Research Article | Open Access

Meihua Huang, Zhan Zhou, "Standing Wave Solutions for the Discrete Coupled Nonlinear Schrödinger Equations with Unbounded Potentials", *Abstract and Applied Analysis*, vol. 2013, Article ID 842594, 6 pages, 2013. https://doi.org/10.1155/2013/842594

# Standing Wave Solutions for the Discrete Coupled Nonlinear Schrödinger Equations with Unbounded Potentials

**Academic Editor:**Chuangxia Huang

#### Abstract

We demonstrate the existence of standing wave solutions of the discrete coupled nonlinear Schrödinger equations with unbounded potentials by using the Nehari manifold approach and the compact embedding theorem. Sufficient conditions are established to show that the standing wave solutions have both of the components not identically zero.

#### 1. Introduction

Consider the coupled discrete Schrödinger system where , are real valued sequences, . is the discrete Laplacian operator defined as .

The system (1) could be viewed as the discretization of the two-component system of time-dependent nonlinear Gross-Pitaevskii system (see [1] for detail)

In this paper, we will study the standing wave solutions of (1), that is, solutions of the form where the amplitude are supposed to be real. Inserting the ansatz of the standing wave solutions (3) into (1), we obtain the following equivalent algebraic equations:

Since Bose-Einstein condensation for a mixture of different interaction atomic species with the same mass was realized in 1997 (see [2]), this stimulated various analytical and numerical results on the standing wave solutions of the system (2). The discrete nonlinear Schrödinger equations (DNLS) have a crucial role in the modeling of a great variety of phenomena, ranging from solid-state and condensed-matter physics to biology. During the last years, there has been a growing interest in approaches to the existence problem for standing waves. We refer to the continuation methods in [3, 4], which have been proved powerful for both theoretical considerations and numerical computations (see [5]), to [6], which exploits spatial dynamics and centre manifold reduction, and to the variational methods in [7–11], which rely on critical point techniques (linking theorems and Nehari manifold).

We noticed that most works on the existence of standing waves solutions are for single discrete nonlinear Schrödinger equation, and less is known for discrete nonlinear Schrödinger system. In the recent paper [12], the authors considered the standing wave solutions of the following system: which is more general than the system (1). However, they make a mistake to obtain the equivalent algebraic equations because may be different from . Hence, there are two ways to correct this mistake. The first method is to study the special standing wave solutions (3) of the system (5) with . The second method is to study the standing wave solutions (3) of the system (5) with . In this paper, we consider the second method. By the way, the proof of the main results in [12] is also not fully corrected.

The paper is organized as follows. In Section 2, we introduce some preliminaries and a discrete version of compact embedding theorem. Some key lemmas on the Nehari manifold are proved in Section 3. In Section 4, the main results are stated and proved.

#### 2. Preliminaries

In this section we describe the functional setting needed for the treatment of the infinite nonlinear system (4). We first introduce a compact embedding theorem.

Consider the real sequence spaces Between spaces the following elementary embedding relation holds: For the case , we need the usual Hilbert space of , endowed with the real scalar product Let us point out that the spectrum of in coincides with the interval . Obviously, we have Assume that the potential , satisfies Without loss of generality we assume that ; that is for . Let which are self-adjoint operators defined on , and

The following lemma can be found in [9].

Lemma 1. *If , satisfy the condition (10), then for any , and are compactly embedded into and denote the best embedding constant and , respectively. Furthermore, the spectra and are discrete, respectively. *

Now we can define the action functional By Lemma 1, it follows that the action functional and (13) corresponds to . So we define and the Nehari manifold

#### 3. Some Lemmas on the Nehari Manifold

Let

To prove the main results, we need some lemmas on the Nehari manifold.

Lemma 2. *Assume that , and (10) holds. Then the Nehari manifold is nonempty in . Furthermore, for , attains a unique maximum point at . *

*Proof. *First we show that .

From (15) and (16), we rewrite
Let ; then by (19)
Notice that and ; by (20), we see that for small enough and for large enough. As a consequence, there exists such that ; that is, .

Let , . Computing the derivative of , we have
This shows that is a unique maximum point. The proof is completed.

Lemma 3. *Assume that , and (10) holds. Then there exists such that , for all . *

*Proof. *Since is the smallest eigenvalue of and is the smallest eigenvalue of , from the definition of the constant and , we get and . For any , we have
where and .

Let

By (22), it is easy to see that
and this implies that
Moreover, we have
Let ; then we get , for all . The proof is completed.

#### 4. Main Results

Now we state our main results in this paper as follows.

Theorem 4. *Assume that , and (10) holds. Then system (13) has a nontrivial solution in ; that is, system (1) has a nontrivial standing wave solution. *

In order to prove Theorem 4, we consider the following constrained minimization problem: From the standard variational method, the proof of Theorem 4 is changed into finding a solution to the minimization problem (27). Now we are ready to prove Theorem 4.

*Proof. *Let be given by (27). By Lemma 2, is nonempty and there exists a sequence such that

By Lemma 3, and . By virtue of (26), we have
Thus, sequences and are bounded in Hilbert spaces and , respectively. Therefore, there exist subsequences of and (denoted by itself) that weakly converge to some and , respectively. By Lemma 1, we get, for any ,
By virtue of (15) and (16), we have
First, we claim that

According to (30), it suffices to show that

In fact,
Thus Hölder inequality and (30) imply the (33) holds.

Next, we show that and .

Since and are Hilbert spaces, by (32) we have
which implies . Through a similar argument to the proof of Lemma 2, we know that is positive as is small enough. Therefore there exists such that which implies . Thus we have and by (32), , where
Clearly, is strictly increasing on . Therefore by (27),
This implies that and .

Finally, we will prove is a nontrivial solution to system (13).

Since is an energy minimizer on Nehari manifold , there exists a Lagrange multiplier such that
for any . Let in (38). implies that
but
Thus, and
for any . Take and in (41) for , where
We see that . Thus, is a nontrivial solution to system (13). The proof is completed.

By Theorem 4, the system (1) has a nontrivial solution. However, it is uncertain if two components of this solution are nonzero. Therefore, we want to find solutions of the system (1) which have both of the components not identically zero. In order to achieve this goal, we consider the system (1) with ; that is, In system (43), we know that , where , is given by (11). By the definition of , in Section 2 of this paper, we obtain that . Hence, . For the sake of simplicity, we let , , and . The notations in Section 2, such as are the same.

Now, we give the second result of this paper as follows.

Theorem 5. *Assume that , , and (10) holds. Then system (43) has a nontrivial standing wave solution in with and . *

*Proof. *By Theorem 4, we know that system (43) has a nontrivial standing wave solution in .

Now we will prove that and .

Since , we know that . If one of the components , say , then . For small enough, we consider ; by a similar argument to the proof of Lemma 2, we know that there exists such that ; that is, .

By (20) and , we have , where
and .

We noticed that and

For the sake of simplicity, we let
If , then
Thus, and (47) yields .

If , then by (45),
Thus, and (48) yields .

From the above arguments, if , then .

For small enough, we have
Hence, by (49), we have
This is a contradiction. So, .

Similarly, if and , then . The proof is completed.

#### Acknowledgments

This work is supported by Program for Changjiang Scholars and Innovative Research Team in University (no. IRT1226), the National Natural Science Foundation of China (no. 11171078), and the Specialized Fund for the Doctoral Program of Higher Education of China (no. 20114410110002).

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#### Copyright

Copyright © 2013 Meihua Huang and Zhan Zhou. 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.