International Journal of Differential Equations

Volume 2018, Article ID 8243180, 23 pages

https://doi.org/10.1155/2018/8243180

## Existence of Asymptotically Almost Automorphic Mild Solutions of Semilinear Fractional Differential Equations

^{1}Department of Mathematics, Guangdong University of Education, Guangzhou 510303, China^{2}School of Mathematical Sciences, Guangxi Teachers Education University, Nanning 530023, China^{3}Department of Mathematics, Morgan State University, Baltimore, MD 21251, USA

Correspondence should be addressed to Gaston M. N’Guérékata; moc.loa@atakereugn

Received 21 December 2017; Revised 18 April 2018; Accepted 10 May 2018; Published 1 August 2018

Academic Editor: Patricia J. Y. Wong

Copyright © 2018 Junfei Cao et al. 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

This paper is concerned with the existence of asymptotically almost automorphic mild solutions to a class of abstract semilinear fractional differential equations where , is a linear densely defined operator of sectorial type on a complex Banach space and is a bounded linear operator defined on , is an appropriate function defined on phase space, and the fractional derivative is understood in the Riemann-Liouville sense. Combining the fixed point theorem due to Krasnoselskii and a decomposition technique, we prove the existence of asymptotically almost automorphic mild solutions to such problems. Our results generalize and improve some previous results since the (locally) Lipschitz continuity on the nonlinearity is not required. The results obtained are utilized to study the existence of asymptotically almost automorphic mild solutions to a fractional relaxation-oscillation equation.

#### 1. Introduction

The almost periodic function introduced seminally by Bohr in 1925 plays an important role in describing the phenomena that are similar to the periodic oscillations which can be observed frequently in many fields, such as celestial mechanics, nonlinear vibration, electromagnetic theory, plasma physics, engineering, and ecosphere. The concept of almost automorphy, which is an important generalization of the classical almost periodicity, was first introduced in the literature [1–4] by Bochner in relation to some aspects of differential geometry. Since then, this pioneer work has attracted more and more attention and has been substantially extended in several different directions. Many authors have made important contributions to this theory (see, for instance, [5–17] and the references therein). Especially, in [5, 6], the authors gave an important overview about the theory of almost automorphic functions and their applications to differential equations.

As a natural extension of almost automorphy, the concept of asymptotic almost automorphy, which is the central issue to be discussed in this paper, was introduced in the literature [18] by N’Guérékata in the early eighties. Since then, this notion has found several developments and has been generalized into different directions. Until now, the asymptotically almost automorphic functions as well as the asymptotically almost automorphic solutions for differential systems have been investigated by many mathematicians; see [19] by Bugajewski and N’Guérékata, [20] by Diagana, Hernández, and dos Santos, and [21] by Ding, Xiao, and Liang for the asymptotically almost automorphic solutions to integrodifferential equations, see [22] by Zhao, Chang, and N’Guérékata for the asymptotically almost automorphic solutions to the nonlinear delay integral equations, and see [23] by Chang and Tang and [24] by Zhao, Chang, and Nieto for the asymptotically almost automorphic solutions to stochastic differential equations, and the existence of asymptotically almost automorphic solutions has become one of the most attractive topics in the qualitative theory of differential equations due to its significance and applications in physics, mathematical biology, control theory, and so on. We refer the reader to the monographs of N’Guérékata [25] for the recently theory and applications of asymptotically almost automorphic functions.

With motivation coming from a wide range of engineering and physical applications, fractional differential equations have recently attracted great attention of mathematicians and scientists. This kind of equations is a generalization of ordinary differential equations to arbitrary noninteger orders. Fractional differential equations find numerous applications in the field of viscoelasticity, feedback amplifiers, electrical circuits, electro analytical chemistry, fractional multipoles, neuron modelling encompassing different branches of physics, chemistry, and biological sciences [26–32]. Many physical processes appear to exhibit fractional order behavior that may vary with time or space. In recent years, there has been a significant development in ordinary and partial differential equations involving fractional derivatives; we only enumerate here the monographs of Kilbas et al. [26, 27], Diethelm [28], Hilfer [29], Podlubny [30], Miller [31], and Zhou [32] and the papers of Agarwal et al. [33, 34], Benchohra et al. [35, 36], El-Borai [37], Lakshmikantham et al. [38–41], Mophou et al. [42–45], N’Guérékata [46], and Zhou et al. [47–50] and the reference therein.

The study of almost periodic and almost automorphic type solutions to fractional differential equations was initiated by Araya and Lizama [11]. In their work, the authors investigated the existence and uniqueness of an almost automorphic mild solution of the semilinear fractional differential equation when is a generator of an -resolvent family and is the Riemann-Liouville fractional derivative. In [51], Cuevas and Lizama considered the fractional differential equation:where is a linear operator of sectorial negative type on a complex Banach space and the fractional derivative is understood in the Riemann-Liouville sense. Under suitable conditions on , the authors proved the existence and uniqueness of an almost automorphic mild solution to (2). Cuevas et al. [52, 53] studied, respectively, the pseudo almost periodic and pseudo almost periodic class infinity mild solutions to (2) assuming that and is a pseudo almost periodic and pseudo almost periodic of class infinity function satisfying suitable conditions in . Agarwal et al. [54] studied the existence and uniqueness of a weighted pseudo almost periodic mild solution to equation (2). Ding et al. [55] investigated the existence and uniqueness of almost automorphic solution to (2) assuming that and is Stepanov-like almost automorphic in satisfying some kind of Lipschitz conditions. Cuevas et al. [56] studied the existence of almost periodic (resp., pseudo almost periodic) mild solutions to equation (2) assuming that and is Stepanov almost (resp., Stepanov-like pseudo almost) periodic in uniformly for . Chang et al. [57] studied the existence and uniqueness of weighted pseudo almost automorphic solution to equation (2) with Stepanov-like weighted pseudo almost automorphic coefficient. He et al. [58] studied also the existence and uniqueness of weighted Stepanov-like pseudo almost automorphic mild solution to (2). Cao et al. [59] studied the existence and uniqueness of antiperiodic mild solution to (2). In [60], Cuevas et al. showed sufficient conditions to ensure the existence and uniqueness of mild solution for (2) in the following classes of vector-valued function spaces: periodic functions, asymptotically periodic functions, pseudo periodic functions, almost periodic functions, asymptotically almost periodic functions, pseudo almost periodic functions, almost automorphic functions, asymptotically almost automorphic functions, pseudo almost automorphic functions, compact almost automorphic functions, asymptotically compact almost automorphic functions, pseudo compact almost automorphic functions, -asymptotically -periodic functions, decay functions, and mean decay functions.

Recently, Xia et al. [61] established some sufficient criteria for the existence and uniqueness of -pseudo almost automorphic solution to the semilinear fractional differential equation where , is a sectorial operator of type on a complex Banach space and is a bounded linear operator. The fractional derivative is understood in the Riemann-Liouville sense. Their discussion is divided into two cases, i.e., , is -pseudo almost automorphic and , and is Stepanov-like -pseudo almost automorphic. Kavitha et al. [62] studied weighted pseudo almost automorphic solutions of the fractional integrodifferential equation where and is a linear densely defined sectorial operator on a complex Banach space , , and is a weighted pseudo almost automorphic function in for each satisfying suitable conditions. The fractional derivative is understood in the Riemann-Liouville sense. Mophou [63] investigated the existence and uniqueness of weighted pseudo almost automorphic mild solution to the fractional differential equation:where is a linear densely operator of sectorial type on a complex Banach space , is a bounded linear operator and , and is a weighted pseudo almost automorphic function in for each satisfying suitable conditions. The fractional derivative is to be understood in Riemann-Liouville sense. Chang et al. [64] investigated some existence results of -pseudo almost automorphic mild solutions to (6) assuming that and is a -pseudo almost automorphic function in for each satisfying suitable conditions. For more on the almost periodicity and almost automorphy for fractional differential equations and related issues, we refer the reader to [65–67] and others.

Equation (6) is motivated by physical problems. Indeed, due to their applications in fields of science where characteristics of anomalous diffusion are presented, type (6) equations are attracting increasing interest (cf. [68–70] and references therein). For example, anomalous diffusion in fractals [69] or in macroeconomics [71] has been recently well studied in the setting of fractional Cauchy problems like (6). For this reason, (6) has gotten a considerable attention in recent years (cf. [51–64, 68–71] and the references therein).

To the best of our knowledge, much less is known about the existence of asymptotically almost automorphic mild solutions to (6) when the nonlinearity as a whole loses the Lipschitz continuity with respect to and . Motivated by the abovementioned works, the purpose of this paper is to establish some new existence results of asymptotically almost automorphic mild solutions to (6). In our results, the nonlinearity , does not have to satisfy a (locally) Lipschitz condition (see Remark 22). However, in many papers (for instance, [11, 51–64]) on almost periodic type and almost automorphic type solutions to fractional differential equations, to be able to apply the well-known Banach contraction principle, a (locally) Lipschitz condition for the nonlinearity of corresponding fractional differential equations is needed. As can be seen, our results generalize those as well as related research and have more broad applications. In particular, as application and to illustrate our main results, we will examine some sufficient conditions for the existence of asymptotically almost automorphic mild solutions to the fractional relaxation-oscillation equation given by with boundary conditions , , where is a function and , , and are positive constants.

The rest of this paper is organized as follows. In Section 2, some concepts, the related notations, and some useful lemmas are introduced and established. In Section 3, we prove the existence of asymptotically almost automorphic mild solutions to such problems. The results obtained are utilized to study the existence of asymptotically almost automorphic mild solutions to a fractional relaxation-oscillation equation given in Section 4.

#### 2. Preliminaries

This section is concerned with some notations, definitions, lemmas, and preliminary facts which are used in what follows.

From now on, let and be two Banach spaces and (resp., ) is the space of all -valued bounded continuous functions (resp., jointly bounded continuous functions ). Furthermore, (resp., ) is the closed subspace of (resp., ) consisting of functions vanishing at infinity (vanishing at infinity uniformly in any compact subset of , in other words, where is an any compact subset of ). Let also be the Banach space of all bounded linear operators from into itself endowed with the norm: For a bounded linear operator , let and stand for the resolvent and domain of , respectively.

First, let us recall some basic definitions and results on almost automorphic and asymptotically almost automorphic functions.

*Definition 1 ((Bochner) [1] (N’Guérékata) [6]). *A continuous function is said to be almost automorphic if for every sequence of real numbers , there exists a subsequence such that is well defined for each and

Denote by the set of all such functions.

*Remark 2 (see [6]). *By the point-wise convergence, the function in Definition 1 is measurable but not necessarily continuous. Moreover, if is continuous, then is uniformly continuous (cf., e.g., [17], Theorem 2.6), and if the convergence in Definition 1 is uniform on , one gets almost periodicity (in the sense of Bochner and von Neumann). Almost automorphy is thus a more general concept than almost periodicity. There exists an almost automorphic function which is not almost periodic. The function given by is an example of such functions [72].

Lemma 3 (see [5]). * is a Banach space with the norm .*

*Definition 4 (see [6]). *A continuous function is said to be almost automorphic in uniformly for all , where is any bounded subset of , if for every sequence of real numbers , there exists a subsequence such that and The collection of those functions is denoted by .

*Remark 5. *The function given by is almost automorphic in uniformly for all , where is any bounded subset of , .

Similar to Lemma 2.2 of [73] and Proposition 3.2 of [63], we have the following result on almost automorphic functions.

Lemma 6. *Let be almost automorphic in uniformly for all , where is any bounded subset of , and assume that is uniformly continuous on uniformly for , that is, for any , there exists such that and imply that Let be almost automorphic. Then the function defined by is almost automorphic.*

*Proof. *Suppose that is a sequence of real numbers. Then by the definition of almost automorphic functions, we can extract a subsequence of such that Write Then Since and are almost automorphic, then , and , and are bounded. Therefore we can choose a bounded subset , such that By , , and the uniform continuity of in , we have Moreover, by , so remembering the above triangle inequality, we deduce that Using the same argument we can prove that This proves that is almost automorphic by the definition.

*Remark 7. *If satisfies a Lipschitz condition with respect to and uniformly in , i.e., for each pair , uniformly in , where is called the Lipschitz constant for the function , then is uniformly continuous on uniformly for , where is any bounded subset of .

*Remark 8. *If satisfies a local Lipschitz condition with respect to and uniformly in , i.e., for each pair , , where , then is uniformly continuous on uniformly for , where is any bounded subset of .

*Definition 9 (see [6]). *A continuous function is said to be asymptotically almost automorphic if it can be decomposed as , where

Denote by the set of all such functions.

*Remark 10. *The function defined by is an asymptotically almost automorphic function with

Lemma 11 (see [6]). * is also a Banach space with the supremum norm .*

*Definition 12 (see [6]). *A continuous function is said to be asymptotically almost automorphic if it can be decomposed as , where

Denote by the set of all such functions.

*Remark 13. *The function given by is asymptotically almost automorphic in uniformly for all , where is any bounded subset of , and

Next we give some basic definitions and properties of the fractional calculus theory which are used further in this paper.

*Definition 14 (see [26]). *The fractional integral of order with the lower limit for a function is defined as provided that the right-hand side is point-wise defined on , where is the Gamma function.

*Definition 15 (see [26]). *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 Riemann-Liouville fractional derivative is that, for and , one has , which means that Riemann-Liouville fractional differentiation operator is a left inverse to the Riemann-Liouville fractional integration operator of the same order .

It is important to define sectorial operator for the definition of mild solution of any fractional abstract equations. So, let us now give the definitions of sectorial linear operators and their associated solution operators.

*Definition 16 ([74] sectorial operator). *A closed and linear operator is said to be sectorial of type and angle if there exist , , and such that its resolvent exists outside the sector and

Sectorial operators are well studied in the literature, usually for the case . For a recent reference including several examples and properties we refer the reader to [74]. Note that an operator is sectorial of type if and only if is sectorial of type 0.

*Definition 17 (see [75]). *Let be a closed and linear operator with domain defined on a Banach space . We call the generator of a solution operator if there are and a strongly continuous function such that and In this case, is called the solution operator generated by .

Note that if is sectorial of type with , then is the generator of a solution operator given by where is a suitable path lying outside the sector (cf. [74]).

Very recently, Cuesta in [74](Theorem 1) has proved that if is a sectorial operator of type for some and , then there exists such that In the border case , this is analogous to saying that is the generator of a exponentially stable -semigroup. The main difference is that in the case the solution family decays like . Cuesta’s result proves that is, in fact, integrable.

In the following, we present the following compactness criterion, which is a special case of the general compactness result of Theorem 2.1 in [76].

Lemma 18 (see [76]). *A set is relatively compact if*(1)* is equicontinuous;*(2)* uniformly for ;*(3)*the set is relatively compact in for every .*

*The following Krasnoselskii’s fixed point theorem plays a key role in the proofs of our main results, which can be found in many books.*

*Lemma 19 (see [77]). Let be a bounded closed and convex subset of and be maps of into such that for every pair . If is a contraction and is completely continuous, then has a solution on .*

*3. Asymptotically Almost Automorphic Mild Solutions*

*In this section, we study the existence of asymptotically almost automorphic mild solutions for the semilinear fractional differential equations of the formwhere is a linear densely defined operator of sectorial type of on a complex Banach space , is a bounded linear operator and , and is a given function to be specified later. The fractional derivative is to be understood in Riemann-Liouville sense.*

*We recall the following definition that will be essential for us.*

*Definition 20 (see [63]). *Assume that generates an integrable solution operator . A continuous function satisfying the integral equation is called a mild solution on to (39).

*In the proofs of our results, we need the following auxiliary result.*

*Lemma 21. Given and , letThen , .*

*Proof. *Firstly, note that Then which implies that is well defined and continuous on . Since , then for any and every sequence of real numbers , there exist a subsequence , a function , and such that Define Then for each and every . This implies thatis well defined for each .

By a similar argument one can obtain Thus .

Since , one can choose an such that for all . This enables us to conclude that, for all , which implies On the other hand, from it follows that there exists an such that . This enables us to conclude that, for all , which implies

*Now we are in position to state and prove our first main result. To prove our main result, let us introduce the following assumptions:*

* with and there exists a constant such that, for all and ,*

*() There exist a function and a nondecreasing function such that, for all and with ,*

*Remark 22. *Assuming that satisfies the assumption (), it is noted that does not have to meet the Lipschitz continuity with respect to and . Such class of asymptotically almost automorphic functions are more complicated than those with Lipschitz continuity with respect to and and little is known about them.

Let be the function involved in assumption (). Define

*Lemma 23. .*

*Proof. *Since , one can choose a such that for all . This enables us to conclude that, for all , which implies On the other hand, from it follows that there exists a such that for all . This enables us to conclude that, for all , which implies

*Theorem 24. Assume that is sectorial of type . Let satisfy the hypotheses () and (). Put . Then (39) has at least one asymptotically almost automorphic mild solution provided that*

*Proof. *The proof is divided into the following five steps.*Step 1*. Define a mapping on byand prove has a unique fixed point .

Firstly, since the function is bounded in and this implies that exists. Moreover from satisfying (54), together with Lemma 6 and Remark 7, it follows that This, together with Lemma 21, implies that is well defined and maps into itself.

In the sequel, we verify that is continuous.

Let be in with as ; then one has Therefore, as and , hence is continuous.

Next, we prove that is a contraction on and has a unique fixed point .

In fact, let be in , and similar to the above proof of the continuity of , one has which implies Together with (61), this proves that is a contraction on . Thus, Banach’s fixed point theorem implies that has a unique fixed point .*Step 2*. Set For the above , define on asand prove that maps into itself, where is a given constant.

Firstly, from (54) it follows that, for all and , which implies that According to (55), one has