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Exponential energy decay and blow-up of solutions for a system of nonlinear viscoelastic wave equations with strong damping

Fei Liang12 and Hongjun Gao1*

Author Affiliations

1 Jiangsu Provincial Key Laboratory for Numerical Simulation of Large Scale Complex Systems, School of Mathematical Sciences, Nanjing Normal University, Nanjing 210046, PR China

2 Department of Mathematics, Anhui Science and Technology University, Feng Yang 233100, Anhui, PR China

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Boundary Value Problems 2011, 2011:22  doi:10.1186/1687-2770-2011-22


The electronic version of this article is the complete one and can be found online at: http://www.boundaryvalueproblems.com/content/2011/1/22


Received:26 April 2011
Accepted:13 September 2011
Published:13 September 2011

© 2011 fei and Hongjun; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M1">View MathML</a>

with initial and Dirichlet boundary conditions. We prove that, under suitable assumptions on the functions gi, fi (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:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M2">View MathML</a>

(1.1)

where Ω is a bounded domain in ℝn with a smooth boundary ∂Ω, and gi(·) : ℝ+ → ℝ+, fi(·, ·): 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:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M3','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M3">View MathML</a>

for a : Ω → ℝ+, a function, which may be null on a part of the domain Ω. Under the conditions that a(x) ≥ a0 > 0 on Ω1 ⊂ Ω, with Ω1 satisfying some geometry restrictions and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M4','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M4">View MathML</a>

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M5','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M5">View MathML</a>

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M6','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M6">View MathML</a>

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M7','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M7">View MathML</a>

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M8','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M8">View MathML</a>

(1.2)

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(ut) = a|ut|m-2ut and f(u) = b|u|p-2u 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(ut) = -Δut and f(u) = |u|p-2u 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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M9">View MathML</a>

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M10','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M10">View MathML</a>

where Ω is a bounded domain with smooth boundary ∂Ω in ℝn, n = 1, 2, 3. Under suitable assumptions on the functions gi, fi (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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M11">View MathML</a>

where Ω is a bounded domain with smooth boundary ∂Ω in ℝn, γ1, γ2 ≥ 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 ℝn, 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 gi, fi (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 Lq(Ω) norm for 1 ≤ q ≤ ∞ and by ||∇ · ||2 the Dirichlet norm in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M12','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M12">View MathML</a> which is equivalent to the H1(Ω)norm. Moreover, we set

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M13">View MathML</a>

as the usual L2(Ω) inner product.

Concerning the functions f1(u, v) and f2(u, v), we take

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M14','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M14">View MathML</a>

where a, b > 0 are constants and p satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M15','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M15">View MathML</a>

(2.1)

One can easily verify that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M16','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M16">View MathML</a>

where

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M17','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M17">View MathML</a>

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

(G1) gi(t) : ℝ+ → ℝ+ belong to C1(ℝ+) and satisfy

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M18','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M18">View MathML</a>

and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M19','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M19">View MathML</a>

(G2) <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M20','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M20">View MathML</a>.

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 f1 and f2, 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 <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M21','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M21">View MathML</a>, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M22','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M22">View MathML</a>. Then problem (1.1) has a unique local solution

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M23','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M23">View MathML</a>

for some T > 0. If T < ∞, then

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M24','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M24">View MathML</a>

(2.2)

Finally, we define

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M25','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M25">View MathML</a>

(2.3)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M26','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M26">View MathML</a>

(2.4)

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M27','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M27">View MathML</a>

(2.5)

where

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M28','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M28">View MathML</a>

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M29','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M29">View MathML</a>

(3.1)

Moreover, the following energy inequality holds:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M30','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M30">View MathML</a>

(3.2)

Lemma 3.2. Let (2.1) hold. Then, there exists η > 0 such that for any <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M31','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M31">View MathML</a>, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M32','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M32">View MathML</a>

(3.3)

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:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M33','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M33">View MathML</a>

(3.4)

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) < E1 and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M34','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M34">View MathML</a>

(3.5)

Then

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M35','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M35">View MathML</a>

(3.6)

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M36','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M36">View MathML</a>

(3.7)

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M37','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M37">View MathML</a>. It is not hard to verify that g is increasing for 0 < α < α*, decreasing for α > α*, g(α) → - ∞ as α → +∞, and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M38','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M38">View MathML</a>

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 t0 ∈ (0, T) such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M39','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M39">View MathML</a>

By (3.7), we observe that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M40','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M40">View MathML</a>

This is impossible since E(t) ≤ E(0) < E1 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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M41','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M41">View MathML</a>

for every t ∈ [0, ∞). Then

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M42','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M42">View MathML</a>

for every t c.

Theorem 3.5. Let (2.1) and (G1) hold. If the initial data <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M21','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M21">View MathML</a>, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M22','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M22">View MathML</a>satisfy E(0) < E1 and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M43','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M43">View MathML</a>

(3.8)

where the constants α*, E1 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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M44','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M44">View MathML</a>

where k = min{k1, k2}, then the energy decay is

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M45','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M45">View MathML</a>

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M46','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M46">View MathML</a>

is bounded independently of t. Since E(0) < E1 and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M47','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M47">View MathML</a>

it follows from Lemma 3.3 that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M48','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M48">View MathML</a>

which implies that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M49','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M49">View MathML</a>

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M50','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M50">View MathML</a>

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M51','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M51">View MathML</a>

(3.9)

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M52','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M52">View MathML</a>

which implies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M53','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M53">View MathML</a>

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 × [t1, t2] (0 ≤ t1 t2), using integration by parts and summing up, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M54','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M54">View MathML</a>

which implies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M55','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M55">View MathML</a>

(3.10)

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M56','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M56">View MathML</a>

(3.11)

Similarly,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M57','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M57">View MathML</a>

(3.12)

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M58','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M58">View MathML</a>

(3.13)

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M59','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M59">View MathML</a>

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M60','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M60">View MathML</a>

where c1 is a constant independent on u and v, from which follows that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M61','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M61">View MathML</a>

(3.14)

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M62','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M62">View MathML</a>

Hence, by Poincaré inequality we get

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M63','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M63">View MathML</a>

(3.15)

where c2 is a constant independent on u and v. In addition, using Young's inequality for convolution ||f * g ||q ≤ || f ||r ||g||s with 1/q = 1/r + 1/s - 1 and 1 ≤ q, r, s ≤ ∞, noting that if q = 1, then r = 1 and s = 1, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M64','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M64">View MathML</a>

(3.16)

and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M65','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M65">View MathML</a>

(3.17)

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M66','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M66">View MathML</a>

(3.18)

From (3.9), we also have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M67','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M67">View MathML</a>

(3.19)

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M68','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M68">View MathML</a>

(3.20)

Finally, we also have the following estimate

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M69','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M69">View MathML</a>

(3.21)

where c3 is a constant independent on u and v. Combining (3.13)-(3.21), we obtain

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M70','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M70">View MathML</a>

(3.22)

where C is a constant independent on u.

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M71','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M71">View MathML</a>

which implies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M72','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M72">View MathML</a>

(3.23)

Note that E(0) < E1, we see that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M73','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M73">View MathML</a>

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M74','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M74">View MathML</a>

that is

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M75','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M75">View MathML</a>

(3.24)

Denote

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M76','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M76">View MathML</a>

We rewrite (3.24)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M77','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M77">View MathML</a>

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M78','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M78">View MathML</a>

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

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M79','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M79">View MathML</a>

(4.1)

From the assumption (G2), we have θi > 0 (i = 1, 2). Similarly Lemma 3.2, we have

Lemma 4.1. Assume (2.1) holds. Then there exists η1 > 0 such that for any <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M31','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M31">View MathML</a>, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M80','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M80">View MathML</a>

(4.2)

where the constants θi (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:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M81','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M81">View MathML</a>

(4.3)

Then we have

Lemma 4.2. Let (G1), G(2) and (2.1) hold. Let (u, v) be the solution of the system (1.1). Assume further that E(0) < E2 and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M82','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M82">View MathML</a>

(4.4)

where the constants θi (i = 1, 2) are defined in (4.1). Then there exists a constant α2 > α* such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M83','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M83">View MathML</a>

(4.5)

Proof. We first note that, by (2.5), (4.2) and the definition of B1, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M84','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M84">View MathML</a>

(4.6)

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M85','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M85">View MathML</a>. It is not hard to verify that g is increasing for 0 < α < α*, decreasing for α > α*, g(α) → - ∞ as α → + ∞, and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M86','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M86">View MathML</a>

where α* is given in (4.3). Since E(0) < E2, there exists α2 > α* such that g(α2) = E(0).

Set <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M87','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M87">View MathML</a>, then by (4.6) we get g(α0) ≤ E(0) = g (α2), which implies that α0 α2. Now, to establish (4.5), we suppose by contradiction that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M88','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M88">View MathML</a>

for some t0 > 0. By the continuity of <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M89','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M89">View MathML</a> we can choose t0 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M90','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M90">View MathML</a>

Again, the use of (4.6) leads to

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M91','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M91">View MathML</a>

This is impossible since E(t) ≤ E(0) for all t ∈ [0, T). Hence (4.5) is established. □

Theorem 4.3. Assume (G1), (G2) and (2.1) hold. Then any solution of problem (1.1) with initial data satisfying

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M92','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M92">View MathML</a>

blows up in finite time, where the constants θi (i = 1, 2) are defined in (4.1) and α*, E2 are defined in (4.3).

Proof. Assume by contradiction that the solution (u, v) is global. Then, for any T > 0 we consider H(t) : [0, T] → ℝ+ defined by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M93','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M93">View MathML</a>

where β and s0 are positive constants to be determined later. A direct computation yields

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M94','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M94">View MathML</a>

and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M95','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M95">View MathML</a>

Multiplying the first equation of system (1.1) by u and the second equation of system (1.1) by v, integrating over Ω, using integration by parts and summing up, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M96','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M96">View MathML</a>

which implies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M97','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M97">View MathML</a>

Therefore, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M98','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M98">View MathML</a>

(4.7)

where Ψ (t), G(t): [0, T] → ℝ+ are the functions defined by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M99','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M99">View MathML</a>

and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M100','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M100">View MathML</a>

Using the Schwarz inequality, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M101','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M101">View MathML</a>

and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M102','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M102">View MathML</a>

Similarly, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M103','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M103">View MathML</a>

and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M104','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M104">View MathML</a>

The previous inequalities entail G(t) ≥ 0 for every [0, T]. Using (4.7), we get

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M105','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M105">View MathML</a>

(4.8)

where

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M106','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M106">View MathML</a>

(4.9)

For the fifth term on the right-hand side of (4.9), we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M107','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M107">View MathML</a>

(4.10)

Similarly,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M108','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M108">View MathML</a>

(4.11)

Combining (4.9), (4.10) with (4.11), we get

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M109','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M109">View MathML</a>

(4.12)

Since

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M110','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M110">View MathML</a>

(4.13)

and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M111','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M111">View MathML</a>

(4.14)

inserting (4.13) and (4.14) into (4.12), we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M112','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M112">View MathML</a>

Using (3.2) for s = 0, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M113','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M113">View MathML</a>

Since

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M114','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M114">View MathML</a>

by Lemma 4.2, there exists a constant α > α* such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M115','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M115">View MathML</a>

(4.15)

which implies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M116','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M116">View MathML</a>

Thus, we can let β satisfy

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M117','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M117">View MathML</a>

which implies that there exists δ > 0 (independent of T) such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M118','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M118">View MathML</a>

(4.16)

From (4.15) and the definition of H(t), there also exists ρ > 0 (independent of T) such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M119','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M119">View MathML</a>

(4.17)

By (4.8), (4.16) and (4.17) it follows that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M120','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M120">View MathML</a>

Moreover, we let s0 satisfy that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M121','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M121">View MathML</a>

which means H' (0) > 0. Thus by H" (t) > 0 we see that H(t) and H'(t) is strictly increasing on [0, T].

Setting y(t) = H(t)-(p+1)/2, then we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M122','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M122">View MathML</a>

and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M123','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M123">View MathML</a>

for all t ∈ [0, T], which implies that y(t) reaches 0 in finite time, say as t T*. Since T* is independent of the initial choice of T, we may assume that T* < T. This tells us that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M124','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M124">View MathML</a>

In turn, this implies that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M125','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M125">View MathML</a>

(4.18)

Indeed, if

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M126','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M126">View MathML</a>

then (4.18) immediately follows. On the contrary, if <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M127','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M127">View MathML</a> remains bounded on [0, T*), then

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M128','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/22/mathml/M128">View MathML</a>

so that again (4.18) is satisfied. This implies a contradiction, i.e. T < ∞. □

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

FL and HGAO carried out all studies in this article.

Acknowledgements

The authors are indebted to the referee for giving some important suggestions which improved the presentations of this article. The study was supported in part by the China NSF Grant No. 10871097, the Qing Lan Project of Jiangsu Province, the Foundation for Young Talents in College of Anhui Province Grant No. 2011SQRL115 and the program sponsored for scientific innovation research of college graduate in Jangsu Province Grant No. 181200000649.

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