Let $f$ be a continuous function on $[0,infty)$ such that $lim_{xto infty}(f(x)+int_0^x f(t)dt)$ exists. Find...
Let $f$ be a continuous function on $[0,infty)$ such that $lim_{xto infty}(f(x)+int_0^x f(t)dt)$ exists. Find $lim_{xto infty}f(x)$.
Useful hints will work. Please give some nice hint to solve the problem.
integration limits functions continuity
|
show 3 more comments
Let $f$ be a continuous function on $[0,infty)$ such that $lim_{xto infty}(f(x)+int_0^x f(t)dt)$ exists. Find $lim_{xto infty}f(x)$.
Useful hints will work. Please give some nice hint to solve the problem.
integration limits functions continuity
Try taking $g=f+K$ with $K$ some constant. What happends? Can you guess the value of the limit?
– xavierm02
Feb 2 '17 at 14:36
@xavierm02 taking $g$=$f+K$ we will get $lim_{xto infty} f(x)+K$=$lim_{xto infty}g(x)$.Is it correct ? If yes whats next?
– Piyush Raut
Feb 2 '17 at 14:39
But what is $lim_{xto infty}(g(x)+int_0^x g(t)dt)$?
– xavierm02
Feb 2 '17 at 14:42
I didn't get that.Did you mean $lim_{xto infty}(f(x)+int_0^x f(t)dt)$?
– Piyush Raut
Feb 2 '17 at 14:45
No I meant with $g$, under the assumption that the limit exists with $f$.
– xavierm02
Feb 2 '17 at 14:45
|
show 3 more comments
Let $f$ be a continuous function on $[0,infty)$ such that $lim_{xto infty}(f(x)+int_0^x f(t)dt)$ exists. Find $lim_{xto infty}f(x)$.
Useful hints will work. Please give some nice hint to solve the problem.
integration limits functions continuity
Let $f$ be a continuous function on $[0,infty)$ such that $lim_{xto infty}(f(x)+int_0^x f(t)dt)$ exists. Find $lim_{xto infty}f(x)$.
Useful hints will work. Please give some nice hint to solve the problem.
integration limits functions continuity
integration limits functions continuity
edited Nov 29 '18 at 21:00
Shaun
8,820113681
8,820113681
asked Feb 2 '17 at 14:32
Piyush RautPiyush Raut
577
577
Try taking $g=f+K$ with $K$ some constant. What happends? Can you guess the value of the limit?
– xavierm02
Feb 2 '17 at 14:36
@xavierm02 taking $g$=$f+K$ we will get $lim_{xto infty} f(x)+K$=$lim_{xto infty}g(x)$.Is it correct ? If yes whats next?
– Piyush Raut
Feb 2 '17 at 14:39
But what is $lim_{xto infty}(g(x)+int_0^x g(t)dt)$?
– xavierm02
Feb 2 '17 at 14:42
I didn't get that.Did you mean $lim_{xto infty}(f(x)+int_0^x f(t)dt)$?
– Piyush Raut
Feb 2 '17 at 14:45
No I meant with $g$, under the assumption that the limit exists with $f$.
– xavierm02
Feb 2 '17 at 14:45
|
show 3 more comments
Try taking $g=f+K$ with $K$ some constant. What happends? Can you guess the value of the limit?
– xavierm02
Feb 2 '17 at 14:36
@xavierm02 taking $g$=$f+K$ we will get $lim_{xto infty} f(x)+K$=$lim_{xto infty}g(x)$.Is it correct ? If yes whats next?
– Piyush Raut
Feb 2 '17 at 14:39
But what is $lim_{xto infty}(g(x)+int_0^x g(t)dt)$?
– xavierm02
Feb 2 '17 at 14:42
I didn't get that.Did you mean $lim_{xto infty}(f(x)+int_0^x f(t)dt)$?
– Piyush Raut
Feb 2 '17 at 14:45
No I meant with $g$, under the assumption that the limit exists with $f$.
– xavierm02
Feb 2 '17 at 14:45
Try taking $g=f+K$ with $K$ some constant. What happends? Can you guess the value of the limit?
– xavierm02
Feb 2 '17 at 14:36
Try taking $g=f+K$ with $K$ some constant. What happends? Can you guess the value of the limit?
– xavierm02
Feb 2 '17 at 14:36
@xavierm02 taking $g$=$f+K$ we will get $lim_{xto infty} f(x)+K$=$lim_{xto infty}g(x)$.Is it correct ? If yes whats next?
– Piyush Raut
Feb 2 '17 at 14:39
@xavierm02 taking $g$=$f+K$ we will get $lim_{xto infty} f(x)+K$=$lim_{xto infty}g(x)$.Is it correct ? If yes whats next?
– Piyush Raut
Feb 2 '17 at 14:39
But what is $lim_{xto infty}(g(x)+int_0^x g(t)dt)$?
– xavierm02
Feb 2 '17 at 14:42
But what is $lim_{xto infty}(g(x)+int_0^x g(t)dt)$?
– xavierm02
Feb 2 '17 at 14:42
I didn't get that.Did you mean $lim_{xto infty}(f(x)+int_0^x f(t)dt)$?
– Piyush Raut
Feb 2 '17 at 14:45
I didn't get that.Did you mean $lim_{xto infty}(f(x)+int_0^x f(t)dt)$?
– Piyush Raut
Feb 2 '17 at 14:45
No I meant with $g$, under the assumption that the limit exists with $f$.
– xavierm02
Feb 2 '17 at 14:45
No I meant with $g$, under the assumption that the limit exists with $f$.
– xavierm02
Feb 2 '17 at 14:45
|
show 3 more comments
2 Answers
2
active
oldest
votes
Let $F(x) = int_0^x f(t)dt$. Then, we have $lim_{xto infty} e^{-x}[e^x F(x)]' = c$ exists. From this, given $epsilon>0$, there is $N>0$ such that for all $x>N$,
$$
(c-epsilon)e^x leq[e^x F(x)]' leq (c+epsilon)e^x.
$$
Now, integrating from $N$ to $t$ gives:
$$(c-epsilon)(e^t-e^N)leq e^tF(t) -e^NF(N) leq (c+epsilon)(e^t-e^N),quadforall t>N
$$and hence
$$
(c-epsilon)(1-e^{N-t})leq F(t) -e^{N-t}F(N) leq (c+epsilon)(1-e^{N-t}), quad forall t>N.
$$Taking $ttoinfty$ yields
$$c-epsilon leq liminf_{tto infty}F(t)leq limsup_{tto infty}F(t)leq c+epsilon.
$$Since $epsilon>0$ was arbitrary, we have $lim_{ttoinfty} F(t) =c$ and thus $lim_{ttoinfty} f(t) = 0$.
add a comment |
One of the easiest approaches is the use of L'Hospital's Rule here. Let $F(x) =int_{0}^{x}f(t),dt$ so that $F'(x) =f(x) $ via Fundamental Theorem of Calculus. We are thus given that $F(x) +F'(x) to L$ as $xtoinfty$.
We have
begin{align}
lim_{xtoinfty} F(x) &=lim_{xtoinfty} frac{e^xF(x)} {e^x}notag\
&=lim_{xto infty} dfrac{e^x{F(x) +F'(x) }} {e^x}text{ (via L'Hospital's Rule)} notag\
&=lim_{xtoinfty} F(x) +F'(x)notag\
&=Lnotag
end{align}
and hence $$lim_{xto infty} f(x) =lim_{xtoinfty} F'(x) =lim_{xtoinfty} {F(x) + F'(x) }-F(x) =L-L=0$$
1
Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
– zhw.
Nov 30 '18 at 17:51
@zhw : I should have added more details. Thanks for putting these details in your comment.
– Paramanand Singh
Nov 30 '18 at 17:55
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%2f2125798%2flet-f-be-a-continuous-function-on-0-infty-such-that-lim-x-to-inftyf%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
Let $F(x) = int_0^x f(t)dt$. Then, we have $lim_{xto infty} e^{-x}[e^x F(x)]' = c$ exists. From this, given $epsilon>0$, there is $N>0$ such that for all $x>N$,
$$
(c-epsilon)e^x leq[e^x F(x)]' leq (c+epsilon)e^x.
$$
Now, integrating from $N$ to $t$ gives:
$$(c-epsilon)(e^t-e^N)leq e^tF(t) -e^NF(N) leq (c+epsilon)(e^t-e^N),quadforall t>N
$$and hence
$$
(c-epsilon)(1-e^{N-t})leq F(t) -e^{N-t}F(N) leq (c+epsilon)(1-e^{N-t}), quad forall t>N.
$$Taking $ttoinfty$ yields
$$c-epsilon leq liminf_{tto infty}F(t)leq limsup_{tto infty}F(t)leq c+epsilon.
$$Since $epsilon>0$ was arbitrary, we have $lim_{ttoinfty} F(t) =c$ and thus $lim_{ttoinfty} f(t) = 0$.
add a comment |
Let $F(x) = int_0^x f(t)dt$. Then, we have $lim_{xto infty} e^{-x}[e^x F(x)]' = c$ exists. From this, given $epsilon>0$, there is $N>0$ such that for all $x>N$,
$$
(c-epsilon)e^x leq[e^x F(x)]' leq (c+epsilon)e^x.
$$
Now, integrating from $N$ to $t$ gives:
$$(c-epsilon)(e^t-e^N)leq e^tF(t) -e^NF(N) leq (c+epsilon)(e^t-e^N),quadforall t>N
$$and hence
$$
(c-epsilon)(1-e^{N-t})leq F(t) -e^{N-t}F(N) leq (c+epsilon)(1-e^{N-t}), quad forall t>N.
$$Taking $ttoinfty$ yields
$$c-epsilon leq liminf_{tto infty}F(t)leq limsup_{tto infty}F(t)leq c+epsilon.
$$Since $epsilon>0$ was arbitrary, we have $lim_{ttoinfty} F(t) =c$ and thus $lim_{ttoinfty} f(t) = 0$.
add a comment |
Let $F(x) = int_0^x f(t)dt$. Then, we have $lim_{xto infty} e^{-x}[e^x F(x)]' = c$ exists. From this, given $epsilon>0$, there is $N>0$ such that for all $x>N$,
$$
(c-epsilon)e^x leq[e^x F(x)]' leq (c+epsilon)e^x.
$$
Now, integrating from $N$ to $t$ gives:
$$(c-epsilon)(e^t-e^N)leq e^tF(t) -e^NF(N) leq (c+epsilon)(e^t-e^N),quadforall t>N
$$and hence
$$
(c-epsilon)(1-e^{N-t})leq F(t) -e^{N-t}F(N) leq (c+epsilon)(1-e^{N-t}), quad forall t>N.
$$Taking $ttoinfty$ yields
$$c-epsilon leq liminf_{tto infty}F(t)leq limsup_{tto infty}F(t)leq c+epsilon.
$$Since $epsilon>0$ was arbitrary, we have $lim_{ttoinfty} F(t) =c$ and thus $lim_{ttoinfty} f(t) = 0$.
Let $F(x) = int_0^x f(t)dt$. Then, we have $lim_{xto infty} e^{-x}[e^x F(x)]' = c$ exists. From this, given $epsilon>0$, there is $N>0$ such that for all $x>N$,
$$
(c-epsilon)e^x leq[e^x F(x)]' leq (c+epsilon)e^x.
$$
Now, integrating from $N$ to $t$ gives:
$$(c-epsilon)(e^t-e^N)leq e^tF(t) -e^NF(N) leq (c+epsilon)(e^t-e^N),quadforall t>N
$$and hence
$$
(c-epsilon)(1-e^{N-t})leq F(t) -e^{N-t}F(N) leq (c+epsilon)(1-e^{N-t}), quad forall t>N.
$$Taking $ttoinfty$ yields
$$c-epsilon leq liminf_{tto infty}F(t)leq limsup_{tto infty}F(t)leq c+epsilon.
$$Since $epsilon>0$ was arbitrary, we have $lim_{ttoinfty} F(t) =c$ and thus $lim_{ttoinfty} f(t) = 0$.
answered Nov 27 '18 at 7:45
SongSong
6,948421
6,948421
add a comment |
add a comment |
One of the easiest approaches is the use of L'Hospital's Rule here. Let $F(x) =int_{0}^{x}f(t),dt$ so that $F'(x) =f(x) $ via Fundamental Theorem of Calculus. We are thus given that $F(x) +F'(x) to L$ as $xtoinfty$.
We have
begin{align}
lim_{xtoinfty} F(x) &=lim_{xtoinfty} frac{e^xF(x)} {e^x}notag\
&=lim_{xto infty} dfrac{e^x{F(x) +F'(x) }} {e^x}text{ (via L'Hospital's Rule)} notag\
&=lim_{xtoinfty} F(x) +F'(x)notag\
&=Lnotag
end{align}
and hence $$lim_{xto infty} f(x) =lim_{xtoinfty} F'(x) =lim_{xtoinfty} {F(x) + F'(x) }-F(x) =L-L=0$$
1
Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
– zhw.
Nov 30 '18 at 17:51
@zhw : I should have added more details. Thanks for putting these details in your comment.
– Paramanand Singh
Nov 30 '18 at 17:55
add a comment |
One of the easiest approaches is the use of L'Hospital's Rule here. Let $F(x) =int_{0}^{x}f(t),dt$ so that $F'(x) =f(x) $ via Fundamental Theorem of Calculus. We are thus given that $F(x) +F'(x) to L$ as $xtoinfty$.
We have
begin{align}
lim_{xtoinfty} F(x) &=lim_{xtoinfty} frac{e^xF(x)} {e^x}notag\
&=lim_{xto infty} dfrac{e^x{F(x) +F'(x) }} {e^x}text{ (via L'Hospital's Rule)} notag\
&=lim_{xtoinfty} F(x) +F'(x)notag\
&=Lnotag
end{align}
and hence $$lim_{xto infty} f(x) =lim_{xtoinfty} F'(x) =lim_{xtoinfty} {F(x) + F'(x) }-F(x) =L-L=0$$
1
Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
– zhw.
Nov 30 '18 at 17:51
@zhw : I should have added more details. Thanks for putting these details in your comment.
– Paramanand Singh
Nov 30 '18 at 17:55
add a comment |
One of the easiest approaches is the use of L'Hospital's Rule here. Let $F(x) =int_{0}^{x}f(t),dt$ so that $F'(x) =f(x) $ via Fundamental Theorem of Calculus. We are thus given that $F(x) +F'(x) to L$ as $xtoinfty$.
We have
begin{align}
lim_{xtoinfty} F(x) &=lim_{xtoinfty} frac{e^xF(x)} {e^x}notag\
&=lim_{xto infty} dfrac{e^x{F(x) +F'(x) }} {e^x}text{ (via L'Hospital's Rule)} notag\
&=lim_{xtoinfty} F(x) +F'(x)notag\
&=Lnotag
end{align}
and hence $$lim_{xto infty} f(x) =lim_{xtoinfty} F'(x) =lim_{xtoinfty} {F(x) + F'(x) }-F(x) =L-L=0$$
One of the easiest approaches is the use of L'Hospital's Rule here. Let $F(x) =int_{0}^{x}f(t),dt$ so that $F'(x) =f(x) $ via Fundamental Theorem of Calculus. We are thus given that $F(x) +F'(x) to L$ as $xtoinfty$.
We have
begin{align}
lim_{xtoinfty} F(x) &=lim_{xtoinfty} frac{e^xF(x)} {e^x}notag\
&=lim_{xto infty} dfrac{e^x{F(x) +F'(x) }} {e^x}text{ (via L'Hospital's Rule)} notag\
&=lim_{xtoinfty} F(x) +F'(x)notag\
&=Lnotag
end{align}
and hence $$lim_{xto infty} f(x) =lim_{xtoinfty} F'(x) =lim_{xtoinfty} {F(x) + F'(x) }-F(x) =L-L=0$$
answered Nov 30 '18 at 2:37
Paramanand SinghParamanand Singh
49.2k555161
49.2k555161
1
Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
– zhw.
Nov 30 '18 at 17:51
@zhw : I should have added more details. Thanks for putting these details in your comment.
– Paramanand Singh
Nov 30 '18 at 17:55
add a comment |
1
Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
– zhw.
Nov 30 '18 at 17:51
@zhw : I should have added more details. Thanks for putting these details in your comment.
– Paramanand Singh
Nov 30 '18 at 17:55
1
1
Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
– zhw.
Nov 30 '18 at 17:51
Nice solution, +1. But the flow is weird because you write $lim$ when it hasn't been shown the limit exists. Also, you don't know the limit of the numerator, $e^xF(x),$ is $infty.$ Now to use L'Hopital in this case, you don't need to know that. But many readers may not know this little fact.
– zhw.
Nov 30 '18 at 17:51
@zhw : I should have added more details. Thanks for putting these details in your comment.
– Paramanand Singh
Nov 30 '18 at 17:55
@zhw : I should have added more details. Thanks for putting these details in your comment.
– Paramanand Singh
Nov 30 '18 at 17:55
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%2f2125798%2flet-f-be-a-continuous-function-on-0-infty-such-that-lim-x-to-inftyf%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
Try taking $g=f+K$ with $K$ some constant. What happends? Can you guess the value of the limit?
– xavierm02
Feb 2 '17 at 14:36
@xavierm02 taking $g$=$f+K$ we will get $lim_{xto infty} f(x)+K$=$lim_{xto infty}g(x)$.Is it correct ? If yes whats next?
– Piyush Raut
Feb 2 '17 at 14:39
But what is $lim_{xto infty}(g(x)+int_0^x g(t)dt)$?
– xavierm02
Feb 2 '17 at 14:42
I didn't get that.Did you mean $lim_{xto infty}(f(x)+int_0^x f(t)dt)$?
– Piyush Raut
Feb 2 '17 at 14:45
No I meant with $g$, under the assumption that the limit exists with $f$.
– xavierm02
Feb 2 '17 at 14:45