- Research Article
- Open Access
Fixed-Point Results for Generalized Contractions on Ordered Gauge Spaces with Applications
© Cristian Chifu and Gabriela Petruşel. 2011
- Received: 6 December 2010
- Accepted: 31 December 2010
- Published: 19 January 2011
The purpose of this paper is to present some fixed-point results for single-valued -contractions on ordered and complete gauge space. Our theorems generalize and extend some recent results in the literature. As an application, existence results for some integral equations on the positive real axis are given.
- Partial Order
- Real Axis
- Comparison Function
- Equivalent Representation
- Nonlinear Integral Equation
Throughout this paper will denote a nonempty set endowed with a separating gauge structure , where is a directed set (see  for definitions). Let and . We also denote by the set of all real numbers and by .
A sequence of elements in is said to be Cauchy if for every and , there is an with for all and . The sequence is called convergent if there exists an such that for every and , there is an with , for all .
A gauge space is called complete if any Cauchy sequence is convergent. A subset of is said to be closed if it contains the limit of any convergent sequence of its elements. See also Dugundji  for other definitions and details.
If is an operator, then is called fixed point for if and only if . The set denotes the fixed-point set of .
On the other hand, Ran and Reurings  proved the following Banach-Caccioppoli type principle in ordered metric spaces.
Theorem 1.1 (Ran and Reurings ).
Let be a partially ordered set such that every pair has a lower and an upper bound. Let be a metric on such that the metric space is complete. Let be a continuous and monotone (i.e., either decreasing or increasing) operator. Suppose that the following two assertions hold:
(1)there exists such that , for each with ;
(2)there exists such that or .
Then has an unique fixed point , that is, , and for each the sequence of successive approximations of starting from converges to .
Since then, several authors considered the problem of existence (and uniqueness) of a fixed point for contraction-type operators on partially ordered sets.
In 2005, Nieto and Rodrguez-López proved a modified variant of Theorem 1.1, by removing the continuity of . The case of decreasing operators is treated in Nieto and Rodrguez-López , where some interesting applications to ordinary differential equations with periodic boundary conditions are also given. Nieto, Pouso, and Rodrguez-López, in a very recent paper, improve some results given by Petruşel and Rus in  in the setting of abstract -spaces in the sense of Fréchet, see, for example, [5, Theorems 3.3 and 3.5]. Another fixed-point result of this type was given by O'Regan and Petruşel in  for the case of -contractions in ordered complete metric spaces.
The aim of this paper is to present some fixed-point theorems for -contractions on ordered and complete gauge space. As an application, existence results for some integral equations on the positive real axis are given. Our theorems generalize the above-mentioned theorems as well as some other ones in the recent literature (see; Ran and Reurings , Nieto and Rodrguez-López [3, 7], Nieto et al. , Petruşel and Rus , Agarwal et al. , O'Regan and Petruşel , etc.).
Let be a nonempty set and be an operator. Then, , , denote the iterate operators of . Let be a nonempty set and let . Let a subset of and an operator. By definition the triple is called an -space (Fréchet ) if the following conditions are satisfied.
(i)If , for all , then and .
(ii)If and , then for all subsequences, , of we have that and .
By definition, an element of is a convergent sequence, is the limit of this sequence and we also write .
In what follow we denote an -space by .
In this setting, if , then an operator is called orbitally -continuous (see ) if [ and , as and for any ] imply [ , as ]. In particular, if , then is called orbitally continuous.
Also, if , with then by we will denote the ordered segment joining and , that is, . In the same setting, consider . Then, is the lower fixed-point set of , while is the upper fixed-point set of . Also, if and , then the cartesian product of and is denoted by , and it is defined in the following way: , .
Let be a nonempty set. By definition is an ordered -space if and only if
(i) is an -space;
(ii) is a partially ordered set;
(iii) , and , for each .
Recall that is said to be a comparison function if it is increasing and , as . As a consequence, we also have , for each , and is right continuous at 0. For example, (where ), and , are comparison functions.
Recall now the following important abstract concept.
Definition 2.2 (Rus ).
Let be an -space. An operator is, by definition, a Picard operator if
(ii) as , for all .
Our first main result is the following existence, uniqueness, and approximation fixed-point theorem.
Let be an ordered complete gauge space and be an operator. Suppose that
(i)for each with there exists such that and ;
(iii)if and , then ;
(iv)there exists such that ;
(v) is orbitally continuous;
Then, is a Picard operator.
Hence is a Cauchy sequence in . From the completeness of the gauge space we have , as .
Let be arbitrarily chosen. Then;
(1)If then and thus, for each , we have , for each . Letting we obtain that .
(2)If then, by (i), there exists such that and . From the second relation, as before, we get, for each , that , for each and hence , as . Then, using the first relation we infer that, for each , we have , for each . Letting again , we conclude .
By the orbital continuity of we get that . Thus . If we have for some , then from above, we must have , so .
If , then plays the role of .
Equivalent representation of condition (iv) are as follows.
(iv)'There exists such that or
Condition (ii) can be replaced by each of the following assertions:
(ii)' is increasing,
(ii)" is decreasing.
However, it is easy to see that assertion (ii) in Theorem 3.1. is more general.
As a consequence of Theorem 3.1, we have the following result very useful for applications.
Let be an ordered complete gauge space and be an operator. One supposes that
(i)for each with there exists such that and ;
(ii) is increasing;
(iii)there exists such that ;
(a) is orbitally continuous or
(b)if an increasing sequence converges to in , then for all ;
(vi)if and , then
Then is a Picard operator.
Hence is a Cauchy sequence in . From the completeness of the gauge space we have that as .
The uniqueness of the fixed point follows by contradiction. Suppose there exists , with . There are two possible cases.
(a)If , then we have as , which is a contradiction. Hence .
(b)If then there exists such that and . The monotonicity condition implies that and are comparable as well as and . Hence , as , which is again a contradiction. Thus .
Consider (4.1). Suppose that
(i) and are continuous;
(ii) is increasing for each ;
Then the integral equation (4.1) has a unique solution in .
Define now for .
Then is an ordered and complete gauge space. Moreover, for any increasing sequence in converging to some we have , for any . Also, for every there exists which is comparable to and .
From (iv) we have that . The conclusion follows now from Theorem 3.4.
Consider (4.9). Suppose that
(i) and are continuous;
(ii) is increasing for each ;
Then the integral equation (4.9) has a unique solution in .
Then is an ordered and complete gauge space. Moreover, for any increasing sequence in converging to a certain we have , for any . Also, for every there exists which is comparable to and . Notice that (ii) implies that is increasing.
As before, from (iv) we have that . The conclusion follows again by Theorem 3.4.
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