Is the norm $p(x) = max_{t in [0,2]}|x(t)| + left(int_0^1|x(t)|^7right)^{1/7}$ on $C[0,2]$ induced by any...
$begingroup$
I have a norm on space $X = C[0,2]$:
$$p(x) = max_{t in [0,2]}|x(t)| + left(int_0^1|x(t)|^7right)^{1/7}$$
Is that norm induced by any scalar product?
I try to find counterexample for parallelogram identity
$$||f+g||^2 + ||f-g||^2 = 2(||f||^2 + ||g||^2)$$
Why I tried to find it? Because I know the norm
$$d(x) = max_{t in [0,2]}|x(t)|$$
can't be induced by any scalar product. Maybe I should use polar identity: $$(f,g) = frac{1}{4}(||f+g||^2 + ||f-g||^2)$$
And then what to do with the norm, where instead of the integral - the sum? For example on space $C[-1,1]$:
$$k(x) = max_{t in [-1,1]}pi^{2t}|x(t)| + 5left(sum_{k=1}^{infty}frac{left|x(-1+frac{3}{2k})right|^2}{k^5}right)^{1/2}$$
Here, too, nothing happened. I know that a sum can be obtained from the scalar product (if $p=2$), if we consider it as a separate norm. And the maximum is impossible. How to work with such norms, where one of two norm can be induced?
Can you tell me a trick or something? Thank!
P.S (Edit): Maybe there is a chance to somehow prove that it is impossible to induce any norm that already includes a norm that is not inducible by anything?
functional-analysis norm normed-spaces
$endgroup$
|
show 1 more comment
$begingroup$
I have a norm on space $X = C[0,2]$:
$$p(x) = max_{t in [0,2]}|x(t)| + left(int_0^1|x(t)|^7right)^{1/7}$$
Is that norm induced by any scalar product?
I try to find counterexample for parallelogram identity
$$||f+g||^2 + ||f-g||^2 = 2(||f||^2 + ||g||^2)$$
Why I tried to find it? Because I know the norm
$$d(x) = max_{t in [0,2]}|x(t)|$$
can't be induced by any scalar product. Maybe I should use polar identity: $$(f,g) = frac{1}{4}(||f+g||^2 + ||f-g||^2)$$
And then what to do with the norm, where instead of the integral - the sum? For example on space $C[-1,1]$:
$$k(x) = max_{t in [-1,1]}pi^{2t}|x(t)| + 5left(sum_{k=1}^{infty}frac{left|x(-1+frac{3}{2k})right|^2}{k^5}right)^{1/2}$$
Here, too, nothing happened. I know that a sum can be obtained from the scalar product (if $p=2$), if we consider it as a separate norm. And the maximum is impossible. How to work with such norms, where one of two norm can be induced?
Can you tell me a trick or something? Thank!
P.S (Edit): Maybe there is a chance to somehow prove that it is impossible to induce any norm that already includes a norm that is not inducible by anything?
functional-analysis norm normed-spaces
$endgroup$
$begingroup$
Trying to find a counterexample for parallelogram identity is a good idea. And that is not working?
$endgroup$
– mathcounterexamples.net
Jan 19 at 14:32
$begingroup$
I considered such functions that the integral was well considered. I tried about five examples that came to mind, but none of them turned out to be a counterexample.
$endgroup$
– mathmaniac
Jan 19 at 14:34
$begingroup$
It seems to be a counter example, used for max separately: $f = cos(frac{{pi}t}{4})$, $g=sin(frac{{pi}t}{4})$ But the integrals there are heavy, counted them with the help of wolframalpha and it seems like it happened. Maybe there is a chance to somehow prove that it is impossible to induce any norm that already includes a norm that is not inducible by anything?
$endgroup$
– mathmaniac
Jan 19 at 14:51
1
$begingroup$
Surely some polynomials of low degree will be good counterexamples for the parallelogram law.
$endgroup$
– GEdgar
Jan 19 at 15:00
2
$begingroup$
for example $f(t)=1$ and $g(t)=t$.
$endgroup$
– GEdgar
Jan 19 at 15:15
|
show 1 more comment
$begingroup$
I have a norm on space $X = C[0,2]$:
$$p(x) = max_{t in [0,2]}|x(t)| + left(int_0^1|x(t)|^7right)^{1/7}$$
Is that norm induced by any scalar product?
I try to find counterexample for parallelogram identity
$$||f+g||^2 + ||f-g||^2 = 2(||f||^2 + ||g||^2)$$
Why I tried to find it? Because I know the norm
$$d(x) = max_{t in [0,2]}|x(t)|$$
can't be induced by any scalar product. Maybe I should use polar identity: $$(f,g) = frac{1}{4}(||f+g||^2 + ||f-g||^2)$$
And then what to do with the norm, where instead of the integral - the sum? For example on space $C[-1,1]$:
$$k(x) = max_{t in [-1,1]}pi^{2t}|x(t)| + 5left(sum_{k=1}^{infty}frac{left|x(-1+frac{3}{2k})right|^2}{k^5}right)^{1/2}$$
Here, too, nothing happened. I know that a sum can be obtained from the scalar product (if $p=2$), if we consider it as a separate norm. And the maximum is impossible. How to work with such norms, where one of two norm can be induced?
Can you tell me a trick or something? Thank!
P.S (Edit): Maybe there is a chance to somehow prove that it is impossible to induce any norm that already includes a norm that is not inducible by anything?
functional-analysis norm normed-spaces
$endgroup$
I have a norm on space $X = C[0,2]$:
$$p(x) = max_{t in [0,2]}|x(t)| + left(int_0^1|x(t)|^7right)^{1/7}$$
Is that norm induced by any scalar product?
I try to find counterexample for parallelogram identity
$$||f+g||^2 + ||f-g||^2 = 2(||f||^2 + ||g||^2)$$
Why I tried to find it? Because I know the norm
$$d(x) = max_{t in [0,2]}|x(t)|$$
can't be induced by any scalar product. Maybe I should use polar identity: $$(f,g) = frac{1}{4}(||f+g||^2 + ||f-g||^2)$$
And then what to do with the norm, where instead of the integral - the sum? For example on space $C[-1,1]$:
$$k(x) = max_{t in [-1,1]}pi^{2t}|x(t)| + 5left(sum_{k=1}^{infty}frac{left|x(-1+frac{3}{2k})right|^2}{k^5}right)^{1/2}$$
Here, too, nothing happened. I know that a sum can be obtained from the scalar product (if $p=2$), if we consider it as a separate norm. And the maximum is impossible. How to work with such norms, where one of two norm can be induced?
Can you tell me a trick or something? Thank!
P.S (Edit): Maybe there is a chance to somehow prove that it is impossible to induce any norm that already includes a norm that is not inducible by anything?
functional-analysis norm normed-spaces
functional-analysis norm normed-spaces
edited Jan 19 at 15:26
Blue
48.5k870154
48.5k870154
asked Jan 19 at 14:25
mathmaniacmathmaniac
18511
18511
$begingroup$
Trying to find a counterexample for parallelogram identity is a good idea. And that is not working?
$endgroup$
– mathcounterexamples.net
Jan 19 at 14:32
$begingroup$
I considered such functions that the integral was well considered. I tried about five examples that came to mind, but none of them turned out to be a counterexample.
$endgroup$
– mathmaniac
Jan 19 at 14:34
$begingroup$
It seems to be a counter example, used for max separately: $f = cos(frac{{pi}t}{4})$, $g=sin(frac{{pi}t}{4})$ But the integrals there are heavy, counted them with the help of wolframalpha and it seems like it happened. Maybe there is a chance to somehow prove that it is impossible to induce any norm that already includes a norm that is not inducible by anything?
$endgroup$
– mathmaniac
Jan 19 at 14:51
1
$begingroup$
Surely some polynomials of low degree will be good counterexamples for the parallelogram law.
$endgroup$
– GEdgar
Jan 19 at 15:00
2
$begingroup$
for example $f(t)=1$ and $g(t)=t$.
$endgroup$
– GEdgar
Jan 19 at 15:15
|
show 1 more comment
$begingroup$
Trying to find a counterexample for parallelogram identity is a good idea. And that is not working?
$endgroup$
– mathcounterexamples.net
Jan 19 at 14:32
$begingroup$
I considered such functions that the integral was well considered. I tried about five examples that came to mind, but none of them turned out to be a counterexample.
$endgroup$
– mathmaniac
Jan 19 at 14:34
$begingroup$
It seems to be a counter example, used for max separately: $f = cos(frac{{pi}t}{4})$, $g=sin(frac{{pi}t}{4})$ But the integrals there are heavy, counted them with the help of wolframalpha and it seems like it happened. Maybe there is a chance to somehow prove that it is impossible to induce any norm that already includes a norm that is not inducible by anything?
$endgroup$
– mathmaniac
Jan 19 at 14:51
1
$begingroup$
Surely some polynomials of low degree will be good counterexamples for the parallelogram law.
$endgroup$
– GEdgar
Jan 19 at 15:00
2
$begingroup$
for example $f(t)=1$ and $g(t)=t$.
$endgroup$
– GEdgar
Jan 19 at 15:15
$begingroup$
Trying to find a counterexample for parallelogram identity is a good idea. And that is not working?
$endgroup$
– mathcounterexamples.net
Jan 19 at 14:32
$begingroup$
Trying to find a counterexample for parallelogram identity is a good idea. And that is not working?
$endgroup$
– mathcounterexamples.net
Jan 19 at 14:32
$begingroup$
I considered such functions that the integral was well considered. I tried about five examples that came to mind, but none of them turned out to be a counterexample.
$endgroup$
– mathmaniac
Jan 19 at 14:34
$begingroup$
I considered such functions that the integral was well considered. I tried about five examples that came to mind, but none of them turned out to be a counterexample.
$endgroup$
– mathmaniac
Jan 19 at 14:34
$begingroup$
It seems to be a counter example, used for max separately: $f = cos(frac{{pi}t}{4})$, $g=sin(frac{{pi}t}{4})$ But the integrals there are heavy, counted them with the help of wolframalpha and it seems like it happened. Maybe there is a chance to somehow prove that it is impossible to induce any norm that already includes a norm that is not inducible by anything?
$endgroup$
– mathmaniac
Jan 19 at 14:51
$begingroup$
It seems to be a counter example, used for max separately: $f = cos(frac{{pi}t}{4})$, $g=sin(frac{{pi}t}{4})$ But the integrals there are heavy, counted them with the help of wolframalpha and it seems like it happened. Maybe there is a chance to somehow prove that it is impossible to induce any norm that already includes a norm that is not inducible by anything?
$endgroup$
– mathmaniac
Jan 19 at 14:51
1
1
$begingroup$
Surely some polynomials of low degree will be good counterexamples for the parallelogram law.
$endgroup$
– GEdgar
Jan 19 at 15:00
$begingroup$
Surely some polynomials of low degree will be good counterexamples for the parallelogram law.
$endgroup$
– GEdgar
Jan 19 at 15:00
2
2
$begingroup$
for example $f(t)=1$ and $g(t)=t$.
$endgroup$
– GEdgar
Jan 19 at 15:15
$begingroup$
for example $f(t)=1$ and $g(t)=t$.
$endgroup$
– GEdgar
Jan 19 at 15:15
|
show 1 more comment
1 Answer
1
active
oldest
votes
$begingroup$
Consider
$$f(x) = begin{cases}
cos pi x & 0 le x le 1/2\
0 & 1/2 le x le 2
end{cases}$$
and $g(x) = f(2-x)$. You have $f,g in mathcal C([0,2],mathbb R)$. Let
$$I = left(int_0^2|f(t)|^7right)^{frac{1}{7}}=left(int_0^2|g(t)|^7right)^{frac{1}{7}} = frac{1}{2}left(int_0^2|f(t)+g(t)|^7right)^{frac{1}{7}}=frac{1}{2}left(int_0^2|f(t)-g(t)|^7right)^{frac{1}{7}}$$ and notice
that
$$1=max_{t in [0,2]}vert f(t)vert=max_{t in [0,2]}vert g(t)vert=max_{t in [0,2]}vert f(t)+g(t)vert=max_{t in [0,2]}vert f(t)-g(t)vert$$
Hence
$$Vert f+g Vert^2+Vert f-g Vert^2 =2(1+2I)^2 neq 4(1+I)^2= left(Vert f Vert^2 + Vert g Vert^2right)^2$$
as $I < frac{1}{sqrt 2}$.
Which allows to conclude.
Using the parallelogram identity is a general way to decide if a norm is induced by an inner product according to THE JORDAN-VON NEUMANN THEOREM
$endgroup$
$begingroup$
Thanks for the elegant counterexample!
$endgroup$
– mathmaniac
Jan 19 at 15:28
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%2f3079401%2fis-the-norm-px-max-t-in-0-2xt-left-int-01xt7-right1%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Consider
$$f(x) = begin{cases}
cos pi x & 0 le x le 1/2\
0 & 1/2 le x le 2
end{cases}$$
and $g(x) = f(2-x)$. You have $f,g in mathcal C([0,2],mathbb R)$. Let
$$I = left(int_0^2|f(t)|^7right)^{frac{1}{7}}=left(int_0^2|g(t)|^7right)^{frac{1}{7}} = frac{1}{2}left(int_0^2|f(t)+g(t)|^7right)^{frac{1}{7}}=frac{1}{2}left(int_0^2|f(t)-g(t)|^7right)^{frac{1}{7}}$$ and notice
that
$$1=max_{t in [0,2]}vert f(t)vert=max_{t in [0,2]}vert g(t)vert=max_{t in [0,2]}vert f(t)+g(t)vert=max_{t in [0,2]}vert f(t)-g(t)vert$$
Hence
$$Vert f+g Vert^2+Vert f-g Vert^2 =2(1+2I)^2 neq 4(1+I)^2= left(Vert f Vert^2 + Vert g Vert^2right)^2$$
as $I < frac{1}{sqrt 2}$.
Which allows to conclude.
Using the parallelogram identity is a general way to decide if a norm is induced by an inner product according to THE JORDAN-VON NEUMANN THEOREM
$endgroup$
$begingroup$
Thanks for the elegant counterexample!
$endgroup$
– mathmaniac
Jan 19 at 15:28
add a comment |
$begingroup$
Consider
$$f(x) = begin{cases}
cos pi x & 0 le x le 1/2\
0 & 1/2 le x le 2
end{cases}$$
and $g(x) = f(2-x)$. You have $f,g in mathcal C([0,2],mathbb R)$. Let
$$I = left(int_0^2|f(t)|^7right)^{frac{1}{7}}=left(int_0^2|g(t)|^7right)^{frac{1}{7}} = frac{1}{2}left(int_0^2|f(t)+g(t)|^7right)^{frac{1}{7}}=frac{1}{2}left(int_0^2|f(t)-g(t)|^7right)^{frac{1}{7}}$$ and notice
that
$$1=max_{t in [0,2]}vert f(t)vert=max_{t in [0,2]}vert g(t)vert=max_{t in [0,2]}vert f(t)+g(t)vert=max_{t in [0,2]}vert f(t)-g(t)vert$$
Hence
$$Vert f+g Vert^2+Vert f-g Vert^2 =2(1+2I)^2 neq 4(1+I)^2= left(Vert f Vert^2 + Vert g Vert^2right)^2$$
as $I < frac{1}{sqrt 2}$.
Which allows to conclude.
Using the parallelogram identity is a general way to decide if a norm is induced by an inner product according to THE JORDAN-VON NEUMANN THEOREM
$endgroup$
$begingroup$
Thanks for the elegant counterexample!
$endgroup$
– mathmaniac
Jan 19 at 15:28
add a comment |
$begingroup$
Consider
$$f(x) = begin{cases}
cos pi x & 0 le x le 1/2\
0 & 1/2 le x le 2
end{cases}$$
and $g(x) = f(2-x)$. You have $f,g in mathcal C([0,2],mathbb R)$. Let
$$I = left(int_0^2|f(t)|^7right)^{frac{1}{7}}=left(int_0^2|g(t)|^7right)^{frac{1}{7}} = frac{1}{2}left(int_0^2|f(t)+g(t)|^7right)^{frac{1}{7}}=frac{1}{2}left(int_0^2|f(t)-g(t)|^7right)^{frac{1}{7}}$$ and notice
that
$$1=max_{t in [0,2]}vert f(t)vert=max_{t in [0,2]}vert g(t)vert=max_{t in [0,2]}vert f(t)+g(t)vert=max_{t in [0,2]}vert f(t)-g(t)vert$$
Hence
$$Vert f+g Vert^2+Vert f-g Vert^2 =2(1+2I)^2 neq 4(1+I)^2= left(Vert f Vert^2 + Vert g Vert^2right)^2$$
as $I < frac{1}{sqrt 2}$.
Which allows to conclude.
Using the parallelogram identity is a general way to decide if a norm is induced by an inner product according to THE JORDAN-VON NEUMANN THEOREM
$endgroup$
Consider
$$f(x) = begin{cases}
cos pi x & 0 le x le 1/2\
0 & 1/2 le x le 2
end{cases}$$
and $g(x) = f(2-x)$. You have $f,g in mathcal C([0,2],mathbb R)$. Let
$$I = left(int_0^2|f(t)|^7right)^{frac{1}{7}}=left(int_0^2|g(t)|^7right)^{frac{1}{7}} = frac{1}{2}left(int_0^2|f(t)+g(t)|^7right)^{frac{1}{7}}=frac{1}{2}left(int_0^2|f(t)-g(t)|^7right)^{frac{1}{7}}$$ and notice
that
$$1=max_{t in [0,2]}vert f(t)vert=max_{t in [0,2]}vert g(t)vert=max_{t in [0,2]}vert f(t)+g(t)vert=max_{t in [0,2]}vert f(t)-g(t)vert$$
Hence
$$Vert f+g Vert^2+Vert f-g Vert^2 =2(1+2I)^2 neq 4(1+I)^2= left(Vert f Vert^2 + Vert g Vert^2right)^2$$
as $I < frac{1}{sqrt 2}$.
Which allows to conclude.
Using the parallelogram identity is a general way to decide if a norm is induced by an inner product according to THE JORDAN-VON NEUMANN THEOREM
edited Jan 19 at 15:39
answered Jan 19 at 15:23
mathcounterexamples.netmathcounterexamples.net
26.8k22157
26.8k22157
$begingroup$
Thanks for the elegant counterexample!
$endgroup$
– mathmaniac
Jan 19 at 15:28
add a comment |
$begingroup$
Thanks for the elegant counterexample!
$endgroup$
– mathmaniac
Jan 19 at 15:28
$begingroup$
Thanks for the elegant counterexample!
$endgroup$
– mathmaniac
Jan 19 at 15:28
$begingroup$
Thanks for the elegant counterexample!
$endgroup$
– mathmaniac
Jan 19 at 15:28
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%2f3079401%2fis-the-norm-px-max-t-in-0-2xt-left-int-01xt7-right1%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
$begingroup$
Trying to find a counterexample for parallelogram identity is a good idea. And that is not working?
$endgroup$
– mathcounterexamples.net
Jan 19 at 14:32
$begingroup$
I considered such functions that the integral was well considered. I tried about five examples that came to mind, but none of them turned out to be a counterexample.
$endgroup$
– mathmaniac
Jan 19 at 14:34
$begingroup$
It seems to be a counter example, used for max separately: $f = cos(frac{{pi}t}{4})$, $g=sin(frac{{pi}t}{4})$ But the integrals there are heavy, counted them with the help of wolframalpha and it seems like it happened. Maybe there is a chance to somehow prove that it is impossible to induce any norm that already includes a norm that is not inducible by anything?
$endgroup$
– mathmaniac
Jan 19 at 14:51
1
$begingroup$
Surely some polynomials of low degree will be good counterexamples for the parallelogram law.
$endgroup$
– GEdgar
Jan 19 at 15:00
2
$begingroup$
for example $f(t)=1$ and $g(t)=t$.
$endgroup$
– GEdgar
Jan 19 at 15:15