For $f : I mapsto mathbb{R}$ convex and of class $C^2$, if $f' geq m$, show an integral inequality regarding...
$begingroup$
Let be $I$ an interval of $mathbb{R}$, $f : I to mathbb{R}$ a convex function of class $C^2$ such that $forall t in I, f'(t) geq m$.
I want to show that:
$begin{equation*}
forall (a, b) in I^2, left lvert displaystyle int_a^b exp left[i f(t)right] textrm{d}t right rvert leq dfrac{2}{m}
end{equation*}$
So my approach was: we have an idea of the speed of $f$ due to its lower bound and its convexity so then we can get, using fundamental theorem of analysis and Taylor theorems get bounds on how much does it spin in the unit circle and extrapolates "its area" (does it makes sense here?).
But I am unable to get anything near the result, I guess, the simplest thing is to work on $I = int_a^b cos (f(t)) textrm{d}t$ beforehand, but I have no idea how to tackle this.
I'd welcome very much any hint rather than a complete solution in order to get rather real understanding of those kind of problems.
real-analysis integration convex-analysis
$endgroup$
add a comment |
$begingroup$
Let be $I$ an interval of $mathbb{R}$, $f : I to mathbb{R}$ a convex function of class $C^2$ such that $forall t in I, f'(t) geq m$.
I want to show that:
$begin{equation*}
forall (a, b) in I^2, left lvert displaystyle int_a^b exp left[i f(t)right] textrm{d}t right rvert leq dfrac{2}{m}
end{equation*}$
So my approach was: we have an idea of the speed of $f$ due to its lower bound and its convexity so then we can get, using fundamental theorem of analysis and Taylor theorems get bounds on how much does it spin in the unit circle and extrapolates "its area" (does it makes sense here?).
But I am unable to get anything near the result, I guess, the simplest thing is to work on $I = int_a^b cos (f(t)) textrm{d}t$ beforehand, but I have no idea how to tackle this.
I'd welcome very much any hint rather than a complete solution in order to get rather real understanding of those kind of problems.
real-analysis integration convex-analysis
$endgroup$
$begingroup$
For that simple result, try to use the 2nd integral mean value theorem, since $f'$ is monotonic due to the convexity of $f$.
$endgroup$
– xbh
Dec 18 '18 at 0:31
add a comment |
$begingroup$
Let be $I$ an interval of $mathbb{R}$, $f : I to mathbb{R}$ a convex function of class $C^2$ such that $forall t in I, f'(t) geq m$.
I want to show that:
$begin{equation*}
forall (a, b) in I^2, left lvert displaystyle int_a^b exp left[i f(t)right] textrm{d}t right rvert leq dfrac{2}{m}
end{equation*}$
So my approach was: we have an idea of the speed of $f$ due to its lower bound and its convexity so then we can get, using fundamental theorem of analysis and Taylor theorems get bounds on how much does it spin in the unit circle and extrapolates "its area" (does it makes sense here?).
But I am unable to get anything near the result, I guess, the simplest thing is to work on $I = int_a^b cos (f(t)) textrm{d}t$ beforehand, but I have no idea how to tackle this.
I'd welcome very much any hint rather than a complete solution in order to get rather real understanding of those kind of problems.
real-analysis integration convex-analysis
$endgroup$
Let be $I$ an interval of $mathbb{R}$, $f : I to mathbb{R}$ a convex function of class $C^2$ such that $forall t in I, f'(t) geq m$.
I want to show that:
$begin{equation*}
forall (a, b) in I^2, left lvert displaystyle int_a^b exp left[i f(t)right] textrm{d}t right rvert leq dfrac{2}{m}
end{equation*}$
So my approach was: we have an idea of the speed of $f$ due to its lower bound and its convexity so then we can get, using fundamental theorem of analysis and Taylor theorems get bounds on how much does it spin in the unit circle and extrapolates "its area" (does it makes sense here?).
But I am unable to get anything near the result, I guess, the simplest thing is to work on $I = int_a^b cos (f(t)) textrm{d}t$ beforehand, but I have no idea how to tackle this.
I'd welcome very much any hint rather than a complete solution in order to get rather real understanding of those kind of problems.
real-analysis integration convex-analysis
real-analysis integration convex-analysis
asked Dec 17 '18 at 21:55
RaitoRaito
686415
686415
$begingroup$
For that simple result, try to use the 2nd integral mean value theorem, since $f'$ is monotonic due to the convexity of $f$.
$endgroup$
– xbh
Dec 18 '18 at 0:31
add a comment |
$begingroup$
For that simple result, try to use the 2nd integral mean value theorem, since $f'$ is monotonic due to the convexity of $f$.
$endgroup$
– xbh
Dec 18 '18 at 0:31
$begingroup$
For that simple result, try to use the 2nd integral mean value theorem, since $f'$ is monotonic due to the convexity of $f$.
$endgroup$
– xbh
Dec 18 '18 at 0:31
$begingroup$
For that simple result, try to use the 2nd integral mean value theorem, since $f'$ is monotonic due to the convexity of $f$.
$endgroup$
– xbh
Dec 18 '18 at 0:31
add a comment |
0
active
oldest
votes
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%2f3044494%2ffor-f-i-mapsto-mathbbr-convex-and-of-class-c2-if-f-geq-m-show-a%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
0
active
oldest
votes
0
active
oldest
votes
active
oldest
votes
active
oldest
votes
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%2f3044494%2ffor-f-i-mapsto-mathbbr-convex-and-of-class-c2-if-f-geq-m-show-a%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
$begingroup$
For that simple result, try to use the 2nd integral mean value theorem, since $f'$ is monotonic due to the convexity of $f$.
$endgroup$
– xbh
Dec 18 '18 at 0:31