#### Abstract

We present some new existence results for singular positone and semipositone nonlinear fractional boundary value problem , , , where , and are continuous, is a real number, and is Riemann-Liouville fractional derivative. Throughout our nonlinearity may be singular in its dependent variable. Two examples are also given to illustrate the main results.

#### 1. Introduction

Fractional calculus has played a significant role in engineering, science, economy, and other fields. Many papers and books on fractional calculus, and fractional differential equations have appeared recently, (see [1–9]). It should be noted that most of papers and books on fractional calculus, are devoted to the solvability of initial fractional differential equations (see [3, 4]). Here, we consider positive solutions of nonlinear fractional differential equation conjugate boundary value problem involving Riemann-Liouville derivative: where , , and are continuous. is a real number, and is the Riemann-Liouville fractional derivative.

It is well known that in mechanics the boundary value problem (1.1) where describes the deflection of an elastic beam rigidly fixed at both ends. The integer order boundary value problem (1.2) has been studied extensively. For details, see for instance, the papers [10–13] and the references therein. In [10, 12], Yao considered and using a Krasnosel'skii fixed-point theorem, derived a -interval such that, for any lying in this interval, the beam equation has existence and multiplicity on positive solution. In this paper, we will consider a more general situation, namely, the boundary value problem (1.1). To the best of our knowledge, there have been few papers which deal with the boundary value problem (1.1) for nonlinear fractional differential equation.

In this paper, in analogy with boundary value problems for differential equations of integer order, we firstly derive the corresponding Green's function named the fractional Green' function. Consequently problem (1.1) is reduced to an equivalent Fredholm integral equation of the second kind. Finally, using Krasnosel'skii's fixed-point theorems, the existence of positive solutions are obtained.

#### 2. Preliminaries

For completeness, in this section, we will demonstrate and study the definitions and some fundamental facts of Riemann-Liouville derivatives of fractional order which can be found in [5].

*Definition 2.1 (see [5, Definition 2.1]). *The integral
where , is called the Riemann-Liouville fractional integral of order .

*Definition 2.2 (see [5, page 36-37]). *For a function given in the interval , the expression
where denotes the integer part of number , is called the Riemann-Liouville fractional derivative of order .

From the definition of Riemann-Liouville derivative, for , we have
giving in particular , where is the smallest integer greater than or equal to .

Lemma 2.3. *Let , then the differential equation
**
has solutions , , , as unique solutions, where is the smallest integer greater than or equal to . **As , from Lemma 2.3, we deduce the following statement.*

Lemma 2.4. *Let , then
**
for some , , is the smallest integer greater than or equal to . **The following Krasnosel'skii's fixed-point theorem will play a major role in our next analysis.*

Theorem 2.5 (see [6]). *Let be a Banach space, and let be a cone in . Assume that are open subsets of with , and let be a completely continuous operator such that, either *(1)*, , , , or*(2)*, , .**Then has a fixed-point in ∖ .*

#### 3. Green's Function and Its Properties

In this section, we derive the corresponding Green's function for boundary-value problem (1.1), and obtain some properties of Green's function.

Lemma 3.1. *Let be a given function, then the boundary-value problem,
**
has a unique solution
**
where
**
Here is called Green's function of boundary-value problem (3.1).*

*Proof. *By means of the Lemma 2.4, we can reduce (3.1) to an equivalent integral equation
From , we have and
Therefore, the unique solution of (3.1) is
The proof is finished.

Lemma 3.2. *The function defined by (3.3) has the following properties: *(1)*;*(2)* and for **where .*

*Proof. *Observing the expression of , it is clear that for . In the following, we consider .

For , we have

and

For , since , we have

Thus , for . Combining , we have
This completes the proof.

We note that is a solution of (1.1) if and only if

For our constructions, we will consider the Banach space equipped with standard norm . We define a cone by Define an integral operator by Notice from (3.13) and Lemma 3.2 that, for , on and then .

On the other hand, we have Thus, . In addition, standard arguments show that is completely continuous.

#### 4. Singular Positone Problems

In this section we present some new result for the singular problem where and nonlinearity may be singular at .

Using Theorem 2.5 we establish the following main result.

Theorem 4.1. *Suppose that the following conditions are satisfied. **
here is Green's function and
**
Then (4.1) has two nonnegative solutions with and for .*

*Proof. *First we will show that there exists a solution to (4.1) with for and Let
We now show
To see this, let . Then and for So for , we have
This together with (4.7) yields
so (4.12) is satisfied.

Next we show

To see this, let so , and let for .

We have

This together with (4.9) yields
Thus so (4.15) is held.

Now Theorem 2.5 implies that has a fixed-point , that is, and for . It follows from (4.12) and (4.15) that so we have

Similarly, if we put

we can show that there exists a solution to (4.1) with for and

This completes the proof of Theorem 4.1.

Similar to the proof of Theorem 4.1, we have the following result.

Theorem 4.2. *Suppose that (4.2)–(4.8) hold. In addition suppose
**
Then (4.1) has a nonnegative solution with and for .*

*Remark 4.3. *If in (4.19) we have , then (4.1) a nonnegative solution with and for .

It is easy to use Theorem 4.2 and Remark 4.3 to write theorems which guarantee the existence of more than two solutions to (4.1). We state one such result.

Theorem 4.4. *Suppose that (4.2)–(4.6) and (4.8) hold. Assume that and constants with , and
**
In addition suppose for each that
**
and
**
hold. Then (4.1) has nonnegative solutions with for .*

*Example 4.5. *Consider the boundary value problem
where is such that
here
Then (4.23) has two solutions with for

To see this we will apply Theorem 4.1 with (here will be chosen below)

Clearly (4.2)–(4.6) and (4.8) hold, and . Now (4.7) holds with since
Finally notice (4.9) is satisfied for small and large since
as , since . Thus all the conditions of Theorem 4.1 are satisfied so existence is guaranteed.

#### 5. Singular Semipositone Problems

In this section we present a new result for the singular semipositone problem: where and nonlinearity may be singular at .

Before we prove our main result, we first state a result.

Lemma 5.1. *Suppose with on . Then the boundary value problem,
**
has a solution with
**
here
**
In fact, from Lemma 3.1, (5.2) has solution
**
According to Lemma 3.2, we have
**
The above Lemma together with Theorem 2.5 establish our main result.*

Theorem 5.2. *Suppose that the following conditions are satisfied. **
here is any constant (choose and fix it) so that (note exists since in fact we can have ) ) and is Green's function and
**
Then (5.1) has a solution with for .*

*Proof. *To show that (5.1) has a nonnegative solution we will look at the boundary value problem
where
( is as in Lemma 5.1).

We will show, using Theorem 2.5, that there exists a solution to (5.16) with for . If this is true then is a nonnegative solution (positive on ) of (5.1), since

As a result, we will concentrate our study on (5.16). Let as in Section 2, and let
Next let be defined by
In addition, standard argument shows that and is completely continuous.

We now show

To see this, let . Then and for Now for , the Lemma 5.1 implies
so for we have
This together with (5.12) yields
so (5.21) is satisfied.

Next we show

To see this let so and for . Also for we have
As a result
We have
This together with (5.14) yields
Thus so (5.25) is held.

Now Theorem 2.5 implies that has a fixed-point , that is, and for . Thus is a solution of (5.16) with for . Thus (5.1) has a positive solution for .

*Example 5.3. *Consider the boundary value problem
where is such that
here

Then (5.30) has a solution with for

To see this we will apply Theorem 5.2 with (here will be chosen later, in fact here we choose so that works, i.e., we choose so that ),

Clearly (5.7)–(5.11) and (5.13) hold. Now (5.12) holds with since
from (5.31). Finally notice (5.14) is satisfied for large since
as , since . Thus all the conditions of Theorem 5.2 are satisfied so existence is guaranteed.

#### Acknowledgments

This paper is supported by Key Subject of Chinese Ministry of Education (no. 109051) and Scientific Research Fund of Heilongjiang Provincial Education Department (no. 11544032).