# Positive solutions for a class of sublinear elliptic systems

Ruyun Ma*, Ruipeng Chen and Yanqiong Lu

Author Affiliations

Department of Mathematics, Northwest Normal University, Lanzhou, 730070, PR China

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Boundary Value Problems 2014, 2014:28  doi:10.1186/1687-2770-2014-28

 Received: 17 October 2013 Accepted: 9 January 2014 Published: 30 January 2014

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 paper, we are concerned with the existence of positive solutions of the semilinear elliptic system

where is a parameter, is a continuous real function for each . Under some appropriate assumptions, we show that the above system has at least one positive solution in certain interval of λ. The proofs of our main results are based upon bifurcation theory.

MSC: 34B15, 34B18.

##### Keywords:
sublinear elliptic systems; positive solutions; eigenvalues; bifurcation theory

### 1 Introduction

Let Ω be a bounded smooth domain in (). In this paper, we study the existence of positive solutions of the semilinear elliptic system

(1.1)

where is a bifurcation parameter, is a continuous real function for each .

A solution of (1.1) is a pair . is called a positive solution of (1.1) if in Ω for each . In the following, also denotes the elements of .

The following definitions will be used in the statement of our main results.

Definition 1.1[1]

Let () be smooth real functions defined on . Define the Jacobian of the vector field as

If () for all , then the semilinear elliptic system

(1.2)

is said to be cooperative. Similarly, H is called a cooperative matrix.

Definition 1.2[2]

An matrix A is reducible if for some permutation matrix Q,

where B and D are square matrices, and is the transpose of Q. Otherwise, A is irreducible.

In the past few years, the existence of positive solutions to sublinear semilinear elliptic systems with two equations have been extensively studied, see for example, [3-6] and the references therein. The sublinear condition plays an important role. Very recently, Wu and Cui [1] considered the existence, uniqueness and stability of positive solutions to the sublinear elliptic system (1.1). By using bifurcation theory and the continuation method, they proved the following.

Theorem AAssume that

(H1) Each () is a smooth real function defined onsatisfying.

(H2) , for all.

(H3) .

(i) If at least one of () is positive and matrixis irreducible, then (1.1) has a unique positive solutionfor all;

(ii) If, for eachand matrixis irreducible, then for some, (1.1) has no positive solution when, and (1.1) has a unique positive solutionfor.

Moreover, (in the first case, we assume) is a smooth curve so thatis strictly increasing inλ, andas.

We are interested in the existence of positive solutions of (1.1) under weaker assumptions. More concretely, we consider the existence of positive solutions of (1.1) from the following two aspects: (a) To obtain the counterpart of Theorem A(ii) under the weaker assumptions than those of [1]. In other words, we will not assume that () are smooth functions any more. (b) Furthermore, we will also consider the case that () may not exist. More precisely, the following two theorems, which are the main results of the present paper, shall be proved.

Theorem 1.1Suppose that

(A1) () are continuous real functions satisfying

(A2) There exist constantssuch that

(A3) , .

If the matrixis irreducible, then there existssuch that (1.1) has no positive solution forand (1.1) has at least one positive solution for.

Theorem 1.2Let (A1) and (A3) hold. Assume the following.

(A2)′ For each, there existsuch that

(A4) The matrixis irreducible, where.

Then for some, (1.1) has at least one positive solution for.

Remark 1.1 It follows from (A2) and (A2)′ that the matrices and are all cooperative.

Remark 1.2 We note that our assumptions in Theorems 1.1 and 1.2 are weaker than those of Theorem A, and, accordingly, our results are weaker than Theorem A. Since we just suppose that is continuous, we can only obtain the continua of positive solutions of (1.1) by applying bifurcation techniques, which are not necessarily curves of positive solutions, and thus the uniqueness and stability of positive solutions are not investigated. In [1], the authors obtained a smooth curve consisting of positive solutions of (1.1) by assuming stronger assumptions, under which the uniqueness and stability of positive solutions can be achieved.

Remark 1.3 For related results, established via bifurcation techniques, for other kind of problems, we refer the readers to [7-9] and the references therein.

The rest of the paper is arranged as follows. In Section 2, we recall some basic knowledges on the maximum principle of cooperative systems as well as the eigenvalues of cooperative matrices. Finally in Section 3, we prove our main results Theorems 1.1 and 1.2 by applying bifurcation theory.

### 2 Preliminaries

We shall essentially work in Banach space , here

The norm of will be defined as , where denotes the norm of . We use and for the standard Sobolev space. We use and to denote the null and the range space of a linear operator L, respectively.

Let be a solution of (1.1). Suppose that () are smooth real functions. Then we can deduce the eigenvalue problem

(2.1)

which can be rewritten as

(2.2)

where

(2.3)

Lemma 2.1[1,4]

Letandwith. Suppose thatL, Hare given as in (2.3), andHis cooperative and irreducible. Then we have the following:

(a) is a real eigenvalue of, whereis the spectrum of.

(b) For, there exists a unique (up a constant multiple) eigenfunction, andin Ω.

(c) For, the equationis uniquely solvable for any, andas long as.

(d) (Maximum principle) For, assume thatsatisfiesin Ω, onΩ, thenin Ω.

(e) If there existssatisfyingin Ω, onΩ, and eitheronΩ orin Ω, then.

For the results and proofs, see Proposition 3.1 and Theorem 1.1 of Sweers [10]. Moreover, from a standard compactness argument, there are countably many eigenvalues of , and as . We notice that () are not necessarily real-valued.

In this section, we also need to consider the adjoint operator of . Let the transpose matrix of H be

Then it is clear that the results in Lemma 2.1 are also true for the eigenvalue problem

which is equivalent to

(2.4)

where . It is easy to verify that is the adjoint operator of , while both are considered as operators defined on subspaces of .

The following lemmas are crucial in the proof of our main results.

Lemma 2.2[1]

LetY, Z, LandHbe the same as in Lemma 2.1. Then the principal eigenvalueofis also a real eigenvalue of, , and for, there exists a unique eigenfunctionof (up a constant multiple), andin Ω.

Lemma 2.3 [[11], Theorem 5.3.1]

LetmatrixAbe a nonnegative irreducible matrix. Thenis a simple eigenvalue ofA, associated to a positive eigenvector, wheredenotes the spectral radius ofA. Moreover, .

Lemma 2.4[12]

LetVbe a real Banach space. Suppose that

is completely continuous andfor all. Let () such thatis the isolated solution of the equation

(2.5)

Furthermore, assume that

whereis an isolated neighborhood of trivial solutions. Let

Then there exists a continuum (i.e., a closed connected set) ofcontaining, and either

(i) is unbounded in; or

(ii) .

Finally, letbe the principal eigen-pair of the linear eigenvalue problem

(2.6)

such thatin Ω and.

### 3 Proof of the main results

Proof of Theorem 1.1. We extend each to be a nonnegative continuous function, which is still denoted by , defined on ℝ in the following way: if , then .

Let us define

(3.1)

where . Then it follows from (A1) that is continuous, and is always a solution of (1.1). Moreover, (A2) implies that F is differentiable at , and

(3.2)

where . By (A2), all entries of J are positive. Therefore Lemma 2.3 yields the result that J has a positive principal eigenvalue , the corresponding eigenvector satisfying (). Moreover, it is not difficult to verify that

(3.3)

where . This implies that is a positive eigenvector of the operator . Similarly, has the same principal eigenvalue and the corresponding eigenvector is , where () is a positive constant. Obviously,

Hence when , is not invertible and is a potential bifurcation point. More precisely, the null space

is one dimensional. In addition, it is easy to see that and exist for .

We divide the rest of the proof into two steps.

Step 1. We show that is actually a bifurcation point.

Indeed, the proof of this is similar to the proof of Theorem A(ii), we state it here for the readers’ convenience.

Suppose . Then there exists such that

(3.4)

Let us consider the adjoint eigenvalue equation

(3.5)

where , . Multiplying the system (3.4) by , multiplying the system (3.5) by , integrating on Ω and subtracting, then we obtain

(3.6)

Thus if and only if (3.6) holds, which implies that is one dimensional.

Next, we verify that

(3.7)

Otherwise, we have

(3.8)

Since

(3.9)

multiplying the system (3.9) by and using (3.8), we can get a contradiction that

By using [[13], Theorem 1.7], we conclude that is a bifurcation point. Furthermore, by the Rabinowitz global bifurcation theorem [14], there exists a continuum of positive solutions of (1.1), which joins to infinity in . Clearly,

(3.10)

since (1.1) has only the trivial solution when .

Step 2: We claim that cannot blow up at some finite .

Otherwise, a sequence can be taken such that

(3.11)

where . Let be the Green operator of −Δ subject to Dirichlet boundary conditions, i.e., if and only if

By the elliptic regularity, satisfies

(3.12)

Here also denotes the Nemytski operator generated by itself. Clearly, (3.12) is equivalent to

(3.13)

where . It is well known that is continuous and compact, and so is continuous and compact on .

Let (). Then in Ω and . Dividing both sides of (3.13) with , we have

(3.14)

For each , from (A2) and (A3) it follows that is bounded in . Moreover, we have

(3.15)

Therefore,

is bounded in X. This together with the compactness of implies that has a subsequence, denoted by itself, satisfying, in X,

Obviously, () in Ω and . In addition, we have . Or else, let , then by (3.14) we get () in Ω, which contradicts .

We define

(3.16)

Then for each ,

by Lebesgue control convergence theorem, we get

which together with (3.15) yields

(3.17)

On the other hand, we know from (A2) and (3.15) that

(3.18)

Hence we conclude from (3.17) and (3.18) that

(3.19)

Now, let in (3.14), using (3.19) and the fact that we can obtain

Finally, by (3.10), the connectness of and above arguments, we can find some such that (1.1) has no positive solution for , and (1.1) has at least one positive solution for . □

To prove Theorem 1.2, we need the following lemmas as required.

By Remark 1.1 and Lemma 2.3, the matrices and have the principal eigenvalues and , respectively, and the corresponding positive eigenvectors are and . Moreover, it is easy to obtain

(3.20)

and

(3.21)

where is given as in (2.6). By Lemma 2.2, the matrices and have principal eigenvalues and , respectively, the associated positive eigenvectors are and . We can easily verify that

(3.22)

and

(3.23)

Let be the closure of the set of positive solutions to (1.1). We extend each to be a function defined on ℝ by

(3.24)

then on ℝ. Let be a solution of

(3.25)

Then by (3.24), for each ,

where K is given as in the proof of Theorem 1.1. Hence is a nonnegative solution of (3.25). Moreover, from (3.24) it follows that is a solution of (1.1). On the other hand, (3.25) has no half-trivial solutions. Otherwise, U must have trivial and nontrivial components, and so there is a such that in Ω, and by the maximum principle of elliptic boundary value problems, we have

which is a contradiction. Therefore, the closure of the set of nontrivial solutions of (3.25) is exactly Σ.

In the following, we shall apply the Leray-Schauder degree theory, mainly to the mapping ,

(3.26)

where is given as in (3.13), and

is the associated Nemytski operator. For , let , let denote the degree of on with respect to 0.

Lemma 3.1Letbe a compact interval with. Then there exists asuch that

Proof Suppose on the contrary that there exist sequences and so that

(3.27)

(3.28)

Apparently, (3.27) is equivalent to

(3.29)

and () in Ω, and therefore , . Furthermore, it follows from (A2)′ that, for sufficiently small, . This together with (3.28) implies that, for k large enough,

(3.30)

Multiplying (3.29) by , multiplying (3.22) by , integrating on Ω and adding, using (3.30) and the fact , , we know that, for k large enough,

(3.31)

and so for k sufficiently large. Similarly, by (3.23) and (3.30), we can deduce that for k large enough. Consequently, for k sufficiently large we get , which contradicts . □

Corollary 3.2Forand, .

Proof Lemma 3.1, applied to the interval , guarantees the existence of such that, for ,

Hence for any ,

□

Lemma 3.3Suppose that. Then there existssuch that

where.

Proof Suppose on the contrary that there exist and a sequence with and such that

which is

(3.32)

Clearly, () in Ω. Multiplying (3.22) by , multiplying (3.32) by , integrating over Ω and adding, then by (3.30) we know that, for k large enough,

Hence for k sufficiently large, which contradicts . □

Corollary 3.4Forand, .

Proof Let , where is the constant given as in Lemma 3.3. Since is bounded in , there exists a constant such that , . By Lemma 3.3, we get

Hence,

□

Proof of Theorem 1.2. For such that , let , and . For any , it is easy to see that the assumptions of Lemma 2.4 are all satisfied. Therefore there exists a continuum of solutions of (3.25) containing , and either

(i) is unbounded in ; or

(ii) .

By Lemma 3.1, the case (ii) cannot occur, and hence is unbounded bifurcated from . Note that (3.25) has only trivial solutions when , and therefore . Moreover, from Lemma 3.1 it follows that for a closed interval , if , then in X is impossible. Thus must be bifurcated from . Finally, applying similar methods to the proof of Step 2 of Theorem 1.1, we can show that

Consequently, (1.1) has at least one positive solution for . □

### Competing interests

The authors declare that they have no competing interests.

### Authors’ contributions

RM and RC completed the main study, carried out the results of this article and drafted the manuscript. YL checked the proofs and verified the calculation. All the authors read and approved the final manuscript.

### Acknowledgements

The authors are very grateful to the anonymous referees for their valuable suggestions. This work was supported by the NSFC (No. 11361054, No. 11201378), SRFDP(No. 20126203110004), Gansu provincial National Science Foundation of China (No. 1208RJZA258).

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