distribution associated with a discontinuous function












1














Let $fcolonmathbb{R}tomathbb{R}$ be such that, for every $ninmathbb{Z}$, $f$ is differentiable on $left(n,n+1right)$ and $n$ is a discontinuity of first kind of $f$. We define
$$T_f(phi)=int_{mathbb{R}}f(x)phi(x)dx,quadtext{where }phitext{ is a test function}.$$
I understand that, when $f$ is continuously differentiable, $T_f$ is well defined and the derivative of $T_f$ is just $T_{f'}$. However, these two do not seem clear in this case.



My attempt: Since $n$ and $n+1$ are discontinuities of first kind of $f$, we can define a new function, say $g_{n}$ such that $g_{n}$ is continuous on $[n,n+1]$ and equal to $f$ a.e. Thus $f$ is (Lebesgue) integrable on $[n,n+1]$. We then conclude that $f$ is locally integrable on $mathbb{R}$ and therefore $T_{f}$ is well defined.



My question: 1) Is the above reasoning (to show that $T_f$ is well defined) correct? And:



2) What would the derivative of $T_f$ look like in this case? I think it is not $T_{f'}$ as $f'$ is not defined at $n$.



Any help/hint is highly appreciated.










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    1














    Let $fcolonmathbb{R}tomathbb{R}$ be such that, for every $ninmathbb{Z}$, $f$ is differentiable on $left(n,n+1right)$ and $n$ is a discontinuity of first kind of $f$. We define
    $$T_f(phi)=int_{mathbb{R}}f(x)phi(x)dx,quadtext{where }phitext{ is a test function}.$$
    I understand that, when $f$ is continuously differentiable, $T_f$ is well defined and the derivative of $T_f$ is just $T_{f'}$. However, these two do not seem clear in this case.



    My attempt: Since $n$ and $n+1$ are discontinuities of first kind of $f$, we can define a new function, say $g_{n}$ such that $g_{n}$ is continuous on $[n,n+1]$ and equal to $f$ a.e. Thus $f$ is (Lebesgue) integrable on $[n,n+1]$. We then conclude that $f$ is locally integrable on $mathbb{R}$ and therefore $T_{f}$ is well defined.



    My question: 1) Is the above reasoning (to show that $T_f$ is well defined) correct? And:



    2) What would the derivative of $T_f$ look like in this case? I think it is not $T_{f'}$ as $f'$ is not defined at $n$.



    Any help/hint is highly appreciated.










    share|cite|improve this question

























      1












      1








      1







      Let $fcolonmathbb{R}tomathbb{R}$ be such that, for every $ninmathbb{Z}$, $f$ is differentiable on $left(n,n+1right)$ and $n$ is a discontinuity of first kind of $f$. We define
      $$T_f(phi)=int_{mathbb{R}}f(x)phi(x)dx,quadtext{where }phitext{ is a test function}.$$
      I understand that, when $f$ is continuously differentiable, $T_f$ is well defined and the derivative of $T_f$ is just $T_{f'}$. However, these two do not seem clear in this case.



      My attempt: Since $n$ and $n+1$ are discontinuities of first kind of $f$, we can define a new function, say $g_{n}$ such that $g_{n}$ is continuous on $[n,n+1]$ and equal to $f$ a.e. Thus $f$ is (Lebesgue) integrable on $[n,n+1]$. We then conclude that $f$ is locally integrable on $mathbb{R}$ and therefore $T_{f}$ is well defined.



      My question: 1) Is the above reasoning (to show that $T_f$ is well defined) correct? And:



      2) What would the derivative of $T_f$ look like in this case? I think it is not $T_{f'}$ as $f'$ is not defined at $n$.



      Any help/hint is highly appreciated.










      share|cite|improve this question













      Let $fcolonmathbb{R}tomathbb{R}$ be such that, for every $ninmathbb{Z}$, $f$ is differentiable on $left(n,n+1right)$ and $n$ is a discontinuity of first kind of $f$. We define
      $$T_f(phi)=int_{mathbb{R}}f(x)phi(x)dx,quadtext{where }phitext{ is a test function}.$$
      I understand that, when $f$ is continuously differentiable, $T_f$ is well defined and the derivative of $T_f$ is just $T_{f'}$. However, these two do not seem clear in this case.



      My attempt: Since $n$ and $n+1$ are discontinuities of first kind of $f$, we can define a new function, say $g_{n}$ such that $g_{n}$ is continuous on $[n,n+1]$ and equal to $f$ a.e. Thus $f$ is (Lebesgue) integrable on $[n,n+1]$. We then conclude that $f$ is locally integrable on $mathbb{R}$ and therefore $T_{f}$ is well defined.



      My question: 1) Is the above reasoning (to show that $T_f$ is well defined) correct? And:



      2) What would the derivative of $T_f$ look like in this case? I think it is not $T_{f'}$ as $f'$ is not defined at $n$.



      Any help/hint is highly appreciated.







      functional-analysis derivatives distribution-theory topological-vector-spaces discontinuous-functions






      share|cite|improve this question













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      asked Nov 27 at 3:59









      weirdo

      420210




      420210






















          2 Answers
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          1














          If $f$ is absolutely continuous on each segment $[n, n + 1]$ when defined to take the values $f(n + 0)$ and $f((n + 1) - 0)$ at the endpoints, integration by parts shows that the distributional derivative is
          $$f'(x) + sum_n (f(n + 0) - f(n - 0)) delta(x - n),$$
          where $f'(x)$ is the ordinary derivative. If the conditions for integration by parts do not hold, the distributional derivative may not be representable in this form. Also, if the test functions do not have finite support, it is necessary to add a condition on how fast $f$ can grow.






          share|cite|improve this answer































            2














            It is important to use the fact that $f$ has left and right limits at integer points. Otherwise $f$ need not be locally integrable and $T_f$ is not a well defined distribution. The fact that these limits exist implies that it is bounded in $(n,n+1)$ and hence it is locally integrable. Any locally integrable function defines a distribution. In 2) there is no guarantee that $f'$ is even locally integrable so $T_f'$ cannot be written in terms of $f'$ without extra assumptions.






            share|cite|improve this answer























            • For the Heaviside function $f(x)=1$ for $xge 0$ and $=0$ else, the derivative is $0$ a.e. but $T_f'=delta_0$ and hence different from $T_{f'}$.
              – Jochen
              Nov 27 at 7:44










            • @Jochen Thank you. I have omitted the last part of my answer.
              – Kavi Rama Murthy
              Nov 27 at 7:46










            • @KaviRamaMurthy So my attempt to show that $f$ is locally integrable was correct? I was not sure which result we can use to deduce that, if $f$ is integrable on every interval $[n,n+1]$, then $f$ is integrable on each of the finite union of these sets...Perhaps I forgot something in measure theory
              – weirdo
              Nov 27 at 16:41






            • 1




              @weirdo $fI_{cup E_i}=sum fI_{E_i}$ if $E_i$'s are disjoint. Finite sum of integrable functions is integrable.
              – Kavi Rama Murthy
              Nov 27 at 23:08













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            2 Answers
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            2 Answers
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            1














            If $f$ is absolutely continuous on each segment $[n, n + 1]$ when defined to take the values $f(n + 0)$ and $f((n + 1) - 0)$ at the endpoints, integration by parts shows that the distributional derivative is
            $$f'(x) + sum_n (f(n + 0) - f(n - 0)) delta(x - n),$$
            where $f'(x)$ is the ordinary derivative. If the conditions for integration by parts do not hold, the distributional derivative may not be representable in this form. Also, if the test functions do not have finite support, it is necessary to add a condition on how fast $f$ can grow.






            share|cite|improve this answer




























              1














              If $f$ is absolutely continuous on each segment $[n, n + 1]$ when defined to take the values $f(n + 0)$ and $f((n + 1) - 0)$ at the endpoints, integration by parts shows that the distributional derivative is
              $$f'(x) + sum_n (f(n + 0) - f(n - 0)) delta(x - n),$$
              where $f'(x)$ is the ordinary derivative. If the conditions for integration by parts do not hold, the distributional derivative may not be representable in this form. Also, if the test functions do not have finite support, it is necessary to add a condition on how fast $f$ can grow.






              share|cite|improve this answer


























                1












                1








                1






                If $f$ is absolutely continuous on each segment $[n, n + 1]$ when defined to take the values $f(n + 0)$ and $f((n + 1) - 0)$ at the endpoints, integration by parts shows that the distributional derivative is
                $$f'(x) + sum_n (f(n + 0) - f(n - 0)) delta(x - n),$$
                where $f'(x)$ is the ordinary derivative. If the conditions for integration by parts do not hold, the distributional derivative may not be representable in this form. Also, if the test functions do not have finite support, it is necessary to add a condition on how fast $f$ can grow.






                share|cite|improve this answer














                If $f$ is absolutely continuous on each segment $[n, n + 1]$ when defined to take the values $f(n + 0)$ and $f((n + 1) - 0)$ at the endpoints, integration by parts shows that the distributional derivative is
                $$f'(x) + sum_n (f(n + 0) - f(n - 0)) delta(x - n),$$
                where $f'(x)$ is the ordinary derivative. If the conditions for integration by parts do not hold, the distributional derivative may not be representable in this form. Also, if the test functions do not have finite support, it is necessary to add a condition on how fast $f$ can grow.







                share|cite|improve this answer














                share|cite|improve this answer



                share|cite|improve this answer








                edited Dec 7 at 15:39

























                answered Dec 1 at 6:42









                Maxim

                4,5031219




                4,5031219























                    2














                    It is important to use the fact that $f$ has left and right limits at integer points. Otherwise $f$ need not be locally integrable and $T_f$ is not a well defined distribution. The fact that these limits exist implies that it is bounded in $(n,n+1)$ and hence it is locally integrable. Any locally integrable function defines a distribution. In 2) there is no guarantee that $f'$ is even locally integrable so $T_f'$ cannot be written in terms of $f'$ without extra assumptions.






                    share|cite|improve this answer























                    • For the Heaviside function $f(x)=1$ for $xge 0$ and $=0$ else, the derivative is $0$ a.e. but $T_f'=delta_0$ and hence different from $T_{f'}$.
                      – Jochen
                      Nov 27 at 7:44










                    • @Jochen Thank you. I have omitted the last part of my answer.
                      – Kavi Rama Murthy
                      Nov 27 at 7:46










                    • @KaviRamaMurthy So my attempt to show that $f$ is locally integrable was correct? I was not sure which result we can use to deduce that, if $f$ is integrable on every interval $[n,n+1]$, then $f$ is integrable on each of the finite union of these sets...Perhaps I forgot something in measure theory
                      – weirdo
                      Nov 27 at 16:41






                    • 1




                      @weirdo $fI_{cup E_i}=sum fI_{E_i}$ if $E_i$'s are disjoint. Finite sum of integrable functions is integrable.
                      – Kavi Rama Murthy
                      Nov 27 at 23:08


















                    2














                    It is important to use the fact that $f$ has left and right limits at integer points. Otherwise $f$ need not be locally integrable and $T_f$ is not a well defined distribution. The fact that these limits exist implies that it is bounded in $(n,n+1)$ and hence it is locally integrable. Any locally integrable function defines a distribution. In 2) there is no guarantee that $f'$ is even locally integrable so $T_f'$ cannot be written in terms of $f'$ without extra assumptions.






                    share|cite|improve this answer























                    • For the Heaviside function $f(x)=1$ for $xge 0$ and $=0$ else, the derivative is $0$ a.e. but $T_f'=delta_0$ and hence different from $T_{f'}$.
                      – Jochen
                      Nov 27 at 7:44










                    • @Jochen Thank you. I have omitted the last part of my answer.
                      – Kavi Rama Murthy
                      Nov 27 at 7:46










                    • @KaviRamaMurthy So my attempt to show that $f$ is locally integrable was correct? I was not sure which result we can use to deduce that, if $f$ is integrable on every interval $[n,n+1]$, then $f$ is integrable on each of the finite union of these sets...Perhaps I forgot something in measure theory
                      – weirdo
                      Nov 27 at 16:41






                    • 1




                      @weirdo $fI_{cup E_i}=sum fI_{E_i}$ if $E_i$'s are disjoint. Finite sum of integrable functions is integrable.
                      – Kavi Rama Murthy
                      Nov 27 at 23:08
















                    2












                    2








                    2






                    It is important to use the fact that $f$ has left and right limits at integer points. Otherwise $f$ need not be locally integrable and $T_f$ is not a well defined distribution. The fact that these limits exist implies that it is bounded in $(n,n+1)$ and hence it is locally integrable. Any locally integrable function defines a distribution. In 2) there is no guarantee that $f'$ is even locally integrable so $T_f'$ cannot be written in terms of $f'$ without extra assumptions.






                    share|cite|improve this answer














                    It is important to use the fact that $f$ has left and right limits at integer points. Otherwise $f$ need not be locally integrable and $T_f$ is not a well defined distribution. The fact that these limits exist implies that it is bounded in $(n,n+1)$ and hence it is locally integrable. Any locally integrable function defines a distribution. In 2) there is no guarantee that $f'$ is even locally integrable so $T_f'$ cannot be written in terms of $f'$ without extra assumptions.







                    share|cite|improve this answer














                    share|cite|improve this answer



                    share|cite|improve this answer








                    edited Nov 27 at 7:46

























                    answered Nov 27 at 6:18









                    Kavi Rama Murthy

                    49.8k31854




                    49.8k31854












                    • For the Heaviside function $f(x)=1$ for $xge 0$ and $=0$ else, the derivative is $0$ a.e. but $T_f'=delta_0$ and hence different from $T_{f'}$.
                      – Jochen
                      Nov 27 at 7:44










                    • @Jochen Thank you. I have omitted the last part of my answer.
                      – Kavi Rama Murthy
                      Nov 27 at 7:46










                    • @KaviRamaMurthy So my attempt to show that $f$ is locally integrable was correct? I was not sure which result we can use to deduce that, if $f$ is integrable on every interval $[n,n+1]$, then $f$ is integrable on each of the finite union of these sets...Perhaps I forgot something in measure theory
                      – weirdo
                      Nov 27 at 16:41






                    • 1




                      @weirdo $fI_{cup E_i}=sum fI_{E_i}$ if $E_i$'s are disjoint. Finite sum of integrable functions is integrable.
                      – Kavi Rama Murthy
                      Nov 27 at 23:08




















                    • For the Heaviside function $f(x)=1$ for $xge 0$ and $=0$ else, the derivative is $0$ a.e. but $T_f'=delta_0$ and hence different from $T_{f'}$.
                      – Jochen
                      Nov 27 at 7:44










                    • @Jochen Thank you. I have omitted the last part of my answer.
                      – Kavi Rama Murthy
                      Nov 27 at 7:46










                    • @KaviRamaMurthy So my attempt to show that $f$ is locally integrable was correct? I was not sure which result we can use to deduce that, if $f$ is integrable on every interval $[n,n+1]$, then $f$ is integrable on each of the finite union of these sets...Perhaps I forgot something in measure theory
                      – weirdo
                      Nov 27 at 16:41






                    • 1




                      @weirdo $fI_{cup E_i}=sum fI_{E_i}$ if $E_i$'s are disjoint. Finite sum of integrable functions is integrable.
                      – Kavi Rama Murthy
                      Nov 27 at 23:08


















                    For the Heaviside function $f(x)=1$ for $xge 0$ and $=0$ else, the derivative is $0$ a.e. but $T_f'=delta_0$ and hence different from $T_{f'}$.
                    – Jochen
                    Nov 27 at 7:44




                    For the Heaviside function $f(x)=1$ for $xge 0$ and $=0$ else, the derivative is $0$ a.e. but $T_f'=delta_0$ and hence different from $T_{f'}$.
                    – Jochen
                    Nov 27 at 7:44












                    @Jochen Thank you. I have omitted the last part of my answer.
                    – Kavi Rama Murthy
                    Nov 27 at 7:46




                    @Jochen Thank you. I have omitted the last part of my answer.
                    – Kavi Rama Murthy
                    Nov 27 at 7:46












                    @KaviRamaMurthy So my attempt to show that $f$ is locally integrable was correct? I was not sure which result we can use to deduce that, if $f$ is integrable on every interval $[n,n+1]$, then $f$ is integrable on each of the finite union of these sets...Perhaps I forgot something in measure theory
                    – weirdo
                    Nov 27 at 16:41




                    @KaviRamaMurthy So my attempt to show that $f$ is locally integrable was correct? I was not sure which result we can use to deduce that, if $f$ is integrable on every interval $[n,n+1]$, then $f$ is integrable on each of the finite union of these sets...Perhaps I forgot something in measure theory
                    – weirdo
                    Nov 27 at 16:41




                    1




                    1




                    @weirdo $fI_{cup E_i}=sum fI_{E_i}$ if $E_i$'s are disjoint. Finite sum of integrable functions is integrable.
                    – Kavi Rama Murthy
                    Nov 27 at 23:08






                    @weirdo $fI_{cup E_i}=sum fI_{E_i}$ if $E_i$'s are disjoint. Finite sum of integrable functions is integrable.
                    – Kavi Rama Murthy
                    Nov 27 at 23:08




















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