Abstract
In this paper, we obtain some common fixed point results for the mappings satisfying rational expressions on a closed ball in complex valued metric spaces. Our results improve several well-known conventional results.
MSC:47H10, 54H25.
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1 Introduction and preliminaries
Azam et al. [1] introduced new spaces called complex valued metric spaces and established the existence of fixed point theorems under the contraction condition. Subsequently, Rouzkard and Imdad [2] established some common fixed point theorems satisfying certain rational expressions in complex valued metric spaces which generalize, unify and complement the results of Azam et al. [1]. Sintunavarat and Kumam [3] obtained common fixed point results by replacing constant of contractive condition to control functions. Recently, Klin-eam and Suanoom [4] extend the concept of complex valued metric spaces and generalized the results of Azam et al. [1] and Rouzkard and Imdad [2]. For more on fixed point theory we refer the reader to [4–26].
The aim of this article is to extend and improve the conditions of contraction from the whole space to closed ball and establish the common fixed point theorems which are more general than the results of Klin-eam and Suanoom [4], Rouzkard and Imdad [2], and Azam et al. [1] on complex valued metric spaces.
Let ℂ be the set of complex numbers and . Define a partial order ≾ on ℂ as follows:
It follows that if and only if one of the following conditions is satisfied:
-
(i)
and ,
-
(ii)
and ,
-
(iii)
and ,
-
(iv)
and .
In particular, we will write if and one of (i), (ii), and (iii) is satisfied and we will write if only (iii) is satisfied. Note that
Definition 1 Let X be a nonempty set. Suppose that the mapping satisfies:
-
(1)
for all ; and if and only if ;
-
(2)
for all ;
-
(3)
for all .
Then d is called a complex valued metric on X and is called a complex valued metric space.
A point is called an interior point of a set whenever there exists such that
where is an open ball. Then is a closed ball.
A point is called a limit point of A whenever for every , we have
A subset is called open whenever each element of A is an interior point of A. A subset is called closed whenever each limit point of B belongs to B. The family
is a sub-basis for a Hausdorff topology τ on X.
Let be a sequence in X and . If for every with there is such that , for all , then is said to be convergent and converges to x. We denote this by or . If for every with there is such that , for all , then is called a Cauchy sequence. If every Cauchy sequence is convergent in , then is called a complete complex valued metric space.
Example 2 Let where
and
Define as follows:
where , . Then is a complete complex valued metric space.
Lemma 3 [1]
Let be a complex valued metric space and let be a sequence in X. Then converges to x if and only if as .
Lemma 4 [1]
Let be a complex valued metric space and let be a sequence in X. Then is a Cauchy sequence if and only if as .
Definition 5 [27]
Two families of self-mappings and are said to be pairwise commuting if:
-
(1)
for all ;
-
(2)
for all ;
-
(3)
for all , .
2 Main result
In our main result, we discuss the existence of the common fixed point of the mappings satisfying a contractive condition on the closed ball. This result is very useful in the sense that it requires the contractiveness of the mappings only on a closed ball instead of the whole space.
Theorem 6 Suppose that is a complete complex valued metric space and . Let and A, B, C, D and E be five nonnegative reals such that . Let satisfy
for all . If
where , then there exists a unique point such that .
Proof Let be an arbitrary point in X and define
We will prove that for all by mathematical induction.
Using inequality (2) and the fact that , we have
It implies that . Let for some . If , where or where , we obtain by using inequality (1)
Now implies that , so we have
This implies that
Since , we have
This implies by the triangular inequality that
Similarly, we get
Putting , we obtain
Now
gives . Hence for all and
for all . Without loss of generality, we take , then
This implies that the sequence is a Cauchy sequence in . Therefore, there exists a point with .
We prove that . Let us consider
Notice that . Hence , that is, . It follows similarly that . For uniqueness, assume that in is a second common fixed point of S and T. Then
Since , so we have
This is contradiction because . Hence . Therefore, u is a unique common fixed point of S and T. □
By setting in Theorem 6, we get the following corollary.
Corollary 7 Suppose that is a complete complex valued metric space and . Let and A, B, C, D and E be five nonnegative reals such that . Let satisfy
for all . If
where , then there exists a unique point such that .
Remark 8 The conclusion of Theorem 6 remains true if the condition (2) is replaced by the condition .
By choosing in Theorem 6, we get the following corollary.
Corollary 9 Suppose that is a complete complex valued metric space and . Let and A, B, C, D be four nonnegative reals such that . Let satisfy
for all . If
where , then there exists a unique point such that .
By setting in Corollary 9, we get the following corollary.
Corollary 10 Suppose that is a complete complex valued metric space and . Let and A, B, C, D be four nonnegative reals such that . Let satisfy
for all . If
where , then there exists a unique point such that .
By choosing in Theorem 6, we get the following corollary.
Corollary 11 Suppose that is a complete complex valued metric space and . Let and A, B, C and E be five nonnegative reals such that . Let satisfy
for all . If
where , then there exists a unique point such that .
By setting in Corollary 11, we get the following corollary.
Corollary 12 Suppose that is a complete complex valued metric space and . Let and A, B, C, and E be five nonnegative reals such that . Let satisfy
for all . If
where , then there exists a unique point such that .
Remark 13 By equating A, B, C, D, and E to 0 in all possible combinations, one can derive a host of corollaries which include the Banach fixed point theorem for self-mappings on the closed ball in complex valued metric spaces.
By choosing in Theorem 6, we get the extension of Theorem 2.1 of [2] to the closed ball as follows.
Corollary 14 Suppose that is a complete complex valued metric space and . Let and A, B, C be three nonnegative reals such that . Let satisfy
for all . If
where , then there exists a unique point such that .
By setting in Corollary 14, we get Corollary 2.3 of [16] on the closed ball as follows.
Corollary 15 Suppose that is a complete complex valued metric space and . Let and A, B, C be three nonnegative reals such that . Let satisfy
for all . If
where , then there exists a unique point such that .
By choosing in Theorem 6, we get the extension of Theorem 4 of [1] to the closed ball as follows.
Corollary 16 Suppose that is a complete complex valued metric space and . Let and A, B be nonnegative reals such that . Let satisfy
for all . If
where , then there exists a unique point such that .
By setting in Corollary 16, we get Corollary 2.3 of [1] on the closed ball as follows.
Corollary 17 Suppose that is a complete complex valued metric space and . Let and A, B be nonnegative reals such that . Let satisfy
for all . If
where , then there exists a unique point such that .
As an application of Theorem 6, we prove the following theorem for two finite families of mappings.
Theorem 18 If and are two finite pairwise commuting finite families of self-mapping defined on a complete complex valued metric space such that the mappings S and T (with and ) satisfy the contractive conditions (1) and (2), then the component maps of the two families and have a unique common fixed point.
Proof From Theorem 6, we can say that the mappings T and S have a unique common fixed point u i.e. . Now our requirement is to show that u is a common fixed point of all the component mappings of both families. In view of pairwise commutativity of the families and (for every ), we can write and which show that (for every k) is also a common fixed point of T and S. By using the uniqueness of common fixed point, we can write (for every k) which shows that u is a common fixed point of the family . Using the same argument one can also show that (for every ) . Thus the component maps of the two families and have a unique common fixed point. □
By setting and in Theorem 18, we get the following corollary.
Corollary 19 Suppose that is a complete complex valued metric space and . Let and A, B, C, D and E be five nonnegative reals such that . Let satisfy
for all and
where , then there exists a unique point such that .
By setting and in Corollary 19, we get the following corollary.
Corollary 20 Suppose that is a complete complex valued metric space and . Let and A, B, C, D and E be five nonnegative reals such that . Let satisfy
for all and
where , then there exists a unique point such that .
Now we give an example satisfying our main result.
Example 21 Let and and let . Consider a metric as follows:
where . Then is a complex valued metric space. Take and . Then
Define by
By a routine calculation, one can verify that the mappings S and T satisfy the conditions (1) and (2) of Theorem 6 with , , , and . Hence S and T are contractions on and is a unique common fixed point of mappings S and T.
It is interesting to notice that S and T are not contractions on the whole space X for as
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Acknowledgements
The authors thank the editor and the referees for their valuable comments and suggestions, which improved greatly the quality of this paper. The research of the third author is supported by the Centre of Excellence in Mathematics, the Commission on Higher Education, Thailand.
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Ahmad, J., Azam, A. & Saejung, S. Common fixed point results for contractive mappings in complex valued metric spaces. Fixed Point Theory Appl 2014, 67 (2014). https://doi.org/10.1186/1687-1812-2014-67
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DOI: https://doi.org/10.1186/1687-1812-2014-67