**Journal of Applied Mathematics and Physics**

Vol.03 No.10(2015), Article ID:60686,11 pages

10.4236/jamp.2015.310156

Existence and Multiplicity of Solutions for Quasilinear p(x)-Laplacian Equations in R^{N}

Honghong Qi, Gao Jia

College of Science, University of Shanghai for Science and Technology, Shanghai, China

Email: qihonghong618@126.com, gaojia89@163.com

Copyright © 2015 by authors and Scientific Research Publishing Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY).

http://creativecommons.org/licenses/by/4.0/

Received 10 September 2015; accepted 25 October 2015; published 28 October 2015

ABSTRACT

We establish some results on the existence of multiple nontrivial solutions for a class of p(x)-Lap- lacian elliptic equations without assumptions that the domain is bounded. The main tools used in the proof are the variable exponent theory of generalized Lebesgue-Sobolev spaces, variational methods and a variant of the Mountain Pass Lemma.

**Keywords:**

p(x)-Laplacian Operator, Generalized Lebesgue-Sobolev Spaces, Variational Method, Multiple Solutions

1. Introduction

The study of differential and partial differential equations involving variable exponent conditions is a new and interesting topic. The interest in studying such problem was stimulated by their applications in elastic mechanics and fluid dynamics. These physical problems were facilitated by the development of Lebesgue and Sobolev spaces with variable exponent.

The existence and multiplicity of solutions of -Laplacian problems have been studied by several authors (see for example [1] [2] , and the references therein).

In [3] , A. R. EL Amrouss and F. Kissi proved the existence of multiple solutions of the following problem

(1)

Also Xiaoyan Lin and X. H. Tang in [4] studied the following quasilinear elliptic equation

(2)

and they proved the multiplicity of solutions for problem (2) by using the cohomological linking method for cones and a new direct sum decomposition of.

In this paper, we consider the following problem

(3)

where is the -Laplacian operator; is a Lipschitz continuous function with

is a given continuous function which satisfies

(B_{0})

here m is the Lebesgue measure on R^{N}.

is a Carathéodory function satisfying the subcritical growth condition

(F_{0})

for some, where, , , and

Define the subspace

and the functional,

where.

Clearly, in order to determine the weak solutions of problem (3), we need to find the critical points of functional Φ. It is well known that under (B_{0}) and (F_{0}), Φ is well defined and is a C^{1} functional. Moreover,

for all.

If for a.e., the constant function is a trivial solution of problem (3). In the following, the key point is to prove the existence of nontrivial solutions for problem (3).

Set

(4)

This paper is to show the existence of nontrivial solutions of problem (3) under the following conditions.

(F_{1})

where as given in (4).

(F_{2}) There exist
and, such that

(F_{3}) There exist
and
such that

for a.e..

(F_{4})
as
and uniformly for, with. Here
is given in the condition (F_{0}).

We have the following results.

Theorem 1.1. If
satisfies (B_{0}),
satisfies (F_{0}), (F_{1}) and (F_{2}), then problem (3) possesses at least one nontrivial solution.

Theorem 1.2. Assume
satisfies (B_{0}),
satisfies (F_{0}), (F_{3}) and (F_{4}), with
for a.e., then problem (3) has at least two nontrivial solutions, in which one is non-negative and another is non-positive.

This paper is divided into three sections. In the second section, we state some basic preliminary results and give some lemmas which will be used to prove the main results. The proofs of Theorem 1.1 and Theorem 1.2 are presented in the third section.

2. Preliminaries

In this section, we recall some results on variable exponent Sobolev space and basic properties of the variable exponent Lebesgue space, we refer to [5] -[8] .

Let,. Define the variable exponent Lebesgue space:

For, we define the following norm

Define the variable exponent Sobolev space:

which is endowed with the norm

It can be proved that the spaces and are separable and reflexive Banach spaces. See [9] for the details.

Proposition 2.1. [10] [11] Let

Then we have

1) For,;

2),

3)

4),

For with, let satisfy

We have the following generalized Hölder type inequality.

Proposition 2.2. [9] [12] For any and, we have

We consider the case that
satisfies (B_{0}). Define the norm

Then is continuously embedding into as a closed subspace. Therefore, is also a separable and reflexive Banach space.

Similar to the Proposition 2.1, we have

Proposition 2.3. [13] The functional defined by

has the following properties:

1);

2)

3)

Lemma 2.4. [13] If
satisfies (B_{0}), then

1) we have a compact embedding;

2) for any measurable function with, we have a compact embedding

. Here means that.

Now, we consider the eigenvalues of the p(x)-Laplacian problem

(5)

For any, define by

For all, set

then is a submanifold of E since t is a regular value of H. Put

where is the genus of I.

Define

We denote by the eigenpair sequences of problem (5) such that

Define

, where

Lemma 2.5. For all, let be an eigenfunction associated with of the problem (5). Then,

Proof. Let. From the definition of, it is easy to see that.

On the other hand, since the functional is coercive and weakly lower semi-continuous and is weakly closed subset of E, there exists such that. Letting, then and. Thus the lemma follows. ,

Lemma 2.6.

Proof. From Lemma 2.5, we have

Since

so we have Then,

Thus we get and.

Similarly, if is the eigenfunction associated with, we get and. Finally, we obtain

On the other hand, it is easy to see that Thus the lemma follows. ,

Now, we consider the truncated problem

where

We denote by and the positive and negative parts of u.

Lemma 2.7.

1) If then and

2) The mappings are continuous on E.

Lemma 2.8. All solutions of (resp.) are non-positive (resp. non-negative) solutions of problem (3).

Proof. Define

where
From Lemma 2.7 and (F_{0}),
is well defined on E, weakly lower semi-con- tinuous and C^{1}-functionals.

Let u be a solution of. Taking in

we have

By virtue of Proposition 2.3, we have, so and, a.e., then u is also a critical point of the functional Φ with critical value.

Similarly, the nontrivial critical points of the functional are non-negative solutions of problem. ,

3. Proof of Main Results

3.1. Proof of Theorem 1.1

To derive the Theorem 1.1, we need the following results.

Proposition 3.1. Φ is coercive on E.

Proof. Put

From (F_{1}) we have, for any, there is
such that

By contradiction, let and such that

(6)

Putting, one has. For a subsequence, we may assume that for some, we have

weakly in E and strongly in.

Consequently,. Let, via the result above we have and

Set

then,

From (6), (F_{1}) and Lemma 2.6, we deduce that

This is a contradiction. Therefore, Φ is coercive on E. ,

Proposition 3.2. Assume
satisfies (F_{0}) and (F_{2}), then zero is local maximum for the functional,
,.

Proof. From (F_{2}), there is a constant
such that

(7)

From (F_{0}) and, there exists
such that

(8)

By (7) and (8), we get

(9)

For we have

Since, there is a such that

(10)

Thus the proposition follows. ,

Proof of Theorem 1.1. From Proposition 3.1, we know Φ is coercive on E. Since Φ has a global minimizer on E, Φ is weakly lower semi-continuous and, then, in order to prove, we need to prove. So we have the Theorem 1.1 following from Proposition 3.2. ,

3.2. Proof of Theorem 1.2

To find the properties of the p(x)-Laplacian operators, we need the following inequalities (see [10] ).

Lemma 3.3. For
and
in R^{N}, then there are the following inequalities

Proposition 3.4. Assume (F_{0}), and let
be a sequence such that
in E and
for all
as, then, for some subsequences,
, a.e. in R^{N}, as
and
for all.

Proof. Let and such that

Let us denote by the following sequence

From Lemma 3.3, we have and

Recalling that in E, we get

and so,

(11)

On the other hand, by (11) and we obtain

Thus,

Combining Hölder’s inequality and Sobolev embedding, we deduce that

(12)

Let us consider the sets

From Lemma 3.3, we get

(13)

(14)

Applying again Hölder’s inequality,

(15)

where

and

Then,

(16)

From (12) and (13), we have

(17)

By (15)-(17), we obtain

(18)

(12) and (14) imply that

(19)

From (18) and (19), a.e. in. Because R is arbitrary, it follows that for some subsequence

.

Combined with Lebesgue’s dominated convergence theorem, we get

(20)

By (20) and, we derive that for all. ,

Proposition 3.5. Assume (F_{0}), and let
and
be a (PS)_{d} sequence in E for
then
is bounded in E.

Proof. From (F_{0}), we have
It is clear that

Assume that for some, then, by Proposition 2.3, Hölder’s inequality and Sobolev embedding, we have

(21)

Since and, (21) implies that is bounded in E. ,

Proposition 3.6. Assume
satisfies (B_{0}),
satisfies (F_{0}) and (F_{4}), and let
be a (PS)_{d} sequence in E, then
satisfies the (PS) condition.

Proof. From Proposition 3.4, we have

(22)

By Lemma 2.4, we get

(23)

On the other hand, Lebesgue’s dominated convergence theorem and the weak convergence of to u in E show

(24)

Moreover, since are bounded in, then we have

Therefore, by virtue of the definition of weak convergence, we obtain

(25)

By (23)-(25), we have

(26)

By (22) and (26), we get

Then combining Lemma 3.3, we obtain

which imply that in E. ,

Proposition 3.7. There exist and such that, for all with.

Proof. The conditions (F_{0}) and (F_{4}) imply that

For small enough, combined with Proposition 2.3, we have

(27)

By the condition (F_{0}), it follows

from Lemma 2.4, which implies the existence of such that

(28)

Using (28) and Proposition 2.1, we deduce

Combining (27), it results in that

here are positives constants. Taking such that we obtain

Since, the function is strictly positive in a neighborhood of zero. It follows that there exist and such that

,

Proposition 3.8. If and, we have for a certain.

Proof. From the condition (F_{3}), we get

For and, we have

Since, we obtain

,

Proof of Theorem 1.2. According to the Mountain Pass Lemma, the functional has a critical point with. But, , that is, , a.e.. Therefore, the problem has a nontrivial solution which, by Lemma 2.8, is a non-positive solution of the problem (3).

Similarly, for functional, we still can show that there exists furthermore a non-negative solution. The proof of Theorem 1.2 is now complete. ,

Acknowledgements

This work has been supported by the Natural Science Foundation of China (No. 11171220) and Shanghai Leading Academic Discipline Project (XTKX2012) and Hujiang Foundation of China (B14005).

Cite this paper

HonghongQi,GaoJia, (2015) Existence and Multiplicity of Solutions for Quasilinear p(x)-Laplacian Equations in R^{N}. *Journal of Applied Mathematics and Physics*,**03**,1270-1281. doi: 10.4236/jamp.2015.310156

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