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Existence of Solutions for point Boundary Value Problems on a HalfLine
Advances in Difference Equations volume 2009, Article number: 609143 (2009)
Abstract
By using the LeraySchauder continuation theorem, we establish the existence of solutions for point boundary value problems on a halfline , where and are given.
1. Introduction
Multipoint boundary value problems (BVPs) for secondorder differential equations in a finite interval have been studied extensively and many results for the existence of solutions, positive solutions, multiple solutions are obtained by use of the LeraySchauder continuation theorem, GuoKrasnosel'skii fixed point theorem, and so on; for details see [1–4] and the references therein.
In the last several years, boundary value problems in an infinite interval have been arisen in many applications and received much attention; see [5, 6]. Due to the fact that an infinite interval is noncompact, the discussion about BVPs on the halfline is more complicated, see [5–14] and the references therein. Recently, in [15], Lian and Ge studied the following threepoint boundary value problem:
where and are given. In this paper, we will study the following point boundary value problems:
where have the same signal, and are given. We first present the Green function for secondorder multipoint BVPs on the halfline and then give the existence results for (1.2) using the properties of this Green function and the LeraySchauder continuation theorem.
We use the space exists, exists with the norm , where is supremum norm on the halfline, and is absolutely integrable on with the norm .
We set
and we suppose are the same signal in this paper and we always assume
2. Preliminary Results
In this section, we present some definitions and lemmas, which will be needed in the proof of the main results.
Definition 2.1 (see [15]).
It holds that is called an SCarathéodory function if and only if

(i)
for each is measurable on

(ii)
for almost every is continuous on ,

(iii)
for each , there exists with on such that implies , for a.e. .
Lemma 2.2.
Suppose if for any with , then the BVP,
has a unique solution. Moreover, this unique solution can be expressed in the form
where is defined by
here note
Proof.
Integrate the differential equation from to , noticing that , then from to and one has
Since , from (2.4), it holds that
For , the unique solution of (2.1) can be stated by
If the unique solution of (2.1) can be stated by
If the unique solution of (2.1) can be stated by
We note then
Therefore, the unique solution of (2.1) is which completes the proof.
Remark of Lemma 2.2 . Obviously satisfies the properties of a Green function, so we call the Green function of the corresponding homogeneous multipoint BVP of (2.1) on the halfline.
Lemma 2.3.
For all , it holds that
Proof.
For each , is nondecreasing in . Immediately, we have
Further, we have
Therefore, we get the result.
Lemma 2.4.
For the Green function , it holds that
Lemma 2.5.
For the function it is satisfied that
and have the same signal, , then there exists satisfying
where .
Proof.
Let are positive, and note , then for every , we have so that is, Because is continuous on the interval , there exists satisfying , where .
Theorem 2.6 (see [5]).
Let . Then is relatively compact in if the following conditions hold:

(a)
is uniformly bounded in ;

(b)
the functions from are equicontinuous on any compact interval of ;

(c)
the functions from are equiconvergent, that is, for any given , there exists a such that , for any .
3. Main Results
Consider the space and define the operator by
The main result of this paper is following.
Theorem 3.1.
Let be an SCarathéodory function. Suppose further that there exists functions with such that
for almost every and all . Then (1.2) has at least one solution provided:
Lemma 3.2.
Let be an SCarathéodory function. Then, for each is completely continuous in .
Proof.
First we show is well defined. Let ; then there exists such that . For each , it holds that
Further, is continuous in so the Lebesgue dominated convergence theorem implies that
where . Thus, .
Obviously, . Notice that
so we can get .
We claim that is completely continuous in , that is, for each , is continuous in and maps a bounded subset of into a relatively compact set.
Let as in . Next we prove that for each , as in . Because is a SCarathéodory function and
where is a real number such that , we have
Also, we can get
Similarly, we have
For any positive number , when , we have
Combining (3.9)(3.13), we can see that is continuous. Let be a bounded subset; it is easy to prove that is uniformly bounded. In the same way, we can prove (3.5),(3.6), and (3.12), we can also show that is equicontinuous and equiconvergent. Thus, by Theorem 2.6, is completely continuous. The proof is completed.
Proof of Theorem 3.1.
In view of Lemma 2.2, it is clear that is a solution of the BVP (1.2) if and only if is a fixed point of Clearly, for each If for each the fixed points in belong to a closed ball of independent of then the LeraySchauder continuation theorem completes the proof. We have known is completely continuous by Lemma 3.2. Next we show that the fixed point of has a priori bound independently of . Assume and set
According to Lemma 2.5, we know that for any , there exists satisfying . Hence, there are three cases as follow.
Case 1 ().
For any holds and, therefore, there exists a such that . Then, we have
and so it holds that
therefore,
At the same time, we have
and so
Set , which is independent of .
Case 2 ().
For any , we have
which implies that for all . In the same way as for Case 1, we can get
Set , which is independent of and is what we need.
Case 3 ().
For , we have
and so for all .
Similarly, we obtain
Set and which is we need. So (1.2) has at least one solution.
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Acknowledgment
The Natural Science Foundation of Hebei Province (A2009000664) and the Foundation of Hebei University of Science and Technology (XL200759) are acknowledged.
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Yu, C., Guo, Y. & Ji, Y. Existence of Solutions for point Boundary Value Problems on a HalfLine. Adv Differ Equ 2009, 609143 (2009). https://doi.org/10.1155/2009/609143
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Keywords
 Differential Equation
 Unique Solution
 Ordinary Differential Equation
 Functional Equation
 Green Function