Continuity of the following function
$begingroup$
Let $f:[a,b] to Bbb R$ be continuous . Define $g:[a,b] to Bbb R$ by $$g(x) = sup[f(t) | t in [a,x]].$$
Then is $g$ continuous? I tried to show the inequality $$|g(x)-g(y)| le sup(f(t) | t in [x,y])$$ but wasn’t able to. Any hints or suggestions will be appreciated.
real-analysis calculus continuity supremum-and-infimum
$endgroup$
add a comment |
$begingroup$
Let $f:[a,b] to Bbb R$ be continuous . Define $g:[a,b] to Bbb R$ by $$g(x) = sup[f(t) | t in [a,x]].$$
Then is $g$ continuous? I tried to show the inequality $$|g(x)-g(y)| le sup(f(t) | t in [x,y])$$ but wasn’t able to. Any hints or suggestions will be appreciated.
real-analysis calculus continuity supremum-and-infimum
$endgroup$
1
$begingroup$
Hint: $g$ is monotone increasing, so if there were a discontinuity at $x = x_0$, it would have to be the case that $lim_{x to x_0^-} g(x) < lim_{x to x_0^+} g(x)$. Apply the Intermediate Value Theorem to $f$.
$endgroup$
– Theo Bendit
Oct 29 '18 at 13:47
$begingroup$
Can you elaborate?
$endgroup$
– John Mitchell
Oct 29 '18 at 16:53
add a comment |
$begingroup$
Let $f:[a,b] to Bbb R$ be continuous . Define $g:[a,b] to Bbb R$ by $$g(x) = sup[f(t) | t in [a,x]].$$
Then is $g$ continuous? I tried to show the inequality $$|g(x)-g(y)| le sup(f(t) | t in [x,y])$$ but wasn’t able to. Any hints or suggestions will be appreciated.
real-analysis calculus continuity supremum-and-infimum
$endgroup$
Let $f:[a,b] to Bbb R$ be continuous . Define $g:[a,b] to Bbb R$ by $$g(x) = sup[f(t) | t in [a,x]].$$
Then is $g$ continuous? I tried to show the inequality $$|g(x)-g(y)| le sup(f(t) | t in [x,y])$$ but wasn’t able to. Any hints or suggestions will be appreciated.
real-analysis calculus continuity supremum-and-infimum
real-analysis calculus continuity supremum-and-infimum
edited Jan 8 at 12:42
Davide Giraudo
127k16151264
127k16151264
asked Oct 29 '18 at 13:25
John MitchellJohn Mitchell
377210
377210
1
$begingroup$
Hint: $g$ is monotone increasing, so if there were a discontinuity at $x = x_0$, it would have to be the case that $lim_{x to x_0^-} g(x) < lim_{x to x_0^+} g(x)$. Apply the Intermediate Value Theorem to $f$.
$endgroup$
– Theo Bendit
Oct 29 '18 at 13:47
$begingroup$
Can you elaborate?
$endgroup$
– John Mitchell
Oct 29 '18 at 16:53
add a comment |
1
$begingroup$
Hint: $g$ is monotone increasing, so if there were a discontinuity at $x = x_0$, it would have to be the case that $lim_{x to x_0^-} g(x) < lim_{x to x_0^+} g(x)$. Apply the Intermediate Value Theorem to $f$.
$endgroup$
– Theo Bendit
Oct 29 '18 at 13:47
$begingroup$
Can you elaborate?
$endgroup$
– John Mitchell
Oct 29 '18 at 16:53
1
1
$begingroup$
Hint: $g$ is monotone increasing, so if there were a discontinuity at $x = x_0$, it would have to be the case that $lim_{x to x_0^-} g(x) < lim_{x to x_0^+} g(x)$. Apply the Intermediate Value Theorem to $f$.
$endgroup$
– Theo Bendit
Oct 29 '18 at 13:47
$begingroup$
Hint: $g$ is monotone increasing, so if there were a discontinuity at $x = x_0$, it would have to be the case that $lim_{x to x_0^-} g(x) < lim_{x to x_0^+} g(x)$. Apply the Intermediate Value Theorem to $f$.
$endgroup$
– Theo Bendit
Oct 29 '18 at 13:47
$begingroup$
Can you elaborate?
$endgroup$
– John Mitchell
Oct 29 '18 at 16:53
$begingroup$
Can you elaborate?
$endgroup$
– John Mitchell
Oct 29 '18 at 16:53
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
Let $aleqslant xlt y$. Then
$$
g(y)=maxleft{sup_{aleqslant tleqslant x}leftlvert f(t)rightrvert,sup_{xleqslant tleqslant y}leftlvert f(t)rightrvertright}= maxleft{g(x),sup_{xleqslant tleqslant y}leftlvert f(t)rightrvertright}.
$$
We use the bound
$$
sup_{xleqslant tleqslant y}leftlvert f(t)rightrvert leqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert+ leftlvert f(x)rightrvertleqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert+g(x)
$$
in order to get
$$
g(y)leqslant maxleft{g(x),g(x)+sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvertright}=g(x)+maxleft{0,sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvertright}
$$
and since $g$ is non-decreasing, it follows that
$$
leftlvert g(y)-g(x)rightrvertleqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert.
$$
Now the continuity of $g$ follows from the uniform continuity of $f$: fix a positive $varepsilon$ and choose $delta$ such that if $aleqslant t_1,t_2leqslant b$ are such that $leftlvert t_2-t_1rightrvertlt delta$, then $leftlvert f(t_2)-f(t_1)rightrvertlt varepsilon$.
If $aleqslant xlt yleqslant b$ and $y-xleqslant delta$, the previous inequality shows that $leftlvert g(y)-g(x)rightrvertleqslantvarepsilon$.
$endgroup$
add a comment |
$begingroup$
Your inequality is false. Suppose that $f:[0,1]rightarrow mathbf{R}$ is given by
$$f(x) = 2x-1.$$
then $g(x) = 2x-1$. What is $|g(1/4)-g(3/4)|$ and what is $sup{f(t)mid tin [1/4,3/4]}$?
For a given $x>0$ consider that $exists xi_x$ such that $f(xi_x) = g(x)$ for every $x$ (why is that?). Suppose that $f(x)<f(xi_x)$ then $f(x+delta)<f(xi_x)$ for $|delta|<d$ where $d$ is sufficiently small (why is that?). Then $g$ is constant on $(x-d,x+d)$ and hence continuous.
Suppose now that $f(x) = f(xi_x) = g(x)$, given $varepsilon>0$ pick $delta>0$ such that $|y-x|<delta$ implies that $|f(x)-f(y)|<varepsilon$. What can then be said about $sup{f(t)mid tin [0,x+delta]}$?
$endgroup$
$begingroup$
I think you’ve made a mistake. It should be g is continuous since g is constant.
$endgroup$
– John Mitchell
Oct 30 '18 at 3:03
$begingroup$
Yes it should be g
$endgroup$
– Olof Rubin
Oct 30 '18 at 6:33
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%2f2976140%2fcontinuity-of-the-following-function%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
$begingroup$
Let $aleqslant xlt y$. Then
$$
g(y)=maxleft{sup_{aleqslant tleqslant x}leftlvert f(t)rightrvert,sup_{xleqslant tleqslant y}leftlvert f(t)rightrvertright}= maxleft{g(x),sup_{xleqslant tleqslant y}leftlvert f(t)rightrvertright}.
$$
We use the bound
$$
sup_{xleqslant tleqslant y}leftlvert f(t)rightrvert leqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert+ leftlvert f(x)rightrvertleqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert+g(x)
$$
in order to get
$$
g(y)leqslant maxleft{g(x),g(x)+sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvertright}=g(x)+maxleft{0,sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvertright}
$$
and since $g$ is non-decreasing, it follows that
$$
leftlvert g(y)-g(x)rightrvertleqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert.
$$
Now the continuity of $g$ follows from the uniform continuity of $f$: fix a positive $varepsilon$ and choose $delta$ such that if $aleqslant t_1,t_2leqslant b$ are such that $leftlvert t_2-t_1rightrvertlt delta$, then $leftlvert f(t_2)-f(t_1)rightrvertlt varepsilon$.
If $aleqslant xlt yleqslant b$ and $y-xleqslant delta$, the previous inequality shows that $leftlvert g(y)-g(x)rightrvertleqslantvarepsilon$.
$endgroup$
add a comment |
$begingroup$
Let $aleqslant xlt y$. Then
$$
g(y)=maxleft{sup_{aleqslant tleqslant x}leftlvert f(t)rightrvert,sup_{xleqslant tleqslant y}leftlvert f(t)rightrvertright}= maxleft{g(x),sup_{xleqslant tleqslant y}leftlvert f(t)rightrvertright}.
$$
We use the bound
$$
sup_{xleqslant tleqslant y}leftlvert f(t)rightrvert leqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert+ leftlvert f(x)rightrvertleqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert+g(x)
$$
in order to get
$$
g(y)leqslant maxleft{g(x),g(x)+sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvertright}=g(x)+maxleft{0,sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvertright}
$$
and since $g$ is non-decreasing, it follows that
$$
leftlvert g(y)-g(x)rightrvertleqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert.
$$
Now the continuity of $g$ follows from the uniform continuity of $f$: fix a positive $varepsilon$ and choose $delta$ such that if $aleqslant t_1,t_2leqslant b$ are such that $leftlvert t_2-t_1rightrvertlt delta$, then $leftlvert f(t_2)-f(t_1)rightrvertlt varepsilon$.
If $aleqslant xlt yleqslant b$ and $y-xleqslant delta$, the previous inequality shows that $leftlvert g(y)-g(x)rightrvertleqslantvarepsilon$.
$endgroup$
add a comment |
$begingroup$
Let $aleqslant xlt y$. Then
$$
g(y)=maxleft{sup_{aleqslant tleqslant x}leftlvert f(t)rightrvert,sup_{xleqslant tleqslant y}leftlvert f(t)rightrvertright}= maxleft{g(x),sup_{xleqslant tleqslant y}leftlvert f(t)rightrvertright}.
$$
We use the bound
$$
sup_{xleqslant tleqslant y}leftlvert f(t)rightrvert leqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert+ leftlvert f(x)rightrvertleqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert+g(x)
$$
in order to get
$$
g(y)leqslant maxleft{g(x),g(x)+sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvertright}=g(x)+maxleft{0,sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvertright}
$$
and since $g$ is non-decreasing, it follows that
$$
leftlvert g(y)-g(x)rightrvertleqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert.
$$
Now the continuity of $g$ follows from the uniform continuity of $f$: fix a positive $varepsilon$ and choose $delta$ such that if $aleqslant t_1,t_2leqslant b$ are such that $leftlvert t_2-t_1rightrvertlt delta$, then $leftlvert f(t_2)-f(t_1)rightrvertlt varepsilon$.
If $aleqslant xlt yleqslant b$ and $y-xleqslant delta$, the previous inequality shows that $leftlvert g(y)-g(x)rightrvertleqslantvarepsilon$.
$endgroup$
Let $aleqslant xlt y$. Then
$$
g(y)=maxleft{sup_{aleqslant tleqslant x}leftlvert f(t)rightrvert,sup_{xleqslant tleqslant y}leftlvert f(t)rightrvertright}= maxleft{g(x),sup_{xleqslant tleqslant y}leftlvert f(t)rightrvertright}.
$$
We use the bound
$$
sup_{xleqslant tleqslant y}leftlvert f(t)rightrvert leqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert+ leftlvert f(x)rightrvertleqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert+g(x)
$$
in order to get
$$
g(y)leqslant maxleft{g(x),g(x)+sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvertright}=g(x)+maxleft{0,sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvertright}
$$
and since $g$ is non-decreasing, it follows that
$$
leftlvert g(y)-g(x)rightrvertleqslant sup_{xleqslant tleqslant y}leftlvert f(t)-f(x)rightrvert.
$$
Now the continuity of $g$ follows from the uniform continuity of $f$: fix a positive $varepsilon$ and choose $delta$ such that if $aleqslant t_1,t_2leqslant b$ are such that $leftlvert t_2-t_1rightrvertlt delta$, then $leftlvert f(t_2)-f(t_1)rightrvertlt varepsilon$.
If $aleqslant xlt yleqslant b$ and $y-xleqslant delta$, the previous inequality shows that $leftlvert g(y)-g(x)rightrvertleqslantvarepsilon$.
answered Jan 8 at 11:35
Davide GiraudoDavide Giraudo
127k16151264
127k16151264
add a comment |
add a comment |
$begingroup$
Your inequality is false. Suppose that $f:[0,1]rightarrow mathbf{R}$ is given by
$$f(x) = 2x-1.$$
then $g(x) = 2x-1$. What is $|g(1/4)-g(3/4)|$ and what is $sup{f(t)mid tin [1/4,3/4]}$?
For a given $x>0$ consider that $exists xi_x$ such that $f(xi_x) = g(x)$ for every $x$ (why is that?). Suppose that $f(x)<f(xi_x)$ then $f(x+delta)<f(xi_x)$ for $|delta|<d$ where $d$ is sufficiently small (why is that?). Then $g$ is constant on $(x-d,x+d)$ and hence continuous.
Suppose now that $f(x) = f(xi_x) = g(x)$, given $varepsilon>0$ pick $delta>0$ such that $|y-x|<delta$ implies that $|f(x)-f(y)|<varepsilon$. What can then be said about $sup{f(t)mid tin [0,x+delta]}$?
$endgroup$
$begingroup$
I think you’ve made a mistake. It should be g is continuous since g is constant.
$endgroup$
– John Mitchell
Oct 30 '18 at 3:03
$begingroup$
Yes it should be g
$endgroup$
– Olof Rubin
Oct 30 '18 at 6:33
add a comment |
$begingroup$
Your inequality is false. Suppose that $f:[0,1]rightarrow mathbf{R}$ is given by
$$f(x) = 2x-1.$$
then $g(x) = 2x-1$. What is $|g(1/4)-g(3/4)|$ and what is $sup{f(t)mid tin [1/4,3/4]}$?
For a given $x>0$ consider that $exists xi_x$ such that $f(xi_x) = g(x)$ for every $x$ (why is that?). Suppose that $f(x)<f(xi_x)$ then $f(x+delta)<f(xi_x)$ for $|delta|<d$ where $d$ is sufficiently small (why is that?). Then $g$ is constant on $(x-d,x+d)$ and hence continuous.
Suppose now that $f(x) = f(xi_x) = g(x)$, given $varepsilon>0$ pick $delta>0$ such that $|y-x|<delta$ implies that $|f(x)-f(y)|<varepsilon$. What can then be said about $sup{f(t)mid tin [0,x+delta]}$?
$endgroup$
$begingroup$
I think you’ve made a mistake. It should be g is continuous since g is constant.
$endgroup$
– John Mitchell
Oct 30 '18 at 3:03
$begingroup$
Yes it should be g
$endgroup$
– Olof Rubin
Oct 30 '18 at 6:33
add a comment |
$begingroup$
Your inequality is false. Suppose that $f:[0,1]rightarrow mathbf{R}$ is given by
$$f(x) = 2x-1.$$
then $g(x) = 2x-1$. What is $|g(1/4)-g(3/4)|$ and what is $sup{f(t)mid tin [1/4,3/4]}$?
For a given $x>0$ consider that $exists xi_x$ such that $f(xi_x) = g(x)$ for every $x$ (why is that?). Suppose that $f(x)<f(xi_x)$ then $f(x+delta)<f(xi_x)$ for $|delta|<d$ where $d$ is sufficiently small (why is that?). Then $g$ is constant on $(x-d,x+d)$ and hence continuous.
Suppose now that $f(x) = f(xi_x) = g(x)$, given $varepsilon>0$ pick $delta>0$ such that $|y-x|<delta$ implies that $|f(x)-f(y)|<varepsilon$. What can then be said about $sup{f(t)mid tin [0,x+delta]}$?
$endgroup$
Your inequality is false. Suppose that $f:[0,1]rightarrow mathbf{R}$ is given by
$$f(x) = 2x-1.$$
then $g(x) = 2x-1$. What is $|g(1/4)-g(3/4)|$ and what is $sup{f(t)mid tin [1/4,3/4]}$?
For a given $x>0$ consider that $exists xi_x$ such that $f(xi_x) = g(x)$ for every $x$ (why is that?). Suppose that $f(x)<f(xi_x)$ then $f(x+delta)<f(xi_x)$ for $|delta|<d$ where $d$ is sufficiently small (why is that?). Then $g$ is constant on $(x-d,x+d)$ and hence continuous.
Suppose now that $f(x) = f(xi_x) = g(x)$, given $varepsilon>0$ pick $delta>0$ such that $|y-x|<delta$ implies that $|f(x)-f(y)|<varepsilon$. What can then be said about $sup{f(t)mid tin [0,x+delta]}$?
edited Oct 30 '18 at 8:02
answered Oct 29 '18 at 19:35
Olof RubinOlof Rubin
1,156316
1,156316
$begingroup$
I think you’ve made a mistake. It should be g is continuous since g is constant.
$endgroup$
– John Mitchell
Oct 30 '18 at 3:03
$begingroup$
Yes it should be g
$endgroup$
– Olof Rubin
Oct 30 '18 at 6:33
add a comment |
$begingroup$
I think you’ve made a mistake. It should be g is continuous since g is constant.
$endgroup$
– John Mitchell
Oct 30 '18 at 3:03
$begingroup$
Yes it should be g
$endgroup$
– Olof Rubin
Oct 30 '18 at 6:33
$begingroup$
I think you’ve made a mistake. It should be g is continuous since g is constant.
$endgroup$
– John Mitchell
Oct 30 '18 at 3:03
$begingroup$
I think you’ve made a mistake. It should be g is continuous since g is constant.
$endgroup$
– John Mitchell
Oct 30 '18 at 3:03
$begingroup$
Yes it should be g
$endgroup$
– Olof Rubin
Oct 30 '18 at 6:33
$begingroup$
Yes it should be g
$endgroup$
– Olof Rubin
Oct 30 '18 at 6:33
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%2f2976140%2fcontinuity-of-the-following-function%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
1
$begingroup$
Hint: $g$ is monotone increasing, so if there were a discontinuity at $x = x_0$, it would have to be the case that $lim_{x to x_0^-} g(x) < lim_{x to x_0^+} g(x)$. Apply the Intermediate Value Theorem to $f$.
$endgroup$
– Theo Bendit
Oct 29 '18 at 13:47
$begingroup$
Can you elaborate?
$endgroup$
– John Mitchell
Oct 29 '18 at 16:53