Embedding a compact manifold in $mathbb{R}^N$
$begingroup$
I have an attempt at solving the following problem. This is not so much a question asking for a solution in general, but more on how to complete my own.
Let $M^n$ be a compact smooth manifold. Show that there exists an embedding of $M$ into $mathbb{R}^N$ for some $N$.
(Recall that an embedding of smooth manifolds is a topological embedding such that each differential is injective.)
My attempt:
Since $M$ is compact, we can cover it by finitely many coordinate neighbourhoods $U_i$ for $i=1,dotsc,k$, where $varphi_i : U_i to mathbb{R}^n$ are the corresponding charts. Choose a subordinate (smooth) partition of unity $psi_i : M to mathbb{R}$. Then the functions
$$ f_i := psi_i cdot varphi_i$$
are smooth, where we interpret $varphi$ as being zero outside of the support of $psi$. Now define
$$ F : M to mathbb{R}^{ncdot k + k} : xmapsto (f_1(x),dotsc,f_k(x),psi_1(x),dotsc,psi_k(x)).$$
My hope was that $F$ is an embedding of smooth manifolds. For this, we verify:
Injectivity. This is why the $psi$'s were stuck at the end of the map. If all the $psi(x)$'s are the same, then all the $varphi(x)$'s are the same, but these are local diffeomorphisms, in particular bijections.
Smoothness. Trivial.
Topological embedding. Immediate since $M$ is compact and $F$ is injective and continuous.
Injective differentials. This is my issue.
Are all the differentials injective for $F$ as defined above? Or would we need more assumptions on the covering or of the partition of unity for this to work (or for this to work more easily)?
differential-geometry smooth-manifolds compact-manifolds
$endgroup$
|
show 1 more comment
$begingroup$
I have an attempt at solving the following problem. This is not so much a question asking for a solution in general, but more on how to complete my own.
Let $M^n$ be a compact smooth manifold. Show that there exists an embedding of $M$ into $mathbb{R}^N$ for some $N$.
(Recall that an embedding of smooth manifolds is a topological embedding such that each differential is injective.)
My attempt:
Since $M$ is compact, we can cover it by finitely many coordinate neighbourhoods $U_i$ for $i=1,dotsc,k$, where $varphi_i : U_i to mathbb{R}^n$ are the corresponding charts. Choose a subordinate (smooth) partition of unity $psi_i : M to mathbb{R}$. Then the functions
$$ f_i := psi_i cdot varphi_i$$
are smooth, where we interpret $varphi$ as being zero outside of the support of $psi$. Now define
$$ F : M to mathbb{R}^{ncdot k + k} : xmapsto (f_1(x),dotsc,f_k(x),psi_1(x),dotsc,psi_k(x)).$$
My hope was that $F$ is an embedding of smooth manifolds. For this, we verify:
Injectivity. This is why the $psi$'s were stuck at the end of the map. If all the $psi(x)$'s are the same, then all the $varphi(x)$'s are the same, but these are local diffeomorphisms, in particular bijections.
Smoothness. Trivial.
Topological embedding. Immediate since $M$ is compact and $F$ is injective and continuous.
Injective differentials. This is my issue.
Are all the differentials injective for $F$ as defined above? Or would we need more assumptions on the covering or of the partition of unity for this to work (or for this to work more easily)?
differential-geometry smooth-manifolds compact-manifolds
$endgroup$
$begingroup$
This is whitney lemma
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:26
$begingroup$
@TsemoAristide Which lemma are you referring to? If it's just about embedding a compact manifold in $mathbb{R}^N$, then like I stated, that's not the point of me raising this question.
$endgroup$
– SvanN
Dec 12 '18 at 18:27
$begingroup$
en.m.wikipedia.org/wiki/Whitney_embedding_theorem
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:29
$begingroup$
@TsemoAristide Thanks for the reference, but again, I am asking this question about my own attempted solution, not a general proof.
$endgroup$
– SvanN
Dec 12 '18 at 18:30
$begingroup$
Read the proof of Whitney's theorem, it starts like your idea, then by using sard lemma it can be shown that $M$ can be embedded in $mathbb{R} ^{2n+1}$ where $n$ is the dimension of $M$.
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:43
|
show 1 more comment
$begingroup$
I have an attempt at solving the following problem. This is not so much a question asking for a solution in general, but more on how to complete my own.
Let $M^n$ be a compact smooth manifold. Show that there exists an embedding of $M$ into $mathbb{R}^N$ for some $N$.
(Recall that an embedding of smooth manifolds is a topological embedding such that each differential is injective.)
My attempt:
Since $M$ is compact, we can cover it by finitely many coordinate neighbourhoods $U_i$ for $i=1,dotsc,k$, where $varphi_i : U_i to mathbb{R}^n$ are the corresponding charts. Choose a subordinate (smooth) partition of unity $psi_i : M to mathbb{R}$. Then the functions
$$ f_i := psi_i cdot varphi_i$$
are smooth, where we interpret $varphi$ as being zero outside of the support of $psi$. Now define
$$ F : M to mathbb{R}^{ncdot k + k} : xmapsto (f_1(x),dotsc,f_k(x),psi_1(x),dotsc,psi_k(x)).$$
My hope was that $F$ is an embedding of smooth manifolds. For this, we verify:
Injectivity. This is why the $psi$'s were stuck at the end of the map. If all the $psi(x)$'s are the same, then all the $varphi(x)$'s are the same, but these are local diffeomorphisms, in particular bijections.
Smoothness. Trivial.
Topological embedding. Immediate since $M$ is compact and $F$ is injective and continuous.
Injective differentials. This is my issue.
Are all the differentials injective for $F$ as defined above? Or would we need more assumptions on the covering or of the partition of unity for this to work (or for this to work more easily)?
differential-geometry smooth-manifolds compact-manifolds
$endgroup$
I have an attempt at solving the following problem. This is not so much a question asking for a solution in general, but more on how to complete my own.
Let $M^n$ be a compact smooth manifold. Show that there exists an embedding of $M$ into $mathbb{R}^N$ for some $N$.
(Recall that an embedding of smooth manifolds is a topological embedding such that each differential is injective.)
My attempt:
Since $M$ is compact, we can cover it by finitely many coordinate neighbourhoods $U_i$ for $i=1,dotsc,k$, where $varphi_i : U_i to mathbb{R}^n$ are the corresponding charts. Choose a subordinate (smooth) partition of unity $psi_i : M to mathbb{R}$. Then the functions
$$ f_i := psi_i cdot varphi_i$$
are smooth, where we interpret $varphi$ as being zero outside of the support of $psi$. Now define
$$ F : M to mathbb{R}^{ncdot k + k} : xmapsto (f_1(x),dotsc,f_k(x),psi_1(x),dotsc,psi_k(x)).$$
My hope was that $F$ is an embedding of smooth manifolds. For this, we verify:
Injectivity. This is why the $psi$'s were stuck at the end of the map. If all the $psi(x)$'s are the same, then all the $varphi(x)$'s are the same, but these are local diffeomorphisms, in particular bijections.
Smoothness. Trivial.
Topological embedding. Immediate since $M$ is compact and $F$ is injective and continuous.
Injective differentials. This is my issue.
Are all the differentials injective for $F$ as defined above? Or would we need more assumptions on the covering or of the partition of unity for this to work (or for this to work more easily)?
differential-geometry smooth-manifolds compact-manifolds
differential-geometry smooth-manifolds compact-manifolds
asked Dec 12 '18 at 18:25
SvanNSvanN
2,0661422
2,0661422
$begingroup$
This is whitney lemma
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:26
$begingroup$
@TsemoAristide Which lemma are you referring to? If it's just about embedding a compact manifold in $mathbb{R}^N$, then like I stated, that's not the point of me raising this question.
$endgroup$
– SvanN
Dec 12 '18 at 18:27
$begingroup$
en.m.wikipedia.org/wiki/Whitney_embedding_theorem
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:29
$begingroup$
@TsemoAristide Thanks for the reference, but again, I am asking this question about my own attempted solution, not a general proof.
$endgroup$
– SvanN
Dec 12 '18 at 18:30
$begingroup$
Read the proof of Whitney's theorem, it starts like your idea, then by using sard lemma it can be shown that $M$ can be embedded in $mathbb{R} ^{2n+1}$ where $n$ is the dimension of $M$.
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:43
|
show 1 more comment
$begingroup$
This is whitney lemma
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:26
$begingroup$
@TsemoAristide Which lemma are you referring to? If it's just about embedding a compact manifold in $mathbb{R}^N$, then like I stated, that's not the point of me raising this question.
$endgroup$
– SvanN
Dec 12 '18 at 18:27
$begingroup$
en.m.wikipedia.org/wiki/Whitney_embedding_theorem
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:29
$begingroup$
@TsemoAristide Thanks for the reference, but again, I am asking this question about my own attempted solution, not a general proof.
$endgroup$
– SvanN
Dec 12 '18 at 18:30
$begingroup$
Read the proof of Whitney's theorem, it starts like your idea, then by using sard lemma it can be shown that $M$ can be embedded in $mathbb{R} ^{2n+1}$ where $n$ is the dimension of $M$.
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:43
$begingroup$
This is whitney lemma
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:26
$begingroup$
This is whitney lemma
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:26
$begingroup$
@TsemoAristide Which lemma are you referring to? If it's just about embedding a compact manifold in $mathbb{R}^N$, then like I stated, that's not the point of me raising this question.
$endgroup$
– SvanN
Dec 12 '18 at 18:27
$begingroup$
@TsemoAristide Which lemma are you referring to? If it's just about embedding a compact manifold in $mathbb{R}^N$, then like I stated, that's not the point of me raising this question.
$endgroup$
– SvanN
Dec 12 '18 at 18:27
$begingroup$
en.m.wikipedia.org/wiki/Whitney_embedding_theorem
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:29
$begingroup$
en.m.wikipedia.org/wiki/Whitney_embedding_theorem
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:29
$begingroup$
@TsemoAristide Thanks for the reference, but again, I am asking this question about my own attempted solution, not a general proof.
$endgroup$
– SvanN
Dec 12 '18 at 18:30
$begingroup$
@TsemoAristide Thanks for the reference, but again, I am asking this question about my own attempted solution, not a general proof.
$endgroup$
– SvanN
Dec 12 '18 at 18:30
$begingroup$
Read the proof of Whitney's theorem, it starts like your idea, then by using sard lemma it can be shown that $M$ can be embedded in $mathbb{R} ^{2n+1}$ where $n$ is the dimension of $M$.
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:43
$begingroup$
Read the proof of Whitney's theorem, it starts like your idea, then by using sard lemma it can be shown that $M$ can be embedded in $mathbb{R} ^{2n+1}$ where $n$ is the dimension of $M$.
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:43
|
show 1 more comment
1 Answer
1
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$begingroup$
Let $x in M$ be such that $T_x F$ is not injective. Then there exists some nonzero $X in T_x M$ such that $T_xF(X)=0$.
Let $i$ be such that $psi_i(x)>0$, then $d_x psi_i(X)=0$ (It is a component of $F$) and $T_x(psi_iphi_i)(X)=0$.
Now, since $d_xpsi_i(X)=0$, $0=T_x(psi_iphi_i)(X) = psi_i(x)T_xphi_i(X)$.
Thus, $T_xphi_i(X)=0$, a contradiction.
So your differentials are injective.
$endgroup$
$begingroup$
How does it follow that $nabla_x psi_i =0$? (And I’m presuming that by $nabla psi$ you mean the differential of $psi$?)
$endgroup$
– SvanN
Dec 12 '18 at 20:24
$begingroup$
Edited, I hope I answered your concern.
$endgroup$
– Mindlack
Dec 12 '18 at 20:31
$begingroup$
You did, yes :) Thank you!
$endgroup$
– SvanN
Dec 12 '18 at 20:49
add a comment |
Your Answer
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1 Answer
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1 Answer
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$begingroup$
Let $x in M$ be such that $T_x F$ is not injective. Then there exists some nonzero $X in T_x M$ such that $T_xF(X)=0$.
Let $i$ be such that $psi_i(x)>0$, then $d_x psi_i(X)=0$ (It is a component of $F$) and $T_x(psi_iphi_i)(X)=0$.
Now, since $d_xpsi_i(X)=0$, $0=T_x(psi_iphi_i)(X) = psi_i(x)T_xphi_i(X)$.
Thus, $T_xphi_i(X)=0$, a contradiction.
So your differentials are injective.
$endgroup$
$begingroup$
How does it follow that $nabla_x psi_i =0$? (And I’m presuming that by $nabla psi$ you mean the differential of $psi$?)
$endgroup$
– SvanN
Dec 12 '18 at 20:24
$begingroup$
Edited, I hope I answered your concern.
$endgroup$
– Mindlack
Dec 12 '18 at 20:31
$begingroup$
You did, yes :) Thank you!
$endgroup$
– SvanN
Dec 12 '18 at 20:49
add a comment |
$begingroup$
Let $x in M$ be such that $T_x F$ is not injective. Then there exists some nonzero $X in T_x M$ such that $T_xF(X)=0$.
Let $i$ be such that $psi_i(x)>0$, then $d_x psi_i(X)=0$ (It is a component of $F$) and $T_x(psi_iphi_i)(X)=0$.
Now, since $d_xpsi_i(X)=0$, $0=T_x(psi_iphi_i)(X) = psi_i(x)T_xphi_i(X)$.
Thus, $T_xphi_i(X)=0$, a contradiction.
So your differentials are injective.
$endgroup$
$begingroup$
How does it follow that $nabla_x psi_i =0$? (And I’m presuming that by $nabla psi$ you mean the differential of $psi$?)
$endgroup$
– SvanN
Dec 12 '18 at 20:24
$begingroup$
Edited, I hope I answered your concern.
$endgroup$
– Mindlack
Dec 12 '18 at 20:31
$begingroup$
You did, yes :) Thank you!
$endgroup$
– SvanN
Dec 12 '18 at 20:49
add a comment |
$begingroup$
Let $x in M$ be such that $T_x F$ is not injective. Then there exists some nonzero $X in T_x M$ such that $T_xF(X)=0$.
Let $i$ be such that $psi_i(x)>0$, then $d_x psi_i(X)=0$ (It is a component of $F$) and $T_x(psi_iphi_i)(X)=0$.
Now, since $d_xpsi_i(X)=0$, $0=T_x(psi_iphi_i)(X) = psi_i(x)T_xphi_i(X)$.
Thus, $T_xphi_i(X)=0$, a contradiction.
So your differentials are injective.
$endgroup$
Let $x in M$ be such that $T_x F$ is not injective. Then there exists some nonzero $X in T_x M$ such that $T_xF(X)=0$.
Let $i$ be such that $psi_i(x)>0$, then $d_x psi_i(X)=0$ (It is a component of $F$) and $T_x(psi_iphi_i)(X)=0$.
Now, since $d_xpsi_i(X)=0$, $0=T_x(psi_iphi_i)(X) = psi_i(x)T_xphi_i(X)$.
Thus, $T_xphi_i(X)=0$, a contradiction.
So your differentials are injective.
edited Dec 12 '18 at 20:30
answered Dec 12 '18 at 20:21
MindlackMindlack
4,760210
4,760210
$begingroup$
How does it follow that $nabla_x psi_i =0$? (And I’m presuming that by $nabla psi$ you mean the differential of $psi$?)
$endgroup$
– SvanN
Dec 12 '18 at 20:24
$begingroup$
Edited, I hope I answered your concern.
$endgroup$
– Mindlack
Dec 12 '18 at 20:31
$begingroup$
You did, yes :) Thank you!
$endgroup$
– SvanN
Dec 12 '18 at 20:49
add a comment |
$begingroup$
How does it follow that $nabla_x psi_i =0$? (And I’m presuming that by $nabla psi$ you mean the differential of $psi$?)
$endgroup$
– SvanN
Dec 12 '18 at 20:24
$begingroup$
Edited, I hope I answered your concern.
$endgroup$
– Mindlack
Dec 12 '18 at 20:31
$begingroup$
You did, yes :) Thank you!
$endgroup$
– SvanN
Dec 12 '18 at 20:49
$begingroup$
How does it follow that $nabla_x psi_i =0$? (And I’m presuming that by $nabla psi$ you mean the differential of $psi$?)
$endgroup$
– SvanN
Dec 12 '18 at 20:24
$begingroup$
How does it follow that $nabla_x psi_i =0$? (And I’m presuming that by $nabla psi$ you mean the differential of $psi$?)
$endgroup$
– SvanN
Dec 12 '18 at 20:24
$begingroup$
Edited, I hope I answered your concern.
$endgroup$
– Mindlack
Dec 12 '18 at 20:31
$begingroup$
Edited, I hope I answered your concern.
$endgroup$
– Mindlack
Dec 12 '18 at 20:31
$begingroup$
You did, yes :) Thank you!
$endgroup$
– SvanN
Dec 12 '18 at 20:49
$begingroup$
You did, yes :) Thank you!
$endgroup$
– SvanN
Dec 12 '18 at 20:49
add a comment |
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$begingroup$
This is whitney lemma
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:26
$begingroup$
@TsemoAristide Which lemma are you referring to? If it's just about embedding a compact manifold in $mathbb{R}^N$, then like I stated, that's not the point of me raising this question.
$endgroup$
– SvanN
Dec 12 '18 at 18:27
$begingroup$
en.m.wikipedia.org/wiki/Whitney_embedding_theorem
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:29
$begingroup$
@TsemoAristide Thanks for the reference, but again, I am asking this question about my own attempted solution, not a general proof.
$endgroup$
– SvanN
Dec 12 '18 at 18:30
$begingroup$
Read the proof of Whitney's theorem, it starts like your idea, then by using sard lemma it can be shown that $M$ can be embedded in $mathbb{R} ^{2n+1}$ where $n$ is the dimension of $M$.
$endgroup$
– Tsemo Aristide
Dec 12 '18 at 18:43