Research

# Approximate controllability of fractional integro-differential equations involving nonlocal initial conditions

NI Mahmudov* and S Zorlu

Author Affiliations

Eastern Mediterranean University, via Mersin 10, Gazimagusa, T.R. North Cyprus, Turkey

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Boundary Value Problems 2013, 2013:118  doi:10.1186/1687-2770-2013-118

 Received: 12 December 2012 Accepted: 22 March 2013 Published: 8 May 2013

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

We discuss the approximate controllability of nonlinear fractional integro-differential system under the assumptions that the corresponding linear system is approximately controllable. Using the fixed-point technique, fractional calculus and methods of controllability theory, a new set of sufficient conditions for approximate controllability of fractional integro-differential equations are formulated and proved. The results in this paper are generalization and continuation of the recent results on this issue. An example is provided to show the application of our result.

### 1 Introduction

Controllability is one of the fundamental concepts in mathematical control theory, which plays an important role in control systems. The controllability of nonlinear systems represented by evolution equations or inclusions in abstract spaces and qualitative theory of fractional differential equations has been extensively studied by several authors. An extensive list of these publications can be found in [1-44] and the references therein. Recently, the approximate controllability for various kinds of (fractional) differential equations has generated considerable interest. A pioneering work on the approximate controllability of deterministic and stochastic systems has been reported by Bashirov and Mahmudov [5], Dauer and Mahmudov [8] and Mahmudov [10]. Sakthivel et al.[28] studied the approximate controllability of nonlinear deterministic and stochastic evolution systems with unbounded delay in abstract spaces. On the other hand, the fractional differential equation has gained more attention due to its demonstrated applications in numerous seemingly diverse and widespread fields of science and engineering. Yan [45] derived a set of sufficient conditions for the controllability of fractional-order partial neutral functional integro-differential inclusions with infinite delay in Banach spaces. Debbouche and Baleanu [1] established the controllability result for a class of fractional evolution nonlocal impulsive quasi-linear delay integro-differential systems in a Banach space using the theory of fractional calculus and fixed point technique. However, there exists only a limited number of papers on the approximate controllability of the fractional nonlinear evolution systems. Sakthivel et al.[28] studied the approximate controllability of deterministic semilinear fractional differential equations in Hilbert spaces. Wang [40] investigated the nonlocal controllability of fractional evolution systems. Surendra Kumar and Sukavanam [33] obtained a new set of sufficient conditions for the approximate controllability of a class of semilinear delay control systems of fractional order using the contraction principle and the Schauder fixed-point theorem. More recently, Sakthivel et al.[27] derived a new set of sufficient conditions for approximate controllability of fractional stochastic differential equations.

In this paper, we discuss the approximate controllability of nonlinear fractional integro-differential system under the assumption that the corresponding linear system is approximately controllable. We consider the following fractional integro-differential control system involving nonlocal conditions,

(1)

in , where , , stands for the Caputo fractional derivative of order β, and , , , are given functions to be specified later. Here, is the infinitesimal generator of a compact analytic semigroup of bounded linear operators , , on a real Hilbert space X. B is a linear bounded operator from a real Hilbert space U to X.

The rest of this paper is organized as follows. In Section 2, we give some preliminary results on the fractional powers of the generator of an analytic compact semigroup and introduce the mild solution of system (1). In Section 3, we study the existence of mild solutions for system (1) under the feedback control defined in (5). We show that the control system (1) is approximately controllable on provided that the corresponding linear system is approximately controllable. Finally, an example is given to demonstrate the applicability of our result.

### 2 Preliminaries

In this section, we introduce some facts about the fractional powers of the generator of a compact analytic semigroup, the Caputo fractional derivative that are used throughout this paper.

We assume that X is a Hilbert space with norm . Let be the Banach space of continuous functions from into X with the norm , here . In this paper, we also assume that is the infinitesimal generator of a compact analytic semigroup , , of uniformly bounded linear operator in X, that is, there exists such that for all . Without loss of generality, let , where is the resolvent set of A. Then for any , we can define by

It follows that each is an injective continuous endomorphism of X. Hence we can define , which is a closed bijective linear operator in X. It can be shown that each has dense domain and that for . Moreover, for every and with , where , I is the identity in X. (For proofs of these facts, we refer to the literature [15,20,22].)

We denote by the Hilbert space of equipped with norm for , which is equivalent to the graph norm of . Then we have , for (with ) and the embedding is continuous. Moreover, has the following basic properties.

Lemma 1[42]

andhave the following properties.

(i) for eachand.

(ii) for eachand.

(iii) For every, is bounded inXand there existssuch that

(iv) is a bounded linear operator for.

Let us recall the following known definitions of fractional calculus. For more details, see [43,44].

Definition 2 The fractional integral of order with the lower limit 0 for a function f is defined as

provided the right-hand side is pointwise defined on , where Γ is the gamma function.

Definition 3 The Caputo derivative of order with the lower limit 0 for a function f can be written as

The Caputo derivative of a constant is equal to zero. If f is an abstract function with values in X then the integrals which appear in Definitions 2 and 3 are taken in Bochner’s sense.

According to Definitions 2 and 3, it is suitable to rewrite the problem (1) in the equivalent integral equation

(2)

provided that the integral in (2) exists. Applying the Laplace transform

to (2) and using the method similar to that used in [38] we get

where

Here, is a probability density function defined on , that is , and .

For , we define two families and of operators by

respectively.

The following lemma follows from the results given in [37-39].

Lemma 4The operatorsandhave the following properties.

(i) For any fixed, and any, we have the operatorsandare linear and bounded operators, i.e. for any,

(ii) The operatorsandare strongly continuous for all.

(iii) andare norm continuous inXfor.

(iv) andare compact operators inXfor.

(v) For every, the restriction oftoand the restriction oftoare norm continuous.

(vi) For every, the restriction oftoand the restriction oftoare compact operators in.

(vii) For alland,

In this paper, we adopt the following definition of mild solution of equation (1).

Definition 5 A function is said to be a mild solution of (1) if for any the integral equation

(3)

is satisfied.

It is clear that is bounded if . In what follows, we assume that .

### 3 Approximate controllability

In this section, we state and prove conditions for the approximate controllability of semilinear fractional control integro-differential systems. To do this, we first prove the existence of a fixed point of the operator defined below using Krasnoselskii’s fixed-point theorem. Secondly, in Theorem 11, we show that under the uniform boundedness of f and g the approximate controllability of fractional systems (1) is implied by the approximate controllability of the corresponding linear system (4).

Let be the state value of (1) at terminal time T corresponding to the control u and the initial value . Introduce the set , which is called the reachable set of system (1) at terminal time T, its closure in is denoted by .

Definition 6 The system (1) is said to be approximately controllable on if , that is, given an arbitrary it is possible to steer from the point to within a distance ε from all points in the state space at time T.

Consider the following linear fractional differential system:

(4)

The approximate controllability for linear fractional system (4) is a natural generalization of approximate controllability of linear first order control system [9,10,12]. It is convenient at this point to introduce the controllability and resolvent operators associated with (4) as

respectively, where denotes the adjoint of B and is the adjoint of . It is straightforward that the operator is a linear bounded operator.

Theorem 7[10]

LetZbe a separable reflexive Banach space and letstands for its dual space. Assume thatis symmetric. Then the following two conditions are equivalent:

1. is positive, that is, for all nonzero.

2. For allstrongly converges to zero as. Here, Jis the duality mapping ofZinto.

Lemma 8The linear fractional control system (4) is approximately controllable onif and only ifasin the strong operator topology.

Proof The lemma is a straightforward consequence of Theorem 7. Indeed, the system (4) is approximately controllable on if and only if for all nonzero , see [7]. By Theorem 7, as for all . □

Remark 9 Notice that positivity of is equivalent to . In other words, since , approximate controllability of the linear system (4) is equivalent to , .

Before proving the main results, let us first introduce our basic assumptions.

(H1) are continuous and for each , there exists a constant and functions , such that

(H2) is a Lipschitz function with Lipschitz constant .

(Hc) The linear system (4) is approximately controllable on .

Using the hypothesis (Hc), for an arbitrary function , we choose the feedback control function as follows:

(5)

Let , where r is a positive constant. Then is clearly a bounded closed and convex subset in . We will show that when using the above control the operator defined by

where

has a fixed point in .

Theorem 10Let the assumptions (H1) and (H2) be satisfied. Then for, the fractional Cauchy problem (1) withhas at least one mild solution provided that

(6)

where

Proof It is easy to see that for any the operator maps into itself.

Let and . Using assumption (H1) yield the following estimations,

and

(7)

From (6) and the assumption (H2), it follows that for any there exists such that

(8)

Therefore, from (7) and (8), it follows that for any there exists such that for every . Therefore, for any the fractional Cauchy problem (1) with the control (5) has a mild solution if and only if the operator has a fixed point in .

In what follows, we will show that and satisfy the conditions of Krasnoselskii’s fixed-point theorem. From (H2) and (6), we infer that is a contraction. Next, we show that is completely continuous on .

Step 1: We first prove that is continuous on . Let be a sequence such that as in . Therefore, it follows from the continuity of f, g and that for each ,

Also, by (H1), we see that

Since

using the Lebesgue dominated convergence theorem that for all , we conclude

implying that as . This proves that is continuous on .

Step 2. is compact on .

For the sake of brevity, we write

Let be fixed and be small enough. For , we define the map

Therefore, from Lemma 4, we see that for each , the set is relatively compact in . Since

approaches to zero as , using the total boundedness, we conclude that for each , the set is relatively compact in .

On the other hand, for and small enough, we have

where

Therefore, it follows from (H1) and Lemma 4 that

and

from which it is easy to see that all , , tend to zero independent of as and . Thus, we can conclude that

and the limit is independent of . The case is trivial. Consequently, the set is equicontinuous. Now applying the Arzela-Ascoli theorem, it results that is compact on .

Therefore, applying Krasnoselskii’s fixed-point theorem, we conclude that has a fixed point, which gives rise to a mild solution of Cauchy problem (1) with control given in (5). This completes the proof. □

Theorem 11Let the assumptions (H1), (H2) and (Hc) be satisfied. Moreover, assume the functionsandare bounded and. Then the semilinear fractional system (3) is approximately controllable on.

Proof It is clear that all assumptions of Theorem 10 are satisfied with . Let be a fixed point of in . Any fixed point of is a mild solution of (3) under the control

and satisfies the equality

(9)

where

Moreover, by the boundedness of the functions f and g and Dunford-Pettis theorem, we have that the sequences and are weakly compact in , so there are subsequences still denoted by and , that weakly converge to, say, f and g in . On the other hand, there exists such that converges to weakly in . Denote

It follows that

as because of compactness of the operator

Then from (9), we obtain

(10)

as . This proves the approximate controllability of (1). □

### 4 Applications

Example 1 As an application to Theorem 11, we study the following simple example. Consider a control system governed by the fractional partial differential equation of the form

(11)

where , , .

Let us take and define the operator A by with the domain . Then

where , ,  . Clearly −A generates a compact analytic semigroup , in X and it is given by

Clearly, the assumption (H1) is satisfied. On the other hand, it can be easily seen that the deterministic linear system corresponding to (11) is approximately controllable on ; see [12].

The operator is given by

where and .

Let , where for . Assume that satisfies the following conditions:

1. The functions , are continuous and uniformly bounded.

2. .

3. is continuously differentiable, and

Denote by , the Mittag-Leffler special function defined by

Therefore,

Define

Then, for each we have

and

It follows that is bounded and Lipschitz continuous. On the other hand, it is not difficult to verify that are continuous.

Next, we show that the linear system corresponding to (11) is approximately controllable on . It is clear that is defined as follows:

By Remark 9, the linear system corresponding to (11) is approximately controllable on if and only if , implies that . This follows from the representation of .

Now, we note that the problem (11) can be reformulated as the abstract problem. Thus, by Theorem 11, the system (11) is approximately controllable on , provided that

### Competing interests

The authors declare that they have no competing interests.

### Authors’ contributions

All authors contributed equally and significantly in writing this paper. All authors read and approved the final manuscript.

### Acknowledgements

Dedicated to Professor Hari M Srivastava.

The authors would like to thank the reviewers for their valuable comments and helpful suggestions that improved the note’s quality.

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