A critical fractional equation with concave–convex power nonlinearities

(1)

ScienceDirect

Ann. I. H. Poincaré – AN 32 (2015) 875–900

www.elsevier.com/locate/anihpc

A critical fractional equation with concave–convex power nonlinearities

^✩

B. Barrios

^a,b

, E. Colorado

^c,b

, R. Servadei

^d

, F. Soria

^a,b,^∗

aDepartamento de Matemáticas, Universidad Autónoma de Madrid, 28049 Madrid, Spain

bInstituto de Ciencias Matemáticas, (ICMAT–CSIC–UAM–UC3M–UCM), C/Nicolás Cabrera, 15, 28049 Madrid, Spain cDepartamento de Matemáticas, Universidad Carlos III de Madrid, 28911 Leganés (Madrid), Spain

dDipartimento di Matematica e Informatica, Università della Calabria, Ponte Pietro Bucci 31 B, 87036 Arcavacata di Rende (Cosenza), Italy

Received 14 June 2013; received in revised form 21 April 2014; accepted 22 April 2014 Available online 2 May 2014

Abstract

In this work we study the following fractional critical problem

(P_λ)=

(−)^su=λu^q+u²^∗^s⁻¹, u>0 inΩ,

u=0 inRⁿ\Ω,

whereΩ⊂Rⁿis a regular bounded domain,λ >0, 0< s <1 andn >2s. Here(−)^s denotes the fractional Laplace operator defined, up to a normalization factor, by

−(−)^su(x)= Rⁿ

u(x+y)+u(x−y)−2u(x)

|y|ⁿ⁺^2s dy, x∈Rⁿ.

Our main results show the existence and multiplicity of solutions to problem(P_λ)for different values of λ. The dependency on this parameter changes according to whether we consider the concave power case (0< q <1) or the convex power case (1< q <2^∗_s−1). These two cases will be treated separately.

MSC:primary 49J35, 35A15, 35S15; secondary 47G20, 45G05

Keywords:Fractional Laplacian; Critical nonlinearities; Convex–concave nonlinearities; Variational techniques; Mountain Pass Theorem

✩ B.B., E.C., and F.S. were partially supported by Research Project of MICINN, Spain (Ref. MTM2010-18128). E.C. was also partially supported by Research Project of MICINN, Spain (Ref. MTM2009-10878). R.S. was supported by the MIUR National Research ProjectVariational and Topological Methods in the Study of Nonlinear Phenomena, by the GNAMPA ProjectNonlocal Problems of Fractional Laplacian Typeand by the FP7-IDEAS-ERC Starting Grant 2011, Ref. #277749 EPSILON.

* Corresponding author.

E-mail addresses:bego.barrios@uam.es(B. Barrios),ecolorad@math.uc3m.es(E. Colorado),servadei@mat.unical.it(R. Servadei), fernando.soria@uam.es(F. Soria).

http://dx.doi.org/10.1016/j.anihpc.2014.04.003

(2)

1. Introduction

In recent years, considerable attention has been given to nonlocal diffusion problems, in particular to the ones driven by the fractional Laplace operator. One of the reasons for this comes from the fact that this operator naturally arises in several physical phenomena like flames propagation and chemical reactions of liquids, in population dynamics and geophysical fluid dynamics, or in mathematical finance (American options). It also provides a simple model to describe certain jump Lévy processes in probability theory. In all these cases, the nonlocal effect is modeled by the singularity at infinity. For more details and applications, see[6,9,20,27,49,50]and the references therein.

In this paper we focus our attention on critical nonlocal fractional problems. To be more precise, we consider the following critical problem with convex–concave nonlinearities

(P_λ)=

⎧⎪

⎨

⎪⎩

(−)^su=λu^q+u²^∗^s⁻¹ inΩ,

u >0 inΩ,

u=0 inRⁿ\Ω,

whereΩ⊂Rⁿis a regular bounded domain,λ >0,n >2s, 0< q <2^∗_s −1 and 2^∗_s = 2n

n−2s (1.1)

is the fractional critical Sobolev exponent. Here(−)^sis the fractional Laplace operator defined, up to a normalization factor, by the Riesz potential as

−(−)^su(x):=

Rⁿ

u(x+y)+u(x−y)−2u(x)

|y|ⁿ⁺^2s dy, x∈Rⁿ, (1.2)

wheres∈(0,1)is a fixed parameter (see[46, Chapter 5]or[22,45]for further details).

One can also define a fractional power of the Laplacian using spectral decomposition. The same problem considered here but for this spectral fractional Laplacian has been treated in[7]. Some related problems involving this operator have been studied in[11,14,19,48]. As in[7] the purpose of this paper is to study the existence of weak solutions for(P_λ). Previous works related to the operator defined in(1.2), or by a more general kernel, can be found in[16,23, 30,34,35,37,38,41–43].

Problems similar to(P_λ)have been also studied in the local setting with different elliptic operators. As far as we know, the first example in this direction was given in[25]for thep-Laplacian operator. Other results, this time for the Laplacian (or essentially the classical Laplacian) operator can be found in[1,4,10,18]. More generally, the case of fully nonlinear operators has been studied in[17].

It is worth noting here that the problem(P_λ), withλ=0, has no solution wheneverΩ is a star-shaped domain.

This has been proved in[24,36]using a Pohozaev identity for the operator(−)^s. This fact motivates the perturbation termλu^q,λ >0, in our work.

We now summarize the main results of the paper. First, in Section2we look at the problem(Pλ)in the concave caseq <1 and prove the following.

Theorem 1.1.Assume0< q <1,0< s <1, andn >2s. Then, there exists0< Λ <∞such that problem(Pλ) (1) has no solution forλ > Λ;

(2) has a minimal solution for any0< λ < Λ; moreover, the family of minimal solutions is increasing with respect toλ;

(3) ifλ=Λthere exists at least one solution;

(4) for0< λ < Λthere are at least two solutions.

The convex case is treated in Section3. The existence result for problem(P_λ)is given by:

Theorem 1.2. Assume1< q <2^∗_s −1, 0< s <1, andn >2s. Then, problem (P_λ)admits at least one solution provided that either

(3)

• n >^2s(q_q₊⁺₁³⁾ andλ >0, or

• n^2s(q_q₊⁺₁³⁾ andλis sufficiently large.

Theorem 1.1corresponds to the nonlocal version of the main result of[4], whileTheorem 1.2may be seen as the nonlocal counterpart of the results obtained for the standard Laplace operator in[13, Subsections 2.3, 2.4 and 2.5]

(see also[25, Theorems 3.2 and 3.3]for the case of the p-Laplacian operator). Note, in particular, that whens=1 one has 2s(q+3)/(q+1)=2(q+3)/(q+1) <4, due to the choice ofq >1.

We will denote byH^s(Rⁿ)the usual fractional Sobolev space endowed with the so-calledGagliardo norm gH^s(Rⁿ)= gL²(Rⁿ)+

Rⁿ×Rⁿ

|g(x)−g(y)|²

|x−y|ⁿ⁺^2s dx dy

1/2

, (1.3)

whileX₀^s(Ω)is the function space defined as X₀^s(Ω)=

u∈H^s Rⁿ

: u=0 a.e. inRⁿ\Ω

. (1.4)

We refer to[40,41]for a general definition ofX^s₀(Ω)and its properties and to[2,22,28]for an account of the properties ofH^s(Rⁿ).

InX^s₀(Ω)we can consider the following norm vX₀^s(Ω)=

Rⁿ×Rⁿ

|v(x)−v(y)|²

1/2

.

We also recall that(X₀^s(Ω), · X^s₀(Ω))is a Hilbert space, with scalar product u, vX^s₀(Ω)=

Rⁿ×Rⁿ

(u(x)−u(y))(v(x)−v(y))

|x−y|ⁿ⁺^2s dx dy. (1.5)

See for instance[40, Lemma 7].

Observe that by[22, Proposition 3.6]we have the following identity uX^s₀(Ω)=(−)^s/2u

L²(Rⁿ). (1.6)

This leads us to establish as a definition that the solutions to our problem in this variational framework are those functions satisfying the relationship(1.8)below.

In our context, the Sobolev constant is given by S(n, s):= inf

v∈H^s(Rⁿ)\{0}Q_n,s(v) >0, (1.7)

where

Q_n,s(v):=

Rⁿ×Rⁿ|v(x)−v(y)|²

|x−y|ⁿ⁺^2s dx dy (

Rⁿ|v(x)|²^∗^s dx)^2/2^∗^s , v∈H^s Rⁿ

,

is the associated Rayleigh quotient. The constantS(n, s)is well defined, as can be seen in[2, Theorem 7.58].

1.1. Variational formulation of the problem

Let us start describing the notion of solution in this context. In order to present the weak formulation of(P_λ)and taking into account that we are looking for positive solutions, we will consider the following Dirichlet problem

P_λ⁺

=

(−)^su=λ(u₊)^q+(u₊)²^∗^s⁻¹ inΩ,

u=0 inRⁿ\Ω,

whereu₊:=max{u,0}denotes the positive part ofu. With this at hand, we can now give the following.

(4)

Definition 1.3.We say thatu∈X^s₀(Ω)is a weak solution of(P_λ⁺)if for everyϕ∈X^s₀(Ω), one has

Rⁿ×Rⁿ

(u(x)−u(y))(ϕ(x)−ϕ(y))

|x−y|ⁿ⁺^2s dx dy=λ

Ω

(u₊)^qϕ dx+

Ω

(u₊)²^∗^s⁻¹ϕ dx. (1.8)

In the sequel we will omit the termweakwhen referring to solutions that satisfy the conditions ofDefinition 1.3.

The crucial observation here is that, by the Maximum Principle[45, Proposition 2.2.8], ifuis a solution of(P_λ⁺)then uis strictly positive inΩand, therefore, it is also a solution of(Pλ).

To find solutions of(P_λ⁺), we will use a variational approach. Hence, we will associate a suitable functional to our problem. More precisely, the Euler–Lagrange functional related to problem(P_λ⁺)is given byJs,λ:X^s₀(Ω)→R defined as follows

Js,λ(u)=1 2

Rⁿ×Rⁿ

|u(x)−u(y)|²

|x−y|ⁿ⁺^2s dx dy− λ q+1

Ω

(u₊)^q⁺¹dx− 1 2^∗_s

Ω

(u₊)²^∗^sdx.

Note thatJs,λisC¹and that its critical points correspond to solutions of(P_λ⁺).

In both cases,q <1 andq >1, we will use the Mountain Pass Theorem (MPT) by Ambrosetti and Rabinowitz (see[5]). In order to do that, we will show thatJs,λsatisfies a compactness property and has suitable geometrical features. The fact that the functional has the suitable geometry is easy to check. Observe that the embeddingX^s₀(Ω) → L²^∗^s(Rⁿ)is not compact (see[2]). This is even true when the nonlocal operator has a more general kernel (see[41, Lemma 9-b)]). Hence, the difficulty to apply MPT lies on proving a local Palais–Smale (PS for short) condition at levelc∈R((PS)c). Moreover, since the PS condition does not hold globally, we have to prove that the Mountain Pass critical level ofJs,λlies below the threshold of application of the (PS)ccondition.

In the concave setting,q <1, the idea is to prove the existence of at least two positive solutions for an admissible small range of λ. For that we are using a contradiction argument, inspired by[4]. The proof is divided into several steps: we first show that we have a solution that is a local minimum for the functionalJs,λ. In the next step, in order to find a second solution, we suppose that this local minimum is the only critical point of the functional, and then we prove a local (PS)ccondition forcunder a critical level related with the best fractional critical Sobolev constant given in(1.7). Also we find a path under this critical level localizing the Sobolev minimizers at the possible concentration on Dirac Deltas. These Deltas are obtained by the concentration–compactness result in[34, Theorem 1.5]inspired in the classical result by P.-L. Lions in[32,33]. Applying the MPT given in[5]and its refined version given in[26], we will reach a contradiction.

In the convex case q >1 we also apply the MPT to obtain the existence of at least one solution for(P_λ⁺)for suitable values ofλdepending on the dimensionn. As before, we prove a local (PS)c condition in an appropriate range related with the constantS(n, s) defined on(1.7). The strategy to obtain a solution follows the ideas given in[13](see also[47,51]) adapted to our nonlocal functional framework.

The linear caseq=1, when the right hand side of the equation is equal toλu+ |u|²^∗^s⁻²u, was treated in[37,38, 41–43]. In these works the authors studied also nonlinearities more general than those given by the power critical function as well as the existence of solutions not necessarily positive.

2. The critical and concave case 0< q <1

This section is devoted to the study of problem(Pλ)in the case of the exponent 0< q <1. We point out that the result ofTheorem 1.1in the subcritical case could be obtained by the arguments given in this paper. However, in this subcritical case the PS condition is easier to prove – it is indeed satisfied for any energy level – and the separation of solutions, presented inLemma 2.3below, is not needed. This approach has been carried out in[8]where the authors obtain the equivalent toTheorem 1.1for a related problem using a technique developed in[3].

We begin with the following result that uses, in its proof, a standard comparison method as well as some ideas given in[4, Lemma 3.1 and Lemma 3.4].

Lemma 2.1.Let0< q <1and letΛbe defined by Λ:=sup

λ >0: problem(P_λ)has solution

. (2.1)

(5)

Then,0< Λ <∞and the critical concave problem(P_λ)has at least one solution for every0< λΛ. Moreover, for 0< λ < Λwe get a family of minimal solutions increasing with respect toλ.

ByLemma 2.1 we easily deduce statements (1)–(3) ofTheorem 1.1. Hence, in the sequel we focus on proving statement (4) of that theorem, that is on the existence of a second solution for(P_λ).

First we prove a regularity result which will be useful in certain parts of this section:

Proposition 2.2.Letube a positive solution to the problem (−)^su=f (x, u) inΩ,

u=0 inRⁿ\Ω,

and assume that|f (x, t )|C(1+ |t|^p), for some1p2^∗_s −1andC >0. Thenu∈L^∞(Ω).

Proof. The proof uses standard techniques for the fractional Laplacian, in particular the following inequality: ifϕis a convex and differentiable function, then

(−)^sϕ(u)ϕ(u)(−)^su.

Let us define, forβ1 andT >0 large, ϕ(t )=ϕ_{T ,β}(t)=

⎧⎨

⎩

0, ift0,

t^β, if 0< t < T , βT^β⁻¹(t−T )+T^β, iftT .

Observe thatϕ(u)∈X₀^s(Ω)sinceϕis Lipschitz with constantK=βT^β⁻¹and, therefore, ϕ(u)X^s₀(Ω)=

Rⁿ×Rⁿ

|ϕ(u(x))−ϕ(u(y))|²

1/2

Rⁿ×Rⁿ

K²|u(x)−u(y)|²

1/2

=KuX^s₀(Ω). By(1.6)and the Sobolev embedding theorem given in[2, Theorem 7.58], we have

Ω

ϕ(u)(−)^sϕ(u)=ϕ(u)²

X₀^s(Ω)S(n, s)ϕ(u)²

L²^∗^s(Ω), (2.2)

whereS(n, s)is defined in(1.7). On the other hand, sinceϕis convex, andϕ(u)ϕ(u)∈X₀^s(Ω),

Ω

ϕ(u)(−)^sϕ(u)

Ω

ϕ(u)ϕ(u)(−)^suC

Ω

ϕ(u)ϕ(u)

1+u²^∗^s⁻¹ .

From(2.2)and the previous inequality we get the following basic estimate:

ϕ(u)²

L^2∗^s(Ω)C

Ω

ϕ(u)ϕ(u)

1+u²^∗^s⁻¹

. (2.3)

Sinceuϕ(u)βϕ(u)andϕ(u)β(1+ϕ(u)), the above estimate(2.3)becomes

Ω

ϕ(u)2^∗_s 2/2^∗_s

Cβ

1+

Ω

ϕ(u)2

+

Ω

ϕ(u)2

u²^∗^s⁻² . (2.4)

It is important to point out here that sinceϕ(u)grows linearly, both sides of(2.4)are finite.

Claim.Letβ₁be such that2β1=2^∗_s. Thenu∈L^β¹²^∗^s.

(6)

To see this, we take R large to be determined later. Then, Hölder’s inequality withp=β₁=2^∗_s/2 and p= 2^∗_s/(2^∗_s−2)gives

Ω

ϕ(u)2

u²^∗^s⁻²=

{uR}

ϕ(u)2

u²^∗^s⁻²+

{u>R}

ϕ(u)2

u²^∗^s⁻²

{uR}

ϕ(u)2

R²^∗^s⁻²+

Ω

ϕ(u)2^∗_s 2/2^∗_s {u>R}

u²^∗^s

(2^∗_s−2)/2^∗_s

.

By the Monotone Convergence Theorem, we may takeRso that

{u>R}

u²^∗^s

(2^∗_s−2)/2^∗_s

1

2Cβ1

.

In this way, the second term above is absorbed by the left hand side of(2.4)to get

Ω

ϕ(u)2^∗_s 2/2^∗_s

2Cβ1

1+

Ω

ϕ(u)2

+

{uR}

ϕ(u)2

R²^∗^s⁻² . (2.5)

Using thatϕT ,β₁(u)u^β¹in the right hand side of(2.5)and then lettingT → ∞in the left hand side, since 2β1=2^∗_s, we obtain

Ω

u²^∗^s^β¹

2/2^∗_s

2Cβ1

1+

Ω

u²^∗^s +R²^∗^s⁻²

Ω

u²^∗^s <∞.

This proves the claim.

We now go back to inequality(2.4)and we use as before thatϕT ,β(u)u^βin the right hand side and then we take T → ∞in the left hand side. Then,

Ω

u²^∗^s^β

2/2^∗_s

Cβ

1+

Ω

u^2β+

Ω

u^2β⁺²^∗^s⁻² .

Since

Ωu^2β |Ω| +

Ωu^2β⁺²^∗^s⁻², we get the following recurrence formula

Ω

u²^∗^s^β

2/2^∗_s

2Cβ

1+ |Ω| 1+

Ω

u^2β⁺²^∗^s⁻² .

Therefore,

1+

Ω

u²^∗^s^β

2∗s (β−1 1)

C

1 2(β−1)

β

1+

Ω

u^2β⁺²^∗^s⁻²

1 2(β−1)

, (2.6)

whereC_β=4Cβ(1+ |Ω|).

Form1 we defineβ_m₊₁inductively so that 2βm+1+2^∗_s −2=2^∗_sβ_m, that is βm+1−1=2^∗_s

2(βm−1)= 2^∗_s

2

m

(β1−1).

Hence, from(2.6)it follows that

1+

Ω

u²^∗^s^β^m⁺¹

1 2∗s (βm+1−1)

C

1 2(βm+1−1)

βm+1

1+

Ω

u²^∗^s^β^m

1 2∗s (βm−1)

,

withC_m₊₁:=C_β_m₊₁=4Cβm+1(1+ |Ω|). Then, defining form1

(7)

A_m:=

1+

Ω

u²^∗^s^β^m

1 2∗s (βm−1)

,

by the Claim proved before, and using a limiting argument, we conclude that there existsC₀>0, independent of m >1, such that

A_m₊₁

m+1 k=2

C

1 2(βk−1)

k A₁C₀A₁. This implies thatuL^∞(Ω)C₀A₁. 2

Coming back to the proof ofTheorem 1.1, as we said in the Introduction, to find the existence of the second solution, we first show that the minimal solutionu_λ>0 given byLemma 2.1is a local minimum for the functionalJs,λ. For that, following the ideas given in[18]we establish a separation lemma in the topology of the class

Cs(Ω):=

w∈C⁰(Ω): w_Cs(Ω):=

w δ^s

L^∞(Ω)

<∞

, (2.7)

whereδ(x)=dist(x, ∂Ω). Then we have the following.

Lemma 2.3.Assume0< λ1< λ0< λ2< Λ. Letuλ₁,uλ₀ anduλ₂ be the corresponding minimal solutions to(Pλ), forλ=λ₁, λ₀andλ₂respectively. If

Z=

u∈Cs(Ω)u_λ₁uu_λ₂ , then there existsε >0such that

{uλ₀} +εB1⊂Z,

withB₁= {w∈C⁰(Ω): _δ^wsL^∞(Ω)<1}.

Proof. Let u be an arbitrary solution of(P_λ)for 0< λ < Λ. Then, by Hopf’s Lemma (see[15, Proposition 2.7]

and[35, Lemma 3.2]) there exists a positive constantcsuch that

u(x)cδ(x)^s, x∈Ω. (2.8)

On the other hand by[35, Proposition 1.1]we get that there exists a positive constantCsuch that

u(x)Cδ(x)^s, x∈Ω. (2.9)

Thus, by(2.8)and(2.9)we finish the proof. 2

Using this previous result we now obtain a local minimum of the functionalJs,λin theCs(Ω)-topology. This is the first step in order to get a local minimum inX^s₀(Ω). That is,

Lemma 2.4.For allλ∈(0, Λ)the minimal solutionu_λis a local minimum of the functionalJs,λin theCs-topology.

Proof. The proof follows in a similar way as in[4](see also Lemma 3.3 of[18]). In our case we have to consider the nonlocal operator(−)^sinstead of(−)and the spaceCs(Ω)instead ofC¹₀(Ω). We omit the details. 2

To prove that we already have a minimum in the spaceX^s₀(Ω)we show that the result obtained by Brezis and Nirenberg in[13]is also valid in our context.

Proposition 2.5.Letz₀∈X^s₀(Ω)be a local minimum ofJs,λinCs(Ω); by this we mean that there existsr₁>0such that

Js,λ(z₀)Js,λ(z₀+z), ∀z∈Cs(Ω)withz_Cs(Ω)r₁. (2.10)

(8)

Then,z₀is also a local minimum ofJs,λinX^s₀(Ω), that is, there existsr₂>0so that Js,λ(z₀)Js,λ(z₀+z), ∀z∈X₀^s(Ω)withzX^s₀(Ω)r₂.

Proof. We follow the ideas given in[18, Theorem 5.1]. Letz0be as in(2.10)and set, forε >0, Bε(z0)=

z∈X^s₀(Ω): z−z0X^s₀(Ω)ε .

Now, we argue by contradiction and we suppose that for everyε >0 we have

v∈minBε(z0)Js,λ(v) <Js,λ(z₀). (2.11)

We pickv_ε∈B_ε(z₀)such that minv∈B_ε(z₀)Js,λ(v)=Js,λ(v_ε). The existence ofv_εcomes from a standard argument of weak lower semi-continuity. We want to prove that

v_ε→z₀ inCs(Ω)asε0, (2.12)

because this would imply that there arez∈Cs(Ω), arbitrarily close toz0in the metric ofCs(Ω)(in fact,z=vεfor someε), such that

Js,λ(z) <Js,λ(z0).

This contradicts our hypothesis(2.10).

Let 0< ε1. Note that the Euler–Lagrange equation satisfied byv_ε involves a Lagrange multiplierξ_ε such that

Js,λ (v_ε), ϕ

=ξ_εv_ε, ϕX₀^s(Ω), ∀ϕ∈X^s₀(Ω). (2.13)

As a consequence, sincev_εis a minimum ofJs,λinB_ε(z₀), we have ξ_ε=J_s,λ (vε), vε

v_ε²_Xs 0(Ω)

0. (2.14)

By(2.13)we easily get thatv_εsatisfies

⎧⎨

⎩

(−)^sv_ε= 1 1−ξε

f_λ(v_ε)=:f_λ^ε(v_ε) inΩ,

vε=0 inRⁿ\Ω,

wheref_λ(t):=λ(t₊)^q+(t₊)²^∗^s⁻¹. Sincev_ε>0 and

v_εX₀^s(Ω)C,

byProposition 2.2there exists a constantC1>0 independent ofεsuch thatv_εL^∞(Ω)C1. Moreover, by(2.14), it follows thatf_λ^ε(vε)L^∞(Ω)C. Therefore, by[35, Proposition 1.1](see also[44, Proposition 5]), we get that vε_C^0,s_(Ω)C2, for someC2independent ofε. HereC^0,s denotes the space of Hölder continuous functions with exponents.

Thus, by the Ascoli–Arzelá Theorem there exists a subsequence, still denoted byvε, such thatvε→z0uniformly asε0. Moreover, by[35, Theorem 1.2], we obtain that for a suitable positive constantC

v_ε−z₀ δ^s

L^∞(Ω)

Csup

Ω

f_λ^ε(v_ε)−f_λ(z₀).

Since the latter tends to zero asε0,(2.12)is proved. 2

Lemma 2.4andProposition 2.5provide us with the existence of a positive local minimum inX^s₀(Ω)ofJs,λthat will be denoted byu₀. We now make a translation as in[4]in order to simplify the calculations.

(9)

For 0< λ < Λ, we consider the functions gλ(x, t)=

λ(u₀+t )^q−λu^q₀+(u₀+t )²^∗^s⁻¹−u²₀^∗^s⁻¹, ift0,

0, ift <0,

(2.15) and

Gλ(x, ξ )=Gλ(ξ )= ξ 0

gλ(x, t) dt. (2.16)

The associated energy functionalJs,λ:X₀^s(Ω)→Ris given by Js,λ(u)=1

2u²_X^s

0(Ω)−

Ω

G_λ(x, u) dx. (2.17)

Sinceu∈X₀^s(Ω),Js,λis well defined. We define the translate problem (Pλ)=

(−)^su=g_λ(x, u) inΩ⊂Rⁿ,

u=0 onRⁿ\Ω.

We know that ifu˜≡0 is a critical point ofJs,λthen it is a solution of(Pλ)and, by the Maximum Principle[45, Proposition 2.2.8], this implies thatu >˜ 0. Therefore u=u0+ ˜u >0 will be a second solution of(P_λ⁺)and con- sequently a second one of (P_λ). Hence, in order to prove statement (4) ofTheorem 1.1, it is enough to study the existence of a non-trivial critical point forJs,λ.

First we have

Lemma 2.6.u=0is a local minimum ofJs,λinX₀^s(Ω).

Proof. The proof follows along the lines of[4, Lemma 4.2], see also[7, Lemma 3.4], so we omit the details. 2 2.1. The Palais–Smale condition forJs,λ

In this subsection assuming that we have a unique critical point, we prove that the functionalJs,λsatisfies a local Palais–Smale condition (seeLemma 2.10). The main tool for proving this fact is an extension of the concentration–

compactness principle by Lions in [32,33]for nonlocal fractional operators, given in [34, Theorem 1.5]. We will also need some technical results related to the behavior of the fractional Laplacian of a product. We start with the following.

Lemma 2.7.Letφbe a regular function that satisfies φ(x) C

1+ |x|ⁿ⁺^s, x∈Rⁿ (2.18)

and

∇φ(x) C

1+ |x|ⁿ⁺^s⁺¹, x∈Rⁿ, (2.19)

for someC > 0. LetB:X^s/2₀ (Ω)×X^s/2₀ (Ω)→Rbe the bilinear form defined by B(f, g)(x):=2

Rⁿ

(f (x)−f (y))(g(x)−g(y))

|x−y|ⁿ⁺^s dy. (2.20)

Then, for everys∈(0,1), there exist positive constantsC1andC2such that forx∈Rⁿone has (−)^s/2φ(x) C₁

1+ |x|ⁿ⁺^s,

(10)

and

B(φ, φ)(x) C₂ 1+ |x|ⁿ⁺^s. Proof. Let

I (x):=

Rⁿ

|φ(x)−φ(y)|

|x−y|ⁿ⁺^s dy.

For anyx∈Rⁿ, it is clear that (−)^s/2φ(x)2I (x).

Also, since|φ(x)|C, we have B(φ, φ)(x)2CI (x).

Hence, it suffices to prove that I (x) C

1+ |x|ⁿ⁺^s, ∀x∈Rⁿ, (2.21)

for a suitable positive constantC.

Sinceφis a regular function, for|x|<1 we obtain that I (x)∇φL^∞(Rⁿ)

|y|<2

dy

|x−y|ⁿ⁺^s⁻¹+C

|y|2

dy

|y|ⁿ⁺^s

C C

1+ |x|ⁿ⁺^s. (2.22)

Let now|x|1. Then

I (x):=I_A₁(x)+I_A₂(x)+I_A₃(x), (2.23)

where

I_A_i(x):=

A_i

|φ(x)−φ(y)|

|x−y|ⁿ⁺^s dy, i=1,2,3, with

A₁:=

y: |x−y||x| 2

, A₂:=

y: |x−y|>|x|

2 ,|y|2|x|

and

A3:=

y: |x−y|>|x|

2 , |y|>2|x|

.

Therefore, since for|x|1 andy∈A₁,|φ(x)−φ(y)||∇φ(ξ )||x−y|with^|^x₂^||ξ|³₂|x|, by(2.19), we obtain that

I_A₁(x) C

|x|ⁿ⁺^s⁺¹

A1

dy

|x−y|ⁿ⁺^s⁻¹C|x|⁻⁽ⁿ⁺^2s). (2.24)

Using now that, for anyx, y∈Rⁿwe have the inequality φ(x)+φ(y) C

1+min{|x|ⁿ⁺^s,|y|ⁿ⁺^s}, we get

(11)

I_A₂(x) C

|x|ⁿ⁺^s

A₂

dy

(1+ |y|ⁿ⁺^s)C|x|⁻⁽ⁿ⁺^s), (2.25)

and

IA₃(x) C

|x|ⁿ⁺^s

A₃

dy

|y|ⁿ⁺^s C|x|⁻⁽ⁿ⁺^2s). (2.26)

Note that the last estimate follows from the fact that(x, y)∈A₃implies|x−y||y|/2. Then, by(2.23)–(2.26), we get that

I (x)C|x|⁻⁽ⁿ⁺^s) C

1+ |x|ⁿ⁺^s, |x|1. (2.27)

Hence, by(2.22)and(2.27), we conclude(2.21). 2

To establish the next auxiliary results we consider a radial, nonincreasing cut-off functionφ∈C^∞₀ (Rⁿ)and

φ_ε(x):=φ(x/ε). (2.28)

Now we get the following.

Lemma 2.8.Let{z_m}be a uniformly bounded sequence inX^s₀(Ω)andφ_εthe function defined in(2.28). Then,

εlim→0 lim

m→∞

Rⁿ

zm(x)(−)^s/2φε(x)(−)^s/2zm(x) dx

=0. (2.29)

Proof. First of all note that, as a consequence of the fact that{z_m}is uniformly bounded in the reflexive spaceX₀^s(Ω), say byM, we get that there existsz∈X₀^s(Ω), such that, up to a subsequence,

zm z weakly inX₀^s(Ω),

z_m→z strongly inL^r(Ω), 1r <2^∗_s,

z_m→z a.e. inΩ. (2.30)

Also it is clear that

(−)^s/2φ_ε(x)=ε⁻^s

(−)^s/2φx

ε Cε⁻^s. (2.31)

Therefore defining I₁:=

Rⁿ

z_m(x)(−)^s/2φ_ε(x)(−)^s/2z_m(x) dx , from(2.31)and the fact thatz_mX^s₀(Ω)< M, we get

I1(−)^s/2zm

L²(Rⁿ)zm(−)^s/2φε

L²(Ω)

M(z_m−z)(−)^s/2φ_ε

L²(Ω)+Mz(−)^s/2φ_ε

L²(Ω)

Cε⁻^sz_m−zL²(Ω)+Mz(−)^s/2φ_ε

L²(Ω). (2.32)

SincezX^s₀(Ω)M thenz_L^2∗s(Ω)C, that isz²∈Lⁿ−ⁿ2s(Ω). Hence, for everyρ >0 there existsη∈C₀^∞(Ω) such that

z²−η

L

n

n−2s(Ω)ρ. (2.33)

Then, by(2.31),(2.33)and Hölder’s inequality withp=n/n−2swe obtain that

(12)

z(−)^s/2φε²

L²(Ω)

Rⁿ

z²(x)−η(x)(−)^s/2φε(x)²dx+

Rⁿ

η(x)(−)^s/2φε(x)²dx z²−η

L

n

n−2s(Ω)(−)^s/2φε²

Lⁿ^s(Rⁿ)+ ηL∞(Ω)(−)^s/2φε²

L²(Rⁿ)

ρε⁻^2s

Rⁿ

(−)^s/2φx ε

n s

dx

2s

n +Cε⁻^2s

Rⁿ

(−)^s/2φx ε ²dx

ρ

Rⁿ

(−)^s/2φ(z)ⁿ^s dz

2s

n +Cεⁿ⁻^2s

Rⁿ

(−)^s/2φ(z)²dz

Cρ+Cεⁿ⁻^2s. (2.34)

Hence, using(2.30), from(2.32),(2.34)and the fact thatn >2s, it follows that

εlim→0 lim

m→∞I₁lim

ε→0C

ρ+εⁿ⁻^2s¹

2 =Cρ¹².

Sinceρ >0 is fixed but arbitrarily small, we conclude the proof ofLemma 2.8. 2 Also, we have the following.

Lemma 2.9.With the same assumptions ofLemma 2.8we have that

εlim→0 lim

m→∞

Rⁿ

(−)^s/2z_m(x)B(z_m, φ_ε)(x) dx

=0, (2.35)

whereBis defined in(2.20).

Proof. Let I₂:=

Rⁿ

(−)^s/2z_m(x)B(z_m, φ_ε)(x) dx . Sincez_mX₀^s(Ω)M, then

I2MB(zm, φε)

L²(Rⁿ)

MB(z_m−z, φ_ε)

L²(Rⁿ)+MB(z, φ_ε)

L²(Rⁿ), (2.36)

wherezis, as inLemma 2.8, the weak limit of the sequence{z_m}inX^s₀(Ω). We estimate each of the summands in the previous inequality. Let

ψ (x):= 1

1+ |x|ⁿ⁺^s and ψ_ε(x):=ψ x

ε . (2.37)

ByLemma 2.7applied toφ, we note that B(φ_ε, φ_ε)(x)=ε⁻^sB(φ, φ)

x

ε Cε⁻^sψ x

ε =C ε⁻^s

1+ |^x_ε|ⁿ⁺^s Cε⁻^s. (2.38)

Therefore, by Cauchy–Schwarz inequality and(2.38), it follows that B(z_m−z, φ_ε)²

L²(Rⁿ)

Rⁿ

B(z_m−z, z_m−z)(x)B(φ_ε, φ_ε)(x) dx (2.39)

Cε⁻^s

Rⁿ

B(z_m−z, z_m−z)(x) dx