An inequality about linear PDE
$begingroup$
I am trying to solve the following problem:
Let $Omega$ be a bounded smooth domain in $mathbb{R}^n, ngeq 2$. Let $uin C^2(overline{Omega})$ be a solution of
$$left{begin{array}{ll}u_t-Delta u=f(x)& text{in } Omegatimes(0,infty)\ u=0&text{on }partial Omegatimes (0,infty) \ u=g(x) & text{on } Omegatimes{0}end{array}right.$$
Show that
$$max_{0leq tleq T} int_Omega u^2(x,t)dx+int_0^Tint_Omega|nabla u(x,t)|^2dx, dtleq Cleft(int_Omega g^2(x)dx+int_0^T|f(x)|^2 dx,dtright)$$
for some constant $C$ independent of $f, g$ and $u$.
My attempt:
Multiplying the first equation by $u$ and taking integration on $Omega$, we have
$$frac{1}{2}frac{d}{dt}||u||^2_{L^2}+int_Omega nabla ucdot nabla u=int_Omega fu.$$
(Here we also use the Green's identity and the boundary condition of $u$)
Taking integration on $[0,s]$ where $sleq T$. Then we have
$$frac{1}{2}||u(x,s)||^2_{L^2}+int_0^sint_Omega|nabla u(x,t)|^2dx, dt= frac{1}{2} int_Omega g^2(x)dx+int_0^sint_Omega fu dt$$
where
$$int_Omega fuleq ||f||_{L^2} ||u||_{L^2}leq epsilon ||u||_{L^2}^2 + C(epsilon ) ||f||_{L^2}$$
Then we have
$$(frac{1}{2}-sepsilon )||u(x,s)||^2_{L^2}+int_0^sint_Omega|nabla u(x,t)|^2dx, dt
leq frac{1}{2} int_Omega g^2(x)dx+int_0^s C(epsilon ) ||f||_{L^2} dt$$
Then I was stuck here. I don't know where can I derive the part
$$max_{0leq tleq T}||u(x,t)||.$$
Also, I am struggling how to make the two coefficients before the two terms on the left the same so that we can get the desired inequality.
real-analysis inequality pde
$endgroup$
add a comment |
$begingroup$
I am trying to solve the following problem:
Let $Omega$ be a bounded smooth domain in $mathbb{R}^n, ngeq 2$. Let $uin C^2(overline{Omega})$ be a solution of
$$left{begin{array}{ll}u_t-Delta u=f(x)& text{in } Omegatimes(0,infty)\ u=0&text{on }partial Omegatimes (0,infty) \ u=g(x) & text{on } Omegatimes{0}end{array}right.$$
Show that
$$max_{0leq tleq T} int_Omega u^2(x,t)dx+int_0^Tint_Omega|nabla u(x,t)|^2dx, dtleq Cleft(int_Omega g^2(x)dx+int_0^T|f(x)|^2 dx,dtright)$$
for some constant $C$ independent of $f, g$ and $u$.
My attempt:
Multiplying the first equation by $u$ and taking integration on $Omega$, we have
$$frac{1}{2}frac{d}{dt}||u||^2_{L^2}+int_Omega nabla ucdot nabla u=int_Omega fu.$$
(Here we also use the Green's identity and the boundary condition of $u$)
Taking integration on $[0,s]$ where $sleq T$. Then we have
$$frac{1}{2}||u(x,s)||^2_{L^2}+int_0^sint_Omega|nabla u(x,t)|^2dx, dt= frac{1}{2} int_Omega g^2(x)dx+int_0^sint_Omega fu dt$$
where
$$int_Omega fuleq ||f||_{L^2} ||u||_{L^2}leq epsilon ||u||_{L^2}^2 + C(epsilon ) ||f||_{L^2}$$
Then we have
$$(frac{1}{2}-sepsilon )||u(x,s)||^2_{L^2}+int_0^sint_Omega|nabla u(x,t)|^2dx, dt
leq frac{1}{2} int_Omega g^2(x)dx+int_0^s C(epsilon ) ||f||_{L^2} dt$$
Then I was stuck here. I don't know where can I derive the part
$$max_{0leq tleq T}||u(x,t)||.$$
Also, I am struggling how to make the two coefficients before the two terms on the left the same so that we can get the desired inequality.
real-analysis inequality pde
$endgroup$
1
$begingroup$
Solved it. Tell me if someone needs the answer. The idea is right. Just need some subtle modification about $epsilon$.
$endgroup$
– Aolong Li
Dec 14 '18 at 7:29
2
$begingroup$
I for one would like to see your solution. Cheers!
$endgroup$
– Robert Lewis
Dec 14 '18 at 7:38
2
$begingroup$
the usual convention is to post your own answer and accept it.
$endgroup$
– dezdichado
Dec 14 '18 at 15:56
$begingroup$
@dezdichado OK thanks! Will do!
$endgroup$
– Aolong Li
Dec 14 '18 at 16:00
add a comment |
$begingroup$
I am trying to solve the following problem:
Let $Omega$ be a bounded smooth domain in $mathbb{R}^n, ngeq 2$. Let $uin C^2(overline{Omega})$ be a solution of
$$left{begin{array}{ll}u_t-Delta u=f(x)& text{in } Omegatimes(0,infty)\ u=0&text{on }partial Omegatimes (0,infty) \ u=g(x) & text{on } Omegatimes{0}end{array}right.$$
Show that
$$max_{0leq tleq T} int_Omega u^2(x,t)dx+int_0^Tint_Omega|nabla u(x,t)|^2dx, dtleq Cleft(int_Omega g^2(x)dx+int_0^T|f(x)|^2 dx,dtright)$$
for some constant $C$ independent of $f, g$ and $u$.
My attempt:
Multiplying the first equation by $u$ and taking integration on $Omega$, we have
$$frac{1}{2}frac{d}{dt}||u||^2_{L^2}+int_Omega nabla ucdot nabla u=int_Omega fu.$$
(Here we also use the Green's identity and the boundary condition of $u$)
Taking integration on $[0,s]$ where $sleq T$. Then we have
$$frac{1}{2}||u(x,s)||^2_{L^2}+int_0^sint_Omega|nabla u(x,t)|^2dx, dt= frac{1}{2} int_Omega g^2(x)dx+int_0^sint_Omega fu dt$$
where
$$int_Omega fuleq ||f||_{L^2} ||u||_{L^2}leq epsilon ||u||_{L^2}^2 + C(epsilon ) ||f||_{L^2}$$
Then we have
$$(frac{1}{2}-sepsilon )||u(x,s)||^2_{L^2}+int_0^sint_Omega|nabla u(x,t)|^2dx, dt
leq frac{1}{2} int_Omega g^2(x)dx+int_0^s C(epsilon ) ||f||_{L^2} dt$$
Then I was stuck here. I don't know where can I derive the part
$$max_{0leq tleq T}||u(x,t)||.$$
Also, I am struggling how to make the two coefficients before the two terms on the left the same so that we can get the desired inequality.
real-analysis inequality pde
$endgroup$
I am trying to solve the following problem:
Let $Omega$ be a bounded smooth domain in $mathbb{R}^n, ngeq 2$. Let $uin C^2(overline{Omega})$ be a solution of
$$left{begin{array}{ll}u_t-Delta u=f(x)& text{in } Omegatimes(0,infty)\ u=0&text{on }partial Omegatimes (0,infty) \ u=g(x) & text{on } Omegatimes{0}end{array}right.$$
Show that
$$max_{0leq tleq T} int_Omega u^2(x,t)dx+int_0^Tint_Omega|nabla u(x,t)|^2dx, dtleq Cleft(int_Omega g^2(x)dx+int_0^T|f(x)|^2 dx,dtright)$$
for some constant $C$ independent of $f, g$ and $u$.
My attempt:
Multiplying the first equation by $u$ and taking integration on $Omega$, we have
$$frac{1}{2}frac{d}{dt}||u||^2_{L^2}+int_Omega nabla ucdot nabla u=int_Omega fu.$$
(Here we also use the Green's identity and the boundary condition of $u$)
Taking integration on $[0,s]$ where $sleq T$. Then we have
$$frac{1}{2}||u(x,s)||^2_{L^2}+int_0^sint_Omega|nabla u(x,t)|^2dx, dt= frac{1}{2} int_Omega g^2(x)dx+int_0^sint_Omega fu dt$$
where
$$int_Omega fuleq ||f||_{L^2} ||u||_{L^2}leq epsilon ||u||_{L^2}^2 + C(epsilon ) ||f||_{L^2}$$
Then we have
$$(frac{1}{2}-sepsilon )||u(x,s)||^2_{L^2}+int_0^sint_Omega|nabla u(x,t)|^2dx, dt
leq frac{1}{2} int_Omega g^2(x)dx+int_0^s C(epsilon ) ||f||_{L^2} dt$$
Then I was stuck here. I don't know where can I derive the part
$$max_{0leq tleq T}||u(x,t)||.$$
Also, I am struggling how to make the two coefficients before the two terms on the left the same so that we can get the desired inequality.
real-analysis inequality pde
real-analysis inequality pde
asked Dec 14 '18 at 7:18
Aolong LiAolong Li
880615
880615
1
$begingroup$
Solved it. Tell me if someone needs the answer. The idea is right. Just need some subtle modification about $epsilon$.
$endgroup$
– Aolong Li
Dec 14 '18 at 7:29
2
$begingroup$
I for one would like to see your solution. Cheers!
$endgroup$
– Robert Lewis
Dec 14 '18 at 7:38
2
$begingroup$
the usual convention is to post your own answer and accept it.
$endgroup$
– dezdichado
Dec 14 '18 at 15:56
$begingroup$
@dezdichado OK thanks! Will do!
$endgroup$
– Aolong Li
Dec 14 '18 at 16:00
add a comment |
1
$begingroup$
Solved it. Tell me if someone needs the answer. The idea is right. Just need some subtle modification about $epsilon$.
$endgroup$
– Aolong Li
Dec 14 '18 at 7:29
2
$begingroup$
I for one would like to see your solution. Cheers!
$endgroup$
– Robert Lewis
Dec 14 '18 at 7:38
2
$begingroup$
the usual convention is to post your own answer and accept it.
$endgroup$
– dezdichado
Dec 14 '18 at 15:56
$begingroup$
@dezdichado OK thanks! Will do!
$endgroup$
– Aolong Li
Dec 14 '18 at 16:00
1
1
$begingroup$
Solved it. Tell me if someone needs the answer. The idea is right. Just need some subtle modification about $epsilon$.
$endgroup$
– Aolong Li
Dec 14 '18 at 7:29
$begingroup$
Solved it. Tell me if someone needs the answer. The idea is right. Just need some subtle modification about $epsilon$.
$endgroup$
– Aolong Li
Dec 14 '18 at 7:29
2
2
$begingroup$
I for one would like to see your solution. Cheers!
$endgroup$
– Robert Lewis
Dec 14 '18 at 7:38
$begingroup$
I for one would like to see your solution. Cheers!
$endgroup$
– Robert Lewis
Dec 14 '18 at 7:38
2
2
$begingroup$
the usual convention is to post your own answer and accept it.
$endgroup$
– dezdichado
Dec 14 '18 at 15:56
$begingroup$
the usual convention is to post your own answer and accept it.
$endgroup$
– dezdichado
Dec 14 '18 at 15:56
$begingroup$
@dezdichado OK thanks! Will do!
$endgroup$
– Aolong Li
Dec 14 '18 at 16:00
$begingroup$
@dezdichado OK thanks! Will do!
$endgroup$
– Aolong Li
Dec 14 '18 at 16:00
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
What I did above tells us that
$$||u(x,s)||^2_{L^2}+frac{1}{frac{1}{2}-sepsilon}int_0^sint_Omega|nabla u(x,t)|^2dx, dt
leq frac{1}{frac{1}{2}-sepsilon}left(frac{1}{2} int_Omega g^2(x)dx+int_0^s C(epsilon ) ||f||_{L^2} dt right) $$
for any $s>0$.
Now, pick up an appropriate $epsilon>0$ such that
$$frac{1}{2}-epsilon>0$$
and
$$int_0^Tint_Omega |nabla u|^2 dx dtleq frac{1}{frac{1}{2}-epsilon}int_0^{t_0}int_Omega |nabla u|^2 dx dt$$
where $t_0$ is the point where $||u(x,t)||^2_{L^2}$ obtain the maximum. Then we have
$$begin{eqnarray}||u(x,t_0)||^2_{L^2}+int_0^Tint_Omega |nabla u|^2 dx dt &leq& ||u(x,t_0)||^2_{L^2} + frac{1}{frac{1}{2}-epsilon}int_0^{t_0}int_Omega |nabla u|^2 dx dt\
&leq& frac{1}{frac{1}{2}-epsilon}left(frac{1}{2} int_Omega g^2(x)dx+int_0^T C(epsilon ) ||f||_{L^2} dtright)\
&leq& C'left( int_Omega g^2(x)dx+int_0^T C(epsilon ) ||f||_{L^2} dtright)
end{eqnarray}$$
for some constant $C'$.
$endgroup$
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3039058%2fan-inequality-about-linear-pde%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
What I did above tells us that
$$||u(x,s)||^2_{L^2}+frac{1}{frac{1}{2}-sepsilon}int_0^sint_Omega|nabla u(x,t)|^2dx, dt
leq frac{1}{frac{1}{2}-sepsilon}left(frac{1}{2} int_Omega g^2(x)dx+int_0^s C(epsilon ) ||f||_{L^2} dt right) $$
for any $s>0$.
Now, pick up an appropriate $epsilon>0$ such that
$$frac{1}{2}-epsilon>0$$
and
$$int_0^Tint_Omega |nabla u|^2 dx dtleq frac{1}{frac{1}{2}-epsilon}int_0^{t_0}int_Omega |nabla u|^2 dx dt$$
where $t_0$ is the point where $||u(x,t)||^2_{L^2}$ obtain the maximum. Then we have
$$begin{eqnarray}||u(x,t_0)||^2_{L^2}+int_0^Tint_Omega |nabla u|^2 dx dt &leq& ||u(x,t_0)||^2_{L^2} + frac{1}{frac{1}{2}-epsilon}int_0^{t_0}int_Omega |nabla u|^2 dx dt\
&leq& frac{1}{frac{1}{2}-epsilon}left(frac{1}{2} int_Omega g^2(x)dx+int_0^T C(epsilon ) ||f||_{L^2} dtright)\
&leq& C'left( int_Omega g^2(x)dx+int_0^T C(epsilon ) ||f||_{L^2} dtright)
end{eqnarray}$$
for some constant $C'$.
$endgroup$
add a comment |
$begingroup$
What I did above tells us that
$$||u(x,s)||^2_{L^2}+frac{1}{frac{1}{2}-sepsilon}int_0^sint_Omega|nabla u(x,t)|^2dx, dt
leq frac{1}{frac{1}{2}-sepsilon}left(frac{1}{2} int_Omega g^2(x)dx+int_0^s C(epsilon ) ||f||_{L^2} dt right) $$
for any $s>0$.
Now, pick up an appropriate $epsilon>0$ such that
$$frac{1}{2}-epsilon>0$$
and
$$int_0^Tint_Omega |nabla u|^2 dx dtleq frac{1}{frac{1}{2}-epsilon}int_0^{t_0}int_Omega |nabla u|^2 dx dt$$
where $t_0$ is the point where $||u(x,t)||^2_{L^2}$ obtain the maximum. Then we have
$$begin{eqnarray}||u(x,t_0)||^2_{L^2}+int_0^Tint_Omega |nabla u|^2 dx dt &leq& ||u(x,t_0)||^2_{L^2} + frac{1}{frac{1}{2}-epsilon}int_0^{t_0}int_Omega |nabla u|^2 dx dt\
&leq& frac{1}{frac{1}{2}-epsilon}left(frac{1}{2} int_Omega g^2(x)dx+int_0^T C(epsilon ) ||f||_{L^2} dtright)\
&leq& C'left( int_Omega g^2(x)dx+int_0^T C(epsilon ) ||f||_{L^2} dtright)
end{eqnarray}$$
for some constant $C'$.
$endgroup$
add a comment |
$begingroup$
What I did above tells us that
$$||u(x,s)||^2_{L^2}+frac{1}{frac{1}{2}-sepsilon}int_0^sint_Omega|nabla u(x,t)|^2dx, dt
leq frac{1}{frac{1}{2}-sepsilon}left(frac{1}{2} int_Omega g^2(x)dx+int_0^s C(epsilon ) ||f||_{L^2} dt right) $$
for any $s>0$.
Now, pick up an appropriate $epsilon>0$ such that
$$frac{1}{2}-epsilon>0$$
and
$$int_0^Tint_Omega |nabla u|^2 dx dtleq frac{1}{frac{1}{2}-epsilon}int_0^{t_0}int_Omega |nabla u|^2 dx dt$$
where $t_0$ is the point where $||u(x,t)||^2_{L^2}$ obtain the maximum. Then we have
$$begin{eqnarray}||u(x,t_0)||^2_{L^2}+int_0^Tint_Omega |nabla u|^2 dx dt &leq& ||u(x,t_0)||^2_{L^2} + frac{1}{frac{1}{2}-epsilon}int_0^{t_0}int_Omega |nabla u|^2 dx dt\
&leq& frac{1}{frac{1}{2}-epsilon}left(frac{1}{2} int_Omega g^2(x)dx+int_0^T C(epsilon ) ||f||_{L^2} dtright)\
&leq& C'left( int_Omega g^2(x)dx+int_0^T C(epsilon ) ||f||_{L^2} dtright)
end{eqnarray}$$
for some constant $C'$.
$endgroup$
What I did above tells us that
$$||u(x,s)||^2_{L^2}+frac{1}{frac{1}{2}-sepsilon}int_0^sint_Omega|nabla u(x,t)|^2dx, dt
leq frac{1}{frac{1}{2}-sepsilon}left(frac{1}{2} int_Omega g^2(x)dx+int_0^s C(epsilon ) ||f||_{L^2} dt right) $$
for any $s>0$.
Now, pick up an appropriate $epsilon>0$ such that
$$frac{1}{2}-epsilon>0$$
and
$$int_0^Tint_Omega |nabla u|^2 dx dtleq frac{1}{frac{1}{2}-epsilon}int_0^{t_0}int_Omega |nabla u|^2 dx dt$$
where $t_0$ is the point where $||u(x,t)||^2_{L^2}$ obtain the maximum. Then we have
$$begin{eqnarray}||u(x,t_0)||^2_{L^2}+int_0^Tint_Omega |nabla u|^2 dx dt &leq& ||u(x,t_0)||^2_{L^2} + frac{1}{frac{1}{2}-epsilon}int_0^{t_0}int_Omega |nabla u|^2 dx dt\
&leq& frac{1}{frac{1}{2}-epsilon}left(frac{1}{2} int_Omega g^2(x)dx+int_0^T C(epsilon ) ||f||_{L^2} dtright)\
&leq& C'left( int_Omega g^2(x)dx+int_0^T C(epsilon ) ||f||_{L^2} dtright)
end{eqnarray}$$
for some constant $C'$.
edited Dec 21 '18 at 7:22
answered Dec 21 '18 at 4:42
Aolong LiAolong Li
880615
880615
add a comment |
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3039058%2fan-inequality-about-linear-pde%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
1
$begingroup$
Solved it. Tell me if someone needs the answer. The idea is right. Just need some subtle modification about $epsilon$.
$endgroup$
– Aolong Li
Dec 14 '18 at 7:29
2
$begingroup$
I for one would like to see your solution. Cheers!
$endgroup$
– Robert Lewis
Dec 14 '18 at 7:38
2
$begingroup$
the usual convention is to post your own answer and accept it.
$endgroup$
– dezdichado
Dec 14 '18 at 15:56
$begingroup$
@dezdichado OK thanks! Will do!
$endgroup$
– Aolong Li
Dec 14 '18 at 16:00