Volume 2013, Issue 1 138068
Research Article
Open Access

Asymptotic Behavior of Solutions to Abstract Stochastic Fractional Partial Integrodifferential Equations

Zhi-Han Zhao

Corresponding Author

Zhi-Han Zhao

Department of Mathematics and Information Engineering, Sanming University, Sanming, Fujian 365004, China

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Yong-Kui Chang

Yong-Kui Chang

Department of Mathematics, Lanzhou Jiaotong University, Lanzhou 730070, China lzjtu.edu.cn

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Juan J. Nieto

Juan J. Nieto

Departamento de Análisis Matemático, Facultad de Matemáticas, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain

Department of Mathematics, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia

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First published: 15 December 2013
Citations: 3
Academic Editor: Dumitru Baleanu

Abstract

The existence of asymptotically almost automorphic mild solutions to an abstract stochastic fractional partial integrodifferential equation is considered. The main tools are some suitable composition results for asymptotically almost automorphic processes, the theory of sectorial linear operators, and classical fixed point theorems. An example is also given to illustrate the main theorems.

1. Introduction

This paper is mainly concerned with the existence and uniqueness of square-mean asymptotically almost automorphic mild solutions to the following stochastic fractional partial integrodifferential equation in the form
()
where 1 < α < 2, A : D(A) ⊂ L2(, ) → L2(, ) is a linear densely defined operator of sectorial type on a Hilbert space L2(, ), W(t) is a two-sided standard one-dimensional Brownian motion defined on the filtered probability space (Ω, , , t), where t = σ{W(u) − W(v); u, vt}, and u0 is an 0-adapted, -valued random variable independent of the Wiener process W. Here f and g are appropriate functions to be specified later. The convolution integral in (1) is understood in the Riemann-Liouville fractional integral (see, e.g., [1, 2]). We notice that fractional order can be complex in viewpoint of pure mathematics and there is much interest in developing the theoretical analysis and numerical methods to fractional equations, because they have recently proven to be valuable in various fields of science and engineering (see, e.g., [311] and references therein).

The concept of asymptotically almost automorphic functions was firstly introduced by N’Guérékata in [12]. Since then these functions have become of great interest to several mathematicians and gained lots of developments and applications, we refer the reader to [1316] and the references listed therein.

Recently, the existence of almost automorphic and pseudo almost automorphic solutions to some stochastic differential equations has been considered in many publications such as [1727] and the references therein. In a very recent paper [28], the authors introduced a new notation of square-mean asymptotically almost automorphic stochastic processes including a composition theorem. However, to the best of our knowledge, the existence of square-mean asymptotically almost automorphic mild solutions to the problem (1) is an untreated topic. Therefore, motivated by the works [16, 28], the main purpose of this paper is to investigate the existence and uniqueness of square-mean asymptotically almost automorphic mild solutions to the problem (1). Then, we present an example as an application of our main results.

The rest of this paper is organized as follows. In Section 2, we recall some basic definitions and facts which will be used throughout this paper. In Section 3, we prove some existence results of square-mean asymptotically almost automorphic mild solutions to the problem (1). Finally, we give an example as an application of our abstract results.

2. Preliminaries

In this section, we introduce some basic definitions, notations, and preliminary facts which will be used in the sequel. For more details on this section, we refer the reader to [2830].

Throughout the paper, (, ∥·∥, 〈·, ·〉) stands for a real separable Hilbert space. (Ω, , ) denotes a complete probability space, and L2(, ) stands for the space of all -valued random variables x such that
()
Note that L2(, ) is a Hilbert space equipped with the norm
()
We denote by C0(+; L2(, )) the collection of all bounded continuous stochastic processes φ from + into L2(, ) such that lim t→+Eφ(t)∥2 = 0. It is then easy to check that C0(+; L2(, )) is a Banach space when it is endowed with the norm . Similarly, C0(+ × L2(, ); L2(, )) stands for the space of the continuous stochastic processes f : + × L2(, ) → L2(, ) such that
()
uniformly for xK, where KL2(, ) is any bounded subset. Additionally, W(t) will be a two-sided standard one-dimensional Brownian motion defined on the filtered probability space (Ω, , , t), where t = σ{W(u) − W(v); u, vt}.

2.1. Sectorial Linear Operators

A closed and linear operator A is said to be sectorial of type ϖ and angle θ if there exist 0 < θ < π/2, M > 0, and ϖ such that its resolvent exists outside the sector ϖ + Sθ : = {ϖ + λ : λ, |arg(−λ)| < θ} and ∥(λA)−1∥ ≤ M/|λϖ|, λϖ + Sθ.

Definition 1 (see [2].)Let A be a closed and linear operator with domain D(A) defined on a Banach space 𝕏. We call A the generator of a solution operator if there exist ϖ and a strongly continuous function Sα : +(𝕏) such that {λα : Re(λ) > ϖ} ⊂ ρ(A) and , Re(λ) > ϖ, x𝕏. In this case, Sα(·) is called the solution operator generated by A.

We note that if A is sectorial of type ϖ with 0 ≤ θ < π(1 − α/2), then A is the generator of a solution operator given by , where γ is a suitable path lying outside the sector ϖ + Sθ. Recently, Cuesta in [1] proved that if A is a sectorial operator of type ϖ < 0 for some M > 0 and 0 ≤ θ < π(1 − α/2), then there exists a constant C > 0 such that
()

Remark 2. Note that Sα(t) is, in fact, integrable. For more details on the solution family Sα(t) and related issues, we refer the reader to [3133].

2.2. Square-Mean Asymptotically Almost Automorphic Processes

We recall some basic facts for a symptotically almost automorphic processes which will be used in the sequel.

Definition 3 (see [22].)A stochastic process x : L2(, ) is said to be stochastically continuous if

()

Definition 4 (see [17].)A stochastically continuous stochastic process x : L2(, ) is said to be square-mean almost automorphic if, for every sequence of real numbers , there exist a subsequence {sn} n and a stochastic process y : L2(, ) such that

()
hold for each t. The collection of all square-mean almost automorphic stochastic processes x : L2(, ) is denoted by AA(; L2(, )).

Definition 5 (see [17].)A function f : × L2(, ) → L2(, ), (t, x) → f(t, x), which is jointly continuous, is said to be square-mean almost automorphic if f(t, x) is square-mean almost automorphic in t uniformly for all x𝕂, where 𝕂 is any bounded subset of L2(, ). That is to say, for every sequence of real numbers , there exists a subsequence {sn} n and a function such that

()
for each t and each x𝕂. Denote by AA( × L2(, ); L2(, )) the set of all such functions.

Lemma 6 (see [22].)(AA(; L2(, )), ∥·∥) is a Banach space equipped with the norm

()
for xAA(; L2(, )).

Lemma 7 (see [17].)Let f : × L2(, ) → L2(, ), (t, x) → f(t, x) be square-mean almost automorphic, and assume that f(t, ·) is uniformly continuous on each bounded subset 𝕂L2(, ) uniformly for t; that is, for all ɛ > 0, there exists δ > 0 such that x, y𝕂 and Exy2 < δ imply that Ef(t,x)−f(t,y)∥2 < ɛ for all t. Then for any square-mean almost automorphic process x : L2(, ), the stochastic process F : L2(, ) given by F(·) : = f(·, x(·)) is square-mean almost automorphic.

Definition 8 (see [25].)A stochastically continuous process f : +L2(, ) is said to be square-mean asymptotically almost automorphic if it can be decomposed as f = g + h, where gAA(; L2(, )) and hC0(+; L2(, )). Denote by AAA(+; L2(, )) the collection of all the square-mean asymptotically almost automorphic processes f : +L2(, ).

Definition 9 (see [28].)A function f : + × L2(, ) → L2(, ), (t, x) → f(t, x), which is jointly continuous, is said to be square-mean asymptotically almost automorphic if it can be decomposed as f = g + h, where gAA( × L2(, ); L2(, )) and hC0(+ × L2(, ); L2(, )). Denote by AAA(+ × L2(, ); L2(, )) the set of all such functions.

Lemma 10 (see [28].)If f, f1, and f2 are all square-mean asymptotically almost automorphic stochastic processes, then the following hold true:

  • (I)

    f1 + f2 is square-mean asymptotically almost automorphic;

  • (II)

    λf is square-mean asymptotically almost automorphic for any scalar λ;

  • (III)

    there exists a constant M > 0 such that .

Lemma 11 (see [28].)Suppose that fAAA(+; L2(, )) admits a decomposition f = g + h, where gAA(; L2(, )) and hC0(+; L2(, )). Then .

Corollary 12 (see [28].)The decomposition of a square-mean asymptotically almost automorphic process is unique.

Lemma 13 (see [28].)AAA(+; L2(, )) is a Banach space when it is equipped with the norm

()
where f = g + hAAA(+; L2(, )) with gAA(; L2(, )), hC0(+; L2(, )).

Lemma 14 (see [28].)AAA(+; L2(, )) is a Banach space with the norm

()

Remark 15 (see [28].)In view of the previous lemmas it is clear that the two norms are equivalent in AAA(+; L2(, )).

Lemma 16 (see [28].)Let fAA( × L2(, ); L2(, )) and let f(t, x) be uniformly continuous in any bounded subset 𝕂L2(, ) uniformly for t+. Then f(t, x) is uniformly continuous in any bounded subset 𝕂L2(, ) uniformly for t.

Lemma 17 (see [28].)Let fAAA(+ × L2(, ); L2(, )) and suppose that f(t, x) is uniformly continuous in any bounded subset 𝕂L2(, ) uniformly for t+. If u(t) ∈ AAA(+; L2(, )), then f(·, u(·)) ∈ AAA(+; L2(, )).

We now give the following concept of mild solution of (1).

Definition 18. Let Sα(t) be an integrable solution operator on L2(, ) with generator A. An t-adapted stochastic process x : [0, +) → L2(, ) is called a mild solution of the problem (1) if x(0) = u0 is 0-measurable and x(t) satisfies the corresponding stochastic integral equation:

()

3. Main Results

In this section, we establish the existence of square-mean asymptotically almost automorphic mild solutions to the problem (1). For that, we need the following technical results.

First, we list the following basic assumptions.
  • (H1)

    The operator A is a sectorial operator of type ϖ < 0 for some M > 0 and 0 ≤ θ < π(1 − α/2), and then there exists  C > 0 such that

    ()

  • where Sα(t) is the solution operator generated by A.

  • (H2)

    The function fAAA(+ × L2(, ); L2(, )) and there exists a continuous and nondecreasing function Lf : [0, +)→[0, +) such that for each r ≥ 0 and for all Ex2r, Ey2r,

    ()

  • for all t+.

  • (H3)

    The function gAAA(+ × L2(, ); L2(, )) and there exists a continuous and nondecreasing function Lg : [0, +) → [0, +) such that for each r ≥ 0 and for all Ex2r, Ey2r,

    ()

  • for all t+.

  • (H4)

    We have

    ()

  • where and .

  • (H5)

    The operator A is a sectorial operator of type ϖ with 0 ≤ θ < π(1 − α/2), and there exists ϕ(·) ∈ L1(+) such that

    ()

  • where Sα(t) is the solution operator generated by A.

Lemma 19. Suppose that assumption (H1) holds and let fAAA(+; L2(, )). If F is the function defined by

()
then FAAA(+; L2(, )).

Proof. Since fAAA(+; L2(, )), we have by definition that f = g + h, where gAA(R;L2 (P, H)) and hC0(+; L2(, )). Then

()
where and .

First we prove that G(t) ∈ AA(; L2(, )). Let be an arbitrary sequence of real numbers. Since gAA(; L2(, )), there exists a subsequence {sn} n of such that for a certain stochastic process

()
hold for each t. Now, let for each σ. Note that is also a Brownian motion and has the same distribution as W. Moreover, if we let , then by making a change of variables σ = ssn to get (see the equation (10.6.6) in [34])
()
and, hence, using the Ito’s isometry property of stochastic integral, we have the following estimations
()
Then by (20), we obtain that for each t. In a similar way, we can show that for each t. Thus we conclude that (·)∈AA(; L2(, )).

Next, let us show that H(·) ∈ C0(+; L2(, )). Since hC0(+; L2(, )) and 1/(1+|ϖ | sα) 2 is integrable in [0, +), for any sufficiently small ɛ > 0, there exists a constant T > 0 such that Eh(s)∥2ɛ and for all sT. Then, for all t ≥ 2T, we obtain

()
This inequality proves the assertion since ɛ is arbitrary. Recalling that F(t) = G(t) + H(t) for all t ≥ 0, we get F(t) ∈ AAA(+; L2(, )). The proof is completed.

It is easy to see that, by arguments similar to those in the proof of Lemma 19, we have the following result.

Lemma 20. Suppose that assumption (H5) holds and let fAAA(+; L2(, )). If F is the function defined by

()
then FAAA(+; L2(, )).

Now, we are ready to establish our main results.

Theorem 21. Assume that (H1)–(H4) hold. Then there exists ɛ > 0 such that for each u0Bɛ(0, L2(, )) there exists a unique square-mean asymptotically almost automorphic mild solution x(·, u0) of the problem (1) on [0, ) such that x(0, u0) = u0.

Proof. We define a nonlinear operator Υ by

()

First we prove that Υ(AAA(+; L2(, )))⊆AAA(+; L2(, )). Given xAAA(+; L2(, )), from the properties of {Sα(t)} t≥0, f, and g, we infer that Υx is well defined and continuous. Since x(t) is bounded, we can choose a bounded subset 𝕂 of L2(, ) such that x(t) ∈ 𝕂 for all t+. It follows from (H2) and (H3) that both f(t, x) and g(t, x) are uniformly continuous on the bounded subset 𝕂 uniformly for t+. Moreover, from Lemmas 17 and 19 and taking into account (H1), it follows that ΥxAAA(+; L2(, )).

Now, by (H4), there exists a constant r > 0 such that

()
Let 0 < λ < 1. We affirm that the assertion holds for ɛ = λr. In fact, let u0Bɛ(0, L2(, )). Define the space 𝔻 = {xAAA(+; L2(, )) : x(0) = u0, Ex(t)∥2r, t ≥ 0}. Then 𝔻 is a closed subspace of AAA(+; L2(, )). We claim that Υ𝔻𝔻. If x𝔻 and t+, we get
()
which from (26) implies that E∥(Υx)(t)∥2r for all t ≥ 0, and so that Υ𝔻𝔻.

Next, to complete the proof, we need to show that Υ(·) is a contraction from 𝔻 into 𝔻. By (26), we know that

()
That is,
()
Then, one has
()
For any x, y𝔻 and t ≥ 0, we have
()
Thus, we get
()
It follows from (30) that Υ is a contraction mapping on 𝔻. So by the Banach contraction mapping principle, we draw a conclusion that there exists a unique fixed point x(·) for Υ in 𝔻. It is clear that x is a square-mean asymptotically almost automorphic mild solution of (1). The proof is complete.

The next result is proved using the similar steps as in the proof of the previous result, so we omit the details.

Theorem 22. Assume that (H1)–(H3) hold. If Lf(r) ≡ Lf and Lg(r) ≡ Lg for all r ≥ 0 and Lf + (CM) 2 | ϖ|−1/α(1 − 1/α)πLg/αsin(π/α) < 1/2, then for every u0L2(, ), there exists a unique square-mean asymptotically almost automorphic mild solution x(·, u0) of the problem (1) on [0, ) such that x(0, u0) = u0.

Theorem 23. Suppose that assumptions (H2), (H3), and (H5) hold. If Lf(r) ≡ Lf and Lg(r) ≡ Lg for all r ≥ 0 and Lf + Lgϕ1 < 1/2, then for every u0L2(, ), there exists a unique square-mean asymptotically almost automorphic mild solution x(·, u0) of the problem (1) on [0, ) such that x(0, u0) = u0.

Proof. Consider the nonlinear operator Υ given by

()

First we prove that Υ maps AAA(+; L2(, )) into itself. Given xAAA(+; L2(, )), from the properties of {Sα(t)} t≥0, f and g, we infer that Υx is well defined and continuous. Since x(t) is bounded, we can choose a bounded subset 𝕂 of L2(, ) such that x(t) ∈ 𝕂 for all t+. It follows from conditions (H2) and (H3) that both f(t, x) and g(t, x) are uniformly continuous on the bounded subset 𝕂 uniformly for t+. Moreover, from Lemmas 17 and 20 and taking into account (H5), it follows that ΥxAAA(+; L2(, )).

Next we prove that Υ is a contraction mapping from AAA(+; L2(, )) into itself. Note that we have already proved Υ(AAA(+; L2(, )))⊆AAA(+; L2(, )). Moreover, for any x, yAAA(+;L2(, )) and t ≥ 0, we have

()
Therefore
()
That is, Υ is a contraction mapping on AAA(+; L2(, )). By the Banach contraction mapping principle, Υ has a unique fixed point x(t) ∈ AAA(+; L2(, )). It is clear that x is a square-mean asymptotically almost automorphic mild solution of (1). The proof is then complete.

4. An Example

In this section, we apply the results obtained previously to investigate the existence of square-mean asymptotically almost automorphic mild solutions for the following partial stochastic fractional differential system:
()
where aAAA(+; ), a1, a2 : + are continuous functions, and ν > 0 is a fixed constant.

Let = L2([0, π]) with the norm ∥·∥ and A : D(A)⊆ be the operator defined by Ax = x′′νx domain D(A) = {x : x′′, x(0) = x(π) = 0}. It is well known that Δx = x′′ is the infinitesimal generator of an analytic semigroup {S(t)} t≥0 on . Furthermore, A is sectorial of type ϖ = −ν < 0.

In the sequel, we assume
()
Now, we can define the functions f, g : + × L2(, ) → L2(, ) by
()
which permits to transform the system (36) into the abstract system (1). Moreover, it is not difficult to see that f, g are continuous and Lipschitz in the second variable with Lipschitz constants Lf and Lg, respectively.

The next result is a consequence of Theorem 22.

Theorem 24. Under the previous assumptions, (36) has a unique mild solution xAAA(+; L2(, )) whenever Lf and Lg are small enough.

Acknowledgments

The first author was supported by Research Fund for Young Teachers of Sanming University (B2011071Q). The second author was supported by NSF of China (11361032), Program for New Century Excellent Talents in University (NCET-10-0022), and NSF of Gansu Province of China (1107RJZA091). And the third author was partially supported by Ministerio de Economia y Competitividad (Spain), project MTM2010-15314, and cofinanced by the European Community fund FEDER.

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