ*-nonexpansive, then we find from Theorem 2.1 the following result.

**Corollary 2.2**

*Let*

*E*

*be a uniformly smooth and strictly convex Banach space which also enjoys the Kadec*-

*Klee property*,

*and let*

*C*

*be a nonempty closed and convex subset of*

*E*.

*Let*Δ

*be an index set*.

*Let*${F}_{i}$

*be a bifunction from*$C\times C$

*to*ℝ

*satisfying*(A1)-(A4)

*for every*$i\in \mathrm{\Delta}$.

*Let*$T:C\to C$

*be an asymptotically quasi*-

*ϕ*-

*nonexpansive mapping*.

*Assume that*

*T*

*is closed asymptotically regular on*

*C*

*and*$\mathrm{\Omega}:=F(T)\cap {\bigcap}_{i\in \mathrm{\Delta}}EF({F}_{i})$

*is nonempty and bounded*.

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

*be a sequence generated in the following manner*:

*where* ${M}_{n}=sup\{\varphi (z,{x}_{n}):z\in \mathrm{\Omega}\}$, $\{{\alpha}_{n}\}$ *is a real number sequence in* $(0,1)$ *such that* ${lim\hspace{0.17em}inf}_{n\to \mathrm{\infty}}{\alpha}_{n}(1-{\alpha}_{n})>0$, $\{{r}_{n,i}\}$ *is a real number sequence in* $[{a}_{i},\mathrm{\infty})$, *where* $\{{a}_{i}\}$ *is a positive real number sequence*. *Then the sequence* $\{{x}_{n}\}$ *converges strongly to* ${\mathrm{\Pi}}_{\mathrm{\Omega}}{x}_{0}$, *where* ${\mathrm{\Pi}}_{\mathrm{\Omega}}$ *is the generalized projection from* *E* *onto* Ω.

**Remark 2.3** Since the index set Δ is arbitrary, Corollary 2.2 is an improvement of the corresponding results in Kim [25].

**Remark 2.4**Corollary 2.2 also improves the corresponding results in Qin

*et al.*[5] in the following aspects:

- (a)
from a uniformly smooth and uniformly convex space to a uniformly smooth and strictly convex Banach space which also enjoys the Kadec-Klee property;

- (b)
from a single bifunction to a family of bifunctions;

- (c)
from a quasi-

*ϕ*-nonexpansive mapping to an asymptotically quasi-*ϕ*-nonexpansive mapping.

In the framework of Hilbert spaces, the theorem is reduced to the following.

**Corollary 2.5**

*Let*

*E*

*be a Hilbert space*,

*and let*

*C*

*be a nonempty closed and convex subset of*

*E*.

*Let*Δ

*be an index set*.

*Let*${F}_{i}$

*be a bifunction from*$C\times C$

*to*ℝ

*satisfying*(A1)-(A4)

*for every*$i\in \mathrm{\Delta}$.

*Let*$T:C\to C$

*be a generalized asymptotically quasi*-

*nonexpansive mapping*.

*Assume that*

*T*

*is closed asymptotically regular on*

*C*

*and*$\mathrm{\Omega}:=F(T)\cap {\bigcap}_{i\in \mathrm{\Delta}}EF({F}_{i})$

*is nonempty and bounded*.

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

*be a sequence generated in the following manner*:

*where* ${M}_{n}=sup\{{\parallel z-{x}_{n}\parallel}^{2}:z\in \mathrm{\Omega}\}$, $\{{\alpha}_{n}\}$ *is a real number sequence in* $(0,1)$ *such that* ${lim\hspace{0.17em}inf}_{n\to \mathrm{\infty}}{\alpha}_{n}(1-{\alpha}_{n})>0$, $\{{r}_{n,i}\}$ *is a real number sequence in* $[{a}_{i},\mathrm{\infty})$, *where* $\{{a}_{i}\}$ *is a positive real number sequence*. *Then the sequence* $\{{x}_{n}\}$ *converges strongly to* ${Proj}_{\mathrm{\Omega}}{x}_{0}$, *where* ${Proj}_{\mathrm{\Omega}}$ *is the metric projection from* *E* *onto* Ω.

*Proof* In the framework of Hilbert spaces, we find that $\varphi (x,y)={\parallel x-y\parallel}^{2}$, *J* is reduced to the identity mapping and the generalized projection ${\mathrm{\Pi}}_{C}$ is reduced to the metric projection ${Proj}_{C}$. This completes the proof. □

For a single bifunction, we also have the following.

**Corollary 2.6**

*Let*

*E*

*be a Hilbert space*,

*and let*

*C*

*be a nonempty closed and convex subset of*

*E*.

*Let*

*F*

*be a bifunction from*$C\times C$

*to*ℝ

*satisfying*(A1)-(A4).

*Let*$T:C\to C$

*be a generalized asymptotically quasi*-

*nonexpansive mapping*.

*Assume that*

*T*

*is closed asymptotically regular on*

*C*

*and*$\mathrm{\Omega}:=F(T)\cap EF(F)$

*is nonempty and bounded*.

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

*be a sequence generated in the following manner*:

*where* ${M}_{n}=sup\{{\parallel z-{x}_{n}\parallel}^{2}:z\in \mathrm{\Omega}\}$, $\{{\alpha}_{n}\}$ *is a real number sequence in* $(0,1)$ *such that* ${lim\hspace{0.17em}inf}_{n\to \mathrm{\infty}}{\alpha}_{n}(1-{\alpha}_{n})>0$, $\{{r}_{n,i}\}$ *is a real number sequence in* $[a,\mathrm{\infty})$, *where* *a* *is a positive real number*. *Then the sequence* $\{{x}_{n}\}$ *converges strongly to* ${Proj}_{\mathrm{\Omega}}{x}_{0}$, *where* ${Proj}_{\mathrm{\Omega}}$ *is the metric projection from* *E* *onto* Ω.

*Proof* In the framework of Hilbert spaces, we find that $\varphi (x,y)={\parallel x-y\parallel}^{2}$, *J* is reduced to the identity mapping, and the generalized projection ${\mathrm{\Pi}}_{C}$ is reduced to the metric projection ${Proj}_{C}$. In view of Corollary 2.5, we may immediately conclude the desired results. □

## Declarations

### Acknowledgements

The author is grateful to the editor and the anonymous reviewers for suggestions which improved the contents of the article.

## Authors’ Affiliations

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