We consider the dynamic behavior of solutions for a nonclassical diffusion equation with memory
ut−ε(t)△ut−△u−∫∞0κ(s)△u(t−s)ds+f(u)=g(x)
on time-dependent space for which the norm of the space depends on the time t explicitly, and the nonlinear term satisfies the critical growth condition. First, based on the classical Faedo-Galerkin method, we obtain the well-posedness of the solution for the equation. Then, by using the contractive function method and establishing some delicate estimates along the trajectory of the solutions on the time-dependent space, we prove the existence of the time-dependent global attractor for the problem. Due to very general assumptions on memory kernel κ and the effect of time-dependent coefficient ε(t), our result will include and generalize the existing results of such equations with constant coefficients. It is worth noting that the nonlinear term cannot be treated by the common decomposition techniques, and this paper overcomes the difficulty by dealing with it as a whole.
Citation: Jing Wang, Qiaozhen Ma, Wenxue Zhou. Attractor of the nonclassical diffusion equation with memory on time- dependent space[J]. AIMS Mathematics, 2023, 8(6): 14820-14841. doi: 10.3934/math.2023757
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We consider the dynamic behavior of solutions for a nonclassical diffusion equation with memory
ut−ε(t)△ut−△u−∫∞0κ(s)△u(t−s)ds+f(u)=g(x)
on time-dependent space for which the norm of the space depends on the time t explicitly, and the nonlinear term satisfies the critical growth condition. First, based on the classical Faedo-Galerkin method, we obtain the well-posedness of the solution for the equation. Then, by using the contractive function method and establishing some delicate estimates along the trajectory of the solutions on the time-dependent space, we prove the existence of the time-dependent global attractor for the problem. Due to very general assumptions on memory kernel κ and the effect of time-dependent coefficient ε(t), our result will include and generalize the existing results of such equations with constant coefficients. It is worth noting that the nonlinear term cannot be treated by the common decomposition techniques, and this paper overcomes the difficulty by dealing with it as a whole.
In this paper, we are concerned with the following nonclassical diffusion equation with memory:
{ut−ε(t)△ut−△u−∫∞0κ(s)△u(t−s)ds+f(u)=g(x),x∈Ω,t≥τ,u|∂Ω=0,t≥τ,u(x,t)=uτ(x),x∈Ω,t≤τ,τ∈R, | (1.1) |
on time-dependent space, where Ω⊂RN(N≥3) is a bounded smooth domain. The model (1.1) describes diffusion in solids of substances which behave as viscous fluid, where unknown function u=u(x,t):Ω×[τ,∞)→R represents the density of the fluid, τ∈R is an initial time, uτ(x,r):Ω×(−∞,τ] is the initial value function that characterizes the past time, g=g(⋅)∈H−1(Ω) represents a forcing term, κ is a nonnegative non-increasing function describing memory effects in the material, the term −△u concerns linear diffusion processes, the term −ε(t)△ut is used to model the effect of viscosity of the diffusing substance, and ε(t) can be interpreted as the coefficient of viscosity. In addition, ε(t)∈C1(R) is a decreasing bounded function satisfying
limt→+∞ε(t)=0, | (1.2) |
and there is a constant L>0 such that
supt∈R(|ε(t)|+|ε′(t)|)≤L. | (1.3) |
The nonlinear function f∈C1(R) with f(0)=0 satisfies the critical growth condition
|f′(s)|≤C(1+|s|4N−2),∀s∈R,N≥3, | (1.4) |
and the dissipation condition
lim inf|s|→∞f(s)s>−λ1,∀s∈R, | (1.5) |
where C is a positive constant and λ1 is the first eigenvalue of −△ with Dirichlet boundary condition in H10(Ω).
The memory kernel κ is a nonnegative summable function satisfying ∫∞0κ(s)ds=1 and having the following form:
κ(s)=∫∞sμ(r)dr, | (1.6) |
where μ∈L1(R+) is a decreasing piecewise absolutely continuous function and is allowed to have infinitely many discontinuity points. We assume
κ(s)≤Θμ(s),∀s∈R+,Θ>0. | (1.7) |
From [19], the above inequality (1.7) is equivalent to the following:
μ(r+s)≤Me−δrμ(s), | (1.8) |
where M≥1, δ>0 and r≥0 are constants. As is well known, the nonclassical diffusion equation is an important mathematical model used to describe several physical phenomena, such as heat conduction, solid mechanics and non-Newtonian flows. In 1980, Aifantis [1] came up with a quite general approach for establishing such partial differential equation models describing different physical phenomena related to diffusion in solids. Among them, the author proposed a pseudo-parabolic equation
ϱt=D∇2ϱ+ˉD∇2ϱt |
under a phenomenon, which is called the nonclassical diffusion equation, where ˉD is a non-negative function. However, the research was focused on the nonclassical diffusion equation with constant coefficient early on. In 1990, the diffusion equation with memory was proposed by Jäckle ([21]) in the study of heat conduction and relaxation of high viscosity liquids. The convolution term represents the influence of past history on its future evolution and describes more accurately the diffusive process in certain materials, such as high viscosity liquids at low temperatures and polymers. Hence, it is necessary and scientifically significant to study the nonclassical diffusion equation with the time-dependent coefficient (i.e., variable coefficient) and memory.
Provided that the function ε(t) is a positive constant in Eq (1.1), the long-time behavior of solutions for this kind of problem has been widely studied, and a lot of excellent results have been obtained when κ(s)=0 or κ(s)≠0. Meanwhile, the research of the nonclassical diffusion equation with memory (i.e., κ(s)≠0) is relatively less. In 2009, Wang and Zhong [41] first considered the nonclassical diffusion equation with memory and applied the condition of memory kernel
μ′(s)+δμ(s)≤0,δ,s≥0,μ∈C1(R+)∩L1(R+), | (1.9) |
introduced in [17]. They obtained the existence and regularity of a uniform attractor for the problem (1.1) with critical growth restriction in H10(Ω)×L2μ(R+,H10(Ω))(Ω∈RN,N≥3). Since then, the condition (1.9) has been used for this type of problem with memory; see [2,3,4,6,7,8,9,10,40,42,45]. Particularly, Wang et al. [42] proved the existence of a global attractor for the problem (1.1) with critical growth restriction in H10(Ω)×L2μ(R+,H10(Ω))(Ω∈RN,N≥3). In [3,6], the authors obtained the existences of the uniform attractor and pullback attractors, respectively. The global attractors were obtained for nonclassical diffusion equations with memory and singularly oscillating external forces in [4,7]. In [8], Conti et al. proved the existence of the global attractor for the problem (1.1) lacking instantaneous damping. In 2014, Conti et al. [9] applied the memory kernel condition (1.7) rather than (1.9) and proved the existence of a global attractor for the problem (1.1) in H10(Ω)×L2μ(R+,H10(Ω))(Ω∈R3). In 2016, the authors of [10] also got the existence of the exponential attractor for the above problem. Later, the authors of [2] obtained the existence of the global attractor for the nonclassical diffusion equations with memory and a new class of nonlinearity. It is worth noting that (1.9) is weaker than (1.8), which shows (1.7) is more general in [2,9,10]. It is also obvious that (1.8) with M=1 boils down to (1.9). On the contrary, when (1.8) holds for some M>1, then it is far more general that (1.9). In fact, any compactly supported decreasing function μ satisfies the condition (1.8) for some M>1, but it does not satisfy (1.9).
When ε(t) is a decreasing function and satisfies (1.2)–(1.3), the equation
ut−ε(t)△ut−△u+λu+f(u)=g(x) | (1.10) |
has been investigated by some authors. The characteristic of this kind of problem is that the phase space depends on time, that is, its norm depends on time t explicitly. So, the problem (1.10) is still non-autonomous although the forcing term g is independent of t. If not, the time-dependent coefficient leads to the loss of the dissipation of the natural energy as t→±∞, which affects the existence of an absorbing set in the general sense. Thereby, in order to overcome the above difficulty, the relevant definitions and theories of the time-dependent global attractor first were introduced in [35]. Then, Conti et al. [11] improved the work in [35] and proved the existence of the time-dependent global attractor for a wave equation. Moreover, Meng et al. [27,28] obtained a new method (i.e., contractive function method) to verify compactness and also got a necessary and sufficient condition for the existence of time-dependent attractors. In recent years, some researchers have extensively studied this type of problem with time-dependent coefficient. In [12,29,36], the authors obtained the corresponding results for wave equations. Liu and Ma [23,24] proved the corresponding results of the time-dependent global attractor for plate equations on the bounded and unbounded domain, respectively. In [30,39], the authors investigated the beam equation and a nonlinear viscoelastic equation with time-dependent memory kernel on time-dependent spaces, respectively. In addition, it should be mentioned that there is a class of studies on variable coefficient equations devoted to studying different solutions or dynamical behavior using numerical simulation methods, which makes the problem more intuitive by graphs. For example, the authors [22,26,32,33] considered, respectively, multiple soliton and M-lump solutions of the variable coefficients Kadomtsev-Petviashvili equation, nonlinear dynamics of different non-autonomous wave structures solutions for a 3D variable-coefficient generalized shallow water wave equation, the stability of the corresponding dynamical system and invariant solutions for a (2+1)-dimensional Kadomtsev-Petviashvili equation with competing dispersion effect and novel multiple soliton solutions of (3+1)-dimensional generalized variable-coefficient B-type Kadomtsev-Petviashvili (VC B-type KP) equation. In [5,18], the authors studied the soliton solutions of the nonlinear diffusive predator-prey system and the diffusion-reaction equations and the Tikhonov regularization method and the inverse source problem for time fractional heat equation, respectively. In [34], various solitons and solutions of the fractional fifth-order Korteweg-de Vries equations were realized. These articles mentioned above, further explained the solutions and dynamics of systems by depicting graphs. In short, it can be seen that the study of variable coefficient partial differential equations has attracted much attention in various fields.
Compared to the studies of the aforementioned other equations, the relevant results of the nonclassical diffusion equations on time-dependent spaces are not abundant. When the forcing term g∈L2(Ω)(Ω⊂R3) and the nonlinearity f(u) satisfies |f′(u)|≤C(1+|u|), Ding and Liu [16] recognized the existence of a time-dependent global attractor for (1.10) by using the decomposition technique. Using the same method, Ma et al. [31] proved the existence, regularity and asymptotic structure of the time-dependent global attractor for (1.10), when the forcing term g∈H−1(Ω) (Ω⊂RN,N≥3) and the nonlinear term f(u) satisfies the critical exponential growth condition. By applying the contractive function method, the authors of [43,46] recognized almost simultaneously the existence of the time-dependent global attractor for (1.10), when the nonlinearity f(u) satisfies a polynomial growth condition of arbitrary order. However, the authors also proved the regularity and asymptotic structure of the time-dependent global attractor in [43]. In [37], Qin and Yang proved the existence and regularity of time-dependent pullback attractors for non-autonomous nonclassical diffusion equations with nonlocal diffusion when the nonlinear term satisfies critical exponential growth and the external force term g∈L2loc(R,H−1(Ω)). Recently, Xie et al. [44] recognized the existence and regularity of time-dependent pullback global attractors for the problem (1.10) with memory and lacking instantaneous damping by using the contractive process and new analytical technique, when the nonlinearity satisfies a polynomial growth condition of arbitrary order and the external force term g∈L2(Ω).
However, we find that the studies of nonclassical diffusion equations with memory are extremely rare on time-dependent spaces. In this paper, based on the idea of [10,27,31] and using the theory framework provided in [27], we discuss the existence of the time-dependent global attractor in Ut for problem (1.1). When the nonlinearity and the external force term satisfy the same condition, our result will generalize the result obtained in [42] because of the generality of memory kernel condition and the time-dependent nature of ε.
In order to obtain the corresponding results for problem (1.1), we need to overcome two difficulties. On the one hand, the weaker memory kernel condition (1.7) makes the energy functional obtained unavailable. On the other hand, due to the influence of the nonlinear term with critical growth condition and the term −ε(t)△ut, it is not easy to get relative compactness of the solution in L2([τ,T],H10(Ω)) when we prove the asymptotic compactness by the contractive function method. To deal with above problems, we construct a new energy functional by introducing a new function related to the memory kernel, and obtain the existence of the time-dependent absorbing set. Then, when applying the contractive function method, we treat the nonlinear term as a whole, which will yield the asymptotic compactness for the corresponding process of the problem (1.1).
The paper is organized as follows. In Section 2, we introduce notations of function spaces involved, some abstract results for the time-dependent global attractor and important lemmas. In Section 3, we will prove the well-posedness of the solution. Based on the existence of the solution, we obtain the process generated by the weak solution. In Section 4, we investigate the existence of the time-dependent global attractor. In Section 5, conclusions and discussion are given.
As in [17], we introduce a new variable which shows the past history of Eq (1.1), that is,
ηt(x,s)=η(x,t,s)=∫s0u(x,t−r)dr,s≥0, | (2.1) |
and
ηtt(x,s)=u(x,t)−ηts(x,s),s≥0, | (2.2) |
where ηt=∂∂tη,ηs=∂∂sη.
Therefore, according to (1.6), (2.1) and (2.2), the problem (1.1) can be transformed into the following system:
{ut−ε(t)△ut−△u−∫∞0μ(s)△ηt(s)ds+f(u)=g,ηtt=−ηts+u, | (2.3) |
with the corresponding initial conditions
{u(x,t)=0,x∈∂Ω,t≥τ,ηt(x,s)=0,(x,s)∈∂Ω×R+,t≥τ,u(x,τ)=uτ(x),x∈Ω,τ∈R,η(x,τ,s)=ητ(x,s)=∫s0uτ(x,τ−r)dr,(x,s)∈Ω×R+,τ∈R. | (2.4) |
First, we give some spaces and the corresponding norms used in the paper. Usually, let ‖⋅‖Lp(Ω) be the norm of Lp(Ω)(p≥1). In particularly, let ⟨⋅,⋅⟩ and ‖⋅‖ be the scalar product and norm of H=L2(Ω), respectively. The Laplacian A=−△ with Dirichlet boundary conditions is a positive operator on H with domain H2(Ω)∩H10(Ω). Then, we consider the family of Hilbert spaces Hs=D(As/2), ∀s∈R, with the standard inner products and norms, respectively,
⟨⋅,⋅⟩s=⟨⋅,⋅⟩D(As/2)=⟨As/2⋅,As/2⋅⟩,‖⋅‖s=‖As/2⋅‖. |
Especially, H−1=H−1(Ω), H0=H,H1=H10(Ω),H2=H2(Ω)∩H10(Ω).
Therefore, we define the space Ht with the time-dependent norm
‖u‖2Ht=‖u‖2+ε(t)‖u‖21. |
According to the definition of memory kernel, we introduce the Hilbert (history) space
M1=L2μ(R+;H1)={ηt:R+→H1:∫∞0μ(s)‖ηt(s)‖1ds<∞} |
with the corresponding inner product and norm
⟨ηt,ξt⟩μ,1=⟨ηt,ξt⟩M1=∫∞0μ(s)⟨ηt(s),ξt(s)⟩1ds, |
‖ηt‖2μ,1=‖ηt‖2M1=∫∞0μ(s)‖ηt(s)‖21ds. |
Now, combining the above spaces, we have the time-dependent space
Ut=Ht×M1 |
endowed with the norm
‖z‖2Ut=‖u‖2+ε(t)‖u‖21+‖ηt‖2μ,1. |
Note that the dual space of X is denoted as X∗. As a convenience, we choose C as a positive constant depending on the subscript that may be different from line to line or in the same line throughout the paper.
Second, we recall some notations, some abstract results and standard conclusions in order to obtain compactness; see [11,13,20,25,27]. For every t∈R, let Xt be a family of normed spaces, and we introduce the R−ball of Xt:
BXt(R)={u∈Xt:‖u‖2Xt≤R}. |
Definition 2.1. [11] Let {Xt}t∈R be a family of normed spaces. A process is a two-parameter family of mappings {U(t,τ):Xτ→Xt,t≥τ∈R} with properties
(i) U(τ,τ)=Id is the identity on Xτ,τ∈R;
(ii) U(t,s)U(s,τ)=U(t,τ),∀t≥s≥τ.
Definition 2.2. [11] A family D={Dt}t∈R of bounded sets Dt⊂Xt is called uniformly bounded if there exists a constant R>0 such that Dt⊂BXt(R),∀t∈R.
Definition 2.3. [11] A time-dependent absorbing set for the process {U(t,τ)}t≥τ is a uniformly bounded family B={Bt}t∈R with the following property: For every R>0 there exists a t0 such that
U(t,τ)BXτ(R)⊂Bt,forallτ≤t−t0. |
Definition 2.4. [11] The time-dependent global attractor for {U(t,τ)}t≥τ is the smallest family A={At}t∈R such that
(i) each At is compact in Xt;
(ii) A is pullback attracting, i.e., it is uniformly bounded, and the limit
limτ→−∞distXt(U(t,τ)Dτ,At)=0 |
holds for every uniformly bounded family D={Dt}t∈R and every fixed t∈R.
Definition 2.5. [11] We say A={At}t∈R is invariant if
U(t,τ)Aτ=At,∀t≥τ. |
Definition 2.6. [27] We say that a process {U(t,τ)}t≥τ in a family of normed spaces {Xt}t∈R is pullback asymptotically compact if and only if for any fixed t∈R, bounded sequence {xn}∞n=1⊂Xτn and any {τn}∞n=1⊂R−t with τn→−∞ as n→∞, the sequence {U(t,τn)xn}∞n=1 has a convergent subsequence, where R−t={τ:τ∈R,τ≤t}.
Definition 2.7. [27] Let {Xt}t∈R be a family of Banach spaces and C={Ct}t∈R be a family of uniformly bounded subsets of {Xt}t∈R. We call a function ψtτ(⋅,⋅) defined on Xt×Xt a contractive function on Cτ×Cτ if for any fixed t∈R and any sequence {xn}∞n=1⊂Cτ, there is a subsequence {xnk}∞n=1⊂{xn}∞n=1 such that
limk→∞liml→∞ψtτ(xnk,xnl)=0. |
Theorem 2.8. [27] Let {U(t,τ)}t≥τ be a process {Xt}t∈R and have a pullback absorbing family B={Bt}t∈R. Moreover, assume that for any ϵ>0 there exist T(ϵ)≤t,ψtT∈C(BT) such that
‖U(t,T)x−U(t,T)y‖Xt≤ϵ+ψtT(x,y),∀x,y∈BT, |
for any fixed t∈R. Then, {U(t,τ)}t≥τ is pullback asymptotically compact.
Theorem 2.9. [27] Let {U(t,τ)}t≥τ be a process in a family of Banach spaces {Xt}t∈R. Then, U(⋅,⋅) has a time-dependent global attractor A={At}t∈R satisfying At=⋂s≤t¯⋃τ≤sU(t,τ)Bτ if and only if
(i) {U(t,τ)}t≥τ has a pullback absorbing family B={Bt}t∈R;
(ii) {U(t,τ)}t≥τ is pullback asymptotically compact.
Lemma 2.10. [13,20] Assume that the memory function κ satisfies (1.6)–(1.8), and then for any T>τ, ηt∈C([τ,T],L2μ(R+;H1)) such that
−⟨ηts,ηt⟩μ,1=−12∫∞0μ(s)dds‖∇ηt(s)‖2ds=[−12μ(s)‖∇ηt(s)‖2]∞0+12∫∞0μ′(s)‖∇ηt(s)‖2ds≤0. | (2.5) |
Lemma 2.11. [25] (Aubin-Lions Lemma) Assume that X,B and Y are three Banach spaces with X↪↪B and B↪Y. Let fn be bounded in Lp([0,T],B)(1≤p<∞). Suppose fn satisfies
(i) fn is bounded in Lp([0,T],X);
(ii) ∂fn∂t is bounded in Lp([0,T],Y).
Then, fn is relatively compact in Lp([0,T],B).
Next, we give the definition of a weak solution and prove well-posedness of the weak solution for the problem (2.3)–(2.4) by using the Faedo-Galerkin method from [14,38].
Definition 3.1. The function z=(u,ηt)=(u(x,t),ηt(x,s)) defined in Ω×[τ,T] is said to be a weak solution for the problem (2.3)–(2.4) with the initial data zτ∈BUτ(R0)⊂Uτ, −∞<τ<T<+∞ if z satisfies
(i) z∈C([τ,T],Ut),(x,t)∈Ω×[τ,T];
(ii) for any θ=(v,ξt)∈Ht×L2μ(R+;H1), the equality
⟨ut,v⟩+ε(t)⟨∇ut,∇v⟩+⟨∇u,∇v⟩+⟨ηt,v⟩μ,1+⟨f(u),v⟩=⟨g,v⟩ |
and
⟨ηtt,ξt⟩μ,1=−⟨ηts,ξt⟩μ,1+⟨u,ξt⟩μ,1 |
hold for a.e.[τ,T].
Theorem 3.2. Assume that (1.2)–(1.8) hold and g∈H−1(Ω), and then for any initial data zτ=(uτ,ητ)∈BUτ(R0)⊂Uτ and any τ∈R, there exists a unique solution z for the problem (2.3)–(2.4) such that z=(u,ηt)∈C([τ,T],Ut) for any fixed T>τ. Furthermore, the solution depends on the initial data continuously in Ut.
Proof. Assume that ωk is the eigenfunction of A=−△ with Dirichlet boundary value in H1, and then {ωk}∞k=1 is a standard orthogonal basis of H and is also an orthogonal basis in H1. The corresponding eigenvalues are denoted by 0<λ1≤λ2≤⋯≤λj≤⋯,λj→∞ with Aωk=λkωk,∀k∈N. Our proof will be finished through the following four steps.
⋆⋆ Faedo-Galerkin scheme.
Given an integer m, we denote by Pm the projection on the subspace span{ω1,⋯,ωm} in H10(Ω) and Qm the projection on the subspace span{e1,⋯,em}⊂L2μ(R+,H1) in L2μ(R+,H1). For every fixed m, we look for function um(t)=Pmu=Σmk=1akm(t)ωk and ηt,m(s)=Qmηt=Σmk=1bkm(t)ek(s) satisfying the following system:
{⟨umt,ωk⟩+⟨ε(t)Aumt,ωk⟩+⟨Aum,ωk⟩+⟨ηt,m,ωk⟩μ,1=⟨g,ωk⟩−⟨f(um),ωk⟩,[3pt]⟨ηt,mt,ek⟩μ,1=−⟨ηt,ms,ek⟩μ,1+⟨um,ek⟩μ,1,[3pt]zmτ=⟨Pmuτ,Qmητ⟩. | (3.1) |
Applying the divergence theorem to the term ⟨∫∞0Δηt,mds,ωk⟩, we obtain a system of ordinary differential equations in the variables akm(t) and bkm(t) of the form
{ddtajm+λjε(t)ddtajm+λjajm+Σmk=1bkm⟨ek,ωj⟩μ,1=⟨g,ωj⟩−⟨f(um),ωj⟩,[3pt]ddtbjm=Σmk=1akm⟨ωk,ej⟩μ,1−Σmk=1bkm⟨e′k,ej⟩μ,1, |
with initial conditions
ajm(τ)=⟨uτ,ωj⟩,bjm(τ)=⟨ητ,ej⟩μ,1,j,k=0,2⋯,m, |
which satisfy
Σmk=1akm(τ)ωj→uτ in Ht, |
Σmk=1bkm(τ)ej→ητ in M1. |
Thereby, there exists a continuous solution of the problem (2.3)–(2.4) on an interval [τ,T] by the standard existence theory for ordinary differential equations. Then, we will prove the convergence of zm(t)=(um,ηt,m).
⋆⋆ Energy estimates.
Multiplying the first and the second equation of (3.1) by akm and bkm respectively and summing from 1 to m about k, we have
ddt(‖um‖2+ε(t)‖um‖21+‖ηt,m‖2μ,1)+(2−ε′(t))‖um‖21=−2⟨ηt,m,ηt,ms⟩μ,1−2⟨f(um),um⟩+2⟨g,um⟩. | (3.2) |
It follows from (1.5) and Poincaré's inequality that there exist c>0 and ν>0 such that
⟨f(um),um⟩≥−12(1−ν)‖um‖21−c. | (3.3) |
According to (2.5), Hölder's inequality and Young's inequality, we have
⟨g,um⟩≤1ν‖g‖2−1+ν4‖um‖21, | (3.4) |
⟨ηt,m,ηt,ms⟩μ,1≥0. | (3.5) |
By (3.2)–(3.5) and the decreasing property of ε(t), we get
ddt(‖um‖2+ε(t)‖um‖21+‖ηt,m‖2μ,1)+ν2‖um‖21≤2ν‖g‖2−1+2c. | (3.6) |
Integrating from τ to t at the sides of (3.6), we obtain
‖zm‖2Ut+ν2∫tτ‖um(s)‖21ds≤R, | (3.7) |
where
R=‖zmτ‖2Uτ+(t−τ)(2ν‖g‖2−1+2c). |
Therefore, we deduce from (3.7) that for any fixed T>t,
{um}∞mis bounded inL∞([τ,T],Ht))∩L2([τ,T],H1), | (3.8) |
{ηt,m}∞mis bounded inL∞([τ,T],L2μ(R+,H1)). | (3.9) |
Combining (1.2), (3.7) and the embedding inequality (c1 is embedding constant), we arrive at
∫Tτ∫Ω|f(um)|2NN+2dxdt≤CN,c1∫Tτ‖um(s)‖2NN−21ds+CN,(T−τ),|Ω|≤CN,c1,ε(T),NRNN−2(T−τ)+CN,(T−τ),|Ω|. | (3.10) |
So, we infer from (3.10) that
{f(um)}∞m=1is bounded inL2NN+2([τ,T],L2NN+2(Ω)). | (3.11) |
Next, we verify the uniform estimate for umt. Multiplying the first equation of (3.1) by ∂takm and summing from 1 to m yields
ddtE(t)=−⟨ηt,m,umt⟩μ,1−‖umt‖2−ε(t)‖umt‖21, | (3.12) |
where
E(t)=12‖um‖21+⟨F(um),1⟩−⟨g,um⟩. |
Applying (1.4), (1.5) and embedding inequality, we have
⟨F(um),1⟩≥−12(1−ν)‖um‖21−c, | (3.13) |
|⟨F(um),1⟩|≤C(‖um‖2+‖um‖2NN−2L2NN−2(Ω))≤C(‖um‖2+c1‖um‖2NN−21). | (3.14) |
Thereby, due to (3.4), (3.13) and (3.14),
E(t)≥ν4‖um‖21−1ν‖g‖2−1−c, | (3.15) |
E(t)≤(12+ν4)‖um‖21+C‖um‖2+Cc1‖um‖2NN−21+1ν‖g‖2−1. | (3.16) |
Moreover,
|⟨ηt,m,umt⟩μ,1|≤κ(0)2ε(t)‖ηt,m‖2μ,1+ε(t)2‖umt‖21. | (3.17) |
Hence, it follows from (3.7), (3.12) and (3.17) that
ddtE(t)+12‖umt‖2+ε(t)2‖umt‖21≤Rκ(0)2ε(t)≤Rκ(0)2ε(T) | (3.18) |
for t∈[τ,T]. Integrating from s to t at the sides of (3.18) and combining with (3.7) yield
E(t)+12∫ts(‖umt(r)‖2+ε(r)‖umt(r)‖21)dr≤E(s)+Rκ(0)2ε(T)(t−s)≤2+ν4ε(T)R+CR+Cc1ε(T)NN−2RNN−2+1ν‖g‖2−1+Rκ(0)2ε(T)(t−s) | (3.19) |
for any s∈(τ,T]. By (3.15) and (3.19), we arrive at
ν4‖um‖21+12∫Tτ(‖umt(r)‖2+ε(r)‖umt(r)‖21)dr≤ρ1 | (3.20) |
for fixed T>t and s→τ, where
ρ1=(2+ν4ε(T)+C)R+Cc1ε(T)NN−2RNN−2++Rκ(0)2ε(T)(T−τ)+2ν‖g‖2−1+c. |
So, (3.20) implies
{umt}∞m=1is bounded inL2([τ,T],Ht). | (3.21) |
⋆⋆ Existence of solutions.
Step 1. Combining (3.8), (3.9) and (3.21), we find that there are u∈L∞([τ,T],Ht)∩L2([τ,T],H1), ηt∈L∞([τ,T],L2μ(R+,H1)), χ∈L2NN+2([τ,T],L2NN+2(Ω)), ut∈L2([τ,T],Ht) and a subsequence of {um}∞m=1 (still denoted as {um}∞m=1) such that
um→uweak-star in L∞([τ,T],Ht)), | (3.22) |
um→uweakly in L2([τ,T],H1), | (3.23) |
ηt,m→ηtweakly in L∞([τ,T],M1), | (3.24) |
f(um)→χweakly in L2NN+2([τ,T],L2NN+2(Ω)), | (3.25) |
umt→utweakly in L2([τ,T],Ht). | (3.26) |
Applying (3.7), (3.20) and Lemma 2.11, we can know that there exists a subsequence of {um}∞m=1 (still denoted as {um}∞m=1) such that
um→uin L2([τ,T],L2(Ω)), |
which shows
um→u,a.e. in Ω×[τ,T]. | (3.27) |
From (3.27) and the continuity of f, we get
f(um)→f(u),a.e. in Ω×[τ,T], |
which combines with Lebesgue term by term integral theorem and the uniqueness of the limit, and we get χ=f(u).
Next, we have
umt−unt−ε(t)△(umt−unt)−△(um−un)=∫∞0μ(s)△(ηt,m(s)−ηt,n(s))ds−(f(um)−f(un)). | (3.28) |
Multiplying Eq (3.28) by um−un and integrating on Ω, we get
ddt(‖um−un‖2+ε(t)‖um−un‖21+‖ηt,m−ηt,n‖2μ,1)+(2−ε′(t))‖um−un‖21=−2⟨ηt,m−ηt,n,ηt,ms−ηt,ns⟩−⟨f(um)−f(un),2(um−un)⟩. | (3.29) |
It follows from (1.4), (2.5), (3.20) and the monotonicity of ε(t) that
ddt(‖um−un‖2+ε(t)‖um−un‖21+‖ηt,m−ηt,n‖2μ,1)≤C(1+‖um‖4N−21+‖un‖4N−21)‖um−un‖21≤Cρ1,ν,NL+1ε(T)(‖um−un‖2+ε(t)‖um−un‖21+‖ηt,m−ηt,n‖2μ,1) | (3.30) |
for any t∈[τ,T]. Using Gronwall's lemma yields
‖zm−zn‖2Ut≤eCρ1,ν,N,ε(T),L(t−τ)‖zm(τ)−zn(τ)‖2Uτ, | (3.31) |
which implies
{zm}∞m=1is a Cauchy sequence inC([τ,T],Ut). |
From the uniqueness of the limit, we know that
zm→zuniformly inC([τ,T],Ut),for allT>τ. |
Therefore, we have the following conclusion:
z∈C([τ,T],Ut). | (3.32) |
Thus, when m→∞, zm(τ)→zτ in Ut.
Step 2. Choose a test function θ(t)=(v,ξt)=(ΣˉNk=1akm(t)ωk,ΣˉNk=1bkm(t)ek)∈C([τ,T],Ut) for fixed ˉN. For m≥ˉN, multiplying the first and the second equation of (3.1) by akm and bkm respectively, summing from 1 to ˉN and integrating from τ to T, we get
{∫Tτ[⟨umt,v⟩+ε(t)⟨∇umt,∇v⟩]dt+∫Tτ⟨∇um,∇v⟩dt+∫Tτ⟨ηt,m,v⟩μ,1dt+∫Tτ⟨f(um),v⟩dt=∫Tτ⟨g,v⟩dt,∫Tτ⟨ηt,mt,ξt⟩μ,1dt=−∫Tτ⟨ηt,ms,ξt⟩μ,1dt+∫Tτ⟨um,ξt⟩μ,1dt. | (3.33) |
By (3.22)–(3.26), (3.33), we have
{∫Tτ[⟨ut,v⟩+ε(t)⟨∇ut,∇v⟩]dt+∫Tτ⟨∇u,∇v⟩dt+∫Tτ⟨ηt,v⟩μ,1dt+∫Tτ⟨f(u),v⟩dt=∫Tτ⟨g,v⟩dt,∫Tτ⟨ηtt,ξt⟩μ,1dt=−∫Tτ⟨ηts,ξt⟩μ,1dt+∫Tτ⟨u,ξt⟩μ,1dt. | (3.34) |
Owing to the arbitrariness of T, for a.e. [τ,T],
⟨ut,v⟩+ε(t)⟨∇ut,∇v⟩+⟨∇u,∇v⟩+⟨ηt,v⟩μ,1+⟨f(u),v⟩=⟨g,v⟩, | (3.35) |
⟨ηtt,ξt⟩μ,1=−⟨ηts,ξt⟩μ,1+⟨u,ξt⟩μ,1. | (3.36) |
Step 3. We now verify z(τ)=zτ. Indeed, it is obvious that z(τ) is meaningful according to z∈C([τ,T],Ut). Choose the function θ(t)∈C1([τ,T],Ut) with (v(T),ξT)=(0,0), and then
{−∫Tτ[⟨u,vt⟩+ε(t)⟨∇ut,∇v⟩]dt+∫Tτ⟨∇u,∇v⟩dt+∫Tτ⟨ηt,v⟩μ,1dt+∫Tτ⟨f(u),v⟩dt=∫Tτ⟨g,v⟩dt+⟨u(τ),v(τ)⟩,−∫Tτ⟨ηt,ξtt⟩μ,1dt=−∫Tτ⟨ηts,ξt⟩μ,1dt+∫Tτ⟨u,ξt⟩μ,1dt+⟨η(τ),ξ(τ)⟩μ,1, | (3.37) |
by (3.34). From (3.33), we can have
{−∫Tτ[⟨um,vt⟩−ε(t)⟨∇umt,∇v⟩]dt+∫Tτ⟨∇um,∇v⟩dt+∫Tτ⟨ηt,m,v⟩μ,1dt+∫Tτ⟨f(um),v⟩dt=∫Tτ⟨g,v⟩dt+⟨ητ,ξ(τ)⟩,−∫Tτ⟨ηt,m,ξtt⟩μ,1dt=−∫Tτ⟨ηt,ms,ξt⟩μ,1dt+∫Tτ⟨um,ξt⟩μ,1dt+⟨ηm(τ),ξ(τ)⟩. | (3.38) |
Because of zm(τ)→zτ(m→∞), it follows from (3.38) that
{−∫Tτ[⟨u,vt⟩+ε(t)⟨∇ut,∇v⟩]dt+∫Tτ⟨∇u,∇v⟩dt+∫Tτ⟨ηt,v⟩μ,1dt+∫Tτ⟨f(u),v⟩dt=∫Tτ⟨g,v⟩dt+⟨uτ,v(τ)⟩,−∫Tτ⟨ηt,ξtt⟩μ,1dt=−∫Tτ⟨ηts,ξt⟩μ,1dt+∫Tτ⟨u,ξt⟩μ,1dt+⟨ητ,ξτ⟩μ,1. | (3.39) |
Combining (3.37), (3.39) and the arbitrariness of θ(τ)=(v(τ),ξ(τ)) yields
z(τ)=zτ. | (3.40) |
Hence, the existence of the weak solution is obtained by (3.32), (3.35), (3.36) and (3.40).
⋆⋆ Uniqueness and continuity of solutions.
Assume that zi=(ui,ηt,i)(i=1,2) are two solutions of the problem (2.3)–(2.4) with the initial data ziτ=(uiτ,ητ,i), respectively. For convenience, define ˉu=u1−u2, ˉηt=ηt,1−ηt,2, and then ˉz(t)=z1(t)−z2(t)=(ˉu,ˉηt) satisfies the following equation:
{ˉut−ε(t)△ˉut−Δˉu−∫∞0μ(s)△ˉηt(s)ds+f(u1)−f(u2)=0,ˉηtt=−ˉηts+ˉu, |
with initial data
ˉz(x,τ)=ˉzτ=z1τ−z2τ. |
Repeating the proof of (3.31) yields
‖ˉz(t)‖2Ut≤eCρ1,ν,N,ε(T),L(t−τ)‖ˉzτ‖2Uτ. | (3.41) |
Thereby, (3.41) implies the uniqueness of the solution as well as the property of continuous dependence of the solution on initial data.
According to Theorem 3.2, we can define a continuous process {U(t,τ)}t≥τ generated by the solution of the problem (2.3)–(2.4), where the mapping
U(t,τ):Uτ→Ut,t≥τ∈R, |
and U(t,τ)zτ=z(t),zτ∈Uτ.
At first, we consider the existence of time-dependent absorbing sets for the solution process {U(t,τ)}t≥τ in Ut.
Lemma 4.1. Assume that (1.2), (1.3), (1.5)–(1.8) hold, g∈H−1(Ω), and zτ=(uτ,ητ)∈BUτ(R0)⊂Uτ, and then there exists R1>0 such that B={Bt}t∈R={BUt(R1)}t∈R is a time-dependent absorbing set in Ut for the process {U(t,τ)}t≥τ corresponding to the problem (2.3)–(2.4).
Proof. Multiplying the first equation of (2.3) by u and repeating the estimate of Theorem 3.2, we conclude that
ddtE1(t)+(1−ε′(t))‖u‖21+ν2‖u‖21≤2ν‖g‖2−1+2c, | (4.1) |
where
E1(t)=‖u‖2+ε(t)‖u‖21+‖ηt‖2μ,1. |
According to (1.2), (1.3), (4.1) and Poincaré's inequality, we get
ddtE1(t)+ε(t)L‖u‖21+ν4‖u‖21+νλ14‖u‖2≤2ν‖g‖2−1+2c. | (4.2) |
To reconstruct E1(t), we introduce a new function
Ψ1(t)=∫∞0κ(s)‖ηt(s)‖21ds | (4.3) |
by using the idea of [15]. Due to (1.7), we have
Ψ1(t)≤Θ‖ηt‖2μ,1≤ΘE1(t). | (4.4) |
In addition, taking the derivative with respect to t at the sides of (4.3) and combining (1.6) and (1.7), we find that
ddtΨ1(t)=−‖ηt‖2μ,1+2∫∞0κ(s)⟨∇ηt(s),∇u(s)⟩ds≤−12‖ηt‖2μ,1+2Θ2κ(0)‖u‖21. | (4.5) |
Therefore, for fixed ν>0, we define the function
Φ1(t)=E1(t)+ν8Θ2κ(0)Ψ1(t). | (4.6) |
Combining (4.2), (4.5) and (4.6), we have
ddtΦ1(t)+ε(t)L‖u‖21+νλ14‖u‖2+ν16Θ2κ(0)‖ηt‖2μ,1≤2ν‖g‖2−1+2c. |
Choose σ1=min{12L,νλ18L,ν32Θ2κ(0)}>0, and then
ddtΦ1(t)+2σ1E1(t)≤2ν‖g‖2−1+2c. | (4.7) |
For small enough ν, we have
E1(t)≤Φ1(t)≤2E1(t). | (4.8) |
It follows from (4.7) and (4.8) that
ddtΦ1(t)+σ1Φ1(t)≤2ν‖g‖2−1+2c. | (4.9) |
By Gronwall's lemma, we see that
Φ1(t)≤e−σ1(t−τ)Φ1(τ)+1σ1(2ν‖g‖2−1+2c). | (4.10) |
Hence, we conclude from (4.8) and (4.10) that
E1(t)≤2e−σ1(t−τ)E1(τ)+1σ1(2ν‖g‖2−1+2c), |
which shows
‖u‖2+ε(t)‖u‖21+‖ηt‖2μ,1≤R1 |
for any t≥t∗=τ+1σ1ln4E1(τ)R1, where
R1=2σ1(2ν‖g‖2−1+2c). |
So, Bt={z=(u,ηt)∈Ut:‖z(t)‖2Ut≤R1} is a time-dependent absorbing set in Ut for the solution process {U(t,τ)}t≥τ. The proof is finished.
We next verify the pullback asymptotical compactness for the process {U(t,τ)}t≥τ corresponding to the problem (2.3)–(2.4).
Theorem 4.2. Assume that (1.2), (1.3) and (1.5)–(1.8) hold, and then the process {U(t,τ)}t≥τ of the problem (2.3)–(2.4) is pullback asymptotic compact in Ut.
Proof. Assume that zn=(un,ηt,n),zm=(um,ηt,m) are two solutions of the problem (2.3)–(2.4) with initial data znτ,zmτ∈BUτ(R0), respectively. Without loss of generality, we assume τ≤T1<t for every fixed T1. As a convenience, let w(t)=un(t)−um(t), ζt=ηt,n−ηt,m, and then (w(t),ζt) satisfies the following equation:
{wt−ε(t)△wt−△w−∫∞0μ(s)△ζt(s)ds+f(un)−f(um)=0,ζtt=−ζts+w,t≥τ, | (4.11) |
with
w(x,T1)=wT1=unT1−umT1,ζT1=ηT1,n−ηT1,m. |
Multiplying the first Eq (4.11) by w and integrating in Ω, we can get
ddtE2(t)+(1−ε′(t))‖w‖21+‖w‖21≤−2⟨f(un)−f(um),w⟩, | (4.12) |
where
E2(t)=‖w‖2+ε(t)‖w‖21+‖ζt‖2μ,1. |
By (1.2), (1.3) and Poincaré's inequality, we have
ddtE2(t)+ε(t)L‖w‖21+12‖w‖21+λ12‖w‖2≤−2⟨f(un)−f(um),w⟩. | (4.13) |
Set
Ψ2(t)=∫∞0κ(s)‖ζt(s)‖21ds, |
Φ2(t)=E2(t)+ν4Θ2κ(0)Ψ2(t). |
Then, applying similar arguments as used in the proof of Theorem 4.1, we get
ddtΦ2(t)+2σ2E2(t)≤−2⟨f(un)−f(um),w⟩, | (4.14) |
E2(t)≤Φ2(t)≤2E2(t), | (4.15) |
where 0<σ2=min{12L,λ14,ν16Θ2κ(0)} and ν is small enough. Combining (4.14) and (4.15), we find that
ddtΦ2(t)+σ2Φ2(t)≤−2⟨f(un)−f(um),w⟩. | (4.16) |
Integrating from s to t at both sides of (4.16), we arrive
Φ2(t)≤Φ2(s)−2∫ts⟨f(un(r))−f(um(r)),w(r)⟩dr. | (4.17) |
At the same time, integrating from T1 to t at both sides of (4.16), we have
∫tT1Φ2(r)dr≤1σ2Φ2(T1)−2σ2∫tT1⟨f(un(r))−f(um(r)),w(r)⟩dr. | (4.18) |
Then, integrating over [T1,t] about variable s at both sides of (4.17), we obtain that
(t−T1)Φ2(t)≤∫tT1Φ2(r)dr−2∫tT1∫ts⟨f(un(r))−f(um(r)),w(r)⟩drds. | (4.19) |
Due to (4.18) and (4.19), we get
Φ2(t)≤Φ2(T1)σ2(t−T1)−2σ2(t−T1)∫tT1⟨f(un(r))−f(um(r)),w(r)⟩dr−2t−T1∫tT1∫ts⟨f(un(r))−f(um(r)),w(r)⟩drds. | (4.20) |
By (4.15) and (4.20), we can see
E2(t)≤2E2(T1)σ2(t−T1)+ψtT1(umT1,umT1), |
where
ψtT1(umT1,umT1)=−2σ2(t−T1)∫tT1⟨f(un(r))−f(um(r)),w(r)⟩dr−2t−T1∫tT1∫ts⟨f(un(r))−f(um(r)),w(r)⟩drds. |
For some fixed t, let t>T1≥τ such that, with t−T1 large enough, we conclude that 2E2(T1)σ2(t−T1)≤ϵ for any ϵ>0. Next, we will prove ψtT1∈C(BT1) for each fixed T1. Indeed, assume that zk=(uk,ηt,k) is a solution of the problem (2.3)–(2.4) with initial data zkτ∈BUτ(R0). Then, ukt∈L2([T1,t],Ht), and uk∈L2([T1,t],H10(Ω)) by using the same arguments of Theorem 3.2. Hence, it follows from Lemma 2.11 that there is a convergent subsequence of uk (denoted as uki) such that
limi→∞limj→∞∫tT1‖uki(r)−ukj(r)‖2dr=0. |
This shows
uki→ukj,a.e.(x,t)∈Ω×[T1,t]. | (4.21) |
In view of (3.25), (4.21) and the continuity of f,
f(uki)→f(ukj),a.e.(x,t)∈Ω×[T1,t]. | (4.22) |
Hence, by (4.21) and (4.22), we have
limi→∞limj→∞∫tT1⟨f(uki(r))−f(ukj(r)),(uki−ukj)⟩dr=0. | (4.23) |
For some fixed t, ∫ts⟨f(uki(r))−f(ukj(r)),(uki−ukj)⟩dr(s∈[T1,t]) is bounded. Using the Lebesgue dominated convergence theorem yields
limi→∞limj→∞∫tT1∫ts⟨f(uki(r))−f(ukj(r)),(uki−ukj)⟩drds=0. | (4.24) |
According to (4.23) and (4.24), we conclude that ψtT1∈C(BT1). Consequently,
‖U(t,T1)|unT1−U(t,T1)umT1‖≤ϵ+ψtT1(unT1,umT1). |
Thereby, it follows from Theorem 2.8 that the process {U(t,τ)}t≥τ is pullback asymptotic compact in Ut.
Theorem 4.3. The process {U(t,τ)}t≥τ generated by the problem (2.3)–(2.4) has an invariant time-dependent global attractor A={At}t∈R in Ut.
Proof. Combining Lemma 4.1 and Theorem 4.2, we get easily the existence of the invariant time-dependent global attractor A={At}t∈R for the problem (2.3)–(2.4).
This research examines the nonclassical diffusion equation with memory and time-dependent coefficient. Using the contractive function method and some delicate estimates, we obtained the existence and uniqueness of the time-dependent global attractor for the problem (1.1). It is well known that attractors, as a powerful tool for studying dynamical systems, can well characterize long-term behaviors. The time-dependent global attractor obtained can describe the asymptotic behavior of Eq (1.1). This study is helpful to more accurately observe the sensitivity of the physical model to the disturbance, the external force and the internal friction, and it provides a better theoretical basis for the study of solid mechanics, heat conduction and relaxation of high viscosity liquids and non-Newtonian fluids.
At present, the study of nonlinear development equations has become one of the important topics in the intersection of dynamic systems, differential equations and nonlinear analysis, while it is a hot topic to explore the dynamic behavior of dissipative partial differential equations with time-dependent coefficients and their related problems in the field of infinite dimensional dynamic systems in recent years. Although some theoretical results and applications on time-dependent space have been obtained, the time-dependent global attractor alone is not a good way to describe the dynamic behavior of the system. Moreover, it is expected that our results will help to describe the observability of attractors in numerical simulation more clearly. Naturally, there are two interesting problems: Can we establish the existence and stability theory of the time-dependent exponential attractor? How does one study the long-term behavior for the nonclassical diffusion equation model by combining with numerical simulation methods, because the nonclassical diffusion equation model we studied is particular and complex?
The authors would like to thank the reviewers and the editors for their valuable suggestions and comments. This work is supported by the National Natural Science Foundation of China (No. 11961059, 11961039) and the Youth Scientific Research Fund of Lanzhou Jiaotong University (No. 2021022).
The authors declare no conflict of interest.
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