In this study, we formulate a reaction-diffusion Zika model which incorporates vector-bias, environmental transmission and spatial heterogeneity. The main question of this paper is the analysis of the threshold dynamics. For this purpose, we establish the mosquito reproduction number R1 and basic reproduction number R0. Then, we analyze the dynamical behaviors in terms of R1 and R0. Numerically, we find that the ignorance of the vector-bias effect will underestimate the infection risk of the Zika disease, ignorance of the spatial heterogeneity effect will overestimate the infection risk, and the environmental transmission is indispensable.
Citation: Liping Wang, Xinyu Wang, Dajun Liu, Xuekang Zhang, Peng Wu. Dynamical analysis of a heterogeneous spatial diffusion Zika model with vector-bias and environmental transmission[J]. Electronic Research Archive, 2024, 32(2): 1308-1332. doi: 10.3934/era.2024061
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In this study, we formulate a reaction-diffusion Zika model which incorporates vector-bias, environmental transmission and spatial heterogeneity. The main question of this paper is the analysis of the threshold dynamics. For this purpose, we establish the mosquito reproduction number R1 and basic reproduction number R0. Then, we analyze the dynamical behaviors in terms of R1 and R0. Numerically, we find that the ignorance of the vector-bias effect will underestimate the infection risk of the Zika disease, ignorance of the spatial heterogeneity effect will overestimate the infection risk, and the environmental transmission is indispensable.
Lattice dynamical systems arise from a variety of applications in electrical engineering, biology, chemical reaction, pattern formation and so on, see, e.g., [4,7,14,19,33]. Many researchers have discussed broadly the deterministic models in [6,12,34,39], etc. Stochastic lattice equations, driven by additive independent white noise, was discussed for the first time in [2], followed by extensions in [8,13,15,16,21,23,27,32,35,36,37,38,40].
In this paper, we will study the long term behavior of the following second order non-autonomous stochastic lattice system driven by additive white noise: for given τ∈R, t>τ and i∈Z,
{¨u+νA˙u+h(˙u)+Au+λu+f(u)=g(t)+a˙ω(t),u(τ)=(uτi)i∈Z=uτ,˙u(τ)=(u1τi)i∈Z=u1τ, | (1.1) |
where u=(ui)i∈Z is a sequence in l2, ν and λ are positive constants, ˙u=(˙ui)i∈Z and ¨u=(¨ui)i∈Z denote the fist and the second order time derivatives respectively, Au=((Au)i)i∈Z, A˙u=((A˙u)i)i∈Z, A is a linear operators defined in (2.2), a=(ai)i∈Z∈l2, f(u)=(fi(ui))i∈Z and h(˙u)=(hi(˙ui))i∈Z satisfy certain conditions, g(t)=(gi(t))i∈Z∈L2loc(R,l2) is a given time dependent sequence, and ω(t)=W(t,ω) is a two-sided real-valued Wiener process on a probability space.
The approximation we use in the paper was first proposed in [18,22] where the authors investigated the chaotic behavior of random equations driven by Gδ(θtω). Since then, their work was extended by many scholars. To the best of my knowledge, there are three forms of Wong-Zakai approximations Gδ(θtω) used recenly, Euler approximation of Brownian [3,10,17,20,25,28,29,30], Colored noise [5,11,26,31] and Smoothed approximation of Brownian motion by mollifiers [9]. In this paper, we will focus on Euler approximation of Brownian and compare the long term behavior of system (1.1) with pathwise deterministic system given by
{¨uδ+νA˙uδ+h(˙uδ)+Auδ+λuδ+f(uδ)=g(t)+aGδ(θtω),uδ(τ)=(uδτi)i∈Z=uδτ,˙uδ(τ)=(uδ,1τi)i∈Z=uδ,1τ, | (1.2) |
for δ∈R with δ≠0, τ∈R, t>τ and i∈Z, where Gδ(θtω) is defined in (3.2). Note that the solution of system (1.2) is written as uδ to show its dependence on δ.
This paper is organized as follows. In Section 2, we prove the existence and uniqueness of random attractors of system (1.1). Section 3 is devoted to consider the the Wong-Zakai approximations associated with system (1.1). In Section 4, we establish the convergence of solutions and attractors for approximate system (1.2) when δ→0.
Throughout this paper, the letter c and ci(i=1,2,…) are generic positive constants which may change their values from line to line.
In this section, we will define a continuous cocycle for second order non-autonomous stochastic lattice system (1.1), and then prove the existence and uniqueness of pullback attractors.
A standard Brownian motion or Wiener process (Wt)t∈R (i.e., with two-sided time) in R is a process with W0=0 and stationary independent increments satisfying Wt−Ws∼N(0,|t−s|I). F is the Borel σ-algebra induced by the compact-open topology of Ω, and P is the corresponding Wiener measure on (Ω,F), where
Ω={ω∈C(R,R):ω(0)=0}, |
the probability space (Ω,F,P) is called Wiener space. Define the time shift by
θtω(⋅)=ω(⋅+t)−ω(t),ω∈Ω, t∈R. |
Then (Ω,F,P,{θt}t∈R) is a metric dynamical system (see [1]) and there exists a {θt}t∈R-invariant subset ˜Ω⊆Ω of full measure such that for each ω∈Ω,
ω(t)t→0ast→±∞. | (2.1) |
For the sake of convenience, we will abuse the notation slightly and write the space ˜Ω as Ω.
We denote by
lp={u|u=(ui)i∈Z,ui∈R, ∑i∈Z|ui|p<+∞}, |
with the norm as
‖u‖pp=∑i∈Z|ui|p. |
In particular, l2 is a Hilbert space with the inner product (⋅,⋅) and norm ‖⋅‖ given by
(u,v)=∑i∈Zuivi,‖u‖2=∑i∈Z|ui|2, |
for any u=(ui)i∈Z, v=(vi)i∈Z∈l2.
Define linear operators B, B∗, and A acting on l2 in the following way: for any u=(ui)i∈Z∈l2,
(Bu)i=ui+1−ui,(B∗u)i=ui−1−ui, |
and
(Au)i=2ui−ui+1−ui−1. | (2.2) |
Then we find that A=BB∗=B∗B and (B∗u,v)=(u,Bv) for all u,v∈l2.
Also, we let Fi(s)=∫s0fi(r)dr, h(˙u)=(hi(˙ui))i∈Z, f(u)=(fi(ui))i∈Z with fi,hi∈C1(R,R) satisfy the following assumptions:
|fi(s)|≤α1(|s|p+|s|), | (2.3) |
sfi(s)≥α2Fi(s)≥α3|s|p+1, | (2.4) |
and
hi(0)=0,0<h1≤h′i(s)≤h2,∀s∈R, | (2.5) |
where p>1, αi and hj are positive constants for i=1,2,3 and j=1,2.
In addition, we let
β=h1λ4λ+h22,β<1ν, | (2.6) |
and
σ=h1λ√4λ+h22(h2+√4λ+h22). | (2.7) |
For any u,v∈l2, we define a new inner product and norm on l2 by
(u,v)λ=(1−νβ)(Bu,Bv)+λ(u,v),‖u‖2λ=(u,u)λ=(1−νβ)‖Bu‖2+λ‖u‖2. |
Denote
l2=(l2,(⋅,⋅),‖⋅‖),l2λ=(l2,(⋅,⋅)λ,‖⋅‖λ). |
Then the norms ‖⋅‖ and ‖⋅‖λ are equivalent to each other.
Let E=l2λ×l2 endowed with the inner product and norm
(ψ1,ψ2)E=(u(1),u(2))λ+(v(1),v(2)),‖ψ‖2E=‖u‖2λ+‖v‖2, |
for ψj=(u(j),v(j))T=((u(j)i),(v(j)i))Ti∈Z∈E, j=1,2,ψ=(u,v)T=((ui),(vi))Ti∈Z∈E.
A family D={D(τ,ω):τ∈R,ω∈Ω} of bounded nonempty subsets of E is called tempered (or subexponentially growing) if for every ϵ>0, the following holds:
limt→−∞eϵt‖D(τ+t,θtω)‖2=0, |
where ‖D‖=supx∈D‖x‖E. In the sequel, we denote by D the collection of all families of tempered nonempty subsets of E, i.e.,
D={D={D(τ,ω):τ∈R,ω∈Ω}:Dis tempered inE}. |
The following conditions will be needed for g when deriving uniform estimates of solutions, for every τ∈R,
∫τ−∞eγs‖g(s)‖2ds<∞, | (2.8) |
and for any ς>0
limt→−∞eςt∫0−∞eγs‖g(s+t)‖2ds=0, | (2.9) |
where γ=min{σ2,α2βp+1}.
Let ˉv=˙u+βu and ˉφ=(u,ˉv)T, then system (1.1) can be rewritten as
˙ˉφ+L1(ˉφ)=H1(ˉφ)+G1(ω), | (2.10) |
with initial conditions
ˉφτ=(uτ,ˉvτ)T=(uτ,u1τ+βuτ)T, |
where
L1(ˉφ)=(βu−ˉv(1−νβ)Au+νAˉv+λu+β2u−βˉv)+(0h(ˉv−βu)), |
H1(ˉφ)=(0−f(u)+g(t)),G1(ω)=(0a˙ω(t)). |
Denote
v(t)=ˉv(t)−aω(t)andφ=(u,v)T. |
By (2.10) we have
˙φ+L(φ)=H(φ)+G(ω), | (2.11) |
with initial conditions
φτ=(uτ,vτ)T=(uτ,u1τ+βuτ−aω(τ))T, |
where
L(φ)=(βu−v(1−νβ)Au+νAv+λu+β2u−βv)+(0h(v−βu+aω(t))), |
H(φ)=(0−f(u)+g(t)),G(ω)=(aω(t)βaω(t)−νAaω(t)). |
Note that system (2.11) is a deterministic functional equation and the nonlinearity in (2.11) is locally Lipschitz continuous from E to E. Therefore, by the standard theory of functional differential equations, system (2.11) is well-posed. Thus, we can define a continuous cocycle Φ0:R+×R×Ω×E→E associated with system (2.10), where for τ∈R, t∈R+ and ω∈Ω
Φ0(t,τ,ω,ˉφτ)=ˉφ(t+τ,τ,θ−τω,ˉφτ)=(u(t+τ,τ,θ−τω,uτ),ˉv(t+τ,τ,θ−τω,ˉvτ))T=(u(t+τ,τ,θ−τω,uτ),v(t+τ,τ,θ−τω,vτ)+a(ω(t)−ω(−τ)))T=φ(t+τ,τ,θ−τω,φτ)+(0,a(ω(t)−ω(−τ)))T, |
where vτ=ˉvτ+aω(−τ).
Lemma 2.1. Suppose that (2.3)–(2.8) hold. Then for every τ∈R, ω∈Ω, and T>0, there exists c=c(τ,ω,T)>0 such that for allt∈[τ,τ+T], the solution φ of system (2.11) satisfies
‖φ(t,τ,ω,φτ)‖2E+∫tτ‖φ(s,τ,ω,φτ)‖2Eds≤c∫tτ(‖g(s)‖2+|ω(s)|2+|ω(s)|p+1)ds+c(‖φτ‖2E+2∑i∈ZFi(uτ,i)). |
Proof. Taking the inner product (⋅,⋅)E on both side of the system (2.11) with φ, it follows that
12ddt‖φ‖2E+(L(φ),φ)E=(H(φ),φ)E+(G(ω),φ)E. | (2.12) |
For the second term on the left-hand side of (2.12), we have
(L(φ),φ)E=β‖u‖2λ+β2(u,v)−β‖v‖2+ν(Av,v)+(h(v−βu+aω(t)),v). |
By the mean value theorem and (2.5), there exists ξi∈(0,1) such that
β2(u,v)+(h(v−βu+aω(t)),v)=β2(u,v)+∑i∈Zh′i(ξi(vi−βui+aiω(t)))(vi−βui+aiω(t))vi≥(β2−h2β)‖u‖‖v‖+h1‖v‖2−h2|(aω(t),v)|. |
Then
(L(φ),φ)E−σ‖φ‖2E−h12‖v‖2≥(β−σ)‖u‖2λ+(h12−β−σ)‖v‖2−βh2√λ‖u‖λ‖v‖−h2|(aω(t),v)|, |
which along with (2.6) and (2.7) implies that
(L(φ),φ)E≥σ‖φ‖2E+h12‖v‖2−σ+h16‖v‖2−c|ω(t)|2‖a‖2. | (2.13) |
As to the first term on the right-hand side of (2.12), by (2.3) and (2.4) we get
(H(φ),φ)E=(−f(u),˙u+βu−aω(t))+(g(t),v)≤−ddt(∑i∈ZFi(ui))−α2β∑i∈ZFi(ui)+α1∑i∈Z(|ui|p+|ui|)|aiω(t)|+(g(t),v)≤−ddt(∑i∈ZFi(ui))−α2βp+1∑i∈ZFi(ui)+c|ω(t)|p+1‖a‖p+1+σλ4‖u‖2+c‖a‖2|ω(t)|2+σ+h16‖v‖2+c‖g(t)‖2. | (2.14) |
The last term of (2.12) is bounded by
(G(ω),φ)E=ω(t)(a,u)λ+βω(t)(a,v)−νω(t)(Aa,v)≤σ4‖u‖2λ+1σ‖a‖2λ|ω(t)|2+σ+h16‖v‖2+c|ω(t)|2‖a‖2. | (2.15) |
It follows from (2.12)–(2.15) that
ddt(‖φ‖2E+2∑i∈ZFi(ui))+γ(‖φ‖2E+2∑i∈ZFi(ui))+γ‖φ‖2E≤c(‖g(t)‖2+|ω(t)|2+|ω(t)|p+1), | (2.16) |
where γ=min{σ2,α2βp+1}. Multiplying (2.16) by eγt and then integrating over (τ,t) with t≥τ, we get for every ω∈Ω
‖φ(t,τ,ω,φτ)‖2E+γ∫tτeγ(s−t)‖φ(s,τ,ω,φτ)‖2Eds≤eγ(τ−t)(‖φτ‖2E+2∑i∈ZFi(uτ,i))+c∫tτeγ(s−t)(‖g(s)‖2+|ω(s)|2+|ω(s)|p+1)ds, | (2.17) |
which implies desired result.
Lemma 2.2. Suppose that (2.3)–(2.9) hold. Then the continuous cocycle Φ0 associated with system (2.10) has a closed measurable D-pullback absorbing set K0={K0(τ,ω):τ∈R,ω∈Ω}∈D, where for every τ∈R and ω∈Ω
K0(τ,ω)={ˉφ∈E:‖ˉφ‖2E≤R0(τ,ω)}, | (2.18) |
where ˉφτ−t∈D(τ−t,θ−tω) and R0(τ,ω) is given by
R0(τ,ω)=c+c|ω(−τ)|2+c∫0−∞eγs(‖g(s+τ)‖2+|ω(s)−ω(−τ)|2+|ω(s)−ω(−τ)|p+1)ds, | (2.19) |
where c is a positive constant independent of τ, ω and D.
Proof. By (2.17), we get for every τ∈R, t∈R+ and ω∈Ω
‖φ(τ,τ−t,θ−τω,φτ−t)‖2E+γ∫ττ−teγ(s−τ)‖φ(s,τ−t,θ−τω,φτ−t)‖2Eds≤e−γt(‖φτ−t‖2E+2∑i∈ZFi(uτ−t,i))+c∫ττ−teγ(s−τ)(‖g(s)‖2+|ω(s−τ)−ω(−τ)|2+|ω(s−τ)−ω(−τ)|p+1)ds≤e−γt(‖φτ−t‖2E+2∑i∈ZFi(uτ−t,i))+c∫0−teγs(‖g(s+τ)‖2+|ω(s)−ω(−τ)|2+|ω(s)−ω(−τ)|p+1)ds. | (2.20) |
By (2.1) and (2.8), the last integral on the right-hand side of (2.20) is well defined. For any s≥τ−t,
ˉφ(s,τ−t,θ−τω,ˉφτ−t)=φ(s,τ−t,θ−τω,φτ−t)+(0,a(ω(s−τ)−ω(−τ)))T, |
which along with (2.20) implies that
‖ˉφ(τ,τ−t,θ−τω,ˉφτ−t)‖2E+γ∫ττ−teγ(s−τ)‖ˉφ(s,τ−t,θ−τω,ˉφτ−t)‖2Eds≤2‖φ(τ,τ−t,θ−τω,φτ−t)‖2E+2γ∫ττ−teγ(s−τ)‖φ(s,τ−t,θ−τω,φτ−t)‖2Eds+2‖a‖2(|ω(−τ)|2+γ∫ττ−teγ(s−τ)|ω(s−τ)−ω(−τ)|2ds)≤4e−γt(‖ˉφτ−t‖2E+‖a‖2|ω(−t)−ω(−τ)|2+∑i∈ZFi(uτ−t,i))+c|ω(−τ)|2+c∫0−∞eγs(‖g(s+τ)‖2+|ω(s)−ω(−τ)|2+|ω(s)−ω(−τ)|p+1)ds. | (2.21) |
By (2.3) and (2.4) we have
∑i∈ZFi(uτ−t,i)≤1α2∑i∈Zfi(uτ−t,i)uτ−t,i≤1α2max−‖uτ−t‖≤s≤‖uτ−t‖|f′i(s)|‖uτ−t‖2. | (2.22) |
Using ˉφτ−t∈D(τ−t,θ−tω), (2.1) and (2.22), we find
lim supt→+∞4e−γt(‖ˉφτ−t‖2E+‖a‖2|ω(−t)−ω(−τ)|2+∑i∈ZFi(uτ−t,i))=0, | (2.23) |
which along with (2.21) implies that there exists T=T(τ,ω,D)>0 such that for all t≥T,
‖ˉφ(τ,τ−t,θ−τω,ˉφτ−t)‖2E+γ∫ττ−teγ(s−τ)‖ˉφ(s,τ−t,θ−τω,ˉφτ−t)‖2Eds≤c+c|ω(−τ)|2+c∫0−∞eγs(‖g(s+τ)‖2+|ω(s)−ω(−τ)|2+|ω(s)−ω(−τ)|p+1)ds, | (2.24) |
where c is a positive constant independent of τ, ω and D. Note that K0 given by (2.18) is closed measurable random set in E. Given τ∈R, ω∈Ω, and D∈D, it follows from (2.24) that for all t≥T,
Φ0(t,τ−t,θ−tω,D(τ−t,θ−tω))⊆K0(τ,ω), | (2.25) |
which implies that K0 pullback attracts all elements in D. By (2.1) and (2.9), one can easily check that K0 is tempered, which along with (2.25) completes the proof.
Next, we will get uniform estimates on the tails of solutions of system (2.10).
Lemma 2.3. Suppose that (2.3)–(2.9) hold. Then for every τ∈R, ω∈Ω, D={D(τ,ω):τ∈R,ω∈Ω}∈D and ε>0, there exist T=T(τ,ω,D,ε)>0 and N=N(τ,ω,ε)>0 such that for allt≥T, the solution ˉφ of system (2.10) satisfies
∑|i|≥N|ˉφi(τ,τ−t,θ−τω,ˉφτ−t,i)|2E≤ε, |
where ˉφτ−t∈D(τ−t,θ−tω) and |ˉφi|2E=(1−νβ)|Bu|2i+λ|ui|2+|ˉvi|2.
Proof. Let η be a smooth function defined on R+ such that 0≤η(s)≤1 for all s∈R+, and
η(s)={0,0≤s≤1;1,s≥2. |
Then there exists a constant C0 such that |η′(s)|≤C0 for s∈R+. Let k be a fixed positive integer which will be specified later, and set x=(xi)i∈Z, y=(yi)i∈Z with xi=η(|i|k)ui, yi=η(|i|k)vi. Note ψ=(x,y)T=((xi),(yi))Ti∈Z. Taking the inner product of system (2.11) with ψ, we have
(˙φ,ψ)E+(L(φ),ψ)E=(H(φ),ψ)E+(G,ψ)E. | (2.26) |
For the first term of (2.26), we have
(˙φ,ψ)E=(1−νβ)∑i∈Z(B˙u)i(Bx)i+λ∑i∈Z˙uixi+∑i∈Z˙viyi=12ddt∑i∈Zη(|i|k)|φi|2E+(1−νβ)∑i∈Z(B˙u)i((Bx)i−η(|i|k)(Bu)i)≥12ddt∑i∈Zη(|i|k)|φi|2E−(1−νβ)C0k∑i∈Z|(B(v−βu+aω(t))i||ui+1|≥12ddt∑i∈Zη(|i|k)|φi|2E−ck‖φ‖2E−ck|ω(t)|2‖a‖2, | (2.27) |
where |φi|2E=(1−νβ)|Bu|2i+λ|ui|2+|vi|2. As to the second term on the left-hand side of (2.26), we get
(L(φ),ψ)E=β(1−νβ)(Au,x)+(1−νβ)((Au,y)−(Av,x))+ν(Av,y)+λβ(u,x)+β2(u,y)−β(v,y)+(h(v−βu+aω(t)),y). |
It is easy to check that
(Au,x)=∑i∈Z(Bu)i(η(|i|k)(Bu)i+(Bx)i−η(|i|k)(Bu)i)≥∑i∈Zη(|i|k)|Bu|2i−2C0k‖u‖2, |
(Av,y)=∑i∈Z(Bv)i(η(|i|k)(Bv)i+(By)i−η(|i|k)(Bv)i)≥∑i∈Zη(|i|k)|Bv|2i−2C0k‖v‖2, |
and
(Au,y)−(Av,x)≥−C0k∑i∈Z|(Bu)i||vi+1|−C0k∑i∈Z|(Bv)i||ui+1|≥−2C0k(‖u‖2+‖v‖2). |
By the mean value theorem and (2.5), there exists ξi∈(0,1) such that
β2(u,y)+(h(v−βu+aω(t)),y)=β2∑i∈Zη(|i|k)uivi+∑i∈Zh′i(ξi(vi−βui+aiω(t)))(vi−βui+aiω(t))η(|i|k)vi≥β(β−h2)∑i∈Zη(|i|k)|uivi|+h1∑i∈Zη(|i|k)|vi|2−h2∑i∈Zη(|i|k)|viaiω(t)|. |
Then
(L(φ),φ)E−σ∑i∈Zη(|i|k)|φi|2E−h12∑i∈Zη(|i|k)|vi|2≥(β−σ)∑i∈Zη(|i|k)((1−νβ)|Bu|2i+λu2i)+(h12−β−σ)∑i∈Zη(|i|k)|vi|2−βh2√λ∑i∈Zη(|i|k)|vi|((1−νβ)(Bu)2i+λ|ui|2)12−h2∑i∈Zη(|i|k)|viaiω(t)|−ck‖φ‖2E, |
which along with (2.6) and (2.7) implies that
(L(φ),φ)E≥σ∑i∈Zη(|i|k)|φi|2E+h12∑i∈Zη(|i|k)|vi|2−ck‖φ‖2E−h2∑i∈Zη(|i|k)|viaiω(t)|≥σ∑i∈Zη(|i|k)|φi|2E+h16∑i∈Zη(|i|k)|vi|2−ck‖φ‖2E−c∑i∈Zη(|i|k)|ai|2|ω(t)|2. | (2.28) |
As to the first term on the right-hand side of (2.26), by (2.3) and (2.4)we get
(H(φ),ψ)E=−∑i∈Zη(|i|k)fi(ui)(˙ui+βui−aiω(t))+∑i∈Zη(|i|k)gi(t)vi≤−ddt(∑i∈Zη(|i|k)Fi(ui))−α2βp+1∑i∈Zη(|i|k)Fi(ui)+c∑i∈Zη(|i|k)|ω(t)|p+1|ai|p+1+σλ4∑i∈Zη(|i|k)|ui|2+c∑i∈Zη(|i|k)|ai|2|ω(t)|2+σ6∑i∈Zη(|i|k)|vi|2+c∑i∈Zη(|i|k)|gi(t)|2. | (2.29) |
For the last term of (2.26), we have
(G,ψ)E=ω(t)(a,x)λ+βω(t)(a,y)−νω(t)(Aa,y)=ω(t)(1−νβ)(Ba,Bx)−νω(t)(Ba,By)+λω(t)(a,x)+βω(t)(a,y), | (2.30) |
As to the first two terms on the right-hand side of (2.30), we get
ω(t)(1−νβ)(Ba,Bx)=ω(t)(1−νβ)∑i∈Z(ai+1−ai)(η(|i+1|k)ui+1−η(|i|k)ui)≤(∑i∈Zη(|i+1|k)u2i+1)12(∑i∈Zη(|i+1|k)(ω(t)(1−νβ)(ai+1−ai))2)12+(∑i∈Zη(|i|k)u2i)12(∑i∈Zη(|i|k)(ω(t)(1−νβ)(ai+1−ai))2)12≤σλ8∑i∈Zη(|i|k)u2i+c|ω(t)|2∑|i|≥ka2i, | (2.31) |
and
−νω(t)(Ba,By)=−νω(t)∑i∈Z(ai+1−ai)(η(|i+1|k)vi+1−η(|i|k)vi)≤σ6∑i∈Zη(|i|k)v2i+c|ω(t)|2∑|i|≥ka2i. | (2.32) |
The last two terms of (2.30) is bounded by
λω(t)(a,x)+βω(t)(a,y)≤σλ8∑i∈Zη(|i|k)u2i+σ+h16∑i∈Zη(|i|k)v2i+c|ω(t)|2∑|i|≥ka2i. | (2.33) |
It follows from (2.26)–(2.33) that
ddt(∑i∈Zη(|i|k)(|φi|2E+2Fi(ui)))+γ(∑i∈Zη(|i|k)(|φi|2E+2Fi(ui)))+γ∑i∈Zη(|i|k)|φ|2E≤ck‖φ‖2E+ck|ω(t)|2+c∑|i|≥k|ai|p+1|ω(t)|p+1+c∑|i|≥k|gi(t)|2+c∑|i|≥k|ai|2|ω(t)|2, | (2.34) |
where γ=min{σ2,α2βp+1}. Multiplying (2.34) by eγt, replacing ω by θ−τω and integrating on (τ−t,τ) with t∈R+, we get for every ω∈Ω
∑i∈Zη(|i|k)(|φi(τ,τ−t,θ−τω,φτ−t,i)|2E+2Fi(ui(τ,τ−t,θ−τω,uτ−t,i)))≤e−γt(∑i∈Zη(|i|k)(|φτ−t,i|2E+2Fi(uτ−t,i)))+ck∫ττ−teγ(s−τ)‖φ(s,τ−t,θ−τω,φτ−t)‖2Eds+ck∫0−∞eγs|ω(s)−ω(−τ)|2ds+c∑|i|≥k|ai|p+1∫0−∞eγs|ω(s)−ω(−τ)|p+1ds+c∑|i|≥k|ai|2∫0−∞eγs|ω(s)−ω(−τ)|2ds+c∫0−∞eγs∑|i|≥k|gi(s+τ)|2ds. | (2.35) |
For any s≥τ−t,
ˉφ(s,τ−t,θ−τω,ˉφτ−t)=φ(s,τ−t,θ−τω,φτ−t)+(0,a(ω(s−τ)−ω(−τ)))T, |
which along with (2.35) implies that
∑i∈Zη(|i|k)(|ˉφi(τ,τ−t,θ−τω,ˉφτ−t,i)|2E+2Fi(ui(τ,τ−t,θ−τω,uτ−t,i)))≤4e−γt(∑i∈Zη(|i|k)(|ˉφτ−t,i|2E+|ai|2|ω(−t)−ω(−τ)|2+Fi(uτ−t,i)))+ck∫ττ−teγ(s−τ)‖ˉφ(s,τ−t,θ−τω,ˉφτ−t)‖2Eds+ck∫0−∞eγs|ω(s)−ω(−τ)|2ds+c∑|i|≥k|ai|p+1∫0−∞eγs|ω(s)−ω(−τ)|p+1ds+c∑|i|≥k|ai|2∫0−∞eγs|ω(s)−ω(−τ)|2ds+c∫0−∞eγs∑|i|≥k|gi(s+τ)|2ds+2∑|i|≥k|ai|2|ω(−τ)|2. | (2.36) |
By (2.1) and (2.8), the last four integrals in (2.36) are well defined. By (2.3) and (2.4), we obtain
∑i∈Zη(|i|k)Fi(ui,τ−t)≤1α2∑i∈Zη(|i|k)fi(uτ−t,i)uτ−t,i≤1α2max−‖uτ−t‖≤s≤‖uτ−t‖|f′i(s)|‖uτ−t‖2, |
which along with ˉφτ−t∈D(τ−t,θ−tω) and (2.1) implies that
lim supt→+∞4e−γt(∑i∈Zη(|i|k)(|ˉφτ−t,i|2E+|ai|2|ω(−t)−ω(−τ)|2+Fi(uτ−t,i)))=0. |
Then there exists T1=T1(τ,ω,D,ε)>0 such that for all t≥T1,
4e−γt(∑i∈Zη(|i|k)(|ˉφτ−t,i|2E+|ai|2|ω(−t)−ω(−τ)|2+Fi(uτ−t,i)))≤ε4. | (2.37) |
By (2.1) and (2.24), there exist T2=T2(τ,ω,D,ε)>T1 and N1=N1(τ,ω,ε)>0 such that for all t≥T2 and k≥N1
ck∫ττ−teγ(s−τ)‖ˉφ(s,τ−t,θ−τω,ˉφτ−t)‖2Eds+ck∫0−∞eγs|ω(s)−ω(−τ)|2ds≤ε4. | (2.38) |
By (2.8), there exists N2=N2(τ,ω,ε)>N1 such that for all k≥N2,
2∑|i|≥k|ai|2|ω(−τ)|2+c∫0−∞eγs∑|i|≥k|gi(s+τ)|2ds≤ε4. | (2.39) |
By (2.1) again, we find that there exists N3=N3(τ,ω,ε)>N2 such that for all k≥N3,
c∑|i|≥k|ai|p+1∫0−∞eγs|ω(s)−ω(−τ)|p+1ds+c∑|i|≥k|ai|2∫0−∞eγs|ω(s)−ω(−τ)|2ds≤ε4. | (2.40) |
Then it follows from (2.36)–(2.40) that for all t≥T2 and k≥N3
∑|i|≥2k|ˉφi(τ,τ−t,θ−τω,ˉφτ−t,i)|2E≤∑i∈Zη(|i|k)|ˉφi(τ,τ−t,θ−τω,ˉφτ−t,i)|2E≤ε. |
This concludes the proof.
As a consequence of Lemma 2.2 and Lemma 2.3, we get the existence of D-pullback attractors for Φ0 immediately.
Theorem 2.1. Suppose that (2.3)–(2.9) hold. Then the continuous cocycle Φ0 associated with system (2.10) has a unique D-pullback attractors A0={A0(τ,ω):τ∈R, ω∈Ω}∈D in E.
In this section, we will approximate the solutions of system (1.1) by the pathwise Wong-Zakai approximated system (1.2). Given δ≠0, define a random variable Gδ by
Gδ(ω)=ω(δ)δ,for allω∈Ω. | (3.1) |
From (3.1) we find
Gδ(θtω)=ω(t+δ)−ω(t)δand∫t0Gδ(θsω)ds=∫t+δtω(s)δds+∫0δω(s)δds. | (3.2) |
By (3.2) and the continuity of ω we get for all t∈R,
limδ→0∫t0Gδ(θsω)ds=ω(t). | (3.3) |
Note that this convergence is uniform on a finite interval as stated below.
Lemma 3.1. ([17]). Let τ∈R, ω∈Ω and T>0. Then for every ε>0, there exists δ0=δ0(ε,τ,ω,T)>0 such that for all 0<|δ|<δ0 and t∈[τ,τ+T],
|∫t0Gδ(θsω)ds−ω(t)|<ε. |
By Lemma 3.1, we find that there exist c=c(τ,ω,T)>0 and ˜δ0=˜δ0(τ,ω,T)>0 such that for all 0<|δ|<˜δ0 and t∈[τ,τ+T],
|∫t0Gδ(θsω)ds|≤c. | (3.4) |
By (3.3) we find that Gδ(θtω) is an approximation of the white noise in a sense. This leads us to consider system (1.2) as an approximation of system (1.1).
Let ˉvδ=˙uδ+βuδ and ˉφδ=(uδ,ˉvδ), the system (1.2) can be rewritten as
˙ˉφδ+Lδ,1(ˉφδ)=Hδ,1(ˉφδ)+Gδ,1(ω), | (3.5) |
with initial conditions
ˉφδ,τ=(uδτ,ˉvδτ)T=(uδτ,uδ,1τ+βuδτ)T, |
where
Lδ,1(ˉφ)=(βuδ−ˉvδ(1−νβ)Auδ+νAˉvδ+λuδ+β2uδ−βˉvδ)+(0h(ˉvδ−βuδ)), |
Hδ,1(¯φδ)=(0−f(uδ)+g(t)),Gδ,1(ω)=(0aGδ(θtω)). |
Denote
vδ(t)=ˉvδ(t)−a∫t0Gδ(θsω)dsandφδ=(uδ,vδ)T. |
By (3.5) we have
˙φδ+Lδ(φδ)=Hδ(φδ)+Gδ(ω), | (3.6) |
with initial conditions
φδ,τ=(uδτ,vδτ)T=(uδτ,uδ,1τ+βuδτ−a∫τ0Gδ(θsω)ds)T, |
where
Lδ(φδ)=(βuδ−vδ(1−νβ)Auδ+νAvδ+λuδ+β2uδ−βvδ)+(0h(vδ−βuδ+a∫t0Gδ(θsω)ds)), |
Hδ(φδ)=(0−f(uδ)+g(t)),Gδ(ω)=(a∫t0Gδ(θsω)dsβa∫t0Gδ(θsω)ds−νAa∫t0Gδ(θsω)ds). |
Note that system (3.6) is a deterministic functional equation and the nonlinearity in (3.6) is locally Lipschitz continuous from E to E. Therefore, by the standard theory of functional differential equations, system (3.6) is well-posed. Thus, we can define a continuous cocycle Φδ:R+×R×Ω×E→E associated with system (3.5), where for τ∈R, t∈R+ and ω∈Ω
Φδ(t,τ,ω,ˉφδ,τ)=ˉφδ(t+τ,τ,θ−τω,ˉφδ,τ)=(uδ(t+τ,τ,θ−τω,uδτ),ˉvδ(t+τ,τ,θ−τω,ˉvδτ))T=(uδ(t+τ,τ,θ−τω,uδτ),vδ(t+τ,τ,θ−τω,vδτ)+a∫t−τGδ(θsω)ds)T=φδ(t+τ,τ,θ−τω,φδ,τ)+(0,a∫t−τGδ(θsω)ds)T, |
where vδτ=ˉvδτ−a∫0−τGδ(θsω)ds.
For later purpose, we now show the estimates on the solutions of system (3.6) on a finite time interval.
Lemma 3.2. Suppose that (2.3)–(2.8) hold. Then for every τ∈R, ω∈Ω, and T>0, there exist δ0=δ0(τ,ω,T)>0 and c=c(τ,ω,T)>0 such that for all 0<|δ|<δ0 andt∈[τ,τ+T], the solution φδ of system (3.6) satisfies
‖φδ(t,τ,ω,φδ,τ)‖2E+∫tτ‖φδ(s,τ,ω,φδ,τ)‖2Eds≤c(‖φδ,τ‖2E+2∑i∈ZFi(uδτ,i))+c∫tτ(‖g(s)‖2+|∫s0Gδ(θlω)dl|2+|∫s0Gδ(θlω)dl|p+1|)ds. |
Proof. Taking the inner product (⋅,⋅)E on both side of the system (3.6) with φδ, it follows that
12ddt‖φδ‖2E+(Lδ(φδ),φδ)E=(Hδ(φδ),φδ)E+(Gδ(ω),φδ)E. | (3.7) |
By the similar calculations in (2.13)–(2.15), we get
(Lδ(φδ),φδ)E≥σ‖φδ‖2E+h12‖vδ‖2−σ+h16‖vδ‖2−c|∫t0Gδ(θsω)ds|2‖a‖2, | (3.8) |
(Hδ(φδ),φδ)E≤−ddt(∑i∈ZFi(uδi))−α2βp+1∑i∈ZFi(uδi)+c|∫t0Gδ(θsω)ds|p+1‖a‖p+1+σλ4‖uδ‖2+c‖a‖2|∫t0Gδ(θsω)ds|2+c‖g(t)‖2+σ+h16‖vδ‖2, | (3.9) |
and
(Gδ(ω),φδ)E≤σ4‖uδ‖2λ+c‖a‖2|∫t0Gδ(θsω)ds|2+σ+h16‖vδ‖2. | (3.10) |
It follows from (3.7)–(3.10) that
ddt(‖φδ‖2E+2∑i∈ZFi(uδi))+γ(‖φδ‖2E+2∑i∈ZFi(uδi))+γ‖φδ‖2E≤c(‖g(t)‖2+|∫t0Gδ(θsω)ds|2+|∫t0Gδ(θsω)ds|p+1), | (3.11) |
where γ=min{σ2,α2βp+1}. Multiplying (3.11) by eγt and integrating on (τ,t) with t≥τ, we get for every ω∈Ω
‖φδ(t,τ,ω,φδ,τ)‖2E+γ∫tτeγ(s−t)‖φδ(s,τ,ω,φδ,τ)‖2Eds≤eγ(τ−t)(‖φδ,τ‖2E+2∑i∈ZFi(uδτ,i))+c∫tτeγ(s−t)(‖g(s)‖2+|∫s0Gδ(θlω)dl|2+|∫s0Gδ(θlω)dl|p+1)ds, |
which implies the desired result.
In what follows, we derive uniform estimates on the solutions of system (3.5) when t is sufficiently large.
Lemma 3.3. Suppose that (2.3)–(2.8) hold. Then for every δ≠0, τ∈R, ω∈Ω, and D={D(τ,ω):τ∈R,ω∈Ω}∈D, there exists T=T(τ,ω,D,δ)>0 such that for allt≥T, the solution ˉφδ of system (3.5) satisfies
‖ˉφδ(τ,τ−t,θ−τω,ˉφδ,τ−t)‖2E+γ∫ττ−teγ(s−τ)‖ˉφδ(s,τ−t,θ−τω,ˉφδ,τ−t)‖2Eds≤Rδ(τ,ω), |
where ˉφδ,τ−t∈D(τ−t,θ−tω) and Rδ(τ,ω) is given by
Rδ(τ,ω)=c∫0−∞eγs(‖g(s+τ)‖2+|∫s−τGδ(θlω)dl|2+|∫s−τGδ(θlω)dl|p+1)ds+c+c|∫0−τGδ(θlω)dl|2, | (3.12) |
where c is a positive constant independent of τ, ω and δ.
Proof. Multiplying (3.11) by eγt, replacing ω by θ−τω and integrating on (τ−t,τ) with t∈R+, we get for every ω∈Ω
‖φδ(τ,τ−t,θ−τω,φδ,τ−t)‖2E+2∑i∈ZFi(uδi(τ,τ−t,θ−τω,uδτ−t,i))+γ∫ττ−teγ(s−τ)‖φδ(s,τ−t,θ−τω,φδ,τ−t)‖2Eds≤e−γt(‖φδ,τ−t‖2E+2∑i∈ZFi(uδτ−t,i))+c∫0−∞eγs(‖g(s+τ)‖2+|∫s−τGδ(θlω)dl|2+|∫s−τGδ(θlω)dl|p+1)ds. | (3.13) |
By (2.1), (2.8) and (3.2), the last integral on the right-hand side of (3.13) is well defined. For any s≥τ−t,
ˉφδ(s,τ−t,θ−τω,ˉφδ,τ−t)=φδ(s,τ−t,θ−τω,φδ,τ−t)+(0,a∫s0Gδ(θl−τω)dl)T, |
which along with (3.13) shows that
‖ˉφδ(τ,τ−t,θ−τω,ˉφδ,τ−t)‖2E+γ∫ττ−teγ(s−τ)‖ˉφδ(s,τ−t,θ−τω,ˉφτ−t)‖2Eds≤4e−γt(‖ˉφδ,τ−t‖2E+‖a‖2|∫−t−τGδ(θlω)dl|2+∑i∈ZFi(uτ−t,i))+c|∫0−τGδ(θlω)dl|2+c∫0−∞eγs(‖g(s+τ)‖2+|∫s−τGδ(θlω)dl|2+|∫s−τGδ(θlω)dl|p+1)ds, | (3.14) |
Note that (2.3) and (2.4) implies that
∑i∈ZFi(uδτ−t,i)≤1α2∑i∈Zfi(uδτ−t,i)uδτ−t,i≤1α2max−‖uδτ−t‖≤s≤‖uδτ−t‖|f′i(s)|‖uδτ−t‖2, |
which along with ˉφδ,τ−t∈D(τ−t,θ−tω), (2.1) and (3.2) implies that
lim supt→+∞4e−γt(‖ˉφδ,τ−t‖2E+‖a‖2|∫−t−τGδ(θlω)dl|2+∑i∈ZFi(uτ−t,i))=0. | (3.15) |
Then (3.14) and (3.15) can imply the desired estimates.
Next, we show that system (3.5) has a D-pullback absorbing set.
Lemma 3.4. Suppose that (2.3)–(2.9) hold. Then the continuous cocycle Φδ associated with system (3.5) has a closed measurable D-pullback absorbing set Kδ={Kδ(τ,ω):τ∈R,ω∈Ω}∈D, where for every τ∈R and ω∈Ω
Kδ(τ,ω)={ˉφδ∈E:‖ˉφδ‖2E≤Rδ(τ,ω)}, | (3.16) |
where Rδ(τ,ω) is given by (3.12).In addition, we have for every τ∈R and ω∈Ω
limδ→0Rδ(τ,ω)=R0(τ,ω), | (3.17) |
where R0(τ,ω) is defined in (2.19).
Proof. Note Kδ given by (3.16) is closed measurable random set in E. Given τ∈R, ω∈Ω, and D∈D, it follows from Lemma 3.3 that there exists T0=T0(τ,ω,D,δ) such that for all t≥T0,
Φδ(t,τ−t,θ−tω,D(τ−t,θ−tω))⊆Kδ(τ,ω), |
which implies that Kδ pullback attracts all elements in D. By (2.1), (2.8) and (3.2), we can prove Kδ(τ,ω) is tempered. The convergence (3.17) can be obtained by Lebesgue dominated convergence as in [17].
We are now in a position to derive uniform estimates on the tail of solutions of system (3.5).
Lemma 3.5. Suppose that (2.3)–(2.8) hold. Then for every τ∈R, ω∈Ω and ε>0, there exist δ0=δ0(ω)>0, T=T(τ,ω,ε)>0 and N=N(τ,ω,ε)>0 such that for allt≥T and 0<|δ|<δ0, the solution ˉφδ of system (3.5) satisfies
∑|i|≥N|ˉφδ,i(τ,τ−t,θ−τω,ˉφδ,τ−t,i)|2E≤ε, |
where ˉφδ,τ−t∈Kδ(τ−t,θ−tω) and |ˉφδ,i|2E=(1−νβ)|Buδ|2i+λ|uδi|2+|ˉvδi|2.
Proof. Let η be a smooth function defined in Lemma 2.3, and set x=(xi)i∈Z, y=(yi)i∈Z with xi=η(|i|k)uδi, yi=η(|i|k)vδi. Note ψ=(x,y)T=((xi),(yi))Ti∈Z. Taking the inner product of system (3.6) with ψ, we have
(˙φδ,ψ)E+(Lδ(φδ),ψ)E=(Hδ(φδ),ψ)E+(Gδ,ψ)E. | (3.18) |
For the first term of (3.18), we have
(˙φδ,ψ)E=(1−νβ)∑i∈Z(B˙uδ)i(Bx)i+λ∑i∈Z˙uδixi+∑i∈Z˙vδiyi=12ddt∑i∈Zη(|i|k)|φδ,i|2E+(1−νβ)∑i∈Z(B˙uδ)i((Bx)i−η(|i|k)(Buδ)i)≥12ddt∑i∈Zη(|i|k)|φδ,i|2E−(1−νβ)C0k∑i∈Z|B(vδ−βuδ+a∫t0Gδ(θsω)ds)|i|uδi+1|≥12ddt∑i∈Zη(|i|k)|φδ,i|2E−ck‖φδ‖2E−ck|∫t0Gδ(θsω)ds|2‖a‖2, | (3.19) |
where |φδ,i|2E=(1−νβ)|Buδ|2i+λ|uδi|2+|vδi|2. By the similar calculations in (2.28)–(2.33), we get
(Lδ(φδ),ψ)E≥σ∑i∈Zη(|i|k)|φδ,i|2E+h16∑i∈Zη(|i|k)|vδi|2−ck‖φδ‖2E−c∑i∈Zη(|i|k)|ai|2|∫t0Gδ(θsω)ds|2, | (3.20) |
(Hδ(φδ),ψ)E≤−ddt(∑i∈Zη(|i|k)Fi(uδi))−α2βp+1∑i∈Zη(|i|k)Fi(uδi)+σλ4∑i∈Zη(|i|k)|uδi|2+σ6∑i∈Zη(|i|k)|vδi|2+c∑i∈Zη(|i|k)|gi(t)|2+c∑i∈Zη(|i|k)|ai|2|∫t0Gδ(θsω)ds|2+c∑i∈Zη(|i|k)|ai|p+1|∫t0Gδ(θsω)ds|p+1, | (3.21) |
and
(Gδ,ψ)E=(1−νβ)∫t0Gδ(θsω)ds(Bx,Ba)λ+β∫t0Gδ(θsω)ds(y,a)≤σλ4∑i∈Zη(|i|k)|uδi|2+(h16+σ3)∑i∈Zη(|i|k)|vδi|2+c|∫t0Gδ(θsω)ds|2∑i∈Zη(|i|k)|ai|2. | (3.22) |
It follows from (3.18)–(3.22) that
ddt(∑i∈Zη(|i|k)(|φδ,i|2E+2Fi(uδi)))+γ(∑i∈Zη(|i|k)(|φδ,i|2E+2Fi(uδi)))+γ∑i∈Zη(|i|k)|φδ,i|2E≤ck‖φδ‖2E+ck|∫t0Gδ(θsω)ds|2+c∑|i|≥k|gi(t)|2+c∑|i|≥k|ai|p+1|∫t0Gδ(θsω)ds|p+1+c∑|i|≥k|ai|2|∫t0Gδ(θsω)ds|2, | (3.23) |
where γ=min{σ2,α2βp+1}. Multiplying (3.23) by eγt, replacing ω by θ−τω and integrating on (τ−t,τ) with t∈R+, we get for every ω∈Ω
∑i∈Zη(|i|k)(|φδ,i(τ,τ−t,θ−τω,φδ,τ−t,i)|2E+2Fi(uδi(τ,τ−t,θ−τω,uδτ−t,i)))≤e−γt∑i∈Zη(|i|k)(|φδ,τ−t,i|2E+2Fi(uδτ−t,i))+ck∫ττ−teγ(s−τ)‖φδ(s,τ−t,θ−τω,φδ,τ−t)‖2Eds+ck∫0−∞eγs|∫s−τGδ(θlω)dl|2ds+c∫0−∞eγs∑|i|≥k|gi(s+τ)|2ds+c∑|i|≥k|ai|2∫0−∞eγs|∫s−τGδ(θlω)dl|2ds+c∑|i|≥k|ai|p+1∫0−∞eγs|∫s−τGδ(θlω)dl|p+1ds. | (3.24) |
For any s≥τ−t,
ˉφδ(s,τ−t,θ−τω,ˉφδ,τ−t)=φδ(s,τ−t,θ−τω,φδ,τ−t)+(0,a∫s0Gδ(θl−τω)dl)T, |
which along with (3.24) shows that
∑i∈Zη(|i|k)|ˉφδ,i(τ,τ−t,θ−τω,ˉφδ,τ−t,i)|2E≤2∑i∈Zη(|i|k)|φδ,i(τ,τ−t,θ−τω,φδ,τ−t,i)|2E+2∑i∈Zη(|i|k)|ai∫τ0Gδ(θl−τω)dl|2≤2∑|i|≥k|ai∫0−τGδ(θlω)dl|2+4e−γt∑i∈Zη(|i|k)(|ˉφδ,τ−t,i|2E+|ai∫−t−τGδ(θlω)dl|2+Fi(uδτ−t,i))+ck∫ττ−teγ(s−τ)‖ˉφδ(s,τ−t,θ−τω,ˉφδ,τ−t)‖2Eds+ck∫0−∞eγs|∫s−τGδ(θlω)dl|2ds+c∫0−∞eγs∑|i|≥k|gi(s+τ)|2ds+c∑|i|≥k|ai|2∫0−∞eγs|∫s−τGδ(θlω)dl|2ds+c∑|i|≥k|ai|p+1∫0−∞eγs|∫s−τGδ(θlω)dl|p+1ds. | (3.25) |
By (2.1) and (2.8), the last four integrals on the right-hand side of (3.24) are well defined. Note that (2.3) and (2.4) implies that
∑i∈Zη(|i|k)Fi(uδτ−t,i)≤1α2∑i∈Zη(|i|k)fi(uδτ−t,i)uδτ−t,i≤1α2max−‖uδτ−t‖≤s≤‖uδτ−t‖|f′i(s)|‖uδτ−t‖2. |
Since ˉφδ,τ−t∈Kδ(τ−t,θ−tω), we find
lim supt→+∞e−γt∑i∈Zη(|i|k)|ˉφδ,τ−t,i|2E≤lim supt→+∞e−γt‖Kδ(τ−t,θ−tω)‖2E=0, |
which along with (2.1) and (3.2) shows that there exist T1=T1(τ,ω,ε)>0 and δ0>0 such that for all t≥T1 and 0<|δ|<δ0,
4e−γt∑i∈Zη(|i|k)(|ˉφδ,τ−t,i|2E+|ai∫−t−τGδ(θlω)dl|2+Fi(uδτ−t,i))≤ε4. | (3.26) |
By Lemma 3.3, (2.1) and (3.2), there exist T2=T2(τ,ω,ε)>T1 and N1=N1(τ,ε)>0 such that for all t≥T2, k≥N1 and 0<|δ|<δ0
ck∫ττ−teγ(s−τ)‖ˉφδ(s,τ−t,θ−τω,ˉφδ,τ−t)‖2Eds+ck∫0−∞eγs|∫s−τGδ(θlω)dl|2ds≤ε4. | (3.27) |
By (2.8), there exists N2=N2(τ,ε)>N1 such that for all k≥N2,
2∑|i|≥k|ai∫0−τGδ(θlω)dl|2+c∫0−∞eγs∑|i|≥k|gi(s+τ)|2ds≤ε4. | (3.28) |
By (2.1) and (3.2) again, we find that there exists N3=N3(τ,ε)>N2 such that for all k≥N3 and 0<|δ|<δ0,
c∑|i|≥k|ai|p+1∫0−∞eγs|∫s−τGδ(θlω)dl|p+1ds+c∑|i|≥k|ai|2∫0−∞eγs|∫s−τGδ(θlω)dl|2ds≤ε4. | (3.29) |
Then it follows from (3.25)–(3.29) that for all t≥T2, k≥N3 and 0<|δ|<δ0,
∑|i|≥2k|ˉφδ,i(τ,τ−t,θ−τω,ˉφδ,τ−t,i)|2E≤∑i∈Zη(|i|k)|ˉφδ,i(τ,τ−t,θ−τω,ˉφδ,τ−t,i)|2E≤ε. |
This concludes the proof.
By Lemma 3.4, Φδ has a closed D-pullback absorbing set, and Lemma 3.5 shows that Φδ is asymptotically null in E with respect to D. Therefore, we get the existence of D-pullback attractors for Φδ.
Lemma 3.6. Suppose that (2.3)–(2.9) hold. Then the continuous cocycle Φδ associated with (3.5) has a unique D-pullback attractors Aδ={Aδ(τ,ω):τ∈R, ω∈Ω}∈D in E.
For the attractor Aδ of Φδ, we have the uniform compactness as showed below.
Lemma 3.7. Suppose that (2.3)–(2.9) hold. Then for every τ∈R, ω∈Ω, there exists δ0=δ0(ω)>0 such that⋃0<|δ|<δ0Aδ(τ,ω) is precompact in E.
Proof. Given ε>0, we will prove that ⋃0<|δ|<δ0Aδ(τ,ω) has a finite covering of balls of radius less than ε. By (3.2) we have
∫0sGδ(θlω)dl=−∫s+δsω(l)δdl+∫δ0ω(l)δdl. | (3.30) |
By limδ→0∫δ0ω(r)δdr=0, there exists δ1=δ1(ω)>0 such that for all 0<|δ|<δ1,
|∫δ0ω(l)δdl|≤1. | (3.31) |
Similarly, there exists l1 between s and s+δ such that ∫s+δsω(l)δdl=ω(l1), which along with (2.1) implies that there exists T1=T1(ω)<0 such that for all s≤T1 and |δ|≤1,
|∫s+δsω(l)δdl|≤1−s. | (3.32) |
Let δ2=min{δ1,1}. By (3.30)–(3.32) we get for all 0<|δ|<δ2 and s≤T1,
|∫0sGδ(θlω)dl|<2−s. | (3.33) |
By (3.4), there exist δ0=δ0(ω)∈(0,δ2) and c1(ω)>0 such that for all 0<|δ|≤δ0 and T1≤s≤0,
|∫0sGδ(θlω)dl|≤c1(ω), |
which along with (3.33) implies that for all 0<|δ|<δ0 and s≤0,
|∫0sGδ(θlω)dl|≤−s+c2(ω), | (3.34) |
where c2(ω)=2+c1(ω). Denote by
B(τ,ω)={ˉφδ∈E:‖ˉφδ‖2≤R(τ,ω)}, |
and
R(τ,ω)=c∫0−∞eγs(‖g(s+τ)‖2+2(c2−s)2+2(|τ|+c2)2+2p(c2−s)p+1+2p(|τ|+c2)p+1)ds+c+2c(|τ|+c2)2, | (3.35) |
with c and c2 being as in (3.12) and (3.34). By (3.12) and (3.35) we find that for all 0<|δ|<δ0,
Rδ(τ,ω)≤R(τ,ω). | (3.36) |
By (3.35) and (3.36), we find that Kδ(τ,ω)⊆B(τ,ω) for all 0<|δ|<δ0, τ∈R and ω∈Ω. Therefore, for every τ∈R, ω∈Ω,
⋃0<|δ|<δ0Aδ(τ,ω)⊆⋃0<|δ|<δ0Kδ(τ,ω)⊆B(τ,ω). | (3.37) |
By Lemma 3.5, there exist T=T(τ,ω,ε)>0 and N=N(τ,ω,ε)>0 such that for all t≥T and 0<|δ|<δ0,
∑|i|≥N|ˉφδ,i(τ,τ−t,θ−τω,ˉφδ,τ−t,i)|2E≤ε4, | (3.38) |
for any ˉφδ,τ−t∈Kδ(τ−t,θ−tω). By (3.38) and the invariance of Aδ, we obtain
∑|i|≥N|ˉφi|2E≤ε4,for allˉφ=(ˉφi)i∈Z∈⋃0<|δ|<δ0Aδ(τ,ω). | (3.39) |
We find that (3.37) implies the set {(ˉφi)|i|<N:ˉφ∈⋃0<|δ|<δ0Aδ(τ,ω)} is bounded in a finite dimensional space and hence is precompact. This along with (3.39) implies ⋃0<|δ|<δ0Aδ(τ,ω) has a finite covering of balls of radius less than ε in E. This completes the proof.
In this section, we will study the limiting of solutions of (3.5) as δ→0. Hereafter, we need an additional condition on f: For all i∈Z and s∈R,
|f′i(s)|≤α4|s|p−1+κi, | (4.1) |
where α4 is a positive constant, κ=(κi)i∈Z∈l2 and p>1.
Lemma 4.1. Suppose that (2.3)–(2.7) and (4.1) hold. Let ˉφ and ˉφδ are the solutions of (2.10) and (3.5), respectively. Then for every τ∈R, ω∈Ω, T>0 and ε∈(0,1), there exist δ0=δ0(τ,ω,T,ε)>0 and c=c(τ,ω,T)>0 such that for allt∈[τ,τ+T] and 0<|δ|<δ0,
‖ˉφδ(t,τ,ω,ˉφδ,τ)−ˉφ(t,τ,ω,ˉφτ)‖2E≤2ec(t−τ)‖ˉφδ,τ−ˉφτ‖2E+cε. |
Proof. Let ˜φ=φδ−φ and ˜φ=(˜u,˜v)T, where ˜u=uδ−u, ˜v=vδ−v, φ and φδ are the solutions of (2.11) and (3.6), respectively. By (2.11) and (3.6) we get
˙˜φ+˜L(˜φ)=˜H(˜φ)+˜G(ω), | (4.2) |
where
˜L(˜φ)=(β˜u−˜v(1−νβ)A˜u+νA˜v+λ˜u+β2˜u−β˜v)+(0h(vδ−βuδ+a∫t0Gδ(θsω)ds)−h(v−βu+aω(t))), |
˜H(˜φ)=(0−f(uδ)+f(u)),˜G(ω)=(a(∫t0Gδ(θsω)ds−ω(t))(βa−νAa)(∫t0Gδ(θsω)ds−ω(t))). |
Taking the inner product of (4.2) with ˜φ in E, we have
12ddt‖˜φ‖2E+(˜L(˜φ),˜φ)E=(˜H(˜φ),˜φ)E+(˜G(ω),˜φ)E. | (4.3) |
For the second term on the left-hand side of (4.3), using the similar calculations in (2.13) we have
(˜L(˜φ),˜φ)E≥σ‖˜φ‖2E+h12‖˜v‖2−h2|(a(∫t0Gδ(θsω)ds−ω(t)),˜v)|≥σ‖˜φ‖2E+h14‖˜v‖2−c|∫t0Gδ(θsω)ds−ω(t))|2‖a‖2. | (4.4) |
For the first term on the right-hand side of (4.3), by (4.1) we get
(f(u)−f(uδ),˜v)=∑i∈Z(fi(ui)−fi(uδi))˜vi=1h1∑i∈Z|fi(ui)−fi(uδi)|2+h14∑i∈Z|˜vi|2≤c(‖φ‖2p−2E+‖φδ‖2p−2E)‖˜φ‖2E+h14‖˜v‖2+2‖κ‖2h1λ‖˜φ‖2E. | (4.5) |
As to the last term of (4.3), we have
(a(∫t0Gδ(θsω)ds−ω(t)),˜u)λ+((βa−νAa)(∫t0Gδ(θsω)ds−ω(t)),˜v)≤σ‖˜u‖2λ+14σ|∫t0Gδ(θsω)ds−ω(t)|2‖a‖2λ+σ‖˜v‖2+c|∫t0Gδ(θsω)ds−ω(t)|2‖a‖2. | (4.6) |
It follows from (4.3)–(4.6) that
ddt‖˜φ‖2E≤c(‖φ‖2p−2E+‖φδ‖2p−2E+1)‖˜φ‖2E+c|∫t0Gδ(θsω)ds−ω(t)|2. | (4.7) |
By Lemma 2.1 and Lemma 3.2, there exists δ1=δ1(τ,ω,T)>0 and c1=c1(τ,ω,T)>0 such that for all 0<|δ|<δ1 and t∈[τ,τ+T],
‖φδ(t,τ,ω,φδ,τ)‖2E+‖φ(t,τ,ω,φτ)‖2E≤c1, |
which along with (4.7) shows that for all 0<|δ|<δ1 and t∈[τ,τ+T]
ddt‖˜φ‖2E≤c‖˜φ‖2E+c|∫t0Gδ(θsω)ds−ω(t)|2. | (4.8) |
Applying Gronwall's inequality and Lemma 3.1 to (4.8), we see that for every ε∈(0,1), there exists δ0=δ0(τ,ω,T,ε)∈(0,δ1) such that for all 0<|δ|<δ0 and t∈[τ,τ+T]
‖˜φ(t,τ,ω,˜φτ)‖2E≤ec(t−τ)‖˜φτ‖2E+cε. | (4.9) |
On the other hand, we have
ˉφδ(t,τ,ω,ˉφδ,τ)−ˉφ(t,τ,ω,ˉφτ)=˜φ+(0,a(∫t0Gδ(θs)ds−ω(t)))T, |
which along with (4.9) implies the desired result.
Finally, we establish the upper semicontinuity of random attractors as δ→0.
Theorem 4.1. Suppose that (2.3)–(2.9) and (4.1) hold. Then for every τ∈R and ω∈Ω,
limδ→0dE(Aδ(τ,ω),A0(τ,ω))=0, | (4.10) |
where dE(Aδ(τ,ω),A0(τ,ω))=supx∈Aδ(τ,ω)infy∈A0(τ,ω)‖x−y‖E.
Proof. Let δn→0 and ˉφδn,τ→ˉφτ in E. Then by Lemma 4.1, we find that for all τ∈R, t≥0 and ω∈Ω,
Φδn(t,τ,ω,ˉφδn,τ)→Φ0(t,τ,ω,ˉφτ)inE. | (4.11) |
By (3.16)–(3.17) we have, for all τ∈R and ω∈Ω,
limδ→0‖Kδ(τ,ω)‖2E≤R0(τ,ω). | (4.12) |
Then by (4.11), (4.12) and Lemma 3.7, (4.10) follows from Theorem 3.1 in [24] immediately.
In this paper we use similar idea in [30] but apply to second order non-autonomous stochastic lattice dynamical systems with additive noise. we establish the convergence of solutions of Wong-zakai approximations and the upper semicontinuity of random attractors of the approximate random system as the step-length of the Wiener shift approaches zero. In addition, as to the second order non-autonomous stochastic lattice dynamical systems with multiplicative noise, we can use the similar method in [29] to get the corresponding results.
The authors would like to thank anonymous referees and editors for their valuable comments and constructive suggestions.
The authors declare no conflict of interest.
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