谭玉鑫
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, 孙义静
中国科学院大学数学科学学院, 北京 100049
摘要: 证明-div(
M(
x)▽
u)=
$\frac{{f\left( x \right)}}{{{u^p}}}$正
H01-解的存在性,其中
M(
x)是有界椭圆矩阵(即存在0 <
α ≤
β满足
M(
x)
ξ·ξ ≥
α|
ξ|
2,|
M(
x)|≤
β,?
x ∈ Ω,
ξ ∈
Rn)和-
p < -1.本工作的关键点在于建立2个密切联系的集合,便于找到相应的能量泛函最小值。
关键词: 有界椭圆矩阵弱解强奇性
In this work, we consider the existence of solutions of the semilinear elliptic problem with a singular nonlinearity,
$\left\{ \begin{array}{l} - {\rm{div}}\left( {\begin{array}{*{20}{c}}{\mathit{\boldsymbol{M}}\left( x \right)}&{\nabla u}\end{array}} \right) = h\left( x \right){u^{ - p}}\;\;\;{\rm{in}}\;\Omega ,\\u > 0\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;{\rm{in}}\;\Omega ,\\u = 0\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;{\rm{on}}\;\Omega ,\end{array} \right.$ | (1) |
where Ω
$\subset $Rn is a bounded open set with smooth boundary
$\partial $Ω,
M(
x) is a real symmetric matrix satisfying
$\begin{array}{*{20}{c}}{\mathit{\boldsymbol{M}}\left( x \right)\mathit{\boldsymbol{\xi }} \cdot \mathit{\boldsymbol{\xi }} \ge \alpha {{\left| \mathit{\boldsymbol{\xi }} \right|}^2},}\\{\left| {\mathit{\boldsymbol{M}}\left( x \right)} \right| \le \beta ,\forall x \in \Omega ,\mathit{\boldsymbol{\xi }} \in {{\bf{R}}^n},}\end{array}$ | (2) |
and
h(
x)>0 a.e.in Ω and
-p < -1.By solutions we mean here weak solutions in
H01(Ω), i.e.,
u∈
H01(Ω) satisfying
u(
x)>0 in Ω and
$\begin{array}{*{20}{c}}{\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla \nu {\rm{d}}x} - \int_\Omega {\frac{{h\left( x \right)}}{{{u^p}}} \cdot \nu {\rm{d}}x} = 0,}\\{\forall \nu \in H_0^1\left( \Omega \right).}\end{array}$ |
Since the work by Stuart
[1], people have paid much attention to the existence and multiplicity of solutions for such singular equations
$ - {\rm{div}}\left( {\begin{array}{*{20}{c}}{\mathit{\boldsymbol{M}}\left( x \right)}&{\nabla u}\end{array}} \right) = f\left( {x,u} \right),$ |
where
f(
x,
s) is singular at
s=0. See Refs.[
2-
5] and the rich list of references provided by these papers for a survey. Recently, Boccardo and Orsina
[6] solved the problem with
f(
x,
u)=
h(
x)
u-p,
h(
x)≥0, -
p < -1 and provided the existence of an
Hloc1(Ω)-solution
u by using approximation arguments when
M(
x) is a real symmetric matrix satisfying
M(
x)
ξ·
ξ≥
α|
ξ|
2, |
M(
x)|≤
β, ?
x∈Ω,
ξ∈
Rn and
${{u}^{\frac{1+p}{2}}}$∈
H01(Ω). Then, under a superlinear perturbation of
uq with
q>1, Boccardo
[7] also proved the existence of
Hloc1(Ω)-solution for each -
p < -1 and
${{u}^{\frac{1+p}{2}}}$∈
H01(Ω). Recently, Boccardo and Casado-Dìaz
[8] studied some properties of the solution of problem (1). They showed that if
M(
x) is a bounded elliptic matrix,
h(
x)∈
Lm(Ω),
m≥(2
*)′, supp(
h(
x)) is compact, then the solution
u of (1) obtained as the limit of the solution
un of -div(
M(
x)Δ
un)=
$\frac{h\left( x \right)}{1/n+u_{n}^{-p}}$ is in
H01(Ω). In this work, we will show a compatible condition on the couple (
h(
x),
p), which is optimal for the existence of
H01-solutions.
We define the singular energy functional
$\begin{array}{l}I\left( u \right) = \frac{1}{2}\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u{\rm{d}}x} + \\\;\;\;\;\;\;\;\;\;\;\frac{1}{{p - 1}}\int_\Omega {h\left( x \right){{\left| u \right|}^{1 - p}}{\rm{d}}x} ,\end{array}$ | (3) |
where -
p < -1. The main difficulty is the absence of integrability of
u-p for
u∈
H01(Ω) when -
p < -1 and any inequality that relates
u∈
H01(Ω) will not be of much help. It should be noted also that there is a sharp contrast between the case -1 < -
p < 0, for which the energy functional is continuous, and the case -
p < -1. Generally, the sub-supersolution method is very effective in dealing with singularity. However, the method cannot be used for such general measurable
h(
x)>0. To reverse this situation, we use constrained sets to restore integrability and recast problem (1) into a variational framework in the spirit of our earlier works
[9-12]. We defined constrained sets
N1 and
N2 as follows:
N1:={
u∈
H01(Ω):
u≥0 in Ω and ∫
ΩM(
x)▽
u·▽
u≥∫
Ωh(
x)|
u|
1-p},
N2:={
u∈
H01(Ω):
u≥0 in Ω and ∫
ΩM(
x)▽
u·▽
u=∫
Ωh(
x)|
u|
1-p}. Here, special care must be taken to establish the validity and connection of the two constraints which simplify the existence of a minimizer for the singular functional
I. It should also be noted that for -
p < -1,
N2 is not closed as usual (certainly not weakly closed) in
H01(Ω).
In this paper we will use the notation,
C,
Ci,
ci,
i=1, 2, …, denoting (possibly different) constants.
We denote the Dirichlet norm in
H01(Ω) by ‖
u‖
2=∫
Ω|▽
u|
2d
x, |
M(
x)|=det
M(
x), and
M(
x)
ξ·
η:=
ξTM(
x)
η.
1 Main results
Theorem 1.1?? Let Ω
$\subset $Rn be bounded open set with smooth boundary
$\partial $Ω,
M(
x) be the real symmetric matrix satisfying
M(
x)
ξ·
ξ≥
α|
ξ|
2, |
M(
x)|≤
β, ?
x∈Ω,
ξ∈
Rn with 0 <
α≤
β,
h(
x)∈
L1(Ω),
h(
x)>0 a.e.in Ω and -
p < -1, then
$ - {\rm{div}}\left( {\begin{array}{*{20}{c}}{\mathit{\boldsymbol{M}}\left( x \right)}&{\nabla u}\end{array}} \right) = h\left( x \right){u^{ - p}}$ |
admits an
H01-solution if there exists
u0∈
H01(Ω) such that
$\int_\Omega {h\left( x \right){{\left| {{u_0}} \right|}^{1 - p}}{\rm{d}}x} < + \infty .$ | (4) |
2 Proof of Theorem 1.1
Proof?? It should be noted that the topology on
H01(Ω) which was generated by the norm
${\left( {\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u} } \right)^{\frac{1}{2}}}$ |
is equivalent to the one that was generated by the norm
${\left( {\int_\Omega {{{\left| {\nabla u} \right|}^2}} } \right)^{\frac{1}{2}}},$ |
since
$\begin{array}{*{20}{c}}{\alpha \int_\Omega {{{\left| {\nabla u} \right|}^2}} \le \int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u} \le }\\{\frac{\beta }{{{\alpha ^{n - 1}}}}\int_\Omega {{{\left| {\nabla u} \right|}^2}} .}\end{array}$ |
Hence
$\begin{array}{*{20}{c}}{{{\left( {H_0^1\left( \Omega \right),{{\left( {\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u} } \right)}^{\frac{1}{2}}}} \right)}^ * } = }\\{{{\left( {H_0^1\left( \Omega \right),{{\left( {\int_\Omega {{{\left| {\nabla u} \right|}^2}} } \right)}^{\frac{1}{2}}}} \right)}^ * }.}\end{array}$ |
Then, it follows that
${u_n} \to u\;{\rm{weakly}}\;{\rm{in}}\;\left( {H_0^1\left( \Omega \right),{{\left( {\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u} } \right)}^{\frac{1}{2}}}} \right)$ |
is equal to
${u_n} \to u\;{\rm{weakly}}\;{\rm{in}}\;\left( {H_0^1\left( \Omega \right),{{\left( {\int_\Omega {{{\left| {\nabla u} \right|}^2}} } \right)}^{\frac{1}{2}}}} \right).$ |
The key to prove (1) depends on a natural interpolation between the constrained sets
Ni,
i=1, 2. Taking
u∈
H01(Ω) with
$\int_\Omega {h\left( x \right){{\left| u \right|}^{1 - p}}} < \infty ,$ |
the function
$U\left( t \right): = {t^{1 + p}}\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u{\rm{d}}x} - \int_\Omega {h\left( x \right){{\left| u \right|}^{1 - p}}} {\rm{d}}x$ |
is increasing on
t>0 with
$\mathop {\lim }\limits_{t \to + \infty } \, U\left( t \right)=+\infty $ and
$\mathop {\lim }\limits_{t \to {0^ + }} \, U\left( t \right) < 0$. Since
$\begin{array}{*{20}{c}}{\frac{{{\rm{d}}I\left( {tu} \right)}}{{{\rm{d}}t}} = t\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u{\rm{d}}x} - {t^{ - p}}\int_\Omega {h\left( x \right){{\left| u \right|}^{1 - p}}} {\rm{d}}x}\\{ = {t^{ - p}}\left( {{t^{1 + p}}\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u{\rm{d}}x} - \int_\Omega {h\left( x \right){{\left| u \right|}^{1 - p}}} {\rm{d}}x} \right),}\end{array}$ |
then it follows that there exists the unique positive minimizer
t(
u)
u such that
$I\left( {tu} \right) \ge I\left( {t\left( u \right)u} \right),\forall t > 0.$ | (5) |
In particular, assumption (4) of Theorem 1.1 implies the existence of
t(
u0)>0 such that
t(
u0)
u0∈
N2 and hence
N1(
$\supset $N2) and
N2 are not empty. Clearly, since
tu0∈
N1 for all ≥
t(
u0),
N1 is unbounded in
H01(Ω). The closeness of
N1 follows easily from Fatou's lemma. However, it should be noted that
N2 is not anymore a closed set in
H01(Ω) since ∫
Ωh(
x)|
u|
(1-p)d
x is not continuous in
H01(Ω) as -
p < -1. Furthermore, unbounded
N1 lies in the exterior of
H01(Ω) (i.e., it stays away from a ball centered at zero). Indeed, since -
p < -1, the reversed H?lder inequality
$\begin{array}{*{20}{c}}{\beta {{\left\| u \right\|}^2} \ge \int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u} \ge \int_\Omega {h\left( x \right){{\left| u \right|}^{1 - p}}} }\\{ \ge {{\left( {\int_\Omega {h{{\left( x \right)}^{1 - p}}} } \right)}^p}{{\left( {\int_\Omega {\left| u \right|} } \right)}^{1 - p}}}\end{array}$ |
and Poincaré inequality
${\left( {\int_\Omega {\left| u \right|} } \right)^{1 - p}} \ge {C_1}{\left\| u \right\|^{1 - p}},$ |
imply that ‖
u‖≥
C for all
u∈
N1.
It should also be noted that
$\begin{array}{*{20}{c}}{\left| {\mathit{\boldsymbol{M}}\left( x \right)\mathit{\boldsymbol{\xi }} \cdot \mathit{\boldsymbol{\eta }}} \right| \le \frac{\beta }{{{\alpha ^{n - 1}}}}\left| \mathit{\boldsymbol{\xi }} \right| \cdot \left| \mathit{\boldsymbol{\eta }} \right|,}\\{\forall \mathit{\boldsymbol{\xi }},\mathit{\boldsymbol{\eta }} \in {{\bf{R}}^n},x \in \Omega .}\end{array}$ | (6) |
Indeed, since
M(
x) is a real symmetric matrix, there exists an orthogonal matrix
Q(
x) such that
${\mathit{\boldsymbol{Q}}^{\rm{T}}}\mathit{\boldsymbol{MQ}} = \left( {\begin{array}{*{20}{c}}{{\lambda _1}}&{}&{}\\{}& \ddots &{}\\{}&{}&{{\lambda _n}}\end{array}} \right),$ |
where
λi,
i=1, …,
n, the eigenvalues of
M(
x), satisfy
λi≥
α since
M(
x)
ξ·
ξ≥
α|
ξ|
2, ?
x∈Ω,
ξ∈
Rn. Then it yields that
$\mathit{\boldsymbol{M}} = \mathit{\boldsymbol{Q}}\left( {\begin{array}{*{20}{c}}{{\lambda _1}}&{}&{}\\{}& \ddots &{}\\{}&{}&{{\lambda _n}}\end{array}} \right){\mathit{\boldsymbol{Q}}^{\rm{T}}}$ |
and
$\begin{array}{l}\mathit{\boldsymbol{M}}\left( x \right)\mathit{\boldsymbol{\xi }} \cdot \mathit{\boldsymbol{\eta }} = {\mathit{\boldsymbol{\xi }}^{\rm{T}}}\mathit{\boldsymbol{M}}\left( x \right)\mathit{\boldsymbol{\eta = }}{\mathit{\boldsymbol{\xi }}^{\rm{T}}}\mathit{\boldsymbol{Q}}\left( {\begin{array}{*{20}{c}}{{\lambda _1}}&{}&{}\\{}& \ddots &{}\\{}&{}&{{\lambda _n}}\end{array}} \right){\mathit{\boldsymbol{Q}}^{\rm{T}}}\mathit{\boldsymbol{\eta }}\\\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\mathit{\boldsymbol{ = }}{\left( {{\mathit{\boldsymbol{Q}}^{\rm{T}}}\mathit{\boldsymbol{\xi }}} \right)^{\rm{T}}}\left( {\begin{array}{*{20}{c}}{{\lambda _1}}&{}&{}\\{}& \ddots &{}\\{}&{}&{{\lambda _n}}\end{array}} \right){\mathit{\boldsymbol{Q}}^{\rm{T}}}\mathit{\boldsymbol{\eta }}.\end{array}$ |
Hence, if one defines
x:=
QTξ,
y:=
QTη, one can obtain
$\begin{array}{l}\mathit{\boldsymbol{M}}\left( x \right)\mathit{\boldsymbol{\xi }} \cdot \mathit{\boldsymbol{\eta }} = {\mathit{\boldsymbol{x}}^{\rm{T}}}\left( {\begin{array}{*{20}{c}}{{\lambda _1}}&{}&{}\\{}& \ddots &{}\\{}&{}&{{\lambda _n}}\end{array}} \right)y\\\;\;\;\;\;\;\;\;\;\;\;\;\;\;\; = \left( {\begin{array}{*{20}{c}}{{x_1}}& \cdots &{{x_n}}\end{array}} \right)\left( {\begin{array}{*{20}{c}}{{\lambda _1}}&{}&{}\\{}& \ddots &{}\\{}&{}&{{\lambda _n}}\end{array}} \right)\left( {\begin{array}{*{20}{c}}{{y_1}}\\ \vdots \\{{y_n}}\end{array}} \right)\\\;\;\;\;\;\;\;\;\;\;\;\;\;\;\; = {\lambda _1}{x_1}{y_1} + {\lambda _2}{x_2}{y_2} + \cdots + {\lambda _n}{x_n}{y_n},\end{array}$ |
which implies that
$\begin{array}{l}\left| {\mathit{\boldsymbol{M}}\left( x \right)\mathit{\boldsymbol{\xi }} \cdot \mathit{\boldsymbol{\eta }}} \right| = \left| {\sum\limits_{i = 1}^n {{\lambda _i}{x_i}{y_i}} } \right|\\\;\;\;\;\; = \left| {\prod\limits_{i = 1}^n {{\lambda _i}\left( {\frac{{{\lambda _1}}}{{\prod {{\lambda _i}} }}{x_1}{y_1} + \frac{{{\lambda _2}}}{{\prod {{\lambda _i}} }}{x_2}{y_2} + \cdots + \frac{{{\lambda _n}}}{{\prod {{\lambda _i}} }}{x_n}{y_n}} \right)} } \right.\\\;\;\;\;\; \le \frac{\beta }{{{\alpha ^{n - 1}}}}\left| x \right| \cdot \left| y \right|.\end{array}$ |
Since
$\begin{array}{l}{\left| x \right|^2} = {\mathit{\boldsymbol{x}}^{\rm{T}}} \cdot \mathit{\boldsymbol{x}} = {\left( {{\mathit{\boldsymbol{Q}}^{\rm{T}}}\mathit{\boldsymbol{\xi }}} \right)^{\rm{T}}} \cdot {\mathit{\boldsymbol{Q}}^{\rm{T}}}\mathit{\boldsymbol{\xi }} = {\mathit{\boldsymbol{\xi }}^{\rm{T}}}\mathit{\boldsymbol{Q}} \cdot {\mathit{\boldsymbol{Q}}^{\rm{T}}}\mathit{\boldsymbol{\xi }}\\\;\;\;\;\; = {\mathit{\boldsymbol{\xi }}^{\rm{T}}} \cdot \mathit{\boldsymbol{\xi }} = {\left| \mathit{\boldsymbol{\xi }} \right|^2},\end{array}$ |
$\begin{array}{l}{\left| y \right|^2} = {\mathit{\boldsymbol{y}}^{\rm{T}}} \cdot \mathit{\boldsymbol{y}} = {\left( {{\mathit{\boldsymbol{Q}}^{\rm{T}}}\mathit{\boldsymbol{\eta }}} \right)^{\rm{T}}} \cdot {\mathit{\boldsymbol{Q}}^{\rm{T}}}\mathit{\boldsymbol{\eta }} = {\mathit{\boldsymbol{\eta }}^{\rm{T}}}\mathit{\boldsymbol{Q}} \cdot {\mathit{\boldsymbol{Q}}^{\rm{T}}}\mathit{\boldsymbol{\eta }}\\\;\;\;\;\; = {\mathit{\boldsymbol{\eta }}^{\rm{T}}} \cdot \mathit{\boldsymbol{\eta }} = {\left| \mathit{\boldsymbol{\eta }} \right|^2},\end{array}$ |
it follows that
$\left| {\mathit{\boldsymbol{M}}\left( x \right)\mathit{\boldsymbol{\xi }} \cdot \mathit{\boldsymbol{\eta }}} \right| \le \frac{\beta }{{{\alpha ^{n - 1}}}}\left| \mathit{\boldsymbol{x}} \right| \cdot \left| \mathit{\boldsymbol{y}} \right| = \frac{\beta }{{{\alpha ^{n - 1}}}}\left| \mathit{\boldsymbol{\xi }} \right| \cdot \left| \mathit{\boldsymbol{\eta }} \right|.$ |
Furthermore, for
u∈
N1,
$\begin{array}{l}\int {h\left( x \right){{\left| u \right|}^{1 - p}}} \le \int {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u} \\\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\; \le \frac{\beta }{{{\alpha ^{n - 1}}}}\int {{{\left| {\nabla u} \right|}^2} < \infty } .\end{array}$ | (7) |
Then there exists
tu>0 such that
tuu∈
N2,
I(
u)≥
I(
tuu)≥
$\mathop {\inf }\limits_{{N_2}} \, I$, and therefore
$\mathop {\inf I}\limits_{{N_1}} \ge \mathop {\inf I}\limits_{{N_2}} .$ | (8) |
However, since
N1 $\supset $ N2, it follows that
$\mathop {\inf I}\limits_{{N_1}} \le \mathop {\inf I}\limits_{{N_2}} .$ | (9) |
In view of (8) and (9), it yields that
$\mathop {\inf I}\limits_{{N_1}} = \mathop {\inf I}\limits_{{N_2}} .$ |
Now, we turn our attention to
$\mathop {\inf }\limits_{{N_1}} \, I$. For
N1, we can assert that it is closed in
H01(Ω). Indeed, as
un→
u in
H01(Ω),
un→
u in
L2(Ω),
un→
u a.e.in Ω, and ∫
M(
x)▽
n·▽
un≥∫
h(
x)|
un|
1-p. Since
h(
x)>0 a.e.in Ω, it follows that
un>0 a.e. in Ω. By (6), we have
$\begin{array}{l}\mathop {\lim \;\rm{inf}}\limits_{n \to \infty } \int {h\left( x \right){{\left| {{u_n}} \right|}^{1 - p}}} \le \mathop {\lim \;\rm{inf}}\limits_{n \to \infty } \int {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla {u_n}} \\\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\; \le \mathop {\lim \;\rm{inf}}\limits_{n \to \infty } \frac{\beta }{{{\alpha ^{n - 1}}}}\int {{{\left| {\nabla {u_n}} \right|}^2} < \infty } ,\end{array}$ |
and based on Fatou' lemma, we obtain
$\begin{array}{l}\int {h\left( x \right){{\left| u \right|}^{1 - p}}} = \int {\mathop {\lim \;\rm{inf}}\limits_{n \to \infty } h\left( x \right){{\left| {{u_n}} \right|}^{1 - p}}} \\ \le \mathop {\lim \;\rm{inf}}\limits_{n \to \infty } \int {h\left( x \right){{\left| {{u_n}} \right|}^{1 - p}}} \le \mathop {\lim \;\rm{inf}}\limits_{n \to \infty } \int {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla {u_n}} \\ = \int {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u} .\end{array}$ |
On the other hand, we can claim that
I(
u) is weakly lower semi-continuous, that is,
$\begin{array}{*{20}{c}}{I\left( u \right) \le \mathop {\lim \;\rm{inf}}\limits_{n \to \infty } I\left( {{u_n}} \right)}\\{{\rm{as}}\;{u_n} \to u\;{\rm{weakly}}\;{\rm{in}}\;\left( {H_0^1\left( \Omega \right),{{\left( {\int {{{\left| {\nabla {u_n}} \right|}^2}} } \right)}^{\frac{1}{2}}}} \right).}\end{array}$ |
as
un→
u weakly in
$\left( {H_0^1\left( \Omega \right),{{\left( {\int {|\nabla u{|^2}} } \right)}^{\frac{1}{2}}}} \right)$. Indeed, there holds that
$\int {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla {u_n}} \to \int {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u} ,$ |
and by Fatou's lemma one can also obtain
$\int {h\left( x \right){{\left| u \right|}^{1 - p}}} \le \mathop {\lim \;\rm{inf}}\limits_{n \to \infty } \int {h\left( x \right){{\left| {{u_n}} \right|}^{1 - p}}} $ |
if
un→
u weakly in
$\left( {H_0^1\left( \Omega \right),{{\left( {\int {|\nabla u{|^2}} } \right)}^{\frac{1}{2}}}} \right)$. Now we can use Ekeland's principle
[13] to exploit the property of the best minimizing sequence for
$\mathop {\inf }\limits_{{N_1}} \, I$, that is, (
un)∈
N1 satisfying
$\begin{align} & \left( \text{ⅰ} \right)I\left( {{u}_{n}} \right) < \underset{{{N}_{1}}}{\mathop{\inf }}\,I+\frac{1}{n} \\ & \left( \text{ⅱ} \right)I\left( {{u}_{n}} \right)\le I\left( v \right)+\frac{1}{n}||{{u}_{n}}-v||,\forall v\in {{N}_{1}} \\ \end{align}$ |
since
N1 is a closed set in
H01(Ω). We may assume
un≥0 as
I(
u)=
I(|
u|). Since -
p < -1,
I(
u) is coercive on
N1 and therefore (
un) is bounded in
H01(Ω). Indeed, by -
p < -1,
$\begin{array}{*{20}{c}}{I\left( u \right) = \frac{1}{2}\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u{\rm{d}}x} + }\\{\frac{1}{{p - 1}}\int_\Omega {h\left( x \right){{\left| u \right|}^{1 - p}}{\rm{d}}x} \ge \frac{\alpha }{2}{{\left\| u \right\|}^2}.}\end{array}$ |
Hence, up to subsequence (still denoted by
un),
un $\rightharpoonup$ u* weakly in
H01(Ω), strongly in
L2(Ω), and pointwise a.e. in Ω. Therefore
u*≥0. More precisely,
${u^ * } > 0\;{\rm{a}}{\rm{.}}\;{\rm{e}}{\rm{.}}\;{\rm{in}}\;\Omega $ | (10) |
as ∫
Ωh(
x)|
u*|
1-p < ∞ by Fatou's lemma. Moreover, we shall show that
u*∈
N2 by evaluating the best minimizing sequence (
un)∈
N1.
Case 1. Suppose that (
un)
N1\
N2 for all
n large. Fix
φ∈
H01(Ω),
φ≥0 and
n by now. Note that, as (
un)
N1\
N2 and
p>1, there holds that
$\int {\mathit{\boldsymbol{M}}\left( x \right)} \nabla {{u}_{n}}\cdot \nabla {{u}_{n}}>\int_{\Omega }{h\left( x \right)}|{{u}_{n}}{{|}^{1-p}}\ge \int_{\Omega }{h\left( x \right)}|{{u}_{n}}+t\varphi {{|}^{1-p}}$ for
t≥0. Subsequently, choose
t>0 sufficiently small such that
$\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla \left( {{u_n} + t\varphi } \right) \cdot \nabla \left( {{u_n} + t\varphi } \right)} > \int_\Omega {h\left( x \right){{\left( {{u_n} + t\varphi } \right)}^{1 - p}}{\rm{d}}x} ,$ |
that is,
${u_n} + t\varphi \in {N_1}.$ |
In virtue of (ⅰ) and (ⅱ), we obtain that
$\begin{array}{l}\frac{t}{n}\left\| \varphi \right\| + \frac{1}{2}\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\left( {{{\left| {\nabla \left( {{u_n} + t\varphi } \right)} \right|}^2} - {{\left| {\nabla {u_n}} \right|}^2}} \right){\rm{d}}x} \\ \ge \frac{1}{{1 - p}}\int_\Omega {h\left( x \right)\left( {{{\left| {\left( {{u_n} + t\varphi } \right)} \right|}^{1 - p}} - {{\left| {{u_n}} \right|}^{1 - p}}} \right){\rm{d}}x} .\end{array}$ |
Dividing by
t>0, passing to the liminf as
t→0
+, we obtain
$\begin{array}{l}\frac{{\left\| \varphi \right\|}}{n} + \int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla \varphi } \\\;\;\;\;\;\;\; \ge \int_\Omega {\mathop {\lim \;\rm{inf}}\limits_{n \to \infty } \frac{{h\left( x \right)}}{{1 - p}}\frac{{{{\left( {{u_n} + t\varphi } \right)}^{1 - p}} - u_n^{1 - p}}}{t}} \\\;\;\;\;\;\;\; = \int_\Omega {h\left( x \right)u_n^{ - p}\varphi } .\end{array}$ |
Using Fatou's lemma again and letting
n tend to infinity, we have
$\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u^ * } \cdot \nabla \varphi } \ge \int_\Omega {h\left( x \right){u^{ * - p}}\varphi } ,\forall \varphi \ge 0.$ |
In view of (10), we obtain that
u*∈
N1, and by the above argument (5) there exists a unique
t(
u*) such that
I(
t(
u*)
u*)=
$\mathop {\min }\limits_{t > 0} \, I\left( t{{u}^{*}} \right)$. So
$\begin{array}{l}\mathop {\inf }\limits_{{N_1}} I = \mathop {\lim }\limits_{n \to \infty } I\left( {{u_n}} \right)\\\; = \mathop {\lim }\limits_{n \to \infty } \left[ {\frac{1}{2}\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla {u_n}} + \frac{1}{{p - 1}}\int_\Omega {h\left( x \right)u_n^{1 - p}} } \right]\\\; \ge \mathop {\lim \;\rm{inf}}\limits_{n \to \infty } \left[ {\frac{1}{2}\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla {u_n}} } \right] + \\\;\;\;\;\mathop {\lim \;\rm{inf}}\limits_{n \to \infty } \left[ {\frac{1}{{p - 1}}\int_\Omega {h\left( x \right)u_n^{1 - p}} } \right]\\\; \ge \frac{1}{2}\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u^ * } \cdot \nabla {u^ * }} + \frac{1}{{p - 1}}\int_\Omega {h\left( x \right){u^{ * 1 - p}}} \\\; = I\left( {{u^ * }} \right) \ge I\left( {t\left( {{u^ * }} \right){u^ * }} \right) \ge \mathop {\inf }\limits_{{N_2}} I \ge \mathop {\inf }\limits_{{N_1}} I,\end{array}$ |
and thus
t(
u*)=1, which means that
$\mathop {\min }\limits_{t > 0} I\left( {t{u^ * }} \right) = I\left( {{u^ * }} \right),{u^ * } \in {N_2}.$ |
Case 2. There exists a subsequence of (
un) (still denoted by
un), which belongs to
N2.
Let
φ∈
H10(Ω),
φ≥0, be fixed. Since -
p < -1,
$\begin{array}{*{20}{c}}{\int_\Omega {h\left( x \right){{\left( {{u_n} + t\varphi } \right)}^{1 - p}}{\rm{d}}x} \le \int_\Omega {h\left( x \right)u_n^{1 - p}{\rm{d}}x} < \infty ,}\\{\forall t \ge 0.}\end{array}$ |
By the previous argument (5), the function
fn, φ(
t):=
t(
un+
tφ), ?
t≥0 exists, and, moreover, using the notation therein,
fn, φ(0)=1 and
fn, φ(
t)(
un+
tφ)∈
N2. The continuity of
fn, φ(
t),
t>0 depends on ∫
Ωh(
x)|
un|
1-p < ∞ and dominates convergence. Indeed,
$\begin{array}{*{20}{c}}{f_{n,\varphi }^2\left( t \right)\int {\mathit{\boldsymbol{M}}\left( x \right)\nabla \left( {{u_n} + t\varphi } \right) \cdot \nabla \left( {{u_n} + t\varphi } \right)} }\\{f_{n,\varphi }^{1 - p}\left( t \right)\int {h\left( x \right){{\left( {{u_n} + t\varphi } \right)}^{1 - p}}} ,}\end{array}$ |
that is,
${f_{n,\varphi }}\left( t \right) = {\left[ {\frac{{\int {h\left( x \right){{\left( {{u_n} + t\varphi } \right)}^{1 - p}}} }}{{\int {\mathit{\boldsymbol{M}}\left( x \right)\nabla \left( {{u_n} + t\varphi } \right) \cdot \nabla \left( {{u_n} + t\varphi } \right)} }}} \right]^{\frac{1}{{p + 1}}}}.$ |
The key to showing that
u*∈
N2 hinges on the estimation of
f′
n, φ(0) defined as
${{f'}_{n,\varphi }}\left( 0 \right) = \mathop {\lim }\limits_{n \to \infty } \frac{{\left( {{f_{n,\varphi }}\left( t \right) - 1} \right)}}{t} \in \left[ { - \infty , + \infty } \right].$ |
If the limit does not exist, we let
tk→0 (instead of
t→0) with
tk>0 chosen in such a way that
$\mathop {\lim }\limits_{k \to \infty } \frac{{\left( {{f_{n, \varphi }}\left( {{t_k}} \right) - 1} \right)}}{{{t_k}}} \in \left[{-\infty, + \infty } \right]$. We deduce that
fn, φ(
t) has uniform behavior at zero with respect to
n, i.e., |
f′
n, φ(0)|≤
C for suitable
C>0 independent of
n. In fact, with
fn, φ(
t)(
un+
tφ)∈
N2,
un∈
N2, we have
$\begin{array}{l}0 = f_{n,\varphi }^2\left( t \right)\int {\mathit{\boldsymbol{M}}\left( x \right)\nabla \left( {{u_n} + t\varphi } \right) \cdot \nabla \left( {{u_n} + t\varphi } \right)} - \\\;\;\;\;\;f_{n,\varphi }^{1 - p}\left( t \right)\int {h\left( x \right){{\left( {{u_n} + t\varphi } \right)}^{1 - p}}} ,\end{array}$ |
$0 = \int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla {u_n}} - \int_\Omega {h\left( x \right){{\left| {{u_n}} \right|}^{1 - p}}} .$ |
By the continuity of
fn, φ(
t),
t>0, it holds
$\begin{array}{l}0 = \left\{ {\left( {{f_{n,\varphi }}\left( t \right) + 1} \right)\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla \left( {{u_n} + t\varphi } \right) \cdot \nabla \left( {{u_n} + t\varphi } \right)} - } \right.\\\left. {\left( {1 - p} \right){{\left[ {{f_{n,\varphi }}\left( 0 \right) + o\left( 1 \right)} \right]}^{ - p}}\int_\Omega {h\left( x \right){{\left( {{u_n} + t\varphi } \right)}^{1 - p}}} } \right\} \cdot \\\frac{{\left( {{f_{n,\varphi }}\left( t \right) - 1} \right)}}{t} - \frac{1}{t}\left\{ {\int_\Omega {h\left( x \right){{\left( {{u_n} + t\varphi } \right)}^{1 - p}}} + h\left( x \right)u_n^{1 - p}{\rm{d}}x - } \right.\\\left. {\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla \left( {{u_n} + t\varphi } \right) \cdot \nabla \left( {{u_n} + t\varphi } \right) - \mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla {u_n}{\rm{d}}x} } \right\},\end{array}$ |
and by letting
t→0
+, then
$\begin{array}{l}0 \ge {{f'}_{n,\varphi }}\left( 0 \right)\left\{ {2\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla {u_n}} + \left( {p - 1} \right)\int_\Omega {h\left( x \right)u_n^{1 - p}} } \right\} + \\\;\;\;\;\;2\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla \varphi } ,\end{array}$ |
which implies that
f′
n, φ(0)≠+∞. Indeed, due to
un∈
N2 $ \subseteq $ N1 and
B(0,
r0)∩
N1=?, it follows that
$\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla {u_n}} \ge \alpha {\left\| {{u_n}} \right\|^2} > \alpha r_0^2 > 0.$ | (11) |
Since -
p < -1, by the reversed H?lder inequality it yields that
$\begin{array}{*{20}{c}}{\int_\Omega {h\left( x \right)u_n^{1 - p}} \ge {{\left( {\int_\Omega {h{{\left( x \right)}^{1/p}}} } \right)}^p}{{\left( {\int_\Omega {{u_n}} } \right)}^{1 - p}}}\\{ \ge {C_2}{{\left( {\int_\Omega {h{{\left( x \right)}^{1/p}}} } \right)}^p}{{\left\| {{u_n}} \right\|}^{1 - p}} > 0.}\end{array}$ |
In addition,
$\begin{array}{*{20}{c}}{\left| {\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla \varphi } } \right| \le \frac{\beta }{{{\alpha ^{n - 1}}}} \cdot }\\{\int_\Omega {\left| {\nabla {u_n}} \right| \cdot \left| \varphi \right| \le {C_3}\left\| {{u_n}} \right\| \cdot \left\| \varphi \right\|} .}\end{array}$ | (12) |
Furthermore, since
r0 is independent of
n, it follows that
${{f'}_{n,\varphi }}\left( 0 \right) \le {c_1}\;{\rm{uniformly}}\;{\rm{in}}\;n.$ | (13) |
On the other hand, we will show that
f′
n, φ(0) cannot go to -∞ as
n→∞, that is,
f′
n, φ(0) is bounded from below uniformly for all
n large. Indeed, by the fact that
u∈
N2, we have
$\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u} = \int_\Omega {h\left( x \right){{\left| {{u_n}} \right|}^{1 - p}}} ,$ |
which imples
$I\left( u \right) = \left( {\frac{1}{2} + \frac{1}{{p - 1}}} \right)\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u} ,$ |
and by condition (ⅱ) we have the additional condition
$\begin{array}{l}\frac{1}{n}\left| {\frac{{1 - {f_{n,\varphi }}\left( t \right)}}{t}} \right| \cdot \left\| {{u_n}} \right\| + \frac{1}{n}{f_{n,\varphi }}\left( t \right)\left\| \varphi \right\|\\ \ge \frac{1}{n}\left\| {{u_n} \cdot {f_{n,\varphi }}\left( t \right)\left( {{u_n} + t\varphi } \right)} \right\|\frac{1}{t}\\ \ge \left[ {I\left( {{u_n}} \right) - I\left( {{f_{n,\varphi }}\left( t \right)\left( {{u_n} + t\varphi } \right)} \right.} \right]\frac{1}{t},\end{array}$ |
that is,
$\begin{array}{l}\frac{{\left\| \varphi \right\|}}{n}{f_{n,\varphi }}\left( t \right) \ge \frac{{{f_{n,\varphi }}\left( t \right) - 1}}{t}\left\{ { - \left( {\frac{1}{2} + \frac{1}{{p - 1}}} \right)\left[ {{f_{n,\varphi }}\left( t \right) + 1} \right] \cdot } \right.\\\left. {\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right){{\left| {\nabla \left( {{u_n} + t\varphi } \right)} \right|}^2}} - \frac{{\left\| {{u_n}} \right\|}}{n} \cdot {\mathop{\rm sgn}} \left( {{f_{n,\varphi }}\left( t \right) - 1} \right)} \right\} - \\\frac{1}{t}\left( {\frac{1}{2} + \frac{1}{{p - 1}}} \right)\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\left[ {{{\left| {\nabla \left( {{u_n} + t\varphi } \right)} \right|}^2} - {{\left| {\nabla {u_n}} \right|}^2}} \right]{\rm{d}}x} .\end{array}$ |
Letting
t→0
+, we obtain that
$\begin{array}{l}\frac{{\left\| \varphi \right\|}}{n} \ge - {{f'}_{n,\varphi }}\left( 0 \right)\left\{ {\left( {\frac{1}{2} + \frac{1}{{p - 1}}} \right)\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla {u_n}} + } \right.\\\left. {\frac{{\left\| {{u_n}} \right\|}}{n} \cdot {\mathop{\rm sgn}} {{f'}_{n,\varphi }}\left( 0 \right)} \right\} - \left( {1 + \frac{1}{{p - 1}}} \right)\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla \varphi } .\end{array}$ |
By (2) and (11) it yields that
$\beta {\left\| u \right\|^2} \ge \int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla u \cdot \nabla u} \ge \alpha {\left\| {{u_n}} \right\|^2} > \alpha r_0^2 > 0,$ |
and in view of (12) it holds that
f′
n, φ is bounded below. More precisely,
${{f'}_{n,\varphi }}\left( 0 \right) \ge {c_2}\;{\rm{uniformly}}\;{\rm{in}}\;{\rm{all}}\;n\;{\rm{large}}$ | (14) |
as
r0 is independent of
n.
Now, applying condition (ⅱ) again, we have that
$\begin{array}{l}\frac{1}{n}\left[ {\frac{{\left| {{f_{n,\varphi }}\left( t \right) - 1} \right|}}{t}\left\| {{u_n}} \right\| + {f_{n,\varphi }}\left( t \right)\left\| \varphi \right\|} \right]\\\;\;\; \ge \frac{1}{n}\left\| {{f_{n,\varphi }}\left( t \right)\left( {{u_n} + t\varphi } \right) - {u_n}} \right\|\frac{1}{t}\\\;\;\; \ge \left[ {I\left( {{u_n}} \right) - I\left( {{f_{n,\varphi }}\left( t \right)\left( {{u_n} + t\varphi } \right)} \right.} \right]\frac{1}{t},\end{array}$ |
that is,
$\begin{array}{l}\frac{{\left\| {{u_n}} \right\|}}{n}\frac{{\left| {{f_{n,\varphi }}\left( t \right) - 1} \right|}}{t} + \frac{{\left\| \varphi \right\|}}{n}{f_{n,\varphi }}\left( t \right)\\ \ge \left\{ { - \frac{{\left[ {{f_{n,\varphi }}\left( t \right) + 1} \right]}}{2}\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla \left( {{u_n} + t\varphi } \right) \cdot \nabla \left( {{u_n} + t\varphi } \right) + } } \right.\\\left. {{{\left[ {{f_{n,\varphi }}\left( 0 \right) + o\left( 1 \right)} \right]}^{ - p}}\int_\Omega {h\left( x \right){{\left( {{u_n} + t\varphi } \right)}^{1 - p}}{\rm{d}}x} } \right\} \cdot \\\frac{{{f_{n,\varphi }}\left( t \right) - 1}}{t} + \frac{1}{{1 - p}}\int_\Omega {\frac{{h\left( x \right)\left[ {{{\left( {{u_n} + t\varphi } \right)}^{1 - p}} - u_n^{1 - p}} \right]}}{t} - } \\\frac{{\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla \left( {{u_n} + t\varphi } \right) \cdot \nabla \left( {{u_n} + t\varphi } \right) - \mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla {u_n}} }}{{2t}}.\end{array}$ |
In other words,
$\begin{array}{l}\frac{1}{{p - 1}}\int_\Omega {\frac{{h\left( x \right)\left[ {u_n^{1 - p} - {{\left( {{u_n} + t\varphi } \right)}^{1 - p}}} \right]}}{t}} \\ \le \left\{ {\frac{{\left[ {{f_{n,\varphi }}\left( t \right) + 1} \right]}}{2}\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla \left( {{u_n} + t\varphi } \right) \cdot \nabla \left( {{u_n} + t\varphi } \right) - } } \right.\\\left. {{{\left[ {{f_{n,\varphi }}\left( 0 \right) + o\left( 1 \right)} \right]}^{ - p}}\int_\Omega {h\left( x \right){{\left( {{u_n} + t\varphi } \right)}^{1 - p}}{\rm{d}}x} } \right\} \cdot \\\frac{{{f_v}\left( t \right) - 1}}{t} + \frac{{\left\| {{u_n}} \right\|}}{n}\frac{{\left| {{f_{n,\varphi }}\left( t \right) - 1} \right|}}{t} + \frac{{\left\| \varphi \right\|}}{n}{f_{n,\varphi }}\left( t \right) + \\\frac{{\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla \left( {{u_n} + t\varphi } \right) \cdot \nabla \left( {{u_n} + t\varphi } \right) - \mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla {u_n}} }}{{2t}}.\end{array}$ |
In view of (13)and (14), it holds that
$\begin{array}{l}\mathop {\lim \inf }\limits_{t \to {0^ + }} \frac{1}{{p - 1}}\int_\Omega {\frac{{h\left( x \right)\left[ {u_n^{1 - p} - {{\left( {{u_n} + t\varphi } \right)}^{1 - p}}} \right]}}{t}} \\ \le {{f'}_{n,\varphi }}\left( 0 \right)\left\{ {\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla {u_n}} - h\left( x \right){{\left( {{u_n} + t\varphi } \right)}^{1 - p}}{\rm{d}}x} \right\} + \\\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla \varphi {\rm{d}}x} + \frac{1}{n}\left[ {{{f'}_{n,\varphi }}\left( 0 \right)\left\| {{u_n}} \right\| + \left\| \varphi \right\|} \right]\\ = \int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla \varphi {\rm{d}}x} + \frac{1}{n}\left[ {{{f'}_{n,\varphi }}\left( 0 \right)\left\| {{u_n}} \right\| + \left\| \varphi \right\|} \right]\\ < \infty .\end{array}$ |
On the other hand, since -
p < -1,
φ≥0,
h(
x)>0, and
t>0, we have
$\frac{{h\left( x \right)\left[ {u_n^{1 - p} - {{\left( {{u_n} + t\varphi } \right)}^{1 - p}}} \right]}}{t} \ge 0,$ |
and by Fatou' lemma, we have
$\begin{array}{l}\int_\Omega {h\left( x \right)u_u^{1 - p}\varphi {\rm{d}}x} \\ \le \mathop {\lim \inf }\limits_{t \to {0^ + }} \frac{1}{{p - 1}}\int_\Omega {\frac{{h\left( x \right)\left[ {u_n^{1 - p} - {{\left( {{u_n} + t\varphi } \right)}^{1 - p}}} \right]}}{t}} \\ \le \int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u_n} \cdot \nabla \varphi } + \frac{1}{n}\left[ {{{f'}_{n,\varphi }}\left( 0 \right)\left\| {{u_n}} \right\| + \left\| \varphi \right\|} \right].\end{array}$ |
Hence, using Fatou's lemma again and
n→∞, we obtain
$\int_\Omega {h\left( x \right){u^{ * 1 - p}}{\rm{d}}x} \le \int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u^ * } \cdot \nabla \varphi ,\forall \varphi } \ge 0.$ |
In other words,
$\int_\Omega {h\left( x \right){u^{ * 1 - p}}{\rm{d}}x} - \int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u^ * } \cdot la\varphi \ge 0,\forall \varphi } \ge 0.$ |
By the same reasoning as in case 1 we derive that
Now it remains to show that
u*∈
H01(Ω) is a weak solution for problem (1) for all -
p < -1. Letting
ψ∈
H01(Ω) be fixed and applying the above inequalities one finds
$\begin{array}{l}0 \le \int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u^ * } \cdot \nabla {{\left( {{u^ * } + t\psi } \right)}^ + }} - \int_\Omega {h\left( x \right){u^{ * - p}}{{\left( {{u^ * } + t\psi } \right)}^ + }} \\ = \int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u^ * } \cdot \nabla \left( {{u^ * } + t\psi } \right)} - \int_\Omega {h\left( x \right){u^{ * - p}}\left( {{u^ * } + t\psi } \right)} - \\\;\;\;\int_{{u^ * } + t\psi < 0} {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u^ * } \cdot \nabla \left( {{u^ * } + t\psi } \right)} + \\\;\;\;\int_\Omega {h\left( x \right){u^{ * - p}}\left( {{u^ * } + t\psi } \right)} \\ \le t\left\{ {\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u^ * } \cdot \nabla \psi } - \int_\Omega {h\left( x \right){u^{ * - p}}\psi } } \right\} - \\\;\;\;\;\int_{{u^ * } + t\psi < 0} {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u^ * } \cdot \nabla {u^ * }} - t\int_{{u^ * } + t\psi < 0} {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u^ * } \cdot \nabla \psi } \\ \le t\left\{ {\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u^ * } \cdot \nabla \psi } - \int_\Omega {h\left( x \right){u^{ * - p}}\psi } - } \right.\\\;\;\;\;\left. {\int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u^ * } \cdot \nabla \psi } } \right\}.\end{array}$ |
Since meas[
u*+
tψ < 0]→0 as
t→0, we may divide the inequality by
t>0 and pass to the limit as
t→0, and we conclude that
$0 \le \int_\Omega {\mathit{\boldsymbol{M}}\left( x \right)\nabla {u^ * } \cdot \nabla \psi } + \int_\Omega {h\left( x \right){u^{ * - p}}\psi {\rm{d}}x} .$ |
By the arbitrariness of
ψ∈
H01(Ω),
u* is indeed a
H01(Ω)-solution of problem (1).
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