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On the Ishikawa iteration processes for multivalued mappings in some CAT(κ) spaces

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Abstract

The purpose of this paper is to prove the strong convergence of the Ishikawa iteration processes for some generalized multivalued nonexpansive mappings in the framework of CAT(1) spaces. Our results extend the corresponding results given by Shahzad and Zegeye (Nonlinear Anal. 71:838-844, 2009), Puttasontiphot (Appl. Math. Sci. 4:3005-3018, 2010), Song and Cho (Bull. Korean Math. Soc. 48:575-584, 2011) and many others.

1 Introduction

Roughly speaking, a CAT(κ) space is a geodesic space of bounded curvature. The precise definition is given below. Here CAT means the initials of three mathematician’s names (E Cartan, AD Alexandrov and A Toponogov) who have made important contributions to the understanding of curvature via inequalities for the distance function, and κ is a real number that we impose as the curvature bound of the space.

Fixed point theory in CAT(κ) spaces was first studied by Kirk [1, 2]. His works were followed by a series of new works by many authors (see, e.g., [312]) mainly focusing on CAT(0) spaces. Since any CAT(κ) space is a CAT( κ ) space for κ κ (see [[13], p.165]), all results for CAT(0) spaces immediately apply to any CAT(κ) space with κ0. Notice also that all CAT(κ) spaces (with appropriate sizes) are uniformly convex metric spaces in the sense of [14]. Thus, the results in [14] concerning uniformly convex metric spaces also hold in CAT(κ) spaces as well.

In 1974, Ishikawa [15] introduced an iteration process for approximating fixed points of a single-valued mapping t on a Hilbert space H by

x n + 1 =(1 α n ) x n + α n t ( ( 1 β n ) x n + β n t ( x n ) ) ,n1,

where { α n } and { β n } are sequences in [0,1] satisfying some certain restrictions. For more details and literature on the convergence of the Ishikawa iteration process for single-valued mappings, see, e.g., [1628].

The first result concerning the convergence of an Ishikawa iteration process for multivalued mappings was proved by Sastry and Babu [29] in a Hilbert space. Panyanak [30] extended the result of Sastry and Babu to a uniformly convex Banach space. Since then the strong convergence of the Ishikawa iteration processes for multivalued mappings has been rapidly developed and many of papers have appeared (see, e.g., [3136]). Among other things, Shahzad and Zegeye [37] defined two types of Ishikawa iteration processes as follows.

Let E be a nonempty closed convex subset of a uniformly convex Banach space X, { α n },{ β n }[0,1], and T:E 2 E be a multivalued mapping whose values are nonempty proximinal subsets of E. For each xE, let P T :E 2 E be a multivalued mapping defined by

P T (x):= { u T ( x ) : x u = inf y T ( x ) x y } .

(A): The sequence of Ishikawa iterates is defined by x 1 E,

y n = β n z n +(1 β n ) x n ,n1,

where z n T( x n ), and

x n + 1 = α n z n +(1 α n ) x n ,n1,

where z n T( y n ).

(B): The sequence of Ishikawa iterates is defined by x 1 E,

y n = β n z n +(1 β n ) x n ,n1,

where z n P T ( x n ), and

x n + 1 = α n z n +(1 α n ) x n ,n1,

where z n P T ( y n ).

They proved, under some suitable assumptions, that the sequence { x n } defined by (A) and (B) converges strongly to a fixed point of T. In 2010, Puttasontiphot [38] gave analogous results to those of Shahzad and Zegeye in complete CAT(0) spaces.

In this paper, we extend Puttasontiphot’s results to the setting of CAT(κ) spaces with κ0.

2 Preliminaries

Let (X,d) be a metric space, and let xX, EX. The distance from x to E is defined by

dist(x,E)=inf { d ( x , y ) : y E } .

The diameter of E is defined by

diam(E)=sup { d ( u , v ) : u , v E } .

The set E is called proximinal if for each xX, there exists an element yE such that d(x,y)=dist(x,E). We shall denote by 2 E the family of nonempty subsets of E, by P(E) the family of nonempty proximinal subsets of E and by C(E) the family of nonempty closed subsets of E. Let H(,) be the Hausdorff (generalized) distance on 2 E , i.e.,

H(A,B)=max { sup a A dist ( a , B ) , sup b B dist ( b , A ) } ,A,B 2 E .

Definition 2.1 Let E be a nonempty subset of a metric space (X,d) and T:E 2 E . Then T is said to

  1. (i)

    be nonexpansive if H(T(x),T(y))d(x,y) for all x,yE;

  2. (ii)

    be quasi-nonexpansive if Fix(T) and

    H ( T ( x ) , T ( p ) ) d(x,p)for all xE and pFix(T);
  3. (iii)

    satisfy condition (I) if there is a nondecreasing function f:[0,)[0,) with f(0)=0, f(r)>0 for r(0,) such that

    dist ( x , T ( x ) ) f ( dist ( x , Fix ( T ) ) ) for all xE;
  4. (iv)

    be hemicompact if for any sequence { x n } in E such that

    lim n dist ( x n , T ( x n ) ) =0,

there exists a subsequence { x n k } of { x n } and qE such that lim k x n k =q.

A point xE is called a fixed point of T if xT(x). We denote by Fix(T) the set of all fixed points of T.

The following lemma can be found in [39]. We observe that the boundedness of the images of T is superfluous.

Lemma 2.2 Let E be a nonempty subset of a metric space (X,d) and T:EP(E) be a multivalued mapping. Then

  1. (i)

    dist(x,T(x))=dist(x, P T (x)) for all xE;

  2. (ii)

    xFix(T)xFix( P T ) P T (x)={x};

  3. (iii)

    Fix(T)=Fix( P T ).

Let (X,d) be a metric space. A geodesic path joining xX to yX (or, more briefly, a geodesic from x to y) is a map c from a closed interval [0,l]R to X such that c(0)=x, c(l)=y, and d(c(t),c( t ))=|t t | for all t, t [0,l]. In particular, c is an isometry and d(x,y)=l. The image c([0,l]) of c is called a geodesic (or metric) segment joining x and y. When it is unique, this geodesic segment is denoted by [x,y]. This means that z[x,y] if and only if there exists α[0,1] such that

d(x,z)=(1α)d(x,y)andd(y,z)=αd(x,y).

In this case, we write z=αx(1α)y. The space (X,d) is said to be a geodesic space if every two points of X are joined by a geodesic, and X is said to be uniquely geodesic if there is exactly one geodesic joining x and y for each x,yX. A subset E of X is said to be convex if E includes every geodesic segment joining any two of its points.

In a geodesic space (X,d), the metric d:X×XR is convex if for any x,y,zX and α[0,1], one has

d ( x , α y ( 1 α ) z ) αd(x,y)+(1α)d(x,z).

Let D(0,], then (X,d) is called a D-geodesic space if any two points of X with their distance smaller than D are joined by a geodesic segment. Notice that (X,d) is a geodesic space if and only if it is a D-geodesic space.

Let nN, we denote by | the Euclidean scalar product in R n , that is,

x|y= x 1 y 1 ++ x n y n ,where x=( x 1 ,, x n ),y=( y 1 ,, y n ).

Let S n denote the n-dimensional sphere defined by

S n = { x = ( x 1 , , x n + 1 ) R n + 1 : x | x = 1 } ,

with metric d(x,y)=arccosx|y, x,y S n (see [[13], Proposition 2.1]).

From now on, we assume that κ0 and define

D κ := π κ if κ>0and D κ :=if κ=0.

We denote by M κ n the following metric spaces:

  1. (i)

    if κ=0 then M 0 n is the Euclidean space R n ;

  2. (ii)

    if κ>0 then M κ n is obtained from S n by multiplying the distance function by the constant 1/ κ .

A geodesic triangle (x,y,z) in the metric space (X,d) consists of three points x, y, z in X (the vertices of ) and three geodesic segments between each pair of vertices (the edges of ). We write p(x,y,z) when p[x,y][y,z][z,x]. For (x,y,z) in a geodesic space X satisfying d(x,y)+d(y,z)+d(z,x)<2 D κ , there exist points x ¯ , y ¯ , z ¯ M κ 2 such that

d(x,y)= d M κ 2 ( x ¯ , y ¯ ),d(y,z)= d M κ 2 ( y ¯ , z ¯ ),andd(z,x)= d M κ 2 ( z ¯ , x ¯ )

(see [[13], Lemma 2.14]). We call the triangle having vertices x ¯ , y ¯ , z ¯ in M κ 2 a comparison triangle of (x,y,z). Notice that it is unique up to an isometry of M κ 2 , and we denote it by ¯ ( x ¯ , y ¯ , z ¯ ). A point p ¯ [ x ¯ , y ¯ ] is called a comparison point for p[x,y] if d(x,p)= d M κ 2 ( x ¯ , p ¯ ).

A geodesic triangle (x,y,z) in X with d(x,y)+d(y,z)+d(z,x)<2 D κ is said to satisfy the CAT(κ) inequality if for any p,q(x,y,z) and for their comparison points p ¯ , q ¯ ¯ ( x ¯ , y ¯ , z ¯ ), one has

d(p,q) d M κ 2 ( p ¯ , q ¯ ).

Definition 2.3 A metric space (X,d) is called a CAT(κ) space if it is D κ -geodesic and any geodesic triangle (x,y,z) in X with d(x,y)+d(y,z)+d(z,x)<2 D κ satisfies the CAT(κ) inequality.

It follows from [[13], Proposition 1.4] that CAT(κ) spaces are uniquely geodesic spaces. In this paper, we consider CAT(κ) spaces with κ0. Since most of the results for such spaces are easily deduced from those for CAT(1) spaces, in what follows, we mainly focus on CAT(1) spaces. The following lemma is a consequence of Proposition 3.1 in [40].

Lemma 2.4 If (X,d) is a CAT(1) space with diam(X)<π/2, then there is a constant K>0 such that

d 2 ( ( 1 α ) x α y , z ) (1α) d 2 (x,z)+α d 2 (y,z) K 2 α(1α) d 2 (x,y)

for any α[0,1] and any points x,y,zX.

The following lemma is also needed.

Lemma 2.5 [30]

Let { α n }, { β n } be two real sequences such that

  1. (i)

    0 α n , β n <1;

  2. (ii)

    β n 0 as n;

  3. (iii)

    α n β n =.

Let { γ n } be a nonnegative real sequence such that n = 1 α n β n (1 β n ) γ n is bounded. Then { γ n } has a subsequence which converges to zero.

3 Main results

We begin this section by proving a crucial lemma.

Lemma 3.1 Let (X,d) be a CAT(1) space with convex metric, E be a nonempty closed convex subset of X, and T:E 2 E be a quasi-nonexpansive mapping with Fix(T) and T(p)={p} for each pFix(T). Let { x n } be the sequence of Ishikawa iterates defined by (A) (replacing + with ). Then lim n d( x n ,p) exists for each pFix(T).

Proof Let pFix(T). For each n1, we have

d ( y n , p ) = d ( β n z n ( 1 β n ) x n , p ) β n d ( z n , p ) + ( 1 β n ) d ( x n , p ) β n H ( T ( x n ) , T ( p ) ) + ( 1 β n ) d ( x n , p ) β n d ( x n , p ) + ( 1 β n ) d ( x n , p ) d ( x n , p )

and

d ( x n + 1 , p ) = d ( α n z n ( 1 α n ) x n , p ) α n d ( z n , p ) + ( 1 α n ) d ( x n , p ) α n H ( T ( y n ) , T ( p ) ) + ( 1 α n ) d ( x n , p ) α n d ( y n , p ) + ( 1 α n ) d ( x n , p ) d ( x n , p ) .

This shows that the sequence {d( x n ,p)} is decreasing and bounded below. Thus lim n d( x n ,p) exists for any pFix(T). □

Now, we prove the strong convergence of the Ishikawa iteration process defined by (A).

Theorem 3.2 Let (X,d) be a complete CAT(1) space with convex metric and diam(X)<π/2, E be a nonempty closed convex subset of X, and T:EC(E) be a quasi-nonexpansive mapping with Fix(T) and T(p)={p} for each pFix(T). Let α n , β n [a,b](0,1) and { x n } be the sequence of Ishikawa iterates defined by (A) (replacing + with ). If T satisfies condition (I), then { x n } converges strongly to a fixed point of T.

Proof Let pFix(T). By using Lemma 2.4, we have

d 2 ( x n + 1 , p ) = d 2 ( α n z n ( 1 α n ) x n , p ) ( 1 α n ) d 2 ( x n , p ) + α n d 2 ( z n , p ) K 2 α n ( 1 α n ) d 2 ( x n , z n ) ( 1 α n ) d 2 ( x n , p ) + α n H 2 ( T ( y n ) , T ( p ) ) K 2 α n ( 1 α n ) d 2 ( x n , z n ) ( 1 α n ) d 2 ( x n , p ) + α n d 2 ( y n , p )

and

d 2 ( y n , p ) = d 2 ( β n z n ( 1 β n ) x n , p ) ( 1 β n ) d 2 ( x n , p ) + β n d 2 ( z n , p ) K 2 β n ( 1 β n ) d 2 ( x n , z n ) ( 1 β n ) d 2 ( x n , p ) + β n H 2 ( T ( x n ) , T ( p ) ) K 2 β n ( 1 β n ) d 2 ( x n , z n ) ( 1 β n ) d 2 ( x n , p ) + β n d 2 ( x n , p ) K 2 β n ( 1 β n ) d 2 ( x n , z n ) d 2 ( x n , p ) K 2 β n ( 1 β n ) d 2 ( x n , z n ) .

So that

d 2 ( x n + 1 ,p)(1 α n ) d 2 ( x n ,p)+ α n d 2 ( x n ,p) K 2 α n β n (1 β n ) d 2 ( x n , z n ).

This implies that

K 2 a 2 (1b) d 2 ( x n , z n ) K 2 α n β n (1 β n ) d 2 ( x n , z n ) d 2 ( x n ,p) d 2 ( x n + 1 ,p)
(1)

and so

n = 1 K 2 a 2 (1b) d 2 ( x n , z n )<.

Thus, lim n d( x n , z n )=0. Also, dist( x n ,T( x n ))d( x n , z n )0 as n. Since T satisfies condition (I), we have lim n dist( x n ,Fix(T))=0. The proof of the remaining part follows the proof of Theorem 3.2 in [38], therefore we omit it. □

Theorem 3.3 Let (X,d) be a complete CAT(1) space with convex metric and diam(X)<π/2, E be a nonempty closed convex subset of X, and T:EC(E) be a quasi-nonexpansive mapping with Fix(T) and T(p)={p} for each pFix(T). Assume that (i) 0 α n , β n <1; (ii) β n 0; (iii) α n β n =, and let { x n } be the sequence of Ishikawa iterates defined by (A) (replacing + with ). If T is hemicompact and continuous, then { x n } converges strongly to a fixed point of T.

Proof Let pFix(T). By (1) we have

K 2 n = 1 α n β n (1 β n ) d 2 ( x n , z n )<.

By Lemma 2.5, there exist subsequences { x n k } and { z n k } of { x n } and { z n } respectively such that lim k d( x n k , z n k )=0. Hence

lim k dist ( x n k , T ( x n k ) ) lim k d( x n k , z n k )=0.

Since T is hemicompact, by passing through a subsequence, we may assume that x n k q for some qE. Since T is continuous,

dist ( q , T ( q ) ) d(q, x n k )+dist ( x n k , T ( x n k ) ) +H ( T ( x n k ) , T ( q ) ) 0as k.

This implies that qFix(T) since T(q) is closed. Thus lim n d( x n ,q) exists by Lemma 3.1 and hence q is the limit of { x n } itself. □

To avoid the restriction of T, that is, T(p)={p} for pFix(T), we use the iteration process defined by (B).

Theorem 3.4 Let (X,d) be a complete CAT(1) space with convex metric and diam(X)<π/2, E be a nonempty closed convex subset of X, and T:EP(E) be a multivalued mapping with Fix(T) and P T is quasi-nonexpansive. Let α n , β n [a,b](0,1) and { x n } be the sequence of Ishikawa iterates defined by (B) (replacing + with ). If T satisfies condition (I), then { x n } converges strongly to a fixed point of T.

Proof It follows from Lemma 2.2 that dist(x, P T (x))=dist(x,T(x)) for all xE,

Fix( P T )=Fix(T)and P T (p)={p}for each pFix( P T ).

Since T satisfies condition (I), for each xE we have

dist ( x , P T ( x ) ) =dist ( x , T ( x ) ) f ( dist ( x , Fix ( T ) ) ) =f ( dist ( x , Fix ( P T ) ) ) .

That is, P T satisfies condition (I). Next, we show that P T (x) is closed for any xE. Let { y n } P T (x) and lim n y n =y for some yE. Then

d(x, y n )=dist ( x , T ( x ) ) and lim n d(x, y n )=d(x,y).

It follows that d(x,y)=dist(x,T(x)) and this implies y P T (x). Applying Theorem 3.2 to the map P T , we can conclude that the sequence { x n } defined by (B) converges to a point zFix( P T )=Fix(T). This completes the proof. □

The following theorem is an analogue of Theorem 3 in [39].

Theorem 3.5 Let (X,d) be a complete CAT(1) space with convex metric and diam(X)<π/2, E be a nonempty closed convex subset of X, and T:EP(E) be a hemicompact mapping with Fix(T) and P T is quasi-nonexpansive and continuous. Assume that (i) 0 α n , β n <1; (ii) β n 0; (iii) α n β n =, and let { x n } be the sequence of Ishikawa iterates defined by (B) (replacing + with ). Then { x n } converges strongly to a fixed point of T.

Proof As in the proof of Theorem 3.4, we have

Fix( P T )=Fix(T)and P T (p)={p}for each pFix( P T ).

The hemicompactness of P T follows from that of T. The conclusion follows from Theorem 3.3. □

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Acknowledgements

The author thanks Chiang Mai University for financial support.

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Correspondence to Bancha Panyanak.

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Panyanak, B. On the Ishikawa iteration processes for multivalued mappings in some CAT(κ) spaces. Fixed Point Theory Appl 2014, 1 (2014) doi:10.1186/1687-1812-2014-1

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Keywords

  • fixed point
  • multivalued mapping
  • strong convergence
  • Ishikawa iteration
  • CAT(κ) space