Home On a Kirchhoff Equation in Bounded Domains
Article Open Access

On a Kirchhoff Equation in Bounded Domains

  • Yisheng Huang and Yuanze Wu EMAIL logo
Published/Copyright: December 22, 2017

Abstract

In this paper, we consider the following Kirchhoff equation:

{ - ( a + b Ω | u | 2 d x ) Δ u = λ u + | u | p - 2 u in  Ω , u = 0 on  Ω ,

where ΩN (N3) is a bounded domain with smooth boundary Ω, 2<p<2*=2NN-2 is the Sobolev exponent and a, b, λ are positive parameters. By the variational method, we obtain some existence and multiplicity results of the sign-changing solutions (including the radial sign-changing solution in the case of Ω=𝔹R) for this problem. Some further properties of these sign-changing solutions, such as the numbers of the nodal domains, the concentration behaviors as b0+, the estimates of the energy values and so on, are also obtained. Our results generalize and improve some known results in the literature. Moreover, we also obtain a uniqueness result of the radial positive solution.

MSC 2010: 35B09; 35B33; 35J20

1 Introduction

1.1 Background

In this paper, we consider the following elliptic equation:

(Pabλ) { - ( a + b Ω | u | 2 d x ) Δ u = λ u + | u | p - 2 u in  Ω , u = 0 on  Ω ,

where ΩN (N3) is a bounded domain with smooth boundary Ω, 2<p<2*=2NN-2 is the Sobolev exponent and a, b, λ are positive parameters.

The elliptic equation (Pabλ) is a so-called Kirchhoff-type equation, since it contains a nonlocal operator u-(a+bΩ|u|2dx)Δu from H01(Ω) to L2(Ω). Such an operator was first proposed by Kirchhoff in 1883 as an extension of the classical D’Alembert’s wave equations for free vibration of elastic strings. Kirchhoff took into account the changes in length of the string produced by transverse vibrations and obtained the following model:

(1.1) { u t t - ( a + b Ω | u | 2 d x ) Δ u = h ( x , u ) in  Ω × ( 0 , T ) , u = 0 on  Ω × ( 0 , T ) , u ( x , 0 ) = u 0 ( x ) , u t ( x , 0 ) = u * ( x ) ,

where T>0 is a constant and u0,u* are continuous functions. Due to this reason, the operators of the form -(a+bΩ|u|2dx)Δu are always called Kirchhoff-type operators, and the equations involving Kirchhoff-type operators are always called Kirchhoff-type equations. In (1.1), u denotes the displacement, the nonlinearity h(x,u) denotes the external force, and the parameter a denotes the initial tension, while the parameter b is related to the intrinsic properties of the string (such as Young’s modulus). For more details on the physical background of the Kirchhoff equation, we refer the readers to [2, 19].

Clearly, (Pabλ) has a natural variational structure in H01(Ω). Indeed, let b:H01(Ω) be defined as

b ( u ) = a 2 u L 2 ( Ω ) 2 + b 4 u L 2 ( Ω ) 4 - λ 2 u L 2 ( Ω ) 2 - 1 p u L p ( Ω ) p ,

where Lr(Ω) is the usual norm in Lr(Ω) (r1). Then it is easy to see that b(u) is of C2 in H01(Ω) and it is the corresponding functional of (Pabλ). This fact implies that the variational method is a powerful tool to deal with (Pabλ). However, since the Kirchhoff term -b(Ω|u|2dx)Δu is non-local and u-b(Ω|u|2dx)Δu is not weakly continuous in H01(Ω), a typical difficulty in studying Kirchhoff-type equations by the variational method is that the weak limit of the (PS) sequence to the corresponding functional is not the weak solution in general. In order to overcome this difficulty, several methods have been developed and various existence and multiplicity results of nontrivial solutions for the Kirchhoff equation involving (Pabλ) in a domain ΩN have been established by the variational method in the literature, see, for example, [9, 17, 16, 22, 26, 27, 29, 39] and the references therein for the bounded Ω and [1, 10, 11, 12, 15, 13, 14, 21, 20, 23, 31, 33, 36, 37] and the references therein for Ω=N.

We note that the study of finding sign-changing solution for elliptic equations is a hot topic in the area of nonlinear analysis, and a lot of excellent results have been obtained in recent years, see, for example, [3, 4, 6, 8, 28, 32, 34] and the references therein. However, to the best of our knowledge, except the references [10, 24, 36, 29, 39], there are few results devoted to the existence of sign-changing solutions of Kirchhoff-type equations. The main reason in our opinion is the difference between decompositions of the corresponding functionals of Kirchhoff-type equations and the related local problem. For example, in the case of b=0, the corresponding functional 0(u) of the local problem (𝒫a,0,λ) has the following decomposition:

0 ( u + + u - ) = 0 ( u + ) + 0 ( u - ) ,

where u±=max{±u,0}. This decomposition plays an important role in many tools, such as the invariant sets of the descending flow, the sign-changing Nihari manifold, the phase separation of elliptic systems, etc., which are used for finding sign-changing solutions to local elliptic equations. However, for the Kirchhoff-type equation (Pabλ), the corresponding functional b(u) has the following decomposition:

b ( u + + u - ) = b ( u + ) + b ( u - ) + b 2 u + L 2 ( Ω ) 2 u - L 2 ( Ω ) 2 .

This different decomposition makes the study on the sign-changing solutions of Kirchhoff-type equations to be nontrivial, even though such study can be seen as a natural extension of that on the related local equations. Actually, by directly applying the results of methods dealing with local elliptic equations stated in the above references to (Pabλ), we have the following results:

  1. If 0<λ<aσ1 and b>0, then (Pabλ) has a least energy sign-changing solution for N=3 and 4<p<6, where σ1 is the first eigenvalue of -Δ in L2(Ω). This result can be obtained by applying the argument in [29] in a direct way.

  2. If Ω=𝔹R is a ball, 0<λ<aσ1 and b>0, then for every k, with k2, (Pabλ) has a least energy radial sign-changing solution which changes sign exactly k times for N=3 and 4<p<6. This result is pointed out by Deng, Peng and Shuai in [10].

  3. (𝒫a,b,0) has infinitely many sign-changing solutions for all b>0, with N=3 and 4<p<6, while for every k, with k2, and N3, there exists bk>0 such that (𝒫a,b,0) has k sign-changing solutions for 0<b<bk and p(2,4](2,2*). These results can be obtained by applying the argument in [36] in a direct way.

Note that the local problem (𝒫a,0,λ) has infinitely many sign-changing solutions and a least energy sign-changing solution for all N3, 0λ<aσ1 and 2<p<2*. Thus, motivated by the above facts, we wonder whether (Pabλ) also has infinitely many sign-changing solutions and a least energy sign-changing solution for all N3, 0λ<aσ1 and 2<p<2*. In this paper, we will explore this question and its related problems, such as the concentration behavior of sign-changing solutions of (Pabλ), the “double energy” property to the least energy sign-changing solution, existence and multiplicity of radial sign-changing solutions of (Pabλ), etc.

1.2 Sign-Changing Solutions

Let us state our first result on the existence and multiplicity of sign-changing solutions of (Pabλ).

Theorem 1.1.

Let a,λ>0 satisfy λ<aσ1. Then we have the following:

  1. If N = 3 and 4 < p < 6 , then ( (Pabλ) ) has a sequence of sign-changing solutions { u k } for all b > 0 , and for each k , uk has at most k+1 nodal domains. In particular, u1 is also a least energy sign-changing solution with exact two nodal domains.

  2. If p ( 2 , 4 ] ( 2 , 2 * ) , then for every k , there exists b k > 0 such that ( (Pabλ) ) has k sign-changing solutions { u m } m = 1 , 2 , , k for 0 < b < b k , and each u m has at most m + 1 nodal domains. In particular, u 1 has two nodal domains. Moreover, u 1 is also a least energy sign-changing solution for p = 4 .

Remark 1.2.

(1) Theorem 1.1 actually extends the known result (R1) mentioned above to the case of N=3 and 4p<6, the known result (R2) to the general bounded domain Ω and the known result (R3) to the case of 0λ<aσ1, respectively. Thus, we partial answer the above question.

(2) Theorem 1.1 gives not only the existence and multiplicity results to the sign-changing solutions of (Pabλ) in general bounded domains but also an estimation of the number of nodal domains to these sign-changing solutions. It is worth to point out that, to the best of our knowledge, the number of nodal domains to the sign-changing solutions was only estimated in [29] for the least energy sign-changing solutions in general bounded domains and in [10] for the radial sign-changing solutions in a ball. Thus, Theorem 1.1 seems the first result on estimating the numbers of nodal domains for high energy sign-changing solutions in general bounded domains.

(3) In the case of p(2,4](2,2*), the existence of bk seems to be necessary for 0λ<aσ1. Indeed, in [17], it has been observed that (Pabλ) only has trivial solution if b>0 is large enough for 0λ<aσ1 and p(2,4](2,2*). On the other hand, if p=4 and 0λ<aσ1, then we can see that (Pabλ) has no sign-changing solution which has at least k+1 nodal domains for b1(k+1)𝒮42, where k and 𝒮4 is the best embedding constant from H01(Ω)L4(Ω), defined by

𝒮 4 = inf { u L 2 ( Ω ) 2 u H 0 1 ( Ω ) , u L 4 ( Ω ) 2 = 1 } .

Indeed, suppose that u is a sign-changing solution of (Pabλ) which has at least k+1 nodal domains for p=4. Let {Ωi}i=1,2,,l (lk+1) be the nodal domains of u and let ui=uχΩi, where χΩi is the characteristic function of Ωi. Then, by [25, Lemma 1], uiH01(Ω) for all i=1,2,,l. Now, multiplying (Pabλ) with ui, we can see that

(1.2) b j = 1 , j i l u j L 2 ( Ω ) 2 u i L 2 ( Ω ) 2 + a u i L 2 ( Ω ) 2 - λ u i L 2 ( Ω ) 2 u i L 4 ( Ω ) 4 - b u i L 2 ( Ω ) 4

for all i=1,2,,l. Since λ<σ1a, we have

a u i L 2 ( Ω ) 2 - λ u i L 2 ( Ω ) 2 > 0 for all  i = 1 , 2 , , l .

This, together with the definition of 𝒮4 and (1.2), implies

b 𝒮 4 2 j = 1 , j i l u j L 2 ( Ω ) 2 < ( 1 - b 𝒮 4 2 ) u i L 2 ( Ω ) 2 , i = 1 , 2 , , l .

It follows that

l b 𝒮 4 2 j = 1 l u j L 2 ( Ω ) 2 < j = 1 l u j L 2 ( Ω ) 2 ,

which implies lb𝒮42<1, so that b<1l𝒮421(k+1)𝒮42.

1.3 Concentration Behavior and “Double” Energy

Since b is the parameter of the Kirchhoff-type nonlocal term in (Pabλ), and it is well known that the solutions of (Pabλ) are related to those of (𝒫a,0,λ), as in the literature, it is natural to study the concentration behavior of solutions to (Pabλ) as b0+. For this purpose, we re-denote the sign-changing solutions {uk} of (Pabλ), given in Theorem 1.1, by {uk,b}. On the other hand, another interesting property for (Pabλ) is the so-called “double energy” property to the least energy sign-changing solution. Let

c 1 ( b ) = b ( u 1 , b ) and m b = inf u 𝒩 b b ( u ) ,

where 𝒩b is defined by

𝒩 b = { u H 0 1 ( Ω ) { 0 } b ( u ) u = 0 } .

Then our results in these aspects can be stated as follows.

Theorem 1.3.

Let a,λ>0 satisfy λ<aσ1. Then we have the following:

  1. For every b n 0 + as n , we have that u k , b n u k , 0 strongly in H 0 1 ( Ω ) as n , where each u k , 0 ( k ) is a sign-changing solution of ( 𝒫 a , 0 , λ ) and has at most k + 1 nodal domains. In particular, u 1 , 0 is a least energy sign-changing solution of ( 𝒫 a , 0 , λ ) with two nodal domains.

  2. If N = 3 , then c 1 ( b ) = inf u b b ( u ) and c 1 ( b ) > 2 m b for all b > 0 in the case of 4 < p < 6 , and there exists b 1 > 0 such that c 1 ( b ) = inf u b ( u ) and c 1 ( b ) > 2 m b for 0 < b < b 1 in the case of p = 4 , where b is the sign-changing Nehari manifold of b ( u ) given by

    (1.3) b = { u ± H 0 1 ( Ω ) { 0 } b ( u ) u ± = 0 } .

  3. We have 1 c 1 ( 0 ) - 1 c 1 ( b ) = O ( b ) as b 0 + , where c 1 ( 0 ) = 0 ( u 1 , 0 ) . Moreover, c 1 ( b ) = O ( b p p - 4 ) as b + in the case of N = 3 and 4 < p < 6 .

Remark 1.4.

  1. We note that the concentration behavior of u1,b stated in Theorem 1.3 (1) has already been proved in [29]. However, the concentration behavior of uk,b for k2 is completely new to the best of our knowledge.

  2. The “double energy” property of (Pabλ) stated in Theorem 1.3 (2) has already been observed in [10] for the radially symmetric situation in the case of N=3 and 4<p<6 if Ω=𝔹R is a ball. Now, Theorem 1.3 extends this result to general bounded domains in the case of N=3 and 4p<6.

  3. To the best of our knowledge, the conclusion of Theorem 1.3 (3) is completely new.

1.4 Radial Sign-Changing Solution

Let Ω=𝔹R be a ball of N. Then it is expected to find out the radially symmetric solutions (radial solution for short) of (Pabλ) by using the radial symmetry of Ω. Now, let us state our results for the radial sign-changing solutions.

Theorem 1.5.

Let a,λ>0 satisfy λ<aσ1 and let Ω=BR be a ball of RN. Then we have the following:

  1. If N = 3 and 4 < p < 6 , then ( (Pabλ) ) has a sequence of radial sign-changing solutions { u k } for all b > 0 , and for each k , uk is also a least energy radial sign-changing solution of ((Pabλ)) which has k+1 nodal domains.

  2. If p ( 2 , 4 ] ( 2 , 2 * ) , then for every k , there exists b k > 0 such that ( (Pabλ) ) has k radial sign-changing solutions { u m } m = 1 , 2 , , k for 0 < b < b k , and u m has m + 1 nodal domains. Moreover, each u m is also a least energy radial sign-changing solution of ( (Pabλ) ) with m + 1 nodal domains for p = 4 .

Remark 1.6.

(1)  Theorem 1.5 (1) is just the known result (R2) mentioned before, which can be directly proved by using the method in [10]. Here, we actually give a different proof. Let us give some words about the differences here. Generally speaking, the argument in [10] is mainly consisted by the studies on two minimizing problems. The first one is

(1.4) γ b , 𝐫 = inf 𝐮 𝐇 𝐫 𝐉 b ( 𝐮 ) ,

where 𝐫=(R0,R1,,Rk-1), with 0=R0<R1<<Rk-1=R, 𝐮=(u1,u2,,uk), with uiH01(𝔹Ri𝔹Ri-1¯), and

𝐇 𝐫 = { 𝐮 i = 1 k ( H 0 1 ( 𝔹 R i 𝔹 R i - 1 ¯ ) { 0 } ) | 𝐉 b ( 𝐮 ) 𝐮 i = 0 , i = 1 , 2 , , k } ,

with 𝐮1=(u1,0,,0), 𝐮i=(0,,0,ui,0,,0) for i=2,,k, and

𝐉 b ( 𝐮 ) = i = 1 k ( a 2 𝔹 R i | u i | 2 d x + b 4 ( 𝔹 R i | u i | 2 d x ) 2 - λ 2 𝔹 R i | u i | 2 d x - 1 p 𝔹 R i | u i | p d x )
+ i , j = 1 ; i j k b 2 𝔹 R i | u i | 2 d x 𝔹 R j | u j | 2 d x .

The second one is

(1.5) γ b = inf 𝐫 Γ γ b , 𝐫 ,

where

Γ = { 𝐫 = ( R 0 , R 1 , , R k - 1 ) 0 = R 0 < R 1 < < R k - 1 = R } .

In such an argument, one first needs to prove that the minimizing problem (1.4) can be attained by some 𝐮𝐫, and then, by using this solution as a building block, construct a radial solution of (Pabλ) that changes sign exactly k times. This construction will be finished in the second step by matching the first derivative of the adjacent components ui𝐫 and ui+1𝐫 with respect to the radial variable, which can be ensured by proving that the minimizing problem (1.5) can be attained. It should be pointed out that this idea can be traced back to [5] to the best of our knowledge. However, in the current paper, we will obtain the same result by studying the phase separation of the “double coupled” elliptic system (3.1), which is mainly inspired by [8, 38, 35].

(2)  Theorem 1.5 (2) is complete new to the best of our knowledge. Moreover, as we stated in Remark 1.2 (3), the existence of bk seems to be necessary.

(3)  Due to the radial symmetry of the ball in N, Theorem 1.5 gives not only the existence of radial sign-changing solutions of (Pabλ) but also the precise number of nodal domains for each radial sign-changing solution, which is different from the circumstances in Theorem 1.1 for general bounded domains.

We re-denote the radial sign-changing solutions {uk} of (Pabλ) given in Theorem 1.5 by {uk,b} and let

c k ( b ) = b ( u k , b ) .

As in Theorem 1.3, we can study the concentration behavior of the radial sign-changing solutions given by Theorem 1.5 and obtain the following results.

Theorem 1.7.

Let a,λ>0 satisfy λ<aσ1 and let Ω=BR be a ball of RN. Then we have the following:

  1. For every b n 0 + as n , we have that u k , b n u k , 0 strongly in H 0 1 ( 𝔹 R ) as n , where each u k , 0 ( k ) is a least energy radial sign-changing solution of (𝒫a,0,λ) with k+1 nodal domains.

  2. If N = 3 , then c k + 1 ( b ) > c k ( b ) and c k ( b ) > k m b for all b > 0 and k in the case of 4 < p < 6 , and for every k , there exists b k > 0 such that c i + 1 ( b ) > c i ( b ) and c i ( b ) > ( i + 1 ) m b for i = 1 , , k and 0 < b < b k in the case of p = 4 . In particular, c 1 ( b ) = inf u b ( u ) for all b > 0 in the case of 4 < p < 6 and for 0 < b < b 1 in the case of p = 4 , where b is the sign-changing Nehari manifold of b ( u ) given by ( 1.3 ).

  3. We have 1 c k ( 0 ) - 1 c k ( b ) = O ( b ) as b 0 + for all k , where c k ( 0 ) = 0 ( u k , 0 ) . Moreover, c k ( b ) = O ( b p p - 4 ) as b + for all k in the case of N = 3 and 4 < p < 6 .

Remark 1.8.

  1. The conclusions (1) and (2) of Theorem 1.7 in the case of 4<p<6 have already been observed in [10]. However, Theorem 1.7 (1) in the case of p(2,4](2,2*) and Theorem 1.7 (2) in the case of p=4 are completely new to the best of our knowledge.

  2. Compared with the concentration behaviors of the energy value obtained in [10], another novelty is Theorem 1.7 (3), which we give some precise estimates for these concentration behaviors. On the other hand, the concentration behaviors of the energy value in Theorem 1.7 are much better than that in Theorem 1.3 due to the radial symmetry of the ball and the mini-max descriptions of the energy values of radial sign-changing solutions found in Theorem 1.5.

1.5 Uniqueness of Radially Positive Solution

It is well known that the local problem (𝒫a,0,λ) has a unique radially positive solution for all 2<p<2* and 0<λ<aσ1 if Ω is a ball of N, hence an interesting question is whether the nonlocal problem (Pabλ) also has a unique radially positive solution for 2<p<2* and 0<λ<aσ1 if Ω is a ball of N. However, under these conditions, the existence and nonexistence of radially positive solutions of (Pabλ) has been discussed in the very recent work [17]. In particular, in [17], we proved that if Ω=𝔹R is ball of N, then for each λ(0,aσ1), (Pabλ) has two radially positive solutions if p(2,4)(2,2*) and has no positive solution if N=3, p=4 and b𝒮421. Hence, to obtain the uniqueness result of radially positive solution of (Pabλ), it seems that the condition that N=3 and 4<p<6 or N=3, p=4 and b𝒮42<1 is necessary. Actually, this condition is also sufficient.

Theorem 1.9.

Let a>0 and let Ω=BR be a ball of RN. Then (Pabλ) has a unique radially positive solution for a.e. λ(0,aσ1) if and only if one of the following two conditions holds:

  1. N = 3 and 4 < p < 6 ,

  2. N = 3 , p=4 and b𝒮42<1.

Remark 1.10.

Theorem 1.9 improves the corresponding result in [17], due to uniqueness.

This paper is organized as follows. In Section 2, we will study the sign-changing solutions of (Pabλ) in a general bounded domain Ω and prove Theorems 1.1 and 1.3. In Section 3, we will study the radial sign-changing solutions of (Pabλ) in 𝔹R and prove Theorems 1.5 and 1.7. In Section 3.3, we will study the uniqueness of the radially positive solution of (Pabλ) in 𝔹R and prove Theorem 1.9.

Throughout this paper, for the sake of simplicity and clarity, we always denote uLr(Ω) by u,r. We use the notation M=(Mi,j)i,j=1,2,,k to denote generic matrices, while the notation 𝐭 is used to denote generic vectors.

2 The Case of General Ω

2.1 An Auxiliary Functional

Let χp(s) be a smooth function in [0,+) such that χp(s)1 in [0,+) for p>4 and

χ p ( s ) = { 1 , 0 s 1 , 0 , s 2 ,

for p4. Moreover, we also request -2χp(s)0 in [0,+) for all p. Now, consider the functional

T , b ( u ) = a 2 u , 2 2 + b 4 χ p ( u , 2 2 T 2 ) u , 2 4 - λ 2 u , 2 2 - 1 p u , p p ,

where T>0 is a constant. Clearly, T,b(u) is of C2 in H01(Ω). Moreover, we have

(2.1) T , b ( u ) v = ( a + b ( χ p ( u , 2 2 T 2 ) + u , 2 2 2 T 2 χ p ( u , 2 2 T 2 ) ) u , 2 2 ) Ω u v d x - λ Ω u v d x - Ω | u | p - 2 u v d x

for all u,vH01(Ω). By the choice of χp, it is easy to see that the critical points of T,b(u) with u,2T1 are also the solutions of (Pabλ). In particular, since χp(s)1 in [0,+) for p>4, the critical points of T,b(u) are always the solutions of (Pabλ) in the case of N=3 and 4<p<6.

Let

(2.2) 𝒬 T , b ( u ) = a + b ( χ p ( u , 2 2 T 2 ) + u , 2 2 2 T 2 χ p ( u , 2 2 T 2 ) ) u , 2 2 .

Then it is easy to see that 𝒬T,b(u) is of C1 in H01(Ω). Moreover, we also have from (2.1) that

(2.3) T , b ( u ) = 𝒬 T , b ( u ) u - λ ( u ) in  H - 1 ( Ω ) ,

where H-1(Ω) is the dual space of H01(Ω) and λ(u)H-1(Ω) is defined by

Ω λ ( u ) v d x = Ω ( λ u + | u | p - 2 u ) v 𝑑 x for all  v H 0 1 ( Ω ) .

It follows that all critical points of T,b(u) satisfy 𝒬T,b(u)u-λ(u)=0 in H-1(Ω).

Lemma 2.1.

Let a>0 and 0<λ<aσ1. Then there exist ε0>0 independent of T and bT>0 such that Lλ,T,b*(Dε±)Dε± for all 0<ε<ε0 and 0<b<bT, where

λ , T , b * ( u ) = 𝒬 T , b ( u ) - 1 λ ( u ) , 𝒟 ε ± = { u H 0 1 ( Ω ) dist ( u , 𝒫 ± ) < ε } 𝑎𝑛𝑑 𝒫 ± = { u H 0 1 ( Ω ) u 0 ( u 0 ) } .

In particular, we have bT=+ in the case of N=3 and 4<p<6.

Proof.

By the choice of χp(s) and the fact that 0<λ<aσ1, there exists bT>0 such that

(2.4) 𝒬 T , b ( u ) 1 2 ( a + λ σ 1 )  for all  u H 0 1 ( Ω ) , with  0 < b < b T .

It follows that λ,T,b*(u) is well defined for all uH01(Ω) if b<bT. Let v=λ,T,b*(u). Then it is easy to see that

dist ( v , 𝒫 ) = inf w 𝒫 ( v - w ) , 2 ( v - v ) , 2 = v ± , 2 .

From (2.3), it follows that

(2.5) dist ( v , 𝒫 ) v ± , 2 v ± , 2 2 = ± Ω v v ± d x = ± 𝒬 T , b ( u ) - 1 Ω ( λ u + | u | p - 2 u ) v ± 𝑑 x .

On the other hand, for all 1r2*, we have

(2.6) inf w 𝒫 v - w , r v - v , r = v ± , r inf w 𝒫 ± v ± - w , r + inf w 𝒫 v - w , r inf w 𝒫 v - w , r .

It follows that v±,r=infw𝒫v-w,r for all 1r2*. Since 0<λ<aσ1, by the definitions of σ1 and 𝒮p, we have

± Ω ( λ u + | u | p - 2 u ) v ± 𝑑 x Ω ( λ u ± + | u ± | p - 2 u ± ) v ± 𝑑 x
λ u ± , 2 v ± , 2 + u ± , p p - 1 v ± , p
(2.7) ( λ σ 1 dist ( u , 𝒫 ) + 𝒮 p - p 2 dist p - 1 ( u , 𝒫 ) ) v ± , 2 ,

where 𝒮p is best embedding constant from H01(Ω)Lp(Ω), defined by

𝒮 p := inf { u , 2 2 u H 0 1 ( Ω ) , u , p 2 = 1 } .

By (2.4), (2.5) and (2.7), we can easily see that

(2.8) dist ( v , 𝒫 ) 2 λ a σ 1 + λ dist ( u , 𝒫 ) + 2 σ 1 ( a σ 1 + λ ) 𝒮 p p 2 dist p - 1 ( u , 𝒫 )

for 0<b<bT. Thus, since 0<λ<aσ1, for 0<b<bT, there exists ε0>0 independent of T such that λ,T,b*(𝒟ε±)𝒟ε±, with 0<ε<ε0. In the case of N=3 and 4<p<6, since χp(s)1, we have 𝒬T,b(u)a for all b>0. Therefore, by repeating the proof above, we actually have that bT=+ in the case of N=3 and 4<p<6. ∎

For every c, we denote the sublevel sets by c={uH01(Ω)T,b(u)c}. Clearly, c is closed and symmetric. For every c, we also denote the set of critical points at level c by

𝒦 c = { u H 0 1 ( Ω ) T , b ( u ) = 0  in  H - 1 ( Ω )  and  T , b ( u ) = c } .

Lemma 2.2.

Let a>0, 0<b<bT and 0<λ<aσ1, where bT is given by Lemma 2.1. Then ET,b(u) satisfies the (PS) condition. That is, if ET,b(un)=on(1) in H-1(Ω) and ET,b(un) is bounded, then {un} has a convergent subsequence in H01(Ω).

Proof.

Let {un}H01(Ω) be a (PS) sequence. Then, by the choice of χp and the definition of σ1, we can see that

T , b ( u n ) - 1 p T , b ( u n ) u n = p - 2 2 p ( a u n , 2 2 - λ u n , 2 2 ) + b ( p - 4 ) 4 p χ p ( u n , 2 2 T 2 ) u n , 2 4 - b 2 p T 2 χ p ( u n , 2 2 T 2 ) u n , 2 4
(2.9) ( p - 2 ) ( a σ 1 - λ ) 2 p σ 1 u n , 2 2 - 4 b T 4 p

for p(2,4](2,2*). In the case of N=3 and 4<p<6, also by the choice of χp, we can see that

(2.10) T , b ( u n ) - 1 4 T , b ( u n ) u n = 1 4 ( a u n , 2 2 - λ u n , 2 2 ) + p - 4 4 p u n , p p a σ 1 - λ 4 σ 1 u n , 2 2 .

Since 0<λ<aσ1, from (2.9) and (2.10), we have that {un} is a bounded sequence in H01(Ω) for all 2<p<2*. Without loss of generality, we assume that unu0 weakly in H01(Ω) and 𝒬T,b(un)0 as n, where 0 is a constant. By (2.4), we actually have 00. It follows from the Sobolev embedding theorem that unu0 strongly in Lr(Ω) for all 1r<2* as n. Consider the functional

( u ) = 0 2 u , 2 2 - λ 2 u , 2 2 - 1 p u , p p .

Then, by T,b(un)=on(1) in H-1(Ω) and 𝒬T,b(un)0 as n, we can see from (2.3) that (un)=on(1) in H-1(Ω). Since unu0 weakly in H01(Ω) as n, it follows that (u0)=0 in H-1(Ω). Therefore,

λ u 0 , 2 2 + u 0 , p p = 0 u 0 , 2 2
0 u n , 2 2 + o n ( 1 )
= 𝒬 T , b ( u n ) u n , 2 2 + o n ( 1 )
= λ u n , 2 2 + u n , p p + o n ( 1 )
= λ u 0 , 2 2 + u 0 , p p .

Thus, we must have u0,22=un,22+on(1), which implies unu0 strongly in H01(Ω) as n. ∎

By Lemma 2.2, we can see that 𝒦c is compact for every c. Set SCc,ε=𝒦c(𝒟¯Dε+𝒟¯Dε-). Then, by Lemma 2.1, we can also see that SCc,ε is compact for all c if 0<ε<ε0.

Lemma 2.3.

Let a>0, 0<b<bT and 0<λ<aσ1, where bT is given by Lemma 2.1. Then any critical value of ET,b(u) must be nonnegative.

Proof.

Let c be a critical value of T,b(u) and let u0 be the corresponding critical point. Then, by (2.10), we can easily see that c0 in the case of N=3 and 4<p<6. For p(2,4](2,2*), we can see from (2.9) that

(2.11) c ( p - 2 ) ( a σ 1 - λ ) 2 p σ 1 u 0 , 2 2 - 4 b T 4 p .

On the other hand, by (2.4) and the definition of σ1, we have from T,b(u0)u0=0 that

1 2 ( a - λ σ 1 ) u 0 , 2 2 u 0 , p p .

From the definition of 𝒮p and 0<λ<aσ1, it follows that

u 0 , 2 ( 1 2 ( a - λ σ 1 ) 𝒮 p p 2 ) 1 p - 2 > 0 .

By choosing bT small enough if necessary, we can see that c0 for 0<b<bT in the case of p(2,4](2,2*). This completes the proof. ∎

Let 0<ε<ε0, where ε0 is given by Lemma 2.1. Define

(2.12) α ~ k = inf { α 0 γ ( α ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) ; 0 ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) , - 1 ) k } ,

where γ(C;B,A) is the relative genus introduced in [3, Definition 4.1]. In what follows, we will show that α~k are critical values of T,b(u). Let us consider the following ODE in H01(Ω)(c𝒦c):

(2.13) { d φ ( t , u ) d t = - 1 2 ( T , b ( φ ( t , u ) ) - T , b ( φ ( t , - u ) ) ) , φ ( 0 , u ) = u .

Clearly, for every uH01(Ω)(c𝒦c), this ODE is unique solvable. We denote the maximum existence time by T(u).

Lemma 2.4.

Let a>0, 0<b<bT and 0<λ<aσ1, where bT is given by Lemma 2.1. Then φ(t,u)D¯𝐷ε+D¯𝐷ε- for all uD¯𝐷ε+D¯𝐷ε- and t[0,T(u)).

Proof.

Due to the semigroup property of φ(t,u), we may assume u𝒟ε+, without loss of generality. Denote 12(T,b(φ(t,u))-T,b(φ(t,-u))) by 𝒱T,b(u). Since (2.13) is unique solvable, by the construction of 𝒱T,b(u), we can see that φ(t,u) is odd in u. It follows from the oddness of T,b(u) that 𝒱T,b(u)=T,b(φ(t,u)). By the Taylor expansion and (2.3), we have

φ ( t , u ) = φ ( 0 , u ) + d φ ( t , u ) d t t + o ( t )
= u - T , b ( φ ( t , u ) ) t + o ( t )
= u - ( 𝒬 T , b ( u ) u - λ ( u ) ) t + o ( t ) ,

which, together with Lemma 2.1 and (2.8), implies

dist ( φ ( t , u ) , 𝒫 + ) ( 1 - 𝒬 T , b ( u ) t ) dist ( u , 𝒫 + ) + t dist ( λ ( u ) , 𝒫 + ) + o ( t )
= ( 1 - 𝒬 T , b ( u ) t ) dist ( u , 𝒫 + ) + 𝒬 T , b ( u ) t dist ( λ , T , b * ( u ) , 𝒫 + ) + o ( t )
( 1 - 𝒬 T , b ( u ) ( a σ 1 - λ ) t a σ 1 + λ ) dist ( u , 𝒫 + ) + 2 σ 1 𝒬 T , b ( u ) t ( a σ 1 + λ ) 𝒮 p p 2 dist ( u , 𝒫 + ) p - 1 + o ( t ) .

Thus, by choosing ε0 small enough if necessary, we can see, from the fact that 0<λ<aσ1 and (2.4), that dist(φ(t,u),𝒫+)<ε for t small enough and 0<ε<ε0. ∎

Since SCc,ε is compact for all c, by the properties of the classical genus (cf. [30, Proposition 5.4]), there exists a neighborhood N of SCc,ε such that γ(N)=γ(SCc,ε). Moreover, by Lemma 2.1, we can also choose N such that N(𝒟¯Dε+𝒟¯Dε-)=.

Lemma 2.5.

Let a>0, 0<b<bT and 0<λ<aσ1, where bT is given by Lemma 2.1. Then for every cR, there exist δ>0 and an odd and continuous map H such that

( ( c + δ N ) ( 𝒟 ¯ 𝐷 ε + 𝒟 ¯ 𝐷 ε - ) ) c - δ ( 𝒟 ¯ 𝐷 ε + 𝒟 ¯ 𝐷 ε - ) .

Proof.

Let u(c+δN)(𝒟¯Dε+𝒟¯Dε-), with δ>0, and we consider the flow φ(t,u) given by (2.13). Since 𝒱T,b(u)=T,b(φ(t,u)), due to the oddness of φ(t,u) in u, we have

(2.14) d T , b ( φ ( t , u ) ) d t = T , b ( φ ( t , u ) ) d φ ( t , u ) d t = - T , b ( φ ( t , u ) ) - 1 2 0 ,

where -1 is the usual norm in H-1(Ω). Due to (2.14), we may assume that u(c+δ(Nc-δ))(𝒟¯Dε+𝒟¯Dε-) without loss of generality. Clearly, one of the following two cases must happen:

  1. there exists t*(u)[0,T(u)) such that φ(t*(u),u)𝒟¯Dε+𝒟¯Dε-,

  2. φ ( t , u ) H 0 1 ( Ω ) ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) for all t[0,T(u)).

If case (1) occurs, then, by Lemma 2.4 and the semigroup property of φ(t,u), we can see that φ(t,u)𝒟¯Dε+𝒟¯Dε- for all t[t*(u),T(u)). In what follows, let us consider case (2). Since N is a neighborhood of SCc,ε, there exists d0>0 such that N2d0N, where N2d0={uH01(Ω)dist(u,SCc,ε)<2d0}. We claim that for some δ>0 small enough, the flow φ(t,u) cannot enter into Nd0¯ before it enters into c-δ. Indeed, suppose the contrary. Since uc+δN, there exist 0<t1<t2<T(u) such that φ(t,u)N2d0¯Nd0¯ for t1<t<t2, φ(t1,u)N2d0 and φ(t2,u)Nd0. We claim that there exists d1>0, independent of δ, such that

(2.15) T , b ( φ ( t , u ) ) - 1 d 1 > 0 for  t 1 t t 2 .

Indeed, suppose the contrary. Since φ(t,u)c-δ, we can see that

(2.16) T , b ( φ ( t 2 , u ) ) - 1 0 and T , b ( φ ( t 2 , u ) ) c as  δ 0 ,

Since d0 is independent of δ, (2.16) contradicts the fact that φ(t2,u)Nd0. Thus, (2.15) holds and d1 is also independent of δ. Now, by (2.13) and (2.14), we can see that

d 0 ( φ ( t 1 , u ) - φ ( t 2 , u ) ) , 2
1 d 1 t 1 t 2 T , b ( φ ( t , u ) ) - 1 2 d t
= 1 d 1 t 1 t 2 - T , b ( φ ( t , u ) ) d φ ( t , u ) d t d t
= 1 d 1 ( T , b ( φ ( t 1 , u ) ) - T , b ( φ ( t 2 , u ) ) )
(2.17) 2 δ d 1 .

Since d0,d1>0 are independent of δ, by (2.17), we can get a contradiction for δ>0 small enough. Now, since the flow φ(t,u) cannot enter into Nd0¯ before it enters into c-δ, we also have

T , b ( φ ( t , u ) ) - 1 d 2 > 0 for  φ ( t , u ) c + δ ( N c - δ ) ,

where d2>0 is a constant. We claim that there exists t(u)(0,T(u)) such that φ(t(u),u)c-δ. Indeed, suppose the contrary. Then φ(t,u)c+δ(Nc-δ) for all t[0,T(u)). Clearly, one of the following two cases must happen:

  1. T ( u ) = + ,

  2. T ( u ) < + .

If case (2.a) occurs, then, by choosing t(u)=3δd22, we have

T , b ( φ ( t ( t ) , u ) ) = T , b ( φ ( 0 , u ) ) + 0 t ( u ) T , b ( φ ( t , u ) ) d φ d t 𝑑 t
c + δ - t ( u ) d 2 2
(2.18) = c - 2 δ .

This is a contradiction. Let us consider case (2.b) in what follows. Since φ(t,u)c+δ(Nc-δ) for all t[0,T(u)), by a similar argument as that used in (2.17), we can see that

(2.19) lim t T ( u ) φ ( t , u ) , 2 u , 2 + 2 δ d 2 < + .

Therefore, we must have T,b(φ(T(u),u))=0 in H-1(Ω). By choosing δ small enough if necessary, we can see from a similar argument as that used for (2.16) that φ(T(u),u)Nd0 for δ small enough. This is impossible, since φ(t,u)c+δ(Nc-δ) for all t[0,T(u)). Thus, for every uc+δ(Nc-δ), there exists t(u)(0,T(u)) such that φ(t(u),u)c-δ. In other words, we actually have that for every uc+δ(N(𝒟¯Dε+𝒟¯Dε-)), there exists t(u)0 such that φ(t(u),u)c-δ(𝒟¯Dε+𝒟¯Dε-). Since φ(t,u) is odd in u, by the uniqueness of φ(t,u), we also have t(u)=t(-u). For every pair ±u(c+δN)(𝒟¯Dε+𝒟¯Dε-), we choose a symmetric neighborhood N±u in (c+δN)(𝒟¯Dε+𝒟¯Dε-). Note that (c+δN)(𝒟¯Dε+𝒟¯Dε-) is paracompact, hence there exists a locally finite partition of unity {πj}jJ to the covering {N±u}. Replacing πj(u) by πj(u)+πj(-u)2 if necessary, we may assume that {πj} are all even, due to the symmetry of {N±u}. Thus, we have that supp(πj)N±vj for some vj(c+δN)(𝒟¯Dε+𝒟¯Dε-). Let

τ ( u ) = j J π j ( u ) t ( v j ) .

Then π is even and continuous on (c+δN)(𝒟¯Dε+𝒟¯Dε-). Let (u)=φ(τ(u),u). Then (u) is odd and continuous. Moreover, we also have ((c+δN)(𝒟¯Dε+𝒟¯Dε-))c-δ(𝒟¯Dε+𝒟¯Dε-). ∎

By the definitions, we can easily see that α~k is nondecreasing for k.

Lemma 2.6.

Let a>0, 0<b<bT and 0<λ<aσ1, where bT is given by Lemma 2.1. Then SC0,ε= and α~1>0.

Proof.

By Lemma 2.3 and (2.11), we can see that 𝒦0={0}. It follows that SC0,ε=. Now, by Lemma 2.5, there exist δ>0 and an odd and continuous map such that

( δ ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) ) - δ ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) 0 ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) .

Thanks to [3, Proposition 4.2], we have γ(δ(𝒟¯Dε+𝒟¯Dε-);0(𝒟¯Dε+𝒟¯Dε-),-1)=0. Thus, by definition, we must have α~1δ>0. ∎

Lemma 2.7.

Let a>0, 0<b<bT and 0<λ<aσ1, where bT is given by Lemma 2.1. If α~k=α~k+1==α~k+l=α~ for some k,lN, then γ(SCα~,ε)l+1.

Proof.

By Lemma 2.5, there exist δ>0 and an odd and continuous map such that

( ( c + δ N ) ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) ) c - δ ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) .

From Lemma 2.6, [3, Proposition 4.2] and the properties of the classical genus (cf. [30, Proposition 5.4]), it follows that

γ ( α ~ + δ ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) ; 0 ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) , - 1 )
γ ( ( α ~ + δ N ) ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) ; 0 ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) , - 1 ) + γ ( N )
(2.20) γ ( ( α ~ - δ N ) ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) ; 0 ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) , - 1 ) + γ ( SC α ~ , ε ) .

Now, by the definition of α~k, we have

γ ( ( α ~ - δ N ) ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) ; 0 ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) , - 1 ) k - 1 .

Suppose that γ(SCα~,ε)l. Then, by (2.20), we have

γ ( ( α ~ + δ N ) ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) ; 0 ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) , - 1 ) k + l - 1 ,

which contradicts the definition of α~k+l. ∎

Now, we can obtain the following proposition.

Proposition 2.8.

Let a>0, 0<b<bT and 0<λ<aσ1, where bT is given by Lemma 2.1. Then ET,b(u) has infinitely many sign-changing critical points.

Proof.

It follows immediately from Lemmas 2.6 and 2.7. ∎

Let

𝒜 0 = { u H 0 1 ( Ω ) φ ( t , u ) 0  as  t T ( u ) } .

Since T,b′′(0)=-aΔ-λ in (H01(Ω)×H01(Ω))-1, 0 is a stable critical point of T,b(u), due to 0<λ<aσ1, where (H01(Ω)×H01(Ω))-1 is the dual space of H01(Ω)×H01(Ω). Thus, 𝒜0 is open, due to Lemma 2.1.

Lemma 2.9.

Let a>0, 0<b<bT and 0<λ<aσ1, where bT is given by Lemma 2.1. Then D¯𝐷ε+D¯𝐷ε-A0 for all ε(0,ε0), and ET,b(u)>0 on D¯𝐷ε+D¯𝐷ε-{0}, where ε0 is also given by Lemma 2.1.

Proof.

For every u𝒟¯Dε+𝒟¯Dε-, we can see from (2.6) and the definition of σ1 and 𝒮p that

u , 2 = u + , 2 + u - , 2 2 ε σ 1 - 1 2

and

u , p = u + , p + u - , p 2 ε 𝒮 p - 1 2 .

It follows that

(2.21) T , b ( u ) - 2 λ ε 2 σ 1 - ( 2 ε ) p p 𝒮 p p 2 > -

for all u𝒟¯Dε+𝒟¯Dε-. Now, suppose on the contrary that there exists u(𝒟¯Dε+𝒟¯Dε-)𝒜0. Then, by Lemma 2.4, φ(t,u)𝒟¯Dε+𝒟¯Dε- for all t[0,T(u)), where φ(t,u) is given by (2.13). Since u𝒜0 and 0 is the only critical point of T,b(u) in 𝒟¯Dε+𝒟¯Dε-, due to Lemma 2.1, T,b(φ(t,u))-1 is bounded below away from 0 for all t[0,T(u)), which, together with (2.21) and a similar argument as that used in (2.18), implies T(u)<+. Now, by (2.21) and a similar argument as that used in (2.19), we also have φ(T(u),u),2<+. This is impossible. Thus, we must have that 𝒟¯Dε+𝒟¯Dε-𝒜0. Finally, from (2.14), we actually have that T,b(u)>0 on 𝒟¯Dε+𝒟¯Dε-{0}. ∎

For every k, we define

(2.22) β k = inf W H 0 1 ( Ω ) dim ( W ) k sup u W T , b ( u ) .

Since 2<p<2*, by the construction of χp(s), for every WH01(Ω) with dim(W)<, there exists R>0 such that WBR-1, where BR={uWu,2<R}. It follows that βk<+ for every k. Moreover, we also have the following lemma.

Lemma 2.10.

Let a>0, 0<b<bT and 0<λ<aσ1, where bT is given by Lemma 2.1. Then α~kβk+1 for all kN.

Proof.

Take any WH01(Ω) with dim(W)=k+1, then there exists R>0 such that WBR-1. Suppose that β<α~k, where β=supuW(u). Then, by the definition of α~k, we have

γ ( β ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) ; 0 ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) , - 1 ) k - 1 .

By the definition of γ(β(𝒟¯Dε+𝒟¯Dε-);0(𝒟¯Dε+𝒟¯Dε-),-1), there exist closed and symmetric subsets U,V of H01(Ω) such that

β ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) U V , 0 ( 𝒟 ¯ D ε + 𝒟 ¯ D ε - ) U and γ ( V ) k - 1 .

Note that Wβ, thus we also have WUV. Moreover, there exists an odd and continuous map h such that h:U0(𝒟¯Dε+𝒟¯Dε-) and h:-1-1. Since U is closed, by Tietze’s theorem, we can extend h to an odd and continuous map on H01(Ω). Now, set 𝒪W={uWh(u)𝒜0}. Since WBR-1 and 𝒜0 is open, we can see that 𝒪W contains 0 and it is a symmetric bounded open set of 0 in W. Thus, by Borsuk’s theorem, we have γ(𝒪W)=k+1. On the other hand, by (2.14) and Lemma 2.9, we can see that h(U𝒪W)¯𝒜0(𝒟¯Dε+𝒟¯Dε-). Note that by the oddness of φ(t,u) in u and Lemma 2.9 once more, we also have that 𝒜0(𝒟¯Dε+𝒟¯Dε-) is symmetric. Thus, we must have γ(h(U𝒪W)¯)1, which, together with the fact that h is an odd and continuous map, implies γ(U𝒪W)1. Since γ(V)k-1, we also have γ(V𝒪W)k-1. Thus, we actually have that γ(𝒪W)k, due to WUV. This is a contradiction. Thus, for any WH01(Ω) with dim(W)=k+1, we must have βα~k, where β=supuW(u). It follows that α~kβk+1, which completes the proof. ∎

Let uk be the sign-changing solution found by Proposition 2.8 with T,b(uk)=α~k. We will estimate the number of nodal domains to uk in what follows. For this purpose, we need the following technical lemma.

Lemma 2.11.

Let a>0, 0<b<bT and 0<λ<aσ1, where bT is given by Lemma 2.1. Let kN. Then for any v=(v1,v2,,vk), with viH01(Ω), vi0 and vivj=0 for all i,j=1,2,,k, with ij, there exists a unique sb=(s1b,s2b,,skb)(R+)k such that

(2.23) T , b ( i = 1 k s i b v i ) = max 𝐭 ( + ) k T , b ( i = 1 k t i v i ) .

Proof.

We first borrow some ideas from [10, Lemma 2.1] to show that the conclusion holds for all b>0 in the case of N=3 and 4<p<6. Let μ[-1,1]. Since χp(s)1 for 4<p<6, we consider the following system in (+)k:

𝐌 * ( μ , 𝐭 ) = 𝟎 ,

where 𝐌*(μ,𝐭)=(Mi,*(μ,𝐭))i=1,2,,k, with

(2.24) M i , * ( μ , 𝐭 ) = μ j = 1 , i j k b t i v i , 2 2 t j v j , 2 2 + a t i v i , 2 2 - λ t i v i , 2 2 + b t i v i , 2 4 - t i v i , p p = 0 , i = 1 , 2 , , k .

Define

𝒵 = { μ [ 0 , 1 ] 𝐌 * ( μ , 𝐭 ) = 𝟎  is uniquely solvable in  ( + ) k } .

Since 4<p<6, by a standard argument, we can see that there exists 𝐭0(+)k such that

a t i 0 v i , 2 2 - λ t i 0 v i , 2 2 + b t i 0 v i , 2 4 - t i 0 v i , p p = 0

for all i=1,2,,k and b>0. It follows that 𝒵 and 0𝒵. For the sake of clarity, we divide the proof into two parts. Claim 1: Z is open in [0,1]. Indeed, let μ0𝒵. Then there exists a unique 𝐬(+)k such that Mi,*(μ0,𝐬)=0 for all i=1,2,,k. Set

M i , * j ( μ 0 , 𝐬 ) = M i , * ( μ 0 , 𝐬 ) t j .

Then, by a direct calculation, we have

M i , * i ( μ 0 , 𝐬 ) = 2 b μ 0 j = 1 , i j k v i , 2 2 s j v j , 2 2 s i + 2 ( a v i , 2 2 - λ v i , 2 2 ) s i + 4 b v i , 2 4 s i 3 - p v i , p p s i p - 1

and

M i , * j ( μ 0 , 𝐬 ) = 2 μ 0 b s i v i , 2 2 v j , 2 2 s j , i , j = 1 , 2 , , k , with  j i .

Consider the matrix Mμ0,*(𝐬)=(Mi,*j(μ0,𝐬))i,j=1,2,,k. Then it is well known that

det ( M μ 0 , * ( 𝐬 ) ) = ( - 1 ) k i = 1 k s i det ( M ~ μ 0 , * ( 𝐬 ) ) ,

where M~μ0,*(𝐬)=(M~i,*j(μ0,𝐬))i,j=1,2,,k, with

M ~ i , * i ( μ 0 , 𝐬 ) = - 2 b μ 0 j = 1 , i j k s i v i , 2 2 s j v j , 2 2 - 2 ( a s i v i , 2 2 - λ s i v i , 2 2 ) - 4 b s i v i , 2 4 + p s i v i , p p

and

(2.25) M ~ i , * j ( μ 0 , 𝐬 ) = - 2 μ 0 b s i v i , 2 2 s j v j , 2 2 , i , j = 1 , 2 , , k , with  j i .

Clearly, M~i,*j(μ0,𝐬)<0 for all i,j=1,2,,k, with ji. Moreover, by (2.24) and p>4, we have

j = 1 k M ~ i , * j ( μ 0 , 𝐬 ) = 2 ( a s i v i , 2 2 - λ s i v i , 2 2 ) + ( p - 4 ) s i u i , p p
(2.26) 2 ( a s i v i , 2 2 - λ s i v i , 2 2 ) > 0

for all i=1,2,,k. Thus, the matrix M~μ0,*(𝐬)=(M~i,*j(μ0,𝐬))i,j=1,2,,k is strictly diagonally dominant. It follows that M~μ0,*(𝐬) is nonsingular and det(M~μ0,*(𝐬))0, which in turn implies that the matrix Mμ0,*(𝐬) is also nonsingular and det(Mμ0,*(𝐬))0. Now, by the implicit function theorem, there exist an open set 𝒰0×𝒯0[0,1]×(+)k, with (μ0,𝐬)𝒰0×𝒯0 and 𝐭(μ)=(t1(μ),t2(μ),,tk(μ))C1(𝒰0,𝒯0), such that 𝐭(μ0)=𝐬 and (μ,𝐭(μ)) is the unique solution of the system Mi,*(μ,𝐭)=0, i=1,2,,k, in 𝒰0×𝒯0. Suppose that 𝒰0𝒵. Then there exists μ1𝒰0 such that the system Mi,*(μ1,𝐭)=0, i=1,2,,k, has a second solution 𝐬1. Clearly, we must have 𝐬1𝒯0. Without loss of generality, we assume μ1>μ0. Note that (μ1,𝐬1) also solves the system Mi,*(μ1,𝐭)=0, i=1,2,,k. Then, by the implicit function theorem once more, we can show that there exists an open set 𝒰1×𝒯1[0,1]×(+)k, with (μ1,𝐬1)𝒰1×𝒯1, 𝒰1𝒰0, 𝒯1𝒯0= and 𝐭*(μ)=(t1*(μ),t2*(μ),,tk*(μ))C1(𝒰1,𝒯1), such that 𝐭*(μ1)=𝐬1 and (μ1,𝐭*(μ)) is the unique solution of the system Mi,*(μ,𝐭)=0, i=1,2,,k, in 𝒰1×𝒯1. By the extension theorem, one of the following three cases must happen:

  1. t i ( μ ) blow up at some μ<μ0 for some i=1,2,,k.

  2. t i ( μ ) 0 as μμ*+ at some μ*(0,μ0) for some i=1,2,,k.

  3. 𝐭 ( μ ) 𝐭 0 as μ0+, where 𝐭0 is the unique one in (+)k satisfying the system Mi,*(0,𝐭)=0.

Note that p>4, Mi,*(μ,𝐭)<0 if |ti(μ)| is large enough and Mi,*(μ,𝐭)>0 if |ti(μ)| is small enough for all i. Thus, we must have 𝐭(μ)𝐭0 as μ0+. Similarly, we also have 𝐭*(μ)𝐭0 as μ0+. Hence, 𝐭(μ) and 𝐭*(μ) are two different branches bifurcated at 0. On the other hand, by similar arguments as those used for (2.25) and (2.26), we can see that M~i,*j(0,𝐭0)=0 for all i,j=1,2,,k, with ji, and

j = 1 k M ~ i , * j ( 0 , 𝐭 0 ) 2 ( a t i 0 v i , 2 2 - λ t i 0 v i , 2 2 ) > 0 for all  i = 1 , 2 , , k .

From the implicit function theorem once more, it follows that there exists only one branch bifurcated at 0, which is a contradiction. Thus, we must have 𝒯0=(+)k and 𝒰0𝒵, that is, 𝒵 is open in [0,1]. Claim 2: Z is closed in [0,1]. Indeed, let {μn}𝒵 be such that μnμ0 as n. Since p>4 and Mi,*(μ,𝐭)<0 for all i=1,2,,k if |𝐭| is large enough, {𝐭n} is bounded in (+)k, where 𝐭n is the unique solution of the system Mi,*(μn,𝐭)=0, i=1,2,,k. Passing to a subsequence, we can see that 𝐭n𝐭0 as n. Clearly, 𝐭0 is a solution of the system Mi,*(μ0,𝐭), i=1,2,,k. By a similar argument as that used in claim 1, we can show that 𝐭0 is also the unique solution of this system. Due to the uniqueness of 𝐭0, we can see that every subsequence of 𝐭n must convergence to 𝐭0. It follows that μ0𝒵, that is, 𝒵 is closed in [0,1].

By claims 1 and 2, 𝒵 is both open and closed in [0,1]. Since 0𝒵, we must have 𝒵=[0,1]. Note that we have

(2.27) t i T , b ( i = 1 k t i v i ) t i = M i , * ( 1 , 𝐭 ) for all  i = 1 , 2 , , k .

Thus, 𝐬b is the unique critical point of T,b(i=1ktivi) in (+)k. On the other hand, since 2<p<2*, by the construction of χp(s), we can see that T,b(i=1ktivi)- as |𝐭|. Thus, T,b(i=1ktivi) has a global maximum point on (+)k¯. By 2<p<2* and the construction of χp(s)once more, we can have, from λ<aσ1 and (2.4), that

M i ( 𝐭 , b ) = 𝒬 T , b ( j = 1 k t j v j ) t i v i , 2 2 - λ t i v i , 2 2 - t i v i , p p
1 2 ( a + λ σ 1 ) t i v i , 2 2 - λ t i v i , 2 2 - t i v i , p p
1 2 ( a - λ σ 1 ) t i 2 v i , 2 2 - t i p 𝒮 p - p 2 v i , 2 p
> 0

if ti>0 is small enough for 0<b<bT and all i=1,2,,k. Therefore, by (2.27), the global maximum point of T,b(i=1ktivi) cannot belong to (+)k¯. Thus, this global maximum point is also a critical point of T,b(i=1ktivi) in (+)k. Note that 𝐬b(+)k is the unique critical point of T,b(i=1ktivi) in (+)k. Hence, 𝐬b(+)k is the global maximum point of T,b(i=1ktivi) in (+)k.

We next consider the case of p(2,2*)(2,4]. Consider the following system in (+)k:

M i ( 𝐭 , b ) = 0 , i = 1 , 2 , , k ,

where

M i ( 𝐭 , b ) = 𝒬 T , b ( j = 1 k t j v j ) t i v i , 2 2 - λ t i v i , 2 2 - t i v i , p p

and 𝒬T,b(u) is given by (2.2). Since χp(s) is of C, for all i=1,2,,k, Mi(𝐭,b) is of C for all tj and b. Note that λ<aσ1. Then, by a standard argument, we can see that the system Mi(𝐭,0)=0, i=1,2,,k, has a unique solution

𝐬 0 = ( s i 0 ) i = 1 , 2 , , k = ( ( a v i , 2 2 - λ v i , 2 2 v i , p p ) 1 p - 2 ) i = 1 , 2 , , k .

On the other hand, we also have

M i ( 𝐬 0 , 0 ) t i = 1 s i 0 [ 2 ( a s i 0 v i , 2 2 - λ s i 0 v i , 2 2 ) - p s i 0 v i , p p ] = 2 - p s i 0 s i 0 v i , p p < 0

and

M i ( 𝐬 0 , 0 ) t j = 0 for all  i , j = 1 , 2 , , k , with  j i .

Thus, the matrix M0(𝐬𝟎)=(Mij(𝐬0,0))i,j=1,2,,k is nonsingular, where

M i j ( 𝐬 0 , 0 ) = M i ( 𝐬 0 , 0 ) t j .

From a similar argument as that used for the case of N=3 and 4<p<6, it follows that the system Mi(𝐭,b)=0, i=1,2,,k, has a unique solution 𝐬b for b small enough. By choosing bT small enough if necessary, we can see that the system Mi(𝐭,b)=0, i=1,2,,k, has a unique solution 𝐬b for 0<b<bT. Note that

t i T , b ( i = 1 k t i v i ) t i = M i ( 𝐭 , b ) for all  i = 1 , 2 , , k .

Thus, by a similar argument as that used for the case of N=3 and 4<p<6, we can also show that (2.23) holds for p(2,2*)(2,4] and 0<b<bT. ∎

Remark 2.12.

By checking the proof of Lemma 2.11, we can see that 𝐬b𝐬0 as b0+.

Proposition 2.13.

Let a>0, 0<b<bT and 0<λ<aσ1, where bT is given by Lemma 2.1. Let kN and let uk be the sign-changing solution found by Proposition 2.8 with ET,b(uk)=α~k. Then uk has at most k+1 nodal domains. In particular, u1 is the least energy sign-changing solution of ET,b(u) and has two nodal domains.

Proof.

Suppose on the contrary that uk has at least k+2 nodal domains. Since uk is a solution of (Pabλ), by the classical elliptic regularity theory, uk is continuous. It follows from [25, Lemma 1] that uk,i=ukχΩ~iH01(Ω), where χΩ~i is the characteristic function of the set Ω~i and {Ω~i}i=1,2,,l (lk+2) are the nodal domains of uk. Moreover, since uk is a sign-changing solution of (Pabλ), we also have that

0 = T , b ( u k ) u k , i
= 𝒬 T , b ( u k ) Ω u k u k , i d x - λ Ω u k u k , i d x - Ω | u k | p - 2 u k u k , i d x
= 𝒬 T , b ( i = 1 l u k , i ) u k , i , 2 2 - λ u k , i , 2 2 - u k , i , p p
(2.28) = T , b ( i = 1 l t i u k , i ) t i | 𝐭 = 𝟏 .

Thus, 𝟏 is a critical point of T,b(i=1ltiuk,i) in (+)l. By the definition of βk+1 and Lemma 2.11, we can see that

(2.29) β k + 1 sup 𝐭 ( + ) k + 1 T , b ( i = 1 k + 1 t i u k , i ) < T , b ( i = 1 l u k , i ) = T , b ( u k ) = α ~ k ,

which contradicts Lemma 2.10. For the sign-changing solution u1, by the above conclusion, we can see that u1 has precisely two nodal domains. Let Ω~1 and Ω~2 be the nodal domains of u1 and set u1,i=u1χΩ~i, i=1,2. Then, by the definition of β2, Lemma 2.11 and (2.28), we have β2=T,b(u1). Now, suppose u* is a sign-changing solution of T,b(u). Then u* has at least two nodal domains. By Lemma 2.11 and a similar argument as that used in (2.29), we can see that T,b(u*)β2. Therefore, u1 is also the least energy sign-changing solution of T,b(u). ∎

2.2 Existence Results and Some Further Properties

Since the functional T,b(u) is dependent on the parameters b andT, we re-denote the sign-changing solutions uk found by Proposition 2.8, the energy value α~k of uk defined by (2.12) and βk defined by (2.22), by ukb,T, α~kb,T and βkb,T, respectively. By (2.22) and the construction of χp(s), we have

(2.30) β k b , T = inf W H 0 1 ( Ω ) dim ( W ) k sup u W T , b ( u ) inf W H 0 1 ( Ω ) dim ( W ) k sup u W 0 ( u ) + b T 4 = : β k * + b T 4

if p(2,4](2,2*). Clearly, by 2<p<2*, βk*<+ for all k and it is independent of b and T.

Proposition 2.14.

Let a>0, 0<b<bT and 0<λ<aσ1, where bT is given by Lemma 2.1. Let kN and ukb,T be the sign-changing solution found by Proposition 2.8 with ET,b(ukb,T)=α~kb,T. Then

u k b , T , 2 T for  T ( 4 p σ 1 β k + 1 * ( p - 2 ) ( a σ 1 - λ ) ) 1 2

in the case of p(2,4](2,2*). That is, ukb,T is a solution of (Pabλ) in one of the following two cases:

  1. p ( 2 , 4 ] ( 2 , 2 * ) and T ( 4 p σ 1 β k + 1 * ( p - 2 ) ( a σ 1 - λ ) ) 1 2 ,

  2. N = 3 , 4<p<6 and T>0.

Proof.

By the construction of χp, it is easy to see that ukb,T is a solution of (Pabλ) in case (2). Thus, let us consider case (1) in what follows. Suppose that ukb,T,2>T in this case. Then, by Lemma 2.10, (2.30) and a similar argument as that used in (2.9), we have

(2.31) ( p - 2 ) ( a σ 1 - λ ) 2 p σ 1 T 2 4 b T 4 p + β k + 1 * + b T 4 .

Since 0<λ<aσ1, by choosing bT small enough if necessary so that

4 b T 4 p + b T 4 < β k + 1 * for  0 < b < b T ,

we can see that (2.31) cannot hold when T(4pσ1βk+1*(p-2)(aσ1-λ))12. Therefore, ukb,T is a solution of (Pabλ) for p(2,4](2,2*) and T(4pσ1βk+1*(p-2)(aσ1-λ))12. ∎

Let us make some observations on the concentration behaviors of ukb,T as b0+ in what follows. For this purpose, without loss of generality, we assume that b1. Let

T k = ( 4 p σ 1 β k + 1 * ( p - 2 ) ( a σ 1 - λ ) ) 1 2 .

Proposition 2.15.

Let a>0 and 0<λ<aσ1. Let kN, let TTk and let ukb,T be the sign-changing solution found by Proposition 2.8 with ET,b(ukb,T)=α~kb,T. Then, for every 0<bn<min{bT,1}, with bn0+ as n, we have ukbn,Tuk* strongly in H01(Ω) for some uk*H01(Ω) as n. Moreover, uk* is a sign-changing solution of (𝒫a,0,λ) and has at most k+1 nodal domains. In particular, u1* is a least energy sign-changing solution of (𝒫a,0,λ) and has two nodal domains.

Proof.

We first consider the case of p(2,4](2,2*). By Proposition 2.14, we can see that ukbn,T,2T. Since T is independent of bn, we have that {ukbn,T} is bounded in H01(Ω) for n. Without loss of generality, we may assume that ukbn,Tuk* weakly in H01(Ω) as n. Thanks to the Sobolev embedding theorem, without loss of generality, we also have that ukbn,Tuk* strongly in Lr(Ω) for all 1r<2* as n. Now, by Proposition 2.14, we have bn(ukbn,T)=0 in H-1(Ω) for all n. It follows that

0 = b n ( u k b n , T ) v
= ( a + o n ( 1 ) ) Ω u k b n , T v d x - λ Ω u k b n , T v d x - Ω | u k b n , T | p - 2 u k b n , T v d x
= 0 ( u k * ) v + o n ( 1 )

for all vH01(Ω), where 0(u) is the corresponding functional of (𝒫a,0,λ). Thus, uk* is a solution of (𝒫a,0,λ). In particular, 0(uk*)uk*=0, which implies

λ u k * , 2 2 + u k * , p p = a u k * , 2 2
a u k b n , T , 2 2 + o n ( 1 )
= λ u k b n , T , 2 2 + u k b n , T , p p + o n ( 1 )
= λ u k * , 2 2 + u k * , p p .

Thus, we actually have that ukbn,Tuk* strongly in H01(Ω) as n. On the other hand, since 0<λ<aσ1 and bn(ukbn,T)=0 in H-1(Ω), we can see from the definitions of σ1 and 𝒮p that

( a - λ σ 1 ) ( u k b n , T ) ± , 2 2 + b n ( u k b n , T ) ± , 2 4 a ( u k b n , T ) ± , 2 2 - λ ( u k b n , T ) ± , 2 2 + b n ( u k b n , T ) ± , 2 4
= ( u k b n , T ) ± , p p
𝒮 p - p 2 ( u k b n , T ) ± , 2 p .

It follows that

0 < ( a - λ σ 1 ) 𝒮 p p 2 ( u k * ) ± , 2 p - 2 .

Therefore, uk* must be sign-changing. Now, suppose that uk* has at least k+2 nodal domains, denoted by {Ω~i}i=1,2,,l (lk+2). Let uk,i*=uk*χΩ~i, where χΩ~i is the characteristic function of the set Ω~i. Then, by the classical elliptic regularity theory and [25, Lemma 1], uk,i*H01(Ω) for all i=1,2,,l. Since 0(u) is local and 0(uk*)=0 in H-1(Ω), by a similar argument as that used in (2.28), we can see that

0 ( i = 1 l t i u k , i * ) t i | 𝐭 = 𝟏 = 0 ( u k , i * ) u k , i * = 0 for all  i = 1 , 2 , , l .

By a standard argument, we can see that 0(uk,i*)>0 for i=k+2,,l and

sup 𝐭 ( + ) l 0 ( i = 1 l t i u k , i * ) = 0 ( i = 1 l u k , i * ) .

Now, since ukbn,T,2T and ukbn,Tuk* strongly in H01(Ω) as n, from Lemmas 2.10 and 2.11, and Remark 2.12, we have that

lim n β k + 1 b n , T lim n sup 𝐭 ( + ) k + 1 T , b n ( i = 1 k + 1 t i u k , i * )
= 0 ( i = 1 k + 1 u k , i * )
< 0 ( u k * )
= lim n T , b n ( u k b n , T )
(2.32) lim n β k + 1 b n , T .

This is a contradiction. Now, by the above conclusion, it is easy to see that u1* has two nodal domains. On the other hand, by a similar argument as that used in the proof of Proposition 2.13, we can see that 0(u1*)=β2*. By a similar argument as that used in (2.32), we can show that 0(u*)β2*, where u* is a sign-changing solution of (𝒫a,0,λ). Thus, u1* is a least energy sign-changing solution of (𝒫a,0,λ). Next, let us consider the case of N=3 and 4<p<6. Since bn<1, by a similar argument as that used in (2.30), we can see that βkbn,Tβk**, where

β k * * = inf W H 0 1 ( Ω ) dim ( W ) k sup u W 1 ( u ) .

Now, since 4<p<6 and χp(s)1, by calculating bn(ukbn,T)-14bn(ukbn,T)ukbn,T in a standard way, we can show that {ukbn,T} is bounded in H01(Ω) for n. Now, we can apply the argument used for p(2,4](2,2*) step by step to obtain the conclusion. ∎

Proposition 2.16.

Let a>0, 0<b<bT and 0<λ<aσ1, where bT is given by Lemma 2.1. Then u1b,T, with TT1, is a least energy sign-changing solution of (Pabλ) in the case of N=3 and 4p<6.

Proof.

The conclusion for p>4 follows immediately from Proposition 2.13 and χp(s)1. In what follows, let us consider the case of p=4. Suppose on the contrary that there exists ub,0H01(Ω) such that b(ub,0)<b(u1b,T), b(ub,0)=0 in H-1(Ω) and ub,0±0. Then, by Proposition 2.14, we have

a T 2 2 + b T T 4 4 b ( u b , 0 ) - 1 4 b ( u b , 0 ) u b , 0 a σ 1 - λ 4 σ 1 u b , 0 , 2 2 .

It follows that {ub,0} is bounded in H01(Ω). Now, by a similar argument as that used in the proof of Proposition 2.15, we can see that ub,0u* strongly in H01(Ω) as b0+, up to a subsequence, where u* is a least energy sign-changing solution of (𝒫a,0,λ). By a standard argument, we can see that

u * , 2 ( 4 σ 1 β 2 * a σ 1 - λ ) 1 2 .

Thus, ub,0,2<T1 for b small enough. Without loss of generality, by choosing bT small enough, we may assume ub,0,2<T1 for 0<b<bT. It follows that ub,0 is also a critical point of T,b(u) for TT1. By Proposition 2.13, we must have T,b(u1b,T)T,b(ub,0). This is impossible due to Proposition 2.14. Thus, u1b,T, with TT1, is a least energy sign-changing solution of (Pabλ) in the case of N=3 and 4p<6. ∎

Let us prove some more properties of the sign-changing solution of (Pabλ). We first give a technical lemma.

Lemma 2.17.

Let N=3 and 0<λ<aσ1. Let W=span{u1,u2,,uk}, with dim(W)=k and uiuj=0 for all i,j=1,2,,k, with ij. If p=4, then for every uAk*, there exists a unique s(R+)k such that

b ( i = 1 k s i u i ) = max 𝐭 ( + ) k b ( i = 1 k t i u i ) ,

where

𝒜 k * = { 𝐮 W | j = 1 , i j k b u i , 2 2 u j , 2 2 + b u i , 2 4 - u i , 4 4 < 0 for all  i = 1 = 2 , , k and  i = 1 k u i , 2 2 = 1 } .

Proof.

The proof is very similar to that of Lemma 2.11 for the case of N=3 and 4<p<6, therefore we only point out the differences. Indeed, for every 𝐮𝒜k*, we consider the matrix M=(Mi,j)i,j=1,2,,k, where

M i , i = u i , 4 4 - b u i , 2 4 and M i , j = - b u i , 2 2 u j , 2 2

for all i,j=1,2,,k, with ij. Clearly, M is symmetric. Moreover, since 𝐮𝒜k*, we can see that Mi,i>0, Mi,j<0 and j=1kMi,j>0 for all i,j=1,2,,k, with ij. It follows that M is diagonally dominant and all eigenvalues of M are positive, which together with μ[0,1], implies

b ( i = 1 k t i u i ) - and i = 1 k M i , * ( μ , 𝐭 ) - as  | 𝐭 |

for all i=1,2,,k, where Mi,*(μ,𝐭) are given by (2.24). Now, we can follow the argument used for the case of N=3 and 4<p<6 in Lemma 2.11 step by step to get the conclusion. ∎

Let the sign-changing Nehari manifold of b(u) be

b = { u ± H 0 1 ( Ω ) { 0 } b ( u ) u ± = 0 } .

Then it is easy to see that all sign-changing solutions of (Pabλ) are contained in b.

Proposition 2.18.

Let N=3, 4p<6, 0<λ<aσ1 and 0<b<bT, where bT is given by Lemma 2.1. Then β2b,T=infMbEb(u) for TT1. Moreover, we also have β2b,T>2mb, where mb=infuNbEb(u) and Nb is the Nehari manifold of Eb(u) given by

𝒩 b = { u H 0 1 ( Ω ) { 0 } b ( u ) u = 0 } .

Proof.

We only prove the case p=4, since the case 4<p<6 is similar and much more simple due to the construction of χp(s). By Proposition 2.16, we can see that u1b,Tb for TT1. It follows that β2b,Tinfbb(u) for TT1. On the other hand, for every ε>0, we can take uεb such that b(uε)<infbb(u)+ε. By (2.30), we have

β 2 * + b T T 4 + ε b ( u ε ) - 1 4 b ( u ε ) u ε a σ 1 - λ 4 σ 1 u ε , 2 2 .

By choosing bT and ε small enough if necessary, we can see that uε,228σ1β1*aσ1-λ=T12. Thus, we also have b,T(uε)<infbb(u)+ε for TT1. Now, since dim(span{uε+,uε-})=2 and uε+uε-=0, by Lemma 2.17, we can see that β2b,Tb,T(uε)<infb(u)+ε for TT1. Thus, we must have β2b,T=infbb(u) for TT1. It remains to show that β2b,T>2mb. Indeed, by similar arguments as those used in [1, Lemma 2.3] and [36, Lemma 4.3], we can see that

m b = inf u H 0 1 ( Ω ) { 0 } sup t 0 b ( t u ) for  p 4 .

Now, thanks to Lemma 2.17once more, we can obtain β2b,T>2mb for TT1 by a standard argument. ∎

By Proposition 2.15, we can see that β2b,Tβ2* as b0+ for TT1. In what follows, let us obtain some more precise concentration behavior of β2b,T as b0+. For this purpose and for simplicity, in what follows, we always assume that TT1, and we re-denote β2b,T by β2(b) and consider it as a function of b. Without loss of generality, we also assume that bT1, where bT is given by Lemma 2.1.

Lemma 2.19.

Let 0<λ<aσ1 and 0<b<bT. Then β2(b) is strictly increasing for b. Moreover,

β 2 ( b ) = 1 4 u 1 b , T , 2 4 for almost  b ( 0 , b T ) .

Proof.

We only give the proof for p(2,4](2,2*), since the case of N=3 and 4<p<6 is similar and much more simple due to χp(s)1 for 4<p<6. Let 0<b1<b2<bT. Since Lemma 2.11 holds, we can see from a standard argument that β2(b1)<β2(b2). It follows that β2(b) is strictly increasing on (0,bT). Thanks to Lebesgue’s lemma, β2(b) exists for almost b(0,bT). Now, consider the following system in (+)2:

0 = f 1 ( μ , 𝐭 )
= a t 1 ( u 1 b , T ) + , 2 2 - λ t 1 ( u 1 b , T ) + , 2 2 - t 1 ( u 1 b , T ) + , p p
+ μ χ p ( 𝒴 ( t 1 ( u 1 b , T ) + , t 2 ( u 1 b , T ) - ) T 2 ) 𝒴 ( t 1 ( u 1 b , T ) + , t 2 ( u 1 b , T ) - ) t 1 ( u 1 b , T ) + , 2 2
+ μ t 1 ( u 1 b , T ) + , 2 2 2 T 2 χ p ( 𝒴 ( t 1 ( u 1 b , T ) + , t 2 ( u 1 b , T ) - ) T 2 ) 𝒴 ( t 1 ( u 1 b , T ) + , t 2 ( u 1 b , T ) - ) 2

and

0 = f 2 ( μ , 𝐭 )
= a t 2 ( u 1 b , T ) - , 2 2 - λ t 2 ( u 1 b , T ) - , 2 2 - t 2 ( u 1 b , T ) - , p p
+ μ χ p ( 𝒴 ( t 1 ( u 1 b , T ) + , t 2 ( u 1 b , T ) - ) T 2 ) 𝒴 ( t 1 ( u 1 b , T ) + , t 2 ( u 1 b , T ) - ) t 2 ( u 1 b , T ) - , 2 2
+ μ t 2 ( u 1 b , T ) - , 2 2 2 T 2 χ p ( 𝒴 ( t 1 ( u 1 b , T ) + , t 2 ( u 1 b , T ) - ) T 2 ) 𝒴 ( t 1 ( u 1 b , T ) + , t 2 ( u 1 b , T ) - ) 2 ,

where

𝒴 ( t 1 ( u 1 b , T ) + , t 2 ( u 1 b , T ) - ) = t 1 ( u 1 b , T ) + , 2 2 + t 2 ( u 1 b , T ) - , 2 2 and 𝐭 = ( t 1 , t 2 ) ( + ) 2 .

Clearly, since 2<p, there exists 𝐭0(+)2 such that fi(0,𝐭0)=0, i=1,2. Moreover, we also have

f i t i | 𝐭 = 𝐭 0 , μ = 0 = 2 ( a t i 0 ( u 1 b , T ) ± , 2 2 - λ t i 0 ( u 1 b , T ) ± , 2 2 ) - p t i 0 ( u 1 b , T ) ± , p p

and

f i t j | 𝐭 = 𝐭 0 , μ = 0 = 0 for all  i , j = 1 , 2 , with  i j .

Thus, since fi(0,𝐭0)=0, i=1,2, we have that

f i t i | 𝐭 = 𝐭 0 , μ = 0 = - ( p - 2 ) ( a t i 0 ( u 1 b , T ) ± , 2 2 - λ t i 0 ( u 1 b , T ) ± , 2 2 ) for all  i = 1 , 2 .

Since p>2, we can see that j=12fitj|𝐭=𝐭0,μ=0<0 for all i=1,2. Thus, the matrix f~=(fi,j)i,j=1,2 is nonsingular, where fi,j=fitj|𝐭=𝐭0,μ=0. By the implicit function theorem, there exists a C1 function 𝐭(μ)=(t1(μ),t2(μ)) near 0 such that fi(μ,𝐭(μ))=0, i=1,2, and 𝐭(0)=𝐭0. By choosing bT small enough, we may assume that 𝐭(b) is C1 for all 0<b<bT. Now, since dim(span{(u1b,T)+,(u1b,T)-})=2 and (u1b,T)+(u1b,T)-=0 for every 0<b<bT, by Lemma 2.11 and u1b,Tb for TT1, we have from the Taylor expansion that

β 2 ( μ ) μ , T ( t 1 ( μ ) ( u 1 b , T ) + + t 2 ( μ ) ( u 1 b , T ) - )
= β 2 ( b ) + b , T ( u 1 b , T ) ( u 1 b , T ) + t 1 ( b ) ( μ - b ) + b , T ( u 1 b , T ) ( u 1 b , T ) - t 2 ( b ) ( μ - b )
(2.33) + 1 4 u 1 b , T , 2 4 ( μ - b ) + o ( μ - b ) .

Note that b,T(u1b,T)=0 in H-1(Ω). Thus, we can see from (2.33) that

lim μ b + β 2 ( μ ) - β 2 ( b ) μ - b 1 4 u 1 b , T , 2 4

and

lim μ b - β 2 ( μ ) - β 2 ( b ) μ - b 1 4 u 1 b , T , 2 4 ,

which implies β2(b)=14u1b,T,24 if β2(b) exists. ∎

Proposition 2.20.

Let 0<λ<aσ1 and 0<b<bT, where bT is given by Lemma 2.1. Then we have

1 β 2 * - 1 β 2 ( b ) = O ( b ) as  b 0 + .

Moreover, if N=3 and 4<p<6, then β2(b)=O(bpp-4) as b+.

Proof.

Since b(u1b,T)=b(u1b,T)-1pb(u1b,T)u1b,T, we have

(2.34) β 2 ( b ) = p - 2 2 p ( a u 1 b , T , 2 2 - λ u 1 b , T , 2 2 ) + b ( p - 4 ) 4 p u 1 b , T , 2 4 .

It follows from the definition of σ1 that

( a σ 1 - λ ) ( p - 2 ) 2 p σ 1 u 1 b , T , 2 2 + b ( p - 4 ) 4 p u 1 b , T , 2 4 β 2 ( b )

and

β 2 ( b ) a ( p - 2 ) 2 p u 1 b , T , 2 2 + b ( p - 4 ) 4 p u 1 b , T , 2 4 ,

which, together with Lemma 2.19, implies

(2.35) ( a σ 1 - λ ) ( p - 2 ) p σ 1 ( β 2 ( b ) ) 1 2 + b ( p - 4 ) p β 2 ( b ) β 2 ( b )

and

(2.36) β 2 ( b ) a ( p - 2 ) p ( β 2 ( b ) ) 1 2 + b ( p - 4 ) p β 2 ( b )

for almost b(0,bT). By Proposition 2.15 and Lemma 2.19 once more, for b small enough, we have that

b ( p - 4 ) p ( β 2 ( b ) ) 1 2 a ( p - 2 ) p for  p 4

and

- ( a σ 1 - λ ) ( p - 2 ) 2 p σ 1 b ( p - 4 ) p ( β 2 ( b ) ) 1 2 for  2 < p < 4 .

Thus, by (2.35) and (2.36), we can see that

( a σ 1 - λ ) ( p - 2 ) 2 p σ 1 ( β 2 ( b ) ) 1 2 β 2 ( b ) 2 a ( p - 2 ) p ( β 2 ( b ) ) 1 2 .

Therefore, 1β2*-1β2(b)=O(b) as b0+. Since bT=+ in Lemma 2.19 for N=3 and 4<p<6, by Proposition 2.15, Lemma 2.19 and (2.34), we can see that b(β2(b))12+ as b+. From (2.35) and (2.36), it follows that

b ( p - 4 ) p β 2 ( b ) β 2 ( b ) ( 1 + o ( 1 ) ) b ( p - 4 ) p β 2 ( b ) ,

where o(1)0 as b+. Hence, we have β2(b)=O(bpp-4) as b+ for N=3 and 4<p<6. ∎

We close this section by noting that the proof of Theorem 1.1 follows immediately from Propositions 2.13, 2.14 and 2.16, and the proof of Theorem 1.3 follows immediately from Propositions 2.15 and 2.182.20.

3 The Case of Ω=𝔹R

Since Ω=𝔹R is radially symmetric, we can consider the radial solution of (Pabλ). Let

H 0 , R 1 ( 𝔹 R ) = { u H 0 1 ( 𝔹 R ) u  is radially symmetric } .

Then by the symmetric criticality principle of Palais, the critical points of b(u) in H0,R1(𝔹R) are also critical points of b(u) in H01(𝔹R). Thus, we consider the functional b(u) in the Hilbert space H0,R1(𝔹R) in what follows. Moreover, for the sake of clarity, we re-denote u,r by u,r,R in this section.

3.1 An Auxiliary Elliptic System of Kirchhoff Type

For a positive integer k2, we consider the following weak coupled elliptic system of Kirchhoff type:

(3.1) { - 𝒬 ~ T , b ( 𝐮 ) Δ u i = λ u i + | u i | p - 2 u i + β ( j = 1 , j i k | u j | p 2 ) | u i | p 2 - 2 u i , u i H 0 , R 1 ( 𝔹 R ) , i = 1 , 2 , , k ,

where p(2,4](2,2*) and

𝒬 ~ T , b ( 𝐮 ) = a + b χ p ( i = 1 k u i , 2 , R 2 T 2 ) i = 1 k u i , 2 , R 2 + b ( i = 1 k u i , 2 , R 2 ) 2 2 T 2 χ p ( i = 1 k u i , 2 , R 2 T 2 ) .

Let be the Hilbert space of H0,R1(𝔹R), equipped with the inner product u,v=𝔹Rauv-λuvdx. Since λ<aσ1, is also a Hilbert space and the corresponding norm is given by u=u,u12. Set k=()k. Then k is a Hilbert space with the inner product 𝐮,𝐯=i=1kui,vi and the corresponding norm is given by 𝐮=𝐮,𝐮12, where ui,vi are the ith component of 𝐮,𝐯, respectively. Define

𝒥 T , b ( 𝐮 ) = i = 1 k ( 1 2 u i 2 - 1 p u i , p , R p ) + b 4 χ p ( i = 1 k u i , 2 , R 2 T 2 ) ( i = 1 k u i , 2 , R 2 ) 2 - 2 β p i , j = 1 , i j k | u i | p 2 | u j | p 2 , 1 , R .

Then it is easy to see that 𝒥T,b(𝐮) is of C2 in k, and the critical points of 𝒥T,b(𝐮) are solutions of the Kirchhoff-type elliptic system (3.1).

Definition 3.1.

We say that 𝐮k is a nontrivial critical point of 𝒥T,b(𝐮) if 𝒥T,b(𝐮)=0 in k-1 and ui0 for all i=1,2,,k, where 𝒥T,b(𝐮) is the Fréchet derivative of 𝒥T,b(𝐮) and k-1 is the dual space of k.

Let k*=({0})k and 𝐮i=(0,,ui,,0), and define

𝒲 T , b = { 𝐮 k * 𝒥 T , b ( 𝐮 ) 𝐮 i = 0  for all  i = 1 , 2 , , k } .

Then it is easy to see that all nontrivial critical points of 𝒥T,b(𝐮) are contained in 𝒲T,b. For every 𝐮k*, we define Φ𝐮,b(𝐭)=𝒥T,b(𝐭𝐮), where 𝐭(+)k is a vector and 𝐭𝐮=(t1u1,t2u2,,tkuk). Since 𝒥T,b(𝐮) is of C2 in k, it is easy to see that Φ𝐮,b(𝐭) is of C2 in (+)k and Φ𝐮,b(𝐭)ti=0 for all i=1,2,,k if and only if 𝐭𝐮𝒲T,b. In particular, Φ𝐮,b(𝟏)ti=0 for all i=1,2,,k if and only if 𝟏𝐮𝒲T,b.

Let

Υ = { 𝐮 k * | β j = 1 , j i k | u i | p 2 | u j | p 2 , 1 , R + u i , p , R p > 0 , i = 1 , 2 , , k } .

Lemma 3.2.

Let 0<λ<aσ1, p(2,4](2,2*) and β<0. Then, for every uΥ, there exists a unique s0(R+)k such that s0uWT,0. Moreover,

Φ 𝐮 , 0 ( 𝐬 0 ) = max 𝐭 ( + ) k Φ 𝐮 , 0 ( 𝐭 ) .

Proof.

Let μ[-1,1] and consider the following system in (+)k:

M i ( μ , 𝐭 ) = - μ j = 1 , i j k β | t i u i | p 2 | t j u j | p 2 , 1 , R + t i u i 2 - t i u i , p , R p = 0 , i = 1 , 2 , , k .

Clearly, Mi(0,𝐭)=0, i=1,2,,k, is unique solvable in (+)k. Define

𝒵 = { μ [ 0 , 1 ] M i ( μ , 𝐭 )  is uniquely solvable in  ( + ) k } .

Then, for every μ𝒵, we have

M i t i = 2 t i u i 2 - p t i p - 1 u i , p , R p - p μ 2 t i p 2 - 1 j = 1 , i j k β | u i | p 2 | t j u j | p 2 , 1 , R
(3.2) = ( 2 - p ) t i u i 2 + p μ 2 t i p 2 - 1 j = 1 , i j k β | u i | p 2 | t j u j | p 2 , 1 , R

and

(3.3) M i t j = - p μ 2 t j p 2 - 1 j = 1 , i j k β | t i u i | p 2 | u j | p 2 , 1 , R

for all i,j=1,2,,k, with ij. Note that 2<p and β<0. We can see from a similar argument as that used in the proof of Lemma 2.11 that the matrix M=(Mi,j)i,j=1,2,,k is nonsingular, where Mi,j=Mitj for all i,j=1,2,,k. On the other hand, let

θ i , j ( 𝐮 ) = β i , j | u i | p 2 | u j | p 2 , 1 , R , i , j = 1 , 2 , , k ,

where βi,i=1 and βi,j=μβ for all i,j=1,2,,k, with ij. Since 𝐮Υ and μ[0,1], we can see that θi,j(𝐮)<0 and j=1kθi,j(𝐮)>0 for all i,j=1,2,,k, with ij. Thus, the matrix θ(𝐮)=(θi,j(𝐮))i,j=1,2,,k is strictly diagonally dominant. It follows that

(3.4) i = 1 k ( j = 1 , j i k μ β | t i ( μ ) u i | p 2 | t j ( μ ) u j | p 2 , 1 , R + t i ( μ ) u i , p , R p ) C i = 1 k ( t i ( μ ) ) p .

Thus, by 2<p and β<0 once more, we can follow the argument used in the proof of Lemma 2.11 for the case of N=3 and 4<p<6 step by step to obtain the conclusion. ∎

Lemma 3.3.

Let 0<λ<aσ1, β<0 and 0<b<bT, where bT is given by Lemma 2.1. Then for every uΥ, there exists a unique sb(R+)k such that sbuWT,b. Moreover, Φu,b(sb)=maxt(R+)kΦu,b(t).

Proof.

Consider the following system in (+)k:

M i ( 𝐭 , b ) = 𝒬 ~ T , b ( 𝐭 𝐮 ) t i u i , 2 , R 2 - λ t i u i , 2 , R 2 - t i u i , p , R p - β j = 1 , i j k | t i u i | p 2 | t j u j | p 2 , 1 , R = 0 , i = 1 , 2 , , k .

Since χp(s) is of C, for all i=1,2,,k, Mi(𝐭,b) is of C for all tj and b. Note that 0<λ<aσ1. Then, by Lemma 3.2, Mi(𝐭,0)=0, i=1,2,,k, is unique solvable in (+)k. On the other hand, by similar arguments as those used in (3.2) and (3.3), we also have

M i t i | b = 0 = ( 2 - p ) t i u i 2 + p μ 2 t i p 2 - 1 j = 1 , i j k β | u i | p 2 | t j u j | p 2 , 1 , R

and

M i t j | b = 0 = - p μ 2 t j p 2 - 1 j = 1 , i j k β | t i u i | p 2 | u j | p 2 , 1 , R

for all i,j=1,2,,k, with ij. Note that the matrix M=(Mi,j)i,j=1,2,,k is nonsingular, where Mi,j=Mitj|b=0 for all i,j=1,2,,k. Thus, by a similar argument as that used in the proof of Lemma 2.11, Mi(𝐭,b)=0, i=1,2,,k, is unique solvable in (+)k for b small enough, due to the construction of χp in the case p(2,4](2,2*). By choosing bT small enough if necessary, we may assume that Mi(𝐭,b)=0, i=1,2,,k, is unique solvable in (+)k for 0<b<bT in the case p(2,4](2,2*). Since p>2 and (3.4) holds for every 𝐮Υ, we can follow the argument used in the proof of Lemma 2.11 step by step to obtain the conclusion in the case p(2,4](2,2*), due to the construction of χp, too. For N=3 and 4<p<6, we define

𝒵 = { μ [ 0 , 1 ] M i ( 𝐭 , μ b )  is uniquely solvable in  ( + ) k } .

Then, for every μ𝒵, by 2<p and the construction of χp, we also have from similar arguments that the matrix M=(Mi,j)i,j=1,2,,k is strictly diagonally dominant and nonsingular, where Mi,j=Mitj for all i,j=1,2,,k and b>0. Thus, by 2<p and the construction of χp once more, we can follow the argument used in the proof of Lemma 2.11 step by step to obtain the conclusion for N=3 and 4<p<6, too. ∎

By Lemma 3.3, we can see that 𝒲T,b for 0<b<bT.

Lemma 3.4.

Let 0<λ<aσ1, β<0 and 0<b<bT, where bT is given by Lemma 2.1. Then WT,b is a natural constraint in Hk.

Proof.

Let i,b(𝐮)=𝒥T,b(𝐮)𝐮i, i=1,2,,k. Clearly, i,b(𝐮) is of C1 in k. Suppose 𝐮 is a critical point of 𝒥T,b|𝒲T,b. Then, by the method of Lagrange multipliers, there exists {ξi}i=1,2,,k()k such that

(3.5) 𝒥 T , b ( 𝐮 ) - j = 1 k ξ j j , b ( 𝐮 ) = 0  in  k - 1 .

Multiplying this equation with 𝐮i, we can see that 𝝃=(ξ1,ξ2,,ξk) is the solution of the following system:

(3.6) j = 1 k ξ j j , b ( 𝐮 ) 𝐮 i = 0 , i = 1 , 2 , , k .

Let Θi,j(b,𝐮)=j,b(𝐮)𝐮i, i,j=1,2,,k. Then we have

Θ i , i ( 0 , 𝐮 ) = ( 2 - p ) u i 2 + p 2 j = 1 , i j k β | u i | p 2 | u j | p 2 , 1 , R

and

Θ i , j ( 0 , 𝐮 ) = - p β 2 j = 1 , i j k | u i | q 2 | u j | q 2 , 1 , R

for all i,j=1,2,,k, ji. Note that 2<p and β<0. We can see that the matrix Θ(0)=(Θi,j(0,𝐮))i,j=1,2,,k is nonsingular. Since the matrix Θ(b) is uniformly continuous for b, due to the construction of χp, Θ(b) is nonsingular for b small enough, independent of 𝐮 in the case p(2,4](2,2*). By choosing bT small enough if necessary, we may assume that Θ(b) is nonsingular for 0<b<bT in the case p(2,4](2,2*). Therefore, 𝟎 is the unique solution of (3.6) due to Cramer’s rule, and we must have 𝝃=𝟎 for 0<b<bT in the case p(2,4](2,2*). Thanks to (3.5), we have 𝒥T,b(𝐮)=0 in k-1 for 0<b<bT in the case p(2,4](2,2*), that is, 𝒲T,b is a natural constraint in k for 0<b<bT in the case p(2,4](2,2*). For N=3 and 4<p<6, since χp(s)1, we have from (3.5) that

0 = j = 1 k ξ j j , b ( 𝐮 ) 𝐮 i
= j = 1 , j i k ξ j ( 2 b u i , 2 , R 2 u j , 2 , R 2 - β p 2 | u i | p 2 | u j | p 2 , 1 , R ) + ξ i ( 2 u i 2 + 4 b u i , 2 , R 4 - p u i , p , R p )
+ ξ i j = 1 , i j k ( 2 b u i , 2 , R 2 u j , 2 , R 2 - β p 2 | u i | p 2 | u j | p 2 , 1 , R )
= j = 1 , j i k ξ j ( 2 b u i , 2 , R 2 u j , 2 , R 2 - β p 2 | u i | p 2 | u j | p 2 , 1 , R ) - ξ i ( 2 u i 2 + ( p - 4 ) u i , p , R p )
- ξ i j = 1 , i j k ( 2 b u i , 2 , R 2 u j , 2 , R 2 + ( p 2 - 4 ) β | u i | p 2 | u j | p 2 , 1 , R )

for all i=1,2,,k. Denote

Θ i , i * = - j = 1 , i j k ( 2 b u i , 2 , R 2 u j , 2 , R 2 + ( p 2 - 4 ) β | u i | p 2 | u j | p 2 , 1 , R ) - ( 2 u i 2 + ( p - 4 ) u i , p , R p )

and

Θ i , j * = ( 2 b u i , 2 , R 2 u j , 2 , R 2 - β p 2 | u i | p 2 | u j | p 2 , 1 , R )

for i,j=1,2,,k, with ji. Since β<0, we have Θi,j*>0. Moreover, we can see that

j = 1 k Θ i , j * = - ( 2 u i 2 + ( p - 4 ) u i , p , R p ) + ( 4 - p ) β i , j = 1 , i j k | u i | p 2 | u j | p 2 , 1 , R = ( 2 - p ) u i 2 < 0 ,

due to 2<p. It follows that the matrix Θ*=(Θi,j*)i,j=1,2,,k is nonsingular. Therefore, 𝟎 is also the unique solution of (3.6) due to Cramer’s rule, and we must have 𝝃=𝟎 for all b>0 in the case of N=3 and 4<p<6. Thanks to (3.5), we also have 𝒥T,b(𝐮)=0 in k-1 for all b>0 in the case of N=3 and 4<p<6, that is, 𝒲T,b is a natural constraint in k for all b>0 in the case of N=3 and 4<p<6. ∎

Let

m T , b = inf 𝐮 𝒲 T , b 𝒥 T , b ( 𝐮 ) .

It is easy to see that if 𝐮0𝒲T,b attains mT,b, then 𝐮0 must be a local minimum point of 𝒥T,b(𝐮) in 𝒲T,b. It follows from Lemma 3.4 that 𝐮0 is also a critical point of 𝒥T,b(𝐮) in k. Let

k = { 𝐫 = ( R i ) i = 1 , 2 , , k ( + ) k 0 < R 1 < R 2 < < R k = R } .

We set R0=0. Then, for every 𝐫k, we define 𝐫,k=i=1kH0,R1(𝔹Ri𝔹Ri-1). For simplicity, we denote H0,R1(𝔹Ri𝔹Ri-1) by 𝐫,ki. Then it is easy to see that 𝐫,kiH0,R1(𝔹R) for all i=1,2,,k. Define

T , b , k 𝐫 = { u ~ 𝐫 , k T , b ( u ) u i = 0 } ,

where ~𝐫,k=i=1k(𝐫,ki{0}) and ui is the projection of u in 𝐫,ki{0}. Then, by Lemma 2.11, T,b,k𝐫 for 0<b<bT. Set

γ T , b 𝐫 = inf u T , b , k 𝐫 T , b ( u ) .

By Lemma 2.11 once more, we can see that γT,b𝐫0 for 0<b<bT. Define

(3.7) γ T , b k = inf 𝐫 k γ T , b 𝐫 .

Note that by a standard argument, we have that

0 ( u ) 0 ( i = 1 k t i u i ) for all  𝐭 = ( t i ) i = 1 , 2 , , k ( + ) k  and  u 0 , k 𝐫 ,

where 0,k𝐫={u~𝐫,k0(u)ui=0}. Then, by Lemma 2.11 and a similar argument as that used in (2.30), we can see that γT,b𝐫γ𝐫+bT4 for p(2,4](2,2*), where

γ 𝐫 = inf u 0 , k 𝐫 0 ( u ) .

Let

(3.8) γ k = inf 𝐫 k γ 𝐫 .

Then γT,bkγk+bT4 for p(2,4](2,2*). Let

b , k 𝐫 = { u 𝐫 , k b ( u ) u i = 0 } .

Then, by Lemma 2.11 once more, b,k𝐫 for all b>0 in the case of N=3 and 4<p<6. Set

γ b 𝐫 = inf u b , k 𝐫 b ( u ) .

By Lemma 2.11 once more, we can see that γb𝐫0 for all b>0 in the case of N=3 and 4<p<6. Define

γ b k = inf 𝐫 k γ b 𝐫 .

Lemma 3.5.

Let 0<λ<aσ1 and β<0. Then we have the following:

  1. 0 < m T , b γ T , b k γ k + b T 4 for 0 < b < b T in the case of p ( 2 , 4 ] ( 2 , 2 * ) , where b T is given by Lemma 2.1.

  2. m T , b is independent of T and 0 < m T , b γ b k for all b > 0 in the case of N = 3 and 4 < p < 6 .

Proof.

(1) Let {𝐮n}𝒲T,b be a minimizing sequence of 𝒥T,b(𝐮) at the energy value mT,b. Then, since p(2,4](2,2*) and β<0, we can see that

𝒥 T , b ( 𝐮 n ) - 1 p 𝒥 T , b ( 𝐮 n ) 𝐮 n = i = 1 k ( a 2 u i n , 2 , R 2 - ( p - 2 ) λ 2 p u i n , 2 , R 2 - 1 p 𝒬 ~ T , b ( 𝐮 n ) u i n , 2 , R 2 )
+ b 4 χ p ( i = 1 k u i n , 2 , R 2 T 2 ) ( i = 1 k u i n , 2 , R 2 ) 2
(3.9) p - 2 2 p i = 1 k ( a - λ σ 1 ) u i n , 2 , R 2 - 4 b T 4 p .

On the other hand, since β<0 and using (2.4), we can see from {𝐮n}𝒲T,b that

1 2 ( a - λ σ 1 ) 𝒮 p u i n , p 2 1 2 ( a - λ σ 1 ) u i n , 2 , R 2
𝒬 ~ T , b ( 𝐮 n ) u i n , 2 , R 2 - λ u i n , 2 , R 2
= u i n , p , R p + β i , j = 1 , i j k | u i n | p 2 | u j n | q 2 , 1 , R
(3.10) u i n , 2 , R p

for all i=1,2,,k. It follows that

u i n , p , R ( 1 2 ( a - λ σ 1 ) 𝒮 p ) 1 p - 2 > 0 for all  i = 1 , 2 , , k ,

which implies

u i n , 2 , R 2 ( 1 2 ( a - λ σ 1 ) ) 2 p - 2 𝒮 p p p - 2 for all  i = 1 , 2 , , k .

Thus, by (3.9), we can see that mT,b>0 for b small enough. By choosing bT small enough if necessary, we may assume that mT,b>0 for 0<b<bT. Take 𝐫k and uT,b,k𝐫. Then, by the construction of ~𝐫,k, we can see that 𝐮𝒲T,b, where 𝐮=(ui)i=1,2,,k and ui is the projection of u in 𝐫,ki{0}. Thus, we must have mT,bγT,bk. It follows that mT,bγk+bT4 for p(2,4](2,2*).

(2) By the construction of χp, it is easy to see that mT,b is independent of T for N=3 and 4<p<6. Moreover, by calculating 𝒥T,b(𝐮n)-14𝒥T,b(𝐮n)𝐮n and by a similar argument as that used for (1), we have 0<mT,bγbk for all b>0. This completes the proof. ∎

Let

T k * = ( 4 p σ 1 γ k ( p - 2 ) ( a σ 1 - λ ) ) 1 2 .

Lemma 3.6.

Let 0<λ<aσ1, β<0 and 0<b<bT, where bT is given by Lemma 2.1. Suppose {un}WT,b is a minimizing sequence of JT,b(u) at the energy value mT,b. Then we have i=1kBuin,2,R2T2 for TTk* in the case of p(2,4](2,2*).

Proof.

Assume that {𝐮n}𝒲T,b is a minimizing sequence of 𝒥T,b(𝐮) at the energy value mT,b. Suppose that i=1kuin,2,R2>T2. Then, by Lemma 3.5 and (3.9), we can see that

(3.11) ( p - 2 ) ( a σ 1 - λ ) 2 p σ 1 T 2 4 b T 4 p + γ k + b T 4 + o n ( 1 ) .

Since 0<λ<aσ1, by choosing bT small enough if necessary so that 4bT4p+bT4<γk, we can see that (3.11) cannot hold for TTk*. ∎

Lemma 3.7.

Let 0<λ<aσ1, β<0 and 0<b<bT, where bT is given by Lemma 2.1. Then mT,b can be attained by some u0,*WT,b for TTk*.

Proof.

Let {𝐮n}𝒲T,b be a minimizing sequence of 𝒥T,b(𝐮) at the energy value mT,b. Then, by (3.9), {𝐮n} is bounded in k for p(2,4](2,2*) if TTk*. For N=3 and 4<p<6, by calculating 𝒥T,b(𝐮n)-14𝒥T,b(𝐮n)𝐮n in a standard way, we can also show that {𝐮n} is bounded in k for all T>0. In other words, we always have that {𝐮n} is bounded in k for TTk*. It follows that 𝐮n𝐮* weakly in k as n, up to a subsequence. Without loss of generality, we may assume that 𝐮n𝐮* weakly in k as n. By (3.10) and the Sobolev embedding theorem, we can see that ui*0 for all i=1,2,,k and 𝐮*k*. By Lemma 3.6 and the construction of χp, we also have that 𝐮*Υ. Now, by Lemma 3.3, there exists 𝐬*(+)k such that 𝐬*𝐮*𝒲T,b. Let

(3.12) f ( t ) = a t 2 + b t 2 4 χ p ( t T 2 ) .

Then, by the construction of χp(s), we can see that f(t) is increasing for b small enough. By choosing bT small enough if necessary, we may assume that f(t) is increasing for 0<b<bT. From the Hölder inequality and Lemma 3.3, it follows that

m T , b + o n ( 1 ) 𝒥 T , b ( 𝐬 * 𝐮 n )
= b 4 χ p ( i = 1 k s i * u i n , 2 , R 2 T 2 ) ( i = 1 k s i * u i n , 2 , R 2 ) 2 + a 2 ( i = 1 k s i * u i n , 2 , R 2 ) - λ 2 s i * u i n , 2 , R 2 - 1 p s i * u i n , p , R p
- 2 β p i , j = 1 , i j k | s i * u i n | p 2 | s j * u j n | p 2 , 1 , R
b 4 χ p ( i = 1 k s i * u i * , 2 , R 2 T 2 ) ( i = 1 k s i * u i * , 2 , R 2 ) 2 + a 2 ( i = 1 k s i * u i * , 2 , R 2 ) - λ 2 s i * u i * , 2 , R 2 - 1 p s i * u i * , p , R p
- 2 β p i , j = 1 , i j k | s i * u i * | p 2 | s j * u j * | p 2 , 1 , R + o n ( 1 )
= 𝒥 T , b ( 𝐬 * 𝐮 * ) + o n ( 1 )
m T , b + o n ( 1 ) ,

which implies that mT,b is attained by 𝐮0,*=𝐬*𝐮*. ∎

Proposition 3.8.

Let 0<λ<aσ1, β<0 and 0<b<bT, where bT is given by Lemma 2.1. Then (3.1) has a nonnegative solution uT,b such that JT,b(uT,b)=mT,b. Here, we say that uT,b is a nonnegative solution of (3.1) if uT,b is a solution of (3.1) with ui0 and ui0 for all i=1,2,,k, where uT,b=(ui)i=1,2,,k.

Proof.

By Lemma 3.7, we can easily see that 𝐮T,b=(|si*ui*|)i=1,2,,k is also the minimizer of 𝒥T,b(𝐮) in 𝒲T,b. It follows from Lemma 3.4 that 𝐮T,b is a critical point of 𝒥T,b(𝐮), which implies that 𝐮T,b is a nonnegative solution of (3.1) such that 𝒥T,b(𝐮T,b)=mT,b. ∎

Since (3.1) is dependent on β, we re-denote 𝒥T,b, 𝒲T,b, mT,b and 𝐮T,b by 𝒥T,b,β, 𝒲T,b,β, mT,b,β and 𝐮T,b,β, respectively, where 𝐮T,b,β=(uT,b,β1,uT,b,β2,,uT,b,βk).

Lemma 3.9.

Let 0<λ<aσ1, TTk* and 0<b<bT, where bT is given by Lemma 2.1. If βn- as n, then we have

β n i , j = 1 , i j k | u T , b , β i | p 2 | u T , b , β j | p 2 , 1 , R 0

and uT,b,βnu^T,b strongly in Hk, with u^T,bi0 and u^T,biu^T,bj=0 in BR for all i,j=1,2,,k, with ij.

Proof.

By a similar argument as that used for (3.9) in the case of p(2,4](2,2*) or by calculating 𝒥T,b(𝐮n)-14𝒥T,b(𝐮n)𝐮n in a standard way in the case of N=3 and 4<p<6, we can show that {𝐮T,b,βn} is bounded in k, due to Lemma 3.5. Thus, 𝐮T,b,βn𝐮^T,b weakly in k as n, up to a subsequence. Without loss of generality, we may assume that 𝐮T,b,βn𝐮^T,b weakly in k as n. Thanks to the Sobolev embedding theorem, we can see that 𝐮T,b,βn𝐮^T,b strongly in r=(Lr(Ω))k for 1r<2* as n. Since βn-, by a similar argument as that used in (3.10), we can show that

u T , b , β n i , p ( 1 2 ( a - λ σ 1 ) 𝒮 p ) 1 p - 2 > 0 for all  i = 1 , 2 , , k .

It follows that u^T,bi0 for all i=1,2,,k. Note that

0 = - β n j = 1 , i j k | u T , b , β n i | p 2 | u T , b , β n j | p 2 , 1 , R - λ u T , b , β n i , 2 , R 2 + 𝒬 ~ T , b ( 𝐮 T , b , β n ) u T , b , β n i , 2 , R 2 - u T , b , β n i , p , R p

for all i=1,2,,k. By the boundedness of {𝐮T,b,βn}, we can see that

| u T , b , β n i | p 2 | u T , b , β n j | p 2 , 1 , R 0 as  n

for all i,j=1,2,,k, with ij, which implies u^T,biu^T,bj=0 in 𝔹R for all i,j=1,2,,k, with ij. By Lemma 3.3, there exists a unique 𝐬n(+)k such that 𝐬n𝐮^T,b𝒲T,b,βn. Thanks to the construction of χp(s), {𝐬n} is bounded from above for b small enough in the case of p(2,4](2,2*). For N=3 and 4<p<6, we set

s i 0 ( n ) = max { s 1 n , s 2 n , , s k n } .

Since β<0, from the construction of χ(s), we have

( s i 0 ( n ) 𝐛 n ) 2 u i 0 ( n ) 2 + ( s i 0 ( n ) n ) 4 b ( j = 1 k u j , 2 , R 2 ) u i 0 ( n ) , 2 , R 2
s i 0 ( n ) 𝐛 n u i 0 ( n ) 2 + b ( j = 1 k s j n u j , 2 , R 2 ) s i 0 ( n ) n u i 0 ( n ) , 2 , R 2
= s i 0 ( n ) n u i 0 ( n ) , p , R p + β j = 1 , j i 0 ( n ) k | s i 0 ( n ) n u i 0 ( n ) | p 2 | s j n u j | p 2 , 1 , R
( s i 0 ( n ) n ) p ( u i 0 ( n ) , p , R p + β j = 1 , j i 0 ( n ) k | u i 0 ( n ) | p 2 | u j | p 2 , 1 , R )
(3.13) ( s i 0 ( n ) n ) p ( u i 0 ( n ) 2 + b ( j = 1 k u j , 2 2 ) u i 0 ( n ) , 2 , R 2 ) .

Note that p>4 and, by (3.13), we have si0(n)n1. It follows that {𝐬n} is bounded from above for all b>0 in the case of N=3 and 4<p<6. Without loss of generality, we may assume that {𝐬n} is bounded from above for 0<b<bT. Now, from the fact that f(t) is increasing, by (3.12), the definition of mT,b,βn, Lemma 3.3, the fact that u^T,biu^T,bj=0 in 𝔹R for all i,j=1,2,,k, with ij, and βn<0, we can see that

m T , b , β n = 𝒥 T , b , β n ( 𝐮 T , b , β n )
𝒥 T , b , β n ( 𝐬 n 𝐮 T , b , β n )
= b 4 χ p ( i = 1 k s i n u ^ T , b , β n i , 2 , R 2 T 2 ) ( i = 1 k s i n u ^ T , b , β n i , 2 , R 2 ) 2 + a 2 ( i = 1 k s i n u ^ T , b , β n i , 2 , R 2 ) - λ 2 s i n u ^ T , b , β n i , 2 , R 2
- 1 p s i n u ^ T , b , β n i , p , R p - 2 β n p i , j = 1 , i j k | s i n u ^ T , b , β n i | p 2 | s j n u ^ T , b , β n j | p 2 , 1 , R
𝒥 T , b , β n ( 𝐬 n 𝐮 T , b ) + o n ( 1 )
m T , b , β n + o n ( 1 ) .

Hence, 𝐮T,b,βn𝐮T,b strongly in k as n and 𝐬0=𝟏, due to Lemma 3.5. Thus, we also have that

β n i , j = 1 , i j k | u ^ T , b , β n i | p 2 | u ^ T , b , β n j | p 2 , 1 0 as  n .

3.2 Existence Results and Some Further Properties

Since βn<0 and Lemma 3.6 holds, by applying the Morse iteration in a standard way, we can see from the boundedness of {u^T,b,βni} in H0,R1(𝔹R) that {u^T,b,βni} is also bounded in L(Ω) for all i=1,2,,k. It follows from Lemma 3.9 and [35, Proposition 5.1] that u^T,bi are continuous on 𝔹R¯ for all i=1,2,,k. Thus, by the fact that u^T,bi0 and u^T,bi0, we have from [25, Lemma 1] that u^T,biH01(Ωi0) for all i=1,2,,k, where Ωi0={xΩu^i(x)>0}. Thanks to the radial symmetry of u^T,b,βni, by Lemma 3.9, u^T,bi are also radially symmetric. It follows that Ωi0 are the units of annuli. Set ~0,k=i=1k(H01(Ωi0){0}) and define

T , b , k 0 = { u ~ 0 , k T , b ( u ) u ~ i = 0  for all  i = 1 , 2 , , k } ,

where u~i is the projection of u in H01(Ωi0){0}. Then, by Lemma 3.9, we can see that u^T,b,𝐚=i=1kaiu^T,biT,b,k0 for all 𝐚=(a1,a2,,ak)(2)k. Let

c k , 0 T , b = inf u T , b , k 0 T , b ( u ) .

Lemma 3.10.

Let 0<λ<aσ1, TTk* and 0<b<bT, where bT is given by Lemma 2.1. Then Ωi0 are the annuli for all i=1,2,,k and i=1kΩi0=BR. Moreover, ET,b(u^T,b,a)=ck,0T,b for all a=(a1,a2,,ak)(Z2)k.

Proof.

Let Ωk0,*=𝔹R(i=1k-1Ωi0). Then, by Lemma 3.9, Ωk0,* is the unit of annuli and Ωk0Ωk0,*. Let

~ * = i = 1 k - 1 ( H 0 1 ( Ω i 0 ) { 0 } ) ( H 0 1 ( Ω i 0 , * ) { 0 } )

and define ck,*=infuk*T,b(u), where k*={u~*T,b(u)u~i=0 for all i=1,2,,k}. Clearly, we have H01(Ωi0)H01(Ωi0,*) and 𝐮=(u~i)i=1,2,,k𝒲T,b,β for all uk* and β<0. Then it is easy to see that ck,*limβ-mβ. Note that, by Lemma 3.9, we also have that ck,0T,blimβ-mβ, thus, ck,0T,b=ck,*, due to ck,0T,bck,*. It follows that ck,* is attained by u^T,b,𝐚. Since T,b(u)u~i is of C1, by a similar argument as that used in the proof of Lemma 3.4, we can show that k* is a natural constraint in ~*. Thus, u^T,b,𝐚 is a least energy critical point of T,b in ~*, which implies that Ωi0 are connected domains for all i=1,2,,k and u^T,b,k is a ground state solution of the following equation:

{ - ( a + b j = 1 k u ^ j , 2 2 ) Δ u ^ k = λ u ^ k + | u ^ k | p - 2 u ^ k in  Ω k 0 , * , u ^ k = 0 on  Ω k 0 , * .

It follows that Ωi0 and Ωk0,* are the annuli for all i=1,2,,k-1. On the other hand, by the maximum principle, we have u^k>0 in Ωk0,*, which implies Ωk0,*=Ωk0.∎

By Lemma 3.10, without loss of generality, we may assume that there exists 𝐫0k such that Ωi0=𝔹Ri0𝔹Ri-10 for all i=1,2,,k. Now, recall the definition of γT,bk given by (3.7).

Lemma 3.11.

Let 0<λ<aσ1, TTk* and 0<b<bT, where bT is given by Lemma 2.1. Then ck,0T,b=γT,bk. Moreover, ck,0T,b is nondecreasing for k.

Proof.

From Lemmas 3.5 and 3.10, we have immediately that ck,0T,b=γT,bk. It remains to show that ck,0 is nondecreasing for k. Indeed, take 𝐫k and set Ri=Ri+1 for i=1,2,,k-1. Then 𝐫=(R1,,Rk-1)k-1. Moreover, we also have

i = 1 2 ( H 0 , R 1 ( 𝔹 R i 𝔹 R i - 1 ) { 0 } ) ( H 0 1 ( 𝔹 R 1 𝔹 R 0 ) { 0 } ) .

It follows that T,b,k𝐫T,b,k-1𝐫, which, together with ck,0T,b=γT,bk, implies ck,0T,bck-1,0T,b. ∎

Proposition 3.12.

Let 0<λ<aσ1, TTk* and 0<b<bT, where bT is given by Lemma 2.1. Then u^T,b,ak is a radial sign-changing solution of (Pabλ) which has k nodal domains, where ak=((-1)i-1)i=1,2,,k.

Proof.

Let v^𝐬=i=1ksi(-1)i-1u^T,bi, where 𝐬(+)k. Then, by a similar argument as that used in the proof to Lemma 3.3, we can show that

(3.14) T , b ( u ^ T , b , 𝐚 k ) = T , b ( v ^ 𝟏 ) = max 𝐬 ( + ) k T , b ( v ^ 𝐬 ) > T , b ( v ^ 𝐬 )

for all 𝐬(+)k and 𝐬𝟏. Suppose that u^T,b,𝐚k is not a solution of (Pabλ). Then, by Lemma 3.6 and the symmetric criticality principle of Palais, there exists φC0,R(𝔹R) such that T,b(u^T,b,𝐚k)φ-1, where C0,R(𝔹R)={uC0(𝔹)Ru is radial symmetric}. From the fact that T,b(u) is of C2, it follows that there exists 0<ε0<110 such that

(3.15) T , b ( v ^ 𝐬 + ε 0 φ ) φ - 1 2 ,

where 𝐬ε0=[1-ε02,1+ε02]k. Consider the function ϕ(𝐬) defined on 12, which satisfies 0ϕ(𝐬)1, ϕ(𝐬)=1 on ε0 and ϕ(𝐬)=0 on 122ε0. Then, by (3.15), we have

T , b ( v ^ 𝐬 + ε 0 ϕ ( 𝐬 ) φ ) φ - 1 2 .

Note that by the mean value theorem, we have

(3.16) T , b ( v ^ 𝐬 + ε 0 ϕ ( 𝐬 ) φ ) = T , b ( v ^ 𝐬 ) + 0 ε 0 T , b ( v ^ 𝐬 + ρ ϕ ( 𝐬 ) φ ) ϕ ( 𝐬 ) φ 𝑑 ρ .

Thus, for 𝐬ε0, from (3.15) and Lemma 3.10, we can see that

T , b ( v ^ 𝐬 + ε 0 ϕ ( 𝐬 ) φ ) c k , 0 T , b - ε 0 2 .

For 𝐬12ε0, from (3.14) and Lemma 3.10, we can also see that there exists δ>0, dependent on ε0, such that T,b(v^𝐬)ck,0T,b-δ. Thus, by (3.16), we have

T , b ( v ^ 𝐬 + ε 0 ϕ ( 𝐬 ) φ ) c k , 0 T , b - δ .

In other words, we always have that T,b(v^𝐬+ε0ϕ(𝐬)φ)<ck,0T,b on 12. On the other hand, by the definition of v^𝐬+ε0ϕ(𝐬)φ, we can see that v^𝐬+ε0ϕ(𝐬)φv^𝐬 uniformly in 𝔹R as ε00. By the construction of v^𝐬, we can see that v^𝐬+ε0ϕ(𝐬)φ has at least k nodal domains for ε0 small enough. Now, let {Ωiε}i=1,2,,l be the nodal domains of v^𝐬+εϕ(𝐬)φ. Then lk for ε<ε0 small enough. It follows that there exists 𝐫εl such that Ωiε𝔹Riε𝔹Ri-1ε for all i=1,2,,k. Consider the maps hε:12~𝐫ε,k and Hε:12()l, respectively given by

h ε ( 𝐬 ) = v ^ 𝐬 + ε ϕ ( 𝐬 ) φ and H ε ( 𝐬 ) = ( T , b ( h ε ( 𝐬 ) ) h ε ( 𝐬 ) i ) i = 1 , 2 , , l .

Since ϕ(𝐬)=0 on 12, we have Hε(𝐬)=H0(𝐬) on 12. It follows from Lemma 2.11 that

1 = deg ( H 0 , 1 2 , 𝟎 ) = deg ( H ε 0 , 1 2 , 𝟎 ) .

Thus, by Lemma 3.11, we must have T,b(v^𝐬+ε0ϕ(𝐬)φ)ck,0T,b for some 𝐬12, which is a contradiction. ∎

As for the general bounded domain Ω, let us prove some more properties of u^T,b,𝐚k in what follows.

Proposition 3.13.

Let 0<λ<aσ1, TTk* and 0<b<bT, where bT is given by Lemma 2.1. Then u^T,b,ak is a least energy radial sign-changing critical point of ET,b(u) which has k nodal domains.

Proof.

Let v be a radial sign-changing critical point of T,b(u) which has k nodal domains and let {Ω~i}i=1,2,,k be the nodal domains of v. By the classical elliptic regularity theory, we can see that v is of C2. Thus, thanks to [25, Lemma 1], we also have vχΩ~iH01(Ω~i). Then it is easy to see from the maximum principle that there exists 𝐫*l such that Ω~i=𝔹Ri*𝔹Ri-1* for all i=1,2,,k. It follows from Lemma 3.11 that T,b(v)ck,0T,b. Hence, by Lemma 3.10, u^T,b,𝐚k is a least energy sign-changing solution of (Pabλ) which has k nodal domains. This completes the proof. ∎

Proposition 3.14.

Let 0<λ<aσ1 and TTk*. If bn0+ as n, then we have u^T,bn,akuk* strongly in H0,R1(BR) as n. Moreover, uk* is a least energy radial sign-changing solution of (𝒫a,0,λ) which has k nodal domains. In particular, u1* is a least energy sign-changing solution (𝒫a,0,λ) and has two nodal domains.

Proof.

Thanks to Lemmas 3.5 and 3.11, we can obtain the conclusion by a similar argument as that used in the proof of Proposition 2.15. ∎

Proposition 3.15.

Let 0<λ<aσ1, TTk* and 0<b<bT, where bT is given by Lemma 2.1. Then u^T,b,ak is a least energy radial sign-changing solution of (Pabλ) which has k nodal domains in the case of N=3 and 4p<6. Moreover, we also have ck,0T,b>kmb, where mb is given in Proposition 2.18.

Proof.

By Proposition 3.14, we can show that u^T,b,𝐚k is a least energy radial sign-changing solution of (Pabλ) which has k nodal domains in the case of N=3 and 4p<6, by applying a similar argument as that used in the proof of Proposition 2.16. Moreover, thanks to Lemmas 2.11 and 2.17, we can see that ck,0T,b has a mini-max description similar to that of βkb,T in the case of N=3 and 4p<6. Thus, by a similar argument as that used in the proof of Proposition 2.18, we have ck,0T,b>kmb in the case of N=3 and 4p<6. ∎

By Proposition 3.14, we can see that ck,0T,bγk as b0+, where γk is given by (3.8). Denote ck,0T,b by ck(b).

Proposition 3.16.

Let 0<λ<aσ1, TTk* and 0<b<bT, where bT is given by Lemma 2.1. Then we have

1 γ k - 1 c k ( b ) = O ( b ) as  b 0 + .

Moreover, if N=3 and 4<p<6, then ck(b)=O(bpp-4) as b+.

Proof.

Since ck,0T,b has a mini-max description similar to that of βkb,T, by Lemmas 2.11 and 2.17, and by a similar argument as that used in the proof of Lemma 2.19, we can see that ck(b) exists for almost b(0,bT) and ck(b)=14u^T,b,𝐚k,2,R4 if ck(b) exists. Now, the conclusion follows from a similar argument as that used in the proof of Lemma 2.20. ∎

We close this section by noting that the proof of Theorem 1.5 follows immediately from Propositions 3.123.13, and the proof of Theorem 1.7 follows immediately from Propositions 3.143.16.

3.3 Uniqueness of the Positive Solution

We re-denote (Pabλ) and b(u) by (𝒫a,b,λ,R) and b,R(u), respectively, for the sake of clarity. Let the Nehari manifold and the sign-changing Nehari manifold of b,R(u), respectively, be

𝒩 R = { u H 0 1 ( 𝔹 R ) { 0 } b , R ( u ) u = 0 }

and

R = { u ± H 0 1 ( Ω ) { 0 } b , R ( u ) u ± = 0 } .

Then by similar arguments as those used in [1, 36], we can show that there exists a radially positive solution uR of (𝒫a,b,λ,R) such that b,R(uR)=mR for λ<aσ1(R) in the following two cases:

  1. 4 < p < 6 ,

  2. p = 4 and b𝒮42<1,

where σ1(R) is the first eigenvalue of -Δ in L2(𝔹R), 𝒮4 is the best embedding constant from H01(𝔹R)L4(Ω), defined by

(3.17) 𝒮 4 = inf { u , 2 , R 2 u H 0 1 ( 𝔹 R ) , u , 4 , R 2 = 1 } ,

and mR=infu𝒩RR(u). Define

𝒦 m R = { u H 0 1 ( 𝔹 R ) b , R ( u ) = 0  in  H - 1 ( 𝔹 R )  and  b , R ( u ) = m R }

and

𝒦 R , + = { u H 0 1 ( 𝔹 R ) b , R ( u ) = 0  in  H - 1 ( 𝔹 R )  and  u > 0  in  𝔹 R } .

Then uR𝒦mR𝒦R,+. Let vt=tu, where u𝒦R,+. Then, by a direct calculation, we have

- Δ v t = t ( - Δ u ) = t a + b u , 2 , R 2 ( λ u + u p - 1 ) = λ a + b u , 2 , R 2 v t + 1 t p - 2 ( a + b u , 2 , R 2 ) v t p - 1 .

Set

t u = ( 1 a + b u , 2 , R 2 ) 1 p - 2 .

Then vtu is a positive solution of (𝒫1,0,α,R) with α=λa+bu,2,R2. Note that since λ<aσ1(R), we can see that α<σ1(R). By a well-known result, we know that vtu is the unique radially positive solution of (𝒫1,0,α,R). It follows that 𝒦R,+𝒦R,0, where

𝒦 R , 0 = { t v R , α v R , α  is the unique radially positive solution of  ( 𝒫 1 , 0 , α , R ) ,
with  α < σ 1 ( R ) , t > 0  and  t v R , α  being a positive solution of  ( 𝒫 a , b , λ , R ) } .

In particular, uR𝒦R,0. Moreover, by [17, Proposition 1.1], u=(λα)1p-2vR,α satisfying

(3.18) a α λ + b ( α λ ) p - 4 p - 2 v R , α , 2 , R 2 = 1

if and only if u𝒦R,0.

Lemma 3.17.

Let λ<aσ1(R). Then KR,+=KR,0=KmR in one of the following two cases:

  1. 4 < p < 6 ,

  2. p = 4 and b 𝒮 4 2 < 1 .

Proof.

For the sake of clarity, we divide the proof into three steps. Step 1: We prove that KR,0KmR. Indeed, let (λα)1p-2vR,α𝒦R,0, then we have

b , R ( ( λ α ) 1 p - 2 v R , α ) = λ 2 p - 2 2 α 2 p - 2 ( a v R , α , 2 , R 2 - λ v R , α , 2 , R 2 ) + b λ 4 p - 2 4 α 4 p - 2 v R , α , 2 , R 4 - λ p p - 2 p α p p - 2 v R , α , p , R p
(3.19) = ( p - 2 ) λ 2 p - 2 2 p α 2 p - 2 ( a v R , α , 2 , R 2 - λ v R , α , 2 , R 2 ) + b ( p - 4 ) λ 4 p - 2 4 p α 4 p - 2 v R , α , 2 , R 4 .

Note that vR,α is a solution of (𝒫1,0,α,R) and

e R ( α ) = p - 2 2 p ( v R , α , 2 , R 2 - α v R , α , 2 , R 2 ) ,

where

e R ( α ) = 𝒥 R , α ( v R , α ) and 𝒥 R , α ( v R , α ) = 1 2 v R , α , 2 , R 2 - α 2 v R , α , 2 , R 2 - 1 p v R , α , p , R p .

Hence, we have from (3.19) that

b , R ( ( λ α ) 1 p - 2 v R , α ) = ( p - 2 ) λ 2 p - 2 2 p α 2 p - 2 ( a v R , α , 2 , R 2 - λ v R , α , 2 , R 2 ) + b ( p - 4 ) λ 4 p - 2 4 p α 4 p - 2 v R , α , 2 , R 4
= ( λ α ) p p - 2 v R , α , 2 , R 2 ( p - 2 2 p ( a α λ - 1 ) + ( p - 4 ) b 4 p ( α λ ) p - 4 p - 2 v R , α , 2 , R 2 ) + ( λ α ) p p - 2 e R ( α ) ,

which, together with (3.18), implies

(3.20) b , R ( ( λ α ) 1 p - 2 v R , α ) = ( λ α ) p p - 2 ( e R ( α ) - 1 4 ( 1 - a α λ ) v R , α , 2 , R 2 ) = ( λ α ) p p - 2 e R ( α ) - 1 4 b ( λ α - a ) 2 .

Set

f ( α ) = ( λ α ) p p - 2 e R ( α ) - 1 4 b ( λ α - a ) 2 .

Then, by a similar argument as that used in Lemma 2.19 (see also [7, Lemma 3.1]), f(α) exists for almost every α(0,σ1(R)) and

f ( α ) = ( λ α ) p p - 2 e R ( α ) - p e R ( α ) λ p p - 2 ( p - 2 ) α 2 p - 2 p - 2 + ( λ α - a ) λ 2 b α 2
= λ p p - 2 α 2 p - 2 p - 2 ( α e R ( α ) - p p - 2 e R ( α ) ) + ( λ α - a ) λ 2 b α 2
= λ p p - 2 α 2 p - 2 p - 2 ( - α 2 v R , α , 2 , R 2 - p p - 2 e R ( α ) ) + ( λ α - a ) λ 2 b α 2 ,

which, together with (3.18) and the fact that eR(α)=p-22p(vR,α,2,R2-αvR,α,2,R2), implies

f ( α ) = - λ p p - 2 2 α 2 p - 2 p - 2 v R , α , 2 , R 2 + ( λ α - a ) λ 2 b α 2 = λ 2 2 b α 3 ( 1 - a α λ - b ( α λ ) p - 4 p - 2 v R , α , 2 , R 2 ) = 0 .

Thus, f(α)=0 for almost every α(0,σ1(R)). It follows that f(α)=d0, independent of α. By (3.20), we can see that b,R((λα)1p-2vR,α)=d0, independent of α for every (λα)1p-2vR,α𝒦R,0. Since uR𝒦R,0, we must have 𝒦R,0𝒦mR. Step 2: We prove that KmRKR,+. We first claim that any nontrivial solution of (𝒫a,b,λ,R) must be positive for 12b𝒮42<1 in the case of p=4. Indeed, suppose that u is a sign-changing solution of (𝒫a,b,λ,R). Then we have

(3.21) { ( a u + , 2 , R 2 - λ u + , 2 , R 2 ) + ( b u + , 2 , R 4 - u + , 4 , R 4 ) + b u + , 2 , R 2 u - , 2 , R 2 = 0 , ( a u - , 2 , R 2 - λ u - , 2 , R 2 ) + ( b u - , 2 , R 4 - u - , 4 , R 4 ) + b u + , 2 , R 2 u - , 2 , R 2 = 0 .

By the fact that λ<aσ1 and (3.17), we can see from (3.21) that

b 𝒮 4 2 u - , 2 2 < ( 1 - b 𝒮 4 2 ) u + , 2 2 and b 𝒮 4 2 u + , 2 2 < ( 1 - b 𝒮 4 2 ) u - , 2 2 .

It follows that b𝒮421-b𝒮42<1, which implies 2b𝒮42<1. It follows from the maximum principle that any nontrivial solution of (𝒫a,b,λ,R) must be positive for 12b𝒮42<1. Thus, we have 𝒦mR𝒦R,+ for 12b𝒮42<1 in the case of p=4. Let us consider the case of p>4 or p=4 and 0<b𝒮42<12 in what follows. Suppose that u is a sign-changing solution of (𝒫a,b,λ,R). Then uR. It follows from (3.21) that

b u ± , 2 , R 2 u , 2 , R 2 + b u ± , 2 , R 4 - u ± , 4 , R 4 < 0

in the case of p=4. Now by Lemmas 2.11 and 2.17, there exists unique (t+,t-)+×+ such that t±u±𝒩R in one of the following two cases:

  1. 4 < p < 6 ,

  2. p = 4 and b𝒮42<1.

This, together with Lemmas 2.11 and 2.17 once more, implies

b , R ( u ) b , R ( t + u + - t - u - ) = b , R ( t + u + ) + b , R ( t - u - ) + b 2 ( t + ) 2 ( s + ) 2 u + , 2 , R 2 u - , 2 , R 2 > 2 m R .

Thus, we must have 𝒦mR𝒦R,+. Step 3: We prove that KR,+=KR,0=KmR. Indeed, since 𝒦R,+𝒦R,0, the conclusion follows immediately from Steps 1 and 2. ∎

We re-denote the positive solution of (𝒫a,b,λ,R), b,R and mR, by uR,λ, b,R,λ and mR(λ), respectively.

Lemma 3.18.

We have that mR(λ) is strictly decreasing for λ(0,aσ1(R)) and mR(λ)=-12BuR,λ,2,R2 for almost every λ(0,aσ1(R)) in one of the following two cases:

  1. 4 < p < 6 ,

  2. p = 4 and b 𝒮 4 2 < 1 ,

where uR,λ is a positive solution of (Pa,b,λ,R).

Proof.

The proof is similar to that of Lemma 2.19. ∎

We close this section by proving Theorem 1.9.

Proof of Theorem 1.9.

Let λ(0,aσ1(R)) so that mR(λ) exists and suppose that uR,λ1 and uR,λ2 are two different positive solutions of (𝒫a,b,λ,R). Then, by Lemma 3.18, we have

u R , λ 1 , 2 , R 2 = - 2 m R ( λ ) = u R , λ 1 , 2 , R 2 .

Note that both uR,λ1 and uR,λ2 belong to 𝒦mR, by Lemma 3.17. Thus, we can see that

u R , λ 1 , 2 , R 2 = u R , λ 2 , 2 , R 2 and u R , λ 1 , p , R p = u R , λ 2 , p , R p ,

due to the fact that 4p<6. On the other hand, by Lemma 3.17 once more, we can see that

u R , λ 1 = ( λ α 1 ) 1 p - 2 v R , α 1 and u R , λ 2 = ( λ α 2 ) 1 p - 2 v R , α 2

for some α1,α2(0,σ1(R)), where vR,α is the unique radially positive solution of (𝒫1,0,α,R). Without loss of generality, we assume α1<α2. Then, by a similar argument as that used in [18, Lemma 5.1], we have eR(α1)>eR(α2), which implies vR,α1,p,Rp>vR,α2,p,Rp. Thus,

u R , λ 1 , p , R p = ( λ α 1 ) p p - 2 v R , α 1 , p , R p > ( λ α 2 ) p p - 2 v R , α 1 , p , R p > ( λ α 2 ) p p - 2 v R , α 2 , p , R p = u R , λ 2 , p , R p .

This is a contradiction. Thus, (𝒫a,b,λ,R) has a unique positive solution for λ(0,aσ1(R)) such that mR(λ) exists. Since mR(λ) exists for almost λ(0,aσ1(R)), (𝒫a,b,λ,R) has a unique positive solution for almost λ(0,aσ1(R)). ∎


Communicated by Paul Rabinowitz


Award Identifier / Grant number: 11471235

Award Identifier / Grant number: 11626226

Award Identifier / Grant number: 11701554

Award Identifier / Grant number: 11771319

Award Identifier / Grant number: 2017XKQY091

Funding statement: Y. Huang was supported by the National Natural Science Foundation of China (11471235). Y. Wu was supported by the Natural Science Foundation of China (11626226, 11701554, 11771319) and the Fundamental Research Funds for the Central Universities (2017XKQY091).

Acknowledgements

The authors thanks Professor Jiangquan Liu for his friendship, encouragement and enlightening discussions.

References

[1] C. O. Alves and G. M. Figueiredo, Nonlinear perturbations of a periodic Kirchhoff equation in N, Nonlinear Anal. 75 (2012), no. 5, 2750–2759. 10.1016/j.na.2011.11.017Search in Google Scholar

[2] A. Azzollini, The elliptic Kirchhoff equation in N perturbed by a local nonlinearity, Differential Integral Equations 25 (2012), no. 5–6, 543–554. 10.57262/die/1356012678Search in Google Scholar

[3] T. Bartsch, Z. Liu and T. Weth, Sign changing solutions of superlinear Schrödinger equations, Comm. Partial Differential Equations 29 (2004), no. 1–2, 25–42. 10.1081/PDE-120028842Search in Google Scholar

[4] T. Bartsch, Z. Liu and T. Weth, Nodal solutions of a p-Laplacian equation, Proc. Lond. Math. Soc. (3) 91 (2005), no. 1, 129–152. 10.1112/S0024611504015187Search in Google Scholar

[5] G. Cerami, S. Solimini and M. Struwe, Some existence results for superlinear elliptic boundary value problems involving critical exponents, J. Funct. Anal. 69 (1986), no. 3, 289–306. 10.1016/0022-1236(86)90094-7Search in Google Scholar

[6] Z. Chen, C.-S. Lin and W. Zou, Sign-changing solutions and phase separation for an elliptic system with critical exponent, Comm. Partial Differential Equations 39 (2014), no. 10, 1827–1859. 10.1080/03605302.2014.908391Search in Google Scholar

[7] Z. Chen and W. Zou, On the Brezis–Nirenberg problem in a ball, Differential Integral Equations 25 (2012), no. 5–6, 527–542. 10.57262/die/1356012677Search in Google Scholar

[8] Z. Chen and W. Zou, Positive least energy solutions and phase separation for coupled Schrödinger equations with critical exponent: Higher dimensional case, Calc. Var. Partial Differential Equations 52 (2015), no. 1–2, 423–467. 10.1007/s00526-014-0717-xSearch in Google Scholar

[9] B. Cheng, X. Wu and J. Liu, Multiple solutions for a class of Kirchhoff type problems with concave nonlinearity, NoDEA Nonlinear Differential Equations Appl. 19 (2012), no. 5, 521–537. 10.1007/s00030-011-0141-2Search in Google Scholar

[10] Y. Deng, S. Peng and W. Shuai, Existence and asymptotic behavior of nodal solutions for the Kirchhoff-type problems in 3, J. Funct. Anal. 269 (2015), no. 11, 3500–3527. 10.1016/j.jfa.2015.09.012Search in Google Scholar

[11] G. M. Figueiredo, N. Ikoma and J. a. R. Santos Júnior, Existence and concentration result for the Kirchhoff type equations with general nonlinearities, Arch. Ration. Mech. Anal. 213 (2014), no. 3, 931–979. 10.1007/s00205-014-0747-8Search in Google Scholar

[12] Z. Guo, Ground states for Kirchhoff equations without compact condition, J. Differential Equations 259 (2015), no. 7, 2884–2902. 10.1016/j.jde.2015.04.005Search in Google Scholar

[13] X. He and W. Zou, Existence and concentration behavior of positive solutions for a Kirchhoff equation in 3, J. Differential Equations 252 (2012), no. 2, 1813–1834. 10.1016/j.jde.2011.08.035Search in Google Scholar

[14] Y. He and G. Li, Standing waves for a class of Kirchhoff type problems in 3 involving critical Sobolev exponents, Calc. Var. Partial Differential Equations 54 (2015), no. 3, 3067–3106. 10.1007/s00526-015-0894-2Search in Google Scholar

[15] Y. He, G. Li and S. Peng, Concentrating bound states for Kirchhoff type problems in 3 involving critical Sobolev exponents, Adv. Nonlinear Stud. 14 (2014), no. 2, 483–510. 10.1515/ans-2014-0214Search in Google Scholar

[16] Y. Huang, Z. Liu and Y. Wu, On a critical Kirchoff problem in high dimensions, preprint (2016), https://arxiv.org/abs/1605.06906v1. Search in Google Scholar

[17] Y. Huang, Z. Liu and Y. Wu, On finding solutions of a Kirchhoff type problem, Proc. Amer. Math. Soc. 144 (2016), no. 7, 3019–3033. 10.1090/proc/12946Search in Google Scholar

[18] Y. Huang, T.-F. Wu and Y. Wu, Multiple positive solutions for a class of concave-convex elliptic problems in N involving sign-changing weight. II, Commun. Contemp. Math. 17 (2015), no. 5, Article ID 1450045. 10.1142/S021919971450045XSearch in Google Scholar

[19] G. Kirchhoff, Mechanik, Teubner, Leipzig, 1883. Search in Google Scholar

[20] G. Li and H. Ye, Existence of positive ground state solutions for the nonlinear Kirchhoff type equations in 3, J. Differential Equations 257 (2014), no. 2, 566–600. 10.1016/j.jde.2014.04.011Search in Google Scholar

[21] Y. Li, F. Li and J. Shi, Existence of a positive solution to Kirchhoff type problems without compactness conditions, J. Differential Equations 253 (2012), no. 7, 2285–2294. 10.1016/j.jde.2012.05.017Search in Google Scholar

[22] Z. Liang, F. Li and J. Shi, Positive solutions to Kirchhoff type equations with nonlinearity having prescribed asymptotic behavior, Ann. Inst. H. Poincaré Anal. Non Linéaire 31 (2014), no. 1, 155–167. 10.1016/j.anihpc.2013.01.006Search in Google Scholar

[23] J. Liu, J.-F. Liao and C.-L. Tang, Positive solutions for Kirchhoff-type equations with critical exponent in N, J. Math. Anal. Appl. 429 (2015), no. 2, 1153–1172. 10.1016/j.jmaa.2015.04.066Search in Google Scholar

[24] A. Mao and Z. Zhang, Sign-changing and multiple solutions of Kirchhoff type problems without the P.S. condition, Nonlinear Anal. 70 (2009), no. 3, 1275–1287. 10.1016/j.na.2008.02.011Search in Google Scholar

[25] E. Müller-Pfeiffer, On the number of nodal domains for eigenfunctions of elliptic differential operators, J. Lond. Math. Soc. (2) 31 (1985), no. 1, 91–100. 10.1112/jlms/s2-31.1.91Search in Google Scholar

[26] D. Naimen, The critical problem of Kirchhoff type elliptic equations in dimension four, J. Differential Equations 257 (2014), no. 4, 1168–1193. 10.1016/j.jde.2014.05.002Search in Google Scholar

[27] K. Perera and Z. Zhang, Nontrivial solutions of Kirchhoff-type problems via the Yang index, J. Differential Equations 221 (2006), no. 1, 246–255. 10.1016/j.jde.2005.03.006Search in Google Scholar

[28] M. Ramos, H. Tavares and W. Zou, A Bahri–Lions theorem revisited, Adv. Math. 222 (2009), no. 6, 2173–2195. 10.1016/j.aim.2009.07.013Search in Google Scholar

[29] W. Shuai, Sign-changing solutions for a class of Kirchhoff-type problem in bounded domains, J. Differential Equations 259 (2015), no. 4, 1256–1274. 10.1016/j.jde.2015.02.040Search in Google Scholar

[30] M. Struwe, Variational Methods. Applications to Nonlinear Partial Differential Equations and Hamiltonian Systems, 2nd ed., Ergeb. Math. Grenzgeb. (3) 34, Springer, Berlin, 1996. Search in Google Scholar

[31] J. Sun and T.-F. Wu, Ground state solutions for an indefinite Kirchhoff type problem with steep potential well, J. Differential Equations 256 (2014), no. 4, 1771–1792. 10.1016/j.jde.2013.12.006Search in Google Scholar

[32] S. Terracini and G. Verzini, Multipulse phases in k-mixtures of Bose–Einstein condensates, Arch. Ration. Mech. Anal. 194 (2009), no. 3, 717–741. 10.1007/s00205-008-0172-ySearch in Google Scholar

[33] J. Wang, L. Tian, J. Xu and F. Zhang, Multiplicity and concentration of positive solutions for a Kirchhoff type problem with critical growth, J. Differential Equations 253 (2012), no. 7, 2314–2351. 10.1016/j.jde.2012.05.023Search in Google Scholar

[34] J. Wei and T. Weth, Radial solutions and phase separation in a system of two coupled Schrödinger equations, Arch. Ration. Mech. Anal. 190 (2008), no. 1, 83–106. 10.1007/s00205-008-0121-9Search in Google Scholar

[35] Y. Wu, On a K-component elliptic system with the Sobolev critical exponent in high dimensions: The repulsive case, Calc. Var. Partial Differential Equations 56 (2017), no. 5, Paper No. 151. 10.1007/s00526-017-1252-3Search in Google Scholar

[36] Y. Wu, Y. Huang and Z. Liu, On a Kirchhoff type problem in N, J. Math. Anal. Appl. 425 (2015), no. 1, 548–564. 10.1016/j.jmaa.2014.12.017Search in Google Scholar

[37] Y. Wu, Y. Huang and Z. Liu, Kirchhoff type problems with potential well and indefinite potential, Electron. J. Differential Equations 2016 (2016), Paper No. 178. Search in Google Scholar

[38] Y. Wu, T.-F. Wu and W. Zou, On a two-component Bose–Einstein condensate with steep potential wells, Ann. Mat. Pura Appl. (4) 196 (2017), no. 5, 1695–1737. 10.1007/s10231-017-0635-6Search in Google Scholar

[39] Z. Zhang and K. Perera, Sign changing solutions of Kirchhoff type problems via invariant sets of descent flow, J. Math. Anal. Appl. 317 (2006), no. 2, 456–463. 10.1016/j.jmaa.2005.06.102Search in Google Scholar

Received: 2017-09-05
Revised: 2017-11-18
Accepted: 2017-11-25
Published Online: 2017-12-22
Published in Print: 2018-08-01

© 2018 Walter de Gruyter GmbH, Berlin/Boston

This work is licensed under the Creative Commons Attribution 4.0 International License.

Downloaded on 18.9.2025 from https://www.degruyterbrill.com/document/doi/10.1515/ans-2017-6042/html
Scroll to top button