Research Article | Open Access

# Existence and Upper Semicontinuity of Attractors for Nonautonomous Stochastic Sine-Gordon Lattice Systems with Random Coupled Coefficients

**Academic Editor:**Bixiang Wang

#### Abstract

We study nonautonomous stochastic sine-Gordon lattice systems with random coupled coefficients and multiplicative white noise. We first consider the existence of random attractors in a weighted space for this system and then establish the upper semicontinuity of random attractors as the intensity of noise approaches zero.

#### 1. Introduction

Stochastic lattice differential equations arise naturally in a wide variety of applications where the spatial structure has a discrete character and uncertainties are taken into account. In recent years, some works have been done regarding the existence of random attractors for stochastic lattice differential equations (see, e.g., [1â€“11]). Of those works, Bates et al. [2] considered the existence of random attractors for first-order nonautonomous stochastic lattice system driven by multiplicative white noise. Zhou and Wei [11] considered the existence of random attractors for second-order lattice system with random coupled coefficients and multiplicative white noise.

Motivated by [2, 11], we will study the asymptotic behavior of solutions of the following nonautonomous stochastic sine-Gordon lattice systems with random coupled coefficients and multiplicative white noise: for every and ,with the initial data where , and are positive constants; ; ; âˆ˜ denotes the Stratonovich sense of the stochastic term; are random variables; is a Brownian motion (Wiener process) on a probability space , where , is the Borel -algebra induced by the compact open topology of , and is the Wiener measure on (see [12]); and is a metric dynamical system defined on , where here for all . Then is an ergodic metric dynamical system.

In practice, the coupled mode between two nodes (say, adjacent nodes) is usually random. It is then of great importance to investigate stochastic lattice system with random coupled coefficients. To the best of our knowledge, there are no results on nonautonomous stochastic sine-Gordon lattice systems with random coupled coefficients and multiplicative white noise.

In this paper, we first consider the existence of a tempered random attractor in a weighted space for stochastic sine-Gordon lattice systems (1), which attracts the random tempered bounded sets in pullback sense. Then, we consider the dependence of attractors on the parameters of the system (1) and establish the upper semicontinuity of the random attractor as the intensity of noise approaches zero.

The rest of this paper is organized as follows. In the next section, we present some mathematical setting for system (1). In Section 3, we mainly consider the existence of a tempered random attractor in a weighted space of infinite sequences for system (1). Then in Section 4, we consider the upper semicontinuity of the tempered random attractor for system (1) as .

#### 2. Mathematical Settings

We first introduce a weighted space of infinite sequences. Let be a positive-valued function from into which satisfieswhere and are positive constants. For example, for , [13] and satisfy condition (3), where . Define and with norm and inner product for . We write as if . Then is a separable Hilbert space with the norm .

For any , define an inner product on by then the norm induced by inner product is equivalent to the norm induced by the inner product . We write ; then and are both separable Hilbert spaces.

Note that system (1) can be rewritten aswhere , is a given time dependent sequence, and is a linear operator defined by

To convert problem (6) into a random differential equation, let , and , which is an Ornstein-Uhlenbeck process on and solve the Ornstein-Uhlenbeck equation , where for . From [12, 14, 15], it is known that the random variables are tempered, and there is a -invariant set of full measure such that is continuous in for every .

Let ; then (6) can be written as the following equivalent random system with random coefficients: for every and ,

We will consider (8) for and write as from now on. In order to obtain the existence and uniqueness of solutions to problem (8), we make the following assumptions on and the coefficients , for :(**A1**)Letting belongs to with respect to for each ,â€‰and is tempered.(**A2**).(**A3**)Let , andâ€‰There exists a positive constant such that

We call a mild solution of the following random lattice differential equations:where , if and

Theorem 1. *Let and ( A1)-(A2) hold. Then for every and and any initial data , problem (8) admits a unique mild solution with , being continuous in ; if . Moreover, (8) generates a continuous cocycle over and with state space : for , and ,*

We can prove Theorem 1 by Theoremâ€‰â€‰6.1.7 in [16] and Definitionâ€‰â€‰2.1 in [2]. We omit it here.

#### 3. Existence of Random Attractors

We first provide some sufficient conditions for the existence of random attractors for nonautonomous RDSs in weighted spaces of infinite sequences in [2].

In the following, let be a separable Banach space and let be the collection of all tempered families of nonempty bounded subsets of .

*Definition 2. *Let be a continuous cocycle on over and .(1)A family is called a random absorbing set for if, for all and and for every , there exists such that(2) A family is called a random attractor for if for all , , and , (i) is compact in and is measurable in with respect to ; (ii) is invariant, that is, ; and (iii) for every ,â€‰where is the Hausdorff semidistance given by , for any .

Theorem 3. *Let be a continuous cocycle on over and . Suppose the following.*(a)*There exists a bounded closed random absorbing set such that, for any , , and , there exists yielding ;.*(b)*For each , and for any , there exist and such that**Then possesses a unique random attractor in given by, for every and ,*

Next, we will use Theorem 3 to prove the existence of a random attractor for the continuous cocycle in under conditions (**A1**)â€“(**A3**).

Theorem 4. *If ( A1)â€“(A3) hold, then, for every , , and and for any , there exists such that, for all and , the solution of (8) satisfieswhere*

*Proof. *For each , there exists a sequence of continuous functions in (see [17]) such thatand for . Consider the following random differential equations:where It is easy to see that (23) has a unique mild solution satisfying (23). Taking the inner product of the first equation in (23) with in and the second equation with in , then we have thatSumming the two equations of (24), we find thatNote thatBy (3), we getwhere

From (25)â€“(28), we obtain that, for ,Recalling that and in (10), then we havewhere is as in (21). Then we obtain from (32) that, for ,From (33) and by replacing by , we haveNote that (34) holds with and being replaced by and ; then we haveLetthen . By (9), we find that there exists such that, for , from which, along with , we haveTherefore, there exists such that, for all ,Since and are tempered, then is tempered. Then by (**A3**), we can verify that the following integrals are convergent:Thus the theorem follows from (35), (39), and (40).

Theorem 5. *Assume that ( A1)â€“(A3) hold. Then the continuous cocycle associated with (8) has a unique random attractor .*

*Proof. *By Theorem 3, it suffices to prove that, for every , , , and and for any , there exist and such that, for all and , the solution of (8) satisfiesChoose a smooth increasing function such thatand there exists a positive constant such that for .

Let be a mild solution of (8) with . For any given define , by if and otherwise.

For any , let , be the solution of (23).

Then taking the inner product of the first equation in (23) with in and the second equation with in , we have that