Research Article | Open Access

# On the Existence of Coincidence and Common Fixed Point of Two Rational Type Contractions and an Application in Dynamical Programming

**Academic Editor:**Pasquale Vetro

#### Abstract

We establish some coincidence point results for self-mappings satisfying rational type contractions in a generalized metric space. Presented coincidence point theorems weaken and extend numerous existing theorems in the literature besides furnishing some illustrative examples for our results. Finally, our results apply, in particular, to the study of solvability of functional equations arising in dynamic programming.

#### 1. Introduction

Banach contraction principle is one of the most important aspects of fixed point theory as a source of the existence and uniqueness of solutions of many problems in various branches inside and outside mathematics (see, [1â€“3]). Some generalizations of this theorem replace the contraction condition by a weaker. For instance, in 1975, Dass and Gupta [4] defined the following rational type contraction which is more general than the contraction condition: where is a mapping from a metric space into itself.

Recently, in 2015, Almeida et al. [5] introduced an extension of condition (1) of Dass and Gupta [4] as follows: where is defined by and is a nondecreasing upper semicontinuous function with for all .

Another offshoot of generalizations of Banachâ€™s theorem is based on extending the axioms of metric spaces. It is worth mentioning that the use of triangle inequality in a metric space is of extreme importance, since it implies that is continuous; each open ball is an open set, a sequence may converge to unique point, and every convergent sequence is a Cauchy sequence and other things. In 2000, Branciari [6] introduced a new concept of generalized metric space by replacing the triangle inequality of a metric space by a so-called rectangular inequality. Since then, various works have dealt with fixed point results in such spaces (see, [7â€“16]). It was not directly noted that such generalized metric spaces (GMS, for short) may fail to satisfy the conditions which were mentioned above in metric spaces.

In this paper, we introduce coincidence point theorems for two contraction self-mappings of rational type in generalized metric spaces. Our results improve the results of Almeida et al. [5]. These theoretical theorems are applied to the study of the existence solutions to a system of functional equations in dynamic programming.

#### 2. Preliminaries

In this section, we present some preliminaries and notations related to rational type contraction and GMS.

*Definition 1 (Branciari [6]). *Suppose that be a nonempty set and be a distance function such that for all and ,(i),(ii),(iii) (quadrilateral inequality). Then we called GMS.

The following example shows that GMS are more general than metric spaces.

*Example 2. *Suppose that Define on as follows: Then is a GMS but not metric space.

*Definition 3 (see [6, 17]). *Suppose that be a GMS and let be a sequence in Then(i) converges to in GMS if ,(ii) is a Cauchy in GMS if, , such that , ,(iii) is called complete GMS if every Cauchy sequence in converges to a point in

*Remark 4 (Sarma et al. [18]). *Definition 1 of GMS does not ensure the following properties:(a) is continuous on its domain.(b)A GMS is Hausdorff.(c)There is a unique limit of a convergence sequence.(d)Any convergent sequence is a Cauchy sequence.In 2009, Samet [19] and Sarma et al. [18] introduced the following example which shows Remark 4.

*Example 5 (see [18, 19]). *Suppose that , where and Define from into as follows: and if and .

Then is a complete GMS. Moreover, one can see that(1) and is not Cauchy sequence,(2)there is no such that ; hence, GMS is not Hausdorff with the respective topology, where ,(3) is not continuous.

Lemma 6 (see [16]). *Any Cauchy sequence in GMS converges to a unique point.*

*Definition 7 (see [17]). *Let and . The mapping is -admissible if, for all such that , we have . If is the identity mapping, then is called -admissible.

*Definition 8 (see [17]). *Let be a GMS and is -regular if, for each sequence in such that for all and , there exists a subsequence of such that

*Definition 9 (see [17]). *Suppose that be two mappings from a nonempty set into itself. The mappings are said to be weakly compatible if implies A point is called point of coincidence of and if there exists a point such that

#### 3. Main Results

In this section we introduce some coincidence point results for two rational contraction self-mappings on GMS.

Theorem 10. *Let be a GMS and let and be self-mappings on such that Suppose that is a complete GMS and the following condition holds:where is defined by and is a continuous nondecreasing function and **Then and have a unique point of coincidence in Moreover, if and are weakly compatible, then and have a unique common fixed point.*

*Proof. *Define the sequence and in defined by If , then is a point of coincidence of and Consequently, we can suppose that for all

Now, by (6), we havewhereWe consider the following cases:(i)If , from (9), we have(ii)If , from (9), we obtain Hence,that is, (11) holds.(iii)If , from (9), we getwhich is impossible.

In any case, we proved that (11) holds. Since is decreasing sequence, it converges to a nonnegative number, If , then, letting in (9), we deducewhich implies that ; that is,Suppose that for all and show that is GMS Cauchy sequence. First, we prove that the sequence is bounded. Since , there exists such that for all If , for all , from (6), we havewhereHence,Thus the sequence is decreasing and hence is bounded. Now, ifdose not hold, then there exists a subsequence of such that Fromwe obtain that Now, by (6), with and , we havewhich tends to as ; hence,

Now, if possible, let be not a Cauchy sequence. Then there exists such that, for , there exist for which we can find subsequences and of such that is the smallest index for which Now, using (24) and the rectangular inequality, we get Letting in the above inequality, using (16) and (20), we obtainFrom letting , we obtain From (6), with and , we get where Now, using the continuity of as , we obtainwhich implies that , a contradiction with Hence, is a GMS Cauchy sequence. Since is complete GMS, there exists such that Let be such that , applying (6) with :where We get from (32) thatwhich implies that ; that is, and so is a point of coincidence for and .

Now, we prove that is the unique point of coincidence of and Let and be arbitrary points of coincidence of and such that and Using condition (6), it follows thatwhich implies that Thus, and have a unique point of coincidence.

Next, we prove that If is a point of coincidence of and as and are weakly compatible, we obtain that and so Consequently, is unique common fixed point of and

*Example 11. * Suppose as in Example 2; let and , defined by , , and .

Then and satisfy all the conditions of Theorem 10. Hence, is unique coincidence and common fixed point of and

Corollary 12. *Replace condition (6) in Theorem 10 with the following condition: where and **Then and have a unique point of coincidence in Moreover, if and are weakly compatible, then and have a unique common fixed point.*

Corollary 13. *Put (the identity mapping) in Theorem 10. Then one can get a unique fixed point of *

*Remark 14 (see [5, Theoremâ€‰â€‰7]). * It is spatial case of Theorem 10. Next, we introduce some coincidence point theorems for two -contractions self-mappings of rational type in complete GMS.

Theorem 15. *Let be a GMS and let be two self-mappings satisfying the following conditions:where is as in Theorem 10; , and is a complete GMS.**Consider also that the next conditions hold:*(i)* such that *(ii)* is -admissible.*(iii)* is -regular and , for each , and *(iv)*Either or whenever and *(v)* is a lower semicontinuous function and ** Then and have a unique point of coincidence in Moreover, if and are weakly compatible, then and have a unique common fixed point.*

*Proof. *Suppose that Define and as two sequences in such that . If , then , which implies that is a coincidence point of and Consequently, we can suppose that for all From (i), we get that Also, by (ii), we have that Continuous with this process, we obtain that Now, by using (37), we getwhereWe consider the following cases:(i)If , from (38), we have Since is nondecreasing, we have(ii)If , from (38), we obtain The nondecreasing property of implies that Hence, (41) is obtained.(iii)If , by (38), we obtainthis is a contradiction.

In any case, we proved that (41) holds. Since is decreasing, it converges to a nonnegative number, If , then, letting in (37), we deducewhich implies that ; that is,Suppose that for all and prove that is GMS Cauchy sequence. First, we show that the sequence is bounded. Since , there exists such that for all If , for all , from (37), we havewhereHence,Thus the sequence is decreasing and hence is bounded. If, for some , we have and , then, from (49), we getwhich is a contradiction. Then is bounded. Now, ifdose not hold, then there exists a subsequence of such that From we obtain that Now, by (37), one can obtain thatwhich implies that

Now, if possible, let be not a Cauchy sequence. Then there exists for which we can find subsequences and of with such thatFurther, corresponding to , we can choose in such a way that it is the smallest integer for whichNow, using (55) and (56) and the rectangular inequality, we getLetting in the above inequality, using (46) and (51), we obtainFromletting , we obtainFrom (37), with and , we getwhereNow, using the continuity of as , we obtainA contradiction is obtained with , and then , and hence is a GMS Cauchy sequence. Since is complete GMS, there exists such that Let be such that , applying (37) with :whereWe get from (64) thatwhich implies that ; that is, and so is a coincidence point for and

Now, we prove that is the unique coincidence point of and Let and be arbitrary coincidence points of and such that and Using condition (37), it follows that