Though boundary value problems for fractional differential equations have been extensively studied, most of the studies focus on scalar equations and the fractional order between 1 and 2. On the other hand, delay is natural in practical systems. However, not much has been done for fractional differential equations with delays. Therefore, in this paper, we consider a boundary value problem of a general delayed nonlinear fractional system. With the help of some fixed point theorems and the properties of the Green function, we establish several sets of sufficient conditions on the existence of positive solutions. The obtained results extend and include some existing ones and are illustrated with some examples for their feasibility.
In the past decades, fractional differential equations have been intensively studied. This is due to the rapid development of the theory of fractional differential equations itself and the applications of such construction in various sciences such as physics, mechanics, chemistry, and engineering [1, 2]. For the basic theory of fractional differential equations, we refer the readers to [3–7].
Recently, many researchers have devoted their attention to studying the existence of (positive) solutions of boundary value problems for differential equations with fractional order [8–23]. We mention that the fractional order involved is generally in with the exception that in [12, 23] and in [8, 17]. Though there have been extensive study on systems of fractional differential equations, not much has been done for boundary value problems for systems of fractional differential equations [18–20].
On the other hand, we know that delay arises naturally in practical systems due to the transmission of signal or the mechanical transmission. Though theory of ordinary differential equations with delays is mature, not much has been done for fractional differential equations with delays [24–31].
As a result, in this paper, we consider the following nonlinear system of fractional order differential equations with delays,
where is the standard Riemann-Liouville fractional derivative of order for some integer , for , for , and is a nonlinear function from to . The purpose is to establish sufficient conditions on the existence of positive solutions to (1.1) by using some fixed point theorems and some properties of the Green function. By a positive solution to (1.1) we mean a mapping with positive components on such that (1.1) is satisfied. Obviously, (1.1) includes the usual system of fractional differential equations when for all and . Therefore, the obtained results generalize and include some existing ones.
The remaining part of this paper is organized as follows. In Section 2, we introduce some basics of fractional derivative and the fixed point theorems which will be used in Section 3 to establish the existence of positive solutions. To conclude the paper, the feasibility of some of the results is illustrated with concrete examples in Section 4.
We first introduce some basic definitions of fractional derivative for the readers' convenience.
The fractional integral of order of a function is defined as
provided that the integral exists on , where is the Gamma function.
Note that has the semigroup property, that is,
The Riemann-Liouville derivative of order of a function is given by
provided that the right-hand side is pointwise defined on , where .
It is well known that if then . Furthermore, if and then for .
The following results on fractional integral and fractional derivative will be needed in establishing our main results.
Lemma 2.3 (see ).
Let . Then solutions to the fractional equation can be written as
where , .
Lemma 2.4 (see ).
Let . Then
for some , .
Now, we cite the fixed point theorems to be used in Section 3.
Lemma 2.5 (the Banach contraction mapping theorem ).
Let be a complete metric space and let be a contraction mapping. Then has a unique fixed point.
Let be a closed and convex subset of a Banach space . Assume that is a relatively open subset of with and is completely continuous. Then at least one of the following two properties holds:
(i) has a fixed point in ;
(ii)there exists and with .
Let be a cone in a Banach space . Assume that and are open subsets of with and . Suppose that is a completely continuous operator such that either
(i) for and for
(ii) for and for .
Then has a fixed point in .
3. Existence of Positive Solutions
Throughout this paper, we let . Then is a Banach space, where
In this section, we always assume that .
System (1.1) is equivalent to the following system of integral equations:
It is easy to see that if satisfies (3.2) then it also satisfies (3.2). So, assume that is a solution to (1.1). Integrating both sides of the first equation of (1.1) of order with respect to gives us
for , . It follows that
for , . This, combined with the boundary conditions in (1.1), yields
Similarly, one can obtain
. Then it follows from (3.8) and the boundary condition that
Therefore, for ,
This completes the proof.
The following two results give some properties of the Green functions .
For is continuous on and for .
Obviously, is continuous on . It remains to show that for . It is easy to see that for . We only need to show that for . For , let
Note that and for . It follows that and hence for .
Therefore, for and the proof is complete.
(i) If , then for .
(ii) If , then for .
(i) Obviously, for . Now, for , we have
where is the function defined by (3.11). It follows that for . In summary, we have proved (i).
(ii) Again, one can easily see that for . When , we have in this case that
which implies that for . To summarize, we have proved (ii) and this completes the proof.
Now, we are ready to present the main results.
Suppose that there exist functions , , 2, , such that
for , ,. If
then (1.1) has a unique positive solution.
It is easy to see that is a complete metric space. Define an operator on by
Because of the continuity of and , it follows easily from Lemma 3.2 that maps into itself. To finish the proof, we only need to show that is a contraction. Indeed, for , by (3.16) we have
This, combined with Lemma 3.3 and (3.17) and (3.18), immediately implies that is a contraction. Therefore, the proof is complete with the help of Lemmas 3.1 and 2.5.
The following result can be proved in the same spirit as that for Theorem 3.4.
For , suppose that there exist nonnegative function and nonnegative constants such that and
for , . If
then (1.1) has a unique positive solution.
For , suppose that there exist nonnegative real-valued functions such that
for almost every and all . If
then (1.1) has at least one positive solution.
Let and be defined by (3.19) and (3.20), respectively. We first show that is completely continuous through the following three steps.
Show that is continuous. Let be a sequence in such that . Then is bounded in . Since is continuous, it is uniformly continuous on any compact set. In particular, for any , there exists a positive integer such that
for and . Then, for , we have
for and . Therefore,
which implies that is continuous.
Show that maps bounded sets of into bounded sets. Let be a bounded subset of . Then is bounded. Since is continuous, there exists an such that
It follows that, for and ,
Immediately, we can easily see that is a bounded subset of .
Show that maps bounded sets of into equicontinuous sets. Let be a bounded subset of . Similarly as in Step 2, there exists such that
Then, for any and and ,
Now the equicontituity of on follows easily from the fact that is continuous and hence uniformly continuous on .
Now we have shown that is completely continuous. To apply Lemma 2.6, let
Fix and define
We claim that there is no such that for some . Otherwise, assume that there exist and such that . Then
If , then
Similarly, we can have if . To summarize, , a contradiction to . This proves the claim. Applying Lemma 2.6, we know that has a fixed point in , which is a positive solution to (1.1) by Lemma 3.1. Therefore, the proof is complete.
As a consequence of Theorem 3.6, we have the following.
If all , , are bounded, then (1.1) has at least one positive solution.
To state the last result of this section, we introduce
Suppose that there exist and positive constants with such that
(ii), for ,
where . Then (1.1) has at least a positive solution.
Let be defined by (3.19) and . Obviously, is a cone in . From the proof of Theorem 3.6, we know that the operator defined by (3.20) is completely continuous on . For any , it follows from Lemma 3.3 and condition (ii) that
On the other hand, for any , it follows from Lemma 3.3 and condition (i) that, for ,
if , whereas
if . In summary,
Therefore, we have verified condition (ii) of Lemma 2.7. It follows that has a fixed point in , which is a positive solution to (1.1). This completes the proof.
In this section, we demonstrate the feasibility of some of the results obtained in Section 3.
One can easily see that (3.16) is satisfied with
It follows from Theorem 3.4 that (4.1) has a unique positive solution on .
Hence, and satisfy (3.25). Moreover, simple calculations give us
Choose , and
Then, for and , we have
for and , we have
By now we have verified all the assumptions of Theorem 3.8. Therefore, (4.6) has at least one positive solution satisfying .
Supported partially by the Doctor Foundation of University of South China under Grant no. 5-XQD-2006-9, the Foundation of Science and Technology Department of Hunan Province under Grant no. 2009RS3019 and the Subject Lead Foundation of University of South China no. 2007XQD13. Research was partially supported by the Natural Science and Engineering Re-search Council of Canada (NSERC) and the Early Researcher Award (ERA) Pro-gram of Ontario.
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