### Abstract

In this paper, we consider the system of nonlinear viscoelastic equations

with initial and Dirichlet boundary conditions. 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.

**2000 Mathematics Subject Classifications: **35L05; 35L55; 35L70.

##### Keywords:

decay; blow-up; positive initial energy; viscoelastic wave equations### 1. Introduction

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

where Ω is a bounded domain in ℝ* ^{n }*with a smooth boundary ∂Ω, and

*g*(·) : ℝ

_{i}_{+ }→ ℝ

_{+},

*f*(·, ·): ℝ

_{i}^{2 }→ 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:

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 Ω_{1 }⊂ Ω, with Ω_{1 }satisfying some geometry restrictions and

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

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

*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

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

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*) = -Δ

_{t}*u*and

_{t }*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

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

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

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

*, we mention the work of Kafini and Messaoudi [10].*

^{n}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.

### 2. Preliminaries

First, let us introduce some notation used throughout this article. We denote by ||
· ||* _{q }*the

*L*(Ω) norm for 1 ≤

^{q}*q*≤ ∞ and by ||∇ · ||

_{2 }the Dirichlet norm in

*H*

^{1}(Ω)norm. Moreover, we set

as the usual *L*^{2}(Ω) inner product.

Concerning the functions *f*_{1}(*u*, *v*) and *f*_{2}(*u*, *v*), we take

where *a*, *b *> 0 are constants and *p *satisfies

One can easily verify that

where

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

and

(G2)

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 *(*G*1) *hold, and that *
*Then problem (1.1) has a unique local solution*

*for some T *> 0. *If T *< ∞, *then*

Finally, we define

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

where

### 3. Global existence and energy decay

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 *(*G*1) *hold. Let *(*u*, *v*) *be the solution of the system (1.1), then the energy functional is a decreasing function,
that is*

*Moreover, the following energy inequality holds:*

**Lemma 3.2**. *Let (2.1) hold. Then, there exists η *> 0 *such that for any *
*we have*

*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:

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 *(*G*1) *hold. Let *(*u*, *v*) *be the solution of the system (1.1). Assume further that E*(0) < *E*_{1 }*and*

Then

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

where
*g *is increasing for 0 < *α *< *α**, decreasing for *α *> *α**, *g*(*α*) → - ∞ as *α *→ +∞, and

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

By (3.7), we observe that

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*

*for every t *∈ [0, ∞). *Then*

*for every t *≥ *c*.

**Theorem 3.5**. *Let (2.1) and *(*G*1) *hold. If the initial data *
*satisfy E*(0) < *E*_{1 }*and*

*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*

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

*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

is bounded independently of *t*. Since *E*(0) < *E*_{1 }and

it follows from Lemma 3.3 that

which implies that

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

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

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

which implies

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

which implies

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

Similarly,

Combining (3.10), (3.11) with (3.12), we have

Now we estimate every term of the right-hand side of the (3.13). First, by Hölder's inequality and Poincaré's inequality

where *λ *being the first eigenvalue of the operator - Δ under homogeneous Dirichlet boundary
conditions. Then, by (3.9), we see that

where *c*_{1 }is a constant independent on *u *and *v*, from which follows that

Since 0 ≤ *J *(*t*) ≤ *E *(*t*), from (3.2) we deduce that

Hence, by Poincaré inequality we get

where *c*_{2 }is a constant independent on *u *and *v*. In addition, using Young's inequality for convolution ||*f ** *g *||* _{q }*≤ ||

*f*||

*||*

_{r }*g*||

*with 1/*

_{s }*q*= 1/

*r*+ 1/

*s*- 1 and 1 ≤

*q*,

*r*,

*s*≤ ∞, noting that if

*q*= 1, then

*r*= 1 and

*s*= 1, we have

and

Hence, combining (3.9), (3.16) with (3.17) we then have

From (3.9), we also have

Combining (3.18) with (3.19), we deduce that

Finally, we also have the following estimate

where *c*_{3 }is a constant independent on *u *and *v*. Combining (3.13)-(3.21), we obtain

where *C *is a constant independent on *u*.

On the other hand, from (3.3) and (3.9), we have

which implies

Note that *E*(0) < *E*_{1}, we see that

Thus, combining (3.22) with (3.23), we have

that is

Denote

We rewrite (3.24)

for every *t *∈ [0, ∞).

Since *a *> 0 from the assumption conditions, by Lemma 3.4, we obtain the following energy decay
for problem (1.1) as

for every *t *≥ *Ca *^{-1}. □

### 4. Blow-up of solution

In this section, we deal with the blow-up solutions of the system (1.1). Set

From the assumption (*G*2), we have *θ _{i }*> 0 (

*i*= 1, 2). Similarly Lemma 3.2, we have

**Lemma 4.1**. *Assume (2.1) holds. Then there exists η*_{1 }*> 0 such that for any *
*we have*

*where the constants **θ _{i }*(

*i*= 1, 2)

*are defined in (4.1)*.

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

Then we have

**Lemma 4.2**. *Let *(*G*1), *G*(2) *and (2.1) hold. Let *(*u, v*) *be the solution of the system (1.1). Assume further that E*(0) < *E*_{2 }*and*

*where the constants **θ _{i }*(

*i*= 1, 2)

*are defined in (4.1)*.

*Then there exists a constant*

*α*

_{2 }>

*α*

_{* }

*such that*

*Proof*. We first note that, by (2.5), (4.2) and the definition of *B*_{1}, we have

where
*g *is increasing for 0 < *α *< *α _{*}*, decreasing for

*α*>

*α*

_{*},

*g*(

*α*) → - ∞ as

*α*→ + ∞, and