Continuous functions of three variables as superpositions of two variable functions
$begingroup$
Could we always locally represent a continuous function $F(x,y,z)$ in the form of $gleft(f(x,y),zright)$ for suitable continuous functions $f$, $g$ of two variables? I am aware of Vladimir Arnold's work on this problem, but it seems that in that context $F(x,y,z)$ is written as a sum of several expressions of this form. Can one reduce it to just a single superposition $gleft(f(x,y),zright)$; or does anyone know a counter example?
real-analysis
$endgroup$
add a comment |
$begingroup$
Could we always locally represent a continuous function $F(x,y,z)$ in the form of $gleft(f(x,y),zright)$ for suitable continuous functions $f$, $g$ of two variables? I am aware of Vladimir Arnold's work on this problem, but it seems that in that context $F(x,y,z)$ is written as a sum of several expressions of this form. Can one reduce it to just a single superposition $gleft(f(x,y),zright)$; or does anyone know a counter example?
real-analysis
$endgroup$
add a comment |
$begingroup$
Could we always locally represent a continuous function $F(x,y,z)$ in the form of $gleft(f(x,y),zright)$ for suitable continuous functions $f$, $g$ of two variables? I am aware of Vladimir Arnold's work on this problem, but it seems that in that context $F(x,y,z)$ is written as a sum of several expressions of this form. Can one reduce it to just a single superposition $gleft(f(x,y),zright)$; or does anyone know a counter example?
real-analysis
$endgroup$
Could we always locally represent a continuous function $F(x,y,z)$ in the form of $gleft(f(x,y),zright)$ for suitable continuous functions $f$, $g$ of two variables? I am aware of Vladimir Arnold's work on this problem, but it seems that in that context $F(x,y,z)$ is written as a sum of several expressions of this form. Can one reduce it to just a single superposition $gleft(f(x,y),zright)$; or does anyone know a counter example?
real-analysis
real-analysis
asked 4 hours ago
KhashFKhashF
986
986
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
Example. The function $F(x,y,z)=x(1-z)+yz$ cannot be represented as $F(x,y,z)=g(f(x,y),z)$.
Proof. Suppose to the contrary that we have such a representation. Let $g_1(t)=g(t,0)$. Then $g_1(f(x,y))=g(f(x,y),0)=F(x,y,0)=x$. That implies that for every $y$, $g_1$ is the inverse function of $xmapsto f(x,y)$ so
$f(x,y)=g_1^{-1}(x)$.
Let $g_2(t)=g(t,1)$. Then $g_2(f(x,y))=g(f(x,y),1)=F(x,y,1)=y$. That implies that for every $x$, $g_2$ is the inverse function of $ymapsto f(x,y)$ so $f(x,y)=g_2^{-1}(y)$.
Let $c=f(0,0)$. Then
$$
f(x,y)=g_1^{-1}(x)=f(x,0)=g_2^{-1}(0)=f(0,0)=c.
$$
Therefore
$$
x(1-z)+yz=F(x,y,z)=g(f(x,y),z)=g(c,z)
$$
which is a contradiction, because the function on the left hand side depends on $x$ and $y$, while the function on the right hand side does not. $Box$
Edit: I modified my proof using a suggestion of user44191.
$endgroup$
$begingroup$
Thanks Piotr! But how did you deduce that $f$ has derivative? Does your example work in the continuous setting as well?
$endgroup$
– KhashF
3 hours ago
$begingroup$
Partial derivatives aren't necessary for this proof; if you let $c = f(0, 0)$, you get that $f(x, y) = g_1^{-1}(x) = f(x, 0) = g_2^{-1}(0) = f(0, 0) = c$.
$endgroup$
– user44191
3 hours ago
$begingroup$
@user44191 Thank you. I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
$begingroup$
@KhashF I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
1
$begingroup$
I’m sorry but where you used continuity of $f$ and $g$? Because without it answer to question in OP is that every function can be represented in such a manner (via bijection between $mathbb{R}$ and $mathbb{R}^2$). And also what is $g_1^{-1}(x)$ really? I see no reasons for $g_1$ to be injective.
$endgroup$
– Aleksei Kulikov
2 hours ago
|
show 1 more 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: "504"
};
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%2fmathoverflow.net%2fquestions%2f322184%2fcontinuous-functions-of-three-variables-as-superpositions-of-two-variable-functi%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
$begingroup$
Example. The function $F(x,y,z)=x(1-z)+yz$ cannot be represented as $F(x,y,z)=g(f(x,y),z)$.
Proof. Suppose to the contrary that we have such a representation. Let $g_1(t)=g(t,0)$. Then $g_1(f(x,y))=g(f(x,y),0)=F(x,y,0)=x$. That implies that for every $y$, $g_1$ is the inverse function of $xmapsto f(x,y)$ so
$f(x,y)=g_1^{-1}(x)$.
Let $g_2(t)=g(t,1)$. Then $g_2(f(x,y))=g(f(x,y),1)=F(x,y,1)=y$. That implies that for every $x$, $g_2$ is the inverse function of $ymapsto f(x,y)$ so $f(x,y)=g_2^{-1}(y)$.
Let $c=f(0,0)$. Then
$$
f(x,y)=g_1^{-1}(x)=f(x,0)=g_2^{-1}(0)=f(0,0)=c.
$$
Therefore
$$
x(1-z)+yz=F(x,y,z)=g(f(x,y),z)=g(c,z)
$$
which is a contradiction, because the function on the left hand side depends on $x$ and $y$, while the function on the right hand side does not. $Box$
Edit: I modified my proof using a suggestion of user44191.
$endgroup$
$begingroup$
Thanks Piotr! But how did you deduce that $f$ has derivative? Does your example work in the continuous setting as well?
$endgroup$
– KhashF
3 hours ago
$begingroup$
Partial derivatives aren't necessary for this proof; if you let $c = f(0, 0)$, you get that $f(x, y) = g_1^{-1}(x) = f(x, 0) = g_2^{-1}(0) = f(0, 0) = c$.
$endgroup$
– user44191
3 hours ago
$begingroup$
@user44191 Thank you. I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
$begingroup$
@KhashF I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
1
$begingroup$
I’m sorry but where you used continuity of $f$ and $g$? Because without it answer to question in OP is that every function can be represented in such a manner (via bijection between $mathbb{R}$ and $mathbb{R}^2$). And also what is $g_1^{-1}(x)$ really? I see no reasons for $g_1$ to be injective.
$endgroup$
– Aleksei Kulikov
2 hours ago
|
show 1 more comment
$begingroup$
Example. The function $F(x,y,z)=x(1-z)+yz$ cannot be represented as $F(x,y,z)=g(f(x,y),z)$.
Proof. Suppose to the contrary that we have such a representation. Let $g_1(t)=g(t,0)$. Then $g_1(f(x,y))=g(f(x,y),0)=F(x,y,0)=x$. That implies that for every $y$, $g_1$ is the inverse function of $xmapsto f(x,y)$ so
$f(x,y)=g_1^{-1}(x)$.
Let $g_2(t)=g(t,1)$. Then $g_2(f(x,y))=g(f(x,y),1)=F(x,y,1)=y$. That implies that for every $x$, $g_2$ is the inverse function of $ymapsto f(x,y)$ so $f(x,y)=g_2^{-1}(y)$.
Let $c=f(0,0)$. Then
$$
f(x,y)=g_1^{-1}(x)=f(x,0)=g_2^{-1}(0)=f(0,0)=c.
$$
Therefore
$$
x(1-z)+yz=F(x,y,z)=g(f(x,y),z)=g(c,z)
$$
which is a contradiction, because the function on the left hand side depends on $x$ and $y$, while the function on the right hand side does not. $Box$
Edit: I modified my proof using a suggestion of user44191.
$endgroup$
$begingroup$
Thanks Piotr! But how did you deduce that $f$ has derivative? Does your example work in the continuous setting as well?
$endgroup$
– KhashF
3 hours ago
$begingroup$
Partial derivatives aren't necessary for this proof; if you let $c = f(0, 0)$, you get that $f(x, y) = g_1^{-1}(x) = f(x, 0) = g_2^{-1}(0) = f(0, 0) = c$.
$endgroup$
– user44191
3 hours ago
$begingroup$
@user44191 Thank you. I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
$begingroup$
@KhashF I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
1
$begingroup$
I’m sorry but where you used continuity of $f$ and $g$? Because without it answer to question in OP is that every function can be represented in such a manner (via bijection between $mathbb{R}$ and $mathbb{R}^2$). And also what is $g_1^{-1}(x)$ really? I see no reasons for $g_1$ to be injective.
$endgroup$
– Aleksei Kulikov
2 hours ago
|
show 1 more comment
$begingroup$
Example. The function $F(x,y,z)=x(1-z)+yz$ cannot be represented as $F(x,y,z)=g(f(x,y),z)$.
Proof. Suppose to the contrary that we have such a representation. Let $g_1(t)=g(t,0)$. Then $g_1(f(x,y))=g(f(x,y),0)=F(x,y,0)=x$. That implies that for every $y$, $g_1$ is the inverse function of $xmapsto f(x,y)$ so
$f(x,y)=g_1^{-1}(x)$.
Let $g_2(t)=g(t,1)$. Then $g_2(f(x,y))=g(f(x,y),1)=F(x,y,1)=y$. That implies that for every $x$, $g_2$ is the inverse function of $ymapsto f(x,y)$ so $f(x,y)=g_2^{-1}(y)$.
Let $c=f(0,0)$. Then
$$
f(x,y)=g_1^{-1}(x)=f(x,0)=g_2^{-1}(0)=f(0,0)=c.
$$
Therefore
$$
x(1-z)+yz=F(x,y,z)=g(f(x,y),z)=g(c,z)
$$
which is a contradiction, because the function on the left hand side depends on $x$ and $y$, while the function on the right hand side does not. $Box$
Edit: I modified my proof using a suggestion of user44191.
$endgroup$
Example. The function $F(x,y,z)=x(1-z)+yz$ cannot be represented as $F(x,y,z)=g(f(x,y),z)$.
Proof. Suppose to the contrary that we have such a representation. Let $g_1(t)=g(t,0)$. Then $g_1(f(x,y))=g(f(x,y),0)=F(x,y,0)=x$. That implies that for every $y$, $g_1$ is the inverse function of $xmapsto f(x,y)$ so
$f(x,y)=g_1^{-1}(x)$.
Let $g_2(t)=g(t,1)$. Then $g_2(f(x,y))=g(f(x,y),1)=F(x,y,1)=y$. That implies that for every $x$, $g_2$ is the inverse function of $ymapsto f(x,y)$ so $f(x,y)=g_2^{-1}(y)$.
Let $c=f(0,0)$. Then
$$
f(x,y)=g_1^{-1}(x)=f(x,0)=g_2^{-1}(0)=f(0,0)=c.
$$
Therefore
$$
x(1-z)+yz=F(x,y,z)=g(f(x,y),z)=g(c,z)
$$
which is a contradiction, because the function on the left hand side depends on $x$ and $y$, while the function on the right hand side does not. $Box$
Edit: I modified my proof using a suggestion of user44191.
edited 3 hours ago
answered 3 hours ago
Piotr HajlaszPiotr Hajlasz
8,18942862
8,18942862
$begingroup$
Thanks Piotr! But how did you deduce that $f$ has derivative? Does your example work in the continuous setting as well?
$endgroup$
– KhashF
3 hours ago
$begingroup$
Partial derivatives aren't necessary for this proof; if you let $c = f(0, 0)$, you get that $f(x, y) = g_1^{-1}(x) = f(x, 0) = g_2^{-1}(0) = f(0, 0) = c$.
$endgroup$
– user44191
3 hours ago
$begingroup$
@user44191 Thank you. I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
$begingroup$
@KhashF I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
1
$begingroup$
I’m sorry but where you used continuity of $f$ and $g$? Because without it answer to question in OP is that every function can be represented in such a manner (via bijection between $mathbb{R}$ and $mathbb{R}^2$). And also what is $g_1^{-1}(x)$ really? I see no reasons for $g_1$ to be injective.
$endgroup$
– Aleksei Kulikov
2 hours ago
|
show 1 more comment
$begingroup$
Thanks Piotr! But how did you deduce that $f$ has derivative? Does your example work in the continuous setting as well?
$endgroup$
– KhashF
3 hours ago
$begingroup$
Partial derivatives aren't necessary for this proof; if you let $c = f(0, 0)$, you get that $f(x, y) = g_1^{-1}(x) = f(x, 0) = g_2^{-1}(0) = f(0, 0) = c$.
$endgroup$
– user44191
3 hours ago
$begingroup$
@user44191 Thank you. I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
$begingroup$
@KhashF I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
1
$begingroup$
I’m sorry but where you used continuity of $f$ and $g$? Because without it answer to question in OP is that every function can be represented in such a manner (via bijection between $mathbb{R}$ and $mathbb{R}^2$). And also what is $g_1^{-1}(x)$ really? I see no reasons for $g_1$ to be injective.
$endgroup$
– Aleksei Kulikov
2 hours ago
$begingroup$
Thanks Piotr! But how did you deduce that $f$ has derivative? Does your example work in the continuous setting as well?
$endgroup$
– KhashF
3 hours ago
$begingroup$
Thanks Piotr! But how did you deduce that $f$ has derivative? Does your example work in the continuous setting as well?
$endgroup$
– KhashF
3 hours ago
$begingroup$
Partial derivatives aren't necessary for this proof; if you let $c = f(0, 0)$, you get that $f(x, y) = g_1^{-1}(x) = f(x, 0) = g_2^{-1}(0) = f(0, 0) = c$.
$endgroup$
– user44191
3 hours ago
$begingroup$
Partial derivatives aren't necessary for this proof; if you let $c = f(0, 0)$, you get that $f(x, y) = g_1^{-1}(x) = f(x, 0) = g_2^{-1}(0) = f(0, 0) = c$.
$endgroup$
– user44191
3 hours ago
$begingroup$
@user44191 Thank you. I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
$begingroup$
@user44191 Thank you. I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
$begingroup$
@KhashF I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
$begingroup$
@KhashF I modified my answer.
$endgroup$
– Piotr Hajlasz
3 hours ago
1
1
$begingroup$
I’m sorry but where you used continuity of $f$ and $g$? Because without it answer to question in OP is that every function can be represented in such a manner (via bijection between $mathbb{R}$ and $mathbb{R}^2$). And also what is $g_1^{-1}(x)$ really? I see no reasons for $g_1$ to be injective.
$endgroup$
– Aleksei Kulikov
2 hours ago
$begingroup$
I’m sorry but where you used continuity of $f$ and $g$? Because without it answer to question in OP is that every function can be represented in such a manner (via bijection between $mathbb{R}$ and $mathbb{R}^2$). And also what is $g_1^{-1}(x)$ really? I see no reasons for $g_1$ to be injective.
$endgroup$
– Aleksei Kulikov
2 hours ago
|
show 1 more comment
Thanks for contributing an answer to MathOverflow!
- 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%2fmathoverflow.net%2fquestions%2f322184%2fcontinuous-functions-of-three-variables-as-superpositions-of-two-variable-functi%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