$L_1$ inequality
$begingroup$
$tilde{f} =lim_{ntoinfty} frac{1}{n} sum_{k=0}^{n-1} f(x+k)$, where $f$ is an $L_1$ function with Lebesgue measure $mu$. Show that $||tilde{f} ||_{L_1} leq ||f ||_{L_1}$
real-analysis functional-analysis measure-theory
$endgroup$
add a comment |
$begingroup$
$tilde{f} =lim_{ntoinfty} frac{1}{n} sum_{k=0}^{n-1} f(x+k)$, where $f$ is an $L_1$ function with Lebesgue measure $mu$. Show that $||tilde{f} ||_{L_1} leq ||f ||_{L_1}$
real-analysis functional-analysis measure-theory
$endgroup$
$begingroup$
Please add your attempts.
$endgroup$
– Jack D'Aurizio
Mar 9 '17 at 13:48
$begingroup$
This is not much more than a straightforward consequence of the triangle inequality and the fact that $int_{mathbb{R}}|f(x+k)|,dx = int_{mathbb{R}}|f(x)|,dx$ if $fin L^1(mathbb{R})$.
$endgroup$
– Jack D'Aurizio
Mar 9 '17 at 13:50
add a comment |
$begingroup$
$tilde{f} =lim_{ntoinfty} frac{1}{n} sum_{k=0}^{n-1} f(x+k)$, where $f$ is an $L_1$ function with Lebesgue measure $mu$. Show that $||tilde{f} ||_{L_1} leq ||f ||_{L_1}$
real-analysis functional-analysis measure-theory
$endgroup$
$tilde{f} =lim_{ntoinfty} frac{1}{n} sum_{k=0}^{n-1} f(x+k)$, where $f$ is an $L_1$ function with Lebesgue measure $mu$. Show that $||tilde{f} ||_{L_1} leq ||f ||_{L_1}$
real-analysis functional-analysis measure-theory
real-analysis functional-analysis measure-theory
asked Mar 9 '17 at 13:43
MAN-MADEMAN-MADE
3,63311031
3,63311031
$begingroup$
Please add your attempts.
$endgroup$
– Jack D'Aurizio
Mar 9 '17 at 13:48
$begingroup$
This is not much more than a straightforward consequence of the triangle inequality and the fact that $int_{mathbb{R}}|f(x+k)|,dx = int_{mathbb{R}}|f(x)|,dx$ if $fin L^1(mathbb{R})$.
$endgroup$
– Jack D'Aurizio
Mar 9 '17 at 13:50
add a comment |
$begingroup$
Please add your attempts.
$endgroup$
– Jack D'Aurizio
Mar 9 '17 at 13:48
$begingroup$
This is not much more than a straightforward consequence of the triangle inequality and the fact that $int_{mathbb{R}}|f(x+k)|,dx = int_{mathbb{R}}|f(x)|,dx$ if $fin L^1(mathbb{R})$.
$endgroup$
– Jack D'Aurizio
Mar 9 '17 at 13:50
$begingroup$
Please add your attempts.
$endgroup$
– Jack D'Aurizio
Mar 9 '17 at 13:48
$begingroup$
Please add your attempts.
$endgroup$
– Jack D'Aurizio
Mar 9 '17 at 13:48
$begingroup$
This is not much more than a straightforward consequence of the triangle inequality and the fact that $int_{mathbb{R}}|f(x+k)|,dx = int_{mathbb{R}}|f(x)|,dx$ if $fin L^1(mathbb{R})$.
$endgroup$
– Jack D'Aurizio
Mar 9 '17 at 13:50
$begingroup$
This is not much more than a straightforward consequence of the triangle inequality and the fact that $int_{mathbb{R}}|f(x+k)|,dx = int_{mathbb{R}}|f(x)|,dx$ if $fin L^1(mathbb{R})$.
$endgroup$
– Jack D'Aurizio
Mar 9 '17 at 13:50
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
since I can't post comments I write this as an answer.
I am sure that this won't work, because you need additionally that $tilde{f} in L_1$.
Then you can follow the hint of Jack D'Aurizio.
But if you can't assume that, then $f = 1_{[0,1)}$ is a counterexample.
Because
$$
tilde{f}_{n} := frac{1}{n} sum_{k=0}^{n-1} 1_{[0,1)}(x+k)
= frac{1}{n} 1_{[-n+1,1)}(x).
$$
Now it is easy to check that $|tilde{f}_n| = 1$ for all $ninmathbb{N}$. So if $tilde{f}_n$ converts in $L_1$ then the norm must be $1$. But if you regard the series $tilde{f}_n$ pointwise then the limit is $0$ which contradicts with the previous statement.
$endgroup$
$begingroup$
But OP didn't say that $lim_n sum_{0le k< n} f(x+k)$ converges in $L^1$. The convergence is only pointwise. So $lim_n tilde{f_n}=0$ does not contradict the statement: $|lim_n tilde{f}_n|le |f|$.
$endgroup$
– Song
Jan 13 at 14:17
$begingroup$
That is true, but I wanted to point out that the given sequence does not automatically converge if you start with an $f in L^1(mathbb{R})$, since the suggested proof of Jack D'Aurizio uses that property.
$endgroup$
– Nathanael Skrepek
Jan 13 at 15:59
$begingroup$
Oh, I just realized that this answer was written almost 2 years ago ... Thank you for your response.
$endgroup$
– Song
Jan 13 at 16:47
add a comment |
$begingroup$
Assuming $g(x)=lim_{ntoinfty} frac{1}{n}sum_{k=0}^{n-1}f(x+k)$ converges almost everywhere pointwise, we can show that $|g|_{L^1}le |f|_{L^1}$ as follows. Let $g_n(x)=frac{1}{n}sum_{k=0}^{n-1}f(x+k)$. Note that
$$
|g_n(x)|le frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|
$$ and hence
$$
|g(x)|=lim_{ntoinfty}|g_n(x)|le liminf_{ntoinfty}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|.
$$ By Fatou's lemma it follows
$$
|g|_{L^1}le int_mathbb{R} left[liminf_{ntoinfty}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|right];dmuleliminf_{ntoinfty}int_mathbb{R}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|dmu=|f|_{L^1}.
$$
$endgroup$
add a comment |
Your Answer
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%2f2179165%2fl-1-inequality%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
$begingroup$
since I can't post comments I write this as an answer.
I am sure that this won't work, because you need additionally that $tilde{f} in L_1$.
Then you can follow the hint of Jack D'Aurizio.
But if you can't assume that, then $f = 1_{[0,1)}$ is a counterexample.
Because
$$
tilde{f}_{n} := frac{1}{n} sum_{k=0}^{n-1} 1_{[0,1)}(x+k)
= frac{1}{n} 1_{[-n+1,1)}(x).
$$
Now it is easy to check that $|tilde{f}_n| = 1$ for all $ninmathbb{N}$. So if $tilde{f}_n$ converts in $L_1$ then the norm must be $1$. But if you regard the series $tilde{f}_n$ pointwise then the limit is $0$ which contradicts with the previous statement.
$endgroup$
$begingroup$
But OP didn't say that $lim_n sum_{0le k< n} f(x+k)$ converges in $L^1$. The convergence is only pointwise. So $lim_n tilde{f_n}=0$ does not contradict the statement: $|lim_n tilde{f}_n|le |f|$.
$endgroup$
– Song
Jan 13 at 14:17
$begingroup$
That is true, but I wanted to point out that the given sequence does not automatically converge if you start with an $f in L^1(mathbb{R})$, since the suggested proof of Jack D'Aurizio uses that property.
$endgroup$
– Nathanael Skrepek
Jan 13 at 15:59
$begingroup$
Oh, I just realized that this answer was written almost 2 years ago ... Thank you for your response.
$endgroup$
– Song
Jan 13 at 16:47
add a comment |
$begingroup$
since I can't post comments I write this as an answer.
I am sure that this won't work, because you need additionally that $tilde{f} in L_1$.
Then you can follow the hint of Jack D'Aurizio.
But if you can't assume that, then $f = 1_{[0,1)}$ is a counterexample.
Because
$$
tilde{f}_{n} := frac{1}{n} sum_{k=0}^{n-1} 1_{[0,1)}(x+k)
= frac{1}{n} 1_{[-n+1,1)}(x).
$$
Now it is easy to check that $|tilde{f}_n| = 1$ for all $ninmathbb{N}$. So if $tilde{f}_n$ converts in $L_1$ then the norm must be $1$. But if you regard the series $tilde{f}_n$ pointwise then the limit is $0$ which contradicts with the previous statement.
$endgroup$
$begingroup$
But OP didn't say that $lim_n sum_{0le k< n} f(x+k)$ converges in $L^1$. The convergence is only pointwise. So $lim_n tilde{f_n}=0$ does not contradict the statement: $|lim_n tilde{f}_n|le |f|$.
$endgroup$
– Song
Jan 13 at 14:17
$begingroup$
That is true, but I wanted to point out that the given sequence does not automatically converge if you start with an $f in L^1(mathbb{R})$, since the suggested proof of Jack D'Aurizio uses that property.
$endgroup$
– Nathanael Skrepek
Jan 13 at 15:59
$begingroup$
Oh, I just realized that this answer was written almost 2 years ago ... Thank you for your response.
$endgroup$
– Song
Jan 13 at 16:47
add a comment |
$begingroup$
since I can't post comments I write this as an answer.
I am sure that this won't work, because you need additionally that $tilde{f} in L_1$.
Then you can follow the hint of Jack D'Aurizio.
But if you can't assume that, then $f = 1_{[0,1)}$ is a counterexample.
Because
$$
tilde{f}_{n} := frac{1}{n} sum_{k=0}^{n-1} 1_{[0,1)}(x+k)
= frac{1}{n} 1_{[-n+1,1)}(x).
$$
Now it is easy to check that $|tilde{f}_n| = 1$ for all $ninmathbb{N}$. So if $tilde{f}_n$ converts in $L_1$ then the norm must be $1$. But if you regard the series $tilde{f}_n$ pointwise then the limit is $0$ which contradicts with the previous statement.
$endgroup$
since I can't post comments I write this as an answer.
I am sure that this won't work, because you need additionally that $tilde{f} in L_1$.
Then you can follow the hint of Jack D'Aurizio.
But if you can't assume that, then $f = 1_{[0,1)}$ is a counterexample.
Because
$$
tilde{f}_{n} := frac{1}{n} sum_{k=0}^{n-1} 1_{[0,1)}(x+k)
= frac{1}{n} 1_{[-n+1,1)}(x).
$$
Now it is easy to check that $|tilde{f}_n| = 1$ for all $ninmathbb{N}$. So if $tilde{f}_n$ converts in $L_1$ then the norm must be $1$. But if you regard the series $tilde{f}_n$ pointwise then the limit is $0$ which contradicts with the previous statement.
edited Jan 12 at 22:59
answered Mar 9 '17 at 16:58
Nathanael SkrepekNathanael Skrepek
1,8021615
1,8021615
$begingroup$
But OP didn't say that $lim_n sum_{0le k< n} f(x+k)$ converges in $L^1$. The convergence is only pointwise. So $lim_n tilde{f_n}=0$ does not contradict the statement: $|lim_n tilde{f}_n|le |f|$.
$endgroup$
– Song
Jan 13 at 14:17
$begingroup$
That is true, but I wanted to point out that the given sequence does not automatically converge if you start with an $f in L^1(mathbb{R})$, since the suggested proof of Jack D'Aurizio uses that property.
$endgroup$
– Nathanael Skrepek
Jan 13 at 15:59
$begingroup$
Oh, I just realized that this answer was written almost 2 years ago ... Thank you for your response.
$endgroup$
– Song
Jan 13 at 16:47
add a comment |
$begingroup$
But OP didn't say that $lim_n sum_{0le k< n} f(x+k)$ converges in $L^1$. The convergence is only pointwise. So $lim_n tilde{f_n}=0$ does not contradict the statement: $|lim_n tilde{f}_n|le |f|$.
$endgroup$
– Song
Jan 13 at 14:17
$begingroup$
That is true, but I wanted to point out that the given sequence does not automatically converge if you start with an $f in L^1(mathbb{R})$, since the suggested proof of Jack D'Aurizio uses that property.
$endgroup$
– Nathanael Skrepek
Jan 13 at 15:59
$begingroup$
Oh, I just realized that this answer was written almost 2 years ago ... Thank you for your response.
$endgroup$
– Song
Jan 13 at 16:47
$begingroup$
But OP didn't say that $lim_n sum_{0le k< n} f(x+k)$ converges in $L^1$. The convergence is only pointwise. So $lim_n tilde{f_n}=0$ does not contradict the statement: $|lim_n tilde{f}_n|le |f|$.
$endgroup$
– Song
Jan 13 at 14:17
$begingroup$
But OP didn't say that $lim_n sum_{0le k< n} f(x+k)$ converges in $L^1$. The convergence is only pointwise. So $lim_n tilde{f_n}=0$ does not contradict the statement: $|lim_n tilde{f}_n|le |f|$.
$endgroup$
– Song
Jan 13 at 14:17
$begingroup$
That is true, but I wanted to point out that the given sequence does not automatically converge if you start with an $f in L^1(mathbb{R})$, since the suggested proof of Jack D'Aurizio uses that property.
$endgroup$
– Nathanael Skrepek
Jan 13 at 15:59
$begingroup$
That is true, but I wanted to point out that the given sequence does not automatically converge if you start with an $f in L^1(mathbb{R})$, since the suggested proof of Jack D'Aurizio uses that property.
$endgroup$
– Nathanael Skrepek
Jan 13 at 15:59
$begingroup$
Oh, I just realized that this answer was written almost 2 years ago ... Thank you for your response.
$endgroup$
– Song
Jan 13 at 16:47
$begingroup$
Oh, I just realized that this answer was written almost 2 years ago ... Thank you for your response.
$endgroup$
– Song
Jan 13 at 16:47
add a comment |
$begingroup$
Assuming $g(x)=lim_{ntoinfty} frac{1}{n}sum_{k=0}^{n-1}f(x+k)$ converges almost everywhere pointwise, we can show that $|g|_{L^1}le |f|_{L^1}$ as follows. Let $g_n(x)=frac{1}{n}sum_{k=0}^{n-1}f(x+k)$. Note that
$$
|g_n(x)|le frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|
$$ and hence
$$
|g(x)|=lim_{ntoinfty}|g_n(x)|le liminf_{ntoinfty}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|.
$$ By Fatou's lemma it follows
$$
|g|_{L^1}le int_mathbb{R} left[liminf_{ntoinfty}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|right];dmuleliminf_{ntoinfty}int_mathbb{R}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|dmu=|f|_{L^1}.
$$
$endgroup$
add a comment |
$begingroup$
Assuming $g(x)=lim_{ntoinfty} frac{1}{n}sum_{k=0}^{n-1}f(x+k)$ converges almost everywhere pointwise, we can show that $|g|_{L^1}le |f|_{L^1}$ as follows. Let $g_n(x)=frac{1}{n}sum_{k=0}^{n-1}f(x+k)$. Note that
$$
|g_n(x)|le frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|
$$ and hence
$$
|g(x)|=lim_{ntoinfty}|g_n(x)|le liminf_{ntoinfty}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|.
$$ By Fatou's lemma it follows
$$
|g|_{L^1}le int_mathbb{R} left[liminf_{ntoinfty}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|right];dmuleliminf_{ntoinfty}int_mathbb{R}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|dmu=|f|_{L^1}.
$$
$endgroup$
add a comment |
$begingroup$
Assuming $g(x)=lim_{ntoinfty} frac{1}{n}sum_{k=0}^{n-1}f(x+k)$ converges almost everywhere pointwise, we can show that $|g|_{L^1}le |f|_{L^1}$ as follows. Let $g_n(x)=frac{1}{n}sum_{k=0}^{n-1}f(x+k)$. Note that
$$
|g_n(x)|le frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|
$$ and hence
$$
|g(x)|=lim_{ntoinfty}|g_n(x)|le liminf_{ntoinfty}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|.
$$ By Fatou's lemma it follows
$$
|g|_{L^1}le int_mathbb{R} left[liminf_{ntoinfty}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|right];dmuleliminf_{ntoinfty}int_mathbb{R}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|dmu=|f|_{L^1}.
$$
$endgroup$
Assuming $g(x)=lim_{ntoinfty} frac{1}{n}sum_{k=0}^{n-1}f(x+k)$ converges almost everywhere pointwise, we can show that $|g|_{L^1}le |f|_{L^1}$ as follows. Let $g_n(x)=frac{1}{n}sum_{k=0}^{n-1}f(x+k)$. Note that
$$
|g_n(x)|le frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|
$$ and hence
$$
|g(x)|=lim_{ntoinfty}|g_n(x)|le liminf_{ntoinfty}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|.
$$ By Fatou's lemma it follows
$$
|g|_{L^1}le int_mathbb{R} left[liminf_{ntoinfty}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|right];dmuleliminf_{ntoinfty}int_mathbb{R}frac{1}{n}sum_{k=0}^{n-1}|f(x+k)|dmu=|f|_{L^1}.
$$
answered Jan 13 at 16:08
SongSong
18.6k21651
18.6k21651
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%2f2179165%2fl-1-inequality%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
$begingroup$
Please add your attempts.
$endgroup$
– Jack D'Aurizio
Mar 9 '17 at 13:48
$begingroup$
This is not much more than a straightforward consequence of the triangle inequality and the fact that $int_{mathbb{R}}|f(x+k)|,dx = int_{mathbb{R}}|f(x)|,dx$ if $fin L^1(mathbb{R})$.
$endgroup$
– Jack D'Aurizio
Mar 9 '17 at 13:50