## Boundary Value Problems

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# Existence of nontrivial solutions for perturbed p-Laplacian system involving critical nonlinearity and magnetic fields

Boundary Value Problems20132013:11

DOI: 10.1186/1687-2770-2013-11

Accepted: 9 January 2013

Published: 24 January 2013

## Abstract

Under the suitable assumptions, we establish the existence of nontrivial solutions for a perturbed p-Laplacian system in with critical nonlinearity and magnetic fields by using the variational method.

MSC:35B33, 35J60, 35J65.

### Keywords

p-Laplacian system critical nonlinearity magnetic fields variational method

## 1 Introduction

In this paper, we consider a class of quasi-linear elliptic systems of the form
(1.1)

where , i is the imaginary unit, is real vector potential, , is a non-negative potential, denotes the Sobolev critical exponent for and is a bounded positive coefficient.

The scalar case corresponding to (1.1) has received considerable attention in recent years. For and , the scalar case corresponding to (1.1) turns into
(1.2)
The equation (1.2) arises in finding standing wave solutions of the nonlinear Schrödinger equation
(1.3)
A standing wave solution of (1.3) is a solution of the form

Then solves (1.3) if and only if solves (1.2) with and .

The equation (1.2) has been extensively investigated in the literature based on various assumptions of the potential and the nonlinearity . See, for example, [115] and the references therein.

There are also many works dealing with the magnetic fields and for the scalar case corresponding to (1.1). In [16], the authors firstly obtained the existence of standing waves for special classes of magnetic fields. For many results, we refer the reader to [1722].

For general , most of the work, as we know, consider the scalar case which corresponds to (1.1) with . See [2327] and the references therein. We especially mention [24] for the existence of positive solutions for a class of p-Laplacian equations. Gloss [24] studied the existence and asymptotic behavior of positive solutions for quasi-linear elliptic equations of the form
(1.4)

where f is a subcritical nonlinearity without some growth conditions such as the Ambrosetti-Rabinowitz condition. The problem (1.4) has also been studied in [2832]. The main difficulty in treating this class of equation (1.4) is a possible lack of compactness due to the unboundedness of the domain.

However, to our best knowledge, it seems there is almost no work on the existence of non-trivial solutions to the problem (1.1) involving critical nonlinearity and magnetic fields. We mainly follow the idea of [7]. Observe that though the idea was used in other problems, the adaption of the procedure to the problem is not trivial at all. Because of the appearance of magnetic fields , we must deal with the problem for complex-valued functions and therefore we need more delicate estimates.

The outline of the paper is as follows. The forthcoming section is the main result and preliminary results including the appropriate space setting to work with. In Section 3, we study the behavior of sequence. Section 4 gets that the functional associated to the problem possesses the mountain geometry structure, and the last section concludes the proof of the main result.

## 2 Main results and preliminaries

Firstly, we make the following assumptions on , , and throughout the paper:

() , and there exists such that the set has finite Lebesgue measure;

() and ;

() , ;

() and as ;

() there exist and such that

() there are , and such that and .

Under the above mentioned conditions, we get the following result.

Theorem 1 Suppose that the assumptions (), (), () and ()-() hold. Then for any , there is such that if , the problem (1.1) has at least one solution which satisfies
Setting , the problem (1.1) is equivalent to the following problem:
(2.1)

We are going to prove the following result.

Theorem 2 Suppose that the assumptions (), (), () and ()-() hold. Then for any , there is such that if , the problem (2.1) has at least one solution which satisfies
(2.2)
For convenience, we quote the following notations. Let denote the Banach space
equipped with the norm

Set and for any .

Similar to the diamagnetic inequality [16], we have
(2.3)

(the bar denotes complex conjugation). This inequality shows that if , then and therefore for any . That is to say, if in , then in for any and a.e. in .

The energy functional associated with (2.1) is defined by

where .

Under the assumptions of Theorem 2, standard arguments [33] show that and its critical points are weak solutions of the equation (2.1).

## 3 condition

We call a sequence a sequence if and strongly in ( is the dual space of E). is said to satisfy the condition if any sequence contains a convergent subsequence.

The main result of Section 3 is the following compactness result.

Proposition 3.1 Let the assumptions of Theorem  2 be satisfied. There exists a constant independent of λ such that, for any sequence for with , either or .

As a consequence, we obtain the following result.

Proposition 3.2 Assume that the assumptions of Proposition  3.1 hold, satisfies the condition for all .

In order to prove Proposition 3.1, we need the following lemmas.

Lemma 3.1 Let the assumptions of Theorem  2 be satisfied. is a sequence of . Then and is bounded in the space E.

Proof

One has
Together with and as , we have

Then is bounded and . □

From Lemma 3.1, we may assume in E and in for any and , a.e. in .

Lemma 3.2 Let . There is a subsequence such that for any , there is with

where .

Proof The proof of Lemma 3.2 is similar to that of Lemma 3.2 of [27], so we omit it. □

Let be a smooth function satisfying , if and if . Define , . Obviously, we have
(3.1)
Lemma 3.3 One has
and

uniformly in with .

Proof The local compactness of Sobolev embedding implies that for any , we have
uniformly in . For any , there exists such that
for all . Together with the assumption () and the Hölder inequality, it follows from Lemma 3.2 that
where () are positive constants. Similarly, we can prove

□

Lemma 3.4 Let and be as defined above. Then the following conclusions hold:
and
Proof By using the similar arguments of [34, 35], we have
By (3.1) and the similar idea of proving the Brézis-Lieb lemma [36], it is easy to get
and
Furthermore, using the fact and , we obtain
In order to prove in , for any , it follows that
It is standard to check that
and
uniformly in with . Together with Lemma 3.3, we have

□

Let , , then , . From (3.1), we get in E if and only if in E.

Observe that
where . Furthermore, we get
(3.2)

Now, we consider the energy level of the functional below which the condition holds.

Let , where b is a positive constant in the assumption (). Since the set has finite measure, we get
(3.3)
In connection with the assumptions ()-() and the Young inequality, there exists such that
(3.4)
Let S be the best Sobolev constant of the immersion
Proof of Proposition 3.1 Assume that , then
and
By the Sobolev embedding inequality and the diamagnetic inequality, we get
This, together with (3.2), gives
Set , then

This completes the proof of Proposition 3.1. □

Proof of Proposition 3.2 Since , we have

In connection with and Proposition 3.1, we complete this proof. □

## 4 The mountain-pass structure

In the following, we always consider . We will prove that possesses the mountain-pass structure which has been carefully discussed in the works [37, 38].

Lemma 4.1 Let the assumptions of Theorem  2 be satisfied. There exist such that
Proof By (3.4), for any , there is such that
Thus,
In connection with , we may choose such that

The fact implies the desired conclusion. □

Lemma 4.2 Under the assumptions of Lemma  4.1, for any finite dimensional subspace , we have
Proof Together with the fact , we have

Since all norms in a finite-dimensional space are equivalent and , we complete the proof. □

In the following, we will find special finite-dimensional subspaces by which we establish sufficiently small mini-max levels.

Define the functional

Obviously, it follows that and for all .

Observe that
and

Then, for any , there are with and such that .

Set . Then . For , we get
where
By direct computation, we have
Since , and , we know that there is such that for all , we have
(4.1)
Lemma 4.3 For any , there is such that , there is with , and

where is defined in Lemma  4.1.

Proof This proof is similar to that of Lemma 4.3 in [7], so we omit the details. □

## 5 Proof of Theorem 2

Proof By using Lemma 4.3, for any with , we choose and define the mini-max level

where .

By Proposition 3.1, we know that satisfies the condition. Hence, by the mountain-pass theorem, there is such that and . This shows is a weak solution of (2.1).

Moreover, note that and . Then

Furthermore, together with the diamagnetic inequality, we prove that satisfies the estimate (2.2). The proof is complete. □

## Declarations

### Acknowledgements

The authors would like to appreciate the referees for their precious comments and suggestions about the original manuscript. This research is supported by the National Natural Science Foundation of China (11271364) and the Fundamental Research Funds for the Central Universities (2012QNA46).

## Authors’ Affiliations

(1)
Department of Mathematics, China University of Mining and Technology

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