Research Article | Open Access
Penumarthy Parvateesam Murthy, Uma Devi Patel, "Common Fixed Point Theorems of Greguš Type -Weak Contraction for -Weakly Commuting Mappings in 2-Metric Spaces", Journal of Operators, vol. 2015, Article ID 195731, 9 pages, 2015. https://doi.org/10.1155/2015/195731
Common Fixed Point Theorems of Greguš Type -Weak Contraction for -Weakly Commuting Mappings in 2-Metric Spaces
The main purpose of this paper is to establish a common fixed point theorem for set valued mappings in 2-metric spaces by generalizing a theorem of Abd EL-Monsef et al. (2009) and Murthy and Tas (2009) by using -weak contraction in view of Greguš type condition for set valued mappings using -weakly commuting maps.
1. Introduction and Preliminaries
The weak contraction condition in Hilbert Space was introduced by Alber and Guerre-Delabriere . Later, Rhoades  has noticed that the results of Alber and Guerre-Delabriere  in Hilbert Spaces are also true in a complete metric space.
Rhoades  established a fixed point theorem in a complete metric space by using the following contraction condition.
Let which satisfies the following condition:where and is a continuous ad nondecreasing function such that if and only if .
Remark 1. In this above result, if , where , then we obtain condition (1) of Banach.
The concept of 2-metric space is a natural generalization of the metric space. The concept of 2-metric spaces has been investigated initially in a series of papers (see Gahler [6–8]) and has been developed extensively by Gahler and many others. Gahler defined a 2-metric space as follows.
Definition 2 (see ). A 2-metric space on a set with at least three points is nonnegative real-valued mapping satisfying the following conditions:(1)For two distinct points , there exists a point such that .(2) if at least two of , , and are equal.(3).(4) for all , , , and in . The function is called a 2-metric for the space and the pair is then called a 2-metric space.
Geometrically, the value of a 2-metric represents the area of a triangle with vertices , , and .
After this, a number of fixed point theorems have been proved for 2-metric spaces by introducing compatible mappings, commuting and weakly commuting mappings. There were some generalizations of metric such as a 2-metric, a -metric, a -metric, a cone metric, and a complex-valued metric. Note that a 2-metric is not a continuous function of its variables, whereas an ordinary metric is. This led Dhage to introduce the notion of a -metric in . But, in 2003, Mustafa and Sims  demonstrated that most of the claims concerning the fundamental topological properties of -metric spaces are incorrect. After that, in 2006, Mustafa and Sims  introduced the notion of -metric spaces. Only a 2-metric space has not been known to be topologically equivalent to an ordinary metric. Then, there was no easy relationship between results obtained in 2-metric spaces and metric spaces. In particular, the fixed point theorems on 2-metric spaces and metric spaces may be unrelated easily. For more fixed point theorems on 2-metric spaces, the researchers may refer to [12–15].
Throughout this paper, is for a 2-metric space and is the class of all nonempty bounded subsets of .
Definition 3 (see ). A sequence in is said to be convergent to a point , denoted by , if for all . The point is called the limit of the sequence in .
Definition 4 (see ). A sequence in is said to be Cauchy sequence if , for all .
Definition 5 (see ). The space is said to be complete if every Cauchy sequence in converges to a point of .
Let , , and be nonempty sets in . Let and be the functions defined by If is a singleton set, then In case and are also singleton sets, then for every , , and From the definition of , we can say that Also, for all . Let us note that if at least two of , , and are equal singleton sets.
Definition 6 (see ). A sequence of subset of a 2-metric space is said to be convergent to a subset of if,(1)given , there is a sequence in such that for and ;(2)given , there exists a positive integer such that for , where is the union of all open spheres with centers in and radius
Definition 7 (see ). Let and . Then, the pair is said to be weakly commuting if and for every , and .
Definition 8 (see ). Let and . Then, the pair is said to be -weakly commuting if for every , and and .
Common fixed points of Greguš type  have been proved by Diviccaro et al. , Fisher and Sessa , Mukherjee and Verma , Murthy et al. , and Singh et al.  under weaker conditions. Later, Murthy and Tas  generalized and extended the results of Singh et al.  and proved a theorem for set valued mapping in 2-metric space.
In this paper, we generalize the results of Abd EL-Monsef et al.  and Murthy and Tas  by using -weak contraction with Greguš type condition in 2-metric spaces for set valued mapping for -weakly commuting maps.
2. Main Results
Let and be mapping of 2-metric space into itself and are two set valued mappings satisfying the following condition:for every , , and ,where, , , and(1) is continuous monotone nondecreasing function with if and only if ;(2) is lower semicontinuous, monotone decreasing function with if and only if and for all
Let be an arbitrary point in . Since , then a point such that . Now again, since , for the point we can find a point such that and so on. Inductively, we can construct a sequence in such that Now we need to prove the following lemma for our main theorem.
Proof. Since we haveSince is nondecreasing function, we can writeHence, we can write a contradiction as for each .
Consider We haveSince is nondecreasing function, From the above conditions, we havea contradiction. Hence, we have Now we are ready to prove a common fixed point theorem by using the concept of -weakly commuting maps theorem as follows.
Theorem 11. Let and be mapping of 2-metric space into itself and are two set valued mappings satisfying conditions (10), (11), and (12) and the following:(a) or is a complete subspace of .(b)The pair and are R-weakly commuting. Then , and have unique common fixed point in .
Proof. Let be an arbitrary point in . Since , then there exists a point such that . Now again, since , for the point we can find a point such that and so on. Inductively, we can construct a sequence in such thatFirstly, we have to prove thatFor this, assume for by using (11):whereBy (26), we getIf we take , in the above equation by using the property of and function, we can write The above implies that a contradiction. So we obtainBy using (28) and (31) and employing the properties of and function, we may write Again, (31) implies thatTherefore, is a monotone decreasing sequence of nonnegative real number. There exists a nonnegative real number such thatLetting limit in (33), we geta contradiction with the property of and function. This implies that . Thus, we haveNow, repeating the above process by putting and , we obtainHence, for all , we can writeNext, we will show that is a Cauchy sequence. If, otherwise, there exists and sequence of natural numbers and such that, for every natural number , corresponding to , we can choose to be the smallest integer such that (41) is satisfied. Then, we havePutting and in (11), we getwhereLetting limit in the above, NowWe have to show that Now, using properties of 2-metric space, we get Letting , we getBy using properties of 2-metric space, we can write Letting limit , we have Again using properties of 2-metric space, we can write Letting limit , we haveUsing (46), (49), (51), and (53), we have Since is nondecreasing function, .
Therefore,a contradiction with function; hence, is a Cauchy sequence.
Assume is a complete subspace . Since the sequence is Cauchy, then its subsequence is Cauchy and converges to a point in . Since is complete subspace of , for some , According to the construction of sequence, we can have Letting limit , The above implies that Therefore, we getSimilarly, Letting limit , Therefore, we getNow we will show that is a coincidence point of and . For and putting and in (11) and (12), we havewhereLetting limit in the above equation, we have Letting limit in (64), we getThis implies that Since is monotone nondecreasing function, we can write a contradiction. Hence, is the coincidence point of and ; that is, Since , for some , we have . If , then, from (11) and (12), putting and , we get where