1 Introduction

In this paper, we investigate the stability of pullback random attractors of the stochastic supercritical wave equation driven by multiplicative noise defined on \({\mathbb {R}}^n\):

$$\begin{aligned}{} & {} \partial _{tt} u - \triangle u + \alpha \partial _t u + \nu u + f(x,u)\nonumber \\{} & {} \quad = g(t,x) + \sigma u \circ \frac{d W(t)}{dt}, \ \ \ t > \tau , \ x \in {\mathbb {R}}^n, \end{aligned}$$
(1.1)

with initial data

$$\begin{aligned} u(\tau , x) = u_0 (x), \ \ \ \ \ \ \partial _t u(\tau , x) = u_{1,0} (x), \ \ x \in {\mathbb {R}}^n, \end{aligned}$$
(1.2)

where \(1 \le n \le 6\), \(\tau \in {\mathbb {R}}\), \(\alpha \) and \(\nu \) are positive constants, \(f: {\mathbb {R}}^n \times {\mathbb {R}} \rightarrow {\mathbb {R}}\) is a nonlinear term, \(g \in L^2_{loc}( {\mathbb {R}}, L^2({\mathbb {R}}^n) )\), \(\sigma \in (0,1) \) is a parameter representing the noise intensity, the symbol \(\circ \) stands for Stratonovich’s integration, and W(t) is a standard real-valued Wiener process on a probability space \(( \Omega , {\mathscr {F}}, {\mathbb {P}} )\).

The long term dynamics of the stochastic wave equation (1.1) depends on the growth rate p of the nonlinear term f(xu) as \( |u| \rightarrow \infty \). In general, \(p={\frac{n}{n-2}}\) for \(n>2\) is called a critical exponent, and the equation is said to be subcritical, critical and supercritical when \(p<{\frac{n}{n-2}}\), \(p={\frac{n}{n-2}}\) and \(p>{\frac{n}{n-2}}\), respectively. In the subcritical or critical case, the nonlinear function f maps \(H^1({{{\mathcal {O}}}}) \) into \( L^2({{{\mathcal {O}}}})\) for a domain \({{{\mathcal {O}}}}\) in \({\mathbb {R}}^n\), which plays a key role for studying the random attractors of the stochastic wave equation, see, e.g., [11, 18, 32, 47, 48] for bounded domains and [40, 42, 43, 46, 49] for unbounded domains.

However, in the supercritical case, the nonlinear function f does not map \(H^1({{{\mathcal {O}}}}) \) into \( L^2({{{\mathcal {O}}}})\) any more, and the uniform Strichartz estimates must be used to study the random attractors in this case, see, e.g., [12, 13, 44, 45]. In particular, the existence of random attractors of system (1.1)–(1.2) with supercritical nonlinearity has been proved in [13] recently. In the present paper, we continue this line of research and further investigate the stability of these random attractors for supercritical stochastic wave equation driven by multiplicative noise as the intensity of noise \(\sigma \rightarrow 0\).

More precisely, we will prove the random attractors of (1.1)–(1.2) are upper semicontinuous at \(\sigma =0\). To that end, we first establish the Strichartz estimates of the solutions which are uniform with respect to noise intensity (see Lemma 3.1). We then prove the pathwise convergence of the solutions of the stochastic equation as \(\sigma \rightarrow 0\) (see Lemma 3.2). The main difficulty of the paper is to show the precompactness of the collection of all random attractors of (1.1)–(1.2) in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\). To overcome the non-compactness of Sobolev embeddings on unbounded domains, we first utilize the invariant property of random attractors as well as the uniform tail-estimates of the solutions to prove that all functions in random attractors are uniformly infinitesimal outside a large bounded domain. We then decompose the solution operator as two parts: one is linear and the other is nonlinear. We prove the linear part is convergent in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\) as \(\sigma \rightarrow 0\). For the nonlinear part in bounded domains, we use the spectral decomposition of the Laplace operator to further split the solutions as a sum of a finite-dimensional component and an infinite-dimensional component. By showing the infinite-dimensional component is uniformly small, we eventually obtain the precompactness of the collection of all random attractors of (1.1)–(1.2) in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\), see Lemma 4.1. Based on above analysis, we then prove by [41, Theorem 3.2] that random attractors of (1.1)–(1.2) are upper semicontinuous as the noise intensity \(\sigma \rightarrow 0\), see Theorem 5.1. For details on upper semicontinuity of attractors, we refer the reader to [1, 10, 23, 24, 30, 35] for deterministic equations and [6, 17, 26, 28, 29, 39, 50] for stochastic equations.

We mention that further information on existence of pathwise random attractors can be found in [5, 7,8,9, 15, 16, 22, 27, 36] and the references therein. For attractors of deterministic wave equations, the reader is referred to [2,3,4, 14, 21, 25, 31, 37, 38] for bounded domains and [19, 20, 33, 34] for unbounded domains. In particular, the existence of global attractors of the supercritical deterministic wave equation on \({\mathbb {R}}^3\) was examined in [20].

The paper is organized as follows. In Sect. 2, we review the existence and uniqueness of random attractors of (1.1)–(1.2). In Sect. 3, we prove the convergence of the solutions of (1.1)–(1.2) with respect to initial data and noise intensity. Section 4 is devoted to the precompactness of the collection of all random attractors of (1.1)–(1.2) in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\). In the last section, we prove the upper semicontinuity of random attractors as the noise intensity \(\sigma \) approaches 0.

Hereafter, we denote the inner product and the norm of \(L^2({\mathbb {R}}^n)\) by \((\cdot , \cdot )\) and \(\Vert \cdot \Vert \), respectively.

2 Preliminaries

In this section, we review existence of random attractors of the stochastic supercritical wave equation (1.1)–(1.2) on \({\mathbb {R}}^n\), which is needed for further proving the upper semicontinuity of these random attractors.

In the sequel, we assume \(f: {\mathbb {R}}^n \times {\mathbb {R}} \rightarrow {\mathbb {R}}\) is continuous and write \(F(x,r)= \int _0^r f(x,s) ds\) for all \(x \in {\mathbb {R}}^n\) and \(r \in {\mathbb {R}}\). Suppose f and F satisfy the conditions: for all \(x \in {\mathbb {R}}^n\) and \(u, u_1, u_2 \in {\mathbb {R}} \),

$$\begin{aligned}&| f (x,u_1) - f (x,u_2)| \le \alpha _1 (\varphi (x) + |u_1|^{p-1} + |u_2|^{p-1} ) |u_1-u_2|, \end{aligned}$$
(2.1)
$$\begin{aligned}&| f (x,u) | \ge \alpha _2 |u|^p - \varphi (x) |u|, \end{aligned}$$
(2.2)
$$\begin{aligned}&f(x,0)=0, \ \ \ \ \ \liminf _{|u|\rightarrow \infty } \inf _{x \in {\mathbb {R}}^n} ( f(x,u) u ) > 0, \end{aligned}$$
(2.3)
$$\begin{aligned}&F(x,u) + \varphi _1(x) \ge \alpha _3 |u|^{p+1}, \end{aligned}$$
(2.4)
$$\begin{aligned}&f(x,u)u- \gamma F(x,u) \ge \varphi _2(x), \end{aligned}$$
(2.5)
$$\begin{aligned}&|\partial _x f(x,s)| \le \varphi _3(x) |s|^{p} + \varphi _4(x), \quad |\partial _s f(x,s)| \le \varphi _5(x) |s|^{p-1} + \varphi _4(x), \end{aligned}$$
(2.6)

where \(p\ge 1\) for \(n=1,2\) and \(1\le p < \frac{n+2}{n-2}\) for \(3\le n \le 6\), \(\alpha _1, \alpha _2, \alpha _3 > 0\), \(\gamma \in (0,1]\), \(\varphi \in L^\infty ({\mathbb {R}}^n)\) for \(n=1,2\) and \(\varphi \in L^\infty ({\mathbb {R}}^n) \cap L^{\frac{2n}{(n-2)(p-1)}}({\mathbb {R}}^n) \) for \(3\le n \le 6\), \(\varphi _1, \varphi _2 \in L^1({\mathbb {R}}^n)\), \(\varphi _3, \varphi _4 \in L^1({\mathbb {R}}^n) \cap L^\infty ({\mathbb {R}}^n)\), and \(\varphi _5\in L^\infty ({\mathbb {R}}^n)\).

We mention that if \(f(x,u) = |u|^{p-1}u\) for \(x\in {\mathbb {R}}^n\) and \(u\in {\mathbb {R}}\), then f satisfies all conditions (2.1)–(2.6) for \(p\ge 1\).

For the probability space, we denote by \(( \Omega , {\mathscr {F}}, {\mathbb {P}} )\) the classical Wiener space, where \(\Omega =\{ \omega : {\mathbb {R}} \rightarrow {\mathbb {R}} \ is \text { continuous}, \ \omega (0) = 0 \}\). Given \(t\in {\mathbb {R}}\), denote by

$$\begin{aligned} \theta _t \omega (\cdot ) = \omega (t+\cdot )- \omega (t), \quad \forall \ \omega \in \Omega . \end{aligned}$$

Let \(y(\theta _t \omega )\) be the unique stationary solution of linear stochastic differential equation

$$\begin{aligned} dy+ \alpha y dt = dW. \end{aligned}$$

Then there exists \(\theta _t\)-invariant set \({\tilde{\Omega }} \subseteq \Omega \) with \({\mathbb {P}}({\tilde{\Omega }}) = 1\) such that \(y(\theta _t \omega )\) is tempered and continuous in t for any \(\omega \in {\tilde{\Omega }}\), For convenience, \({\tilde{\Omega }}\) will be written as \(\Omega \).

Denote by \(V(t) = u(t) e^{-\sigma \int _0^t y(\theta _s \omega ) ds } \). Then equation (1.1) can be rewritten as follows:

$$\begin{aligned}&\partial _{tt} V - \triangle V + \alpha \partial _t V + \nu V + f( x, e^{\sigma \int _0^t y(\theta _s \omega ) ds} V ) \ e^{ - \sigma \int _0^t y(\theta _s \omega ) ds } \nonumber \\&\quad = g(t,x) e^{ - \sigma \int _0^t y(\theta _s \omega ) ds } - \sigma ^2 y^2(\theta _t \omega ) V - 2 \sigma y(\theta _t \omega ) \partial _t V, \end{aligned}$$
(2.7)

with initial data

$$\begin{aligned} V(\tau , x) = V_0(x) \ \ \text {and} \ \ \partial _t V(\tau , x) = V_{1,0}(x), \ x \in {\mathbb {R}}^n, \end{aligned}$$
(2.8)

where \(V_0(x) = u_0(x) e^{ -\sigma \int _0^{\tau } y(\theta _s \omega ) ds }\) and \(V_{1,0}(x) = \left( u_{1,0}(x) - \sigma y(\theta _\tau \omega ) u_0(x) \right) e^{ -\sigma \int _0^{\tau } y(\theta _s \omega ) ds }.\)

As usual, a solution of (2.7)–(2.8) will be understood in the following sense.

Definition 2.1

Given \(\tau \in {\mathbb {R}}\), \(\omega \in \Omega \), \((V_0,V_{1,0}) \in H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\), a function \(V(\cdot ; \tau , \omega , (V_0, V_{1,0})): [\tau , \tau +T] \rightarrow H^1({\mathbb {R}}^n)\) is called a solution of (2.7) and (2.8) if

  1. (i)

    \( V \in L^\infty (\tau , \tau +T; H^1({\mathbb {R}}^n) ) \bigcap C([\tau , \tau +T], L^2({\mathbb {R}}^n) ), \) \( \partial _t V \in L^\infty (\tau , \tau +T; L^2({\mathbb {R}}^n) ) \bigcap C([\tau , \tau +T], H^{-1}({\mathbb {R}}^n) ), \) \(V(\tau )=V_0\) and \(\partial _t V(\tau ) = V_{1,0}\).

  2. (ii)

    \(V(t; \tau , \cdot , (V_0, V_{1,0})): \Omega \rightarrow H^1({\mathbb {R}}^n)\) is \(({\mathcal {F}}, {\mathcal {B}}(H^1({\mathbb {R}}^n))\)-measurable, and \(\partial _t V(t; \tau , \cdot , (V_0, V_{1,0})): \Omega \rightarrow L^2({\mathbb {R}}^n)\) is \(({\mathcal {F}}, {\mathcal {B}}(L^2({\mathbb {R}}^n))\)-measurable.

  3. (iii)

    For each \(\zeta \in C_0^\infty ( (\tau ,\tau +T) \times {\mathbb {R}}^n )\) and a.s. \(\omega \in \Omega \),

    $$\begin{aligned}&- \int _\tau ^{\tau +T} \left( \partial _t V, \partial _t \zeta \right) dt + \int _\tau ^{\tau +T} \left( \nabla V, \nabla \zeta \right) dt + \alpha \int _\tau ^{\tau +T} \left( \partial _t V, \zeta \right) dt + \nu \int _\tau ^{\tau +T} \left( V, \zeta \right) dt \nonumber \\&\qquad + \int _\tau ^{\tau +T} \int _{{\mathbb {R}}^n} f( x, e^{\sigma \int _0^t y(\theta _s \omega ) ds} V(t,x) ) \ e^{ - \sigma \int _0^t y(\theta _s \omega ) ds } \zeta (t,x) dxdt\ \nonumber \\&\quad = \int _\tau ^{\tau +T} e^{ - \sigma \int _0^t y(\theta _s \omega ) ds } \left( g(t,\cdot ), \zeta \right) dt - \sigma ^2 \int _\tau ^{\tau +T} y^2(\theta _t \omega ) \left( V(t), \zeta \right) dt \nonumber \\&\qquad - 2 \sigma \int _\tau ^{\tau +T} y(\theta _t \omega ) \left( \partial _t V(t), \zeta \right) dt. \end{aligned}$$

We recall from [13] the following existence and uniqueness of solutions to (2.7)–(2.8).

Theorem 2.1

Assume (2.1)–(2.4) hold, \(V_0 \in H^1({\mathbb {R}}^n)\), \(V_{1, 0} \in L^2({\mathbb {R}}^n)\), \(\tau \in {\mathbb {R}}\) and \(T>0\). Then problem (2.7)–(2.8) has a unique solution V on \([\tau , \tau +T]\) in the sense of Definition 2.1 which further satisfies Strichartz’s inequality:

$$\begin{aligned} \int _\tau ^{\tau +T} \Vert V(t) \Vert _{L^{p*}({\mathbb {R}}^n)}^{q*} dt <\infty , \end{aligned}$$
(2.9)

for any \(p^*\) and \(q^*\) with

$$\begin{aligned} \frac{n-3}{2n}< \frac{1}{p^*} < \frac{n-2}{2n}, \quad \quad \frac{1}{q*} = \frac{n-2}{2} - \frac{n}{p*}, \quad 3\le n \le 6. \end{aligned}$$
(2.10)

In addition, the solution V with (2.9) is continuous with respect to initial data \((V_0, V_{1,0})\) in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\), \( V \in C([\tau , \tau +T], H^1({\mathbb {R}}^n))\), \( V_t \in C([\tau , \tau +T], L^2({\mathbb {R}}^n))\), and for a.e. \(t\in ( \tau , \tau +T)\),

$$\begin{aligned}&\frac{d}{dt} \left( \Vert \partial _t V (t) \Vert ^2 + \Vert \nabla V (t) \Vert ^2 + \nu \Vert V (t) \Vert ^2 + 2 \int _{{\mathbb {R}}^n } F( x, e^{\sigma \int _0^t y(\theta _s \omega ) ds} V(t,x) ) \ e^{ - 2\sigma \int _0^t y(\theta _s \omega ) ds } dx \right) \nonumber \\&\quad = - 4\sigma \int _{{\mathbb {R}}^n } F( x, e^{\sigma \int _0^t y(\theta _s \omega ) ds} V(t,x) ) \ y(\theta _t\omega ) e^{ - 2\sigma \int _0^t y(\theta _s \omega ) ds } dx \nonumber \\&\qquad + 2 \sigma \int _{{\mathbb {R}}^n } f( x, e^{\sigma \int _0^t y(\theta _s \omega ) ds} V(t,x) ) \ V(t,x) y(\theta _t\omega ) e^{ - \sigma \int _0^t y(\theta _s \omega ) ds } dx \nonumber \\&\qquad + 2 \left( e^{ - \sigma \int _0^t y(\theta _s \omega ) ds } g(t,\cdot ) , \partial _t V(t) \right) - 2\alpha \Vert \partial _t V(t) \Vert ^2 - 4 \sigma y(\theta _t \omega ) \Vert \partial _t V(t) \Vert ^2 \nonumber \\&\qquad - 2 \sigma ^2 y^2(\theta _t \omega ) \left( V(t), \partial _t V(t) \right) . \end{aligned}$$
(2.11)

By Theorem 2.1, we find that for every \(\sigma \in (0, 1)\), \(\tau \in {\mathbb {R}}\) and \( (u_0, u_{1,0} ) \in H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\), problem (1.1)–(1.2) has a solution \(u_\sigma (t) = u_\sigma (t; \tau , \omega , (u_0, u_{1,0}) ) \) which is pathwise unique. Then a cocycle \(\Phi _\sigma : {\mathbb {R}}^+ \times {\mathbb {R}} \times \Omega \times H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n) \rightarrow H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\) associated with (1.1)–(1.2) is given by

$$\begin{aligned} \Phi _\sigma (s, \tau , \omega , (u_0, u_{1,0}) )&:= \left( u_\sigma (s+\tau ; \tau , \theta _{-\tau }\omega , u_0 ),\ \partial _t u_\sigma (s+\tau ; \tau , \theta _{-\tau }\omega , u_{1,0} ) \right) , \end{aligned}$$
(2.12)

for all \(s\in {\mathbb {R}}^+\), \(\tau \in {\mathbb {R}}\), \(\omega \in \Omega \) and \( (u_0, u_{1,0} ) \in H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\).

Denote by \( c_1 = 2 (1+\nu ^{-1}) \left( 16 + 2\alpha _1^2 \alpha _3^{-2} + \alpha _1^2 \Vert \varphi \Vert ^2_{L^\infty ({\mathbb {R}}^n) } \right) , \ c_2 = 8 (1+\nu ^{-1}). \) Since \(\alpha >0\) and \(\nu > 0\), it follows that there exists \(\varepsilon >0\) such that

$$\begin{aligned} \frac{1}{2}\alpha - \varepsilon -\varepsilon \gamma>0, \quad \frac{1}{2}\nu - \alpha \varepsilon + \frac{1}{2}\varepsilon ^2\gamma > 0 \quad \text {and} \quad \varepsilon \le \min \left\{ \ {\frac{32}{c_1}}, \ {\frac{8}{c_2}} \right\} . \end{aligned}$$
(2.13)

Let \({\mathcal {D}}\) be the collection of all families \(D = \left\{ D(\tau ,\omega ):\tau \in {\mathbb {R}}, \omega \in \Omega \right\} \) of bounded nonempty subsets of \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\) such that

$$\begin{aligned} \lim _{t\rightarrow +\infty } e^{- \frac{1}{4(p+1) } \varepsilon \gamma t} \Vert D(\tau -t, \theta _{-t}\omega ) \Vert _{H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)}^2 =0 \end{aligned}$$
(2.14)

for every \(\tau \in {\mathbb {R}}\) and \(\omega \in \Omega \). For the function g, we assume

$$\begin{aligned} \int _{-\infty }^\tau e^{ {\frac{1}{32(p+1)} } {\varepsilon }\gamma t } \Vert g(t)\Vert ^2 dt < \infty , \quad \tau \in {\mathbb {R}}. \end{aligned}$$
(2.15)

For convenience, we set

$$\begin{aligned} \sigma _0 = \min \left\{ 1, \ \frac{\varepsilon \gamma \alpha }{4 \alpha c_1 + 4 c_2}, \ {\frac{(4p+3){\varepsilon }\gamma }{8(p+1)c_1}}, \ {\frac{ {\varepsilon }\gamma \alpha }{16(p+1) c_2}}, \ {\frac{p\alpha {\varepsilon }\gamma \nu }{(p+1)(\nu +1) (24+50\alpha ) }} \right\} . \end{aligned}$$
(2.16)

Then under (2.1)–(2.6) and (2.15), it follows from [13, Lemma 4.2] that for every \(\sigma \in (0, \sigma _0]\), the cocycle \(\Phi _\sigma \) has a closed measurable \({\mathcal {D}}\)-pullback absorbing set as given by

$$\begin{aligned} K_\sigma (\tau ,\omega ) = \left\{ ( u, v) \in H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n): \Vert u\Vert _{H^1({\mathbb {R}}^n)}^2 + \Vert v \Vert ^2 \le L_\sigma (\tau ,\omega ) \right\} , \end{aligned}$$
(2.17)

where

$$\begin{aligned} L_\sigma (\tau , \omega )&= M_1 (1 + R_\sigma (\tau , \omega )) (1+\sigma ^2 y^2(\omega ) ) e^{2\sigma \int _{-\tau }^0 y(\theta _s\omega ) ds}, \end{aligned}$$
(2.18)

with \(M_1>0\) being a number independent of \(\tau , \omega \) and \(\sigma \), and

$$\begin{aligned} R_\sigma (\tau , \omega )&= \int _{-\infty }^0 e^{ \int _0^s \left[ \frac{1}{2} \varepsilon \gamma - \sigma \left( c_1+ c_2 y^2(\theta _{r} \omega ) \right) \right] dr } \left( 1 + \sigma y^2(\theta _{s} \omega ) + \Vert g(s+\tau )\Vert ^2 \right) \nonumber \\ {}&\quad e^{ - 2\sigma \int _{-\tau }^s y(\theta _{r} \omega ) dr } ds \end{aligned}$$
(2.19)

which satisfies

$$\begin{aligned} \lim _{t\rightarrow \infty } e^{- {\frac{{\varepsilon }\gamma t}{8(p+1)}} } R_\sigma (\tau -t, \theta _{-t}\omega )=0, \quad \forall \ \tau \in {\mathbb {R}}, \omega \in \Omega . \end{aligned}$$
(2.20)

Furthermore, by [13, Theorem 6.1] we know that the cocycle \(\Phi _\sigma \) possesses a unique \({\mathcal {D}}\)-pullback random attractor \({\mathcal {A}}_\sigma = \left\{ {\mathcal {A}} _\sigma (\tau ,\omega ): \tau \in {\mathbb {R}}, \omega \in \Omega \right\} \in {\mathcal {D}}\) in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\). We will investigate the limit of the family \(\{{\mathcal {A}} _\sigma \}\) as \(\sigma \rightarrow 0\).

For \(\sigma =0\), the stochastic equation (1.1) becomes a deterministic equation on \({\mathbb {R}}^n\):

$$\begin{aligned} \partial _{tt} u -\triangle u + \alpha \partial _t u + \nu u + f(x,u) = g(t,x), \ \ \ t > \tau , \ x \in {\mathbb {R}}^n, \end{aligned}$$
(2.21)

with initial data

$$\begin{aligned} u(\tau , x)=u_0 (x), \quad \partial _t u(\tau ,x) = u_{1,0} (x),\ \ x\in {\mathbb {R}}^n. \end{aligned}$$
(2.22)

Denote by \({\mathcal {D}}_0\) a collection of some families of bounded nonempty subsets of \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\) as given by

$$\begin{aligned} {\mathcal {D}}_0 = \bigg \{ D = \{D(\tau ):\tau \in {\mathbb {R}}\}: \lim _{t\rightarrow +\infty } e^{- \frac{1}{4(p+1) } \varepsilon \gamma t} \Vert D(\tau -t)\Vert ^2_{H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)}=0 \bigg \}. \end{aligned}$$

As for system (1.1)–(1.2), under (2.1)–(2.6) and (2.15), we know that for every \(\tau \in {\mathbb {R}}\) and \((u_0, u_{1,0}) \in H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\), problem (2.21)–(2.22) has a unique solution \((u, u_t) \in C([\tau , \infty ), H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n))\), which generates a continuous deterministic cocycle \(\Phi _0\) in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\). Moreover, the cocycle \(\Phi _0\) has a unique \({\mathcal {D}}_0\)-pullback attractor \({\mathcal {A}}_0 = \{ {\mathcal {A}}_0(\tau ): \tau \in {\mathbb {R}}\}\) in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\), and has a \({\mathcal {D}}_0\)-pullback absorbing set \(K_0 =\{ K_0(\tau ): \tau \in {\mathbb {R}}\}\) where \(K_0(\tau )\) is given by

$$\begin{aligned} K_0 (\tau ) = \left\{ (u, v) \in H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n): \Vert u \Vert _{H^1({\mathbb {R}}^n)}^2 + \Vert v \Vert ^2 \le L_0 (\tau ) \right\} , \end{aligned}$$
(2.23)

and

$$\begin{aligned} L_0 (\tau )&= M_1 + M_1 \int _{-\infty }^0 e^{ \frac{1}{2} \varepsilon \gamma s } \left( 1 + \Vert g(s+\tau )\Vert ^2 \right) ds, \end{aligned}$$
(2.24)

with \(M_1>0\) being the same number as in (2.18).

By (2.17)–(2.19) and (2.23)–(2.24), we see that

$$\begin{aligned} \limsup _{\sigma \rightarrow 0} \Vert K_\sigma (\tau ,\omega ) \Vert ^2_{ H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n) }= & {} \limsup _{\sigma \rightarrow 0} L_\sigma (\tau ,\omega ) = L_0(\tau ) \nonumber \\= & {} \Vert K_0 (\tau ) \Vert ^2_{ H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n) }. \end{aligned}$$
(2.25)

In order to investigate the upper semicontinuity of the pullback attractors \({\mathcal {A}}_\sigma \) as \(\sigma \rightarrow 0\), we first establish the convergence of solutions to (2.7)–(2.8) as \(\sigma \rightarrow 0\) in the next section.

3 Pathwise Convergence of Solutions

For convenience, for every \(\sigma \in (0,1)\), we write the solution of (2.7)–(2.8) as \(V_\sigma (t; \tau , \omega , (V_0, V_{1,0}) )\) = \(V_\sigma (t)\), and write the solution of (2.21)–(2.22) as \(u(t; \tau , (u_0, u_{1,0}) )=u(t)\). We first derive the uniform estimates of the solutions of (2.7)–(2.8).

Lemma 3.1

Assume (2.1)–(2.5) hold. Then for every \(\tau \in {\mathbb {R}}\), \(\omega \in \Omega \) and \(T>0\), there exists \(C_1=C_1( \tau , T, \omega )>0\) such that for all \((V_{0}, V_{1,0}) \in H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\), \(\sigma \in (0,1)\) and \(t \in [\tau , \tau +T]\), the solution \(V_\sigma (t)\) of (2.7)–(2.8) satisfies

$$\begin{aligned}&\Vert V_\sigma (t) \Vert _{H^1({\mathbb {R}}^n)}^2 +\Vert \partial _t V_\sigma (t)\Vert ^2 \le C_1 \left( 1+ \Vert V_{1,0} \Vert ^2 + \Vert V_{0} \Vert _{H^1({\mathbb {R}}^n)}^{p+1} +\Vert g\Vert ^2_{L^2(\tau , \tau +T; L^2({\mathbb {R}}^n))} \right) , \end{aligned}$$
(3.1)

and if \(\frac{n}{n-2}< p < \frac{n+2}{n-2}\) and \( q={\frac{2p}{(n-2)p -n}}\) with \(3\le n \le 6\), then

$$\begin{aligned} \Vert V_\sigma \Vert _ {L^q (\tau ,\tau +T; L^{2p}({\mathbb {R}}^n) ) } \le C_1 \left( 1+ \Vert V_{1,0} \Vert + \Vert V_{0} \Vert _{H^1({\mathbb {R}}^n)} ^{\frac{p+1}{2} } + \Vert g \Vert _{L^2(\tau ,\tau +T; L^2( {\mathbb {R}}^n) )} \right) . \end{aligned}$$
(3.2)

Proof

By Theorem 2.1, we have

$$\begin{aligned}&\frac{d}{dt} \left. \Bigg ( \Vert \partial _t V_\sigma (t) \Vert ^2 + \nu \Vert V_\sigma (t) \Vert ^2 + \Vert \nabla V_\sigma (t) \Vert ^2\right. \nonumber \\&\left. \qquad + 2 \int _{ {\mathbb {R}}^n } F(x, e^{\sigma \int _0^t y(\theta _s \omega ) ds } V_\sigma (t,x) ) \ e^{- 2\sigma \int _0^t y(\theta _s \omega ) ds } dx \right) \nonumber \\&\quad = - 2\alpha \Vert \partial _t V_\sigma (t)\Vert ^2 - 4\sigma y(\theta _t \omega ) \int _{ {\mathbb {R}}^n } F(x, e^{\sigma \int _0^t y(\theta _s \omega ) ds } V_\sigma (t,x) ) \ e^{- 2\sigma \int _0^t y(\theta _s \omega ) ds } dx \nonumber \\&\qquad + 2 \sigma y(\theta _t \omega ) \int _{ {\mathbb {R}}^n } f(x, e^{\sigma \int _0^t y(\theta _s \omega ) ds } V_\sigma (t,x) ) \ V_\sigma (t,x) \ e^{- \sigma \int _0^t y(\theta _s \omega ) ds } dx \nonumber \\&\qquad + 2 \left( g(t) e^{- \sigma \int _0^t y(\theta _s \omega ) ds}, \partial _t V_\sigma (t) \right) - 2 \sigma ^2 y^2(\theta _t \omega ) \left( V_\sigma (t), \partial _t V_\sigma (t) \right) - 4 \sigma y(\theta _t \omega ) \Vert \partial _t V_\sigma (t) \Vert ^2. \end{aligned}$$
(3.3)

For the third term on the right-hand side of (3.3), by (2.1), (2.3) and (2.4), we have

$$\begin{aligned}&2 \sigma y(\theta _t \omega ) \int _{ {\mathbb {R}}^n } f(x, e^{\sigma \int _0^t y(\theta _s \omega ) ds } V_\sigma (t,x) ) \ V_\sigma (t,x) \ e^{- \sigma \int _0^t y(\theta _s \omega ) ds } dx \nonumber \\&\quad \le 2 \sigma \alpha _1 |y(\theta _t \omega ) | \ \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n) } \Vert V_\sigma (t)\Vert ^2 \nonumber \\&\qquad + 2 \alpha _1 \sigma |y(\theta _t \omega )| \int _{ {\mathbb {R}}^n } \left| e^{ \sigma \int _0^t y(\theta _s \omega ) ds } V_\sigma (t,x) \right| ^{p+1} e^{- 2\sigma \int _0^t y(\theta _s \omega ) ds } dx \nonumber \\&\quad \le 2 \sigma \alpha _1 |y(\theta _t \omega ) | \ \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n) } \Vert V_\sigma (t)\Vert ^2 \nonumber \\&\qquad + \frac{2 \sigma \alpha _1}{\alpha _3} |y(\theta _t \omega )| \int _{ {\mathbb {R}}^n } F(x, e^{ \sigma \int _0^t y(\theta _s \omega ) ds } V_\sigma (t,x) ) \ e^{- 2\sigma \int _0^t y(\theta _s \omega ) ds } dx \nonumber \\&\qquad + \frac{2 \sigma \alpha _1}{\alpha _3} |y(\theta _t \omega )| e^{- 2\sigma \int _0^t y(\theta _s \omega ) ds } \Vert \varphi _1 \Vert _{L^1({\mathbb {R}}^n) }. \end{aligned}$$
(3.4)

For the fourth term on the right-hand side of (3.3), by Hölder’s inequality, we obtain

$$\begin{aligned} 2 \left( g(t) e^{- \sigma \int _0^t y(\theta _s \omega ) ds}, \partial _t V_\sigma (t) \right) \le \alpha \Vert \partial _t V_\sigma (t) \Vert ^2 + \frac{1}{\alpha } \Vert g(t) \Vert ^2 e^{- 2\sigma \int _0^t y(\theta _s \omega ) ds}. \end{aligned}$$
(3.5)

From (3.3)–(3.5), it follows that

$$\begin{aligned}&\frac{d}{dt} \left( \Vert \partial _t V_\sigma (t) \Vert ^2 + \nu \Vert V_\sigma (t) \Vert ^2 + \Vert \nabla V_\sigma (t) \Vert ^2 + 2 \int _{ {\mathbb {R}}^n } F(x, e^{\sigma \int _0^t y(\theta _s \omega ) ds } V_\sigma (t,x) ) \ e^{- 2\sigma \int _0^t y(\theta _s \omega ) ds } dx \right) \nonumber \\&\quad \le \left( 4 \sigma + \frac{2\sigma \alpha _1}{\alpha _3} \right) |y(\theta _t \omega )| \int _{ {\mathbb {R}}^n } \left( F(x, e^{\sigma \int _0^t y(\theta _s \omega ) ds } V_\sigma (t,x) ) + \varphi _1(x) \right) e^{- 2\sigma \int _0^t y(\theta _s \omega ) ds } dx \nonumber \\&\qquad + \left( 2 \sigma \alpha _1 |y(\theta _t \omega ) | \ \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n) } + \sigma ^2 y^2(\theta _t \omega ) \right) \Vert V_\sigma (t) \Vert ^2 \nonumber \\&\qquad + \left( \sigma ^2 y^2(\theta _t \omega ) + 4 \sigma |y(\theta _t \omega )| \right) \Vert \partial _t V_\sigma (t) \Vert ^2 \nonumber \\&\qquad + \frac{1}{\alpha } \Vert g(t) \Vert ^2 e^{- 2\sigma \int _0^t y(\theta _s \omega ) ds} + \left( 4\sigma + \frac{4 \sigma \alpha _1}{\alpha _3} \right) |y(\theta _t \omega )| e^{- 2\sigma \int _0^t y(\theta _s \omega ) ds } \Vert \varphi _1 \Vert _{L^1({\mathbb {R}}^n) }. \end{aligned}$$
(3.6)

By (3.6), we find that there exists \(\lambda _1=\lambda _1 (\tau , T, \omega )>0\) such that for all \(t\in (\tau , \tau +T)\) and \(\sigma \in (0,1)\),

$$\begin{aligned}&\frac{d}{dt} \left( \Vert \partial _t V_\sigma (t) \Vert ^2 + \nu \Vert V_\sigma (t) \Vert ^2 + \Vert \nabla V_\sigma (t) \Vert ^2 \right. \nonumber \\&\left. \quad + 2 \int _{ {\mathbb {R}}^n } F(x, e^{\sigma \int _0^t y(\theta _s \omega ) ds } V_\sigma (t,x) ) \ e^{- 2\sigma \int _0^t y(\theta _s \omega ) ds } dx \right) \nonumber \\&\quad \le 2 \lambda _1 \int _{ {\mathbb {R}}^n } F(x, e^{\sigma \int _0^t y(\theta _s \omega ) ds } V_\sigma (t,x) ) e^{- 2\sigma \int _0^t y(\theta _s \omega ) ds} dx + \lambda _1 \left( \nu \Vert V_\sigma (t) \Vert ^2 + \Vert \partial _t V_\sigma (t) \Vert ^2 \right) \nonumber \\&\qquad + \lambda _1 \left( 1 + \Vert g(t) \Vert ^2 \right) . \end{aligned}$$
(3.7)

By (2.1), (2.3), (2.5), (3.7) and Gronwall’s inequality, we obtain that for all \(t \in [\tau , \tau +T]\) and \(\sigma \in (0,1)\),

$$\begin{aligned}&\Vert \partial _t V_\sigma (t) \Vert ^2 + \nu \Vert V_\sigma (t) \Vert ^2 + \Vert \nabla V_\sigma (t) \Vert ^2\nonumber \\&\qquad + 2 \int _{ {\mathbb {R}}^n } F(x, e^{\sigma \int _0^t y(\theta _s \omega ) ds } V_\sigma (t,x) ) \ e^{- 2\sigma \int _0^t y(\theta _s \omega ) ds } dx \nonumber \\&\quad \le e^{\lambda _1 (t-\tau )} \left[ \Vert V_{1,0} \Vert ^2 + (\nu +1) \Vert V_{0} \Vert _{H^1{({\mathbb {R}}^n)}}^2 \right. \nonumber \\&\left. \qquad + 2 \int _{ {\mathbb {R}}^n } F(x, e^{\sigma \int _0^{\tau } y(\theta _s \omega ) ds } V_{0} (x) ) \ e^{- 2\sigma \int _0^{\tau } y(\theta _s \omega ) ds } dx \right] \nonumber \\&\qquad + \lambda _1 \int _\tau ^t e^{\lambda _1 (t-s)} (1+ \Vert g(s) \Vert ^2) ds \nonumber \\&\quad \le e^{\lambda _1 (t-\tau )} \left[ \Vert V_{1,0} \Vert ^2 + (\nu +1) \Vert V_{0} \Vert _{H^1({\mathbb {R}}^n)}^2 \right] \nonumber \\&\qquad + \frac{2\alpha _1}{\gamma } e^{\lambda _1 (t-\tau )} \left( \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n)} \Vert V_{0} \Vert ^2 + \Vert V_{0} \Vert _{L^{p+1}({\mathbb {R}}^n)}^{p+1} \ e^{\sigma (p-1) \int _0^{\tau } y(\theta _s \omega ) ds } \right) \nonumber \\&\qquad + \lambda _1 \int _\tau ^t e^{\lambda _1 (t-s)} (1+ \Vert g(s) \Vert ^2 ) ds \nonumber \\&\qquad + \frac{2}{\gamma } e^{\lambda _1 (t-\tau )} \Vert \varphi _2\Vert _{L^1({\mathbb {R}}^n)} \ e^{-2 \sigma \int _0^{\tau } y(\theta _s \omega ) ds } \nonumber \\&\quad \le e^{\lambda _1 (t-\tau )} \left[ \Vert V_{1,0} \Vert ^2 + (\nu +1) \Vert V_{0} \Vert _{H^1({\mathbb {R}}^n)}^2 \right] \nonumber \\&\qquad + \frac{2\alpha _1}{\gamma } e^{\lambda _1 (t-\tau )} \left( \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n)} \Vert V_{0} \Vert ^2 + \Vert V_{0} \Vert _{H^1({\mathbb {R}}^n)}^{p+1} \ e^{(p-1) \left| \int _0^{\tau } y(\theta _s \omega ) ds \right| } \right) \nonumber \\&\qquad + \lambda _1 \int _\tau ^t e^{\lambda _1 (t-s)} (1+ \Vert g(s) \Vert ^2 ) ds + \frac{2}{\gamma } e^{\lambda _1 (t-\tau )} \Vert \varphi _2\Vert _{L^1({\mathbb {R}}^n)} \ e^{2 \left| \int _0^{\tau } y(\theta _s \omega ) ds \right| }, \end{aligned}$$
(3.8)

where we have used the Sobolev embedding \(H^1({\mathbb {R}}^n) \hookrightarrow L^r({\mathbb {R}}^n)\) for \(2\le r < \frac{2n}{(n-2)} \).

Then it follows from (2.4) and (3.8) that there exists a constant \(C_1 =C_1(\tau , T, \omega )>0\) such that for all \(t \in [\tau , \tau +T]\) and \(\sigma \in (0,1)\),

$$\begin{aligned}&\Vert \partial _t V_\sigma (t) \Vert ^2 + \Vert V_\sigma (t) \Vert _{H^1({\mathbb {R}}^n)}^2 \le C_1 \left( 1+ \Vert V_{1,0} \Vert ^2 + \Vert V_{0} \Vert _{H^1({\mathbb {R}}^n)}^{p+1} +\Vert g\Vert ^2_{L^2(\tau , \tau +T; L^2({\mathbb {R}}^n))} \right) , \end{aligned}$$
(3.9)

which yields (3.1).

Next, we prove (3.2). By the Strichartz inequality given by Theorem 2.1, we find

$$\begin{aligned} f( \cdot , e^{\sigma \int _0^t y(\theta _s \omega ) ds} V_\sigma (t) ) e^{ - \sigma \int _0^t y(\theta _s \omega ) ds } \in L^1(\tau , \tau +T; L^2({\mathbb {R}}^n) ), \end{aligned}$$

and hence by [44, Lemma 3.4] we get for \(t \in [\tau , \tau +T]\) and \(\frac{n}{n-2}< p < \frac{n+2}{n-2}\),

$$\begin{aligned}&\Vert V_\sigma (t) \Vert _{H^1({\mathbb {R}}^n)} + \Vert \partial _t V_\sigma (t) \Vert + \Vert V_\sigma \Vert _{L^q (\tau ,t; L^{2p}({\mathbb {R}}^n) ) } \nonumber \\&\quad \le C_2 \left( \Vert V_{0} \Vert _{H^1({\mathbb {R}}^n)} + \Vert V_{1,0} \Vert \right) + C_2 \int _{\tau }^{t} \left\| f(\cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} V_\sigma (s) ) e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } \right\| ds \nonumber \\&\qquad + C_2 \int _{\tau }^{t} \left\| g(s) e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } \right\| ds + C_2 \sigma ^2 \int _{\tau }^{t} y^2(\theta _s \omega ) \Vert V_\sigma (s) \Vert ds \nonumber \\&\qquad + 2C_2 \sigma \int _{\tau }^{t} | y(\theta _s \omega )| \Vert \partial _s V_{\sigma }(s) \Vert ds, \end{aligned}$$
(3.10)

where \(C_2\) is a positive constant depending on T and \(\omega \), but independent of \(\tau , \sigma \) and initial data.

For the second term on the right-hand side of (3.10), by (2.1) and (2.3) we have

$$\begin{aligned}&\Vert f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} V_\sigma (s,x) ) \ e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } \Vert \nonumber \\&\quad \le \alpha _1 \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n)} \Vert V_\sigma (s) \Vert + \alpha _1 e^{ \sigma (p-1) \int _0^s y(\theta _r \omega ) dr } \Vert V_\sigma (s)\Vert _{L^{2p}({\mathbb {R}}^n)}^p. \end{aligned}$$
(3.11)

By (3.10), (3.11), and the Hölder inequality, we obtain that for all \(t\in [\tau , \tau +T]\) and \(\sigma \in (0,1)\),

$$\begin{aligned}&\Vert V_\sigma \Vert _{L^q (\tau ,t; L^{2p}({\mathbb {R}}^n) ) } \nonumber \\&\quad \le C_2 \left( \Vert V_{0} \Vert _{H^1({\mathbb {R}}^n)} + \Vert V_{1,0} \Vert \right) \nonumber \\&\qquad + C_2 \alpha _1 \int _\tau ^{t} \left( \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n)} \Vert V_\sigma (s) \Vert + e^{ \sigma (p-1) \int _0^s y(\theta _r \omega ) dr } \Vert V_\sigma (s) \Vert _{L^{2p}({\mathbb {R}}^n)}^p \right) ds \nonumber \\&\qquad + C_2 \int _{\tau }^{t} \left\| g(s) e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } \right\| ds + C_2 \sigma ^2 \int _{\tau }^{t} y^2(\theta _s \omega ) \Vert V_\sigma (s) \Vert ds \nonumber \\&\qquad + 2C_2 \sigma \int _{\tau }^{t} | y(\theta _s \omega )| \Vert \partial _s V_{\sigma } (s) \Vert ds \nonumber \\&\quad \le C_2 \left( \Vert V_{0} \Vert _{H^1({\mathbb {R}}^n)} + \Vert V_{1,0} \Vert \right) + C_2 \left( \alpha _1 \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n)} + \sup _{s\in [\tau ,\tau +T]} y^2(\theta _s \omega ) \right) \int _\tau ^t \Vert V_\sigma (s) \Vert ds \nonumber \\&\qquad + 2C_2 \sup _{s\in [\tau ,\tau +T]} | y(\theta _s \omega )| \int _\tau ^t \Vert \partial _s V_\sigma (s) \Vert ds + C_2 \sup _{s\in [\tau ,\tau +T]} e^{ | \int _0^{s} y(\theta _r \omega ) dr |} \int _{\tau }^{t} \left\| g(s) \right\| ds \nonumber \\&\qquad + C_2 \alpha _1 \sup _{s\in [\tau ,\tau +T]} e^{ (p-1) | \int _0^{s} y(\theta _r \omega ) dr | } (t-\tau )^{\frac{q-p}{q}} \Vert V_\sigma \Vert _{L^q (\tau ,t; L^{2p}({\mathbb {R}}^n) )}^p. \end{aligned}$$
(3.12)

Denote

$$\begin{aligned} \lambda _2&= T \left( \alpha _1 \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n)} + \sup _{s\in [\tau ,\tau +T]} y^2(\theta _s \omega ) \right) + 2 T \sup _{s\in [\tau ,\tau +T]} | y(\theta _s \omega )| \nonumber \\&\quad + (1+\alpha _1) \sup _{s\in [\tau ,\tau +T]} e^{ (p-1) | \int _0^{s} y(\theta _r \omega ) dr |}. \end{aligned}$$
(3.13)

Then by (3.9) and (3.12), we have

$$\begin{aligned}&\Vert V_\sigma \Vert _{L^q (\tau ,t; L^{2p}({\mathbb {R}}^n) ) } \nonumber \\&\quad \le C_2 \left( \Vert V_{0} \Vert _{H^1({\mathbb {R}}^n)} + \Vert V_{1,0} \Vert \right) \nonumber \\&\qquad + C_2 \lambda _2 \left( \sup _{s\in [\tau ,\tau +T]} \Vert V_\sigma (s) \Vert + \sup _{s\in [\tau ,\tau +T]} \Vert \partial _s V_{\sigma } (s) \Vert + \int _{\tau }^{\tau + T} \Vert g(s) \Vert ds \right) \nonumber \\&\qquad + C_2 \lambda _2 (t-\tau )^{\frac{q-p}{q}} \Vert V_\sigma \Vert _{L^q (\tau ,t; L^{2p}({\mathbb {R}}^n) )}^p \nonumber \\&\quad \le C_2(1+\lambda _2) \left( \sup _{s\in [\tau ,\tau +T]} \Vert V_\sigma (s) \Vert + \sup _{s\in [\tau ,\tau +T]} \Vert \partial _s V_\sigma (s) \Vert \right) \nonumber \\&\qquad + C_2 \lambda _2 \int _{\tau }^{\tau + T} \Vert g(s) \Vert ds + C_2 \lambda _2 (t-\tau )^{\frac{q-p}{q}} \Vert V_\sigma \Vert _{L^q (\tau ,t; L^{2p}({\mathbb {R}}^n) )}^p \nonumber \\&\quad \le C_1^{\frac{1}{2}} C_2(1+\lambda _2) \left( 1+ \Vert V_{1,0} \Vert + \Vert V_{0} \Vert _{H^1({\mathbb {R}}^n)}^{\frac{p+1}{2} } +\Vert g\Vert _{L^2(\tau , \tau +T; L^2({\mathbb {R}}^n))} \right) \nonumber \\&\qquad + C_2 \lambda _2 \int _{\tau }^{\tau + T} \Vert g(s) \Vert ds + C_2 \lambda _2 (t-\tau )^{\frac{q-p}{q}} \Vert V_\sigma \Vert _{L^q (\tau ,t; L^{2p} ({\mathbb {R}}^n) )}^p. \end{aligned}$$
(3.14)

Since \(p<{\frac{n+2}{n-2}}\), we see that \(q>p\). Then by (3.14) we infer that there exists \(\delta \in (0, T)\) independent of \(\sigma \) such that for all \(t\in [\tau , \tau +\delta ]\) and \(\sigma \in (0,1)\),

$$\begin{aligned} \Vert V_\sigma \Vert _{L^q (\tau ,t; L^{2p}({\mathbb {R}}^n) ) }&\le 2 C_1^{\frac{1}{2}} C_2(1+\lambda _2) \left( 1 + \Vert V_{1,0} \Vert + \Vert V_{0} \Vert _{H^1({\mathbb {R}}^n)}^{\frac{p+1}{2} } \right) \nonumber \\&\quad + \left( 2 C_1^{\frac{1}{2}} C_2 + 2 C_2\lambda _2 ( T^{\frac{1}{2}} + C_1^{\frac{1}{2}} ) \right) \Vert g\Vert _{L^2(\tau , \tau +T; L^2({\mathbb {R}}^n))} . \end{aligned}$$
(3.15)

Repeating the above argument in \([\tau +\delta , \tau +2\delta ]\) if necessary, and after a finite number of steps, we can extend the inequality (3.15) to the entire interval \([\tau , \tau +T]\), which concludes the proof. \(\square \)

In what follows, we investigate the convergence of solutions of (2.7)–(2.8) as the noise intensity \(\sigma \) approaches zero.

Lemma 3.2

Assume (2.1)–(2.5) hold. Then for every \(\eta > 0\), \(\tau \in {\mathbb {R}}\), \(\omega \in \Omega \) and \(T>0\), there exist \(c=c(T,\omega )>0\), \(\sigma _1=\sigma _1(\eta , \tau ,\omega ,T) \in (0, \sigma _0)\) and \(M=M(\tau ,\omega ,T, u_0, u_{1,0})>0\) such that for all \(t\in [\tau ,\tau +T]\) and \(\sigma \in (0, \sigma _1)\),

$$\begin{aligned}&\Vert V_\sigma (t; \tau , \omega , (V_0, V_{1,0}) ) - u(t; \tau , (u_0, u_{1,0})) \Vert _{H^1({\mathbb {R}}^n)} \\&\qquad + \Vert \partial _t V_\sigma (t; \tau , \omega , (V_0, V_{1,0}) ) - \partial _t u(t; \tau , (u_0, u_{1,0})) \Vert \nonumber \\&\quad \le M e^{ c (t-\tau ) } \bigg ( \Vert V_{0} - u_0 \Vert _{H^1({\mathbb {R}}^n)} + \Vert V_{1,0} - u_{1,0} \Vert + \eta + \sigma \bigg ), \end{aligned}$$

where \((u_0, u_{1,0})\), \((V_0, V_{1,0}) \in H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\) with \(\Vert V_{0} - u_0\Vert _{H^1({\mathbb {R}}^n)} + \Vert V_{1,0} - u_{1,0}\Vert \le 1\).

Proof

Let \(z(t) = V_\sigma (t) - u(t)\). By (2.7) and (2.21) we have

$$\begin{aligned}&\partial _{tt} z - \triangle z + \alpha \partial _t z + \nu z + f( x, e^{\sigma \int _0^t y(\theta _s \omega ) ds} V_\sigma ) \ e^{ - \sigma \int _0^t y(\theta _s \omega ) ds } - f(x,u) \nonumber \\&\quad = g(t,x) \left( e^{ - \sigma \int _0^t y(\theta _s \omega ) ds } - 1 \right) - \sigma ^2 y^2(\theta _t \omega ) V_\sigma - 2 \sigma y(\theta _t \omega ) \partial _t V_\sigma . \end{aligned}$$
(3.16)

Since both \(V_\sigma \) and u satisfy the Strichartz estimates, we see

$$\begin{aligned} f( \cdot , u ), \ \ f( \cdot , e^{\sigma \int _0^t y(\theta _s \omega ) ds} V_\sigma (t) ) e^{ - \sigma \int _0^t y(\theta _s \omega ) ds } \in L^1(\tau , \tau +T; L^2({\mathbb {R}}^n) ). \end{aligned}$$

Then by [44, Lemma 3.4] we find that for all \(t \in [\tau , \tau +T]\), \(\frac{n}{n-2}< p < \frac{n+2}{n-2}\) and \(q = {\frac{2p}{(n-2)p-n} }\),

$$\begin{aligned}&\Vert z(t) \Vert _{H^1({\mathbb {R}}^n)} + \Vert \partial _t z(t) \Vert + \Vert z \Vert _{L^q (\tau ,t; L^{2p}({\mathbb {R}}^n) ) } \nonumber \\&\quad \le C_2 \left( \Vert z(\tau ) \Vert _{H^1({\mathbb {R}}^n)} + \Vert \partial _t z(\tau ) \Vert \right) \nonumber \\&\qquad + C_2 \int _{\tau }^{t} \left\| f(\cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} V_\sigma (s) ) e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } - f(\cdot , u(s) ) \right\| ds \nonumber \\&\qquad + C_2 \int _{\tau }^{t} \left\| g(s) \left( e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } - 1 \right) \right\| ds \nonumber \\&\qquad + C_2 \sigma ^2 \int _{\tau }^{t} y^2(\theta _s \omega ) \Vert V_\sigma (s) \Vert ds + 2C_2 \sigma \int _{\tau }^{t} | y(\theta _s \omega )| \Vert \partial _s V_\sigma (s) \Vert ds, \end{aligned}$$
(3.17)

where \(C_2\) is a positive constant depending on T and \(\omega \), but independent of \(\tau , \sigma \) and initial data.

For the second term on the right-hand side of (3.17), we have

$$\begin{aligned}&C_2 \int _{\tau }^{t} \left\| f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} V_\sigma (s) ) e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } - f( \cdot , u(s) ) \right\| ds \nonumber \\&\quad \le C_2 \int _{\tau }^{t} \left\| f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} V_\sigma (s) ) - f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} u(s) ) \right\| e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } ds \nonumber \\&\qquad + C_2 \int _{\tau }^{t} \left\| \left( e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } - 1 \right) f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} u(s) ) \right\| ds \nonumber \\&\qquad + C_2 \int _{\tau }^{t} \left\| f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} u(s) ) - f( \cdot , u(s) ) \right\| ds. \end{aligned}$$
(3.18)

Applying the assumption (2.1) to the first term on the right-hand side of (3.18), we have

$$\begin{aligned}&C_2 \int _{\tau }^{t} \left\| f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} V_\sigma (s) ) - f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} u(s) ) \right\| e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } ds \nonumber \\&\quad \le \alpha _1 C_2 \int _{\tau }^{t} \left\| \left( |V_\sigma (s)|^{p-1} + |u(s)|^{p-1} \right) z(s) \right\| e^{ (p-1) \sigma \int _0^s y(\theta _r \omega ) dr } ds \nonumber \\ {}&\qquad + \alpha _1 C_2 \int _{\tau }^{t} \left\| \varphi z(s) \right\| ds. \end{aligned}$$
(3.19)

For the first term on the right-hand side of (3.19), by the Hölder inequality, we obtain

$$\begin{aligned}&\alpha _1 C_2 \int _{\tau }^{t} \left\| \left( |V_\sigma (s)|^{p-1} + |u(s)|^{p-1} \right) z(s) \right\| e^{ (p-1) \sigma \int _0^s y(\theta _r \omega ) dr } ds \nonumber \\&\quad \le \alpha _1 C_2 \int _{\tau }^{t} \left\| |V_\sigma (s)|^{p-1} z(s) \right\| e^{ (p-1) \sigma \int _0^s y(\theta _r \omega ) dr } ds\nonumber \\&\qquad + \alpha _1 C_2 \int _{\tau }^{t} \left\| |u(s)|^{p-1} z(s) \right\| e^{ (p-1) \sigma \int _0^s y(\theta _r \omega ) dr } ds \nonumber \\&\quad \le \alpha _1 C_2 \sup _{s\in [\tau , \tau +T]} e^{ (p-1) \sigma \int _0^s y(\theta _r \omega ) dr } \int _{\tau }^{t} \left\| V_\sigma (s) \right\| _{L^{2p}({\mathbb {R}}^n)}^{p-1} \Vert z(s)\Vert _{L^{2p}({\mathbb {R}}^n)} ds \nonumber \\&\qquad + \alpha _1 C_2 \sup _{s\in [\tau , \tau +T]} e^{ (p-1) \sigma \int _0^s y(\theta _r \omega ) dr } \int _{\tau }^{t} \left\| u(s) \right\| _{L^{2p}({\mathbb {R}}^n)}^{p-1} \Vert z(s)\Vert _{L^{2p}({\mathbb {R}}^n)} ds \nonumber \\&\quad \le \alpha _1 C_2 \sup _{s\in [\tau , \tau +T]} e^{ (p-1) \sigma \int _0^s y(\theta _r \omega ) dr } (t-\tau )^{\frac{q-p}{q} } \Vert z\Vert _{L^q(\tau , t; L^{2p}({\mathbb {R}}^n) ) } \nonumber \\&\qquad \cdot \left( \Vert V_\sigma \Vert _{L^q(\tau , t; L^{2p}({\mathbb {R}}^n) ) }^{p-1} + \Vert u \Vert _{L^q(\tau , t; L^{2p}({\mathbb {R}}^n) ) }^{p-1} \right) . \end{aligned}$$
(3.20)

Since \(\Vert V_{0} - u_0 \Vert _{H^1({\mathbb {R}}^n)} + \Vert V_{1,0} - u_{1,0} \Vert \le 1 \), we find by (3.2) that there exists a constant \(C_3>0\), depending on \(\tau , T, \omega \) and \((u_0, u_{1,0})\) but not on \(\sigma \) or \((V_{0}, V_{1,0})\), such that

$$\begin{aligned} \Vert V_\sigma \Vert _{L^q(\tau , \tau +T; L^{2p}({\mathbb {R}}^n) ) } + \Vert u \Vert _{L^q(\tau , \tau +T; L^{2p}({\mathbb {R}}^n) ) } \le C_3. \end{aligned}$$
(3.21)

Then by (3.20) and (3.21), we obtain

$$\begin{aligned}&\alpha _1 C_2 \int _{\tau }^{t} \left\| \left( |V_\sigma (s)|^{p-1} + |u(s)|^{p-1} \right) z(s) \right\| e^{ (p-1) \sigma \int _0^s y(\theta _r \omega ) dr } ds \nonumber \\&\quad \le 2 \alpha _1 C_2 C_3^{p-1} \sup _{s\in [\tau , \tau +T]} e^{ (p-1) | \int _0^s y(\theta _r \omega ) dr| } (t-\tau )^{\frac{q-p}{q} } \Vert z\Vert _{L^q(\tau , t; L^{2p}({\mathbb {R}}^n) ) }, \end{aligned}$$

which implies that there exists \(\delta _1>0\), depending only on \(\tau , T, \omega \) and \((u_0, u_{1,0})\) but not on \(\sigma \) or \((V_{0}, V_{1,0})\), such that for all \(t \in [\tau , \tau +\delta _1]\) and \(\sigma \in (0,1)\),

$$\begin{aligned} \alpha _1 C_2 \int _{\tau }^{t} \left\| \left( |V_\sigma (s)|^{p-1} + |u(s)|^{p-1} \right) z(s) \right\| e^{ (p-1) \sigma \int _0^s y(\theta _r \omega ) dr } ds \le \frac{1}{2} \Vert z\Vert _{L^q(\tau , t; L^{2p}({\mathbb {R}}^n) ) }. \end{aligned}$$
(3.22)

Then by (3.19) and (3.22), we have

$$\begin{aligned}&C_2 \int _{\tau }^{t} \left\| f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} V_\sigma (s) ) - f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} u(s) ) \right\| e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } ds \nonumber \\&\quad \le \frac{1}{2} \Vert z\Vert _{L^q(\tau , t; L^{2p}({\mathbb {R}}^n) ) } + \alpha _1 C_2 \int _{\tau }^{t} \left\| \varphi z(s) \right\| ds. \end{aligned}$$
(3.23)

For the second term on the right-hand side of (3.18), by (2.1), we have

$$\begin{aligned}&C_2 \int _{\tau }^{t} \left\| \left( e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } - 1 \right) f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} u(s) ) \right\| ds \nonumber \\&\quad \le \alpha _1 C_2 \int _{\tau }^{t} \big | e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } - 1 \big | \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n)} e^{\sigma \int _0^s y(\theta _r \omega ) dr} \Vert u(s)\Vert ds \nonumber \\&\qquad + \alpha _1 C_2 \int _{\tau }^{t} \big | e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } - 1 \big | e^{\sigma p \int _0^s y(\theta _r \omega ) dr} \Vert u(s)\Vert _{L^{2p}({\mathbb {R}}^n)}^p ds, \quad t \in [\tau , \tau +T]. \end{aligned}$$
(3.24)

Given \(\tau \in {\mathbb {R}}\), \(\omega \in \Omega \), \(T > 0\) and \(\eta \in (0, 1)\), by the continuity of \(\int _0^s y(\theta _r \omega ) dr\) in \(s\in {\mathbb {R}}\), we find that there exists \(\sigma _1 \in (0, \sigma _0)\), depending only on \(\tau , \omega , T\) and \(\eta \), such that for all \(\sigma \in (0, \sigma _1)\) and \(s \in [\tau , \tau +T]\),

$$\begin{aligned} \left| e^{ \sigma \int _0^s y(\theta _r \omega ) dr } - 1 \right| + \left| e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } - 1 \right| < \eta . \end{aligned}$$
(3.25)

Then by (3.24) and (3.25), we obtain for all \(t \in [\tau , \tau +T]\) and \(\sigma \in (0, \sigma _1)\),

$$\begin{aligned}&C_2 \int _{\tau }^{t} \left\| \left( e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } - 1 \right) f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} u(s) ) \right\| ds \nonumber \\&\quad \le \eta \alpha _1 C_2 \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n)} \sup _{s \in [\tau , \tau +T]} e^{ \left| \int _0^s y(\theta _r \omega ) dr \right| } \int _{\tau }^{t} \Vert u(s)\Vert ds \nonumber \\&\qquad + \eta \alpha _1 C_2 \sup _{s \in [\tau , \tau +T]} e^{ p \left| \int _0^s y(\theta _r \omega ) dr \right| } T^{\frac{q-p}{q}} \Vert u \Vert _{L^q(\tau , t; L^{2p}({\mathbb {R}}^n) ) }^{p}. \end{aligned}$$
(3.26)

Similar to (3.1), we find that there exists \(C_4=C_4(\tau , T)>0\) such that for all \(t\in [\tau , \tau +T]\),

$$\begin{aligned} \Vert u(t)\Vert _{H^1({\mathbb {R}}^n)} + \Vert \partial _t u(t)\Vert \le C_4 \left( 1+ \Vert u_{1,0}\Vert + \Vert u_0\Vert _{H^1({\mathbb {R}}^n)} ^{\frac{p+1}{2}} \right) . \end{aligned}$$
(3.27)

Then by (3.21), (3.26) and (3.27), we obtain for all \(t \in [\tau , \tau +T]\) and \(\sigma \in (0, \sigma _1)\),

$$\begin{aligned}&C_2 \int _{\tau }^{t} \left\| \left( e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } - 1 \right) f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} u(s) ) \right\| ds \nonumber \\&\quad \le \eta \alpha _1 C_2C_4T \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n)} \sup _{s \in [\tau , \tau +T]} e^{ \left| \int _0^s y(\theta _r \omega ) dr \right| } \left( 1+ \Vert u_{1,0}\Vert + \Vert u_0\Vert _ {H^1({\mathbb {R}}^n)}^{\frac{p+1}{2}} \right) \nonumber \\&\qquad + \eta \alpha _1 C_2 C_3^{p} T^{\frac{q-p}{q}} \sup _{s \in [\tau , \tau +T]} e^{ p \left| \int _0^s y(\theta _r \omega ) dr \right| }. \end{aligned}$$
(3.28)

For the third term on the right-hand side of (3.18), by (2.1) and (3.25), we get

$$\begin{aligned}&C_2 \int _{\tau }^{t} \left\| f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} u(s) ) - f( \cdot , u(s) ) \right\| ds \nonumber \\&\quad \le \eta \alpha _1 C_2 \left( 1+ \sup _{s\in [\tau , \tau +T]} e^{\sigma (p-1) \int _0^s y(\theta _r \omega ) dr} \right) \int _{\tau }^{t} \Vert u(s)\Vert _{L^{2p}({\mathbb {R}}^n)}^p ds \\&\qquad + \eta \alpha _1 C_2 \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n)} \int _{\tau }^{t} \Vert u(s)\Vert ds, \end{aligned}$$

which together with (3.21) and (3.27) yields that for all \(t \in [\tau , \tau +T]\) and \(\sigma \in (0, \sigma _1)\),

(3.29)

From (3.18), (3.23), (3.28) and (3.29), it follows that there exists \(L=L(\tau , T, \omega )>0\) such that for all \(t\in [\tau , \tau +\delta _1]\) and \(\sigma \in (0, \sigma _1)\),

$$\begin{aligned}&C_2 \int _{\tau }^{t} \left\| f( \cdot , e^{\sigma \int _0^s y(\theta _r \omega ) dr} V_\sigma (s) ) e^{ - \sigma \int _0^s y(\theta _r \omega ) dr } - f( \cdot , u(s) ) \right\| ds \nonumber \\&\quad \le \frac{1}{2} \Vert z\Vert _{L^q(\tau , t; L^{2p}({\mathbb {R}}^n) ) } + \alpha _1 C_2 \int _{\tau }^{t} \left\| \varphi z(s) \right\| ds + \eta C_2 L \left( 1 + \Vert u_{1,0}\Vert + \Vert u_0\Vert _{H^1({\mathbb {R}}^n)} ^{\frac{p+1}{2}} \right) . \end{aligned}$$
(3.30)

By (3.17), (3.25) and (3.30), we obtain that for any \(\sigma \in (0, \sigma _1)\) and \(t \in [ \tau , \tau +\delta _1 ]\),

$$\begin{aligned}&\Vert z(t) \Vert _{H^1({\mathbb {R}}^n)} + \Vert \partial _t z(t) \Vert + \frac{1}{2} \Vert z \Vert _{L^q (\tau ,t; L^{2p}({\mathbb {R}}^n) ) } \nonumber \\&\quad \le C_2 \left( \Vert z(\tau ) \Vert _{H^1({\mathbb {R}}^n)} + \Vert \partial _t z(\tau ) \Vert \right) + \alpha _1 C_2 \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n)} \int _{\tau }^{t} \Vert z(s) \Vert ds \nonumber \\&\qquad + \eta C_2 L \left( 1 + \Vert u_{1,0}\Vert + \Vert u_0\Vert _{H^1({\mathbb {R}}^n)}^{\frac{p+1}{2}} \right) + \eta C_2 \int _{\tau }^{\tau +T} \Vert g(s)\Vert ds \nonumber \\&\qquad + C_2 \sigma ^2 \int _{\tau }^{\tau +T} y^2(\theta _s \omega ) \Vert V_\sigma (s) \Vert ds + 2C_2 \sigma \int _{\tau }^{\tau +T} | y(\theta _s \omega )| \Vert \partial _s V_\sigma (s) \Vert ds. \end{aligned}$$
(3.31)

Then from the Gronwall inequality, it follows that for all \(\sigma \in (0, \sigma _1)\) and \(t \in [ \tau , \tau +\delta _1 ]\),

$$\begin{aligned}&\Vert z(t) \Vert _{H^1({\mathbb {R}}^n)} + \Vert \partial _t z(t) \Vert \nonumber \\&\quad \le C_2 e^{ \alpha _1 C_2 \Vert \varphi \Vert _{L^\infty ({\mathbb {R}}^n)} (t-\tau ) } \bigg [ \Vert z(\tau ) \Vert _{H^1({\mathbb {R}}^n)} + \Vert \partial _t z(\tau ) \Vert + \eta L \left( 1 + \Vert u_{1,0}\Vert + \Vert u_0\Vert _{H^1({\mathbb {R}} ^n)}^{\frac{p+1}{2}} \right) \nonumber \\&\qquad + \eta \int _{\tau }^{\tau +T} \Vert g(s)\Vert ds + \sigma ^2 \int _{\tau }^{\tau +T} y^2(\theta _s \omega ) \Vert V_\sigma (s) \Vert ds + 2 \sigma \int _{\tau }^{\tau +T} | y(\theta _s \omega )| \Vert \partial _s V_\sigma (s) \Vert ds \bigg ], \end{aligned}$$

which along with (3.1) implies that there exists \(C_3=C_3(\tau , T, \omega , u_0, u_{1,0})>0\) such that for all \(\sigma \in (0, \sigma _1)\) and \(t \in [ \tau , \tau +\delta _1 ]\),

$$\begin{aligned}&\Vert z(t) \Vert _{H^1({\mathbb {R}}^n)} + \Vert \partial _t z(t) \Vert \le C_3 \bigg ( \Vert z(\tau ) \Vert _{H^1({\mathbb {R}}^n)} + \Vert \partial _s z(\tau ) \Vert + \eta + \sigma \bigg ). \end{aligned}$$
(3.32)

Repeating the above argument in \([\tau +\delta _1, \tau +2\delta _1]\) if necessary, and after a finite number of steps, we can extend the inequality (3.32) to the entire interval \([\tau , \tau +T]\), which concludes the lemma when \(p\in ( {\frac{n}{n-2}}, \ {\frac{n+2}{n-2}} )\) for \(3\le n \le 6\).

If \(1\le p\le {\frac{n}{n-2}}\) for \(3\le n \le 6\) or \(p\ge 1\) for \(1\le n \le 2\), then \(H^1({\mathbb {R}}^n) \hookrightarrow L^{2p} ({\mathbb {R}}^n)\) and hence the proof is much simpler in this case and thus is omitted. \(\square \)

In the next section, we prove the uniform precompactness of the family of random attractors \(\{{\mathcal {A}}_\sigma \}\) in \(H^1({\mathbb {R}}^n)\times L^2({\mathbb {R}}^n)\).

4 Uniform Precompactness of Random Attractors

To prove the precompactness of the set \(\{{\mathcal {A}}_\sigma \} _{\sigma \in (0, \sigma _0)} \), we introduce a family of subsets of \(H^1({\mathbb {R}}^n)\times L^2({\mathbb {R}}^n)\): for every \(\tau \in {\mathbb {R}}\) and \(\omega \in \Omega \), setting

$$\begin{aligned} K(\tau ,\omega ) = \left\{ ( u_0, u_{1,0}) \in H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n): \Vert u_0\Vert _{H^1({\mathbb {R}}^n)}^2 + \Vert u_{1,0} \Vert ^2 \le L(\tau ,\omega ) \right\} , \end{aligned}$$

where

$$\begin{aligned} L(\tau , \omega )&= M_1 (1+ \sigma _0^2 y^2(\omega )) e^{ 2 \sigma _0 \left| \int _{-\tau } ^0 y(\theta _s \omega ) ds \right| } \bigg [ 1 + \int _{-\infty }^0 e^{ \int _0^s \left[ \frac{1}{2} \varepsilon \gamma - \sigma _0 \left( c_1+ c_2 y^2(\theta _{r} \omega ) \right) \right] dr } \nonumber \\&\quad \cdot \left( 1 + \sigma _0 y^2(\theta _{s} \omega ) + \Vert g(s+\tau )\Vert ^2 \right) e^{ 2 \sigma _0 \left| \int _{-\tau }^s y(\theta _{r} \omega ) dr \right| } ds \bigg ], \end{aligned}$$
(4.1)

and \(M_1\) is the same number as in (2.18).

Then by (2.20) and \(\lim _{t \rightarrow +\infty } \frac{1}{t} \int _{-t}^{0} y(\theta _{r} \omega ) dr = 0\), we know that

$$\begin{aligned} \lim _{t\rightarrow +\infty } e^{ -\frac{3}{16(p+1)} \varepsilon \gamma t } \Vert K(\tau -t, \theta _{-t}\omega ) \Vert _{ H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n) }^2 = 0, \end{aligned}$$
(4.2)

which implies \( K = \{ K(\tau ,\omega ): \tau \in {\mathbb {R}}, \omega \in \Omega \} \in {\mathcal {D}}. \)

Moreover, by comparing (2.17) with (4.1), we have for any \(\tau \in {\mathbb {R}}\) and \(\omega \in \Omega \),

$$\begin{aligned} \bigcup _{\sigma \in (0,\sigma _0] } {\mathcal {A}}_\sigma (\tau ,\omega ) \subseteq \bigcup _{\sigma \in (0,\sigma _0] } K_\sigma (\tau ,\omega ) \subseteq K(\tau ,\omega ). \end{aligned}$$
(4.3)

We now prove the following compactness results on the family of attractors \(\left\{ {\mathcal {A}}_{\sigma } \right\} _{\sigma \in (0,\sigma _0)}\). To avoid confusion, if necessary, the derivatives of the solutions u and V with respect to time are also written as \({\dot{u}}\) and \({\dot{V}}\), respectively.

Lemma 4.1

Assume (2.1)–(2.6) and (2.15) hold, \(\tau \in {\mathbb {R}}\), \(\omega \in \Omega \), and \(\sigma _k \rightarrow 0\). If \((u_k,v_k) \in {\mathcal {A}}_{\sigma _k}(\tau , \omega )\), then the sequence \(\{ (u_k,v_k) \}_{k=1}^\infty \) is precompact in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\).

Proof

Without loss of generality, we may assume \(\sigma _k\le \sigma _0\) for all \(k\in {\mathbb {N}}\). It follows from the invariance of \({\mathcal {A}}_{\sigma _k}\) that for every \(k\in {\mathbb {N}}\), there exists \((\varphi _k, \psi _k ) \in {\mathcal {A}}_{\sigma _k} (\tau -k,\theta _{-k} \omega )\) such that

$$\begin{aligned} ( u_k, v_k )&= \Phi _{\sigma _k}(k, \tau -k, \theta _{-k} \omega , (\varphi _k ,\psi _k ) ) \nonumber \\&= \left( u_{\sigma _k} (\tau ; \tau -k ,\theta _{-\tau }\omega , (\varphi _k ,\psi _k) ), \ {\dot{u}}_{\sigma _k} (\tau ; \tau -k ,\theta _{-\tau }\omega , (\varphi _k ,\psi _k ) ) \right) . \end{aligned}$$
(4.4)

Claim 1. \(\{(u_k, v_k)\}_{k=1}^\infty \) is uniformly infinitesimal outside a large ball in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\).

By the transform \(V(t) = u(t) e^{-\sigma \int _0^t y(\theta _s \omega ) ds }\), we have

$$\begin{aligned}&u_{\sigma _k} ( \tau ; \tau -k , \theta _{-\tau }\omega , (\varphi _k ,\psi _k ) ) = V_{\sigma _k}( \tau ; \tau -k ,\theta _{-\tau }\omega , ( {\widetilde{\varphi }}_k , {\widetilde{\psi }}_k ) ) \ e^{ \sigma _k \int _0^{\tau } y(\theta _{s-\tau } \omega ) ds } \end{aligned}$$
(4.5)

and

$$\begin{aligned}&{\dot{u}}_{\sigma _k} (\tau ; \tau -k, \theta _{-\tau }\omega , (\varphi _k ,\psi _k ) ) \nonumber \\&\quad = {\dot{V}}_{\sigma _k} ( \tau ; \tau -k ,\theta _{-\tau }\omega , ( {\widetilde{\varphi }}_k , {\widetilde{\psi }}_k ) ) \ e^{ \sigma _k \int _0^\tau y(\theta _{s-\tau } \omega ) ds } \nonumber \\&\qquad + \sigma _k y(\omega ) V_{\sigma _k}( \tau ; \tau -k, \theta _{-\tau }\omega , ( {\widetilde{\varphi }}_k , {\widetilde{\psi }}_k ) ) \ e^{ \sigma _k \int _0^{\tau } y(\theta _{s-\tau } \omega ) ds }, \end{aligned}$$
(4.6)

where \({\widetilde{\varphi }}_k = \varphi _k e^{-\sigma _k \int _0^{\tau -k} y(\theta _{s-\tau } \omega ) ds}\) and \({\widetilde{\psi }}_k = e^{-\sigma _k \int _0^{\tau -k} y(\theta _{s-\tau } \omega ) ds} \left( \psi _k - \sigma _k y(\theta _{-k} \omega ) \varphi _k \right) \).

For any \(\tau \in {\mathbb {R}}\) and \(\omega \in \Omega \), let

$$\begin{aligned} {\bar{K}}(\tau ,\omega ) = \left\{ ( v_0, v_{1,0}) \in H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n): \ \Vert v_0\Vert _{H^1({\mathbb {R}}^n)}^2 + \Vert v_{1,0} \Vert ^2 \le {\bar{L}}(\tau ,\omega ) \right\} , \end{aligned}$$
(4.7)

where

$$\begin{aligned} {\bar{L}}(\tau , \omega ) = 2 e^{2\sigma _0 | \int _0^\tau y (\theta _{r-\tau } \omega ) dr| } \left( 1+ \sigma _0^2 |y(\omega )|^2 \right) L(\tau ,\omega ). \end{aligned}$$

Then \( {\bar{K}} = \{ {\bar{K}}(\tau ,\omega ): \tau \in {\mathbb {R}},\omega \in \Omega \} \in {\mathcal {D}} \) due to (4.2). By \((\varphi _k, \psi _k ) \in {\mathcal {A}}_{\sigma _k} (\tau -k,\theta _{-k} \omega ) \subseteq K(\tau -k, \theta _{-k}\omega )\), we find \(( {\widetilde{\varphi }}_k, {\widetilde{\psi }}_k ) \in {\bar{K}}(\tau -k,\theta _{-k} \omega )\). Then together with (4.5) and (4.6), it follows from [13, Lemma 5.1] on the uniform tail-estimates of the solutions of (2.7) that for every \(\epsilon '>0, \tau \in {\mathbb {R}}\) and \(\omega \in \Omega \), there exist \(T_0 = T_0(\epsilon ', \tau , \omega , K)>0\) and \(m_0 = m_0(\epsilon ', \tau , \omega ) \ge 1\) such that for all \(k \ge T_0\), \(m \ge m_0\) and \(k \in {\mathbb {N}}\),

$$\begin{aligned}&\int _{ |x | \ge m} \left( | u_{ \sigma _k} ( \tau , x; \tau -k, \theta _{-\tau }\omega , (\varphi _k, \psi _k ) ) |^2 + | \nabla u_{\sigma _k} ( \tau , x; \tau -k, \theta _{-\tau }\omega , (\varphi _k, \psi _k ) ) |^2 \right) dx \nonumber \\&\quad + \int _{ |x | \ge m} | {\dot{u}}_{\sigma _k} ( \tau , x; \tau -k, \theta _{-\tau }\omega , (\varphi _k, \psi _k) ) |^2 dx < \frac{\epsilon '}{2}. \end{aligned}$$
(4.8)

So by (4.4) and (4.8), we obtain that for all \(k \ge T_0\) and \(k \in {\mathbb {N}}\),

$$\begin{aligned} \left\| \Phi _{\sigma _k} ( k, \tau -k, \theta _{-k}\omega , (\varphi _k, \psi _k) ) |_{{\mathcal {O}}_{m_0}^c} \right\| ^2_{H^1({\mathcal {O}}_{m_0}^c) \times L^2({\mathcal {O}}_{m_0}^c)} < \frac{\epsilon '}{2}, \end{aligned}$$
(4.9)

where \({\mathcal {O}}_{m_0}^c\) is the complement of \({\mathcal {O}}_{m_0} = \{ x \in {\mathbb {R}}^n: \ |x| \le m_0\}\). By (4.4) and (4.9), we obtain for all \(k \in {\mathbb {N}}\),

$$\begin{aligned} \left\| (u_k, v_k) |_{{\mathcal {O}}_{m_0}^c} \right\| _{H^1({\mathcal {O}}_{m_0}^c) \times L^2({\mathcal {O}}_{m_0}^c)} < \epsilon '. \end{aligned}$$
(4.10)

Claim 2. \(\{(u_k, v_k) |_{{\mathcal {O}}_{m_0}}\}_{k=1}^\infty \) is precompact in \(H^1({\mathcal {O}}_{m_0}) \times L^2({\mathcal {O}}_{m_0})\).

Let \(V_{\sigma _k}(t) =: V_{\sigma _k} ( t; \tau -k, \theta _{-\tau }\omega , ({\widetilde{\varphi }}_k, {\widetilde{\psi }}_k ) ) \). Then by the uniqueness of solutions of (2.7)–(2.8), \(V_{\sigma _k}(t)\) can be decomposed into \(V_{\sigma _k}(t) = {\widetilde{V}}_{\sigma _k}(t) + {\widehat{V}}_{\sigma _k}(t)\), where \({\widetilde{V}}_{\sigma _k}(t)\) and \({\widehat{V}}_{\sigma _k}(t)\) are the solutions of the following random wave equations:

$$\begin{aligned} \left\{ \begin{array}{ll} &{} \partial _{tt} {\widetilde{V}} - \triangle {\widetilde{V}} + \alpha \partial _t {\widetilde{V}} + \nu {\widetilde{V}} = g(t,x)\ e^{ - \sigma _k \int _0^t y(\theta _{s-\tau } \omega ) ds } \\ &{} \qquad \qquad \qquad \qquad \qquad \qquad \qquad \quad - 2 \sigma _k y(\theta _{t-\tau } \omega ) \partial _t {\widetilde{V}} - \sigma _k^2 y^2(\theta _{t-\tau } \omega ) {\widetilde{V}}, \\ &{} {\widetilde{V}}(\tau -k) = {\widetilde{\varphi }}_k , \quad \partial _t {\widetilde{V}}(\tau -k) = {\widetilde{\psi }}_k , \end{array} \right. \end{aligned}$$
(4.11)

and

$$\begin{aligned} \left\{ \begin{array}{ll} &{} \partial _{tt} {\widehat{V}} - \triangle {\widehat{V}} + \alpha \partial _t {\widehat{V}} + \nu {\widehat{V}} = - f( x, e^{\sigma _k \int _0^t y(\theta _{s-\tau } \omega ) ds} V_{\sigma _k} )\ e^{ - \sigma _k \int _0^t y(\theta _{s-\tau } \omega ) ds } \\ &{} \qquad \qquad \qquad \qquad \qquad \qquad \qquad \quad - 2 \sigma _k y(\theta _{t-\tau } \omega ) \partial _t {\widehat{V}} - \sigma _k^2 y^2(\theta _{t-\tau } \omega ) {\widehat{V}}, \\ &{} {\widehat{V}}(\tau -k) = 0, \quad \partial _t {\widehat{V}}(\tau -k) = 0. \end{array} \right. \end{aligned}$$
(4.12)

In what follows, we will prove Claim 2 in three steps.

Step 1: prove \(\{ ({\widetilde{V}}_{\sigma _k}(\tau ), \partial _t {\widetilde{V}}_{\sigma _k}(\tau )) \}_{k=1}^\infty \) is a Cauchy sequence in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\).

For any \(k,l \in {\mathbb {N}}\) with \(k < l\), let \( V_{k,l}(t) = {\widetilde{V}}_{\sigma _k}(t) - {\widetilde{V}}_{\sigma _l}(t). \) Then \(V_{k,l}\) satisfies the following equation:

$$\begin{aligned}&\partial _{tt} V_{k,l}(t) - \triangle V_{k,l}(t) + \alpha \partial _t V_{k,l}(t) + \nu V_{k,l}(t) \nonumber \\&\quad = g(t,x) \left( e^{ - \sigma _k \int _0^t y(\theta _{s-\tau } \omega ) ds } - e^{ - \sigma _l \int _0^t y(\theta _{s-\tau } \omega ) ds } \right) \nonumber \\&\qquad - 2 \sigma _k y(\theta _{t-\tau } \omega ) \partial _t V_{k,l}(t) - \sigma _k^2 y^2(\theta _{t-\tau } \omega ) V_{k,l}(t) \nonumber \\&\qquad - 2 \left( \sigma _k -\sigma _l \right) y(\theta _{t-\tau } \omega ) \partial _t {\widetilde{V}}_{\sigma _l}(t) - \left( \sigma _k^2 - \sigma _l^2 \right) y^2(\theta _{t-\tau } \omega ) {\widetilde{V}}_{\sigma _l}(t), \quad t > \tau - k, \end{aligned}$$
(4.13)

with initial data

$$\begin{aligned} V_{k,l}(\tau - k)&= V_{\sigma _k}(\tau - k) - V_{\sigma _l}(\tau - k), \nonumber \\ {\dot{V}}_{k,l}(\tau - k)&= {\dot{V}}_{\sigma _k}(\tau - k) - {\dot{V}}_{\sigma _l}(\tau - k). \end{aligned}$$

Since \(g \in L^2(\tau , \tau +T; L^2({\mathbb {R}}^n) )\) and \({\widetilde{V}}_{\sigma _l} \in L^\infty (\tau , \tau +T; H^1({\mathbb {R}}^n) ), \partial _t {\widetilde{V}}_{\sigma _l} \in L^\infty ( \tau , \tau +T; L^2({\mathbb {R}}^n) ) \), it follows from the energy equation associated with (4.13) that

$$\begin{aligned}&\frac{d}{dt} \left( \Vert \partial _t V_{k,l}(t) \Vert ^2 + \nu \Vert V_{k,l}(t) \Vert ^2 + \Vert \nabla V_{k,l}(t) \Vert ^2 + \varepsilon \left( V_{k,l}(t),\ \partial _t V_{k,l}(t) \right) \right) \nonumber \\&\qquad + (2 \alpha - \varepsilon ) \Vert \partial _t V_{k,l}(t) \Vert ^2 + \varepsilon \Vert \nabla V_{k,l}(t) \Vert ^2 + \varepsilon \nu \Vert V_{k,l}(t) \Vert ^2 + \varepsilon \alpha \left( V_{k,l}(t),\ \partial _t V_{k,l}(t) \right) \nonumber \\&\quad = \left( e^{ - \sigma _k \int _0^t y(\theta _{s-\tau } \omega ) ds } - e^{ - \sigma _l \int _0^t y(\theta _{s-\tau } \omega ) ds } \right) \left( g(t, \cdot ), \ 2 \partial _t V_{k,l}(t) + \varepsilon V_{k,l}(t) \right) \nonumber \\&\qquad - 2 \sigma _k y(\theta _{t-\tau } \omega ) \left( \partial _t V_{k,l}(t), \ 2 \partial _t V_{k,l}(t) + \varepsilon V_{k,l}(t) \right) \nonumber \\&\qquad - \sigma _k^2 y^2(\theta _{t-\tau } \omega ) \left( V_{k,l}(t),\ 2 \partial _t V_{k,l}(t) + \varepsilon V_{k,l}(t) \right) \nonumber \\&\qquad - 2 \left( \sigma _k -\sigma _l \right) y(\theta _{t-\tau } \omega ) \left( \partial _t {\widetilde{V}}_{\sigma _l}(t), \ 2 \partial _t V_{k,l}(t) + \varepsilon V_{k,l}(t) \right) \nonumber \\&\qquad - \left( \sigma _k^2 - \sigma _l^2 \right) y^2(\theta _{t-\tau } \omega ) \left( {\widetilde{V}}_{\sigma _l}(t), \ 2 \partial _t V_{k,l}(t) + \varepsilon V_{k,l}(t) \right) . \end{aligned}$$
(4.14)

For the first term on the right-hand side of (4.14), we have

$$\begin{aligned}&\left( e^{ - \sigma _k \int _0^t y(\theta _{s-\tau } \omega ) ds } - e^{ - \sigma _l \int _0^t y(\theta _{s-\tau } \omega ) ds } \right) \left( g(t, \cdot ), \ 2 \partial _t V_{k,l}(t) + \varepsilon V_{k,l}(t) \right) \nonumber \\&\quad \le \frac{\alpha }{3} \Vert \partial _{t} V_{k,l}(t) \Vert ^2 + \frac{\varepsilon \nu }{12} \Vert V_{k,l}(t) \Vert ^2 \nonumber \\&\qquad + 3 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \Vert g(t,\cdot )\Vert ^2 \left( e^{ - \sigma _k \int _0^t y(\theta _{s-\tau } \omega ) ds } - e^{ - \sigma _l \int _0^t y(\theta _{s-\tau } \omega ) ds } \right) ^2. \end{aligned}$$
(4.15)

For the last two terms on the right-hand side of (4.14), we have

$$\begin{aligned}&- 2 \left( \sigma _k -\sigma _l \right) y(\theta _{t-\tau } \omega ) \left( \partial _t {\widetilde{V}}_{\sigma _l}(t), \ 2 \partial _t V_{k,l}(t) + \varepsilon V_{k,l}(t) \right) \nonumber \\&\qquad - \left( \sigma _k^2 - \sigma _l^2 \right) y^2(\theta _{t-\tau } \omega ) \left( {\widetilde{V}}_{\sigma _l}(t), \ 2 \partial _t V_{k,l}(t) + \varepsilon V_{k,l}(t) \right) \nonumber \\&\quad \le \frac{2\alpha }{3} \Vert \partial _{t} V_{k,l}(t) \Vert ^2 + \frac{\varepsilon \nu }{6} \Vert V_{k,l}(t) \Vert ^2 + 12 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \left( \sigma _k -\sigma _l \right) ^2 y^2(\theta _{t-\tau } \omega )\ \Vert \partial _t {\widetilde{V}}_{\sigma _l}(t) \Vert ^2 \nonumber \\&\qquad + 3 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \left( \sigma _k^2 - \sigma _l^2 \right) ^2 y^4(\theta _{t-\tau } \omega )\ \Vert {\widetilde{V}}_{\sigma _l}(t) \Vert ^2. \end{aligned}$$
(4.16)

For the second and third terms on the right-hand side of (4.14), we have

$$\begin{aligned}&- 2 \sigma _k y(\theta _{t-\tau } \omega ) \left( \partial _t V_{k,l}(t), \ 2 \partial _t V_{k,l}(t) + \varepsilon V_{k,l}(t) \right) \nonumber \\&\qquad - \sigma _k^2 y^2(\theta _{t-\tau } \omega ) \left( V_{k,l}(t),\ 2 \partial _t V_{k,l}(t) + \varepsilon V_{k,l}(t) \right) \nonumber \\&\quad = - 4 \sigma _k y(\theta _{t-\tau } \omega ) \Vert \partial _t V_{k,l}(t) \Vert ^2 - 2 \varepsilon \sigma _k y(\theta _{t-\tau } \omega ) \left( \partial _t V_{k,l}(t),\ V_{k,l}(t) \right) \nonumber \\&\qquad - 2 \sigma _k^2 y^2(\theta _{t-\tau } \omega ) \left( \partial _t V_{k,l}(t),\ V_{k,l}(t) \right) - \varepsilon \sigma _k^2 y^2(\theta _{t-\tau } \omega ) \Vert V_{k,l}(t) \Vert ^2 \nonumber \\&\quad \le \left( 5 \sigma _k |y(\theta _{t-\tau } \omega )| + \sigma _k^2 y^2(\theta _{t-\tau } \omega ) \right) \Vert \partial _t V_{k,l}(t) \Vert ^2 \nonumber \\&\qquad + \left( \sigma _k |y(\theta _{t-\tau } \omega )| +2 \sigma _k^2 y^2(\theta _{t-\tau } \omega ) \right) \Vert V_{k,l}(t) \Vert ^2 \nonumber \\&\quad \le \sigma _k \left( \frac{25}{4} + 3 y^2(\theta _{t-\tau } \omega ) \right) \left( \Vert \partial _t V_{k,l}(t) \Vert ^2 + \Vert V_{k,l}(t) \Vert ^2 \right) \nonumber \\&\quad \le (1+\nu ^{-1}) \sigma _k \left( \frac{25}{4} + 3 y^2(\theta _{t-\tau } \omega ) \right) \left( \Vert \partial _t V_{k,l}(t) \Vert ^2 + \nu \Vert V_{k,l}(t) \Vert ^2 \right) . \end{aligned}$$
(4.17)

Then it follows from (4.14)–(4.17) that

$$\begin{aligned}&\frac{d}{dt} \left( \Vert \partial _t V_{k,l}(t) \Vert ^2 + \nu \Vert V_{k,l}(t) \Vert ^2 + \Vert \nabla V_{k,l}(t) \Vert ^2 + \varepsilon \left( V_{k,l}(t),\ \partial _t V_{k,l}(t) \right) \right) \nonumber \\&\qquad + \left( \alpha - \varepsilon - \frac{\varepsilon \gamma }{2} \right) \Vert \partial _t V_{k,l}(t) \Vert ^2 + \left( \varepsilon - \frac{\varepsilon \gamma }{2} \right) \Vert \nabla V_{k,l}(t) \Vert ^2 + \varepsilon \nu \left( \frac{3 }{4} - \frac{\gamma }{2} \right) \Vert V_{k,l}(t) \Vert ^2 \nonumber \\&\qquad + \left[ \frac{\varepsilon \gamma }{2} - 2 \sigma _k (1+\nu ^{-1}) \left( \frac{25}{4} + 3 y^2(\theta _{t-\tau } \omega ) \right) \right] \nonumber \\&\qquad \quad \cdot \left( \Vert \partial _t V_{k,l}(t) \Vert ^2 + \nu \Vert V_{k,l}(t) \Vert ^2 + \Vert \nabla V_{k,l}(t) \Vert ^2 \right) \nonumber \\&\qquad + \varepsilon \left[ \frac{\varepsilon \gamma }{2} - 2 \sigma _k (1+\nu ^{-1}) \left( \frac{25}{4} + 3 y^2(\theta _{t-\tau } \omega ) \right) \right] \left( V_{k,l}(t),\ \partial _t V_{k,l}(t) \right) \nonumber \\&\qquad + \sigma _k \left( \frac{25}{4} + 3 y^2(\theta _{t-\tau } \omega ) \right) \left( \Vert \partial _t V_{k,l}(t) \Vert ^2 + \Vert V_{k,l}(t) \Vert ^2 \right) \nonumber \\&\quad \le 3 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \Vert g(t,\cdot )\Vert ^2 \left( e^{ - \sigma _k \int _0^t y(\theta _{s-\tau } \omega ) ds } - e^{ - \sigma _l \int _0^t y(\theta _{s-\tau } \omega ) ds } \right) ^2 \nonumber \\&\qquad + 12 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \left( \sigma _k -\sigma _l \right) ^2 y^2(\theta _{t-\tau } \omega )\ \Vert \partial _t {\widetilde{V}}_{\sigma _l}(t) \Vert ^2 \nonumber \\&\qquad + 3 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \left( \sigma _k^2 - \sigma _l^2 \right) ^2 y^4(\theta _{t-\tau } \omega )\ \Vert {\widetilde{V}}_{\sigma _l}(t) \Vert ^2 \nonumber \\&\qquad + \varepsilon \left[ \frac{\varepsilon \gamma }{2} - \alpha - 2 \sigma _k (1+\nu ^{-1}) \left( \frac{25}{4} + 3 y^2(\theta _{t-\tau } \omega ) \right) \right] \left( V_{k,l}(t),\ \partial _t V_{k,l}(t) \right) . \end{aligned}$$
(4.18)

Since \(\varepsilon (1+\nu ^{-1}) \le 1\) due to (2.13), we can obtain

$$\begin{aligned}&\varepsilon \left[ \frac{\varepsilon \gamma }{2} - \alpha - 2 \sigma _k (1+\nu ^{-1}) \left( \frac{25}{4} + 3 y^2(\theta _{t-\tau } \omega ) \right) \right] \left( V_{k,l}(t),\ \partial _t V_{k,l}(t) \right) \nonumber \\&\quad \le \left( \frac{\alpha }{2} - \frac{\varepsilon \gamma }{4} \right) \Vert \partial _t V_{k,l}(t) \Vert ^2 + \varepsilon ^2 \left( \frac{\alpha }{2} - \frac{\varepsilon \gamma }{4} \right) \Vert V_{k,l}(t) \Vert ^2 \nonumber \\&\qquad + \sigma _k \left( \frac{25}{4} + 3 y^2(\theta _{t-\tau } \omega ) \right) \left( \Vert \partial _t V_{k,l}(t) \Vert ^2 + \Vert V_{k,l}(t)\Vert ^2 \right) . \end{aligned}$$
(4.19)

Then it follows from (2.13), (4.18) and (4.19) that

$$\begin{aligned}&\frac{d}{dt} \left( \Vert \partial _t V_{k,l}(t) \Vert ^2 + \nu \Vert V_{k,l}(t) \Vert ^2 + \Vert \nabla V_{k,l}(t) \Vert ^2 + \varepsilon \left( V_{k,l}(t),\ \partial _t V_{k,l}(t) \right) \right) \nonumber \\&\qquad + \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{t-\tau } \omega ) \right) \right] \left( \Vert \partial _t V_{k,l}(t) \Vert ^2 + \nu \Vert V_{k,l}(t) \Vert ^2 + \Vert \nabla V_{k,l}(t) \Vert ^2 \right) \nonumber \\&\qquad + \varepsilon \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{t-\tau } \omega ) \right) \right] \left( V_{k,l}(t),\ \partial _t V_{k,l}(t) \right) \nonumber \\&\quad \le 3 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \Vert g(t,\cdot )\Vert ^2 \left( e^{ - \sigma _k \int _0^t y(\theta _{s-\tau } \omega ) ds } - e^{ - \sigma _l \int _0^t y(\theta _{s-\tau } \omega ) ds } \right) ^2 \nonumber \\&\qquad + 12 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \left( \sigma _k -\sigma _l \right) ^2 \left( y^2(\theta _{t-\tau } \omega ) + y^4(\theta _{t-\tau } \omega )\ \right) \left( \Vert \partial _t {\widetilde{V}}_{\sigma _l}(t) \Vert ^2 + \Vert {\widetilde{V}}_{\sigma _l}(t) \Vert ^2 \right) . \end{aligned}$$
(4.20)

Solving the above inequality (4.20) on \([\tau -k, \tau ]\), we get

$$\begin{aligned}&\Vert {\dot{V}}_{k,l}(\tau ) \Vert ^2 + \nu \Vert V_{k,l}(\tau ) \Vert ^2 + \Vert \nabla V_{k,l}(\tau ) \Vert ^2 + \varepsilon \left( V_{k,l}(\tau ),\ {\dot{V}}_{k,l}(\tau ) \right) \nonumber \\&\quad \le e^{- \int _{\tau -k}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{s-\tau } \omega ) \right) \right] ds } \nonumber \\&\quad \cdot \bigg ( \Vert {\dot{V}}_{k,l}(\tau -k) \Vert ^2 + \nu \Vert V_{k,l}(\tau -k) \Vert ^2 + \Vert \nabla V_{k,l}(\tau -k) \Vert ^2 \nonumber \\&\qquad + \varepsilon \left( V_{k,l}(\tau -k),\ {\dot{V}}_{k,l}(\tau -k) \right) \bigg ) \nonumber \\&\qquad + 3 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \int _{\tau -k}^{\tau } e^{-\int _{s}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \Vert g(s,\cdot )\Vert ^2 \nonumber \\&\qquad \cdot \left( e^{ - \sigma _k \int _0^s y(\theta _{r-\tau } \omega ) dr } - e^{ - \sigma _l \int _0^s y(\theta _{r-\tau } \omega ) dr } \right) ^2 ds \nonumber \\&\qquad + 12 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \left( \sigma _k -\sigma _l \right) ^2 \int _{\tau -k}^{\tau } e^{-\int _{s}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \nonumber \\&\qquad \cdot \left( y^2(\theta _{s-\tau } \omega ) + y^4(\theta _{s-\tau } \omega )\ \right) \left( \Vert \partial _s {\widetilde{V}}_{\sigma _l}(s) \Vert ^2 + \Vert {\widetilde{V}}_{\sigma _l}(s) \Vert ^2 \right) ds. \end{aligned}$$
(4.21)

Since \({\varepsilon }\le 1\) and \({\varepsilon }\le \nu \) we have

$$\begin{aligned} \varepsilon \left| \left( V_{k,l}(\tau ),\ {\dot{V}}_{k,l}(\tau ) \right) \right| \le {\frac{1}{2}} \nu \Vert V_{k,l}(\tau ) \Vert ^2 +{\frac{1}{2}} \Vert {\dot{V}}_{k,l}(\tau ) \Vert ^2. \end{aligned}$$
(4.22)

By (4.21)–(4.22) we get

$$\begin{aligned}&\Vert {\dot{V}}_{k,l}(\tau ) \Vert ^2 + \nu \Vert V_{k,l}(\tau ) \Vert ^2 + 2\Vert \nabla V_{k,l}(\tau ) \Vert ^2 \nonumber \\&\quad \le 3 (1+\nu ) e^{- \int _{\tau -k}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{s-\tau } \omega ) \right) \right] ds } \big ( \Vert {\dot{V}}_{k,l}(\tau -k) \Vert ^2 + \Vert V_{k,l}(\tau -k) \Vert ^2_{H^1({\mathbb {R}}^n)} \big ) \nonumber \\&\qquad + 6 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \int _{\tau -k}^{\tau } e^{-\int _{s}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \Vert g(s,\cdot )\Vert ^2 \nonumber \\&\qquad \cdot \left( e^{ - \sigma _k \int _0^s y(\theta _{r-\tau } \omega ) dr } - e^{ - \sigma _l \int _0^s y(\theta _{r-\tau } \omega ) dr } \right) ^2 ds \nonumber \\&\qquad + 24 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \left( \sigma _k -\sigma _l \right) ^2 \int _{\tau -k}^{\tau } e^{-\int _{s}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \nonumber \\&\qquad \cdot \left( y^2(\theta _{s-\tau } \omega ) + y^4(\theta _{s-\tau } \omega )\ \right) \left( \Vert \partial _s {\widetilde{V}}_{\sigma _l}(s) \Vert ^2 + \Vert {\widetilde{V}}_{\sigma _l}(s) \Vert ^2 \right) ds. \end{aligned}$$
(4.23)

We now deal with the first term on the right-hand side of (4.23), for which we have

$$\begin{aligned}&3 (1+\nu ) e^{- \int _{\tau -k}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{s-\tau } \omega ) \right) \right] ds } \left( \Vert {\dot{V}}_{k,l}(\tau -k) \Vert ^2 + \Vert V_{k,l}(\tau -k) \Vert ^2_{H^1({\mathbb {R}}^n)} \right) \nonumber \\&\quad \le 6 (1+\nu ) e^{- \int _{\tau -k}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{s-\tau } \omega ) \right) \right] ds } \left( \Vert {\widetilde{\psi }}_k \Vert ^2 + \Vert {\widetilde{\varphi }}_k \Vert ^2_{H^1({\mathbb {R}}^n)} \right) \nonumber \\&\qquad + 6 (1+\nu ) e^{ - \int _{\tau -k}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{s-\tau } \omega ) \right) \right] ds }\nonumber \\&\qquad \cdot \left( \Vert {\dot{V}}_{\sigma _l}(\tau -k) \Vert ^2 + \Vert V_{\sigma _l }(\tau -k) \Vert ^2_{H^1({\mathbb {R}}^n)} \right) . \end{aligned}$$
(4.24)

As in [13, (6.14)], one can verify

$$\begin{aligned}&\Vert {\dot{V}}_{\sigma _l}(\tau -k; \tau -l, \theta _{-\tau } \omega , {\widetilde{\psi }}_l ) \Vert ^2 + \nu \Vert V_{\sigma _l }(\tau -k; \tau -l, \theta _{-\tau } \omega , {\widetilde{\varphi }}_l ) \Vert ^2\nonumber \\&\qquad + 2 \Vert \nabla V_{\sigma _l }(\tau -k; \tau -l, \theta _{-\tau } \omega , {\widetilde{\varphi }}_l ) \Vert ^2 \nonumber \\&\quad \le 3(1+\nu ) e^{\int _{\tau -k}^{\tau -l} \left[ {\frac{1}{2}} {\varepsilon }\gamma - \sigma _l (1+\nu ^{-1}) \left( {\frac{25}{2}} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \left( \Vert {\widetilde{\psi }}_l\Vert ^2 + \Vert {\widetilde{\varphi }}_l\Vert ^2_{H^1({\mathbb {R}}^n)} \right) \nonumber \\&\qquad + 2 ({\varepsilon }\nu ^{-1} + \alpha ^{-1}) \int _{\tau -l}^{\tau -k} e^{\int _{\tau -k}^s \left[ {\frac{1}{2}} {\varepsilon }\gamma - \sigma _l (1+\nu ^{-1}) \left( {\frac{25}{2}} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \Vert g(s)\Vert ^2 e^{-2 \sigma _l \int _0^s y(\theta _{r-\tau } \omega ) dr} ds. \end{aligned}$$
(4.25)

By (4.24)–(4.25) we obtain

$$\begin{aligned}&3 (1+\nu ) e^{- \int _{\tau -k}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{s-\tau } \omega ) \right) \right] ds } \left( \Vert {\dot{V}}_{k,l}(\tau -k) \Vert ^2 + \Vert V_{k,l}(\tau -k) \Vert ^2_{H^1({\mathbb {R}}^n)} \right) \nonumber \\&\quad \le 6 (1+\nu ) e^{- \int _{\tau -k}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{s-\tau } \omega ) \right) \right] ds } \left( \Vert {\widetilde{\psi }}_k \Vert ^2 + \Vert {\widetilde{\varphi }}_k \Vert ^2_{H^1({\mathbb {R}}^n)} \right) \nonumber \\&\qquad + C_1 e^{ ( \sigma _k-\sigma _l) ( 1 + \nu ^{-1} ) \int _{\tau -k}^{\tau } \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) dr } e^{ \int ^{\tau -l}_{\tau } \left[ {\frac{1}{2} {\varepsilon }\gamma } - \sigma _l (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \nonumber \\&\qquad \cdot \left( \Vert {\widetilde{\psi }} _l\Vert ^2 + \Vert {\widetilde{\varphi }} _l\Vert ^2_{H^1({\mathbb {R}}^n)} \right) \nonumber \\&\qquad + C_1 e^{ - \int _{\tau -k}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{s-\tau } \omega ) \right) \right] ds } \nonumber \\&\qquad \cdot \int _{\tau -l}^{\tau -k} e^{ \int ^{s}_{\tau -k} \left[ {\frac{1}{2} {\varepsilon }\gamma } - \sigma _l (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \Vert g(s)\Vert ^2 e^{ -2 \sigma _l \int _0^s y (\theta _{r-\tau } \omega ) dr } ds, \end{aligned}$$
(4.26)

where \(C_1=C_1(\nu , \alpha )>0\).

By (2.16) we see that if \(\sigma \le \sigma _0\), then \(\sigma \le {\frac{p\alpha {\varepsilon }\gamma \nu }{(p+1)(1+\nu ) (24+50\alpha )}} \), and hence

$$\begin{aligned} -{\frac{1}{4}}{\varepsilon }\gamma +\left( {\frac{25}{2}} +{\frac{6}{\alpha }} \right) (1+\nu ^{-1}) \sigma \le - {\frac{{\varepsilon }\gamma }{4(p+1)}}, \quad \forall \ \sigma \in (0, \sigma _0). \end{aligned}$$
(4.27)

Since \( \lim \limits _{t\rightarrow \infty } {\frac{1}{t}} \int ^0_{-t} y^2(\theta _r \omega ) dr = {\frac{1}{2\alpha }} \), we know that there exists \(T_1=T_1(\omega )>0\) such that

$$\begin{aligned} {\frac{1}{t}} \int ^0_{-t} y^2(\theta _r \omega ) dr < {\frac{1}{\alpha }}, \quad \forall \ t\ge T_1. \end{aligned}$$
(4.28)

Since \(\sigma _k\le \sigma _0\), by (4.27)–(4.28) we get for \(k\ge T_1\),

$$\begin{aligned} e^{- \int _{\tau -k}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{s-\tau } \omega ) \right) \right] ds } \le e^{-{\frac{{\varepsilon }\gamma k}{4}}} e^{-{\frac{{\varepsilon }\gamma k }{4(p+1)}}}. \end{aligned}$$
(4.29)

For the first term on the right-hand side of (4.26), by (4.29) we have for all \(k\ge T_1\),

$$\begin{aligned}&6 (1+\nu ) e^{- \int _{\tau -k}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{s-\tau } \omega ) \right) \right] ds } \left( \Vert {\widetilde{\psi }} _k \Vert ^2 + \Vert {\widetilde{\varphi }} _k \Vert ^2_{H^1({\mathbb {R}}^n)} \right) \nonumber \\&\quad \le 6 (1+\nu ) e^{-{\frac{{\varepsilon }\gamma k}{4}}} e^{-{ \frac{{\varepsilon }\gamma k }{4(p+1)} } } \Vert {\bar{K}}(\tau -k,\theta _{-k} \omega ) \Vert ^2 \rightarrow 0, \ \ \text { as } \ k \rightarrow \infty . \end{aligned}$$
(4.30)

For the second term on the right-hand side of (4.26), by (4.28) and (4.29) we have for all \(l > k \ge T_1\),

$$\begin{aligned}&C_1 e^{ ( \sigma _k-\sigma _l) (1+\nu ^{-1}) \int _{\tau -k}^{\tau } \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) dr } e^{ \int ^{\tau -l}_{\tau } \left[ {\frac{1}{2} {\varepsilon }\gamma } - \sigma _l (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \nonumber \\&\quad \cdot \left( \Vert {\widetilde{\psi }}_l\Vert ^2 + \Vert {\widetilde{\varphi }}_l \Vert ^2_{H^1({\mathbb {R}}^n)} \right) \nonumber \\&\quad \le C_1 e^{ ( \sigma _k-\sigma _l) (1+\nu ^{-1}) \int _{\tau -k}^{\tau } \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) dr } e^{-{\frac{{\varepsilon }\gamma l}{4} } } e^{-{\frac{{\varepsilon }\gamma l }{4(p+1)} } } \Vert {\bar{K}}(\tau -l,\theta _{-l} \omega )\Vert ^2 \nonumber \\&\quad \le C_1 e^{ ( \sigma _k-\sigma _l) (1+\nu ^{-1}) ({\frac{25}{2}} +{\frac{6}{\alpha }})k } e^{-{\frac{{\varepsilon }\gamma l}{4}}} e^{-{\frac{{\varepsilon }\gamma l }{4(p+1)}}} \Vert {\bar{K}}(\tau -l,\theta _{-l} \omega )\Vert ^2. \end{aligned}$$
(4.31)

Since \(\sigma _k \rightarrow 0\), there exists \(k_0=k_0(\omega )\ge T_1\) such that for all \(l>k\ge k_0\),

$$\begin{aligned} |\sigma _k-\sigma _l| (1+\nu ^{-1}) \left( {\frac{25}{2}} +{\frac{6}{\alpha }} \right)< {\frac{{\varepsilon }\gamma }{8}} \quad \text { and } \quad \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + {\frac{6}{\alpha }} \right) < {\frac{{\varepsilon }\gamma }{8}}, \end{aligned}$$
(4.32)

and thus for all \(l>k\ge k_0\),

$$\begin{aligned}&C_1 e^{ ( \sigma _k-\sigma _l) (1+\nu ^{-1}) ({\frac{25}{2}} +{\frac{6}{\alpha }})k } e^{-{\frac{{\varepsilon }\gamma l}{4} } } e^{-{\frac{{\varepsilon }\gamma l }{4(p+1)} } } \Vert {\bar{K}}(\tau -l,\theta _{-l} \omega ) \Vert ^2 \nonumber \\&\quad \le C_1 e^{-{\frac{{\varepsilon }\gamma l }{4(p+1)} } } \Vert {\bar{K}}(\tau -l,\theta _{-l} \omega ) \Vert ^2 \rightarrow 0, \ \ \text {as } \ l \rightarrow \infty . \end{aligned}$$
(4.33)

By (4.31) and (4.33) we obtain as \(l > k \rightarrow \infty \),

$$\begin{aligned}&C_1 e^{ ( \sigma _k-\sigma _l) (1+\nu ^{-1}) \int _{\tau -k}^{\tau } \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) dr } e^{ \int ^{\tau -l}_{\tau } \left[ {\frac{1}{2} {\varepsilon }\gamma } - \sigma _l (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \nonumber \\&\quad \cdot \left( \Vert {\widetilde{\psi }}_l\Vert ^2 + \Vert {\widetilde{\varphi }}_l\Vert ^2_{H^1({\mathbb {R}}^n)} \right) \rightarrow 0. \end{aligned}$$
(4.34)

By (4.28) and (4.32) for \(l > k \ge k_0 \), we have

$$\begin{aligned}&\int _{\tau -l}^{\tau -k} e^{ \int ^{s}_{\tau -k} \left[ {\frac{1}{2} {\varepsilon }\gamma } - \sigma _l (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \Vert g(s)\Vert ^2 e^{-2 \sigma _l \int _0^s y (\theta _{r-\tau } \omega ) dr } ds \nonumber \\&\quad = \int _{-l}^{-k} e^{ {\frac{1}{2} {\varepsilon }\gamma } (s+k) + \sigma _l (1+\nu ^{-1}) \int _{s }^{ -k} \left( \frac{25}{2} + 6 y^2(\theta _{r } \omega ) \right) dr } \Vert g(s+\tau )\Vert ^2 e^{-2\sigma _l\int _{-\tau }^{s } y (\theta _{r } \omega ) dr } ds \nonumber \\&\quad \le \int _{-l}^{-k} e^{ {\frac{1}{2} {\varepsilon }\gamma } (s+k) + \sigma _l (1+\nu ^{-1}) \int _{s }^{ 0} \left( \frac{25}{2} + 6 y^2(\theta _{r } \omega ) \right) dr } \Vert g(s+\tau ) \Vert ^2 e^{ -2 \sigma _l \int _{-\tau }^{s } y (\theta _{r } \omega ) dr } ds \nonumber \\&\quad \le \int _{-l}^{-k} e^{ {\frac{1}{4} {\varepsilon }\gamma } (s+k) - \sigma _l (1+\nu ^{-1}) \left( \frac{25}{2} + {\frac{6}{\alpha }} \right) s } \Vert g(s+\tau ) \Vert ^2 e^{ -2 \sigma _l \int _{-\tau }^{s } y (\theta _{r } \omega ) dr } ds \nonumber \\&\quad \le e^{ {\frac{1}{4} {\varepsilon }\gamma } k } \int _{-l}^{-k} e^{ {\frac{1}{8} {\varepsilon }\gamma } s } \Vert g(s+\tau ) \Vert ^2 e^{ -2 \sigma _l \int _{-\tau }^{s } y (\theta _{r } \omega ) dr } ds. \end{aligned}$$
(4.35)

Similarly, by (4.28) and (4.32) for \( k \ge k_0 \), we have

$$\begin{aligned} C_1 e^{- \int _{\tau -k}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{s-\tau } \omega ) \right) \right] ds } \le C_1 e^{ - {\frac{3}{8} {\varepsilon }\gamma } k }. \end{aligned}$$
(4.36)

By (2.15), (4.35) and (4.36) for \( l>k\ge k_0 \), we obtain

$$\begin{aligned}&C_1 e^{ - \int _{\tau -k}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{s-\tau } \omega ) \right) \right] ds } \nonumber \\&\qquad \cdot \int _{\tau -l}^{\tau -k} e^{ \int ^{s}_{\tau -k} \left[ {\frac{1}{2} {\varepsilon }\gamma } - \sigma _l (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \Vert g(s) \Vert ^2 e^{ -2 \sigma _l \int _0^s y (\theta _{r-\tau } \omega ) dr } ds \nonumber \\&\quad \le C_1 e^{ - {\frac{1}{8} {\varepsilon }\gamma } k } \int _{-l}^{-k} e^{ {\frac{1}{8} {\varepsilon }\gamma } s } \Vert g(s+\tau ) \Vert ^2 e^{ -2 \sigma _l \int _{-\tau }^{s} y (\theta _{r } \omega ) dr } ds \rightarrow 0 \quad \text { as } \ k \rightarrow \infty . \end{aligned}$$
(4.37)

It follows from (4.26), (4.30) (4.34) and (4.37) that for \(l>k\ge k_0\),

$$\begin{aligned} 3 (1+\nu ) e^{- \int _{\tau -k}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{s-\tau } \omega ) \right) \right] ds } \big ( \Vert {\dot{V}}_{k,l}(\tau -k) \Vert ^2 + \Vert V_{k,l}(\tau -k) \Vert ^2_{H^1({\mathbb {R}}^n)} \big ) \rightarrow 0,\nonumber \\ \end{aligned}$$
(4.38)

as \(k\rightarrow \infty \).

For the second term on the right-hand side of (4.23), by (2.15), (4.32) and the Lebesgue dominated convergence theorem we obtain

$$\begin{aligned}&6 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \int _{\tau -k}^{\tau } e^{ - \int _{s}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \Vert g(s,\cdot )\Vert ^2 \nonumber \\&\quad \cdot \left( e^{ - \sigma _k \int _0^s y(\theta _{r-\tau } \omega ) dr } - e^{ - \sigma _l \int _0^s y(\theta _{r-\tau } \omega ) dr } \right) ^2 ds \rightarrow 0, \ \text { as } \ k\rightarrow \infty . \end{aligned}$$
(4.39)

For the last term on the right-hand side of (4.23), we have

$$\begin{aligned}&24 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \left( \sigma _k -\sigma _l \right) ^2 \int _{\tau -k}^{\tau } e^{-\int _{s}^{\tau } [ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) ]dr } \nonumber \\&\qquad \cdot \left( y^2(\theta _{s-\tau } \omega ) + y^4(\theta _{s-\tau } \omega )\ \right) \left( \Vert \partial _s {\widetilde{V}}_{\sigma _l}(s) \Vert ^2 + \Vert {\widetilde{V}}_{\sigma _l}(s) \Vert ^2 \right) ds \nonumber \\&\quad = 24 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \left( \sigma _k -\sigma _l \right) ^2 \int _{-k}^{0} e^{-\int _{s}^{0} \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r} \omega ) \right) \right] dr } \nonumber \\&\qquad \ \cdot \left( y^2(\theta _{s} \omega ) + y^4(\theta _{s} \omega )\ \right) \left( \Vert \partial _s {\widetilde{V}}_{\sigma _l}(\tau + s) \Vert ^2 + \Vert {\widetilde{V}}_{\sigma _l}(\tau + s) \Vert ^2 \right) ds. \end{aligned}$$
(4.40)

As in [13, (4.16)], we also have

$$\begin{aligned}&\Vert \partial _s {\widetilde{V}}_{\sigma _l}(\tau +s; \tau -l, \theta _{-\tau } \omega , {\widetilde{\psi }}_l ) \Vert ^2 + \nu \Vert {\widetilde{V}}_{\sigma _l} (\tau +s; \tau -l, \theta _{-\tau } \omega , {\widetilde{\varphi }}_l ) \Vert ^2\nonumber \\&\qquad + 2 \Vert \nabla {\widetilde{V}}_{\sigma _l} (\tau +s; \tau -l, \theta _{-\tau } \omega , {\widetilde{\varphi }}_l ) \Vert ^2 \nonumber \\&\quad \le C_2 e^{ \int _s^{-l} \left[ {\frac{1}{2}} {\varepsilon }\gamma - \sigma _l (1+\nu ^{-1}) \left( {\frac{25}{2}} + 6 y^2(\theta _r \omega ) \right) \right] dr } \left( \Vert {\widetilde{\psi }}_l \Vert ^2 + \Vert {\widetilde{\varphi }}_l \Vert ^2_{H^1({\mathbb {R}}^n)} \right) \nonumber \\&\qquad + C_2 \int _{-l}^s e^{ \int _s^z \left[ {\frac{1}{2}} {\varepsilon }\gamma - \sigma _l (1+\nu ^{-1}) \left( {\frac{25}{2}} + 6 y^2(\theta _r \omega ) \right) \right] dr } \Vert g(\tau +z)\Vert ^2 e^{ -2 \sigma _l \int _{-\tau }^z y(\theta _r \omega ) dr } dz, \end{aligned}$$
(4.41)

where \(C_2=C_2(\nu , \alpha )>0\). By (4.40)–(4.41) we get

$$\begin{aligned}&24 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \left( \sigma _k -\sigma _l \right) ^2 \int _{\tau -k}^{\tau } e^{ - \int _{s}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \nonumber \\&\qquad \cdot \left( y^2(\theta _{s-\tau } \omega ) + y^4(\theta _{s-\tau } \omega )\ \right) \left( \Vert \partial _s {\widetilde{V}}_{\sigma _l}(s) \Vert ^2 + \Vert {\widetilde{V}}_{\sigma _l}(s) \Vert ^2 \right) ds \nonumber \\&\quad \le C_3 \left( \sigma _k -\sigma _l \right) ^2 \int _{-k}^{0} e^{-\int _{s}^{0} \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r} \omega ) \right) \right] dr } \nonumber \\&\qquad \cdot \left( y^2(\theta _{s} \omega ) + y^4(\theta _{s} \omega )\ \right) e^{ \int _s^{-l} \left[ {\frac{1}{2}} {\varepsilon }\gamma - \sigma _l (1+\nu ^{-1}) \left( {\frac{25}{2}} + 6 y^2(\theta _r \omega ) \right) \right] dr } \left( \Vert {\widetilde{\psi }} _l\Vert ^2 + \Vert {\widetilde{\varphi }} _l \Vert ^2_{H^1({\mathbb {R}}^n)} \right) ds \nonumber \\&\qquad + C_3 \left( \sigma _k -\sigma _l \right) ^2 \int _{-k}^{0} e^{-\int _{s}^{0} \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1 + \nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r} \omega ) \right) \right] dr } \nonumber \\&\qquad \cdot \left( y^2(\theta _{s} \omega ) + y^4(\theta _{s} \omega )\ \right) \left( \int _{-l}^s e^{ \int _s^z \left[ {\frac{1}{2}} {\varepsilon }\gamma - \sigma _l (1+\nu ^{-1}) \left( {\frac{25}{2}} + 6 y^2(\theta _r \omega ) \right) \right] dr } \Vert g(\tau +z) \Vert ^2\right. \nonumber \\&\qquad \left. e^{ -2 \sigma _l \int _{-\tau }^z y(\theta _r \omega ) dr } dz \right) ds. \end{aligned}$$
(4.42)

For the first term on the right-hand side of (4.42), we have for \(l > k \ge k_0\),

$$\begin{aligned}&C_3 \left( \sigma _k -\sigma _l \right) ^2 \int _{-k}^{0} e^{-\int _{s}^{0} \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r} \omega ) \right) \right] dr } \nonumber \\&\qquad \cdot \left( y^2(\theta _{s} \omega ) + y^4(\theta _{s} \omega )\ \right) e^{ \int _s^{-l} \left[ {\frac{1}{2}} {\varepsilon }\gamma - \sigma _l (1+\nu ^{-1}) \left( {\frac{25}{2}} + 6 y^2(\theta _r \omega ) \right) \right] dr } \left( \Vert {\widetilde{\psi }} _l\Vert ^2 + \Vert {\widetilde{\varphi }} _l \Vert ^2_{H^1({\mathbb {R}}^n)} \right) ds \nonumber \\&\quad = C_3 \left( \sigma _k -\sigma _l \right) ^2 e^{ \int _0^{-l} \left[ {\frac{1}{2}} {\varepsilon }\gamma - \sigma _l (1+\nu ^{-1}) \left( {\frac{25}{2}} + 6 y^2(\theta _r \omega ) \right) \right] dr } \left( \Vert {\widetilde{\psi }}_l \Vert ^2 + \Vert {\widetilde{\varphi }}_l \Vert ^2_{H^1({\mathbb {R}}^n)} \right) \nonumber \\&\qquad \cdot \int _{-k}^{0} \left( y^2(\theta _{s} \omega ) + y^4(\theta _{s} \omega ) \right) e^{ \int _{s}^{0} (\sigma _k - \sigma _l) (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r} \omega ) \right) dr } ds \nonumber \\&\quad \le C_3 \left( \sigma _k -\sigma _l \right) ^2 e^{ \int _0^{-l} \left[ {\frac{1}{4}} {\varepsilon }\gamma - \sigma _l (1+\nu ^{-1}) \left( {\frac{25}{2}} + 6 y^2(\theta _r \omega ) \right) \right] dr } \left( \Vert {\widetilde{\psi }}_l \Vert ^2 + \Vert {\widetilde{\varphi }}_l \Vert ^2_{H^1({\mathbb {R}}^n)} \right) \nonumber \\&\qquad \cdot \int _{-k}^{0} \left( y^2(\theta _{s} \omega ) + y^4(\theta _{s} \omega ) \right) e^{{\frac{1}{4}} {\varepsilon }\gamma s} e^{ \int _{s}^{0} ( \sigma _k -\sigma _l) (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r} \omega ) \right) dr } ds \rightarrow 0, \ \text { as } k \rightarrow \infty , \end{aligned}$$
(4.43)

which can be verified by (4.32) as before.

For the second term on the right-hand side of (4.42), we have for \(l > k \ge k_0\),

$$\begin{aligned}&C_3 \left( \sigma _k -\sigma _l \right) ^2 \int _{-k}^{0} e^{-\int _{s}^{0} \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r} \omega ) \right) \right] dr } \nonumber \\&\qquad \cdot \left( y^2(\theta _{s} \omega ) + y^4(\theta _{s} \omega )\ \right) \left( \int _{-l}^s e^{ \int _s^z \left[ {\frac{1}{2}} {\varepsilon }\gamma - \sigma _l (1+\nu ^{-1}) \left. \big ( {\frac{25}{2}} + 6 y^2(\theta _r \omega ) \right) \right] dr } \Vert g(\tau +z) \Vert ^2\right. \nonumber \\ {}&\qquad \left. e^{ -2 \sigma _l \int _{-\tau }^z y(\theta _r \omega ) dr } dz \right) ds \nonumber \\&\quad = C_3 \left( \sigma _k - \sigma _l \right) ^2 \int _{-k}^{0} e^{ \int _{s}^{0} \left[ ( \sigma _k - \sigma _l) (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r} \omega ) \right) \right] dr } \nonumber \\&\qquad \cdot \left( y^2(\theta _{s} \omega ) + y^4(\theta _{s} \omega ) \ \right) \left. \bigg ( \int _{-l}^s e^{ \int _0^z \left[ {\frac{1}{2}} {\varepsilon }\gamma - \sigma _l (1+\nu ^{-1}) \left( {\frac{25}{2}} + 6 y^2(\theta _r \omega ) \right) \right] dr } \Vert g(\tau + z) \Vert ^2\right. \nonumber \\ {}&\qquad \left. e^{ -2 \sigma _l \int _{-\tau }^z y(\theta _r \omega ) dr } dz \right. \bigg ) ds \nonumber \\&\quad \le C_3 \left( \sigma _k - \sigma _l \right) ^2 \int _{-k}^{0} e^{{\frac{1}{4}} {\varepsilon }\gamma s} e^{ \int _{s}^{0} \left[ ( \sigma _k - \sigma _l) (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r} \omega ) \right) \right] dr } \nonumber \\&\qquad \cdot \left( y^2(\theta _{s} \omega ) + y^4(\theta _{s} \omega )\ \right) \left. \bigg ( \int _{-l}^s e^{ \int _0^z \left[ {\frac{1}{4}} {\varepsilon }\gamma - \sigma _l (1+\nu ^{-1}) \left( {\frac{25}{2}} + 6 y^2(\theta _r \omega ) \right) \right] dr } \Vert g(\tau +z)\Vert ^2\right. \nonumber \\ {}&\qquad \left. e^{ -2 \sigma _l \int _{-\tau }^z y(\theta _r \omega ) dr } dz \right. \bigg ) ds \nonumber \\&\quad = C_3 \left( \sigma _k - \sigma _l \right) ^2 \int _{-k}^{0} e^{ {\frac{1}{4}} {\varepsilon }\gamma s} e^{ \int _{s}^{0} \left[ ( \sigma _k - \sigma _l) (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r} \omega ) \right) \right] dr } \left( y^2(\theta _{s} \omega ) + y^4(\theta _{s} \omega ) \right) ds \nonumber \\&\qquad \cdot \int _{-l}^0 e^{ \int _0^z \left[ {\frac{1}{4}} {\varepsilon }\gamma - \sigma _l (1+\nu ^{-1}) \left( {\frac{25}{2}} + 6 y^2(\theta _r \omega ) \right) \right] dr } \Vert g(\tau +z) \Vert ^2 e^{ -2 \sigma _l \int _{-\tau }^z y(\theta _r \omega ) dr } dz \rightarrow 0, \ \text { as } k \rightarrow \infty , \end{aligned}$$
(4.44)

which can be verified by (2.15) and (4.32). By (4.42)–(4.44) we see that for \(l>k\) with \(k\rightarrow \infty \),

$$\begin{aligned}&24 \left( \frac{1}{\alpha } + \frac{\varepsilon }{\nu } \right) \left( \sigma _k -\sigma _l \right) ^2 \int _{\tau -k}^{\tau } e^{-\int _{s}^{\tau } \left[ \frac{\varepsilon \gamma }{2} - \sigma _k (1+\nu ^{-1}) \left( \frac{25}{2} + 6 y^2(\theta _{r-\tau } \omega ) \right) \right] dr } \nonumber \\&\quad \cdot \left( y^2(\theta _{s-\tau } \omega ) + y^4(\theta _{s-\tau } \omega )\ \right) \left( \Vert \partial _s {\widetilde{V}}_{\sigma _l}(s) \Vert ^2 + \Vert {\widetilde{V}}_{\sigma _l}(s) \Vert ^2 \right) ds \rightarrow 0. \end{aligned}$$
(4.45)

It follows from (4.23), (4.38), (4.39) and (4.45) that

$$\begin{aligned} \Vert {\dot{V}}_{k,l}(\tau ) \Vert ^2 + \nu \Vert V_{k,l}(\tau ) \Vert ^2 + 2\Vert \nabla V_{k,l}(\tau ) \Vert ^2 \rightarrow 0, \quad \text {as} \ k, l \rightarrow \infty , \end{aligned}$$

and hence \( \left\{ \big ( {\widetilde{V}}_{\sigma _k}(\tau ), \dot{{\widetilde{V}}}_{\sigma _k}(\tau ) \big ) \right\} _{k=1}^\infty \) is a Cauchy sequence in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\).

Step 2: prove for every \(\tau \in {\mathbb {R}}\), \(k\in {\mathbb {N}}\) and \(\omega \in \Omega \), \( \Big \{ \Big ( {\widehat{V}}_{\sigma _k} (\tau ;\tau -k,\omega ,0), \ \dot{{\widehat{V}}}_{\sigma _k} (\tau ;\tau -k,\omega ,0) \Big ) \Big \}_{k=1}^\infty \) is precompact in \({H^1( {\mathcal {O}}_{m} )} \times {L^2( {\mathcal {O}}_{m} )}\) for each \(m \in {\mathbb {N}}\).

The idea of the proof is to first truncate the solutions in a bounded domain and then use the spectral decomposition of the Laplace operator with homogeneous Dirichlet boundary condition.

Let \(\xi \) be a smooth function on \({\mathbb {R}}^n\) such that \(0 \le \xi (x) \le 1\) for all \(x \in {\mathbb {R}}^n\), and \(\xi (x)=1\) for \(|x| \le 1\) and \(\xi (x)=0\) for \(|x| \ge {\frac{3}{2}}\). For each \(m \in {\mathbb {N}}\), let \(\xi _m (x) = \xi (\frac{x}{m})\), and \({\mathcal {O}}_m=\{ x \in {\mathbb {R}}^n: |x| < m \}\).

Let \({\widehat{V}}_{k,m} (t) = \xi _m {\widehat{V}}_{\sigma _k}(t)\), then by (4.12) we find \({\widehat{V}}_{k,m}\) satisfies the following initial and boundary value problem:

$$\begin{aligned} \left\{ \begin{array}{ll} &{} \partial _{tt} {\widehat{V}}_{k,m} - \triangle {\widehat{V}}_{k,m} + \alpha \partial _t {\widehat{V}}_{k,m} + \nu {\widehat{V}}_{k,m} = - \xi _m (x) f( x, e^{\sigma _k \int _0^t y(\theta _{s-\tau } \omega ) ds} V_{\sigma _k} )\ e^{ - \sigma _k \int _0^t y(\theta _{s-\tau } \omega ) ds } \\ &{} \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \quad - 2 \sigma _k y(\theta _{t-\tau } \omega ) \partial _t {\widehat{V}}_{k,m} - \sigma _k^2 y^2(\theta _{t-\tau } \omega ) {\widehat{V}}_{k,m} \\ &{} \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad \quad - {\widehat{V}}_{\sigma _k} \triangle \xi _m(x) - 2 \nabla \xi _m (x) \cdot \nabla {\widehat{V}}_{\sigma _k}, \\ &{} {\widehat{V}}_m(t,x) = 0, \quad \partial _t {\widehat{V}}_m(t,x) = 0, \quad \text {for } |x|=2m,\ t > \tau - k, \\ &{} {\widehat{V}}_m (\tau - k, x) = 0, \quad \partial _t {\widehat{V}}_m (\tau - k, x) = 0, \quad |x| \le 2m.\\ \end{array} \right. \end{aligned}$$
(4.46)

Let \(A = - \triangle \) with domain \(H^2({\mathcal {O}}_{2\,m})\bigcap H^1_0({\mathcal {O}}_{2\,m})\). Then \(L^2({\mathcal {O}}_{2\,m})\) has an orthonormal basis \(\{e_j\}_{j=1}^\infty \) such that \(A e_j=\lambda _j e_j\) and \( 0<\lambda _1\le \lambda _2 \le \dots \le \lambda _n \rightarrow \infty . \) Given \(n\in {\mathbb {N}}\), let \(P_n: L^2({\mathcal {O}}_{2\,m}) \rightarrow \text {span}\{e_1,\cdots , e_n\}\) be the projection operator.

Since \({\widehat{V}}_{\sigma _k}\) is the solution of (4.12), as in [44, Lemma 3.4] we have

$$\begin{aligned} \Vert {\widehat{V}}_{\sigma _k} (t;\tau -k, \omega ,0) \Vert _{H^{1} ({\mathbb {R}}^n)} + \Vert \partial _t {\widehat{V}}_{\sigma _k} (t;\tau -k, \omega ,0) \Vert \le C_4, \quad \forall \ t \in [\tau -k, \tau -k+T], \end{aligned}$$
(4.47)

where \(C_4 > 0\) depends on \(\tau , k, T\) and \(\omega \), but not on \(\sigma _k\), n or m. As in [13], after detailed calculations, we find that for every \(\eta >0\), there exists \(n_1=n_1(\eta , \tau , k, T, \omega )\ge 1\) and \(m_1=m_1(\eta , \tau , k, T, \omega ) \ge 1\) such that for all \(n\ge n_1\) and \(m\ge m_1\),

$$\begin{aligned}&\sup _{\tau -k \le r \le \tau -k+T} \Vert (I-P_n) {\widehat{V}}_{k,m}(r;\tau -k,\omega ,0)\Vert _{H^1( {\mathcal {O}}_{2m} )} \nonumber \\&\quad + \sup _{\tau -k \le r \le \tau -k+T} \Vert (I-P_n) \partial _r {\widehat{V}}_ {k,m}(r;\tau -k, \omega ,0)\Vert _{L^2( {\mathcal {O}}_{2m} )} \le \frac{1}{4} \eta . \end{aligned}$$
(4.48)

By (4.47) we have

$$\begin{aligned} \Vert P_n {\widehat{V}}_{k,m} (t;\tau -k,\omega ,0) \Vert _{H^1( {\mathcal {O}}_{2m} )} + \Vert P_n \partial _t {\widehat{V}}_{k,m} (t;\tau -k,\omega ,0)\Vert _{L^2( {\mathcal {O}}_{2m} )} \le C_5 \end{aligned}$$
(4.49)

for all \(t \in [\tau -k, \tau -k+T]\), where \(C_5\) is a positive constant depending on \(\tau , k, T, \omega \), but not on \(\sigma _k\), n or m. Then for every \(\tau \), k, \(\omega \), n and m, by (4.49) we find that the set

$$\begin{aligned} \left\{ \left( P_n {\widehat{V}}_{k,m}(\tau ;\tau -k,\omega ,0),\ P_n \dot{{\widehat{V}}}_{k,m}(\tau ;\tau -k,\omega ,0) \right) \right\} _{k=1}^\infty \end{aligned}$$

is precompact in a finite-dimensional space, which along with (4.48) shows that the set

$$\begin{aligned} \left\{ \left( {\widehat{V}}_{k,m}(\tau ;\tau -k,\omega ,0), \ \dot{{\widehat{V}}}_{k,m}(\tau ;\tau -k,\omega ,0) \right) \right\} _{k=1}^\infty \end{aligned}$$

has a finite cover of radius \(\eta \) in \({H^1( {\mathcal {O}}_{2\,m} )} \times {L^2( {\mathcal {O}}_{2\,m} )}\) for \(m\ge m_1\).

Since \({\widehat{V}}_{k,m} (\tau ;\tau -k,\omega ,0) = {\widehat{V}}_{\sigma _k} (\tau ;\tau -k,\omega ,0)\) for \(|x| \le m\), we see that the set

$$\begin{aligned} \left\{ \left( {\widehat{V}}_{\sigma _k} (\tau ;\tau -k,\omega ,0), \ \dot{{\widehat{V}}}_k (\tau ;\tau -k,\omega ,0) \right) \right\} _{k=1}^\infty \end{aligned}$$

has a finite cover of radius \(\eta \) in \({H^1( {\mathcal {O}}_{m} )} \times {L^2( {\mathcal {O}}_{m} )}\) for \(m\ge m_1\). Therefore

$$\begin{aligned} \left\{ \left( {\widehat{V}}_{\sigma _k} (\tau ;\tau -k,\omega ,0), \ \dot{{\widehat{V}}}_{\sigma _k} (\tau ;\tau -k,\omega ,0) \right) \right\} _{k=1}^\infty \end{aligned}$$

is precompact in \({H^1( {\mathcal {O}}_{m} )} \times {L^2( {\mathcal {O}}_{m} )}\) for each \(m \in {\mathbb {N}}\).

Step 3: prove \(\{(u_k, v_k) |_{{\mathcal {O}}_{m}}\}_{k=1}^\infty \) is precompact in \(H^1({\mathcal {O}}_{m}) \times L^2({\mathcal {O}}_{m})\) for \(m\in {\mathbb {N}}\).

Since \(V_{\sigma _k}(t) = {\widetilde{V}}_{\sigma _k}(t) + {\widehat{V}}_{\sigma _k}(t)\), by Steps 1 and 2 we see that for each \(\tau \) and \(\omega \), the sequence

$$\begin{aligned} \left\{ \left( V_{\sigma _k} (\tau ; \tau -k, \omega , {\widetilde{\varphi }}_k ), \ {\dot{V}}_{\sigma _k} (\tau ; \tau -k, \omega , {\widetilde{\psi }}_k ) \right) \right\} _{k=1}^\infty \end{aligned}$$

is precompact in \({H^1( {\mathcal {O}}_{m} )} \times {L^2( {\mathcal {O}}_{m} )}\) for every \(m \in {\mathbb {N}}\). Since \(\sigma _k \rightarrow 0\), it follows from (4.5) and (4.6) that for every \(\tau \) and \(\omega \), the sequence

$$\begin{aligned} \left\{ \left( u_{\sigma _k} (\tau ; \tau -k, \omega , \varphi _k ), \ {\dot{u}}_{\sigma _k} (\tau ; \tau -k, \omega , \psi _k ) \right) \right\} _{k=1}^\infty \end{aligned}$$

is precompact in \({H^1( {\mathcal {O}}_{m} )} \times {L^2( {\mathcal {O}}_{m} )}\) for each \(m \in {\mathbb {N}}\), which along with (4.4) yields that \(\{ (u_k, v_k) |_{{\mathcal {O}}_{m_0}} \}_{k=1}^\infty \) is precompact in \(H^1({\mathcal {O}}_{m_0}) \times L^2({\mathcal {O}}_{m_0})\). This concludes Claim 2.

Therefore, by Claims 1 and 2, we find \(\{(u_k, v_k)\}_{k=1}^\infty \) has a finite cover of radius \(3 \epsilon '\) in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\) for every \(\epsilon '>0\), and hence is precompact in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\), as desired. \(\square \)

5 Upper Semicontinuity of Random Attractors

In this section, we prove the upper semicontinuity of random pullback attractors of (1.1) when the noise intensity \(\sigma \rightarrow 0\).

Theorem 5.1

Assume (2.1)–(2.6) and (2.15) hold. Then for every \(\tau \in {\mathbb {R}}\) and \(\omega \in \Omega \),

$$\begin{aligned} \lim \limits _{\sigma \rightarrow 0} dist_{H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)} ({\mathcal {A}}_\sigma (\tau ,\omega ), {\mathcal {A}}_0(\tau )) = 0. \end{aligned}$$

Proof

We first note that if a sequence \(\sigma _k \rightarrow 0\) and \((u_{0}^{\sigma _k}, u_{1,0}^{\sigma _k}) \rightarrow (u_0, u_{1,0})\) in \(H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)\), then for any \(t \ge 0\), \(\tau \in {\mathbb {R}}\) and \(\omega \in \Omega \),

$$\begin{aligned} \Phi _{\sigma _k} (t, \tau , \omega , (u_0^{\sigma ^k}, u_{1,0}^{\sigma _k}) ) \rightarrow \Phi _{0} (t, \tau , \omega , (u_0, u_{1,0}) ), \quad \text { as } \ k \rightarrow \infty , \end{aligned}$$
(5.1)

which follows immediately from (2.12), Lemma 3.2 and the transformation

$$\begin{aligned} u_\sigma (t; \tau , \omega , (u_0, u_{1,0})) = V_\sigma (t; \tau , \omega , (V_0, V_{1,0})) e^{\sigma \int _0^t y(\theta _s \omega ) ds}. \end{aligned}$$

By (2.25), (5.1) and Lemma 4.1, we find that all conditions of [41, Theorem 3.2] are fulfilled, from which we obtain that for every \(\tau \in {\mathbb {R}}\) and \(\omega \in \Omega \),

$$\begin{aligned}\lim \limits _{\sigma \rightarrow 0} dist_{H^1({\mathbb {R}}^n) \times L^2({\mathbb {R}}^n)} ({\mathcal {A}}_\sigma (\tau ,\omega ), {\mathcal {A}}_0(\tau ))=0. \end{aligned}$$

This completes the proof. \(\square \)