Asymptotically Almost Periodic Solutions for Abstract Partial Neutral Integro-Differential Equation
© José Paulo C. dos Santos et al. 2010
Received: 24 November 2009
Accepted: 24 February 2010
Published: 1 March 2010
The existence of asymptotically almost periodic mild solutions for a class of abstract partial neutral integro-differential equations with unbounded delay is studied.
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
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 , Hale and Lunel , Wu , 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  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.  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  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.
for every continuous and bounded; see [27, Proposition ] for details.
for each . Observe that this condition is verified, for example, if is a fading memory, see [27, Proposition ].
Let , and let be a nonnegative measurable function which satisfies the conditions (g-5) and (g-6) in the terminology of . 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.
The next lemmas are useful characterizations of a.p and a.a.p functions.
Lemma 2.7 (see [28, Theorem ]).
endowed with the norms and respectively. We know from the result in  that and are Banach spaces.
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).
In this work, we always assume that the following conditions are verified.
(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 .
We next study some preliminary properties needed to establish the existence of a resolvent operator for the problem (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
This complete the proof.
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.
In the next result, we denote by the fractional power of the operator (see  for details).
which ends the proof.
In the remainder of this section, we discuss the existence and regularity of solutions of
The next result has been established in .
Theorem 3.15 ([30, Theorem ]).
An immediate consequence of the above theorem is the uniqueness of classical solutions.
Motivated by (3.55), we introduce the following concept.
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.
Motivated by the theory of resolvent operator, we introduce the following concept of mild solution for (1.1).
Now, we can establish our existence result.
In this section, we study the existence of asymptotically almost periodic solutions of the partial neutral integro-differential system
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  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
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.
José Paulo C. dos Santos is partially supported by FAPEMIG/Brazil under Grant CEX-APQ-00476-09.
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