# Asymptotically Almost Periodic Solutions for Abstract Partial Neutral Integro-Differential Equation

- José Paulo C. dos Santos
^{1}Email author, - Sandro M. Guzzo
^{2}and - Marcos N. Rabelo
^{3}

**2010**:310951

**DOI: **10.1155/2010/310951

© José Paulo C. dos Santos et al. 2010

**Received: **24 November 2009

**Accepted: **24 February 2010

**Published: **1 March 2010

## Abstract

The existence of asymptotically almost periodic mild solutions for a class of abstract partial neutral integro-differential equations with unbounded delay is studied.

## 1. Introduction

In this paper, we study the existence of asymptotically almost periodic mild solutions for a class of abstract partial neutral integro-differential equations modelled in the form

where and , , are closed linear operators; is a Banach space; the history , , belongs to some abstract phase space defined axiomatically are appropriated functions.

The study of abstract neutral equations is motivated by different practical applications in different technical fields. The literature related to ordinary neutral functional differential equations is very extensive and we refer the reader to Chukwu [1], Hale and Lunel [2], Wu [3], and the references therein. As a practical application, we note that the equation

arises in the study of the dynamics of income, employment, value of capital stock, and cumulative balance of payment; see [1] for details. In the above system, is a real number, the state , , are continuous functions matrices, is a constant matrix, represents the government intervention, and the private initiative. We note that by assuming the solution is known on , we can transform this system into an abstract system with unbounded delay described as (1.1).

Abstract partial neutral differential equations also appear in the theory of heat conduction. In the classic theory of heat conduction, it is assumed that the internal energy and the heat flux depend linearly on the temperature and on its gradient . Under these conditions, the classic heat equation describes sufficiently well the evolution of the temperature in different types of materials. However, this description is not satisfactory in materials with fading memory. In the theory developed in [4, 5], the internal energy and the heat flux are described as functionals of and . The next system, see for instance [6–9], has been frequently used to describe this phenomenon,

In this system, is open, bounded, and with smooth boundary; ; represents the temperature in at the time ; is a physical constant , are the internal energy and the heat flux relaxation, respectively. By assuming that the solution is known on and , we can transform this system into an abstract system with unbounded delay described in the form (1.1).

Recent contributions on the existence of solutions with some of the previously enumerated properties or another type of almost periodicity to neutral functional differential equations have been made in [10, 11], for the case of neutral ordinary differential equations, and in [12–15] for partial functional differential systems.

The purpose of this work is to study the existence of asymptotically almost periodic mild solutions for the neutral system (1.1). To this end, we study the existence and qualitative properties of an exponentially stable resolvent operator for the integro-differential system

There exists an extensive literature related to the existence and qualitative properties of resolvent operator for integro-differential equations. We refer the reader to the book by Gripenberg et al. [16] which contains an overview of the theory for the case where the underlying space has finite dimension. For abstract integro-differential equations described on infinite dimensional spaces, we cite the Prüss book [17] and the papers [18–20], Da Prato et al. [21, 22], and Lunardi [9, 23]. To finish this short description of the related literature, we cite the papers [24–26] where some of the above topics for the case of abstract neutral integro-differential equations with unbounded delay are treated.

To the best of our knowledge, the study of the existence of asymptotically almost periodic solutions of neutral integro-differential equations with unbounded delay described in the abstract form (1.1) is an untreated topic in the literature and this is the main motivation of this article.

To finish this section, we emphasize some notations used in this paper. Let and be Banach spaces. We denote by the space of bounded linear operators from into endowed with norm of operators, and we write simply when . By , we denote the range of a map , and for a closed linear operator , the notation represents the domain of endowed with the graph norm, , . In the case , the notation stands for the resolvent set of and is the resolvent operator of . Furthermore, for appropriate functions and , the notation denotes the Laplace transform of and the convolution between and , which is defined by . The notation stands for the closed ball with center at and radius in . As usual, represents the subspace of formed by the functions which vanish at infinity.

## 2. Preliminaries

- (A)

(C2) If is a sequence in formed by functions with compact support such that uniformly on compact, then and as

Remark 2.1.

for every continuous and bounded; see [27, Proposition ] for details.

Definition 2.2.

Let be the -semigroup defined by on and on . The phase space is called a fading memory if as for each with .

Remark 2.3.

for each . Observe that this condition is verified, for example, if is a fading memory, see [27, Proposition ].

Example 2.4 (The phase space ).

Let , and let be a nonnegative measurable function which satisfies the conditions (g-5) and (g-6) in the terminology of [27]. Briefly, this means that is locally integrable, and there exists a nonnegative, locally bounded function on such that for all and , where is a set with Lebesgue measure zero. The space consists of all classes of functions such that is continuous on , Lebesgue-measurable, and is Lebesgue integrable on . The seminorm in is defined by

The space satisfies axioms (A), (A-1), and (B). Moreover, when and , we can take , , and , for ; see [27, Theorem ] for details.

Now, we need to introduce some concepts, definitions, and technicalities on almost periodical functions.

Definition 2.5.

Definition 2.6.

A function is asymptotically almost periodic (a.a.p.) if there exists an almost periodic function and such that .

The next lemmas are useful characterizations of a.p and a.a.p functions.

Lemma 2.7 (see [28, Theorem ]).

In this paper, and are the spaces

endowed with the norms and respectively. We know from the result in [28] that and are Banach spaces.

Next, and are abstract Banach spaces.

Definition 2.8.

- (c)

The next lemma summarizes some properties which are fundamental to obtain our results.

Lemma 2.9 (see [29, Theorem ]).

## 3. Resolvent Operators

In this section, we study the existence and qualitative properties of an exponentially resolvent operator for the integro-differential abstract Cauchy problem

The results obtained for the resolvent operator in this section are similar to those that can be found, for instance, in the papers [21, 23, 30]. In this paper, we prove the necessary estimates for the proof of an existence theorem of asymptotically almost periodic solutions for (1.1). For the better comprehension of the subject, we will introduce the following definitions, hypothesis, and results.

We introduce the following concept of resolvent operator for integro-differential problem (3.1).

Definition 3.1.

- (a)
- (b)

Definition 3.2.

A resolvent operator of (3.1) is called exponentially stable if there exist positive constants such that

In this work, we always assume that the following conditions are verified.

(H1) The operator is the infinitesimal generator of an analytic semigroup on , and there are constants , and such that and for all .

(H2) For all is a closed linear operator, , and is strongly measurable on for each . There exists such that exists for and for all and . Moreover, the operator valued function has an analytical extension (still denoted by ) to such that for all , and as .

(H3) There exist a subspace dense in and positive constants , such that , , and for every and all .

In the sequel, for , , and , set

with are oriented counterclockwise. In addition, is the set

We next study some preliminary properties needed to establish the existence of a resolvent operator for the problem (3.1).

Lemma 3.3.

Proof.

for sufficiently large. This proves (3.9) and completes the proof.

Observation 1.

If is a resolvent operator for (3.1), it follows from (3.3) that for all . Applying Lemma 3.3 and the properties of the Laplace transform, we conclude that is the unique resolvent operator for (3.1).

In the remainder of this section, and are numbers such that and . Moreover, we denote by a generic constant that represents any of the constants involved in the statements of Lemma 3.3 as well as any other constant that arises in the estimate that follows. We now define the operator family by

We will next establish that is a resolvent operator for (3.1).

Lemma 3.4.

The function is exponentially bounded in .

Proof.

This complete the proof.

Lemma 3.5.

The operator function is exponentially bounded in .

Proof.

From before and Lemma 3.4, we infer that is exponentially bounded in . The proof is finished.

Lemma 3.6.

The function is strongly continuous.

Proof.

where represent the curve for .

we can affirm that for all , which completes and the proof since is dense in and is bounded on .

Notice that the sectors from Lemma 3.3, is analytic. Consider the contours

and proceeding as before, we obtain for all which ends the proof.

The following result can be proved with an argument similar to that used in the proof of the preceding lemma with changing by

Lemma 3.7.

The function is strongly continuous.

Lemma 3.8.

Proof.

This property allows us to define the extension by this integral.

Similarly, the integral on the right hand side of (3.34) is also absolutely convergent in and strong, continuous on for . For ,

where is integrable for . From the Lebesgue dominated convergence theorem, we obtain that verifies (3.34). The proof is ended.

Lemma 3.9.

Proof.

Theorem 3.10.

The function is a resolvent operator for the system (3.1).

Proof.

Arguing as above but using the equality we obtain that (3.2) holds.

On the other hand, by Lemma 3.8 we infer that . Next, we analyze the differentiability on . Let and for all we can choose such that

which proves the existence of the right derivative of at zero and that This proves that resolvent equation (3.3) is valid for every and for every . This completes the proof.

Corollary 3.11.

If then the function is an exponentially stable resolvent operator for the system (3.1).

In the next result, we denote by the fractional power of the operator (see [32] for details).

Theorem 3.12.

Proof.

which ends the proof.

Corollary 3.13.

In the remainder of this section, we discuss the existence and regularity of solutions of

where . In the sequel, is the operator function defined by (3.17). We begin by introducing the following concept of classical solution.

Definition 3.14.

A function , , is called a classical solution of (3.53)-(3.54) on if , the condition (3.54) holds and (3.53) is verified on .

The next result has been established in [30].

Theorem 3.15 ([30, Theorem ]).

An immediate consequence of the above theorem is the uniqueness of classical solutions.

Corollary 3.16.

If are classical solutions of (3.53)-(3.54) on , then on .

Motivated by (3.55), we introduce the following concept.

Definition 3.17.

## 4. Existence Result of Asymptotically Almost Periodic Solutions

In this section, we study the existence of asymptotically almost periodic mild solutions for the abstract integro-differential system (1.1). To establish our existence result, motivated by the previous section we introduce the following assumptions.

(P_{1}) There exists a Banach space
continuously included in
such that the following conditions are verified.

(P_{3}) The continuous function
is p.a.a.p, and there exists a continuous function
such that

Motivated by the theory of resolvent operator, we introduce the following concept of mild solution for (1.1).

Definition 4.1.

Lemma 4.2.

Let condition hold and let be a function in . If is the function defined by then .

Proof.

Now, from inequality (4.6) and Lemma 2.7, we conclude that is a.a.p. The proof is complete.

Lemma 4.3.

Proof.

From inequality (4.11) and Lemma 2.7, we conclude that is a.a.p., which ends the proof.

Now, we can establish our existence result.

Theorem 4.4.

Assume that is a fading memory space and , and are held. If and for every , then there exists such that for each , there exists a mild solution, , of (1.1) on such that and .

Proof.

where is the function defined by the relation and on . From the hypothesis , and , we obtain that is well defined and that Moreover, from Lemmas 4.2 and 4.3 it follows that .

Next, we prove that is a contraction from into . If and , we get

which shows that is a contraction from into The assertion is now a consequence of the contraction mapping principle. The proof is complete.

## 5. Applications

In this section, we study the existence of asymptotically almost periodic solutions of the partial neutral integro-differential system

for , , and Moreover, we have identified .

To represent this system in the abstract form (1.1), we choose the spaces and ; see Example 2.4 for details. We also consider the operators , , given by , for Moreover, has discrete spectrum, the eigenvalues are , with corresponding eigenvectors , and the set of functions is an orthonormal basis of and for . For from [32] we can define the fractional power of is given by where In the next theorem, we consider . We observe that and for from [33, Proposition ], we obtain that is a sectorial operator satisfying Moreover, it is easy to see that conditions (H2)-(H3) in Section 3 are satisfied with , and is the space of infinitely differentiable functions that vanish at and . Under the above conditions, we can represent the system

in the abstract form

for all . This allows us to rewrite the system (5.1) in the abstract form (1.1) with

Theorem 5.1.

Assume that the previous conditions are verified. Let and such that then there exists a mild solution of (5.1) with .

Proof.

since . By using a similar procedure as in the proofs of Lemma 3.3 and Theorem 3.10, we obtain the existence of resolvent operator for (5.2). From the hypothesis, we obtain by the Lemma 3.3, Corollaries 3.11 and 3.13, the assumption is satisfied. From Theorem 4.4, the proof is complete.

## Declarations

### Acknowledgment

José Paulo C. dos Santos is partially supported by FAPEMIG/Brazil under Grant CEX-APQ-00476-09.

## Authors’ Affiliations

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