Research Article

# Nonlinear Contractive Conditions for Coupled Cone Fixed Point Theorems

Wei-Shih Du

Author Affiliations

Department of Mathematics, National Kaohsiung Normal University, Kaohsiung 802, Taiwan

Fixed Point Theory and Applications 2010, 2010:190606  doi:10.1155/2010/190606

 Received: 19 April 2010 Revisions received: 8 June 2010 Accepted: 5 July 2010 Published: 21 July 2010

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 establish some new coupled fixed point theorems for various types of nonlinear contractive maps in the setting of quasiordered cone metric spaces which not only obtain several coupled fixed point theorems announced by many authors but also generalize them under weaker assumptions.

### 1. Introduction

The existence of fixed point in partially ordered sets has been studied and investigated recently in [113] and references therein. Since the various contractive conditions are important in metric fixed point theory, there is a trend to weaken the requirement on contractions. Nieto and Rodríguez-López in [8, 10] used Tarski's theorem to show the existence of solutions for fuzzy equations and fuzzy differential equations, respectively. The existence of solutions for matrix equations or ordinary differential equations by applying fixed point theorems are presented in [2, 6, 9, 11, 12]. In [3, 13], the authors proved some fixed point theorems for a mixed monotone mapping in a metric space endowed with partial order and applied their results to problems of existence and uniqueness of solutions for some boundary value problems.

In 2006, Bhaskar and Lakshmikantham [2] first proved the following interesting coupled fixed point theorem in partially ordered metric spaces.

Let be a partially ordered set and a metric on such that is a complete metric space. Let be a continuous mapping having the mixed monotone property on . Assume that there exists a with

(11)

If there exist such that and , then, there exist , such that and .

Let be a topological vector space (t.v.s. for short) with its zero vector . A nonempty subset of is called a convex cone if and for . A convex cone is said to be if . For a given proper, pointed, and convex cone in , we can define a partial ordering with respect to by

(12)

will stand for and while will stand for , where denotes the interior of .

In the following, unless otherwise specified, we always assume that is a locally convex Hausdorff t.v.s. with its zero vector , a proper, closed, convex, and pointed cone in with , a partial ordering with respect to , and .

Very recently, Du [14] first introduced the concepts of -cone metric and -cone metric space to improve and extend the concept of cone metric space in the sense of Huang and Zhang [15].

Definition 1.1 . (see [14]).

Let be a nonempty set. A vector-valued function is said to be a -cone metric if the following conditions hold:

(C1) for all and if and only if ;

(C2) for all ;

(C3) for all .

The pair is then called a -cone metric space.

Definition 1.2 . (see [14]).

Let be a -cone metric space, , and a sequence in .

(i) is said to -cone converge to if for every with there exists a natural number such that for all . We denote this by cone- or as and call the -cone limit of .

(ii) is said to be a -cone Cauchy sequence if for every with there is a natural number such that for all , .

(iii) is said to be -cone complete if every -cone Cauchy sequence in is -cone convergent in .

In [14], the author proved the following important results.

Theorem 1.3 . (see [14]).

Let be a -cone metric space. Then defined by is a metric, where is defined by

(13)

Theorem 1.4 . (see [14]).

Let be a -cone metric space, , and a sequence in . Then the following statements hold:

(a)if -cone converges to (i.e., as , then as (i.e., as ;

(b)if is a -cone Cauchy sequence in , then is a Cauchy sequence (in usual sense) in .

In this paper, we establish some new coupled fixed point theorems for various types of nonlinear contractive maps in the setting of quasiordered cone metric spaces. Our results generalize and improve some results in [2, 4, 9, 11] and references therein.

### 2. Preliminaries

Let be a nonempty set and "" a quasiorder (preorder or pseudoorder, i.e., a reflexive and transitive relation) on . Then is called a quasiordered set. A sequence is called -nondecreasing (resp., -nonincreasing) if (resp., ) for each . In this paper, we endow the product space with the following quasiorder :

(21)

Recall that the nonlinear scalarization function is defined by

(22)

Theorem 2.1 . (see [14, 16, 17]).

For each and , the following statements are satisfied:

(i);

(ii);

(iii);

(iv);

(v) is positively homogeneous and continuous on ;

(vi)if , then ;

(vii) for all .

Remark 2.2.

(a) Clearly, .

(b) The reverse statement of (vi) in Theorem 2.1 (i.e., ) does not hold in general. For example, let , and . Then is a proper, closed, convex, and pointed cone in with and . For , it is easy to see that , and . By applying (iii) and (iv) of Theorem 2.1, we have but indeed .

For any -cone metric space , we can define the map by

(23)

It is obvious that is also a -cone metric on , and if and as , then (i.e., TVS-cone converges to .

By Theorem 1.3, we know that is a metric on . Hence the function : , defined by

(24)

is a metric on .

A map is said to be -continuous at if any sequence with implies that . is said to be -continuous on if is continuous at every point of .

Definition 2.3 . (see [2, 4]).

Let be a quasiordered set and a map. one says that has the mixed monotone property on if is monotone nondecreasing in and is monotone nonincreasing in , that is, for any , ,

(25)

Definition 2.4 . (see [2, 4]).

Let be a nonempty set and a map. One calls an element a coupled fixed point of if

(26)

Definition 2.5.

Let be a -cone metric space with a quasi-order ( for short). A nonempty subset of is said to be

(i)-cone sequentially- if every -nondecreasing -cone Cauchy sequence in converges,

(ii)-cone sequentially- if every -nonincreasing -cone Cauchy sequence in converges,

(iii)-cone sequentially- if it is both -cone sequentially -complete and -cone sequentially -.

Definition 2.6 . (see [4, 18]).

A function is said to be a - if it satisfies Mizoguchi-Takahashi's condition (i.e., for all ).

Clearly, if is a nondecreasing function, then is a -function. Notice that is a -function if and only if for each there exist and such that for all ; for more detail, see [4, Remark (iii)].

Very recently, Du and Wu [5] introduced and studied the concept of functions of contractive factor.

Definition 2.7 . (see [5]).

One says that is a function of contractive factor if for any strictly decreasing sequence in , one has

(27)

The following result tells us the relationship between -functions and functions of contractive factor.

Theorem 2.8.

Any -function is a function of contractive factor.

Proof.

Let be a -function, and let be a strictly decreasing sequence in . Then exists. Since is a -function, there exist and such that for all . On the other hand, there exists , such that

(28)

for all with . Hence for all . Let

(29)

Then for all , and hence . Therefore is a function of contractive factor.

### 3. Coupled Fixed Point Theorems for Various Types of Nonlinear Contractive Maps

Definition 3.1.

One says that is a function of strong contractive factor if for any strictly decreasing sequence in , one has

(31)

It is quite obvious that if is a function of strong contractive factor, then is a function of contractive factor but the reverse is not always true.

The following results are crucial to our proofs in this paper.

Lemma 3.2.

A function of strong contractive factor can be structured by a function of contractive factor.

Proof.

Let be a function of contractive factor. Define , . We claim that is a function of strong contractive factor. Clearly, for all . Let be a strictly decreasing sequence in . Since is a function of contractive factor, . Thus it follows that

(32)

Hence is a function of strong contractive factor.

Lemma 3.3.

Let be a t.v.s., a convex cone with in , and . Then the following statements hold.

(i)If and , then ;

(ii)If and , then ;

(iii)If and , then .

Proof.

To see (i), since the set is open in and is a convex cone, we have

(33)

Since and , it follows that

(34)

which means that . The proofs of conclusions (ii) and(iii) are similar to (i).

Lemma 3.4 (see [4]).

Let be a quasiordered set and a multivalued map having the mixed monotone property on . Let . Define two sequences and by

(35)

for each . If and , then is -nondecreasing and is -nonincreasing.

In this section, we first present the following new coupled fixed point theorem for functions of contractive factor in quasiordered cone metric spaces which is one of the main results of this paper.

Theorem 3.5.

Let be a -cone sequentially -complete metric space, a map having the mixed monotone property on , and . Assume that there exists a function of contractive factor such that for any with ,

(36)

and there exist such that and . Define the iterative sequence in by and for . Then the following statements hold.

(a)There exists a nonempty subset of , such that is a complete metric space.

(b)There exists a nonempty subset of , such that is a complete metric space, where for any . Moreover, if is -continuous on , then -cone converges to a coupled fixed point in of .

Proof.

Since is a locally convex Hausdorff t.v.s. with its zero vector , let denote the topology of and let be the base at consisting of all absolutely convex neighborhood of . Let

(37)

Then is a family of seminorms on . For each , let

(38)

and let

(39)

Then is a base at , and the topology generated by is the weakest topology for such that all seminorms in are continuous and . Moreover, given any neighborhood of , there exists such that (see, e.g., [19, Theorem in II.12, Page 113]).

By Lemma 3.2, we can define a function of strong contractive factor by . Then for all . For any , let and . Then, by Lemma 3.4, is -nondecreasing and is -nonincreasing. So and for each . By (3.6), we obtain

(310)

(311)

By (3.10) and Theorem 2.1,

(312)

Similarly, by (3.11) and Theorem 2.1, we also have

(313)

Combining (3.12) and (3.13), we get

(314)

For each , let . Then . By induction, we can obtain the following. For each ,

(315)

(316)

(317)

(318)

(319)

Since for all , the sequence is strictly decreasing in from (3.19). Since is a function of strong contractive factor, we have

(320)

So for all . We want to prove that is a -nondecreasing -cone Cauchy sequence and is a -nonincreasing -cone Cauchy sequence in . For each , by (3.15), we have

(321)

Similarly, by (3.16), we obtain

(322)

From (3.21) and (3.22), we get

(323)

Hence it follows from (3.21), (3.22), and (3.23) that

(324)

Therefore, for with , we have

(325)

(326)

Given with (i.e., , there exists a neighborhood of such that . Therefore, there exists with such that , where

(327)

for some and , . Let

(328)

If , since each is a seminorm, we have and

(329)

for all and all . If , since , , and hence there exists such that for all . So, for each and any , we obtain

(330)

Therefore for any , for all , and hence . So we obtain

(331)

or

(332)

for all . For with , by (3.25), (3.26), (3.32), and Lemma 3.3, we obtain

(333)

Hence is a -nondecreasing -cone Cauchy sequence and is a -nonincreasing -cone Cauchy sequence in . By the -cone sequential -completeness of , there exist such that -cone converges to and -cone converges to . Therefore -cone converges to .

On the other hand, applying Theorem 1.4, we have the following:

(334)

(335)

(336)

(337)

Since for all , by (3.36) and (3.37), we have as . Let , , and . Then , , and are also complete metric spaces. Hence conclusion (a) holds.

Finally, in order to complete the proof of conclusion (b), we need to verify that is a coupled fixed point of . Let be given. Since is -continuous on and , is -continuous at . So there exists such that

(338)

whenever with . Since and as , for there exists such that

(339)

So, for each with , by (3.39),

(340)

and hence we have from (3.38) that

(341)

Therefore

(342)

Since is arbitrary, or . Similarly, we can also prove that . So is a coupled fixed point of . The proof is finished.

The following conclusions are immediate from Theorems 2.8 and 3.5.

Theorem 3.6.

Let be a -cone sequentially -complete metric space, a map having the mixed monotone property on , and . Assume that there exists a -function such that for any with ,

(343)

and there exist such that and . Define the iterative sequence in by and for . Then the following statements hold.

(a)There exists a nonempty subset of , such that is a complete metric space.

(b)There exists a nonempty subset of , such that is a complete metric space. Moreover, if is -continuous on , then -cone converges to a coupled fixed point in of .

Theorem 3.7.

Let be a -cone sequentially -complete metric space, a map having the mixed monotone property on , and . Assume that there exists a nonnegative number such that for any with ,

(344)

and there exist such that and . Define the iterative sequence in by and for . Then the following statements hold.

(a)There exists a nonempty subset of , such that is a complete metric space.

(b)There exists a nonempty subset of , such that is a complete metric space. Moreover, if is -continuous on , then -cone converges to a coupled fixed point in of .

Remark 3.8.

(a) Theorems 3.5 and 3.6 all generalize and improve [4, Theorem ] and some results in [2, 9, 11].

(b) Theorems 3.5–3.7 all generalize Bhaskar-Lakshmikantham's coupled fixed points theorem (i.e., Theorem BL).

Finally, we focus our research on -cone metric spaces.

Theorem 3.9.

Let be a -cone complete metric space, a map, and . Assume that there exists a function of contractive factor such that for any

(345)

Let . Define the iterative sequence in by and for . Then the following statements hold.

(a)There exists a nonempty subset of , such that is a complete metric space.

(b)There exists a nonempty subset of , such that is a complete metric space.

(c) has a unique coupled fixed point in . Moreover, -cone converges to the coupled fixed point of .

Proof.

For any , by (3.45) and Theorem 2.1, we obtain

(346)

From (3.46), we know that is -continuous on . Following the same argument as in the proof of Theorem 3.5, we can prove that conclusions (a) and (b) hold and there exists , such that -cone converges to and is a coupled fixed point of . To complete the proof, it suffices to show the uniqueness of the coupled fixed point of . On the contrary, suppose that there exists , such that and . By (3.46), we have

(347)

So, it follows from (3.47) that

(348)

The following results are immediate from Theorem 3.9.

Theorem 3.10.

Let be a -cone complete metric space, a map, and . Assume that there exists a -function such that for any ,

(349)

Let . Define the iterative sequence in by and for . Then the following statements hold.

(a)There exists a nonempty subset of , such that is a complete metric space.

(b)There exists a nonempty subset of , such that is a complete metric space.

(c) has a unique coupled fixed point in . Moreover, -cone converges to the coupled fixed point of .

Theorem 3.11.

Let be a -cone complete metric space and a map. Assume that there exists a nonnegative number such that for any ,

(350)

Let . Define the iterative sequence in by and for . Then the following statements hold.

(a)There exists a nonempty subset of , such that is a complete metric space.

(b) There exists a nonempty subset of , such that is a complete metric space.

(c) has a unique coupled fixed point in . Moreover, -cone converges to the coupled fixed point of .

Remark 3.12.

(a) Theorems 3.9 and 3.10 all generalize and improve [4, Theorem ].

(b) Theorems 3.9–3.11 all generalize some results in [2, 9, 11].

### Acknowledgment

This research was supported by the National Science Council of the Republic of China.

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