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On Fixed Point Theorems of Mixed Monotone Operators
Fixed Point Theory and Applications volume 2011, Article number: 563136 (2011)
Abstract
We obtain some new existence and uniqueness theorems of positive fixed point of mixed monotone operators in Banach spaces partially ordered by a cone. Some results are new even for increasing or decreasing operators.
1. Introduction
Mixed monotone operators were introduced by Guo and Lakshmikantham in [1] in 1987. Thereafter many authors have investigated these kinds of operators in Banach spaces and obtained a lot of interesting and important results. They are used extensively in nonlinear differential and integral equations. In this paper, we obtain some new existence and uniqueness theorems of positive fixed point of mixed monotone operators in Banach spaces partially ordered by a cone. Some results are new even for increasing or decreasing operators.
Let the real Banach space be partially ordered by a cone of , that is, if and only if . is said to be a mixed monotone operator if is increasing in and decreasing in , that is, implies . Element is called a fixed point of if .
Recall that cone is said to be solid if the interior is nonempty, and we denote if . is normal if there exists a positive constant such that implies , is called the normal constant of .
For all , the notation means that there exist and such that . Clearly, ~ is an equivalence relation. Given (i.e., and ), we denote by the set . It is easy to see that is convex and for all . If and , it is clear that .
All the concepts discussed above can be found in [2, 3]. For more facts about mixed monotone operators and other related concepts, the reader could refer to [4–9] and some of the references therein.
2. Main Results
In this section, we present our main results. To begin with, we give the definition of --concave-convex operators.
Definition 2.1.
Let be a real Banach space and a cone in . We say an operator is --concave-convex operator if there exist two positive-valued functions on interval such that
is a surjection,
, for all ,
, for all , .
Theorem 2.2.
Let be normal cone of , and let be a mixed monotone and --concave-convex operator. In addition, suppose that there exists such that , then has exactly one fixed point in . Moreover, constructing successively the sequence
for any initial , one has
Proof.
We divide the proof into 3 steps.
Step 1.
We prove that has a fixed point in .
Since , we can choose a sufficiently small number such that
It follows from that there exists such that , and hence
By , we know that . So, we can take a positive integer such that
It is clear that
Let . Evidently, and . By the mixed monotonicity of , we have . Further, combining the condition with (2.4) and (2.6), we have
For , from , we get
and hence
Thus, we have
Construct successively the sequences
It follows from (2.7), (2.10), and the mixed monotonicity of that
Note that , so we can get , . Let
Thus, we have , and then
Therefore, , that is, is increasing with . Suppose that as , then . Indeed, suppose to the contrary that . By , there exists such that . We distinguish two cases.
Case 1.
There exists an integer such that . In this case, we know that for all . So, for , we have
By the definition of , we get , which is a contradiction.
Case 2.
If for all integer , , then . By , there exists such that . So, we have
By the definition of , we have
Let , we get , which is also a contradiction. Thus, . For any natural number , we have
Since is normal, we have
Here, is the normality constant.
So, and are Cauchy sequences. Because is complete, there exist such that . By (2.12), we know that and
Further,
and thus . Let , we obtain
Let , we get . That is, is a fixed point of in .
Step 2.
We prove that is the unique fixed point of in .
In fact, suppose that is a fixed point of in . Since , there exist positive numbers such that . Let . Evidently, . We now prove that . If otherwise, . From , there exists such that . Then,
Since , this contradicts the definition of . Hence, , thus, . Therefore, has a unique fixed point in .
Step 3.
We prove (2.2).
For any , we can choose a small number such that
Also from , there is such that , and hence
We can choose a sufficiently large integer , such that
Let . It is easy to see that and . Put , , , ,. Similarly to Step 1, it follows that there exists such that , . By the uniqueness of fixed points of operator in , we get . And by induction, . Since is normal, we have , .
3. Concerned Remarks and Corollaries
If we suppose that the operator or with is a solid cone, then is automatically satisfied. This proves the following corollaries.
Corollary 3.1.
Let be a normal cone of , and let be a mixed monotone and --concave-convex operator, then has exactly one fixed point in . Moreover, constructing successively the sequence
for any initial , one has
Corollary 3.2.
Let be a normal solid cone of , and let be a mixed monotone and --concave-convex operator, then has exactly one fixed point in . Moreover, constructing successively the sequence
for any initial , one has
When , , , , the conditions and are automatically satisfied. So, one has
Corollary 3.3.
Let be a normal cone of a real Banach space is a mixed monotone operator. In addition, suppose that for all , there exists such that
then has exactly one fixed point in . Moreover, constructing successively the sequence
for any initial , one has
Remark 3.4.
Corollary 3.3 is the main result in [5]. So, our results generalized the result in [5].
When , the conditions and are automatically satisfied. So, we have the following.
Corollary 3.5.
Let be normal solid cone of , and let be a mixed monotone operator. In addition, suppose that there exists such that
then has exactly one fixed point in . Moreover, constructing successively the sequence
for any initial , one has
Remark 3.6.
Corollary 3.5 is the main result in [4]. So, our results generalized the result in [4].
References
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Acknowledgments
This research was supported by the National Natural Science Foundation of China (nos. 10871116, 11001151), the Natural Science Foundation of Shandong Province (no. Q2008A03), the Doctoral Program Foundation of Education Ministry of China (no. 20103705120002), and the Youth Foundation of Qufu Normal University (nos. XJ200910, XJ03003).
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Du, X., Zhao, Z. On Fixed Point Theorems of Mixed Monotone Operators. Fixed Point Theory Appl 2011, 563136 (2011). https://doi.org/10.1155/2011/563136
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DOI: https://doi.org/10.1155/2011/563136