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Common solutions of equilibrium and fixed point problems

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.

1 Introduction and preliminaries

Let E be a real Banach space. Let U E ={xE:x=1} be the unit sphere of E. E is said to be smooth iff lim t 0 x + t y x t exists for each x,y U E . It is also said to be uniformly smooth iff the above limit is attained uniformly for x,y U E . E is said to be strictly convex iff x + y 2 <1 for all x,yE with x=y=1 and xy. It is said to be uniformly convex iff lim n x n y n =0 for any two sequences { x n } and { y n } in E such that x n = y n =1 and lim n x n + y n 2 =1.

Recall that the normalized duality mapping J from E to 2 E is defined by

Jx= { f E : x , f = x 2 = f 2 } ,

where , 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 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 }E, and xE with x n x, and x n x, then x n x0 as n. It is well known that if E is a uniformly convex Banach space, then E enjoys the Kadec-Klee property.

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

ϕ(x,y)= x 2 2x,Jy+ y 2 x,yE.

Observe that, in a Hilbert space H, the equality is reduced to ϕ(x,y)= x y 2 , x,yH. As we all know, if C is a nonempty closed convex subset of a Hilbert space H and P C :HC 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 Π C in a Banach space E, which is an analogue of the metric projection P C in Hilbert spaces. Recall that the generalized projection Π C :EC is a map that assigns to an arbitrary point xE the minimum point of the functional ϕ(x,y), that is, Π C x= x ¯ , where x ¯ is the solution to the minimization problem ϕ( x ¯ ,x)= min y C ϕ(y,x). Existence and uniqueness of the operator Π C follows from the properties of the functional ϕ(x,y) and strict monotonicity of the mapping J. If E is a reflexive, strictly convex and smooth Banach space, then ϕ(x,y)=0 if and only if x=y; for more details, see [1] and the references therein. In Hilbert spaces, Π C = P C . It is obvious from the definition of a function ϕ that

ϕ(x,y)=ϕ(x,z)+ϕ(z,y)+2xz,JzJy,x,y,zE,
(1.1)

and

( x y ) 2 ϕ(x,y) ( y + x ) 2 ,x,yE.
(1.2)

Let C be a nonempty subset of E, and let T:CC 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 }C such that lim n x n = x 0 and lim n 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,

lim sup n { T n + 1 x T n x : x K } =0.

Recall that a point 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 x n T x n =0. The set of asymptotic fixed points of T will be denoted by F ˜ (T). T is said to be relatively nonexpansive iff

F ˜ (T)=F(T),ϕ(p,Tx)ϕ(p,x),xC,pF(T).

T is said to be relatively asymptotically nonexpansive iff

F ˜ (T)=F(T),ϕ ( p , T n x ) (1+ μ n )ϕ(p,x),xC,pF(T),n1,

where { μ n }[0,) is a sequence such that μ n 0 as n.

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].

Recall that T is said to be quasi-ϕ-nonexpansive iff

F(T),ϕ(p,Tx)ϕ(p,x),xC,pF(T).

Recall that T is said to be asymptotically quasi-ϕ-nonexpansive iff there exists a sequence { μ n }[0,) with μ n 0 as n such that

F(T),ϕ ( p , T n x ) (1+ μ n )ϕ(p,x),xC,pF(T),n1.

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)= 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.

Recall that T is said to be generalized asymptotically quasi-ϕ-nonexpansive iff F(T), and there exist two nonnegative sequences { μ n }[0,) with μ n 0, and { ξ n }[0,) with ξ n 0 as n such that

ϕ ( p , T n x ) (1+ μ n )ϕ(p,x)+ ξ n ,xC,pF(T),n1.

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].

Let F be a bifunction from C×C to , where denotes the set of real numbers. Recall the following equilibrium problem. Find pC such that F(p,y)0, yC. We use EP(F) to denote the solution set of the equilibrium problem. Given a mapping Q:C E , let

F(x,y)=Qx,yx,x,yC.

Then pEP(F) if and only if p is a solution of the following variational inequality. Find p such that

Qp,yp0,yC.

Numerous problems in physics, optimization and economics reduce to finding a solution of the equilibrium problem; see [936] and the related references therein. In [25], Kim studied a sequence { x n } which is generated in the following manner:

{ x 0 E , chosen arbitrarily , C 1 = C , x 1 = Π C 1 x 0 , y n = J 1 ( α n J x n + ( 1 α n ) J T n x n ) , u n C  such that  F ( u n , y ) + 1 r n y u n , J u n J y n 0 , y C , C n + 1 = { z C n : ϕ ( z , u n ) ϕ ( z , x n ) + ( k n 1 ) M n } , x n + 1 = Π C n + 1 x 0 ,

where M n =sup{ϕ(z, x n ):zF} for each n1, { α n } is a real sequence in [0,1], { r n } is a real sequence in [a,), 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]R such that g(0)=0 and

t x + ( 1 t ) y 2 t x 2 +(1t) y 2 t(1t)g ( x y )

for all x,y B r ={xE:xr} and t[0,1].

Lemma 1.7 [1]

Let C be a nonempty closed convex subset of a smooth Banach space E and xE. Then x 0 = Π C x if and only if

x 0 y,JxJ x 0 0yC.

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 xE. Then

ϕ(y, Π C x)+ϕ( Π C x,x)ϕ(y,x)yC.

Lemma 1.9 [5, 22]

Let C be a closed convex subset of a smooth, strictly convex and reflexive Banach space E. Let F be a bifunction from C×C to satisfying (A1)-(A4). Let r>0 and xE. Then there exists zC such that F(z,y)+ 1 r yz,JzJx0, yC. Define a mapping T r :EC by

S r x= { z C : f ( z , y ) + 1 r y z , J z J x , y C } .

Then the following conclusions hold:

  1. (1)

    S r is a single-valued firmly nonexpansive-type mapping, i.e., for all x,yE,

    S r x S r y,J S r xJ S r y S r x S r y,JxJy;
  2. (2)

    F( S r )=EP(F) is closed and convex;

  3. (3)

    S r is quasi-ϕ-nonexpansive;

  4. (4)

    ϕ(q, S r x)+ϕ( S r x,x)ϕ(q,x), qF( 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:CC 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×C to satisfying (A1)-(A4) for every iΔ. Let T:CC be a generalized asymptotically quasi-ϕ-nonexpansive mapping. Assume that T is closed asymptotically regular on C and Ω:=F(T) i Δ EF( F i ) is nonempty and bounded. Let { x n } be a sequence generated in the following manner:

{ x 0 E , chosen arbitrarily , C 1 , i = C , C 1 = i Δ C 1 , i , x 1 = Π C 1 x 0 , y n = J 1 ( α n J x n + ( 1 α n ) J T n x n ) , u n , i C  such that  F i ( u n , i , y ) + 1 r n , i y u n , i , J u n , i J y n 0 , y C , C n + 1 , i = { z C n : ϕ ( z , u n , i ) ϕ ( z , x n ) + μ n M n + ξ n } , C n + 1 = i Δ C n + 1 , i , x n + 1 = Π C n + 1 x 0 ,

where M n =sup{ϕ(z, x n ):zΩ}, { α n } is a real number sequence in (0,1) such that lim inf n α n (1 α n )>0, { r n , i } is a real number sequence in [ a i ,), where { a i } is a positive real number sequence. Then the sequence { x n } converges strongly to Π Ω x 0 , where Π Ω 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 iΔ, 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 j1. 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, a,b C j + 1 , i , we see that a,b C j , i , and

ϕ(a, u j , i )ϕ(a, x j )+ μ j M j + ξ j ,

and

ϕ(b, u j , i )ϕ(b, x j )+ μ j M j + ξ j .

Notice that the two inequalities above are equivalent to the following inequalities, respectively:

2a,J x j J u j , i x j 2 u j , i 2 + μ j M j + ξ j ,

and

2b,J x j J u j , i x j 2 u j , i 2 + μ j M j + ξ j .

These imply that

2 t a + ( 1 t ) b , J x j J u j , i x j 2 u j , i 2 + μ j M j + ξ j ,t(0,1).

Since C j , i is convex, we see that ta+(1t)b C j , i . Notice that the above inequality is equivalent to

ϕ ( t a + ( 1 t ) b , u j , i ) ϕ ( t a + ( 1 t ) b , x j ) + μ j M j + ξ j .

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

Next, we prove that Ω C n . It suffices to claim that Ω C n , i for every iΔ. Note that Ω C 1 , i =C. Suppose that Ω C j , i for some j and for every iΔ. Then, for wΩ C j , i , we have

ϕ ( w , u j , i ) = ϕ ( w , S r j , i y j ) = ϕ ( w , J 1 ( α j J x j + ( 1 α j ) J T j x j ) ) = w 2 2 w , α j J x j + ( 1 α j ) J T j x j + α j J x j + ( 1 α j ) J T j x j 2 w 2 2 α j w , J x j 2 ( 1 α j ) w , J T j x j + α j x j 2 + ( 1 α j ) T j x j 2 = α j ϕ ( w , x j ) + ( 1 α j ) ϕ ( w , T j x j ) α j ϕ ( w , x j ) + ( 1 α j ) ( 1 + μ j ) ϕ ( w , x j ) + ξ j ( 1 α j ) ϕ ( w , x j ) + μ j ϕ ( w , x j ) + ξ j ϕ ( w , x j ) + μ j M j + ξ j .

This shows that w C j + 1 , i . This implies that Ω C n for every n1.

On the other hand, it follows from Lemma 1.8 that

ϕ( x n , x 0 )=ϕ( Π C n x 0 , x 0 )ϕ(w, x 0 )ϕ(w, x n )ϕ(w, x 0 ),wΩ C n .

This shows that the sequence ϕ( x n , x 0 ) is bounded. In view of (1.2), we see that the sequence { x n } is also bounded. Since the space is reflexive, we may, without loss of generality, assume that x n p C n . Note that ϕ( x n , x 0 )ϕ(p, x 0 ). It follows that

ϕ(p, x 0 ) lim inf n ϕ( x n , x 0 ) lim sup n ϕ( x n , x 0 )ϕ(p, x 0 ).

This implies that

lim n ϕ( x n , x 0 )=ϕ(p, x 0 ).

Hence, we have x n p as n. In view of the Kadec-Klee property of E, we obtain that x n p as n.

Next, we show that pF(T). By the construction of C n , we have that C n + 1 C n and x n + 1 = Π C n + 1 x 0 C n . It follows that

ϕ ( x n + 1 , x n ) = ϕ ( x n + 1 , Π C n x 0 ) ϕ ( x n + 1 , x 0 ) ϕ ( Π C n x 0 , x 0 ) = ϕ ( x n + 1 , x 0 ) ϕ ( x n , x 0 ) .

Letting n, we obtain that ϕ( x n + 1 , x n )0. In view of x n + 1 C n + 1 , we see that

ϕ( x n + 1 , u n , i )ϕ( x n + 1 , x n )+ μ n M n + ξ n .

It follows that

lim n ϕ( x n + 1 , u n , i )=0.

From (1.2), we see that lim n u n , i =p. It follows that lim n J u n , i =Jp. This implies that {J u n , i } is bounded. Note that E is reflexive and E is also reflexive. We may assume that J u n , i x , i E . In view of the reflexivity of E, we see that J(E)= E . This shows that there exists an x i E such that J x i = x , i . It follows that

ϕ ( x n + 1 , u n , i ) = x n + 1 2 2 x n + 1 , J u n + u n 2 = x n + 1 2 2 x n + 1 , J u n + J u n 2 .

Taking lim inf n on the both sides of the equality above yields that

0 p 2 2 p , x , i + x , i 2 = p 2 2 p , J x i + J x i 2 = p 2 2 p , J x i + x i 2 = ϕ ( p , x i ) .

That is, p= x i , which in turn implies that x , i =Jp. It follows that J u n , i Jp E . Since E enjoys the Kadec-Klee property, we obtain that J u n , i Jp0 as n. Note that J 1 : E E is demi-continuous. It follows that u n , i p. Since E enjoys the Kadec-Klee property, we obtain that u n , i p as n. Note that

x n u n , i x n p+p u n , i .

It follows that

lim n x n u n , i =0.
(2.1)

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

lim n J x n J u n , i =0.
(2.2)

Let r= sup n 0 { x n , T n x n }. Since E is uniformly smooth, we know that E is uniformly convex. In view of Lemma 1.6, we see that

ϕ ( w , u n , i ) = ϕ ( w , S r n , i y n ) = ϕ ( w , J 1 ( α n J x n + ( 1 α j ) J T n x n ) ) = w 2 2 w , α n J x n + ( 1 α n ) J T n x n + α n J x n + ( 1 α n ) J T n x n 2 w 2 2 α n w , J x n 2 ( 1 α n ) w , J T n x n + α n x n 2 + ( 1 α n ) T n x n 2 α n ( 1 α n ) g ( J x n J T n x n ) = α n ϕ ( w , x n ) + ( 1 α n ) ϕ ( w , T n x n ) α n ( 1 α n ) g ( J x n J T n x n ) α n ϕ ( w , x n ) + ( 1 α n ) ( 1 + μ n ) ϕ ( w , x n ) + ξ n ( 1 α n ) α n ( 1 α n ) g ( J x n J T n x n ) ϕ ( w , x n ) + μ n ϕ ( w , x n ) + ξ n α n ( 1 α n ) g ( J x n J T n x n ) ϕ ( w , x n ) + μ n M n + ξ n α n ( 1 α n ) g ( J x n J T n x n ) .

It follows that

α n (1 α n )g ( J x n J T n x n ) ϕ(w, x n )ϕ(w, u n , i )+ μ n M n + ξ n .

Notice that

ϕ ( w , x n ) ϕ ( w , u n , i ) = x n 2 u n , i 2 2 w , J x n J u n , i x n u n , i ( x n + u n , i ) + 2 w J x n J u n , i .

It follows from (2.1) and (2.2) that ϕ(w, x n )ϕ(w, u n , i )0 as n. In view of lim inf n α n (1 α n )>0, we see that lim n g(J x n J T n x n )=0. It follows from the property of g that

lim n J x n J T n x n =0.
(2.3)

Since x n p as n and J:E E is demi-continuous, we obtain that J x n Jp E . Note that

| J x n J p | = | x n p | x n p.

This implies that J x n Jp as n. Since E enjoys the Kadec-Klee property, we see that

lim n J x n Jp=0.
(2.4)

Notice that

J T n x n J p J T n x n J x n +J x n Jp.

It follows from (2.3) and (2.4) that

lim n J T n x n J p =0.
(2.5)

Note that J 1 : E E is demi-continuous. It follows that T n x n p. On the other hand, we have

| T n x n p | = | J T n x n J p | J T n x n J p .

In view of (2.5), we obtain that T n x n p as n. Since E enjoys the Kadec-Klee property, we obtain that

lim n T n x n p =0.
(2.6)

Note that

T n + 1 x n p T n + 1 x n T n x n + T n x n p .

It follows from the asymptotic regularity of T and (2.6) that

lim n T n + 1 x n p =0.

That is, T T n x n p0 as n. It follows from the closedness of T that Tp=p.

Next, we show that p i Δ EF( F i ). Notice that ϕ(w, y n )ϕ(w, x n )+ μ n M n + ξ n . In view of u n , i = S r n , i y n , we find from Lemma 1.8 that

ϕ ( u n , i , y n ) = ϕ ( S r n , i y n , y n ) ϕ ( w , y n ) ϕ ( w , S r n , i y n ) ϕ ( w , x n ) ϕ ( w , S r n , i y n ) + μ n M n + ξ n = ϕ ( w , x n ) ϕ ( w , u n , i ) + μ n M n + ξ n .

This in turn implies that

lim n ϕ( u n , i , y n )=0.

It follows from (1.2) that u n , i y n 0 as n. In view of u n , i p as n, we arrive at lim n y n =p. It follows that lim n J y n =Jp. Since E is reflexive, we may assume that J y n f E . In view of J(E)= E , we see that there exists fE such that Jf= f . It follows that

ϕ( u n , i , y n )= u n , i 2 2 u n , i ,J y n + J y n 2 .

Taking lim inf n on the both sides of the equality above yields that

0 p 2 2 p , f + f 2 = p 2 2 p , J f + J f 2 = p 2 2 p , J f + f 2 = ϕ ( p , f ) .

That is, p=f, which in turn implies that f =Jp. It follows that J y n Jp E . Since E enjoys the Kadec-Klee property, we obtain that J y n Jp0 as n. Note that J 1 : E E is demi-continuous. It follows that y n p. Since E enjoys the Kadec-Klee property, we obtain that y n p as n. Notice that u n , i y n u n , i p+p y n . It follows that

lim n u n , i y n =0.

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

lim n J u n , i J y n =0.

From the assumption r n , i a i , we see that

lim n J u n , i J y n r n , i =0.

Notice that

F i ( u n , i ,y)+ 1 r n , i y u n , i ,J u n , i J y n 0,yC.

It follows from condition (A2) that

y u n , i J u n , i J y n r n , i 1 r n , i y u n , i ,J u n , i J y n F i (y, u n , i ),yC.

By taking the limit as n in the above inequality, from condition (A4) we obtain that

F i (y,p)0,yC.

For 0< t i <1 and yC, define y t i = t i y+(1 t i )p. It follows that y t , i C, which yields that F i ( y t , i ,p)0. It follows from conditions (A1) and (A4) that

0= F i ( y t , i , y t , i ) t i F i ( y t , i ,y)+(1 t i ) F i ( y t , i ,p) t i F i ( y t , i ,y).

That is,

F i ( y t , i ,y)0.

Letting t i 0, we find from condition (A3) that F i (p,y)0, yC. This implies that pEP( F i ). This completes the proof that pΩ.

Finally, we prove that p= Π Ω x 0 . From x n = Π C n x 0 , we see that

x n z,J x 0 J x n 0,z C n .

In view of Ω C n , we find that

x n w,J x 0 J x n 0,wΩ.

Letting n in the above inequality, we see that

pw,J x 0 Jp0,wΩ.

In view of Lemma 1.7, we can obtain that p= Π Ω 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×C to satisfying (A1)-(A4) for every iΔ. Let T:CC be an asymptotically quasi-ϕ-nonexpansive mapping. Assume that T is closed asymptotically regular on C and Ω:=F(T) i Δ EF( F i ) is nonempty and bounded. Let { x n } be a sequence generated in the following manner:

{ x 0 E , chosen arbitrarily , C 1 , i = C , C 1 = i Δ C 1 , i , x 1 = Π C 1 x 0 , y n = J 1 ( α n J x n + ( 1 α n ) J T n x n ) , u n , i C  such that  F i ( u n , i , y ) + 1 r n , i y u n , i , J u n , i J y n 0 , y C , C n + 1 , i = { z C n : ϕ ( z , u n , i ) ϕ ( z , x n ) + μ n M n } , C n + 1 = i Δ C n + 1 , i , x n + 1 = Π C n + 1 x 0 ,

where M n =sup{ϕ(z, x n ):zΩ}, { α n } is a real number sequence in (0,1) such that lim inf n α n (1 α n )>0, { r n , i } is a real number sequence in [ a i ,), where { a i } is a positive real number sequence. Then the sequence { x n } converges strongly to Π Ω x 0 , where Π Ω 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:

  1. (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;

  2. (b)

    from a single bifunction to a family of bifunctions;

  3. (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×C to satisfying (A1)-(A4) for every iΔ. Let T:CC be a generalized asymptotically quasi-nonexpansive mapping. Assume that T is closed asymptotically regular on C and Ω:=F(T) i Δ EF( F i ) is nonempty and bounded. Let { x n } be a sequence generated in the following manner:

{ x 0 E , chosen arbitrarily , C 1 , i = C , C 1 = i Δ C 1 , i , x 1 = Π C 1 x 0 , y n = α n x n + ( 1 α n ) T n x n , u n , i C  such that  F i ( u n , i , y ) + 1 r n , i y u n , i , u n , i y n 0 , y C , C n + 1 , i = { z C n : z u n , i 2 z x n 2 + μ n M n + ξ n } , C n + 1 = i Δ C n + 1 , i , x n + 1 = Proj C n + 1 x 0 ,

where M n =sup{ z x n 2 :zΩ}, { α n } is a real number sequence in (0,1) such that lim inf n α n (1 α n )>0, { r n , i } is a real number sequence in [ a i ,), where { a i } is a positive real number sequence. Then the sequence { x n } converges strongly to Proj Ω x 0 , where Proj Ω is the metric projection from E onto Ω.

Proof In the framework of Hilbert spaces, we find that ϕ(x,y)= x y 2 , J is reduced to the identity mapping and the generalized projection Π 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×C to satisfying (A1)-(A4). Let T:CC be a generalized asymptotically quasi-nonexpansive mapping. Assume that T is closed asymptotically regular on C and Ω:=F(T)EF(F) is nonempty and bounded. Let { x n } be a sequence generated in the following manner:

{ x 0 E , chosen arbitrarily , C 1 = C , x 1 = Π C 1 x 0 , y n = α n x n + ( 1 α n ) T n x n , u n C  such that  F ( u n , y ) + 1 r n y u n , u n y n 0 , y C , C n + 1 = { z C n : z u n 2 z x n 2 + μ n M n + ξ n } , x n + 1 = Proj C n + 1 x 0 ,

where M n =sup{ z x n 2 :zΩ}, { α n } is a real number sequence in (0,1) such that lim inf n α n (1 α n )>0, { r n , i } is a real number sequence in [a,), where a is a positive real number. Then the sequence { x n } converges strongly to Proj Ω x 0 , where Proj Ω is the metric projection from E onto Ω.

Proof In the framework of Hilbert spaces, we find that ϕ(x,y)= x y 2 , J is reduced to the identity mapping, and the generalized projection Π C is reduced to the metric projection Proj C . In view of Corollary 2.5, we may immediately conclude the desired results. □

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Zhang, QN. Common solutions of equilibrium and fixed point problems. J Inequal Appl 2013, 425 (2013). https://doi.org/10.1186/1029-242X-2013-425

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Keywords

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