/ / Article

Research Article | Open Access

Volume 2020 |Article ID 5393521 | https://doi.org/10.1155/2020/5393521

F. Sabetghadam, H. P. Masiha, H. Aydi, "New Results for Set-Valued Mappings on Ordered Metric Spaces", Journal of Function Spaces, vol. 2020, Article ID 5393521, 5 pages, 2020. https://doi.org/10.1155/2020/5393521

# New Results for Set-Valued Mappings on Ordered Metric Spaces

Accepted22 Aug 2020
Published24 Sep 2020

#### Abstract

We establish some new fixed point results for order-closed multivalued mappings in complete metric spaces endowed with a partial order.

#### 1. Introduction

In 1976, an order relation was defined in metric spaces . Later, many researchers proved various fixed point results in this setting (see ), while the authors in [4, 5, 9] considered coupled questions for so-called monotone conditions. Zhang  proved some (coupled) fixed point theorems for multivalued mappings with monotone conditions in metric spaces with a partial order. Since then, Agarwal and Khamsi  extended Caristi’s fixed point to vector-valued metric spaces. Also, Chung  considered nonlinear contraction mappings. Nadler  defined multivalued contractions, and Assad and Kirk  proved fixed point theorems for set-valued mappings (see also the related works ).

In this paper, we present some fixed point results in ordered metric spaces for order-closed multivalued operators. First, we need some facts.

Lemma 1.  Let be a metric space and be a functional. Consider a nondecreasing, continuous, and subadditive function so that Take the relation “” on given as Then, “” is a partial order relation on . Apparently, then .

Here, we state some definitions. Let be a topological space. Denote by the family of nonempty subsets of . Let be a partial order on .

Definition 2.  Given two nonempty subsets of .

Note that and are different relations between and (see Remark 114 of ). Also, , , and are not partial orders on (see Remark 2 of ).

Definition 3. If satisfies or , then is called a monotone sequence.

Definition 4. A multivalued operator is said to be order-closed if, for monotone sequences and , we have , , and imply .

Definition 5. A function is said to be order upper (lower) semicontinuous, if, for a monotone sequence with , we have

Note that an upper (lower) semicontinuous function is an order upper (lower) semicontinuous. But the converse is not true (see Remark 3 of ).

#### 2. Main Results

Let be a metric space. For , define the partial order “” on induced by and in Lemma 1.

Theorem 6. Let be a complete ordered metric space and be a bounded below function. Suppose that is order-closed with respect to “” so that Then, there is a monotone sequence , so that for , and converges to , which is a fixed point of . If in addition, is order lower semicontinuous on , then for each .

Proof. By the condition , take . From , there is so that . Again, from , there is with . Continuing this procedure, we have an increasing sequence so that . Therefore, That is, the real sequence is decreasing, so it is a convergent sequence because is bounded from below on .
Using Remark 3 of  and Remark 2 of , we find that provided that is subadditive. Thus, By (7), there are and so that Since is nondecreasing, we have for each .
Let . Then i.e., Therefore, we have which together with (8) implies that Note that is convergent; then, there is so that for all , Moreover, by (12), we get The convergence of implies that is a Cauchy sequence. By the completeness of , there is so that as . Since is order-closed, is monotone and . Consequently, i.e., is a fixed point of .
If is order lower semicontinuous on , by definition of “,” then we have for each It yields that . The proof is completed.

The proof of the following theorem carries over in the same manner as for Theorem 6.

Theorem 7. Let be a complete ordered metric space and be a bounded below function. Suppose that is order-closed with respect to “.” Assume that Then, there is a monotone sequence , , , such that converges to , which is a fixed point of . If, in addition, is order upper semicontinuous on , then for all .

Theorem 8. Let be a complete ordered metric space and be bounded below function. Suppose that is order-closed with respect to “.” Assume that (i)for each with (ii)there exists such that, .Then, has a fixed point , and there is a sequence with for , such that . Moreover, if is order lower semicontinuous, then for all .

Proof. Since , by , we can choose so that . This implies that , by definition of , there is so that . Continuing this procedure, we can find an increasing sequence such that . The rest of the proof is similar as in Theorem 6.

The following supports Theorem 8.

Example 9. Let and , for . The metric space is complete. Consider the multivalued mapping given as where is the set of rational numbers. Let and . Note that is bounded from below. Take the order induced by , that is, given as follows: Mention that verifies the following assertions: (a)for each with , we have (b)(c) is order-closed on .Hence, has a fixed point on , which is .

Theorem 10. Let be a complete ordered metric space and be a bounded below function. Suppose is order-closed with respect to “.” If the following conditions hold: (i)for each with (ii)there exists such that, then has a fixed point . Also, there is a sequence with for any such that . Moreover, if is order upper semicontinuous, then for all .
The following example illustrates Theorem 10.

Example 11. Let , , for and for each . Here, is a complete metric space, and is a bounded below function. Consider the order induced by : Clearly, this partial order is the usual order on . Define by . It is obvious that satisfies the following: (a)for each with (b)(c) is order-closed on .Hence, has a fixed point on , which is .

Now, a multivalued version of Theorem 2 of  may be obtained. Here, the considered multivalued mapping is not necessarily continuous.

Theorem 12. Let be a complete ordered metric space and be a continuous function bounded below. Let be a multivalued mapping. Suppose that Then, has a fixed point.

Proof. Set We claim that has a maximum element. For a directed set , let be a totally ordered subset in . For with , the fact that yields . Due to the fact that is bounded below, is a convergent set in . Consider As in proof of Theorem 6, is Cauchy in , which is complete, so converges to in . For , Hence, for each . By , for each , there is so that . By the compactness of , there is a convergence subset of . Assume that converges to . Take such that implies . One writes So for all . Also, So and . Thus, has an upper bound in .
By Zorn’s Lemma, there is a maximum element . also, there is so that . Using , there is so that . Hence, . The element is maximum in , so and . That is, is a fixed point of .

Theorem 13. Let be a complete ordered metric space and be a continuous function bounded below. Let be a multivalued mapping. Assume that Then, has a fixed point.

Remark 14. If is a continuous single-valued mapping in Theorem 6 (resp., Theorem 7), we can replace the condition by and we can obtain the same result.

Remark 15. If in Theorem 8 (resp., Theorem 10), is assumed to be a continuous single-valued mapping, then we get the same result when replacing the conditions and by (i) is monotone increasing (resp., decreasing), that is, for , we have (ii)there exists with (resp., ).If in addition is order upper (resp., lower) semicontinuous on , then is the smallest (resp., largest) fixed point of in (resp., ).

Proof. Let be a fixed point of in , i.e., . Since , we get . Hence, , i.e., . Suppose , then , i.e., . By a mathematical induction, we have for all , then That is, .
Note that if we omit the continuity of and add above condition on , the results remain true.

#### Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

#### Conflicts of Interest

The authors declare that they have no competing interests regarding the publication of this paper.

#### Authors’ Contributions

All authors contributed equally and significantly in writing this article. All authors read and approved the final manuscript.

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