- Research Article
- Open Access
Fixed Point Results in Quasimetric Spaces
© A. Latif and S. A. Al-Mezel. 2011
- Received: 21 August 2010
- Accepted: 5 October 2010
- Published: 14 October 2010
In the setting of quasimetric spaces, we prove some new results on the existence of fixed points for contractive type maps with respect to -function. Our results either improve or generalize many known results in the literature.
- Fixed Point Theorem
- Lower Semicontinuous
- Cauchy Sequence
- Point Result
- Contraction Principle
where is the distance from the point to the subset
For a multivalued map , we say
A point is called a fixed point of a multivalued map if . We denote
A sequence in is called an of at if for all integer . A real valued function on is called lower semicontinuous if for any sequence with implies that
Using the Hausdorff metric, Nadler Jr.  has established a multivalued version of the well-known Banach contraction principle in the setting of metric spaces as follows.
Let be a complete metric space, then each contraction map has a fixed point.
Without using the Hausdorff metric, Feng and Liu  generalized Nadler's contraction principle as follows.
Let be a complete metric space and let be a weakly contractive map, then has a fixed point in provided the real valued function on is a lower semicontinuous.
In , Kada et al. introduced the concept of -distance in the setting of metric spaces as follows.
A function is called a -distance on if it satisfies the following:
(w1) for all
(w2) is lower semicontinuous in its second variable;
(w3)for any there exists , such that and imply
Note that in general for , and not either of the implications necessarily holds. Clearly, the metric is a -distance on . Many other examples and properties of -distances are given in .
In , Suzuki and Takahashi improved Nadler contraction principle (Theorem 1.1) as follows.
then has a fixed point.
Recently, Latif and Albar  generalized Theorem 1.2 with respect to -distance (see, Theorem in ), and Latif  proved a fixed point result with respect to -distance ( see, Theorem in ) which contains Theorem 1.3 as a special case.
A nonempty set together with a quasimetric (i.e., not necessarily symmetric) is called a quasimetric space. In the setting of a quasimetric spaces, Al-Homidan et al.  introduced the concept of a -function on quasimetric spaces which generalizes the notion of a -distance.
A function is called a -function on if it satisfies the following conditions:
(Q1) for all
(Q2)If is a sequence in such that and for , for some , then
(Q3)for any there exists , such that and imply
Let be a complete quasimetric space and let be a -function on . Let and be sequences in . Let and be sequences in converging to , then the following hold for any :
(i)if and for all then in particular, if and , then
(ii)if and for all then converges to ;
(iii)if for any with then is a Cauchy sequence;
(iv)if for any then is a Cauchy sequence.
Using the concept -function, Al-Homidan et al.  recently studied an equilibrium version of the Ekeland-type variational principle. They also generalized Nadler's fixed point theorem (Theorem 1.1) in the setting of quasimetric spaces as follows.
then has a fixed point.
In the sequel, we consider as a quasimetric space with quasimetric .
where is a function of to , such that for all
Clearly, t he class of weakly - contractive maps contains the class of weakly contractive maps, and the class of generalized -contractive maps contains the classes of generalized -contraction maps , -contractive maps , and -contractive maps .
In this paper, we prove some new fixed point results in the setting of quasimetric spaces for weakly -contractive and generalized -contractive multivalued maps. Consequently, our results either improve or generalize many known results including the above stated fixed point results.
First, we prove a fixed point theorem for weakly -contractive maps in the setting of quasimetric spaces.
Let be a complete quasimetric space and let be a weakly - contractive map. If a real valued function on is lower semicontinuous, then there exists , such that Further, if then is a fixed point of .
and thus, It follows that there exists a sequence in , such that Now, if then by Lemma 1.4, . Since is closed, we get
Now, we prove the following useful lemma.
Let be a complete quasimetric space and let be a generalized -contractive map, then there exists an orbit of at , such that the sequence of nonnegative numbers is decreasing to zero and is a Cauchy sequence.
and thus by Lemma 1.4, is a Cauchy sequence.
Applying Lemma 2.2, we prove a fixed point result for generalized -contractive maps.
Let be a complete quasimetric space then each generalized q -contractive map has a fixed point.
Thus, it follows from Lemma 1.4 that . Since is closed, we get
where is a monotonic increasing function from to , then has a fixed point.
Finally, we conclude with the following remarks concerning our results related to the known fixed point results.
(3)Theorem 2.3 also generalizes Theorem in  in several ways.
(4)Corollary 2.4 improves and generalizes Theorem in .
The authors thank the referees for their kind comments. The authors also thank King Abdulaziz University and the Deanship of Scientific Research for the research Grant no. 3-35/429.
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