The smallest compactification for continuous extension of $sin(x)$
I would like to ask what the smallest Hausdorff compactification of $mathbb{R}$ is, onto which we may continuously extend the function $g: mathbb{R} rightarrow mathbb{R}, x mapsto text{sin}(x)$.
A weaker / related version of this question has been asked here, and even that only has a partial answer: Smallest compactification for continuous extension of $sin(x)$
There the answer is about minimal compactifications; I would like to know about a smallest one. I believe, mimicking the answer given there, that the compactification I am looking for is $h=(f,g): mathbb{R} rightarrow [-1,1] times [-1,1], x mapsto (text{tanh}(x),text{sin}(x)) $.
(The first component is just an embedding of $mathbb{R}$ into an interval.) Let's call this compactification $(h,tilde{mathbb{R}})$. It is my candidate for being a smallest compactification onto which we may extend $g(x)=$ sin$(x)$.
SUMMARY: I would like to show that given any compactification $(j, gamma mathbb{R})$ such that the function $g(x)=$ sin$(x)$ extends continuously onto it, there exists a continuous map $k$ from $gamma mathbb{R}$ to $tilde{mathbb{R}}$ such that $kj=h$.
Idea for proof: I'm sure it suffices (and is necessary!) to have that $f(x)= $ tanh$(x)$ extends continuously to $(j, gamma mathbb{R})$, since by assumption $g(x)$ already extends continuously to it. However the idea I had for constructing an extension of $f(x)$ involves using sequential compactness rather than compactness, which is not allowed because in general the two notions don't coincide. For instance, I've tried to define the extension $tilde{f}: gamma mathbb{R} rightarrow [-1,1]$ as being equal to $f$ on the image of $j$ (obviously!), but then the remaining points of $gamma mathbb{R}$ have to go to either $-1$ or $1$ depending on some condition that I've not worked out. I tried to say, let $tilde{f}$ take the value $1$ (respectively $-1$) on $p in gamma mathbb{R} backslash j(mathbb{R})$ if there exists a sequence of distinct real numbers $(x_n)$ such that cos$(x_n)$ tends to $1$ (respectively $-1$) and $j(x_n)$ tends to $p$, but that's not well-defined for every $p$.
general-topology
add a comment |
I would like to ask what the smallest Hausdorff compactification of $mathbb{R}$ is, onto which we may continuously extend the function $g: mathbb{R} rightarrow mathbb{R}, x mapsto text{sin}(x)$.
A weaker / related version of this question has been asked here, and even that only has a partial answer: Smallest compactification for continuous extension of $sin(x)$
There the answer is about minimal compactifications; I would like to know about a smallest one. I believe, mimicking the answer given there, that the compactification I am looking for is $h=(f,g): mathbb{R} rightarrow [-1,1] times [-1,1], x mapsto (text{tanh}(x),text{sin}(x)) $.
(The first component is just an embedding of $mathbb{R}$ into an interval.) Let's call this compactification $(h,tilde{mathbb{R}})$. It is my candidate for being a smallest compactification onto which we may extend $g(x)=$ sin$(x)$.
SUMMARY: I would like to show that given any compactification $(j, gamma mathbb{R})$ such that the function $g(x)=$ sin$(x)$ extends continuously onto it, there exists a continuous map $k$ from $gamma mathbb{R}$ to $tilde{mathbb{R}}$ such that $kj=h$.
Idea for proof: I'm sure it suffices (and is necessary!) to have that $f(x)= $ tanh$(x)$ extends continuously to $(j, gamma mathbb{R})$, since by assumption $g(x)$ already extends continuously to it. However the idea I had for constructing an extension of $f(x)$ involves using sequential compactness rather than compactness, which is not allowed because in general the two notions don't coincide. For instance, I've tried to define the extension $tilde{f}: gamma mathbb{R} rightarrow [-1,1]$ as being equal to $f$ on the image of $j$ (obviously!), but then the remaining points of $gamma mathbb{R}$ have to go to either $-1$ or $1$ depending on some condition that I've not worked out. I tried to say, let $tilde{f}$ take the value $1$ (respectively $-1$) on $p in gamma mathbb{R} backslash j(mathbb{R})$ if there exists a sequence of distinct real numbers $(x_n)$ such that cos$(x_n)$ tends to $1$ (respectively $-1$) and $j(x_n)$ tends to $p$, but that's not well-defined for every $p$.
general-topology
add a comment |
I would like to ask what the smallest Hausdorff compactification of $mathbb{R}$ is, onto which we may continuously extend the function $g: mathbb{R} rightarrow mathbb{R}, x mapsto text{sin}(x)$.
A weaker / related version of this question has been asked here, and even that only has a partial answer: Smallest compactification for continuous extension of $sin(x)$
There the answer is about minimal compactifications; I would like to know about a smallest one. I believe, mimicking the answer given there, that the compactification I am looking for is $h=(f,g): mathbb{R} rightarrow [-1,1] times [-1,1], x mapsto (text{tanh}(x),text{sin}(x)) $.
(The first component is just an embedding of $mathbb{R}$ into an interval.) Let's call this compactification $(h,tilde{mathbb{R}})$. It is my candidate for being a smallest compactification onto which we may extend $g(x)=$ sin$(x)$.
SUMMARY: I would like to show that given any compactification $(j, gamma mathbb{R})$ such that the function $g(x)=$ sin$(x)$ extends continuously onto it, there exists a continuous map $k$ from $gamma mathbb{R}$ to $tilde{mathbb{R}}$ such that $kj=h$.
Idea for proof: I'm sure it suffices (and is necessary!) to have that $f(x)= $ tanh$(x)$ extends continuously to $(j, gamma mathbb{R})$, since by assumption $g(x)$ already extends continuously to it. However the idea I had for constructing an extension of $f(x)$ involves using sequential compactness rather than compactness, which is not allowed because in general the two notions don't coincide. For instance, I've tried to define the extension $tilde{f}: gamma mathbb{R} rightarrow [-1,1]$ as being equal to $f$ on the image of $j$ (obviously!), but then the remaining points of $gamma mathbb{R}$ have to go to either $-1$ or $1$ depending on some condition that I've not worked out. I tried to say, let $tilde{f}$ take the value $1$ (respectively $-1$) on $p in gamma mathbb{R} backslash j(mathbb{R})$ if there exists a sequence of distinct real numbers $(x_n)$ such that cos$(x_n)$ tends to $1$ (respectively $-1$) and $j(x_n)$ tends to $p$, but that's not well-defined for every $p$.
general-topology
I would like to ask what the smallest Hausdorff compactification of $mathbb{R}$ is, onto which we may continuously extend the function $g: mathbb{R} rightarrow mathbb{R}, x mapsto text{sin}(x)$.
A weaker / related version of this question has been asked here, and even that only has a partial answer: Smallest compactification for continuous extension of $sin(x)$
There the answer is about minimal compactifications; I would like to know about a smallest one. I believe, mimicking the answer given there, that the compactification I am looking for is $h=(f,g): mathbb{R} rightarrow [-1,1] times [-1,1], x mapsto (text{tanh}(x),text{sin}(x)) $.
(The first component is just an embedding of $mathbb{R}$ into an interval.) Let's call this compactification $(h,tilde{mathbb{R}})$. It is my candidate for being a smallest compactification onto which we may extend $g(x)=$ sin$(x)$.
SUMMARY: I would like to show that given any compactification $(j, gamma mathbb{R})$ such that the function $g(x)=$ sin$(x)$ extends continuously onto it, there exists a continuous map $k$ from $gamma mathbb{R}$ to $tilde{mathbb{R}}$ such that $kj=h$.
Idea for proof: I'm sure it suffices (and is necessary!) to have that $f(x)= $ tanh$(x)$ extends continuously to $(j, gamma mathbb{R})$, since by assumption $g(x)$ already extends continuously to it. However the idea I had for constructing an extension of $f(x)$ involves using sequential compactness rather than compactness, which is not allowed because in general the two notions don't coincide. For instance, I've tried to define the extension $tilde{f}: gamma mathbb{R} rightarrow [-1,1]$ as being equal to $f$ on the image of $j$ (obviously!), but then the remaining points of $gamma mathbb{R}$ have to go to either $-1$ or $1$ depending on some condition that I've not worked out. I tried to say, let $tilde{f}$ take the value $1$ (respectively $-1$) on $p in gamma mathbb{R} backslash j(mathbb{R})$ if there exists a sequence of distinct real numbers $(x_n)$ such that cos$(x_n)$ tends to $1$ (respectively $-1$) and $j(x_n)$ tends to $p$, but that's not well-defined for every $p$.
general-topology
general-topology
edited Dec 9 at 7:46
user1101010
7571630
7571630
asked Dec 9 at 6:53
SSF
362110
362110
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
Consider the injective map $jcolonBbb Rto Bbb R^3$, $tmapsto (sin t,cos t, sin sqrt 2t)$ and let $gamma Bbb Rsubset Bbb R^2$ be the closure of the image of $j$. Clearly, $gamma Bbb R$ is the cylinder $S^1times [-1,1]$. Then $gamma Bbb Rto Bbb R$, $(x,y,z)mapsto x$ is a continuous extension of the sine function, but we cannot map the cylinder suitably to $tilde{Bbb R}$ because the points $j(2npi)$ and $j(-2npi)$ are both dense in ${(0,1)}times [-1,1]$
Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
– SSF
Dec 9 at 11:32
Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
– SSF
Dec 9 at 11:38
Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
– SSF
Dec 9 at 13:48
@SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
– Henno Brandsma
Dec 9 at 15:32
1
See these notes for some more theory. Warning: exercise E2 is wrong...
– Henno Brandsma
Dec 9 at 15:45
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%2f3032092%2fthe-smallest-compactification-for-continuous-extension-of-sinx%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
Consider the injective map $jcolonBbb Rto Bbb R^3$, $tmapsto (sin t,cos t, sin sqrt 2t)$ and let $gamma Bbb Rsubset Bbb R^2$ be the closure of the image of $j$. Clearly, $gamma Bbb R$ is the cylinder $S^1times [-1,1]$. Then $gamma Bbb Rto Bbb R$, $(x,y,z)mapsto x$ is a continuous extension of the sine function, but we cannot map the cylinder suitably to $tilde{Bbb R}$ because the points $j(2npi)$ and $j(-2npi)$ are both dense in ${(0,1)}times [-1,1]$
Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
– SSF
Dec 9 at 11:32
Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
– SSF
Dec 9 at 11:38
Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
– SSF
Dec 9 at 13:48
@SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
– Henno Brandsma
Dec 9 at 15:32
1
See these notes for some more theory. Warning: exercise E2 is wrong...
– Henno Brandsma
Dec 9 at 15:45
add a comment |
Consider the injective map $jcolonBbb Rto Bbb R^3$, $tmapsto (sin t,cos t, sin sqrt 2t)$ and let $gamma Bbb Rsubset Bbb R^2$ be the closure of the image of $j$. Clearly, $gamma Bbb R$ is the cylinder $S^1times [-1,1]$. Then $gamma Bbb Rto Bbb R$, $(x,y,z)mapsto x$ is a continuous extension of the sine function, but we cannot map the cylinder suitably to $tilde{Bbb R}$ because the points $j(2npi)$ and $j(-2npi)$ are both dense in ${(0,1)}times [-1,1]$
Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
– SSF
Dec 9 at 11:32
Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
– SSF
Dec 9 at 11:38
Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
– SSF
Dec 9 at 13:48
@SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
– Henno Brandsma
Dec 9 at 15:32
1
See these notes for some more theory. Warning: exercise E2 is wrong...
– Henno Brandsma
Dec 9 at 15:45
add a comment |
Consider the injective map $jcolonBbb Rto Bbb R^3$, $tmapsto (sin t,cos t, sin sqrt 2t)$ and let $gamma Bbb Rsubset Bbb R^2$ be the closure of the image of $j$. Clearly, $gamma Bbb R$ is the cylinder $S^1times [-1,1]$. Then $gamma Bbb Rto Bbb R$, $(x,y,z)mapsto x$ is a continuous extension of the sine function, but we cannot map the cylinder suitably to $tilde{Bbb R}$ because the points $j(2npi)$ and $j(-2npi)$ are both dense in ${(0,1)}times [-1,1]$
Consider the injective map $jcolonBbb Rto Bbb R^3$, $tmapsto (sin t,cos t, sin sqrt 2t)$ and let $gamma Bbb Rsubset Bbb R^2$ be the closure of the image of $j$. Clearly, $gamma Bbb R$ is the cylinder $S^1times [-1,1]$. Then $gamma Bbb Rto Bbb R$, $(x,y,z)mapsto x$ is a continuous extension of the sine function, but we cannot map the cylinder suitably to $tilde{Bbb R}$ because the points $j(2npi)$ and $j(-2npi)$ are both dense in ${(0,1)}times [-1,1]$
edited Dec 9 at 8:02
Henno Brandsma
105k346113
105k346113
answered Dec 9 at 7:39
Hagen von Eitzen
276k21268495
276k21268495
Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
– SSF
Dec 9 at 11:32
Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
– SSF
Dec 9 at 11:38
Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
– SSF
Dec 9 at 13:48
@SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
– Henno Brandsma
Dec 9 at 15:32
1
See these notes for some more theory. Warning: exercise E2 is wrong...
– Henno Brandsma
Dec 9 at 15:45
add a comment |
Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
– SSF
Dec 9 at 11:32
Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
– SSF
Dec 9 at 11:38
Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
– SSF
Dec 9 at 13:48
@SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
– Henno Brandsma
Dec 9 at 15:32
1
See these notes for some more theory. Warning: exercise E2 is wrong...
– Henno Brandsma
Dec 9 at 15:45
Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
– SSF
Dec 9 at 11:32
Why does the fact that those points are dense prove the cylinder can't map suitably to $tilde{mathbb{R}}$?
– SSF
Dec 9 at 11:32
Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
– SSF
Dec 9 at 11:38
Is this argument correct: If such a suitable mapping $k$ exists, then $k$ has to map that strip ${(0,1)} times [-1,1]$ to a curve in $tilde{mathbb{R}}$ on which the points $kj(2npi)=h(2npi)$, $kj(-2npi)=h(-2npi)$ are dense. But no such curve exists... (why)?
– SSF
Dec 9 at 11:38
Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
– SSF
Dec 9 at 13:48
Also, what you wrote shows that my guess of a smallest compactification was incorrect, but we still don't know that a smallest one doesn't exist, right?
– SSF
Dec 9 at 13:48
@SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
– Henno Brandsma
Dec 9 at 15:32
@SSF There is no theorem to tell us that a minimal such compactification exists, no. I believe the set of compactifications of a Tychonoff space $X$ always forms a poset for which sups exist. And $beta X$ always is maximal. There is a minimal compactification iff $X$ is locally compact. Beyond this not much is known.
– Henno Brandsma
Dec 9 at 15:32
1
1
See these notes for some more theory. Warning: exercise E2 is wrong...
– Henno Brandsma
Dec 9 at 15:45
See these notes for some more theory. Warning: exercise E2 is wrong...
– Henno Brandsma
Dec 9 at 15:45
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.
Some of your past answers have not been well-received, and you're in danger of being blocked from answering.
Please pay close attention to the following guidance:
- 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.
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%2f3032092%2fthe-smallest-compactification-for-continuous-extension-of-sinx%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