# Fixed Point Theorems for Nonlinear Operators with and without Monotonicity in Partially Ordered Banach Spaces

- Hui-Sheng Ding
^{1}, - Jin Liang
^{2}and - Ti-Jun Xiao
^{3}Email author

**2010**:108343

**DOI: **10.1155/2010/108343

© Hui-Sheng Ding et al. 2010

**Received: **30 September 2009

**Accepted: **6 December 2009

**Published: **12 January 2010

## Abstract

We establish two fixed point theorems for nonlinear operators on Banach spaces partially ordered by a cone. The first fixed point theorem is concerned with a class of mixed monotone operators. In the second fixed point theorem, the nonlinear operators are neither monotone nor mixed monotone. We also provide an illustrative example for our second result.

## 1. Introduction

Fixed point theorems for nonlinear operators on partially ordered Banach spaces have many applications in nonlinear equations and many other subjects (cf., e.g., [1–7] and references therein); in particular, various kinds of fixed point theorems for mixed monotone operators are proved and applied (see, e.g., [1, 3, 5, 7] and references therein).

Stimulated by [7, 8], we investigate further, in this paper, the existence of fixed points of nonlinear operators with and without monotonicity in partially ordered Banach spaces.

In Section 2, a fixed point theorem for a class of mixed monotone operators is established. In Section 3, without any monotonicity assumption for a class of nonlinear operators, we obtain a fixed point theorem by using Hilbert's projection metric.

Let us recall some basic notations about cone (for more details, we refer the reader to [2]). Let be a real Banach space. A closed convex set in is called a convex cone if the following conditions are satisfied:

(i)if , then for any ,

(ii)if and , then .

A cone induces a partial ordering in by

For any given ,

A cone is called normal if there exists a constant such that

where is the norm on .

Throughout this paper, we denote by the set of nonnegative integers, the set of real numbers, a real Banach space, a convex cone in , an element in ( is the zero element of ), and the following set:

## 2. Monotonic Operators

Theorem 2.1.

Suppose that the operator satisfies the following.

(S1) is increasing, is decreasing, and is decreasing.

(S2) There exist a constant and a function such that for each and , and

(S3)There exist such that , , and

(S4)There exists a constant such that, for all with ,

Then has a unique fixed point in , that is, .

Proof.

The proof is divided into 4 steps.

Step 1.

Hence, in the following proof, one can assume that in (S2) and (S3) without loss.

Step 2.

that .

Step 3.

Next, by making some needed modifications in the proof of [3, Theorem ], one can show that has a fixed point . Suppose that is a fixed point of . It follows from the definition of and that for all . Then, by the normality of , we get . So is the unique fixed point of in .

Step 4.

Let such that . Then it follows from Step 2 that , that is, is a fixed point of in . Thus, by Step 3, , which means that is the unique fixed point of in .

Remark 2.2.

Compared with [7,Remark ], the nonlinear operator in Theorem 2.1 is more general, and so Theorem 2.1 may have a wider range of applications.

## 3. Nonmonotonic Case

First, let us recall some definitions and basic results about Hilbert's projection metric (for more details, see [6]).

Definition 3.1.

**linked**if there exist such that

This defines an equivalence relation on
and divides
into disjoint subsets which we call **constituents** of
.

Let and be linked. Define

Then, the following holds.

Theorem 3.2.

defines a complete metric on each constituent of .

Proof.

See [6].

We will also need the following result.

Theorem 3.3.

where is upper semicontinuous from the right and satisfies for all . Then has a unique fixed point in .

Theorem 3.3 is a generalization of the classical Banach's contraction mapping principle. There are many generalizations of the classical Banach's contraction mapping principle (see, e.g., [10, 11] and references therein), and these generalizations play an important role in research work about fixed points of nonlinear operators in partially ordered Banach spaces; see, for example, [1] and the proof of the following theorem.

Now, we are ready to present our fixed point theorem, in which no monotone condition is assumed on the nonlinear operator.

Theorem 3.4.

for all and satisfying . Then has a unique fixed point in .

Proof.

We divided the proof into 2 steps.

Step 1.

for all and satisfying .

Step 2.

On the other hand, is obviously a constituent of , and thus is complete by Theorem 3.2. Now, Theorem 3.3 yields that has a unique fixed point in .

Corollary 3.5.

for all and . Then has a unique fixed point in .

Proof.

for all and satisfying . Then, Theorem 3.4 yields the conclusion.

Remark 3.6.

Corollary 3.5 is an improvement of [1,Corollary ] in the sense that there is lower semicontinuous on , and the corresponding conditions need to hold on the whole interval .

## 4. An Example

In this section, we give an example to illustrate Theorem 3.4. Let us consider the following nonlinear delay integral equation:

which is a classical model for the spread of some infectious disease (cf. [12]). In fact, (4.1) has been of great interest for many authors (see, e.g., [3, 8] and references therein).

In the rest of this paper, let and

Next, let us investigate the existence of positive almost periodic solution to (4.1). For the reader's convenience, we recall some definitions and basic results about almost periodic functions (for more details, see [13]).

Definition 4.1.

Denote by the set of all such functions.

Lemma 4.2.

Assume that , . Then the following hold.

(a)The range is precompact in , and so is bounded.

(b) provided that is continuous on .

- (d)
Equipped with the sup norm

turns out to be a Banach space.

Now, let , and is defined by . It is not difficult to verify that is a normal cone in , and

Define a nonlinear operator on by

By Lemma 4.2 and [3, Corollary ], it is not difficult to verify that is an operator from to . In addition, in view of (4.2), one can verify that

that is, for all and with . Then, by Theorem 3.4, has a unique fixed point in , that is, (4.1) has a unique almost periodic solution with positive infimum.

## Declarations

### Acknowledgments

The authors are very grateful to the referees for valuable suggestions and comments. In addition, Hui-Sheng Ding acknowledges support from the NSF of China (10826066), the NSF of Jiangxi Province of China (2008GQS0057), and the Youth Foundation of Jiangxi Provincial Education Department(GJJ09456); Jin Liang and Ti-Jun Xiao acknowledge support from the NSF of China (10771202), the Research Fund for Shanghai Key Laboratory for Contemporary Applied Mathematics (08DZ2271900), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (2007035805).

## Authors’ Affiliations

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