Asymptotic Behavior of Solutions to Abstract Stochastic Fractional Partial Integrodifferential Equations
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
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 [13–16] 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 [17–27] 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 [28–30].
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 ∈ ℝ such that its resolvent exists outside the sector ϖ + Sθ : = {ϖ + λ : λ ∈ ℂ, |arg(−λ)| < θ} and ∥(λ−A)−1∥ ≤ M/|λ − ϖ|, λ ∉ ϖ + 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 ∈ ℝ and a strongly continuous function Sα : ℝ+ → ℒ(𝕏) such that {λα : Re(λ) > ϖ} ⊂ ρ(A) and , Re(λ) > ϖ, x ∈ 𝕏. In this case, Sα(·) is called the solution operator generated by A.
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 : ℝ → L2(ℙ, ℍ) is said to be stochastically continuous if
Definition 4 (see [17].)A stochastically continuous stochastic process x : ℝ → L2(ℙ, ℍ) is said to be square-mean almost automorphic if, for every sequence of real numbers , there exist a subsequence {sn} n∈ℕ and a stochastic process y : ℝ → L2(ℙ, ℍ) such that
Definition 5 (see [17].)A function f : ℝ × L2(ℙ, ℍ) → L2(ℙ, ℍ), (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 ∈ ℝ uniformly for all x ∈ 𝕂, where 𝕂 is any bounded subset of L2(ℙ, ℍ). That is to say, for every sequence of real numbers , there exists a subsequence {sn} n∈ℕ and a function such that
Lemma 6 (see [22].)(AA(ℝ; L2(ℙ, ℍ)), ∥·∥∞) is a Banach space equipped with the norm
Lemma 7 (see [17].)Let f : ℝ × L2(ℙ, ℍ) → L2(ℙ, ℍ), (t, x) → f(t, x) be square-mean almost automorphic, and assume that f(t, ·) is uniformly continuous on each bounded subset 𝕂 ⊂ L2(ℙ, ℍ) uniformly for t ∈ ℝ; that is, for all ɛ > 0, there exists δ > 0 such that x, y ∈ 𝕂 and E∥x−y∥2 < δ imply that E∥f(t,x)−f(t,y)∥2 < ɛ for all t ∈ ℝ. Then for any square-mean almost automorphic process x : ℝ → L2(ℙ, ℍ), the stochastic process F : ℝ → L2(ℙ, ℍ) given by F(·) : = f(·, x(·)) is square-mean almost automorphic.
Definition 8 (see [25].)A stochastically continuous process f : ℝ+ → L2(ℙ, ℍ) is said to be square-mean asymptotically almost automorphic if it can be decomposed as f = g + h, where g ∈ AA(ℝ; L2(ℙ, ℍ)) and h ∈ C0(ℝ+; L2(ℙ, ℍ)). Denote by AAA(ℝ+; L2(ℙ, ℍ)) the collection of all the square-mean asymptotically almost automorphic processes f : ℝ+ → L2(ℙ, ℍ).
Definition 9 (see [28].)A function f : ℝ+ × L2(ℙ, ℍ) → L2(ℙ, ℍ), (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 g ∈ AA(ℝ × L2(ℙ, ℍ); L2(ℙ, ℍ)) and h ∈ C0(ℝ+ × L2(ℙ, ℍ); L2(ℙ, ℍ)). Denote by AAA(ℝ+ × L2(ℙ, ℍ); L2(ℙ, ℍ)) 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 f ∈ AAA(ℝ+; L2(ℙ, ℍ)) admits a decomposition f = g + h, where g ∈ AA(ℝ; L2(ℙ, ℍ)) and h ∈ C0(ℝ+; L2(ℙ, ℍ)). Then .
Corollary 12 (see [28].)The decomposition of a square-mean asymptotically almost automorphic process is unique.
Lemma 13 (see [28].)AAA(ℝ+; L2(ℙ, ℍ)) is a Banach space when it is equipped with the norm
Lemma 14 (see [28].)AAA(ℝ+; L2(ℙ, ℍ)) 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(ℝ+; L2(ℙ, ℍ)).
Lemma 16 (see [28].)Let f ∈ AA(ℝ × L2(ℙ, ℍ); L2(ℙ, ℍ)) and let f(t, x) be uniformly continuous in any bounded subset 𝕂 ⊂ L2(ℙ, ℍ) uniformly for t ∈ ℝ+. Then f(t, x) is uniformly continuous in any bounded subset 𝕂 ⊂ L2(ℙ, ℍ) uniformly for t ∈ ℝ.
Lemma 17 (see [28].)Let f ∈ AAA(ℝ+ × L2(ℙ, ℍ); L2(ℙ, ℍ)) and suppose that f(t, x) is uniformly continuous in any bounded subset 𝕂 ⊂ L2(ℙ, ℍ) uniformly for t ∈ ℝ+. If u(t) ∈ AAA(ℝ+; L2(ℙ, ℍ)), then f(·, u(·)) ∈ AAA(ℝ+; L2(ℙ, ℍ)).
We now give the following concept of mild solution of (1).
Definition 18. Let Sα(t) be an integrable solution operator on L2(ℙ, ℍ) with generator A. An ℱt-adapted stochastic process x : [0, +∞) → L2(ℙ, ℍ) is called a mild solution of the problem (1) if x(0) = u0 is ℱ0-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.
- (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 f ∈ AAA(ℝ+ × L2(ℙ, ℍ); L2(ℙ, ℍ)) and there exists a continuous and nondecreasing function Lf : [0, +∞)→[0, +∞) such that for each r ≥ 0 and for all E∥x∥2 ≤ r, E∥y∥2 ≤ r,
() -
for all t ∈ ℝ+.
- (H3)
The function g ∈ AAA(ℝ+ × L2(ℙ, ℍ); L2(ℙ, ℍ)) and there exists a continuous and nondecreasing function Lg : [0, +∞) → [0, +∞) such that for each r ≥ 0 and for all E∥x∥2 ≤ r, E∥y∥2 ≤ r,
() -
for all t ∈ ℝ+.
- (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(ℝ+) such that
() -
where Sα(t) is the solution operator generated by A.
Lemma 19. Suppose that assumption (H1) holds and let f ∈ AAA(ℝ+; L2(ℙ, ℍ)). If F is the function defined by
Proof. Since f ∈ AAA(ℝ+; L2(ℙ, ℍ)), we have by definition that f = g + h, where g ∈ AA(R;L2 (P, H)) and h ∈ C0(ℝ+; L2(ℙ, ℍ)). Then
First we prove that G(t) ∈ AA(ℝ; L2(ℙ, ℍ)). Let be an arbitrary sequence of real numbers. Since g ∈ AA(ℝ; L2(ℙ, ℍ)), there exists a subsequence {sn} n∈ℕ of such that for a certain stochastic process
Next, let us show that H(·) ∈ C0(ℝ+; L2(ℙ, ℍ)). Since h ∈ C0(ℝ+; L2(ℙ, ℍ)) and 1/(1+|ϖ | sα) 2 is integrable in [0, +∞), for any sufficiently small ɛ > 0, there exists a constant T > 0 such that E∥h(s)∥2 ≤ ɛ and for all s ≥ T. Then, for all t ≥ 2T, we obtain
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 f ∈ AAA(ℝ+; L2(ℙ, ℍ)). If F is the function defined by
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 u0∈Bɛ(0, L2(ℙ, ℍ)) 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(ℝ+; L2(ℙ, ℍ)))⊆AAA(ℝ+; L2(ℙ, ℍ)). Given x ∈ AAA(ℝ+; L2(ℙ, ℍ)), 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(ℙ, ℍ) such that x(t) ∈ 𝕂 for all t ∈ ℝ+. 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 ∈ ℝ+. Moreover, from Lemmas 17 and 19 and taking into account (H1), it follows that Υx ∈ AAA(ℝ+; L2(ℙ, ℍ)).
Now, by (H4), there exists a constant r > 0 such that
Next, to complete the proof, we need to show that Υ(·) is a contraction from 𝔻 into 𝔻. By (26), we know that
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 u0 ∈ L2(ℙ, ℍ), 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 + Lg∥ϕ∥1 < 1/2, then for every u0 ∈ L2(ℙ, ℍ), 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(ℝ+; L2(ℙ, ℍ)) into itself. Given x ∈ AAA(ℝ+; L2(ℙ, ℍ)), 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(ℙ, ℍ) such that x(t) ∈ 𝕂 for all t ∈ ℝ+. 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 ∈ ℝ+. Moreover, from Lemmas 17 and 20 and taking into account (H5), it follows that Υx ∈ AAA(ℝ+; L2(ℙ, ℍ)).
Next we prove that Υ is a contraction mapping from AAA(ℝ+; L2(ℙ, ℍ)) into itself. Note that we have already proved Υ(AAA(ℝ+; L2(ℙ, ℍ)))⊆AAA(ℝ+; L2(ℙ, ℍ)). Moreover, for any x, y ∈ AAA(ℝ+;L2(ℙ, ℍ)) and t ≥ 0, we have
4. An Example
Let ℍ = L2([0, π]) with the norm ∥·∥ and A : D(A)⊆ℍ → ℍ be the operator defined by Ax = x′′ − νx domain D(A) = {x ∈ ℍ : x′′ ∈ ℍ, 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 ℍ. Furthermore, A is sectorial of type ϖ = −ν < 0.
The next result is a consequence of Theorem 22.
Theorem 24. Under the previous assumptions, (36) has a unique mild solution x ∈ AAA(ℝ+; L2(ℙ, ℍ)) 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.