`Abstract and Applied AnalysisVolume 2012 (2012), Article ID 736214, 19 pageshttp://dx.doi.org/10.1155/2012/736214`
Research Article

## Common Fixed Point Theorems for a Class of Twice Power Type Contraction Maps in G-Metric Spaces

Institute of Applied Mathematics and Department of Mathematics, Hangzhou Normal University, Hangzhou Zhejiang 310036, China

Received 5 February 2012; Accepted 27 July 2012

Academic Editor: Svatoslav Staněk

Copyright © 2012 Hongqing Ye and Feng Gu. 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

We introduce a new twice power type contractive condition for three mappings in G-metric spaces, and several new common fixed point theorems are established in complete G-metric space. An example is provided to support our result. The results obtained in this paper differ from other comparable results already known.

#### 1. Introduction

The study of fixed points of mappings satisfying certain contractive conditions has been in the center of rigorous research activity. In 2006, a new structure of generalized metric space was introduced by Mustafa and Sims [1] as an appropriate notion of generalized metric space called -metric space. Abbas and Rhoades [2] initiated the study of common fixed point in generalized metric space. Recently, many fixed point theorems for certain contractive conditions have been established in -metric spaces, and for more details one can refer to [327]. Fixed point problems have also been considered in partially ordered -metric spaces [2831], cone metric spaces [32], and generalized cone metric spaces [33].

In 2006, Gu and He [34] introduced a class of twice power type contractive condition in metric space, proving some common fixed point theorems for four self-maps with twice power type -contractive condition.

In this paper, motivated and inspired by the above results, we introduce a new twice power type contractive condition in -metric space, and we prove some new common fixed point theorems in complete -metric spaces. Our results obtained in this paper differ from other comparable results already known.

Throughout the paper, we mean by the set of all natural numbers. Consistent with Mustafa and Sims [1], the following definitions and results will be needed in the sequel.

Definition 1.1 (see [1]). Let be a nonempty set, and let be a function satisfying the following axioms: if ;, for all with ;  , for all with y;   (symmetry in all three variables); for all , (rectangle inequality);then the function is called a generalized metric, or, more specifically, a -metric on and the pair are called a -metric space.

Definition 1.2 (see [1]). Let be a -metric space, and let be a sequence of points in , a point in is said to be the limit of the sequence if , and one says that sequence is -convergent to .

Thus, if in a -metric space , then for any , there exists such that , for all .

Proposition 1.3 (see [1]). Let be a -metric space, then the followings are equivalent.(1) is -convergent to .(2) as .(3) as .(4) as .

Definition 1.4 (see [1]). Let be a -metric space. A sequence is called -Cauchy sequence if for each there exists a positive integer such that for all ; that is, if as .

Definition 1.5 (see [1]). A -metric space is said to be -complete if every -Cauchy sequence in is -convergent in .

Proposition 1.6 (see [1]). Let be a -metric space. Then the following are equivalent.(1)The sequence is -Cauchy.(2)For every , there exists such that , for all .

Proposition 1.7 (see [1]). Let be a -metric space. Then the function is jointly continuous in all three of its variables.

Definition 1.8 (see [1]). Let and be -metric space, and be a function. Then is said to be -continuous at a point if and only if for every , there is such that and implies . A function is -continuous at if and only if it is -continuous at all .

Proposition 1.9 (see [1]). Let and be -metric space. Then is -continuous at if and only if it is -sequentially continuous at ; that is, whenever is -convergent to , is -convergent to .

Proposition 1.10 (see, [1]). Let be a -metric space. Then, for any in it follows that:(i)if , then ;(ii); (iii); (iv); (v);(vi).

#### 2. Main Results

Theorem 2.1. Let be a complete -metric space. Suppose the three self-mappings satisfy the following condition: for all , where are nonnegative real numbers and . Then , and have a unique common fixed point (say ) and are all -continuous at .

Proof . We will proceed in two steps.
Step 1. We prove any fixed point of is a fixed point of and and conversely. Assume that is such that . However, by (2.1), we have Now we discuss the above inequality in three cases.
Case (i). If and , then, by , we have So, the above inequality becomes Since , hence we have ; however, it contradicts with , so we get .
Case (ii). If , then we have Hence we have and so .
Case (iii). If , we can also get . Hence we have . Therefore is a common fixed point of and .
The same conclusion holds if or .
Step 2. We prove that , , and have a unique common fixed point.
Let be an arbitrary point, and define the sequence by , . If , for some , with , then is a fixed point of and, by the first step, is a common fixed point of , , and . The same holds if or . Without loss of generality, we can assume that , for all .
Next, we prove sequence is a -Cauchy sequence. In fact, by (2.1) and , we have Which gives that It follows that From we know that . Then, we have On the other hand, by using (2.1) and , we have Which implies that It follows that Form the condition , we know that . Therefore, we have Again, using (2.1) and , we can get Which implies that It follows that By the condition , we know that . Hence, we have Let , then from we know that . Combining (2.9), (2.13), and (2.17), we have Thus, by and , for every , noting that , we have Which implies that , as . Thus is a -Cauchy sequence. Due to the completeness of , there exists , such that is -convergent to .
Next we prove is a common fixed point of , and . By using (2.1), we have Letting , and using the fact that is continuous on its variables, we can get Which gives that , that is is a fixed point of . By using (2.1) again, we have Letting at both sides, for is continuous on its variables, it follows that Therefore, ; that is, is a fixed point of . Similarly, by (2.1), we can also get On taking at both sides, since is continuous on its variables, we get that Which gives that , therefore, is fixed point of . Consequently, we have , and is a common fixed point of and . Suppose is another common fixed point of , and , and we have , then by (2.1), we have Which implies that , hence, . Then we know the common fixed point of , and is unique.
To show that is -continuous at , let be any sequence in such that is -convergent to . For , we have Which implies that . Hence is -convergent to . So is -continuous at . Similarly, we can also prove that are -continuous at . Therefore, we complete the proof.

Corollary 2.2. Let be a complete -metric space. Suppose the three self-mappings satisfy the condition: for all , where are nonnegative real numbers and . Then , and have a unique common fixed point (say ) and are all -continuous at .

Proof. From Theorem 2.1 we know that have a unique common fixed point (say ); that is, , and are -continuous at . Since , so is another fixed point of , , so is another fixed point of , and , so is another fixed point of . By and the condition (2.28) in Corollary 2.2, we have Since , we can get . That means , hence is another common fixed point of -and . Since the common fixed point of -and is unique, we deduce that . By the same argument, we can prove . Thus, we have . Suppose is another common fixed point of , and , then , and by using the condition (2.28) in Corollary 2.2 again, we have Which implies that , hence . So the common fixed of , and is unique. It is obvious that every fixed point of is a fixed point of and and conversely.

Corollary 2.3. Let be a complete -metric space. Suppose the self-mapping satisfies the following condition: for all , where are nonnegative real numbers and . Then has a unique fixed point (say ) and is -continuous at .

Proof. Let in Theorem 2.1, we can get this conclusion holds.

Corollary 2.4. Let be a complete -metric space. Suppose the self-mapping satisfies the following condition: for all , where are nonnegative real numbers and . Then has a unique fixed point (say ) and is -continuous at .

Proof. Let in Corollary 2.2, we can get this conclusion holds.

Theorem 2.5. Let be a complete -metric space, and let be three self-mappings in , which satisfy the following condition. for all , are nonnegative real numbers and . Then and have a unique common fixed point (say ) and are all -continuous at .

Proof. We will proceed in two steps.
Step 1. We prove any fixed point of is a fixed point of and and conversely. Assume that is such that . Now we prove that and . If it is not the case, then for and , by (2.33) and we have It follows that Since , hence we have , however it contradicts with the condition , so we can have , hence is a common fixed point of , and .
Analogously, following the similar arguments to those given above, we can obtain a contradiction for and or and . Hence in all the cases, we conclude that . The same conclusion holds if or .
Step 2. We prove that , and have a unique common fixed point. Let be an arbitrary point, and define the sequence by , . If , for some , with , then is a fixed point of and, by the first step, is a common fixed point of , , and . The same holds if or . Without loss of generality, we can assume that , for all . We first prove the sequence is a -Cauchy sequence. In fact, by using (2.33) and , we have Which gives that It follows that From , we know that . Then, we have On the other hand, by using (2.33) and , we have Which implies that It follows that Since , we know that . So, we have Again, using (2.33) and , we can get Which implies that It follows that Since , we know that . So we have Let , then , and by combining (2.39), (2.43), and (2.47), we have Thus, by and , for every , if , noting that , we have Which implies that , as . Thus is a -Cauchy sequence. Due to the completeness of , there exists , such that is -convergent to .
Now we prove is a common fixed point of , and . By using (2.33), we have Letting , and using the fact that is continuous on its variables and , we can get Which gives that , hence is a fixed point of . By using (2.33) again, we have Letting at both sides, for is continuous in its variables, it follows that For , Therefore, we can get , hence , hence is a fixed point of . Similarly, by (2.33), we can also get On taking at both sides, since is continuous in its variables, we get that Since , so we get , hence , therefore, is a fixed point of . Consequently, we have , and is a common fixed point of , and . Suppose is another common fixed point of , and , and we have , then by (2.33), we have Which gives that Hence, we can get . By using (2.33) again, we get Which implies that Hence, we can get By combining , we can have Since , so we have that . Since , we know , so it’s a contradiction. Hence, we get . Then we know the common fixed point of , and is unique.
To show that is -continuous at , let be any sequence in such that is -convergent to . For , we have Which implies that On taking at both sides, considering , we get . Hence is -convergent to . So is -continuous at . Similarly, we can also prove that are -continuous at . Therefore, we complete the proof.

Now we introduce an example to support Theorem 2.5.

Example 2.6. Let , and let be a -metric space defined by , for all in . Let , , and be three self-mappings defined by
Next we proof the mappings , , and are satisfying Condition (2.33) of Theorem 2.5 with , and .

Case 1. If , , then Thus, we have

Case 2. If , then we can get Thus, we have

Case 3. If , then we have Thus, we have

Case 4. If , then we have Thus, we have