In this article, we study the following system of viscoelastic equations:

u t t - Δ u + ∫ 0 t g 1 ( t - τ ) Δ u ( τ ) d τ - Δ u t = f 1 ( u , v ) , ( x , t ) ∈ Ω × ( 0 , T ) , v t t - Δ v + ∫ 0 t g 2 ( t - τ ) Δ v ( τ ) d τ - Δ v t = f 2 ( u , v ) , ( x , t ) ∈ Ω × ( 0 , T ) , u ( x , t ) = v ( x , t ) = 0 , x ∈ ∂ Ω × ( 0 , T ) , u ( x , 0 ) = u 0 ( x ) , u t ( x , 0 ) = u 1 ( x ) , x ∈ Ω , v ( x , 0 ) = v 0 ( x ) , v t ( x , 0 ) = v 1 ( x ) , x ∈ Ω ,

where Ω is a bounded domain in ℝ ⁿ with a smooth boundary ∂Ω, and g_i (·) : ℝ₊ → ℝ₊, f_i (·, ·): ℝ² → ℝ (i = 1, 2) are given functions to be specified later. Here, u and v denote the transverse displacements of waves. This problem arises in the theory of viscoelastic and describes the interaction of two scalar fields, we can refer to Cavalcanti et al. 1 , Messaoudi and Tatar 2 , Renardy et al. 3 .

To motivate this study, let us recall some results regarding single viscoelastic wave equation. Cavalcanti et al. 4 studied the following equation:

u t t - Δ u + ∫ 0 t g ( t - τ ) Δ u ( τ ) d τ + a ( x ) u t + | u | γ u = 0 , i n Ω × ( 0 , ∞ )

for a : Ω → ℝ⁺, a function, which may be null on a part of the domain Ω. Under the conditions that a(x) ≥ a ₀ > 0 on Ω₁ ⊂ Ω, with Ω₁ satisfying some geometry restrictions and

- ξ 1 g ( t ) ≤ g ′ ( t ) ≤ - ξ 2 g ( t ) , t ≥ 0 ,

the authors established an exponential rate of decay. This latter result has been improved by Cavalcanti and Oquendo 5 and Berrimi and Messaoudi 6 . In their study, Cavalcanti and Oquendo 5 considered the situation where the internal dissipation acts on a part of Ω and the viscoelastic dissipation acts on the other part. They established both exponential and polynomial decay results under the conditions on g and its derivatives up to the third order, whereas Berrimi and Messaoudi 6 allowed the internal dissipation to be nonlinear. They also showed that the dissipation induced by the integral term is strong enough to stabilize the system and established an exponential decay for the solution energy provided that g satisfies a relation of the form

g ′ ( t ) ≤ - ξ g ( t ) , t ≥ 0 .

Cavalcanti et al. 1 also studied, in a bounded domain, the following equation:

| u t | ρ u t t - Δ u - Δ u t t + ∫ 0 t g 1 ( t - τ ) Δ u ( τ ) d τ - γ Δ u t = 0 ,

ρ > 0, and proved a global existence result for γ ≥ 0 and an exponential decay for γ > 0. This result has been extended by Messaoudi and Tatar 2 7 to the situation where γ = 0 and exponential and polynomial decay results in the absence, as well as in the presence, of a source term have been established. Recently, Messaoudi 8 9 considered

u t t - Δ u + ∫ 0 t g 1 ( t - τ ) Δ u ( τ ) d τ = b | u | γ u , ( x , t ) ∈ Ω × ( 0 , ∞ ) ,

for b = 0 and b = 1 and for a wider class of relaxation functions. He established a more general decay result, for which the usual exponential and polynomial decay results are just special cases.

For the finite time blow-up of a solution, the single viscoelastic wave equation of the form

u t t - Δ u + ∫ 0 t g ( t - τ ) Δ u ( τ ) d τ + h ( u t ) = f ( u )

in Ω × (0, ∞) with initial and boundary conditions has extensively been studied. See in this regard, Kafini and Messaoudi 10 , Messaoudi 11 12 , Song and Zhong 13 , Wang 14 . For instance, Messaoudi 11 studied (1.2) for h(u_t ) = a|u_t | ^m-2 u _t and f(u) = b|u| ^p-2 u and proved a blow-up result for solutions with negative initial energy if p > m ≥ 2 and a global result for 2 ≤ p ≤ m. This result has been later improved by Messaoudi 12 to accommodate certain solutions with positive initial energy. Song and Zhong 13 considered (1.2) for h(u_t ) = -Δu_t and f(u) = |u| ^p-2 u and proved a blow-up result for solutions with positive initial energy using the ideas of the "potential well'' theory introduced by Payne and Sattinger 15 .

This study is also motivated by the research of the well-known Klein-Gordon system

u t t - Δ u + m 1 u + k 1 u v 2 = 0 , v t t - Δ v + m 2 v + k 2 u 2 v = 0 ,

which arises in the study of quantum field theory 16 . See also Medeiros and Miranda 17 , Zhang 18 for some generalizations of this system and references therein. As far as we know, the problem (1.1) with the viscoelastic effect described by the memory terms has not been well studied. Recently, Han and Wang 19 considered the following problem

u t t - Δ u + ∫ 0 t g 1 ( t - τ ) Δ u ( τ ) d τ + | u t | m - 1 u t = f 1 ( u , v ) , ( x , t ) ∈ Ω × ( 0 , T ) , v t t - Δ v + ∫ 0 t g 2 ( t - τ ) Δ v ( τ ) d τ + | v t | r - 1 v t = f 2 ( u , v ) , ( x , t ) ∈ Ω × ( 0 , T ) , u ( x , t ) = v ( x , t ) = 0 , x ∈ ∂ Ω × ( 0 , T ) , u ( x , 0 ) = u 0 ( x ) , u t ( x , 0 ) = u 1 ( x ) , x ∈ Ω , v ( x , 0 ) = v 0 ( x ) , v t ( x , 0 ) = v 1 ( x ) , x ∈ Ω ,

where Ω is a bounded domain with smooth boundary ∂Ω in ℝ ⁿ , n = 1, 2, 3. Under suitable assumptions on the functions g_i , f_i (i = 1, 2), the initial data and the parameters in the equations, they established several results concerning local existence, global existence, uniqueness, and finite time blow-up (the initial energy E(0) < 0) property. This latter blow-up result has been improved by Messaoudi and Said-Houari 20 , to certain solutions with positive initial energy. Liu 21 studied the following system

| u t | ρ u t t - Δ u - γ 1 Δ u t t + ∫ 0 t g 1 ( t - τ ) Δ u ( τ ) d τ + f ( u , v ) = 0 , ( x , t ) ∈ Ω × ( 0 , T ) , | v t | ρ v t t - Δ v - γ 2 Δ v t t + ∫ 0 t g 2 ( t - τ ) Δ v ( τ ) d τ + k ( u , v ) = 0 , ( x , t ) ∈ Ω × ( 0 , T ) , u ( x , t ) = v ( x , t ) = 0 , x ∈ ∂ Ω × ( 0 , T ) , u ( x , 0 ) = u 0 ( x ) , u t ( x , 0 ) = u 1 ( x ) , x ∈ Ω , v ( x , 0 ) = v 0 ( x ) , v t ( x , 0 ) = v 1 ( x ) , x ∈ Ω ,

where Ω is a bounded domain with smooth boundary ∂Ω in ℝ ⁿ , γ ₁, γ ₂ ≥ 0 are constants and ρ is a real number such that 0 < ρ ≤ 2/(n - 2) if n ≥ 3 or ρ > 0 if n = 1, 2. Under suitable assumptions on the functions g(s), h(s), f(u, v), k(u, v), they used the perturbed energy method to show that the dissipations given by the viscoelastic terms are strong enough to ensure exponential or polynomial decay of the solutions energy, depending on the decay rate of the relaxation functions g(s) and h(s). For the problem (1.1) in ℝ ⁿ , we mention the work of Kafini and Messaoudi 10 .

Motivated by the above research, we consider in this study the coupled system (1.1). We prove that, under suitable assumptions on the functions g_i , f_i (i = 1, 2) and certain initial data in the stable set, the decay rate of the solution energy is exponential. Conversely, for certain initial data in the unstable set, there are solutions with positive initial energy that blow up in finite time.

This article is organized as follows. In Section 2, we present some assumptions and definitions needed for this study. Section 3 is devoted to the proof of the uniform decay result. In Section 4, we prove the blow-up result.

First, let us introduce some notation used throughout this article. We denote by || · || _q the L^q (Ω) norm for 1 ≤ q ≤ ∞ and by ||∇ · ||₂ the Dirichlet norm in H 0 1 ( Ω ) which is equivalent to the H ¹(Ω)norm. Moreover, we set

( φ , ψ ) = ∫ Ω φ ( x ) ψ ( x ) d x

as the usual L ²(Ω) inner product.

Concerning the functions f ₁(u, v) and f ₂(u, v), we take

f 1 ( u , v ) = [ a | u + v | 2 ( p + 1 ) ( u + v ) + b | u | p u | v | ( p + 2 ) ] , f 2 ( u , v ) = [ a | u + v | 2 ( p + 1 ) ( u + v ) + b | u | ( p + 2 ) | v | p v ] ,

where a, b > 0 are constants and p satisfies

p > - 1 , i f n = 1 , 2 , - 1 < p ≤ 1 , i f n = 3 .

One can easily verify that

u f 1 ( u , v ) + v f 2 ( u , v ) = 2 ( p + 2 ) F ( u , v ) , ∀ ( u , v ) ∈ ℝ 2 ,

where

F ( u , v ) = 1 2 ( p + 2 ) [ a | u + v | 2 ( p + 2 ) + 2 b | u v | p + 2 ] .

For the relaxation functions g_i (t) (i = 1, 2), we assume

(G1) g_i (t) : ℝ₊ → ℝ₊ belong to C ¹(ℝ₊) and satisfy

g i ( t ) ≥ 0 , g i ′ ( t ) ≤ 0 , f o r t ≥ 0

and

1 - ∫ 0 ∞ g i ( s ) d s = k i > 0 .

(G2) max ∫ 0 ∞ g 1 ( s ) d s , ∫ 0 ∞ g 2 ( s ) d s < 4 ( p + 1 ) ( p + 2 ) 4 ( p + 1 ) ( p + 2 ) + 1 .

We next state the local existence and the uniqueness of the solution of problem (1.1), whose proof can be found in Han and Wang 19 (Theorem 2.1) with slight modification, so we will omit its proof. In the proof, the authors adopted the technique of Agre and Rammaha 22 which consists of constructing approximations by the Faedo-Galerkin procedure without imposing the usual smallness conditions on the initial data to handle the source terms. Unfortunately, due to the strong nonlinearities on f ₁ and f ₂, the techniques used by Han and Wang 19 and Agre and Rammaha 22 allowed them to prove the local existence result only for n ≤ 3. We note that the local existence result in the case of n > 3 is still open. For related results, we also refer the reader to Said-Houari and Messaoudi 23 and Messaoudi and Said-Houari 20 . So throughout this article, we have assumed that n ≤ 3.

Theorem 2.1. Assume that (2.1) and (G1) hold, and that ( u 0 , u 1 ) ∈ H 0 1 ( Ω ) × L 2 ( Ω ) , ( v 0 , v 1 ) ∈ H 0 1 ( Ω ) × L 2 ( Ω ) . Then problem (1.1) has a unique local solution

u , v ∈ C ( [ 0 , T ) ; H 0 1 ( Ω ) ) , u t , v t ∈ C ( [ 0 , T ) ; L 2 ( Ω ) ) ∩ L 2 ( [ 0 , T ) ; H 0 1 ( Ω ) )

for some T > 0. If T < ∞, then

lim t → T ( k 1 | | ∇ u ( t ) | | 2 2 + | | u t ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 + | | v t ( t ) | | 2 2 ) = ∞ .

Finally, we define

I ( t ) = ( 1 - ∫ 0 t g 1 ( τ ) d τ ) | | ∇ u ( t ) | | 2 2 + 1 - ∫ 0 t g 2 ( τ ) d τ | | ∇ v ( t ) | | 2 2 (1) + [ ( g 1 ∘ ∇ u ) ( t ) + ( g 2 ∘ ∇ v ) ( t ) ] - 2 ( p + 2 ) ∫ Ω F ( u , v ) d x , (2) (3)

J ( t ) = 1 2 1 - ∫ 0 t g 1 ( τ ) d τ | | ∇ u ( t ) | | 2 2 + 1 - ∫ 0 t g 2 ( τ ) d τ | | ∇ v ( t ) | | 2 2 (1) + 1 2 [ ( g 1 ∘ ∇ u ) ( t ) + ( g 2 ∘ ∇ v ) ( t ) ] - ∫ Ω F ( u , v ) d x , (2) (3)

such functionals we could refer to Muñoz Rivera 24 25 . We also define the energy function as follows

E ( t ) = 1 2 | | u t ( t ) | | 2 2 + | | v t ( t ) | | 2 2 + J ( t ) ,

where

( g i ∘ w ) ( t ) = ∫ 0 t g i ( t - τ ) | | w ( t ) - w ( τ ) | | 2 2 d τ .

In this section, we deal with the uniform exponential decay of the energy for system (1.1) by using the perturbed energy method. Before we state and prove our main result, we need the following lemmas.

Lemma 3.1. Assume (2.1) and (G1) hold. Let (u, v) be the solution of the system (1.1), then the energy functional is a decreasing function, that is

E ′ ( t ) = - | | ∇ u t ( t ) | | 2 2 - | | ∇ v t ( t ) | | 2 2 + 1 2 ( g ′ 1 ∘ u ) ( t ) + 1 2 ( g ′ 2 ∘ v ) ( t ) (1) - 1 2 g 1 ( t ) | | ∇ u ( t ) | | 2 2 - 1 2 g 2 ( t ) | | ∇ v ( t ) | | 2 2 ≤ 0 . (2) (3)

Moreover, the following energy inequality holds:

E ( t ) + ∫ s t ( | | ∇ u t ( τ ) | | 2 2 + | | ∇ v t ( τ ) | | 2 2 ) d τ ≤ E ( s ) , f o r 0 ≤ s ≤ t < T .

Lemma 3.2. Let (2.1) hold. Then, there exists η > 0 such that for any ( u , v ) ∈ H 0 1 ( Ω ) × H 0 1 ( Ω ) , we have

| | u + v | | 2 ( p + 2 ) 2 ( p + 2 ) + 2 | | u v | | p + 2 p + 2 ≤ η ( k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 ) p + 2 .

Proof. The proof is almost the same that of Said-Houari 26 , so we omit it here. □

To prove our result and for the sake of simplicity, we take a = b = 1 and introduce the following:

B = η 1 2 ( p + 2 ) , α * = B - p + 2 P + 1 , E 1 = 1 2 - 1 2 ( p + 2 ) α * 2 ,

where η is the optimal constant in (3.3). The following lemma will play an essential role in the proof of our main result, and it is similar to a lemma used first by Vitillaro 27 , to study a class of a single wave equation, which introduces a potential well.

Lemma 3.3. Let (2.1) and (G1) hold. Let (u, v) be the solution of the system (1.1). Assume further that E(0) < E ₁ and

( k 1 | | ∇ u 0 | | 2 2 + k 2 | | ∇ v 0 | | 2 2 ) 1 ∕ 2 < α * ,

Then

( k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 + ( g 1 ∘ ∇ u ) ( t ) + ( g 2 ∘ ∇ v ) ( t ) ) 1 ∕ 2 < α * , f o r t ∈ [ 0 , T ) .

Proof. We first note that, by (2.5), (3.3) and the definition of B, we have

E ( t ) ≥ 1 2 ( k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 + ( g 1 ∘ ∇ u ) ( t ) + ( g 2 ∘ ∇ v ) ( t ) ) (1) - 1 2 ( p + 2 ) ( | | u + v | | 2 ( p + 2 ) 2 ( p + 2 ) + 2 | | u v | | p + 2 p + 2 ) (2) ≥ 1 2 ( k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 + ( g 1 ∘ ∇ u ) ( t ) + ( g 2 ∘ ∇ v ) ( t ) ) (3) - B 2 ( p + 2 ) 2 ( p + 2 ) ( k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 ) p + 2 (4) ≥ 1 2 α 2 - B 2 ( p + 2 ) 2 ( p + 2 ) α 2 ( p + 2 ) = g ( α ) , (5) (6)

where α = ( k 1 | | ∇ u ( t ) | | 2 2 +   k 2 | | ∇ v ( t ) | | 2 2 +   ( g 1 ∘ ∇ u ) ( t ) + ( g 2 ∘ ∇ v ) ( t ) ) 1 / 2 . It is not hard to verify that g is increasing for 0 < α < α*, decreasing for α > α*, g(α) → - ∞ as α → +∞, and

g ( α * ) = 1 2 α * 2 - B 2 ( p + 2 ) 2 ( p + 2 ) α * 2 ( p + 2 ) = E 1 ,

where α* is given in (3.4). Now we establish (3.6) by contradiction. Suppose (3.6) does not hold, then it follows from the continuity of (u(t), v(t)) that there exists t ₀ ∈ (0, T) such that

( k 1 | | ∇ u ( t 0 ) | | 2 2 + k 2 | | ∇ v ( t 0 ) | | 2 2 + ( g 1 ∘ ∇ u ) ( t 0 ) + ( g 2 ∘ ∇ v ) ( t 0 ) ) 1 ∕ 2 = α * .

By (3.7), we observe that

E ( t 0 ) ≥ g ( k 1 | | ∇ u ( t 0 ) | | 2 2 + k 2 | | ∇ v ( t 0 ) | | 2 2 + ( g 1 ∘ ∇ u ) ( t 0 ) + ( g 2 ∘ ∇ v ) ( t 0 ) ) 1 ∕ 2 = g ( α * ) = E 1 .

This is impossible since E(t) ≤ E(0) < E ₁ for all t ∈ [0, T). Hence (3.6) is established. □

The following integral inequality plays an important role in our proof of the energy decay of the solutions to problem (1.1).

Lemma 3.4. 28 Assume that the function φ : ℝ⁺ ∪ {0} → ℝ⁺ ∪ {0} is a non-increasing function and that there exists a constant c > 0 such that

∫ t ∞ φ ( s ) d s ≤ c φ ( t )

for every t ∈ [0, ∞). Then

φ ( t ) ≤ φ ( 0 ) exp ( 1 - t / c )

for every t ≥ c.

Theorem 3.5. Let (2.1) and (G1) hold. If the initial data ( u 0 , u 1 ) ∈ H 0 1 ( Ω ) × L 2 ( Ω ) , ( v 0 , v 1 ) ∈ H 0 1 ( Ω ) × L 2 ( Ω ) satisfy E(0) < E ₁ and

( k 1 | | ∇ u 0 | | 2 2 + k 2 | | ∇ v 0 | | 2 2 ) 1 ∕ 2 < α * ,

where the constants α _*, E ₁ are defined in (3.4), then the corresponding solution to (1.1) globally exists, i.e. T = ∞. Moreover, if the initial energy E(0) and k such that

1 - η ( 2 ( p + 2 ) ( p + 1 ) E ( 0 ) ) ( p + 1 ) - 5 ( 1 - k ) ( p + 2 ) 2 k ( p + 1 ) > 0 ,

where k = min{k ₁, k ₂}, then the energy decay is

E ( t ) ≤ E ( 0 ) exp ( 1 - a C - 1 t )

for every t ≥ aC ^-1, where C is some positive constant.

Proof. In order to get T = ∞, by (2.2), it suffices to show that

| | u t ( t ) | | 2 2 + | | v t ( t ) | | 2 2 + k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2

is bounded independently of t. Since E(0) < E ₁ and

( k 1 | | ∇ u 0 | | 2 2 + k 2 | | ∇ v 0 | | 2 2 ) 1 ∕ 2 < α * ,

it follows from Lemma 3.3 that

k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 ≤ k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 + ( g 1 ∘ ∇ u ) ( t ) + ( g 2 ∘ ∇ v ) ( t ) < α * 2 ,

which implies that

I ( t ) ≥ k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 + [ ( g 1 ∘ ∇ u ) ( t ) + ( g 2 ∘ ∇ v ) ( t ) ] - 2 ( p + 2 ) ∫ Ω F ( u , v ) d x (1) ≥ k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 - 2 ( p + 2 ) ∫ Ω F ( u , v ) d x (2) = k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 - ( | | u + v | | 2 ( p + 2 ) 2 ( p + 2 ) + 2 | | u v | | p + 2 p + 2 ) (3) ≥ k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 - η ( k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 ) p + 2 ≥ 0 , f o r t ∈ [ 0 , T ) , (4) (5) 

where we have used (3.3). Furthermore, by (2.3) and (2.4), we get

J ( t ) ≥ 1 2 - 1 2 ( p + 2 ) ( 1 - ∫ 0 t g 1 ( s ) d s ) | | ∇ u ( t ) | | 2 2 + 1 - ∫ 0 t g 2 ( s ) d s | | ∇ v ( t ) | | 2 2 (1) + 1 2 - 1 2 ( p + 2 ) ( g 1 ∘ ∇ u ) ( t ) + ( g 2 ∘ ∇ v ) ( t ) + 1 2 ( p + 2 ) I ( t ) (2) ≥ p + 1 2 ( p + 2 ) k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 + ( g 1 ∘ ∇ u ) ( t ) + ( g 2 ∘ ∇ v ) ( t ) + 1 2 ( p + 2 ) I ( t ) ≥ 0 , (3) (4) 

from which, the definition of E(t) and E(t) ≤ E(0), we deduce that

k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 ≤ 2 ( p + 2 ) ( p + 1 ) J ( t ) ≤ 2 ( p + 2 ) ( p + 1 ) E ( t ) ≤ 2 ( p + 2 ) ( p + 1 ) E ( 0 ) ,

for t ∈ [0, T). So it follows from (16) and Lemma 3.1 that

p + 1 2 ( p + 2 ) k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 + 1 2 ( | | u t ( t ) | | 2 2 + | | v t ( t ) | | 2 2 ) ≤ J ( t ) + 1 2 ( | | u t ( t ) | | 2 2 + | | v t ( t ) | | 2 2 ) (1) = E ( t ) ≤ E ( 0 ) < E 1 , ∀ t ∈ [ 0 , T ) , (2) (3) 

which implies

| | u t ( t ) | | 2 2 + | | v t ( t ) | | 2 2 + k 1 | | ∇ u ( t ) | | 2 2 + k 2 | | ∇ v ( t ) | | 2 2 < C E 1 ,

where C is a positive constant depending only on p.

Next we want to derive the decay rate of energy function for problem (1.1). By multiplying the first equation of system (1.1) by u and the second equation of system (1.1) by v, integrating over Ω × [t ₁, t ₂] (0 ≤ t ₁ ≤ t ₂), using integration by parts and summing up, we have

∫ Ω u t ( t ) u ( t ) d x | t 1 t 2 - ∫ t 1 t 2 | | u t ( t ) | | 2 2 d t + ∫ Ω v t ( t ) v ( t ) d x | t 1 t 2 - ∫ t 1 t 2 | | v t ( t ) | | 2 2 d t = - ∫ t 1 t 2 ( ∇ u ( t ) , ∇ u t ( t ) ) d t - ∫ t 1 t 2 ( ∇ v ( t ) , ∇ v t ( t ) ) d t - ∫ t 1 t 2 | | ∇ u ( t ) | | 2 2 d t - ∫ t 1 t 2 | | ∇ v ( t ) | | 2 2 d t - ∫ t 1 t 2 ∫ Ω ∫ 0 t g 1 ( t - τ ) Δ u ( τ ) d τ u ( t ) d x d t - ∫ t 1 t 2 ∫ Ω ∫ 0 t g 2 ( t - τ ) Δ v ( τ ) d τ v ( t ) d x d t + 2 ( p + 2 ) ∫ t 1 t 2 ∫ Ω F ( u , v ) d x d t ,

which implies

2 ∫ t 1 t 2 E ( t ) d t - 2 ( p + 1 ) ∫ t 1 t 2 ∫ Ω F ( u , v ) d x d t = - ∫ Ω u t ( t ) u ( t ) d x | t 1 t 2 - ∫ Ω v t ( t ) v ( t ) d x | t 1 t 2 + 2 ∫ t 1 t 2 | | u t ( t ) | | 2 2 d t + 2 ∫ t 1 t 2 | | v t ( t ) | | 2 2 d t + ∫ t 1 t 2 ( g 1 ∘ ∇ u ) ( t ) d t + ∫ t 1 t 2 ( g 2 ∘ ∇ v ) ( t ) d t - ∫ t 1 t 2 ∫ 0 t g 1 ( τ ) d τ | | ∇ u ( t ) | | 2 2 d t - ∫ t 1 t 2 ∫ 0 t g 2 ( τ ) d τ | | ∇ v ( t ) | | 2 2 d t - ∫ t 1 t 2 ( ∇ u ( t ) , ∇ u t ( t ) ) d t - ∫ t 1 t 2 ( ∇ v ( t ) , ∇ v t ( t ) ) d t - ∫ t 1 t 2 ∫ Ω ∫ 0 t g 1 ( t - τ ) Δ u ( τ ) d τ u ( t ) d x d t - ∫ t 1 t 2 ∫ Ω ∫ 0 t g 2 ( t - τ ) Δ v ( τ ) d τ v ( t ) d x d t .

For the 11th term on the right-hand side of (3.10), one has

- 2 ∫ Ω ∫ 0 t g 1 ( t - τ ) Δ u ( τ ) d τ u ( t ) d x = 2 ∫ Ω ∫ 0 t g 1 ( t - τ ) ∇ u ( τ ) ∇ u ( t ) d τ d x = ∫ 0 t g 1 ( t - τ ) ( | | ∇ u ( t ) | | 2 2 + | | ∇ u ( τ ) | | 2 2 ) d τ - ∫ 0 t g 1 ( t - τ ) ( | | ∇ u ( t ) - ∇ u ( τ ) | | 2 2 ) d τ .