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# Strong Stability and Asymptotical Almost Periodicity of Volterra Equations in Banach Spaces

*Advances in Difference Equations***volume 2011**, Article number: 414906 (2011)

## Abstract

We study strong stability and asymptotical almost periodicity of solutions to abstract Volterra equations in Banach spaces. Relevant criteria are established, and examples are given to illustrate our results.

## 1. Introduction

Owing to the *memory behavior* (cf., e.g., [1, 2]) of materials, many practical problems in engineering related to *viscoelasticity* or *thermoviscoelasticity* can be reduced to the following Volterra equation:

in a Banach space , with being the infinitesimal generator of a -semigroup defined on , and a scalar function ( and ), which is often called *kernel function* or *memory kernel* (cf., e.g., [1]). It is known that the above equation is well-posed. This implies the existence of the *resolvent* operator , and the mild solution is then given by

which is actually a classical solution if . In the present paper, we investigate strong stability and asymptotical almost periodicity of the solutions. For more information and related topics about the two concepts, we refer to the monographs [3, 4]. In particular, their connections with the vector-valued Laplace transform and theorems of Widder type can be found in [4–6]. Recall the following.

Definition 1.1.

Let be a Banach space and : a bounded uniformly continuous function.

(i) is called *almost periodic* if it can be uniformly approximated by linear combinations of (). Denote by the space of all almost periodic functions on .

(ii) is called *asymptotically almost periodic* if with and . Denote by the space of all asymptotically almost periodic functions on .

(iii)We call (1.1) or *strongly stable* if, for each . We call (1.1) or asymptotically almost periodic if for each .

The following two results on -semigroup will be used in our investigation, among which the first is due to Ingham (see, e.g., [7, Section 1] and the second is known as *Countable Spectrum Theorem* [3, Theorem ]. As usual, the letter denotes the imaginary unit and the imaginary axis.

Lemma 1.2.

Suppose that generates a bounded -semigroup on a Banach space . If , then

Lemma 1.3.

Let be a bounded -semigroup on a reflexive Banach space with generator . If is countable, then is asymptotically almost periodic.

## 2. Results and Proofs

Asymptotic behaviors of solutions to the special case of have been studied systematically, see, for example, [3, Chapter 4] and [8, Chapter V]. The following example shows that asymptotic behaviors of solutions to (1.1) are more complicated even in the finite-dimensional case.

Example 2.1.

Let in (1.1). Then taking Laplace transform we can calculate

It is clear that the following assertions hold.

(a)The corresponding semigroup is exponentially stable.

(b)Each solution with initial value is *not* strongly stable and hence not exponentially stable.

(c)Each solution with is asymptotically almost periodic.

It is well known that the *semigroup approach* is useful in the study of (1.1). More information can be found in the book [8, Chapter VI.7] or the papers [9–11].

Let be the product Banach space with the norm

for each and . Then the operator matrix

generates a -semigroup on Here, is the vector-valued Sobolev space and the Dirac distribution, that is, for each ; the operator is given by

Denote by the -semigroup generated by . It follows that, for each , the first coordinate of

is the unique solution of (1.1).

Theorem 2.2.

Let be the generator of a -semigroup on the Banach space and with . Assume that

is a left-shift invariant closed subspace of such that for all ;

and

for some constant . Here, : .

Then

(1.1) is strongly stable if ;

if is reflexive and , then every solution to (1.1) is asymptotically almost periodic provided is countable.

Proof.

Since the first coordinate of (2.5) is the unique solution of (1.1), it is easy to see that the strong stability and asymptotic almost periodicity of (1.1) follows from the strong stability and asymptotic almost periodicity of , respectively.

Moreover, from [9, Proposition 2.8]) we know that if is a closed subspace of such that is -invariant and for all , then (the restriction of to ) generates the -semigroup

which is defined on the Banach space

Thus, by assumptions (i), (ii) and the well-known Hille-Yosida theorem for -semigroups, we know that is bounded. Hence, in view of Lemma 1.2, we get

Clearly

for each . So, combining (2.5) with (2.9), we have

This means that (a) holds.

On the other hand, we note that, to get (b), it is sufficient to show that is asymptotically almost periodic. Actually, if is reflexive and , then it is not hard to verify that is reflexive. Hence, is reflexive. By assumption (i), is a closed subspace of . Thus, Pettis's theorem shows that is also reflexive. Hence, in view of Lemma 1.3, we get (b). This completes the proof.

Corollary 2.3.

Let be the generator of a -semigroup on the Banach space and . Assume that

(i)for each ,

(ii)there exists a constant satisfying

with

Then

if and

for each , then (1.1) is strongly stable;

if is reflexive and , then (1.1) is asymptotically almost periodic provided

is countable.

Proof.

As in [9, Section 3], we take

In view of the discussion in [8, Lemma ], we can infer that

Moreover, we have

Hence,

with being defined as in (2.13). Thus, it is clear that is bounded if (2.12) is satisfied.

Next, for , we consider the eigenequation

Writing and , we see easily that (2.20) is equivalent to

Thus, if and

then by (2.21) we obtain

By the closed graph theorem, the operator

in the second equality of (2.23) is bounded. Hence, noting that

we have

Consequently, in view of (a) of Theorem 2.2, we know that (1.1) is strongly stable if (2.14) holds.

Furthermore, by [9, Lemma 3.3], we have

Combining this with (b) of Theorem 2.2, we conclude that (1.1) is asymptotically almost periodic if is reflexive, , and the set in (2.15) is countable.

Theorem 2.4.

Let be the generator of a -semigroup on the Banach space and with . Assume that

for all ,

is bounded on , where ,

is analytic on and are bounded on , where

for each is the set of half-line spectrum of and ,

for each and , the limit

exists uniformly for .

Then every solution to (1.1) is asymptotically almost periodic. Moreover, if for each and the limit in (2.30) equals 0 uniformly for , then (1.1) is strongly stable.

Proof.

Take . Then the solution to (1.1) is Lipschitz continuous and hence uniformly continuous. Actually, by assumption (i), we know that

and that

is analytic on . Thus, and

On the other hand, if (2.28) holds, then there exists such that

Hence, from [4, Chapter 1] (or [5]) and the uniqueness of the Laplace transform, it follows that

satisfies

and that

Moreover, by [3, Corollary ], the assumption (i) implies the boundedness of . Therefore,

is bounded and uniformly continuous on . In addition, the half-line spectrum set of is just the following set:

Write . Then

From assumption (ii) and [3, Corollary ], it follows that is bounded, which implies

uniformly for . Finally, combining (2.40) with Theorem [7, Theorem 4.1], we complete the proof.

## 3. Applications

In this section, we give some examples to illustrate our results.

First, we apply Corollary 2.3 to Example 2.1. As one will see, the previous result will be obtained by a different point of view.

Example 3.1.

We reconsider Example 2.1. First, we note that

This implies that condition is not satisfied. Therefore, part (a) of Corollary 2.3 is not applicable, and this explains partially why the corresponding Volterra equation is not strongly stable. However, it is easy to check that conditions (i) and (ii) in Corollary 2.3 are satisfied. In particular, we have accordingly

and hence the estimate

Note and

Applying part (b) of Corollary 2.3, we know that the corresponding Volterra equation is asymptotically almost periodic.

Example 3.2.

Consider the Volterra equation

where the constants satisfy

Let , and define

Then (3.5) can be formulated into the abstract form (1.1). It is well known that is self-adjoint (see, e.g., [12, page 280, (b) of Example 3]) and that generates an analytic -semigroup. The self-adjointness of implies

On the other hand, we can compute

It follows immediately that condition (i) in Corollary 2.3 holds. Moreover, corresponding to (2.13), we have

Combining this with (3.6), (3.8), and (3.9), we estimate

Note that (2.15) becomes

Applying part (b) of Corollary 2.3, by (3.11), we conclude that (3.5) is asymptotically almost periodic (cf. [9, Remark 3.6]).

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## Acknowledgments

This work was supported partially by the NSF of China (11071042) and the Research Fund for Shanghai Key Laboratory for Contemporary Applied Mathematics (08DZ2271900).

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### Keywords

- Banach Space
- Mild Solution
- Strong Stability
- Infinitesimal Generator
- Volterra Equation