Advances in Mathematical Physics

Volume 2015, Article ID 523254, 5 pages

http://dx.doi.org/10.1155/2015/523254

## Growth of Solutions with Positive Initial Energy to Systems of Nonlinear Wave Equations with Damping and Source Terms

Department of Mathematics, Dicle University, 21280 Diyarbakir, Turkey

Received 27 November 2014; Accepted 15 February 2015

Academic Editor: Ivan Avramidi

Copyright © 2015 Erhan Pişkin. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We consider initial-boundary conditions for coupled nonlinear wave equations with damping and source terms. We prove that the solutions of the problem are unbounded when the initial data are large enough in some sense.

#### 1. Introduction

In this work, we consider the following initial-boundary value problem: where is a bounded domain with smooth boundary in , ; ; are given functions to be specified later.

Throughout this paper, we define by where , are nonnegative constants and .

This type of problems not only is important from the theoretical point of view, but also arises in material science and physics that deal with system of nonlinear wave equations.

Ye [1] obtained the local existence and the blowup of the solution of problem (1), for . In the absence of the strong damping terms, problem (1) becomes Wu et al. [2] obtained the global existence and blowup of the solution of problem (3) under some suitable conditions. Fei and Hongjun [3] considered problem (3) and improved the blowup result obtained in [2], for a large class of initial data in positive initial energy, using the same techniques as in Payne and Sattinger [4] and some estimates used firstly by Vitillaro [5]. Recently, Pişkin and Polat [6, 7] studied the local and global existence, energy decay, and blowup of the solution of problem (3). Also, for more information about (1) and (3), see [2, 3, 7].

The many problems associated with (1) are studied from various aspects in many papers [8–13].

In this work, we will consider the blowup property in infinity time, that is, exponential growth.

This work is organized as follows. In Section 2, we state the local existence result. In Section 3, we establish that the energy will grow up as an exponential as time goes to infinity, provided that the initial data are large enough or , where and are defined in (9) and (15).

#### 2. Preliminaries

In this section, we introduce some notations and lemmas and local existence theorem needed in the proof of our main results. Let and denote the usual norm and norm, respectively.

Concerning the functions and , we take where are constants and satisfies According to the above equalities they can easily verify that where

We have the following result.

Lemma 1 (see [14]). *There exist two positive constants and such that
**
is satisfied.*

We define the energy function as follows: where , .

Lemma 2 (see [7]). * is a nonincreasing function for and
*

*Lemma 3 (Sobolev-Poincare inequality [15]). Let be a number with or , then there is a constant such that
*

*Next, we state the local existence theorem [1, 7].*

*Theorem 4 (local existence). Suppose that (5) holds. Then there exist , satisfying
and further , . Then problem (1) has a unique local solution
*

*3. Exponential Growth*

*3. Exponential Growth**In this section, we will prove that the energy is unbounded when the initial data are large enough in some sense. Our techniques of proof follow very carefully the techniques used in [16].*

*Lemma 5 (see [3]). Suppose that (5) holds. Then there exists such that for any the inequality
holds.*

*For the sake of simplicity and to prove our result, we take and introduce
where is the optimal constant in (14). Next, we will state a lemma which is similar to the one introduced firstly by Vitillaro in [5] to study a class of a single wave equation.*

*Lemma 6 (see [3]). Suppose that (5) holds. Let be the solution of problem (1). Assume further that and
Then there exists a constant such that
for all .*

*Theorem 7. Suppose that (5) and hold. Then any solution of problem (1) with initial data satisfying
grows exponentially.*

*Proof. *We set
From (10) and (20) we get
hence we have .

We consider the following functional:
for small to be specified later.

Our goal is to show that satisfies a differential inequality of the form
This, of course, will lead to exponential growth.

By taking a derivative of (22) and using (1), it follows that
From (9) and (20), it follows that
Inserting (25) into (24), we get
Then using (18), we obtain
where . It is clear that , since . In order to estimate the last two terms in (27), we use the following Young inequality:
where , , such that . Consequently, applying the above inequality we have
Inserting estimates (29) into (27), we have
where .

Since , from the embedding and embedding , we have
for some positive constants and . Using the algebraic inequality
and since , we get
where . Similarly
Inserting (33) and (34) into (30), we have
Now, once and are fixed, we can choose small enough such that
Consequently (35) takes the form
where .

Then we have
On the other hand, applying Hölder inequality, we obtain
Young inequality gives
Since , algebraic inequality (32) yields
Note that
Combining with (37) and (42), we arrive at
Integrating differential inequality (43) between and gives the following estimate for :
The proof of Theorem 7 is completed.

*Remark 8. *When , by setting , the similar result is obtained by applying the same arguments in the proof of Theorem 7.

*Conflict of Interests*

*Conflict of Interests*

*The author declares that there is no conflict of interests regarding the publication of this paper.*

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