Home Mathematics On the fractional p-Laplacian equations with weight and general datum
Article Open Access

On the fractional p-Laplacian equations with weight and general datum

  • EMAIL logo , and
Published/Copyright: December 2, 2016

Abstract

The aim of this paper is to study the following problem:

{ ( - Δ ) p , β s u = f ( x , u ) in  Ω , u = 0 in  N Ω ,

where Ω is a smooth bounded domain of N containing the origin,

( - Δ ) p , β s u ( x ) := PV N | u ( x ) - u ( y ) | p - 2 ( u ( x ) - u ( y ) ) | x - y | N + p s d y | x | β | y | β

with 0 β < N - p s 2 , 1 < p < N , s ( 0 , 1 ) , and p s < N . The main purpose of this work is to prove the existence of a weak solution under some hypotheses on f. In particular, we will consider two cases:

  1. f ( x , σ ) = f ( x ) ; in this case we prove the existence of a weak solution, that is, in a suitable weighted fractional Sobolev space for all f L 1 ( Ω ) . In addition, if f 0 , we show that the problem above has a unique entropy positive solution.

  2. f ( x , σ ) = λ σ q + g ( x ) , σ 0 ; in this case, according to the values of λ and q, we get the largest class of data g for which the problem above has a positive solution.

MSC 2010: 49J35; 35A15; 35S15

1 Introduction and motivations

We consider the following problem:

(1.1) { ( - Δ ) p , β s u = f ( x , u ) in  Ω , u = 0 in  N Ω ,

where

( - Δ ) p , β s u ( x ) := PV N | u ( x ) - u ( y ) | p - 2 ( u ( x ) - u ( y ) ) | x - y | N + p s d y | x | β | y | β ,

Ω is a smooth bounded domain containing the origin and f belongs to a suitable Lebesgue space.

This class of operators appear in a natural way when dealing with the improved Hardy inequality, namely, for all ϕ 𝒞 0 ( Ω ) and for all q < p we have

G s , p ( ϕ ) C Ω Ω | v ( x ) - v ( y ) | p | x - y | N + q s w ( x ) p 2 w ( y ) p 2 𝑑 x 𝑑 y ,

where

G s , p ( ϕ ) N N | ϕ ( x ) - ϕ ( y ) | p | x - y | N + p s 𝑑 x 𝑑 y - Λ N , p , s N | ϕ ( x ) | p | x | p s 𝑑 x ,

Λ N , p , s is the optimal Hardy constant, w ( x ) = | x | - ( N - p s ) / p , and v ( x ) = ϕ ( x ) w ( x ) . We refer to [18, 2, 4, 1] for a complete discussion about this fact.

In the same way, we can consider ( - Δ ) p , β s as an extension of the local operator - div ( | x | - β | u | p - 2 u ) . This last one is strongly related to the classical Caffarelli–Khon–Nirenberg inequalities given in [11] and it was deeply analyzed in the literature. Notice that, as a consequence of the Caffarelli–Khon–Nirenberg inequalities, it is known that the weight | x | - β , with β < N - p , is an admissible weight in the sense that, if u is a weak positive supersolution to the problem

- div ( | x | - β | u | p - 2 u ) = 0 ,

then it satisfies a weak Harnack inequality.

More precisely, there exists a positive constant κ > 1 such that for all 0 < q < κ ( p - 1 ) we have

( B 2 ρ ( x 0 ) u q ( x ) | x | - p β 𝑑 x ) 1 q C inf B ρ ( x 0 ) u ,

where B 2 ρ ( x 0 ) Ω and C > 0 depends only on B.

We refer to [16, 19] and the references therein for a complete discussion and the proof of the Harnack inequality and its generalization of admissible weights.

Our objective in this work is to analyze the properties of the operator ( - Δ ) p , β s and to get the existence of a solution, in a suitable sense, to problem (1.1) for the largest class of the datum f.

The case of p-Laplacian equations is well known in the literature; we refer for example to [8, 7] where the authors proved the existence and the uniqueness of an entropy solution for L 1 datum. The case of measure datum was treated in [13], where the existence of a renormalized solution is obtained.

The local case with weight was considered in [3]. The authors proved the existence and the uniqueness of an entropy solution for datum in L 1 .

For the operator ( - Δ ) p , β s the case p = 2 and β = 0 was analyzed in [22, 20]. Using a duality argument, in the sense of Stampacchia, the authors were able to prove the existence of a solution for any datum in L 1 . A more general semilinear problem was considered in [6], where the existence and the uniqueness of the solution is studied.

The case p 2 and β = 0 , with regular data and variational structure, was treated in the year 2016 in [14, 12].

For general datum, based on some generalization of the Wolff potential theory, Kuusi, Mingione and Sire succeeded in [21] in obtaining the existence of a weak solution belonging to a suitable fractional Sobolev space.

In this paper, we will treat the case p 2 and β > 0 . The argument considered in [21] seems to be too complicated to be adapted to our case.

Our approach is more simple and it is based on a suitable choice of a test function’s family and on some algebraic inequalities.

In the first part of the present paper, we will consider the case f ( x , σ ) = f ( x ) . We prove the existence of a weak solution that is in an appropriate fractional Sobolev space. More precisely, we get the following existence result.

Theorem 1.1.

Assume that f L 1 ( Ω ) . Then problem (1.1) has a weak solution u such that

(1.2) Ω Ω | u ( x ) - u ( y ) | q | x - y | N + q s 1 1 | x | β | y | β 𝑑 y 𝑑 x M for all  q < N ( p - 1 ) N - s and for all  s 1 < s ,

and T k ( u ) W β , 0 s , p ( Ω ) for all k > 0 , where

T k ( a ) = { a if  | a | k , k a | a | if  | a | > k .

If p > 2 - s N , then u W β , 0 s 1 , q ( Ω ) for all 1 q < N ( p - 1 ) N - s and for all s 1 < s .

It is clear that for β = 0 , we reach the same existence and regularity result as was obtained in [21]. However, it seems that our approach is more simple and can be adapted for a large class of weighted nonlocal operators.

Next, assuming that f 0 , we show the existence of a positive entropy solution in the sense of Definition 2.9. The statement of our result is the following.

Theorem 1.2.

Assume that f L 1 ( Ω ) is such that f 0 . Then problem (1.1) has a unique entropy positive solution u in the sense of Definition 2.9 given below. Moreover, if u n is the unique solution to the approximating problem

{ ( - Δ ) p , β s u n = f n ( x ) in  Ω , u n = 0 in  N Ω

with f n = T n ( f ) , then T k ( u n ) T k ( u ) strongly in W β , 0 s , p ( Ω ) .

In the second part of the paper, we consider the case f ( x , σ ) = λ σ q + g ( x ) . According to the values of q and λ, we prove the existence of an entropy solution for the largest class of the datum g.

The paper is organized as follows. In Section 2, we introduce some useful tools and preliminaries that we will use throughout the paper, like the weighted fractional Sobolev spaces and some related inequalities, a weak comparison principle and some algebraic inequalities. We also specify the sense in which the solutions to problem (1.1) are defined.

In Section 3, we begin by proving Theorem 1.1, namely, the case where f ( x , σ ) f ( x ) . The main idea is to proceed by approximation and to pass to the limit using suitable test functions. In the second part of the section, we prove Theorem 1.2, more precisely, if f 0 , we are able to show that problem (1.1) has a unique positive entropy solution. In the same way, setting u n as the solution of (1.1) with datum f n T n ( f ) , we will prove that the sequence { T k ( u n ) } n converges to T k ( u ) strongly in the corresponding weighted fractional Sobolev space.

In Section 4, we study the case where f ( x , σ ) = λ σ q + g ( x ) with λ > 0 and g 0 . According to the values of q and λ, we get the largest class of the data g such that the problem (1.1) has a positive solution.

2 Functional setting and main tools

In this section, we give some functional settings that will be used below. We refer to [15, 23] for more details.

Let s ( 0 , 1 ) , p 1 and 0 β < N - p s 2 . For simplicity of notation, we will set

d μ := d x | x | 2 β  and  d ν := d x d y | x - y | N + p s | x | β | y | β .

Let Ω N ; the weighted fractional Sobolev space W β s , p ( Ω ) is defined by

W β s , p ( Ω ) { ϕ L p ( Ω , d μ ) : Ω Ω | ϕ ( x ) - ϕ ( y ) | p 𝑑 ν < + } .

Furthermore, W β s , p ( Ω ) is a Banach space endowed with the norm

ϕ W β s , p ( Ω ) = ( Ω | ϕ ( x ) | p 𝑑 μ ) 1 p + ( Ω Ω | ϕ ( x ) - ϕ ( y ) | p 𝑑 ν ) 1 p .

In the same way, we define the space W β , 0 s , p ( Ω ) as the completion of 𝒞 0 ( Ω ) with respect to the previous norm.

As in [5] (see also [15]) we can prove the following extension result.

Lemma 2.1.

Assume that Ω R N is a regular domain. Then for all w W β s , p ( Ω ) there exists w ~ W β s , p ( R N ) such that w ~ | Ω = w and

w ~ W β s , p ( N ) C w W β s , p ( Ω ) ,

where C C ( N , s , p , Ω ) > 0 .

The following weighted Sobolev inequality is obtained in [1] and will be used systematically in this paper.

Theorem 2.2 (Weighted fractional Sobolev inequality).

Assume that 0 < s < 1 and p > 1 are such that p s < N . Let β < N - p s 2 . Then there exists a positive constant S ( N , s , β ) such that for all v C 0 ( R N ) we have

N N | v ( x ) - v ( y ) | p | x - y | N + p s d x | x | β d y | y | β S ( N , s , β ) ( N | v ( x ) | p s * | x | 2 β p s * p ) p p s * ,

where p s * = p N N - p s .

Moreover, if Ω R N is a bounded domain and β = N - p s 2 , then for all q < p there exists a positive constant C ( Ω ) such that

N N | v ( x ) - v ( y ) | p | x - y | N + p s d x | x | β d y | y | β C ( Ω ) ( N | v ( x ) | p s , q * | x | 2 β p s , q * p ) p p s , q *

for all v C 0 ( Ω ) , where p s , q * = p N N - q s .

Remark 2.3.

As in the case β = 0 , if Ω is a bounded smooth domain of N , we can endow W β , 0 s , p ( Ω ) with the equivalent norm

| ϕ | W β , 0 s , p ( Ω ) = ( Ω Ω | ϕ ( x ) - ϕ ( y ) | p | x - y | N + p s d x d y | x | β | y | β ) 1 p .

Now, for w W β s , p ( N ) , we set

( - Δ ) p , β s w ( x ) = PV N | w ( x ) - w ( y ) | p - 2 ( w ( x ) - w ( y ) ) | x - y | N + p s d y | x | β | y | β .

It is clear that for all w , v W β s , p ( N ) we have

( - Δ ) p , β s w , v = 1 2 N N | w ( x ) - w ( y ) | p - 2 ( w ( x ) - w ( y ) ) ( v ( x ) - v ( y ) ) | x - y | N + p s d x d y | x | β | y | β .

In the case where β = 0 , we denote ( - Δ ) p , β s by ( - Δ ) p s .

The following comparison principle extends the classical one obtained by Brezis–Kamin in [10]. See [22, 1] for the proof.

Lemma 2.4.

Let Ω be a bounded domain and let h be a non-negative continuous function such that h ( x , σ ) > 0 if σ > 0 and h ( x , σ ) σ p - 1 is decreasing. Let u , v W β , 0 s , p ( Ω ) be such that u , v > 0 in Ω and

{ ( - Δ ) p , β s u h ( x , u ) in  Ω , ( - Δ ) p , β s v h ( x , v ) in  Ω .

Then u v in Ω.

The following algebraic inequalities can be proved using a suitable rescaling argument.

Lemma 2.5.

Assume that p 1 , a , b R + and α > 0 . Then there exist positive constants c, c 1 , c 2 such that

(2.1) ( a + b ) α c 1 a α + c 2 b α

and

(2.2) | a - b | p - 2 ( a - b ) ( a α - b α ) c | a p + α - 1 p - b p + α - 1 p | p .

In the case where α 1 , under the same conditions on a, b, p as above, we have

(2.3) | a + b | α - 1 | a - b | p c | a p + α - 1 p - b p + α - 1 p | p .

Since we are considering a solution with datum in L 1 , we need to use the concept of truncation. Recall that, for k > 0 we have

T k ( a ) = { a if  | a | k , k a | a | if  | a | > k .

Define G k ( a ) = a - T k ( a ) ; if we take the above definition into consideration, it is not difficult to show the algebraic inequalities

(2.4) | a - b | p - 2 ( a - b ) ( T k ( a ) - T k ( b ) ) | T k ( a ) - T k ( b ) | p

and

| a - b | p - 2 ( a - b ) ( G k ( a ) - G k ( b ) ) | G k ( a ) - G k ( b ) | p ,

where a , b and p 1 .

In the same way, we will use the classical weighted Marcinkiewicz spaces.

Definition 2.6.

For a measurable function u we set

Φ u ( k ) = μ { x Ω : | u ( x ) | > k } ,

where d μ = | x | - 2 β d x .

We say that u is in the Marcinkiewicz space q ( Ω , d μ ) if Φ u ( k ) C k - q .

Since Ω is a bounded domain,

L q ( Ω , d μ ) q ( Ω , d μ ) L q - ε ( Ω , d μ )

for all ε > 0 .

Since we are considering problems with general datum, we need to specify the concept of solution. We begin by the following definitions.

Definition 2.7.

Let u be a measurable function. We say that u 𝒯 β , 0 1 , p ( Ω ) if for all k > 0 we have T k ( u ) W β , 0 s , p ( Ω ) .

Now, we are able to state what we mean by a solution to problem (1.1).

Definition 2.8.

Assume that f L 1 ( Ω ) . We say that u is a weak solution to problem (1.1) if for all ϕ 𝒞 0 ( Ω ) we have

1 2 D Ω | u ( x ) - u ( y ) | p - 2 ( u ( x ) - u ( y ) ) ( ϕ ( x ) - ϕ ( y ) ) 𝑑 ν = Ω f ( x ) ϕ ( x ) 𝑑 x .

Following [6], we define the notion of entropy solution as follows.

Definition 2.9.

Consider f L 1 ( Ω ) . We say that u 𝒯 0 , β 1 , p ( Ω ) is an entropy solution to problem (1.1) if

(2.5) R h | u ( x ) - u ( y ) | p - 1 𝑑 ν 0 as  h ,

where

R h = { ( x , y ) N × N : h + 1 max { | u ( x ) | , | u ( y ) | }  with  min { | u ( x ) | , | u ( y ) | } h  or  u ( x ) u ( y ) < 0 } ,

and for all k > 0 and ϕ W β , 0 s , p ( Ω ) L ( Ω ) we have

1 2 D Ω | u ( x ) - u ( y ) | p - 2 ( u ( x ) - u ( y ) ) [ T k ( u ( x ) - ϕ ( x ) ) - T k ( u ( y ) - ϕ ( y ) ) ] 𝑑 ν Ω f ( x ) T k ( u ( x ) - ϕ ( x ) ) 𝑑 x .

Remark 2.10.

Notice that for h k , choosing ϕ = T h - 1 ( u ) , we obtain that

1 2 D Ω | u ( x ) - u ( y ) | p - 2 ( u ( x ) - u ( y ) ) [ T k ( G h - 1 ( u ( x ) ) ) - T k ( G h - 1 ( u ( y ) ) ) ] 𝑑 ν
Ω f ( x ) T k ( G h - 1 ( u ( x ) ) ) 𝑑 x k | u | > h - k - 1 | f ( x ) | 𝑑 x .

Since

| u ( x ) - u ( y ) | p - 2 ( u ( x ) - u ( y ) ) [ T k ( G h - 1 ( u ( x ) ) ) - T k ( G h - 1 ( u ( y ) ) ) ] 0

in D Ω , setting

R ~ h = { ( x , y ) N × N : u ( x ) u ( y ) 0  with  | u ( x ) | h  and  h - k - 1 | u ( y ) | h }

and

R ^ h = { ( x , y ) N × N : u ( x ) u ( y ) 0  with  | u ( x ) | h  and  h - k - 1 | u ( x ) | h } ,

we reach that

1 2 R ~ h | u ( x ) - u ( y ) | p - 1 ( h - u ( y ) ) 𝑑 ν k | u | > h - k - 1 | f ( x ) | 𝑑 x

and

(2.6) 1 2 R ^ h | u ( x ) - u ( y ) | p - 1 ( h - u ( x ) ) 𝑑 ν k | u | > h - k - 1 | f ( x ) | 𝑑 x .

It is clear that

1 2 { h - k - 1 u ( y ) < u ( x ) h } ( u ( x ) - u ( y ) ) p 𝑑 ν k | u | > h - k - 1 | f ( x ) | 𝑑 x

and

(2.7) 1 2 { h - k - 1 u ( x ) < u ( y ) h } ( u ( y ) - u ( x ) ) p 𝑑 ν k | u | > h - k - 1 | f ( x ) | 𝑑 x .

3 Existence results: Proofs of Theorems 1.1 and 1.2

In this section, we consider the problem

(3.1) { ( - Δ ) p , β s u = f in  Ω , u = 0 in  N Ω ,

where f L 1 ( Ω ) .

The main goal of this section is to show that problem (3.1) has a weak solution u in the sense of Definition 2.8. As in the local case, the main idea is to proceed by approximation and then pass to the limit by using suitable a priori estimates.

Before proving the main existence results, we need several lemmas.

Let { f n } n L ( Ω ) be such that f n f strongly in L 1 ( Ω ) and define u n as the unique solution to the approximated problem

(3.2) { ( - Δ ) p , β s u n = f n ( x ) in  Ω , u n = 0 in  N Ω .

Notice that the existence and the uniqueness of u n follows by using a classical variational argument in the space W β , 0 s , p ( Ω ) .

The first a priori estimate is given by the following Lemma.

Lemma 3.1.

Let { u n } n be defined as above. Then { u n } n is bounded in the space M p 1 ( Ω , d μ ) with p 1 = ( p - 1 ) N N - p s .

Proof.

Using T k ( u n ) as a test function in (3.2), we reach that

1 2 D Ω | u n ( x ) - u n ( y ) | p - 2 ( u n ( y ) - u n ( x ) ) [ T k ( u n ( x ) ) - T k ( u n ( y ) ) ] 𝑑 ν k Ω | f n ( x ) | 𝑑 x .

Thus,

(3.3) D Ω | u n ( x ) - u n ( y ) | p - 2 ( u n ( y ) - u n ( x ) ) [ T k ( u n ( x ) ) - T k ( u n ( y ) ) ] d ν C k .

Recall that u n = T k ( u n ) + G k ( u n ) . Then by inequalities (2.4) and (3.3), we reach that

1 k D Ω | T k ( u n ( x ) ) - T k ( u n ( y ) ) | p 𝑑 ν M for all  k > 0 .

Now, using the weighted Sobolev inequality in Theorem 2.2, we get

S ( N | T k ( u n ( x ) ) | p s * | x | - 2 β p s * p 𝑑 x ) p / p s * D Ω | T k ( u n ( x ) ) - T k ( u n ( y ) ) | p 𝑑 ν C k .

Since { | u n | k } = { | T k ( u n ) | = k } , we obtain that

μ { x Ω : | u n | k } μ { x Ω : | T k ( u n ) | = k } Ω | T k ( u n ( x ) ) | p s * k p s * | x | - 2 β p s * p 𝑑 x .

Hence,

μ { x Ω : | u n | > k } C M p s * p k - ( p s * - p s * p ) .

Setting p 1 = p s * - p s * p = N ( p - 1 ) N - p s , we conclude that the sequence { u n } n is bounded in the space p 1 ( Ω , d μ ) and the result follows. ∎

As a consequence we easily get that the sequence { | u n | p - 2 u n } n is bounded in the space L σ ( Ω , d μ ) for all σ < N N - p s .

As in the local case, we prove now that the sequence { u n } n is bounded in a suitable fractional Sobolev space. More precisely we have the following lemma.

Lemma 3.2.

Assume that { u n } n is defined as in Lemma 3.1. Then

(3.4) Ω Ω | u n ( x ) - u n ( y ) | q | x - y | N + q s 1 1 | x | β | y | β 𝑑 y 𝑑 x M for all  q < N ( p - 1 ) N - s and for all  s 1 < s .

Proof.

Let q < N ( p - 1 ) N - s be fixed. Since Ω is a bounded domain, it is sufficient to prove (3.4) for s 1 very close to s. In particular, we fix s 1 such that

p q ( s - s 1 ) p - q < β .

Define w n ( x ) = 1 - 1 ( u n + ( x ) + 1 ) α , where α > 0 is to be chosen later and u n + ( x ) = max { u n ( x ) , 0 } .

In what follows, we denote by C 1 , C 2 , any positive constants that are independent of u n and can change from one line to another.

Using w n as a test function in (3.2), we get

1 2 D Ω | u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) ( u n + ( x ) + 1 ) α - ( u n + ( y ) + 1 ) α ( u n + ( x ) + 1 ) α ( u n + ( y ) + 1 ) α 𝑑 ν Ω f n ( x ) 𝑑 x .

Hence,

(3.5) D Ω | u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) ( u n + ( x ) + 1 ) α - ( u n + ( y ) + 1 ) α ( u n + ( x ) + 1 ) α ( u n + ( y ) + 1 ) α 𝑑 ν C 1 .

Let v n ( x ) = u n + ( x ) + 1 . Since

| u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) ( ( u n + ( x ) + 1 ) α - ( u n + ( y ) + 1 ) α )
| u n + ( x ) - u n + ( y ) | p - 2 ( u n + ( x ) - u n + ( y ) ) ( ( u n + ( x ) + 1 ) α - ( u n + ( y ) + 1 ) α )
= | v n ( x ) - v n ( y ) | p - 2 ( v n ( x ) - v n ( y ) ) ( v n α ( x ) - v n α ( y ) )

by (3.5), it follows that

D Ω | v n ( x ) - v n ( y ) | p - 2 ( v n ( x ) - v n ( y ) ) v n α ( x ) - v n α ( y ) v n α ( x ) v n α ( y ) 𝑑 ν C 1 .

Now, using the fact that v n 1 and by inequality (2.2), we get

(3.6) D Ω | v n p + α - 1 p ( x ) - v n p + α - 1 p ( y ) | p v n α ( x ) v n α ( y ) 𝑑 ν C 2 .

Defining q 1 = q s 1 s < q and using the Hölder inequality, we find

Ω Ω | v n ( x ) - v n ( y ) | q | x - y | N + q s 1 d y d x | x | β | y | β
= Ω Ω | v n ( x ) - v n ( y ) | q | x - y | q s ( v n ( x ) + v n ( y ) ) α - 1 ( v n ( x ) v n ( y ) ) α ( v n ( x ) v n ( y ) ) α ( v n ( x ) + v n ( y ) ) α - 1 | x - y | ( q - q 1 ) s d y d x | x | β | y | β | x - y | N
( Ω Ω | v n ( x ) - v n ( y ) | p ( v n ( x ) + v n ( y ) ) α - 1 | x - y | N + p s ( v ( x ) v ( y ) ) α | x | β | y | β 𝑑 y 𝑑 x ) q p
(3.7)     × ( Ω Ω ( v n ( x ) + v n ( y ) ) α - 1 ( v ( x ) v ( y ) ) α ( v n ( x ) v n ( y ) ) α p p - q ( v n ( x ) + v n ( y ) ) ( α - 1 ) p p - q | x - y | ( q - q 1 ) s p p - q d y d x | x - y | N | x | β | y | β ) p - q q .

Now, using the algebraic inequality (2.3), we have

| v n ( x ) - v n ( y ) | p ( v n ( x ) + v n ( y ) ) α - 1 C | v n ( x ) p + α - 1 p - v n ( y ) p + α - 1 p | p .

Hence, taking into consideration that Ω × Ω D Ω and by (3.6), we get

( Ω Ω | v n ( x ) - v n ( y ) | p ( v n ( x ) + v n ( y ) ) α - 1 | x - y | N + p s ( v ( x ) v ( y ) ) α | x | β | y | β 𝑑 y 𝑑 x ) q p C ( D Ω | v n ( x ) p + α - 1 p - v n ( y ) p + α - 1 p | p | x - y | N + p s ( v n ( x ) v n ( y ) ) α | x | β | y | β 𝑑 y 𝑑 x ) q p C 3 .

So, going back to (3.7), we reach that

Ω Ω | v n ( x ) - v n ( y ) | q | x - y | N + q s 1 d y d x | x | β | y | β C 4 ( Ω Ω ( ( v n ( x ) v n ( y ) ) α ( v n ( x ) + v n ( y ) ) α ) q p - q ( v n ( x ) + v n ( y ) ) q p - q 1 | x - y | N - p s ( q - q 1 ) p - q d y d x | x | β | y | β ) p - q q .

By inequality (2.1), we have

( v n ( x ) + v n ( y ) ) ( v n ( x ) v n ( y ) v n ( x ) + v n ( y ) ) α c ( v n ( x ) + v n ( y ) ) α + 1 C 5 ( v n α + 1 ( x ) + v n α + 1 ( y ) ) .

Therefore,

Ω Ω | v n ( x ) - v n ( y ) | q | x - y | N + q s 1 d x d y | x | β | y | β
(3.8) C 5 ( Ω Ω v n ( α + 1 ) q p - q ( x ) d x d y | x - y | N - p s ( q - q 1 ) p - q | x | β | y | β ) p - q q + C 5 ( Ω Ω v n ( α + 1 ) q p - q ( y ) d x d y | x - y | N - p s ( q - q 1 ) p - q | x | β | y | β ) p - q q .

It is clear that

Ω Ω v n ( α + 1 ) q p - q ( x ) d x d y | x - y | N - p s ( q - q 1 ) p - q | x | β | y | β = Ω Ω v n ( α + 1 ) q p - q ( y ) d x d y | x - y | N - p s ( q - q 1 ) p - q | x | β | y | β ,

hence we just have to estimate the first term. We have

Ω Ω v n ( α + 1 ) q p - q ( x ) d x d y | x - y | N - p s ( q - q 1 ) p - q | x | β | y | β = Ω v n ( α + 1 ) q p - q ( x ) | x | β 𝑑 x Ω d y | x - y | N - p s ( q - q 1 ) p - q | y | β .

Since Ω is a bounded domain, Ω B R ( 0 ) . Thus

Ω Ω v n ( α + 1 ) q p - q ( x ) d x d y | x - y | N - p s ( q - q 1 ) p - q | x | β | y | β B R ( 0 ) v n ( α + 1 ) q p - q ( x ) | x | β 𝑑 x B R ( 0 ) d y | x - y | N - p s ( q - q 1 ) p - q | y | β ,

where v n = 1 in B R ( 0 ) Ω . We set r = | x | and ρ = | y | . Then x = r x , y = ρ y , where | x | = | y | = 1 .

Let

(3.9) τ = ( α + 1 ) q p - q and θ = p s ( q - q 1 ) p - q .

Then

Ω Ω v n τ ( x ) d x d y | x - y | N - θ | x | β | y | β B R ( 0 ) v n τ ( x ) d x | x | β 0 R ρ N - 1 ρ β r N - θ ( | y | = 1 d H N - 1 ( y ) | x - ρ r y | N - θ ) 𝑑 ρ .

We set σ = ρ r ; hence

Ω Ω v n τ ( x ) d x d y | x - y | N - θ | x | β | y | β B R ( 0 ) v n τ ( x ) d x | x | 2 β - θ 0 R r σ N - β - 1 ( | y | = 1 d H N - 1 ( y ) | x - σ y | N - θ ) 𝑑 σ .

Defining

K θ ( σ ) = | y | = 1 d H N - 1 ( y ) | x - σ y | N - θ

as in [17], we find

K θ ( σ ) = 2 π N - 1 2 β ( N - 1 2 ) 0 π sin N - 2 ( ξ ) ( 1 - 2 σ cos ( ξ ) + σ 2 ) N - θ 2 𝑑 ξ .

Notice that K θ ( σ ) C | 1 - σ | - 1 + θ as σ 1 and K θ ( σ ) σ θ - N as σ .

Therefore, there holds

(3.10) Ω Ω v n τ ( x ) d x d y | x - y | N - θ | x | β | y | β B R 3 ( 0 ) v n τ ( x ) d x | x | 2 β - θ 0 R r σ N - β - 1 K θ ( σ ) 𝑑 σ + B R ( 0 ) B R 3 ( 0 ) v n τ ( x ) d x | x | 2 β - θ 0 R r σ N - β - 1 K θ ( σ ) 𝑑 σ .

Recall that r = | x | . Then if x B R ( 0 ) B R 3 ( 0 ) , we have R r < 3 . Hence taking into consideration that θ > 0 and the behavior of K θ near 1, we reach that

0 R r σ N - β - 1 K θ ( σ ) 𝑑 σ C 1 0 3 σ N - β - 1 | 1 - σ | 1 - θ 𝑑 σ = C 2 < .

Now, if r | x | < R 3 , there holds

0 R r σ N - β - 1 K θ ( σ ) 𝑑 σ = 0 3 σ N - β - 1 K θ ( σ ) 𝑑 σ + 3 R r σ N - β - 1 K θ ( σ ) 𝑑 σ
C 2 + ( R r ) a 3 R r σ N - β - a - 1 K θ ( σ ) 𝑑 σ ,

where a > 0 is to be chosen later.

Since

3 R r σ N - β - a - 1 K θ ( σ ) 𝑑 σ 3 σ N - β - a - 1 K θ ( σ ) 𝑑 σ ,

using the fact that σ N - β - a - 1 K θ ( σ ) σ - 1 - β - a + θ as σ and choosing a > θ , it follows that

0 σ N - β - a - 1 K ( σ ) 𝑑 σ C 3 < .

Now, going back to (3.10), there holds

Ω Ω v n τ ( x ) d x d y | x - y | N - θ | x | β | y | β C 1 B R ( 0 ) v n τ ( x ) d x | x | 2 β - θ 𝑑 x + C 2 R a B R 3 ( 0 ) v n τ ( x ) d x | x | 2 β + a - θ 𝑑 x
C ( R ) B R ( 0 ) v n τ ( x ) d x | x | 2 β + a - θ 𝑑 x .

Since q < ( p - 1 ) N N - s , we can choose α > 0 in (3.9) such that τ < ( p - 1 ) N N - p s . By Lemma 3.1, choosing a > θ , very close to θ and using the Hölder inequality, we reach that

(3.11) Ω Ω v n τ ( x ) d y d x | x - y | N - θ | x | β | y | β C 6 B R ( 0 ) v n τ ( x ) d x | x | 2 β + a - θ C 7 for all  n .

Hence by (3.8) and (3.11), we conclude that

Ω Ω | u n + ( x ) - u n + ( y ) | q | x - y | N + q s 1 d y d x | x | β | y | β = Ω Ω | v n ( x ) - v n ( y ) | q | x - y | N + q s 1 d x d y | x | β | y | β C 8 .

In the same way, by using 1 - 1 / ( ( u n - ( x ) + 1 ) α ) as a test function in (3.2), we obtain that

Ω Ω | u n - ( x ) - u n - ( y ) | q | x - y | N + q s 1 d y d x | x | β | y | β C 9 .

Combining the above estimates, we reach that

Ω Ω | u n ( x ) - u n ( y ) | q | x - y | N + q s 1 d y d x | x | β | y | β C

and the result follows. ∎

Remark 3.3.

As a consequence we get the existence of a measurable function u such that T k ( u ) W β , 0 s , p ( Ω ) , | u | p - 2 u L σ ( Ω , | x | - 2 β d x ) for all σ < N N - p s , and T k ( u n ) T k ( u ) weakly in W β , 0 s , p ( Ω ) .

It is clear that u n u a.e. in Ω. Since u n = 0 a.e. in N Ω , we have u = 0 a.e. in N Ω .

Notice that by Lemma 3.1 we conclude that

| u n | p - 2 u n | u | p - 2 u strongly in  L a ( Ω , d μ )  for all  a < N N - p s .

Let

U n ( x , y ) = | u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) and U ( x , y ) = | u ( x ) - u ( y ) | p - 2 ( u ( x ) - u ( y ) ) .

Since Ω is a bounded domain, by the result of Lemma 3.2 and using Vitali’s lemma, we obtain that

U n U strongly in  L 1 ( Ω × Ω , d ν ) .

We are now able to prove the first existence result.

Proof of Theorem 1.1.

It is clear that estimate (1.2) follows by using Lemma 3.2 and Fatou’s lemma.

Let ϕ 𝒞 0 ( Ω ) . Then, by using ϕ as a test function in (3.2), it follows that

(3.12) 1 2 D Ω | u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) ( ϕ ( x ) - ϕ ( y ) ) 𝑑 ν = Ω f n ( x ) ϕ ( x ) 𝑑 x .

We set Φ ( x , y ) = ϕ ( x ) - ϕ ( y ) . By (3.12), we have

(3.13) 1 2 D Ω U ( x , y ) Φ ( x , y ) 𝑑 ν + 1 2 D Ω ( U n ( x , y ) - U ( x , y ) ) Φ ( x , y ) 𝑑 ν = Ω f n ( x ) ϕ ( x ) 𝑑 x .

It is clear that

Ω f n ( x ) ϕ ( x ) 𝑑 x Ω f ( x ) ϕ ( x ) 𝑑 x as  n .

We claim that

(3.14) D Ω ( U n ( x , y ) - U ( x , y ) ) Φ ( x , y ) 𝑑 ν 0 as  n .

Since u n u a.e. in Ω, it follows that

U n ( x , y ) Φ ( x , y ) | x - y | N + p s | x | β | y | β U ( x , y ) Φ ( x , y ) | x - y | N + p s | x | β | y | β a.e. in  D Ω .

Using the fact that u ( x ) = u n ( x ) = ϕ ( x ) = 0 for all x N Ω , we obtain

N Ω N Ω ( U n ( x , y ) - U ( x , y ) ) Φ ( x , y ) 𝑑 ν = 0 .

Thus,

D Ω ( U n ( x , y ) - U ( x , y ) ) Φ ( x , y ) 𝑑 ν
= Ω × Ω ( U n ( x , y ) - U ( x , y ) ) Φ ( x , y ) 𝑑 ν + N Ω Ω ( U n ( x , y ) - U ( x , y ) ) Φ ( x , y ) 𝑑 ν
    + Ω N Ω ( U n ( x , y ) - U ( x , y ) ) Φ ( x , y ) 𝑑 ν
= I 1 ( n ) + I 2 ( n ) + I 3 ( n ) .

Using Lemma 3.2 and Remark 3.3, we easily find that

I 1 ( n ) 0 as  n .

We now deal with I 2 ( n ) . It is clear that for ( x , y ) Ω × B R Ω we have

| ( U n ( x , y ) - U ( x , y ) ) Φ ( x , y ) | ( | u n ( x ) | p - 1 + | u ( x ) | p - 1 ) | ϕ ( x ) | .

Since

sup { x Supp ϕ , y B R Ω } 1 | x - y | N + p s C ,

we have

| ( U n ( x , y ) - U ( x , y ) ) Φ ( x , y ) | x - y | N + p s | x | β | y | β | ( | u n ( x ) | p - 1 + | u ( x ) | p - 1 ) | ϕ ( x ) | | x | β | y | β Q n ( x , y ) .

Using Lemma 3.1 and Remark 3.3, we get Q n Q strongly in L 1 ( Ω × B R Ω ) with

Q ( x , y ) = 2 | u ( x ) | p - 1 | ϕ ( x ) | | x | - β | y | - β .

Thus by the dominated convergence theorem we arrive to I 2 ( n ) 0  as  n . In the same way, we obtain that I 3 ( n ) 0  as  n . Hence (3.14) follows and the claim is proved.

Therefore, passing to the limit in (3.13), we arrive at

1 2 D Ω U ( x , y ) Φ ( x , y ) 𝑑 ν = Ω f ( x ) ϕ ( x ) 𝑑 x .

Remark 3.4.

(i) It is clear that the same existence result holds if we replace f by a bounded Radon measure ν.

(ii) In the case where β = 0 , we get the same existence and regularity results as were obtained in [21].

3.1 The case of positive datum: Existence and uniqueness of the positive entropy solution

If f 0 , we choose f n = T n ( f ) , thus { u n } n is an increasing sequence. In this case, we are able to prove that problem (3.1) has a unique entropy positive solution in the sense of Definition 2.9. Before beginning with the proof of Theorem 1.2, let us prove the following compactness result.

Lemma 3.5.

Let { u n } n and u be defined as above. Then

T k ( u n ) T k ( u ) strongly in  W β , 0 s , p ( Ω ) .

The proof of Lemma 3.5 will be a consequence of the following more general compactness result.

Lemma 3.6.

Let { u n } n W β , 0 s , p ( Ω ) be an increasing sequence such that u n 0 and ( - Δ ) p , β s u n 0 . Assume further that { T k ( u n ) } n is bounded in W β , 0 s , p ( Ω ) for all k > 0 . Then there exists a measurable function u such that u n u a.e. in Ω, T k ( u ) W β , 0 s , p ( Ω ) for all k > 0 and

T k ( u n ) T k ( u ) strongly in  W β , 0 s , p ( Ω ) .

Proof.

Since { T k ( u n ) } n is bounded in W β , 0 s , p ( Ω ) , using the monotony of the sequence { u n } n , we get the existence of a measurable function u such that u n u a.e. in Ω, T k ( u ) W β , 0 s , p ( Ω ) and T k ( u n ) T k ( u ) weakly in W β , 0 s , p ( Ω ) . Since ( - Δ ) p , β s u n 0 , we have

( - Δ ) p , β s u n , T k ( u n ) - T k ( u ) 0 .

Thus,

D Ω | u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) ( T k ( u n ( x ) ) - T k ( u n ( y ) ) ) 𝑑 ν
(3.15) D Ω | u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) ( T k ( u ( x ) ) - T k ( u ( y ) ) ) 𝑑 ν .

Define

I 1 , n D Ω | u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) ( T k ( u n ( x ) ) - T k ( u n ( y ) ) ) 𝑑 ν

and

I 2 , n D Ω | u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) ( T k ( u ( x ) ) - T k ( u ( y ) ) ) 𝑑 ν .

For simplicity of notation, we set

T n , k ( x , y ) | T k ( u n ( x ) ) - T k ( u n ( y ) ) | p - 2 ( T k ( u n ( x ) ) - T k ( u n ( y ) ) ) .

We have

I 1 , n = D Ω | T k ( u n ( x ) ) - T k ( u n ( y ) ) | p 𝑑 ν + D Ω [ U n ( x , y ) - T n , k ( x , y ) ] ( T k ( u n ( x ) ) - T k ( u n ( y ) ) ) 𝑑 ν .

In the same way, using Young inequality, we obtain that

I 2 , n = D Ω T n , k ( x , y ) ( T k ( u ( x ) ) - T k ( u ( y ) ) ) 𝑑 ν + D Ω [ U n ( x , y ) - T n , k ( x , y ) ] ( T k ( u ( x ) ) - T k ( u ( y ) ) ) 𝑑 ν
p - 1 p D Ω | T k ( u n ( x ) ) - T k ( u n ( y ) ) | p 𝑑 ν + 1 p D Ω | T k ( u ( x ) ) - T k ( u ( y ) ) | p 𝑑 ν
+ D Ω [ U n ( x , y ) - T n , k ( x , y ) ] ( T k ( u ( x ) ) - T k ( u ( y ) ) ) 𝑑 ν .

Combining the above estimates and going back to (3.15), we find

1 p D Ω | T k ( u n ( x ) ) - T k ( u n ( y ) ) | p 𝑑 ν + D Ω [ U n ( x , y ) - T n , k ( x , y ) ]
× [ ( T k ( u n ( x ) ) - T k ( u ( x ) ) ) - ( T k ( u n ( y ) ) - T k ( u ( y ) ) ) ] d ν
(3.16)     1 p D Ω | T k ( u ( x ) ) - T k ( u ( y ) ) | p d ν .

Define

(3.17) K n ( x , y ) [ U n ( x , y ) - T n , k ( x , y ) ] [ ( T k ( u n ( x ) ) - T k ( u ( x ) ) ) - ( T k ( u n ( y ) ) - T k ( u ( y ) ) ) ] .

We claim that K n ( x , y ) 0 a.e. in D Ω . We set

D 1 = { ( x , y ) D Ω : u n ( x ) k , u n ( y ) k } , D 2 = { ( x , y ) D Ω : u n ( x ) k , u n ( y ) k } ,
D 3 = { ( x , y ) D Ω : u n ( x ) k , u n ( y ) k } , D 4 = { ( x , y ) D Ω : u n ( x ) k , u n ( y ) k } .

Then D Ω = D 1 D 2 D 3 D 4 .

In D 1 we have U n ( x , y ) - T n , k ( x , y ) = 0 . Then K n ( x , y ) = 0 . In the same way, if ( x , y ) D 2 , we have u ( x ) u n ( x ) k and u ( y ) u n ( y ) k . Then

[ ( T k ( u n ( x ) ) - T k ( u ( x ) ) ) - ( T k ( u n ( y ) ) - T k ( u ( y ) ) ) ] = 0 .

Thus, K n ( x , y ) = 0 in D 2 .

Assume that ( x , y ) D 3 . Then

U n ( x , y ) - T n , k ( x , y ) = ( u n ( x ) - u n ( y ) ) p - 1 - ( k - u n ( y ) ) p - 1 0 .

Since

[ ( T k ( u n ( x ) ) - T k ( u ( x ) ) ) - ( T k ( u n ( y ) ) - T k ( u ( y ) ) ) ] = - ( T k ( u n ( y ) ) - T k ( u ( y ) ) ) 0

by (3.17), it follows that K n ( x , y ) 0 in D 3 . In the same way, we can prove that K n ( x , y ) 0 in D 4 . Thus K n ( x , y ) 0 a.e. in D Ω and the claim follows.

Going back to (3.16), there results that

lim sup n D Ω | T k ( u n ( x ) ) - T k ( u n ( y ) ) | p 𝑑 ν D Ω | T k ( u ( x ) ) - T k ( u ( y ) ) | p 𝑑 ν .

Since T k ( u n ) T k ( u ) weakly in W β , 0 s , p ( Ω ) , we obtain T k ( u n ) T k ( u ) strongly in W β , 0 s , p ( Ω ) . ∎

Remark 3.7.

(i) As a consequence of the previous strong convergence we reach that

D Ω K n ( x , y ) 𝑑 ν 0 as  n .

(ii) Letting w n = 1 - 1 1 + u n and w = 1 - 1 1 + u , and using w n as a test function in (3.2), we have

D Ω | u n ( x ) - u n ( y ) | p ( 1 + u n ( x ) ) ( 1 + u n ( y ) ) 𝑑 ν = Ω f n ( x ) w n ( x ) 𝑑 x Ω f ( x ) w ( x ) 𝑑 x as  n .

For k > 0 fixed, we define the sets

A n = D Ω { u n ( x ) 2 k , u n ( y ) k } and A = D Ω { u ( x ) 2 k , u ( y ) k } .

It is clear that for ( x , y ) A n we have u n ( x ) - u n ( y ) 1 2 u n ( x ) . Thus,

(3.18) D Ω u n p - 1 ( x ) χ A n ( x , y ) 𝑑 ν C ( k ) D Ω | u n ( x ) - u n ( y ) | p ( 1 + u n ( x ) ) ( 1 + u n ( y ) ) 𝑑 ν < C ¯ ( k ) .

Since u n χ { A n } ( x , y ) u χ { A } a.e. in D Ω , if p > 2 , we get

u n χ { A n } u χ { A } weakly in  L p - 1 ( D Ω , d ν ) .

(iii) From (3.18) we conclude that

ν { D Ω A n } D Ω A n 𝑑 ν C ~ ( k ) .

Hence by Fatou’s lemma, we reach that

ν { D Ω A } D Ω A 𝑑 ν C ~ ( k ) .

Now, we are in the position to prove the existence and the uniqueness of the entropy solution.

Proof of Theorem 1.2: Existence part.

It is clear that the existence of u follows by using Theorem 1.1, however the strong convergence of { T k ( u n ) } n in the space W β , 0 s , p ( Ω ) is a consequence of Lemma 3.5. To finish we just need to show that u is an entropy solution to problem (3.1) in the sense of Definition 2.9.

Let us begin by proving that (2.5) holds.

Since u , u n 0 , the set R h given in Definition 2.9 is reduced to

R h = { ( x , y ) N × N : h + 1 max { u ( x ) , u ( y ) }  with  min { u ( x ) , u ( y ) } h } .

Using T 1 ( G h ( u n ) ) as a test function in (3.2), we obtain

1 2 D Ω | u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) [ T 1 ( G h ( u n ( x ) ) ) - T 1 ( G h ( u n ( y ) ) ) ] 𝑑 ν
(3.19) = Ω f n ( x ) T 1 ( G h ( u n ( x ) ) ) 𝑑 x u n h f n ( x ) 𝑑 x .

It is not difficult to show that, for ( x , y ) R h , we have

| u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) [ T 1 ( G h ( u n ( x ) ) ) - T 1 ( G h ( u n ( y ) ) ) ] 0 .

Thus, using Fatou’s lemma and by (3.19), we conclude that

1 2 D Ω | u ( x ) - u ( y ) | p - 2 ( u ( x ) - u ( y ) ) [ T 1 ( G h ( u ( x ) ) ) - T 1 ( G h ( u ( y ) ) ) ] 𝑑 ν
lim inf n 1 2 D Ω | u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) [ T 1 ( G h ( u n ( x ) ) ) - T 1 ( G h ( u n ( y ) ) ) ] 𝑑 ν
(3.20) Ω f ( x ) T 1 ( G h ( u ( x ) ) ) 𝑑 x u h f ( x ) 𝑑 x .

It is clear that for all ( x , y ) R h we have

| u ( x ) - u ( y ) | p - 2 ( u ( x ) - u ( y ) ) [ T 1 ( G h ( u ( x ) ) ) - T 1 ( G h ( u ( y ) ) ) ] | u ( x ) - u ( y ) | p - 1 .

Therefore, using the fact that

u h f ( x ) 𝑑 x 0 as  h

and by (3.20), we conclude that

R h | u ( x ) - u ( y ) | p - 1 𝑑 ν 0 as  h .

Hence (2.5) holds.

Recall that

U n ( x , y ) = | u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) and U ( x , y ) = | u ( x ) - u ( y ) | p - 2 ( u ( x ) - u ( y ) ) .

Let v W β , 0 s , p ( Ω ) L ( Ω ) . Taking T k ( u n - v ) as a test function in (3.2), we reach that

1 2 D Ω U n ( x , y ) [ T k ( u n ( x ) - v ( x ) ) - T k ( u n ( y ) - v ( y ) ) ] 𝑑 ν = Ω f n ( x ) T k ( u n ( x ) - v ( x ) ) 𝑑 x .

One immediately sees that

Ω f n ( x ) T k ( u n ( x ) - v ( x ) ) 𝑑 x Ω f ( x ) T k ( u ( x ) - v ( x ) ) 𝑑 x as  n .

We now deal with the first term. We have

(3.21) U n ( x , y ) [ T k ( u n ( x ) - v ( x ) ) - T k ( u n ( y ) - v ( y ) ) ] = : K 1 , n ( x , y ) + K 2 , n ( x , y ) ,

where

K 1 , n ( x , y ) = | ( u n ( x ) - v ( x ) ) - ( u n ( y ) - v ( y ) ) | p - 2 ( ( u n ( x ) - v ( x ) ) - ( u n ( y ) - v ( y ) ) )
× [ T k ( u n ( x ) - v ( x ) ) - T k ( u n ( y ) - v ( y ) ) ]

and

K 2 , n ( x , y ) = [ U n ( x , y ) - | ( u n ( x ) - v ( x ) ) - ( u n ( y ) - v ( y ) ) | p - 2 ( ( u n ( x ) - v ( x ) ) - ( u n ( y ) - v ( y ) ) ) ]
× [ T k ( u n ( x ) - v ( x ) ) - T k ( u n ( y ) - v ( y ) ) ] .

It is clear that K 1 , n ( x , y ) 0 a.e. in D Ω . Since

K 1 , n ( x , y ) | ( u ( x ) - v ( x ) ) - ( u ( y ) - v ( y ) ) | p - 2 ( ( u ( x ) - v ( x ) ) - ( u ( y ) - v ( y ) ) )
× [ T k ( u ( x ) - v ( x ) ) - T k ( u ( y ) - v ( y ) ) ] a.e. in  D Ω

as n , using Fatou’s lemma, we obtain that

D Ω K 1 , n ( x , y ) 𝑑 ν D Ω [ | ( u ( x ) - v ( x ) ) - ( u ( y ) - v ( y ) ) | p - 2 ( ( u ( x ) - v ( x ) ) - ( u ( y ) - v ( y ) ) ) ]
(3.22) × [ T k ( u ( x ) - v ( x ) ) - T k ( u ( y ) - v ( y ) ) ] d ν .

We now deal with K 2 , n .

We set

w n = u n - v , w = u - v , σ 1 ( x , y ) = u n ( x ) - u n ( y ) , σ 2 ( x , y ) = w n ( x ) - w n ( y ) .

Then

K 2 , n ( x , y ) = [ | σ 1 ( x , y ) | p - 2 σ 1 ( x , y ) - | σ 2 ( x , y ) | p - 2 σ 2 ( x , y ) ] × [ T k ( u n ( x ) - v ( x ) ) - T k ( u n ( y ) - v ( y ) ) ] .

We claim that

D Ω K 2 , n ( x , y ) 𝑑 ν D Ω [ U ( x , y ) - | ( u ( x ) - v ( x ) ) - ( u ( y ) - v ( y ) ) | p - 2 ( ( u ( x ) - v ( x ) ) - ( u ( y ) - v ( y ) ) ) ]
(3.23) × [ T k ( u ( x ) - v ( x ) ) - T k ( u ( y ) - v ( y ) ) ] d ν as  n .

We divide the proof of the claim into two cases according to the value of p.

The singular case: p ( 1 , 2 ] .

In this case, we have

| | σ 1 ( x , y ) | p - 2 σ 1 ( x , y ) - | σ 2 ( x , y ) | p - 2 σ 2 ( x , y ) | C | σ 1 ( x , y ) - σ 2 ( x , y ) | p - 1 = C | v ( x ) - v ( y ) | p - 1 .

Thus,

(3.24) | K 2 , n ( x , y ) | C | v ( x ) - v ( y ) | p - 1 | T k ( u n ( x ) - v ( x ) ) - T k ( u n ( y ) - v ( y ) ) | K ~ 2 , n ( x , y ) .

Since T k ( u n ) T k ( u ) strongly in W β , 0 s , p ( Ω ) , we get

K ~ 2 , n C | v ( x ) - v ( y ) | p - 1 | T k ( u ( x ) - v ( x ) ) - T k ( u ( y ) - v ( y ) ) | strongly in  L 1 ( D Ω , d ν )

as v W β , 0 s , p ( Ω ) L ( Ω ) . Using the dominated convergence theorem, we reach that

D Ω K 2 , n ( x , y ) 𝑑 ν D Ω [ U ( x , y ) - | ( u ( x ) - v ( x ) ) - ( u ( y ) - v ( y ) ) | p - 2 ( ( u ( x ) - v ( x ) ) - ( u ( y ) - v ( y ) ) ) ]
× [ T k ( u ( x ) - v ( x ) ) - T k ( u ( y ) - v ( y ) ) ] d ν

as n and then (3.23) follows in this case.

The degenerate case: p > 2 . This case is more relevant. As in the previous case, we have

| | σ 1 ( x , y ) | p - 2 σ 1 ( x , y ) - | σ 2 ( x , y ) | p - 2 σ 2 ( x , y ) | C 1 | σ 1 ( x , y ) - σ 2 ( x , y ) | p - 1 + C 2 | σ 2 ( x , y ) | p - 2 | σ 1 ( x , y ) - σ 2 ( x , y ) |
C 1 | v ( x ) - v ( y ) | p - 1 + C 2 | v ( x ) - v ( y ) | | w n ( x ) - w n ( y ) | p - 2
C 1 | v ( x ) - v ( y ) | p - 1 + C 2 | v ( x ) - v ( y ) | | u n ( x ) - u n ( y ) | p - 2 .

Thus,

| K 2 , n ( x , y ) | C 1 | v ( x ) - v ( y ) | p - 1 | T k ( u n ( x ) - v ( x ) ) - T k ( u n ( y ) - v ( y ) ) |
+ C 2 | v ( x ) - v ( y ) | | u n ( x ) - u n ( y ) | p - 2 | T k ( w n ( x ) ) - T k ( w n ( y ) ) |
K ¯ 2 , n ( x , y ) + K ˇ 2 , n ( x , y ) .

The term K ¯ 2 , n ( x , y ) can be treated as K ~ 2 , n defined in (3.24). Hence it remains to deal with K ˇ 2 , n ( x , y ) .

We define

D 1 = { ( x , y ) D Ω : u n ( x ) k ~ , u n ( y ) k ~ } ,

where k ~ k + v is a large constant. Using the fact that T k ~ ( u n ) T k ~ ( u ) strongly in W β , 0 s , p ( Ω ) , we obtain that

K ˇ 2 , n ( x , y ) χ { D 1 } C 2 | v ( x ) - v ( y ) | | u ( x ) - u ( y ) | p - 2 | T k ( w ( x ) ) - T k ( w ( y ) ) | χ { u ( x ) k ~ , u ( y ) k ~ }

strongly in L 1 ( D Ω , d ν ) .

Now, consider the set

D 2 = { ( x , y ) D Ω : u n ( x ) k 1 , u n ( y ) k 1 } ,

where k 1 > k + v . Then K ˇ 2 , n ( x , y ) χ { D 2 } = 0 .

Hence we just have to deal with sets of the form

D 3 = { ( x , y ) D Ω : u n ( x ) 2 k , u n ( y ) k }

or

D 4 = { ( x , y ) D Ω : u n ( y ) 2 k , u n ( x ) k } .

We will use Remark 3.7 and a duality argument.

It is clear that for ( x , y ) D 3 we have

K ˇ 2 , n ( x , y ) χ { D 3 } ( x , y ) C ( k ) | v ( x ) - v ( y ) | | T k ( w n ( x ) ) - T k ( w n ( y ) ) | u n p - 2 ( x ) χ { D 3 } ( x , y ) .

From Remark 3.7, we know that

(3.25) u n p - 2 χ { D 3 } u p - 2 χ { u ( x ) 2 k , u ( y ) k } weakly in  L p - 1 p - 2 ( D Ω , d ν ) .

Notice that

[ | v ( x ) - v ( y ) | | T k ( w n ( x ) ) - T k ( w n ( y ) ) | ] p - 1 p - 1 p | T k ( w n ( x ) ) - T k ( w n ( y ) ) | p + 1 p | v ( x ) - v ( y ) | p ( p - 1 )
p - 1 p | T k ( w n ( x ) ) - T k ( w n ( y ) ) | p + 1 p ( 2 v ) p ( p - 2 ) | v ( x ) - v ( y ) | p
= : L n ( x , y ) .

It is clear that L n L  strongly in  L 1 ( D Ω , d ν ) with

(3.26) L ( x , y ) = p - 1 p | T k ( w ( x ) ) - T k ( w ( y ) ) | p + 1 p ( 2 v ) p ( p - 2 ) | v ( x ) - v ( y ) | p .

Thus by (3.25), (3.26) and using a duality argument, we find that

K ˇ 2 , n χ { D 3 } C 2 | v ( x ) - v ( y ) | | u ( x ) - u ( y ) | p - 2 | T k ( w ( x ) ) - T k ( w ( y ) ) | χ { u ( x ) 2 k , u ( y ) k }

strongly in L 1 ( D Ω , d ν ) .

We can treat the set D 4 in the same way.

Therefore, combining the above estimates and using the dominated convergence theorem, we conclude that

D Ω K 2 , n ( x , y ) 𝑑 ν D Ω [ U ( x , y ) - | ( u ( x ) - v ( x ) ) - ( u ( y ) - v ( y ) ) | p - 2 ( ( u ( x ) - v ( x ) ) - ( u ( y ) - v ( y ) ) ) ]
× [ T k ( u ( x ) - v ( x ) ) - T k ( u ( y ) - v ( y ) ) ] d ν as  n ,

and the claim follows.

Hence by (3.21)–(3.23), we conclude that

(3.27) 1 2 D Ω U ( x , y ) [ T k ( u ( x ) - v ( x ) ) - T k ( u ( y ) - v ( y ) ) ] 𝑑 ν Ω f ( x ) T k ( u ( x ) - v ( x ) ) 𝑑 x

and the result follows at once. ∎

It is clear that if u is an entropy solution of (3.1), then for all w C 0 ( Ω ) we have

(3.28) 1 2 D Ω U ( x , y ) ( w ( x ) - w ( y ) ) 𝑑 ν = Ω f ( x ) w ( x ) 𝑑 x ,

where U ( x , y ) = | u ( x ) - u ( y ) | p - 2 ( u ( x ) - u ( y ) ) .

Moreover, we can prove that (3.28) holds for all w W β , 0 s , p ( Ω ) L ( Ω ) such that w 0 in the set { u > k } for some k > 0 . More precisely we have the following lemma.

Lemma 3.8.

Assume that u is an entropy solution to (3.1) with f 0 . Then for all w W β , 0 s , p ( Ω ) L ( Ω ) such that for some k > 0 we have w 0 in the set { u > k } , we obtain

(3.29) 1 2 D Ω U ( x , y ) ( w ( x ) - w ( y ) ) 𝑑 ν = Ω f ( x ) w ( x ) 𝑑 x .

Proof.

Let w W β , 0 s , p ( Ω ) L ( Ω ) be such that w 0 in the set { u > k 0 } for some k 0 > 0 . Define v h = T h ( u - w ) with h k 0 + w + 1 .

Since u is an entropy solution to (3.1), by (3.27), for k fixed such that k max { k 0 , w } , we have

(3.30) 1 2 D Ω U ( x , y ) [ T k ( u ( x ) - v h ( x ) ) - T k ( u ( y ) - v h ( y ) ) ] 𝑑 ν Ω f ( x ) T k ( u ( x ) - v h ( x ) ) 𝑑 x .

It is clear that

Ω f ( x ) T k ( u ( x ) - v h ( x ) ) 𝑑 x Ω f ( x ) w 𝑑 x as  h .

Notice that for h w we have { u w - h } = , thus for h as above there results that

{ | u ( x ) - w ( x ) | h } { ( u ( x ) - w ( x ) ) h } .

Define

A h { ( x , y ) D Ω : | u ( x ) - w ( x ) | < h , | u ( y ) - w ( y ) | < h } ,
B h { ( x , y ) D Ω : | u ( x ) - w ( x ) | h , | u ( y ) - w ( y ) | h }
= { ( x , y ) D Ω : ( u ( x ) - w ( x ) ) h , ( u ( y ) - w ( y ) ) h } ,
E h { ( x , y ) D Ω : ( u ( x ) - w ( x ) ) h , | u ( y ) - w ( y ) | h } ,
F h { ( x , y ) D Ω : | u ( x ) - w ( x ) | < h , ( u ( y ) - w ( y ) ) > h } .

Then

D Ω U ( x , y ) [ T k ( u ( x ) - v h ( x ) ) - T k ( u ( y ) - v h ( y ) ) ] 𝑑 ν = A h + B h + E h + F h = I A h + I B h + I E h + I F h .

It is clear that

I A h = A h U ( x , y ) [ T k ( w ( x ) ) - T k ( w ( y ) ) ] 𝑑 ν = A h U ( x , y ) [ w ( x ) - w ( y ) ] 𝑑 ν
= A h { u ( x ) < k 0 , u ( y ) < k 0 } U ( x , y ) [ w ( x ) - w ( y ) ] 𝑑 ν + A h { u ( x ) > k 0 , u ( y ) > k 0 } U ( x , y ) [ w ( x ) - w ( y ) ] 𝑑 ν
+ A h { u ( x ) > k 0 , u ( y ) k 0 } U ( x , y ) [ w ( x ) - w ( y ) ] 𝑑 ν + A h { u ( x ) k 0 , u ( y ) > k 0 } U ( x , y ) [ w ( x ) - w ( y ) ] 𝑑 ν
= I 1 ( h ) + I 2 ( h ) + I 3 ( h ) + I 4 ( h ) .

Since T k ( u ) W β , 0 s , p ( Ω ) , we have

I 1 ( h ) { u ( x ) < k 0 , u ( y ) < k 0 } U ( x , y ) [ w ( x ) - w ( y ) ] 𝑑 ν as  h .

Using the properties of w, we have I 2 ( h ) = 0 . Let us consider now I 3 ( h ) . We have

I 3 ( h ) = A h { k 0 < u ( x ) < 2 k 0 , u ( y ) k 0 } U ( x , y ) [ w ( x ) - w ( y ) ] 𝑑 ν + A h { u ( x ) > 2 k 0 , u ( y ) k 0 } U ( x , y ) [ w ( x ) - w ( y ) ] 𝑑 ν
= J 1 ( h ) + J 2 ( h ) .

As above, since T k ( u ) W β , 0 s , p ( Ω ) , we obtain

(3.31) J 1 ( h ) { k 0 < u ( x ) < 2 k 0 , u ( y ) < k 0 } U ( x , y ) [ w ( x ) - w ( y ) ] 𝑑 ν as  h .

For J 2 ( h ) we have

| U ( x , y ) [ w ( x ) - w ( y ) ] | w | U ( x , y ) | = w | u ( x ) - u ( y ) | p - 1 .

Using the fact that

{ u ( x ) > 2 k 0 , u ( y ) k 0 } | U ( x , y ) | 𝑑 ν < ,

by the dominated convergence theorem, we conclude that

(3.32) J 2 ( h ) { u ( x ) 2 k 0 , u ( y ) < k 0 } U ( x , y ) [ w ( x ) - w ( y ) ] 𝑑 ν as  h .

Hence by (3.31) and (3.32), we obtain that

I 3 ( h ) { k 0 < u ( x ) < 2 k 0 , u ( y ) < k 0 } U ( x , y ) [ w ( x ) - w ( y ) ] 𝑑 ν + { u ( x ) 2 k 0 , u ( y ) < k 0 } U ( x , y ) [ w ( x ) - w ( y ) ] 𝑑 ν

as h .

We can treat I 4 ( h ) in the same way. Hence,

I A h D Ω U ( x , y ) [ w ( x ) - w ( y ) ] 𝑑 ν as  h .

We now deal with I B h . It is clear that if ( x , y ) B h , then v h ( x ) = v h ( y ) = h , hence

I B h = B h U ( x , y ) [ T k ( u ( x ) - h ) - T k ( u ( y ) - h ) ] 𝑑 ν 0 .

Now, for ( x , y ) E h , we have u ( x ) h - w > k 0 , thus w ( x ) = 0 . Hence,

E h E h { u ( y ) < h - w - 1 } E h { h u ( x ) , u ( y ) h - w - 1 }
E 1 ( h ) E 2 ( h ) .

It is clear that for ( x , y ) E 2 ( h ) we have w ( x ) = w ( y ) = 0 . Then

U ( x , y ) [ T k ( u ( x ) - v ( x ) ) - T k ( u ( y ) - v h ( y ) ) ] = U ( x , y ) [ T k ( G h ( u ( x ) ) ) - T k ( G h ( u ( y ) ) ) ] 0 .

Thus,

E 2 ( h ) U ( x , y ) [ T k ( u ( x ) - v ( x ) ) - T k ( u ( y ) - v h ( y ) ) ] 𝑑 ν 0 .

Therefore, we conclude that

I E h E 1 ( h ) U ( x , y ) [ T k ( G h ( u ( x ) ) ) - T k ( w ( y ) ) ] 𝑑 ν - 2 k E 1 ( h ) | U ( x , y ) | 𝑑 ν .

Let h 1 = h - w - 1 . Then by (2.5) we reach that

E 1 ( h ) | U ( x , y ) | 𝑑 ν u ( x ) > h 1 , u ( y ) < h 1 - 1 | U ( x , y ) | 𝑑 ν 0 as  h .

Thus, I E h o ( h ) .

In the same way, we can prove that I F h o ( h ) . Therefore, we reach that

lim inf h 1 2 D Ω U ( x , y ) [ T k ( u ( x ) - v ( x ) ) - T k ( u ( y ) - v h ( y ) ) ] 𝑑 ν 1 2 D Ω U ( x , y ) ( w ( x ) - w ( y ) ) 𝑑 ν .

As a conclusion, and going back to (3.30), we have proved that

1 2 D Ω U ( x , y ) ( w ( x ) - w ( y ) ) 𝑑 ν Ω f ( x ) w ( x ) 𝑑 x .

Substituting w by - w in the above inequality, we obtain that

1 2 D Ω U ( x , y ) ( w ( x ) - w ( y ) ) 𝑑 ν = Ω f ( x ) w ( x ) 𝑑 x ,

which is the desired result. ∎

Now we are in a position to prove the uniqueness result in Theorem 1.2.

Proof of Theorem 1.2: Uniqueness part.

Let u be the entropy positive solution defined in Theorem 1.1. Recall that u = lim sup u n , where u n is the unique solution to the approximated problem (3.2).

Assume that v is another entropy positive solution to problem (3.1). We claim that u n v for all n. To prove the claim, we fix n and define w n = ( u n - v ) + . Then w n = ( u n - T k ( v ) ) + , where k u n . Hence w n W β , 0 s , p ( Ω ) L ( Ω ) and w n 0 in the set { v > u n } . Therefore, using w n as a test function in (3.2) and taking into consideration the identity (3.29) in Lemma 3.8, we reach that

1 2 D Ω U n ( x , y ) ( w n ( x ) - w n ( y ) ) 𝑑 ν = Ω f n ( x ) w n ( x ) 𝑑 x
Ω f ( x ) w n ( x ) 𝑑 x = 1 2 D Ω V ( x , y ) ( w n ( x ) - w n ( y ) ) 𝑑 ν ,

where

U n ( x , y ) = | u n ( x ) - u n ( y ) | p - 2 ( u n ( x ) - u n ( y ) ) and V ( x , y ) = | v ( x ) - v ( y ) | p - 2 ( v ( x ) - v ( y ) ) .

Thus,

1 2 D Ω ( U n ( x , y ) - V ( x , y ) ) ( w n ( x ) - w n ( y ) ) 𝑑 ν 0 .

Using the fact that

( U n ( x , y ) - V ( x , y ) ) ( w n ( x ) - w n ( y ) ) C | w n ( x ) - w n ( y ) | p ,

it follows that w n 0 , hence u n v for all n and the claim follows. As a consequence we obtain that u v .

Let us now prove that v u . To this end, we will follow closely the argument used in [7].

Since u and v are entropy solutions to (3.1), for h k , we have

(3.33) 1 2 D Ω U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] 𝑑 ν Ω f ( x ) T k ( u ( x ) - T h ( v ( x ) ) ) 𝑑 x

and

(3.34) 1 2 D Ω V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] 𝑑 ν Ω f ( x ) T k ( v ( x ) - T h ( u ( x ) ) ) 𝑑 x .

It is clear that

Ω f ( x ) T k ( u ( x ) - T h ( v ( x ) ) ) 𝑑 x + Ω f ( x ) T k ( v ( x ) - T h ( u ( x ) ) ) 𝑑 x 0 as  h .

Thus from (3.33) and (3.34), we have

I ( h ) 1 2 D Ω U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] 𝑑 ν
+ 1 2 D Ω V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] 𝑑 ν
(3.35) = P ( h ) + Q ( h ) o ( h ) .

Let

D Ω 1 ( h ) { ( x , y ) D Ω : u ( x ) < h  and  u ( y ) < h }

and

D Ω 2 ( h ) { ( x , y ) D Ω : v ( x ) < h  and  v ( y ) < h } .

Then

P ( h ) = D Ω 1 ( h ) U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] 𝑑 ν
+ D Ω D Ω 1 ( h ) U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] 𝑑 ν
= P 1 ( h ) + P 2 ( h )

and

Q ( h ) = D Ω 2 ( h ) V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] 𝑑 ν
+ D Ω D Ω 2 ( h ) V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] 𝑑 ν
= Q 1 ( h ) + Q 2 ( h ) .

We claim that P 2 ( h ) o ( h ) and Q 2 ( h ) o ( h ) .

Let us begin by proving that P 2 ( h ) o ( h ) . Recall that u v . Then

D Ω D Ω 1 ( h ) = { ( x , y ) D Ω : u ( x ) h } { ( x , y ) D Ω : u ( y ) h } .

If u ( x ) h and u ( y ) h , then v ( x ) h and v ( y ) h . Thus,

U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] = U ( x , y ) [ T k ( u ( x ) - h ) - T k ( u ( y ) - h ) ] 0 .

On the other hand, by (2.5), we find

{ u ( x ) > h , u ( y ) < h - 1 } | U ( x , y ) | 𝑑 ν = o ( h ) .

Hence,

{ u ( x ) h } U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] 𝑑 ν
(3.36) { u ( x ) h , h - 1 u ( y ) h } U ( x , y ) [ T k ( u ( x ) - h ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] 𝑑 ν + o ( h ) .

Notice that for ( x , y ) { u ( x ) h , h - 1 u ( y ) h } we have

U ( x , y ) [ T k ( u ( x ) - h ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] = U ( x , y ) [ T k ( u ( x ) - h ) + T k ( T h ( v ( y ) ) - u ( y ) ) ] 0 .

Then by (3.36) there results that

(3.37) { u ( x ) h } U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] 𝑑 ν o ( h ) .

In the same way, we can prove that

(3.38) { u ( y ) h } U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] 𝑑 ν o ( h ) .

Thus combining (3.37) and (3.38), we arrive at P 2 ( h ) o ( h ) as claimed.

We now deal with Q 2 ( h ) . Recall that

Q 2 ( h ) = D Ω D Ω 2 ( h ) V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] 𝑑 ν .

As above, we have

D Ω D Ω 2 ( h ) = { ( x , y ) D Ω : v ( x ) h } { ( x , y ) D Ω : v ( y ) h } M 1 ( h ) M 2 ( h ) M 3 ( h ) ,

where

M 1 ( h ) = { ( x , y ) D Ω : v ( x ) h  and  v ( y ) h } , M 2 ( h ) = { ( x , y ) D Ω : v ( x ) h  and  v ( y ) < h } ,

and

M 3 ( h ) = { ( x , y ) D Ω : v ( x ) < h  and  v ( y ) h } .

Let

Z 1 ( h ) = { ( x , y ) D Ω : v ( x ) - T h ( u ( x ) ) k } and Z 2 ( h ) = { ( x , y ) D Ω : v ( y ) - T h ( u ( y ) ) k } .

If ( x , y ) Z 1 ( h ) Z 2 ( h ) , we have

V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] = 0 .

Hence we can assume that

( x , y ) ( Z 1 ( h ) Z 2 ( h ) ) ( Z 2 ( h ) Z 1 ( h ) ) Y 1 ( h ) Y 2 ( h ) .

Therefore, we conclude that

Q 2 ( h ) = M 1 ( h ) Y 1 ( h ) + M 2 ( h ) Y 1 ( h ) + M 3 ( h ) Y 1 ( h ) + M 1 ( h ) Y 2 ( h ) + M 2 ( h ) Y 2 ( h ) + M 3 ( h ) Y 2 ( h )
(3.39) = J 1 ( h ) + J 2 ( h ) + J 3 ( h ) + T 1 ( h ) + T 2 ( h ) + T 3 ( h ) .

Let us begin by proving that J 1 ( h ) o ( h ) . Notice that

M 1 ( h ) Y 1 ( h ) = { ( x , y ) D Ω : v ( x ) h , v ( y ) h  and  v ( x ) - T h ( u ( x ) ) k , v ( y ) - T h ( u ( y ) ) < k } .

Then for ( x , y ) M 1 ( h ) Y 1 ( h ) we have

V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] = V ( x , y ) [ k - ( v ( y ) - T h ( u ( y ) ) ) ]

If v ( x ) v ( y ) , then V ( x , y ) [ k - ( v ( y ) - T h ( u ( y ) ) ) ] 0 . Therefore, we just have to consider the case where ( x , y ) M 1 ( h ) Y 1 ( h ) with v ( x ) < v ( y ) . Thus,

| V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] | = ( v ( y ) - v ( x ) ) p - 1 [ k - ( v ( y ) - T h ( u ( y ) ) ) ] .

Now taking into consideration that ( x , y ) M 1 ( h ) Y 1 ( h ) , we get

0 ( v ( y ) - v ( x ) ) T h ( u ( y ) ) + k - ( T h ( u ( x ) ) + k ) T h ( u ( y ) ) - T h ( u ( x ) ) u ( y ) - u ( x ) .

Therefore, we conclude that

| V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] | 2 k ( u ( y ) - u ( x ) ) p - 1 .

If u ( x ) h , then u ( y ) h , hence

V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] = V ( x , y ) [ T k ( v ( x ) ) - T k ( v ( y ) ) ] 0 .

It then remains to consider the case u ( x ) < h . We distinguish the following three cases:

  1. If u ( y ) > ( h + 1 ) , by (2.5), we reach that

    { u ( y ) > h + 1 , u ( x ) < h } | U ( x , y ) | 𝑑 ν = o ( h ) .

  2. If h < u ( y ) ( h + 1 ) , then 0 k - ( v ( y ) - T h ( u ( y ) ) ) u ( y ) - u ( x ) , thus

    | V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] | ( u ( y ) - u ( x ) ) p .

    Now, by (2.7), we get

    { h < u ( y ) h + 1 , u ( x ) < h } ( u ( y ) - u ( x ) ) p 𝑑 ν = o ( h ) .

  3. We now deal with the set u ( y ) h . Since ( x , y ) M 1 ( h ) Y 1 ( h ) , we have u ( y ) ( h - k ) . Therefore, if u ( x ) < ( h - k - 1 ) , using again (2.6), we reach that

    { u ( y ) > ( h - k ) , u ( x ) < ( h - k - 1 ) } | U ( x , y ) | 𝑑 ν = o ( h ) .

    Let us assume that ( h - k - 1 ) < u ( x ) u ( y ) < h . In this case, we have

    [ k - ( v ( y ) - T h ( u ( y ) ) ) ] = u ( y ) - ( v ( y ) - k ) u ( y ) - ( v ( x ) - k ) u ( y ) - u ( x ) .

    So for ( x , y ) M 1 ( h ) Y 1 ( h ) with h - k - 1 < u ( x ) u ( y ) < h we get

    | V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] | = ( v ( y ) - v ( x ) ) p - 1 [ k - ( v ( y ) - T h ( u ( y ) ) ) ] ( u ( y ) - u ( x ) ) p .

    Now, by using again (2.7), it follows that

    { h - k - 1 < u ( x ) u ( y ) < h } | V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] | 𝑑 ν
    = { h - k - 1 < u ( x ) u ( y ) < h } ( u ( y ) - u ( x ) ) p 𝑑 ν = o ( h ) .

Therefore, combining the above estimates, we obtain J 1 ( h ) o ( h ) .

For J 2 ( h ) we have v ( y ) h < v ( x ) and

T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) k - ( v ( y ) - T h ( u ( x ) ) ) 0 .

Thus,

V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] 0 for all  ( x , y ) M 2 ( h ) Y 1 ( h ) .

Hence, J 2 ( h ) 0 .

We now deal with J 3 ( h ) . We have v ( x ) h < v ( y ) and for all ( x , y ) M 3 ( h ) Y 1 ( h ) we have

| V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] | = ( v ( y ) - v ( x ) ) p - 1 [ k - ( v ( y ) - T h ( u ( y ) ) ) ] .

If v ( x ) ( h - 1 ) , then by (2.6) we have

{ v ( y ) > h , v ( x ) < h - 1 } | V ( x , y ) [ T k ( v ( x ) - T h ( u ( x ) ) ) - T k ( v ( y ) - T h ( u ( y ) ) ) ] | 𝑑 ν
(3.40) 2 k { v ( y ) > h , v ( x ) < h - 1 } | V ( x , y ) | 𝑑 ν = o ( h ) .

Now, assume ( h - 1 ) < v ( x ) h . Since ( x , y ) M 3 ( h ) Y 1 ( h ) , we obtain v ( y ) k + T h ( u ( y ) ) k + u ( y ) . Thus, u ( y ) > ( h - k ) . It is clear that

(3.41) 0 v ( y ) - v ( x ) T h ( u ( y ) ) - T h ( u ( x ) ) u ( y ) - u ( x ) .

Hence following the same discussion as in case (iii) in the analysis of J 1 ( h ) and using (3.40) and (3.41), we obtain that J 3 ( h ) 0 .

Notice that in a symmetric way we can prove that T 1 ( h ) + T 2 ( h ) + T 3 ( h ) o ( h ) .

Going back to (3.39), it holds that Q 2 ( h ) o ( h ) and then the claim follows.

Therefore, going back to the definition of I ( h ) given in (3.35) and taking into consideration that u < h in the set { v < h } , we get

I ( h ) 1 2 D Ω 1 ( h ) U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] 𝑑 ν
+ 1 2 D Ω 2 ( h ) V ( x , y ) [ T k ( v ( x ) - u ( x ) ) - T k ( v ( y ) - u ( y ) ) ] 𝑑 ν + o ( h )
1 2 D Ω 2 ( h ) ( V ( x , y ) - U ( x , y ) ) [ T k ( v ( x ) - u ( x ) ) - T k ( v ( y ) - u ( y ) ) ] 𝑑 ν
+ 1 2 D Ω 1 ( h ) D Ω 2 ( h ) U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] 𝑑 ν + o ( h )
I 1 ( h ) + I 2 ( h ) + o ( h ) .

It is clear that

I 1 ( h ) C D Ω 2 ( h ) | T k ( v ( x ) - u ( x ) ) - T k ( v ( y ) - u ( y ) ) | p 𝑑 ν .

We claim that I 2 ( h ) o ( h ) .

Notice that

D Ω 1 ( h ) D Ω 2 ( h ) = N 1 ( h ) N 2 ( h ) N 3 ( h ) ,

where

N 1 ( h ) { ( x , y ) D Ω : u ( x ) h , u ( y ) h , v ( x ) > h , v ( y ) > h } ,
N 2 ( h ) { ( x , y ) D Ω : u ( x ) h , u ( y ) h , v ( x ) > h , v ( y ) h } ,
N 3 ( h ) { ( x , y ) D Ω : u ( x ) h , u ( y ) h , v ( x ) h , v ( y ) > h } .

Since

1 2 N 1 ( h ) U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] 𝑑 ν 0 ,

we conclude that

I 2 ( h ) 1 2 N 2 ( h ) + 1 2 N 3 ( h ) = I 21 ( h ) + I 22 ( h ) .

For ( x , y ) N 2 ( h ) we will consider the following three main cases:

(I) If h - u ( x ) v ( y ) - u ( y ) , then 0 h - v ( y ) u ( x ) - u ( y ) . Hence,

U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] = U ( x , y ) [ T k ( v ( y ) - u ( y ) ) - T k ( h - u ( x ) ) ] 0 .

(II) If u ( x ) - u ( y ) 0 h - v ( y ) , then u ( x ) - u ( y ) 0 and h - u ( x ) v ( y ) - u ( y ) . Thus,

U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] = U ( x , y ) [ T k ( v ( y ) - u ( y ) ) - T k ( h - u ( x ) ] 0 .

(III) Now consider the case where 0 u ( x ) - u ( y ) h - v ( y ) . It is clear that 0 u ( x ) - u ( y ) v ( x ) - v ( y ) . Hence,

| U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] |
= ( u ( x ) - u ( y ) ) p - 1 [ T k ( h - u ( x ) ) - T k ( v ( y ) - u ( y ) ) ] 2 k ( v ( x ) - v ( y ) ) p - 1 .

If v ( y ) h - 1 or v ( x ) h + 1 , by (2.5), we get

N 2 ( h ) { { v ( x ) > h , v ( y ) < h - 1 } { v ( x ) > h + 1 , v ( y ) < h } } | V ( x , y ) | 𝑑 ν = o ( h ) .

Thus, we deal with the set { h - 1 < v ( y ) h  and  v ( x ) h + 1 } .

It is clear that if v ( y ) - u ( y ) k , then h - u ( x ) k . Thus,

| U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] | = 0 .

Assume that h - u ( x ) k . Then v ( y ) - u ( y ) k , hence

| U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] |
( v ( x ) - v ( y ) ) p - 1 [ ( h - u ( x ) ) - ( v ( y ) - u ( y ) ) ] ( v ( x ) - v ( y ) ) p .

Therefore, using (2.7), we obtain

N 2 ( h ) { h - 1 < v ( y ) h v ( x ) h + 1 } { h - k u ( x ) } | U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] | 𝑑 ν
N 2 ( h ) { h - 1 < v ( y ) h v ( x ) h + 1 } ( v ( x ) - v ( y ) ) p 𝑑 ν = o ( h ) .

We now consider the set { v ( y ) - u ( y ) < k < h - u ( x ) } . Then u ( x ) < h - k , and thus u ( y ) < h - k . As above, we have

| U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] |
( v ( x ) - v ( y ) ) p - 1 [ ( k - ( v ( y ) - u ( y ) ) ] ( v ( x ) - v ( y ) ) p

Thus using again (2.7), we obtain

N 2 ( h ) { h - 1 < v ( y ) h v ( x ) h + 1 } { u ( x ) < h - k } | U ( x , y ) [ T k ( u ( x ) - T h ( v ( x ) ) ) - T k ( u ( y ) - T h ( v ( y ) ) ) ] | 𝑑 ν
N 2 ( h ) { h - 1 < v ( y ) h v ( x ) h + 1 } ( v ( x ) - v ( y ) ) p 𝑑 ν = o ( h ) .

Therefore, we conclude that I 21 ( h ) o ( h ) . In the same way and using a symmetric argument, we can prove that I 22 ( h ) o ( h ) .

Hence I 2 ( h ) o ( h ) and the claim follows.

In conclusion, we have proved that

C D Ω 2 ( h ) | T k ( v ( x ) - u ( x ) ) - T k ( v ( y ) - u ( y ) ) | p 𝑑 ν o ( h ) .

Let h . There results that

D Ω | T k ( v ( x ) - u ( x ) ) - T k ( v ( y ) - u ( y ) ) | p 𝑑 ν = 0 .

Thus T k ( u ) = T k ( v ) for all k. Then u = v . ∎

4 Problem with reaction term and general datum

In this section, we consider the problem

(4.1) { ( - Δ ) p , β s u = λ u q + g ( x ) in  Ω , u 0 in  Ω , u = 0 in  N Ω ,

where λ , q > 0 and g 0 . According to the values of q and λ, we will prove that problem (4.1) has an entropy solution in the sense of Definition 2.9.

Let us begin with the case q < p - 1 . We have the following existence result.

Theorem 4.1.

Assume that q < p - 1 . Then for all g L 1 ( Ω ) and for all λ > 0 problem (4.1) has a positive entropy solution.

Proof.

Without loss of generality we can assume that λ = 1 . We set g n = T n ( g ) . Then g n 0 and g n g strongly in L 1 ( Ω ) . Define u n to be the unique solution to the approximated problem

{ ( - Δ ) p , β s u n = u n q + g n in  Ω , u n 0 in  Ω , u n = 0 in  N Ω .

Notice that the existence of u n can be obtained as a critical point of the functional

J ( u ) = 1 2 p D Ω | u ( x ) - u ( y ) | p 𝑑 ν - 1 q + 1 Ω u + q + 1 𝑑 x - Ω g n u 𝑑 x .

However, the uniqueness follows by using the comparison result in Lemma 2.4. It is clear that by the same comparison principle we obtain that u n u n + 1 .

We claim that { u n p - 1 } n is uniformly bounded in L 1 ( Ω ) . To prove the claim we argue by contradiction. Assume that C n u n p - 1 L 1 ( Ω ) as n . We set

v n = u n C n 1 p - 1 .

Then v n p - 1 L 1 ( Ω ) = 1 and v n solves the problem

(4.2) { ( - Δ ) p , β s v n = C n q - p + 1 p - 1 v n q + C n - 1 g n in  Ω , v n 0 in  Ω , v n = 0 in  N Ω .

We set

G n C n q - p + 1 p - 1 v n q + C n - 1 g n .

Then G n L 1 ( Ω ) 0 as n . Taking into consideration the results of Lemmas 3.1 and 3.2, we get the existence of a measurable function v such that T k ( v ) W β , 0 s , p ( Ω ) , v p - 1 L σ ( Ω , | x | - 2 β d x ) for all σ < N N - p s , and T k ( v n ) T k ( v ) weakly in W β , 0 s , p ( Ω ) .

Since σ > 1 , using Vitali’s lemma, we can prove that v n p - 1 v p - 1 strongly in L 1 ( Ω ) . Thus, v p - 1 L 1 ( Ω ) = 1 .

Now taking T k ( v n ) as a test function in (4.2) and using the fact that G n L 1 ( Ω ) 0 , we conclude that

T k ( v n ) W β , 0 s , p ( Ω ) 0

as n . Hence T k ( v ) = 0 for all k. Then v 0 . Thus we reach a contradiction with the fact that v p - 1 L 1 ( Ω ) = 1 .

Therefore, u n p - 1 L 1 ( Ω ) C for all n and the claim follows.

Since q < p - 1 , we conclude that the sequence { u n q + g n } n is bounded in L 1 ( Ω ) , and thus we get the existence of a measurable function u such that u n q u q , u p - 1 L σ ( Ω , | x | - 2 β d x ) for all σ < N N - p s and T k ( u n ) T k ( u ) weakly in W β , 0 s , p ( Ω ) .

Since { u n q + f n } n is an increasing sequence, using Lemma 3.6, we conclude that T k ( u n ) T k ( u ) strongly in W β , 0 s , p ( Ω ) . Now, by Theorem 1.2 we obtain that u is an entropy solution to problem (4.1) in the sense of Definition 2.9.

We now prove that u is the minimal solution of (4.1).

Let u ¯ be another entropy positive solution to problem (4.1). Recall that u = lim n u n , so to finish we have to show that u n u ¯ for all n. Fix n and consider the sequence { w n , i } i defined by w n , 0 = 0 with w n , i + 1 being the unique solution to the problem

{ ( - Δ ) p , β s w n , i + 1 = w n , i q + g n in  Ω , w n , i + 1 0 in  Ω , w n , i + 1 = 0 in  N Ω .

It is clear that the sequence { w n , i } i is increasing in i with w n , i u n for all i. Hence w n , i w ¯ n is a solution to problem (4.4). Now, by the comparison principle in Lemma 2.4 we conclude that w ¯ n = u n , and by an iteration argument we can prove that w n , i u ¯ for all i. Hence u n u ¯ and the result follows. ∎

In the case where q = p - 1 , the problem is related to the first eigenvalue of the operator ( - Δ ) p , β s . More precisely, we set

λ 1 = inf ϕ W β , 0 s , p ( Ω ) , ϕ 0 1 2 D Ω | ϕ ( x ) - ϕ ( y ) | p 𝑑 ν Ω | ϕ | p 𝑑 x .

As in the case β = 0 , it is not difficult to show that λ 1 > 0 and that λ 1 is attained.

Now, we can formulate our existence result.

Theorem 4.2.

Assume that q = p - 1 . If λ < λ 1 , then for all g L 1 ( Ω ) problem (4.1) has a minimal entropy positive solution.

To prove Theorem 4.2, we need the following classical regularity result.

Lemma 4.3.

Let u be the unique solution to the problem

(4.3) { ( - Δ ) p , β s u = f in  Ω , u = 0 in  N Ω ,

where | f | | x | p s * β L m ( Ω , | x | - p s * β d x ) for some m > N p s . Then u L ( Ω ) .

Proof.

We follow closely the Stampacchia argument given in [24]. Using G k ( u ( x ) ) , with k > 0 , as a test function (4.3), and taking into consideration that

U ( x , y ) ( G k ( u ( x ) ) - G k ( u ( y ) ) ) | G k ( u ( x ) ) - G k ( u ( y ) ) | p ,

where U ( x , y ) = | u ( x ) - u ( y ) | p - 2 ( u ( x ) - u ( y ) ) , we reach that

1 2 D Ω | G k ( u ( x ) ) - G k ( u ( y ) ) | p | x - y | N + p s d x | x | β d y | y | β Ω | f | | G k ( u ( x ) ) | 𝑑 x .

By the Weighted Fractional Sobolev Inequality in Theorem 2.2, it follows that

S G k ( u ) L p s * ( Ω , | x | - p s * β d x ) p A k | f | | G k ( u ( x ) ) | 𝑑 x ,

where A k = { x Ω : | u ( x ) | k } . We set d ω = d x | x | p s * β . Then

A k | f | | G k ( u ( x ) ) | 𝑑 x = A k ( | f | | x | p s * β ) | G k ( u ( x ) ) | 𝑑 ω
G k ( u ) L p s * ( Ω , d ω ) p ( | f | | x | p s * β ) L m ( Ω , d ω ) | A k | d ω 1 - 1 m - 1 p s * .

Thus,

C G k ( u ) L p s * ( Ω , d ω ) p - 1 p s * ( | f | | x | p s * β ) L m ( Ω , d ω ) | A k | d ω 1 - 1 m - 1 p s * .

Let h > k . Since A h A k , there results that

( h - k ) | A h | d ω p - 1 p s * ( | f | | x | p s * β ) L m ( Ω , d ω ) | A k | d ω 1 - 1 m - 1 p s * .

Hence,

| A h | d ω C ( | f | | x | p s * β ) L m ( Ω , d ω ) p s * p - 1 | A k | d ω p s * p - 1 ( 1 - 1 m - 1 p s * ) ( h - k ) p s * p - 1 .

We set Φ ( k ) = | A h | d ω . Then

Φ ( h ) C Φ p s * p - 1 ( 1 - 1 m - 1 p s * ) ( k ) ( h - k ) p s * p - 1 .

Since m > N p s , we have

p s * p - 1 ( 1 - 1 m - 1 p s * ) > 1 .

By the classical result of Stampacchia (see [24]) we get the existence of k 0 > 0 such that Φ ( h ) = 0 for all h k 0 , hence we conclude the proof. ∎

Proof of Theorem 4.2.

We follow closely the argument used in the proof of Theorem 4.1.

Define u n to be the unique solution to the approximated problem

(4.4) { ( - Δ ) p , β s u n = λ u n p - 1 + g n in  Ω , u n 0 in  Ω , u n = 0 in  N Ω .

Notice that, since λ < λ 1 , the existence of u n can be obtained as a critical point of the functional

J ( u n ) = 1 2 p D Ω | u ( x ) - u ( y ) | p 𝑑 ν - λ p Ω | u | p 𝑑 x - Ω g n u 𝑑 x .

However, the uniqueness follows by using the comparison result in Lemma 2.4. It is clear that by using the same comparison principle we obtain that u n u n + 1 .

We claim that { u n p - 1 } n is uniformly bounded in L 1 ( Ω ) . We argue by contradiction. Assume that

C n u n p - 1 L 1 ( Ω )

as n . We set

v n = u n C n 1 p - 1 .

Then v n p - 1 L 1 ( Ω ) = 1 and v n solves the problem

{ ( - Δ ) p , β s v n = λ v n p - 1 + g n C n in  Ω , v n 0 in  Ω , v n = 0 in  N Ω .

We set G n v n p - 1 + g n C n , thus G n L 1 ( Ω ) C . Taking into consideration the results of Lemmas 3.1 and 3.2, we get the existence of a measurable function v such that T k ( v ) W β , 0 s , p ( Ω ) , v p - 1 L σ ( Ω , | x | - 2 β d x ) for all σ < N N - p s and T k ( v n ) T k ( v ) weakly in W β , 0 s , p ( Ω ) .

Since σ > 1 , using Vitali’s lemma, we can prove that v n p - 1 v p - 1 strongly in L 1 ( Ω ) . Thus, v p - 1 L 1 ( Ω ) = 1 . It is clear that G n λ v p - 1 strongly in L 1 ( Ω ) . Thus v solves

(4.5) { ( - Δ ) p , β s v = λ v p - 1 in  Ω , v 0 in  Ω , v = 0 in  N Ω .

We claim that v L ( Ω ) . From the previous discussion we know that v p - 1 L σ ( Ω , | x | - 2 β d x ) for all σ < N N - p s . Thus setting a 1 = ( p - 1 ) N N - p s - ( p - 1 ) - ε , with ε very small, and using an approximation argument, we can take v a 1 as a test function in (4.5) to conclude that

D Ω | v ( x ) - v ( y ) | p - 2 ( v ( x ) - v ( y ) ) ( v a 1 ( x ) - v a 1 ( y ) ) 𝑑 ν C .

Hence by using inequality (2.2), it follows that

D Ω | v a 1 + p - 1 p ( x ) - v a 1 + p - 1 p ( y ) | p 𝑑 ν C .

Using the weighted Sobolev inequality in Theorem 2.2, we reach that

Ω | v ( x ) | ( a 1 + p - 1 ) p s * p | x | p s * β 𝑑 x < .

Now we set a 2 = ( a 1 + p - 1 ) p s * p - ( p - 1 ) . Then using v a 2 as a test function in (4.5) and following the same argument as above, we conclude that

Ω | v ( x ) | ( a 2 + p - 1 ) p s * p | x | p s * β 𝑑 x < .

Now consider the sequence a n + 1 = ( a n + p - 1 ) p s * p - ( p - 1 ) . It is clear that a n and by an induction argument we can prove that Ω v a n 𝑑 x < for all n. Thus using Theorem 4.3, we conclude that v is an energy solution to problem (4.5) and that v L ( Ω ) . Now, by using v as a test function in (4.5) and taking into consideration that λ < λ 1 , it follows that v W β , 0 s , p ( Ω ) = 0 , which is a contradiction to the fact that v p - 1 L 1 ( Ω ) = 1 . Therefore, the claim follows. The rest of the proof follows exactly the same argument as in the proof of Theorem 4.1. ∎

Let us now consider the case q > p - 1 . We follow closely the argument used in [9]. It is clear that in this case additional conditions on g are needed in order to guarantee the existence of a positive solution.

More precisely, if g L 1 ( Ω ) , we define w to be the unique positive solution to the problem

{ ( - Δ ) p , β s w = g in  Ω , w = 0 in  N Ω .

We are able to prove the following result.

Theorem 4.4.

Assume that g L 1 ( Ω ) verifies w q ( x ) g ( x ) a.e. in Ω. Then there exists a positive constant λ ¯ such that for all λ < λ ¯ problem (4.1) has a minimal entropy positive solution.

Proof.

Recall that by the results of Lemmas 3.1 and 3.2 we know that w p - 1 L σ ( Ω , | x | - 2 β d x ) for all σ < N N - p s and T k ( w ) W β , 0 s , p ( Ω ) .

Let v be the minimal solution to the problem

{ ( - Δ ) p , β s v = g + w q in  Ω , v = 0 in  N Ω .

It is not difficult to show that v 2 p - 1 w , hence by using the hypothesis on g, it follows that

( - Δ ) p , β s v = g + w q g + 2 q 1 - p v q .

Then v is a supersolution to (4.1) for λ λ ¯ = 2 q 1 - p . Fixing λ as above and defining the sequence { u n } n by u 0 = 0 , we have that u n + 1 is the unique solution to the following problem:

{ ( - Δ ) p , β s u n + 1 = u n q + g n + 1 in  Ω , u n + 1 = 0 in  N Ω .

By an induction argument, we can prove that u n v for all n and that the sequence { u n } n is increasing in n. Thus { u n q + g n } n is increasing and bounded in L 1 ( Ω ) . Now, using the same compactness argument as in the proofs of Theorems 4.1 and 4.2, we get the existence result. ∎

Award Identifier / Grant number: MTM2013–40846-P

Funding statement: Work partially supported by Project MTM2013–40846-P, MINECO, Spain.

Acknowledgements

The authors would like to express their gratitude to the anonymous referee for their comments and suggestions that improved the last version of the manuscript.

References

[1] B. Abdellaoui and R. Bentifour, Caffarelli–Kohn–Nirenberg type inequalities of fractional order and applications, submitted. 10.1016/j.jfa.2017.02.007Search in Google Scholar

[2] B. Abdellaoui, M. Medina, I. Peral and A. Primo, A note on the effect of the Hardy potential in some Calderon–Zygmund properties for the fractional Laplacian, J. Differential Equations 260 (2016), 8160–8206. 10.1016/j.jde.2016.02.016Search in Google Scholar

[3] B. Abdellaoui and I. Peral, On quasilinear elliptic equation related to some Caffarelli–Khon–Nirenberg inequalities, Comm. Pure Appl. Math. 3 (2003), no. 3, 539–566. 10.3934/cpaa.2003.2.539Search in Google Scholar

[4] B. Abdellaoui, I. Peral and A. Primo, A remark on the fractional Hardy inequality with a remainder term, C. R. Acad. Sci. Paris Ser. I 352 (2014), 299–303. 10.1016/j.crma.2014.02.003Search in Google Scholar

[5] R. A. Adams, Sobolev Spaces, Academic Press, New York, 1975. Search in Google Scholar

[6] N. Alibaud, B. Andreianov and M. Bendahmane, Renormalized solutions of the fractional Laplace equation, C. R. Acad. Sci. Paris Ser. I 348 (2010), 759–762. 10.1016/j.crma.2010.05.006Search in Google Scholar

[7] P. Bénilan, L. Boccardo, T. Gallouët, R. Gariepy, M. Pierre and J. L. Vázquez, An L 1 theory of existence and uniqueness of solutions of nonlinear elliptic equations, Ann. Sc. Norm. Super. Pisa Cl. Sci. (4) 22 (1995), no. 2, 240–273. Search in Google Scholar

[8] L. Boccardo, T. Galloüet and L. Orsina, Existence and uniqueness of entropy solutions for nonlinear elliptic equations involving measure data, Ann. Inst. H. Poincar é Anal. Non Linéaire 13 (1996), no. 13, 539–551. 10.1016/s0294-1449(16)30113-5Search in Google Scholar

[9] H. Brezis and X. Cabré, Some simple nonlinear PDEs without solution, Boll. Unione. Mat. Ital. Sez. B 8 (1998), no. 1, 223–262. Search in Google Scholar

[10] H. Brezis and S. Kamin, Sublinear elliptic equations in N , Manuscripta Math. 74 (1992), 87–106. 10.1007/BF02567660Search in Google Scholar

[11] L. Caffarelli, R. Kohn and L. Nirenberg, First order interpolation inequality with weights, Compos. Math. 53 (1984), 259–275. Search in Google Scholar

[12] M. Caponi and P. Pucci, Existence theorems for entire solutions of stationary Kirchhoff fractional p-Laplacian equations, Ann. Mat. Pura Appl. (4) 195 (2016), no. 6, 2099–2129. 10.1007/s10231-016-0555-xSearch in Google Scholar

[13] G. Dal Maso, F. Murat, L. Orsina and A. Prignet, Renormalized solutions of elliptic equations with general measure data, Ann. Sc. Norm. Super. Pisa Cl. Sci. (4) 28 (1999), no. 4, 741–808. Search in Google Scholar

[14] A. Di Castro, T. Kuusi and G. Palatucci, Local behavior of fractional p-minimizers, Ann. Inst. H. Poincar é Anal. Non Linéaire 33 (2016), no. 5, 1279–1299. 10.1016/j.anihpc.2015.04.003Search in Google Scholar

[15] E. Di Nezza, G. Palatucci and E. Valdinoci, Hitchhiker’s guide to the fractional Sobolev spaces, Bull. Sci. Math. 136 (2012), no. 5, 521–573. 10.1016/j.bulsci.2011.12.004Search in Google Scholar

[16] E. B. Fabes, C. E. Kenig and R. P. Serapioni, The local regularity of solutions of degenerate elliptic equations, Comm. Partial Differential Equations 7 (1982), no. 1, 77–116. 10.1080/03605308208820218Search in Google Scholar

[17] F. Ferrari and I. Verbitsky, Radial fractional Laplace operators and Hessian inequalities, J. Differential Equations 253 (2012), no. 1, 244–272. 10.1016/j.jde.2012.03.024Search in Google Scholar

[18] R. L. Frank, E. H. Lieb and R. Seiringer, Hardy–Lieb–Thirring inequalities for fractional Schrödinger operators, J. Amer. Math. Soc. 21 (2008), 925–950. 10.1090/S0894-0347-07-00582-6Search in Google Scholar

[19] J. Heinonen, T. Kilpelainen and O. Martio, Nonlinear Potential Theory of Degenerate Elliptic Equations, Clarendon Press, Oxford, 1993. Search in Google Scholar

[20] K. H. Karlsen, F. Petitta and S. Ulusoy, A duality approach to the fractional Laplacian with measure data, Publ. Mat. 55 (2011), no. 1, 151–161. 10.5565/PUBLMAT_55111_07Search in Google Scholar

[21] T. Kuusi, G. Mingione and Y. Sire, Nonlocal equations with measure data, Comm. Math. Phys. 337 (2015), 1317–1368. 10.1007/s00220-015-2356-2Search in Google Scholar

[22] T. Leonori, I. Peral, A. Primo and F. Soria, Basic estimates for solutions of a class of nonlocal elliptic and parabolic equations, Discrete Contin. Dyn. Syst. 35 (2015), no. 12, 6031–6068. 10.3934/dcds.2015.35.6031Search in Google Scholar

[23] V. Maz’ya, Sobolev Spaces with Applications to Elliptic Partial Differential Equations, 2nd ed., Grundlehren Math. Wiss. 342, Springer, Heidelberg, 2011. 10.1007/978-3-642-15564-2Search in Google Scholar

[24] G. Stampacchia, Le problème de Dirichlet pour les équations elliptiques du second ordre à coefficients discontinus, Ann. Inst. Fourier (Grenoble) 15 (1965), 189–258. 10.5802/aif.204Search in Google Scholar

Received: 2016-03-29
Revised: 2016-09-13
Accepted: 2016-10-07
Published Online: 2016-12-02

© 2019 Walter de Gruyter GmbH, Berlin/Boston

This work is licensed under the Creative Commons Attribution 4.0 Public License.

Articles in the same Issue

  1. Frontmatter
  2. Asymptotic behavior of evolution systems in arbitrary Banach spaces using general almost periodic splittings
  3. Solvability of a product-type system of difference equations with six parameters
  4. On Dirichlet problem for fractional p-Laplacian with singular non-linearity
  5. Absence of Lavrentiev gap for non-autonomous functionals with (p,q)-growth
  6. On a class of fully nonlinear parabolic equations
  7. On sign-changing solutions for (p,q)-Laplace equations with two parameters
  8. Weighted Caffarelli–Kohn–Nirenberg type inequalities related to Grushin type operators
  9. On the fractional p-Laplacian equations with weight and general datum
  10. An elliptic equation with an indefinite sublinear boundary condition
  11. Liouville-type theorems for elliptic equations in half-space with mixed boundary value conditions
  12. Well/ill-posedness for the dissipative Navier–Stokes system in generalized Carleson measure spaces
  13. Hypercontractivity, supercontractivity, ultraboundedness and stability in semilinear problems
  14. Theoretical analysis of a water wave model with a nonlocal viscous dispersive term using the diffusive approach
  15. A multiplicity result for asymptotically linear Kirchhoff equations
  16. Higher-order anisotropic models in phase separation
  17. Well-posedness and maximum principles for lattice reaction-diffusion equations
  18. Existence of a bound state solution for quasilinear Schrödinger equations
  19. Existence and concentration behavior of solutions for a class of quasilinear elliptic equations with critical growth
  20. Homoclinics for strongly indefinite almost periodic second order Hamiltonian systems
  21. A new method for converting boundary value problems for impulsive fractional differential equations to integral equations and its applications
  22. Diffusive logistic equations with harvesting and heterogeneity under strong growth rate
  23. On viscosity and weak solutions for non-homogeneous p-Laplace equations
  24. Periodic impulsive fractional differential equations
  25. A result of uniqueness of solutions of the Shigesada–Kawasaki–Teramoto equations
  26. Solutions of vectorial Hamilton–Jacobi equations are rank-one absolute minimisers in L
  27. Large solutions to non-divergence structure semilinear elliptic equations with inhomogeneous term
  28. The elliptic sinh-Gordon equation in a semi-strip
  29. The Gelfand problem for the 1-homogeneous p-Laplacian
  30. Boundary layers to a singularly perturbed Klein–Gordon–Maxwell–Proca system on a compact Riemannian manifold with boundary
  31. Subharmonic solutions of Hamiltonian systems displaying some kind of sublinear growth
  32. Multiple solutions for an elliptic system with indefinite Robin boundary conditions
  33. New solutions for critical Neumann problems in ℝ2
  34. A fractional Kirchhoff problem involving a singular term and a critical nonlinearity
  35. Existence and non-existence of solutions to a Hamiltonian strongly degenerate elliptic system
  36. Characterizing the strange term in critical size homogenization: Quasilinear equations with a general microscopic boundary condition
  37. Nonlocal perturbations of the fractional Choquard equation
  38. A pathological example in nonlinear spectral theory
  39. Infinitely many solutions for cubic nonlinear Schrödinger equations in dimension four
  40. On Cauchy–Liouville-type theorems
  41. Maximal Lp -Lq regularity to the Stokes problem with Navier boundary conditions
  42. Besov regularity for solutions of p-harmonic equations
  43. The classical theory of calculus of variations for generalized functions
  44. On the Cauchy problem of a degenerate parabolic-hyperbolic PDE with Lévy noise
  45. Hölder gradient estimates for a class of singular or degenerate parabolic equations
  46. Critical and subcritical fractional Trudinger–Moser-type inequalities on
  47. Multiple nonradial solutions for a nonlinear elliptic problem with singular and decaying radial potential
  48. Quantization of energy and weakly turbulent profiles of solutions to some damped second-order evolution equations
  49. An elliptic system with logarithmic nonlinearity
  50. The Caccioppoli ultrafunctions
  51. Equilibrium of a production economy with non-compact attainable allocations set
  52. Exact behavior around isolated singularity for semilinear elliptic equations with a log-type nonlinearity
  53. The higher integrability of weak solutions of porous medium systems
  54. Classification of stable solutions for boundary value problems with nonlinear boundary conditions on Riemannian manifolds with nonnegative Ricci curvature
  55. Regularity results for p-Laplacians in pre-fractal domains
  56. Carleman estimates and null controllability of a class of singular parabolic equations
  57. Limit profiles and uniqueness of ground states to the nonlinear Choquard equations
  58. On a measure of noncompactness in the space of regulated functions and its applications
  59. p-fractional Hardy–Schrödinger–Kirchhoff systems with critical nonlinearities
  60. On the well-posedness of a multiscale mathematical model for Lithium-ion batteries
  61. Global existence of a radiative Euler system coupled to an electromagnetic field
  62. On the existence of a weak solution for some singular p ( x ) -biharmonic equation with Navier boundary conditions
  63. Choquard-type equations with Hardy–Littlewood–Sobolev upper-critical growth
  64. Clustered solutions for supercritical elliptic equations on Riemannian manifolds
  65. Ground state solutions for the Hénon prescribed mean curvature equation
  66. Quasilinear equations with indefinite nonlinearity
  67. Concentrating solutions for a planar elliptic problem with large nonlinear exponent and Robin boundary condition
  68. Retraction of: Concentrating solutions for a planar elliptic problem with large nonlinear exponent and Robin boundary condition
Downloaded on 24.3.2026 from https://www.degruyterbrill.com/document/doi/10.1515/anona-2016-0072/html
Scroll to top button