What is $intdelta(x-y)delta(y-z)f(y):{rm d}y$?












2












$begingroup$


Let $(Omega,mathcal A,mu)$ be a measurable space and $delta$ denote the Dirac delta function. If $finmathcal L^1(mu)$ and $x,zinOmega$, what is $$intdelta(x-y)delta(y-z)f(y):mu({rm d}y)?$$ I've found that in a paper, but isn't that undefined?










share|cite|improve this question









$endgroup$












  • $begingroup$
    Indeed, I would say it's undefined as well.
    $endgroup$
    – Crostul
    Dec 30 '18 at 14:48






  • 1




    $begingroup$
    Well providing the reference (paper) would be useful! If well-defined then I would guess this should equal to something like $delta(x-z)f(x)$
    $endgroup$
    – Winther
    Dec 30 '18 at 14:50












  • $begingroup$
    Unfortunately "a paper" is not enough for us to find it. Some papers are pure nonsense. Other papers have definitions in them revealing what their notation means.
    $endgroup$
    – GEdgar
    Dec 30 '18 at 14:51










  • $begingroup$
    @Winther It's occuring implicitly in the definition of $kappa_n^circ$ on page 4 here.
    $endgroup$
    – 0xbadf00d
    Dec 30 '18 at 14:52










  • $begingroup$
    Don't know how to rigorously define and prove the results above (products of distributions are troublesome, but my guess is that this one can make sense). One way of justifying a formula like the one above is to consider a smooth approximation to the Dirac $delta$ like for example $delta_epsilon(x) equiv frac{1}{sqrt{2pi epsilon}}e^{-frac{x^2}{2epsilon}}$. In this case it's easy to check that $int delta_epsilon(x-y)delta_epsilon(y-z){rm d}y = delta_{epsilon/2}(x-z)$ so atleast for cases where the delta is just an approximation for a sharp pulse the formula above should hold.
    $endgroup$
    – Winther
    Dec 30 '18 at 15:15


















2












$begingroup$


Let $(Omega,mathcal A,mu)$ be a measurable space and $delta$ denote the Dirac delta function. If $finmathcal L^1(mu)$ and $x,zinOmega$, what is $$intdelta(x-y)delta(y-z)f(y):mu({rm d}y)?$$ I've found that in a paper, but isn't that undefined?










share|cite|improve this question









$endgroup$












  • $begingroup$
    Indeed, I would say it's undefined as well.
    $endgroup$
    – Crostul
    Dec 30 '18 at 14:48






  • 1




    $begingroup$
    Well providing the reference (paper) would be useful! If well-defined then I would guess this should equal to something like $delta(x-z)f(x)$
    $endgroup$
    – Winther
    Dec 30 '18 at 14:50












  • $begingroup$
    Unfortunately "a paper" is not enough for us to find it. Some papers are pure nonsense. Other papers have definitions in them revealing what their notation means.
    $endgroup$
    – GEdgar
    Dec 30 '18 at 14:51










  • $begingroup$
    @Winther It's occuring implicitly in the definition of $kappa_n^circ$ on page 4 here.
    $endgroup$
    – 0xbadf00d
    Dec 30 '18 at 14:52










  • $begingroup$
    Don't know how to rigorously define and prove the results above (products of distributions are troublesome, but my guess is that this one can make sense). One way of justifying a formula like the one above is to consider a smooth approximation to the Dirac $delta$ like for example $delta_epsilon(x) equiv frac{1}{sqrt{2pi epsilon}}e^{-frac{x^2}{2epsilon}}$. In this case it's easy to check that $int delta_epsilon(x-y)delta_epsilon(y-z){rm d}y = delta_{epsilon/2}(x-z)$ so atleast for cases where the delta is just an approximation for a sharp pulse the formula above should hold.
    $endgroup$
    – Winther
    Dec 30 '18 at 15:15
















2












2








2


0



$begingroup$


Let $(Omega,mathcal A,mu)$ be a measurable space and $delta$ denote the Dirac delta function. If $finmathcal L^1(mu)$ and $x,zinOmega$, what is $$intdelta(x-y)delta(y-z)f(y):mu({rm d}y)?$$ I've found that in a paper, but isn't that undefined?










share|cite|improve this question









$endgroup$




Let $(Omega,mathcal A,mu)$ be a measurable space and $delta$ denote the Dirac delta function. If $finmathcal L^1(mu)$ and $x,zinOmega$, what is $$intdelta(x-y)delta(y-z)f(y):mu({rm d}y)?$$ I've found that in a paper, but isn't that undefined?







real-analysis functional-analysis dirac-delta






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Dec 30 '18 at 14:46









0xbadf00d0xbadf00d

1,88241533




1,88241533












  • $begingroup$
    Indeed, I would say it's undefined as well.
    $endgroup$
    – Crostul
    Dec 30 '18 at 14:48






  • 1




    $begingroup$
    Well providing the reference (paper) would be useful! If well-defined then I would guess this should equal to something like $delta(x-z)f(x)$
    $endgroup$
    – Winther
    Dec 30 '18 at 14:50












  • $begingroup$
    Unfortunately "a paper" is not enough for us to find it. Some papers are pure nonsense. Other papers have definitions in them revealing what their notation means.
    $endgroup$
    – GEdgar
    Dec 30 '18 at 14:51










  • $begingroup$
    @Winther It's occuring implicitly in the definition of $kappa_n^circ$ on page 4 here.
    $endgroup$
    – 0xbadf00d
    Dec 30 '18 at 14:52










  • $begingroup$
    Don't know how to rigorously define and prove the results above (products of distributions are troublesome, but my guess is that this one can make sense). One way of justifying a formula like the one above is to consider a smooth approximation to the Dirac $delta$ like for example $delta_epsilon(x) equiv frac{1}{sqrt{2pi epsilon}}e^{-frac{x^2}{2epsilon}}$. In this case it's easy to check that $int delta_epsilon(x-y)delta_epsilon(y-z){rm d}y = delta_{epsilon/2}(x-z)$ so atleast for cases where the delta is just an approximation for a sharp pulse the formula above should hold.
    $endgroup$
    – Winther
    Dec 30 '18 at 15:15




















  • $begingroup$
    Indeed, I would say it's undefined as well.
    $endgroup$
    – Crostul
    Dec 30 '18 at 14:48






  • 1




    $begingroup$
    Well providing the reference (paper) would be useful! If well-defined then I would guess this should equal to something like $delta(x-z)f(x)$
    $endgroup$
    – Winther
    Dec 30 '18 at 14:50












  • $begingroup$
    Unfortunately "a paper" is not enough for us to find it. Some papers are pure nonsense. Other papers have definitions in them revealing what their notation means.
    $endgroup$
    – GEdgar
    Dec 30 '18 at 14:51










  • $begingroup$
    @Winther It's occuring implicitly in the definition of $kappa_n^circ$ on page 4 here.
    $endgroup$
    – 0xbadf00d
    Dec 30 '18 at 14:52










  • $begingroup$
    Don't know how to rigorously define and prove the results above (products of distributions are troublesome, but my guess is that this one can make sense). One way of justifying a formula like the one above is to consider a smooth approximation to the Dirac $delta$ like for example $delta_epsilon(x) equiv frac{1}{sqrt{2pi epsilon}}e^{-frac{x^2}{2epsilon}}$. In this case it's easy to check that $int delta_epsilon(x-y)delta_epsilon(y-z){rm d}y = delta_{epsilon/2}(x-z)$ so atleast for cases where the delta is just an approximation for a sharp pulse the formula above should hold.
    $endgroup$
    – Winther
    Dec 30 '18 at 15:15


















$begingroup$
Indeed, I would say it's undefined as well.
$endgroup$
– Crostul
Dec 30 '18 at 14:48




$begingroup$
Indeed, I would say it's undefined as well.
$endgroup$
– Crostul
Dec 30 '18 at 14:48




1




1




$begingroup$
Well providing the reference (paper) would be useful! If well-defined then I would guess this should equal to something like $delta(x-z)f(x)$
$endgroup$
– Winther
Dec 30 '18 at 14:50






$begingroup$
Well providing the reference (paper) would be useful! If well-defined then I would guess this should equal to something like $delta(x-z)f(x)$
$endgroup$
– Winther
Dec 30 '18 at 14:50














$begingroup$
Unfortunately "a paper" is not enough for us to find it. Some papers are pure nonsense. Other papers have definitions in them revealing what their notation means.
$endgroup$
– GEdgar
Dec 30 '18 at 14:51




$begingroup$
Unfortunately "a paper" is not enough for us to find it. Some papers are pure nonsense. Other papers have definitions in them revealing what their notation means.
$endgroup$
– GEdgar
Dec 30 '18 at 14:51












$begingroup$
@Winther It's occuring implicitly in the definition of $kappa_n^circ$ on page 4 here.
$endgroup$
– 0xbadf00d
Dec 30 '18 at 14:52




$begingroup$
@Winther It's occuring implicitly in the definition of $kappa_n^circ$ on page 4 here.
$endgroup$
– 0xbadf00d
Dec 30 '18 at 14:52












$begingroup$
Don't know how to rigorously define and prove the results above (products of distributions are troublesome, but my guess is that this one can make sense). One way of justifying a formula like the one above is to consider a smooth approximation to the Dirac $delta$ like for example $delta_epsilon(x) equiv frac{1}{sqrt{2pi epsilon}}e^{-frac{x^2}{2epsilon}}$. In this case it's easy to check that $int delta_epsilon(x-y)delta_epsilon(y-z){rm d}y = delta_{epsilon/2}(x-z)$ so atleast for cases where the delta is just an approximation for a sharp pulse the formula above should hold.
$endgroup$
– Winther
Dec 30 '18 at 15:15






$begingroup$
Don't know how to rigorously define and prove the results above (products of distributions are troublesome, but my guess is that this one can make sense). One way of justifying a formula like the one above is to consider a smooth approximation to the Dirac $delta$ like for example $delta_epsilon(x) equiv frac{1}{sqrt{2pi epsilon}}e^{-frac{x^2}{2epsilon}}$. In this case it's easy to check that $int delta_epsilon(x-y)delta_epsilon(y-z){rm d}y = delta_{epsilon/2}(x-z)$ so atleast for cases where the delta is just an approximation for a sharp pulse the formula above should hold.
$endgroup$
– Winther
Dec 30 '18 at 15:15












3 Answers
3






active

oldest

votes


















4












$begingroup$

Talking non-rigorously, $delta(x-y) delta(y-z) f(y)$ will be non-zero only when $x-y=0$ and $y-z=0$, i.e. when $x=y=z.$ Therefore the integral over $y$ would be non-zero only when $x=z.$ We can thus expect the integral to be a multiple of $delta(x-z).$



So, let $phi$ be a nice function and study the formal integral
$$
int left( int delta(x-y) delta(y-z) f(y) , dy right) phi(z) , dz.
$$



Swapping the order of integration gives
$$
int delta(x-y) left( int delta(y-z) phi(z) , dz right) f(y) , dy
= int delta(x-y) phi(y) f(y) , dy
= phi(x) f(x).
$$



Thus,
$$
int delta(x-y) delta(y-z) f(y) , dy = f(x) delta(z-x).
$$






share|cite|improve this answer









$endgroup$





















    0












    $begingroup$

    Recall the usual $intdelta(x-y)g(y)dy=g(x)$ (assuming it's a definite integral over $A$), in this case with $g(y):=f(y)delta(y-z)$, giving $color{blue}{f(x)delta(x-z)}$ for your in-title integral (again, if it's definite over $A$). Here we've eliminated the $delta$ containing $x$, effectively imposing $y=x$. Of course we could have used the other $delta$ to impose $y=z$ instead, obtaining $color{blue}{f(z)delta(x-z)}$. But by inspection, these two answers are equivalent.






    share|cite|improve this answer









    $endgroup$





















      0












      $begingroup$

      $$color{green}{delta(x!-!z)}tag{1a}$$and$$color{red}{int_{mathbb{R}} !mathrm{d}y ~f(y)delta(x!-!y)delta(y!-!z)}tag{1b}$$ are informal notations for the distributions
      $u,vin D^{prime}(mathbb{R}^2)$ given by$^1$
      $$u[varphi]~:=~int_{mathbb{R}} !mathrm{d}y~varphi(y,y),tag{2a}$$
      $$v[varphi]~:=~int_{mathbb{R}} !mathrm{d}y ~f(y)~varphi(y,y),tag{2b} $$
      respectively, where $varphiin D(mathbb{R}^2)$ is a test function. The distributions are written in the informal notation as
      $$u[varphi]~=~int_{mathbb{R}^2} !mathrm{d}x~mathrm{d}z~color{green}{delta(x!-!z)}~ varphi(x,z), tag{3a}$$
      $$v[varphi]~:=~int_{mathbb{R}^2} !mathrm{d}x~mathrm{d}z~color{red}{int_{mathbb{R}} !mathrm{d}y ~f(y)delta(x!-!y)delta(y!-!z)} ~varphi(x,z),tag{3b} $$
      respectively.



      --



      $^1$Disclaimer: Here we assume for simplicity that OP's space is $Omega=mathbb{R}$ and the measure $mu$ is the Lebesque measure. Note that e.g. boundaries $partial Omega$ often requires extra attention in distribution theory.






      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%2f3056887%2fwhat-is-int-deltax-y-deltay-zfy-rm-dy%23new-answer', 'question_page');
        }
        );

        Post as a guest















        Required, but never shown

























        3 Answers
        3






        active

        oldest

        votes








        3 Answers
        3






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes









        4












        $begingroup$

        Talking non-rigorously, $delta(x-y) delta(y-z) f(y)$ will be non-zero only when $x-y=0$ and $y-z=0$, i.e. when $x=y=z.$ Therefore the integral over $y$ would be non-zero only when $x=z.$ We can thus expect the integral to be a multiple of $delta(x-z).$



        So, let $phi$ be a nice function and study the formal integral
        $$
        int left( int delta(x-y) delta(y-z) f(y) , dy right) phi(z) , dz.
        $$



        Swapping the order of integration gives
        $$
        int delta(x-y) left( int delta(y-z) phi(z) , dz right) f(y) , dy
        = int delta(x-y) phi(y) f(y) , dy
        = phi(x) f(x).
        $$



        Thus,
        $$
        int delta(x-y) delta(y-z) f(y) , dy = f(x) delta(z-x).
        $$






        share|cite|improve this answer









        $endgroup$


















          4












          $begingroup$

          Talking non-rigorously, $delta(x-y) delta(y-z) f(y)$ will be non-zero only when $x-y=0$ and $y-z=0$, i.e. when $x=y=z.$ Therefore the integral over $y$ would be non-zero only when $x=z.$ We can thus expect the integral to be a multiple of $delta(x-z).$



          So, let $phi$ be a nice function and study the formal integral
          $$
          int left( int delta(x-y) delta(y-z) f(y) , dy right) phi(z) , dz.
          $$



          Swapping the order of integration gives
          $$
          int delta(x-y) left( int delta(y-z) phi(z) , dz right) f(y) , dy
          = int delta(x-y) phi(y) f(y) , dy
          = phi(x) f(x).
          $$



          Thus,
          $$
          int delta(x-y) delta(y-z) f(y) , dy = f(x) delta(z-x).
          $$






          share|cite|improve this answer









          $endgroup$
















            4












            4








            4





            $begingroup$

            Talking non-rigorously, $delta(x-y) delta(y-z) f(y)$ will be non-zero only when $x-y=0$ and $y-z=0$, i.e. when $x=y=z.$ Therefore the integral over $y$ would be non-zero only when $x=z.$ We can thus expect the integral to be a multiple of $delta(x-z).$



            So, let $phi$ be a nice function and study the formal integral
            $$
            int left( int delta(x-y) delta(y-z) f(y) , dy right) phi(z) , dz.
            $$



            Swapping the order of integration gives
            $$
            int delta(x-y) left( int delta(y-z) phi(z) , dz right) f(y) , dy
            = int delta(x-y) phi(y) f(y) , dy
            = phi(x) f(x).
            $$



            Thus,
            $$
            int delta(x-y) delta(y-z) f(y) , dy = f(x) delta(z-x).
            $$






            share|cite|improve this answer









            $endgroup$



            Talking non-rigorously, $delta(x-y) delta(y-z) f(y)$ will be non-zero only when $x-y=0$ and $y-z=0$, i.e. when $x=y=z.$ Therefore the integral over $y$ would be non-zero only when $x=z.$ We can thus expect the integral to be a multiple of $delta(x-z).$



            So, let $phi$ be a nice function and study the formal integral
            $$
            int left( int delta(x-y) delta(y-z) f(y) , dy right) phi(z) , dz.
            $$



            Swapping the order of integration gives
            $$
            int delta(x-y) left( int delta(y-z) phi(z) , dz right) f(y) , dy
            = int delta(x-y) phi(y) f(y) , dy
            = phi(x) f(x).
            $$



            Thus,
            $$
            int delta(x-y) delta(y-z) f(y) , dy = f(x) delta(z-x).
            $$







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Dec 30 '18 at 15:09









            md2perpemd2perpe

            8,31111028




            8,31111028























                0












                $begingroup$

                Recall the usual $intdelta(x-y)g(y)dy=g(x)$ (assuming it's a definite integral over $A$), in this case with $g(y):=f(y)delta(y-z)$, giving $color{blue}{f(x)delta(x-z)}$ for your in-title integral (again, if it's definite over $A$). Here we've eliminated the $delta$ containing $x$, effectively imposing $y=x$. Of course we could have used the other $delta$ to impose $y=z$ instead, obtaining $color{blue}{f(z)delta(x-z)}$. But by inspection, these two answers are equivalent.






                share|cite|improve this answer









                $endgroup$


















                  0












                  $begingroup$

                  Recall the usual $intdelta(x-y)g(y)dy=g(x)$ (assuming it's a definite integral over $A$), in this case with $g(y):=f(y)delta(y-z)$, giving $color{blue}{f(x)delta(x-z)}$ for your in-title integral (again, if it's definite over $A$). Here we've eliminated the $delta$ containing $x$, effectively imposing $y=x$. Of course we could have used the other $delta$ to impose $y=z$ instead, obtaining $color{blue}{f(z)delta(x-z)}$. But by inspection, these two answers are equivalent.






                  share|cite|improve this answer









                  $endgroup$
















                    0












                    0








                    0





                    $begingroup$

                    Recall the usual $intdelta(x-y)g(y)dy=g(x)$ (assuming it's a definite integral over $A$), in this case with $g(y):=f(y)delta(y-z)$, giving $color{blue}{f(x)delta(x-z)}$ for your in-title integral (again, if it's definite over $A$). Here we've eliminated the $delta$ containing $x$, effectively imposing $y=x$. Of course we could have used the other $delta$ to impose $y=z$ instead, obtaining $color{blue}{f(z)delta(x-z)}$. But by inspection, these two answers are equivalent.






                    share|cite|improve this answer









                    $endgroup$



                    Recall the usual $intdelta(x-y)g(y)dy=g(x)$ (assuming it's a definite integral over $A$), in this case with $g(y):=f(y)delta(y-z)$, giving $color{blue}{f(x)delta(x-z)}$ for your in-title integral (again, if it's definite over $A$). Here we've eliminated the $delta$ containing $x$, effectively imposing $y=x$. Of course we could have used the other $delta$ to impose $y=z$ instead, obtaining $color{blue}{f(z)delta(x-z)}$. But by inspection, these two answers are equivalent.







                    share|cite|improve this answer












                    share|cite|improve this answer



                    share|cite|improve this answer










                    answered Dec 30 '18 at 15:12









                    J.G.J.G.

                    31.8k23250




                    31.8k23250























                        0












                        $begingroup$

                        $$color{green}{delta(x!-!z)}tag{1a}$$and$$color{red}{int_{mathbb{R}} !mathrm{d}y ~f(y)delta(x!-!y)delta(y!-!z)}tag{1b}$$ are informal notations for the distributions
                        $u,vin D^{prime}(mathbb{R}^2)$ given by$^1$
                        $$u[varphi]~:=~int_{mathbb{R}} !mathrm{d}y~varphi(y,y),tag{2a}$$
                        $$v[varphi]~:=~int_{mathbb{R}} !mathrm{d}y ~f(y)~varphi(y,y),tag{2b} $$
                        respectively, where $varphiin D(mathbb{R}^2)$ is a test function. The distributions are written in the informal notation as
                        $$u[varphi]~=~int_{mathbb{R}^2} !mathrm{d}x~mathrm{d}z~color{green}{delta(x!-!z)}~ varphi(x,z), tag{3a}$$
                        $$v[varphi]~:=~int_{mathbb{R}^2} !mathrm{d}x~mathrm{d}z~color{red}{int_{mathbb{R}} !mathrm{d}y ~f(y)delta(x!-!y)delta(y!-!z)} ~varphi(x,z),tag{3b} $$
                        respectively.



                        --



                        $^1$Disclaimer: Here we assume for simplicity that OP's space is $Omega=mathbb{R}$ and the measure $mu$ is the Lebesque measure. Note that e.g. boundaries $partial Omega$ often requires extra attention in distribution theory.






                        share|cite|improve this answer









                        $endgroup$


















                          0












                          $begingroup$

                          $$color{green}{delta(x!-!z)}tag{1a}$$and$$color{red}{int_{mathbb{R}} !mathrm{d}y ~f(y)delta(x!-!y)delta(y!-!z)}tag{1b}$$ are informal notations for the distributions
                          $u,vin D^{prime}(mathbb{R}^2)$ given by$^1$
                          $$u[varphi]~:=~int_{mathbb{R}} !mathrm{d}y~varphi(y,y),tag{2a}$$
                          $$v[varphi]~:=~int_{mathbb{R}} !mathrm{d}y ~f(y)~varphi(y,y),tag{2b} $$
                          respectively, where $varphiin D(mathbb{R}^2)$ is a test function. The distributions are written in the informal notation as
                          $$u[varphi]~=~int_{mathbb{R}^2} !mathrm{d}x~mathrm{d}z~color{green}{delta(x!-!z)}~ varphi(x,z), tag{3a}$$
                          $$v[varphi]~:=~int_{mathbb{R}^2} !mathrm{d}x~mathrm{d}z~color{red}{int_{mathbb{R}} !mathrm{d}y ~f(y)delta(x!-!y)delta(y!-!z)} ~varphi(x,z),tag{3b} $$
                          respectively.



                          --



                          $^1$Disclaimer: Here we assume for simplicity that OP's space is $Omega=mathbb{R}$ and the measure $mu$ is the Lebesque measure. Note that e.g. boundaries $partial Omega$ often requires extra attention in distribution theory.






                          share|cite|improve this answer









                          $endgroup$
















                            0












                            0








                            0





                            $begingroup$

                            $$color{green}{delta(x!-!z)}tag{1a}$$and$$color{red}{int_{mathbb{R}} !mathrm{d}y ~f(y)delta(x!-!y)delta(y!-!z)}tag{1b}$$ are informal notations for the distributions
                            $u,vin D^{prime}(mathbb{R}^2)$ given by$^1$
                            $$u[varphi]~:=~int_{mathbb{R}} !mathrm{d}y~varphi(y,y),tag{2a}$$
                            $$v[varphi]~:=~int_{mathbb{R}} !mathrm{d}y ~f(y)~varphi(y,y),tag{2b} $$
                            respectively, where $varphiin D(mathbb{R}^2)$ is a test function. The distributions are written in the informal notation as
                            $$u[varphi]~=~int_{mathbb{R}^2} !mathrm{d}x~mathrm{d}z~color{green}{delta(x!-!z)}~ varphi(x,z), tag{3a}$$
                            $$v[varphi]~:=~int_{mathbb{R}^2} !mathrm{d}x~mathrm{d}z~color{red}{int_{mathbb{R}} !mathrm{d}y ~f(y)delta(x!-!y)delta(y!-!z)} ~varphi(x,z),tag{3b} $$
                            respectively.



                            --



                            $^1$Disclaimer: Here we assume for simplicity that OP's space is $Omega=mathbb{R}$ and the measure $mu$ is the Lebesque measure. Note that e.g. boundaries $partial Omega$ often requires extra attention in distribution theory.






                            share|cite|improve this answer









                            $endgroup$



                            $$color{green}{delta(x!-!z)}tag{1a}$$and$$color{red}{int_{mathbb{R}} !mathrm{d}y ~f(y)delta(x!-!y)delta(y!-!z)}tag{1b}$$ are informal notations for the distributions
                            $u,vin D^{prime}(mathbb{R}^2)$ given by$^1$
                            $$u[varphi]~:=~int_{mathbb{R}} !mathrm{d}y~varphi(y,y),tag{2a}$$
                            $$v[varphi]~:=~int_{mathbb{R}} !mathrm{d}y ~f(y)~varphi(y,y),tag{2b} $$
                            respectively, where $varphiin D(mathbb{R}^2)$ is a test function. The distributions are written in the informal notation as
                            $$u[varphi]~=~int_{mathbb{R}^2} !mathrm{d}x~mathrm{d}z~color{green}{delta(x!-!z)}~ varphi(x,z), tag{3a}$$
                            $$v[varphi]~:=~int_{mathbb{R}^2} !mathrm{d}x~mathrm{d}z~color{red}{int_{mathbb{R}} !mathrm{d}y ~f(y)delta(x!-!y)delta(y!-!z)} ~varphi(x,z),tag{3b} $$
                            respectively.



                            --



                            $^1$Disclaimer: Here we assume for simplicity that OP's space is $Omega=mathbb{R}$ and the measure $mu$ is the Lebesque measure. Note that e.g. boundaries $partial Omega$ often requires extra attention in distribution theory.







                            share|cite|improve this answer












                            share|cite|improve this answer



                            share|cite|improve this answer










                            answered Jan 5 at 12:10









                            QmechanicQmechanic

                            5,17711858




                            5,17711858






























                                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%2f3056887%2fwhat-is-int-deltax-y-deltay-zfy-rm-dy%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

                                Probability when a professor distributes a quiz and homework assignment to a class of n students.

                                Aardman Animations

                                Are they similar matrix