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Elliptic Equations with Hardy Potential and Gradient-Dependent Nonlinearity

  • Konstantinos T. Gkikas and Phuoc-Tai Nguyen EMAIL logo
Published/Copyright: February 18, 2020

Abstract

Let ΩN (N3) be a C2 bounded domain, and let δ be the distance to Ω. We study equations (E±), -Lμu±g(u,|u|)=0 in Ω, where Lμ=Δ+μδ2, μ(0,14] and g:×++ is nondecreasing and locally Lipschitz in its two variables with g(0,0)=0. We prove that, under some subcritical growth assumption on g, equation (E+) with boundary condition u=ν admits a solution for any nonnegative bounded measure on Ω, while equation (E-) with boundary condition u=ν admits a solution provided that the total mass of ν is small. Then we analyze the model case g(s,t)=|s|ptq and obtain a uniqueness result, which is even new with μ=0. We also describe isolated singularities of positive solutions to (E+) and establish a removability result in terms of Bessel capacities. Various existence results are obtained for (E-). Finally, we discuss existence, uniqueness and removability results for (E±) in the case g(s,t)=|s|p+tq.

1 Introduction and Main Results

Let Ω be a C2 bounded domain in N (N3), μ(0,14] and δ(x)=δΩ(x):-dist(x,Ω). In this paper, we investigate the boundary value problem with measure data for equation

(E$\pm$) - L μ u ± g ( u , | u | ) = 0 in Ω ,

where Lμ=LμΩ:-Δ+μδ2 and g:×++ is nondecreasing and locally Lipschitz in its two variables with g(0,0)=0. The term μδ2 is called Hardy potential since it is related to the Hardy inequality. The nonlinearity g(u,|u|) is called absorption (resp. source) if the “plus sign” (resp. “minus sign”) appears in (E$\pm$). One prototype model to keep in mind is g(u,|u|)=|u|p|u|q.

1.1 Background and Main Contributions

The boundary value problem for (E$\pm$) without Hardy potential, i.e. μ=0, has received substantial attention over the last decades, starting from the pioneering work of Brezis [10]. In particular, Brezis proved that, for every prescribed L1 boundary datum, the semilinear equation with absorption term

(1.1) - Δ u + g ( u ) = 0 in Ω

admits a unique solution. Afterwards, equation (1.1) in measure framework was first considered by Gmira and Véron in [18] where they showed that boundary value problem for (1.1) is not always solvable for every measure boundary datum. Because of its applications in many areas, equation (1.1) with g(u,|u|)=|u|p-1u has been intensively studied in many works, among them is the celebrated series of papers of Marcus and Véron [26, 27, 28]. These results were then extended to the equation with gradient-dependent absorption term

- Δ u + g ( u , | u | ) = 0 in Ω .

We refer to [32] for the case when g depends only on u and to [24, 30] for the case when g depends on both u and u.

Equation (E-) with μ=0, i.e.

(1.2) - Δ u - g ( u , | u | ) = 0 in Ω ,

has been studied in various directions. Necessary and sufficient criteria in terms of capacities for the existence of a solution with measure boundary data were obtained in [9]. Singular solutions of (1.2) with g(u,|u|)=|u|q in a perturbation of the ball was studied in [2]. Recently, Bidaut-Véron, Garcia-Huidobro and Véron have established a priori estimates for solutions of (1.2) with g(u,|u|)=|u|p|u|q or g(u,|u|)=|u|p+M|u|q (see [8, 7]).

The case with Hardy potential has been intensively studied over the last decade. See e.g. Bandle, Moroz and Reichel [5], Bandle, Marcus and Moroz [4], Marcus and Nguyen [25], Gkikas and Véron [17], Marcus and Moroz [23], Nguyen [31], Gkikas and Nguyen [16]. In the aforementioned papers, the best constant in the Hardy inequality

(1.3) C H ( Ω ) :- inf φ H 0 1 ( Ω ) { 0 } Ω | φ | 2 d x Ω ( φ / δ ) 2 d x

is deeply involved in the analysis. It is well known that CH(Ω)(0,14] and CH(Ω)=14 if Ω is convex (see [22, Theorem 11]) or if -Δδ0 in the sense of distributions (see [6, Theorem A]). Moreover, the infimum in (1.3) is achieved if and only if CH(Ω)<14.

Moreover, Brezis and Marcus [11, Remark 3.2] proved that, for any μ<14, the eigenvalue problem

(1.4) λ μ :- inf φ H 0 1 ( Ω ) { 0 } Ω ( | φ | 2 - μ δ 2 φ 2 ) d x Ω φ 2 d x

admits a minimizer φμ in H01(Ω), and hence λμ is the first eigenvalue of -Lμ in H01(Ω). Moreover, -Lμφμ=λμφμ in Ω. When μ=14, there is no minimizer of (1.4) in H01(Ω), but there exists a nonnegative function φ14Hloc1(Ω) such that -L14φ14=λ14φ14 in Ω in the sense of distributions.

We see from (1.3) and (1.4) that λμ>0 if μ<CH(Ω), λμ=0 if μ=CH(Ω)<14, while λμ<0 when μ>CH(Ω). It is not known if λμ>0 when μ=CH(Ω)=14. However, if Ω is convex or if -Δδ0 in the sense of distributions (in these cases CH(Ω)=14), then λ14>0 (see [11, Theorem II] and [6, Theorem A] with k=1 and p=2).

Throughout the present paper, we assume that

(1.5) μ ( 0 , 1 4 ] and λ μ > 0 .

This assumption implies the validity of the representation theorem which states that every positive Lμ-harmonic function u in Ω (i.e. u is a solution of Lμu=0 in Ω in the sense of distributions) can be uniquely represented in the form u=𝕂μ[ν] for some positive measure ν𝔐+(Ω) (the space of positive bounded measure on Ω), where 𝕂μ denotes the Martin operator (see Subsection 2.2 for more details). The representation theorem is derived from Ancona [3] (see also [25, page 70]) in the case μ<CH(Ω) and was proved by Gkikas and Véron [17, Theorem 2.33] in the case μ=14 and λ14>0.

In order to investigate the boundary behavior of Lμ-harmonic functions, Gkikas and Véron [17] introduced a notion of boundary trace in a dynamic way which is recalled below.

Let DΩ and x0D. If hC(D), then the problem

{ - L μ u = 0 in D , u = h on D

admits a unique solution which allows to define the Lμ-harmonic measure ωDx0 on D by

u ( x 0 ) = D h ( y ) d ω D x 0 ( y ) .

A sequence of domains {Ωn} is called a smooth exhaustion of Ω if ΩnC2, Ωn¯Ωn+1, nΩn=Ω and N-1(Ωn)N-1(Ω). For each n, let ωΩnx0 be the LμΩn-harmonic measure on Ωn.

Definition 1.1.

Let μ(0,14]. We say that a function u possesses a boundary trace if there exists a measure ν𝔐(Ω) (the space of bounded measure on Ω) such that, for any smooth exhaustion {Ωn} of Ω, it holds

lim n Ω n ϕ u d ω Ω n x 0 = Ω ϕ d ν for all ϕ C ( Ω ¯ ) .

The boundary trace of u is denoted by tr(u), and we write tr(u)=ν.

In [17, Proposition 2.34], Gkikas and Véron proved that if tr(𝕂μ[ν])=ν for every ν𝔐(Ω). This fact and the representation theorem allow to characterize Lμ-harmonic functions in terms of their boundary behavior. It was shown in [15] that, when μ(0,CH(Ω)), the notion of boundary trace in Definition 1.1 coincides with the notion of normalized boundary trace introduced by Marcus and Nguyen in [25, Definition 1.2]. This notion was employed to formulate the boundary value problem

(1.6) { - L μ u ± | u | p - 1 u = 0 in Ω , tr ( u ) = ν .

A complete description of the structure of positive solutions to (1.6) with “plus sign” was established in [25, 17], and various existence results for (1.6) with “minus sign” were given in [15, 31] in connection to the critical exponent

(1.7) p μ :- N + α N + α - 2 with α :- 1 2 + 1 4 - μ .

In particular, it was proved that, when 1<p<pμ, equation (1.6) with “plus sign” admits a unique solution for every finite measure ν𝔐+(Ω), while the existence phenomenon occurs for (1.6) with “minus sign” only with boundary measure of small total mass. When ppμ, the nonexistence phenomenon happens, i.e. equations (E$\pm$) do not admit any solution with an isolated singularity. Related results were obtained in [5, 4, 23] and references therein.

Very recently, a thorough study of the boundary value problem

(1.8) { - L μ u + | u | q = 0 in Ω , tr ( u ) = ν

was carried out in [16], revealing that the value

q μ :- N + α N + α - 1

is a critical exponent for the solvability of (1.8). This means that if 1<q<qμ, then, for every ν𝔐+(Ω), there is a unique solution of (1.8); otherwise, if qμq<2, singularities are removable.

Motivated by the aforementioned works, in the present paper, we aim to investigate related issues for (E$\pm$). Main features of a boundary value problem for (E$\pm$) with measure are

  1. the presence of the Hardy potential which blowups strongly at the boundary,

  2. the dependence of the nonlinearity on both solution and its gradient,

  3. rough data which cause the invalidity of some classical results.

The interplay between the features leads to new essential difficulties, hence complicates drastically the analysis and produces interestingly new phenomena.

Our contributions are the following.

  1. We establish the existence of weak solutions of (E$\pm$) with prescribed boundary trace ν under sharp assumption on g. In particular, by using the standard approximation method, combined with the estimates of the Green kernel and the Martin kernel as well as their gradient [15], the sub- and supersolutions theorem and the Vitali convergence theorem, we show that, for every measure ν𝔐+(Ω), equation (E+) admits a solution. Unlike the absorption case, thanks to the Schauder fixed point theorem, we can construct a solution to (E$\pm$) under the smallness assumption on the total mass of the boundary datum.

  2. We prove the comparison principle for (E+), which in turn implies the uniqueness. This result, which is obtained by developing the method in [24, 16] and the theory of linear equations [25, 17, 15], is new, even in the case without Hardy potential.

  3. We show sharp a priori estimates for singular solutions of (E$\pm$). This allows to study solutions with an isolated singularity. As a matter of fact, we show that there are two types of solutions with an isolated singularity of (E+): weakly singular ones and strongly singular one. Moreover, the strongly singular solution can be obtained as the limit of the weakly singular solutions. It is interesting that this phenomenon does not occur for (E-). The interaction of up and |u|q is a source of difficulties, which requires a delicate analysis and heavy computations.

  4. We demonstrate removability results of singularities in terms of capacities. The absorption case and source case are treated differently using different types of capacities (see [9, 16]).

Our results cover and refine most of the aforementioned works in the literature and provide a full understanding of equations with Hardy potential and gradient-dependent nonlinearity.

1.2 Main Results

First we are concerned with a boundary value problem for equations with absorption term of the form

(P+ν) { - L μ u + g ( u , | u | ) = 0 in Ω , tr ( u ) = ν .

Before stating the main results, let us give the definition of weak solutions of (P+ν).

Definition 1.2.

Let ν𝔐(Ω). A function u is called a weak solution of (P+ν) if

u L 1 ( Ω , δ α ) , g ( u , | u | ) L 1 ( Ω , δ α )

and

- Ω u L μ ζ d x + Ω g ( u , | u | ) ζ d x = - Ω 𝕂 μ [ ν ] L μ ζ d x for all ζ 𝐗 μ ( Ω ) ,

where the space of test function 𝐗μ(Ω) is defined by

(1.9) 𝐗 μ ( Ω ) :- { ζ H loc 1 ( Ω ) : δ - α ζ H 1 ( Ω , δ 2 α ) , δ - α L μ ζ L ( Ω ) } .

We notice that this definition is inspired by the definition in [17, Section 3.2]. For more properties of the space of test functions 𝐗μ(Ω), we refer to [17].

Our first result is the existence of a weak solution of (P+ν) under an integral condition on g.

Theorem 1.3 (Existence).

Assume g satisfies

(1.10) Λ g :- 1 g ( s , s p μ q μ ) s - 1 - p μ d s < .

Then, for any νM+(Ω), (P+ν) admits a positive weak solution 0uKμ[ν] in Ω.

Remark 1.4.

We remark the following.

  1. If g(s,t)=|s|ptq for s, t+, p,q0, p+q>1, then g satisfies (1.10) if

    (1.11) ( N + α - 2 ) p + ( N + α - 1 ) q < N + α .

  2. If g(s,t)=|s|p+tq for s, t+, p>1, q>1, then g satisfies (1.10) if

    (1.12) 1 < p < p μ and 1 < q < q μ .

It is worth noticing that this theorem is established by developing the sub- and supersolutions method in [16], in combination with the Schauder fixed point theorem and the Vitali convergence theorem.

It seems infeasible to obtain the uniqueness in case of general nonlinearity; however, when

g ( u , | u | ) = | u | p | u | q ,

we are able to prove the comparison principle, which in turn implies the uniqueness. The method is delicate, relying on a regularity result (see Proposition 4.1), maximum principle (see Lemma 4.2), estimates on the gradient of subsolutions of a nonhomogeneous linear equation (see Lemma 4.4). We emphasize that this result is new even in the case without Hardy potential, i.e. μ=0.

Theorem 1.5 (Comparison Principle).

Assume g(u,|u|)=|u|p|u|q with q1 and p and q satisfying (1.11). Let νiM+(Ω), i=1,2, and let ui be a nonnegative solution of (P+ν) with ν=νi. If ν1ν2, then u1u2 in Ω.

Assume 0Ω, and denote by δ0 the Dirac measure concentrated at 0. A complete picture of isolated singularities concentrated at 0 is depicted in the next theorem.

Theorem 1.6.

Assume g(u,|u|)=|u|p|u|q with q1 and p and q satisfying (1.11).

  1. Weak singularity. For any k > 0 , let u 0 , k Ω be the solution of

    (1.13) { - L μ u + g ( u , | u | ) = 0 𝑖𝑛 Ω tr ( u ) = k δ 0 .

    Then there exists a constant c = c ( N , μ , Ω ) > 0 such that u 0 , k Ω ( x ) c k δ ( x ) α | x | 2 - N - 2 α for every x Ω and

    | u 0 , k Ω ( x ) | c k δ ( x ) α - 1 | x | 2 - N - 2 α for all x Ω .

    Moreover,

    (1.14) lim Ω x y u 0 , k Ω ( x ) K μ Ω ( x , 0 ) = k .

    Furthermore, the mapping k u 0 , k Ω is increasing.

  2. Strong singularity. Put u 0 , Ω :- lim k u 0 , k Ω . Then u 0 , Ω is a solution of

    (1.15) { - L μ u + g ( u , | u | ) = 0 𝑖𝑛 Ω , u = 0 𝑜𝑛 Ω { 0 } .

    There exists a constant c = c ( N , μ , p , q , Ω ) > 0 such that

    c - 1 δ ( x ) α | x | - 2 - q p + q - 1 - α u 0 , Ω ( x ) c δ ( x ) α | x | - 2 - q p + q - 1 - α for all x Ω ,
    | u 0 , Ω ( x ) | c δ ( x ) α - 1 | x | - 2 - q p + q - 1 - α for all x Ω .

    Moreover,

    (1.16) lim Ω x 0 x | x | = σ S + N - 1 | x | 2 - q p + q - 1 u 0 , Ω ( x ) = ω ( σ ) ,

    locally uniformly on the upper hemisphere S + N - 1 = + N S N - 1 , where ω is the unique solution of problem (4.35). Here +N={x=(x1,,xN)=(x,xN):xN>0}, and SN-1 is the unit sphere in N.

Let us discuss briefly the proof of Theorem 1.6. The main ingredients in the proof of convergence (1.14) are the estimates on the Green kernel (2.1) and the Martin kernel (2.2) and condition (1.11). From the monotonicity of the sequence {u0,kΩ}, universal estimate (4.15) and a standard argument, we deduce that u0,Ω is a solution of (1.15). The proof of convergence (1.16) relies strongly on the similarity transformation T (see (4.19)) and the study of problem (4.35) in the upper hemisphere S+N-1. The existence and uniqueness result for (4.35) is provided in Section 4.4.

When g(u,|u|)=|u|p|u|q with q1, in order to deal with a wider range of p and q (i.e. p and q may not satisfy (1.11)), we make use of Bessel capacities (see Section 5). A necessary condition for the existence of a solution to (P+ν) and a removability result are stated in the following theorems.

Theorem 1.7 (Absolute Continuity).

Assume g(u,|u|)=|u|p|u|q with

p 0 , 1 q < 2 , p + q > 1 𝑎𝑛𝑑 ( N + α - 2 ) p + ( N + α - 1 ) q N + α .

Let νM+(Ω) and assume that problem (P+ν) has a nonnegative solution uC2(Ω).

  1. If q α + 1 , then ν is absolutely continuous with respect to C1-α+α+1-qp+q,(p+q)N-1, i.e. ν(K)=0 for any Borel set KΩ such that C1-α+α+1-qp+q,(p+q)N-1(K)=0. Here (p+q) denotes the conjugate exponent of p+q.

  2. If q = α + 1 , then, for any ε ( 0 , min { α + 1 , ( N - 1 ) α α + 1 - ( 1 - α ) } ) , ν is absolutely continuous with respect to C1-α+εp+α+1,(p+α+1)N-1. Here the capacity Cs,κN-1 is defined in Section 5.

Put

(1.17) W ( x ) :- { δ ( x ) 1 - α if μ < 1 4 , δ ( x ) 1 2 | ln δ ( x ) | if μ = 1 4 .

We note that, by [17, Propositions 2.17, 2.18], for any hC(Ω), there exists a unique Lμ-harmonic function uhC(Ω¯)L1(Ω,δα) such that

(1.18) lim x Ω , x ξ u h ( x ) W ( x ) = h ( ξ ) for all ξ Ω .

In addition, tr(uh)=hωx0, where x0Ω is a fixed reference point and ωx0 is the Lμ-harmonic measure in Ω (see [17, Subsection 2.3] for further details). It is worth mentioning that (1.18) can be viewed as the boundary condition in the case with Hardy potential. If μ=0, then α=1 and W(x)1, in which case (1.18) becomes the boundary condition in the classical sense.

The following result provides a removability result for solutions with “zero boundary condition” on ΩK (see (1.20)).

Theorem 1.8 (Removability).

Assume p0, 1q<2, p+q>1 and (N+α-2)p+(N+α-1)qN+α. Let KΩ be compact such that

  1. C 1 - α + α + 1 - q p + q , ( p + q ) N - 1 ( K ) = 0 if q α + 1 or

  2. C 1 - α + ε p + α + 1 , ( p + α + 1 ) N - 1 ( K ) = 0 for some ε ( 0 , min { α + 1 , ( N - 1 ) α α + 1 - ( 1 - α ) } ) if q = α + 1 .

Then any nonnegative solution uC2(Ω)C(Ω¯K) of

(1.19) - L μ u + | u | p | u | q = 0 𝑖𝑛 Ω

such that

(1.20) lim x Ω , x ξ u ( x ) W ( x ) = 0 for all ξ Ω K

is identically zero.

Next we deal with the boundary value problem for equations with source term of the form

(P$-$ν) { - L μ u - g ( u , | u | ) = 0 in Ω , tr ( u ) = ν .

Weak solutions of (P$-$ν) are defined similarly to Definition 1.2.

Phenomena occurring in this case are different from those in the case of absorption nonlinearity. This is reflected in Theorem 1.9 which ensures the existence of a weak solution under a smallness assumption on the total mass of the boundary data.

In order to make the statement clear and lucid, we rewrite equation (P$-$ν) as

(P$-$ρν) { - L μ u - g ( u , | u | ) = 0 in Ω , tr ( u ) = ϱ ν ,

where ϱ is a positive parameter and ν𝔐+(Ω) with ν𝔐(Ω)=1.

Theorem 1.9 (Existence Result for (P$-$ρν) in Subcritical Case).

Let νM+(Ω) with νM(Ω)=1. Assume g satisfies (1.10) and

(1.21) g ( a s , b t ) k ~ ( a p ~ + b q ~ ) g ( s , t ) for all ( a , b , s , t ) + 4 ,

for some p~>1, q~>1, k~>0. Then there exists ϱ0>0 depending on N,μ,Ω,Λg,k~,p~,q~ such that, for any ϱ(0,ϱ0), problem (P$-$ρν) admits a positive weak solution uϱKμ[ν] in Ω.

This result is established by combining an idea in [34] and the Schauder fixed point theorem.

Remark 1.10.

It is easy to see that if g(s,t)=|s|ptq or g(s,t)=|s|p+tq, then (1.21) holds.

The next result provides sufficient conditions for the existence of a solution to (P$-$ρν) with g(u,|u|)=|u|p|u|q in terms of capacities. See the definition of the capacities CapΩ and 2α-1,1 in Section 7.

Theorem 1.11 (Existence Result for (P$-$ρν)).

Assume that g(u,|u|)=|u|p|u|q with p0, q0, p+q>1 and q<1+α+(1-α)pα. Assume one of the following conditions holds.

  1. There exists a constant C > 0 such that

    ν ( E ) C Cap 1 - α + α + 1 - q p + q , ( p + q ) Ω ( E ) for every Borel set E Ω .

    Here ( p + q ) denotes the conjugate exponent of p + q .

  2. There exists a positive constant C > 0 such that

    (1.22) 2 α - 1 , 1 [ δ α p + ( α - 1 ) q + α 2 α - 1 , 1 [ ν ] p + q ] C 2 α - 1 , 1 [ ν ] < a.e. in Ω .

Then there exists ϱ0=ϱ0(N,μ,p,q,C,Ω)>0 such that, for any ϱ(0,ϱ0), problem (P$-$ρν) admits a weak solution u satisfying

(1.23) | u | C δ α 2 α - 1 , 1 [ ϱ ν ] , | u | C δ α - 1 2 α - 1 , 1 [ ϱ ν ] 𝑖𝑛 Ω ,

where C=C(N,μ,Ω) is a positive constant.

Organization of the paper

In Section 2, we recall main properties of the first eigenvalue and the corresponding eigenfunction of -Lμ in Ω and collect estimates on the Green kernel and the Martin kernel, as well as their gradient. In Section 3, we prove Theorem 1.3, and in Section 4, we demonstrate Theorem 1.5 and Theorem 1.6. In Section 5, we give the proof of Theorem 1.7 and Theorem 1.8. Section 6 is devoted to the proof of Theorem 1.9, and in Section 7, the proof of Theorem 1.11 is provided. In Appendix A, we construct a barrier in the case g(u,|u|)=|u|p|u|q. Finally, in Appendix B, we discuss the case g(u,|u|)=|u|p+|u|q and state main results without proofs since the arguments are similar to those in the case g(u,|u|)=up|u|q.

1.3 Notations

We list below some notations that we use frequently in the present paper.

  1. For ϕ0, denote by Lκ(Ω,ϕ) (κ>1) the space of functions v satisfying Ω|v|κϕdx<. We denote by H1(Ω,ϕ) the space of functions v such that vL2(Ω,ϕ) and vL2(Ω,ϕ). Let 𝔐(Ω,ϕ) be the space of Radon measures τ on Ω satisfying Ωϕd|τ|<, and let 𝔐+(Ω,ϕ) be the positive cone of 𝔐(Ω,ϕ). Denote by 𝔐(Ω) the space of bounded Radon measures on Ω and by 𝔐+(Ω) the positive cone of 𝔐(Ω).

  2. Denote Lwκ(Ω,τ), 1κ<, τ𝔐+(Ω), the weak Lκ space (or Marcinkiewicz space) with weight τ. The subscript w is an abbreviation of “weak”. See Subsection 2.2 for more details.

  3. We denote by λμ the first eigenvalue of -Lμ and by φμ the corresponding eigenfunction (see Subsection 1.4).

  4. For κ>1, we denote by κ the conjugate exponent.

  5. Throughout the paper, c,c1,c2,C,C1,C denote positive constants which may vary from line to line. We write C=C(a,b) to emphasize the dependence of C on the data a,b.

  6. The notation fh means that there exist positive constants c1,c2 such that c1h<f<c2h.

  7. Denote by χE the indicator function of a set E.

  8. For zΩ, denote by 𝐧z the outer unit normal vector at z.

2 Preliminaries

We recall that, throughout the paper, we assume that μ(0,14] and λμ>0.

2.1 Eigenvalue and Eigenfunction

We recall important facts of the eigenvalue λμ of -Lμ and the associated eigenfunction φμ which can be found in [13, 14]. If 0<μ<14, then the minimizer φμH01(Ω) of (1.4) exists and satisfies φμδα, where α is defined in (1.7). If μ=14, there is no minimizer of (1.4) in H01(Ω), but there exists a nonnegative function φ14Hloc1(Ω) such that φ14δ12 and it satisfies -L14φ14=λμφ14 in Ω in the sense of distributions. In addition, we have δ-12φ14H01(Ω,δ).

2.2 Green Kernel and Martin Kernel

Denote by GμΩ and KμΩ the Green kernel and the Martin kernel of -Lμ in Ω respectively (see [25, 17]). The Green operator and the Martin operator are defined as follows:

𝔾 μ Ω [ τ ] ( x ) :- Ω G μ Ω ( x , y ) d τ ( y ) for every τ 𝔐 ( Ω , δ α ) ,
𝕂 μ Ω [ ν ] ( x ) :- Ω K μ Ω ( x , z ) d ν ( z ) for every ν 𝔐 ( Ω ) .

When there is no ambiguity, we will drop the superscript Ω, i.e. we write Gμ, Kμ, 𝔾μ, 𝕂μ instead of GμΩ, KμΩ, 𝔾μΩ, 𝕂μΩ.

By [14, Theorem 4.11], it holds

(2.1) G μ ( x , y ) min { | x - y | 2 - N , δ ( x ) α δ ( y ) α | x - y | 2 - N - 2 α } for every x , y Ω , x y .

Since (1.5) holds, by [3] and [17, Proposition 2.29], the Martin kernel Kμ exists. Moreover, it holds (see [25, (2.7), page 76] for μ<CH(Ω) and [17, Theorem 2.30] for μ=14)

(2.2) K μ ( x , y ) δ ( x ) α | x - y | 2 - N - 2 α for every x Ω , y Ω .

For estimates on the Green kernel and the Martin kernel of a more general Schrödinger operator, we refer to [21].

Next we recall estimates of Green kernel and Martin kernel in weak Lκ spaces. Let τ𝔐+(Ω). For κ>1, κ=κκ-1 and uLloc1(Ω,τ), we set

u L w κ ( Ω , τ ) :- inf { c [ 0 , ] : E | u | d τ c ( E d τ ) 1 κ for any Borel set E Ω }

and

L w κ ( Ω , τ ) :- { u L loc 1 ( Ω , τ ) : u L w κ ( Ω , τ ) < } .

L w κ ( Ω , τ ) is called weak Lκ space (or Marcinkiewicz space with exponent κ) with quasi-norm Lwκ(Ω,τ). See [29] for more details. Notice that, for every s>-1,

(2.3) L w κ ( Ω , δ s ) L r ( Ω , δ s ) for every r [ 1 , κ ) .

Moreover, for any uLwκ(Ω,δs) (s>-1),

(2.4) { | u | λ } δ s d x λ - κ u L w κ ( Ω , δ s ) κ for all λ > 0 .

Proposition 2.1 ([15, Proposition 2.4]).

The following statements hold.

  1. Let γ ( - α N N + 2 α - 2 , α N N - 2 ) . There exists a constant c = c ( N , μ , γ , Ω ) such that

    (2.5) 𝔾 μ [ τ ] L w N + γ N + α - 2 ( Ω , δ γ ) c τ 𝔐 ( Ω , δ α ) for all τ 𝔐 ( Ω , δ α ) .

  2. Let γ > - 1 . Then there exists a constant c = c ( N , μ , γ , Ω ) such that

    (2.6) 𝕂 μ [ ν ] L w N + γ N + α - 2 ( Ω , δ γ ) c ν 𝔐 ( Ω ) for all ν 𝔐 ( Ω ) .

Proposition 2.2 ([16, Proposition A]).

The following statements hold.

  1. Let θ [ 0 , α ] and γ [ 0 , θ N N - 1 ) . Then there exists a positive constant c = c ( N , μ , θ , γ , Ω ) such that

    (2.7) 𝔾 μ [ | τ | ] L w N + γ N + θ - 1 ( Ω , δ γ ) c τ 𝔐 ( Ω , δ θ ) for all τ 𝔐 ( Ω , δ θ ) ,

    where 𝔾 μ [ τ ] ( x ) = Ω x G μ ( x , y ) d τ ( y ) .

  2. Let γ 0 . Then there exists a positive constant c = c ( N , μ , γ , Ω ) such that

    𝕂 μ [ | ν | ] L w N + γ N + α - 1 ( Ω , δ γ ) c ν 𝔐 ( Ω ) for all ν 𝔐 ( Ω ) ,

    where 𝕂 μ [ ν ] ( x ) = Ω x K μ ( x , z ) d ν ( z ) .

2.3 Linear Equations

The Green kernel and the Martin kernel play an important role in the study of the boundary value problem for the linear equation

(2.8) { - L μ u = τ in Ω , tr ( u ) = ν .

Definition 2.3.

Assume (τ,ν)𝔐(Ω,δα)×𝔐(Ω). We say that u is a weak solution of (2.8) if uL1(Ω,δα) and

- Ω u L μ ζ d x = Ω ζ d τ - Ω 𝕂 μ [ ν ] L μ ζ d x for all ζ 𝐗 μ ( Ω ) ,

where 𝐗μ(Ω) is defined in (1.9).

Proposition 2.4 ([15, Proposition 2.11]).

Assume that (τ,ν)M(Ω,δα)×M(Ω). Then u is a weak solution of (2.8) if and only if u=Gμ[τ]+Kμ[ν] in Ω. Moreover, there exists a constant C=C(N,μ,Ω)>0 such that uL1(Ω,δα)C(τM(Ω,δα)+νM(Ω)).

For DΩ, denote by GμD and KμD the Green kernel and the Poisson kernel of -Lμ in D. Consider the problem

(2.9) { - L μ u = φ in D , u = η on D .

A similar result has been established for (2.9).

Proposition 2.5 ([17]).

For any (φ,η)M(D,δD)×M(D) (where δD=dist(,D)), there exists a unique weak solution u=uφ,ηL1(D,δD) of (2.9), i.e.

- D u L μ ζ d x = D ζ d φ - D 𝕂 μ D [ η ] L μ ζ d x for all ζ 𝐗 0 ( D ) ,

where X0(D) is defined similarly to Xμ(Ω) with μ=0 and Ω=D. It holds

(2.10) u = 𝔾 μ D [ φ ] + 𝕂 μ D [ η ] ,

and there exists a constant c=c(N,μ,D)>0 such that uL1(D,δD)c(φM(Ω,δD)+ηM(D)).

Finally, we will need the following classical properties of C2 domains.

Proposition 2.6 ([29]).

There exists a positive constant β0 such that δC2(Ω¯4β0). Moreover, for any xΩ4β0, there exists a unique ξxΩ such that

  1. δ ( x ) = | x - ξ x | and 𝐧 ξ x = - δ ( x ) = - x - ξ x | x - ξ x | , where 𝐧 ξ x denotes the outer unit normal vector at ξ x Ω ,

  2. x ( s ) :- x + s δ ( x ) Ω β 0 and δ ( x ( s ) ) = | x ( s ) - ξ x | = δ ( x ) + s for any 0 < s < 4 β 0 - δ ( x ) .

3 Nonlinear Equations with Subcritical Absorption

In this section, we establish the existence of a positive solution of (P+ν). The approach is based on a combination of the idea in [20], estimates on the Green kernel, the Martin kernel, their gradient and the Vitali convergence theorem.

Proof of Theorem 1.3.

We divide the proof into three steps.

Step 1. In this step, we assume that

(3.1) M :- sup s , t + | g ( s , t ) | < + .

Let D⊂⊂Ω be a smooth open domain, and consider the equation

(3.2) - L μ v + g ( v + 𝕂 μ [ ν ] , | ( v + 𝕂 μ [ ν ] ) | ) = 0 in D .

First we note that u1=0 is a supersolution of (3.2) and u2=-𝕂μ[ν] is a solution of (3.2). Let

(3.3) 𝒯 ( u ) :- { 0 if  0 u , u if u 2 u 0 , u 2 if u u 2 .

In this step, we use the idea in [20] in order to construct a solution vW1,(D) of the problem

(3.4) { - L μ v + g ( v + 𝕂 μ [ ν ] , | ( v + 𝕂 μ [ ν ] ) | ) = 0 in D , v = 0 on D ,

which satisfies

(3.5) - 𝕂 μ [ ν ] v 0 for all x D .

Let dD,Ω:-dist(D,Ω) and uW1,1(D). By the standard elliptic theory, there exists a unique solution of the problem

(3.6) { - Δ w + ( d D , Ω - 2 - μ δ - 2 ) w = - g ( v + 𝕂 μ [ ν ] , | ( v + 𝕂 μ [ ν ] ) | ) + d D , Ω - 2 𝒯 ( v ) in D , w = 0 on D .

Recall that δ=dist(,Ω).

We define an operator 𝔸 as follows: to each uW1,1(D), we associate the unique solution 𝔸[u] of (3.6). Furthermore, since

d D , Ω - 2 - μ δ ( x ) - 2 ( 1 - μ ) δ ( x ) - 2 for all x D ,

by standard elliptic estimates, there exists a constant C1=C1(N,μ,dD,Ω,D)>0 such that

sup x D | 𝔸 [ u ] ( x ) | C 1 ( M + ν 𝔐 ( Ω ) ) -: A 1 .

Also, by (3.3) and standard elliptic estimates, there exists a positive constant C2=C2(N,μ,dD,Ω,D) such that

sup x D | 𝔸 [ u ] ( x ) | C 2 ( M + ν 𝔐 ( Ω ) ) -: A 2 .

By using an argument similar to the proof of [16, Theorem B, Step 1], we can show that

𝔸 : W 1 , 1 ( D ) W 1 , 1 ( D )

is continuous and compact. Now set 𝒦:-{ξW1,1(D):ξW1,(D)A1+A2}. Then 𝒦 is a closed, convex subset of W1,1(D) and 𝔸(𝒦)𝒦. Thus we can apply the Schauder fixed point theorem to obtain the existence of a function v𝒦 such that 𝔸[v]=v. This means v is a weak solution of (3.6). By the standard elliptic theory, we can easily deduce that v,u2C2(D)C(D¯). Moreover, it can be seen that v0.

Now we allege that vu2 in D by employing an argument of contradiction. Suppose by contradiction that there exists x0D such that infxD(v(x)-u2(x))=v(x0)-u2(x0)<0. Then we have v(x0)=u2(x0), -Δ(v-u2)(x0)0, 𝒯[v](x0)=𝒯[u2](x0)=u2(x0). But

- Δ ( v - u 2 ) ( x 0 ) = - ( d D , Ω - 2 - μ δ ( x 0 ) - 2 ) ( v ( x 0 ) - u 2 ( x 0 ) ) - g ( v ( x 0 ) + 𝕂 μ [ ν ] ( x 0 ) , | v ( x 0 ) + 𝕂 μ [ ν ] ( x 0 ) | ) + g ( u 2 ( x 0 ) + 𝕂 μ [ ν ] ( x 0 ) , | u 2 ( x 0 ) + 𝕂 μ [ ν ] ( x 0 ) | ) > 0 ,

which is clearly a contradiction. Therefore, vu2 in D.

As a consequence, 𝒯(v)=v, and therefore v is a solution of (3.4).

Step 2. In this step, we still assume that (3.1) holds. Let {Ωn} be a smooth exhaustion of Ω, and let vn be the solution of (3.4) in D=Ωn (constructed in Step 1) satisfying (3.5). Then there exists a constant C=C(N,μ,Ω)>0 such that

| v n ( x ) | 𝔾 μ [ χ Ω n g ( v n + 𝕂 μ [ ν ] , | ( v n + 𝕂 μ [ ν ] ) | ) ] ( x ) C M δ ( x ) α for all x Ω n .

This implies that there exists a subsequence, still denoted by {vn}, such that vnv in Wloc1,p(Ω) and v satisfies

{ - L μ v + g ( v + 𝕂 μ [ ν ] , | ( v + 𝕂 μ [ ν ] ) | ) = 0 in Ω , tr ( v ) = 0 .

Furthermore, -𝕂μ[ν]v0 for all xΩ. Setting u=v+𝕂μ[ν], then u is a solution of (P+ν) satisfying 0u𝕂μ[ν] in Ω.

Step 3. In this step, we drop condition (3.1). Set gn:-min(g,n), and let un be a nonnegative solution (the existence of un is guaranteed in Step 2) of

{ - L μ u n + g n ( u n , | u n | ) = 0 in Ω , tr ( u n ) = ν

satisfying

(3.7) 0 u n 𝕂 μ [ ν ] in Ω .

Then un satisfies

(3.8) - Ω u n L μ ζ d x + Ω g n ( u n , | u n | ) ζ d x = - Ω 𝕂 μ [ ν ] L μ ζ d x for all ζ 𝐗 μ ( Ω ) ,
(3.9) u n + 𝔾 μ [ g n ( u n , | u n | ) ] = 𝕂 μ [ ν ] .

Choosing ζ=φμ, where φμ is an eigenfunction associated to the first eigenvalue of -Lμ, by (3.8), we have

(3.10) λ μ Ω | u n | φ μ d x + Ω g n ( u n , | u n | ) φ μ d x λ μ Ω 𝕂 μ [ | ν | ] φ μ d x .

Now, by (3.9) and Proposition 2.2, we obtain

u n L w q μ ( Ω , δ α ) c ( N , μ , Ω ) ( g n ( u n , | u n | ) L 1 ( Ω , δ α ) + ν 𝔐 ( Ω ) ) .

This, together with (3.10) and (2.6), implies

u n L w q μ ( Ω , δ α ) c ( N , μ , Ω ) ν 𝔐 ( Ω ) .

Similarly, we can show that

u n L w p μ ( Ω , δ α ) c ( N , μ , Ω ) ν 𝔐 ( Ω ) .

Next we prove that

(3.11) g n ( u n , | u n | ) g ( u , | u | ) in L 1 ( Ω , δ α ) .

For λ>0 and any function w, set

(3.12)

𝐀 λ w :- { x Ω : | w ( x ) | > λ } , 𝐚 w ( λ ) :- 𝐀 λ w δ α d x ,
𝐁 λ w :- { x Ω : | w ( x ) | > λ p μ q μ } , 𝐛 w ( λ ) :- 𝐁 λ w δ α d x ,
𝐂 λ w :- 𝐀 λ w 𝐁 λ w , 𝐜 w ( λ ) :- 𝐂 λ w δ α d x .

Then, for λ>0 and n, put

𝐀 n , λ = 𝐀 λ u n , 𝐚 n ( λ ) = 𝐚 u n ( λ ) ,
𝐁 n , λ = 𝐁 λ u n , 𝐛 n ( λ ) = 𝐛 u n ( λ ) ,
𝐂 n , λ = 𝐂 λ u n , 𝐜 n ( λ ) = 𝐜 u n ( λ ) .

For any Borel set EΩ,

(3.13) E g n ( u n , | u n | ) δ α d x = E 𝐂 n , λ g n ( u n , | u n | ) δ α d x + E 𝐀 n , λ c 𝐁 n , λ g n ( u n , | u n | ) δ α d x + E 𝐀 n , λ 𝐁 n , λ c g n ( u n , | u n | ) δ α d x + E 𝐀 n , λ c 𝐁 n , λ c g n ( u n , | u n | ) δ α d x C λ g ( s , s p μ q μ ) s - 1 - p μ d s + g ( λ , λ p μ q μ ) E δ α d x .

Note that the first term on the right-hand side of (3.13) tends to 0 as λ. Therefore, for any ε>0, there exists λ>0 such that the first term on the right-hand side of (3.13) is smaller than ε2. Fix such λ, and put

η = ε 2 max { g ( λ , λ p μ q μ ) , 1 } .

Then, by (3.13),

E δ α d x η E g n ( u n , | u n | ) δ α d x < ε .

Therefore, the sequence {gn(un,|un|)} is equi-integrable in L1(Ω,δα). Thus, by invoking the Vitali convergence theorem, we derive (3.11).

From (3.7), we deduce that 0unu in L1(Ω,δα). Therefore, letting n in (3.8), we deduce that u is a weak solution of (P+ν). ∎

4 Absorption g(u,|u|)=|u|p|u|q: Subcritical Case

In this section, we assume g(u,|u|)=|u|p|u|q with p0, q>0, p+q>1. We recall that (see Remark 1.4) g satisfies (1.10) if and only if (1.11) holds. Moreover, g satisfies (1.21). Therefore, by Theorem 1.3, for any ν𝔐+(Ω), the problem

(4.1) { - L μ u + | u | p | u | q = 0 in Ω , tr ( v ) = ν

admits a positive weak solution.

Next we prove the following regularity result.

Proposition 4.1.

Assume p0 and 0<q<NN-1. If u is a nonnegative solution of

(4.2) - L μ u + | u | p | u | q = 0 𝑖𝑛 Ω ,

then uC2(Ω).

Proof.

Let D⊂⊂Ω be a smooth open domain. Since u is a nonnegative solution of (4.1), by (2.10), we can easily obtain 0u(x)𝕂μ[ν](x)CD for all xD. Consequently, |u|p|u|qCDp|u|q in D. Hence, by invoking [16, Lemma 4.2], we can derive the desired result. ∎

4.1 Comparison Principle

Lemma 4.2.

Let uC2(Ω) be a nonnegative solution of (4.2). If there exists x0Ω such that u(x0)=0, then u0.

Proof.

By Young’s inequality, |u|p|u|q|u|p+q+|u|p+q in Ω. As a consequence, u satisfies

(4.3) - L μ u + | u | p + q + | u | p + q 0 in Ω .

Now set 𝐚(x):-|u(x)|p+q-2u(x) and b(x):-|u(x)|p+q-1. Let β(0,β0) be small enough such that x0Dβ. Since uC2(Ω), there exists a constant Cβ such that supxDβ|𝐚(x)|+supxDβb(x)Cβ. From (4.3), we deduce -Δu+𝐚u+buμδ2u0 in Dβ. Since b(x)0, by the maximum principle, u cannot achieve a nonpositive minimum in Dβ. Thus the result follows straightforward. ∎

Next we state the comparison principle for (4.2).

Lemma 4.3.

Let p0, q1 and DΩ. We assume that u1,u2C2(D) are respectively nonnegative subsolution and positive supersolution of (4.2) in D such that

(4.4) lim sup x D u 1 ( x ) u 2 ( x ) < 1 .

Then u1u2 in D.

Proof.

Suppose by contradiction that

m :- sup x D u 1 ( x ) u 2 ( x ) > 1 .

By (4.4), we deduce that there exists x0D such that

u 1 ( x 0 ) u 2 ( x 0 ) = sup x D u 1 ( x ) u 2 ( x ) = m .

Let r>0 be such that B(x0,r)D. Then we see that

(4.5) - Δ ( m - 1 u 1 - u 2 ) + ( m - 1 u 1 ) p | m - 1 u 1 | q - u 2 p | u 2 | q μ δ 2 ( m - 1 u 1 - u 2 ) 0 in B ( x 0 , r 2 ) .

Now note that

(4.6) ( m - 1 u 1 ) p | m - 1 u 1 | q - u 2 p | u 2 | q = ( m - 1 u 1 ) p | m - 1 u 1 | q - ( m - 1 u 1 ) p | u 2 | q + ( m - 1 u 1 ) p | u 2 | q - u 2 p | u 2 | q = 𝐚 ~ ( x ) ( m - 1 u 1 - u 2 ) + b ~ ( x ) ( m - 1 u 1 - u 2 ) ,

where

𝐚 ~ ( x ) = ( m - 1 u 1 ) p | m - 1 u 1 | q - | u 2 | q | m - 1 u 1 - u 2 | 2 ( m - 1 u 1 - u 2 )

and

b ~ ( x ) :- | u 2 | q ( ( m - 1 u 1 ) p - u 2 p m - 1 u 1 - u 2 ) 0 .

Since u1,u2C2(D), u2(x)>0 for any xD and q1, there exists a positive constant C>0 such that

sup x B ( x 0 , r 2 ) | 𝐚 ~ ( x ) | + sup x B ( x 0 , r 2 ) b ~ ( x ) < C .

Combining (4.5) and (4.6), we have

- Δ ( m - 1 u 1 - u 2 ) + 𝐚 ~ ( m - 1 u 1 - u 2 ) + b ~ ( m - 1 u 1 - u 2 ) 0 in B ( x 0 , r 2 ) .

Hence, by the maximum principle, m-1u1-u2 cannot achieve a nonnegative maximum in B(x0,r2). This is a contradiction. Thus u1u2 in D. ∎

In order to prove the comparison principle for (4.1), we need the following result.

Lemma 4.4.

[ 16 , Lemma 4.5] Let τM(Ω,δα), and v0 satisfies

{ - L μ v τ 𝑖𝑛 Ω , tr ( v ) = 0 .

Then, for any 1<κ<qμ, there exists a constant c=c(N,Ω,μ) such that vLκ(Ω,δα)cτM(Ω,δα).

Proof of Theorem 1.5.

Since ui is a nonnegative solution of (4.1), |ui|p|ui|qL1(Ω,δα), i=1,2. Moreover, from Propositions 2.1 and 2.2, we deduce that

u i L p 1 ( Ω , δ α ) + u i L q 1 ( Ω , δ α ) c 1 ( | u i | p | u i | q L 1 ( Ω , δ α ) + ν i 𝔐 ( Ω ) )

for any 1<p1<pμ, 1<q1<qμ and i=1,2.

Without loss of generality, we assume that ν20; thus, by Lemma 4.2, u2>0 in Ω. In addition, by Proposition 4.1, uiC2(Ω) for i=1,2. Finally, by the representation formula, we have

u i + 𝔾 μ [ | u i | p | u i | q ] = 𝕂 μ [ ν i ] , i = 1 , 2 .

Let 0<ε1. Then

( ε u 1 - u 2 ) + ( 𝔾 μ [ | u 2 | p u 2 | q ] - ε 𝔾 μ [ | u 1 | p | u 1 | q ] ) + 𝔾 μ [ | | u 2 | p | u 2 | q - | ε u 1 | p | u 1 | q | ] -: v ,

which implies tr((εu1-u2)+)tr(v)=0. Hence tr((εu1-u2)+)=0.

Note that εu1 is a subsolution of (4.2). Also, since uiC2(Ω) and u2>0 in Ω, it follows that, for small enough β>0,

C β :- sup x D β u 1 u 2 < .

Without loss of generality, we assume that Cβ>1. Set εβ=1Cβ<1. Then εβu1-u20 in Dβ. Moreover, in view of the proof of Lemma 4.3, we derive that

(4.7) ε β u 1 - u 2 < 0 in D β .

Put Eβ:-{xΩ:εβu1-u2>0}. Due to Kato’s inequality [29], we get

(4.8) - L μ ( ε β u 1 - u 2 ) + ( u 2 p | u 2 | q - ( ε u 1 ) p | ε u 1 | q ) χ E β ( ( ε u 1 ) p ( | u 2 | q - | ε u 1 | q ) ) χ E β .

By (4.7), we derive that EβΩβ.

Let κ>1 and

max { 1 , N + α N + α - p ( N + α - 2 ) } < κ < N + α q ( N + α - 1 ) .

Note that, for this choice of κ, we have

κ κ - 1 p < p μ and κ q < q μ .

Using (4.8), Lemma 4.4 and Hölder’s inequality, we get

(4.9) Ω | ( ε β u 1 - u 2 ) + | κ q δ α d x c ( Ω ( ε β u 1 ) p | | u 2 | q - | ε β u 1 | q | χ E β δ α d x ) κ q c ( E β ( ε β u 1 ) p ( ε β q - 1 | u 1 | q - 1 + | u 2 | q - 1 ) | ( ε β u 1 - u 2 ) | δ α d x ) κ q c ( E β ( ε β u 1 ) κ κ - 1 p δ α d x ) q ( κ - 1 ) ( E β ( ( ε β q - 1 | u 1 | q - 1 + | u 2 | q - 1 ) | ( ε β u 1 - u 2 ) | ) κ δ α d x ) q c ( E β ( ε β u 1 ) κ κ - 1 p δ α d x ) q ( κ - 1 ) ( E β ( | u 1 | κ q + | u 2 | κ q ) δ α d x ) q - 1 ( E β ( | ( ε β u 1 - u 2 ) | ) κ q δ α d x ) .

Since EβΩβ, u1Lκκ-1p(Ω,δα) and |ui|Lκq(Ω,δα), we can choose β* small enough such that

c ( E β * ( ε β * u 1 ) κ κ - 1 p δ α d x ) q ( κ - 1 ) ( E β * ( | u 1 | κ q + | u 2 | κ q ) δ α d x ) q - 1 < 1 4 .

By the above inequality and (4.9), we obtain

( ε β * u 1 - u 2 ) + = 0 ( ε β * u 1 - u 2 ) + = c *

for some constant c*0, and since (εβ*u1-u2)+=0 on D¯β*, we have c*=0, namely εβ*u1u2 in Ω. As a consequence,

(4.10) sup x Ω u 1 ( x ) u 2 ( x ) = sup x D β * u 1 ( x ) u 2 ( x ) = sup x D β * u 1 ( x ) u 2 ( x ) = ε β * - 1 > 1 .

This implies the existence of x*Dβ* such that

(4.11) ( ε β * u 1 - u 2 ) ( x * ) = 0 .

Next we take β<β*; then εβεβ*. On the other hand, we infer from (4.10) that εβεβ* and hence εβ=εβ*. Therefore, (4.11) contradicts (4.7). The proof is complete. ∎

Lemma 4.5.

Let p0, 1<q<NN-1 and p+q>1. If u is a nonnegative solution of (4.2), then

(4.12) u ( x ) C δ ( x ) - 2 - q q + p - 1 + M β 0 for all x Ω ,
(4.13) | u ( x ) | C δ ( x ) - 1 + p p + q - 1 for all x Ω ,

where Mβ0:-supD¯β0u, C=C(N,μ,q,p,β0,Mβ0) and C=C(N,μ,q,p,β0,Mβ0).

Proof.

The proof is similar to that of [16, Lemma 4.6], and hence we omit it. ∎

4.2 Isolated Singularities

In this section, we assume the origin 0Ω and study the behavior near 0 of solutions of (4.2) which vanish on Ω{0}. We first establish pointwise a priori estimates for solutions with an isolated singularity at 0, as well as their gradient.

Proposition 4.6.

Assume 0Ω, p0, q1, p+q>1 and p and q satisfy (1.11). Let u be a positive solution of (4.2) in Ω such that

(4.14) lim x Ω , x ξ u ( x ) W ( x ) = 0 for all ξ Ω { 0 } ,

locally uniformly in Ω{0}. Here W is defined in (1.17). Then there exists a constant C=C(N,μ,q,p,Ω)>0 such that

(4.15) u(x) C δ ( x ) α | x | - 2 - q p + q - 1 - α for all x Ω ,
| u ( x ) | C δ ( x ) α - 1 | x | - 2 - q p + q - 1 - α for all x Ω .

Proof.

We split the proof into two steps.

Step 1. Let β0 be the constant in Proposition 2.6. Let xiΩ be such that |xi|β016,

Ω B ( 0 , β 0 4 ) i = 1 n B ( x i , β 0 32 ) -: 𝒜 for some n .

Notice that there exists a constant ε0=ε0(β0)>0 such that dist(𝒜,Ω)>ε0.

Let wi be the function constructed in Proposition A.1 in B(xi,β016) for R=β016, i=1,,n. Then, by the maximum principle (see [17, Propositions 2.13 and 2.14]), we have

u ( x ) w i ( x ) for all x B ( x i , β 0 16 ) , i = 1 , , n .

As a consequence, there is a constant C0=C0(N,μ,q,p,Ω,β0)>1 such that

u ( x ) C 0 for all x i = 1 n B ( x i , β 0 32 ) .

Set

v ( x ) :- C 1 ( | x | - β 0 4 ) - 2 - q p + q - 1 ,

where C1>0 will be chosen later. We will show that v(x)u(x) for every xΩ𝒜. Indeed, by a direct computation, we can show that there is a constant C1>0 such that, for all xΩ𝒜¯,

(4.16) - Δ v = 2 - q q + p - 1 ( ( N - 1 ) | x | - 1 ( | x | - β 0 4 ) - 1 - p + 1 p + q - 1 ( | x | - β 0 4 ) - 2 ) v - C 1 ( 2 - q ) ( 1 + p ) ( p + q - 1 ) 2 ( | x | - β 0 4 ) - 2 p + q p + q - 1 ,
(4.17) | v | q = C 1 q ( 2 - q p + q - 1 ) q ( | x | - β 0 4 ) - q ( 1 + p ) p + q - 1 ,
(4.18) μ v ( x ) δ ( x ) 2 C 1 ε 0 - 2 ( sup x Ω | x | ) 2 ( | x | - β 0 4 ) - 2 p + q p + q - 1 .

Gathering estimates (4.16)–(4.18) leads to, for C1=C1(N,μ,p,q,β0,Ω)>0 large enough,

- L μ v + v p | v | q ( | x | - β 0 4 ) - 2 p + q p + q - 1 [ - C 1 ( 2 - q ) ( 1 + p ) ( p + q - 1 ) 2 - C 1 ε 0 - 2 ( sup x Ω | x | ) 2 + C 1 p + q ( 2 - q p + q - 1 ) q ] 0 for all x Ω 𝒜 ¯ .

Moreover, we can choose C1=C1(C0,N,μ,q,p,Ω,β0) large enough such that lim supx(Ω𝒜¯)(u-v)<0. By Lemma 4.3, we deduce that uv in Ω𝒜¯, which implies that uC2 in Dβ0 for some positive constant C2=C2(N,μ,q,p,Ω,β0). Thus, by Lemma 4.5, there exists C3=C3(Ω,N,μ,q,p,β0)>0 such that

u ( x ) C 3 δ ( x ) - 2 - q p + q - 1 for all x Ω .

Step 2. For >0, put

(4.19) T [ u ] ( x ) :- 2 - q p + q - 1 u ( x ) , x Ω :- - 1 Ω .

Let ξΩ{0}, and put d=d(ξ):-12|ξ|. We assume that d1. Denote ud:-Td[u]. Then ud is a solution of (4.2) in Ωd=1dΩ. Let R0=β016, where β0 is the constant in Proposition 2.6. Then the solution wξ,3R04 mentioned in Proposition A.1 satisfies ud(y)wξ,3R04(y) for all yB3R04(ξ)Ωd. Thus ud is bounded above in B3R05(ξ)Ωd by a constant C>0 depending only on N,μ,p,q and the C2 characteristic of Ωd (see [29] for the definition of the C2 characteristic of Ω). As d1, a C2 characteristic of Ω is also a C2 characteristic of Ωd; therefore, the constant C can be taken to be independent of ξ. We note here that the constant R0(0,1) depends on the C2 characteristic of Ω. The rest of the proof can proceed similarly to the proof of [16, Proposition E], and we omit it. ∎

4.3 Weak Singularities

Proof of Theorem 1.6.

We use the same idea as in the proof of [16, Theorem F]. Let u=u0,kΩ be the positive solution of (1.13). By Theorem 1.3 and Lemma 4.2, 0<ukKμ(,0) in Ω. Moreover,

(4.20) u + 𝔾 μ [ u p | u | q ] = k K μ ( , 0 ) .

This and (2.2) imply that

(4.21) u ( x ) k K μ ( x , 0 ) c k δ ( x ) α | x | 2 - N - 2 α for all x Ω .

By proceeding as in the proof of (4.13), we obtain

(4.22) | u ( x ) | c k δ ( x ) α - 1 | x | 2 - N - 2 α for all x Ω .

It follows from (2.1), (4.21) and (4.22) that

(4.23) 𝔾 μ [ u p | u | q ] ( x ) c k p + q Ω δ ( y ) α p + ( α - 1 ) q G μ ( x , y ) | y | ( 2 - N - 2 α ) ( p + q ) d y .

Case 1: α+αp+(α-1)q0. By the assumption and (2.1), we have

(4.24) 𝔾 μ [ u p | u | q ] ( x ) c k p + q δ ( x ) α Ω | x - y | 2 - N - 2 α | y | α - ( N + α - 2 ) p - ( N + α - 1 ) q d y .

Since p and q satisfy (1.11), it follows that

(4.25) Ω | x - y | 2 - N - 2 α | y | α - ( N + α - 2 ) p - ( N + α - 1 ) q d y c | x | 2 - α - ( N + α - 2 ) p - ( N - 1 + α ) q .

Combining (4.24), (4.25) and (2.2) yields

𝔾 μ [ u p | u | q ] ( x ) c k p + q | x | N + α - ( N + α - 2 ) p - ( N + α - 1 ) q K μ ( x , 0 ) .

As a consequence,

(4.26) lim | x | 0 𝔾 μ [ u p | u | q ] ( x ) K μ ( x , 0 ) = 0 .

Case 2: -1+α<α+αp+(α-1)q<0. By (4.23) and (2.1), we have

𝔾 μ [ u p | u | q ] ( x ) c k p + q Ω δ ( y ) α p + ( α - 1 ) q F μ ( x , y ) | y | ( 2 - N - 2 α ) ( p + q ) d y ,

where

(4.27) F μ ( x , y ) :- | x - y | 2 - N min { 1 , δ ( x ) α δ ( y ) α | x - y | - 2 α } for all x , y Ω , x y .

Let β(0,β0) be such that δC2(Ωβ¯). We consider the cut-off function ϕC(Ωβ2¯) such that 0ϕ1, ϕ=1 in Ωβ4 and ϕ=0 in ΩΩβ2¯. Then

(4.28) Ω δ ( y ) α p + ( α - 1 ) q F μ ( x , y ) | y | - ( N + 2 α - 2 ) ( p + q ) d y = Ω δ ( y ) α p + ( α - 1 ) q F μ ( x , y ) | y | - ( N + 2 α - 2 ) ( p + q ) ϕ ( y ) d y + Ω δ ( y ) α p + ( α - 1 ) q F μ ( x , y ) | y | - ( N + 2 α - 2 ) ( p + q ) ( 1 - ϕ ( y ) ) d y .

We first deal with the first term on the right-hand side of (4.28). By the definition of ϕ and the inequality (which follows from (4.27))

F μ ( x , y ) δ ( x ) α | x - y | 2 - N - α ,

we deduce that there exists C=C(N,μ,p,q,Ω,β) such that

(4.29) Ω δ ( y ) α p + ( α - 1 ) q F μ ( x , y ) | y | - ( N + 2 α - 2 ) ( p + q ) ( 1 - ϕ ( y ) ) d y C δ ( x ) α .

Now we deal with the second term on the right-hand side of (4.28). Let β~(0,β4) be such that |x-y|>r0>0 for any yΩβ~ and some r0>0. Let ε>0 be such that

( N + α - 2 ) p + ( N + α - 1 ) q = N + α - ε ,

and let ε~(0,ε) be such that αp+(α-1)q+1-ε~>0. Then, by (4.27), we have

Σ β ~ δ ( y ) α p + ( α - 1 ) q + 1 F μ ( x , y ) | y | - ( N + 2 α - 2 ) ( p + q ) d S ( y ) δ ( x ) α r 0 2 - N - 2 α Σ β ~ δ ( y ) ε ~ | y | - N + 1 + ( ε - ε ~ ) d S ( y ) .

Note that, by the choice of ε~, N-2-N+1+(ε-ε~)>-1, which implies

sup β ~ ( 0 , β 4 ) Σ β ~ | y | - N + 1 + ( ε - ε ~ ) d S ( y ) < C .

Combining the above estimates, we deduce

(4.30) lim β ~ 0 Σ β ~ δ ( y ) α p + ( α - 1 ) q + 1 F μ ( x , y ) | y | - ( N + 2 α - 2 ) ( p + q ) d S ( y ) = 0 .

Now note that

- Ω β δ ( y ) y F μ ( x , y ) δ ( y ) α p + ( α - 1 ) q + 1 | y | - ( N + 2 α - 2 ) ( p + q ) ϕ ( y ) d y = ( N - 2 ) Ω β δ ( y ) ( x - y ) | x - y | N min { 1 , δ ( x ) α δ ( y ) α | x - y | 2 α } δ ( y ) α p + ( α - 1 ) q + 1 | y | - ( N + 2 α - 2 ) ( p + q ) ϕ ( y ) d y - Ω β δ ( y ) y ( min { 1 , δ ( x ) α δ ( y ) α | x - y | 2 α } ) δ ( y ) α p + ( α - 1 ) q + 1 | x - y | 2 - N | y | - ( N + 2 α - 2 ) ( p + q ) ϕ ( y ) d y .

On the other hand,

- δ ( y ) y ( min { 1 , δ ( x ) α δ ( y ) α | x - y | - 2 α } ) 2 α | x - y | - 1 min { 1 , δ ( x ) α δ ( y ) α | x - y | - 2 α } a.e. in Ω .

By collecting the above estimates, we obtain

(4.31) - Ω β δ ( y ) F μ ( x , y ) δ ( y ) α p + ( α - 1 ) q + 1 | y | - ( N + 2 α - 2 ) ( p + q ) ϕ ( y ) d y C δ ( x ) α Ω | x - y | - ( N + α - 1 ) | y | - ( N + α - 2 ) p - ( N + α - 1 ) q + 1 d y .

It follows from integration by parts, (4.30) and (4.31) that

(4.32) Ω β δ ( y ) α p + ( α - 1 ) q F μ ( x , y ) | y | - ( N + 2 α - 2 ) ( p + q ) ϕ ( y ) d y = 1 α p + ( α - 1 ) q + 1 Ω β ( δ ( y ) α p + ( α - 1 ) q + 1 ) δ ( y ) F μ ( x , y ) | y | - ( N + 2 α - 2 ) ( p + q ) ϕ ( y ) d y C δ ( x ) α Ω | x - y | - ( N + 2 α - 2 ) | y | - ( N + α - 2 ) p - ( N + α - 1 ) q + α d y + C δ ( x ) α Ω | x - y | - ( N + α - 1 ) | y | - ( N + α - 2 ) p - ( N + α - 1 ) q + 1 d y -: M ( x ) + N ( x ) .

Since 0<α<1, N3 and p and q satisfy (1.11), we infer from (2.2) that

(4.33) max { M ( x ) , N ( x ) } C | x | N + α - ( N + α - 2 ) p - ( N + α - 1 ) q K μ ( x , 0 ) .

Combining (4.23), (4.28), (4.29), (4.32) and (4.33) implies that there exists a positive constant

C = C ( N , μ , p , q , Ω ) > 0

such that

(4.34) 𝔾 μ [ u p | u | q ] ( x ) C k q | x | N + α - ( N + α - 2 ) p - ( N + α - 1 ) q K μ ( x , 0 ) for all x Ω .

Since p and q satisfy (1.11), we deduce (4.26) from (4.34).

Thus, from (4.26) and (4.20), we obtain (1.14). Finally, the monotonicity comes from the comparison principle. ∎

4.4 Strong Singularities

Let SN-1 be the unit sphere in N and +N={x=(x1,,xN)=(x,xN):xN>0}. We denote by

x = ( r , σ ) + × S N - 1 with r = | x | and σ = r - 1 x

the spherical coordinates in N, and we recall the representation

u = u r 𝐞 + 1 r u , Δ u = u r r + N - 1 r u r + 1 r 2 Δ u ,

where denotes the covariant derivative on SN-1 identified with the tangential derivative and Δ is the Laplace–Beltrami operator on SN-1.

We look for a particular positive solution of

{ - L μ u + | u | p | u | q = 0 in + N , u = 0 on + N { 0 } = N - 1 { 0 } ,

under the separable form

u ( x ) = u ( r , σ ) = r - 2 - q p + q - 1 ω ( σ ) ( r , σ ) ( 0 , ) × S + N - 1 .

It follows from a straightforward computation that ω>0 satisfies

(4.35) { - μ ω - N , p , q ω + J ( ω , ω ) = 0 in S + N - 1 , ω = 0 on S + N - 1 ,

where

μ ω :- Δ ω + μ ( 𝐞 N σ ) 2 w , N , p , q :- 2 - q p + q - 1 ( 2 p + q p + q - 1 - N ) , J ( s , ξ ) :- s p ( ( 2 - q p + q - 1 ) 2 s 2 + | ξ | 2 ) q 2 , ( s , ξ ) + × N .

Let κμ be the first eigenvalue of -μ in S+N-1 and ϕμ the corresponding eigenfunction ϕμ(σ)=(𝐞Nσ)α for σS+N-1, where 𝐞N is the unit vector pointing toward the north pole.

Notice that the eigenvalue κμ is explicitly determined by

κ μ = α ( N + α - 2 ) ,

and the corresponding eigenfunction ϕμ(σ)=(xN|x||S+N-1)α=(𝐞Nσ)α solves

(4.36) { - μ ϕ μ = κ μ ϕ μ in S + N - 1 , ϕ μ = 0 on S + N - 1 .

Notice that equation (4.36) admits a unique positive solution with supremum 1, and if μ=0, then α=1, which means that ϕ0(σ)=𝐞Nσ is the first eigenfunction of -Δ in H01(S+N-1).

We could have defined the first eigenvalue κμ of the operator μ by

κ μ = inf { S + N - 1 ( | w | 2 - μ ( 𝐞 N σ ) - 2 w 2 ) d S S + N - 1 w 2 d S : w H 0 1 ( S + N - 1 ) , w 0 } .

By [12, Theorem 6.1], the infimum exists since ϕ0(σ)=𝐞Nσ is the first eigenfunction of -Δ in H01(S+N-1). The minimizer ϕμ belongs to H01(S+N-1) only if 1<μ<14.

By (4.36), the following expression holds:

(4.37) | ϕ 0 ( σ ) | 2 = 1 - ϕ 0 ( σ ) 2 for all σ S + N - 1 .

Indeed, since ϕ14=ϕ012, we have

- Δ ϕ 0 1 2 = 1 4 ϕ 0 - 3 2 | ϕ 0 | 2 + N - 1 2 ϕ 0 1 2 = 1 4 ϕ 0 - 3 2 + κ 1 4 ϕ 0 1 2 .

Taking into account that κ14-N-12=-14, from the above equalities, we obtain (4.37).

Denote

𝐘 μ ( S + N - 1 ) :- { ϕ H loc 1 ( S N - 1 ) : ϕ 0 - α ϕ H 1 ( S + N - 1 , ϕ 0 2 α ) } .

It is asserted below that condition (1.11) is sharp for the existence of a positive solution of (4.35).

Theorem 4.7.

Assume p0, q0 and p+q>1.

  1. If ( 1.11 ) does not hold, then there exists no positive solution of ( 4.35 ).

  2. If ( 1.11 ) holds and q 1 , then problem ( 4.35 ) admits a unique positive solution ω 𝐘 μ ( S + N - 1 ) . Moreover, there exists a positive constant C = C ( N , μ , p , q ) such that

    ω ( σ ) ( N , p , q - κ μ α q ) 1 p + q - 1 ϕ μ ( σ ) for all σ S + N - 1 ,
    | ω ( σ ) | C ϕ μ ( σ ) α - 1 α for all σ S + N - 1 .

Proof.

(i) By multiplying (4.35) by ϕμ, we obtain

(4.38) ( κ μ - N , p , q ) S + N - 1 ω ϕ μ d σ + S + N - 1 J ( ω , ω ) ) ϕ μ d σ = 0 .

Note that the second term on the left-hand side of (4.38) is nonnegative. Thus, if (1.11) does not hold, or equivalently N,p,qκμ, then no positive solution of (4.35) exists.

(ii) The proof is split into two steps.

Step 1: Existence. Set

γ 1 :- ( N , p , q - κ μ α q ) 1 p + q - 1 .

Then the function ω¯=γ1ϕμ is a supersolution of (4.35). Indeed, by (4.36) and (4.37),

- μ ω ¯ - N , p , q ω ¯ + J ( ω ¯ , ω ¯ ) = γ 1 ( κ μ - N , p , q ) ϕ μ + γ 1 p + q ϕ μ p ( ( 2 - q p + q - 1 ) 2 ϕ μ 2 + | ϕ μ | 2 ) q 2 = γ 1 ( κ μ - N , p , q ) ϕ 0 α + γ 1 p + q ϕ 0 α p ( ( ( 2 - q p + q - 1 ) 2 - α 2 ) ϕ 0 2 α + α 2 ϕ 0 2 ( α - 1 ) ) q 2 γ 1 ( κ μ - N , p , q ) ϕ 0 α + α q γ 1 p + q ϕ 0 α p + q ( α - 1 ) [ γ 1 ( κ μ - N , p , q ) + α q γ 1 p + q ] ϕ 0 α = 0 .

In the above estimates, we note that (1.11) implies 2-qp+q-1>α.

Let α0(α,1) be such that

q < N + α 0 N + α 0 - 1 < q μ .

We note that ϕμ0=ϕ0α0, where μ0=14-(α0-12)2<μ.

We allege that there exists a positive constant γ2=γ2(N,q,μ,μ0)γ1 such that the function ω¯=γ2ϕμ0 is a subsolution of (4.35). Indeed, since q1, by (4.36) and (4.37), we have

- μ ω ¯ - N , p , q ω ¯ + J ( ω ¯ , ω ¯ ) = γ 2 ( μ 0 - μ ) ϕ μ 0 ( 𝐞 N σ ) 2 + γ 2 ( κ μ 0 - N , p , q ) ϕ μ 0 + γ 2 p + q ϕ μ 0 p ( ( 2 - q p + q - 1 ) 2 ϕ μ 0 2 + | ϕ μ 0 | 2 ) q 2 ( γ 2 ( μ 0 - μ ) + γ 2 q + p α 0 q ) ϕ 0 α 0 - 2 + ( γ 2 ( κ μ 0 - N , p , q ) + γ 2 q + p | ( 2 - q p + q - 1 ) 2 - α 0 2 | q 2 ) ϕ 0 α 0 0 ,

provided γ2 is small enough. Notice that we can choose γ2γ1.

For t(0,1), set St:-{σS+N-1:ϕ0(σ)<t}, S~t:-S+N-1St. In view of the proof of [20, Theorem 6.5], there exists a solution ωtW2,p(S~t) to (4.35) such that

(4.39) ω ¯ ( σ ) ω t ( σ ) ω ¯ ( σ ) for all σ S ~ t .

Therefore, by the standard elliptic theory, there exist a function w~ and a sequence tn0 such that ωtnω~ locally uniformly in C1(S+N-1) and ω~ satisfies -μω~-N,p,qω~+J(ω~,ω~)=0 in S+N-1. Furthermore, by (4.39), we have ω¯(σ)ω~(σ)ω¯(σ) for all σS+N-1.

Set u~(x)=|x|-2-qp+q-1ω~(σ). Then u~ satisfies -Lμu~+u~p|u~|q=0 in +N and

| u ~ ( x ) | ( N , p , q - κ μ α q ) 1 p + q - 1 x N α | x | - 2 - q p + q - 1 - α for all x + N .

Let x0=(x0,0) be such that |x0|=1. Then, in view of the proof of (4.13), there exists a constant C1=C(N,μ,q) such that |u~(x)|C1xNα-1 for all xB(x0,12). This implies

(4.40) | ω ~ ( σ ) | C ϕ 0 ( σ ) α - 1 for all σ S + N - 1 .

Step 2: Uniqueness. Let ωi𝐘μ(S+N-1), i=1,2, be two positive solutions of (4.35). Let x0=(x0,0) be such that |x0|=1. Put ui(x)=|x|-2-qp+q-1ωi. Then uiH1(B(x0,23),xN2α), and it satisfies -Lμui+uip|ui|q=0 in +N, which implies -Lμui0 in +N.

Since 0<vi:-xN-αuiH1(B(x0,12),xN2α), and it satisfies -div(xN2αv)0 in +N, by [14, Theorem 2.12], there exists a positive constant Ci>0 such that ui(x)CixNα for all xB(x0,12). Therefore, in view of the proof of (4.40), there exists a positive constant C0 such that

(4.41) w i ( σ ) C 0 ϕ 0 ( σ ) α for all σ S + N - 1 , i = 1 , 2 ,
(4.42) | w i ( σ ) | C 0 ϕ 0 ( σ ) α - 1 for all σ S + N - 1 , i = 1 , 2 .

Set bt:-infc>1{c:cω1ω2,σS~t}<. Without loss of generality, we may assume that bt0>1 for some t0(0,1); thus, by (4.41), we have

1 < b t 0 b t for all t ( 0 , t 0 ) .

In the sequel, we consider t(0,t0).

Put ψ:-ϕ0α-12ϕ0α+ε, where ε(0,1-α) is a parameter that will be determined later. Then we have 12ϕ0αψϕ0α. We recall that ϕ0α=ϕμ and ϕ0α+ε=ϕμε, where με:-14-(α+ε-12)2. From the definition of ψ, it is easy to check that

(4.43) - μ ψ = μ - μ ε 2 ϕ 0 α + ε - 2 + ϕ 0 α ( κ μ - κ μ ε 2 ϕ 0 ε ) .

Now let ωt=bt-1ω2. We remark that ωt is a subsolution of (4.35) and ωt-ω10 in S~t. Also, we have

(4.44) - μ ( ω t - ω 1 ) + | - ( ω 1 ω t ) p J ( ω t , ω t ) + J ( ω 1 , ω 1 ) | + N , p , q | ω t - ω 1 | .

Since 1q<2, the following inequality holds for any nonnegative number h1,h2,k1,k2:

(4.45) - ( h 1 2 + h 2 2 ) q 2 + ( k 1 2 + k 2 2 ) q 2 ( h 1 q - 1 + h 2 q - 1 + k 1 q - 1 + k 2 q - 1 ) ( | h 1 - k 1 | + | h 2 - k 2 | ) .

By applying (4.45) with h1=(2-qp+q-1)ωt, h2=|ωt|, k1=(2-qp+q-1)ω1 and k2=|ω1| and keeping in mind estimates (4.41) and (4.42), we obtain

(4.46) - ( ω 1 ω t ) p J ( ω t , ω t ) + J ( ω 1 , ω 1 ) C ( q , C 0 ) ϕ 0 α p + ( q - 1 ) ( α - 1 ) ( | ω t - ω 1 | + | ( ω t - ω 1 ) | ) .

Now set Vt:-ψ-1(ωt-ω1)+. By (4.44), (4.46) and the definition of ψ, we can easily deduce the existence of a positive constant C=C(N,μ,q,C0) such that

- div ( ψ 2 V t ) + ψ V t ( - μ ψ ) C ( ϕ 0 α p + q ( α - 1 ) + α | ψ - 1 ( ω t - ω 1 ) | + ϕ 0 α p + ( q - 1 ) ( α - 1 ) + 2 α | ( ψ - 1 ( ω t - ω 1 ) ) | ) .

Now, since ψVt𝐘μ(S+N-1) and Vt(σ)0 for any σS~t, multiplying the above inequality by (Vt)+ and integrating over S+N-1, we get

(4.47) S t | ( V t ) + | 2 ψ 2 d S ( σ ) + S t ψ ( V t ) + 2 ( - μ ψ ) d S ( σ ) C ( S t ϕ 0 α p + q ( α - 1 ) + α ( V t ) + 2 d S ( σ ) + S t ϕ 0 α p + ( q - 1 ) ( α - 1 ) + 2 α | ( V t ) + | ( V t ) + d S ( σ ) ) .

By the definition of ψ and (4.43), we have

(4.48) S t | ( V t ) + | 2 ψ 2 d S ( σ ) + S t ψ ( V t ) + 2 ( - μ ψ ) d S ( σ ) 1 4 S t | ( V t ) + | 2 ϕ 0 2 α d S ( σ ) + μ - μ ε 4 S t ( V t ) + 2 ϕ 0 2 α + ε - 2 d S ( σ ) - N - 1 2 S t ( V t ) + 2 ϕ 0 2 α d S ( σ ) .

Here we note that if ε<1-α, then q<2<2-α-ε1-α. This leads to

(4.49) 2 - α - ε - q ( 1 - α ) > 0 and 4 - 2 α - ε - 2 q ( 1 - α ) > 0 .

By Young’s inequality, we deduce that

(4.50) C S t ϕ 0 α p + ( q - 1 ) ( α - 1 ) + 2 α | ( V t ) + | ( V t ) + d S ( σ ) 1 8 S t ϕ 0 2 α | ( V t ) + | 2 d S ( σ ) + C ^ S t ϕ 0 2 α p + 2 ( q - 1 ) ( α - 1 ) + 2 α ( V t ) + 2 d S ( σ ) ,

where C is the constant in (4.47) and C^=C^(N,μ,p,q).

Gathering (4.47), (4.48) and (4.50) yields

1 8 S t ϕ 0 2 α | ( V t ) + | 2 d S ( σ ) - μ - μ ε 4 S t ϕ 0 2 α + ε - 2 ( V t ) + 2 d S ( σ ) + C 1 S t ( ϕ 0 α p + q ( α - 1 ) + α + ϕ 0 2 α p + 2 ( q - 1 ) ( α - 1 ) + 2 α + ϕ 0 2 α ) ( V t ) + 2 d S ( σ ) S t ϕ 0 2 α + ε - 2 ( μ ε - μ 4 + C 1 ( t 2 + α p - α - ε - q ( 1 - α ) + t 4 + 2 α p - 2 α - ε - 2 q ( 1 - α ) + t 2 - ε ) ) ( V t ) + 2 d S ( σ ) ,

where C1=C(N,μ,p,q). By (4.49) and the above inequality, we can find a positive constant

t 1 = t 1 ( N , q , μ , ε , C 0 ) such that 1 8 S t 1 ϕ 0 2 α | ( V t 1 ) + | 2 d S ( σ ) 0 ,

which implies (Vt1)+=0 in St1 since (Vt1)+=0 on {σS+N-1:ϕ0(σ)=t1}. Hence bt1-1ω2ω1 for all σSt1.

Thus we have proved that

b t 1 = inf c > 1 { c : c ω 1 ω 2 , σ S ~ t 1 } = inf c > 1 { c : c ω 1 ω 2 , σ S + N - 1 } .

This means that (ω1-ωt1)(σ)0 for any σS+N-1 and

(4.51) ω 1 ( σ 0 ) - ω t 1 ( σ 0 ) = 0 for some σ 0 S ~ t 1 .

But -μ(ω1-ωt1)-N,q(ω1-ωt1)+J(ω1,ω1)-J(ωt1,ωt1)0, which implies

- Δ ( ω 1 - ω t 1 ) + J ( ω 1 , ω 1 ) - J ( ω t , ω 1 ) + J ( ω t , ω 1 ) - J ( ω t 1 , ω t 1 ) 0 .

By the above inequality, the fact that min(ω1,ωt)>0 in S~t12 and the mean value theorem, there exists Λ¯>0 such that

- Δ ( ω 1 - ω t 1 ) + J ( s ¯ , ξ ¯ ) ξ ( ω 1 - ω t 1 ) + Λ ¯ ( ω 1 - ω t 1 ) 0 in S ~ t 1 2 ,

where s¯ and ξ¯ are functions of σS~t12 such that J(s¯,ξ¯)ξL(S~t12). By the maximum principle, ω1-ωt1 cannot achieve a nonpositive minimum in S~t12S~t12, which clearly contradicts (4.51).

The result follows by exchanging the role of ω1 and ω2. ∎

5 Absorption g(u,|u|)=|u|p|u|q: Supercritical Case

Let us recall the following result in [25, 16].

Proposition 5.1.

Let νM+(Ω), and let β0 be the constant in Proposition 2.6. Then the following inequalities hold:

sup 0 < β β 0 β α - 1 Σ β 𝕂 μ [ ν ] d S C ( β 0 , α , Ω ) ν 𝔐 ( Ω ) 𝑖𝑓 μ < 1 4 ,
sup 0 < β β 0 ( β | log β | 2 ) - 1 2 Σ β 𝕂 μ [ ν ] d S C ( β 0 , α , Ω ) ν 𝔐 ( Ω ) 𝑖𝑓 μ = 1 4 .

Lemma 5.2.

Assume νM+(Ω), p0, 1q<2, and let uC2(Ω) be a nonnegative solution of (4.1).

  1. If q α + 1 , then there exists a constant β 1 = β 1 ( N , μ , p , q , Ω ) > 0 such that

    (5.1) Ω δ α - q u p + q d x C ( Ω δ α u p | u | q d x + 1 ) ,

    where C depends only on N, μ, p,q, Ω and supΣβ1(𝕂μ[ν])p+q.

  2. If q = α + 1 , then, for any ε > 0 small enough, there exists a constant β 1 = β 1 ( N , μ , p , Ω , ε ) > 0 such that

    (5.2) Ω δ ε - 1 u p + α + 1 d x C ( Ω δ α u p | u | α + 1 d x + 1 ) ,

    where C depends only on N, μ, p, Ω, ε and supΣβ1(𝕂μ[ν])p+α+1.

Proof.

Since u is a nonnegative solution of (4.1) we have upq|u|Lq(Ω,δα). Let β1(0,β0), where β0 is the constant in Proposition 2.6.

(i) First we assume that q>1, qα+1, and let γ-1. Then, for β(0,β1),

D β D β 1 δ γ u p + q d x = ( γ + 1 ) - 1 D β D β 1 δ γ + 1 δ u p + q d x = ( γ + 1 ) - 1 ( - D β D β 1 δ γ + 1 Δ δ u p + q d x - ( p + q ) D β D β 1 δ γ + 1 u p + q - 1 δ u d x + Σ β 1 δ γ + 1 δ 𝐧 β 1 u p + q d x + Σ β δ γ + 1 δ 𝐧 β u p + q d x ) C | γ + 1 | - 1 ( D β D β 1 δ γ + 1 u p + q d x + D β D β 1 δ γ + 1 u p + q - 1 | u | d x + β 1 γ + 1 sup Σ β 1 ( 𝕂 μ [ ν ] ) p + q + Σ β δ γ + 1 u p + q d x ) .

Observe that, for any γ(α-q,max{α-1-q2,2(α-q)+1}), we have |γ+1|-1<2|α+1-q|-1. Therefore, for such γ, we can choose β1=β1(N,q,μ,Ω) such that

C | γ + 1 | - 1 D β D β 1 δ γ + 1 u p + q d x 2 C | α + 1 - q | - 1 D β D β 1 δ γ + 1 u p + q d x 1 4 D β D β 1 δ γ u p + q d x .

Consequently, by Young’s inequality, we can find a constant C1=C1(N,μ,p,q,Ω) such that

C | γ + 1 | - 1 D β D β 1 δ γ + 1 u p + q - 1 | u | d x = C | γ + 1 | - 1 D β D β 1 δ γ + 1 u p + q - 1 - p q u p q | u | d x 1 4 D β D β 1 δ γ u p + q d x + C 1 D β D β 1 δ γ + q u p | u | q d x .

By the above estimates, there is a positive constant C2=C2(N,μ,p,q,Ω) such that

(5.3) D β D β 1 δ γ u p + q d x C 2 ( D β D β 1 δ γ + q u p | u | q d x + β 1 γ + 1 sup Σ β 1 ( 𝕂 μ [ ν ] ) q + Σ β δ γ + 1 u p + q d x ) .

By (4.12), Proposition 5.1 and taking into account that γ+q-1>α-1, we obtain

Σ β δ γ + 1 u p + q d S C β γ + q - 1 Σ β u d S C β γ + q - 1 Σ β 𝕂 μ [ ν ] d S 0 as β 0 .

Therefore, by letting β0 in (5.3), we obtain

(5.4) Ω β 1 δ γ u p + q d x C 2 ( Ω β 1 δ γ + q u p | u | q d x + β 1 γ + 1 sup Σ β 1 ( 𝕂 μ [ ν ] ) p + q ) .

By the dominated convergence theorem, we can send γα-q in (5.4) to obtain

Ω β 1 δ α - q u p + q d x C 2 ( Ω β 1 δ α u p | u | q d x + β 1 α - q + 1 sup Σ β 1 ( 𝕂 μ [ ν ] ) p + q ) .

This implies (5.1).

The proof of (5.2) follows by arguments similar to the proof of (5.1) (with γ=ε-1) with some modifications, and we omit it.∎

We recall below some notations concerning the Besov space (see e.g. [1, 33]). For σ>0, 1κ<, we denote by Wσ,κ(d) the Sobolev space over d. If σ is not an integer, the Besov space Bσ,κ(d) coincides with Wσ,κ(d). When σ is an integer, we denote Δx,yf:-f(x+y)+f(x-y)-2f(x) and

B 1 , κ ( d ) :- { f L κ ( d ) : Δ x , y f | y | 1 + d κ L κ ( d × d ) }

with norm

f B 1 , κ :- ( f L κ κ + d × d | Δ x , y f | κ | y | κ + d d x d y ) 1 κ .

Then

B m , κ ( d ) :- { f W m - 1 , κ ( d ) : D x θ f B 1 , κ ( d ) for all θ d , | θ | = m - 1 }

with norm

f B m , κ :- ( f W m - 1 , κ κ + | θ | = m - 1 d × d | D x θ Δ x , y f | κ | y | κ + d d x d y ) 1 κ .

These spaces are fundamental because they are stable under the real interpolation method developed by Lions and Petree. For s, we defined the Bessel kernel of order s by Gs(ξ)=-1(1+||2)-s2(ξ), where is the Fourier transform of moderate distributions in d. The Bessel space Ls,κ(d) is defined by

L s , κ ( d ) :- { f = G s g : g L κ ( d ) }

with norm fLs,κ:-gLκ=G-sfLκ. It is known that if 1<κ< and s>0, Ls,κ(d)=Ws,κ(d) if s, and Ls,κ(d)=Bs,κ(d) if s, always with equivalent norms. The Bessel capacity is defined for compact subsets Kd by

C s , κ d ( K ) :- inf { f L s , κ κ , f 𝒮 ( d ) , f χ K } .

It is extended to open sets and then Borel sets by the fact that it is an outer measure.

Proof of Theorem 1.7.

Let ε0, and let uC2(Ω) be the solution of (4.1). Put Σ=Ω. If

η L ( Ω ) B 1 - α + α + 1 - q p + q + ε p + q , ( p + q ) ( Ω ) ,

we denote by H:-H[η] the solution of

{ H s + Δ Σ H = 0 in ( 0 , ) × Ω , H ( 0 , ) = η on Ω .

Let hC(+) be such that 0h1, h0, h1 on [0,β02], h0 on [β0,]. The lifting we consider is expressed by

(5.5) R [ η ] ( x ) :- { H [ η ] ( δ 2 , σ ( x ) ) h ( δ ) if x Ω ¯ β 0 , 0 if x D β 0 ,

with x=(δ,σ)=(δ(x),σ(x)).

Case 1: qα+1. Set ε=0 and ζ=φμR[η](p+q), where φμ is the eigenfunction associated to the first eigenvalue λμ of -Lμ in Ω (see Subsection 2.1). By proceeding as in the proof of [17, Lemma 3.8, (3.46)], we deduce that there exists C0=C0(N,μ,Ω,ν𝔐(Ω)) such that

(5.6) C 0 ( Ω η d ν ) ( p + q ) Ω u p | u | q ζ d x + λ μ Ω u ζ d x + ( p + q ) ( Ω u p + q φ μ - q α ζ d x ) 1 p + q ( Ω L [ η ] ( p + q ) d x ) 1 ( p + q ) ,

where

L [ η ] :- ( 2 φ μ q α ( p + q ) - 1 p + q | φ μ R [ η ] | + φ μ 1 + q α ( p + q ) - 1 p + q | Δ R [ η ] | ) .

Following the arguments of the proof of [17, Lemma 3.9, (3.48)], we can obtain

(5.7) Ω L [ η ] ( p + q ) d x c η L ( Ω ) ( p + q ) - 1 η B 1 - α + α + 1 - q p + q , ( p + q ) ( Ω ) .

We infer from (5.1) that

(5.8) Ω u p + q φ μ - q α ζ d x C η L ( Ω ) ( p + q ) Ω δ α - q u p + q d x C η L ( Ω ) ( p + q ) ( 1 + Ω u p | u | q δ α d x ) ,

where the constant C depends on N,μ,p,q and Ω. Combining (5.6), (5.7) and (5.8), we obtain

(5.9) C 0 ( Ω η d ν ) ( p + q ) Ω u p | u | q ζ d x + λ μ Ω u ζ d x + C η L ( Ω ) ( p + q ) p + q ( 1 + Ω u p | u | q δ α d x ) 1 p + q ( η L ( Ω ) ( p + q ) - 1 η B 1 - α + α + 1 - q p + q , ( p + q ) ( Ω ) ) 1 ( p + q ) .

Let KΩ be a compact set. Since (N+α-2)p+(N+α-1)qN+α, if

C 1 - α + α + 1 - q p + q , ( p + q ) N - 1 ( K ) = 0 ,

then there exists a sequence {ηn} in C02(Ω) with the following properties:

(5.10) 0 η n 1 , η n = 1 in a neighborhood of K and lim n η n = 0 in B 1 - α + α + 1 - q p + q , ( p + q ) ( Ω ) .

This implies that 0R[ηn]1 and limnR[ηn]=0 a.e. in Ω. Put ζn=φμR[ηn](p+q). Then

(5.11) lim n Ω u p | u | q ζ n d x = 0 and lim n Ω u ζ n d x = 0 .

From (5.9)–(5.11), we obtain

ν ( K ) Ω η n 𝑑 ν 0 as n .

This implies that ν(K)=0. Thus ν is absolutely continuous with respect to C1-α+α+1-qp+q,(p+q)N-1.

Case 2: q=α+1. Let 0<ε<α+1 and ζ=φμR[η](p+α+1). Proceeding as in the proof of (5.6), we can prove

C 0 ( Ω η d ν ) ( p + α + 1 ) Ω u p | u | α + 1 ζ d x + λ μ Ω u ζ d x + ( p + α + 1 ) ( Ω u p + α + 1 φ μ - α + 1 - ε α ζ d x ) 1 p + α + 1 ( Ω L [ η ] ( p + α + 1 ) d x ) ( p + 1 + α ) ,

where

L [ η ] = ( 2 φ μ α + 1 - ε α ( p + α + 1 ) - 1 p + α + 1 | φ μ R [ η ] | + φ μ 1 + α + 1 - ε α ( p + α + 1 ) - 1 p + α + 1 | Δ R [ η ] | ) .

Using (5.2) and the ideas of the proof of (5.9), we can obtain the inequality

C 0 ( Ω η d ν ) ( p + α + 1 ) Ω u p | u | α + 1 ζ d x + λ μ Ω u ζ d x + C η L ( Ω ) ( p + α + 1 ) p + α + 1 ( 1 + Ω u p | u | α + 1 δ α d x ) 1 p + α + 1 × ( η L ( Ω ) ( p + α + 1 ) - 1 η B 1 - α + ε p + α + 1 , ( p + α + 1 ) ( Ω ) ) 1 ( p + α + 1 ) ,

where the constant C depends on N,μ,p,Ω and ε.

The rest of the proof follows by using an argument similar to the first case. ∎

Proposition 5.3.

Let uC2(Ω) be a positive solution of (1.19). If up|u|Lq(Ω,δα), then u possesses a boundary trace νM+(Ω), i.e. u is the solution of boundary value problem (4.1) with boundary trace ν.

Proof.

If v:-𝔾μ[up|u|q], then vL1(Ω,δα) and u+v is a positive Lμ-harmonic function. Hence we have u+vL1(Ω,δα), and there exists a measure ν𝔐+(Ω) such that u+v=𝕂μ[ν]. By [16, Proposition 2.2], we obtain the result. ∎

Proof of Theorem 1.8.

In view of the proof of [17, Proposition A.2], we can obtain the estimates

| u ( x ) | C δ ( x ) α dist ( x , K ) - 2 - q p + q - 1 - α for all x Ω ,
| u ( x ) | C δ ( x ) α - 1 dist ( x , K ) - 2 - q p + q - 1 - α for all x Ω ,

where C depends on N,μ,p,q,Ω and supΣβ0u.

Case 1. Assume that

q α + 1 and C 1 - α + α + 1 - q p + q , ( p + q ) N - 1 ( K ) = 0 .

Then there exists a sequence {ηn} in C02(Ω) satisfying (5.10). In particular, there exists a decreasing sequence {𝒪n} of relatively open subsets of Ω, containing K such that ηn=1 on 𝒪n, and thus ηn=1 on Kn:-𝒪¯n. We set

η ~ n = 1 - η n and ζ ~ n = φ μ R [ η ~ n ] 2 ( p + q ) ,

where R is defined by (5.5). Then 0η~n1 and η~n=0 on Kn. Therefore,

ζ ~ n ( x ) ϕ μ min { 1 , c δ ( x ) 1 - N e - ( 4 δ ( x ) ) - 2 ( dist ( x , K n c ) ) 2 } for all x Ω .

Furthermore,

| R [ η ~ n ] | c min { 1 , δ ( x ) - 2 - N e - ( 4 δ ( x ) ) - 2 ( dist ( x , K n c ) ) 2 } for all x Ω ,
| Δ R [ η ~ n ] | c min { 1 , δ ( x ) - 4 - N e - ( 4 δ ( x ) ) - 2 ( dist ( x , K n c ) ) 2 } for all x Ω .

Proceeding as in the proof of [17, Theorem 3.10, (3.65)], we have

(5.12) Ω ( u L μ ζ ~ n + u p | u | q ζ ~ n ) d x = 0 .

Using the expression of Lμζ~n, we derive from (5.12) that

Ω u p | u | q ζ ~ n d x = Ω ( - λ μ φ μ R [ η ~ n ] 2 ( p + q ) + 4 ( p + q ) R [ η ~ n ] 2 ( p + q ) - 1 φ μ R [ η ~ n ] + 2 ( p + q ) R [ η ~ n ] 2 ( p + q ) - 2 φ μ ( R [ η ~ n ] Δ R [ η ~ n ] + ( 2 ( p + q ) - 1 ) | R [ η ~ n ] | 2 ) ) u d x c ( Ω u p + q φ μ - q α ζ ~ n d x ) 1 p + q ( Ω L ~ [ η n ] ( p + q ) d x ) 1 ( p + q ) ,

where

L ~ [ η ] = φ μ q α ( p + q ) - 1 p + q | φ μ R [ η n ] | + φ μ 1 + q α ( p + q ) - 1 p + q | Δ R [ η ~ n ] | + φ μ 1 + q α ( p + q ) - 1 p + q | R [ η ~ n ] | 2 .

By proceeding as in the proof of [17, Theorem 3.10, (3.75)], we can prove

Ω | u | p | u | q φ μ R [ η ~ n ] 2 ( p + q ) d x C η n B 1 - α + α + 1 - q p + q , ( p + q ) ( Ω ) ( Ω δ α - q u q R [ η ~ n ] 2 q d x ) 1 q .

The rest of the proof is similar to the proof of [16, Theorem J], and we omit it.

Case 2. Assume that

q = α + 1 and C 1 - α + ε p + α + 1 , ( p + α + 1 ) N - 1 ( K ) = 0

for ε as in statement (ii). Then we can obtain the desired result by combining the ideas in Case 1 of this theorem and in Case 2 of Theorem 1.7. ∎

6 Nonlinear Equations with Subcritical Source

In this section, we prove Theorem 1.9. We first establish an existence result for the case when g is smooth and bounded.

Lemma 6.1.

Let νM+(Ω) with νM(Ω)=1 and gC1(R×R+)L(R×R+). Assume (1.10) and (1.21) are satisfied. Then there exists ϱ0>0 depending on N,μ,Ω,Λg,k~ such that, for every ϱ(0,ϱ0), the problem

(6.1) { - L μ v = g ( v + ϱ 𝕂 μ [ ν ] , | ( v + ϱ 𝕂 μ [ ν ] ) | ) 𝑖𝑛 Ω , tr ( v ) = 0

admits a positive weak solution v satisfying

v L w p μ ( Ω , δ α ) + v L w q μ ( Ω , δ α ) t 0 ,

where t0>0 depends on N,μ,Ω,Λg,k~,p~,q~. Here Λg is defined in (1.10) and k~,p~,q~ are as in (1.21).

Proof.

We shall use the Schauder fixed point theorem to show the existence of a positive weak solution of (6.1). Define the operator 𝕊 by

𝕊 ( v ) :- 𝔾 μ [ g ( v + ϱ 𝕂 μ [ ν ] , | ( v + ϱ 𝕂 μ [ ν ] ) | ) ] , v W 1 , 1 ( Ω , δ α ) .

Fix 1<κ<min{p~,q~,qμ},

Q 1 ( v ) :- v L w p μ ( Ω , δ α ) for v L w p μ ( Ω , δ α ) , Q 2 ( v ) :- v L w q μ ( Ω , δ α ) for | v | L w q μ ( Ω , δ α ) , Q 3 ( v ) :- v L κ ( Ω , δ α ) for v L κ ( Ω , δ α ) , Q 4 ( v ) :- v L κ ( Ω , δ α ) for | v | L κ ( Ω , δ α )

and

Q ( v ) :- Q 1 ( v ) + Q 2 ( v ) + Q 3 ( v ) + Q 4 ( v ) .

Step 1: Estimate the L1(Ω,δα)-norm of g(v+ϱKμ[ν],|(v+ϱKμ[ν])|). For λ>0 and any function w, we use the notation as in (3.12). For the sake of simplicity, when w=v+ϱ𝕂μ[ν], we drop the superscript v+ϱ𝕂μ[ν] in the above notations. For instance, we use the notations 𝐀λ and 𝐚(λ) instead of 𝐀λv+ϱ𝕂μ[ν] and 𝐚v+ϱ𝕂μ[ν](λ).

Then, by (2.4), we have

𝐚 ( λ ) λ - p μ v + ϱ 𝕂 μ [ ν ] L w p μ ( Ω , δ α ) p μ ,
𝐛 ( λ ) λ - p μ ( v + ϱ 𝕂 μ [ ν ] ) L w q μ ( Ω , δ α ) q μ ,
𝐜 ( λ ) λ - p μ min { v + ϱ 𝕂 μ [ ν ] L w p μ ( Ω , δ α ) p μ , ( v + ϱ 𝕂 μ [ ν ] ) L w q μ ( Ω , δ α ) q μ } .

With the above notations, we split

(6.2) g ( v + ϱ 𝕂 μ [ ν ] , | ( v + ϱ 𝕂 μ [ ν ] ) | ) L 1 ( Ω , δ α ) 𝐂 1 g ( v + ϱ 𝕂 μ [ ν ] , | ( v + ϱ 𝕂 μ [ ν ] ) | ) δ α d x + 𝐀 1 c 𝐁 1 g ( v + ϱ 𝕂 μ [ ν ] , | ( v + ϱ 𝕂 μ [ ν ] ) | ) δ α d x + 𝐀 1 c 𝐁 1 c g ( v + ϱ 𝕂 μ [ ν ] , | ( v + ϱ 𝕂 μ [ ν ] ) | ) δ α d x + 𝐀 1 𝐁 1 c g ( v + ϱ 𝕂 μ [ ν ] , | ( v + ϱ 𝕂 μ [ ν ] ) | ) δ α d x -: I 1 + I 2 + I 3 + I 4 .

First we estimate I3. Since |v+ϱ𝕂μ[ν]|1 and |(v+ϱ𝕂μ[ν])|1 in 𝐀1c𝐁1c and 1<κ<min{p~,q~,qμ}, we obtain

(6.3) I 3 k ~ ( v + ϱ 𝕂 μ [ ν ] L κ ( Ω , δ α ) κ + ( v + ϱ 𝕂 μ [ ν ] ) L κ ( Ω , δ α ) κ ) g ( 1 , 1 ) .

Next I1 is estimated as follows:

(6.4) I 1 - 1 g ( λ , λ p μ q μ ) d 𝐜 ( λ ) = g ( 1 , 1 ) 𝐜 ( 1 ) + 1 𝐜 ( λ ) d g ( λ , λ p μ q μ ) p μ min { v + ϱ 𝕂 μ [ ν ] L w p μ ( Ω , δ α ) p μ , ( v + ϱ 𝕂 μ [ ν ] ) L w q μ ( Ω , δ α ) q μ } 1 g ( λ , λ p μ q μ ) λ - 1 - p μ d λ .

We bound I4 from above as follows:

(6.5) I 4 - 1 g ( λ , 1 ) d 𝐚 ( λ ) p μ v + ϱ 𝕂 μ [ ν ] L w p μ ( Ω , δ α ) p μ 1 g ( λ , λ p μ q μ ) λ - 1 - p μ d λ .

Similarly, we can estimate I2 as follows:

(6.6) I 2 p μ ( v + ϱ 𝕂 μ [ ν ] ) L w q μ ( Ω , δ α ) q μ 1 g ( λ , λ p μ q μ ) λ - 1 - p μ d λ .

By combining (6.2)–(6.6), we obtain (assuming ϱ1)

(6.7) g ( v + ϱ 𝕂 μ [ ν ] , | ( v + ϱ 𝕂 μ [ ν ] ) | ) L 1 ( Ω , δ α ) C ( Q 1 ( v ) p μ + Q 2 ( v ) q μ + Q 3 ( v ) κ + Q 4 ( v ) κ + ϱ κ ) ,

where C=C(N,μ,Ω,k~,Λg).

Step 2: Estimate Q1,Q2,Q3,Q4 and Q. By (2.5), we have

Q 1 ( 𝕊 ( v ) ) c g ( v + ϱ 𝕂 μ [ ν ] , | ( v + ϱ 𝕂 μ [ ν ] ) | ) L 1 ( Ω , δ α ) .

This and (6.7) imply that

Q 1 ( 𝕊 ( v ) ) C ( Q 1 ( v ) p μ + Q 2 ( v ) q μ + Q 3 ( v ) κ + Q 4 ( v ) κ + ϱ κ ) ,

where C=C(N,μ,Ω,k~,Λg). Next we deduce from (2.7) that

Q 2 ( 𝕊 ( v ) ) c g ( v + ϱ 𝕂 μ [ ν ] , | ( v + ϱ 𝕂 μ [ ν ] ) | ) L 1 ( Ω , δ α ) ,

which in turn implies

Q 2 ( 𝕊 ( v ) ) C ( Q 1 ( v ) p μ + Q 2 ( v ) q μ + Q 3 ( v ) κ + Q 4 ( v ) κ + ϱ κ ) ,

where C=C(N,μ,Ω,k~,Λg). By (2.3), (2.5) and (2.7), we can easily deduce that

Q 3 ( 𝕊 ( v ) ) c g ( v + ϱ 𝕂 μ [ ν ] , | ( v + ϱ 𝕂 μ [ ν ] ) | ) L 1 ( Ω , δ α ) ,
Q 4 ( 𝕊 ( v ) ) c g ( v + ϱ 𝕂 μ [ ν ] , | ( v + ϱ 𝕂 μ [ ν ] ) | ) L 1 ( Ω , δ α ) .

Thus,

Q 3 ( 𝕊 ( v ) ) + Q 4 ( 𝕊 ( v ) ) C ( Q 1 ( v ) p μ + Q 2 ( v ) q μ + Q 3 ( v ) κ + Q 4 ( v ) κ + ϱ κ ) ,

where C(N,μ,Ω,k~,Λg). Consequently,

Q ( 𝕊 ( v ) ) C ( Q 1 ( v ) p μ + Q 2 ( v ) q μ + Q 3 ( v ) κ + Q 4 ( v ) κ + ϱ κ ) .

Therefore, if Q(v)t, then

Q ( 𝕊 ( v ) ) C ( t p μ + t q μ + 2 t κ + ϱ κ ) .

Since pμ>qμ>κ>1, there exists ϱ0>0 depending on N,μ,Ω,k~,Λg such that, for any ϱ(0,ϱ0), the equation C(tpμ+tqμ+2tκ+ϱκ)=t admits a largest root t0>0 which depends on N,μ,Ω,Λg,k~. Therefore,

(6.8) Q ( v ) t 0 Q ( 𝕊 ( v ) ) t 0 .

Step 3. We apply the Schauder fixed point theorem to our setting. By a standard argument, we can show that 𝕊:W1,1(Ω,δα)W1,1(Ω,δα) is continuous and compact. Set

(6.9) 𝒪 :- { ξ W 1 , 1 ( Ω , δ α ) : Q ( u ) t 0 } .

Then 𝒪 is a closed, convex subset of W1,1(Ω,δα), and by (6.8), 𝕊(𝒪)𝒪. Thus we can apply the Schauder fixed point theorem to obtain the existence of a function v𝒪 such that 𝕊(v)=v. This means that v is a nonnegative solution of (6.1), and hence it holds

- Ω v L μ ζ d x = Ω g ( v + ϱ 𝕂 μ [ ν ] , | ( v + ϱ 𝕂 μ [ ν ] ) | ) ζ d x for every ζ 𝐗 μ ( Ω ) .

Proof of Theorem 1.9.

Let {gn} be a sequence of C1 nonnegative functions defined on +2 such that

g n ( 0 , 0 ) = g ( 0 , 0 ) = 0 , g n g n + 1 g , sup × + g n = n and lim n g n - g L loc ( × + ) = 0 .

We observe that ΛgnΛg<, where Λgn is defined as in (1.10) with g replaced by gn. Therefore, the constant ϱ0 in Lemma 6.1 can be chosen to depend on Λg (and N,μ,Ω,k~,p~,q~), but independent of n. Similarly, the constant t0 in Lemma 6.1 can be chosen to depend on Λg (and also N,μ,Ω,k~,p~,q~), but independent of n. By Lemma 6.1, for any ϱ(0,ϱ0) and n, there exists a solution vn𝒪 (where 𝒪 is defined in (6.9)) of

{ - L μ v n = g n ( v n + ϱ 𝕂 μ [ ν ] , | ( v n + ϱ 𝕂 μ [ ν ] ) | ) in Ω , tr ( v n ) = 0 .

Set un=vn+ϱ𝕂μ[ν]. Then tr(un)=ϱν and

(6.10) - Ω u n L μ ζ d x = Ω g n ( u n , | u n | ) ζ d x - ϱ Ω 𝕂 μ [ ν ] L μ ζ d x for every ζ 𝐗 μ ( Ω ) .

Since {vn}𝒪, the sequence {gn(vn+ϱ𝕂μ[ν],|(vn+ϱ𝕂μ[ν])|)} is uniformly bounded in L1(Ω,δα), and the sequence {μδ2vn} is uniformly bounded in Lp1(G) for every compact subset GΩ for some p1>0. As a consequence, {Δvn} is uniformly bounded in L1(G). By a standard regularity result for elliptic equations, {vn} is uniformly bounded in W1,p2(G) for some p2>1. Consequently, there exists a subsequence, still denoted by {vn}, and a function v such that vnv a.e. in Ω and vnv a.e. in Ω. Therefore, unu a.e. in Ω, where u=v+ϱ𝕂μ[ν] and gn(un,|un|)g(u,|u|) a.e. in Ω.

We show that unu in L1(Ω,δα). Since {vn} is uniformly bounded in Lp(Ω,δα), by (2.6), we derive that {un} is uniformly bounded in Lp(Ω,δα). Due to Hölder’s inequality, {un} is equi-integrable in L1(Ω,δα). We invoke Vitali’s convergence theorem to derive that unu in L1(Ω,δα).

Next proceeding as in the proof of (3.11), we obtain that gn(un,|un|)g(u,|u|) in L1(Ω,δα). Therefore, by sending n in each term of (6.10), we obtain

- Ω u L μ ζ d x = Ω g ( u , | u | ) ζ d x - ϱ Ω 𝕂 μ [ ν ] L μ ζ d x for every ζ 𝐗 μ ( Ω ) .

This means u is a nonnegative weak solution of (P$-$ρν). Therefore,

u = 𝔾 μ [ g ( u , | u | ) ] + ϱ 𝕂 μ [ ν ] in Ω ,

which implies that uϱ𝕂μ[ν] in Ω. ∎

7 Nonlinear Equations with Supercritical Source

7.1 Capacities and Existence Results

In this subsection, we introduce the definition of some capacities and provide related results which will be employed to prove Theorem 1.11 in the next subsection.

For 0θβ<N, set

(7.1) N θ , β ( x , y ) :- 1 | x - y | N - β max { | x - y | , δ ( x ) , δ ( y ) } θ for all ( x , y ) Ω ¯ × Ω ¯ , x y ,
(7.2) θ , β [ τ ] ( x ) :- Ω ¯ N θ , β ( x , y ) d τ ( y ) for all τ 𝔐 + ( Ω ¯ ) .

For a>-1, 0θβ<N and s>1, define Capθ,β,sa by

Cap θ , β , s a ( E ) :- inf { Ω ¯ δ a ϕ s d x : ϕ 0 , θ , β [ δ a ϕ ] χ E } for any Borel set E Ω ¯ .

Here χE denotes the indicator function of E.

Let 𝐙 be a metric space and ω𝔐+(𝐙). Let J:𝐙×𝐙(0,] be a Borel positive kernel such that J is symmetric and J-1 satisfies a quasi-metric inequality, i.e. there is a constant C>1 such that, for all x,y,z,𝐙,

1 J ( x , y ) C ( 1 J ( x , z ) + 1 J ( z , y ) ) .

Under these conditions, one can define the quasi-metric d by

d ( x , y ) :- 1 J ( x , y )

and denote by r(x):-{y𝐙:d(x,y)<r} the open d-ball of radius r>0 and center x. Note that this set can be empty.

For ω𝔐+(𝐙), we define the potentials 𝕁[ω] and 𝕁[ϕ,ω] by

𝕁 [ ω ] ( x ) :- 𝐙 J ( x , y ) d ω ( y ) and 𝕁 [ ϕ , ω ] ( x ) :- 𝐙 J ( x , y ) ϕ ( y ) d ω ( y ) .

For t>1, the capacity Cap𝕁,tω in 𝐙 is defined by

Cap 𝕁 , t ω ( E ) :- inf { 𝐙 ϕ ( x ) t d ω ( x ) : ϕ 0 , 𝕁 [ ϕ , ω ] χ E } for any Borel E 𝐙 .

Proposition 7.1 ([19]).

Let p>1 and τ,ωM+(Z) such that

(7.3) 0 2 r ω ( s ( x ) ) s 2 d s C 0 r ω ( s ( x ) ) s 2 d s ,
(7.4) sup y r ( x ) 0 r ω ( s ( y ) ) s 2 d s C 0 r ω ( s ( x ) ) s 2 d s

for any r>0, xZ, where C>0 is a constant. Then the following statements are equivalent.

  1. The equation u = 𝕁 [ u p , ω ] + σ 𝕁 [ τ ] has a solution for σ > 0 small.

  2. For any Borel set E 𝐙 , it holds E 𝕁 [ τ E ] p d ω C τ ( E ) , where τ E = χ E τ .

  3. For any Borel set E 𝐙 , it holds τ ( E ) C Cap 𝕁 , p ω ( E ) .

  4. The inequality 𝕁 [ 𝕁 [ τ ] p , ω ] C 𝕁 [ τ ] < holds ω -a.e.

We point out below that θ,β defined in (7.2) satisfies all assumptions of 𝕁 in Proposition 7.1.

Proposition 7.2 ([9, Lemma 2.2]).

N θ , β is symmetric and satisfies the quasi-metric inequality.

Next we give sufficient conditions for (7.3), (7.4) to hold.

Proposition 7.3.

Let ω=δ(x)aχΩ(x)dx with a>-1. Then (7.3) and (7.4) hold.

Proof.

If a0, then the statement follows from [9, Lemma 2.3]. We now treat the case -1<a<0. We claim that, for any 0<s<8diam(Ω¯) and any xΩ¯, we have

(7.5) ω ( B s ( x ) ) max { δ ( x ) , s } a s N .

Indeed, in order to obtain (7.5), we consider four cases.

Case 1: 4sδ(x). Then δ(x)δ(y) for any yBs(x), and the proof of (7.5) can be obtained easily.

Case 2: s>δ(x)4. Then δ(y)5s; thus

B s ( x ) Ω ¯ δ ( y ) a d y C s a + N C max { δ ( x ) , s } a s N .

Case 3: δ(x)4s4δ(x). Since Ω is smooth, there exists r*>0 such that

(7.6) B r 0 ( x i ) Ω ¯ δ ( y ) a d y C a + 1 r 0 a + N for all r 0 r * 8 and δ ( x i ) < r * 4 .

Set

r 0 :- r * δ ( x ) 32 diam ( Ω ¯ ) .

Then there exist xiBs(x), i=1,,k, such that Bs(x)i=1kBr0(xi). We note that k does depend neither on x, nor on δ(x). Thus we have

B s ( x ) Ω ¯ δ ( y ) a d y i = 1 k B r 0 ( x i ) Ω ¯ δ ( y ) a d y .

Now, by (7.6), we get

B r 0 ( x i ) Ω ¯ δ ( y ) a d y C δ ( x ) a + N C max { δ ( x ) , s } a s N if δ ( x i ) < r * 4 ,
B r 0 ( x i ) Ω ¯ δ ( y ) a d y C ( r * ) a δ ( x ) N C max { δ ( x ) , s } a s N if δ ( x i ) r * 4 ,

and hence (7.5) follows.

Case 4: s4δ(x). Set

r 0 :- r * s 32 diam ( Ω ¯ ) .

Then the proof of (7.5) follows due to an argument similar to Case 3.

The rest of the proof can proceed as in the proof of [9, Lemma 2.3], and we omit it. ∎

We recall below the definition of the capacity associated to θ,β (see [19]).

Definition 7.4.

Let a>-1, 0θβ<N and s>1. For any Borel set EΩ¯, define Capθ,β,sa by

Cap θ , β , s a ( E ) :- inf { Ω ¯ δ a ϕ s d y : ϕ 0 , θ , β [ δ a ϕ ] χ E } .

Clearly, for any Borel set EΩ¯, we have

Cap θ , β , s a ( E ) = inf { Ω ¯ δ - a ( s - 1 ) ϕ s d y : ϕ 0 , θ , β [ ϕ ] χ E } .

Furthermore, by [1, Theorem 2.5.1], we have

( Cap θ , β , s a ( E ) ) 1 s = inf { ω ( E ) : ω 𝔐 b + ( Ω ¯ ) , θ , β [ ω ] L s ( Ω ¯ ; δ a ) 1 }

for any compact set EΩ¯, where s is the conjugate exponent of s.

Thanks to Propositions 7.2 and 7.3, we can apply Proposition 7.1 to obtain the following result.

Proposition 7.5.

Let τM+(Ω¯), a>-1, 0θβ<N, p>1. Then the following statements are equivalent.

  1. For any Borel set E Ω ¯ , it holds τ ( E ) C Cap θ , β , p a ( E ) .

  2. The inequality θ , β [ δ a θ , β [ τ ] p ] C θ , β [ τ ] < holds a.e. in Ω.

Recall the capacity Capθ,sΩ introduced in [9] which is used to deal with boundary measures. Let θ(0,N-1), and denote by θ the Bessel kernel in N-1 with order θ. For s>1, define

Cap θ , s ( F ) :- inf { N - 1 δ s d y : ϕ 0 , θ * ϕ χ F } for any Borel set F N - 1 .

Since Ω is a bounded smooth domain in N, there exist open sets O1,,Om in N, diffeomorphisms Ti:OiB1(0) and compact sets K1,,Km in Ω such that

  1. K i O i , 1im, and Ωi=1mKi,

  2. T i ( O i Ω ) = B 1 ( 0 ) { x N = 0 } , Ti(OiΩ)=B1(0){xN>0},

  3. for any xOiΩ, there exists yOiΩ such that δ(x)=|x-y|.

We then define the Capθ,sΩ-capacity of a compact set FΩ by

Cap θ , s Ω ( F ) :- i = 1 m Cap θ , s ( T ~ i ( F K i ) ) ,

where Ti(FKi)=T~i(FKi)×{xN=0}.

The following result is obtained by the same argument as in the proof of [9, Proposition 2.9].

Proposition 7.6.

Let a>-1, 0θβ<N and s>1. Assume that -1+s(1+θ-β)<a<-1+s(N+θ-β). Then it holds

Cap θ , β , s a ( E ) Cap β - θ + a + 1 s - 1 , s Ω ( E ) for any Borel  E Ω .

7.2 Case g(u,|u|)=|u|p|u|q

Proof of Theorem 1.11.

We see that, under the assumption on p and q, from Proposition 7.5 and Proposition 7.6, conditions (i) and (ii) are equivalent. Therefore, we will prove the existence of a solution by assuming (ii). For uWloc1,1(Ω), put

[ u ] ( x ) :- 𝔾 μ [ | u | p | u | q ] ( x ) + 𝕂 μ [ ϱ ν ] ( x ) a.e. in Ω .

From (2.1), (2.2) and (7.1), we have

(7.7)

G μ ( x , y ) C 1 δ ( x ) α δ ( y ) α N 2 α , 2 ( x , y ) C 1 δ ( x ) α δ ( y ) α N 2 α - 1 , 1 ( x , y ) for all x , y Ω , x y ,
K μ ( x , y ) C 1 δ ( x ) α N 2 α - 1 , 1 ( x , y ) for all x Ω , y Ω ,

and

(7.8)

| x G μ ( x , y ) | C 1 δ ( x ) α - 1 δ ( y ) α N 2 α - 1 , 1 ( x , y ) for all x , y Ω , x y ,
| x K μ ( x , y ) | C 1 δ ( x ) α - 1 N 2 α - 1 , 1 ( x , y ) for all x Ω , y Ω .

From (7.7) and (7.8), we obtain

| [ u ] | C 1 δ α 2 α - 1 , 1 [ δ α | u | p | u | q ] + C 1 δ α 2 α - 1 , 1 [ ϱ ν ] ,
| [ u ] | C 1 δ α - 1 2 α - 1 , 1 [ δ α | u | p | u | q ] + C 1 δ α - 1 2 α - 1 , 1 [ ϱ ν ] .

Put

:- { u W loc 1 , 1 ( Ω ) : | u | 2 C 1 δ α 2 α - 1 , 1 [ ϱ ν ] , | u | 2 C 1 δ α - 1 2 α - 1 , 1 [ ϱ ν ] } .

Then, by using (1.22), we deduce that there exists ϱ0=ϱ0(p,q,C1,C)>0 such that if ϱ(0,ϱ0), then ().

Define 𝒱 the space of functions vWloc1,1(Ω) with the norm

v 𝒱 = v L p + q ( Ω , δ - q + α ) + v L p + q ( Ω , δ p + α ) .

We can see that 𝒱 and is convex and closed under the strong topology of 𝒱. Moreover, it can be justified that is a continuous and compact operator. Therefore, by invoking the Schauder fixed point theorem, we conclude that there exists u such that [u]=u. Therefore, u is a weak solution of problem (P$-$ρν) satisfying (1.23) with C=2C1. ∎


Dedicated to Laurent Véron on his 70th birthday.



Communicated by Julián López-Gómez and Patrizia Pucci


Award Identifier / Grant number: GJ19-14413Y

Funding statement: P.-T. Nguyen was supported by Czech Science Foundation, project GJ19-14413Y.

A Barrier

In this section, we will provide a barrier which plays an important role. This barrier will have the same properties as the barrier in [17, Proposition 6.1]. Let β0 be the constant in Proposition 2.6.

Proposition A.1.

Let ΩRN be a C2 domain, 0<μ14, q>0 and p+q>1. Then, for any zΩ and 0<Rβ016, there exists a supersolution w:-wz,R of (4.2) in ΩBR(z) such that wC(Ω¯BR(z)), w(x) when dist(x,K)0, for any compact subset KΩBR(z), and w vanishes on ΩBR(z). More precisely,

w ( x ) = { c ( R 2 - | x - z | 2 ) - b δ ( x ) γ for all γ ( 1 - α , α ) 𝑖𝑓  0 < μ < 1 4 , c ( R 2 - | x - z | 2 ) - b δ ( x ) 1 2 ( ln diam ( Ω ) δ ( x ) ) 1 2 𝑖𝑓 μ = 1 4 ,

where b is a constant such that bmax{4-q-pq+p-1+γ,N-22,1} and c=c(N,μ,p,q,b,γ).

Proof.

The proof is similar to that of [17, Proposition 6.1] with some minor modifications, and hence we omit it. ∎

B Case g(u,|u|)=|u|p+|u|q

In this section, we assume that g(u,|u|)=|u|p+|u|q with p>1 and 1<q<2. We will state main results for this case without proving since the proofs are similar, even simpler, to those for the case g(u,|u|)=|u|p|u|q.

B.1 Absorption Case

This subsection is devoted to the study of the equation

(B.1) - L μ u + | u | p + | u | q = 0 in Ω .

When g(u,|u|)=|u|p+|u|q with p,q>1, then g satisfies (1.10) if p and q satisfy (1.12). Moreover, g satisfies (1.21). Hence, if p and q satisfy (1.12), then, for any ν𝔐+(Ω), the problem

(B.2) { - L μ u + | u | p + | u | q = 0 in Ω , tr ( v ) = ν

admits a positive weak solution.

Theorem B.1.

Assume p and q satisfy (1.12). Let νiM+(Ω), i=1,2, and let ui be a nonnegative solution of (B.2) with ν=νi. If ν1ν2, then u1u2 in Ω.

Set

m p , q :- max { p , q 2 - q } .

Lemma B.2.

Let p>1 and 1<q<NN-1. If u is a nonnegative solution of (B.1), then

u ( x ) C δ ( x ) - 2 m p , q - 1 for all x Ω ,
| u ( x ) | C δ ( x ) - 2 m p , q - 1 - 1 for all x Ω .

Lemma B.3.

Let p and q satisfy (1.12). Assume u is a positive solution of (B.1) in Ω such that (4.14) holds locally uniformly in Ω{0}. Then there exists a constant C=C(N,μ,p,q,Ω) such that

u ( x ) C δ ( x ) α | x | - 2 m p , q - 1 - α for all x Ω ,
| u ( x ) | C δ ( x ) α - 1 | x | - 2 m p , q - 1 - α for all x Ω .

Theorem B.4.

Assume g(u,|u|)=|u|p+|u|q with p and q satisfying (1.12).

  1. Weak singularity. For any k > 0 , let u 0 , k Ω be the solution of ( 1.13 ). Then ( 1.14 ) holds. Furthermore, the mapping k u 0 , k Ω is increasing.

  2. Strong singularity. Put u 0 , Ω :- lim k u 0 , k Ω . Then u 0 , Ω is a solution of ( 1.15 ). Then there exists a constant c = c ( N , μ , p , q , Ω ) > 0 such that

    c - 1 δ ( x ) α | x | - 2 m p , q - 1 u 0 , Ω ( x ) c δ ( x ) α | x | - 2 m p , q - 1 for all x Ω ,
    | u 0 , Ω ( x ) | c δ ( x ) α - 1 | x | - 2 m p , q - 1 - α for all x Ω .

    Moreover,

    lim Ω x 0 x | x | = σ S + N - 1 | x | 2 m p , q - 1 u 0 , Ω ( x ) = ω ~ ( σ )

    locally uniformly on upper hemisphere S + N - 1 = + N S N . Here ω ~ is the unique positive solution of

    { - μ ω - N , p , q ω + J ( ω , ω ) = 0 𝑖𝑛 S + N - 1 , ω = 0 𝑜𝑛 S + N - 1 ,

    where

    μ ω :- Δ ω + μ ( 𝐞 N σ ) 2 ω , N , p , q :- 2 m p , q ( 2 m p , q + 2 - N ) , J ( s , ξ ) :- { ( ( 2 m p , q ) 2 s 2 + | ξ | 2 ) q 2 𝑖𝑓 p < q 2 - q , ( s , ξ ) + × N , s p + ( ( 2 m p , q ) 2 s 2 + | ξ | 2 ) q 2 𝑖𝑓 p = q 2 - q , ( s , ξ ) + × N , s p 𝑖𝑓 p > q 2 - q , ( s , ξ ) + × N .

Theorem B.5.

Let νM+(Ω), ppμ or qμq<2. Assume problem (B.2) admits a weak solution.

  1. If p p μ , then ν is absolutely continuous with respect to C2-1+αp,pN-1.

  2. If q μ q < 2 , then the following occurs.

    1. If q α + 1 , then ν is absolutely continuous with respect to Cα+1q-α,qN-1.

    2. If q = α + 1 , then, for any ε ( 0 , min { α + 1 , ( N - 1 ) α α + 1 - ( 1 - α ) } ) , ν is absolutely continuous with respect to Cε+1-α,α+1αN-1.

Theorem B.6.

Assume ppμ or qμq<2. Let KΩ be compact such that one of the following holds:

C 2 - 1 + α p , p N - 1 ( K ) = 0 𝑖𝑓 p p μ ,
C α + 1 q - α , q N - 1 ( K ) = 0 𝑖𝑓 q μ q < 2 𝑎𝑛𝑑 q α + 1 ,
C ε + 1 - α , q N - 1 ( K ) = 0 𝑖𝑓 q = α + 1 for some ε ( 0 , min { α + 1 , ( N - 1 ) α α + 1 - ( 1 - α ) } ) .

Then any nonnegative solution uC2(Ω)C(Ω¯K) of equation (B.1) satisfying (1.20) is identically zero.

B.2 Source Case

Theorem B.7.

Assume g(u,|u|)=|u|p+|u|q with p>1 and α+1N+α-1<q<1+αα. Assume one of the following conditions holds.

  1. There exists a constant C > 0 such that, for every Borel set E Ω ,

    ν ( E ) C min { Cap 1 - α + α + 1 p , p Ω ( E ) , Cap - α + α + 1 q , q Ω ( E ) } .

  2. There exists a positive constant C > 0 such that

    2 α , 2 [ δ α ( p + 1 ) 2 α , 2 [ ν ] p ] C 2 α , 2 [ ν ] < a.e. in Ω ,
    2 α - 1 , 1 [ δ ( α - 1 ) q + α 2 α - 1 , 1 [ ν ] q ] C 2 α - 1 , 1 [ ν ] < a.e. in Ω .

Then there exists ϱ0=ϱ0(N,μ,p,q,C,Ω)>0 such that if ϱ(0,ϱ0), then problem (P$-$ρν) admits a weak solution u satisfying (1.23).

Acknowledgements

The authors wish to thank Professor L. Véron for useful discussions. The authors are also grateful to the anonymous referee for the valuable comments which help to improve the manuscript.

References

[1] D. R. Adams and L. I. Hedberg, Function Spaces and Potential Theory, Grundlehren Math. Wiss. 314, Springer, Berlin, 1996. 10.1007/978-3-662-03282-4Search in Google Scholar

[2] A. Aghajani, C. Cowan and S. H. Lui, Singular solutions of elliptic equations involving nonlinear gradient terms on perturbations of the ball, J. Differential Equations 264 (2018), no. 4, 2865–2896. 10.1016/j.jde.2017.11.009Search in Google Scholar

[3] A. Ancona, Negatively curved manifolds, elliptic operators, and the Martin boundary, Ann. of Math. (2) 125 (1987), no. 3, 495–536. 10.2307/1971409Search in Google Scholar

[4] C. Bandle, M. Marcus and V. Moroz, Boundary singularities of solutions of semilinear elliptic equations in the half-space with a Hardy potential, Israel J. Math. 222 (2017), no. 1, 487–514. 10.1007/s11856-017-1598-4Search in Google Scholar

[5] C. Bandle, V. Moroz and W. Reichel, “Boundary blowup” type sub-solutions to semilinear elliptic equations with Hardy potential, J. Lond. Math. Soc. (2) 77 (2008), no. 2, 503–523. 10.1112/jlms/jdm104Search in Google Scholar

[6] G. Barbatis, S. Filippas and A. Tertikas, A unified approach to improved Lp Hardy inequalities with best constants, Trans. Amer. Math. Soc. 356 (2004), no. 6, 2169–2196. 10.1090/S0002-9947-03-03389-0Search in Google Scholar

[7] M. F. Bidaut-Véron, M. Garcia-Huidobro and L. Véron, A priori estimates for elliptic equations with reaction terms involving the function and its gradient, preprint (2018), https://arxiv.org/abs/1810.12230. 10.1007/s00208-019-01872-xSearch in Google Scholar

[8] M.-F. Bidaut-Véron, M. García-Huidobro and L. Véron, Estimates of solutions of elliptic equations with a source reaction term involving the product of the function and its gradient, Duke Math. J. 168 (2019), no. 8, 1487–1537. 10.1215/00127094-2018-0067Search in Google Scholar

[9] M.-F. Bidaut-Véron, G. Hoang, Q.-H. Nguyen and L. Véron, An elliptic semilinear equation with source term and boundary measure data: The supercritical case, J. Funct. Anal. 269 (2015), no. 7, 1995–2017. 10.1016/j.jfa.2015.06.020Search in Google Scholar

[10] H. Brezis, Une équation semilinéaire avec conditions aux limites dans L1, unpublished note (1972). Search in Google Scholar

[11] H. Brezis and M. Marcus, Hardy’s inequalities revisited, Ann. Sc. Norm. Super. Pisa Cl. Sci. (4) no. 25 (1997), 217–237. Search in Google Scholar

[12] L. D’Ambrosio and S. Dipierro, Hardy inequalities on Riemannian manifolds and applications, Ann. Inst. H. Poincaré Anal. Non Linéaire 31 (2014), no. 3, 449–475. 10.1016/j.anihpc.2013.04.004Search in Google Scholar

[13] J. Dávila and L. Dupaigne, Hardy-type inequalities, J. Eur. Math. Soc. (JEMS) 6 (2004), no. 3, 335–365. 10.4171/JEMS/12Search in Google Scholar

[14] S. Filippas, L. Moschini and A. Tertikas, Sharp two-sided heat kernel estimates for critical Schrödinger operators on bounded domains, Comm. Math. Phys. 273 (2007), no. 1, 237–281. 10.1007/s00220-007-0253-zSearch in Google Scholar

[15] K. T. Gkikas and P.-T. Nguyen, On the existence of weak solutions of semilinear elliptic equations and systems with Hardy potentials, J. Differential Equations 266 (2019), no. 1, 833–875. 10.1016/j.jde.2018.07.060Search in Google Scholar

[16] K. T. Gkikas and P. T. Nguyen, Semilinear elliptic equations with Hardy potential and gradient nonlinearity, preprint (2019), https://arxiv.org/abs/1903.11090; to appear in Rev. Mat. Iberoam. 10.4171/rmi/1164Search in Google Scholar

[17] K. T. Gkikas and L. Véron, Boundary singularities of solutions of semilinear elliptic equations with critical Hardy potentials, Nonlinear Anal. 121 (2015), 469–540. 10.1016/j.na.2015.03.004Search in Google Scholar

[18] A. Gmira and L. Véron, Boundary singularities of solutions of some nonlinear elliptic equations, Duke Math. J. 64 (1991), no. 2, 271–324. 10.1215/S0012-7094-91-06414-8Search in Google Scholar

[19] N. J. Kalton and I. E. Verbitsky, Nonlinear equations and weighted norm inequalities, Trans. Amer. Math. Soc. 351 (1999), no. 9, 3441–3497. 10.1090/S0002-9947-99-02215-1Search in Google Scholar

[20] J. L. Kazdan and R. J. Kramer, Invariant criteria for existence of solutions to second-order quasilinear elliptic equations, Comm. Pure Appl. Math. 31 (1978), no. 5, 619–645. 10.1002/cpa.3160310505Search in Google Scholar

[21] M. Marcus, Estimates of Green and Martin kernels for Schrödinger operators with singular potential in Lipschitz domains, Ann. Inst. H. Poincaré Anal. Non Linéaire 36 (2019), no. 5, 1183–1200. 10.1016/j.anihpc.2018.09.003Search in Google Scholar

[22] M. Marcus, V. J. Mizel and Y. Pinchover, On the best constant for Hardy’s inequality in 𝐑n, Trans. Amer. Math. Soc. 350 (1998), no. 8, 3237–3255. 10.1090/S0002-9947-98-02122-9Search in Google Scholar

[23] M. Marcus and V. Moroz, Moderate solutions of semilinear elliptic equations with Hardy potential under minimal restrictions on the potential, Ann. Sc. Norm. Super. Pisa Cl. Sci. (5) 18 (2018), no. 1, 39–64. 10.2422/2036-2145.201604_002Search in Google Scholar

[24] M. Marcus and P.-T. Nguyen, Elliptic equations with nonlinear absorption depending on the solution and its gradient, Proc. Lond. Math. Soc. (3) 111 (2015), no. 1, 205–239. 10.1112/plms/pdv020Search in Google Scholar

[25] M. Marcus and P.-T. Nguyen, Moderate solutions of semilinear elliptic equations with Hardy potential, Ann. Inst. H. Poincaré Anal. Non Linéaire 34 (2017), no. 1, 69–88. 10.1016/j.anihpc.2015.10.001Search in Google Scholar

[26] M. Marcus and L. Véron, The boundary trace of positive solutions of semilinear elliptic equations: The subcritical case, Arch. Ration. Mech. Anal. 144 (1998), no. 3, 201–231. 10.1007/s002050050116Search in Google Scholar

[27] M. Marcus and L. Veron, The boundary trace of positive solutions of semilinear elliptic equations: The supercritical case, J. Math. Pures Appl. (9) 77 (1998), no. 5, 481–524. 10.1016/S0021-7824(98)80028-7Search in Google Scholar

[28] M. Marcus and L. Veron, Removable singularities and boundary traces, J. Math. Pures Appl. (9) 80 (2001), no. 9, 879–900. 10.1016/S0021-7824(01)01209-0Search in Google Scholar

[29] M. Marcus and L. Véron, Nonlinear Second Order Elliptic Equations Involving Measures, De Gruyter Ser. Nonlinear Anal. Appl. 21, De Gruyter, Berlin, 2014. 10.1515/9783110305319Search in Google Scholar

[30] P.-T. Nguyen, Isolated singularities of positive solutions of elliptic equations with weighted gradient term, Anal. PDE 9 (2016), no. 7, 1671–1692. 10.2140/apde.2016.9.1671Search in Google Scholar

[31] P.-T. Nguyen, Semilinear elliptic equations with Hardy potential and subcritical source term, Calc. Var. Partial Differential Equations 56 (2017), no. 2, Article ID 44. 10.1007/s00526-017-1144-6Search in Google Scholar

[32] T. Nguyen Phuoc and L. Véron, Boundary singularities of solutions to elliptic viscous Hamilton–Jacobi equations, J. Funct. Anal. 263 (2012), no. 6, 1487–1538. 10.1016/j.jfa.2012.05.019Search in Google Scholar

[33] E. M. Stein, Singular Integrals and Differentiability Properties of Functions, Princeton Math. Ser. 30, Princeton University, Princeton, 1970. 10.1515/9781400883882Search in Google Scholar

[34] L. Véron, Local and Global Aspects of Quasilinear Degenerate Elliptic Equations. Quasilinear Elliptic Singular Problems, World Scientific, Hackensack, 2017. 10.1142/9850Search in Google Scholar

Received: 2019-09-27
Revised: 2020-01-08
Accepted: 2020-01-10
Published Online: 2020-02-18
Published in Print: 2020-05-01

© 2020 Walter de Gruyter GmbH, Berlin/Boston

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