An inequality about linear PDE












6












$begingroup$


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.










share|cite|improve this question









$endgroup$








  • 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
















6












$begingroup$


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.










share|cite|improve this question









$endgroup$








  • 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














6












6








6


2



$begingroup$


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.










share|cite|improve this question









$endgroup$




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






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










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














  • 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










1 Answer
1






active

oldest

votes


















1












$begingroup$

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 $C'$.






share|cite|improve this answer











$endgroup$













    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
    });


    }
    });














    draft saved

    draft discarded


















    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









    1












    $begingroup$

    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 $C'$.






    share|cite|improve this answer











    $endgroup$


















      1












      $begingroup$

      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 $C'$.






      share|cite|improve this answer











      $endgroup$
















        1












        1








        1





        $begingroup$

        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 $C'$.






        share|cite|improve this answer











        $endgroup$



        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 $C'$.







        share|cite|improve this answer














        share|cite|improve this answer



        share|cite|improve this answer








        edited Dec 21 '18 at 7:22

























        answered Dec 21 '18 at 4:42









        Aolong LiAolong Li

        880615




        880615






























            draft saved

            draft discarded




















































            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.




            draft saved


            draft discarded














            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





















































            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







            Popular posts from this blog

            Plaza Victoria

            Brian Clough

            Cáceres