Research Article | Open Access

# Complete Controllability for Fractional Evolution Equations

**Academic Editor:**Simeon Reich

#### Abstract

The paper is concerned with the complete controllability of fractional evolution equation with nonlocal condition by using a more general concept for mild solution. By contraction fixed point theorem and Krasnoselskii's fixed point theorem, we obtain some sufficient conditions to ensure the complete controllability. Our obtained results are more general to known results.

#### 1. Introduction

Fractional differential equations have recently been proved to be valuable tools in the modeling of many phenomena in various fields of science and engineering. It draws a great application in nonlinear oscillations of earthquakes and many physical phenomena such as seepage flow in porous media and in fluid dynamic traffic model. There has been a significant development in fractional differential equations in recent years, see the monographs of Kilbas et al. [1], Miller and Ross [2], Podlubny [3], Lakshmikantham et al. [4], and the papers [5–14] and the references therein.

Some recent papers investigated the problem of the existence of a mild solution for abstract differential equation with fractional derivative [15–23]. However, the results in [15, 16, 18, 19] are incorrect since the considered variation of constant formulas is not appropriate [17]. Zhou and Jiao [22, 23] introduced two characteristic solution operators and gave a suitable concept on a mild solution by applying Laplace transform and probability density functions. But the condition that the analytic semigroup was uniformly bounded was too strong. Shu et al. [20] researched the existence of mild solutions for impulsive fractional partial differential equation. But, Fekan et al. [24] had pointed out that the definition of solution of impulsive fractional differential equation was not correct. By using Laplace transform, Shu and Wang [25] gave a definition of mild solution for fractional differential equation with order and investigated its existence. Agarwal et al. [26] studied the existence and dimension of the set for mild solutions of semilinear fractional differential equations inclusions.

In 1960, Kalman first introduced the concept of controllability which leads to some very important results regarding the behavior of linear and nonlinear dynamical systems. There are various works of complete controllability of systems represented by differential equations, integrodifferential equations, differential inclusions, neutral functional differential equations, and impulsive differential inclusions in Banach spaces (see [8, 27–29] and the references therein). Recently, more and more researchers also pay attention to study the controllability of fractional order evolution systems (see [21, 30, 31] and the references and therein). Unfortunately, the concept of mild solutions used in [30, 31] was not suitable for fractional evolution systems at all and the corresponding definition of mild solutions is only a simple extension of the mild solutions of integer order systems. Wang and Zhou [21] investigated the complete controllability of fractional evolution systems with two characteristic solution operators introduced by them.

The nonlocal condition can be applied in physics with better effect than the classical initial condition . Nonlocal condition was initiated by Byszewski [32] when he proved the existence and uniqueness of mild and classical solutions of nonlocal Cauchy problems. As remarked by Byszewski and Lakshmikantham [33], the nonlocal condition can be more useful than the standard initial condition to describe some physical phenomena.

Inspired by the above discussions, in this paper, we consider a class of fractional evolution equations. By using a more general definition of mild solution, we obtain some sufficient conditions to ensure the complete controllability.

We consider the following fractional evolution equations: where is the Caputo fractional derivative of order , the state takes value in a Banach space , the control function is given in , with as a Banach space, is a bounded linear operator from into , is a sectorial operator on , and are given functions satisfying some assumptions, and .

The rest of this paper is organized as follows. In Section 2, some notations and preparations are given. A suitable concept on a mild solution for our problem is introduced. In Section 3, the complete controllability results are obtained by using fixed point theorems. Some conclusions are given in Section 4.

#### 2. Preliminaries

In this section, we will firstly introduce fractional integral and derivative, some notations about sectorial operators, solution operators, and analytic solution operators and then give the definition of a mild solution of system (1).

Throughout this paper, , denote the sets of real and complex numbers, respectively, and . By , we denote the space of all continuous functions from to . is the space of all bounded linear operators from to . denotes domain of , while means resolvent set of and stands for the resolvent operator of .

*Definition 1 (see [3]). *The fractional integral of order with the lower limit for a function is defined as
provided that the right side is point-wise defined on , where is the gamma function.

*Definition 2 (see [3]). *The Riemann-Liouville derivative of order for a function is defined as

*Definition 3 (see [3]). *The Caputo derivative of order for a function is defined as
Let .

*Remark 4. *(i) If , then

The Caputo derivative of a constant is equal to zero.

If is an abstract function with values in , then the integrals which appear in Definitions 1 and 2 are taken in Bochner’s sense.

*Definition 5. *An operator is said to be sectorial if there are constants , , and such that the resolvent of exists outside the sector
with

Consider the following Cauchy problem for the Caputo derivative evolution equation of order :

*Definition 6 (see [7]). *A family is called a solution operator for system (8), if the following conditions are satisfied:(a) is strongly continuous, for and .(b) and , for all and .(c) is a solution of the following integral equation:
for all and .

*Remark 7 (see [7]). *If is the solution operator of system (8), then
where consists of those for which this limit exists. We call the infinitesimal generator of or say that generates .

*Remark 8 (see [7]). *The solution operator of system (8) is defined as follows:
where is a suitable path such that , for .

A operator is said to belong to or , if system (8) has solution operator satisfying , . Denote and .

*Definition 9 (see [7]). *A solution operator of system (8) is called analytic, if admits an analytic extension to a sectorial for some . An analytic solution operator is said to be of analyticity type , if, for each and , there is an such that , . Denote

Lemma 10 (see [7]). *Let ; a linear closed densely defined operator belongs to , if , for each and, for any , , there is a constant such that
**
Next, we consider the definition of the mild solution of system (1).*

According to Definitions 1 and 2, it is suitable to rewrite the nonlocal Cauchy problem (1) in the equivalent integral equation provided that the integral in (14) exists.

The following Lemma 11 is discussed in [20]; for the sake of completeness, we outline its proof here.

Lemma 11. *If (14) holds and is a sectorial operator, then we have
**
where**
and is a suitable path such that , for .*

*Proof. *By applying the Laplace transform to (14), we have
Since exists, that is, , from the above equation, we obtain
Therefore, by the Laplace inverse transform, we have

Lemma 12. *If and , then the operators and are continuous on .*

*Proof. *For , by Lemma 10, we have
choose the integration path as follows:
such that is oriented counterclockwise, where , , and .

From (20), we have
By noticing that , by the dominated convergence theorem, we have as , which implies that is continuous on . For the same reason, is too continuous on . The proof is complete.

Lemma 13 (see [20]). *If and , then, for any , we have
*

If , then and , for all . Let we have

Lemma 14 (Krasnoselskii’s fixed point theorem). *Let be a Banach space, let be a bounded closed and convex subset of , and let and be maps of into such that for every pair . If is a contraction and is completely continuous, then the equation has a solution on .*

In [34], Reich gave a general fixed point theorem which contained Krasnoselskii’s fixed point theorem, for more details we can see the reference.

*Definition 15. *A function is called a mild solution of system (1), if satisfies the following equation
where
and is a suitable path such that , for .

*Remark 16. *When , is a semigroup and system (1) degenerates into 1 order evolution equation. However, the limits of and in [21–23] did not exist as .

*Remark 17. *When generates a semigroup in system (1), we have
where and is a probability density function defined on in [21–23]. So, this definition is more general to that in [21–23].

*Remark 18. *It is easy to verify that a classical solution of system (1) is a mild solution of the same system.

*Definition 19. *The system (1) is said to be completely controllable on , if, for every , there exists a control , such that a mild solution of system (1) satisfies .

In this paper, we assume the following. is continuous and there exist constant and function such that
for all and ; is continuous and there exists a constant such that
for all .

It is easy to see that if holds, then the following assumption holds: is continuous and there exist positive constants and such that
for all ; the operator family is compact;the linear operator is bounded; defined by
has an inverse operator which takes values in and there exist two positive constants such that

#### 3. Complete Controllability Results

Theorem 20. *Suppose that , , and are satisfied; then system (1) is completely controllable on , provided that and
*

*Proof. *Using hypothesis for an arbitrary function , we defined the control function by
We show that using this control, the operator on by
has a fixed point , which is a mild solution of system (1).

It is obvious that , which means that steers the mild from to in finite time . This implies that system (1) is completely controllable on . Next, we will prove that has a fixed point on .

Taking and, for all , we have, from , , and (35),
by (25), we have
Hence, is a contraction mapping and has a unique fixed point . Therefore, this is a mild solution of system (1). The proof is complete.

Theorem 21. *Suppose that , , , and are satisfied; then system (1) is completely controllable on provided that and
*

*Proof. *Define
for and any . Taking into account (35), by , , and , we have
In order to make the following process clear, we divide it into several steps.* I*. For and any , we have
By the condition , we can find such that, for ,
* II*. is a contraction mapping on .

For any and , we have
From the condition , we obtain , which implies that is a contraction mapping.* III*. is a completely continuous operator.

First, we will prove that is continuous on . Let with . By , we have
So, we have
which implies that is continuous.

Next, we will show that is relatively compact. It suffices to show that the family of function is uniformly bounded and equicontinuous and, for any , is relatively compact.

For any , we have which implies that is uniformly bounded. In the following, we will show that is a family of equicontinuous functions.

For any and , we have
From Lemma 12, we have independently of as , which means that is equicontinuous.

By the compactness of , we know that is relatively compact. Therefore, is relatively compact by Arzela-Ascoli theorem. The continuity of and relative compactness of imply that is a completely continuous operator. By using Krasnoselskii’s fixed point theorem, we obtain that has a fixed point on . Therefore, the nonlocal Cauchy problem (1) has at least one mild solution. The proof is complete.

When there is no control term, system (1) degenerates to the following system: As a direct result of Theorems 20 and 21, we have the following corollaries.

Corollary 22. *Suppose that , , and are satisfied; then system (48) has a unique mild solution, if and
*

Corollary 23. *Suppose that , , , and are satisfied; then system (48) has at least one mild solution, if and
*

#### 4. Conclusions

In this paper, we introduce a more general definition for mild solution of fractional evolution equation with nonlocal condition based on solution operator. By contraction fixed point theorem and Krasnoselskii’s fixed point theorem, we obtain some sufficient conditions to ensure the complete controllability for system (1). Here, we do not require the operator to be the infinitesimal generator of an analytic semigroup of uniform boundedness. So, the results we obtained are more general. For fractional evolution equation with Riemann-Liouville derivative, since it is equipped with a singular initial, it will be a difficult problem.

#### Conflict of Interests

The authors declare that there is no conflict of interests regarding the publishing of this paper.

#### Acknowledgments

The authors would like to thank the Editor and the anonymous reviewers for their helpful comments and suggestions. This work was supported by the National Natural Science Foundation of China (11271309 and 11301451), the Specialized Research Fund for the Doctoral Program of Higher Education (20114301110001), and the Key Projects of Hunan Provincial Natural Science Foundation of China (12JJ2001).

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#### Copyright

Copyright © 2014 Xia Yang and Haibo Gu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.