Continuous distribution-valued function induces distribution












2












$begingroup$


Suppose that the map $mathbb{R}^n to mathcal{D}'(mathbb{R}^n), hspace{3mm}etamapsto E_eta$ is continuous. Furthermore let $mu$ be a Radon-measure with compact support.



I'm having trouble showing that the functional $E$ on $mathcal{D}(mathbb{R}^n)$, whose action on a test function $psi$ is defined by
begin{equation}
langle E,psirangle = int_{mathbb{R}^n} langle E_eta, psi rangle dmu(eta),
end{equation}

is in fact a distribution.










share|cite|improve this question











$endgroup$

















    2












    $begingroup$


    Suppose that the map $mathbb{R}^n to mathcal{D}'(mathbb{R}^n), hspace{3mm}etamapsto E_eta$ is continuous. Furthermore let $mu$ be a Radon-measure with compact support.



    I'm having trouble showing that the functional $E$ on $mathcal{D}(mathbb{R}^n)$, whose action on a test function $psi$ is defined by
    begin{equation}
    langle E,psirangle = int_{mathbb{R}^n} langle E_eta, psi rangle dmu(eta),
    end{equation}

    is in fact a distribution.










    share|cite|improve this question











    $endgroup$















      2












      2








      2





      $begingroup$


      Suppose that the map $mathbb{R}^n to mathcal{D}'(mathbb{R}^n), hspace{3mm}etamapsto E_eta$ is continuous. Furthermore let $mu$ be a Radon-measure with compact support.



      I'm having trouble showing that the functional $E$ on $mathcal{D}(mathbb{R}^n)$, whose action on a test function $psi$ is defined by
      begin{equation}
      langle E,psirangle = int_{mathbb{R}^n} langle E_eta, psi rangle dmu(eta),
      end{equation}

      is in fact a distribution.










      share|cite|improve this question











      $endgroup$




      Suppose that the map $mathbb{R}^n to mathcal{D}'(mathbb{R}^n), hspace{3mm}etamapsto E_eta$ is continuous. Furthermore let $mu$ be a Radon-measure with compact support.



      I'm having trouble showing that the functional $E$ on $mathcal{D}(mathbb{R}^n)$, whose action on a test function $psi$ is defined by
      begin{equation}
      langle E,psirangle = int_{mathbb{R}^n} langle E_eta, psi rangle dmu(eta),
      end{equation}

      is in fact a distribution.







      functional-analysis lebesgue-integral distribution-theory






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Jan 22 at 16:44









      Abdelmalek Abdesselam

      647311




      647311










      asked Jan 18 at 12:20









      Joseph ExpoJoseph Expo

      465




      465






















          1 Answer
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          $begingroup$

          The question is a bit ambiguous because the topology on $mathcal{D}'(mathbb{R}^n)$ is not specified when mentioning that $etarightarrow E_{eta}$ is continuous.
          I don't see immediately if $E$ is indeed continuous when the chosen topology is the weak-$ast$ topology. However, if the topology is the strong one (a better choice anyway) then the answer is yes as is shown below.



          Firstly, by writing $mu$ as a difference of positive measures one can reduce the problem to the case where $mu$ is positive. Moreover, by assumption there is a compact set $K$ on which it is supported so
          $$
          langle E,psirangle=int_K langle E_eta,psirangle dmu(eta) .
          $$

          Let $A$ be a bounded set in $mathcal{D}(mathbb{R}^n)$ and for a distribution $T$ let us use the notation
          $$
          ||T||_A=sup_{psiin A} |langle T,psirangle| .
          $$

          These seminorms define the strong topology on $mathcal{D}'(mathbb{R}^n)$ and are thus continuous. By composition, the map $Krightarrow [0,infty)$, $etamapsto ||E_eta||_A$ is continuous and therefore, by compactness, is bounded by some constant $M$.



          Now let $(psi_n)$ be a sequence that converges to $psi$ in $mathcal{D}(mathbb{R}^n)$. Let $A={psi_n : ninmathbb{N}}$ which is bounded.
          We have $|langle E_{eta},psi_nrangle|le M$ for all $ninmathbb{N}$ and $etain K$. Moreover, for fixed $eta$, $langle E_eta,psi_nranglerightarrow
          langle E_eta,psirangle$
          . Therefore, by dominated convergence
          $$
          langle E,psi_nranglerightarrow langle E,psirangle
          $$

          and $E$ is a distribution.





          Addendum recalling some prerequisites:



          Here the space of smooth compactly supported functions $mathcal{D}(mathbb{R}^n)$ is a topological vector space. Its topology is defined by an uncountable collection of seminorms which are described explicitly here. As for the notion of bounded set in $mathcal{D}(mathbb{R}^n)$, the following are equivalent, for $Asubsetmathcal{D}(mathbb{R}^n)$:





          1. $A$ is bounded

          2. For every open set $Vsubsetmathcal{D}(mathbb{R}^n)$ which contains the origin, there exists $lambda>0$ such that $lambda Asubset V$.

          3. For every continuous seminorm $||cdot||$ on $mathcal{D}(mathbb{R}^n)$, we have $sup_{psiin A}||psi||<infty$.

          4. For every seminorm $||cdot||$ in a defining set of seminorms (e.g., the ones in the MO link I gave above), we have $sup_{psiin A}||psi||<infty$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Thanks for the reply. Could you specify what you mean when a set is bounded in $mathcal{D}(mathbb{R}^n)$? Are you using the supremum norm or is the topology also induced by a family of seminorms?
            $endgroup$
            – Joseph Expo
            Jan 24 at 9:23












          • $begingroup$
            I edited my answer to recall what a bounded set is. If by supremum norm you mean $sup_{xinmathbb{R}^n}|psi(x)|$, that's not what I was using. The notion of bounded set here involves a supremum over a set of test functions, not over a set of points in $mathbb{R}^n$.
            $endgroup$
            – Abdelmalek Abdesselam
            Jan 24 at 15:37













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          $begingroup$

          The question is a bit ambiguous because the topology on $mathcal{D}'(mathbb{R}^n)$ is not specified when mentioning that $etarightarrow E_{eta}$ is continuous.
          I don't see immediately if $E$ is indeed continuous when the chosen topology is the weak-$ast$ topology. However, if the topology is the strong one (a better choice anyway) then the answer is yes as is shown below.



          Firstly, by writing $mu$ as a difference of positive measures one can reduce the problem to the case where $mu$ is positive. Moreover, by assumption there is a compact set $K$ on which it is supported so
          $$
          langle E,psirangle=int_K langle E_eta,psirangle dmu(eta) .
          $$

          Let $A$ be a bounded set in $mathcal{D}(mathbb{R}^n)$ and for a distribution $T$ let us use the notation
          $$
          ||T||_A=sup_{psiin A} |langle T,psirangle| .
          $$

          These seminorms define the strong topology on $mathcal{D}'(mathbb{R}^n)$ and are thus continuous. By composition, the map $Krightarrow [0,infty)$, $etamapsto ||E_eta||_A$ is continuous and therefore, by compactness, is bounded by some constant $M$.



          Now let $(psi_n)$ be a sequence that converges to $psi$ in $mathcal{D}(mathbb{R}^n)$. Let $A={psi_n : ninmathbb{N}}$ which is bounded.
          We have $|langle E_{eta},psi_nrangle|le M$ for all $ninmathbb{N}$ and $etain K$. Moreover, for fixed $eta$, $langle E_eta,psi_nranglerightarrow
          langle E_eta,psirangle$
          . Therefore, by dominated convergence
          $$
          langle E,psi_nranglerightarrow langle E,psirangle
          $$

          and $E$ is a distribution.





          Addendum recalling some prerequisites:



          Here the space of smooth compactly supported functions $mathcal{D}(mathbb{R}^n)$ is a topological vector space. Its topology is defined by an uncountable collection of seminorms which are described explicitly here. As for the notion of bounded set in $mathcal{D}(mathbb{R}^n)$, the following are equivalent, for $Asubsetmathcal{D}(mathbb{R}^n)$:





          1. $A$ is bounded

          2. For every open set $Vsubsetmathcal{D}(mathbb{R}^n)$ which contains the origin, there exists $lambda>0$ such that $lambda Asubset V$.

          3. For every continuous seminorm $||cdot||$ on $mathcal{D}(mathbb{R}^n)$, we have $sup_{psiin A}||psi||<infty$.

          4. For every seminorm $||cdot||$ in a defining set of seminorms (e.g., the ones in the MO link I gave above), we have $sup_{psiin A}||psi||<infty$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Thanks for the reply. Could you specify what you mean when a set is bounded in $mathcal{D}(mathbb{R}^n)$? Are you using the supremum norm or is the topology also induced by a family of seminorms?
            $endgroup$
            – Joseph Expo
            Jan 24 at 9:23












          • $begingroup$
            I edited my answer to recall what a bounded set is. If by supremum norm you mean $sup_{xinmathbb{R}^n}|psi(x)|$, that's not what I was using. The notion of bounded set here involves a supremum over a set of test functions, not over a set of points in $mathbb{R}^n$.
            $endgroup$
            – Abdelmalek Abdesselam
            Jan 24 at 15:37


















          1












          $begingroup$

          The question is a bit ambiguous because the topology on $mathcal{D}'(mathbb{R}^n)$ is not specified when mentioning that $etarightarrow E_{eta}$ is continuous.
          I don't see immediately if $E$ is indeed continuous when the chosen topology is the weak-$ast$ topology. However, if the topology is the strong one (a better choice anyway) then the answer is yes as is shown below.



          Firstly, by writing $mu$ as a difference of positive measures one can reduce the problem to the case where $mu$ is positive. Moreover, by assumption there is a compact set $K$ on which it is supported so
          $$
          langle E,psirangle=int_K langle E_eta,psirangle dmu(eta) .
          $$

          Let $A$ be a bounded set in $mathcal{D}(mathbb{R}^n)$ and for a distribution $T$ let us use the notation
          $$
          ||T||_A=sup_{psiin A} |langle T,psirangle| .
          $$

          These seminorms define the strong topology on $mathcal{D}'(mathbb{R}^n)$ and are thus continuous. By composition, the map $Krightarrow [0,infty)$, $etamapsto ||E_eta||_A$ is continuous and therefore, by compactness, is bounded by some constant $M$.



          Now let $(psi_n)$ be a sequence that converges to $psi$ in $mathcal{D}(mathbb{R}^n)$. Let $A={psi_n : ninmathbb{N}}$ which is bounded.
          We have $|langle E_{eta},psi_nrangle|le M$ for all $ninmathbb{N}$ and $etain K$. Moreover, for fixed $eta$, $langle E_eta,psi_nranglerightarrow
          langle E_eta,psirangle$
          . Therefore, by dominated convergence
          $$
          langle E,psi_nranglerightarrow langle E,psirangle
          $$

          and $E$ is a distribution.





          Addendum recalling some prerequisites:



          Here the space of smooth compactly supported functions $mathcal{D}(mathbb{R}^n)$ is a topological vector space. Its topology is defined by an uncountable collection of seminorms which are described explicitly here. As for the notion of bounded set in $mathcal{D}(mathbb{R}^n)$, the following are equivalent, for $Asubsetmathcal{D}(mathbb{R}^n)$:





          1. $A$ is bounded

          2. For every open set $Vsubsetmathcal{D}(mathbb{R}^n)$ which contains the origin, there exists $lambda>0$ such that $lambda Asubset V$.

          3. For every continuous seminorm $||cdot||$ on $mathcal{D}(mathbb{R}^n)$, we have $sup_{psiin A}||psi||<infty$.

          4. For every seminorm $||cdot||$ in a defining set of seminorms (e.g., the ones in the MO link I gave above), we have $sup_{psiin A}||psi||<infty$.






          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            Thanks for the reply. Could you specify what you mean when a set is bounded in $mathcal{D}(mathbb{R}^n)$? Are you using the supremum norm or is the topology also induced by a family of seminorms?
            $endgroup$
            – Joseph Expo
            Jan 24 at 9:23












          • $begingroup$
            I edited my answer to recall what a bounded set is. If by supremum norm you mean $sup_{xinmathbb{R}^n}|psi(x)|$, that's not what I was using. The notion of bounded set here involves a supremum over a set of test functions, not over a set of points in $mathbb{R}^n$.
            $endgroup$
            – Abdelmalek Abdesselam
            Jan 24 at 15:37
















          1












          1








          1





          $begingroup$

          The question is a bit ambiguous because the topology on $mathcal{D}'(mathbb{R}^n)$ is not specified when mentioning that $etarightarrow E_{eta}$ is continuous.
          I don't see immediately if $E$ is indeed continuous when the chosen topology is the weak-$ast$ topology. However, if the topology is the strong one (a better choice anyway) then the answer is yes as is shown below.



          Firstly, by writing $mu$ as a difference of positive measures one can reduce the problem to the case where $mu$ is positive. Moreover, by assumption there is a compact set $K$ on which it is supported so
          $$
          langle E,psirangle=int_K langle E_eta,psirangle dmu(eta) .
          $$

          Let $A$ be a bounded set in $mathcal{D}(mathbb{R}^n)$ and for a distribution $T$ let us use the notation
          $$
          ||T||_A=sup_{psiin A} |langle T,psirangle| .
          $$

          These seminorms define the strong topology on $mathcal{D}'(mathbb{R}^n)$ and are thus continuous. By composition, the map $Krightarrow [0,infty)$, $etamapsto ||E_eta||_A$ is continuous and therefore, by compactness, is bounded by some constant $M$.



          Now let $(psi_n)$ be a sequence that converges to $psi$ in $mathcal{D}(mathbb{R}^n)$. Let $A={psi_n : ninmathbb{N}}$ which is bounded.
          We have $|langle E_{eta},psi_nrangle|le M$ for all $ninmathbb{N}$ and $etain K$. Moreover, for fixed $eta$, $langle E_eta,psi_nranglerightarrow
          langle E_eta,psirangle$
          . Therefore, by dominated convergence
          $$
          langle E,psi_nranglerightarrow langle E,psirangle
          $$

          and $E$ is a distribution.





          Addendum recalling some prerequisites:



          Here the space of smooth compactly supported functions $mathcal{D}(mathbb{R}^n)$ is a topological vector space. Its topology is defined by an uncountable collection of seminorms which are described explicitly here. As for the notion of bounded set in $mathcal{D}(mathbb{R}^n)$, the following are equivalent, for $Asubsetmathcal{D}(mathbb{R}^n)$:





          1. $A$ is bounded

          2. For every open set $Vsubsetmathcal{D}(mathbb{R}^n)$ which contains the origin, there exists $lambda>0$ such that $lambda Asubset V$.

          3. For every continuous seminorm $||cdot||$ on $mathcal{D}(mathbb{R}^n)$, we have $sup_{psiin A}||psi||<infty$.

          4. For every seminorm $||cdot||$ in a defining set of seminorms (e.g., the ones in the MO link I gave above), we have $sup_{psiin A}||psi||<infty$.






          share|cite|improve this answer











          $endgroup$



          The question is a bit ambiguous because the topology on $mathcal{D}'(mathbb{R}^n)$ is not specified when mentioning that $etarightarrow E_{eta}$ is continuous.
          I don't see immediately if $E$ is indeed continuous when the chosen topology is the weak-$ast$ topology. However, if the topology is the strong one (a better choice anyway) then the answer is yes as is shown below.



          Firstly, by writing $mu$ as a difference of positive measures one can reduce the problem to the case where $mu$ is positive. Moreover, by assumption there is a compact set $K$ on which it is supported so
          $$
          langle E,psirangle=int_K langle E_eta,psirangle dmu(eta) .
          $$

          Let $A$ be a bounded set in $mathcal{D}(mathbb{R}^n)$ and for a distribution $T$ let us use the notation
          $$
          ||T||_A=sup_{psiin A} |langle T,psirangle| .
          $$

          These seminorms define the strong topology on $mathcal{D}'(mathbb{R}^n)$ and are thus continuous. By composition, the map $Krightarrow [0,infty)$, $etamapsto ||E_eta||_A$ is continuous and therefore, by compactness, is bounded by some constant $M$.



          Now let $(psi_n)$ be a sequence that converges to $psi$ in $mathcal{D}(mathbb{R}^n)$. Let $A={psi_n : ninmathbb{N}}$ which is bounded.
          We have $|langle E_{eta},psi_nrangle|le M$ for all $ninmathbb{N}$ and $etain K$. Moreover, for fixed $eta$, $langle E_eta,psi_nranglerightarrow
          langle E_eta,psirangle$
          . Therefore, by dominated convergence
          $$
          langle E,psi_nranglerightarrow langle E,psirangle
          $$

          and $E$ is a distribution.





          Addendum recalling some prerequisites:



          Here the space of smooth compactly supported functions $mathcal{D}(mathbb{R}^n)$ is a topological vector space. Its topology is defined by an uncountable collection of seminorms which are described explicitly here. As for the notion of bounded set in $mathcal{D}(mathbb{R}^n)$, the following are equivalent, for $Asubsetmathcal{D}(mathbb{R}^n)$:





          1. $A$ is bounded

          2. For every open set $Vsubsetmathcal{D}(mathbb{R}^n)$ which contains the origin, there exists $lambda>0$ such that $lambda Asubset V$.

          3. For every continuous seminorm $||cdot||$ on $mathcal{D}(mathbb{R}^n)$, we have $sup_{psiin A}||psi||<infty$.

          4. For every seminorm $||cdot||$ in a defining set of seminorms (e.g., the ones in the MO link I gave above), we have $sup_{psiin A}||psi||<infty$.







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Jan 24 at 15:34

























          answered Jan 22 at 15:56









          Abdelmalek AbdesselamAbdelmalek Abdesselam

          647311




          647311












          • $begingroup$
            Thanks for the reply. Could you specify what you mean when a set is bounded in $mathcal{D}(mathbb{R}^n)$? Are you using the supremum norm or is the topology also induced by a family of seminorms?
            $endgroup$
            – Joseph Expo
            Jan 24 at 9:23












          • $begingroup$
            I edited my answer to recall what a bounded set is. If by supremum norm you mean $sup_{xinmathbb{R}^n}|psi(x)|$, that's not what I was using. The notion of bounded set here involves a supremum over a set of test functions, not over a set of points in $mathbb{R}^n$.
            $endgroup$
            – Abdelmalek Abdesselam
            Jan 24 at 15:37




















          • $begingroup$
            Thanks for the reply. Could you specify what you mean when a set is bounded in $mathcal{D}(mathbb{R}^n)$? Are you using the supremum norm or is the topology also induced by a family of seminorms?
            $endgroup$
            – Joseph Expo
            Jan 24 at 9:23












          • $begingroup$
            I edited my answer to recall what a bounded set is. If by supremum norm you mean $sup_{xinmathbb{R}^n}|psi(x)|$, that's not what I was using. The notion of bounded set here involves a supremum over a set of test functions, not over a set of points in $mathbb{R}^n$.
            $endgroup$
            – Abdelmalek Abdesselam
            Jan 24 at 15:37


















          $begingroup$
          Thanks for the reply. Could you specify what you mean when a set is bounded in $mathcal{D}(mathbb{R}^n)$? Are you using the supremum norm or is the topology also induced by a family of seminorms?
          $endgroup$
          – Joseph Expo
          Jan 24 at 9:23






          $begingroup$
          Thanks for the reply. Could you specify what you mean when a set is bounded in $mathcal{D}(mathbb{R}^n)$? Are you using the supremum norm or is the topology also induced by a family of seminorms?
          $endgroup$
          – Joseph Expo
          Jan 24 at 9:23














          $begingroup$
          I edited my answer to recall what a bounded set is. If by supremum norm you mean $sup_{xinmathbb{R}^n}|psi(x)|$, that's not what I was using. The notion of bounded set here involves a supremum over a set of test functions, not over a set of points in $mathbb{R}^n$.
          $endgroup$
          – Abdelmalek Abdesselam
          Jan 24 at 15:37






          $begingroup$
          I edited my answer to recall what a bounded set is. If by supremum norm you mean $sup_{xinmathbb{R}^n}|psi(x)|$, that's not what I was using. The notion of bounded set here involves a supremum over a set of test functions, not over a set of points in $mathbb{R}^n$.
          $endgroup$
          – Abdelmalek Abdesselam
          Jan 24 at 15:37




















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