Abstract and Applied Analysis

Volume 2008 (2008), Article ID 636843, 13 pages

http://dx.doi.org/10.1155/2008/636843

## Differentiable Solutions of Equations Involving Iterated Functional Series

Deparment of Mathematics, Harbin Institute of Technology, Harbin 150001, China

Received 12 October 2008; Accepted 28 December 2008

Academic Editor: John Rassias

Copyright © 2008 Wei Song. 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.

#### Abstract

The nonmonotonic differentiable solutions of equations involving iterated functional series are investigated. Conditions for the existence, uniqueness, and stability of such solutions are given. These extend earlier results due to Murugan and Subrahmanyam.

#### 1. Introduction

Let be a self-mapping on a topological space and denote the *m*th iterate of ,
that is, Let be the set of all continuous self-mappings on .
Equations having iteration as their main operation, that is, including iterates
of the unknown mapping, are called iterative equations. It is one of the most
interesting classes of functional equations [1–4], because it concludes the problem of iterative roots
[1, 5, 6], that is, finding such that is identical to a given .
As a natural generalization of the problem of iterative roots, a class of
iterative equations named as polynomial-like iterative equationhad fascinated many scholars,
such as Dhombres [7], Zhao [8], Mukherjea and Ratti [9]. Despite their nice
constructive proofs, the classical methods prevented them from obtaining more
fruitful results. In 1986, Zhang [10] constructed an interesting operator called “structural operator” for (1.1) and used
the fixed point theory in Banach space to get the solutions of (1.1).
Hence he overcame the difficulties encountered by the formers. By means of this
method, Zhang and Si made a series of work concerning these
qualitative problems, such as [11–15]. In 2002, Kulczycki and Tabor [16] improved Zhang's method and
investigated the existence of Lipschitzian solutions of the iterative
functional equationwhere is a compact convex subset of and are a given Lipschitz function. It is easy to
see that (1.1) is the special case of (1.2) with , and

Recently Zhang et al. [17] and Xu et al. [18] developed this method and they have got the nonmonotonic, convex, and decreasing continuous solutions of (1.1). In fact they have answered the open problem 2 which was proposed by J. Zhang et al. [19].

The problem of differentiable solutions of iterative equation had also fascinated many scholars' attentions. In Zhang [12] and Si [15], the and solutions of (1.1) are considered. In Wang and Si [20] the differentiable solutions of the below equation are considered. Murugan and Subrahmanyam [21, 22] discussed the existence and uniqueness of solutions of the more general equationswhich involve iterated functional series. All the above references only got the increasing differentiable solutions for the above equations because they only considered the case that is increasing. Li and Deng [23] considered the solutions of the (1.2). In [24] solutions of the equationwhere is a compact convex subset of and are discussed. Li and Deng [23] and Li [24] work in higher dimensional case, they do not require monotonicity. It should be pointed out that Mai and Liu [25] made an important contribution to solutions of iterative equations. Mai and Liu proved the existence, uniqueness of solutions of a relatively general kind of iterative equationswhere is a connected closed subset of and Here denotes the set of all mappings from to .

Inspired by the above work, we will investigate (1.4) and extend earlier results due to Murugan and Subrahmanyam in two directions. In [21] the authors only get increasing solutions of (1.4), so the present paper will investigate the nonmonotonic differentiable solutions of (1.4) and give conditions for the existence, uniqueness, and stability of such solutions. In [21] the authors require not only all coefficients are nonnegative but also are all increasing, but we will find that those conditions are not necessary.

#### 2. Preliminaries

Let and be two compact intervals. Let be the set of all continuously differentiable functions from to . Then is a closed subset of the Banach space consisting of all continuously differentiable functions from to with norm . Here the norm is defined by where and is the derivative of . Following Zhang [12], we define the families of functionswhere , are all constants.

Lemma 2.1. *Both and are compact convex subsets of .*

The Lemma above can be proved by a method which is contained in the proof of Theorem 3.1 in [12].

Lemma 2.2 (see [12]). *Suppose that (or ). Then for **where as and denotes .*

We can get the following Lemma from [12]. In [12] the author proved that the lemma is valid for , but we find it is also valid for

Lemma 2.3 (see [12]). * Suppose that satisfies that**where , are positive constants. Then*

Lemma 2.4 (see [18]). * If both are homeomorphisms from to such that**where is a positive constant. Then*

#### 3. Differentiable Solutions of (1.4)

##### 3.1. Existence of Solutions

Let be coefficients of (1.4) and . For any define and . It is easy to see that both the convergence and the value of have nothing to do with the choice of .

Theorem 3.1. *Suppose , are positive constants and for where and are positive constants for Assume further the following
conditions:** hold. Then for any given (1.4) has a solution where , and , .*

As in [22], firstly we give the following three Lemmas which lead directly to the proof of Theorem 3.1. In the sequel we denote

Lemma 3.2. *Under the assumptions of Theorem 3.1
the following series: ** are all convergent.*

*Proof. *The convergence of , , , , and is easy to be verified. As mentioned in
[21], the equalityholds. We get thatBy the convergence of and , is also convergent.

Lemma 3.3. *Under the assumptions of Theorem 3.1,
for each the mapping defined by**has the following
properties:
*(i)*;*(ii)*,**
where and *

*Proof. * For any ,
we haveIt is easy to see that for any We have for any and for any Thus is an orientation-preserving diffeomorphism.

By Lemma 2.2 we can see that for any , By (3.9), (3.11), and Lemma 2.3 we get for any :

Lemma 3.4. *Under the assumptions of Theorem 3.1,
for each the mappings satisfy the following inequalities:
*(i)*;*(ii)* .*

*Proof. * Firstly we have and thusSecondly we getNotice thatand for ,thenFinally we get

*Proof of Theorem 3.1.. * For any we define as follows:and denote for convenience. Clearly , , , (or ),
and (3.10) yields that for any , Furthermore by (3.12) we get that
for any , So ,
which means that

Secondly we prove that is continuous. For any we denote It is easy to see thatBy Lemma 3.4 we getBy the discussion above we
getwhere

Hence is continuous. By Schauder fixed point theorem, there exists a function such thatThat means is a solution of (1.4) in .

#### 4. Uniqueness and Stability of Solutions

Theorem 4.1. *Let be coefficient of (1.4) and , positive constants. Suppose that the conditions in (3.1)
are valid. Further one assumes that**Then for any ,
there exists a unique function satisfying (1.4), where . Furthermore the solution depends continuously on the given function *

*Proof. *If ,
then by (3.26) the map defined in Theorem 3.1 becomes a strict
contraction. The fix point of ,
which is a solution of (1.4), is unique by Banach's contraction
principle.

Let be the solutions of (1.4) for the
corresponding functions First, sincewe getSecond, we have

By the above discussion we getwhich means thatSo the solution depends continuously on the given function

##### 4.1. Examples

*Example 4.2. *Let and The equationwhere ,
has a unique solution

*Proof. * It is easy to
see that , and .
By simple calculation we get that by Theorem 3.1 the equation has a
solution Further we get thatthis means .
By Theorem 4.1 the solution is unique.

By similar discussion we have the following example.

*Example 4.3. *Let , and The equationwhere
has a unique solution

#### Acknowledgements

The author would like to thank the referees for their detailed comments and helpful suggestions. He is supported by Project HITC200706 supported by Science Research Foundation in Harbin Institute of Technology.

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