Hindawi Publishing Corporation
Fixed Point Theory and Applications
Volume 2009, Article ID 892691, 14 pages
doi:10.1155/2009/892691
Research Article
On Series-Like Iterative Equation with
a General Boundary Restriction
Wei Song,1 Guo-qiu Yang,1 and Feng-chun Lei2
1
2
Department of Mathematics, Harbin Institute of Technology, Harbin 150001, China
Department of Applied Mathematics, Dalian University of Technology, Dalian 116024, China
Correspondence should be addressed to Wei Song, dawenhxi@126.com
Received 26 August 2008; Revised 19 November 2008; Accepted 4 February 2009
Recommended by Tomas Dominguez Benavides
By means of Schauder fixed point theorem and Banach contraction principle, we investigate the
existence and uniqueness of Lipschitz solutions of the equation Pf ◦ f F. Moreover, we get
that the solution f depends continuously on F. As a corollary, we investigate
the existence and
n
uniqueness of Lipschitz solutions of the series-like iterative equation ∞
a
f
x
Fx, x ∈ B
n
n1
with a general boundary restriction, where F : B → A is a given Lipschitz function, and B, A are
compact convex subsets of RN with nonempty interior.
Copyright q 2009 Wei Song 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.
1. Introduction
Let f be a self-mapping on a topological space X. For integer n ≥ 0 define the nth iterate
of f by f n f ◦ f n−1 and f 0 id, where id denotes the identity mapping on X, and ◦
denotes the composition of mappings. Let CX, X be the set of all continuous self-mappings
on X. An equation with iteration as its main operation is simply called an iterative equation.
It is one of the most interesting classes of functional equations 1–4 because it concludes
the problem of iterative roots 2, 5, 6, that is, finding f ∈ CX, X such that f n is identical
to a given F ∈ CX, X and the problem of invariant curves 7. Iteration equations also
appear in the study on transversal homoclinic intersection for diffeomorphisms 8, normal
form of dynamical systems 9, and dynamics of a quadratic mapping 10. The well-known
Feigenbaum equation fx −1/λffλx, arising in the discussion of period-doubling
bifurcations 11, 12, is also an iterative equation.
As a natural generalization of the problem of iterative roots, a class of iterative
equations which is called polynomial-like iterative equation:
λ1 fx λ2 f 2 x · · · λn f n x Fx,
x ∈ I a, b,
1.1
2
Fixed Point Theory and Applications
always fascinates many scholars’ attentions 3, 13. It is more difficult than the analogous
differential equation, where each f j is replaced with the jth derivative f j of f because
differentiation is a linear operator but iteration is not.
In 1986, Zhang 14 constructed an interesting operator called “structural operator” L :
f → Lf for 1.1 and used the fixed point theory in Banach spaces to get the solutions of 1.1.
By means of this method, Zhang and Si made a series of works concerning these qualitative
problems such as 15–19. Recently, Zhang et al. 20, 21 developed this method and made
a series of works on 1.1. Furthermore, they have got the nonmonotonic and decreasing
solutions of 1.1, and the convexity of solutions is also considered.
In 2002, Kulczycki and Tabor 22 investigated iterative functional equations in the
class of Lipschitz functions. In 2004, Tabor and Żołdak 23 studied the iterative equations in
Banach spaces. In the above references, the authors first gave theorems for the existence of
solutions of
Pf ◦ f F.
1.2
By virtue of these theorems, in 22, the authors considered the existence of Lipschitz
solutions of the iterative functional equation:
∞
an f n x Fx,
x ∈ B,
1.3
n1
where B is a compact convex subset of RN with nonempty interior, and F : B → B is a given
Lipschitz function. In 23, the existence of solutions of
Ai f i x Fx,
i
fφi x Fx,
x∈B
1.4
i
is investigated, where B is a nonempty closed subset of a Banach Space X. But they all
considered the case F|∂B id|∂B .
It is easy to see that 1.1 is the special case of 1.3 with ai 0, i n 1, n 2, . . . ,
and B a, b. Since the left-hand side of 1.3 is a functional series, in this paper we call it
series-like iterative equation. In 14–20, the authors considered the solutions of 1.1 with
F : I → I, Fa a, Fb b. In 22, the authors considered the solutions of 1.3 with
F : B → B, F|∂B id∂B . In fact, the above authors had studied the solutions of
∞
an f n x Fx,
x ∈ B,
n1
F : B −→ B,
F|∂B id∂B ,
1.5
Fixed Point Theory and Applications
3
where B ⊂ RN is a convex compact set with nonempty interior. In 21, the authors considered
the solutions of 1.1 with F : I → J, Fa d, Fb c, I a, b, J c, d. Obviously,
the more general case is
∞
an f n x Fx,
x ∈ B,
1.6
n1
F : B −→ A,
F|∂B g,
where B, A are convex compact subsets of RN with nonempty interior, and g : ∂B → ∂A
is a continuous surjective map. Since g could be any map, in this paper we call 1.6 serieslike iterative equation with a general boundary restriction. It is easy to see that 14–22 all
considered one special case of 1.6.
The problem of differentiable solutions of iterative equations has also fascinated many
scholars’ attentions. In Zhang 16 and Si 19, the C1 and C2 solutions of 1.1 are considered.
In Wang and Si 24, the differentiable solutions of
H x, φn1 x, . . . , φni x Fx,
x ∈ I a, b
1.7
are considered. Murugan and Subrahmanyam 25–27 offered theorems on the existence and
uniqueness of differentiable solutions to the iterative equations involving iterated functional
series:
∞
λi Hi f i x Fx,
x ∈ I a, b,
i1
∞
λi Hi x, φ x, . . . , φ
ai1
aini
1.8
x Fx,
x ∈ I a, b.
i1
But the references above only considered the case that Fa a, Fb b.
The problem of differentiable solutions of higher dimensional iterative equations is
also interesting. By constructing a new operator for the structure of 1.3, which simplifies
the procedure of applying fixed point theorems in some sense, Li 28 studies the smoothness
of solutions of 1.3. In 29, C1 solutions of
∞
λn xf n x Fx,
x∈B
1.9
n1
are discussed, where B is a compact convex subset of RN and for any n ≥ 1, λn x : B → R is
continuous. The boundary restrictions are not considered in the two references above because
they only consider the case that FB ⊆ B.
It should be pointed out that Mai and Liu 30 made an important contribution to
Cm solutions of iterative equations. Using the method of approximating fixed points by
small shift of maps, choosing suitable metrics, and finding a relation between uniqueness
4
Fixed Point Theory and Applications
and stability of fixed points of maps of general spaces, Mai and Liu proved the existence,
uniqueness of Cm solutions of a relatively general kind of iterative equations:
G x, fx, . . . , f n x 0,
x ∈ J,
1.10
where J is a connected closed subset of R and G ∈ Cm J n1 , R, n ≥ 2. Here, Cm J n1 , R
denotes the set of all Cm mappings from J n1 to R.
Inspired and motivated by the above work as well as 14–30, we will study 1.6 and
investigate the existence and uniqueness of Lipschitz solution of this equation.
The rest of this paper is organized as follows. In Section 2, we will give some
definitions and lemmas. In Section 3, we will give a main theorem concerning the existence
and uniqueness of solution of
Pf ◦ f F.
1.11
In Sections 4 and 5, we will study some special cases of 1.6 by means of the above main
theorem.
2. Preliminary
Let B be a compact convex subset of RN with nonempty interior. Let CB, RN {f : B →
RN | f is continuous}, N ∈ Z . In CB, RN , we use the supremum norm
fB sup fx,
for f ∈ C B, RN ,
2.1
x∈B
where · denotes the usual metric of RN . Obviously, CB, RN is a complete metric space.
Definition 2.1. Let A, B be two convex compact subsets of RN with nonempty interior. For
m ∈ 0, 1, M ∈ 1, ∞, define
LipB, A, m, M : {f : B −→ A | f is continuous, fB A, ∀x, y ∈ B,
mx − y ≤ fx − fy ≤ Mx − y}.
2.2
Let g : ∂B → ∂A be a continuous surjective map, and let CB, A, m, M, g denote the
subset of LipB, A, m, M whose elements satisfy f∂B ∂A and f|∂B g.
Lemmas 2.2–2.4 can be proved by a corresponding method which is contained in the
proofs of Observation 2.2, Lemma 2.3, and Lemma 2.4 of 22.
Lemma 2.2. Let m ∈ 0, 1, M ∈ 1, ∞, and f ∈ LipB, A, m, M be arbitrary, then f −1 ∈
LipA, B, 1/M, 1/m.
Fixed Point Theory and Applications
5
Lemma 2.3. For every m > 0, the mapping
L : f ∈ LipB, A, m, ∞ −→ f
−1
1
∈ Lip A, B, 0,
m
2.3
is well defined and Lipschitz with constant 1/m.
Lemma 2.4. For 1 ≤ K, M < ∞ and F ∈ LipB, A, 0, K, the mapping
SF : f ∈ LipA, B, 0, M −→ f ◦ F ∈ LipB, B, 0, K, M
2.4
is Lipschitz with constant 1.
Lemmas 2.5 and 2.6 can be proved by a method which is contained in the proof of
Proposition 1 in 23.
Lemma 2.5. If H, G are homeomorphisms from B to A with Lipschitz constant L, then H − GB ≤
LH −1 − G−1 A .
Lemma 2.6. If f, g ∈ LipB, B, m, M, then f k − g k B ≤
k−1
j0
Mj f − gB .
Lemma 2.7. For any M ∈ 1, ∞ and g : ∂B → ∂B, which is a surjective map, CB, B, 0, M, g is
a compact subset of CB, RN .
Proof. It is easy to see that CB, B, 0, M, g is uniformly bounded and equicontinuous. By
Ascoli-Arzelá lemma for any sequence {fn }∞
n1 ⊂ CB, B, 0, M, g, there exists a subsequence
∞
of
{f
}
which
converges
to
a
continuous
map f ∈ CB, RN . Without any loss of
{fnk }∞
n
k1
n1
generality, we suppose limn → ∞ fn f. We can easily get f|∂B g, f∂B ∂B, and fx −
fy ≤ Mx − y, ∀x, y ∈ B. We only need to prove fB B. Since fn B B, so for any
y ∈ B there is a xn ∈ B with fn xn y. By the compactness of B, we suppose limn → ∞ xn x.
Noticing that
y − fx fn xn − fx ≤ fn xn − fn x fn x − fx,
2.5
we can get fx y. Then, CB, A, 0, M, g is compact.
Let DN {x | x ∈ RN , x ≤ 1}. Then, ∂DN SN−1 . Obviously, B is homeomorphic to
D , and ∂B is homeomorphic to SN−1 .
N
Lemma 2.8. If f : DN → DN is continuous and fSN−1 ⊂ SN−1 . Let f0 denote f|SN−1 . If
degf0 / 0, then f is surjective, where degf0 denotes the degree of f0 .
Proof. Suppose that f is not surjective. Let x0 ∈ DN \fDN . If x0 ∈ SN−1 , then f0 is homotopic
∈ SN−1 , then there exists a retraction
to a constant and degf0 0, a contradiction. So x0 /
N
N−1
N
. Thus, r ◦ f : D → SN−1 is a continuous mapping, and
mapping r : D \ {x0 } → S
f0 r ◦ f|SN−1 . This means that f0 ∗N−1 is trivial, then degf0 0. So f is surjective.
6
Fixed Point Theory and Applications
Lemma 2.9. Let M ∈ 1, ∞ and CB, B, 0, M, g be defined as above, where the surjective map g :
∂B → ∂B is the restriction of the elements of CB, B, 0, M, g. If degg /
0, then CB, B, 0, M, g
is a convex subset of CB, RN .
Proof. For ∀t ∈ 0, 1 and ∀f, h ∈ CB, B, 0, M, g, tf 1 − th is continuous and
tf 1 − th|∂B tg 1 − tg g.
2.6
It is easy to see that
tf 1 − thx − tf 1 − thy ≤ Mx − y,
∀x, y ∈ B.
2.7
By lemma 2.8, tf 1 − th is surjective. Thus, tf 1 − th ∈ CB, B, 0, M, g, that is,
CB, B, 0, M, g is convex.
3. Main Result
Theorem 3.1. Give M, K ∈ 1, ∞ and A, B which are compact convex subsets of RN with
nonempty interior. Suppose that both g : ∂B → ∂B and T : ∂B → ∂A are continuous surjective
maps and degg /
0. If there exist a decreasing function α : 1, ∞ → 0, 1 and a continuous map
P defined on CB, B, 0, M, g such that
Pf ∈ CB, A, αM, ∞, T ,
∀f ∈ CB, B, 0, M, g
M · αM ≥ K.
3.1
Then, for any F ∈ CB, A, 0, K, T ◦ g, there exists a f ∈ CB, B, 0, M, g such that
Pf ◦ f F.
3.2
Furthermore, if P is Lipschitz with a Lipschitz constant d which satisfies d/αM < 1, then f is
unique, and f depends continuously on F.
Proof. Firstly, we prove that Pf : B → A is a homeomorphism for all f ∈ CB, B, 0, M, g.
Since the interior of A is nonempty, αM > 0 otherwise we would get that F is a constant,
while we suppose FB A. Then, by Lemma 2.2, Pf is a homeomorphism. We also get
M ≥ K/αM.
By Lemmas 2.3 and 2.4, the mapping
1
,
L : f ∈ LipB, A, αM, ∞ −→ f −1 ∈ Lip A, B, 0,
αM
K
1
SF : f ∈ Lip A, B, 0,
−→ f ◦ F ∈ Lip B, B, 0,
αM
αM
are both well defined and continuous.
3.3
Fixed Point Theory and Applications
7
From the above discussions, for ∀f ∈ CB, B, 0, M, g, we can get that
Pf ∈ CB, A, αM, ∞, T ,
1
, T −1 ,
Pf−1 L ◦ Pf ∈ C A, B, 0,
αM
K
SF ◦ L ◦ Pf ∈ C B, B, 0,
, g ⊂ CB, B, 0, M, g.
αM
3.4
These mean that SF ◦ L ◦ P : CB, B, 0, M, g → CB, B, 0, M, g is well defined and
continuous.
By Lemmas 2.7, 2.9 and Schauder’s fixed points theorem, SF ◦ L ◦ P has a fixed point
f in CB, B, 0, M, g. Then,
SF ◦ L ◦ Pfx fx,
∀x ∈ B,
3.5
which implies
Pf−1 ◦ F f.
3.6
This means that f satisfies the assertion of the theorem.
For f1 , f2 ∈ CB, B, 0, M, g, by Lemma 2.5,
SF ◦ L ◦ P f1 − SF ◦ L ◦ P f2 B
−1
−1
P f1
◦ F − P f2
◦F B
−1
−1 ≤ P f1
− P f2 A
1
P f1 − P f2 ≤
B
αM
d
f1 − f2 .
≤
B
αM
3.7
Since d/αM < 1, the mapping SF ◦ L ◦ P is a contraction on CB, B, 0, M, g. By Banach
contraction principle, f is unique.
Suppose F1 , F2 ∈ CB, A, 0, K, T ◦ g and f1 , f2 ∈ CB, B, 0, M, g such that Pf1 ◦ f1 F1 and Pf2 ◦ f2 F2 , then,
f1 − f2 SF ◦ L ◦ P f1 − SF ◦ L ◦ P f2 1
2
B
B
−1
−1
P f1
◦ F1 − P f2
◦ F2 B
−1
−1
−1
−1
≤ P f1
◦ F1 − P f2
◦ F1 B P f2
◦ F1 − P f2
◦ F2 B
d
1
F1 − F2 .
f1 − f2 B ≤
B
αM
αM
3.8
8
Fixed Point Theory and Applications
This means that
f1 − f2 ≤
B
1
αM
1−
d
αM
−1
F1 − F2 ,
B
3.9
then f depends continuously on F.
Theorem 3.2. Let the sequence {ak }∞
k0 ⊂ R satisfy that
all M ∈ 1, ∞ the mapping
P : f ∈ LipB, B, 0, M −→
∞
∞
k0
ak is absolutely convergent, then for
ak f k ∈ C B, RN
3.10
k0
is well defined and continuous.
k ∞
Proof. Since ∞
k0 ak is absolutely convergent and {f }k0 is a uniformly bounded sequence of
continuous maps on the compact space B to itself, by Weierstrass M-test, Pf is well defined
and continuous.
By Lemma 2.6 and the absolute convergence of ∞
k0 ak , the continuity of the mapping
P can be easily got.
4. Iterative Equation in R
Let I a, b and J c, d be two compact intervals. Let h1 id|∂I and h2 r1 be the
antipodal maps on ∂I. Let g1 , g2 : ∂I → ∂J satisfy g1 a c, g1 b d and g2 a d, g2 b c. Obviously, g1 g1 ◦ h1 and g2 g1 ◦ h2 . Obviously, degh1 1 and degh2 −1.
Theorem 4.1. Suppose that the sequence {ak }∞
k1 ⊂ R satisfy a1 > 0 and
convergent and M, K ≥ 1, if
∞ an Mn−1 ≥ K ,
M
n2
∞
∞
ai hi−1
ai hi−1
k 1, 2.
k a <
k b,
1 ≥ a1 −
i1
∞
k1
ak is absolutely
4.1
4.2
i1
Then, for any F ∈ CI, J, 0, K, gk , 1.6 has a solution in CI, I, 0, K, hk , where I a, b and
∞
i−1
i−1
J ∞
i1 ai hk a,
i1 ai hk b, k 1, 2. Moreover, if
∞ k−2 j ak M
k2
j0
αM
f is unique and depends continuously on F.
< 1,
4.3
Fixed Point Theory and Applications
9
i−1
Proof. For t ∈ 1, ∞ define αt by αt min{maxa1 − ∞
i2 |ai |t , 0, 1}. Since 0 ≤ αt ≤ 1,
we obtain that α : 1, ∞ → 0, 1. From 4.1, we have M · αM ≥ K. By Theorem 3.2, we
can define P : CI, I, 0, M, hk → CI, R, k 1, 2 by
Pfx ∞
ai f i−1 x,
∀x ∈ I,
4.4
i1
where f ∈ CI, I, 0, M, hk , k 1, 2. It is easy to see that Pfa i−1
Pfb ∞
i1 ai hk b. For x, y ∈ I with y > x, one can check that
0 < αMy − x ≤ Pfy − Pfx ≤ 2a1 y − x.
∞
i1
ai hi−1
a and
k
4.5
This means that PfI J, and Pf is an orientation preserving homeomorphism of I.
The above discussions imply that
Pf ∈ C I, J, αM, ∞, g1 .
4.6
For any f, g ∈ CI, I, 0, M, hk by Lemma 2.6, we get that
∞
∞
ai g i−1 x
Pf − PgI sup ai f i−1 x −
x∈I i1
i1
≤ sup
∞ ai · f i−1 x − g i−1 x
x∈I i1
∞ ai · f i−1 − g i−1 ≤
I
i2
≤
4.7
∞ i−2
ai ·
Mj f − gI .
i2
j0
By Theorem 3.1, the assertion is true.
Example 4.2.
∞
54
−1i−2 i
fx f x x2 ,
i−1
55
i2 54
F1 x x : I −→ I,
2
F1 x ∈ I 0, 1,
∂I
g1 .
4.8
10
Fixed Point Theory and Applications
Obviously, F1 x x2 ∈ CI, I, 0, 2, g1 . Let M 4 since
∞ ∞
4i−1
248 2
an Mn−1 54 −
> ,
i−1
55 i2 54
275 4
n2
∞ ∞ k−2 j
∞ k−1 k−2 k−1 k−2 j
·4
/54k−1
275
j0 M
j0 4
k2 ak
k2 1/54
k2 2
≤
.
αM
αM
αM
23 × 248
4.9
αM a1 −
Then, by Theorem 4.1, the equation has an unique strictly increasing solution in CI, I, 0,
4, h1 . For
⎧
⎪
⎪
⎪2x,
⎪
⎪
⎪
⎪
⎨1
F2 x ,
⎪
2
⎪
⎪
⎪
⎪
⎪
⎪
⎩2x − 1,
1
x ∈ 0, ,
4
1 3
x∈
, ,
4 4
3
x∈
,1 ,
4
4.10
it is easy to see that F2 ∈ CI, I, 0, 2, g1 . Then,
∞
54
−1i−2 i
fx f x F2 x, x ∈ I 0, 1,
i−1
55
i2 54
F2 : I −→ I, F2 ∂I g1
4.11
has an unique increasing solution in CI, I, 0, 4, h1 .
For a nonmonotonic example, we consider F3 x x 1/2 sin2πx ∈ CI, I, 0, 5, g1 .
As mentioned in 20, F3 has a local maximum at a point x1 and a local minimum at a point
x2 in 0, 1. The equation
∞
210
−1i−2 i
fx f x F3 x, x ∈ I 0, 1,
i−1
211
i2 210
F3 : I −→ I, F3 ∂I g1
4.12
has an unique nonmonotonic solution in CI, I, 0, 10, h1 .
Example 4.3. For convenience, we only consider {ak }∞
k1 with a2i 0, i 1, 2, . . . . Obviously,
for I 0, 1, F1 x 1 − x2 ∈ CI, I, 0, 2, g2 . By Theorem 4.1,
∞
254
−1i−2 2i−1
fx f x F1 x,
i−1
255
i2 256
F1 : I −→ I, F1 ∂I g2
x ∈ 0, 1,
4.13
Fixed Point Theory and Applications
11
has an unique strictly decreasing solution in CI, I, 0, 4, h2 . For
⎧
⎪
⎪
1 − 2x,
⎪
⎪
⎪
⎪
⎪
⎨1
,
F2 x ⎪
2
⎪
⎪
⎪
⎪
⎪
⎪
⎩2 − 2x,
1
x ∈ 0, ,
4
1 3
x∈
, ,
4 4
3
x∈
,1 ,
4
4.14
it is easy to see that F2 ∈ CI, I, 0, 2, g2 . Then,
∞
254
−1i−2 2i−1
fx f x F2 x,
i−1
255
i2 256
F2 : I −→ I, F2 ∂I g2
x ∈ 0, 1,
4.15
has an unique decreasing solution in CI, I, 0, 4, h2 .
For a nonmonotonic example, we consider F3 x 1 − x − 1/2 sin2πx ∈ CI, I, 0,
5, g2 . F3 has a local maximum at a point x1 and a local minimum at a point x2 in 0, 1. The
equation
∞
−1i−2 2i−1
1000
fx f x F3 x,
i−1
1001
i2 1000
F3 : I −→ I, F3 ∂I g2
x ∈ 0, 1,
4.16
has an unique nonmonotonic solution in CI, I, 0, 10, h2 .
5. Iterative Equation in RN N ≥ 2
In 22, Kulczycki and Tabor got the existence of solutions of the iterative 1.6 on compact
convex subsets of RN , but they only discussed the case F|∂B id∂B . In this section, we will
continue the work of 22 and discuss the solutions for a special case of 1.6 on unit closed
ball of RN .
Let ξ : SN−1 → SN−1 be a homeomorphism which satisfies ξ ◦ ξ idSN−1 . Obviously,
degξ ±1.
Theorem 5.1. Let {ai }∞
i1 ⊂ 0, 1 with
with
∞
i1
a1 −
ai 1, a1 > 0 and there exist two constants M, K ≥ 1
∞
i2
ai M2i−2 ≥
K
.
M
5.1
12
Fixed Point Theory and Applications
Then, for any F ∈ CDN , DN , 0, K, ξ,
∞
ai f 2i−1 x Fx,
i1
F:D
N
x ∈ DN ,
F
−→ D ,
N
has a solution f ∈ CDN , DN , 0, M, ξ. Moreover, if unique and depends continuously on F.
Proof. For t ∈ 1, ∞, define αt max{a0 −
P : CDN , DN , 0, M, ξ → CDN , RN by
Pfx ∞
∞
ξ
k2
ak
2k−3
j0
Mj /αM < 1, then f is
ai t2i−2 , 0}, then 0 ≤ αt ≤ a0 ≤ 1. Define
i1
∞
SN−1
5.2
ai f 2i−2 x.
5.3
i1
Then, we get
∞
∞
2i−2
2i−2
Pfx − Pfy ai f x −
ai f y
i1
i1
≥ a1 x − y −
ai f 2i−2 x − f 2i−2 y
i2
≥
∞
a1 −
∞
ai M
2i−2
x − y
5.4
i2
αMx − y,
∞
2i−2
x − y.
ai M
Pfx − Pfy ≤ a1 i2
For all x ∈ DN , we have
∞
ai f 2i−2 x ∈ conv x, f 2 x, f 4 x, . . . ⊂ DN .
5.5
i1
2i−2 N−1
Since Pf|SN−1 ∞
|S id|SN−1 and degid|SN−1 1, then PfDN DN
i1 ai ξ
and Pf is a homeomorphism from DN to DN . By the above discussion, we get that
∞
2i−1
ξ. For f, g ∈
Pf ∈ CDN , DN , αM, ∞, id|SN−1 . But Pf ◦ f|SN−1 i1 ai ξ
N
N
CD , D , 0, M, ξ, by Lemma 2.6, we get that
Pf − PgDN ≤
∞
k2
ak
2k−3
Mj f − gDN .
5.6
j0
Obviously, the maps id|SN−1 and ξ are the concrete forms of the maps T and g in Theorem 3.1.
By Theorem 3.1, the assertion is true.
Fixed Point Theory and Applications
13
Example 5.2. For Fx −x1 , x2 distx, S1 1, 0 1 − x1 − x12 x22 , −x2 , where x x1 , x2 ∈ D2 , and distx, S1 denotes the distance of the point x from S1 . Obviously, F|S1 r1 ,
where r1 denotes the antipodal map on S1 . By simple calculation, we get that for any x, y ∈
D2 ,
Fx − Fy ≤ 2x − y,
Fx ≤ x dist x, S1 1
5.7
hold. By the above discussion, we get that F ∈ CD2 , D2 , 0, 2, r1 . Then, by Theorem 5.1,
∞
254
1
fx f 2i−1 x Fx,
i−1
255
i2 256
F : D −→ D ,
2
2
x ∈ D2 ,
5.8
F|S1 r1
has a unique solution in CD2 , D2 , 0, 4, r1 .
Acknowledgments
The authors would like to thank the referees for their valuable comments and suggestions
that led to truly significant improvement of the manuscript. Project HITC200706 is supported
by Science Research Foundation in Harbin Institute of Technology.
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