Exponential of the product between $x$ the derivative operator of $x$ acting in a $f(x)$












3












$begingroup$


The question



I'm stuck here trying to figure out how to compute and prove, the following operator action in a function:



$exp(varepsilon x partial_x) f(x) = f(x exp(varepsilon) )$



where $varepsilon$ is a constant.



I saw this result and I failed in my attempt to reproduced it. What I did was to expand $exp(varepsilon x partial_x)$ in Taylor's series as:



$begin{align*}
exp(varepsilon x partial_x)f(x)& = sum_{m=0}^{infty}frac{1}{m!}(varepsilon x partial_x)^m f(x)\
&= sum_{m=0}^{infty}frac{1}{m!}(varepsilon x)^mfrac{partial^m}{partial x^m}f(x) \
end{align*}$



I took this procedure because I already know how to compute $e^{partial_x}f(x)$. Let me show you what I did in this case:



The translation operator



The Taylor series of a function f is



begin{equation}
f(x)=sum_{n=0}^inftyfrac{(partial_x^nf)(a)}{n!}(x-a)^n
end{equation}



Expanding about $x+b$ and letting $a=x$:



begin{equation}
f(x+b)=sum_{n=0}^inftyfrac{(partial_x^nf)(x)}{n!}b^n=sum_{n=0}^inftyfrac{((bpartial_x)^nf)(x)}{n!}
end{equation}



By definition:



begin{equation}
e^{bpartial_x}=sum_{n=0}^inftyfrac{(bpartial_x)^n}{n!}
end{equation}



Hence



begin{equation}
f(x+b)=(e^{bpartial_x}f)(x)
end{equation}



Returning to my question



I tried to generalize or make something similar for the previous case I discussed but I didn't get anywhere. Anyone can give me a tip or recommend a book or paper?



Thanks!!










share|cite|improve this question









$endgroup$

















    3












    $begingroup$


    The question



    I'm stuck here trying to figure out how to compute and prove, the following operator action in a function:



    $exp(varepsilon x partial_x) f(x) = f(x exp(varepsilon) )$



    where $varepsilon$ is a constant.



    I saw this result and I failed in my attempt to reproduced it. What I did was to expand $exp(varepsilon x partial_x)$ in Taylor's series as:



    $begin{align*}
    exp(varepsilon x partial_x)f(x)& = sum_{m=0}^{infty}frac{1}{m!}(varepsilon x partial_x)^m f(x)\
    &= sum_{m=0}^{infty}frac{1}{m!}(varepsilon x)^mfrac{partial^m}{partial x^m}f(x) \
    end{align*}$



    I took this procedure because I already know how to compute $e^{partial_x}f(x)$. Let me show you what I did in this case:



    The translation operator



    The Taylor series of a function f is



    begin{equation}
    f(x)=sum_{n=0}^inftyfrac{(partial_x^nf)(a)}{n!}(x-a)^n
    end{equation}



    Expanding about $x+b$ and letting $a=x$:



    begin{equation}
    f(x+b)=sum_{n=0}^inftyfrac{(partial_x^nf)(x)}{n!}b^n=sum_{n=0}^inftyfrac{((bpartial_x)^nf)(x)}{n!}
    end{equation}



    By definition:



    begin{equation}
    e^{bpartial_x}=sum_{n=0}^inftyfrac{(bpartial_x)^n}{n!}
    end{equation}



    Hence



    begin{equation}
    f(x+b)=(e^{bpartial_x}f)(x)
    end{equation}



    Returning to my question



    I tried to generalize or make something similar for the previous case I discussed but I didn't get anywhere. Anyone can give me a tip or recommend a book or paper?



    Thanks!!










    share|cite|improve this question









    $endgroup$















      3












      3








      3


      1



      $begingroup$


      The question



      I'm stuck here trying to figure out how to compute and prove, the following operator action in a function:



      $exp(varepsilon x partial_x) f(x) = f(x exp(varepsilon) )$



      where $varepsilon$ is a constant.



      I saw this result and I failed in my attempt to reproduced it. What I did was to expand $exp(varepsilon x partial_x)$ in Taylor's series as:



      $begin{align*}
      exp(varepsilon x partial_x)f(x)& = sum_{m=0}^{infty}frac{1}{m!}(varepsilon x partial_x)^m f(x)\
      &= sum_{m=0}^{infty}frac{1}{m!}(varepsilon x)^mfrac{partial^m}{partial x^m}f(x) \
      end{align*}$



      I took this procedure because I already know how to compute $e^{partial_x}f(x)$. Let me show you what I did in this case:



      The translation operator



      The Taylor series of a function f is



      begin{equation}
      f(x)=sum_{n=0}^inftyfrac{(partial_x^nf)(a)}{n!}(x-a)^n
      end{equation}



      Expanding about $x+b$ and letting $a=x$:



      begin{equation}
      f(x+b)=sum_{n=0}^inftyfrac{(partial_x^nf)(x)}{n!}b^n=sum_{n=0}^inftyfrac{((bpartial_x)^nf)(x)}{n!}
      end{equation}



      By definition:



      begin{equation}
      e^{bpartial_x}=sum_{n=0}^inftyfrac{(bpartial_x)^n}{n!}
      end{equation}



      Hence



      begin{equation}
      f(x+b)=(e^{bpartial_x}f)(x)
      end{equation}



      Returning to my question



      I tried to generalize or make something similar for the previous case I discussed but I didn't get anywhere. Anyone can give me a tip or recommend a book or paper?



      Thanks!!










      share|cite|improve this question









      $endgroup$




      The question



      I'm stuck here trying to figure out how to compute and prove, the following operator action in a function:



      $exp(varepsilon x partial_x) f(x) = f(x exp(varepsilon) )$



      where $varepsilon$ is a constant.



      I saw this result and I failed in my attempt to reproduced it. What I did was to expand $exp(varepsilon x partial_x)$ in Taylor's series as:



      $begin{align*}
      exp(varepsilon x partial_x)f(x)& = sum_{m=0}^{infty}frac{1}{m!}(varepsilon x partial_x)^m f(x)\
      &= sum_{m=0}^{infty}frac{1}{m!}(varepsilon x)^mfrac{partial^m}{partial x^m}f(x) \
      end{align*}$



      I took this procedure because I already know how to compute $e^{partial_x}f(x)$. Let me show you what I did in this case:



      The translation operator



      The Taylor series of a function f is



      begin{equation}
      f(x)=sum_{n=0}^inftyfrac{(partial_x^nf)(a)}{n!}(x-a)^n
      end{equation}



      Expanding about $x+b$ and letting $a=x$:



      begin{equation}
      f(x+b)=sum_{n=0}^inftyfrac{(partial_x^nf)(x)}{n!}b^n=sum_{n=0}^inftyfrac{((bpartial_x)^nf)(x)}{n!}
      end{equation}



      By definition:



      begin{equation}
      e^{bpartial_x}=sum_{n=0}^inftyfrac{(bpartial_x)^n}{n!}
      end{equation}



      Hence



      begin{equation}
      f(x+b)=(e^{bpartial_x}f)(x)
      end{equation}



      Returning to my question



      I tried to generalize or make something similar for the previous case I discussed but I didn't get anywhere. Anyone can give me a tip or recommend a book or paper?



      Thanks!!







      taylor-expansion lie-algebras operator-algebras differential-operators






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked Feb 1 at 13:43









      Alexssandre de Oliveira JAlexssandre de Oliveira J

      163




      163






















          2 Answers
          2






          active

          oldest

          votes


















          1












          $begingroup$

          You are assuming that multiplication by $x$ and the derivative commute, and that's not the case. For clarity, let me write $M_x$ for the operator of multiplication by $x$. If $f(x)=x^k$, then
          $$
          [M_xpartial_x f](x)=kx^k, [(M_xpartial_x)^2f](x)=k^2x^k, cdots , [(M_xpartial_x)^mf](x)=k^mx^k.
          $$



          Then
          begin{align*}
          [exp(varepsilon x partial_x)f](x)& = sum_{m=0}^{infty}frac{1}{m!}(varepsilon x partial_x)^m f(x)\
          &= sum_{m=0}^{infty}frac{1}{m!}(varepsilon )^mk^mx^k \
          &=x^k,exp(varepsilon k)=x^k (exp(varepsilon)^k\
          &=f(xexp(varepsilon)).
          end{align*}



          Thus we get by linearity that, for any polynomial $p$,
          $$tag1
          [exp(varepsilon x partial_x)p](x)=p(xexp(varepsilon)).
          $$

          Analytic functions are differentiable term by term, so the differential operator gets inside the series. This allows us to extend $(1)$ to $f$ analytic.






          share|cite|improve this answer









          $endgroup$













          • $begingroup$
            When I tried to perform the solution, I didn't notice the commutation relation between $x$ and $partial_x$. Very thank you for the solution, Martin.
            $endgroup$
            – Alexssandre de Oliveira J
            Feb 1 at 19:30



















          0












          $begingroup$

          Before Martin Argerami came up with a solution I friend of mine thought in something but he's not so sure if it's right... I like to think that it is a physicist solution. I'll explain why, but first, let me show you what he thought.



          If we assume that $f(x)$ has a Fourier Transform, we can write:



          begin{align*}
          e^{varepsilon x partial_x}f(x) &= int d^3k , f(k)e^{varepsilon x partial_x}e^{ikx}\
          &= int d^3k, f(k) sum_{n,m} frac{(varepsilon x partial_x)}{m!}frac{ikx}{n!}
          end{align*}



          Notice that



          begin{align*}
          (x partial_x)^m x^n = n^m x^n , , ,
          end{align*}



          where commutation relation is respect! And then we can resum the series in $m$ and $n$ to obtain:



          begin{align*}
          e^{varepsilon x partial_x}f(x) &= int d^3k, f(k) sum_{n} e^{(varepsilon)^n}frac{(ikx)^n}{n!}\
          &= int d^3k, f(k) sum_{n} frac{(e^{varepsilon}ikx)^n}{n!} \
          &= int d^3k , f(k) e^{ike^varepsilon x} = f(e^epsilon x)
          end{align*}



          It's a physicist solution because we're assuming that every operation is well defined.






          share|cite|improve this answer









          $endgroup$














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            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            1












            $begingroup$

            You are assuming that multiplication by $x$ and the derivative commute, and that's not the case. For clarity, let me write $M_x$ for the operator of multiplication by $x$. If $f(x)=x^k$, then
            $$
            [M_xpartial_x f](x)=kx^k, [(M_xpartial_x)^2f](x)=k^2x^k, cdots , [(M_xpartial_x)^mf](x)=k^mx^k.
            $$



            Then
            begin{align*}
            [exp(varepsilon x partial_x)f](x)& = sum_{m=0}^{infty}frac{1}{m!}(varepsilon x partial_x)^m f(x)\
            &= sum_{m=0}^{infty}frac{1}{m!}(varepsilon )^mk^mx^k \
            &=x^k,exp(varepsilon k)=x^k (exp(varepsilon)^k\
            &=f(xexp(varepsilon)).
            end{align*}



            Thus we get by linearity that, for any polynomial $p$,
            $$tag1
            [exp(varepsilon x partial_x)p](x)=p(xexp(varepsilon)).
            $$

            Analytic functions are differentiable term by term, so the differential operator gets inside the series. This allows us to extend $(1)$ to $f$ analytic.






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              When I tried to perform the solution, I didn't notice the commutation relation between $x$ and $partial_x$. Very thank you for the solution, Martin.
              $endgroup$
              – Alexssandre de Oliveira J
              Feb 1 at 19:30
















            1












            $begingroup$

            You are assuming that multiplication by $x$ and the derivative commute, and that's not the case. For clarity, let me write $M_x$ for the operator of multiplication by $x$. If $f(x)=x^k$, then
            $$
            [M_xpartial_x f](x)=kx^k, [(M_xpartial_x)^2f](x)=k^2x^k, cdots , [(M_xpartial_x)^mf](x)=k^mx^k.
            $$



            Then
            begin{align*}
            [exp(varepsilon x partial_x)f](x)& = sum_{m=0}^{infty}frac{1}{m!}(varepsilon x partial_x)^m f(x)\
            &= sum_{m=0}^{infty}frac{1}{m!}(varepsilon )^mk^mx^k \
            &=x^k,exp(varepsilon k)=x^k (exp(varepsilon)^k\
            &=f(xexp(varepsilon)).
            end{align*}



            Thus we get by linearity that, for any polynomial $p$,
            $$tag1
            [exp(varepsilon x partial_x)p](x)=p(xexp(varepsilon)).
            $$

            Analytic functions are differentiable term by term, so the differential operator gets inside the series. This allows us to extend $(1)$ to $f$ analytic.






            share|cite|improve this answer









            $endgroup$













            • $begingroup$
              When I tried to perform the solution, I didn't notice the commutation relation between $x$ and $partial_x$. Very thank you for the solution, Martin.
              $endgroup$
              – Alexssandre de Oliveira J
              Feb 1 at 19:30














            1












            1








            1





            $begingroup$

            You are assuming that multiplication by $x$ and the derivative commute, and that's not the case. For clarity, let me write $M_x$ for the operator of multiplication by $x$. If $f(x)=x^k$, then
            $$
            [M_xpartial_x f](x)=kx^k, [(M_xpartial_x)^2f](x)=k^2x^k, cdots , [(M_xpartial_x)^mf](x)=k^mx^k.
            $$



            Then
            begin{align*}
            [exp(varepsilon x partial_x)f](x)& = sum_{m=0}^{infty}frac{1}{m!}(varepsilon x partial_x)^m f(x)\
            &= sum_{m=0}^{infty}frac{1}{m!}(varepsilon )^mk^mx^k \
            &=x^k,exp(varepsilon k)=x^k (exp(varepsilon)^k\
            &=f(xexp(varepsilon)).
            end{align*}



            Thus we get by linearity that, for any polynomial $p$,
            $$tag1
            [exp(varepsilon x partial_x)p](x)=p(xexp(varepsilon)).
            $$

            Analytic functions are differentiable term by term, so the differential operator gets inside the series. This allows us to extend $(1)$ to $f$ analytic.






            share|cite|improve this answer









            $endgroup$



            You are assuming that multiplication by $x$ and the derivative commute, and that's not the case. For clarity, let me write $M_x$ for the operator of multiplication by $x$. If $f(x)=x^k$, then
            $$
            [M_xpartial_x f](x)=kx^k, [(M_xpartial_x)^2f](x)=k^2x^k, cdots , [(M_xpartial_x)^mf](x)=k^mx^k.
            $$



            Then
            begin{align*}
            [exp(varepsilon x partial_x)f](x)& = sum_{m=0}^{infty}frac{1}{m!}(varepsilon x partial_x)^m f(x)\
            &= sum_{m=0}^{infty}frac{1}{m!}(varepsilon )^mk^mx^k \
            &=x^k,exp(varepsilon k)=x^k (exp(varepsilon)^k\
            &=f(xexp(varepsilon)).
            end{align*}



            Thus we get by linearity that, for any polynomial $p$,
            $$tag1
            [exp(varepsilon x partial_x)p](x)=p(xexp(varepsilon)).
            $$

            Analytic functions are differentiable term by term, so the differential operator gets inside the series. This allows us to extend $(1)$ to $f$ analytic.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered Feb 1 at 18:24









            Martin ArgeramiMartin Argerami

            129k1184185




            129k1184185












            • $begingroup$
              When I tried to perform the solution, I didn't notice the commutation relation between $x$ and $partial_x$. Very thank you for the solution, Martin.
              $endgroup$
              – Alexssandre de Oliveira J
              Feb 1 at 19:30


















            • $begingroup$
              When I tried to perform the solution, I didn't notice the commutation relation between $x$ and $partial_x$. Very thank you for the solution, Martin.
              $endgroup$
              – Alexssandre de Oliveira J
              Feb 1 at 19:30
















            $begingroup$
            When I tried to perform the solution, I didn't notice the commutation relation between $x$ and $partial_x$. Very thank you for the solution, Martin.
            $endgroup$
            – Alexssandre de Oliveira J
            Feb 1 at 19:30




            $begingroup$
            When I tried to perform the solution, I didn't notice the commutation relation between $x$ and $partial_x$. Very thank you for the solution, Martin.
            $endgroup$
            – Alexssandre de Oliveira J
            Feb 1 at 19:30











            0












            $begingroup$

            Before Martin Argerami came up with a solution I friend of mine thought in something but he's not so sure if it's right... I like to think that it is a physicist solution. I'll explain why, but first, let me show you what he thought.



            If we assume that $f(x)$ has a Fourier Transform, we can write:



            begin{align*}
            e^{varepsilon x partial_x}f(x) &= int d^3k , f(k)e^{varepsilon x partial_x}e^{ikx}\
            &= int d^3k, f(k) sum_{n,m} frac{(varepsilon x partial_x)}{m!}frac{ikx}{n!}
            end{align*}



            Notice that



            begin{align*}
            (x partial_x)^m x^n = n^m x^n , , ,
            end{align*}



            where commutation relation is respect! And then we can resum the series in $m$ and $n$ to obtain:



            begin{align*}
            e^{varepsilon x partial_x}f(x) &= int d^3k, f(k) sum_{n} e^{(varepsilon)^n}frac{(ikx)^n}{n!}\
            &= int d^3k, f(k) sum_{n} frac{(e^{varepsilon}ikx)^n}{n!} \
            &= int d^3k , f(k) e^{ike^varepsilon x} = f(e^epsilon x)
            end{align*}



            It's a physicist solution because we're assuming that every operation is well defined.






            share|cite|improve this answer









            $endgroup$


















              0












              $begingroup$

              Before Martin Argerami came up with a solution I friend of mine thought in something but he's not so sure if it's right... I like to think that it is a physicist solution. I'll explain why, but first, let me show you what he thought.



              If we assume that $f(x)$ has a Fourier Transform, we can write:



              begin{align*}
              e^{varepsilon x partial_x}f(x) &= int d^3k , f(k)e^{varepsilon x partial_x}e^{ikx}\
              &= int d^3k, f(k) sum_{n,m} frac{(varepsilon x partial_x)}{m!}frac{ikx}{n!}
              end{align*}



              Notice that



              begin{align*}
              (x partial_x)^m x^n = n^m x^n , , ,
              end{align*}



              where commutation relation is respect! And then we can resum the series in $m$ and $n$ to obtain:



              begin{align*}
              e^{varepsilon x partial_x}f(x) &= int d^3k, f(k) sum_{n} e^{(varepsilon)^n}frac{(ikx)^n}{n!}\
              &= int d^3k, f(k) sum_{n} frac{(e^{varepsilon}ikx)^n}{n!} \
              &= int d^3k , f(k) e^{ike^varepsilon x} = f(e^epsilon x)
              end{align*}



              It's a physicist solution because we're assuming that every operation is well defined.






              share|cite|improve this answer









              $endgroup$
















                0












                0








                0





                $begingroup$

                Before Martin Argerami came up with a solution I friend of mine thought in something but he's not so sure if it's right... I like to think that it is a physicist solution. I'll explain why, but first, let me show you what he thought.



                If we assume that $f(x)$ has a Fourier Transform, we can write:



                begin{align*}
                e^{varepsilon x partial_x}f(x) &= int d^3k , f(k)e^{varepsilon x partial_x}e^{ikx}\
                &= int d^3k, f(k) sum_{n,m} frac{(varepsilon x partial_x)}{m!}frac{ikx}{n!}
                end{align*}



                Notice that



                begin{align*}
                (x partial_x)^m x^n = n^m x^n , , ,
                end{align*}



                where commutation relation is respect! And then we can resum the series in $m$ and $n$ to obtain:



                begin{align*}
                e^{varepsilon x partial_x}f(x) &= int d^3k, f(k) sum_{n} e^{(varepsilon)^n}frac{(ikx)^n}{n!}\
                &= int d^3k, f(k) sum_{n} frac{(e^{varepsilon}ikx)^n}{n!} \
                &= int d^3k , f(k) e^{ike^varepsilon x} = f(e^epsilon x)
                end{align*}



                It's a physicist solution because we're assuming that every operation is well defined.






                share|cite|improve this answer









                $endgroup$



                Before Martin Argerami came up with a solution I friend of mine thought in something but he's not so sure if it's right... I like to think that it is a physicist solution. I'll explain why, but first, let me show you what he thought.



                If we assume that $f(x)$ has a Fourier Transform, we can write:



                begin{align*}
                e^{varepsilon x partial_x}f(x) &= int d^3k , f(k)e^{varepsilon x partial_x}e^{ikx}\
                &= int d^3k, f(k) sum_{n,m} frac{(varepsilon x partial_x)}{m!}frac{ikx}{n!}
                end{align*}



                Notice that



                begin{align*}
                (x partial_x)^m x^n = n^m x^n , , ,
                end{align*}



                where commutation relation is respect! And then we can resum the series in $m$ and $n$ to obtain:



                begin{align*}
                e^{varepsilon x partial_x}f(x) &= int d^3k, f(k) sum_{n} e^{(varepsilon)^n}frac{(ikx)^n}{n!}\
                &= int d^3k, f(k) sum_{n} frac{(e^{varepsilon}ikx)^n}{n!} \
                &= int d^3k , f(k) e^{ike^varepsilon x} = f(e^epsilon x)
                end{align*}



                It's a physicist solution because we're assuming that every operation is well defined.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Feb 1 at 20:01









                Alexssandre de Oliveira JAlexssandre de Oliveira J

                163




                163






























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