In this paper, we consider a Neumann problem of Kirchhoff type equation
{−(a+b∫Ω|∇u|2dx)Δu+u=Q(x)|u|4u+λP(x)|u|q−2u,in Ω,∂u∂v=0,on ∂Ω,
where Ω ⊂ R3 is a bounded domain with a smooth boundary, a,b>0, 1<q<2, λ>0 is a real parameter, Q(x) and P(x) satisfy some suitable assumptions. By using the variational method and the concentration compactness principle, we obtain the existence and multiplicity of nontrivial solutions.
Citation: Jun Lei, Hongmin Suo. Multiple solutions of Kirchhoff type equations involving Neumann conditions and critical growth[J]. AIMS Mathematics, 2021, 6(4): 3821-3837. doi: 10.3934/math.2021227
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In this paper, we consider a Neumann problem of Kirchhoff type equation
{−(a+b∫Ω|∇u|2dx)Δu+u=Q(x)|u|4u+λP(x)|u|q−2u,in Ω,∂u∂v=0,on ∂Ω,
where Ω ⊂ R3 is a bounded domain with a smooth boundary, a,b>0, 1<q<2, λ>0 is a real parameter, Q(x) and P(x) satisfy some suitable assumptions. By using the variational method and the concentration compactness principle, we obtain the existence and multiplicity of nontrivial solutions.
We study the following Neumann problem of Kirchhoff type equation with critical growth
{−(a+b∫Ω|∇u|2dx)Δu+u=Q(x)|u|4u+λP(x)|u|q−2u,in Ω,∂u∂v=0,on ∂Ω, | (1.1) |
where Ω ⊂ R3 is a bounded domain with a smooth boundary, a,b>0, 1<q<2, λ>0 is a real parameter. We assume that Q(x) and P(x) satisfy the following conditions:
(Q1) Q(x)∈C(ˉΩ) is a sign-changing;
(Q2) there exists xM∈Ω such that QM=Q(xM)>0 and
|Q(x)−QM|=o(|x−xM|)asx→xM; |
(Q3) there exists 0∈∂Ω such that Qm=Q(0)>0 and
|Q(x)−Qm|=o(|x|)asx→0; |
(P1) P(x) is positive continuous on ˉΩ and P(x0)=maxx∈ˉΩP(x);
(P2) there exist σ>0, R>0 and 3−q<β<6−q2 such that P(x)≥σ|x−y|−β for |x−y|≤R, where y is xM∈Ω or 0∈∂Ω.
In recent years, the following Dirichlet problem of Kirchhoff type equation has been studied extensively by many researchers
{−(a+b∫Ω|∇u|2dx)Δu=f(x,u),in Ω,u=0,on ∂Ω, | (1.2) |
which is related to the stationary analogue of the equation
utt−(a+b∫Ω|∇u|2dx)Δu=f(x,u) | (1.3) |
proposed by Kirchhoff in [13] as an extension of the classical D'Alembert's wave equation for free vibrations of elastic strings. Kirchhoff's model takes into account the changes in length of the string produced by transverse vibrations. In (1.2) and (1.3), u denotes the displacement, b is the initial tension and f(x,u) stands for the external force, while a is related to the intrinsic properties of the string (such as Young's modulus). We have to point out that such nonlocal problems appear in other fields like biological systems, such as population density, where u describes a process which depends on the average of itself (see Alves et al. [2]). After the pioneer work of Lions [18], where a functional analysis approach was proposed. The Kirchhoff type Eq (1.2) with critical growth began to call attention of researchers, we can see [1,9,14,17,23,24,28,30] and so on.
Recently, the following Kirchhoff type equation has been well studied by various authors
{−(a+b∫R3|∇u|2dx)Δu+V(x)u=f(x,u),inR3,u>0,u∈H1(R3). | (1.4) |
There has been much research regarding the concentration behavior of the positive solutions of (1.4), we can see [10,11,12,25,33]. Many papers studied the existence of ground state solutions of (1.4), for example [5,8,15,16,21,22,24]. In addition, the authors established the existence of sign-changing solutions of (1.4) in [20,31]. In papers [27,32] proved the existence and multiplicity of nontrivial solutions of (1.4) by using mountain pass theorem.
In particular, Chabrowski in [6] studied the solvability of the Neumann problem
{−Δu=Q(x)|u|2∗−2u+λf(x,u),in Ω,∂u∂v=0,on ∂Ω, |
where Ω ⊂ RN is a smooth bounded domain, 2∗=2NN−2(N≥3) is the critical Sobolev exponent, λ>0 is a parameter. Assume that Q(x)∈C(¯Ω) is a sign-changing function and ∫ΩQ(x)dx<0, under the condition of f(x,u). Using the space decomposition H1(Ω)=span1⊕V, where V={v∈H1(Ω):∫Ωvdx=0}, the author obtained the existence of two distinct solutions by the variational method.
In [14], Lei et al. considered the following Kirchhoff type equation with critical exponent
{−(a+b∫Ω|∇u|2dx)Δu=u5+λuq−1|x|β,in Ω,u=0,on ∂Ω, |
where Ω ⊂ R3 is a smooth bounded domain, a,b>0, 1<q<2, λ>0 is a parameter. They obtained the existence of a positive ground state solution for 0≤β<2 and two positive solutions for 3−q≤β<2 by the Nehari manifold method.
In [34], Zhang obtained the existence and multiplicity of nontrivial solutions of the following equation
{−(a+b∫Ω|∇u|2dx)Δu+u=λ|u|q−2u+f(x,u)+Q(x)u5,in Ω,∂u∂v=0,on ∂Ω, | (1.5) |
where Ω is an open bounded domain in R3, a,b>0, 1<q<2, λ≥0 is a parameter, f(x,u) and Q(x) are positive continuous functions satisfying some additional assumptions. Moreover, f(x,u)∼|u|p−2u with 4<p<6.
Comparing with the above mentioned papers, our results are different and extend the above results to some extent. Specially, motivated by [34], we suppose Q(x) changes sign on Ω and f(x,u)≡0 for (1.5). Since (1.1) is critical growth, which leads to the cause of the lack of compactness of the embedding H1(Ω)↪L6(Ω), we overcome this difficulty by using P.Lions concentration compactness principle [19]. Moreover, note that Q(x) changes sign on Ω, how to estimate the level of the mountain pass is another difficulty.
We define the energy functional corresponding to problem (1.1) by
Iλ(u)=12‖u‖2+b4(∫Ω|∇u|2dx)2−16∫ΩQ(x)|u|6dx−λq∫ΩP(x)|u|qdx. |
A weak solution of problem (1.1) is a function u∈H1(Ω) and for all φ∈H1(Ω) such that
∫Ω(a∇u∇φ+uφ)dx+b∫Ω|∇u|2dx∫Ω∇u∇φdx=∫ΩQ(x)|u|4uφdx+λ∫ΩP(x)|u|q−2uφdx. |
Our main results are the following:
Theorem 1.1. Assume that 1<q<2 and Q(x) changes sign on Ω. Then there exists Λ0>0 such that for every λ∈(0,Λ0), problem (1.1) has at least one nontrivial solution.
Theorem 1.2. Assume that 1<q<2, 3−q<β<6−q2 and Q(x) changes sign on Ω, there exists Λ∗>0 such that for all λ∈(0,Λ∗). Then problem (1.1) has at least two nontrivial solutions.
Throughout this paper, we make use of the following notations:
● The space H1(Ω) is equipped with the norm ‖u‖2H1(Ω)=∫Ω(|∇u|2+u2)dx, the norm in Lp(Ω) is denoted by ‖⋅‖p.
● Define ‖u‖2=∫Ω(a|∇u|2+u2)dx for u∈H1(Ω). Note that ‖⋅‖ is an equivalent norm on H1(Ω) with the standard norm.
● Let D1,2(R3) is the completion of C∞0(R3) with respect to the norm ‖u‖2D1,2(R3)=∫R3|∇u|2dx.
● 0<QM=maxx∈ˉΩQ(x), 0<Qm=maxx∈∂ΩQ(x).
● Ω+={x∈Ω:Q(x)>0} and Ω−={x∈Ω:Q(x)<0}.
● C,C1,C2,… denote various positive constants, which may vary from line to line.
● We denote by Sρ (respectively, Bρ) the sphere (respectively, the closed ball) of center zero and radius ρ, i.e. Sρ={u∈H1(Ω):‖u‖=ρ}, Bρ={u∈H1(Ω):‖u‖≤ρ}.
● Let S be the best constant for Sobolev embedding H1(Ω)↪L6(Ω), namely
S=infu∈H1(Ω)∖{0}∫Ω(a|∇u|2+u2)dx(∫Ω|u|6dx)1/3. |
● Let S0 be the best constant for Sobolev embedding D1,2(R3)↪L6(R3), namely
S0=infu∈D1,2(R3)∖{0}∫R3|∇u|2dx(∫R3|u|6dx)1/3. |
In this section, we firstly show that the functional Iλ(u) has a mountain pass geometry.
Lemma 2.1. There exist constants r,ρ,Λ0>0 such that the functional Iλ satisfies the following conditions for each λ∈(0,Λ0):
(i) Iλ|u∈Sρ≥r>0; infu∈BρIλ(u)<0.
(ii) There exists e∈H1(Ω) with ‖e‖>ρ such that Iλ(e)<0.
Proof. (i) From (P1), by the H¨older inequality and the Sobolev inequality, for all u∈H1(Ω) one has
∫ΩP(x)|u|qdx≤P(x0)∫Ω|u|qdx≤P(x0)|Ω|6−q6S−q2‖u‖q, | (2.1) |
and there exists a constant C>0, we get
|∫ΩQ(x)|u|6dx|≤C∫Ω|u|6dx≤CS−3‖u‖6. | (2.2) |
Hence, combining (2.1) and (2.2), we have the following estimate
Iλ(u)=12‖u‖2+b4(∫Ω|∇u|2dx)2−16∫ΩQ(x)|u|6dx−λq∫ΩP(x)|u|qdx≥12‖u‖2−C6∫Ω|u|6dx−λqP(x0)|Ω|6−q6S−q2‖u‖q≥‖u‖q(12‖u‖2−q−C6S−3‖u‖6−q−λqP(x0)|Ω|6−q6S−q2). |
Set h(t)=12t2−q−C6S−3t6−q for t>0, then there exists a constant ρ=(3(2−q)S3C(6−q))14>0 such that maxt>0h(t)=h(ρ)>0. Letting Λ0=qSq2P(x0)|Ω|6−q6h(ρ), there exists a constant r>0 such that Iλ|u∈Sρ≥r for every λ∈(0,Λ0). Moreover, for all u∈H1(Ω)∖{0}, we have
limt→0+Iλ(tu)tq=−λq∫ΩP(x)|u|qdx<0. |
So we obtain Iλ(tu)<0 for every u≠0 and t small enough. Therefore, for ‖u‖ small enough, one has
m≜infu∈BρIλ(u)<0. |
(ii) Let v∈H1(Ω) be such that supp v⊂Ω+, v≢0 and t>0, we have
Iλ(tv)=t22‖v‖2+bt44(∫Ω|∇v|2dx)2−t66∫ΩQ(x)|v|6dx−λtqq∫ΩP(x)|v|qdx→−∞ |
as t→∞, which implies that Iλ(tv)<0 for t>0 large enough. Therefore, we can find e∈H1(Ω) with ‖e‖>ρ such that Iλ(e)<0. The proof is complete.
Denote
{Θ1=abS304QM+b3S6024Q2M+aS0√b2S40+4aS0QM6QM+b2S40√b2S40+4aS0QM24Q2M,Θ2=abS3016Qm+b3S60384Q2m+aS0√b2S40+16aS0Qm24Qm+b2S40√b2S40+16aS0Qm384Q2m. |
Then we have the following compactness result.
Lemma 2.2. Suppose that 1<q<2. Then the functional Iλ satisfies the (PS)cλ condition for every cλ<c∗= min {Θ1−Dλ22−q,Θ2−Dλ22−q}, where D=2−q3q(6−q4P(x0)S−q2|Ω|6−q6)22−q.
Proof. Let {un}⊂H1(Ω) be a (PS)cλ sequence for
Iλ(un)→cλandI′λ(un)→0asn→∞. | (2.3) |
It follows from (2.1), (2.3) and the H¨older inequality that
cλ+1+o(‖un‖)≥Iλ(un)−16⟨I′λ(un),un⟩≥13‖un‖2+b12(∫Ω|∇un|2dx)2−λ(1q−16)P(x0)S−q2|Ω|6−q6‖un‖q≥13‖un‖2−λ(6−q)6qP(x0)S−q2|Ω|6−q6‖un‖q. |
Therefore {un} is bounded in H1(Ω) for all 1<q<2. Thus, we may assume up to a subsequence, still denoted by {un}, there exists u∈H1(Ω) such that
{un⇀u,weaklyinH1(Ω),un→u,stronglyinLp(Ω)(1≤p<6),un(x)→u(x),a.e.inΩ, | (2.4) |
as n→∞. Next, we prove that un→u strongly in H1(Ω). By using the concentration compactness principle (see [19]), there exist some at most countable index set J, δxj is the Dirac mass at xj⊂ˉΩ and positive numbers {νj}, {μj}, j∈J, such that
|un|6dx⇀dν=|u|6dx+∑j∈Jνjδxj,|∇un|2dx⇀dμ≥|∇u|2dx+∑j∈Jμjδxj. |
Moreover, numbers νj and μj satisfy the following inequalities
S0ν13j≤μjifxj∈Ω,S0223ν13j≤μjifxj∈∂Ω. | (2.5) |
For ε>0, let ϕε,j(x) be a smooth cut-off function centered at xj such that 0≤ϕε,j≤1, |∇ϕε,j|≤2ε, and
ϕε,j(x)={1, in B(xj,ε2)∩ˉΩ,0, in Ω∖B(xj,ε). |
There exists a constant C>0 such that
limε→0limn→∞∫ΩP(x)|un|qϕε,jdx≤P(x0)limε→0limn→∞∫B(xj,ε)|un|qdx=0. |
Since |∇ϕε,j|≤2ε, by using the H¨older inequality and L2(Ω)-convergence of {un}, we have
limε→0limn→∞(a+b∫Ω|∇un|2dx)∫Ω⟨∇un,∇ϕε,j⟩undx≤Climε→0limn→∞(∫Ω|∇un|2dx)12(∫Ω|un|2|∇ϕε,j|2dx)12≤Climε→0(∫B(xj,ε)|u|6dx)16(∫B(xj,ε)|∇ϕε,j|3dx)13≤Climε→0(∫B(xj,ε)|u|6dx)16(∫B(xj,ε)(2ε)3dx)13≤C1limε→0(∫B(xj,ε)|u|6dx)16=0, |
where C1>0, and we also derive that
limε→0limn→∞∫Ω|∇un|2ϕε,jdx≥limε→0∫Ω|∇u|2ϕε,jdx+μj=μj, |
limε→0limn→∞∫ΩQ(x)|un|6ϕε,jdx=limε→0∫ΩQ(x)|u|6ϕε,jdx+Q(xj)νj=Q(xj)νj, |
limε→0limn→∞∫Ωu2nϕε,jdx=limε→0∫Ωu2ϕε,jdx≤limε→0∫B(xj,ε)u2dx=0. |
Noting that unϕε,j is bounded in H1(Ω) uniformly for n, taking the test function φ=unϕε,j in (2.3), from the above information, one has
0=limε→0limn→∞⟨I′λ(un),unϕε,j⟩=limε→0limn→∞{(a+b∫Ω|∇un|2dx)∫Ω⟨∇un,∇(unϕε,j)⟩dx+∫Ωu2nϕε,jdx−∫ΩQ(x)|un|6ϕε,jdx−λ∫ΩP(x)|un|qϕε,jdx}=limε→0limn→∞{(a+b∫Ω|∇un|2dx)∫Ω(|∇un|2ϕε,j+⟨∇un,∇ϕε,j⟩un)dx−∫ΩQ(x)|un|6ϕε,jdx}≥limε→0{(a+b∫Ω|∇u|2dx+bμj)(∫Ω|∇u|2ϕε,jdx+μj)−∫ΩQ(x)|u|6ϕε,jdx−Q(xj)νj}≥(a+bμj)μj−Q(xj)νj, |
so that
Q(xj)νj≥(a+bμj)μj, |
which shows that {un} can only concentrate at points xj where Q(xj)>0. If νj>0, by (2.5) we get
ν13j≥bS20+√b2S40+4aS0QM2QMifxj∈Ω,ν13j≥bS20+√b2S40+16aS0Qm273Qmifxj∈∂Ω. | (2.6) |
From (2.5) and (2.6), we have
μj≥bS30+√b2S60+4aS30QM2QMifxj∈Ω,μj≥bS30+√b2S60+16aS30Qm8Qmifxj∈∂Ω. | (2.7) |
To proceed further we show that (2.7) is impossible. To obtain a contradiction assume that there exists j0∈J such that μj0≥bS30+√b2S60+4aS30QM2QM and xj0∈Ω. By (2.1), (2.3) and (2.4), one has
cλ=limn→∞{Iλ(un)−16⟨I′λ(un),un⟩}=limn→∞{a3∫Ω|∇un|2dx+b12(∫Ω|∇un|2dx)2+13∫Ωu2ndx−λ6−q6q∫ΩP(x)|un|qdx}≥a3(∫Ω|∇u|2dx+∑j∈Jμj)+b12(∫Ω|∇u|2dx+∑j∈Jμj)2+13∫Ωu2dx−λ6−q6qP(x0)S−q2|Ω|6−q6‖u‖q≥a3μj0+b12μ2j0+13‖u‖2−λ6−q6qP(x0)S−q2|Ω|6−q6‖u‖q. |
Set
g(t)=13t2−λ6−q6qP(x0)S−q2|Ω|6−q6tq,t>0, |
then
g′(t)=23t−λ6−q6P(x0)S−q2|Ω|6−q6tq−1=0, |
we can deduce that mint≥0g(t) attains at t0>0 and
t0=(λ6−q4P(x0)S−q2|Ω|6−q6)12−q. |
Consequently, we obtain
cλ≥abS304QM+b3S6024Q2M+aS0√b2S40+4aS0QM6QM+b2S40√b2S40+4aS0QM24Q2M−Dλ22−q=Θ1−Dλ22−q, |
where D=2−q3q(6−q4P(x0)S−q2|Ω|6−q6)22−q. If μj0≥bS30+√b2S60+16aS30Qm8Qm and xj0∈∂Ω, then, by the similar calculation, we also get
cλ≥abS3016Qm+b3S60384Q2m+aS0√b2S40+16aS0Qm24Qm+b2S40√b2S40+16aS0Qm384Q2m−Dλ22−q=Θ2−Dλ22−q. |
Let c∗=min{Θ1−Dλ22−q,Θ2−Dλ22−q}, from the above information, we deduce that cλ≥c∗. It contradicts our assumption, so it indicates that νj=μj=0 for every j∈J, which implies that
∫Ω|un|6dx→∫Ω|u|6dx | (2.8) |
as n→∞. Now, we may assume that ∫Ω|∇un|2dx→A2 and ∫Ω|∇u|2dx≤A2, by (2.3), (2.4) and (2.8), one has
0=limn→∞⟨I′λ(un),un−u⟩=limn→∞[(a+b∫Ω|∇un|2dx)(∫Ω|∇un|2dx−∫Ω∇un∇udx)+∫Ωun(un−u)dx−∫ΩQ(x)|un|5(un−u)dx−λ∫ΩP(x)|un|q−1(un−u)dx]=(a+bA2)(A2−∫Ω|∇u|2dx). |
Then, we obtain that un→u in H1(Ω). The proof is complete.
As well known, the function
Uε,y(x)=(3ε2)14(ε2+|x−y|2)12,foranyε>0, |
satisfies
−ΔUε,y=U5ε,yinR3, |
and
∫R3|∇Uε,y|2dx=∫R3|Uε,y|6dx=S320. |
Let ϕ∈C1(R3) such that ϕ(x)=1 on B(xM,R2), ϕ(x)=0 on R3−B(xM,R) and 0≤ϕ(x)≤1 on R3, we set vε(x)=ϕ(x)Uε,xM(x). We may assume that Q(x)>0 on B(xM,R) for some R>0 such that B(xM,R)⊂Ω. From [4], we have
{‖∇vε‖22=S320+O(ε),‖vε‖66=S320+O(ε3),‖vε‖22=O(ε),‖vε‖2=aS320+O(ε). | (2.9) |
Moreover, by [28], we get
{‖∇vε‖42≤S30+O(ε),‖∇vε‖82≤S60+O(ε),‖∇vε‖122≤S90+O(ε). | (2.10) |
Then we have the following Lemma.
Lemma 2.3. Suppose that 1<q<2, 3−q<β<6−q2, QM>4Qm, (Q1) and (Q2), then supt≥0Iλ(tvε)<Θ1−Dλ22−q.
Proof. By Lemma 2.1, one has Iλ(tvε)→−∞ as t→∞ and Iλ(tvε)<0 as t→0, then there exists tε>0 such that Iλ(tεvε)=supt>0Iλ(tvε)≥r>0. We can assume that there exist positive constants t1,t2>0 and 0<t1<tε<t2<+∞. Let Iλ(tεvε)=β(tεvε)−λψ(tεvε), where
β(tεvε)=t2ε2‖vε‖2+bt4ε4‖∇vε‖42−t6ε6∫ΩQ(x)|vε|6dx, |
and
ψ(tεvε)=tqεq∫ΩP(x)|vε|qdx. |
Now, we set
h(t)=t22‖vε‖2+bt44‖∇vε‖42−t66∫ΩQ(x)|vε|6dx. |
It is clear that limt→0h(t)=0 and limt→∞h(t)=−∞. Therefore there exists T1>0 such that h(T1)=maxt≥0h(t), that is
h′(t)|T1=T1‖vε‖2+bT31‖∇vε‖42−T51∫ΩQ(x)|vε|6dx=0, |
from which we have
‖vε‖2+bT21‖∇vε‖42=T41∫ΩQ(x)|vε|6dx. | (2.11) |
By (2.11) we obtain
T21=b‖∇vε‖42+√b2‖∇vε‖82+4‖vε‖2∫ΩQ(x)|vε|6dx2∫ΩQ(x)|vε|6dx. |
In addition, by (Q2), for all η>0, there exists ρ>0 such that |Q(x)−QM|<η|x−xM| for 0<|x−xM|<ρ, for ε>0 small enough, we have
|∫ΩQ(x)v6εdx−∫ΩQMv6εdx|≤∫Ω|Q(x)−QM|v6εdx<∫B(xM,ρ)η|x−xM|(3ε2)32(ε2+|x−xM|2)3dx+C∫Ω∖B(xM,ρ)(3ε2)32(ε2+|x−xM|2)3dx≤Cηε3∫ρ0r3(ε2+r2)3dr+Cε3∫Rρr2(ε2+r2)3dr≤Cηε∫ρ/ε0t3(1+t2)3dt+C∫R/ερ/εt2(1+t2)3dt≤C1ηε+C2ε3, |
where C1,C2>0 (independent of η, ε). From this we derive that
lim supε→0|∫ΩQ(x)v6εdx−∫ΩQMv6εdx|ε≤C1η. | (2.12) |
Then from the arbitrariness of η>0, by (2.9) and (2.12), one has
∫ΩQ(x)|vε|6dx=QM∫Ω|vε|6dx+o(ε)=QMS320+o(ε). | (2.13) |
Hence, it follows from (2.9), (2.10) and (2.13) that
β(tεvε)≤h(T1)=T21(13‖vε‖2+bT2112‖∇vε‖42)=b‖∇vε‖42‖vε‖24∫ΩQ(x)|vε|6dx+b3‖∇vε‖12224(∫ΩQ(x)|vε|6dx)2+‖vε‖2√b2‖∇vε‖82+4‖vε‖2∫ΩQ(x)|vε|6dx6∫ΩQ(x)|vε|6dx+b2‖∇vε‖82√b2‖∇vε‖82+4‖vε‖2∫ΩQ(x)|vε|6dx24(∫ΩQ(x)|vε|6dx)2≤b(S30+O(ε))(aS320+O(ε))4(QMS320+o(ε))+b3(S90+O(ε))24(QMS320+o(ε))2+(aS320+O(ε))√b2(S60+O(ε))+4(aS320+O(ε))(QMS320+o(ε))6(QMS320+o(ε))+b2(S60+O(ε))√b2(S60+O(ε))+4(aS320+O(ε))(QMS320+o(ε))24(QMS320+o(ε))2≤abS304QM+b3S6024Q2M+aS0√b2S40+4aS0QM6QM+b2S40√b2S40+4aS0QM24Q2M+C3ε=Θ1+C3ε, |
where the constant C3>0. According to the definition of vε, from [29], for R2>ε>0, there holds
ψ(tεvε)≥1q3q4tq1∫B(xM,R2)σεq2(ε2+|x−xM|2)q2|x−xM|βdx≥Cεq2∫R/20r2(ε2+r2)q2rβdr=Cε6−q2−β∫R/2ε0t2(1+t2)q2tβdt≥Cε6−q2−β∫10t2−βdt=C4ε6−q2−β, | (2.14) |
where C4>0 (independent of ε,λ). Consequently, from the above information, we obtain
Iλ(tεvε)=β(tεvε)−λψ(tεvε)≤Θ1+C3ε−C4λε6−q2−β<Θ1−Dλ22−q. |
Here we have used the fact that β>3−q and let ε=λ22−q, 0<λ<Λ1=min{1,(C3+DC4)2−q6−2q−2β}, then
C3ε−C4λε6−q2−β=C3λ22−q−C4λ8−2q−2β2−q=λ22−q(C3−C4λ6−2q−2β2−q)<−Dλ22−q. | (2.15) |
The proof is complete.
We assume that 0∈∂Ω and Qm=Q(0). Let φ∈C1(R3) such that φ(x)=1 on B(0,R2), φ(x)=0 on R3−B(0,R) and 0≤φ(x)≤1 on R3, we set uε(x)=φ(x)Uε(x), the radius R is chosen so that Q(x)>0 on B(0,R)∩Ω. If H(0) denotes the mean curvature of the boundary at 0, then the following estimates hold (see [6] or [26])
{‖uε‖22=O(ε),‖∇uε‖22‖uε‖26≤S0223−A3H(0)εlog1ε+O(ε), | (2.16) |
where A3>0 is a constant. Then we have the following lemma.
Lemma 2.4. Suppose that 1<q<2, 3−q<β<6−q2, QM≤4Qm, H(0)>0, Q is positive somewhere on ∂Ω, (Q1) and (Q3), then supt≥0Iλ(tuε)<Θ2−Dλ22−q.
Proof. Similar to the proof of Lemma 2.3, we also have by Lemma 2.1, there exists tε>0 such that Iλ(tεuε)=supt>0Iλ(tuε)≥r>0. We can assume that there exist positive constants t1,t2>0 such that 0<t1<tε<t2<+∞. Let Iλ(tεuε)=A(tεuε)−λB(tεuε), where
A(tεuε)=t2ε2‖uε‖2+bt4ε4‖∇uε‖42−t6ε6∫ΩQ(x)|uε|6dx, |
and
B(tεuε)=tqεq∫ΩP(x)|uε|qdx. |
Now, we set
f(t)=t22‖uε‖2+bt44‖∇uε‖42−t66∫ΩQ(x)|uε|6dx. |
Therefore, it is easy to see that there exists T2>0 such that f(T2)=maxf≥0f(t), that is
f′(t)|T2=T2‖uε‖2+bT32‖∇uε‖42−T52∫ΩQ(x)|uε|6dx=0. | (2.17) |
From (2.17) we obtain
T22=b‖∇uε‖42+√b2‖∇uε‖82+4‖uε‖2∫ΩQ(x)|uε|6dx2∫ΩQ(x)|uε|6dx. |
By the assumption (Q3), we have the expansion formula
∫ΩQ(x)|uε|6dx=Qm∫Ω|uε|6dx+o(ε). | (2.18) |
Hence, combining (2.16) and (2.18), there exists C5>0, such that
A(tεuε)≤f(T2)=T22(13‖uε‖2+bT2212‖∇uε‖42)=b‖∇uε‖42‖uε‖24∫ΩQ(x)|uε|6dx+b3‖∇uε‖12224(∫ΩQ(x)|uε|6dx)2+‖uε‖2√b2‖∇uε‖82+4‖uε‖2∫ΩQ(x)|uε|6dx6∫ΩQ(x)|uε|6dx+b2‖∇uε‖82√b2‖∇uε‖82+4‖uε‖2∫ΩQ(x)|uε|6dx24(∫ΩQ(x)|uε|6dx)2≤ab4Qm(‖∇uε‖62∫Ω|uε|6dx+O(ε))+b324Q2m(‖∇uε‖122(∫Ω|uε|6dx)2+O(ε))+a6Qm(‖∇uε‖22(∫Ω|uε|6dx)13√b2‖∇uε‖82(∫Ω|uε|6dx)43+4aQm‖∇uε‖22(∫Ω|uε|6dx)13+O(ε))+b224Q2m(‖∇uε‖82(∫Ω|uε|6dx)43√b2‖∇uε‖82(∫Ω|uε|6dx)43+4aQm‖∇uε‖22(∫Ω|uε|6dx)13+O(ε))≤abS3016Qm+b3S60384Q2m+aS0√b2S40+16aS0Qm24Qm+b2S40√b2S40+16aS0Qm384Q2m+C5ε=Θ2+C5ε. |
Consequently, by (2.14) and (2.15), similarly, there exists Λ2>0 such that 0<λ<Λ2, we get
Iλ(tεuε)=A(tεuε)−λB(tεuε)≤Θ2+C5ε−C6λε6−q2−β<Θ2−Dλ22−q. |
where C6>0 (independent of ε,λ). The proof is complete.
Theorem 2.5. Assume that 0<λ<Λ0 (Λ0 is as in Lemma 2.1) and 1<q<2. Then problem (1.1) has a nontrivial solution uλ with Iλ(uλ)<0.
Proof. It follows from Lemma 2.1 that
m≜infu∈¯Bρ(0)Iλ(u)<0. |
By the Ekeland variational principle [7], there exists a minimizing sequence {un}⊂¯Bρ(0) such that
Iλ(un)≤infu∈¯Bρ(0)Iλ(u)+1n,Iλ(v)≥Iλ(un)−1n‖v−un‖,v∈¯Bρ(0). |
Therefore, there holds Iλ(un)→m and I′λ(un)→0. Since {un} is a bounded sequence and ¯Bρ(0) is a closed convex set, we may assume up to a subsequence, still denoted by {un}, there exists uλ∈¯Bρ(0)⊂H1(Ω) such that
{un⇀uλ,weaklyinH1(Ω),un→uλ,stronglyinLp(Ω),1≤p<6,un(x)→uλ(x),a.e.inΩ. |
By the lower semi-continuity of the norm with respect to weak convergence, one has
m≥lim infn→∞[Iλ(un)−16⟨I′λ(un),un⟩]=lim infn→∞[13∫Ω(a|∇un|2+u2n)dx+b12(∫Ω|∇un|2dx)2+λ(16−1q)∫ΩP(x)|un|qdx]≥13∫Ω(a|∇uλ|2+u2λ)dx+b12(∫Ω|∇uλ|2dx)2+λ(16−1q)∫ΩP(x)|uλ|qdx=Iλ(uλ)−16⟨I′λ(uλ),uλ⟩=Iλ(uλ)=m. |
Thus Iλ(uλ)=m<0, by m<0<cλ and Lemma 2.2, we can see that ∇un→∇uλ in L2(Ω) and uλ≢0. Therefore, we obtain that uλ is a weak solution of problem (1.1). Since Iλ(|uλ|)=Iλ(uλ), which suggests that uλ≥0, then uλ is a nontrivial solution to problem (1.1). That is, the proof of Theorem 1.1 is complete.
Theorem 2.6. Assume that 0<λ<Λ∗(Λ∗=min{Λ0,Λ1,Λ2}), 1<q<2 and 3−q<β<6−q2. Then the problem (1.1) has a nontrivial solution u1∈H1(Ω) such that Iλ(u1)>0.
Proof. Applying the mountain pass lemma [3] and Lemma 2.2, there exists a sequence {un}⊂H1(Ω) such that
Iλ(un)→cλ>0andI′λ(un)→0asn→∞, |
where
cλ=infγ∈Γmaxt∈[0,1]Iλ(γ(t)), |
and
Γ={γ∈C([0,1],H1(Ω)):γ(0)=0,γ(1)=e}. |
According to Lemma 2.2, we know that {un}⊂H1(Ω) has a convergent subsequence, still denoted by {un}, such that un→u1 in H1(Ω) as n→∞,
Iλ(u1)=limn→∞Iλ(un)=cλ>r>0, |
which implies that u1≢0. Therefore, from the continuity of I′λ, we obtain that u1 is a nontrivial solution of problem (1.1) with Iλ(u1)>0. Combining the above facts with Theorem 2.5 the proof of Theorem 1.2 is complete.
In this paper, we consider a class of Kirchhoff type equations with Neumann conditions and critical growth. Under suitable assumptions on Q(x) and P(x), using the variational method and the concentration compactness principle, we proved the existence and multiplicity of nontrivial solutions.
This research was supported by the National Natural Science Foundation of China (Grant Nos. 11661021 and 11861021). Authors are grateful to the referees for their very constructive comments and valuable suggestions.
The authors declare no conflict of interest in this paper.
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