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Nonlinear quasi-contractions in non-normal cone metric spaces
Fixed Point Theory and Applications volume 2014, Article number: 165 (2014)
Abstract
In the paper, we prove a new fixed point theorem of nonlinear quasi-contractions in non-normal cone metric spaces, which partially improve the recent results of Arandelović and Kečkić’s and of Li and Jiang since some of the essential conditions therein are removed. A suitable example is presented to show the usability of our theorem. It is worth mentioning that the results in this paper could not be derived from the corresponding results in the setting of metric spaces by using a scalarization function or a Minkowski functional.
MSC:06A07, 47H10.
1 Introduction
In 2007, Huang and Zhang [1] introduced the concept of cone metric spaces, as a generalization of metric spaces, and gave the version of the Banach contraction principle and other basic theorems in the setting of cone metric spaces. Later on, by omitting the assumption of normality of the cone, Rezapour and Hamlbarani [2] improved the relevant results of [1], and presented a number of examples to support the existence of non-normal cones, which shows that such generalizations are meaningful. Since then, many authors have been interested in the study of fixed point results in non-normal cone metric spaces; see [3–17]. In the preceding references except [3–5, 16, 17], the involving contractions are always assumed to be restricted with a constant.
There are some references concerned with the problem of whether cone metric spaces are equivalent to metric spaces in terms of the existence of the fixed points of the mappings in cone metric spaces; see [7–10]. Actually, it has been shown that each cone metric space is equivalent to a usual metric space , where the real-valued metric function is defined by a nonlinear scalarization function [8] or by a Minkowski functional [9]. Besides, it has been pointed out in [18] that many fixed point generalizations obtained in cone metric spaces are not real generalizations, and the authors should take care in obtaining real fixed point generalizations in cone metric spaces.
In 1974, Ćirić [11] introduced Ćirić’s quasi-contractions in metric spaces as one of the most general classes of contractive-type mappings, and proved the well-known theorem that every Ćirić’s quasi-contraction T has a unique fixed point, which was then generalized to cone metric spaces by [13–15]. There were many works concerned with the fixed point results of contractions or quasi-contractions restricted with nonlinear comparison functions, we refer the readers to [19–25]. Recently, Arandelović and Kečkić [16] considered nonlinear quasi-contractions in cone metric spaces, and by using the nonlinear scalarization method of Du [8], they obtained several fixed point theorems of nonlinear quasi-contractions and quasi-contractions restricted with linear contractive bounded mappings in cone metric spaces over locally convex Hausdorff topological vector spaces with the assumption that . Very recently, Li and Jiang [17] removed the contractive condition of linear bounded mappings appearing in [16], and they proved a fixed point result of quasi-contractions restricted with linear bounded mappings in non-normal cone metric spaces at the expense of
In this paper, we first show that every nondecreasing mapping satisfies the condition (H) provided that it is continuous at θ and (see Lemma 3), and consequently, the condition (H) in [17] is superfluous and could be omitted; see Remark 1. Then by using Lemma 3, we prove a new fixed point theorems of nonlinear quasi-contractions in non-normal cone metric spaces, which improved the relevant results of [16, 17] since the conditions and (H) are removed. In addition, a suitable example is presented to show the usability of our theorem.
It is worth mentioning that the results in this paper could not be derived from the corresponding results in the setting of metric spaces by the methods of [8, 9] and also cannot be obtained by any existing fixed point results in cone metric spaces. Hence the results in this paper are real generalizations.
2 Preliminaries
Let be a normed vector space. A cone of E is a nonempty closed subset P of E such that for each and each , and , where θ is the zero element of E. A cone P of E determines a partial order ⪯ on E by for each . In this case E is called an ordered normed vector space.
A cone P of a normed vector space E is solid if , where intP is the interior of P. For each with , we write . Let P be a solid cone of a normed vector space E. A sequence of E weakly converges [5] to (denote ) if for each , there exists a positive integer such that for all .
A cone P of E is normal if the unit ball is order-convex, which is equivalent to the condition that there is some positive number N such that and implies that , and the minimal N is called a normal constant of P. Another equivalent condition is that
Then it is not hard to conclude that P is non-normal if and only if there exists a sequence such that
which implies that the sandwich theorem does not hold in the case that P is non-normal. However, in the sense of weak convergence, the sandwich theorem still holds even if P is non-normal, and we have the following lemma.
Let P be a solid cone of a normed vector space and . If
and there exists some such that and , then .
Lemma 2 (see [5])
Let P be a solid cone of a normed vector space . Then for each sequence , implies .
Lemma 3 Let P be a solid cone of a normed vector space and a nondecreasing mapping. If A is continuous at θ and , then it satisfies (H).
Proof Let be a sequence of P such that . It suffices to show .
Fix . It is clear that for each m. From we find that, for each m, there exists such that for each . Since A is nondecreasing, for each . Note that (), then () since A is continuous at θ and . Hence by Lemma 2, (), which implies that, for each , there exists such that for each . Therefore we have for each , i.e., (). The proof is complete. □
Remark 1 Every linear bounded mapping is certainly nondecreasing and continuous at θ, and hence it satisfies the condition (H) by Lemma 3. Therefore in Theorem 1 of [17], the condition (H) is superfluous and could be omitted.
Let X be a nonempty set and P be a cone of a topological vector space E. A cone metric on X is a mapping such that, for each ,
(d1) ;
(d2) ;
(d3) .
The pair is called a cone metric space over P. A cone metric d on X over a solid cone P generates a topology on X which has a base of the family of open d-balls , where for each and each .
Let be a cone metric space over a solid cone P of a normed vector space E. A sequence of X converges [1, 5] to (denote by ) if . A sequence of X is Cauchy [1, 5], if . The cone metric space is complete [1, 5], if each Cauchy sequence of X converges to a point .
3 Main results
Let P be a solid cone of a normed vector space . A mapping is called a quasi-contraction, if there exists a mapping such that
where . In particular when A is a linear bounded mapping, T is reduced to the one considered in [17].
Some slight modifications of the proof of [[17], Theorem 1] yield the following result.
Theorem 1 Let be a complete cone metric space over a solid cone P of a normed vector space and a quasi-contraction. Assume that is a nondecreasing and subadditive (i.e., for each ) mapping with such that
If A and B are continuous at θ, where for each . Then T has a unique fixed point , and for each , the Picard iterative sequence converges to , where for each n.
Remark 2 In particular when is a linear bounded mapping with the spectra radius , then (2) is naturally satisfied and B is continuous on P since and is a linear bounded mapping, where is the inverse of .
The following example shows that there exists some nonlinear mapping such that (2) is satisfied and B is continuous at θ.
Example 1 Let be endowed with the norm and which is a non-normal cone [26]. Let for each and each , where .
For each , we have for each and each , and so for each . Note that for each and each , then for each and . Thus for each , we have
and so
which implies that (2) is satisfied since the series and are convergent.
Note that , then for each we have
which implies that B is continuous at θ.
Proof of Theorem 1 It follows from (2) that the mapping B is well defined. Clearly, and since and . By (2), we get
Since A and B are commutative,
We claim that, for all ,
In the following we shall show this claim by induction.
If , then , and so the claim is trivial.
Assume that (5) holds for n. To prove (5) holds for , it suffices to show
By (1),
where
Consider the case that .
If , then by the triangle inequality, the nondecreasing property of A, (5), and (7),
i.e., (6) holds.
If , then by (7) and ,
i.e., (6) holds.
If , then by the triangle inequality, the nondecreasing property and subadditivity of A, (7), and ,
which implies that
Act on the above inequality with B, then by (4) and ,
i.e., (6) holds.
If , then by (5), (7), and ,
i.e., (6) holds.
If , we set , and then by (7),
Consider the case that .
If , or , or , then by (5), (7), and ,
i.e., (6) holds.
If , or , we set , or , respectively, and then (8) follows.
From the above discussions of both cases, we have the result that either (6) holds, and so the proof of our claim is complete, or there exists such that (8) holds. For the latter situation, continue in a similar way, and we will have the result that either
which together with (8) forces
i.e., (6) holds, and so the proof of our claim is complete, or there exists such that
If the above procedure ends by the k th step with , that is, there exist integers such that
then by ,
i.e. (6) holds, and so the proof of our claim is complete.
If the above procedure continues more than n steps, then there exist integers such that
It is clear that implies there exist two integers with such that , then by the nondecreasing property of A and (9),
and so
Set for each . By (2), is well defined. Clearly, and
Act on (10) with , then by (11), and we get , and hence (6) holds by (9). The proof of our claim is complete.
For each and each , set
From (1), it follows that, for each , there exists some such that . Consequently for all , there exist () such that
since A is nondecreasing. Note that , then by (5),
and so by (12),
It follows from (3) that (), and hence () since B is continuous at θ. This together with Lemma 2 implies that (). Moreover, by (13) and Lemma 1, we get
i.e., is a Cauchy sequence of X. Therefore by the completeness of X, there exists some such that (), i.e.,
By (1),
where .
If , or , or , then by (14), (15), (16), Lemma 1, and Lemma 3, we get since A is continuous at θ.
If , then by (16),
and hence by (15), for each , there exists such that, for each ,
which implies that
Act on (18) with B, then by and we get .
If , then by the triangle inequality, the nondecreasing property, and subadditivity of A and (16), we have
and so
Thus it follows from (15) and Lemma 3 that (17) holds for each since A is continuous at θ. Consequently, we get (18). Act on (18) with B, then by and we get . This shows that is a fixed point of T.
If x is another fixed point of T, then by (1),
where . If , or , then , and hence . If , or or , then we must have , and hence . Act on it with B, then by and we get . This shows is the unique fixed point of T. The proof is complete. □
The following example shows the usability of Theorem 1.
Example 2 Let E and P be the same ones as those in Example 1 and . Define a mapping by
Clearly, is a complete cone metric space.
Let and for each and each .
Clearly, is a nondecreasing mapping with , and A is continuous at θ. From Example 1 we know that (2) is satisfied and B is continuous at θ. For each , we have for each , and so for each , i.e., A is subadditive.
Note that + = ≤ = for each and each , i.e., for each , then
i.e., (1) is satisfied with .
Hence all the assumptions of Theorem 1 are satisfied, and so T has a unique fixed point. In fact, θ is the unique fixed point of T.
Remark 3
-
(i)
Since in Example 2 the underlying mapping A is nonlinear, we can conclude that any of the theorems in [12–15, 17] cannot cope with Example 2.
-
(ii)
Let for each in Example 2. Clearly, and for each . Take , we have , and so , i.e., . Note that it is necessarily assumed that in [16], then Theorem 2 of [16] is not applicable.
In what follows, we shall show that the subadditivity of A assumed in Theorem 1 could be removed in the case that (1) is satisfied for .
Theorem 2 Let be a complete cone metric space over a solid cone P of a normed vector space and . Assume that
where is a nondecreasing mapping with such that (2) is satisfied. If A and B are continuous at θ, where for each . Then T has a unique fixed point , and for each , the Picard iterative sequence converges to .
Proof By the nondecreasing property of A and (19), we have
and so, by the triangle inequality,
Since B is continuous at θ, it follows from (3) that (), which together with Lemma 2 implies that (). Moreover, by (20) and Lemma 1, we get
i.e., is a Cauchy sequence of X. Therefore by the completeness of X, there exists some such that (15) is satisfied. By the triangle inequality and (19), we get
which together with (15), Lemma 1, and Lemma 3 implies that since A is continuous at θ. Hence is a fixed point of T. Let x be another fixed point of T, then by (19),
and so . Act on it with B, then by and we get . This shows is the unique fixed point of T. The proof is complete. □
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Acknowledgements
The work was supported by Natural Science Foundation of China (11161022), Natural Science Foundation of Jiangxi Province (20114BAB211006, 20122BAB201015), Educational Department of Jiangxi Province (GJJ12280, GJJ13297, KJLD14034), Program for Excellent Youth Talents of JXUFE (201201) and Candidate of Jiangxi Youth Scientist.
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An erratum to this article is available at http://dx.doi.org/10.1186/1687-1812-2014-196.
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Jiang, S., Li, Z. Nonlinear quasi-contractions in non-normal cone metric spaces. Fixed Point Theory Appl 2014, 165 (2014). https://doi.org/10.1186/1687-1812-2014-165
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DOI: https://doi.org/10.1186/1687-1812-2014-165