Abstract
In the present paper, we give some fixed point results for generalized Ćirić type strong almost contractions on partial metric spaces which generalizes some recent results appearing in the literature. Particularly, our result has as a particular case, mappings satisfying a general contractive condition of integral type.
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Introduction
Partial metric spaces were introduced by Matthews in [20] as a part of the study of denotational semantics of dataflow networks. These spaces are a generalization of usual metric spaces where the self distance for any point need not be equal to zero.
Let us recall that a partial metric on a set is a function such that for all (1) (-separation axiom), (2) (small self-distance axiom), (3) (symmetry), (4) (modified triangular inequality).
A partial metric space (for short PMS) is a pair such that is a nonempty set and is a partial metric on .
It is clear that, if then . But if , may not be .
At this point it seems interesting to remark the fact that partial metric spaces play an important role in constructing models in the theory of computation (see for instance [15–17], etc).
Example 1
Let and for all . Then is a PMS.
Example 2
Let denote the set of all intervals for some real numbers . Let be the function such that . Then is a PMS.
Example 3
Let and for all . Then is a PMS.
Other examples of partial metric spaces may be found in [16, 18, 20, 22], etc.
Each partial metric on generates a topology on which has as a base the family open -balls
where
for all and .
Observe that a sequence in a PMS converges to a point , with respect to if and only if .
If is a partial metric on , then the functions given by
and
are ordinary metrics on . It is easy to see that and are equivalent metrics on .
According to [20], a sequence in a PMS converges, with respect to , to a point if and only if
A sequence in a PMS is called a Cauchy sequence if exists (and is finite). is called complete if every Cauchy sequence in converges, with respect to , to a point such that .
Finally, the following crucial facts are shown in [20]:
-
1.
is a Cauchy sequence in if and only if it is a Cauchy sequence in the metric space .
-
2.
is complete if and only if is complete.
Matthews obtained, among other results, a partial metric version of the Banach fixed point theorem ([20, Theorem 5.3]) as follows.
Theorem 1
([20]) Let be a complete partial metric space and let be a contraction mapping, that is, there exists such that
for all . Then has a unique fixed point . Moreover, .
Later on, Abdeljawad et al. [1], Acar et al. [2, 3], Altun et al. [6–8], Karapinar and Erhan [19], Oltra and Valero [21] and Valero [27], gave some generalizations of the result of Matthews. Also, Ćirić et al. [14], Samet et al. [25] and Shatanawi et al. [26] proved some common fixed point results in partial metric spaces. The best two generalizations of it were given by Romaguera [23, 24].
Theorem 2
Let be a complete partial metric space and let be a map such that
for all , where
and satisfies one of the following:
-
1.
is [23] an upper semicontinuous from the right such that for all
-
2.
is a [24] nondecreasing function such that as for all
Then has a unique fixed point . Moreover,
In [22], Romaguera defined the -complete PMS as follows: A sequence in a PMS is called -Cauchy if
and is called -complete if every -Cauchy sequence in converges, with respect to , to a point such that . It is clear that every complete PMS is -complete, but as it was shown in [22] the converse is not true.
On the other hand Berinde [9–11] defined weak contraction (or -weak contraction) mappings in a metric space as follows.
Definition 1
Let be a metric space and be a self operator. is said to be a weak contraction (or -weak contraction) if there exists a constant and some such that
for all .
Note that, by the symmetry property of the distance, the weak contraction condition implicitly includes the following dual one
for all . So, in order to check the weak contractiveness of a mapping , it is necessary to check both (1.2) and (1.3).
In [9] and [11], Berinde showed that any Banach, Kannan, Chatterjea and Zamfirescu mappings are weak contraction. Using the concept of weak contraction mappings, Berinde [9] proved that if is a -weak contraction self mapping of a complete metric space , then has a fixed point. Also, Berinde shows that any -weak contraction mapping is a Picard operator. Then, Berinde [12] introduced the nonlinear type weak contraction using a comparison function and proved the following fixed point theorem. A map , where , is called comparison function if it satisfies:
-
1.
is monotone increasing,
-
2.
for all .
If satisfies (1) and
-
3.
converges for all
then is said to be -comparison function.
It is clear that -comparison function implies comparison function, but the converse may not be true. We can find some properties and examples of comparison and -comparison functions in [11].
Definition 2
Let be a metric space and is a self operator. be said to be a weak -contraction (or -weak contraction) if there exists a comparison function and some such that
for all .
Similar to the case of weak contraction, in order to check the weak -contractiveness of a mapping , it is necessary to check both (1.4) and
for all .
Clearly any weak contraction is a weak -contraction, but the converse may not be true. Also the class of weak -contractions includes Matkowski type nonlinear contractions.
Theorem 3
Let be a complete metric space and be -weak contraction with is -comparison function. Then has a fixed point.
Let be a metric space and be a map such that
for all , where ,
Then is called Ciric type strong almost contraction [13].
In light of the above information, Altun and Acar [5] introduced the concepts of weak and weak -contractions in the sense of Berinde on partial metric space, showed that any Banach, Kannan, Chatterjea and Zamfirescu mappings are weak contraction and proved some fixed point theorems in this interesting space.
Let be a partial metric space. A map is called -weak contraction if there exists a comparison function and some such that
for all .
As above, because of the symmetry of the distance, the -weak contraction condition implicitly includes the following dual one
for all . Consequently, in order to check the -weak contractiveness of , it is necessary to check both (1.6) and (1.7).
Theorem 4
Let be a -complete partial metric space and be weak contraction with a -comparison function. Then has a fixed point.
Later, Acar et al generalized Theorem 4 to Ciric type strong almost contractions and they proved the following results.
Theorem 5
Let be a -complete partial metric space and be a map such that
for all , where , is a -comparison function and as in Theorem 2.
Then has a fixed point in .
Theorem 6
Let be a -complete partial metric space and be a map such that
for all , where , as in Theorem 2 and is an upper semicontinuous from the right function such that for all .
Then has a fixed point in .
The purpose of this paper is to present a generalization of Theorem 5 which has as a particular case mappings satisfying an integral type almost contraction condition.
Main results
Let be the class of functions defined by
Some examples of functions belonging to are: with and .
Our main result is the following.
Theorem 7
Let be a -complete partial metric space and be a mapping satisfying
for all , where , is defined as in Theorem 2, and is a -comparison function.
Then has a fixed point in such that .
Proof
We take and consider for any . If for some , then is a fixed point of and the proof is finished. Suppose that for any
Since
then we have
Applying the contractive condition (2.1) we have
If for some , then from (2.2) we obtain,
and, since is nondecreasing,
which is a contradiction. Therefore for all . From (2.2), we get
and, since is nondecreasing,
By using mathematical induction, we obtain
for all . By triangle rule, for , we have
Since is a -comparison function, then is convergent and so is a -Cauchy sequence in . Since is -complete, converges, with respect to , to a point such that
Now we claim that . Suppose on contrary As is a -comparison function, for , As and , there exists such that for ,
and there exists such that for ,
If we take then, by (2.3), (2.4) and triangular inequality, we have
Now for , then, by (2.3), (2.4) and (2.5), we have
Letting in the last inequality, we have and since is nondecreasing which is a contradiction. Therefore and .
We can obtain the following corollaries from our main theorem.
Corollary 1
Theorem 4.
Proof
Consider identity mapping in Theorem 7.
Notice that if is a Lebesgue-integrable mapping then the function defined by
belongs to . Therefore we can obtain the following corollary.
Corollary 2
Let be a -complete partial metric space and be a mapping satisfying
for all , where , as in Theorem 2, is a -comparison function and is a Lebesgue-integrable mapping.
Then has a fixed point in .
Now we give an illustrative example.
Example 4
Let , where and
Then is a partial metric space and it is also -complete. Define by
We show that the contractive condition (2.1) of Theorem 7 is satisfied for , and .
Now consider the following cases.
Case 1. If then and so the result is clear. Therefore we will assume in the following cases.
Case 2. Let . Then (note that if or then with If and then )
Case 3. Let . Then
Case 4. Let and . Then
Case 5. Let and . This case is similar to Case 4.
Hence, all conditions of Theorem 7 are satisfied. Therefore has a fixed point in .
Note that , then the condition of (1.1) is not satisfied, because we can not find a function satisfying
and the condition (1) or (2) of Theorem 2. Therefore Theorem 2 is not applicable to this example.
In the above, we show that if is a generalized almost contraction then it has a fixed point. But in order to guarantee the uniqueness of the fixed point of , we have to consider an additional condition, as in the following theorem.
Theorem 8
Let be a -complete partial metric space and be a map such that (2.1) holds. Suppose also satisfies the following condition: there exists a comparison function some and with for such that
holds, for all . Then has a unique fixed point in .
Proof
Suppose that, there are two fixed points and of . If , it is clear that . Assume that By (2.6) with and , we have
since is nondecreasing
which is a contradiction. Therefore has a unique fixed point.
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The authors would like to thank the referees for their helpful advice which led them to present this paper.
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Altun, I., Sadarangani, K. Fixed point theorems for generalized almost contractions in partial metric spaces. Math Sci 8, 122 (2014). https://doi.org/10.1007/s40096-014-0122-9
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DOI: https://doi.org/10.1007/s40096-014-0122-9