Theorem 5.8 Baby Rudin. Some questions
$begingroup$
This is with reference to the page number 107 of Baby Rudin.
Theorem 5.8 Let $f$ be defined on $[a,b]$.If a point $xin(a,b)$ is a
local maximum of function $f$, and if $f'(x)$ exist, then $f'(x)=0$.
I have some questions. Thanks in advance for reading and helping out.
What about the converse of this theorem? Suppose $f'(x)=0$ at some point, can we always conclude that it is local maximum?
The theorem add a condition that $f'(x)$ should exist. This raised a question, Can we have a function function with local maximum/minimum at some point say $p$ but derivative undefined at the point.
Edits:
We can take $f(x)=x^2$ on $[-1,1]$. $f'(0)=0$ but $0$ is not local maximum. I think this works for (1).
real-analysis
$endgroup$
add a comment |
$begingroup$
This is with reference to the page number 107 of Baby Rudin.
Theorem 5.8 Let $f$ be defined on $[a,b]$.If a point $xin(a,b)$ is a
local maximum of function $f$, and if $f'(x)$ exist, then $f'(x)=0$.
I have some questions. Thanks in advance for reading and helping out.
What about the converse of this theorem? Suppose $f'(x)=0$ at some point, can we always conclude that it is local maximum?
The theorem add a condition that $f'(x)$ should exist. This raised a question, Can we have a function function with local maximum/minimum at some point say $p$ but derivative undefined at the point.
Edits:
We can take $f(x)=x^2$ on $[-1,1]$. $f'(0)=0$ but $0$ is not local maximum. I think this works for (1).
real-analysis
$endgroup$
add a comment |
$begingroup$
This is with reference to the page number 107 of Baby Rudin.
Theorem 5.8 Let $f$ be defined on $[a,b]$.If a point $xin(a,b)$ is a
local maximum of function $f$, and if $f'(x)$ exist, then $f'(x)=0$.
I have some questions. Thanks in advance for reading and helping out.
What about the converse of this theorem? Suppose $f'(x)=0$ at some point, can we always conclude that it is local maximum?
The theorem add a condition that $f'(x)$ should exist. This raised a question, Can we have a function function with local maximum/minimum at some point say $p$ but derivative undefined at the point.
Edits:
We can take $f(x)=x^2$ on $[-1,1]$. $f'(0)=0$ but $0$ is not local maximum. I think this works for (1).
real-analysis
$endgroup$
This is with reference to the page number 107 of Baby Rudin.
Theorem 5.8 Let $f$ be defined on $[a,b]$.If a point $xin(a,b)$ is a
local maximum of function $f$, and if $f'(x)$ exist, then $f'(x)=0$.
I have some questions. Thanks in advance for reading and helping out.
What about the converse of this theorem? Suppose $f'(x)=0$ at some point, can we always conclude that it is local maximum?
The theorem add a condition that $f'(x)$ should exist. This raised a question, Can we have a function function with local maximum/minimum at some point say $p$ but derivative undefined at the point.
Edits:
We can take $f(x)=x^2$ on $[-1,1]$. $f'(0)=0$ but $0$ is not local maximum. I think this works for (1).
real-analysis
real-analysis
edited Jan 5 at 9:57
StammeringMathematician
asked Jan 5 at 9:52


StammeringMathematicianStammeringMathematician
2,3441322
2,3441322
add a comment |
add a comment |
4 Answers
4
active
oldest
votes
$begingroup$
Answer to question 1
The derivative of $f(x)=x^3$ vanishes at $0$ but $f$ doesn’t have a minimum nor at maximum at $0$.
Answer to question 2
Take $g(x)=vert x vert$, again look at zero.
$endgroup$
add a comment |
$begingroup$
If $f'(x)=0$, then $x$ is said to be a stationary point. It can be something like $x=0$ in $f(x)=x^3$ too, for example.
For (2), we can even have a function with a global maximum at $p$ but without a derivative in this point. See, for example, $f(x)=-|x|$.
$endgroup$
add a comment |
$begingroup$
- What about the converse of this theorem? Suppose $f'(x)=0$ at some point, can we always conclude that it is local maximum?
No. It could be a minimum, or it could be neither. Such examples:
$f(x) = x^2$. Here, $f'(x) = 2x$ thus implying $f'(x) = 0$ for $x = 0$. It is immediately clear on looking at the graph that this is a local minimum for some intervals of the function.
$f(x) = x^3$. Here, $f'(x) = 3x^2$. Thus, $f'(x) = 0$ only if $x=0$. On looking at the graph, it is clearly not a local extremum in either respect, up to choice of the interval.
(I say "up to choice of the interval" because $x=0$ yields a local minimum on the choice of interval $[0,1]$ for both functions, and on $[-1,0]$ for example the latter has a maximum instead. If we generally state intervals $[a,b]$ with $a<0<b$, though, they respectively have a minimum and neither respectively at $x=0$.)
This touches on the nature of "critical points" you might have learned in your introductory calculus courses: points where $f'(x) = 0$ or is undefined are candidates for local extrema.
- The theorem add a condition that $f'(x)$ should exist. This raised a question, Can we have a function with local maximum/minimum at some point say $p$ but derivative undefined at the point?
Yes. Consider $f(x) = |x|$ on the interval $[a,b]$ where $a<0<b$. The function clearly has a minimum at $x=0$, but the derivative is undefined at that point.
$endgroup$
add a comment |
$begingroup$
Let me try:
1) $f'(x)=0; $
Example:$ f(x)=x^3$, at $x= 0$, inflection point, not an extremum
2) $f(x)= |x|$ , has a local minimum at $x=0.$
$f(x)= -|x|$ has a local maximum at $x=0$
(f'(0) does not exist)(Why?)
$endgroup$
1
$begingroup$
$f'(0)$ does not exist because RHD and LHD are not equal. If we take difference coefficient $f(x)-f(0)/(x-0) $ then depending upon from where we approach $0$ we get two different limits. Thanks a lot.
$endgroup$
– StammeringMathematician
Jan 5 at 10:10
$begingroup$
Stammering.Yes,exactly!!Greetings.
$endgroup$
– Peter Szilas
Jan 5 at 10:14
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%2f3062567%2ftheorem-5-8-baby-rudin-some-questions%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
4 Answers
4
active
oldest
votes
4 Answers
4
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Answer to question 1
The derivative of $f(x)=x^3$ vanishes at $0$ but $f$ doesn’t have a minimum nor at maximum at $0$.
Answer to question 2
Take $g(x)=vert x vert$, again look at zero.
$endgroup$
add a comment |
$begingroup$
Answer to question 1
The derivative of $f(x)=x^3$ vanishes at $0$ but $f$ doesn’t have a minimum nor at maximum at $0$.
Answer to question 2
Take $g(x)=vert x vert$, again look at zero.
$endgroup$
add a comment |
$begingroup$
Answer to question 1
The derivative of $f(x)=x^3$ vanishes at $0$ but $f$ doesn’t have a minimum nor at maximum at $0$.
Answer to question 2
Take $g(x)=vert x vert$, again look at zero.
$endgroup$
Answer to question 1
The derivative of $f(x)=x^3$ vanishes at $0$ but $f$ doesn’t have a minimum nor at maximum at $0$.
Answer to question 2
Take $g(x)=vert x vert$, again look at zero.
answered Jan 5 at 9:59


mathcounterexamples.netmathcounterexamples.net
26k21955
26k21955
add a comment |
add a comment |
$begingroup$
If $f'(x)=0$, then $x$ is said to be a stationary point. It can be something like $x=0$ in $f(x)=x^3$ too, for example.
For (2), we can even have a function with a global maximum at $p$ but without a derivative in this point. See, for example, $f(x)=-|x|$.
$endgroup$
add a comment |
$begingroup$
If $f'(x)=0$, then $x$ is said to be a stationary point. It can be something like $x=0$ in $f(x)=x^3$ too, for example.
For (2), we can even have a function with a global maximum at $p$ but without a derivative in this point. See, for example, $f(x)=-|x|$.
$endgroup$
add a comment |
$begingroup$
If $f'(x)=0$, then $x$ is said to be a stationary point. It can be something like $x=0$ in $f(x)=x^3$ too, for example.
For (2), we can even have a function with a global maximum at $p$ but without a derivative in this point. See, for example, $f(x)=-|x|$.
$endgroup$
If $f'(x)=0$, then $x$ is said to be a stationary point. It can be something like $x=0$ in $f(x)=x^3$ too, for example.
For (2), we can even have a function with a global maximum at $p$ but without a derivative in this point. See, for example, $f(x)=-|x|$.
answered Jan 5 at 10:00
Gabriel RibeiroGabriel Ribeiro
1,424522
1,424522
add a comment |
add a comment |
$begingroup$
- What about the converse of this theorem? Suppose $f'(x)=0$ at some point, can we always conclude that it is local maximum?
No. It could be a minimum, or it could be neither. Such examples:
$f(x) = x^2$. Here, $f'(x) = 2x$ thus implying $f'(x) = 0$ for $x = 0$. It is immediately clear on looking at the graph that this is a local minimum for some intervals of the function.
$f(x) = x^3$. Here, $f'(x) = 3x^2$. Thus, $f'(x) = 0$ only if $x=0$. On looking at the graph, it is clearly not a local extremum in either respect, up to choice of the interval.
(I say "up to choice of the interval" because $x=0$ yields a local minimum on the choice of interval $[0,1]$ for both functions, and on $[-1,0]$ for example the latter has a maximum instead. If we generally state intervals $[a,b]$ with $a<0<b$, though, they respectively have a minimum and neither respectively at $x=0$.)
This touches on the nature of "critical points" you might have learned in your introductory calculus courses: points where $f'(x) = 0$ or is undefined are candidates for local extrema.
- The theorem add a condition that $f'(x)$ should exist. This raised a question, Can we have a function with local maximum/minimum at some point say $p$ but derivative undefined at the point?
Yes. Consider $f(x) = |x|$ on the interval $[a,b]$ where $a<0<b$. The function clearly has a minimum at $x=0$, but the derivative is undefined at that point.
$endgroup$
add a comment |
$begingroup$
- What about the converse of this theorem? Suppose $f'(x)=0$ at some point, can we always conclude that it is local maximum?
No. It could be a minimum, or it could be neither. Such examples:
$f(x) = x^2$. Here, $f'(x) = 2x$ thus implying $f'(x) = 0$ for $x = 0$. It is immediately clear on looking at the graph that this is a local minimum for some intervals of the function.
$f(x) = x^3$. Here, $f'(x) = 3x^2$. Thus, $f'(x) = 0$ only if $x=0$. On looking at the graph, it is clearly not a local extremum in either respect, up to choice of the interval.
(I say "up to choice of the interval" because $x=0$ yields a local minimum on the choice of interval $[0,1]$ for both functions, and on $[-1,0]$ for example the latter has a maximum instead. If we generally state intervals $[a,b]$ with $a<0<b$, though, they respectively have a minimum and neither respectively at $x=0$.)
This touches on the nature of "critical points" you might have learned in your introductory calculus courses: points where $f'(x) = 0$ or is undefined are candidates for local extrema.
- The theorem add a condition that $f'(x)$ should exist. This raised a question, Can we have a function with local maximum/minimum at some point say $p$ but derivative undefined at the point?
Yes. Consider $f(x) = |x|$ on the interval $[a,b]$ where $a<0<b$. The function clearly has a minimum at $x=0$, but the derivative is undefined at that point.
$endgroup$
add a comment |
$begingroup$
- What about the converse of this theorem? Suppose $f'(x)=0$ at some point, can we always conclude that it is local maximum?
No. It could be a minimum, or it could be neither. Such examples:
$f(x) = x^2$. Here, $f'(x) = 2x$ thus implying $f'(x) = 0$ for $x = 0$. It is immediately clear on looking at the graph that this is a local minimum for some intervals of the function.
$f(x) = x^3$. Here, $f'(x) = 3x^2$. Thus, $f'(x) = 0$ only if $x=0$. On looking at the graph, it is clearly not a local extremum in either respect, up to choice of the interval.
(I say "up to choice of the interval" because $x=0$ yields a local minimum on the choice of interval $[0,1]$ for both functions, and on $[-1,0]$ for example the latter has a maximum instead. If we generally state intervals $[a,b]$ with $a<0<b$, though, they respectively have a minimum and neither respectively at $x=0$.)
This touches on the nature of "critical points" you might have learned in your introductory calculus courses: points where $f'(x) = 0$ or is undefined are candidates for local extrema.
- The theorem add a condition that $f'(x)$ should exist. This raised a question, Can we have a function with local maximum/minimum at some point say $p$ but derivative undefined at the point?
Yes. Consider $f(x) = |x|$ on the interval $[a,b]$ where $a<0<b$. The function clearly has a minimum at $x=0$, but the derivative is undefined at that point.
$endgroup$
- What about the converse of this theorem? Suppose $f'(x)=0$ at some point, can we always conclude that it is local maximum?
No. It could be a minimum, or it could be neither. Such examples:
$f(x) = x^2$. Here, $f'(x) = 2x$ thus implying $f'(x) = 0$ for $x = 0$. It is immediately clear on looking at the graph that this is a local minimum for some intervals of the function.
$f(x) = x^3$. Here, $f'(x) = 3x^2$. Thus, $f'(x) = 0$ only if $x=0$. On looking at the graph, it is clearly not a local extremum in either respect, up to choice of the interval.
(I say "up to choice of the interval" because $x=0$ yields a local minimum on the choice of interval $[0,1]$ for both functions, and on $[-1,0]$ for example the latter has a maximum instead. If we generally state intervals $[a,b]$ with $a<0<b$, though, they respectively have a minimum and neither respectively at $x=0$.)
This touches on the nature of "critical points" you might have learned in your introductory calculus courses: points where $f'(x) = 0$ or is undefined are candidates for local extrema.
- The theorem add a condition that $f'(x)$ should exist. This raised a question, Can we have a function with local maximum/minimum at some point say $p$ but derivative undefined at the point?
Yes. Consider $f(x) = |x|$ on the interval $[a,b]$ where $a<0<b$. The function clearly has a minimum at $x=0$, but the derivative is undefined at that point.
answered Jan 5 at 10:03


Eevee TrainerEevee Trainer
5,7471936
5,7471936
add a comment |
add a comment |
$begingroup$
Let me try:
1) $f'(x)=0; $
Example:$ f(x)=x^3$, at $x= 0$, inflection point, not an extremum
2) $f(x)= |x|$ , has a local minimum at $x=0.$
$f(x)= -|x|$ has a local maximum at $x=0$
(f'(0) does not exist)(Why?)
$endgroup$
1
$begingroup$
$f'(0)$ does not exist because RHD and LHD are not equal. If we take difference coefficient $f(x)-f(0)/(x-0) $ then depending upon from where we approach $0$ we get two different limits. Thanks a lot.
$endgroup$
– StammeringMathematician
Jan 5 at 10:10
$begingroup$
Stammering.Yes,exactly!!Greetings.
$endgroup$
– Peter Szilas
Jan 5 at 10:14
add a comment |
$begingroup$
Let me try:
1) $f'(x)=0; $
Example:$ f(x)=x^3$, at $x= 0$, inflection point, not an extremum
2) $f(x)= |x|$ , has a local minimum at $x=0.$
$f(x)= -|x|$ has a local maximum at $x=0$
(f'(0) does not exist)(Why?)
$endgroup$
1
$begingroup$
$f'(0)$ does not exist because RHD and LHD are not equal. If we take difference coefficient $f(x)-f(0)/(x-0) $ then depending upon from where we approach $0$ we get two different limits. Thanks a lot.
$endgroup$
– StammeringMathematician
Jan 5 at 10:10
$begingroup$
Stammering.Yes,exactly!!Greetings.
$endgroup$
– Peter Szilas
Jan 5 at 10:14
add a comment |
$begingroup$
Let me try:
1) $f'(x)=0; $
Example:$ f(x)=x^3$, at $x= 0$, inflection point, not an extremum
2) $f(x)= |x|$ , has a local minimum at $x=0.$
$f(x)= -|x|$ has a local maximum at $x=0$
(f'(0) does not exist)(Why?)
$endgroup$
Let me try:
1) $f'(x)=0; $
Example:$ f(x)=x^3$, at $x= 0$, inflection point, not an extremum
2) $f(x)= |x|$ , has a local minimum at $x=0.$
$f(x)= -|x|$ has a local maximum at $x=0$
(f'(0) does not exist)(Why?)
answered Jan 5 at 10:07
Peter SzilasPeter Szilas
11k2821
11k2821
1
$begingroup$
$f'(0)$ does not exist because RHD and LHD are not equal. If we take difference coefficient $f(x)-f(0)/(x-0) $ then depending upon from where we approach $0$ we get two different limits. Thanks a lot.
$endgroup$
– StammeringMathematician
Jan 5 at 10:10
$begingroup$
Stammering.Yes,exactly!!Greetings.
$endgroup$
– Peter Szilas
Jan 5 at 10:14
add a comment |
1
$begingroup$
$f'(0)$ does not exist because RHD and LHD are not equal. If we take difference coefficient $f(x)-f(0)/(x-0) $ then depending upon from where we approach $0$ we get two different limits. Thanks a lot.
$endgroup$
– StammeringMathematician
Jan 5 at 10:10
$begingroup$
Stammering.Yes,exactly!!Greetings.
$endgroup$
– Peter Szilas
Jan 5 at 10:14
1
1
$begingroup$
$f'(0)$ does not exist because RHD and LHD are not equal. If we take difference coefficient $f(x)-f(0)/(x-0) $ then depending upon from where we approach $0$ we get two different limits. Thanks a lot.
$endgroup$
– StammeringMathematician
Jan 5 at 10:10
$begingroup$
$f'(0)$ does not exist because RHD and LHD are not equal. If we take difference coefficient $f(x)-f(0)/(x-0) $ then depending upon from where we approach $0$ we get two different limits. Thanks a lot.
$endgroup$
– StammeringMathematician
Jan 5 at 10:10
$begingroup$
Stammering.Yes,exactly!!Greetings.
$endgroup$
– Peter Szilas
Jan 5 at 10:14
$begingroup$
Stammering.Yes,exactly!!Greetings.
$endgroup$
– Peter Szilas
Jan 5 at 10:14
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%2f3062567%2ftheorem-5-8-baby-rudin-some-questions%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