$L^p$ Dominated Convergence Theorem
$begingroup$
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
real-analysis lp-spaces
$endgroup$
add a comment |
$begingroup$
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
real-analysis lp-spaces
$endgroup$
$begingroup$
$|f_n| < g$ 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
add a comment |
$begingroup$
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
real-analysis lp-spaces
$endgroup$
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
real-analysis lp-spaces
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
add a comment |
$begingroup$
$|f_n| < g$ 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
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
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
$endgroup$
add a comment |
$begingroup$
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 $h_i$.
$endgroup$
add a comment |
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
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f623708%2flp-dominated-convergence-theorem%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
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
$endgroup$
add a comment |
$begingroup$
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
$endgroup$
add a comment |
$begingroup$
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
$endgroup$
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
answered Jan 1 '14 at 4:54
ir7ir7
4,19811115
4,19811115
add a comment |
add a comment |
$begingroup$
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 $h_i$.
$endgroup$
add a comment |
$begingroup$
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 $h_i$.
$endgroup$
add a comment |
$begingroup$
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 $h_i$.
$endgroup$
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 $h_i$.
edited Jan 31 at 17:33
answered Jan 30 at 22:45
Barak OhanaBarak Ohana
225
225
add a comment |
add a comment |
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.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f623708%2flp-dominated-convergence-theorem%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
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
$begingroup$
$|f_n| < g$ 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