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Multiplicity solutions of a class fractional Schrödinger equations

  • Li-Jiang Jia , Bin Ge EMAIL logo , Ying-Xin Cui and Liang-Liang Sun
Published/Copyright: August 8, 2017

Abstract

In this paper, we study the existence of nontrivial solutions to a class fractional Schrödinger equations

(Δ)su+V(x)u=λf(x,u)inRN,

where (Δ)su(x)=2limε0RNBε(X)u(x)u(y)|xy|N+2sdy,xRN is a fractional operator and s ∈ (0, 1). By using variational methods, we prove this problem has at least two nontrivial solutions in a suitable weighted fractional Sobolev space.

MSC 2010: 35P15; 35P30; 35R11

1 Introduction

There are a lot of interesting problems in the standard framework of the Laplacian (and, more generally, of uniformly elliptic operators), widely studied in the literature. A natural question is whether or not the existence results got in this classical context can be extended to the non-local framework of the fractional Laplacian type operators.

First, we focus on the so-called fractional Schrödinger equation

iψt=(Δ)sψ+V(x)ψ|ψ|p1ψ,(1)

where (x, t) ∈ ℝ × (0, +∞), and V : ℝN → ℝ an external potential function. The fractional Laplacian operator (−Δ)su with 0 < s < 1 of a function ϕ ∈ ℓ is defined by

((Δ)sϕ)(ξ)=|ξ|2s(ϕ)ξ,

where ℓ denotes the Schwartz space of rapidly decreasing C functions in ℝN ℑ is the Fourier transform, i.e.,

((Δ)sϕ)(ξ)=|ξ|2s(ϕ)ξ=1(2π)N2RNe2πiξ.xϕ(x)dx.

This equation was introduced by Laskin ([21, 22]), and comes from an expansion of the Feynman path integral from Brownian-like to Lévy-like quantum mechanical paths. When s = 1, the Lévy dynamics becomes the Brownian dynamics, and (1) reduces to the classical Schrödinger equation

iψt=Δψ+V(x)ψ|ψ|p1ψ.

Standing wave solutions to this equation are solutions of the form

ψ(x,t)=eiωtu(x),

where u solves the elliptic equation

Δψ+V(x)ψ|ψ|p1ψ=0.

In this paper we study the following fractional Schrödinger equation

(Δ)su+V(x)u=λf(x,u)inRN(2)

where λ is a parameter.

The fractional Schrödinger equations are an important model in the study of the fractional quantum mechanics. Recently, this has been widely investigated by many authors in the last decades, see [3-15, 18-20, 22-26] and references therein. In most of the papers mentioned above the existence of positive solutions has been considered under different assumptions on V and f. We refer the reader to [16, 17] and to the references included for a selfcontained overview of the basic properties of fractional Sobolev spaces.

In [3], the author used the Ekeland variational principle and the mountain pass theorem to obtain a nontrivial solution for (2) with the Ambrosetti-Rabinowitz condition:

there is a constant μ > 2 such that

0μ0sf(x,t)dtsf(x,s),forallxRN,sR{0}.(AR)

In [4-6], the authors used variant fountain theorems and the ℤ2 version of mountain pass theorem to establish the existence of infinitely many nontrivial high-energy or small-energy solutions for (2). In [7], the authors used the concentration compactness principle to show that (2) (V(x) = 1) has at least two nontrivial radial solutions without the (AR) condition.

In [25], Bisci and Rǎdulescu studied the following equation

(Δ)su+V(x)u=λ(f(x,u)+κg(x,u))inRN

when the potential VC(ℝN) satisfies

(V)infRNV(x)>0

and

(V0)lim|y|+μ{xB(y,r):V(x)M}<+,M>0,

where μ denotes the Lebesgue measure in ℝN, B(y, r) denotes the open ball in ℝN with center y and radius r > 0, and established two existence theorems for two nontrivial solutions when the nonlinearity f and g satisfy

(h)max{|f(x,t)|,|g(x,t)|}W(x)|t|q,

where WL1(ℝN)∩L(ℝN) and q ∈ (0, 1).

Motivated by the above papers, we shall assume that f(x, t) satisfies the following conditions:

(f1) fC(ℝN+1, ℝ), and

limt0f(x,t)t=0.

(f2)

lim|t||f(x,t)|t=0.

(f3)

suptRF(t)>0.

where F(x,u)=0uf(x,s)ds.

And on the potential function V we assume

(V1) VC(ℝN, ℝ) is a positive weight and there exists a constant V0 > 0 such that V(x) ≥ V0 for all x ∈ ℝN.

(V2)

1V(x)L1(RN).

The main purpose of this paper is to generalize the main results of [24, 25]. Now we state our main results:

Theorem 1.1

Assume that f and V satisfy (f1) (f2), (f3), (V1), and (V2). Then problem (1.2) admits at least two nontrivial nonnegative solutions.

This paper is organized as follows. In Section 2, we will give some notation and introduce our main idea. In Section 3, we prove Theorem 1.1.

Remark 1.2

If g(x, u) = 0, then (h) ⇒ (f2). However, there are many functions which satisfy (f2), but do not satisfy the condition (h). For example, the function

f(x,t)={ t2,M112M02M1M0(tM0)+M02,t12,0,if0tM0,M0>1ifM0tM1,iftM1,ifx<0,

does not satisfy (h), but it satisfies our conditions (f2).

2 Preliminary

In this section, for the reader’s convenience, we collect some basic results that will be used in the forthcoming sections. In the following, we denote the N-dimensional Lebesgue measure of a set A ⊂ ℝN by meas(A). We use " ⇀ " and " → " to denote weak and strong convergence in the related function space. For any Eucliden space (ℝN ; |·|) we will denote

Bρ={uRN:|uu0|ρ}(u0RN,ρ>0).

2.1 Variational formulation of the problem

First we introduce a variational setting for problem (2). The Gagliardo seminorm is defined for all measurable function u : ℝN → ℝ by

u:[u]2,s=(R2N|u(x)u(y)|2|xy|N+2sdxdy)12,

we can define the fractional Sobolev space

Hs(RN)={uL2(RN):umeasurable,[u]2,s<},

with

||u||Hs=(||u||22+[u]2,s2)12,

where ||u||ϑ=(RN|υ|ϑds)1ϑ, which is the norm of Lϑ(ℝN). The space Hs(ℝN) is a Hilbert space with the inner product

u,υ=R2N(u(x)u(y))(υ(x)υ(y))|xy|N+2xdxdy+RNuυdx.

The space Hs (ℝN) can be described by means of the Fourier transform. Indeed, it is defined by

Hs(RN)={uL2(RN):RN(1+|ξ|2)s|u(ξ)|2dξ<+}.

In this case, the inner product and the norm are defined as

u,υ=RN(1+|ξ|2)su(ξ)υ(ξ)dξ,||u||Hs=(RN(1+|ξ|2)s|u(ξ)|2dξ)12.

In order to give the relationship of the above two norms, we introduce the definition of Schwartz function ℓ, that is, the rapidly decreasing C function on ℝN . If u ∈ ℓ, the fractional Laplacian (−Δ)s acts on u as

(Δ)su(x)=C(N,s)limε0+RNB(0,ε)u(x)u(y)|xy|N+2sdy,

where C(N, s) is the following constant

C(N,s)=(RN1cos(ζ1)|ζ|n+2s)1.

In [17], it is proved that

(Δ)su=1(|ξ|2su),[u]2,s=2C(N,s)RN|ξ|2s|u|2dξ,

and that

[u]2,s=2C(N,s)||(Δ)s2u||22.

As a consequence, the norms on Hs(ℝN) defined above

u(R2N(u(x)u(y))2|xy|N+2sdxdy+RNu2dx)12,u(RN(1+|ξ|2)s|u(ξ)|2dξ)12,u(RN|ξ|2s|u(ξ)|2dξ+RNu2dx)12,u(||(Δ)s2u||22+RNu2dx)12.

are all equivalent.

Moreover, it is easy to see that (Hs(ℝN), ||u||2,s) is a uniformly convex Banach space and the embedding Hs(ℝN) ↪ Lϑ(ℝN) is continuous for any ϑ[2,2s] by Theorem 6.7 of [17], that is, there exists a positive constant C* such that

||u||Lϑ(RN)C||u||2,sforalluHs(RN).

We will work in the following linear subspace

E={uHs(RN),RNV(x)|u|2dx<+},

which denotes the completion of C0(RN) with respect to the norm

||u||E=[u]2,s2+||u||2,V212,||u||2,V2=RNV(x)|u|2dx.

We also know that (E, ||·||E) is a uniformly convex Banach space, see ([27] Lemma 10). The dual space of (E, ||·||E) is denoted by (E*, ||·||E*). 〈·, ·〉 denotes the pairing between E and its dual space E*. We define the nonlinear operator Φ : EE* as

Φ(u),υ=R2N(u(x)u(y))(υ(x)υ(y))|xy|N+2sdxdy+RNV(x)u(x)υ(x)dx.

It can be seen that a weak solution of problem (2) is a function uE such that

Φ(u),υ=RNf(x,u)υdx.(3)

for all υE. Clearly, for all uE, Φ(u),υ=||u||E2.

Now we introduce the minimal hypotheses on the reaction term of (2):

(H)f : ℝN × ℝ → ℝ is a Carathéodory mapping, F(x,t)=0tf(x,s)ds for all (x,t) ∈ ℝN × ℝ and q(2,2s) such that

|f(x,u)|a(1+|u|q1),

a.e. in ℝN and for all t ∈ ℝ(a > 0).

We set for all uE

I(u)=||u||E22RNF(x,u)dx.(4)

defined on E, and for any uE, it holds that:

I(u),υ=Φ(u),υRNf(x,u)υdx.(5)

By hypotheses (H), we have IC1(E). We denote by K(I) the set of all critical points of I. If uK(I), then (4) hold for all υE, i.e., u is a weak solution to (2).

2.2 Some preliminary lemmas

We first recall some embedding results related to the fractional Sobolev space E, for more details, see [27].

Lemma 2.1

([27, Lemma 1]). Let (V1) hold. Then the embeddings EHs(ℝN) ↪ Lϑ(ℝN) are continuous with

min{1,V0}||u||2,s2||u||E2,(6)

for all uE and ϑ[2,2s]. Moreover, for any R > 0 and ϑ[2,2s), the embedding ELϑ(BR(0)) is compact.

Lemma 2.2

Suppose that (V1) and (V2) are fulfilled. If {υj} is a bounded sequence in E, then there exists υELϑ(ℝN) such that up to a subsequence,

υjυstronglyinLϑ(RN)

as j → ∞, for any ϑ[2,2s).

Proof

Since υj is bounded in E, by lemma 2.1 we have υj is bounded in Lϑ(ℝN). Then by the reflexivity of E, up to a subsequence, we get that υjυ weakly in ELϑ(ℝN) as j → ∞. Next we prove that

υjυstronglyinLϑ(RN).

Now, for any ɛ > 0, there exists R1 > 0 such that

RNBR(0)1V(x)dx<ε

for all RR1, since 1V(x)L1(RN) by assumption (V2). Then, by Hölder inequality, we can get that

RNBR(0)V(x)12V(x)1212s|υjυ|2dx(RNBR(0)V(x)|υjυ|2dx)12(RNBR(0)|υjυ|2sdx)12s(RNBR(0)1V(x)dx)2s222sε,(7)

for all RR1.

Fix R1 > 0, we have

υjυweaklyinL2s(BR1(0))Hs(BR1(0)),

by Theorem 6.7 of [17]. Since 2ϑ<2s, by Corollary 7.2 of [17], we obtain υjυ strongly in Lϑ(BR1(0)), i.e. for above ɛ > 0, there exists N1 > 0 such that

BR1(0)|υjυ|ϑdx<ε(8)

for all jN1. Combining (7) and (8), for all jN1, by interpolation inequality we have

RN|υjυ|ϑdx=BR1(0)|υjυ|ϑdx+RNBR1(0)|υjυ|ϑdxε+(RNBR1(0)|υjυ|2dx)θϑ2(RNBR1(0)|υjυ|2sdx)(1θ)ϑ2sε+(RNBR1(0)V(x)12V012sV(x)1212s|υjυ|2dx)θϑ2(RNBR1(0)|υjυ|2sdx)(1θ)ϑ2sε+1V0θϑ22s(RNBR1(0)V(x)|υjυ|2dx)θϑ4(RNBR1(0)|υjυ|2sdx)(1θ)θϑ22s2s×(RNBR1(0)1V(x)dx)θϑ(2s2)42sCε,

where C denotes various positive constants, and θ ∈ (0, 1) such that

1ϑ=θ2+1θ2s.

Therefore, υjυ strongly in Lϑ(ℝN ). □

Proposition 2.3

Let (E, ||·||E) be a Banach space and its dual space (E*, ||·||E*) and IC1(E, ℝ1)

  1. For c ∈ ℝ1, we say that I satisfies the Cc condition if for any sequence {xn} ⊂ E with

    I(xn)c,||I(xn)||E(1+||xn||E)0.
  2. For c ∈ ℝ1, we say that I satisfies the (PS) condition if for any sequence {xn} ⊂ E with

    I(xn)c,I(xn)0inE.
The following critical points theorem was established in [1, 2].

Lemma 2.4

([1, 2]). Let (E, ||·||) be a separable and reflexive real Banach space, and let Φ, ψ : E → ℝ be two continuously Gâteaux differentiable functionals. Assume that there exists z0E such that Φ(z0) = ψ(z0) = 0 and infzEΦ(z)0 and that there exist z1E, ϱ > 0 such that

  1. ϱ < Φ(z1);

  2. supΦ(z)<ϱψ(z)<ϱψ(z1)Φ(z1). Further, put

    a¯=ζϱϱψ(z1)Φ(z1)supΦ(Z)<ϱψ(z),

    with ς > 1, assume that the functional

    J(z)=Φ(z)λψ(z),(zE)

    is sequentially weakly lower semicontinuous, satisfies the (PS) condition, and

  3. lim||z||+J(z)=+, for every λ ∈ [0, ā].

Then there is an open interval Λ ⊂ [0, ā] and a number κ > 0 such that for each λ ∈ Λ, the equation J′ (z) = 0 admits at least three solutions in E having norm less than κ.

3 The main results and its proofs

In this section, we are ready to prove the Theorem 1.1. In the sequel, for the sake of clarity, we divide the proof of the theorem into several steps. We write the functional J as follows:

J(u)=Φ(u)λψ(u),

where

Φ(u)=12R2N(u(x)u(y))2|xy|N+2sdxdy+12RNV(x)u2dx,

and

ψ(u)=RNF(x,u)dx.

We first give two preliminary lemmas.

Lemma 3.1

If (f1), (f2) and (V1) hold, then we have that

limϱ0+supuSϱRNF(x,u)dxϱ=0

where Sϱ = Φ-1(−∞,ϱ).

Proof

By (f1) and (f2), there is a positive constants δ such that

|f(x,t)|min(1,V0)ε|t|2C2+cmin(1,V0)ϑ2|t|ϑ1Cϑ|F(x,t)|min(1,V0)ε|t|24C2+cmin(1,V0)ϑ2|t|ϑCϑ

for a fixed ϑ[2,2s) and for all t ∈ ℝN.

Moreover, by using lemma 2.1, we have

supuSϱRNF^(x,u)dxRNmin(1,V0)ε|u|24C2+cmin(1,V0)ϑ2|u|ϑCϑdx=min(1,V0)ε||u||224C2+cmin(1,V0)ϑ2|u|ϑϑCϑε2ϱ+c2ϑ2ϱϑ2.

Thus, there exists ϱ(ɛ) > 0 such that, for every 0 < ϱ < ϱ(ɛ), we have

0supuSϱRNF^(x,u)dxϱε2+c2ϑ2ϱϑ22ε.

The proof is complete. □

Lemma 3.2

Let σ ∈ [0, 1], t0 ∈ ℝ, and

Γ(t)=0+zt1ezdz,(t>0)

be the usual Gamma function, then we have

||uσt0||2<t02(1σ)2πN2τN2(1σN)Γ(1+N2)S0(9)

where

uσt0=0,ifxRNB(x0,τ),t0(1σ)τ(τ|xx0|),ifxB(x0,τ)B(x0,στ),t0,ifxB(x0,στ),

and

S0=max{(2π)N(1+1λ1),maxxB(x0,τ)V(x)λ1}.

Proof

Computing the standard seminorm of the function uσt0 in H1 (ℝN), we have

[uσt0]H1(RN)2=RN|uσt0|2dx=B(x0,τ)B(x0,στ)t02(1σ)2τ2dx=t02(1σ)2τ2[meas(B(x0,τ))meas(B(x0,στ))]=t02(1σ)2πN2τN2(1σN)Γ(1+N2),

where meas (B(x0, τ)) and meas (B(x0, στ)) denote the Lebesgue measure of B(x0, τ) and B(x0, στ).

Now, by standard arguments on the Fourier transform, we have

uσt0L2(RN)ifandonlyifuσt0L2(RN),

and

||uσt0||L2(RN)2=(2π)N||uσt0||L2(RN)2,

as well as

|uσt0|L2(RN)ifandonlyif|ξ|uσt0L2(RN),

and

||uσt0||L2(RN)2=(2π)N|||ξ|uσt0||L2(RN)2.

Then, we have

||uσt0||2=RN|ξ|2s|uσt0|2dξ+RNV(x)|uσt0|2dx<RN(1+|ξ|2)|uσt0|2dξ+RNV(x)|uσt0|2dx=(2π)N(||uσt0||L2(RN)2+||uσt0||L2(RN)2)+RNV(x)|uσt0|2dx(2π)N(1+1λ1)||uσt0||L2(RN)2+maxxB(x0,τ)V(x)||uσt0||L2(RN)2<max{(2π)N(1+1λ1),maxxB(x0,τ)V(x)λ1}[uσt0]H1(RN)2,

where

λ1=infuH1(B(x0,τ))0||u||L2(RN)2||u||L2(RN)2.

Thus (9) holds. □

Now, we prove Theorem 1.1.

Proof of Theorem 1.1

Step 1. We prove for every λ ∈ ℝ, the functional J is coercive and satisfies the compactness (PS) condition.

Let us fix λ ∈ ℝ. By (f2), there is a positive constant δ such that

|f(t)|min(1,V0)|t|C2(1+|λ|),

for every |t| ≥ δ.

So, we get

|F(t)|min(1,V0)|t|22C2(1+|λ|)+max|t|δ|f(t)||t|,

for every t ∈ ℝ.

Thus

J(u)=12||u||E2λRNF(x,u)dx12||u||E2|λ||RNF(x,u)dx|12||u||E2|λ|RN|F(x,u)|dx12(1+|λ|)||u||E2|λ|RNmax|t|δ|f(t)||u|dx=12(1+|λ|)||u||E2|λ|RNmax|t|δV(x)12V(x)12|f(t)||u|dx12(1+|λ|)||u||E2|λ|max|t|δ|f(t)|(RNV(x)|u|2dx)12(RN1V(x)dx)1212(1+|λ|)||u||E2C|λ|max|t|δ|f(t)|||u||E,

for every uE.

Then the functional J is bounded from below and J(u) → +∞ when ||u||E → +∞. Hence J is coercive.

Now we prove that J satisfies the (PS) condition. Let {un} ⊂ E be a (PS) sequence for J(u), that is

J(un)c,J(un)0inE.

Taking into account the coercivity of J, the sequence {un} is necessarily bounded in E. Assume without loss of generality that {un} converges to u weakly in E, and by Lemma 2.2, we may assume that

{un(x)u(x)a.e.inRunuinLϑ(RN) (10)

where ϑ[2,2s).

To prove that {un} converges strongly to u in E, we first introduce a simple notation. Let ϕE be fixed and denote by Bϕ the linear functional on E defined by

Bφ(υ)=R2N(φ(x)φ(y))(υ(x)υ(y))|xy|N+2sdxdy

for all υE.

Due to (f1) and (f2), there exists Cɛ > 0 such that

|f(t)|ε|t|+Cε|t|ϑ1.

Then, by (10), we get

R N ( f ( x , u n ) f ( x , u ) ) ( u n u ) d x R N ( ε | u n | + C ε | u n | ϑ 1 + ε | u | + C ε | u | ϑ 1 ) ( u n u ) d x ε | | u n | | 2 | | u n u | | 2 + ε | | u | | 2 | | u n u | | 2 + C ε | | u n | | ϑ ϑ 1 | | u n u | | ϑ + C ε | | u | | ϑ ϑ 1 | | u n u | | ϑ 0. (11)

Obviously, 〈J′(un) − J′(u), unu) → 0 as n → ∞, since unu in E and J′(un) → 0 in E*. Hence, (10) and (11) give as n → ∞

o(1)=J(un)J(u),unu=Bun(unu)Bu(unu)+RNV(x)(unu)2dxRN(f(x,un)f(x,u))(unu)dx=Bun(unu)Bu(unu)+RNV(x)(unu)2dx+o(1).

That is

||unu||E2=Bun(unu)Bu(unu)+RNV(x)(unu)2dx0

as n → ∞.

Therefore, J satisfies the (PS) condition.

Step 2. We claim that the functional J(u) is weakly lower semicontinuous on E.

The functional

uR2N(u(x)u(y))2|xy|N+2sdxdy+RNV(x)u2dx,

is sequentially weakly lower semicontinuous on E.

Thus it is enough to prove that the map

uRNF(x,u)dx,

is sequentially weakly continuous on E. To this aim fix unE and uE such that unu in E as n → ∞. Then, by Lemma 2.2, without loss of generality, we can assume that unu strongly in Lϑ(ℝN) for 2 ϑ < 2 s and a.e. in ℝN. It is dominated by some function hϑLϑ(ℝN), that is,

|un|hϑ(x)a.e.xRN

for any nN and for any ϑ[2,2s).

By (f1) and (f2), there exists Cɛ such that

|f(t)|ε|t|+Cε|t|ϑ1.

Then, by the continuity of F, it follows that

|F(x,un)|h(x),

a.e. x ∈ ℝN, for some hL1(ℝN), and

F(x,un)F(x,u),

a.e. in ℝN.

Hence, by Lebesgue dominated convergence theorem, we have

limn+RNF(x,un)dx=RNF(x,u)dx,

that is the map

uRNF(x,u)dx,

is weakly continuous in E. Thus, J is weakly semicontinuous in E.

Step 3. We show that there exists a u0E, ϱ > 0 such that ϱ < Φ(u0) and supΦ(u)<ϱΨ(u)<ϱΨ(u0)Φ(u0).

By (f3), there exists t0 ∈ ℝN such that F(t0) > 0. Further, let σ0 [0, 1] be such that

F(t0)σ0N(1σ0N)max|t||t0||F(t)|>0.

Indeed, since

||uσt0||=maxxB(x0,τ)|uσt0||t0|,

(recalled the definition of uσt0 ), it follows that

B(x0,τ)B(x0,σ0τ)F(uσt0)dxmax|t||t0||F(t0)|B(x0,τ)B(x0,σ0τ)dx=(1σ0N)max|t||t0||F(t)|τNωN,

where ωN denotes the volume of the unit ball in ℝN.

Then, we have that

Ψ(uσt0)=B(x0,τ)F(uσt0)dx=B(x0,σ0τ)F(uσt0)dx+B(x0,τ)B(x0,σ0τ)F(uσt0)dxF(t0)σ0NτNωN+B(x0,τ)B(x0,σ0τ)F(uσt0)dx[F(t0)σ0N(1σ0N)max|t||t0||F(t)|]τNωN. (12)

By Lemma 3.2, we have

Φ(uσt0)C, (13)

where

C=t022(1σ)2πN2τN2(1σN)Γ(1+N2)S0.

By Lemma 3.1, there exists ϱ > 0 such that the function uσt0E verifies the following conditions:

uσt0RSϱ, (14)

and

supuSϱΦ(u)ϱ<[F(t0)σ0N(1σ0N)max|t||t0||F(t)|]τNωNC. (15)

Then by (14), we have

ϱ<Φ(uσt0),

as well as, by (12) and (13), it follows that:

[F(t0)σ0N(1σ0N)max|t||t0||F(t)|]τNωNCΨ(uσt0)Φ(uσt0). (16)

Hence, (15) and (16) give

supuSϱΦ(u)Ψ(uσt0)Φ(uσt0). (17)

By choosing u0=uσt0, we get that ϱ < Φ(u0), and sup Φ ( u ) ϱ Ψ ( u ) < ϱ Ψ ( u 0 ) Φ ( u 0 ) .

Set

a¯=(1+ϱ)ϱϱΨ(u0)Φ(u0)supΦ(u)<ϱΨ(u),

Clearly, 1 + ϱ > 1, and ā is a positive constant.

It’s easy to see that infxEΦ(u)=0, and by choosing u1 = 0, we have

Φ(u1)=Ψ(u1)=0.

Then, by Step 1, Step 2, and Step 3, all the assumptions of Lemma 2.4 are verified. Thus there is an open interval Λ ⊆ [0, ā] and a number κ > 0 such for all λ ∈ Λ, the functional J admits three solutions in E having norm less than κ. Since one of them may be a trivial one, we have at least two distinct, nontrivial weak solutions of problem (2). ☐

Remark 3.3

Our hypotheses are similar to those employed by Bisci-Rădulescu [24] of (2) on bounded subset Ω ofN . Moreover, (f2) is weaker than the condition (h). So Theorem 1.1 extends Theorem 1 of [24] and Theorem 2 of [25].

Acknowledgement

Supported by the National Natural Science Foundation of China (Nos. 11201095, 71373059), Postdoctoral research startup foundation of Heilongjiang (No. LBH-Q14044) and the Science Research Funds for Overseas Returned Chinese Scholars of Heilongjiang Province (No. LC201502), Research Fund for the Doctoral Program of Higher Education of China (No. 20122304110018).

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Received: 2016-11-8
Accepted: 2017-6-19
Published Online: 2017-8-8

© 2017 Jia et al.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

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