Hindawi Publishing Corporation
Journal of Applied Mathematics
Volume 2012, Article ID 327878, 9 pages
doi:10.1155/2012/327878
Research Article
Necessary and Sufficient Condition for
Mann Iteration Converges to a Fixed Point of
Lipschitzian Mappings
Chang-He Xiang,1 Jiang-Hua Zhang,2 and Zhe Chen1
1
2
College of Mathematics, Chongqing Normal University, Chongqing 400047, China
School of Management, Shandong University, Shandong Jinan 250100, China
Correspondence should be addressed to Jiang-Hua Zhang, zhangjianghua28@163.com
Received 18 July 2012; Revised 2 September 2012; Accepted 2 September 2012
Academic Editor: Jian-Wen Peng
Copyright q 2012 Chang-He Xiang et al. 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.
Suppose that E is a real normed linear space, C is a nonempty convex subset of E, T : C → C is a
Lipschitzian mapping, and x∗ ∈ C is a fixed point of T . For given x0 ∈ C, suppose that the sequence
sequence
defined by xn1 1 − αn xn αn T xn , n ≥ 0, where {αn } is a
{xn } ⊂ C is the Mann
iterative
∞
2
sequence in 0, 1, ∞
n0 αn < ∞,
n0 αn ∞. We prove that the sequence {xn } strongly converges
to x∗ if and only if there exists a strictly increasing function Φ : 0, ∞ → 0, ∞ with Φ0 0
such that lim supn → ∞ infjxn −x∗ ∈Jxn −x∗ {T xn − x∗ , jxn − x∗ − xn − x∗ 2 Φ
xn − x∗ } ≤ 0.
1. Introduction
Let E be an arbitrary real normed linear space with dual space E∗ , and let C be a nonempty
∗
subset of E. We denote by J the normalized duality mapping from E to 2E defined by
2 Jx f ∈ E∗ : x, f x
2 f ,
∀ x ∈ E,
1.1
where ·, · denotes the generalized duality pairing.
A mapping T : C → E is called strongly pseudocontractive if there exists a constant
k ∈ 0, 1 such that, for all x, y ∈ C, there exists jx − y ∈ Jx − y satisfying
2
T x − T y, j x − y ≤ 1 − kx − y .
1.2
2
Journal of Applied Mathematics
T is called φ-strongly pseudocontractive if there exists a strictly increasing function φ : 0, ∞ →
0, ∞ with φ0 0 such that, for all x, y ∈ C, there exists jx − y ∈ Jx − y satisfying
2
T x − T y, j x − y ≤ x − y − φ x − y x − y.
1.3
T is called generalized Φ-pseudocontractive see, e.g., 1 if there exists a strictly increasing
function Φ : 0, ∞ → 0, ∞ with Φ0 0 such that
2
T x − T y, j x − y ≤ x − y − Φ x − y
1.4
holds for all x, y ∈ C and for some jx − y ∈ Jx − y.
Let FT {x ∈ C : T x x} denote the fixed point set of T . If FT /
∅, and 1.3
and 1.4 hold for all x ∈ C and y ∈ FT , then the corresponding mapping T is called
φ-hemicontractive and generalized Φ-hemicontractive, respectively. It is well known that these
kinds of mappings play important roles in nonlinear analysis.
φ-hemicontractive resp., generalized Φ-hemicontractive mapping is also called uniformly pseudocontractive resp., uniformly hemicontractive mapping in 2, 3. It is easy
to see that if T is generalized Φ-hemicontractive mapping, then FT is singleton.
It is known see, e.g., 4, 5 that the class of strongly pseudocontractive mappings is a
proper subset of the class of φ-strongly pseudocontractive mappings. By taking Φs sφs,
where φ : 0, ∞ → 0, ∞ is a strictly increasing function with φ0 0, we know that
the class of φ-strongly pseudocontractive mappings is a subset of the class of generalized Φpseudocontractive mappings. Similarly, the class of φ-hemicontractive mappings is a subset
of the class of generalized Φ-hemicontractive mappings. The example in 6 demonstrates
that the class of Lipschitzian φ-hemicontractive mappings is a proper subset of the class of
Lipschitzian generalized Φ-hemicontractive mappings.
It is well known see, e.g., 7 that if C is a nonempty closed convex subset of a real
Banach space E and T : C → C is a continuous strongly pseudocontractive mapping, then
T has a unique fixed point p ∈ C. In 2009, it has been proved in 8 that if C is a nonempty
closed convex subset of a real Banach space E and T : C → C is a continuous generalized
Φ-pseudocontractive mappings, then T has a unique fixed point p ∈ C.
Many results have been proved on convergence or stability of Ishikawa iterative
sequences with errors or Mann iterative sequences with errors for Lipschitzian φhemicontractive mappings or Lipschitzian generalized Φ-hemicontractive mapping see, e.g.,
4–6, 9–12 and the references therein. In 2010, Xiang et al. 6 proved the following result.
Theorem XCZ see 6, Theorem 3.2. Let E be a real normed linear space, let C be a nonempty
convex subset of E, and let T : C → C be a Lipschitzian generalized Φ-hemicontractive mapping. For
given x0 ∈ C, suppose that the sequence {xn } ⊂ C is the Mann iterative sequence defined by
xn1 1 − βn xn βn T xn ,
n ≥ 0,
1.5
Journal of Applied Mathematics
3
where {βn } is a sequence in 0, 1 satisfying the following conditions:
1 ∞
n0 βn ∞,
∞ 2
2 n0 βn < ∞.
Then {xn } converges strongly to the unique fixed point of T in C.
The main purpose of this paper is to give necessary and sufficient condition for the
Mann iterative sequence which converges to a fixed point of general Lipschitzian mappings
in an arbitrary real normed linear space. As an immediate consequence, we will obtain
necessary and sufficient condition for the Mann iterative sequence which converges to a
solution of a general Lipschitzian operator equation T x f.
2. Preliminaries
The following lemmas will be used in the proof of our main results.
Lemma 2.1 see, e.g., 12. Let E be a real normed linear space. Then for all x, y ∈ E, we have
x y2 ≤ x
2 2 y, j x y ,
∀j x y ∈ J x y .
2.1
Lemma 2.2 see, e.g., 13. Let {an }, {bn }, {cn } be three nonnegative sequences satisfying the
following condition:
an1 ≤ 1 bn an cn ,
where n0 is some nonnegative integer,
exists.
∞
nn0
bn < ∞, and
∀ n ≥ n0 ,
∞
nn0
2.2
cn < ∞. Then the limit limn → ∞ an
Lemma 2.3. Suppose that ϕ : 0, ∞ → 0, ∞ is a strictly increasing function with ϕ0 0 and
there exists a natural number n0 such that an , bn , εn , and αn are nonnegative real numbers for all
n ≥ n0 satisfying the following conditions:
i an1 ≤ 1 bn an − αn ϕan1 αn εn , for all n ≥ n0 ,
ii ∞
nn bn < ∞, limn → ∞ εn 0,
∞ 0
iii nn0 αn ∞.
Then limn → ∞ an 0.
Proof. Without loss of generality, let limn → ∞ inf an a. Now, we will show that a 0.
Consider its contrary: a > 0 or a ∞. For any given r ∈ 0, a, there exists a nonnegative
integer n1 ≥ n0 such that an ≥ r > 0 and εn < 1/2ϕr ≤ 1/2ϕan1 for all n ≥ n1 . By
condition i, we have
1
an1 ≤ 1 bn an − αn ϕan1 αn · ϕan1 2
1
1 bn an − αn ϕan1 2
≤ 1 bn an ,
∀n ≥ n1 .
2.3
4
Journal of Applied Mathematics
Using Lemma 2.2 and condition ii, we obtain that limn → ∞ an exists and {an } is bounded.
Suppose that an ≤ M for all n ≥ n1 , where M is a nonnegative constant. It follows that
1
1
an1 ≤ 1 bn an − αn ϕan1 ≤ an − αn ϕr Mbn ∀n ≥ n1 .
2
2
2.4
Thus,
∞
∞
∞
1
ϕr
αn ≤ an1 M
bn < ∞,
2
nn1
nn1
2.5
which is a contradiction. Therefore,
lim inf an 0.
2.6
n→∞
By condition ii, for all ε > 0, there exists a nonnegative integer n2 ≥ n0 such that
εn < ϕε
∞
∀n ≥ n2 ,
nn2
bn < ln 2.
2.7
By 2.6, there exists a natural number N ≥ n2 such that aN < ε. Now, we prove the following
inequality 2.8 holds for all k ≥ N:
ak ≤ ε · exp
k−1
bn .
2.8
nN
It is obvious that 2.8 holds for k N. Assuming 2.8 holds for some k ≥ N, we prove that
2.8 holds for k 1. Suppose this is not true, that is, ak1 > ε · exp knN bn . Then ak1 ≥ ε
and so ϕak1 ≥ ϕε. Noting that 1 bk ≤ expbk , it follows from condition i, 2.7, and
2.8 that
ak1 ≤ 1 bk ak − αk ϕak1 αk εk
≤ 1 bk ak − αk ϕε αk ϕε
k−1
≤ ε · 1 bk exp
bn
≤ ε · exp
nN
k
nN
bn ,
2.9
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5
which is a contradiction. This implies that 2.8 holds for k 1. By induction, 2.8 holds for
all k ≥ N. From 2.7, and 2.8, we have
lim sup ak ≤ ε · exp
k→∞
∞
bn
< 2ε.
2.10
nN
Taking ε → 0, we obtain limn → ∞ sup ak 0. By 2.6, we have limn → ∞ an 0. This completes
the proof.
Remark 2.4. Lemma 2.3 is different from Lemma 3 in 14, which requires that bn 0 for all
n ≥ 0. It is also different from Lemma 2.3 in 6, which requires that ∞
nn0 αn εn < ∞.
3. Main Results
Theorem 3.1. Let E be a real normed linear space, C be a nonempty convex subset of E, let T : C → C
be a Lipschitzian mapping, and let x∗ ∈ C be a fixed point of T . For given x0 ∈ C, suppose that the
sequence {xn } ⊂ C is the Mann iterative sequence defined by
xn1 1 − αn xn αn T xn ,
n ≥ 0,
3.1
where {αn } is a sequence in 0, 1 satisfying the following conditions:
2
i ∞
n0 αn < ∞,
∞
ii n0 αn ∞.
Then {xn } converges strongly to x∗ if and only if there exists a strictly increasing function Φ :
0, ∞ → 0, ∞ with Φ0 0 such that
lim sup
n → ∞ jxn
inf
−x∗ ∈Jx
n
−x∗ T xn − x∗ , jxn − x∗ − xn − x∗ 2 Φ
xn − x∗ ≤ 0.
3.2
Proof. First, we prove the sufficiency of Theorem 3.1.
Suppose there exists a strictly increasing function Φ : 0, ∞ → 0, ∞ with Φ0 0
such that 3.2 holds. Let
γn jxn
inf
−x∗ ∈Jx
n
−x∗ T xn − x∗ , jxn − x∗ − xn − x∗ 2 Φ
xn − x∗ .
3.3
Then there exists jxn − x∗ ∈ Jxn − x∗ such that
1
T xn − x∗ , jxn − x∗ − xn − x∗ 2 Φ
xn − x∗ < γn ,
n
∀n ≥ 1.
3.4
By 3.2, we obtain limn → ∞ sup γn ≤ 0. Taking εn 1/n 1 max{γn1 , 0} for all n ≥ 0,
then
lim εn 0.
n→∞
3.5
6
Journal of Applied Mathematics
From 3.1 and 3.4, by using Lemma 2.1, we obtain
xn1 − x∗ 2
1 − αn xn − x∗ αn T xn − x∗ 2
≤ 1 − αn 2 xn − x∗ 2 2αn T xn − x∗ , jxn1 − x∗ ≤ 1 − αn 2 xn − x∗ 2 2αn T xn1 − x∗ , jxn1 − x∗ 2αn T xn − T xn1 , jxn1 − x∗ ≤ 1 − αn 2 xn − x∗ 2 2αn xn1 − x∗ 2 − Φ
xn1 − x∗ γn1 1
n1
3.6
2Lαn xn − xn1 · xn1 − x∗ ≤ 1 − αn 2 xn − x∗ 2 2αn xn1 − x∗ 2 − Φ
xn1 − x∗ εn
2Lαn xn − xn1 · xn1 − x∗ ,
where L is the Lipschitzian constant of T . It follows from 3.1 that
xn − xn1 αn xn − T xn ≤ αn xn − x∗ T x∗ − T xn 3.7
≤ αn 1 L
xn − x∗ .
Substituting 3.7 into 3.6, we have
xn1 − x∗ 2 ≤ 1 − αn 2 xn − x∗ 2 2αn xn1 − x∗ 2 − 2αn Φ
xn1 − x∗ 2αn εn 2L1 Lα2n xn − x∗ · xn1 − x∗ ≤ 1 − αn 2 xn − x∗ 2 2αn xn1 − x∗ 2 − 2αn Φ
xn1 − x∗ 2αn εn L1 Lα2n xn − x∗ 2 xn1 − x∗ 2 .
3.8
√
Setting an xn − x∗ 2 for all n ≥ 0, ϕs 2Φ s, it follows from 3.8 that
an1 ≤ 1 − αn 2 an 2αn an1 − αn ϕan1 2αn εn
L1 Lα2n an an1 1 − 2αn α2n L1 Lα2n an 2αn L1 Lα2n an1
− αn ϕan1 2αn εn .
3.9
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7
It follows from condition i that limn → ∞ 2αn L1 Lα2n 0. Thus, there exists a natural
number n0 such that 2αn L1 Lα2n ≤ 1/2 for all n ≥ n0 . Let
bn 1 − 2αn α2n L1 Lα2n
1 − 2αn − L1 Lα2n
−1
α2n 2L1 Lα2n
1 − 2αn − L1 Lα2n
,
∀n ≥ n0 .
3.10
Since 1/2 ≤ 1 − 2αn − L1 Lα2n ≤ 1 for all n ≥ n0 , by 3.9 and 3.10, we have
an1 ≤ 1 bn an − αn ϕan1 4αn εn ,
0 ≤ bn ≤ 21 2L1 Lα2n ,
∀n ≥ n0 ,
∀n ≥ n0 .
3.11
It follows from condition i that ∞
nn0 bn < ∞. Therefore, by 3.5, condition ii, and
Lemma 2.3, we obtain that limn → ∞ an limn → ∞ xn − x∗ 2 0. That is, {xn } converges
strongly to x∗ .
Finally, we prove the necessity of Theorem 3.1.
Assume that {xn } converges strongly to x∗ . Let L be the Lipschitzian constant of T . For
all jxn − x∗ ∈ Jxn − x∗ , we have
T xn − x∗ , jxn − x∗ ≤ L
xn − x∗ 2 .
3.12
√
Taking Φs s, then Φ : 0, ∞ → 0, ∞ is a strictly increasing function with Φ0 0,
and limn → ∞ Φ
xn − x∗ 0. From 3.12, we obtain
lim
inf
∗
n → ∞jxn −x ∈Jxn −x∗ T xn − x∗ , jxn − x∗ − xn − x∗ 2 Φ
xn − x∗ 0,
3.13
which implies 3.2 holds. This completes the proof of Theorem 3.1.
Remark 3.2. If T : C → C is a generalized Φ-hemicontractive mapping, then 3.2 holds. By
Theorem 3.1, we obtain Theorem XCZ.
Theorem 3.3. Let E be a real Banach space, let C be a nonempty closed convex subset of E, and let
T : C → C be a Lipschitzian generalized Φ-pseudocontractive mapping. For given x0 ∈ C, suppose
that the sequence {xn } ⊂ C is the Mann iterative sequence defined by
xn1 1 − αn xn αn T xn ,
n ≥ 0,
3.14
where {αn } is a sequence in 0, 1 satisfying the following conditions:
1 ∞
n0 αn ∞,
∞ 2
2 n0 αn < ∞.
Then {xn } converges strongly to the unique fixed point of T in C.
Proof. By Theorem 2.1 in 8, T has a unique fixed point x∗ in C. By Theorem 3.1, {xn }
converges strongly to x∗ . This completes the proof of Theorem 3.3.
8
Journal of Applied Mathematics
Theorem 3.4. Let E be a real normed linear space, let S : E → E be a Lipschitzian operator, and let
f ∈ E and x∗ be a solution of the equation Sx f. For given x0 ∈ E, suppose that the sequence {xn }
is the Mann iterative sequence defined by
xn1 1 − αn xn αn f xn − Sxn ,
n ≥ 0,
3.15
where {αn } is a sequences in 0, 1 satisfying the following conditions:
i
ii
∞
n0
α2n < ∞,
n0
αn ∞.
∞
Then {xn } converges strongly to x∗ if and only if there exists a strictly increasing function Φ :
0, ∞ → 0, ∞ with Φ0 0 such that
lim inf
n→∞
sup
jxn −x∗ ∈Jxn −x∗ Sxn − Sx∗ , jxn − x∗ − Φ
xn − x∗ ≥ 0.
3.16
Proof. Define T : E → E by T x f x − Sx. Since Sx∗ f, we have T x∗ x∗ . From 3.15,
we obtain xn1 1 − αn xn αn T xn , n ≥ 0. Since
Sxn − Sx∗ , jxn − x∗ − Φ
xn − x∗ − T xn − x∗ , jxn − x∗ − xn − x∗ 2 Φ
xn − x∗ .
3.17
Therefore,
lim inf
n→∞
sup
jxn −x∗ ∈Jxn −x∗ −lim sup
inf
∗
Sxn − Sx∗ , jxn − x∗ − Φ
xn − x∗ ∗
n → ∞ jxn −x ∈Jxn −x T xn − x∗ , jxn − x∗ − xn − x∗ Φ
xn − x∗ .
3.18
The condition 3.16 is equivalent to condition 3.2. Since S is a Lipschitzian operator, T
is a Lipschitzian mapping. By Theorem 3.1, Theorem 3.4 holds. This completes the proof of
Theorem 3.4.
Acknowledgments
This work was partially supported by the National Natural Science Foundation of China
NSFC Grant no. 71201093, no. 11001289, and no. 11171363, Humanities and Social
Sciences Foundation of Ministry of Education of China Grant no. 10YJCZH217, Promotive
Research Fund for Excellent Young and Middle-Aged Scientists of Shandong Province Grant
no. BS2012SF012, and Independent Innovation Foundation of Shandong University, IIFSDU
Grant no. 2012TS194.
Journal of Applied Mathematics
9
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