$L^p$ Dominated Convergence Theorem












6












$begingroup$


I want to prove the $L^p$ Dominated Convergence Theorem which says :



Let ${ f_n }$ be a sequence of measurable functions that converges pointwise a.e. on E to $f$. For$ 1 leq p < infty$, suppose that there is a function $g$ in $L^p(E)$ such that for all $n$, $|f_n|<g$ a.e. on $E$. Prove that ${ f_n } to f$ in $L^p(E)$.



This is my attempt :



$$ |f_n-f|<|g|+|f| since |f_n - f|leq |f_n| + |f| leq |f|+|g| leq 2 max{|f|,|g|} Rightarrow |f_n - f|^p<2^p(|g|^p + |f|^p) $$



If I show that $2^p(|g|^p + |f|^p)$ is integrable over $E$ then the result follows by Lebesgue's Dominated Convergence theorem. But this is where I am having a question : If ${ f_n }$ is a sequence in $L^p(E)$ and ${ f_n } to f$ pointwise a.e. on $E$, does this necessarily mean that $f in L^p(E) $ ? If yes, the result follows after using $2^p(|g|^p + |f|^p)$ as the dominating function in LDC..



ADDED LATER
For the bold-faced question, I tried like this $$|f|=|f-f_n+f_n|leq |f-f_n|+|f_n| Rightarrow int_E |f|^p leq int_E|f-f_n|^p+int_E|f_n|^p$$ Is there a theorem that says if ${f_n}→f$ pointwise a.e. on E, ${f_n}→f$ uniformly on $Esetminus E_0$ where $m(E_0)$ ? If thats true, for any $epsilon$ and for some $N$ , we can say $|f-f_N|^p < epsilon^{p}$ and since $f_n in L^p(E)$, this ensures that the $int_E |f|^p < infty$.



I'd appreciate if you tell me what is going wrong in my statement. Thank you










share|cite|improve this question











$endgroup$












  • $begingroup$
    $|f_n| < g$ does not imply $|f_n - f| < |g - f|$.
    $endgroup$
    – Michael Albanese
    Jan 1 '14 at 2:17






  • 2




    $begingroup$
    $|f_n|<g$ implies $|f| leq g$. $|f_n-f|^p<2^p g^p$ so we can apply LDCT.
    $endgroup$
    – PVAL-inactive
    Jan 1 '14 at 3:16












  • $begingroup$
    That's true...I made that too complicated.Thanks.
    $endgroup$
    – the8thone
    Jan 1 '14 at 3:23
















6












$begingroup$


I want to prove the $L^p$ Dominated Convergence Theorem which says :



Let ${ f_n }$ be a sequence of measurable functions that converges pointwise a.e. on E to $f$. For$ 1 leq p < infty$, suppose that there is a function $g$ in $L^p(E)$ such that for all $n$, $|f_n|<g$ a.e. on $E$. Prove that ${ f_n } to f$ in $L^p(E)$.



This is my attempt :



$$ |f_n-f|<|g|+|f| since |f_n - f|leq |f_n| + |f| leq |f|+|g| leq 2 max{|f|,|g|} Rightarrow |f_n - f|^p<2^p(|g|^p + |f|^p) $$



If I show that $2^p(|g|^p + |f|^p)$ is integrable over $E$ then the result follows by Lebesgue's Dominated Convergence theorem. But this is where I am having a question : If ${ f_n }$ is a sequence in $L^p(E)$ and ${ f_n } to f$ pointwise a.e. on $E$, does this necessarily mean that $f in L^p(E) $ ? If yes, the result follows after using $2^p(|g|^p + |f|^p)$ as the dominating function in LDC..



ADDED LATER
For the bold-faced question, I tried like this $$|f|=|f-f_n+f_n|leq |f-f_n|+|f_n| Rightarrow int_E |f|^p leq int_E|f-f_n|^p+int_E|f_n|^p$$ Is there a theorem that says if ${f_n}→f$ pointwise a.e. on E, ${f_n}→f$ uniformly on $Esetminus E_0$ where $m(E_0)$ ? If thats true, for any $epsilon$ and for some $N$ , we can say $|f-f_N|^p < epsilon^{p}$ and since $f_n in L^p(E)$, this ensures that the $int_E |f|^p < infty$.



I'd appreciate if you tell me what is going wrong in my statement. Thank you










share|cite|improve this question











$endgroup$












  • $begingroup$
    $|f_n| < g$ does not imply $|f_n - f| < |g - f|$.
    $endgroup$
    – Michael Albanese
    Jan 1 '14 at 2:17






  • 2




    $begingroup$
    $|f_n|<g$ implies $|f| leq g$. $|f_n-f|^p<2^p g^p$ so we can apply LDCT.
    $endgroup$
    – PVAL-inactive
    Jan 1 '14 at 3:16












  • $begingroup$
    That's true...I made that too complicated.Thanks.
    $endgroup$
    – the8thone
    Jan 1 '14 at 3:23














6












6








6


3



$begingroup$


I want to prove the $L^p$ Dominated Convergence Theorem which says :



Let ${ f_n }$ be a sequence of measurable functions that converges pointwise a.e. on E to $f$. For$ 1 leq p < infty$, suppose that there is a function $g$ in $L^p(E)$ such that for all $n$, $|f_n|<g$ a.e. on $E$. Prove that ${ f_n } to f$ in $L^p(E)$.



This is my attempt :



$$ |f_n-f|<|g|+|f| since |f_n - f|leq |f_n| + |f| leq |f|+|g| leq 2 max{|f|,|g|} Rightarrow |f_n - f|^p<2^p(|g|^p + |f|^p) $$



If I show that $2^p(|g|^p + |f|^p)$ is integrable over $E$ then the result follows by Lebesgue's Dominated Convergence theorem. But this is where I am having a question : If ${ f_n }$ is a sequence in $L^p(E)$ and ${ f_n } to f$ pointwise a.e. on $E$, does this necessarily mean that $f in L^p(E) $ ? If yes, the result follows after using $2^p(|g|^p + |f|^p)$ as the dominating function in LDC..



ADDED LATER
For the bold-faced question, I tried like this $$|f|=|f-f_n+f_n|leq |f-f_n|+|f_n| Rightarrow int_E |f|^p leq int_E|f-f_n|^p+int_E|f_n|^p$$ Is there a theorem that says if ${f_n}→f$ pointwise a.e. on E, ${f_n}→f$ uniformly on $Esetminus E_0$ where $m(E_0)$ ? If thats true, for any $epsilon$ and for some $N$ , we can say $|f-f_N|^p < epsilon^{p}$ and since $f_n in L^p(E)$, this ensures that the $int_E |f|^p < infty$.



I'd appreciate if you tell me what is going wrong in my statement. Thank you










share|cite|improve this question











$endgroup$




I want to prove the $L^p$ Dominated Convergence Theorem which says :



Let ${ f_n }$ be a sequence of measurable functions that converges pointwise a.e. on E to $f$. For$ 1 leq p < infty$, suppose that there is a function $g$ in $L^p(E)$ such that for all $n$, $|f_n|<g$ a.e. on $E$. Prove that ${ f_n } to f$ in $L^p(E)$.



This is my attempt :



$$ |f_n-f|<|g|+|f| since |f_n - f|leq |f_n| + |f| leq |f|+|g| leq 2 max{|f|,|g|} Rightarrow |f_n - f|^p<2^p(|g|^p + |f|^p) $$



If I show that $2^p(|g|^p + |f|^p)$ is integrable over $E$ then the result follows by Lebesgue's Dominated Convergence theorem. But this is where I am having a question : If ${ f_n }$ is a sequence in $L^p(E)$ and ${ f_n } to f$ pointwise a.e. on $E$, does this necessarily mean that $f in L^p(E) $ ? If yes, the result follows after using $2^p(|g|^p + |f|^p)$ as the dominating function in LDC..



ADDED LATER
For the bold-faced question, I tried like this $$|f|=|f-f_n+f_n|leq |f-f_n|+|f_n| Rightarrow int_E |f|^p leq int_E|f-f_n|^p+int_E|f_n|^p$$ Is there a theorem that says if ${f_n}→f$ pointwise a.e. on E, ${f_n}→f$ uniformly on $Esetminus E_0$ where $m(E_0)$ ? If thats true, for any $epsilon$ and for some $N$ , we can say $|f-f_N|^p < epsilon^{p}$ and since $f_n in L^p(E)$, this ensures that the $int_E |f|^p < infty$.



I'd appreciate if you tell me what is going wrong in my statement. Thank you







real-analysis lp-spaces






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Jan 1 '14 at 2:52







the8thone

















asked Jan 1 '14 at 2:10









the8thonethe8thone

1,838935




1,838935












  • $begingroup$
    $|f_n| < g$ does not imply $|f_n - f| < |g - f|$.
    $endgroup$
    – Michael Albanese
    Jan 1 '14 at 2:17






  • 2




    $begingroup$
    $|f_n|<g$ implies $|f| leq g$. $|f_n-f|^p<2^p g^p$ so we can apply LDCT.
    $endgroup$
    – PVAL-inactive
    Jan 1 '14 at 3:16












  • $begingroup$
    That's true...I made that too complicated.Thanks.
    $endgroup$
    – the8thone
    Jan 1 '14 at 3:23


















  • $begingroup$
    $|f_n| < g$ does not imply $|f_n - f| < |g - f|$.
    $endgroup$
    – Michael Albanese
    Jan 1 '14 at 2:17






  • 2




    $begingroup$
    $|f_n|<g$ implies $|f| leq g$. $|f_n-f|^p<2^p g^p$ so we can apply LDCT.
    $endgroup$
    – PVAL-inactive
    Jan 1 '14 at 3:16












  • $begingroup$
    That's true...I made that too complicated.Thanks.
    $endgroup$
    – the8thone
    Jan 1 '14 at 3:23
















$begingroup$
$|f_n| < g$ does not imply $|f_n - f| < |g - f|$.
$endgroup$
– Michael Albanese
Jan 1 '14 at 2:17




$begingroup$
$|f_n| < g$ does not imply $|f_n - f| < |g - f|$.
$endgroup$
– Michael Albanese
Jan 1 '14 at 2:17




2




2




$begingroup$
$|f_n|<g$ implies $|f| leq g$. $|f_n-f|^p<2^p g^p$ so we can apply LDCT.
$endgroup$
– PVAL-inactive
Jan 1 '14 at 3:16






$begingroup$
$|f_n|<g$ implies $|f| leq g$. $|f_n-f|^p<2^p g^p$ so we can apply LDCT.
$endgroup$
– PVAL-inactive
Jan 1 '14 at 3:16














$begingroup$
That's true...I made that too complicated.Thanks.
$endgroup$
– the8thone
Jan 1 '14 at 3:23




$begingroup$
That's true...I made that too complicated.Thanks.
$endgroup$
– the8thone
Jan 1 '14 at 3:23










2 Answers
2






active

oldest

votes


















2












$begingroup$

For your additional question, we have Egorov (sometimes spelled Egoroff) theorem:



On a probability measure space $(Omega, {cal F}, mu)$, if $(X_n)_{ngeq1}$ is a sequence of random variables convergent to a random variable $X$ $mu$-a.e, then for any given $delta>0$ there is an event $Ain {cal F}$ such that $mu(A) < delta$ and $(X_n)_{ngeq1}$ converges uniformly to $X$ on $Omega setminus A$.



Best regards






share|cite|improve this answer









$endgroup$





















    1












    $begingroup$

    I think I have simpler proof.



    If we know that $ f_n to f $ in measure this means that there is subsequence $f_{n_k}$ converging to $f$ point wise a.e. We still have $left| f_{n_k} right| < g $ so the Dominated Convergence (for $L_1$) implies that
    $$
    intop left| f_{n_k} - f right|dmu to 0
    $$



    We know (form the point-wise converges) that $left| f_{n_k} - f right| to 0 $ a.e so we can assume that $left| f_{n_k} - f right| < 1 $ a.e. This implies us



    $$
    0leq intop left| f_{n_k} - f right| ^ p dmu leq intop left| f_{n_k} - f right|dmu to 0
    $$
    Thus
    $$
    leftVert f_{n_k} -f rightVert _p to 0
    $$



    If we won't have $ leftVert f_{n} -f rightVert _p to 0 $ (i.e. converges in $L^p$) we could construct subsequence of $f_n$ $h_i = f_{n_i}$ such that



    $$
    leftVert h_{i} -f rightVert _p > epsilon
    $$

    for some $epsilon > 0 $. We would still have $leftvert h_i rightvert < g$ and $h_i to f$ is measure, so we may construct subsequence $h_{i_k}$ (as done before) of $h_i$ such that
    $$
    leftVert h_{i_k} - f rightVert _p to 0
    $$



    which contradict the condition that defined $h_i$.






    share|cite|improve this answer











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      2 Answers
      2






      active

      oldest

      votes








      2 Answers
      2






      active

      oldest

      votes









      active

      oldest

      votes






      active

      oldest

      votes









      2












      $begingroup$

      For your additional question, we have Egorov (sometimes spelled Egoroff) theorem:



      On a probability measure space $(Omega, {cal F}, mu)$, if $(X_n)_{ngeq1}$ is a sequence of random variables convergent to a random variable $X$ $mu$-a.e, then for any given $delta>0$ there is an event $Ain {cal F}$ such that $mu(A) < delta$ and $(X_n)_{ngeq1}$ converges uniformly to $X$ on $Omega setminus A$.



      Best regards






      share|cite|improve this answer









      $endgroup$


















        2












        $begingroup$

        For your additional question, we have Egorov (sometimes spelled Egoroff) theorem:



        On a probability measure space $(Omega, {cal F}, mu)$, if $(X_n)_{ngeq1}$ is a sequence of random variables convergent to a random variable $X$ $mu$-a.e, then for any given $delta>0$ there is an event $Ain {cal F}$ such that $mu(A) < delta$ and $(X_n)_{ngeq1}$ converges uniformly to $X$ on $Omega setminus A$.



        Best regards






        share|cite|improve this answer









        $endgroup$
















          2












          2








          2





          $begingroup$

          For your additional question, we have Egorov (sometimes spelled Egoroff) theorem:



          On a probability measure space $(Omega, {cal F}, mu)$, if $(X_n)_{ngeq1}$ is a sequence of random variables convergent to a random variable $X$ $mu$-a.e, then for any given $delta>0$ there is an event $Ain {cal F}$ such that $mu(A) < delta$ and $(X_n)_{ngeq1}$ converges uniformly to $X$ on $Omega setminus A$.



          Best regards






          share|cite|improve this answer









          $endgroup$



          For your additional question, we have Egorov (sometimes spelled Egoroff) theorem:



          On a probability measure space $(Omega, {cal F}, mu)$, if $(X_n)_{ngeq1}$ is a sequence of random variables convergent to a random variable $X$ $mu$-a.e, then for any given $delta>0$ there is an event $Ain {cal F}$ such that $mu(A) < delta$ and $(X_n)_{ngeq1}$ converges uniformly to $X$ on $Omega setminus A$.



          Best regards







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Jan 1 '14 at 4:54









          ir7ir7

          4,19811115




          4,19811115























              1












              $begingroup$

              I think I have simpler proof.



              If we know that $ f_n to f $ in measure this means that there is subsequence $f_{n_k}$ converging to $f$ point wise a.e. We still have $left| f_{n_k} right| < g $ so the Dominated Convergence (for $L_1$) implies that
              $$
              intop left| f_{n_k} - f right|dmu to 0
              $$



              We know (form the point-wise converges) that $left| f_{n_k} - f right| to 0 $ a.e so we can assume that $left| f_{n_k} - f right| < 1 $ a.e. This implies us



              $$
              0leq intop left| f_{n_k} - f right| ^ p dmu leq intop left| f_{n_k} - f right|dmu to 0
              $$
              Thus
              $$
              leftVert f_{n_k} -f rightVert _p to 0
              $$



              If we won't have $ leftVert f_{n} -f rightVert _p to 0 $ (i.e. converges in $L^p$) we could construct subsequence of $f_n$ $h_i = f_{n_i}$ such that



              $$
              leftVert h_{i} -f rightVert _p > epsilon
              $$

              for some $epsilon > 0 $. We would still have $leftvert h_i rightvert < g$ and $h_i to f$ is measure, so we may construct subsequence $h_{i_k}$ (as done before) of $h_i$ such that
              $$
              leftVert h_{i_k} - f rightVert _p to 0
              $$



              which contradict the condition that defined $h_i$.






              share|cite|improve this answer











              $endgroup$


















                1












                $begingroup$

                I think I have simpler proof.



                If we know that $ f_n to f $ in measure this means that there is subsequence $f_{n_k}$ converging to $f$ point wise a.e. We still have $left| f_{n_k} right| < g $ so the Dominated Convergence (for $L_1$) implies that
                $$
                intop left| f_{n_k} - f right|dmu to 0
                $$



                We know (form the point-wise converges) that $left| f_{n_k} - f right| to 0 $ a.e so we can assume that $left| f_{n_k} - f right| < 1 $ a.e. This implies us



                $$
                0leq intop left| f_{n_k} - f right| ^ p dmu leq intop left| f_{n_k} - f right|dmu to 0
                $$
                Thus
                $$
                leftVert f_{n_k} -f rightVert _p to 0
                $$



                If we won't have $ leftVert f_{n} -f rightVert _p to 0 $ (i.e. converges in $L^p$) we could construct subsequence of $f_n$ $h_i = f_{n_i}$ such that



                $$
                leftVert h_{i} -f rightVert _p > epsilon
                $$

                for some $epsilon > 0 $. We would still have $leftvert h_i rightvert < g$ and $h_i to f$ is measure, so we may construct subsequence $h_{i_k}$ (as done before) of $h_i$ such that
                $$
                leftVert h_{i_k} - f rightVert _p to 0
                $$



                which contradict the condition that defined $h_i$.






                share|cite|improve this answer











                $endgroup$
















                  1












                  1








                  1





                  $begingroup$

                  I think I have simpler proof.



                  If we know that $ f_n to f $ in measure this means that there is subsequence $f_{n_k}$ converging to $f$ point wise a.e. We still have $left| f_{n_k} right| < g $ so the Dominated Convergence (for $L_1$) implies that
                  $$
                  intop left| f_{n_k} - f right|dmu to 0
                  $$



                  We know (form the point-wise converges) that $left| f_{n_k} - f right| to 0 $ a.e so we can assume that $left| f_{n_k} - f right| < 1 $ a.e. This implies us



                  $$
                  0leq intop left| f_{n_k} - f right| ^ p dmu leq intop left| f_{n_k} - f right|dmu to 0
                  $$
                  Thus
                  $$
                  leftVert f_{n_k} -f rightVert _p to 0
                  $$



                  If we won't have $ leftVert f_{n} -f rightVert _p to 0 $ (i.e. converges in $L^p$) we could construct subsequence of $f_n$ $h_i = f_{n_i}$ such that



                  $$
                  leftVert h_{i} -f rightVert _p > epsilon
                  $$

                  for some $epsilon > 0 $. We would still have $leftvert h_i rightvert < g$ and $h_i to f$ is measure, so we may construct subsequence $h_{i_k}$ (as done before) of $h_i$ such that
                  $$
                  leftVert h_{i_k} - f rightVert _p to 0
                  $$



                  which contradict the condition that defined $h_i$.






                  share|cite|improve this answer











                  $endgroup$



                  I think I have simpler proof.



                  If we know that $ f_n to f $ in measure this means that there is subsequence $f_{n_k}$ converging to $f$ point wise a.e. We still have $left| f_{n_k} right| < g $ so the Dominated Convergence (for $L_1$) implies that
                  $$
                  intop left| f_{n_k} - f right|dmu to 0
                  $$



                  We know (form the point-wise converges) that $left| f_{n_k} - f right| to 0 $ a.e so we can assume that $left| f_{n_k} - f right| < 1 $ a.e. This implies us



                  $$
                  0leq intop left| f_{n_k} - f right| ^ p dmu leq intop left| f_{n_k} - f right|dmu to 0
                  $$
                  Thus
                  $$
                  leftVert f_{n_k} -f rightVert _p to 0
                  $$



                  If we won't have $ leftVert f_{n} -f rightVert _p to 0 $ (i.e. converges in $L^p$) we could construct subsequence of $f_n$ $h_i = f_{n_i}$ such that



                  $$
                  leftVert h_{i} -f rightVert _p > epsilon
                  $$

                  for some $epsilon > 0 $. We would still have $leftvert h_i rightvert < g$ and $h_i to f$ is measure, so we may construct subsequence $h_{i_k}$ (as done before) of $h_i$ such that
                  $$
                  leftVert h_{i_k} - f rightVert _p to 0
                  $$



                  which contradict the condition that defined $h_i$.







                  share|cite|improve this answer














                  share|cite|improve this answer



                  share|cite|improve this answer








                  edited Jan 31 at 17:33

























                  answered Jan 30 at 22:45









                  Barak OhanaBarak Ohana

                  225




                  225






























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