# Fixed Point Results in Quasimetric Spaces

- Abdul Latif
^{1}Email author and - Saleh A Al-Mezel
^{1}

**2011**:178306

https://doi.org/10.1155/2011/178306

© A. Latif and S. A. Al-Mezel. 2011

**Received: **21 August 2010

**Accepted: **5 October 2010

**Published: **14 October 2010

## Abstract

## Keywords

## 1. Introduction and Preliminaries

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. [1] has established a multivalued version of the well-known Banach contraction principle in the setting of metric spaces as follows.

Theorem 1.1.

Let be a complete metric space, then each contraction map has a fixed point.

Without using the Hausdorff metric, Feng and Liu [2] generalized Nadler's contraction principle as follows.

Theorem 1.2.

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 [3], 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:

(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 [3].

In [4], Suzuki and Takahashi improved Nadler contraction principle (Theorem 1.1) as follows.

Theorem 1.3.

Recently, Latif and Albar [5] generalized Theorem 1.2 with respect to -distance (see, Theorem in [5]), and Latif [6] proved a fixed point result with respect to -distance ( see, Theorem in [6]) 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. [7] 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:

(Q2)If is a sequence in such that and for , for some , then

(Q3)for any there exists , such that and imply

Note that every -distance is a -function, but the converse is not true in general [7]. Now, we state some useful properties of -function as given in [7].

Lemma 1.4.

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. [7] 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.

Theorem 1.5.

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 [6],
-contractive maps [4], and
-contractive maps [7].

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.

## 2. The Results

First, we prove a fixed point theorem for weakly -contractive maps in the setting of quasimetric spaces.

Theorem 2.1.

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 .

Proof.

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.

Lemma 2.2.

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.

Proof.

and thus by Lemma 1.4, is a Cauchy sequence.

Applying Lemma 2.2, we prove a fixed point result for generalized -contractive maps.

Theorem 2.3.

Let be a complete quasimetric space then each generalized q -contractive map has a fixed point.

Proof.

Thus, it follows from Lemma 1.4 that . Since is closed, we get

Corollary 2.4.

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.

Remark 2.5.

(1)Theorem 2.1 generalizes Theorem 1.2 according to Feng and Liu [2] and Latif and Albar [5, Theorem ].

(2)Theorem 2.3 generalizes Theorem 1.3 according to Suzuki and Takahashi [4] and Theorem 1.5 according to Al-Homidan et al. [7] and contains Latif's Theorem in [6].

(3)Theorem 2.3 also generalizes Theorem in [8] in several ways.

(4)Corollary 2.4 improves and generalizes Theorem in [9].

## Declarations

### Acknowledgments

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.

## Authors’ Affiliations

## References

- Nadler SB Jr.:
**Multi-valued contraction mappings.***Pacific Journal of Mathematics*1969,**30:**475–488.MATHMathSciNetView ArticleGoogle Scholar - Feng Y, Liu S:
**Fixed point theorems for multi-valued contractive mappings and multi-valued Caristi type mappings.***Journal of Mathematical Analysis and Applications*2006,**317**(1):103–112. 10.1016/j.jmaa.2005.12.004MATHMathSciNetView ArticleGoogle Scholar - Kada O, Suzuki T, Takahashi W:
**Nonconvex minimization theorems and fixed point theorems in complete metric spaces.***Mathematica Japonica*1996,**44**(2):381–391.MATHMathSciNetGoogle Scholar - Suzuki T, Takahashi W:
**Fixed point theorems and characterizations of metric completeness.***Topological Methods in Nonlinear Analysis*1997,**8**(2):371–382.MathSciNetMATHGoogle Scholar - Latif A, Albar WA:
**Fixed point results in complete metric spaces.***Demonstratio Mathematica*2008,**41**(1):145–150.MATHMathSciNetGoogle Scholar - Latif A:
**A fixed point result in complete metric spaces.***JP Journal of Fixed Point Theory and Applications*2007,**2**(2):169–175.MATHMathSciNetGoogle Scholar - Al-Homidan S, Ansari QH, Yao J-C:
**Some generalizations of Ekeland-type variational principle with applications to equilibrium problems and fixed point theory.***Nonlinear Analysis: Theory, Methods & Applications*2008,**69**(1):126–139. 10.1016/j.na.2007.05.004MATHMathSciNetView ArticleGoogle Scholar - Siddiqi AH, Ansari QH:
**An iterative method for generalized variational inequalities.***Mathematica Japonica*1989,**34**(3):475–481.MATHMathSciNetGoogle Scholar - Kaneko H:
**Generalized contractive multivalued mappings and their fixed points.***Mathematica Japonica*1988,**33**(1):57–64.MATHMathSciNetGoogle Scholar

## Copyright

This article is published under license to BioMed Central Ltd. 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.