# Uniqueness of positive solutions to a class of semilinear elliptic equations

- Chunming Li
^{1}and - Yong Zhou
^{1}Email author

**2011**:38

**DOI: **10.1186/1687-2770-2011-38

© Li and Zhou; licensee Springer. 2011

**Received: **2 June 2011

**Accepted: **24 October 2011

**Published: **24 October 2011

## Abstract

In this article, we consider the uniqueness of positive radial solutions to the Dirichlet boundary value problem

where Ω denotes an annulus in ℝ^{
n
}(*n* ≥ 3). The uniqueness criterion is established by applying shooting method.

### Keywords

positive solution semilinear elliptic equation uniqueness## 1 Introduction

where Ω: = {*x* | *x* ∈ ℝ^{
n
}, *a* < |*x*| < *b*}, *a* and *b* are positive real numbers, *f* ∈ *C*^{1}((0, + ∞) × [0, + ∞)) and *g* : [0, + ∞) → ℝ is differentiable. Equation 1.1 describes stationary states for many reaction-diffusion equations. The absence of positive solutions to the elliptic equations also means that the existing solutions oscillate, which is also important information in applications.

*g*(|

*x*|) =

*0*, i.e.,

When the nonlinear term just depends on *u*, the uniqueness of (1.2) has been exhaustively studied (see [1–6]). In 1985, the uniqueness of (1.2) was discussed in different domains by Ni and Nussbaum [7] to the case when *f* depends on |*x*| and *u, f*(|*x*|,*u*) > 0 and *f*(|*x*|,*u*) satisfies some growth conditions. Erbe and Tang [8] presented a new uniqueness criterion using a shooting method and Sturm comparison theorem.

So far it seems that nobody considers the uniqueness to problem (1.1). Inspired by the above articles, the aim of the present article is to establish some simple criteria for the uniqueness of positive radial solutions to problem (1.1). Obviously, what we investigate in this article has a more general form than (1.2). Although due to technical reasons, when *g*(|*x*|) = 0 it does not hold in this article, there exist many other *g*(|*x*|) which satisfy our main result.

where *α* > 0. Our method is the Schauder-Tikhonov fixed point theory. The existence and uniqueness of this initial problem is important to prove our main result. In Section 3, we will give the proof of our main result, i.e., show the uniqueness of positive solutions to Equation 1.1, using a shooting method and Sturm theorem.

## 2 Preliminaries

*t*= |

*x*|. For giving a proof of uniqueness of problem (1.1), let us consider the initial problem

where $\alpha >0,h\left(t\right)=\frac{n-1}{t}+tg\left(t\right)$. We shall show that problem (2.1) has a unique positive solution. By a solution to problem (1.2), we mean *u* ∈ *C*^{
2
}and *u* > 0 for all *t* ∈ (*a, b*). First of all, we give a well-known lemma.

**Lemma 2.1** *(The Schauder-Tikhonov fixed point theorem* [9]*). Let × be a Banach space and K* ⊂ *X be a nonempty, closed, bounded and convex set. If the operator T* : *K* → *X continuously maps K into itself and T*(*K*) *is relatively compact in X, then T has a fixed point x* ∈ *K*.

**Theorem 2.1**

*If there exist m and M, such that*0 <

*m*≤

*u*≤

*M for u*∈

*C*([

*a, b*], (0, ∞))

*and*

*Then, Equation* 2.*1 has a unique positive solution*.

**Proof**. We assume that

*u*|| = sup

_{a≤t≤b}|

*u*(

*t*)|. Let

*T*:

*K*→

*X*, by

We shall apply the Schauder-Tikhonov theorem to prove that there exists a fixed point *u*(*t*), which is a positive solution of problem (2.1), for the operator *T* in the non-empty closed convex set *K.*

We shall do it by several steps as follows:

**Step 1**: Check that

*T*:

*K*→

*K*is well defined. Obviously, by (2.2), we have

thus *T* : *K* → *K* is well defined.

**Step 2**: Verify that

*T*:

*K*→

*K is*continuous. Note that

*h*(

*t*)

*, f*(

*t, u*) are continuous, they are integrable on [

*a, b*], there exists a constant

*M*

_{ 1 }such that

*f*(

*t,u*) is continuous, thus for ∀

*ε*> 0, there exists

*δ*> 0 such that for any

*u*(

*t*),

*v*(

*t*) ∈

*K*with ||

*u-v*|| ≤

*δ*,

Thus, *T* is continuous on *K.*

**Step 3**: We check that *T*(*K*) is relatively compact in *X.*

*TK*⊂

*K*,

*TK*is uniformly bounded. Now, verify that

*TK*is equicontinuous. Let

*u*∈

*K*, then we have

*M*

_{2}such that

*u*

_{ n }} ⊂

*K*, by the mean value theorem, we have

Thus, *TK* is equicontinuous. Arzela-Ascoli theorem [9] implies *TK* is relatively compact. Now, we have verified that *T* : *K* → *K* satisfies all assumptions of the Schauder-Tikhonov theorem. Thus, there exists a fixed point *u* which is a positive solution of problem (2.1).

*u*and

*v*are two different solutions of problem (2.1), then the function

*ψ*=

*f*(

*t,v*)

*- f*(

*t,u*). It follows that

*M*

_{3}is a constant, such that

*f*(

*t, u*) is Hölder continuous with respect to the second variable on (0, + ∞), we obtain, for appropriate values

*t*

_{0},

*L*>

*0*,

From this, we have $\left|\omega \left(t\right)\right|\phantom{\rule{2.77695pt}{0ex}}\le {M}_{3}L{\int}_{a}^{t}\left|\omega \left(s\right)\right|ds$ for *t* ≤ *t*_{0}. It now follows from Gronwall's inequality that *ω* ≡ 0 for *a* < *t* ≤ *t*_{0}, consequently *u*' ≡ *v'* for *t* ≤ *t*_{0}. We find *u*(*t*) ≡ *v*(*t*) for all *t* ∈ (*a,t*_{0}]. With the initial point *t*_{0} replace by *ρ* > *t*_{0}, for an appropriate value *ρ*, the same proof can be reapplied as often as necessary to give uniqueness of any continuation of the solution whose values lie in (*a, b*). The proof is complete.

## 3 Uniqueness

**Theorem 3.1**

*Assume that h*(

*t*)

*and f*(

*t,u*)

*for a*<

*t*<

*b, u*(

*t*) > 0,

*satisfy inequality*(

*2.2*)

*and*

*where* $v\left(t\right)={\int}_{a}^{t}{e}^{-{\int}_{a}^{r}h\left(s\right)ds}dr$, *then problem* (*1.1*) *has at most one positive radial solution*.

**Example 3.1**For the equation

*t*= |

*x*|, then

Therefore, Theorem 3.1 ensures that there exists at most one positive radial solution.

Before proving our main result, we will do some preliminaries and give some useful lemmas.

*u*(

*t,α*) denote the unique solution of Equation 2.1. If

*α*> 0, then the solution

*u*(

*t,α*) is positive for

*t*slightly larger than

*a*. When it vanishes in (

*a, b*), we define

*b*(

*α*) to be the first zero of

*u*(

*t, α*). More precisely,

*b*(

*α*) is a function of

*α*which has the property that

*u*(

*t, α*) > 0 for

*t*∈ (

*a, b*(

*α*)) and

*u*(

*b*(

*α*)

*, α*) = 0. Let

*N*denote the set of

*α*> 0 for which the solution

*u*(

*t, α*) has a finite zero

*b*(

*α*). The variation of

*u*(

*t, α*) is defined by

*L*be the linear operator given by

By (2.4), it is easy to show that *u*(*t, α*) has a unique critical point *c*(*α*) in (*a, b*(*α*)), and at this point, *u*(*t, α*) obtains a local maximum value.

**Lemma 3.1** *Assume that* (*F2*) *holds, then ϕ*(*t, α*) > 0 *for all t* ∈ (*a, c*(*α*)).

**Proof**. We introduce a function

*Q(t, α)*with respect to

*t*, we get

*F*2), we obtain

Since Q(*t*,*α*) > 0 in *t* ∈ (*a*,*c*(*α*)) and inequality (3.3) holds, by the Sturm comparison principle (see [2]), we see that *Q*(*t*,*α*) oscillates faster that *ϕ*(*t*,*α*). Hence, *ϕ*(*t*,*α*) has no zero in *t* ∈ (*a*,*c*(*α*)). From *ϕ*(*a*,*α*) = 0 and *ϕ'*(*a*,*α*) = 1, it follows that *ϕ*(*t, α*) > 0 for all *t* ∈ (*a, c*(*α*)). The proof is complete.

**Remark 3.1** Lemma 3.1 was already proved in [11]. Here we give a simpler proof, directly using Sturm comparison principle.

Now, we present a lemma which has been given to the case *g*(|*x*|) = 0 (see [8]). To make the article as self-contained as possible, we will give a simple proof with a slight modification to [8].

**Lemma 3.2** *Assume α* ∈ *N and f*(*t,u*) *satisfies* (*F* 1)*, then*

(*H* 1) *ϕ*(*t,α*) *vanishes at least once and at most finitely many times in* (*a,b*(*α*)),

(*H2*) *if* 0 < *α*_{1} < *α*_{2}*, and at least one of u*(*t,α*_{1}) *and u*(*t,α*_{2}) *has a finite zero, then they intersect in* (*a*,min{*b*(*α*_{1})*,b*(*α*_{2})}).

**Proof**. We shall prove this by contradiction. Suppose to the contrary that

*ϕ*(

*t, α*) does not vanish in (

*a, b*(

*α*)), then

*ϕ*(

*t, α*) > 0,

*t*∈ (

*a, b*(

*α*)). Note that

*L*(

*ϕ(t, α*)) = 0, so we have

*L*, we have

*u*(

*t, α*) and (3.5) by

*ϕ*(

*t, α*), then subtract the resulting identities and we have

*F1*), we have the right side of (3.6) is positive in (

*a, b*(

*α*)). The left side of (3.6) is then a strictly increasing function of

*t*in (3.6). We get

Thus, ${e}^{{\int}_{a}^{b\left(\alpha \right)}h\left(s\right)ds}\varphi \left(b\left(\alpha \right),\alpha \right){u}^{\prime}\left(b\left(\alpha \right),\alpha \right)>0$. However, it contradicts *u*'(*b*(*α*),*α*) < 0 and *ϕ*(*b*(*α*),*α*) ≥ 0.

Since the rest of proof can be completed by the same argument as [8], we omit them.

**Lemma 3.3** *If* (*F1*) *and* (*F* 3) *hold, then ϕ*(*b*(*α*)*, α*) ≠ 0.

**Proof**. We shall prove this by contradiction. Suppose to the contrary that

*ϕ*(

*b*(

*α*)

*, α*) = 0. Now, we may as well define

*τ*(

*α*) to be the last zero of

*ϕ*(

*t, α*) in (

*a, b*(

*α*)). By Lemma 3.1, it is easy to get

*c*(

*α*) ≤

*τ*(

*α*), thus

*u*'(

*τ*(

*α*),

*α*) ≤ 0 and

*u*'(

*t, α*) < 0 for all

*t*∈ (

*τ*(

*α*),

*b*(

*α*)]. We introduce a function

*G*(

*t, α*) with respect to

*t*, we get

*G*(

*t, α*), and (3.7) by

*ϕ*(

*t, α*) then we have

*ϕ*(

*b*(

*α*)

*, α*) = 0, thus integrating both sides of (3.8) from

*τ*(

*α*) to

*b*(

*α*), we obtain

*τ*(

*α*) to be the last zero of

*ϕ*(

*t, α*) in (

*a, b*(

*α*)), the behavior of

*ϕ*(

*t, α*) in (

*τ*(

*α*)

*, b*(

*α*)) can be classified into two cases as follows:

- (i)
*ϕ*(*t, α*) > 0 in (*τ*(*α*)*, b*(*α*)), then the left side of (3.9) is negative, but by (*F*3) the right side is positive.

- (ii)
*ϕ*(*t, α*) < 0 in (*τ*(*α*)*, b*(*α*)), then the left side of (3.9) is positive, but by (*F*3) the right side is negative.

It is also impossible. The proof is complete.

**The proof of Theorem 3.1**We will prove it as a standard process. Assume that

*N*is a nonempty set, otherwise we have nothing to prove. Let

*α*∈

*N*, then

*u*(

*b*(

*α*)

*, α*) = 0. It is easy to see that

*u'*(

*b*(

*α*)

*, α*) ≤ 0. If

*u'*(

*b*(

*α*)

*, α*) = 0, then the assumption

*f*(

*t*, 0) ≡ 0 for all

*t*≥ 0, and the uniqueness of solution of initial value problems for ordinary differential equations imply that

*u*(

*t, α*) ≡ 0 for all

*t*∈ [

*a, b*(

*α*)], which contradicts the initial condition of

*u*(

*t, α*). Hence, we have

and the implicit function theorem implies that *b*(*α*) is well-defined as a function of *α* in *N* and *b*(*α*) ∈ *C*^{1}(*N*). Furthermore, it follows from (3.10) that *N* is an open set. By Lemma 3.2, we have *N* is an open interval (see [8]).

*u*(

*b*(

*α*)

*, α*) = 0 with respect to

*α*, we obtain

From above Lemma 3.3, we have *ϕ*(*b*(*α*)*,α*) ≠ 0. Thus, *b*'(*α*) ≠ 0, *α* ∈ *N*. It means that *b*'(*α*) does not change sign, i.e., *b*(*α*) is monotone. The proof is complete.

**Remark 3.2**Actually, if the functions

*f*(|

*x*|,

*u*) and

*g*(|

*x*|) satisfy some suitable conditions, it is not difficult to get the existence of positive radial solutions to the Dirichlet boundary value problem (1.1). We just need that for Equation 2.1, the functions

*f*(|

*x*|,

*u*) and

*g*(|

*x*|) satisfy inequality (2.2) and

However, it seems that these assumptions are too strict.

## Declarations

### Acknowledgements

Li thanks Zhou for enthusiastic guidance and constant encouragement. The authors were very grateful to the anonymous referees for careful reading and valuable comments. This study was partially supported by the Zhejiang Innovation Project (Grant No. T200905), ZJNSF (Grant No. R6090109) and NSFC (Grant No. 10971197).

## Authors’ Affiliations

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