Does Young's inequality hold only for conjugate exponents?
$begingroup$
Suppose that $ab leq frac{1}{p}a^p+frac{1}{q}b^q$ holds for every real numbers $a,bge 0$. (where $p,q>0$ are some fixed numbers).
Is it true that $ frac{1}{p}+frac{1}{q}=1$?
I guess so, and I would like to find an easy proof of that fact. Plugging in $a=b=1$, we get $ frac{1}{p}+frac{1}{q}ge1$. Is there an easy way to see that the converse inequality must hold?
To be clear, I am not looking for proofs of Young's inequality; only for a way to see why the relation between the conjugate exponents is the only possible option.
Edit:
Here is a comment to help future readers (including future me) to see how can one come with Nicolás's nice idea to apply the inequality for $a=lambda^{frac{1}{p}}, b=lambda^{frac{1}{q}}$.
The idea is that it is inconvenient to compare a sum with another number; By using the specific choice of $a,b$ above, the sum is simplified, since the scales of the auxiliary parameter $lambda$ in both summands are now identical.
Comment: I know that the relation $ frac{1}{p}+frac{1}{q}=1$ is necessary for Holder's inequality in general; this can be seen by scaling the measure. However, I don't think this approach is applicable here.
real-analysis inequality symmetry young-inequality
$endgroup$
add a comment |
$begingroup$
Suppose that $ab leq frac{1}{p}a^p+frac{1}{q}b^q$ holds for every real numbers $a,bge 0$. (where $p,q>0$ are some fixed numbers).
Is it true that $ frac{1}{p}+frac{1}{q}=1$?
I guess so, and I would like to find an easy proof of that fact. Plugging in $a=b=1$, we get $ frac{1}{p}+frac{1}{q}ge1$. Is there an easy way to see that the converse inequality must hold?
To be clear, I am not looking for proofs of Young's inequality; only for a way to see why the relation between the conjugate exponents is the only possible option.
Edit:
Here is a comment to help future readers (including future me) to see how can one come with Nicolás's nice idea to apply the inequality for $a=lambda^{frac{1}{p}}, b=lambda^{frac{1}{q}}$.
The idea is that it is inconvenient to compare a sum with another number; By using the specific choice of $a,b$ above, the sum is simplified, since the scales of the auxiliary parameter $lambda$ in both summands are now identical.
Comment: I know that the relation $ frac{1}{p}+frac{1}{q}=1$ is necessary for Holder's inequality in general; this can be seen by scaling the measure. However, I don't think this approach is applicable here.
real-analysis inequality symmetry young-inequality
$endgroup$
add a comment |
$begingroup$
Suppose that $ab leq frac{1}{p}a^p+frac{1}{q}b^q$ holds for every real numbers $a,bge 0$. (where $p,q>0$ are some fixed numbers).
Is it true that $ frac{1}{p}+frac{1}{q}=1$?
I guess so, and I would like to find an easy proof of that fact. Plugging in $a=b=1$, we get $ frac{1}{p}+frac{1}{q}ge1$. Is there an easy way to see that the converse inequality must hold?
To be clear, I am not looking for proofs of Young's inequality; only for a way to see why the relation between the conjugate exponents is the only possible option.
Edit:
Here is a comment to help future readers (including future me) to see how can one come with Nicolás's nice idea to apply the inequality for $a=lambda^{frac{1}{p}}, b=lambda^{frac{1}{q}}$.
The idea is that it is inconvenient to compare a sum with another number; By using the specific choice of $a,b$ above, the sum is simplified, since the scales of the auxiliary parameter $lambda$ in both summands are now identical.
Comment: I know that the relation $ frac{1}{p}+frac{1}{q}=1$ is necessary for Holder's inequality in general; this can be seen by scaling the measure. However, I don't think this approach is applicable here.
real-analysis inequality symmetry young-inequality
$endgroup$
Suppose that $ab leq frac{1}{p}a^p+frac{1}{q}b^q$ holds for every real numbers $a,bge 0$. (where $p,q>0$ are some fixed numbers).
Is it true that $ frac{1}{p}+frac{1}{q}=1$?
I guess so, and I would like to find an easy proof of that fact. Plugging in $a=b=1$, we get $ frac{1}{p}+frac{1}{q}ge1$. Is there an easy way to see that the converse inequality must hold?
To be clear, I am not looking for proofs of Young's inequality; only for a way to see why the relation between the conjugate exponents is the only possible option.
Edit:
Here is a comment to help future readers (including future me) to see how can one come with Nicolás's nice idea to apply the inequality for $a=lambda^{frac{1}{p}}, b=lambda^{frac{1}{q}}$.
The idea is that it is inconvenient to compare a sum with another number; By using the specific choice of $a,b$ above, the sum is simplified, since the scales of the auxiliary parameter $lambda$ in both summands are now identical.
Comment: I know that the relation $ frac{1}{p}+frac{1}{q}=1$ is necessary for Holder's inequality in general; this can be seen by scaling the measure. However, I don't think this approach is applicable here.
real-analysis inequality symmetry young-inequality
real-analysis inequality symmetry young-inequality
edited 7 hours ago
Asaf Shachar
asked 9 hours ago
Asaf ShacharAsaf Shachar
5,2423941
5,2423941
add a comment |
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
If you apply the inequality for $a=lambda^{frac{1}{p}}, b=lambda^{frac{1}{q}}$, you get something like
$$ lambda^{frac{1}{p}+frac{1}{q}}leq left( frac{1}{p}+frac{1}{q} right)lambda, qquad forall lambda>0. $$
If you take $lambda to infty$, it's clear that $frac{1}{p}+frac{1}{q}leq 1$. Otherwise, after dividing by $lambda$ you get
$$ lambda^{frac{1}{p}+frac{1}{q}-1} leq frac{1}{p}+frac{1}{q}, $$
which is false for $lambda$ large enough, as the right side is constant. Taking $lambda to 0$ gets the other inequality.
$endgroup$
add a comment |
$begingroup$
I think the following can help.
By AM-GM $$frac{1}{p}a^p+frac{1}{q}b^qgeqleft(frac{1}{p}+frac{1}{q}right)left(left(a^pright)^{frac{1}{p}}left(b^qright)^{frac{1}{q}}right)^{frac{1}{frac{1}{p}+frac{1}{q}}}=left(frac{1}{p}+frac{1}{q}right)(ab)^{frac{1}{frac{1}{p}+frac{1}{q}}}.$$
The equality occurs, of course.
$endgroup$
$begingroup$
Thanks, but I do not understand; How exactly do you deduce from that something on the sum $frac{1}{p}+frac{1}{q}$? I tried putting $a^p=b^q$ (this is when we have equality in the AM-GM inequality) but I don't see how it helps.
$endgroup$
– Asaf Shachar
3 hours ago
$begingroup$
@Asaf Shachar The equality, which I got is true for all positives $a$, $b$, $p$ and $q$. If we'll assume that $frac{1}{p}+frac{1}{q}neq1$ then your inequality would wrong.
$endgroup$
– Michael Rozenberg
3 hours ago
add a comment |
$begingroup$
I found an argument that works for $p,qge 1$, since I need convexity of $xto x^p,x^q$.
Assume by contradiction that $1/p+1/q>1$. Then, if $p'$ is the conjugate of $q$ we know that $p'>p$.
Now, for any $a>0$ we have
$$
a^{p'}/p'=maxlimits_{b>0}{ab-b^q/q}
$$
(just because $ato a^{p'}/p'$ is the conjugate (Legendre transform) of $bto b^q/q$). Then we can pick $a$ large enough such that
$$
frac{a^p}{p}<frac{a^{p'}}{p'}
$$
and for such $a$ we would have
$$
a^{p}/p<maxlimits_{b>0}{ab-b^q/q}
$$
which implies that for some $b>0$ (and the given $a$) Young's inequality is violated.
Hope this helps.
New contributor
$endgroup$
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%2f3075429%2fdoes-youngs-inequality-hold-only-for-conjugate-exponents%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
If you apply the inequality for $a=lambda^{frac{1}{p}}, b=lambda^{frac{1}{q}}$, you get something like
$$ lambda^{frac{1}{p}+frac{1}{q}}leq left( frac{1}{p}+frac{1}{q} right)lambda, qquad forall lambda>0. $$
If you take $lambda to infty$, it's clear that $frac{1}{p}+frac{1}{q}leq 1$. Otherwise, after dividing by $lambda$ you get
$$ lambda^{frac{1}{p}+frac{1}{q}-1} leq frac{1}{p}+frac{1}{q}, $$
which is false for $lambda$ large enough, as the right side is constant. Taking $lambda to 0$ gets the other inequality.
$endgroup$
add a comment |
$begingroup$
If you apply the inequality for $a=lambda^{frac{1}{p}}, b=lambda^{frac{1}{q}}$, you get something like
$$ lambda^{frac{1}{p}+frac{1}{q}}leq left( frac{1}{p}+frac{1}{q} right)lambda, qquad forall lambda>0. $$
If you take $lambda to infty$, it's clear that $frac{1}{p}+frac{1}{q}leq 1$. Otherwise, after dividing by $lambda$ you get
$$ lambda^{frac{1}{p}+frac{1}{q}-1} leq frac{1}{p}+frac{1}{q}, $$
which is false for $lambda$ large enough, as the right side is constant. Taking $lambda to 0$ gets the other inequality.
$endgroup$
add a comment |
$begingroup$
If you apply the inequality for $a=lambda^{frac{1}{p}}, b=lambda^{frac{1}{q}}$, you get something like
$$ lambda^{frac{1}{p}+frac{1}{q}}leq left( frac{1}{p}+frac{1}{q} right)lambda, qquad forall lambda>0. $$
If you take $lambda to infty$, it's clear that $frac{1}{p}+frac{1}{q}leq 1$. Otherwise, after dividing by $lambda$ you get
$$ lambda^{frac{1}{p}+frac{1}{q}-1} leq frac{1}{p}+frac{1}{q}, $$
which is false for $lambda$ large enough, as the right side is constant. Taking $lambda to 0$ gets the other inequality.
$endgroup$
If you apply the inequality for $a=lambda^{frac{1}{p}}, b=lambda^{frac{1}{q}}$, you get something like
$$ lambda^{frac{1}{p}+frac{1}{q}}leq left( frac{1}{p}+frac{1}{q} right)lambda, qquad forall lambda>0. $$
If you take $lambda to infty$, it's clear that $frac{1}{p}+frac{1}{q}leq 1$. Otherwise, after dividing by $lambda$ you get
$$ lambda^{frac{1}{p}+frac{1}{q}-1} leq frac{1}{p}+frac{1}{q}, $$
which is false for $lambda$ large enough, as the right side is constant. Taking $lambda to 0$ gets the other inequality.
answered 8 hours ago
Nicolás VilchesNicolás Vilches
49128
49128
add a comment |
add a comment |
$begingroup$
I think the following can help.
By AM-GM $$frac{1}{p}a^p+frac{1}{q}b^qgeqleft(frac{1}{p}+frac{1}{q}right)left(left(a^pright)^{frac{1}{p}}left(b^qright)^{frac{1}{q}}right)^{frac{1}{frac{1}{p}+frac{1}{q}}}=left(frac{1}{p}+frac{1}{q}right)(ab)^{frac{1}{frac{1}{p}+frac{1}{q}}}.$$
The equality occurs, of course.
$endgroup$
$begingroup$
Thanks, but I do not understand; How exactly do you deduce from that something on the sum $frac{1}{p}+frac{1}{q}$? I tried putting $a^p=b^q$ (this is when we have equality in the AM-GM inequality) but I don't see how it helps.
$endgroup$
– Asaf Shachar
3 hours ago
$begingroup$
@Asaf Shachar The equality, which I got is true for all positives $a$, $b$, $p$ and $q$. If we'll assume that $frac{1}{p}+frac{1}{q}neq1$ then your inequality would wrong.
$endgroup$
– Michael Rozenberg
3 hours ago
add a comment |
$begingroup$
I think the following can help.
By AM-GM $$frac{1}{p}a^p+frac{1}{q}b^qgeqleft(frac{1}{p}+frac{1}{q}right)left(left(a^pright)^{frac{1}{p}}left(b^qright)^{frac{1}{q}}right)^{frac{1}{frac{1}{p}+frac{1}{q}}}=left(frac{1}{p}+frac{1}{q}right)(ab)^{frac{1}{frac{1}{p}+frac{1}{q}}}.$$
The equality occurs, of course.
$endgroup$
$begingroup$
Thanks, but I do not understand; How exactly do you deduce from that something on the sum $frac{1}{p}+frac{1}{q}$? I tried putting $a^p=b^q$ (this is when we have equality in the AM-GM inequality) but I don't see how it helps.
$endgroup$
– Asaf Shachar
3 hours ago
$begingroup$
@Asaf Shachar The equality, which I got is true for all positives $a$, $b$, $p$ and $q$. If we'll assume that $frac{1}{p}+frac{1}{q}neq1$ then your inequality would wrong.
$endgroup$
– Michael Rozenberg
3 hours ago
add a comment |
$begingroup$
I think the following can help.
By AM-GM $$frac{1}{p}a^p+frac{1}{q}b^qgeqleft(frac{1}{p}+frac{1}{q}right)left(left(a^pright)^{frac{1}{p}}left(b^qright)^{frac{1}{q}}right)^{frac{1}{frac{1}{p}+frac{1}{q}}}=left(frac{1}{p}+frac{1}{q}right)(ab)^{frac{1}{frac{1}{p}+frac{1}{q}}}.$$
The equality occurs, of course.
$endgroup$
I think the following can help.
By AM-GM $$frac{1}{p}a^p+frac{1}{q}b^qgeqleft(frac{1}{p}+frac{1}{q}right)left(left(a^pright)^{frac{1}{p}}left(b^qright)^{frac{1}{q}}right)^{frac{1}{frac{1}{p}+frac{1}{q}}}=left(frac{1}{p}+frac{1}{q}right)(ab)^{frac{1}{frac{1}{p}+frac{1}{q}}}.$$
The equality occurs, of course.
edited 8 hours ago
answered 8 hours ago
Michael RozenbergMichael Rozenberg
98.2k1590188
98.2k1590188
$begingroup$
Thanks, but I do not understand; How exactly do you deduce from that something on the sum $frac{1}{p}+frac{1}{q}$? I tried putting $a^p=b^q$ (this is when we have equality in the AM-GM inequality) but I don't see how it helps.
$endgroup$
– Asaf Shachar
3 hours ago
$begingroup$
@Asaf Shachar The equality, which I got is true for all positives $a$, $b$, $p$ and $q$. If we'll assume that $frac{1}{p}+frac{1}{q}neq1$ then your inequality would wrong.
$endgroup$
– Michael Rozenberg
3 hours ago
add a comment |
$begingroup$
Thanks, but I do not understand; How exactly do you deduce from that something on the sum $frac{1}{p}+frac{1}{q}$? I tried putting $a^p=b^q$ (this is when we have equality in the AM-GM inequality) but I don't see how it helps.
$endgroup$
– Asaf Shachar
3 hours ago
$begingroup$
@Asaf Shachar The equality, which I got is true for all positives $a$, $b$, $p$ and $q$. If we'll assume that $frac{1}{p}+frac{1}{q}neq1$ then your inequality would wrong.
$endgroup$
– Michael Rozenberg
3 hours ago
$begingroup$
Thanks, but I do not understand; How exactly do you deduce from that something on the sum $frac{1}{p}+frac{1}{q}$? I tried putting $a^p=b^q$ (this is when we have equality in the AM-GM inequality) but I don't see how it helps.
$endgroup$
– Asaf Shachar
3 hours ago
$begingroup$
Thanks, but I do not understand; How exactly do you deduce from that something on the sum $frac{1}{p}+frac{1}{q}$? I tried putting $a^p=b^q$ (this is when we have equality in the AM-GM inequality) but I don't see how it helps.
$endgroup$
– Asaf Shachar
3 hours ago
$begingroup$
@Asaf Shachar The equality, which I got is true for all positives $a$, $b$, $p$ and $q$. If we'll assume that $frac{1}{p}+frac{1}{q}neq1$ then your inequality would wrong.
$endgroup$
– Michael Rozenberg
3 hours ago
$begingroup$
@Asaf Shachar The equality, which I got is true for all positives $a$, $b$, $p$ and $q$. If we'll assume that $frac{1}{p}+frac{1}{q}neq1$ then your inequality would wrong.
$endgroup$
– Michael Rozenberg
3 hours ago
add a comment |
$begingroup$
I found an argument that works for $p,qge 1$, since I need convexity of $xto x^p,x^q$.
Assume by contradiction that $1/p+1/q>1$. Then, if $p'$ is the conjugate of $q$ we know that $p'>p$.
Now, for any $a>0$ we have
$$
a^{p'}/p'=maxlimits_{b>0}{ab-b^q/q}
$$
(just because $ato a^{p'}/p'$ is the conjugate (Legendre transform) of $bto b^q/q$). Then we can pick $a$ large enough such that
$$
frac{a^p}{p}<frac{a^{p'}}{p'}
$$
and for such $a$ we would have
$$
a^{p}/p<maxlimits_{b>0}{ab-b^q/q}
$$
which implies that for some $b>0$ (and the given $a$) Young's inequality is violated.
Hope this helps.
New contributor
$endgroup$
add a comment |
$begingroup$
I found an argument that works for $p,qge 1$, since I need convexity of $xto x^p,x^q$.
Assume by contradiction that $1/p+1/q>1$. Then, if $p'$ is the conjugate of $q$ we know that $p'>p$.
Now, for any $a>0$ we have
$$
a^{p'}/p'=maxlimits_{b>0}{ab-b^q/q}
$$
(just because $ato a^{p'}/p'$ is the conjugate (Legendre transform) of $bto b^q/q$). Then we can pick $a$ large enough such that
$$
frac{a^p}{p}<frac{a^{p'}}{p'}
$$
and for such $a$ we would have
$$
a^{p}/p<maxlimits_{b>0}{ab-b^q/q}
$$
which implies that for some $b>0$ (and the given $a$) Young's inequality is violated.
Hope this helps.
New contributor
$endgroup$
add a comment |
$begingroup$
I found an argument that works for $p,qge 1$, since I need convexity of $xto x^p,x^q$.
Assume by contradiction that $1/p+1/q>1$. Then, if $p'$ is the conjugate of $q$ we know that $p'>p$.
Now, for any $a>0$ we have
$$
a^{p'}/p'=maxlimits_{b>0}{ab-b^q/q}
$$
(just because $ato a^{p'}/p'$ is the conjugate (Legendre transform) of $bto b^q/q$). Then we can pick $a$ large enough such that
$$
frac{a^p}{p}<frac{a^{p'}}{p'}
$$
and for such $a$ we would have
$$
a^{p}/p<maxlimits_{b>0}{ab-b^q/q}
$$
which implies that for some $b>0$ (and the given $a$) Young's inequality is violated.
Hope this helps.
New contributor
$endgroup$
I found an argument that works for $p,qge 1$, since I need convexity of $xto x^p,x^q$.
Assume by contradiction that $1/p+1/q>1$. Then, if $p'$ is the conjugate of $q$ we know that $p'>p$.
Now, for any $a>0$ we have
$$
a^{p'}/p'=maxlimits_{b>0}{ab-b^q/q}
$$
(just because $ato a^{p'}/p'$ is the conjugate (Legendre transform) of $bto b^q/q$). Then we can pick $a$ large enough such that
$$
frac{a^p}{p}<frac{a^{p'}}{p'}
$$
and for such $a$ we would have
$$
a^{p}/p<maxlimits_{b>0}{ab-b^q/q}
$$
which implies that for some $b>0$ (and the given $a$) Young's inequality is violated.
Hope this helps.
New contributor
New contributor
answered 8 hours ago
GReyesGReyes
2563
2563
New contributor
New contributor
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%2f3075429%2fdoes-youngs-inequality-hold-only-for-conjugate-exponents%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