Abstract and Applied Analysis

Volume 2015 (2015), Article ID 243753, 12 pages

http://dx.doi.org/10.1155/2015/243753

## A Coincidence Best Proximity Point Problem in -Metric Spaces

^{1}Department of Mathematics, Lahore University of Management Sciences, Pakistan^{2}Department of Mathematics, University of Sargodha, Pakistan^{3}Department of Mathematics, Faculty of Science, King Mongkut’s University of Technology Thonburi (KMUTT), 126 Pracha Uthit Road, Bang Mod, Thung Khru, Bangkok 10140, Thailand

Received 2 June 2014; Accepted 3 August 2014

Academic Editor: Sehie Park

Copyright © 2015 M. Abbas et al. 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 aim of this paper is to initiate the study of coincidence best proximity point problem in the setup of generalized metric spaces. Some results dealing with existence and uniqueness of a coincidence best proximity point of mappings satisfying certain contractive conditions in such spaces are obtained. An example is provided to support the result proved herein. Our results generalize, extend, and unify various results in the existing literature.

#### 1. Introduction and Preliminaries

Let be any nonempty subset of a metric space and . A fixed point problem defined by and is to find a point in such that . A point in , where , is attained; that is, holds and is called an approximate fixed point of . In case it is not possible to solve , it could be interesting to study the conditions that assure existence and uniqueness of approximate fixed point of a mapping .

Let and be two nonempty subsets of and . Suppose that is the measure of a distance between two sets and . A point is called the best proximity point of if . Thus the best proximity point problem defined by a mapping and a pair of sets is to find a point in such that . If , the fixed point problem defined by and has no solution. If , the best proximity point problem reduces to a fixed point problem. In this way, the best proximity point problem can be viewed as a natural generalization of a fixed point problem. Furthermore, results dealing with existence and uniqueness of the best proximity point of certain mappings are more general than the ones dealing with fixed point problem of those mappings. A coincidence best proximity point problem is defined as follows: find a point in such that , where is a self-mapping on . This is an extension of the best proximity point problem. There are several results dealing with proximity point problem in the setup of metric spaces (see, e.g., [1–11] and references mentioned therein).

Mustafa and Sims [12] introduced the concept of a -metric space as a substantial generalization of metric space. They [13] obtained some fixed point theorems for mappings satisfying different contractive conditions in such spaces. Based on the notion of generalized metric spaces, Mustafa et al. [14–16] obtained several fixed point theorems for mappings satisfying different contractive conditions. Mustafa et al. [17–19] obtained some fixed point theorems for mappings satisfying different contractive conditions. Chugh et al. [20] obtained some fixed point results for maps satisfying property in -metric spaces. Saadati et al. [21] studied fixed point of contractive mappings in partially ordered -metric spaces. Shatanawi [22] obtained fixed points of -maps in -metric spaces. For more details, we refer to, for example, [22–39] and references therein.

A study of the best proximity point problem in the setup of -metric space is a recent development by Hussain et al. [40]. This motivates us to extend the scope of this investigation and extend this study to coincidence proximity point problem of certain mappings in the framework of generalized metric spaces.

Consistent with Mustafa and Sims [12], the following definitions and results will be needed in the sequel.

*Definition 1. *Let be a nonempty set. Suppose that a mapping satisfies(*G1*) for all and if and only if ,(*G2*) for all , with ,(*G3*) for all , with ,(*G4*) (symmetric in all three variables),(*G5*) for all (rectangle inequality).Then is called a generalized metric on or -metric on and is called a -*metric space*.

*Definition 2. *Let be a -metric space, a sequence in , and . One says that is(i)a -*Cauchy* sequence if, for any , there exists a natural number such that, for all ;(ii)a -*convergent* sequence if, for any , there exists a natural number such that, for all for some in .A -metric space is said to be complete if every -Cauchy sequence in is convergent in . It is known that converges to if and only if as .

Proposition 3. *Let be a -metric space; then the following are equivalent.*(1)* converges to .*(2)*, as .*(3)*, as .*(4)*, as .*

*Definition 4. *A -metric on is said to be symmetric if for all .

*Proposition 5. Every -metric on will define a metric on by
*

*Remark 6. *Let be a sequence in -metric space . If and is not a Cauchy sequence, then there exist and two subsequences and such that, for all , , , and for all . If , then
for all . Indeed, if , then, for all , we have
From (3) we have
Taking limit as , we obtain that . To prove
for all , we use induction on . Equation (5) for holds obviously. Suppose that (5) holds for some . Consider
Also,
From (6) and (7), we obtain that
Taking limit as , we have .

*Definition 7. *Let be a -metric space and and two nonempty subsets of . Define

Now we define the concept of -best proximity point of a mapping in the setup of -metric spaces.

*Definition 8. *Let be a -metric space and and two nonempty subsets of . Suppose that , and . A point is called -best proximity point of if .

Note that if is an identity mapping on , then in above definition becomes the best proximity point of .

Consistent with [41], we consider the following classes of mappings.

such that, for all , the series converges}. Elements in are called (c)-comparison functions.

such that and for all .

such that if one or more arguments take the value zero and is continuous}.

such that if one or more arguments take the value zero}.

such that .

such that , whenever the sequences , , , are such that at least one of them is convergent to zero}.

*Definition 9. *Let be a -metric space and and two nonempty subsets of , , and . A mapping is said to be -contraction if, for all with and , one has
where
and .

*Definition 10. *Let be a -metric space and and two nonempty subsets of , , and . A mapping is said to be -proximinal and admissible if , , , , and

*Definition 11. *Let be a -metric space and and two subsets of such that is nonempty, , and . For , the quadruple has(1)weak -property of the first kind if
(2)weak -property of the second kind if
(3)weak -property of the third kind if

*Definition 12 (see [41]). *Let and be two mappings and let , . One will say that is -transitive on if , .

Indeed, we will only use the notion of -transitive mapping on ; that is, , , , and

*2. Coincidence Best Proximity Point Results*

*2. Coincidence Best Proximity Point Results*

*In this section, we obtain several coincidence best proximity results in the setup of generalized metric spaces.*

*Theorem 13. Let be a complete -metric space, and two closed subsets of , and a continuous self-mapping on such that . Suppose that is continuous -proximal and admissible and -contraction, where , , and . If the following conditions hold:(a)quadruple satisfies weak -property of the first kind;(b)if a sequence in such that is Cauchy, then is also a Cauchy;(c)there exists such that and .*

Then there exists a convergent sequence which satisfies and the limit of is a -best proximity point of .

*Proof. *Let . Then . Hence there is such that which implies that . As , there is such that , so . In a similar way, there is such that . Inductively we construct a sequence such that
If there exists some , such that , then implies that is a -best proximity point of . If we define for all , then converges to a -best proximity point of . The proof is complete. Assume that
Note that and for all . We claim that
If , then holds by given hypothesis. Suppose that for some . As is -proximal and admissible, for , , , and , we have . Thus (20) holds.

Use weak -property of the first kind, for all , , , imply the following inequality:
Now by (20), (21), and -contractive property of , we have
for all , where
That is,
From (22) and (24), we have
for all .

If there exists some such that
then, using (19) and the fact that for all , we have
which is a contradiction. Hence
for all . Now (25) implies that
for all .

In particular, for all , we have
Fix and . Since , converges. In particular, there exists some such that . Hence, for , we have
This implies that is a Cauchy sequence. By given hypothesis, is a Cauchy sequence. By completeness of , there exists such that . As for all , so . Since and are continuous mappings, and . Taking limit in (18) as , we conclude that is a -best proximity point of .

*Remark 14. *If is an identity map in Theorem 13, then we obtain the best proximity point of mapping .

*Corollary 15. Let be a complete -metric space, and two closed subsets of , and a continuous self-mapping on such that . Suppose that is continuous -proximal and admissible and -contraction, where , , and . If following conditions hold:(a)quadruple satisfies weak -property of the first kind,(b)for with and , the following holds:
(c)if a sequence in with is Cauchy, then is Cauchy,(d)there is such that and .*

Then there exists a convergent sequence which satisfies and converges to -best proximity point of .

*Example 16. *Let and defined by
It is known that is a complete -metric space. Let and . Obviously and are closed subsets of and . Take . Define the mapping by
Obviously is continuous and . A mapping defined by is continuous. Define by . Clearly
As , so is -proximal and admissible. Since
and , therefore is -contraction. Now
imply that . Hence quadruple has weak -property of the first kind. Note that with and . Thus has -best proximity point ( and are -best proximity point of ).

*Lemma 17. Let be a mapping and let be a sequence. If and for all , then .*

*Theorem 18. If condition () in Theorem 13 is replaced by the following: () and is -transitive, then there exists a sequence which satisfies
and converges to a -best proximity point of .*

*Proof. *Following arguments similar to those in the proof of Theorem 13, we have
By Lemma 17, we have
Next, we show that is a Cauchy sequence. Assume on the contrary that is not a Cauchy sequence. Then, by Remark 6, there exist and two subsequences and such that the following hold:
. Note that
Therefore
Similarly,
Furthermore,
where
Taking limit as in (49) and using (45), we obtain that
Taking limit as in (48) and using (43), (45), (46), and (50), we have
Thus a sequence converges to and terms of this sequence are strictly greater than . ln particular, since ,
From the fact that for all and is -transitive, we deduce that
As has the weak -property of the first kind, so, for all ,
This implies that
As is -contraction, so we have
Using (45), the third and the fourth arguments of converge to zero as . Since , all the terms tend to zero as . Taking limit as in (56), using (45) and (52), we have
which is an absurd statement. Hence is a Cauchy sequence. The rest follows from Theorem 13.

*Theorem 19. Theorem 13 also holds if contractive condition (10) is valid for all and ; conditions () and () are replaced by the following: ()quadruple has the weak -property of the second kind;()for a sequence converging to and for all , there exists a subsequence of such that for all .*

*Proof. *Following similar arguments to those given in proof of Theorem 13, we deduce that and are Cauchy sequences in closed subset of . So we obtain an in such that and . We show that is a -best proximity point of .

Given that has the weak -property of the second kind, for all ,
imply that
It follows that is also a Cauchy sequence in . Hence, there is such that . Thus
Since for all , we deduce that
that is, and . Using condition (), we conclude that there exists a subsequence of such that
Note that
Therefore
The first and the second arguments of
tend to zero, while the last argument gives
Therefore,
Suppose that ; that is,
Since the first and the second terms in (65) tend to zero, and the fourth term tends to , there exists such that
Using the contractivity condition, we have