Research

# Existence of a positive solution for quasilinear elliptic equations with nonlinearity including the gradient

Mieko Tanaka

Author Affiliations

Department of Mathematics, Tokyo University of Science, Kagurazaka 1-3, Shinjyuku-ku, Tokyo, 162-8601, Japan

Boundary Value Problems 2013, 2013:173  doi:10.1186/1687-2770-2013-173

 Received: 15 May 2013 Accepted: 10 July 2013 Published: 24 July 2013

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

We provide the existence of a positive solution for the quasilinear elliptic equation

in Ω under the Dirichlet boundary condition. As a special case (), our equation coincides with the usual p-Laplace equation. The solution is established as the limit of a sequence of positive solutions of approximate equations. The positivity of our solution follows from the behavior of as t is small. In this paper, we do not impose the sign condition to the nonlinear term f.

MSC: 35J92, 35P30.

##### Keywords:
nonhomogeneous elliptic operator; positive solution; the first eigenvalue with weight; approximation

### 1 Introduction

In this paper, we consider the existence of a positive solution for the following quasilinear elliptic equation:

where is a bounded domain with boundary Ω. Here, is a map which is strictly monotone in the second variable and satisfies certain regularity conditions (see the following assumption (A)). Equation (P) contains the corresponding p-Laplacian problem as a special case. However, in general, we do not suppose that this operator is -homogeneous in the second variable.

Throughout this paper, we assume that the mapAand the nonlinear termfsatisfy the following assumptions (A) and (f), respectively.

(A) , where for all , and there exist positive constants , , , , and such that

(i) ;

(ii) for every , and ;

(iii) for every , and ;

(iv) for every , ;

(v) for every , with .

(f) f is a continuous function on satisfying for every and the following growth condition: there exist , and a continuous function on such that

(1)

for every .

In this paper, we say that is a (weak) solution of (P) if

for all .

A similar hypothesis to (A) is considered in the study of quasilinear elliptic problems (see [[1], Example 2.2.], [2-5] and also refer to [6,7] for the generalized p-Laplace operators). From now on, we assume that, which is without any loss of generality as can be seen from assumptions (A)(ii), (iii).

In particular, for , that is, stands for the usual p-Laplacian , we can take in (A). Conversely, in the case where holds in (A), by the inequalities in Remark 3(ii) and (iii), we see that whence . Hence, our equation contains the p-Laplace equation as a special case.

In the case where f does not depend on the gradient of u, there are many existence results because our equation has the variational structure (cf.[1,4,8]). Although there are a few results for our equation (P) with f including ∇u, we can refer to [7,9] and [10] for the existence of a positive solution in the case of the -Laplacian or m-Laplacian (). In particular, in [9] and [7], the nonlinear term f is imposed to be nonnegative. The results in [7] and [10] are applied to the m-Laplace equation with an -superlinear term f w.r.t. u. Here, we mention the result in [9] for the p-Laplacian. Faria, Miyagaki and Motreanu considered the case where f is -sublinear w.r.t. u and ∇u, and they supposed that for some and . The purpose of this paper is to remove the sign condition and to admit the condition like for large as . Concerning the condition for f as , Zou in [10] imposed that there exists an satisfying as for the m-Laplace problem. Hence, we cannot apply the result of [10] and [9] to the case of as for and (admitting sign changes), but we can do our result if is large.

In [9], the positivity of a solution is proved by the comparison principle. However, since we are not able to do it for our operator in general, after we provide a non-negative and non-trivial solution as a limit of positive approximate solutions (in Section 2), we obtain the positivity of it due to the strong maximum principle for our operator.

#### 1.1 Statements

To state our first result, we define a positive constant by

(2)

which is equal to 1 in the case of (i.e., the case of the p-Laplacian) because we can choose . Then, we introduce the hypothesis (f1) to the function in (f) as t is small.

(f1) There exist and such that the Lebesgue measure of is positive and

(3)

where is the continuous function in (f) and is the first positive eigenvalue of the p-Laplacian with the weight function m obtained by

(4)

Theorem 1Assume (f1). Then equation (P) has a positive solution, where

andνdenotes the outward unit normal vector onΩ.

Next, we consider the case where A is asymptotically -homogeneous near zero in the following sense:

(AH0) There exist a positive function and such that

(5)

(6)

Under (AH0), we can replace the hypothesis (f1) with the following (f2):

(f2) There exist and such that (3) and the Lebesgue measure of is positive, where is the first positive eigenvalue of with a weight function m obtained by

(7)

Theorem 2Assume (AH0) and (f2). Then equation (P) has a positive solution.

Throughout this paper, we may assume that for every , and because we consider the existence of a positive solution only. In what follows, the norm on is given by , where denotes the usual norm of for (). Moreover, we denote .

#### 1.2 Properties of the map A

Remark 3 The following assertions hold under condition (A):

(i) for all , is maximal monotone and strictly monotone in y;

(ii) for every ;

(iii) for every ,

where and are the positive constants in (A).

Proposition 4 ([[3], Proposition 1])

Letbe a map defined by

for. ThenAis maximal monotone, strictly monotone and hasproperty, that is, any sequenceweakly convergent touwithstrongly converges tou.

### 2 Constructing approximate solutions

Choose a function . In this section, for such ψ and , we consider the following elliptic equation:

In [7], the case in the above equation is considered.

Lemma 5Suppose (f1) or (f2). Then there existssuch thatfor every, and.

Proof From the growth condition of and (3), it follows that

holds, where is a positive constant independent of . Therefore, for , we easily see that for every , and holds. □

Proposition 6Ifis a non-negative solution of () for, then. Moreover, for any, there exists a positive constantsuch thatholds for every.

Proof Set if , and in the case of , is an arbitrarily fixed constant. Let be a non-negative solution of () with (some ). For , choose a smooth increasing function such that if , if and if for some . Define for .

If , then by taking as a test function (note that is bounded), we have

(8)

due to Remark 3(iii) and . Putting , we see that provided (note ). Similarly, if , then , and if , then (note ). Thus, according to Young’s inequality, for every , there exists such that

(9)

where and (>1). As a result, because of , according to Hölder’s inequality and the monotonicity of with respect to r on , taking a and setting , we obtain

(10)

provided by (8) and (9), where , comes from the continuous embedding of into and is a positive constant independent of , ε and r. Consequently, Moser’s iteration process implies our conclusion. In fact, we define a sequence by and . Then, we see that holds if by applying Fatou’s lemma to (10) and letting . Here, we also see as . Therefore, by the same argument as in Theorem C in [4], we can obtain and for some positive constant D independent of and ε. □

Lemma 7Suppose (f1) or (f2). Ifis a solution of () for, then.

Proof Taking as a test function in (), we have

because of if and by Remark 3(iii). Hence, follows. Because Proposition 6 guarantees that , we have (for some ) by the regularity result in [11]. Note that because of and . In addition, Lemma 5 implies the existence of such that in the distribution sense. Therefore, according to Theorem A and Theorem B in [4], in Ω and on Ω, namely, . □

The following result can be shown by the same argument as in [[9], Theorem 3.1].

Proposition 8Suppose (f1) or (f2). Then, for every, () has a positive solution.

Proof Fix any and let be a Schauder basis of (refer to [12] for the existence). For each , we define the m-dimensional subspace of by . Moreover, set a linear isomorphism by , and let be a dual map of . By identifying and , we may consider that maps from to . Define maps and from to as follows:

for u, . We claim that for every , there exists such that in . Indeed, by the growth condition of f, Remark 3(iii) and Hölder’s inequality, we easily have

(11)

for every , where . This implies that is coercive on by . Set a homotopy for and . By recalling that is coercive on , we see that there exists an such that for every and because and the norm of are equivalent on . Therefore, we have

where is the identity map on , and denotes the degree on for a continuous map (cf.[13]). Hence, this yields the existence of such that , and so the desired is obtained by setting since is injective.

Because (11) with leads to the boundedness of by , we may assume, by choosing a subsequence, that converges to some weakly in and strongly in . Let be a natural projection onto , that is, for . Since and in , by noting that on for a map A defined in Proposition 4, we obtain

as , where we use the boundedness of , the growth condition of f and in . In addition, since is bounded, by the boundedness of , we see that as , whence as holds. As a result, it follows from the property of A that in as .

Finally, we shall prove that is a solution of (). Fix any and . For each , by letting in , we have

(12)

Since l is arbitrary, (12) holds for every . Moreover, the density of in guarantees that (12) holds for every . This means that is a solution of (). Consequently, our conclusion follows from Lemma 7. □

### 3 Proof of theorems

Lemma 9Let. Then

holds, whereis the positive constant defined by (2).

Proof Because of , there exist such that in . Thus, and in Ω. Hence, hold. Therefore, we have

(13)

in Ω by (ii) and (iii) in Remark 3 and Young’s inequality. □

Lemma 10Assume thatand let. Then

holds.

Proof First, we note that hold by the same reason as in Lemma 9. Applying Young’s inequality to the second term of the right-hand side in (14) (refer to (13) with ), we obtain

(14)

(15)

in Ω. Similarly, we also have

(16)

The conclusion follows from (15) and (16). □

Under (f1) or (f2), we denote a solution of () for each obtained by Proposition 8.

Lemma 11Assume (f1) or (f2). Let. Thenis bounded in.

Proof Taking as a test function in (), we have

by Remark 3(iii), the growth condition of f, Hölder’s inequality and the continuity of the embedding of into , where (<p) and is a positive constant independent of . Because of , this yields the boundedness of (). □

Lemma 12Assume (f1) or (f2). Thenandhold for every, wheredenotes the Lebesgue measure of Ω, and whereandare positive constants as in (A) and Lemma 5, respectively.

Proof Fix any and choose any . By taking as a test function, we obtain

(17)

by Lemma 5 and . On the other hand, by Remark 3(iii) and , we have

(18)

Therefore, (17) and (18) imply the inequality for every . As a result, by letting , our conclusion is shown. □

Lemma 13Assume (f2) and (AH0). Let. Ifinas, then

holds, whereis a continuous function as in (AH0).

Proof Note that hold (as in the proof of Lemma 9). Because we easily see that for every with some independent of u (see (6)), it is sufficient to show as . Here, we fix any . By the property of (see (6)) and because we are assuming that in as , we have for every provided sufficiently small . Therefore, for such sufficiently small , we obtain

because of by Lemma 12. Since is arbitrary, our conclusion is shown. □

#### Proof of Theorems

Let . Due to Proposition 6 and Lemma 11, we have for some independent of . Hence, there exist and such that and for every by the regularity result in [11]. Because the embedding of into is compact and by , there exists a sequence and such that and in as . If occurs, then by the same reason as in Lemma 7, and hence our conclusion is proved. Now, we shall prove by contradiction for each theorem. So, we suppose that , whence in as .

Proof of Theorem 1 Let be an eigenfunction corresponding to the first positive eigenvalue (cf.[14,15], it is well known that we can obtain φ as the minimizer of (4)), namely, φ is a positive solution of in Ω and on Ω. Since p-Laplacian is -homogeneous, we may assume that φ satisfies , and hence holds by taking φ as a test function. Choose satisfying (note that as in (f1)). Due to (f1), there exists a such that for every and . Since we are assuming in as , occurs for sufficiently large n. Then, for such sufficiently large n, according to Lemma 9, (1) and , we obtain

Proof of Theorem 2 Since holds, by the standard argument as in the p-Laplacian, we see that and it is the first positive eigenvalue of in Ω and on Ω. Therefore, by the well-known argument, there exists a positive eigenfunction corresponding to (we can obtain as the minimizer of (7)). Hence, by taking as a test function, we have . Thus, follows. Because is a solution of and is an eigenfunction corresponding to , according to Lemma 11 and Lemma 13 (note as in (AH0)), we obtain

(19)

as since we are assuming in , where we use the facts that and in Ω. Furthermore, by Fatou’s lemma and (3), we have

As a result, by taking a limit superior with respect to n in (19), we have . This is a contradiction. □

### Competing interests

The author declares that she has no competing interests.

### Acknowledgements

The author would like to express her sincere thanks to Professor Shizuo Miyajima for helpful comments and encouragement. The author thanks Professor Dumitru Motreanu for giving her the opportunity of this work. The author thanks referees for their helpful comments.

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