Hindawi Publishing Corporation
Fixed Point Theory and Applications
Volume 2010, Article ID 596952, 12 pages
doi:10.1155/2010/596952
Research Article
On Some Geometric Constants and the Fixed Point
Property for Multivalued Nonexpansive Mappings
Jingxin Zhang
1
and Yunan Cui
2
1
Department of Mathematics, Harbin Institute of Technology, Harbin 150001, China
2
Department of Mathematics, Harbin University of Science and Technology, Harbin 150080, China
Correspondence should be addressed to Jingxin Zhang, zhjx
Received 30 July 2010; Accepted 5 October 2010
Academic Editor: L. G
´
orniewicz
Copyright q 2010 J. Zhang and Y. Cui. This is an open access article distributed under the Creative
Commons Attribution License, which permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
We show some geometric conditions on a Banach space X concerning the Jordan-von Neumann
constant, Zb
˘
aganu constant, characteristic of separation noncompact convexity, and the
coefficient R1, X, the weakly convergent sequence coefficient, which imply the existence of fixed
points for multivalued nonexpansive mappings.
1. Introduction
< 1
WCS
X
2
4
1.1
implies property D.
3 Satit Saejung 5 proved that the condition ε
0
X < WCSX implies property D.
4 Gavira 6 showed that the condition
J
X
< 1
1
R
1,X
1.2
implies DL condition.
In 2007, Dom
´
ınguez Benavides and Gavira 7 have established FFP for multivalued
nonexpansive mappings in terms of the modulus of squareness, universal infinite-dimension-
al modulus, and Opia modulus. Attapol Kaewkhao 8 has established FFP for multivalued
A, B
: max
sup
x∈A
inf
y∈B
x − y
, sup
y∈B
inf
x∈A
x − y
,A,B∈ CB
X
.
2.2
Fixed Point Theory and Applications 3
,
A
C,
{
x
n
}
x ∈ C : lim sup
n
x
n
− x
r
C,
{
x
n
}
.
2.3
It is known that AC, {x
}. In Banach spaces,
we have the following results:
1Goebel 10 and Lim 2 there always exists a subsequence of {x
n
} which is
regular relative to C;
2Kirk 11 if C is separable, then {x
n
} contains a subsequence which is asymptoti-
cally uniform relative to C.
If D is a bounded subset of X, the Chebyshev radius of D relative to C is defined by
r
C
D
inf
x∈C
sup
y∈D
x − y
.
2.4
In 2006, Dhompongsa et al. 3 introduced the Domnguez-Lorenzo condition DL
condition, in short in the following way.
Definition 2.1 see 3. We say that a Banach space X satisfies the DL condition if there
Theorem 2.2 see 3. Let C be a weakly compact convex subset of Banach space X;ifC satisfies
(DL) condition, then multivalued nonexpansive mapping T : C → KCC has a fixed point.
4 Fixed Point Theory and Applications
Definition 2.3 see 4. A Banach space X is said to have property D if there exists λ ∈ 0, 1
such that for every weakly compact convex subset C of X and for every sequence {x
n
}⊂C
and for every {y
n
}⊂AC, {x
n
} which are regular asymptotically uniform relative to C,
r
C,
y
n
≤ λr
C,
{
x
n
}
. 2.6
It was observed that property D is weaker than the DL condition and stronger
than weak normal structure, and Dhompongsa et al. 4 proved that property D implies the
x
n
: lim sup
n →∞
lim sup
m →∞
x
n
− x
m
≤ 1.
2.8
Obviously, 1 ≤ R1,X ≤ 2.
The weakly convergent sequence coefficient WCSX is equivalently defined by see
13
WCS
X
inf
lim
n
/
m
n
in X convergers to x with respect to F, denoted by
lim
F
x
n
x, if for each neighborhood U of x, {i ∈ I : x
i
∈ U}∈F.AfilterU on N is called an
ultrafilter if it is maximal with respect to the set inclusion. An ultrafilter is called trivial if it is
of the form {A ⊂ N,i
0
∈ A} for some fixed i
0
∈ N; otherwise, it is called nontrivial. Let l
∞
X
denote the subspace of the product space Π
i∈N
X
i
equipped with the norm
x
n
: sup
n∈N
The ultrapower of X, denoted by
X, is t he quotient space l
∞
X/N
U
equipped with
the quotient norm. Write x
n
U
to denote the elements of ultrapower. It follows from the
definition of the quotient norm that
x
n
U
lim
U
x
n
.
2.12
Note that if U is nontrivial, then X can be embedded into
x
2
y
2
: x, y ∈ X,
x
y
/
0
⎫
⎬
⎭
.
3.1
1
r
C,
{
x
n
}
.
3.2
Proof. Denote r rC, {x
n
} and A AC, {x
n
}. We can assume that r>0. Since {x
n
}⊂C is
bounded and C is a weakly compact set, by passing through a subsequence if necessary, we
can also assume that x
n
converges weakly to some element in x ∈ C and d lim
n
/
m
x
n
− x
m
n
/
m
x
n
− x
m
d.
3.3
Let η>0; taking a subsequence if necessary, we can assume that x
n
− x < d η for all n.
Let z ∈ A. Then we have lim sup
n
x
n
−z r and x−z≤lim inf
n
x
n
−z≤r. Denote
R R1,X; by definition, we have
R ≥ lim inf
n
6 Fixed Point Theory and Applications
On the other hand, observe that the convexity of C implies R −1/R 1x 2/R
1z ∈ C; since the norm is weak lower semicontinuity, we have
lim inf
n
1
r
x
n
− z
1
R
x
n
− x
d η
−
x − z
r
Rr
x −
1
r
−
1
Rr
z
≥
1
r
−
1
Rr
x
2
R 1
z − z
≥
1
r
1
Rr
r
C
A
,
lim inf
n
1
r
x
1
R
d η
x
n
− x
−
1
r
1
Rr
z − x
≥
1
x
n
− z
}
U
∈ S
X
, v
1
R
x
n
− x
d η
−
x − z
r
U
∈ B
X
.
3.6
Using the above estimates, we obtain
u v
1
r
1
Rr
r
C
A
,
u − v
lim
U
1
r
x
n
− z
−
Therefore, we have
C
NJ
X
≥
u v
2
u − v
2
2
u
2
v
2
≥
2
.
3.8
Fixed Point Theory and Applications 7
Since Jordan-von Neumann constant C
NJ
X of
X equals to C
NJ
X of X,weobtain
C
NJ
X
≥
1
2
1
r
1
Rr
2
6, Theorem 3 and Corollary 3.2 includes 6, Corollary 2.
To characterize Hilbert space, Zb˘aganu defined the following Zb˘aganu constant: see
16
C
Z
X
sup
x y
x − y
x
2
y
Z
X ≤ C
Z
X, suppose x, y ∈
X are not all zero.
Without loss of generality, we assume x a>0.
Let us choose η ∈ 0,a. Since x lim
U
x
n
a and
c :
x y x y
x
2
y
2
lim
U
x
n
y
n
x
n
− y
n
x y
x
2
y
2
<
x
n
y
n
x
n
− y
n
8 Fixed Point Theory and Applications
Theorem 3.5. Let X be a Banach space and C a weakly compact convex subset of X. Assume that
{x
n
} is a bounded sequence in C which is regulary relative to C.Then
r
C
A
C,
{
x
n
}
≤
R
1,X
2C
Z
X
R
1,X
u − v
≥
1
r
1
Rr
r
C
A
. 3.14
Therefore, by the definition of Zb˘aganu constant, we have
C
Z
X
≥
u tv
u − tv
X of
X equals to C
Z
X of X,weobtain
C
Z
X
≥
1
2
1
r
1
Rr
2
r
C
A
2
.
−→ y
0
∈ C and d lim
n
/
m
y
n
−y
m
exists.
Let r rC, {x
n
}. Again passing to a subsequence of {x
n
}, still denoted by {x
n
},we
assume in addition that
lim
n →∞
x
n
− y
2n
lim
r. 3.17
Fixed Point Theory and Applications 9
Let us consider an ultrapower
X of X.Put
u
1
r
x
n
− y
2n
U
, v
1
r
x
n
− y
2n1
r
2, 3.19
u − v
lim
U
x
n
− y
2n
r
−
x
n
− y
2n1
r
u v
u − v
u
2
v
2
≥
d
r
.
3.21
Since the Zb˘aganu constants of X and of
X are the same, we obtain C
Z
X ≥ d/r.Now
we estimate d as follows:
d lim
n
/
m
lim sup
n
y
n
− y
0
≥ WCS
X
r
C,
y
n
.
3.22
Hence rC, {y
n
} ≤ C
Z
X/WCSXrC, {x
n
for any bounded subset B of a Banach space X, where
sep
{
x
n
}
inf
{
x
n
− x
m
: n
/
m
}
. 3.24
The modulus of noncompact convexity associated to β is defined in the following way:
Δ
X,β
ε
inf
1 − d
When X is a reflexive Banach space, we have the following alternative expression for
the modulus of noncompact convexity associated with β,
ε
β
X
inf
1 −
x
:
{
x
n
}
⊂ B
X
,x w − lim
n
x
n
, sep
{
x
n
}
/
l
y
k
− y
l
exists.
Let r rC, {x
n
}.
Since {y
0
,y
j
}⊂AC, {x
n
}, we have
lim sup
n
x
n
− y
0
r, lim sup
n
/r η}⊂B
X
;noticethat
β
x
N
− y
j
r η
≥
d − η
r η
,
x
N
− y
j
r η
w
−→
x
N
− y
0
r η
.
3.29
By the definition of Δ
≤ 1 −
r − η
r η
.
3.30
Since the last inequality is true for any η>0, we obtain Δ
X,β
d/r0; thus ε
β
X ≥ d/r.
Now we estimate d as follows:
d lim
k
/
l
y
k
− y
l
lim
k
/
l
X
r
C,
y
n
.
3.31
Fixed Point Theory and Applications 11
Hence,
r
C,
y
n
≤
ε
β
X
WCS
X
β
X < WCSX and let T : C → KCC be a nonexpansive mapping. Then T has a fixed
point.
Noticing WCSX ≥ 1, obviously, Corollary 3.11 extends the following well-known
result.
Theorem 3.12 see 18, Theorem 3.5. Let C be a nonempty bounded closed convex subset of a
reflexive Banach space X such that ε
β
X < 1 and let T : C → KCC be a nonexpansive mapping.
Then T has a fixed point.
Acknowledgments
The authors would like to thank the anonymous referee for providing some suggestions to
improve the manuscript. This work was supported by China Natural Science Fund under
grant 10571037.
References
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488, 1969.
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´
ınguez-Lorenzo condition and
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5, pp. 958–970, 2006.
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˘
aganu, “An inequality of M. R
˘
adulescu and S. R
˘
adulescu which characterizes the inner product
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ematiques Pures et Appliqu
´
ees, vol. 47, no. 2, pp. 253–257, 2002.
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opez Acedo, Measures of Noncompactness
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auser, Basel,
Switzerland, 1997.
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´
ınguez Benavides and P. Lorenzo Ram
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ırez, “Asymptotic centers and fixed points for
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