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A modified Mann iterative scheme by generalized f-projection for a countable family of relatively quasi-nonexpansive mappings and a system of generalized mixed equilibrium problems Fixed

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A modified Mann iterative scheme by generalized f-projection for a countable

family of relatively quasi-nonexpansive mappings and a system of generalized

mixed equilibrium problems

Fixed Point Theory and Applications 2011, 2011:104 doi:10.1186/1687-1812-2011-104

Siwaporn Saewan (si_wa_pon@hotmail.com)Poom Kumam (poom.kum@kmutt.ac.th)

ISSN 1687-1812

Article type Research

Submission date 23 July 2011

Acceptance date 21 December 2011

Publication date 21 December 2011

Article URL http://www.fixedpointtheoryandapplications.com/content/2011/1/104

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A modified Mann iterative scheme by

generalized f -projection for a

countable family of relatively

quasi-nonexpansive mappings and a

system of generalized mixed

equilibrium problems

Siwaporn Saewan∗1 and Poom Kumam∗1,2

1Department of Mathematics, Faculty of Science

King Mongkut’s University of Technology Thonburi (KMUTT)

Bangmod, Bangkok 10140, Thailand

2Centre of Excellence in Mathematics, CHE

Si Ayutthaya Rd., Bangkok 10400, Thailand

∗Corresponding authors: si wa pon@hotmail.com

Email address:

PK: kumampoom@hotmail.com; poom.kum@kmutt.ac.th

AbstractThe purpose of this paper is to introduce a new hybrid projection

method based on modified Mann iterative scheme by the

general-ized f -projection operator for a countable family of relatively

quasi-nonexpansive mappings and the solutions of the system of generalized

mixed equilibrium problems Furthermore, we prove the strong

con-vergence theorem for a countable family of relatively quasi-nonexpansivemappings in a uniformly convex and uniform smooth Banach space

Finally, we also apply our results to the problem of finding zeros of

B-monotone mappings and maximal monotone operators The results

presented in this paper generalize and improve some well-known sults in the literature.

re-Keywords: The generalized f -projection operator; relatively nonexpansive mapping; B-monotone mappings; maximal monotone

quasi-operator; system of generalized mixed equilibrium problems

2000 Mathematics Subject Classification: 47H05; 47H09; 47H10

The theory of equilibrium problems, the development of an efficient and plementable iterative algorithm, is interesting and important This theorycombines theoretical and algorithmic advances with novel domain of applica-tions Analysis of these problems requires a blend of techniques from convexanalysis, functional analysis, and numerical analysis

im-Equilibrium problems theory provides us with a natural, novel, and fied framework for studying a wide class of problems arising in economics,finance, transportation, network, and structural analysis, image reconstruc-tion, ecology, elasticity and optimization, and it has been extended and gen-eralized in many directions The ideas and techniques of this theory are beingused in a variety of diverse areas and proved to be productive and innovative

uni-In particular, generalized mixed equilibrium problem and equilibrium lems are related to the problem of finding fixed points of nonlinear mappings

prob-Let E be a real Banach space with norm k · k, C be a nonempty closed convex subset of E and let E ∗ denote the dual of E Let {θ i } i∈Λ : C ×C → R

be a bifunction, {ϕ i } i∈Λ : C → R be a real-valued function, and {A i } i∈Λ :

system of generalized mixed equilibrium problems is to find x ∈ C such that

θ i (x, y) + hA i x, y − xi + ϕ i (y) − ϕ i (x) ≥ 0, i ∈ Λ, ∀y ∈ C. (1.1)

If Λ is a singleton, then problem (1.1) reduces to the generalized mixed

equi-librium problem is to find x ∈ C such that

The set of solutions to (1.2) is denoted by GMEP(θ, A, ϕ), i.e.,

GMEP(θ, A, ϕ) = {x ∈ C : θ(x, y)+hAx, y−xi+ϕ(y)−ϕ(x) ≥ 0, ∀y ∈ C}.

(1.3)

If A ≡ 0, the problem (1.2) reduces to the mixed equilibrium problem for θ, denoted by MEP(θ, ϕ) is to find x ∈ C such that

If θ ≡ 0, the problem (1.2) reduces to the mixed variational inequality of Browder type, denoted by V I(C, A, ϕ) is to find x ∈ C such that

If A ≡ 0 and ϕ ≡ 0 the problem (1.2) reduces to the equilibrium problem for

θ, denoted by EP(θ) is to find x ∈ C such that

noncooperative games In other words, the GMEP(θ, A, ϕ), MEP(θ, ϕ) and EP(θ) are an unifying model for several problems arising in physics, engi-

neering, science, optimization, economics, etc Many authors studied and

constructed some solution methods to solve the GMEP(θ, A, ϕ), MEP(θ, ϕ), EP(θ) [1–16, and references therein].

Let C be a closed convex subset of E and recall that a mapping T : C → C

assigns to each x ∈ H, the unique point in P C x ∈ C satisfying the property

kx − P C xk = min y∈C kx − yk, then we also have P C is nonexpansive Thisfact actually characterizes Hilbert spaces and consequently, it is not available

in more general Banach spaces We consider the functional defined by

where J is the normalized duality mapping In this connection, Alber [17]

introduced a generalized projection ΠC from E in to C as follows:

ΠC (x) = arg min

It is obvious from the definition of functional φ that

(kyk − kxk)2 ≤ φ(y, x) ≤ (kyk + kxk)2, ∀x, y ∈ E. (1.10)

If E is a Hilbert space, then φ(y, x) = ky − xk2 and ΠC becomes the metric

projection of E onto C The generalized projection Π C : E → C is a map that assigns to an arbitrary point x ∈ E the minimum point of the functional

φ(y, x), that is, Π C x = ¯ x, where ¯ x is the solution to the minimization problem

φ(¯ x, x) = inf

The existence and uniqueness of the operator ΠC follow from the properties

of the functional φ(y, x) and strict monotonicity of the mapping J [17–21] It

is well known that the metric projection operator plays an important role innonlinear functional analysis, optimization theory, fixed point theory, nonlin-ear programming, game theory, variational inequality, and complementarityproblems, etc [17, 22] In 1994, Alber [23] introduced and studied the gen-eralized projections from Hilbert spaces to uniformly convex and uniformlysmooth Banach spaces Moreover, Alber [17] presented some applications

of the generalized projections to approximately solve variational inequalitiesand von Neumann intersection problem in Banach spaces In 2005, Li [22]extended the generalized projection operator from uniformly convex and uni-formly smooth Banach spaces to reflexive Banach spaces and studied someproperties of the generalized projection operator with applications to solvethe variational inequality in Banach spaces Later, Wu and Huang [24] in-

troduced a new generalized f -projection operator in Banach spaces They

extended the definition of the generalized projection operators introduced by

Abler [23] and proved some properties of the generalized f -projection

oper-ator In 2009, Fan et al [25] presented some basic results for the generalized

f -projection operator and discussed the existence of solutions and

approxi-mation of the solutions for generalized variational inequalities in noncompactsubsets of Banach spaces

Let h·, ·i denote the duality pairing of E ∗ and E Next, we recall the concept of the generalized f -projection operator Let G : C × E ∗ −→ R ∪ {+∞} be a functional defined as follows:

where ξ ∈ C, $ ∈ E ∗ , ρ is positive number and f : C → R∪ {+∞} is proper,

convex, and lower semicontinuous By the definitions of G, it is easy to see

the following properties:

(1) G(ξ, $) is convex and continuous with respect to $ when ξ is fixed; (2) G(ξ, $) is convex and lower semicontinuous with respect to ξ when $

is fixed

Definition 1.1 Let E be a real Banach space with its dual E ∗ Let C

be a nonempty closed convex subset of E We say that π C f : E ∗ → 2 C is

generalized f -projection operator if

π C f $ = {u ∈ C : G(u, $) = inf

ξ∈C G(ξ, $)}, ∀$ ∈ E ∗

Observe that, if f (x) = 0, then the generalized f -projection operator

(1.12) reduces to the generalized projection operator (1.9)

For the generalized f -projection operator, Wu and Hung [24] proved the

following basic properties:

Lemma 1.2 [24] Let E be a real reflexive Banach space with its dual E ∗

and C a nonempty closed convex subset of E Then the following statement holds:

(1) π C f $ is a nonempty closed convex subset of C for all $ ∈ E ∗ ;

(2) if E is smooth, then for all $ ∈ E ∗ , x ∈ π C f $ if and only if

hx − y, $ − Jxi + ρf (y) − ρf (x) ≥ 0, ∀y ∈ C;

(3) if E is strictly convex and f : C → R ∪ {+∞} is positive homogeneous

(i.e., f (tx) = tf (x) for all t > 0 such that tx ∈ C where x ∈ C), then

π C f $ is single-valued mapping.

Recently, Fan et al [25] show that the condition f is positive homogeneous

which appeared in [25, Lemma 2.1 (iii)] can be removed

Lemma 1.3 [25] Let E be a real reflexive Banach space with its dual E ∗

and C a nonempty closed convex subset of E If E is strictly convex, then

Next, we give the following example [27] of metric projection, generalized

projection operator and generalized f -projection operator do not coincide.

Example 1.5 Let X = R3 be provided with the norm k(x1, x2, x3)k =

This is a smooth strictly convex Banach space and C = {x ∈ R3|x2 = 0, x3 =

0} is a closed and convex subset of X It is a simple computation; we get

Recall that a point p in C is said to be an asymptotic fixed point of T [28] if C contains a sequence {x n } which converges weakly to p such that

limn→∞ kx n − T x n k = 0 The set of asymptotic fixed points of T will be

denoted by bF (T ) A mapping T from C into itself is said to be relatively nonexpansive mapping [29–31] if

(R1) F (T ) is nonempty;

(R2) φ(p, T x) ≤ φ(p, x) for all x ∈ C and p ∈ F (T );

(R3) bF (T ) = F (T ).

A mapping T is said to be relatively quasi-nonexpansive ( or quasi-φ-nonexpansive)

if the conditions (R1) and (R2) are satisfied The asymptotic behavior of arelatively nonexpansive mapping was studied in [32–34] The class of rela-tively quasi-nonexpansive mappings is more general than the class of rela-tively nonexpansive mappings [11, 32–35] which requires the strong restric-

tion: F (T ) = b F (T ) In order to explain this better, we give the following

example [36] of relatively quasi-nonexpansive mappings which is not tively nonexpansive mapping It is clearly by the definition of relatively

rela-quasi-nonexpansive mapping T is equivalent to F (T ) 6= ∅ and G(p, JT x) ≤

G(p, Jx) for all x ∈ C and p ∈ F (T ).

element of E.

We define a mapping T : E → E by

T (x) =

½(1

Example 1.7 Let E be a uniformly smooth and strictly convex Banach space

and A ⊂ E × E ∗ be a maximal monotone mapping such that its zero set

A −1 0 6= ∅ Then, J r = (J + rA) −1 JJ is a closed quasi-φ-nonexpansive

map-ping from E onto D(A) and F (J r ) = A −10

Proof By Matsushita and Takahashi [35, Theorem 4.3], we see that J r is

rel-atively nonexpansive mapping from E onto D(A) and F (J r ) = A −10

There-fore, J r is quasi-φ-nonexpansive mapping from E onto D(A) and F (J r) =

A −1 0 On the other hand, we can obtain the closedness of J r easily from the

continuity of the mapping J and the maximal monotonicity of A; see [35] for

Example 1.8 Let C be the generalized projection from a smooth, strictly

convex, and reflexive Banach space E onto a nonempty closed convex subset

C of E Then, C is a closed quasi-φ-nonexpansive mapping from E onto C

with F (Π C ) = C.

In 1953, Mann [37] introduced the iteration as follows: a sequence {x n }

defined by

x n+1 = α n x n + (1 − α n )T x n , (1.14)

where the initial guess element x1 ∈ C is arbitrary and {α n } is real sequence

in [0, 1] Mann iteration has been extensively investigated for sive mappings One of the fundamental convergence results is proved byReich [38] In an infinite-dimensional Hilbert space, Mann iteration can con-

nonexpan-clude only weak convergence [39,40] Attempts to modify the Mann iteration

method (1.14) so that strong convergence is guaranteed have recently beenmade Nakajo and Takahashi [41] proposed the following modification ofMann iteration method as follows:

They proved that if the sequence {α n } bounded above from one, then {x n }

defined by (1.15) converges strongly to P F (T ) x.

In 2007, Aoyama et al [42, Lemma 3.1] introduced {T n } is a sequence

of nonexpansive mappings of C into itself with ∩ ∞

n=1 F (T n ) 6= ∅ satisfy the following condition: if for each bounded subset B of C, P∞ n=1 sup{kT n+1 z −

T n zk : z ∈ B < ∞} Assume that if the mapping T : C → C defined by

T x = lim n→∞ T n x for all x ∈ C, then lim n→∞ sup{kT z − T n zk : z ∈ C} = 0.

They proved that the sequence {T n } converges strongly to some point of C

for all x ∈ C.

In 2009, Takahashi et al [43] studied and proved a strong convergencetheorem by the new hybrid method for a family of nonexpansive mappings

in Hilbert spaces as follows: x0 ∈ H, C1 = C and x1 = P C1x0 and

where 0 ≤ α n ≤ a < 1 for all n ∈ N and {T n } is a sequence of nonexpansive

mappings of C into itself such that ∩ ∞

n=1 F (T n ) 6= ∅ They proved that if

{T n } satisfies some appropriate conditions, then {x n } converges strongly to

P ∩ ∞

n=1 F (T n) x0

The ideas to generalize the process (1.14) from Hilbert spaces have cently been made By using available properties on a uniformly convex anduniformly smooth Banach space, Matsushita and Takahashi [35] proposed thefollowing hybrid iteration method with generalized projection for relatively

re-nonexpansive mapping T in a Banach space E:

They proved that {x n } converges strongly to Π F (T ) x0, where ΠF (T ) is the

generalized projection from C onto F (T ) Plubtieng and Ungchittrakool [44]

introduced and proved the processes for finding a common fixed point of

a countable family of relatively nonexpansive mappings in a Banach space.They proved the strong convergence theorems for a common fixed point of

a countable family of relatively nonexpansive mappings {T n } provided that {T n } satisfies the following condition:

• if for each bounded subset D of C, there exists a continuous

increas-ing and convex function h : R+ → R+ such that h(0) = 0 and

limk,l→∞supz∈D h(kT k z − T l zk) = 0.

Motivated by the results of Takahashi and Zembayashi [13], Cholumjiakand Suantai [2] proved the following strong convergence theorem by the hy-brid iterative scheme for approximation of common fixed point of countable

families of relatively quasi-nonexpansive mappings {T i } on C into itself in a

uniformly convex and uniformly smooth Banach space: x0 ∈ E, x1 = ΠC1x0,

r i,n , i = 1, 2, 3, , m defined in Lemma 2.8 Then, they proved

that under certain appropriate conditions imposed on {α n }, and {r n,i }, the

sequence {x n } converges strongly to Π C n+1 x0

Recently, Li et al [26] introduced the following hybrid iterative schemefor approximation of fixed point of relatively nonexpansive mapping using

the properties of generalized f -projection operator in a uniformly smooth real Banach space which is also uniformly convex: x0 ∈ C,

They obtained a strong convergence theorem for finding an element in the

fixed point set of T The results of Li et al [26] extended and improved on

the results of Matsushita and Takahashi [35]

Very recently, Shehu [45] studied and obtained the following strong vergence theorem by the hybrid iterative scheme for approximation of com-mon fixed point of finite family of relatively quasi-nonexpansive mappings

con-in a uniformly convex and uniformly smooth Banach space: let x0 ∈ C,

where T n = T n (mod N) He proved that the sequence {x n } converges strongly

to ΠC n+1 x0 under certain appropriate conditions

Recall that a mapping T : C → C is closed if for each {x n } in C, if x n → x

and T x n → y, then T x = y Let {T n } be a family of mappings of C into

itself with F := ∩ ∞

n=1 F (T n ) 6= ∅, {T n } is said to satisfy the (∗)-condition [46]

if for each bounded sequence {z n } in C,

lim

It follows directly from the definitions above that if T n ≡ T and T is closed,

then {T n } satisfies (∗)-condition [46] Next, we give the following example:

n , if x > 1

n ,

for all n ≥ 0 and for each x ∈ R Hence, T∞ n=1 F (T n ) = F (T n ) = {0} and

φ(0, T n x) = k0 − T n xk ≤ k0 − xk = φ(0, x), ∀x ∈ R Then, T is a relatively

quasi-nonexpansive mapping but not a relatively nonexpansive mapping

Moreover, for each bounded sequence z n ∈ E, we observe that T n z n= 1

n → 0

as n → ∞, and hence z = lim n→∞ z n = limn→∞ T n z n = 0 as n → ∞; this implies that z = 0 ∈ F (T n ) Therefore, T n is a relatively quasi-nonexpansive

mapping and satisfies the (∗)-condition.

In 2010, Shehu [47] introduced a new iterative scheme by hybrid methodsand proved strong convergence theorem for approximation of a common fixedpoint of two countable families of weak relatively nonexpansive mappingswhich is also a solution to a system of generalized mixed equilibrium problems

in a uniformly convex real Banach space which is also uniformly smooth using

the properties of generalized f -projection operator.

The following questions naturally arise in connection with the above

re-sults using the (∗)-condition:

Question 1 : Can the Mann algorithms (1.20) of [45] still be valid for an

infinite family of relatively quasi-nonexpansive mappings?

Question 2 : Can an iterative scheme (1.19) to solve a system of generalized

mixed equilibrium problems?

Question 3 : Can the Mann algorithms (1.20) be extended to more

general-ized f -projection operator?

The purpose of this paper is to solve the above questions We introduce

a new hybrid iterative scheme of the generalized f -projection operator for

finding a common element of the fixed point set for a countable family of atively quasi-nonexpansive mappings and the set of solutions of the system

rel-of generalized mixed equilibrium problem in a uniformly convex and

uni-formly smooth Banach space by using the (∗)-condition Furthermore, we

show that our new iterative scheme converges strongly to a common element

of the aforementioned sets Our results extend and improve the recent result

of Li et al [26], Matsushita and Takahashi [35], Takahashi et al [43], Nakajoand Takahashi [41] and Shehu [45] and others

A Banach space E is said to be strictly convex if k x+y2 k < 1 for all x, y ∈ E

with kxk = kyk = 1 and x 6= y Let U = {x ∈ E : kxk = 1} be the unit sphere of E Then a Banach space E is said to be smooth if the limit

lim

t→0

kx+tyk−kxk

t exists for each x, y ∈ U It is also said to be uniformly smooth

if the limit exists uniformly in x, y ∈ U Let E be a Banach space The

modulus of smoothness of E is the function ρ E : [0, ∞) → [0, ∞) defined by

ρ E (t) = sup{ kx+yk+kx−yk2 − 1 : kxk = 1, kyk ≤ t} The modulus of convexity

of E is the function δ E : [0, 2] → [0, 1] defined by δ E (ε) = inf{1 − k x+y2 k :

x, y ∈ E, kxk = kyk = 1, kx − yk ≥ ε} The normalized duality mapping

J : E → 2 E ∗

is defined by J(x) = {x ∗ ∈ E ∗ : hx, x ∗ i = kxk2, kx ∗ k = kxk} If

E is a Hilbert space, then J = I, where I is the identity mapping.

It is also known that if E is uniformly smooth, then J is uniformly to-norm continuous on each bounded subset of E.

norm-Remark 2.1 If E is a reflexive, strictly convex and smooth Banach space,

then for x, y ∈ E, φ(x, y) = 0 if and only if x = y It is sufficient to show that if φ(x, y) = 0 then x = y From (1.8), we have kxk = kyk This implies that hx, Jyi = kxk2 = kJyk2 From the definition of J, one has Jx = Jy.

Therefore, we have x = y; see [19, 21] for more details.

We also need the following lemmas for the proof of our main results:

Lemma 2.2 [20] Let E be a uniformly convex and smooth Banach space

and let {x n } and {y n } be two sequences of E If φ(x n , y n ) → 0 and either

{x n } or {y n } is bounded, then kx n − y n k → 0.

Lemma 2.3 [48] Let E be a Banach space and f : E → R ∪ {+∞} be a

lower semicontinuous convex functional Then there exist x ∗ ∈ E ∗ and α ∈ R such that

f (x) ≥ hx, x ∗ i + α, ∀x ∈ E.

Lemma 2.4 [26] Let E be a reflexive smooth Banach space and C be a

nonempty closed convex subset of E The following statements hold:

1 Π f C x is nonempty closed convex subset of C for all x ∈ E;

2 for all x ∈ E, ˆ x ∈ Π f C x if and only if

hˆ x − y, Jx − J ˆ xi + ρf (y) − ρf (ˆ x) ≥ 0, ∀y ∈ C;

3 if E is strictly convex, then Π f C is a single-valued mapping.

Lemma 2.5 [26] Let E be a real reflexive smooth Banach space, let C be a

nonempty closed convex subset of E, and let x ∈ E, ˆ x ∈ Π f C x Then

φ(y, ˆ x) + G(ˆ x, Jx) ≤ G(y, Jx), ∀y ∈ C.

Remark 2.6 Let E be a uniformly convex and uniformly smooth Banach

space and f (x) = 0 for all x ∈ E; then Lemma 2.5 reduces to the property

of the generalized projection operator considered by Alber [17]

Lemma 2.7 [4] Let E be a real uniformly smooth and strictly convex

Ba-nach space, and C be a nonempty closed convex subset of E Let T : C → C

be a closed and relatively quasi-nonexpansive mapping Then F (T ) is a closed and convex subset of C.

For solving the equilibrium problem for a bifunction θ : C × C → R, let

us assume that θ satisfies the following conditions:

(A1) θ(x, x) = 0 for all x ∈ C;

(A2) θ is monotone, i.e., θ(x, y) + θ(y, x) ≤ 0 for all x, y ∈ C;

(A3) for each x, y, z ∈ C,

lim

t↓0 θ(tz + (1 − t)x, y) ≤ θ(x, y);

(A4) for each x ∈ C, y 7→ θ(x, y) is convex and lower semi-continuous For example, let A be a continuous and monotone operator of C into E ∗ anddefine

θ(x, y) = hAx, y − xi, ∀x, y ∈ C.

Then, θ satisfies (A1)–(A4) The following result is in Blum and Oettli [1].

Motivated by Combettes and Hirstoaga [3] in a Hilbert space and hashi and Zembayashi [12] in a Banach space, Zhang [16] obtain the followinglemma:

Taka-Lemma 2.8 Let C be a closed convex subset of a smooth, strictly convex

and reflexive Banach space E Assume that θ be a bifunction from C × C to

R satisfying (A1)–(A4), A : C → E ∗ be a continuous and monotone mapping and ϕ : C → R be a semicontinuous and convex functional For r > 0 and let x ∈ E Then, there exists z ∈ C such that

In this section, by using the (∗)-condition, we prove the new convergence

theorems for finding a common fixed points of a countable family of relativelyquasi-nonexpansive mappings, in a uniformly convex and uniformly smoothBanach space

Theorem 3.1 Let C be a nonempty closed and convex subset of a

n=1 be a countable family of relatively quasi-nonexpansive mappings of C into E sat- isfy the (∗)-condition and f : E → R be a convex lower semicontinuous mapping with C ⊂ int(D(f ), where D(f ) is a domain of f For each

j = 1, 2, , m let θ j be a bifunction from C × C to R which satisfies tions (A1)–(A4), A j : C → E ∗ be a continuous and monotone mapping, and

condi-ϕ j : C → R be a lower semicontinuous and convex function Assume that

n=1 ⊂ [d, ∞) for some d > 0 (j = 1, 2, , m) If lim inf n→∞ (1 − α n ) >

0, then {x n } converges strongly to p ∈ F, where p = Π fFx0.

Proof We split the proof into five steps.

Step 1 : We first show that C n is closed and convex for each n ∈ N.

Clearly C1 = C is closed and convex Suppose that C n is closed and

convex for each n ∈ N Since for any z ∈ C n , we know G(z, Ju n ) ≤

G(z, Jx n) is equivalent to

2hz, Jx n − Ju n i ≤ kx n k2− ku n k2.

So, C n+1is closed and convex This implies that Πf C n+1 x0is well defined

Step 2 : We show that F ⊂ C n for all n ∈ N.

Next, we show by induction that F ⊂ C n for all n ∈ N It is obvious that F ⊂ C = C1 Suppose that F ⊂ C n for some n ∈ N Let q ∈ F and