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Mathematical Problems in Engineering

Volume 2013 (2013), Article ID 916369, 17 pages

http://dx.doi.org/10.1155/2013/916369

## On Attractivity and Positivity of Solutions for Functional Integral Equations of Fractional Order

Department of Mathematics, Guangdong University of Education, Guangzhou 510310, China

Received 20 December 2012; Accepted 20 February 2013

Academic Editor: Sotiris Ntouyas

Copyright © 2013 Xianyong Huang and Junfei Cao. 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.

#### Abstract

We investigate a class of functional integral equations of fractional order given by , : sufficient conditions for the existence, global attractivity, and ultimate positivity of solutions of the equations are derived. The main tools include the techniques of measures of noncompactness and a recent measure theoretic fixed point theorem of Dhage. Our investigations are placed in the Banach space of continuous and bounded real-valued functions defined on unbounded intervals. Moreover, two examples are given to illustrate our results.

#### 1. Introduction

Nonlinear functional integral equations with bounded intervals have been studied extensively in the literature as regards various qualitative properties. This includes existence, uniqueness, stability, boundedness, monotonicity and extremality of solutions. But the study of nonlinear functional integral equations with unbounded intervals is relatively rare and exploited for the characteristics of attractivity and asymptotic attractivity of solutions. There are two methods for dealing with these characteristics of solutions, namely, classical fixed point theorems involving the hypotheses from analysis and topology and the fixed point theorems involving the use of measure of noncompactness. Each one of these methods has some advantages and disadvantages over the others [1–11].

The theory of integral equations of fractional order plays a very important role in describing some real world problems; it has recently received a lot of attention and now constitutes a significant branch of nonlinear analysis. In recent years, differential and integral equations of fractional order have found wide applications in physics, mechanics, engineering, electro chemistry, economics, and other fields [12–19]; meanwhile, numerous research papers and monographs have appeared devoted to differential and integral equations of fractional order [20–38]. These papers contain various qualitative properties such as existence, uniqueness, stability, and asymptotic behavior for equations of fractional order.

The aim of this paper is to study the existence, global attractivity, and positivity of solutions for a functional integral equation of fractional order. The mentioned equation has rather general form and contains, as particular cases, a lot of fractional functional equations and nonlinear fractional integral equations of Volterra type. The main technique used in our considerations is the measures of noncompactness and a fixed point theorem of Dhage [3]. Our investigations will be situated in the Banach space of real functions which are defined, continuous, and bounded on the right-hand real half axis .

The measures of noncompactness used in this paper allow us not only to obtain the existence of solutions of the mentioned fractional functional integral equations but also to characterize those solutions in terms of global attractivity and positivity on unbounded intervals. This assertion means that all possible solutions of the equations in question are globally uniformly attractive and positive in the sense which will be defined further on.

It is worthwhile mentioning that the novelty of our approach consists mainly in the possibility of obtaining the global attractivity, asymptotic attractivity, and positivity of solutions for the considered fractional functional integral equations. We hope that the concept of measure of noncompactness considered here may be a stimulant for further investigations concerning solutions of nonlinear fractional differential and integral equations of other types.

#### 2. Notations, Definitions, and Auxiliary Facts

This section is devoted to collect some notations, definitions, and auxiliary facts which will be used in the further considerations of this paper.

Let be a Banach space, , a class of subsets of and let denote the class of all nonempty subsets of with property . Here may be (in short cl), (in short bd), (in short rcp), and so forth. Thus, , , , and denote, respectively, the classes of closed, bounded, closed and bounded, and relatively compact subsets of . A function defined by satisfies all the conditions of a metric on and is called a Hausdorff-Pompeiu metric on , where . It is known that the hyperspace is a complete metric space.

The auxiliary way of defining the measures of noncompactness has been adopted in several papers in the literature; see Akhmerov et al. [39], Appell [40], Banaśand Goebel [41], in the works Väth [42] and the references therein. In this paper, we adopt the following axiomatic definition of the measure of noncompactness in a Banach space given by Dhage [3]. The other useful forms appear in the work of Banaś and Goebel [41] and the references therein.

Before giving definition of measure of noncompactness, we need the following definitions.

*Definition 1 (see [43]). *A sequence of nonempty sets in is said to converge to a set , called the limiting set if as . A mapping is called continuous if for any sequence in one has that

*Definition 2 (see [43]). *A mapping is called nondecreasing, if for with , then , where is an order relation by inclusion .

Now we are equipped with the necessary details to define the measures of noncompactness for a bounded subset of the Banach space .

*Definition 3 (see [43]). *A function is called a measure of noncompactness if it satisfies , , where is the closure of , , where is the convex hull of and is nondecreasing, and if is a decreasing sequence of sets in such that , then the limiting set is nonempty.

The family described in is said to be the kernel of and

A measure is called complete or full if the kernel of consists of all possible relatively compact subsets of . Next, a measure is called sublinear if it satisfies

There do exist the sublinear measures of noncompactness on Banach spaces . Indeed, the Kuratowskii and Hausdorff measures of noncompactness are sublinear in . A good collection of different types of measures of noncompactness appears in Appell [40].

Observe that the limiting set from is a member of the family . In fact, since
one infers that . This yields that . This simple observation will be essential in our further investigations.

Now we state a key fixed point theorem of Dhage [3] which will be used in the sequel. Before stating this fixed point result, we give a useful definition.

*Definition 4 (see [43]). *A mapping is called -set-Lipschitz if there exists a continuous nondecreasing function such that for all with , where . Sometimes we call the function to be a -function of on . In the special case, when , , is called a -set-Lipschitz mapping, and if , then is called a -set-contraction on . Further, if for , then is called a nonlinear -set-contraction on .

Theorem 5 (see, Dhage [43]). *Let be a nonempty, closed, convex, and bounded subset of a Banach space and let be a continuous and nonlinear -set-contraction. Then has a fixed point.*

*Remark 6. *Denote by the set of all fixed points of the operator which belong to . It can be shown that the set existing in Theorem 5 belongs to the family . In fact if , then and . Now from nonlinear -set-contraction it follows that which is a contradiction, since for . Hence, .

Our further considerations will be placed in the Banach space consisting of all real functions defined, continuous, and bounded on . This space is equipped with the standard supremum norm .

For our purposes we will use the Hausdorff or ball measure of noncompactness in . A handy formula for Hausdorff measure of noncompactness useful in application is defined as follows. Fix a nonempty and bounded subset of the space and a positive number . For and , denote by the modulus of continuity of the function on the closed and bounded interval defined by
Next, put

It is known that is a measure of noncompactness in the Banach space of continuous and real-valued functions defined on a closed and bounded interval in which is equivalent to Hausdorff or ball measure of noncompactness in it. In fact, one has for any bounded subset of (see Banaś and Goebel [41] and the references therein). Finally, define .

Now, for a fixed number , denote
Finally, consider the functions ’s defined on the family by the formulas

Let be fixed. Then for any , define . Similarly, for any bounded subset of , define
Define the functions by the formulas
for all .

*Remark 7. *It can be shown as in Banaś and Goebel [41] that the functions , , , , , and are measures of noncompactness in the space . The kernels , , and of the measures , , and consist of nonempty and bounded subsets of such that functions from are locally equicontinuous on and the thickness of the bundle formed by functions from tends to zero at infinity. Moreover, the functions from come closer along a line and the functions from come closer along the line as increases to through . A similar situation is also true for the kernels , , and of the measures of noncompactness , , and . Moreover, these measures , , and characterize the ultimate positivity of the functions belonging to the kernels of , , and . The above expressed property of , , , and , , permits us to characterize solutions of the fractional functional integral equations considered in the sequel.

In order to introduce further concepts used in this paper, let us assume that and let be a subset of . Let be an operator and consider the following operator equation in :

Below we give different characterizations of the solutions for (14) on .

*Definition 8. *We say that solutions of (14) are locally attractive if there exists a closed ball in the space for some such that for arbitrary solutions and of (14) belonging to one has that
In the case when the limit (15) is uniform with respect to the set , that is, when for each there exists such that
for all being solutions of (14) and for all , we will say that solutions of (14) are uniformly locally attractive on .

*Definition 9. *The solution of (14) is said to be globally attractive if (15) holds for each solution of (14) on . In other words, we may say that solutions of (14) are globally attractive, if for arbitrary solutions and of (14) on , the condition (15) is satisfied. In the case when the condition (15) is satisfied uniformly with respect to the set , that is, if for every there exists such that the inequality (16) is satisfied for all being the solutions of (14) and for all , we will say that solutions of (14) are uniformly globally attractive on .

The following definitions appear in the work of Dhage [7].

*Definition 10. *A line , where is a real number, is called an attractor for a solution to (14) if . In this case the solution to (14) is also called to be asymptotic to the line and the line is an asymptote for the solution on .

Now we introduce the following definitions which are useful in the sequel.

*Definition 11. *The solutions of (14) are said to be globally asymptotically attractive if for any two solutions and of (14), the condition (15) is satisfied, and there is a line which is a common attractor to them on . In the case when condition (15) is satisfied uniformly, that is, if for every there exists such that the inequality (16) is satisfied for all and for all being the solutions of (14) and having a line as a common attractor, we will say that solutions of (14) are uniformly globally asymptotically attractive on .

*Definition 12. *A solution of (14) is called locally ultimately positive if there exists a closed ball in for some such that and
In the case when the limit (17) is uniform with respect to the solution set of the operator equation (14), that is, when for each there exists such that
for all being solutions of (14) and for all , we will say that solutions of (14) are uniformly locally ultimately positive on .

*Definition 13. *A solution of (14) is called globally ultimately positive if (17) is satisfied. In the case when the limit (17) is uniform with respect to the solution set of the operator equation (14) in , that is, when for each there exists such that (18) is satisfied for all being solutions of (14) and for all , we will say that solutions of (14) are uniformly globally ultimately positive on .

*Remark 14. *Note that the global attractivity and global asymptotic attractivity imply, respectively, the local attractivity and local asymptotic attractivity of the solutions for the operator equation (14) on . Similarly, global ultimate positivity implies local ultimate positivity of the solutions for the operator equation (14) on unbounded intervals. However, the converse of the above two statements may not be true. A few details of ultimate positivity are given in the work of Dhage [44].

Finally, we introduce the concept of the fraction integral and the Riemann-Liouville fractional derivative.

*Definition 15 (see [45, 46]). *The fractional integral of order with the lower limit for a function is defined as
provided the right-hand side is pointwise on , where is the Gamma function.

*Definition 16 (see [45, 46]). *The Riemann-Liouville derivative of order with the lower limit for a function can be written as

The first and maybe the most important property of the Riemann-Liouville fractional derivative is that for and , one has , which means that the Riemann-Liouville fractional differentiation operator is a left inverse to the Riemann-Liouville fractional integration operator of the same order .

In the following section we prove the main results of this paper.

#### 3. Attractivity and Positivity Results

In this section we will investigate the following functional integral equation of fractional order with deviating arguments: where , , , , , , and denotes the Gamma function.

Equation (21) has rather general form, when Equation (21) reduces to the following quadratic Volterra integral equation of fractional order Equation (23) has been studied in the work of Banaś and O’Regan [47] for the existence and local attractivity of solutions via classical hybrid fixed point theory, when Equation (21) reduces to the following functional integral equation of fractional order considered in Balachandran et al. [48] for the local attractivity of solutions Therefore, (21) is more general and contains as particular cases a lot of fractional functional equations and nonlinear fractional integral equations of Volterra type.

By a solution of (21) we mean a function that satisfies (21), where is the space of continuous real-valued functions defined on .

Equation (21) will be considered under the following assumptions.

the functions are continuous and satisfy

the function is continuous and bounded.

the function is continuous and there exists a bounded function with bound and a positive constant such that

for all and . Moreover, we assume that .

the function is continuous and there exists a function being continuous on and such that

for all and .

the function is bounded with .

the function is continuous and there exists a function being continuous on and a function being continuous and nondecreasing on with such that

for all and .

For further purposes, define the functions by putting and by putting . Obviously the functions and are continuous on .

the functions defined by the formulas

are bounded on and the functions , , , vanish at infinity, that is, Keeping in mind assumption , define the following finite constants:

Now we formulate the last assumption.

There exists a positive solution of the inequality

*Remark 17. *Hypothesis is satisfied if in particular satisfies the condition,
for all and , where , and the function is defined as in hypothesis which further yields the usual Lipschitz condition on the function ,
for all and provided . As mentioned in the work of Dhage [11], our hypothesis is more general than that existing in the literature.

Now, consider the operators , , , and defined on the space :

Then one has the following lemma.

Lemma 18. *Under the above assumptions the operator transforms the ball in the space into itself. Moreover, all solutions of (21) belonging to the space are fixed points of the operator .*

*Proof. *Observe that for any function , and are continuous on . We show that the same holds also for . Take an arbitrary function and fix , . Next assume that such that . Without loss of generality one can assume that . Then, in view of imposed assumptions, one has
where
Obviously, in view of the uniform continuity of on the set , one has that as . In what follows, denote
Then, keeping in mind the estimate (37) one obtains
From the above inequality one can infer that the function is continuous on the interval for any . This yields the continuity of on .

Finally, combining the continuity of the functions , , and , one deduces that the function is continuous on on .

Now, taking an arbitrary function , then, using our assumptions, for a fixed , one has
Hence, in view of assumption one can infer that the function is bounded on . This assertion in conjunction with the continuity of on allows us to conclude that . Moreover, from the estimate (41) one obtains
Linking this estimate with assumption , one deduces that there exists such that the operator transforms the ball into itself.

Finally, let us notice that the second assertion of our lemma is obvious in the light of the fact that the operator transforms the space into itself. The proof is complete.

Now, we are prepared to state and prove our main theorem of this section.

Theorem 19. *Under the above assumptions (H _{0})–(H_{7}), (21) has at least one solution in the space . Moreover, these solutions are globally uniformly attractive on .*

*Proof. *In what follows we will consider the operator as a mapping from into itself. Now we show that the operator is continuous on the ball . To do this, fix arbitrarily and take such that . Then one gets
Now, linking the above established facts one concludes that the operator maps continuously the closed ball into itself.

Further, taking a nonempty subset of the ball , fixing arbitrarily and and choosing and with , without loss of generality we may assume that . Then, taking into account our assumptions and , one gets
where
Moreover, mention that other notations used in the above estimate were introduced earlier.

From the above estimate one can derive the following inequality:
Observe that , , and as , which is a simple consequence of the uniform continuity of the functions , , , and on the sets