#### Abstract

We consider the properties of Green’s function for the nonlinear fractional differential equation boundary value problem: , where , is the standard Riemann-Liouville derivative. Here our nonlinearity may be singular at . As applications of Green’s function, we give some multiple positive solutions for singular boundary value problems by means of Schauder fixed-point theorem.

#### 1. Introduction

Fractional differential equations have been of great interest recently. This is due to the intensive development of the theory of fractional calculus itself as well as its applications. Apart from diverse areas of mathematics, fractional differential equations arise in rheology, dynamical processes in selfsimilar and porous structures, fluid flows, electrical networks, viscoelasticity, chemical physics, and many other branches of science. For details, see [1–10].

It should be noted that most of the papers and books on fractional calculus are devoted to the solvability of linear initial fractional differential equations on terms of special functions. Recently, there are some papers dealing with the existence and multiplicity of solution to the nonlinear fractional differential equations boundary value problems, see [11–17].

Bai [14] investigated the existence and uniqueness of positive solutions for a nonlocal boundary value problem of fractional differential equation by contraction map principle and fixed-point index theory, where , , , is the standard Riemann-Liouville derivative. The function is continuous on .

Li et al. [17] investigated the the existence and multiplicity results of positive solutions for the nonlinear differential equation of fractional order by using some fixed-point theorems, where , , , , , , is the standard Riemann-Liouville derivative.

Xu and Fei [18] considered the properties of Green’s function for the nonlinear fractional differential equation boundary value problem where , , , , , is the standard Riemann-Liouville derivative. Here the nonlinearity may be singular at . As applications of Green’s function, they give some existence of positive solutions for singular boundary value problems by means of Schauder fixed-point theorem. Here they consider the case: , , .

In this paper, we consider the singular boundary value problem where , , , is a real constant, is the standard Riemann-Liouville fractional derivative. We will deduce a property of Green’s function. The result we establish in Section 2 can be stated as follow.

Theorem 1. *The Function defined by (12) is continuous and satisfies
**
where .*

In this paper, we give some existence of positive solutions for singular boundary value problems by means of Schauder fixed-point theorem for the case: , , , .

The paper is organized as follows. In Section 2, we state some known results and give a property of Green’s function. In Section 3, using Schauder fixed-point theorem, the existence of positive solutions to singular problems are obtained.

#### 2. Background Materials

For the convenience of the reader, we present here the necessary definitions from fractional calculus theory.

*Definition 2 (see [7]). *The Riemann-Liouville fractional integral of order of a function is given by
provided the right side is pointwise defined on .

*Definition 3 (see [7]). *The Riemann-Liouville fractional derivative of order of a continuous function is given by
where denotes the integer part of number , provided that the right side is pointwise defined on .

From the definition of Riemann-Liouville’s derivative, we can obtain the statement.

Lemma 4 (see [7]). *Let , if one assumes that , then the fractional differential equation
**
has , , , as unique solutions, where is the smallest integer greater than or equal to .*

Lemma 5 (see [7]). *Assume that with a fractional derivative of order that belongs to . Then,
**
for some , , is the smallest integer greater than or equal to .*

Lemma 6. *Given the problem
**
is equivalent to
**
where
*

*Proof. *We can apply Lemma 5 to reduce (10) to an equivalent integral equation
for some . Consequently, the general solution of (10) is
By , this is .

On the other hand, combining with
yields
Therefor, the unique solution of problem (10) is
For , we have
For , we have
The proof is complete.

*Proof of Theorem 1. *It is easy to prove that is continuous on , here we omit it. In the following, we consider . When , , let
We have
When , let
We have
When , let , we have
When , , we have

It is easy to see that . Thus, the proof is complete.

Let us fix some notations to be used in the following. “For a.e.” means “for almost every”. Given , we write if for a.e. , and it is positive in a set of positive measure, we write if is a -caratheodory function, that is, the map is continuous for a.e. , and the map is measurable for all .

Let us define Then,

#### 3. Main Results

In this section, we establish the existence of positive solutions for equation where , , , , , , is the standard Riemann-Liouville derivative. The following is the first main result in this section.

Theorem 7. *Suppose that the following conditions are satisfied.*(H_{1})*For each , there exists a function such that for a.e. , all .*(H_{2})* There exist , , and , such that
here
* (H_{3})* There exist two positive constants such that
and here
**Then, (28) has at least one positive solution.*

*Proof. *Let , and is a closed convex set defined as
here is the Banach space of continuous functions defined on with the norm
and are positive constants to be given below.

Now, we define an operator by

Then, (28) is equivalent to the fixed-point problem
Let be the positive constant satisfying (H_{3}) and
Then, we have . Now, we prove .

In fact, for each and for all , by (H_{1}) and (H_{3})
On the other hand, by conditions (H_{2}) and (H_{3}), we have
In conclusion, .

Finally, it is standard that is a continuous and completely continuous operator. By a direct application of Schauder’s fixed-point theorem, (28) has at least one positive solution , the proof is finished.

*Case 1 (). *As an application of Theorem 7, we consider the case . The following corollary is a direct result of Theorem 7 with .

Corollary 8. *Suppose that satisfies conditions -. Furthermore, assume the following.*()* There exists a positive constant such that
and here
**Then, (28) has at least one positive solution. *

From now on, let us define

*Example 9. *Suppose that the nonlinearity in (28) is
where , and
If , then (28) has at least one positive solution.

*Proof. *We will apply Corollary 8. To this end, we take
then (H_{1}) and (H_{2}) are satisfied since , and the existence condition () becomes
for some . Since , we can choose large enough such that (46) is satisfied, and the proof is finished.

*Example 10. *Suppose that the nonlinearity in (28) is
where , and is a nonnegative parameter. For each with ,(i)if , then (28) has at least one positive solution for each .(ii)If , then (28) has at least one positive solution for each , where is some positive constant.

*Proof. *We will apply Corollary 8. To this end, we take
Then, (H_{1})-(H_{2}) are satisfied since . Now, the existence condition () becomes , and
for some with . So, (28) has at least one positive solution for
Note that if and if , set
then, we have
Let the function possess a maximum at , then
so we have
it is easy to find that since , and . Finally, it would remain to prove . This is easily verified through elementary computations since . We have the desired results (i) and (ii).

*Case 2 (). *The next result explores the case when . In this case .

Corollary 11. *Suppose that satisfies . Furthermore, assume the following.*(H_{4})* There exists , , such that
**If , then (28) has at least one positive solution.*

*Example 12. *Suppose that the nonlinearity in (28) be (43) with , . If , , then (28) has at least one positive solution.

*Proof. *We will apply Corollary 11. Take , , and as the same in the proof of Example 9. Then, (H_{2}) is satisfied, and the existence condition (H_{4}) is satisfied if we take with , and .

*Example 13. *Let the nonlinearity in (28) be (47) with and . For each with , ,(i)if , then (28) has at least one positive solution for each .(ii)If , then (28) has at least one positive solution for each , where is some positive constant.

*Proof. *We will apply Corollary 11. To this end, we take , , and as the same in the proof of Example 10, then (H_{2}) is satisfied, and the existence condition (H_{4}) becomes ,
for some . So, (28) has at least one positive solution for
Note that if and if , and if set
The function possesses a maximum at
then . We have the desired results (i) and (ii).

*Case 3 (). *The next result considers the case .

Corollary 14. *Suppose that satisfies -. Furthermore, assume the following.*(H_{5})* There exist two positive constants such that
here
**Then, (28) has at least one positive solution.*

*Example 15. *Suppose that the nonlinearity in (28) be (43) with , . If , ,
then (28) has at least one positive solution.

*Proof. *We will apply Corollary 14. Take , as the same in the proof of Example 9. Then, (H_{2}) is satisfied, and the existence condition (H_{5}) is satisfied if we take with
and . If we fix , then the first inequality holds if satisfies
or equivalently
The function possesses a minimum at
Taking , then the first inequality in (63) holds if , which is just condition (62). The second inequality holds directly from the choice of , so it remains to prove that . This is easily verified through elementary computations.

*Example 16. *Let the nonlinearity in (28) be (47) with and . If ,
here is the unique solution of the equation
Then (28) has at least one positive solution.

*Proof. *We will apply Corollary 14. To this end, we take , , and as the same in the proof of Example 10, then (H_{2}) is satisfied, and the existence condition (H_{5}) is satisfied if we take with
and . If we fix , then the first inequality holds if satisfies
Let , then

Then, we have

Let , then we have

Now, let us define by

It is easy to see that is a nondecreasing function for and , as . Thus, has a unique solution such that
and .

So, it remains to prove that , that is,

In fact, by (75), we have
that is,

Also we have
that is,

Thus, we have
since , and .

We have the desired results.

*Case 4 (). ** *

*Example 17. *Suppose that the nonlinearity in (28) be (43) with . If ,
and here is the unique solution of the equation
then (28) has at least one positive solution.

*Proof. *We will apply Theorem 7. Take as the same in the proof of Example 9. Then, (H_{2}) is satisfied, and the existence condition (H_{3}) is satisfied if we take with
and . If we fix , then the first inequality holds if satisfies
Taking , then for , we have

Then, we have

Let , then we have