Keller-Osserman estimates for some quasilinear elliptic systems

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Keller-Osserman estimates for some quasilinear elliptic systems

Marie-Françoise Bidaut-Véron, Marta Garcia-Huidobro, Cecilia Yarur

To cite this version:

Marie-Françoise Bidaut-Véron, Marta Garcia-Huidobro, Cecilia Yarur. Keller-Osserman estimates

for some quasilinear elliptic systems. Communications in Pure and Applied Analysis, 2013, 12 (4),

pp.1547-1568. �10.3934/cpaa.2012.12�. �hal-00565280v2�

Keller-Osserman estimates for some quasilinear elliptic systems

Marie-Fran¸coise BIDAUT-VERON

^∗

Marta GARC´IA-HUIDOBRO

^†

Cecilia YARUR

^‡

.

Abstract

In this article we study quasilinear systems of two types, in a domain Ω ofR^N : with absorption terms, or mixed terms:

(A)

Apu=v^δ,

Aqv=u^µ, (M)

Apu=v^δ,

−Aqv=u^µ,

where δ, µ > 0 and 1 < p, q < N, and D = δµ−(p−1)(q−1) > 0; the model case is Ap= ∆p,Aq = ∆q.Despite of the lack of comparison principle, we prove a priori estimates of Keller-Osserman type:

u(x)≤Cd(x, ∂Ω)^{−p(q−1)+qδD} , v(x)≤Cd(x, ∂Ω)^{−q(p−1)+pµD} .

Concerning system (M), we show thatv always satisfies Harnack inequality. In the case Ω = B(0,1)\ {0}, we also study the behaviour near 0 of the solutions of more general weighted systems, giving a priori estimates and removability results. Finally we prove the sharpness of the results.

Keywords.Quasilinear elliptic systems, a priori estimates, large solutions, asymptotic behaviour, Harnack inequality.

Mathematic Subject Classification (2010)35B40, 35B45, 35J47, 35J92, 35M30

∗Laboratoire de Math´ematiques et Physique Th´eorique, CNRS UMR 6083, Facult´e des SCiences, 37200 Tours France. E-mail address: veronmf@univ-tours.fr

†Departamento de Matem´aticas, Pontificia Universidad Cat´olica de Chile, Casilla 306, Correo 22, Santiago de Chile. E-mail address: mgarcia@mat.puc.cl

‡Departamento de Matem´atica y C.C., Universidad de Santiago de Chile, Casilla 307, Correo 2, Santiago de Chile. E-mail address: cecilia.yarur@usach.cl

1 Introduction

In this article we study the nonnegative solutions of quasilinear systems in a domain Ω of

R^N

, either with absorption terms, or mixed terms, that is,

(A)

A

p

u = v

^δ

,

A

q

v = u

^µ

, (M)

A

p

u = v

^δ

,

−A

q

v = u

^µ

, (1.1)

where

δ, µ > 0 and 1 < p, q < N.

The operators are given in divergence form by

A

p

u := div [A

_p

(x, u, ∇u)] , A

q

v := div [A

_q

(x, v, ∇v)] ,

where A

_p

and A

_q

are Carath´eodory functions. In our main results, we suppose that A

_p

is S-p-C (strongly-p-coercive), that means (see [8])

A

_p

(x, u, η).η ≥ K

_1,p

|η|

^p

≥ K

_2,p

|A

_p

(x, u, η)|

^p′

, ∀(x, u, η) ∈ Ω ×

R⁺

×

R^N

.

for some K

_1,p

, K

_2,p

> 0, and similarly for A

q

. The model type for A

p

is the p-Laplace operator u 7−→ ∆

_p

u = div(|∇u|

^p−2

∇u).

We prove a priori estimates of Keller-Osserman type for such operators, under a natural con- dition of ”superlinearity”:

D = δµ − (p − 1)(q − 1) > 0, (1.2)

and we deduce Liouville type results of nonexistence of entire solutions. We also study the behaviour near 0 of nonnegative solutions of possibly weighted systems of the form

(A

_w

)

A

p

u = |x|

^a

v

^δ

,

A

_q

v = |x|

^b

u

^µ

, (M

_w

)

A

p

u = |x|

^a

v

^δ

,

−A

_q

v = |x|

^b

u

^µ

, in Ω\ {0} , where

a, b ∈

R

, a > −p, b > −q.

In particular we discuss about the Harnack inequality for u or v.

Recall some classical results in the scalar case. For the model equation with an absorption term

∆

_p

u = u

^Q

, (1.3)

in Ω, with Q > p− 1, the first estimate was obtained by Keller [19] and Osserman [24] for p = 2, and extended to the case p 6= 2 in [29]: any nonnegative solution u ∈ C

²

(Ω) satisfies

u(x) ≤ Cd(x, ∂Ω)

^{−p/(Q−p+1)}

, (1.4)

where d(x, ∂Ω) is the distance to the boundary, and C = C(N, p, Q). For the equation with a source term

−∆

p

u = u

^Q

,

up to now estimate (1.4), valid for any Q > p − 1 in the radial case, has been obtained only for Q < Q

^∗

, where Q

^∗

=

^N(p−1)+p_N−p

is the Sobolev exponent, with difficult proofs, see [18], [9]

in the case p = 2 and [27] in the general case p > 1. For p = 2, the estimate, with a universal constant, is not true for Q =

^N_N⁺¹₋₃

, and the problem is open between Q

^∗

and

^N+1_N−3

.

Up to our knowledge all the known estimates for systems are related with systems for which some comparison properties hold, of competitive type, see [16], or of cooperative type, see [11];

or with quasilinear operators in [17], [32]. Problems (A) and (M) have been the object of very few works because such properties do not hold. The main ones concern systems (A

w

) and (M

w

) in the linear case p = q = 2, see [5] and [6]; the proofs rely on the inequalities satisfied by the mean values u and v on spheres of radius r, they cannot be extended to the quasilinear case.

A radial study of system (A) was introduced in [15], and recently in [7].

The problem with two source terms (S)

−A

p

u = |x|

^a

v

^δ

,

−A

q

v = |x|

^b

u

^µ

,

was analyzed in [8]. The results are based on integral estimates, still valid under weaker assumptions: from [8], A

p

is called W-p-C (weakly-p-coercive) if

A

_p

(x, u, η).η ≥ K

_p

|A

p

(x, u, η)|

^p′

, ∀(x, u, η) ∈ Ω ×

R⁺

×

R^N

(1.5) for some K

_p

> 0; similarly for A

q

. When δ, µ < Q

₁

, where Q

₁

=

^N(p−1)_N−p

, punctual estimates were deduced for S-p-C, S-q-C operators and it was shown that u and v satisfy the Harnack inequality.

In Section 2, we give our main tools for obtaining a priori estimates. First we show that the technique of integral estimates if fundamental, and can be used also for systems (A) and (M). In Proposition 2.1 we consider both equations with absorption or source terms

−A

_p

u + f = 0, or − A

_p

u = f, (1.6)

in a domain Ω, where f ∈ L

¹_loc

(Ω), f ≥ 0, and obtain local integral estimates of f with respect to u in a ball B(x

₀

, ρ). When A

_p

is S-p-C, they imply minorizations by the W¨ olf potential of f in the ball

W

_1,p^f

(B (x

₀

, ρ)) =

Z ρ

0

t

^p I

B(x0,t)

f

!_p−1¹

dt

t , (1.7)

extending the first results of [20], [21]. The second tool is the well known weak Harnack inequalities for solutions of (1.6) in case of S-p-C operators, and a more general version in case of equation with absorption, which appears to be very useful. The third one is a boostrap argument given in [5] which remains essential.

In Section 3 we study both systems (A) and (M). When A

p

= ∆

_p

and A

q

= ∆

_q

, they admit particular radial solutions

u

^∗

(x) = A

^∗

|x|

^−γ

, v

^∗

(r) = B

^∗

|x|

^−ξ

,

where

γ = p(q − 1) + qδ

D , ξ = q(p − 1) + pµ

D , (1.8)

whenever

γ > N − p

p − 1 and ξ > N − q

q − 1 for system (A), γ > N − p

p − 1 and ξ < N − q

q − 1 for system (M ).

Our main result for the system with absorption term (A) extends precisely the Osserman- Keller estimate of the scalar case (1.3):

Theorem 1.1 Assume that

A

_p

is S-p-C, A

_q

is S-q-C, (1.9)

and (1.2) holds. Let u ∈ W

_loc^1,p

(Ω) ∩ C (Ω) , v ∈ W

_loc^1,q

(Ω) ∩ C (Ω) be nonnegative solutions of

−A

p

u + v

^δ

≤ 0,

−A

q

v + u

^µ

≤ 0, in Ω.

Then for any x ∈ Ω

u(x) ≤ Cd(x, ∂Ω)

^−γ

, v(x) ≤ Cd(x, ∂Ω)

^−ξ

, (1.10) with C = C(N, p, q, δ, µ, K

_1,p

, K

_2,p

, K

_1,q

, K

_2,q

).

Our second result shows that the mixed system (M) also satisfies the Osserman-Keller estimate, without any restriction on δ and µ, and moreover the second function v always satisfies Harnack inequality:

Theorem 1.2 Assume (1.2),(1.9). Let u ∈ W

_loc^1,p

(Ω) ∩ C (Ω) , v ∈ W

_loc^1,q

(Ω) ∩ C (Ω) be nonneg- ative solutions of

−A

_p

u + v

^δ

≤ 0,

−A

q

v

≧

u

^µ

, in Ω.

Then (1.10) still holds for any x ∈ Ω.

Moreover, if u, v are any nonnegative solution of system (M), then v satisfies Harnack inequality in Ω, and there exists another C > 0 as above, such that the punctual inequality holds

u

^µ

(x) ≤ Cv

^q−1

(x)d(x, ∂Ω)

^−q

. (1.11) Notice that the results are new even for p = q = 2. As a consequence we deduce Liouville properties:

Corollary 1.3 Assume (1.2),(1.9). Then there exist no entire nonnegative solutions of systems

(A) or (M).

Section 4 concerns the behaviour near 0 of systems with possible weights (A

_w

) and (M

_w

), where γ, ξ are replaced by

γ

_a,b

= (p + a)(q − 1) + (q + b)δ

D , ξ

_a,b

= (q + b)(p − 1) + (p + a)µ

D , (1.12)

in other terms δξ

_a,b

= (p − 1)γ

_a,b

+ p + a, µγ

_a,b

= (q − 1)ξ

_a,b

+ q + b. We set B

_r

= B(0, r) and B

_r^′

= B

r

\ {0} for any r > 0. Our results extend and simplify the results of [5], [6] in a significant way:

Theorem 1.4 Assume (1.2),(1.9). Let u ∈ W

_loc^1,p

(B

₁^′

) ∩ C (B

₁^′

) , v ∈ W

_loc^1,q

(B

₁^′

) ∩ C (B

₁^′

) be nonnegative solutions of

−A

p

u + |x|

^a

v

^δ

≤ 0,

−A

q

v + |x|

^b

u

^µ

≤ 0, in B

₁^′

. (1.13) Then there exists C = C(N, p, q, a, b, δ, µ, K

_1,p

, K

_2,p

, K

_1,q

, K

_2,q

) > 0 such that

u(x) ≤ C |x|

^−γa,b

, v(x) ≤ C |x|

^−ξa,b

in B

^′1 2

. (1.14)

Theorem 1.5 Assume (1.2),(1.9). Let u ∈ W

_loc^1,p

(B

₁^′

) ∩ C (B

₁^′

) , v ∈ W

_loc^1,q

(B

₁^′

) ∩ C (B

₁^′

) be nonnegative solutions of

−A

p

u + |x|

^a

v

^δ

≤ 0,

−A

_q

v ≥ |x|

^b

u

^µ

, in B

₁^′

. (1.15) in B

₁^′

. Then there exists C > 0 as in theorem 1.4 such that

u(x) ≤ C |x|

^−γa,b

, v(x) ≤ C min(|x|

^−ξa,b

, |x|

^{−N−qq−1}

), in B

^′1 2

. (1.16)

Moreover if (u, v) is any nonnegative solution of (M

_w

), then v satisfies Harnack inequality in B

^′₁

2

, and there exist another C > 0 as above, such that

|x|

^b+q

u

^µ

(x) ≤ Cv

^q−1

(x), in B

^′1

2

. (1.17)

Moreover we give removability results for the two systems (A

_w

) and (M

_w

), see Theorems 4.1, 4.2, whenever A

_p

and A

_q

satisfy monotonicity and homogeneity properties, extending to the quasilinear case [5, Corollary 1.2] and [6, Theorem 1.1].

In Section 5 we show that our results on Harnack inequality are optimal, even in the radial case. And we prove the sharpness of the removability conditions.

2 Main tools

For any x ∈

R^N

and r > 0, we set B(x, r) =

y ∈

R^N

/ |y − x| < r and B

_r

= B(0, r).

For any function w ∈ L

¹

(Ω), and for any weight function ϕ ∈ L

^∞

(Ω) such that ϕ ≥ 0, ϕ 6= 0, we denote by

I

ϕ

w = 1

R

Ω

ϕ

Z

Ω

wϕ

the mean value of w with respect to ϕ and by

I

Ω

w = 1

|Ω|

Z

Ω

w =

I

1

w.

For any function g ∈ L

¹_loc

(Ω), we say that a function u ∈ W

_loc^1,p

(Ω) satisfies

−A

p

u

≧

g in Ω, (resp.

≦,

resp. =) if A

p

(x, u, ∇u) ∈ L

^p_loc^′

(Ω) and

−

Z

Ω

A

_p

(x, u, ∇u).∇φ

≧ Z

Ω

gφ, (resp.

≦,

resp. =) (2.1)

for any nonnegative φ ∈ W

^1,∞

(Ω) with compact support in Ω.

2.1 Integral estimates under weak conditions

Next we prove integral inequalities on the second member f of equations (1.6) in terms of the function u, for either with source or with absorption terms, obtained by multiplication by u

^α

with α < 0 for the source case, α > 0 for the absorption case. The method is now classical, initiated by Serrin [26] and Trudinger [28], leading to Harnack inequalities for S-p-C operators.

These estimates were developped for the p-Laplace operator in [20]. Under weak conditions on the operator, this technique of multiplication by u

^α

was used with specific f for obtaining Liouville results in [23]. It was developped for general f in [8, Proposition 2.1] where the notion of W-p-C operator was introduced. More recent Liouville results were given in [10, Theorem 2.1], and in [14] for the case of absorption terms.

Proposition 2.1 Let A

_p

be W-p-C. Let f ∈ L

¹_loc

(Ω), f ≥ 0 and let u ∈ W

_loc^1,p

(Ω) be any nonnegative solution of inequality

−A

p

u

≧

f, in Ω, (2.2)

or of inequality

−A

p

u + f

≦

0, in Ω. (2.3)

Let ξ ∈ D(Ω), with values in [0, 1] , and ϕ = ξ

^λ

, λ > 0, and S

_ξ

=supp|∇ξ|.

Then for any ℓ > p − 1, there exists λ(p, ℓ) such that for λ ≥ λ(p, ℓ), there exists C = C(N, p, K

_p

, ℓ, λ) > 0 such that

Z

Ω

f ϕ

≦

C |S

ξ

| max

Ω

|∇ξ|

^p I

Sξ

u

^ℓ

ϕ

!^p−1_ℓ

. (2.4)

Proof. (i) First assume that ℓ > p − 1 + α, with α ∈ (1 − p, 0) in case of equation (2.2), α ∈ (0, 1) (any α > 0 if u ∈ L

^∞_loc

(Ω)) in case of equation (2.3). We claim that there exists λ(p, α, ℓ) such that for any λ ≥ λ(p, α, ℓ)

Z

Ω

f u

^α

ϕ

≦

C |S

ξ

| max

Ω

|∇ξ|

^p I

Sξ

u

^ℓ

ϕ

!^p−1+α_ℓ

, (2.5)

for some C = C(N, p, K

_p

, α, ℓ, λ). For proving (2.5), one can assume that u

^ℓ

∈ L

¹

(B(x

₀

, ρ)).

Let ϕ = ξ

^λ

, where λ > 0 will be chosen after. Let δ > 0, k ≥ 1, and (η

_n

) be a sequence of mollifiers; we set u

_δ

= u + δ, u

_δ,k

= min(u, k) + δ and approximate u by u

_δ,k,n

= u

_δ,k

∗ η

_n

, and we take φ = u

^α_δ,k,n

ϕ as a test function. Then in any case, from (1.5) and H¨ older inequality,

|α|

Z

Ω

u

^α−1_δ,k,n

ϕA

p

(x, u, ∇u).∇u

_δ,k,n

+

Z

Ω

f u

^α_δ,k,n

ϕ

≤ λ

Z

Sξ

u

^α_δ,k,n

ξ

^λ−1

|A

p

(x, u, ∇u)||∇ξ|

≤ λK

_p^−1/p′ Z

Sξ

u

^α_δ,k,n

ξ

^λ−1

(A

p

(x, u, ∇u).∇u)

^1/p′

|∇ξ|

≤ λK

_p^−1/p′ Z

Sξ

u

^α−1_δ,k,n

ξ

^λ

A

_p

(x, u, ∇u).∇u

!¹

p′ Z

Sξ

u

^α+p−1_δ,k,n

ξ

^λ−p

|∇ξ|

^p

!¹_p

.

Otherwise (∇u

δ,k,n

) tends to χ

_{u≤k}

∇u in L

^p_loc

(Ω), and up to subsequence a.e. in Ω, and A

_p

(x, u, ∇u) ∈ L

^p_loc^′

(Ω). By letting n → ∞, we obtain

|α|

Z

{u≤k}

u

^α−1_δ,k

ξ

^λ

A

p

(x, u, ∇u).∇u +

Z

Ω

f u

^α_δ,k

ξ

^λ

≤ λK

_p^−1/p′ Z

Sξ

u

^α−1_δ,k

ξ

^λ

A

_p

(x, u, ∇u).∇u

!_p′¹ Z

Sξ

u

^α+p−1_δ,k

ξ

^λ−p

|∇ξ|

^p

!_p¹

≤ |α|

2

Z

Sξ

u

^α−1_δ,k

ξ

^λ

A

_p

(x, u, ∇u).∇u + C

Z

Sξ

u

^α+p−1_δ,k

ξ

^λ−p

|∇ξ|

^p

,

with C = C(α, K

_p

, p, λ); otherwise, for α < 1 (or u ∈ L

^∞_loc

(Ω) and taking k ≥ sup

_Sξ

u)

Z

Ω

u

^α−1_δ,k

ξ

^λ

A

p

(x, u, ∇u).∇u =

Z

{u≤k}

u

^α−1_δ,k

ξ

^λ

A

p

(x, u, ∇u).∇u +

Z

{u>k}

u

^α−1_δ,k

ξ

^λ

A

p

(x, u, ∇u).∇u

≤

Z

{u≤k}

u

^α−1_δ,k

ξ

^λ

A

_p

(x, u, ∇u).∇u + M k

^α−1

where M =

R

Ω

ξ

^λ

A

_p

(x, u, ∇u).∇u (or M = 0) is independent of k and δ. Then, for any θ > 1,

|α|

2

Z

{u≤k}

u

^α−1_δ,k

ξ

^λ

A

_p

(x, u, ∇u).∇u +

Z

Ω

f u

^α_δ,k

ξ

^λ

≤ C

Z

Sξ

u

^α+p−1_δ,k

ξ

^λ−p

|∇ξ|

^p

+ M |α| k

^α−1

≤ C

Z

Sξ

u

^(α+p−1)θ_δ,k

ξ

^λ

!¹_θ Z

Sξ

ξ

^λ−pθ′

|∇ξ|

^pθ′

!_θ′¹

+ M |α| k

^α−1

.

Choosing θ = ℓ/(α + p − 1) > 1, and λ ≥ λ(p, α, ℓ) = pθ

^′

, we find

|α|

2

Z

{u≤k}

u

^α−1_δ,k

ξ

^λ

A

_p

(x, u, ∇u).∇u +

Z

Ω

f u

^α_δ,k

ξ

^λ

≤ C

Z

Sξ

u

^ℓ_δ,k

ϕ

!^α+p−1_ℓ Z

Sξ

|∇ξ|

^pθ′

!¹

θ′

+ M |α| k

^α−1

≤ C |S

ξ

|

^θ1′

max

Ω

|∇ξ|

^p Z

Sξ

u

^ℓ_δ

ϕ

!^α+p−1_ℓ

+ M |α| k

^α−1

, with a new constant C = C(N, p, K, α, ℓ). As k → ∞, we deduce

|α|

2

Z

Ω

u

^α−1_δ

ϕA

_p

(x, u, ∇u).∇u +

Z

Ω

f u

^α_δ

ϕ ≤ C |S

ξ

|

^θ1′

max

Ω

|∇ξ|

^p Z

Sξ

u

^ℓ_δ

ϕ

!^α+p−1_ℓ

. (2.6) Finally as δ → 0 we get (2.5) with a new constant C. Moreover we deduce an estimate of the gradient terms:

|α|

2

Z

Ω

u

^α−1

ϕA

_p

(x, u, ∇u).∇u ≤ C |S

ξ

|

^θ1′

max

Ω

|∇ξ|

^p Z

Ω

u

^ℓ

ϕ

^α+p−1_ℓ

. (2.7)

(ii) Next we only assume that ℓ > p − 1, u

^ℓ

∈ L

¹

(B(x

₀

, ρ)). Let ϕ as above, and fix some α = α(p, ℓ) such that α ∈ (1 − p, 0) and (1 − α)(p − 1) < ℓ for (2.2), α ∈ (0, 1) and α + p − 1 <

ℓ for (2.3). In any case τ = ℓ/(1 − α)(p − 1) > 1, and 1/θp

^′

+ 1/pτ = (p − 1)/ℓ. Let λ ≥ λ(p, α(p, ℓ), ℓ) ≥ pτ

^′

. We take ϕ as a test function and from (2.6) we deduce successively, with new constants C,

Z

Ω

f ϕ ≤ λ

Z

Ω

ξ

^λ−1

|A

p

(x, u, ∇u)| |∇ξ| ≤ C

Z

Ω

ξ

^λ−1

|A

p

(x, u, ∇u)| |∇ξ| u

α−1 p′

δ

u

1−α p′

δ

≤ C

Z

Sξ

u

^α−1_δ

|A

p

(x, u, ∇u)|

^p′

ϕ

!¹

p′ Z

Sξ

u

^{(1−α)(p−1)}_δ

ξ

^λ−p

|∇ξ|

^p

!¹_p

≤ C

Z

Sξ

u

^α−1_δ

ϕA

_p

(x, u, ∇u).∇u

!¹

p′ Z

Sξ

u

^ℓ_δ

ϕ

!_pτ¹ Z

Sξ

ξ

^λ−pτ′

|∇ξ|

^pτ′

! ¹

pτ′

≤ C |S

ξ

|

^{θ′1p′+pτ1′}

max

Ω

|∇ξ|

^p Z

Sξ

u

^ℓ_δ

ϕ

! ¹

p′θ+_pτ¹

≦

C |S

ξ

|

^1−p−1ℓ

max

Ω

|∇ξ|

^p Z

Sξ

u

^ℓ_δ

ϕ

!^p−1_ℓ

;

and (2.4) follows as δ → 0.

Corollary 2.2 Under the assumptions of Proposition 2.1, consider any ball B (x

₀

, 2ρ) ⊂ Ω, and any ε ∈ 0,

¹₂

. Let ϕ = ξ

^λ

with ξ such that

ξ = 1 in B(x

₀

, ρ), ξ = 0 in Ω\ B ¯ (x

₀

, ρ(1 + ε)) |∇ξ| ≤ C

₀

ερ . (2.8)

Then for any ℓ > p − 1, there exists λ(p, ℓ) > 0 such that for λ ≥ λ(p, ℓ), there exists C = C(N, p, K, ℓ, λ) > 0 such that

I

ϕ

f ≤ C(ερ)

^−p I

ϕ

u

^{ℓ p−1}_ℓ

. (2.9)

Remark 2.3 If S

_ξ

= ∪

^k_i=1

S

_ξⁱ

where the S

_ξⁱ

are 2 by 2 disjoint, then (2.4) can be replaced by

Z

Ω

f ϕ

≦

C

k

X

i=1

S

_ξⁱ

max

S_ξⁱ

|∇ξ|

^p I

Sⁱ_ξ

u

^ℓ

!^p−1_ℓ

. (2.10)

2.2 Punctual estimates under strong conditions

When A

_p

is S-p-C, the estimate (2.7) of the gradient is the beginning of the proof of the well-known weak Harnack inequalities:

Theorem 2.4 ([25], [28]) (i) Let A

p

be S-p-C, and u ∈ W

_loc^1,p

(Ω) be nonnegative, such that

−A

p

u

≦

0 in Ω;

then for any ball B (x

₀

, 3ρ) ⊂ Ω, and any ℓ > p − 1, sup

B(x0,ρ)

u ≤ C

I

B(x0,2ρ)

u

^ℓ

!¹_ℓ

, (2.11)

with C = C(N, p, ℓ, K

_1,p

, K

_2,p

).

(ii) Let w ∈ W

_loc^1,p

(Ω) be nonnegative, such that

−A

_p

w ≥ 0 in Ω;

then for any ball B (x

0

, 3ρ) ⊂ Ω, for any ℓ ∈ (0, N (p − 1)/(N − p))

I

B(x0,2ρ)

v

^ℓ

!¹_ℓ

≤ C inf

B(x0,ρ)

v. (2.12)

Next we give a more precise version of weak Harnack inequality (2.11). Such a kind of

inequality was first established in the parabolic case in [12].

Lemma 2.5 Let A

p

be S-p-C, and u ∈ W

_loc^1,p

(Ω) be nonnegative, such that

−A

_p

u

≦

0 in Ω;

then for any s > 0, there exists a constant C = C(N, p, s, K

_1,p

, K

_2,p

), such that for any ball B (x

₀

, 2ρ) ⊂ Ω and any ε ∈ 0,

¹₂

,

sup

B(x0,ρ)

u ≤ Cε

^{−N p}

2 s2

I

B(x0,ρ(1+ε))

u

^s

!¹_s

. (2.13)

Proof. From a slight adaptation of the usual case where ε =

¹₂

, for any ℓ > p − 1, there exists C = C(N, ℓ) > 0 such that for any ε ∈ 0,

¹₂

,

sup

B(x0,ρ)

u

≦

Cε

^−N I

B(x0,ρ(1+ε))

u

^ℓ

!¹_ℓ

. (2.14)

Thus we can assume s ≤ p − 1. We fix for example ℓ = p, and define a sequence (ρ

_n

) by ρ

₀

= ρ, and ρ

_n

= ρ(1 +

^ε₂

+ ... + (

^ε₂

)

ⁿ

) for any n ≥ 1, and we set M

_n

= sup

_B(x0,ρn)

u

^p

. From (2.14) we obtain, with new constants C = C(N, p),

M

_n≦

C( ρ

_n+1

ρ

_n

− 1)

^{−N p} I

B(x0,ρn+1)

u

^p

≤ C( ε

2 )

^{−(n+1)N p} I

B(x0,ρn+1)

u

^p

. From the Young inequality, for any δ ∈ (0, 1), and any r < 1, we obtain

M

n≦

C( ε

2 )

^{−(n+1)N p}

M

_n+1^1−r I

B(x0,ρn+1)

u

^pr

≦

δM

_n+1

+ rδ

^1−1/r

(C( ε

2 )

^{−(n+1)N p}

)

^1r I

B(x0,ρn+1)

u

^pr

!¹_r

.

Defining κ = rδ

^1−1/r

C

^1r

and b = (

^ε₂

)

^{−N p/r}

, we find

M

n≦

δM

n+1

+ b

ⁿ⁺¹

κ

I

B(x0,ρn+1)

u

^pr

!¹_r

. Taking δ =

_2b¹

and iterating, we obtain

M

₀

= sup

B(x0,ρ)

u

^p≦

δ

ⁿ⁺¹

M

_n+1

+ bκ

n

X

i=0

(δb)

ⁱ I

B(x0,ρn+1)

u

^pr

!¹_r

≦

δ

ⁿ⁺¹

M

_n+1

+ 2bκ

I

B(x0,ρn+1)

u

^pr

!¹_r

.

Since B(x

₀

, ρ

_n+1

) ⊂ B(x

₀

, ρ(1+ε)), going to the limit as n → ∞, and returning to u, we deduce sup

B(x0,ρ)

u ≤ (2bκ)

^1/p I

B(x0,ρ(1+ε))

u

^pr

!_rp¹

, and the conclusion follows by taking r = s/p.

It is interesting to make the link between Proposition 2.1, with the powerful estimates issued from the potential theory, involving W¨ olf potentials, proved in [20], [21] and [22]. Here we show that the lower estimates hold for any S-p-C operator.

Corollary 2.6 Suppose that A

_p

is S-p-C. Let f ∈ L

¹_loc

(Ω), f ≥ 0 and u ∈ W

_loc^1,p

(Ω) be any nonnegative such that

−A

p

u

≧

f, in Ω;

then for any ball B (x

₀

, 2ρ) ⊂ Ω,

CW

_1,p^f

(B(x

0

, ρ)) + inf

B(x0,2ρ)

u ≤ lim inf

x→x0

u(x), (2.15)

where W

_1,p^f

is the W¨ olf potential of f defined at (1.7), and C = C(N, p, K

_1,p

, K

_2,p

). If u satisfies (2.3), then

CW

_1,p^f

(B(x

₀

, ρ)) + lim sup

x→x0

u(x) ≤ sup

B(x0,2ρ)

u. (2.16)

Proof. (i) The function w = u− m

_2ρ

, where m

_ρ

= inf

_B(x0,ρ)

u, is nonnegative in B (x

₀

, 2ρ), and satisfies the inequality −B

p

w ≥ f, where

w 7−→ B

p

w = div A

_p

(x, w + m

_2ρ

, ∇w)

is also a S-p-C operator. Then from Proposition 2.1 with ξ as in (2.8), fixing ℓ ∈

0,

^N(p−1)_N−p

and ε =

¹₂

, and applying Harnack inequality (2.12), there exists C = C(N, p, K

_1,p

, K

_2,p

) such that

2C ρ

^1−N Z

B(x0,ρ)

f

!_p−1¹

≤ ρ

⁻¹ I

B(x0,2ρ)

(u − m

_2ρ

)

^ℓ

!¹_ℓ

≤ ρ

⁻¹

(m

_ρ

− m

_2ρ

).

Setting ρ

_j

= 2

^1−j

ρ, as in [20], CW

_1,p^f

(B(x

₀

, ρ)) ≤

∞

X

j=1

(m

_ρj

− m

_ρj−1

) = lim m

_ρj

− inf

B(x0,2ρ)

u = lim inf

x→x0

u − inf

B(x0,2ρ)

u.

(ii) The function y = M

_2ρ

− u where M

_2ρ

= sup

_B(x0,2ρ)

u satisfies the inequality −C

p

w ≥ f in B (x

₀

, 2ρ), where

w 7−→ C

p

w := div [A

_p

(x, M

_2ρ

− w, ∇w)]

is still S-p-C. Then

W

_1,p^f

(B(x

₀

, ρ)

≦

C( sup

B(x0,2ρ)

u − lim sup

x→x0

u),

and (2.16) follows.

Remark 2.7 The minorizations by W¨ olf potentials (2.15) and (2.16) have been proved in [20]

and [22] for S-p-C operators of type A

p

u := div [A

_p

(x, ∇u)] independent of u, satisfying more- over monotonicity and homogeneity properties, in particular A

_p

(−u) = −A

_p

u. The solutions are defined in the sense of potential theory, and may not belong to W

_loc^1,p

(Ω) , f can be a Radon measure; majorizations by W¨ olf potentials are also given, with weighted operators, see [21] and [22]. In the same way Proposition 2.1 can also be extended to weighted operators, see [8, Re- mark 2.4] and [14], or to the case of a Radon measure when A

p

is S-p-C by using the notion of local renormalized solution introduced in [3].

2.3 A bootstrap result

Finally we give a variant of a result of [5, Lemma 2.2]:

Lemma 2.8 Let d, h ∈

R

with d ∈ (0, 1) and y, Φ be two positive functions on some interval (0, R] , and y is nondecreasing. Assume that there exist some K, M > 0 and ε

₀

∈ 0,

¹₂

such that, for any ε ∈ (0, ε

₀

],

y(ρ)

≦

Kε

^−h

Φ(ρ)y

^d

[ρ(1 + ε)] and max

τ∈

[

^ρ,3ρ₂

] Φ(τ )

≦

M Φ(ρ), ∀ρ ∈

0, R 2

. Then there exists C = C(K, M, d, h, ε

₀

) > 0 such that

y(ρ)

≦

CΦ(ρ)

^1−d1

, ∀ρ ∈

0, R 2e

. (2.17)

Proof. Let ε

_m

= ε

₀

/2

^m

(m ∈

N

), and P

_m

= (1 + ε

₁

)..(1 + ε

_m

). Then (P

_m

) has a finite limit P > 0, and more precisely P ≤ e

^2ε0

≤ e. For any ρ ∈ 0,

_2e^R

and any m ≥ 1, y(ρP

_m−1

) ≤ Kε

^−h_m

Φ(ρP

_m−1

)y

^d

(ρP

_m

).

By induction, for any m ≥ 1,

y(ρ) ≤ K

^{1+d+..+dm−1}

ε

^−h₁

ε

^−hd₂

..ε

^−hd_m ^m−1

Φ(ρ)Φ

^d

(ρP

1

)..Φ

^dm−1

(ρP

m−1

)y

^dm

(ρP

m

).

Hence from the assumption on Φ,

y(ρ) ≤ (Kε

^−h₀

)

^{1+d+..+dm−1}

2

k(1+2d+..+md^m−1)

M

^{d+2d2+..+(m−1)dm−1}

Φ(ρ)

^{1+d+..+dm−1}

y

^dm

(ρP

_m

);

and y

^dm

(ρP

_m

) ≤ y

^dm

(eρ) ≤ y

^dm

(

^R₂

), and lim y

^dm

(

^R₂

) = 1, because d < 1. Hence (2.17) follows with C = (Kε

^−h₀

)

^1/(1−d)

2

^h/(1−d)2

M

^d/(1−d)2

.

3 Keller-Osserman estimates

3.1 The scalar case

First consider the solutions of inequality

−A

p

u + cu

^Q

≤ 0, in Ω, (3.1)

with Q > p − 1 and c > 0. From the integral estimates of Proposition 2.1 we get easily Keller- Osserman estimates in the scalar case of the equation with absorption, without any hypothesis of monotonicity on the operator:

Proposition 3.1 Let Q > p − 1, c > 0. If A

_p

is S-p-C, and u ∈ W

_loc^1,p

(Ω) ∩ C (Ω) is a nonnegative solution of (3.1), there exists a constant C = C(N, p, K

_1,p

, K

_2,p

, Q) > 0 such that, for any x ∈ Ω,

u(x) ≤ Cc

^{−1/(Q+1−p)}

d(x, ∂Ω)

^{−p/(Q+1−p)}

. (3.2) Proof. Let B (x

₀

, ρ

₀

) ⊂ Ω, and u ∈ W

^1,p

(B(x

₀

, ρ

₀

)) . From Corollary 2.2 with ρ ≤

^ρ₂⁰

, ε =

¹₂

, and ℓ = Q and a function ϕ satisfying (2.8), we obtain for λ = λ(p, Q)

I

ϕ

u

^Q

≤ c

⁻¹

Cρ

^−p I

ϕ

u

^{Q p−1}_Q

, (3.3)

where C = C(N, p, K

_1,p

, K

_2,p

, Q). Then with another C > 0 as above,

I

B(x0,ρ)

u

^Q

!_Q¹

≤ Cc

^−Q+1−p1

ρ

^−Q+1−pp

.

Since A

p

is S-p-C, from the weak Harnack inequality (2.11), with another constant C as above,

u(x

₀

) ≤ C

I

B(x0,ρ)

u

^Q

!_Q¹

≤ c

^−Q+1−p1

ρ

^−Q+1−pp

, and (3.2) follows by taking ρ

₀

= d(x

₀

, ∂Ω).

3.2 The systems (A) and (M )

Here we prove theorems 1.1, 1.2, and Corollary 1.3. We recall that γ and ξ are defined by (1.8) under the condition (1.2) of superlinearity:

γ = p(q − 1) + qδ

D , ξ = q(p − 1) + pµ

D , D = δµ − (p − 1)(q − 1) > 0.

Proof of Theorem 1.1. Consider a ball B(x

₀

, ρ

₀

) ⊂ Ω, ε ∈ 0,

¹₂

, and a function ϕ satisfying (2.8) with λ large enough.

(i) Case µ > p − 1, δ > q − 1. Here C denotes different constants which only depend on N, p, q, δ, µ, and K

_1,p

, K

_2,p

, K

_1,q

, K

_2,q

. We take ε =

¹₂

and apply Corollary 2.2 with ρ ≤

^ρ₂⁰

to the solution u with f = v

^δ

, and with ℓ = µ > p − 1. since A

_p

is W-p-C, from (2.9), we obtain

I

ϕ

v

^δ

≤ Cρ

^−p I

ϕ

u

^{µ p−1}_µ

, (3.4)

and similarly we apply it to the solution v with now f = u

^µ

and ℓ = δ > q − 1 : since A

q

is W-q-C, we obtain

I

ϕ

u

^µ

≤ Cρ

^−q I

ϕ

v

^{δ q−1}_δ

. (3.5)

We can assume that

H

ϕ

u

^µ

> 0. Indeed if

H

ϕ

u

^µ

= 0, then u = 0 in B(x

₀

, ρ

₀

). Then ∇u = 0, thus v

^δ

= 0 and then the estimates are trivially verified. Replacing (3.5) in (3.4) we deduce

I

ϕ

v

^δ

≤ Cρ

^{−p−qp−1µ} I

ϕ

v

^δ

^{(q−1)(p−1)}_µδ

, and similarly for u, hence

I

ϕ

v

^{δ 1}_δ

≤ Cρ

^−ξ

,

I

ϕ

u

^µ _µ¹

≤ Cρ

^−γ

. (3.6)

Moreover, since A

q

is S-q-C, then from the usual weak Harnack inequality, since v ∈ L

^∞_loc

(Ω), and ϕ(x) = 1 in B (x

₀

, ρ), with values in [0, 1] ,

sup

B(x0,^ρ₂)

v ≤ C

I

B(x0,ρ)

v

^δ

!¹_δ

≤

I

ϕ

v

^{δ 1}_δ

≤ Cρ

^−ξ

. Similarly

sup

B(x0,^ρ₂)

u ≤ Cρ

^−γ

, because A

p

is S-p-C.

(ii) Case µ > p − 1, and δ ≤ q − 1. Here we still apply Corollary 2.2 with ρ ≤

^ρ₂⁰

, ε ∈ (0, 1/4] , and a function ϕ satisfying (2.8). Since µ > p − 1, we still obtain (3.4); and for any k > q − 1, and λ large enough,

I

ϕ

u

^µ

≤ C(ερ)

^−q I

ϕ

v

^k

(q−1)/k

, (3.7)

and from Lemma 2.5,

I

ϕ

v

^k 1/k

≤ sup

B(x0,ρ(1+ε))

v ≤ Cε

^{−N q}

2 δ2

I

B(x0,ρ(1+2ε))

v

^δ

!¹_δ

. Then with new constants C, setting m = q + δ

⁻²

N q

²

(q − 1), and h = (p − 1)µ

⁻¹

m,

I

ϕ

u

^µ

≤ Cε

^−m

ρ

^−q I

B(x0,ρ(1+2ε))

v

^δ

!^(q−1)_δ

, (3.8)

hence from (3.4) and (3.8),

I

B(x0,ρ)

v

^δ

≤ C

I

ϕ

v

^δ

≤ Cρ

^−p I

ϕ

u

^{µ p−1}_µ

≤ Cε

^−h

ρ

^{−pµ+q(p−1)µ} I

B(x0,ρ(1+2ε))

v

^δ

!^{(p−1)(q−1)}_δµ

,

for any ρ ≤

^ρ₂⁰

. Next we apply the boostrap Lemma 2.8 with R = ρ

₀

, y(ρ) =

H

B(x0,ρ)

v

^δ

, Φ(r) = r

^{−pµ+q(p−1)µ}

and 2ε. We deduce that

I

B(x0,ρ)

v

^δ

!1/δ

≤ Cρ

^−ξ

, for any ρ <

^ρ₂⁰

e, and thus also

sup

B(x0,^ρ₂)

v ≤ C

I

B(x0,ρ)

v

^δ

!¹_δ

≤ Cρ

^−ξ

, sup

B(x0,^ρ₂)

u ≤ C

I

B(x0,ρ)

u

^µ

!1/µ

≤ Cρ

^−γ

. In particular

u(x

₀

) ≤ Cρ

^−γ₀

, v(x

₀

) ≤ Cρ

^−ξ₀

, (3.9) for any ball B(x

₀

, ρ

₀

) ⊂ Ω, and the estimates (1.10) follow by taking ρ

₀

= d(x

₀

, ∂Ω).

Proof of Theorem 1.2. We consider a ball B(x

0

, ρ

0

) such that B(x

0

, 2ρ

0

) ⊂ Ω. From Propo- sition 2.1, we have the same estimates: for any ℓ > p − 1, k > q − 1, ρ ≤ ρ

₀

,

I

ϕ

u

^µ

≤ Cρ

^−q I

ϕ

v

^{k q−1}_k

,

I

ϕ

v

^δ

≤ Cρ

^−p I

ϕ

u

^{ℓ p−1}_ℓ

. From Lemma 2.5 (even if µ < p − 1), we have

sup

B(x0,^ρ₂)

u

^µ

≤ C

I

B(x0,ρ)

u

^µ

.

Taking k <

^N(q−1)_N−q

, and using the weak Harnack inequality for v, we obtain sup

B(x0,^ρ₂)

u

^µ

≤ C

I

B(x0,ρ)

u

^µ

≤ C

I

ϕ

u

^µ

≤ Cρ

^−q I

ϕ

v

^{k q−1}_k

≤ Cρ

^−q I

B(x0,2ρ)

v

^k

!^q−1_k

≤ Cρ

^−q

inf

B(x0,ρ)

v

^(q−1)

;

hence (1.11) holds in B(x

0

,

^ρ₂

). Moreover if v(x

0

) = 0, then u = 0 in B (x

0

,

^ρ₂

), then also v = 0 in B(x

₀

,

^ρ₂

). Since Ω is connected, it implies that v ≡ 0, and then u ≡ 0. If v 6≡ 0, then v stays positive in Ω, and we can write

−A

q

v = dv

^q−1

, in Ω, (3.10)

with d(x) = u

^µ

/v

^(q−1)

≤ Cρ

^−q

in B(x

₀

,

^ρ₂

); in particular d(x

₀

) = u

^µ

(x

₀

)

v

^q−1

(x

₀

) ≤ Cρ

^−q

, (3.11)