Open Access Research

General decay for a wave equation of Kirchhoff type with a boundary control of memory type

Shun-Tang Wu

Author affiliations

General Education Center, National Taipei University of Technology, Taipei 106, Taiwan

Citation and License

Boundary Value Problems 2011, 2011:55  doi:10.1186/1687-2770-2011-55


The electronic version of this article is the complete one and can be found online at: http://www.boundaryvalueproblems.com/content/2011/1/55


Received:30 July 2011
Accepted:23 December 2011
Published:23 December 2011

© 2011 Wu; 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

A nonlinear wave equation of Kirchhoff type with memory condition at the boundary in a bounded domain is considered. We establish a general decay result which includes the usual exponential and polynomial decay rates. Furthermore, our results allow certain relaxation functions which are not necessarily of exponential and polynomial decay. This improves earlier results in the literature.

MSC: 35L05; 35L70; 35L75; 74D10.

Keywords:
general decay; wave equation; relaxation; memory type; Kirchhoff type; nondissipative

1 Introduction

In this article, we study the asymptotic behavior of the energy function related to a nonlinear wave equation of Kirchhoff type subject to memory condition at the boundary as follows:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M1">View MathML</a>

(1.1)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M2">View MathML</a>

(1.2)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M3','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M3">View MathML</a>

(1.3)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M4','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M4">View MathML</a>

(1.4)

where Ω is a bounded domain with smooth boundary ∂Ω = Γ0 ∪ Γ1. The partition Γ0 and Γ1 are closed and disjoint, with meas0) > 0, ν represents the unit normal vector directed towards the exterior of Ω, u is the transverse displacement, and g is the relaxation function considered positive and nonincreasing belonging to W1,2 (Ω).

From the physical point of view, we know that the memory effect described in integral equation (1.3) can be caused by the interaction with another viscoelastic element. In fact, the boundary condition (1.3) signifies that Ω is composed of a material which is clamped in a rigid body in the portion Γ0 of its boundary and is clamped in a body with viscoelastic properties in the portion of Γ1.

When Γ1 = ϕ, problem (1.1) has its origin in describing the nonlinear vibrations of an elastic string. More precisely, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M5','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M5">View MathML</a>

(1.5)

for 0 < × < L, t ≥ 0; where u is the lateral deflection, x the space coordinate, t the time, E the Young modulus, ρ the mass density, h the cross section area, L the length, p0 the initial axial tension and f the external force. Kirchhoff [1] was the first one who introduced (1.5) to study the oscillations of stretched strings and plates, so that (1.5) is called the wave equation of Kirchhoff type after him. In this direction, problem (1.1) with ∂Ω = Γ0 and l(t) = 0 has been investigated by many authors in recent years, and many results concerning existence, nonexistence and asymptotic behavior have been established, see [2-13].

On the other hand, regarding the viscoelastic wave equations with memory term acting in the boundary or in the domain, there are numerous results related to asymptotic behavior of solutions. For example, in the case where M(s) = 1, Cavalcanti et al. [14] investigated the existence and uniform decay of strong solutions of wave equation (1.1) with a nonlinear boundary damping of memory type and a nonlinear boundary source when l(t) = 0. Cavalcanti and Guesmia [15] considered the following system:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M6','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M6">View MathML</a>

(1.6)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M7','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M7">View MathML</a>

(1.7)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M8','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M8">View MathML</a>

(1.8)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M9">View MathML</a>

(1.9)

where Ω is a bounded domain with smooth boundary ∂Ω = Γ0 ∪ Γ1. They obtained the general decay result which depends on the relaxation function g. In particular, if the relaxation function g decays exponentially (or polynomially), then the solution also decays exponentially (or polynomially) and with the same decay rate. Moreover, when u0 = 0 on Γ1, they obtained exponential or polynomial decay of solutions, even if the relaxation function g does not converge to 0 at ∞. Later, Messaoudi and Soufyane [16] generalized this result to the case of a system of Timoshenko type. They established general decay rate results, from which the usual exponential and polynomial decay rates are only special cases. Recently, Messaoudi and Soufyane [17] studied the following problem:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M10','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M10">View MathML</a>

in a bounded domain with boundary conditions (1.7)-(1.9). They improved the results of [15] by applying the multiplier techniques. Indeed, they obtained not only a general decay result, but their works also allowed certain relaxation functions which are not necessarily of exponential or polynomial decay. For other related works, we refer the reader to [18-20] and references therein.

Conversely, in the case where M is not a constant function, Santos [21] considered

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M11">View MathML</a>

where μ(t) is a nonincreasing function satisfying μ(t) ≥ μ0 > 0. By denoting k the resolvent kernel of g', he showed that the solution decays exponentially (or polynomially) to zero provided k decays exponentially (or polynomially) to zero. Later on, Santos et al. [22] generalized this result to a nonlinear n-dimensional equation of Kirchhoff type of the form

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M12','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M12">View MathML</a>

(1.10)

in a bounded domain with boundary conditions (1.2)-(1.3). In that article, they proved that the energy decays with the same rate of decay of the relaxation function. This latter result improved an earlier one by Park et al. [23], where the authors considered (1.10) in a bounded domain with nonlinear boundary damping and memory term and M(s) = 1 + s and f = 0.

We note that stability of problems with the nonlinear term h(∇u) requires a careful treatment because we do not have any information about the influence of the integral <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M13">View MathML</a> about the sign of the derivative E'(t). Although the subject is important, there are few mathematical results in the presence of the nonlinearity given by h(∇u), see [24-26]. In light of this and previous articles [17,22], it is interesting to investigate whether we still have the similar general decay result as in [17] for nondissipative distributed system (1.1) with the memory-type damping acting on a part of the boundary. Hence, the main purpose of this article is to answer the above question for system (1.1)-(1.4). Consequently, by following the arguments close to the one in [17] with necessary modification required the nature of our problem, we establish a general decay result which includes the usual exponential and polynomial decay rates. Furthermore, our results allow a larger class of relax functions which are not necessarily of exponential and polynomial decay. Therefore, this improves earlier results in the literature [22,27].

In order to obtain our results, we consider system (1.1)-(1.4), under some assumptions on a(x), l(t), M and f. Precisely, we state the general assumptions:

(A1) a(x): Ω → R+ is a function.

(A2) f C1(R) is a function and satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M14','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M14">View MathML</a>

(1.11)

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M15','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M15">View MathML</a> with

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M16','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M16">View MathML</a>

d > 0 and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M17','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M17">View MathML</a>.

(A3) M is a C1 function on [0, ∞) satisfying

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M18','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M18">View MathML</a>

(1.12)

Where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M19','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M19">View MathML</a>.

(A4) h : Rn R is a C1 function such that ∇h is bounded and there exists β1 > 0 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M20','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M20">View MathML</a>

(1.13)

and l(t) is a positive and nonincreasing function.

The remainder of this article is organized as follows. In Section 2, we introduce some notations, present Lemma 2.1 to describe more general relations between the relaxation function g and the corresponding resolvent kernel k and state the existence result to system (1.1)-(1.4). In Section 3, we give the proof of our main result Theorem 3.5.

2 Preliminaries

In this section, we introduce some notations and establish the existence of solutions of the problem (1.1)-(1.4). In what follows, let ||·||p denote the usual Lp (Ω) norm <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M21','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M21">View MathML</a>, for 1 ≤ p ≤ ∞. We define the convolution product operator by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M22','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M22">View MathML</a>

(2.1)

and set

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M23','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M23">View MathML</a>

(2.2)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M24','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M24">View MathML</a>

(2.3)

Using Hölder's inequality, we observe that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M25','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M25">View MathML</a>

(2.4)

Next, we shall use Equation 1.3 to estimate the boundary term <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M26','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M26">View MathML</a>. Differentiating (1.3), we obtain

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M27','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M27">View MathML</a>

Assume the function k is the resolvent kernel of the relaxation function g, then

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M28','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M28">View MathML</a>

Applying Volterra's inverse operator yields

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M29','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M29">View MathML</a>

which implies that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M30','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M30">View MathML</a>

(2.5)

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M31','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M31">View MathML</a>. Reciprocally, taking u0 = 0 on Γ1, identity (2.5) implies (1.3). As we are interested in relaxation functions of more general decay and the resolvent k appeared in Equation 2.5, we want to know if the resolvent k has the same property with the relaxation function g involved in (1.3). The following lemma answers this question. Let h be a relaxation function and k its resolvent kernel, that is

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M32','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M32">View MathML</a>

Lemma 2.1. [15,17,22]If h : [0, ∞) → R+ is continuous, then k is also a positive continuous function. Moreover,

(1) If there exists a positive constant c0 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M33','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M33">View MathML</a>

where γ : [0, ∞) → R+, is a nonincreasing function satisfying, for some positive constant ε < 1,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M34','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M34">View MathML</a>

Then, k satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M35','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M35">View MathML</a>

(2) Suppose that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M36','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M36">View MathML</a>

for c0 < p - 1. Then, there exists a positive constant ε < 1 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M37','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M37">View MathML</a>

where β > 0 is a constant.

Based on this lemma, we will use (2.5) instead of (1.3), i.e., we can consider system (1.1)-(1.4) as follows:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M38','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M38">View MathML</a>

We notice that, due to the condition (1.2), the solution of system (1.1)-(1.4) must belong to the following space:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M39','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M39">View MathML</a>

which endowed with the norm ||∇·||2 is a Hilbert space. Now, we are ready to give the well-posedness of system (1.1)-(1.4).

Theorem 2.2. Let k W2,1 (R+) ∩ W1,∞ (R+), (u0, u1) ∈ (H2 (Ω) ∩ V)2 and satisfy the compatibility condition

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M40','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M40">View MathML</a>

Assume further that (A1)-(A4) hold. Then, there exists a unique solution u of system (1.1)-(1.4) such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M41','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M41">View MathML</a>

Proof. Using the Galerkin method and procedures similar to that of [22,28], we can obtain the result.    □

3 Decay of solutions

In this section, we study the asymptotic behavior of the solutions of system (1.1)-(1.4) when the resolvent kernel k satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M42','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M42">View MathML</a>

(3.1)

where γ : [0, ∞) → R+ is a function satisfying the following condition:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M43','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M43">View MathML</a>

(3.2)

To get our result, we further assume that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M44','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M44">View MathML</a>

(3.3)

Let x0 be a fixed point in Rn. Set

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M45','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M45">View MathML</a>

and partition the boundary ∂Ω into two sets

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M46','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M46">View MathML</a>

(3.4)

Define the first-order energy function of system (1.1)-(1.4) by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M47','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M47">View MathML</a>

(3.5)

The following lemma is associated with the property of the convolution operator, which is used to estimate the energy identity.

Lemma 3.1. If g, ϕ C1(R+), then

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M48','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M48">View MathML</a>

(3.6)

Proof. Our conclusion is followed by differentiating the term g ϕ.   □

Lemma 3.2. Under the assumptions of (A1)-(A4), the energy function E(t) satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M49','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M49">View MathML</a>

(3.7)

Proof. Multiplying Equation 1.1 by ut, and integrating by parts over Ω, we get

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M50','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M50">View MathML</a>

Exploiting (2.5), (3.6) and the definition of E(t) by (3.5), we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M51','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M51">View MathML</a>

Then, using Hölder's inequality and Young's inequality, the inequality (3.7) is obtained.    □

Next, we construct a Lyapunov functional which is equivalent to E(t). To do so, for N > 0 large enough, let

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M52','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M52">View MathML</a>

(3.8)

where

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M53','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M53">View MathML</a>

(3.9)

for 0 < θ < 1.

For the purpose of achieving our main result, we need the following lemmas.

Lemma 3.3. There exist two positive constants α1 and α2 such that the relation

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M54','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M54">View MathML</a>

holds for all t ≥ 0.

Proof. From (3.9) and using Young's inequality, we get

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M55','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M55">View MathML</a>

where we have used the fact that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M56','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M56">View MathML</a> by (3.5) and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M57','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M57">View MathML</a>

(3.10)

due to <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M58','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M58">View MathML</a> by (A3) and (3.5). Here B1 > 0 is the smallest constant such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M59','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M59">View MathML</a>

(3.11)

Thus, from (3.8), we deduce that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M60','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M60">View MathML</a>

Hence, selecting

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M61','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M61">View MathML</a>

(3.12)

there exist two positive constants α1 and α2 such that the relation

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M62','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M62">View MathML</a>

holds.    □

Lemma 3.4. Let (A1)-(A4) and (3.1)-(3.3) hold, with β1 (given by (A4)) small enough and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M63','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M63">View MathML</a>

(3.13)

Then, for some t0 large enough, the functional L(t) verifies, along the solution u of (1.1)-(1.4),

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M64','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M64">View MathML</a>

(3.14)

for all t t0, where α = min {2θ, 1 - θ} and ci are positive constants given in the proof, i = 4, 5.

Proof. First, we are going to estimate the derivative of ψ(t). From (3.9) and using Equation 1.1, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M65','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M65">View MathML</a>

Performing integration by parts and using Young's inequality, we obtain

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M66','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M66">View MathML</a>

(3.15)

where ε > 0, cε and c0 are some positive constants. In the following, we will estimate the last two terms on the right-hand side of (3.15). It follows from (1.13), Hölder's inequality, (3.11), (3.3) and (3.10) that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M67','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M67">View MathML</a>

(3.16)

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M68','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M68">View MathML</a>. Taking (1.11) and (3.4) into account, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M69','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M69">View MathML</a>

(3.17)

A substitution of (3.16)-(3.17) into (3.15), we obtain

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M70','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M70">View MathML</a>

(3.18)

Now, we analyze the boundary term on the right-hand side of (3.18). Applying Young's inequality and M(λ) ≥ m0 > 0 by (1.12), we have, for ε1 > 0,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M71','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M71">View MathML</a>

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M72','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M72">View MathML</a> is a positive constant and B* > 0 is the constant such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M73','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M73">View MathML</a>

(3.19)

Thus, (3.18) becomes

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M74','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M74">View MathML</a>

(3.20)

By rewriting the boundary condition (2.5) as

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M75','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M75">View MathML</a>

and, then, combining (3.7) and (3.20), we deduce that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M76','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M76">View MathML</a>

Similarly as in deriving (3.16), we note that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M77','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M77">View MathML</a>

(3.21)

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M78','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M78">View MathML</a>. This implies that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M79','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M79">View MathML</a>

At this point, we choose

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M80','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M80">View MathML</a>

Once ε = ε1 is fixed (hence cε and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M81','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M81">View MathML</a> are also fixed), we pick N large satisfying (3.12) and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M82','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M82">View MathML</a>

(3.22)

at the same time. Then, from the properties of k(t) by (3.1) and noting that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M83','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M83">View MathML</a> by (2.4), we see that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M84','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M84">View MathML</a>

Utilizing the inequality <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M85','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M85">View MathML</a> by (1.12) and the definition of E(t) by (3.5), we obtain

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M86','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M86">View MathML</a>

which together with (3.19) and (3.10) infers that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M87','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M87">View MathML</a>

where α = min{2θ, 1 - θ}. Besides, we note that there exists t0 large enough satisfying

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M88','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M88">View MathML</a>

(3.23)

because of limt→∞ k(t) = 0 by (3.13). Therefore, taking β1 small enough such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M89','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M89">View MathML</a>

(3.24)

then,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M90','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M90">View MathML</a>

(3.25)

for all t t0, where ci are positive constants, i = 4, 5. This completes the proof.    □

Theorem 3.5. Given that (u0, u1) ∈ (H2 (Ω) ∩ V)2, assume that (A1)-(A4), (3.1)-(3.3) and (3.13)hold, with β1 (given by (A4)) small enough. Assume further that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M91','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M91">View MathML</a>

(3.26)

Then, for some t0 large enough, we have, ∀t t0,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M92','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M92">View MathML</a>

(3.27)

otherwise (if u0 ≠ 0 on Γ1),

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M93','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M93">View MathML</a>

(3.28)

where a1 is a fixed positive constant and cis a generic positive constant.

Proof. Multiplying (3.25) by γ(t) and exploiting (3.26), (3.1) and (3.7), we derive that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M94','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M94">View MathML</a>

(3.29)

where c6 = c4γ(0) + c5 and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M95','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M95">View MathML</a>. Employing (3.21) again, (3.29) becomes

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M96','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M96">View MathML</a>

where

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M97','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M97">View MathML</a>

which is equivalent to E(t) due to Lemma 3.3 and γ(t) is nonincreasing by (3.2). In addition to (3.24), we further require

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M98','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M98">View MathML</a>

then, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M99','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M99">View MathML</a>

(3.30)

where a1 is a positive constant.

Case I: If u0 = 0 on Γ1, then (3.30) reduces to

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M100','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M100">View MathML</a>

Integrating the above inequality over (t0, t) to get

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M101','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M101">View MathML</a>

Then, using the fact F1(t) is equivalent to E(t), we obtain, for some positive constant c,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M102','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M102">View MathML</a>

Thus, (3.27) is proved.

Case II: If u0 ≠ 0 on Γ1, then (3.30) gives

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M103','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M103">View MathML</a>

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M104','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M104">View MathML</a>. Direct computations give

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M105','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M105">View MathML</a>

An integration over (t0, t) yields

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M106','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M106">View MathML</a>

Again using the fact F1(t) is equivalent to E(t), we obtain, for some positive constant c,

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M107','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M107">View MathML</a>

This completes the proof of Theorem 3.5.    □

4 Conclusion and suggestions

Santos et al. [22] considered problem (1.1)-(1.4) with a = 1 and without a function of the gradient term. They showed the solution decays exponentially (or polynomially) to zero provided the kernel decays exponentially (or polynomially) to zero. Recently, Messaoudi and Soufyane in 2010 [17] considered a semi-linear wave equation, in a bounded domain, where the memory-type damping is acting on the boundary. They established a general decay result, from which the usual exponential and polynomial decay rate are only special cases. Motivated by this, we intended to investigate the decay properties of problem (1.1)-(1.4) using the work of Messaaoudi and Soufyane [17]. Since stability of problems with the nonlinear term h(∇u) requires a careful treatment, it is interesting to investigate whether we still have the similar general decay result as that of [16] in the presence of a function of the gradient term. This is our motivation to consider problem (1.1)-(1.4). And, this problem is not considered before.

By adopting and modifying the method proposed by Messaoudi and Soufyane in 2010 [17], we establish a general decay result, from which the usual exponential and polynomial decay rate are only special cases. Further, our result allows certain kernels which are not necessarily of exponential or polynomial decay. In this way, we improved the results of Santos et al. [22], in which they considered problem (1.1)-(1.4) with a = 1 and in the absence of l(t)h (∇u). Moreover, we note that our result also holds for problem (1.1)-(1.4) with a = 1 and l(t) = 0 and without imposing strong damping term, thus our result improves the one of Bae et al. [27]. More precisely, the estimate (3.27) and (3.28) generalizes the exponential and polynomial decay result given in [22,27]. Indeed, we obtain exponential decay for γ(t) = c and polynomial decay for γ(t) = c(1 + t)-1, where c is a positive constant. Additionally, as in [17], our result allows kernels which satisfy k″(t) ≥ c (-k′)1+q, for 0 < q < 1 instead of the usual assumption <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M108','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M108">View MathML</a>. It suffices to take, for example, k(t) = (1 + t)-λ, for λ > 0. Direct computations yield

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M109','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M109">View MathML</a>

It is clear that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M110','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2011/1/55/mathml/M110">View MathML</a>, for λ > 0.

Though we consider the conditions on the term involving the gradient are too restrictive and we combine some known ideas to obtain our result, our findings extend those decay results in [22,27] and these findings are interesting to those with closely concerns. For future work, we will consider not necessarily decreasing kernels and relax the condition of h(∇u).

Competing interests

The author declare that they have no competing interests.

Acknowledgements

The author would like to thank the anonymous referees for their valuable and constructive suggestions which improve this work.

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