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International Journal of Partial Differential Equations

Volume 2013 (2013), Article ID 748761, 10 pages

http://dx.doi.org/10.1155/2013/748761

## An Initial Boundary Value Problem for the Zakharov Equation

^{1}Institute of Mathematics, School of Mathematical Sciences, Nanjing Normal University, Nanjing 210008, China^{2}Department of Mathematics, Nanjing Xiaozhuang College, Nanjing 210008, China^{3}Department of Mathematics, Wellesley College, Wellesley, MA 02481, USA

Received 15 February 2013; Accepted 20 June 2013

Academic Editor: Nikolai A. Kudryashov

Copyright © 2013 Quankang Yang and Charles Bu. 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

This paper studies an inhomogeneous initial boundary value problem for the one-dimensional Zakharov equation. Existence and uniqueness of the global strong solution are proved by Galerkin’s method and integral estimates.

#### 1. Introduction

In this paper, we consider the following inhomogeneous initial boundary value problem for the Zakharov equations in one dimension:

Zakharov equations play an important role in the turbulence theory for plasma waves and resemble closely to the nonlinear Schrödinger equations. There has been extensive study both theoretically and numerically on these equations (e.g., see [1–17]). Most investigation have been focused on the Cauchy problem of the system of Zakharov equations (SZEs), sometimes with a homogeneous boundary condition. It is well known that Zakharov equations possess 1D soliton solutions. Some numerical experiments suggest that the solutions of 2D and 3D SZE may become singular in finite time [7, 18]. Global existence of solutions of the -dimensional SZE () has been only proved for small initial data [5, 6]. It has been shown that solutions with large initial data may blow up in finite time [8].

The system (1)-(2) above describes the interaction of a Langmuir wave and an ion acoustic wave in a plasma (Dendy [19] and Bellan [20]). Here, is an unknown complex vector-valued function that denotes a slowly varying envelope of a highly oscillatory electric field. Meanwhile, is an unknown real function that denotes the fluctuation in the ion density about its equilibrium value (see [19, 21]). We assume that , are given smooth functions.

Let be any function on with compact support satisfying , and we define Thus, the problem (1)–(4) is equivalent to

In Section 2, we obtain the existence of a local weak solution via Galerkin’s method and the principle of compactness. In Section 3, we derive estimates of higher-order derivatives of Galerkin’s approximate solution to obtain the existence and uniqueness of the local strong solution. In Section 3, we prove the existence and uniqueness of the global strong solution.

#### 2. Existence of a Local Strong Solution

We first work on Galerkin’s approximation solution for the problem (6)–(9) by choosing the basic functions as follows:

The approximate solution for the problem (6)–(9) can be written as

According to Galerkin’s method, these undetermined coefficients must satisfy the following initial value problem for a system of ordinary differential equations: with , , and

To obtain existence of a local weak solution, we need the following lemmas.

Lemma 1. *Assume that , then . *

*Proof. *We multiply (13) by and sum up in to obtain
Since , taing imaginary parts of (16) yields . Therefore, is a constant, and (16) is proved.

From Gagliardo-Nirenberg inequality [22], one has (if )
which will be used in the following estimates.

Lemma 2. *Let , let , let , and let . Then, there is such that
**
for any , where is a positive constant depending only on data and .*

*Proof. *We multiply (13) by , multiply (14) by , and sum up in to get
By taking the imaginary parts of (19), we get
From (20), we have
Combining (21) and (22), we find

We multiply (13) by and sum up to obtain
Therefore,
Differentiating (13) in , we have
By taking the imaginary parts of (27), we obtain the following inequalities:
From (23), we have
Combining (28), (29), and (25), we get
By Gagliardo-Nirenberg’s inequality with , we obtain the following estimates:
From (25), (31), (32), and (33), we have
with . Put the above inequalities into (30), and use Young’s inequality to obtain
Hence, there exist such that for any . Here, only depends on data and but not .

Lemma 3. *Under the conditions of Lemma 2, there exist a positive constant , such that
**
for any , and is a positive constant that depends only on data and .*

*Proof. *By Lemma 2 and (31), (34), and (35), we have

Lemma 4. *Assume that , , , and , then there exist a positive constant such that
**
for any , where is a positive constant that depends only on data and .*

*Proof. *By differentiating (13) twice in , we have
Multiply (40) by , and sum up in to obtain
Taking the imaginary parts of (41), we have
By Lemmas 2 and 3, we have
Substituting (43) and (44) into (42), we obtain
Differentiating (14) in , we have
Multiplying (46) by and summing up in , we obtain
That is
where
Differentiating (13) in , we have
Multiplying (51) by and summing up the above formulas for form to , we obtain
Taking the real parts of (52), we get
Multiplying (26) by and summing up in , we obtain
By the above lemmas, we have
Substituting (55)–(58) into (54), we have
Substituting (59) into (50), we get
Substituting (49) and (60) into (48), we obtain

Combining (43), (45), (53), and (61), we have
That is

Therefore, there exist a positive constant , such that
for any , and is a positive constant that depends only on data and (not on ).

Multiplying (14) by and then summing up the above formulas for form to , we obtain
That is
where

This implies that
By the above lemmas, we get
Hence,

This completes the proof.

Lemma 5 (see [23, Lemma 4]). *Let be Banach spaces. If the embedding is compact, then the following embeddings are also compact:*(1)*,*(2)*. *

Theorem 6. *Under the conditions of Lemma 4, then there exists a local strong solution for (6)–(9).*

*Proof. *According to those estimates outlined in the above lemmas, we know that , , , and are all bounded uniformly in . By the principle of compactness, there exist subsequences of , , , and (we still denote these by the same letter) such that

Since , are uniformly bounded in , , we can choose subsequences of , (we still denote these by the same index) such that

Therefore,
In fact, we have

Since

we get

On the other hand, since
and for all , , we see that

As is dense in , this establishes the local strong solutions and completes the proof of Theorem 6.

#### 3. Existence and Uniqueness of a Global Strong Solution

In the following, in order to obtain the global strong solution we will give a priori estimates for the solution which we get in Theorem 6.

Lemma 7. *Assume that , then
*

*Proof. *Multiply both sides of (1) by , integrate the equation on (), and then take the real parts. It is easy to see that . This implies that is a conserved quantity in respect to time. Therefore, Lemma 7 is proved.

Lemma 8. *Assume that , , , and ; then for , , one has
**
where is a positive constant that depends only on data and .*

*Proof. *From (7), we have
Integrating (81) over , we obtain
Taking the inner product of (6) and , we get
Taking the real parts of (83), it follows that

Substituting (82) into (84), we have
By (82), we get

Integrating by parts yields that

Combining (85), (86), and (87), we have

Integrating (88) over , we obtain
where

Substituting (82) into (90), we have

Therefore,
Since
we have the following estimate after combining (89) and (92):

We may choose such that to obtain

By Groonwall’s lemma, we get
for all , for any given .

This implies that

Taking the inner product of (6) and , we have

By (98), (99), and Lemma 2, we obtain

Taking the inner product of (7) and , we get

Differentiating (6) with respect to then taking the inner product of the result and , we have
Taking the imaginary parts of (105), we obtain
Combining (104) and (106), we get
By Gronwall’s lemma, we obtain
From (95), we get

This completes the proof of Lemma 8. So, we have the following theorem.

Theorem 9. *The solution given by Theorem 6 exists for all time, that is, .*

The proof of uniqueness is done via standard integral estimates, so, we omit it here.

#### Acknowledgments

This work is supported by Qing Lan and “333” Project of Jiangsu Province and the NSF of the Jiangsu Higher Education Committee of China (11KJA110001). It is supported by the Brachman Hoffman Small Grant and Wellesley College Faculty Award.

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