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Strong convergence theorems for a finite family of total asymptotically strict pseudocontractive semigroups in Banach spaces

Fixed Point Theory and Applications20132013:178

https://doi.org/10.1186/1687-1812-2013-178

  • Received: 9 October 2012
  • Accepted: 19 June 2013
  • Published:

Abstract

The purpose of this paper is to introduce the concepts of total asymptotically strictly pseudocontractive semigroup, asymptotically strictly pseudocontractive semigroup etc., and to prove some strong convergence theorems of the explicit iteration process for these kinds of semigroups in arbitrary Banach spaces. The results presented in the paper extend and improve some recent results announced in the current literature.

MSC:47H20, 47H10.

Keywords

  • total asymptotically strict pseudocontractive semigroup
  • asymptotically strict pseudocontractive semigroup
  • asymptotically demicontractive semigroup
  • fixed point
  • normalized duality mapping

1 Introduction and preliminaries

Let E be a real Banach space, E be the dual space of E. In the sequel we assume that C is a nonempty closed convex subset of E, + is the set of nonnegative real numbers and J : E 2 E is the normalized duality mapping defined by
J ( x ) = { f E : x , f = x f , x = f } , x E .
(1.1)

Let T : C C be a mapping, we denote by F ( T ) the set of fixed points of T. We also use ‘→’ to stand for strong convergence and ‘’ for weak convergence.

We first recall some definitions.

A one-parameter family : = { T ( t ) : t 0 } of self-mappings of C is said to be a nonexpansive semigroup if the following conditions are satisfied:
  1. (i)

    T ( t 1 + t 2 ) x = T ( t 1 ) T ( t 2 ) x , for any t 1 , t 2 + and x C ;

     
  2. (ii)

    T ( 0 ) x = x for each x C ;

     
  3. (iii)

    for each x C , t T ( t ) x is continuous;

     
  4. (iv)
    for any t 0 , T ( t ) is a nonexpansive mapping on C, that is, for any x , y C ,
    T ( t ) x T ( t ) y x y .
    (1.2)
     
If the family : = { T ( t ) : t 0 } satisfies conditions (i)-(iii), then it is:
  1. (a)
    Pseudocontractive semigroup if for any x , y C , there exists j ( x y ) J ( x y ) such that
    T ( t ) x T ( t ) y , j ( x y ) x y 2 .
    (1.3)
     
  2. (b)
    Uniformly Lipschitzian semigroup if there exists a bounded measurable function L : [ 0 , ) ( 0 , ) such that, for any x , y C and t 0 ,
    T n ( t ) x T n ( t ) y L ( t ) x y , n 1 .
    (1.4)
     
In this case, we also say that is a uniformly L ( t ) -Lipschitzian semigroup.
  1. (c)
    Strictly pseudocontractive semigroup if there exists a bounded function λ : [ 0 , ) ( 0 , ) and for any given x , y C , there exists j ( x y ) J ( x y ) such that
    T ( t ) x T ( t ) y , j ( x y ) x y 2 λ ( t ) ( I T ( t ) ) x ( I T ( t ) ) y 2
    (1.5)
     

for any t 0 .

It is easy to see that such a semigroup is ( ( 1 + λ ( t ) ) / λ ( t ) ) -Lipschitzian and pseudocontractive semigroup.
  1. (d)
    Demicontractive semigroup if t 0 F ( T ( t ) ) , and there exists a bounded function λ : [ 0 , ) ( 0 , ) for any t 0 , x C and y t 0 F ( T ( t ) ) , there exists j ( x y ) J ( x y ) such that
    T ( t ) x y , j ( x y ) x y 2 λ ( t ) ( I T ( t ) ) x 2 .
    (1.6)
     

In this paper, we introduce the following semigroups.

Definition 1.1 A one-parameter family : = { T ( t ) : t 0 } of self-mappings of C satisfies conditions (i)-(iii), then it is:
  1. (e)
    Total asymptotically strictly pseudocontractive semigroup if there exist a bounded function λ : [ 0 , ) ( 0 , ) and sequences { μ n } [ 0 , ) and { ξ n } [ 0 , ) with μ n 0 and ξ n 0 as n . For any given x , y C , there exists j ( x y ) J ( x y ) such that
    T n ( t ) x T n ( t ) y , j ( x y ) x y 2 λ ( t ) ( I T n ( t ) ) x ( I T n ( t ) ) y 2 + μ n ϕ ( x y ) + ξ n , n 1 , t 0 ,
    (1.7)
     

where ϕ : [ 0 , ) [ 0 , ) is a continuous and strictly increasing function with ϕ ( 0 ) = 0 .

In this case, we also say that is a ( λ ( t ) , { μ n } , { ξ n } , ϕ ) -total asymptotically strict pseudocontractive semigroup.
  1. (f)
    Asymptotically strictly pseudocontractive semigroup if there exist a bounded function λ : [ 0 , ) ( 0 , ) and a sequence { k n } [ 1 , ) with k n 1 as n ; for any given x , y C , there exists j ( x y ) J ( x y ) such that
    T n ( t ) x T n ( t ) y , j ( x y ) k n x y 2 λ ( t ) ( I T n ( t ) ) x ( I T n ( t ) ) y 2 , n 1 ,
    (1.8)
     

for any t 0 .

In this case, we also say that is a ( { k n } , λ ( t ) ) -asymptotically strict pseudocontractive semigroup.
  1. (g)
    Asymptotically demicontractive semigroup if t 0 F ( T ( t ) ) and there exist a bounded function λ : [ 0 , ) ( 0 , ) and a sequence { k n } [ 1 , ) with k n 1 as n ; for any t 0 , x C and y t 0 F ( T ( t ) ) , there exists j ( x y ) J ( x y ) such that
    T n ( t ) x y , j ( x y ) k n x y 2 λ ( t ) ( I T n ( t ) ) x 2 , n 1 ,
    (1.9)
     

for any t 0 .

In this case, we also say that is a ( { k n } , λ ( t ) ) -asymptotically demicontractive semigroup.

Remark 1.2 If ϕ ( λ ) = λ 2 and ξ n = 0 , then a total asymptotically strict pseudocontractive semigroup is an asymptotically strict pseudocontractive semigroup. Every asymptotically strict pseudocontractive semigroup with t > 0 F ( T ( t ) ) ϕ is an asymptotically demicontractive semigroup. If k n = 1 , n 1 , an asymptotically strict pseudocontractive semigroup is a strict pseudocontractive semigroup, an asymptotically demicontractive semigroup is a demicontractive semigroup.

It is easy to see that condition (1.7) is equivalent to the following condition: for any t 0 , x C and y F ( T ( t ) ) , there exists j ( x y ) J ( x y ) such that
x T n ( t ) x , j ( x y ) λ ( t ) x T n ( t ) x 2 μ n ϕ ( x y ) ξ n .
(1.10)

The convergence problem of implicit and explicit iterative sequences for nonexpansive semigroups to a common fixed point has been considered by some authors in various spaces; see, for example, [111].

In 1998, Shioji-Takahashi [1] introduced and studied a Halpern-type scheme for a common fixed point of a family of asymptotically nonexpansive semigroups in the framework of a real Hilbert space.

In 2003, Suzuki [2] proved that the implicit scheme defined by u , x 1 C
x n = α n u + ( 1 α n ) T ( t n ) ( x n ) , n 1 ,
(1.11)

converges strongly to a common fixed point of the family of nonexpansive semigroups in a real Hilbert space. Xu [3] extended the result of Suzuki to a more general real uniformly convex Banach space having a weakly sequentially continuous duality mapping.

In 2005, Aleyner and Reich [4] proved the strong convergence of an explicit Halpern-type scheme defined by u , x 1 C
x n + 1 = α n u + ( 1 α n ) T ( t n ) x n , n 0 ,
(1.12)

to a common fixed point of the family { T ( t ) : t 0 } of nonexpansive semigroups in a reflexive Banach space with a uniformly Gâteaux differentiable norm.

Recently, Chang et al. [11] introduced the following explicit iteration process:
x 1 C , x n + 1 = ( 1 α n ) x n + α n T ( t n ) x n , n 1 ,
(1.13)

for the Lipschitzian and demicontractive semigroup : = { T ( t ) : t 0 } in general Banach spaces. Under appropriate assumptions imposed upon the parameter sequences { α n } and { t n } , they proved that the sequence { x n } defined by (1.13) converges strongly to some point in F = : t 0 F ( T ( t ) ) .

Inspired and motivated by the above works of Shioji-Takahashi [1], Suzuki [2], Xu [3], Aleyner-Reich [4] and Chang [11], in this paper we aim to study the strong convergence to a common fixed point for a finite family of uniformly Lipschitzian and total asymptotically strict pseudocontractive semigroups i : = { T i ( t ) : t 0 } , i { 1 , 2 , , N } in general Banach spaces. The results presented in the paper extend and improve some recent results given in [4, 5, 7, 9].

The following lemmas will be needed in proving our main results.

Lemma 1.3 Let { a n } , { b n } and { δ n } be sequences of nonnegative real numbers satisfying
a n + 1 ( 1 + δ n ) a n + b n , n n 0 ,
(1.14)

where n 0 is some nonnegative integer. If n = 1 δ n < and n = 1 b n < , then the limit lim n a n exists.

Lemma 1.4 [12]

Let E be any real Banach space, let E be the dual space of E and let J : E 2 E be the normalized duality mapping. Then, for any x , y E , we have
x + y 2 x 2 + 2 y , j ( x + y ) , j ( x + y ) J ( x + y ) .
(1.15)

2 Main results

Let E be a real Banach space, and let C be a nonempty, closed convex subset of E. For some fixed i N , let i : = { T i ( t ) : t 0 } be a uniformly L i ( t ) -Lipschitzian with a bounded measurable function L i : [ 0 , ) ( 0 , ) and ( λ i ( t ) , { μ i n } , { ξ i n } , ϕ i ) -total asymptotically strictly pseudocontractive semigroup with a bounded function λ i : [ 0 , ) ( 0 , ) , sequences { μ i n } , { ξ i n } [ 0 , ) with μ i n 0 and ξ i n 0 as n , such that
L i : = sup t 0 L i ( t ) < , λ i : = inf t 0 λ i ( t ) > 0 , F i : = t 0 F ( T i ( t ) ) .
(2.1)
Then, for x , y C , p F i and t 0 ,
T i n ( t ) x p , j ( x p ) x p 2 λ i x T i n ( t ) x 2 + μ i n ϕ i ( x p ) + ξ i n , n 1 ,
(2.2)
where ϕ i : [ 0 , ) [ 0 , ) is a continuous and strictly increasing function with ϕ i ( 0 ) = 0 , and
T i n ( t ) x T i n ( t ) y L i x y , n 1 .
(2.3)
Consider a family { i } i = 1 N of uniformly L i ( t ) -Lipschitzian and ( λ i ( t ) , { μ i n } , { ξ i n } , ϕ i ) -total asymptotically strict pseudocontractive semigroups of C and let
L : = max 1 i N L i < , λ : = min 1 i N λ i > 0 , F : = 1 i N t 0 F ( T i ( t ) ) , μ n = max 1 i N μ i n , ξ n = max 1 i N ξ i n , ϕ = max 1 i N ϕ i .
(2.4)
For x , y C , p F , t 0 and any i { 1 , 2 , , N } ,
T i n ( t ) x p , j ( x p ) x p 2 λ x T i n ( t ) x 2 + μ n ϕ ( x p ) + ξ n , n 1
(2.5)
and
T i n ( t ) x T i n ( t ) y L x y , n 1 .
(2.6)

Now, we are ready to give our main results.

Theorem 2.1 Let E be a real Banach space, and let C be a nonempty, closed convex subset of E. Let { i } i = 1 N be a finite family of uniformly L i ( t ) -Lipschitzian and ( λ i ( t ) , { μ i n } , { ξ i n } , ϕ i ) -total asymptotically strictly pseudocontractive semigroups of C, L i ( t ) , λ i ( t ) , { μ i n } , { ξ i n } , ϕ i , L i , λ i , F i , L, λ, F, ϕ, { μ n } and { ξ n } be the same as above. In addition, there exist positive constants M and M such that ϕ ( λ ) M λ 2 for all λ M . Let { x n } be the sequence generated by
x 1 C , x n + 1 = ( 1 α n ) x n + α n T n n ( t n ) x n , n 1 ,
(2.7)
where T n n ( t n ) = T n ( mod N ) n ( t n ) , n 1 , { α n } is a sequence in ( 0 , 1 ) and { t n } is an increasing sequence in [ 0 , ) . If the following conditions are satisfied:
  1. (1)

    n = 1 α n 2 < , n = 1 α n = , n = 1 α n μ n < , n = 1 α n ξ n < .

     
  2. (2)
    Assume for any i { 1 , 2 , , N } and for any bounded subset D C ,
    lim n sup x D , s R + T i n ( s + t n ) x T i n ( t n ) x = 0 .
    (2.8)
     
  3. (3)

    There exists a compact subset G of E such that t 0 T l ( t ) ( C ) G for some l { 1 , 2 , , N } .

     

Then the sequence { x n } converges strongly to some element in F.

Proof The proof of Theorem 2.1 is divided into four steps.

Step 1. First we prove that lim n x n p exists for all p F .

For any p F , by (1.4) we have
T n n ( t n ) x n p = T n n ( t n ) x n T n n ( t n ) p L x n p .
(2.9)
It follows from (2.7) and (2.9) that
x n + 1 p = ( 1 α n ) x n + α n T n n ( t n ) x n ( 1 α n ) p α n p ( 1 α n ) x n p + α n T n n ( t n ) x n p ( 1 α n ) x n p + α n L x n p ( 1 + L ) x n p
(2.10)
and
x n + 1 x n = α n T n n ( t n ) x n x n α n ( T n n ( t n ) x n p + x n p ) α n ( 1 + L ) x n p .
(2.11)
By (1.10), for the point x n + 1 and p, there exists j ( x n + 1 p ) J ( x n + 1 p ) such that
T n n ( t n ) x n + 1 x n + 1 , j ( x n + 1 p ) λ T n n ( t n ) x n + 1 x n + 1 2 + μ n ϕ ( x n + 1 p ) + ξ n , n 1 .
(2.12)
Since ϕ is an increasing function, it results that ϕ ( λ ) ϕ ( M ) if λ M and ϕ ( λ ) M λ 2 if λ M . In either case, we can obtain that
ϕ ( λ ) ϕ ( M ) + M λ 2 .
(2.13)
Thus, by Lemma 1.4, (2.10)-(2.13), we have
x n + 1 p 2 x n p + α n ( T n n ( t n ) x n x n ) 2 x n p 2 + 2 α n T n n ( t n ) x n x n , j ( x n + 1 p ) = x n p 2 + 2 α n T n n ( t n ) x n T n n ( t n ) x n + 1 , j ( x n + 1 p ) + 2 α n T n n ( t n ) x n + 1 x n + 1 , j ( x n + 1 p ) + 2 α n x n + 1 x n , j ( x n + 1 p ) x n p 2 + 2 α n L x n + 1 x n x n + 1 p 2 α n λ T n n ( t n ) x n + 1 x n + 1 2 + 2 α n μ n ϕ ( x n + 1 p ) + 2 α n ξ n + 2 α n x n + 1 x n x n + 1 p ( 1 + 2 α n 2 ( 1 + L ) 3 + 2 α n μ n M ( 1 + L ) 2 ) x n p 2 2 α n λ T n n ( t n ) x n + 1 x n + 1 2 + 2 α n μ n ϕ ( M ) + 2 α n ξ n ( 1 + 2 α n 2 ( 1 + L ) 3 + 2 α n μ n M ( 1 + L ) 2 ) x n p 2 + 2 α n μ n ϕ ( M ) + 2 α n ξ n .
(2.14)

By condition (1), it follows from Lemma 1.3 that the limit lim n x n p exists and so the sequence { x n } is bounded in C.

Step 2. Now we prove that
lim inf n x n T n n ( t ) x n = 0 .
(2.15)
In fact, it follows from (2.14) that
2 α n λ T n n ( t n ) x n + 1 x n + 1 2 x n p 2 x n + 1 p 2 + 2 α n 2 ( 1 + L ) 3 K 2 + ( 2 M ( 1 + L ) 2 K 2 + 2 ϕ ( M ) ) α n μ n + 2 α n ξ n ,
(2.16)
where K = sup n 1 x n p . Hence, for some m 1 ,
2 λ n = 1 m α n T n n ( t n ) x n + 1 x n + 1 2 n = 1 m ( x n p 2 x n + 1 p 2 ) + 2 ( 1 + L ) 3 K 2 n = 1 m α n 2 + ( 2 M ( 1 + L ) 2 K 2 + 2 ϕ ( M ) ) n = 1 m α n μ n + 2 n = 1 m α n ξ n x 1 p 2 + 2 ( 1 + L ) 3 K 2 n = 1 m α n 2 + ( 2 M ( 1 + L ) 2 K 2 + 2 ϕ ( M ) ) n = 1 m α n μ n + 2 n = 1 m α n ξ n .
(2.17)
Letting m , we have
2 λ n = 1 α n T n n ( t n ) x n + 1 x n + 1 2 x 1 p 2 + 2 ( 1 + L ) 3 K 2 n = 1 α n 2 + ( 2 M ( 1 + L ) 2 K 2 + 2 ϕ ( M ) ) n = 1 α n μ n + 2 n = 1 α n ξ n .
(2.18)
By condition (1), we obtain
n = 1 α n T n n ( t n ) x n + 1 x n + 1 2 < ,
(2.19)
which implies
lim inf n x n + 1 T n n ( t n ) x n + 1 = 0 .
(2.20)
Since lim n x n p exists for all p F and lim n α n = 0 , using (2.11), we have
lim n x n + 1 x n = 0 .
(2.21)
This implies that
lim n x n + i x n = 0 , i { 1 , 2 , , N } .
(2.22)
It follows from (2.20) and (2.21) that
lim inf n x n T n n ( t n ) x n lim inf n { x n x n + 1 + x n + 1 T n n ( t n ) x n + 1 + T n n ( t n ) x n + 1 T n n ( t n ) x n } lim inf n { ( 1 + L ) x n x n + 1 + x n + 1 T n n ( t n ) x n + 1 } = 0 .
(2.23)
For any t 0 , we have
x n T n n ( t ) x n x n T n n ( t n ) x n + T n n ( t n ) x n T n n ( t n + t ) x n + T n n ( t n + t ) x n T n n ( t ) x n ( 1 + L ) x n T n n ( t n ) x n + sup z { x n } , t R + T n n ( t + t n ) z T n n ( t n ) z ,
(2.24)

from (2.8) and (2.23), conclusion (2.15) is proved.

Step 3. Now we prove that
lim inf n x n T m ( t ) x n = 0 , m { 1 , 2 , , N } .
(2.25)
For each j = 1 , 2 , , N , let
η i N + j : = x i N + j T j i N + j ( t ) x i N + j .
Since T j is uniformly L j ( t ) -Lipschitzian continuous, it follows from (2.1) and (2.4) that
x i N + j T j ( t ) x i N + j x i N + j T j i N + j ( t ) x i N + j + T j i N + j ( t ) x i N + j T j ( t ) x i N + j η i N + j + L T j i N + j 1 ( t ) x i N + j x i N + j η i N + j + L { T j i N + j 1 ( t ) x i N + j T j i N + j 1 ( t ) x i N + j 1 + T j i N + j 1 ( t ) x i N + j 1 x i N + j 1 + x i N + j 1 x i N + j } η i N + j + L ( 1 + L ) x i N + j x i N + j 1 + L η i N + j 1 ,
from (2.15) and (2.21), we have
lim inf n x i N + j T j ( t ) x i N + j = 0 .
(2.26)
This implies that
lim inf n x n T n ( t ) x n = 0 ,
(2.27)

where T n ( t ) = T n ( mod N ) ( t ) , n 1 .

For l { 1 , 2 , , N } , we have
x n T n + l ( t ) x n x n x n + l + x n + l T n + l ( t ) x n + l + T n + l ( t ) x n + l T n + l ( t ) x n ( 1 + L ) x n x n + l + x n + l T n + l ( t ) x n + l ,
from (2.22) and (2.27), we obtain
lim inf n x n T n + l ( t ) x n = 0 .
(2.28)

It follows from (2.28) that lim inf n x n T m ( t ) x n = 0 , m { 1 , 2 , , N } . This completes the proof.

Step 4. Finally, we prove the sequence { x n } converges strongly to some element in F.

By (2.25), we have lim inf n x n T m ( t ) x n = 0 , m { 1 , 2 , , N } . If t 0 T l ( t ) ( C ) G for some compact subset G of E and some l { 1 , 2 , , N } , then there exists a subsequence { x n k } of { x n } and q C such that
lim k T l ( t ) x n k = q , lim k x n k T l ( t ) x n k = 0 .
(2.29)
Hence, it follows from (2.29) that
lim k x n k = q .
(2.30)

Now, for any m { 1 , 2 , , N } , since lim inf k x n k T m ( t ) x n k = 0 , there exists a subsequence { x n k j } of { x n k } such that lim j x n k j T m ( t ) x n k j = 0 . Using (2.30) and the fact that T m is Lipschitzian, we get q t 0 F ( T m ( t ) ) . Since m { 1 , 2 , , N } is arbitrarily chosen, we have q F .

Since x n k q as k and the limit lim n x n q exists, this implies that x n q F as n . This completes the proof. □

The following theorem can be obtained from Theorem 2.1 immediately.

Theorem 2.2 Let E be a real Banach space, and let C be a nonempty, closed convex subset of E. Let { i } i = 1 N be a finite family of uniformly L i ( t ) -Lipschitzian and ( { k i n } , λ i ( t ) ) -asymptotically strictly pseudocontractive semigroups of C, L i ( t ) , λ i ( t ) , L i , λ i , F i , L, λ, F be as in Theorem  2.1. k n = m a x 1 i N { k i n } . Let { x n } be the sequence defined by (2.7), { α n } is a sequence in ( 0 , 1 ) and let { t n } be an increasing sequence in [ 0 , ) . If the following conditions are satisfied:
  1. (1)

    n = 1 α n = , n = 1 α n 2 < , n = 1 α n ( k n 1 ) < .

     
  2. (2)
    Assume for any i { 1 , 2 , , N } and for any bounded subset D C ,
    lim n sup x D , s R + T i n ( s + t n ) x T i n ( t n ) x = 0 .
     
  3. (3)

    There exists a compact subset G of E such that t 0 T l ( t ) ( C ) G for some l { 1 , 2 , , N } .

     

Then the sequence { x n } converges strongly to some element in F.

Proof Taking ϕ ( λ ) = λ 2 , ξ n = 0 , μ n = k n 1 in Theorem 2.1, since all conditions in Theorem 2.1 are satisfied. It follows from Theorem 2.1 that the sequence x n q F : = t 0 F ( T ( t ) ) as n .

This completes the proof of Theorem 2.2. □

Remark 2.3 Theorems 2.1 and 2.2 extend and improve the corresponding results of Chang et al. [11], Shioji and Takahashi [1], Suzuki [2], Xu [3], Aleyner and Reich [4] and others.

Open problem It may be interesting to post the following open problem: Can Theorem 2.1 be generalized to a finite family of semigroups of mappings S which are representation, so commutative or left reversible (see Liu and Zhang [13] and related references there), which is not necessarily the positive real number with addition?

Declarations

Acknowledgements

The authors would like to express their thanks to the editor and the referees for their helpful comments and suggestions. This work was supported by the Natural Science Foundation of Sichuan Province (No. 08ZA008).

Authors’ Affiliations

(1)
School of Science, South West University of Science and Technology, Mianyang, Sichuan, China
(2)
College of Statistics and Mathematics, Yunnan University of Finance and Economics, Kunming, Yunnan, China

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© Yang et al.; licensee Springer 2013

This article is published under license to BioMed Central Ltd. 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.

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