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Research article

Multiple solutions of Kirchhoff type equations involving Neumann conditions and critical growth

  • Received: 13 December 2020 Accepted: 25 January 2021 Published: 29 January 2021
  • MSC : 35B33, 35B35, 35J33

  • In this paper, we consider a Neumann problem of Kirchhoff type equation

    {(a+bΩ|u|2dx)Δu+u=Q(x)|u|4u+λP(x)|u|q2u,in  Ω,uv=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+bΩ|u|2dx)Δu+u=Q(x)|u|4u+λP(x)|u|q2u,in  Ω,uv=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+bΩ|u|2dx)Δu+u=Q(x)|u|4u+λP(x)|u|q2u,in  Ω,uv=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 xMΩ such that QM=Q(xM)>0 and

    |Q(x)QM|=o(|xxM|)asxxM;

    (Q3) there exists 0Ω such that Qm=Q(0)>0 and

    |Q(x)Qm|=o(|x|)asx0;

    (P1) P(x) is positive continuous on ˉΩ and P(x0)=maxxˉΩP(x);

    (P2) there exist σ>0, R>0 and 3q<β<6q2 such that P(x)σ|xy|β for |xy|R, where y is xMΩ or 0Ω.

    In recent years, the following Dirichlet problem of Kirchhoff type equation has been studied extensively by many researchers

    {(a+bΩ|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+bΩ|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+bR3|u|2dx)Δu+V(x)u=f(x,u),inR3,u>0,uH1(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|22u+λf(x,u),in  Ω,uv=0,on  Ω,

    where Ω RN is a smooth bounded domain, 2=2NN2(N3) 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)=span1V, where V={vH1(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+bΩ|u|2dx)Δu=u5+λuq1|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 0β<2 and two positive solutions for 3qβ<2 by the Nehari manifold method.

    In [34], Zhang obtained the existence and multiplicity of nontrivial solutions of the following equation

    {(a+bΩ|u|2dx)Δu+u=λ|u|q2u+f(x,u)+Q(x)u5,in  Ω,uv=0,on  Ω, (1.5)

    where Ω is an open bounded domain in R3, a,b>0, 1<q<2, λ0 is a parameter, f(x,u) and Q(x) are positive continuous functions satisfying some additional assumptions. Moreover, f(x,u)|u|p2u 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)=12u2+b4(Ω|u|2dx)216ΩQ(x)|u|6dxλqΩP(x)|u|qdx.

    A weak solution of problem (1.1) is a function uH1(Ω) and for all φH1(Ω) such that

    Ω(auφ+uφ)dx+bΩ|u|2dxΩuφdx=ΩQ(x)|u|4uφdx+λΩP(x)|u|q2uφdx.

    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, 3q<β<6q2 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 u2H1(Ω)=Ω(|u|2+u2)dx, the norm in Lp(Ω) is denoted by p.

    ● Define u2=Ω(a|u|2+u2)dx for uH1(Ω). Note that is an equivalent norm on H1(Ω) with the standard norm.

    ● Let D1,2(R3) is the completion of C0(R3) with respect to the norm u2D1,2(R3)=R3|u|2dx.

    0<QM=maxxˉΩQ(x), 0<Qm=maxxΩQ(x).

    Ω+={xΩ:Q(x)>0} and Ω={xΩ: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={uH1(Ω):u=ρ}, Bρ={uH1(Ω):uρ}.

    ● Let S be the best constant for Sobolev embedding H1(Ω)L6(Ω), namely

    S=infuH1(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=infuD1,2(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|uSρr>0; infuBρIλ(u)<0.

    (ii) There exists eH1(Ω) with e>ρ such that Iλ(e)<0.

    Proof. (i) From (P1), by the H¨older inequality and the Sobolev inequality, for all uH1(Ω) one has

    ΩP(x)|u|qdxP(x0)Ω|u|qdxP(x0)|Ω|6q6Sq2uq, (2.1)

    and there exists a constant C>0, we get

    |ΩQ(x)|u|6dx|CΩ|u|6dxCS3u6. (2.2)

    Hence, combining (2.1) and (2.2), we have the following estimate

    Iλ(u)=12u2+b4(Ω|u|2dx)216ΩQ(x)|u|6dxλqΩP(x)|u|qdx12u2C6Ω|u|6dxλqP(x0)|Ω|6q6Sq2uquq(12u2qC6S3u6qλqP(x0)|Ω|6q6Sq2).

    Set h(t)=12t2qC6S3t6q for t>0, then there exists a constant ρ=(3(2q)S3C(6q))14>0 such that maxt>0h(t)=h(ρ)>0. Letting Λ0=qSq2P(x0)|Ω|6q6h(ρ), there exists a constant r>0 such that Iλ|uSρr for every λ(0,Λ0). Moreover, for all uH1(Ω){0}, we have

    limt0+Iλ(tu)tq=λqΩP(x)|u|qdx<0.

    So we obtain Iλ(tu)<0 for every u0 and t small enough. Therefore, for u small enough, one has

    minfuBρIλ(u)<0.

    (ii) Let vH1(Ω) be such that supp vΩ+, v0 and t>0, we have

    Iλ(tv)=t22v2+bt44(Ω|v|2dx)2t66ΩQ(x)|v|6dxλtqqΩP(x)|v|qdx

    as t, which implies that Iλ(tv)<0 for t>0 large enough. Therefore, we can find eH1(Ω) with e>ρ such that Iλ(e)<0. The proof is complete.

    Denote

    {Θ1=abS304QM+b3S6024Q2M+aS0b2S40+4aS0QM6QM+b2S40b2S40+4aS0QM24Q2M,Θ2=abS3016Qm+b3S60384Q2m+aS0b2S40+16aS0Qm24Qm+b2S40b2S40+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 {Θ1Dλ22q,Θ2Dλ22q}, where D=2q3q(6q4P(x0)Sq2|Ω|6q6)22q.

    Proof. Let {un}H1(Ω) be a (PS)cλ sequence for

    Iλ(un)cλandIλ(un)0asn. (2.3)

    It follows from (2.1), (2.3) and the H¨older inequality that

    cλ+1+o(un)Iλ(un)16Iλ(un),un13un2+b12(Ω|un|2dx)2λ(1q16)P(x0)Sq2|Ω|6q6unq13un2λ(6q)6qP(x0)Sq2|Ω|6q6unq.

    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 uH1(Ω) such that

    {unu,weaklyinH1(Ω),unu,stronglyinLp(Ω)(1p<6),un(x)u(x),a.e.inΩ, (2.4)

    as n. Next, we prove that unu 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 xjˉΩ and positive numbers {νj}, {μj}, jJ, such that

    |un|6dxdν=|u|6dx+jJνjδxj,|un|2dxdμ|u|2dx+jJμjδxj.

    Moreover, numbers νj and μj satisfy the following inequalities

    S0ν13jμjifxjΩ,S0223ν13jμjifxjΩ. (2.5)

    For ε>0, let ϕε,j(x) be a smooth cut-off function centered at xj such that 0ϕε,j1, |ϕε,j|2ε, and

    ϕε,j(x)={1, in B(xj,ε2)ˉΩ,0, in ΩB(xj,ε).

    There exists a constant C>0 such that

    limε0limnΩP(x)|un|qϕε,jdxP(x0)limε0limnB(xj,ε)|un|qdx=0.

    Since |ϕε,j|2ε, by using the H¨older inequality and L2(Ω)-convergence of {un}, we have

    limε0limn(a+bΩ|un|2dx)Ωun,ϕε,jundxClimε0limn(Ω|un|2dx)12(Ω|un|2|ϕε,j|2dx)12Climε0(B(xj,ε)|u|6dx)16(B(xj,ε)|ϕε,j|3dx)13Climε0(B(xj,ε)|u|6dx)16(B(xj,ε)(2ε)3dx)13C1limε0(B(xj,ε)|u|6dx)16=0,

    where C1>0, and we also derive that

    limε0limnΩ|un|2ϕε,jdxlimε0Ω|u|2ϕε,jdx+μj=μj,
    limε0limnΩQ(x)|un|6ϕε,jdx=limε0ΩQ(x)|u|6ϕε,jdx+Q(xj)νj=Q(xj)νj,
    limε0limnΩu2nϕε,jdx=limε0Ωu2ϕε,jdxlimε0B(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=limε0limnIλ(un),unϕε,j=limε0limn{(a+bΩ|un|2dx)Ωun,(unϕε,j)dx+Ωu2nϕε,jdxΩQ(x)|un|6ϕε,jdxλΩP(x)|un|qϕε,jdx}=limε0limn{(a+bΩ|un|2dx)Ω(|un|2ϕε,j+un,ϕε,jun)dxΩQ(x)|un|6ϕε,jdx}limε0{(a+bΩ|u|2dx+bμj)(Ω|u|2ϕε,jdx+μj)ΩQ(x)|u|6ϕε,jdxQ(xj)νj}(a+bμj)μjQ(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

    ν13jbS20+b2S40+4aS0QM2QMifxjΩ,ν13jbS20+b2S40+16aS0Qm273QmifxjΩ. (2.6)

    From (2.5) and (2.6), we have

    μjbS30+b2S60+4aS30QM2QMifxjΩ,μjbS30+b2S60+16aS30Qm8QmifxjΩ. (2.7)

    To proceed further we show that (2.7) is impossible. To obtain a contradiction assume that there exists j0J such that μj0bS30+b2S60+4aS30QM2QM and xj0Ω. By (2.1), (2.3) and (2.4), one has

    cλ=limn{Iλ(un)16Iλ(un),un}=limn{a3Ω|un|2dx+b12(Ω|un|2dx)2+13Ωu2ndxλ6q6qΩP(x)|un|qdx}a3(Ω|u|2dx+jJμj)+b12(Ω|u|2dx+jJμj)2+13Ωu2dxλ6q6qP(x0)Sq2|Ω|6q6uqa3μj0+b12μ2j0+13u2λ6q6qP(x0)Sq2|Ω|6q6uq.

    Set

    g(t)=13t2λ6q6qP(x0)Sq2|Ω|6q6tq,t>0,

    then

    g(t)=23tλ6q6P(x0)Sq2|Ω|6q6tq1=0,

    we can deduce that mint0g(t) attains at t0>0 and

    t0=(λ6q4P(x0)Sq2|Ω|6q6)12q.

    Consequently, we obtain

    cλabS304QM+b3S6024Q2M+aS0b2S40+4aS0QM6QM+b2S40b2S40+4aS0QM24Q2MDλ22q=Θ1Dλ22q,

    where D=2q3q(6q4P(x0)Sq2|Ω|6q6)22q. If μj0bS30+b2S60+16aS30Qm8Qm and xj0Ω, then, by the similar calculation, we also get

    cλabS3016Qm+b3S60384Q2m+aS0b2S40+16aS0Qm24Qm+b2S40b2S40+16aS0Qm384Q2mDλ22q=Θ2Dλ22q.

    Let c=min{Θ1Dλ22q,Θ2Dλ22q}, from the above information, we deduce that cλc. It contradicts our assumption, so it indicates that νj=μj=0 for every jJ, which implies that

    Ω|un|6dxΩ|u|6dx (2.8)

    as n. Now, we may assume that Ω|un|2dxA2 and Ω|u|2dxA2, by (2.3), (2.4) and (2.8), one has

    0=limnIλ(un),unu=limn[(a+bΩ|un|2dx)(Ω|un|2dxΩunudx)+Ωun(unu)dxΩQ(x)|un|5(unu)dxλΩP(x)|un|q1(unu)dx]=(a+bA2)(A2Ω|u|2dx).

    Then, we obtain that unu in H1(Ω). The proof is complete.

    As well known, the function

    Uε,y(x)=(3ε2)14(ε2+|xy|2)12,foranyε>0,

    satisfies

    ΔUε,y=U5ε,yinR3,

    and

    R3|Uε,y|2dx=R3|Uε,y|6dx=S320.

    Let ϕC1(R3) such that ϕ(x)=1 on B(xM,R2), ϕ(x)=0 on R3B(xM,R) and 0ϕ(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

    {vε22=S320+O(ε),vε66=S320+O(ε3),vε22=O(ε),vε2=aS320+O(ε). (2.9)

    Moreover, by [28], we get

    {vε42S30+O(ε),vε82S60+O(ε),vε122S90+O(ε). (2.10)

    Then we have the following Lemma.

    Lemma 2.3. Suppose that 1<q<2, 3q<β<6q2, QM>4Qm, (Q1) and (Q2), then supt0Iλ(tvε)<Θ1Dλ22q.

    Proof. By Lemma 2.1, one has Iλ(tvε) as t and Iλ(tvε)<0 as t0, 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)=t2ε2vε2+bt4ε4vε42t6ε6ΩQ(x)|vε|6dx,

    and

    ψ(tεvε)=tqεqΩP(x)|vε|qdx.

    Now, we set

    h(t)=t22vε2+bt44vε42t66ΩQ(x)|vε|6dx.

    It is clear that limt0h(t)=0 and limth(t)=. Therefore there exists T1>0 such that h(T1)=maxt0h(t), that is

    h(t)|T1=T1vε2+bT31vε42T51ΩQ(x)|vε|6dx=0,

    from which we have

    vε2+bT21vε42=T41ΩQ(x)|vε|6dx. (2.11)

    By (2.11) we obtain

    T21=bvε42+b2vε82+4vε2ΩQ(x)|vε|6dx2ΩQ(x)|vε|6dx.

    In addition, by (Q2), for all η>0, there exists ρ>0 such that |Q(x)QM|<η|xxM| for 0<|xxM|<ρ, for ε>0 small enough, we have

    |ΩQ(x)v6εdxΩQMv6εdx|Ω|Q(x)QM|v6εdx<B(xM,ρ)η|xxM|(3ε2)32(ε2+|xxM|2)3dx+CΩB(xM,ρ)(3ε2)32(ε2+|xxM|2)3dxCηε3ρ0r3(ε2+r2)3dr+Cε3Rρr2(ε2+r2)3drCηερ/ε0t3(1+t2)3dt+CR/ερ/εt2(1+t2)3dtC1ηε+C2ε3,

    where C1,C2>0 (independent of η, ε). From this we derive that

    lim supε0|ΩQ(x)v6εdxΩQMv6εdx|εC1η. (2.12)

    Then from the arbitrariness of η>0, by (2.9) and (2.12), one has

    ΩQ(x)|vε|6dx=QMΩ|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(13vε2+bT2112vε42)=bvε42vε24ΩQ(x)|vε|6dx+b3vε12224(ΩQ(x)|vε|6dx)2+vε2b2vε82+4vε2ΩQ(x)|vε|6dx6ΩQ(x)|vε|6dx+b2vε82b2vε82+4vε2ΩQ(x)|vε|6dx24(ΩQ(x)|vε|6dx)2b(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))2abS304QM+b3S6024Q2M+aS0b2S40+4aS0QM6QM+b2S40b2S40+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)1q3q4tq1B(xM,R2)σεq2(ε2+|xxM|2)q2|xxM|βdxCεq2R/20r2(ε2+r2)q2rβdr=Cε6q2βR/2ε0t2(1+t2)q2tβdtCε6q2β10t2βdt=C4ε6q2β, (2.14)

    where C4>0 (independent of ε,λ). Consequently, from the above information, we obtain

    Iλ(tεvε)=β(tεvε)λψ(tεvε)Θ1+C3εC4λε6q2β<Θ1Dλ22q.

    Here we have used the fact that β>3q and let ε=λ22q, 0<λ<Λ1=min{1,(C3+DC4)2q62q2β}, then

    C3εC4λε6q2β=C3λ22qC4λ82q2β2q=λ22q(C3C4λ62q2β2q)<Dλ22q. (2.15)

    The proof is complete.

    We assume that 0Ω and Qm=Q(0). Let φC1(R3) such that φ(x)=1 on B(0,R2), φ(x)=0 on R3B(0,R) and 0φ(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])

    {uε22=O(ε),uε22uε26S0223A3H(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, 3q<β<6q2, QM4Qm, H(0)>0, Q is positive somewhere on Ω, (Q1) and (Q3), then supt0Iλ(tuε)<Θ2Dλ22q.

    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)=t2ε2uε2+bt4ε4uε42t6ε6ΩQ(x)|uε|6dx,

    and

    B(tεuε)=tqεqΩP(x)|uε|qdx.

    Now, we set

    f(t)=t22uε2+bt44uε42t66ΩQ(x)|uε|6dx.

    Therefore, it is easy to see that there exists T2>0 such that f(T2)=maxf0f(t), that is

    f(t)|T2=T2uε2+bT32uε42T52ΩQ(x)|uε|6dx=0. (2.17)

    From (2.17) we obtain

    T22=buε42+b2uε82+4uε2ΩQ(x)|uε|6dx2ΩQ(x)|uε|6dx.

    By the assumption (Q3), we have the expansion formula

    ΩQ(x)|uε|6dx=QmΩ|uε|6dx+o(ε). (2.18)

    Hence, combining (2.16) and (2.18), there exists C5>0, such that

    A(tεuε)f(T2)=T22(13uε2+bT2212uε42)=buε42uε24ΩQ(x)|uε|6dx+b3uε12224(ΩQ(x)|uε|6dx)2+uε2b2uε82+4uε2ΩQ(x)|uε|6dx6ΩQ(x)|uε|6dx+b2uε82b2uε82+4uε2ΩQ(x)|uε|6dx24(ΩQ(x)|uε|6dx)2ab4Qm(uε62Ω|uε|6dx+O(ε))+b324Q2m(uε122(Ω|uε|6dx)2+O(ε))+a6Qm(uε22(Ω|uε|6dx)13b2uε82(Ω|uε|6dx)43+4aQmuε22(Ω|uε|6dx)13+O(ε))+b224Q2m(uε82(Ω|uε|6dx)43b2uε82(Ω|uε|6dx)43+4aQmuε22(Ω|uε|6dx)13+O(ε))abS3016Qm+b3S60384Q2m+aS0b2S40+16aS0Qm24Qm+b2S40b2S40+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+C5εC6λε6q2β<Θ2Dλ22q.

    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

    minfu¯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)1nvun,v¯Bρ(0).

    Therefore, there holds Iλ(un)m and Iλ(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

    {unuλ,weaklyinH1(Ω),unuλ,stronglyinLp(Ω),1p<6,un(x)uλ(x),a.e.inΩ.

    By the lower semi-continuity of the norm with respect to weak convergence, one has

    mlim infn[Iλ(un)16Iλ(un),un]=lim infn[13Ω(a|un|2+u2n)dx+b12(Ω|un|2dx)2+λ(161q)ΩP(x)|un|qdx]13Ω(a|uλ|2+u2λ)dx+b12(Ω|uλ|2dx)2+λ(161q)ΩP(x)|uλ|qdx=Iλ(uλ)16Iλ(uλ),uλ=Iλ(uλ)=m.

    Thus Iλ(uλ)=m<0, by m<0<cλ and Lemma 2.2, we can see that unuλ in L2(Ω) and uλ0. Therefore, we obtain that uλ is a weak solution of problem (1.1). Since Iλ(|uλ|)=Iλ(uλ), which suggests that uλ0, 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 3q<β<6q2. Then the problem (1.1) has a nontrivial solution u1H1(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>0andIλ(un)0asn,

    where

    cλ=infγΓmaxt[0,1]Iλ(γ(t)),

    and

    Γ={γC([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 unu1 in H1(Ω) as n,

    Iλ(u1)=limnIλ(un)=cλ>r>0,

    which implies that u10. Therefore, from the continuity of Iλ, 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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