Research

# Sub-super solutions for (p-q) Laplacian systems

Somayeh Haghaieghi1* and Ghasem Alizadeh Afrouzi2

### Author affiliations

1 Department of Mathematics, Science and Research Branch, Islamic Azad University, Tehran, Iran

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

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Boundary Value Problems 2011, 2011:52  doi:10.1186/1687-2770-2011-52

 Received: 13 August 2011 Accepted: 2 December 2011 Published: 2 December 2011

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

In this work, we consider the system:

{ - Δ p u = λ [ g ( x ) a ( u ) + f ( v ) ] in Ω - Δ q v = λ [ g ( x ) b ( v ) + h ( u ) ] in Ω u = v = 0 on Ω ,

where Ω is a bounded region in RN with smooth boundary ∂Ω, Δp is the p-Laplacian operator defined by Δpu = div (|∇u|p-2u), p, q > 1 and g (x) is a C1 sign-changing the weight function, that maybe negative near the boundary. f, h, a, b are C1 non-decreasing functions satisfying a(0) ≥ 0, b(0) ≥ 0. Using the method of sub-super solutions, we prove the existence of weak solution.

### 1 Content

In this paper, we study the existence of positive weak solution for the following system:

{ - Δ p u = λ [ g ( x ) a ( u ) + f ( v ) ] in Ω - Δ q v = λ [ g ( x ) b ( v ) + h ( u ) ] in Ω u = v = 0 on Ω , (1)

where Ω is a bounded region in RN with smooth boundary ∂Ω, Δp is the p-Laplacian operator defined by Δpu = div(|∇u|p-2 u), p, q > 1 and g(x) is a C1 sign-changing the weight function, that maybe negative near the boundary. f, h, a, b are C1 non-decreasing functions satisfying a(0) ≥ 0, b(0) ≥ 0.

This paper is motivated by results in [1-5]. We shall show the system (1) with sign-changing weight functions has at least one solution.

### 2 Preliminaries

(Al) lim f M ( h ( s ) ) 1 q - 1 s p - 1 = 0 as s → ∞, ∀M > 0

(A2) lim f (s) = lim h (s) = ∞ as s → ∞.

(A3) lim a ( s ) s p - 1 = l i m b ( s ) s q - 1 = 0 as s → ∞.

Let λp, λq be the first eigenvalue of -Δp, -Δq with Dirichlet boundary conditions and φp, φq be the corresponding positive eigenfunctions with ||φp||= ||φq||= 1.

Let m, δ, γ, μp, μq > 0 be such that

| φ p | p - λ p φ p m in Ω ¯ δ φ p μ p on Ω - Ω δ (2)

and

{ | φ q | q - λ q φ q m in Ω ¯ δ φ p μ p on Ω - Ω δ . (3)

Ω ¯ δ = { x Ω ; d ( x , Ω ) δ } .

We assume that the weight function g(x) take negative values in Ωδ, but it requires to be strictly positive in Ω-Ωδ. To be precise, we assume that there exist positive constants β and η such that g(x) ≥-β on Ω ¯ δ and g(x) ≥ η on Ω-Ωδ. Let s0 ≥ 0 such that ηa(s) + f (s) > 0, ηb(s) + h(s) > 0 for s > s0 and

f 0 = max { 0 , - f ( 0 ) } , h 0 = max { 0 , - h ( 0 ) } .

For γ such that γr-1 t > s0; t = min {αp, αq}, r = min{p, q} we define

A = max [ γ λ p η a ( γ 1 p - 1 α p ) + f ( γ 1 q - 1 α q ) , γ λ q η b ( γ 1 q - 1 α q ) + h ( γ 1 p - 1 α p ) ] B = min [ m γ β a ( γ 1 p - 1 ) + f 0 , m γ β b ( γ 1 q - 1 ) + h 0 ]

where α p = p - 1 p μ p p p - 1 and α q = q - 1 q μ q q q - 1 .

We use the following lemma to prove our main results.

Lemma 1.1 [6]. Suppose there exist sub and supersolutions (ψ1, ψ2) and (z1, z2) respectively of (1) such that (ψ1, ψ2) ≤ (z1, z2). then (1) has a solution (u, v) such that (u, v) ∈ [(ψ1, ψ2), (z1, z2)].

### 3 Main result

Theorem 3.1Suppose that (A1)-(A3) hold, then for every λ ∈ [A, B], system (1) has at least one positive solution.

Proof of Theorem 3.1 We shall verify that (ψ1, ψ2) is a sub solution of (1.1) where

ψ 1 = γ 1 p - 1 p - 1 p φ p p p - 1 ψ 2 = γ 1 q - 1 q - 1 q φ q q q - 1 .

Let W H 0 1 ( Ω ) with w ≥ 0. Then

Ω | ψ 1 | p - 2 ψ 1 w d x = γ Ω ( λ p φ p p - | φ p | p ) w d x (4)

Now, on Ω ¯ δ by (2),(3) we have

γ ( λ p φ p p - | φ p | p ) - m γ

Since λ B then

λ m γ β a ( γ 1 p - 1 ) + f 0 .

thus

γ ( λ p φ p p - | φ p | p ) - m γ λ - β a ( γ 1 p - 1 ) - f 0 λ g ( x ) a ( γ 1 p - 1 ) - f 0 λ g ( x ) a p - 1 p γ 1 p - 1 φ p 1 p - 1 + f q - 1 q γ 1 q - 1 φ q 1 q - 1

then by (4)

Ω ¯ δ | ψ 1 | p - 2 ψ 1 w d x Ω ¯ δ λ g ( x ) a p - 1 p γ 1 p - 1 φ p p p - 1 + f q - 1 q γ 1 q - 1 φ q q q - 1 w d x

A similar argument shows that

Ω ¯ δ | ψ 2 | q - 2 ψ 2 w d x Ω ¯ δ λ g ( x ) b q - 1 q γ 1 q - 1 φ q 1 q - 1 + h p - 1 p γ 1 p - 1 φ p 1 p - 1 w d x

Next, on Ω - Ω ¯ δ . Since λ A, then

λ γ λ p η a γ 1 p - 1 α p + f γ 1 q - 1 α q

so we have

γ ( λ p φ p p - | φ p | p ) γ λ p λ η a γ 1 p - 1 α p + f γ 1 q - 1 α q λ [ g ( x ) a ( ψ 1 ) + f ( ψ 2 ) ] , Ω - Ω δ ¯

Then by (4) on we have

- Δ p ψ 1 λ [ g ( x ) a ( ψ 1 ) + f ( ψ 2 ) ] on Ω - Ω δ ¯

A similar argument shows that

- Δ q ψ 2 λ [ g ( x ) b ( ψ 2 ) + h ( ψ 1 ) ]

We suppose that κp and κq be solutions of

- Δ p κ p = 1 in Ω κ p = 0 on Ω - Δ q κ q = 1 in Ω κ q = 0 on Ω

respectively, and μ'p = ||κp||κ, ||κq||κ = μ'q.

Let

( z 1 , z 2 ) = c μ p λ 1 p - 1 κ p , 2 h c λ 1 q - 1 1 q - 1 λ 1 q - 1 κ q .

Let W H 0 1 ( Ω ) with w ≥ 0.

For sufficient C large

μ p p - 1 [ | | g | | a ( C λ 1 p - 1 ) + f ( ( 2 h ( C λ 1 p - 1 ) ) 1 q - 1 λ 1 q - 1 μ q ) ] C p - 1 1

then

| z 1 | p - 2 z 1 w d x = λ C μ p p - 1 w d x λ | | g | | a ( C λ 1 p - 1 ) + f ( 2 h ( C λ 1 p - 1 ) ) 1 q - 1 λ 1 q - 1 μ q d x λ g ( x ) a ( C λ 1 p - 1 κ p μ p ) + f ( 2 h ( C λ 1 p - 1 ) ) 1 q - 1 λ 1 q - 1 κ q d x = [ g ( x ) a ( z 1 ) + f ( z 2 ) ] w d x

Similarly, choosing C large so that

| | g | | b 2 h C λ 1 p - 1 1 q - 1 λ 1 q - 1 μ q h C λ 1 p - 1 1

then

| z 2 | q - 2 z 2 w d x = 2 λ h C λ 1 p - 1 w d x λ | | g | | b ( z 2 ) + h ( z 1 ) w d x .

Hence by Lemma (1.1), there exist a positive solution (u, v) of (1) such that (ψ1, ψ2) ≤ (u, v) ≤ (z1, z2).

### Competing interests

The authors declare that they have no competing interests.

### Authors' contributions

SH has presented the main purpose of the article and has used GAA contribution due to reaching to conclusions. All authors read and approved the final manuscript.

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