Open Access Research

Existence of positive solutions for variable exponent elliptic systems

Samira Ala1*, Ghasem Alizadeh Afrouzi2, Qihu Zhang3 and Asadollah Niknam4

Author affiliations

1 Department of Mathematics, Sciences and Research, Islamic Azad University (IAU) Tehran, Iran

2 Department of Mathematics, Faculty of Mathematical Sciences, University of Mazandaran, Babolsar, Iran

3 Department of Mathematics and Information Science, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, China

4 Department of Mathematics, Ferdowsi University of Mashhad, Mashhad, Iran

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Citation and License

Boundary Value Problems 2012, 2012:37  doi:10.1186/1687-2770-2012-37


The electronic version of this article is the complete one and can be found online at: http://www.boundaryvalueproblems.com/content/2012/1/37


Received:30 October 2011
Accepted:3 April 2012
Published:3 April 2012

© 2012 Ala et al; licensee Springer.

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 consider the system of differential equations

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M1">View MathML</a>

where Ω ⊂ ℝN is a bounded domain with C2 boundary ∂Ω, 1 < p(x) ∈C1 <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M2">View MathML</a> is a function. <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M3','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M3">View MathML</a> is called p(x)-Laplacian. We discuss the existence of positive solution via sub-super solutions without assuming sign conditions on f(0), h(0).

MSC: 35J60; 35B30; 35B40.

Keywords:
positive solutions; p(x)-Laplacian problems; sub-supersolution

1. Introduction

The study of diferential equations and variational problems with variable exponent has been a new and interesting topic. It arises from nonlinear elasticity theory, electrorheological fluids, etc., (see[1-3]). Many results have been obtained on this kind of problems, for example [1,3-8]. In [7], Fan gives the regularity of weak solutions for differential equations with variable exponent. On the existence of solutions for elliptic systems with variable exponent, we refer to [8,9]. In this article, we mainly consider the existence of positive weak solutions for the system

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M4','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M4">View MathML</a>

where Ω ⊂ ℝN is a bounded domain with C2 boundary Ω, 1 < p(x) ∈ C1 <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M2">View MathML</a> is a function. The operator <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M3','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M3">View MathML</a> is called p(x)-Laplacian. Especially, if p(x) ≡ p (a constant), (P) is the well-known p-Laplacian system. There are many articles on the existence of solutions for p-Laplacian elliptic systems, for example [5,10]. Owing to the nonhomogeneity of p(x)-Laplacian problems are more complicated than those of p-Laplacian, many results and methods for p-Laplacian are invalid for p(x)-Laplacian; for example, if Ω is bounded, then the Rayleigh quotient

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M5','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M5">View MathML</a>

is zero in general, and only under some special conditions λp(x) > 0 (see [11]), and maybe the first eigenvalue and the first eigenfunction of p(x)-Laplacian do not exist, but the fact that the first eigenvalue λp > 0 and the existence of the first eigenfunction are very important in the study of p-Laplacian problems. There are more difficulties in discussing the existence of solutions of variable exponent problems.

Hai and Shivaji [10], consider the existence of positive weak solutions for the following p-Laplacian problems

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M6','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M6">View MathML</a>

the first eigenfunction is used to construct the subsolution of p-Laplacian problems success-fully. On the condition that λ is large enough and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M7','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M7">View MathML</a>

the authors give the existence of positive solutions for problem (I).

Chen [5], considers the existence and nonexistence of positive weak solution to the following quasilinear elliptic system:

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M8','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M8">View MathML</a>

the first eigenfunction is used to construct the subsolution of problem(II), the main results are as following

(i) If α, β ≥ 0, γ, δ > 0, θ = (p - 1 - α)(q - 1 - β) - γδ > 0, then problem (II) has a positive weak solution for each λ > 0;

(ii) If θ = 0 and pγ = q(p - 1 - α), then there exists λ0 > 0 such that for 0 < λ < λ0, then problem (II) has no nontrivial nonnegative weak solution.

On the p(x)-Laplacian problems, maybe the first eigenvalue and the first eigenfunction of p(x)-Laplacian do not exist. Even if the first eigenfunction of p(x)-Laplacian exist, because of the nonhomogeneity of p(x)-Laplacian, the first eigenfunction cannot be used to construct the subsolution of p(x)-Laplacian problems. Zhang [12] investigated the existence of positive solutions of the system

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M9">View MathML</a>

In this article, we consider the existence of positive solutions of the system

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M10','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M10">View MathML</a>

where p(x) ∈ C1 <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M2">View MathML</a> is a function, F(x, u, v) = [g(x)a(u) + f(v)], G(x, u, v) = [g(x)b(v) +h(u)], λ is a positive parameter and Ω ⊂ ℝN is a bounded domain.

To study p(x)-Laplacian problems, we need some theory on the spaces Lp(x)(Ω), W1,p(x)(Ω) and properties of p(x)-Laplacian which we will use later (see [6,13]). If Ω ⊂ ℝN is an open domain, write

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M11">View MathML</a>

Throughout the article, we will assume that:

(H1) Ω ⊂ ℝN is an open bounded domain with C2 boundary Ω.

(H2) p(x) ∈ C1 <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M2">View MathML</a> and 1 < p- ≤ p+.

(H3) a, b C1([0, )) are nonnegative, nondecreasing functions such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M12','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M12">View MathML</a>

(H4) f, h : [0, +) → R are C1, monotone functions, limu→+∞ f(u) = +, limu→+∞ h(u) = +, and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M13">View MathML</a>

(H5) g : [0, +) (0, +) is a continuous function such that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M14','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M14">View MathML</a>, and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M15','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M15">View MathML</a>

Denote

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M16','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M16">View MathML</a>

We introduce the norm on Lp(x)(Ω) by

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M17','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M17">View MathML</a>

and (Lp(x)(Ω), |.|p(x)) becomes a Banach space, we call it generalized Lebesgue space. The space (Lp(x)(Ω), |.|p(x)) is a separable, reflexive, and uniform convex Banach space (see [[6], Theorems 1.10 and 1.14]).

The space W1,p(x)(Ω) is defined by W1,p(x)(Ω) = {u Lp(x) : | u| Lp(x)}, and it is equipped with the norm

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M18','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M18">View MathML</a>

We denote by W01,p(x)(Ω) is the closure of <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M19','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M19">View MathML</a> in W1,p(x)(Ω). W1,p(x)(Ω) and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M20','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M20">View MathML</a> are separable, reflexive, and uniform convex Banach space (see [[6], Theorem 2.1] We define

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M21','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M21">View MathML</a>

then <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M22','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M22">View MathML</a> is a continuous, bounded, and strictly monotone operator, and it is a homeomorphism (see [[14], Theorem 3.1]).

If <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M23','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M23">View MathML</a> is called a weak solution of (P) if it satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M24','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M24">View MathML</a>

Define <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M25','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M25">View MathML</a>as

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M26','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M26">View MathML</a>

where l(x, u) is continuous on <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M27','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M27">View MathML</a>, and l(x, .) is increasing. It is easy to check that A is a continuous bounded mapping. Copying the proof of [15], we have the following lemma.

Lemma 1.1. (Comparison Principle). Let u, v W1,p(x)(Ω) satisfying Au - Av ≥ 0 in <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M28','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M28">View MathML</a>. If <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M29','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M29">View MathML</a>(i.e., u ≥ v on ∂Ω ), then u ≥ v a.e. in Ω.

Here and hereafter, we will use the notation d(x, Ω) to denote the distance of x ∈ Ω to the boundary of Ω.

Denote d(x) = d(x, Ω) and <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M30','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M30">View MathML</a>. Since Ω is C2 regularly, then there exists a constant δ ∈ (0, 1) such that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M31','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M31">View MathML</a>, and |d(x)| ≡ 1.

Denote

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M32','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M32">View MathML</a>

Obviously, <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M33','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M33">View MathML</a>. Considering

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M34','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M34">View MathML</a>

(1)

we have the following result

Lemma 1.2. (see [16]). If positive parameter η is large enough and w is the unique solution of (1), then we have

(i) For any θ∈ (0, 1) there exists a positive constant C1 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M35','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M35">View MathML</a>

(ii) There exists a positive constant C2 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M36','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M36">View MathML</a>

2. Existence results

In the following, when there be no misunderstanding, we always use Ci to denote positive constants.

Theorem 2.1. On the conditions of (H1) - (H5), then (P) has a positive solution when λ is large enough.

Proof. We shall establish Theorem 2.1 by constructing a positive subsolution (Φ1, Φ2) and supersolution (z1, z2) of (P), such that Φ1 ≤ z1 and Φ2 ≤ z2. That is (Φ1, Φ2) and (z1, z2) satisfies

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M37','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M37">View MathML</a>

and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M38','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M38">View MathML</a>

for all <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M39','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M39">View MathML</a> with q ≥ 0. According to the sub-supersolution method for p(x)-Laplacian equations (see [16]), then (P) has a positive solution.

Step 1. We construct a subsolution of (P).

Let σ ∈ (0, δ) is small enough. Denote

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M40','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M40">View MathML</a>

It is easy to see that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M41','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M41">View MathML</a>. Denote

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M42','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M42">View MathML</a>

By computation

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M43','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M43">View MathML</a>

From (H3) and (H4), there exists a positive constant M > 1 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M44','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M44">View MathML</a>

Let <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M45','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M45">View MathML</a>, then

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M46','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M46">View MathML</a>

(2)

If k is sufficiently large, from (2), we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M47','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M47">View MathML</a>

(3)

Let -λζ = , then

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M48','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M48">View MathML</a>

from (3), then we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M49','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M49">View MathML</a>

(4)

Since <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M50','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M50">View MathML</a>, then there exists a positive constant C3 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M51','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M51">View MathML</a>

If k is sufficiently large, let -λζ = , we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M52','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M52">View MathML</a>

then

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M53','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M53">View MathML</a>

Since ϕ (x) 0 and a, f are monotone, when λ is large enough, then we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M54','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M54">View MathML</a>

(5)

Obviously

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M55','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M55">View MathML</a>

(6)

Combining (4), (5), and (6), we can conclude that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M56','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M56">View MathML</a>

(7)

Similarly

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M57','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M57">View MathML</a>

(8)

From (7) and (8), we can see that (ϕ1, ϕ2) = (ϕ, ϕ) is a subsolution of (P).

Step 2. We construct a supersolution of (P).

We consider

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M58','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M58">View MathML</a>

where <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M59','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M59">View MathML</a>. We shall prove that (z1, z2) is a supersolution for (p).

For <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M60','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M60">View MathML</a> with q ≥ 0, it is easy to see that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M61','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M61">View MathML</a>

(9)

Since <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M62','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M62">View MathML</a>,when μ is sufficiently large, combining Lemma 1.2 and (H3), then we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M63','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M63">View MathML</a>

(10)

Hence

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M64','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M64">View MathML</a>

(11)

Also

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M65','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M65">View MathML</a>

By (H3), (H4), when μ is sufficiently large, combining Lemma 1.2 and (H3), we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M66','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M66">View MathML</a>

Then

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M67','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M67">View MathML</a>

(12)

According to (11) and (12), we can conclude that (z1, z2) is a supersolution for (P).

It only remains to prove that ϕ1 ≤ z1 and ϕ2 ≤ z2.

In the definition of v1(x), let <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M68','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M68">View MathML</a>. We claim that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M69','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M69">View MathML</a>

(13)

From the definition of v1, it is easy to see that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M70','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M70">View MathML</a>

and

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M71','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M71">View MathML</a>

It only remains to prove that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M72','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M72">View MathML</a>

Since <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M73','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M73">View MathML</a> then there exists a point <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M74','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M74">View MathML</a>such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M75','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M75">View MathML</a>

If v1(x0) - ϕ(x0) < 0, it is easy to see that 0 < d(x0) < δ, and then

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M76','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M76">View MathML</a>

From the definition of v1, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M77','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M77">View MathML</a>

It is a contradiction to ∇v1(x0) - ϕ(x0) = 0. Thus (13) is valid.

Obviously, there exists a positive constant C3 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M78','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M78">View MathML</a>

Since <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M79','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M79">View MathML</a>, according to the proof of Lemma 1.2, then there exists a positive constant C4 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M80','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M80">View MathML</a>

When <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M81','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M81">View MathML</a> is large enough, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M82','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M82">View MathML</a>

According to the comparison principle, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M83','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M83">View MathML</a>

(14)

From (13) and (14), when <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M84','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M84">View MathML</a> and λ ≥ 1 is sufficiently large, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M85','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M85">View MathML</a>

(15)

According to the comparison principle, when μ is large enough, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M86','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M86">View MathML</a>

Combining the definition of v1(x) and (15), it is easy to see that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M87','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M87">View MathML</a>

When μ ≥ 1 and λ is large enough, from Lemma 1.2, we can see that <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M88','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M88">View MathML</a> is large enough, then <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M89','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M89">View MathML</a> is large enough. Similarly, we have ϕ2 ≤ z2. This completes the proof. □

3. Asymptotic behavior of positive solutions

In this section, when parameter λ → +, we will discuss the asymptotic behavior of maximum of solutions about parameter λ, and the asymptotic behavior of solutions near boundary about parameter λ.

Theorem 3.1. On the conditions of (H1)-(H5), if (u, v) is a solution of (P) which has been given in Theorem 2.1, then

(i) There exist positive constants C1 and C2 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M90','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M90">View MathML</a>

(16)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M91','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M91">View MathML</a>

(17)

(ii) for any θ ∈ (0, 1), there exist positive constants C3 and C4 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M92','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M92">View MathML</a>

(18)

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M93','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M93">View MathML</a>

(19)

where μ satisfies (10).

Proof. (i) Obviously, when 2δ ≤ d(x), we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M94','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M94">View MathML</a>

then there exists a positive constant C1 such that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M95','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M95">View MathML</a>

It is easy to see

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M96','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M96">View MathML</a>

then

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M97','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M97">View MathML</a>

Similarly

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M98','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M98">View MathML</a>

Thus (16) and (17) are valid.

(ii) Denote

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M99','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M99">View MathML</a>

where θ ∈ (0, 1) is a positive constant, ρ ∈ (0, δ) is small enough.

Obviously, v3(x) ∈ C1ρ), By computation

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M100','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M100">View MathML</a>

where

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M101','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M101">View MathML</a>

Let <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M102','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M102">View MathML</a> where ρ > 0 is small enough, it is easy to see that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M103','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M103">View MathML</a>

where ρ > 0 is small enough, then we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M104','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M104">View MathML</a>

Obviously v3(x) ≥ z1(x) on Ωρ. According to the comparison principle, we have v3(x) ≥ z1 (x) on Ωρ. Thus

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M105','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M105">View MathML</a>

Let <a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M106','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M106">View MathML</a> when ρ > 0 is small enough, it is easy to see that

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M107','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M107">View MathML</a>

Similarly, when ρ > 0 is small enough, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M108','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M108">View MathML</a>

Obviously, when d(x) < σ, we have

<a onClick="popup('http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M109','MathML',630,470);return false;" target="_blank" href="http://www.boundaryvalueproblems.com/content/2012/1/37/mathml/M109">View MathML</a>

Thus (18) and (19) are valid. This completes the proof. □

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

All authors typed, read and approved the final manuscript.

Acknowledgements

The authors would like to appreciate the referees for their helpful comments and suggestions. The third author partly supported by the National Science Foundation of China (10701066 & 10971087).

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