$C^{infty}(mathbb{R})$ as a Fréchet space
I think I'm misunderstanding what is being asked of me in the following question:
The space $C^{infty}(mathbb{R})$ has a Fréchet space topology with respect to which $f_n rightarrow f$ iff $f^{(k)}_n rightarrow f^{(k)}$ uniformly on all compact subsets ($k$, non-negative, is the order of derivative).
To show the backwards implication I need to show that convergence ($f^{(k)}_n rightarrow f^{(k)}$ uniformly on all compact subsets) give me seminorms, there are countably many such semi norms, and these seminorms give me a complete Hausdorff TVS. My book gives me some facts to help me in this direction.
I'm confused by the forward implication though - if endow the space of $C^{infty}(mathbb{R})$ with a topology given by countable seminorms which makes $C^{infty}(mathbb{R})$ a complete Hausdorff TVS -- how do I know this implies $f^{(k)}_n rightarrow f^{(k)}$ uniformly on all compact subsets? What if the seminorms are really strange and have nothing to do with the derivatives of $f$ over compact subsets?
real-analysis general-topology functional-analysis topological-vector-spaces
add a comment |
I think I'm misunderstanding what is being asked of me in the following question:
The space $C^{infty}(mathbb{R})$ has a Fréchet space topology with respect to which $f_n rightarrow f$ iff $f^{(k)}_n rightarrow f^{(k)}$ uniformly on all compact subsets ($k$, non-negative, is the order of derivative).
To show the backwards implication I need to show that convergence ($f^{(k)}_n rightarrow f^{(k)}$ uniformly on all compact subsets) give me seminorms, there are countably many such semi norms, and these seminorms give me a complete Hausdorff TVS. My book gives me some facts to help me in this direction.
I'm confused by the forward implication though - if endow the space of $C^{infty}(mathbb{R})$ with a topology given by countable seminorms which makes $C^{infty}(mathbb{R})$ a complete Hausdorff TVS -- how do I know this implies $f^{(k)}_n rightarrow f^{(k)}$ uniformly on all compact subsets? What if the seminorms are really strange and have nothing to do with the derivatives of $f$ over compact subsets?
real-analysis general-topology functional-analysis topological-vector-spaces
1
The seminirms are $p_{n,k}(f):= sup_{[-k,k]} |f^{(n)}|$. The key is that any compact set is contained in some interval $[-k,k]$
– Tito Eliatron
Nov 30 at 16:58
I see that, but that's for a particular choice of seminorms. Why should it be true for any arbitrary countable family of seminorms though?
– yoshi
Nov 30 at 17:21
1
For fixed k,n,m, The set ${p_{n,k}(f,0)<m}$ is open, so for your Family of seminorms, you can put a p- Bail inside.
– Tito Eliatron
Nov 30 at 17:31
Can you elaborate on this statement, what is a p-ball and why is it suffient for one p-ball to be inside?
– yoshi
Dec 1 at 15:20
add a comment |
I think I'm misunderstanding what is being asked of me in the following question:
The space $C^{infty}(mathbb{R})$ has a Fréchet space topology with respect to which $f_n rightarrow f$ iff $f^{(k)}_n rightarrow f^{(k)}$ uniformly on all compact subsets ($k$, non-negative, is the order of derivative).
To show the backwards implication I need to show that convergence ($f^{(k)}_n rightarrow f^{(k)}$ uniformly on all compact subsets) give me seminorms, there are countably many such semi norms, and these seminorms give me a complete Hausdorff TVS. My book gives me some facts to help me in this direction.
I'm confused by the forward implication though - if endow the space of $C^{infty}(mathbb{R})$ with a topology given by countable seminorms which makes $C^{infty}(mathbb{R})$ a complete Hausdorff TVS -- how do I know this implies $f^{(k)}_n rightarrow f^{(k)}$ uniformly on all compact subsets? What if the seminorms are really strange and have nothing to do with the derivatives of $f$ over compact subsets?
real-analysis general-topology functional-analysis topological-vector-spaces
I think I'm misunderstanding what is being asked of me in the following question:
The space $C^{infty}(mathbb{R})$ has a Fréchet space topology with respect to which $f_n rightarrow f$ iff $f^{(k)}_n rightarrow f^{(k)}$ uniformly on all compact subsets ($k$, non-negative, is the order of derivative).
To show the backwards implication I need to show that convergence ($f^{(k)}_n rightarrow f^{(k)}$ uniformly on all compact subsets) give me seminorms, there are countably many such semi norms, and these seminorms give me a complete Hausdorff TVS. My book gives me some facts to help me in this direction.
I'm confused by the forward implication though - if endow the space of $C^{infty}(mathbb{R})$ with a topology given by countable seminorms which makes $C^{infty}(mathbb{R})$ a complete Hausdorff TVS -- how do I know this implies $f^{(k)}_n rightarrow f^{(k)}$ uniformly on all compact subsets? What if the seminorms are really strange and have nothing to do with the derivatives of $f$ over compact subsets?
real-analysis general-topology functional-analysis topological-vector-spaces
real-analysis general-topology functional-analysis topological-vector-spaces
edited Nov 30 at 17:37
Bernard
118k638111
118k638111
asked Nov 30 at 16:27
yoshi
1,170817
1,170817
1
The seminirms are $p_{n,k}(f):= sup_{[-k,k]} |f^{(n)}|$. The key is that any compact set is contained in some interval $[-k,k]$
– Tito Eliatron
Nov 30 at 16:58
I see that, but that's for a particular choice of seminorms. Why should it be true for any arbitrary countable family of seminorms though?
– yoshi
Nov 30 at 17:21
1
For fixed k,n,m, The set ${p_{n,k}(f,0)<m}$ is open, so for your Family of seminorms, you can put a p- Bail inside.
– Tito Eliatron
Nov 30 at 17:31
Can you elaborate on this statement, what is a p-ball and why is it suffient for one p-ball to be inside?
– yoshi
Dec 1 at 15:20
add a comment |
1
The seminirms are $p_{n,k}(f):= sup_{[-k,k]} |f^{(n)}|$. The key is that any compact set is contained in some interval $[-k,k]$
– Tito Eliatron
Nov 30 at 16:58
I see that, but that's for a particular choice of seminorms. Why should it be true for any arbitrary countable family of seminorms though?
– yoshi
Nov 30 at 17:21
1
For fixed k,n,m, The set ${p_{n,k}(f,0)<m}$ is open, so for your Family of seminorms, you can put a p- Bail inside.
– Tito Eliatron
Nov 30 at 17:31
Can you elaborate on this statement, what is a p-ball and why is it suffient for one p-ball to be inside?
– yoshi
Dec 1 at 15:20
1
1
The seminirms are $p_{n,k}(f):= sup_{[-k,k]} |f^{(n)}|$. The key is that any compact set is contained in some interval $[-k,k]$
– Tito Eliatron
Nov 30 at 16:58
The seminirms are $p_{n,k}(f):= sup_{[-k,k]} |f^{(n)}|$. The key is that any compact set is contained in some interval $[-k,k]$
– Tito Eliatron
Nov 30 at 16:58
I see that, but that's for a particular choice of seminorms. Why should it be true for any arbitrary countable family of seminorms though?
– yoshi
Nov 30 at 17:21
I see that, but that's for a particular choice of seminorms. Why should it be true for any arbitrary countable family of seminorms though?
– yoshi
Nov 30 at 17:21
1
1
For fixed k,n,m, The set ${p_{n,k}(f,0)<m}$ is open, so for your Family of seminorms, you can put a p- Bail inside.
– Tito Eliatron
Nov 30 at 17:31
For fixed k,n,m, The set ${p_{n,k}(f,0)<m}$ is open, so for your Family of seminorms, you can put a p- Bail inside.
– Tito Eliatron
Nov 30 at 17:31
Can you elaborate on this statement, what is a p-ball and why is it suffient for one p-ball to be inside?
– yoshi
Dec 1 at 15:20
Can you elaborate on this statement, what is a p-ball and why is it suffient for one p-ball to be inside?
– yoshi
Dec 1 at 15:20
add a comment |
1 Answer
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The question doesn't exclude that there are different topologies on $C^infty(mathbb{R})$ such that $C^infty(mathbb{R})$ is a Frechet space with this topology. However, since the open mapping theorem is also true for Frechet spaces, we cannot have a finer (or weaker) topology (as the usual one) on $C^infty(mathbb{R})$ in which $C^infty(mathbb{R})$ is also a Frechet space and $f^{(k)}_n rightarrow f^{(k)}$ uniformly on compact sets for each $k in mathbb{N}$.
One can proof that $C^infty(mathbb{R})$ has dimension $2^{aleph_0}$ in the sense of Hamel basis. Thus you can take e.g. the Banach-space $L^1(mathbb{R})$ and transport the norm to $C^infty(mathbb{R})$. With these norm $C^infty(mathbb{R})$ is a Banach space. Since the topology of local convergence of all derivates is not normable, this weird topology is different than the usual one.
why does "no finer topology" follow from the open mapping theorem?
– yoshi
Dec 1 at 15:22
1
If $tau_1$, $tau_2$ are topologies on $C^infty(mathbb{R})$ with $tau_1 subset tau_2$, i.e. $tau_2$ is finer than $tau_1$, then $mathrm{id} colon (C^infty(mathbb{R}),tau_2) rightarrow (C^infty(mathbb{R}),tau_1)$ the map is continuous. If both topologies are Frechet, then the open mapping theorem already implies that this map is open, i.e. the inverse map $mathrm{id} colon (C^infty(mathbb{R}),tau_1) rightarrow (C^infty(mathbb{R}),tau_2)$ is continuous and thus $tau_1 = tau_2$.
– p4sch
Dec 1 at 21:27
I like this, but my only problem is that we haven't covered the open mapping theorem for Frechet spaces. Is there a way to adapt Tito Aliatron's hints into this? I'm looking through the proof of the open mapping theorem for Frechet spaces to see if I can do it too.
– yoshi
Dec 1 at 21:44
1
I don't think that you can prove this much easier. It is a feature coming from the completeness of the space. You may have a look in Rudin's book on functional analysis. There you can find a proof of the open mapping theorem - that is Theorem 2.11. In fact, the idea of the proof is the same as in the case of banach spaces.
– p4sch
Dec 2 at 11:03
add a comment |
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The question doesn't exclude that there are different topologies on $C^infty(mathbb{R})$ such that $C^infty(mathbb{R})$ is a Frechet space with this topology. However, since the open mapping theorem is also true for Frechet spaces, we cannot have a finer (or weaker) topology (as the usual one) on $C^infty(mathbb{R})$ in which $C^infty(mathbb{R})$ is also a Frechet space and $f^{(k)}_n rightarrow f^{(k)}$ uniformly on compact sets for each $k in mathbb{N}$.
One can proof that $C^infty(mathbb{R})$ has dimension $2^{aleph_0}$ in the sense of Hamel basis. Thus you can take e.g. the Banach-space $L^1(mathbb{R})$ and transport the norm to $C^infty(mathbb{R})$. With these norm $C^infty(mathbb{R})$ is a Banach space. Since the topology of local convergence of all derivates is not normable, this weird topology is different than the usual one.
why does "no finer topology" follow from the open mapping theorem?
– yoshi
Dec 1 at 15:22
1
If $tau_1$, $tau_2$ are topologies on $C^infty(mathbb{R})$ with $tau_1 subset tau_2$, i.e. $tau_2$ is finer than $tau_1$, then $mathrm{id} colon (C^infty(mathbb{R}),tau_2) rightarrow (C^infty(mathbb{R}),tau_1)$ the map is continuous. If both topologies are Frechet, then the open mapping theorem already implies that this map is open, i.e. the inverse map $mathrm{id} colon (C^infty(mathbb{R}),tau_1) rightarrow (C^infty(mathbb{R}),tau_2)$ is continuous and thus $tau_1 = tau_2$.
– p4sch
Dec 1 at 21:27
I like this, but my only problem is that we haven't covered the open mapping theorem for Frechet spaces. Is there a way to adapt Tito Aliatron's hints into this? I'm looking through the proof of the open mapping theorem for Frechet spaces to see if I can do it too.
– yoshi
Dec 1 at 21:44
1
I don't think that you can prove this much easier. It is a feature coming from the completeness of the space. You may have a look in Rudin's book on functional analysis. There you can find a proof of the open mapping theorem - that is Theorem 2.11. In fact, the idea of the proof is the same as in the case of banach spaces.
– p4sch
Dec 2 at 11:03
add a comment |
The question doesn't exclude that there are different topologies on $C^infty(mathbb{R})$ such that $C^infty(mathbb{R})$ is a Frechet space with this topology. However, since the open mapping theorem is also true for Frechet spaces, we cannot have a finer (or weaker) topology (as the usual one) on $C^infty(mathbb{R})$ in which $C^infty(mathbb{R})$ is also a Frechet space and $f^{(k)}_n rightarrow f^{(k)}$ uniformly on compact sets for each $k in mathbb{N}$.
One can proof that $C^infty(mathbb{R})$ has dimension $2^{aleph_0}$ in the sense of Hamel basis. Thus you can take e.g. the Banach-space $L^1(mathbb{R})$ and transport the norm to $C^infty(mathbb{R})$. With these norm $C^infty(mathbb{R})$ is a Banach space. Since the topology of local convergence of all derivates is not normable, this weird topology is different than the usual one.
why does "no finer topology" follow from the open mapping theorem?
– yoshi
Dec 1 at 15:22
1
If $tau_1$, $tau_2$ are topologies on $C^infty(mathbb{R})$ with $tau_1 subset tau_2$, i.e. $tau_2$ is finer than $tau_1$, then $mathrm{id} colon (C^infty(mathbb{R}),tau_2) rightarrow (C^infty(mathbb{R}),tau_1)$ the map is continuous. If both topologies are Frechet, then the open mapping theorem already implies that this map is open, i.e. the inverse map $mathrm{id} colon (C^infty(mathbb{R}),tau_1) rightarrow (C^infty(mathbb{R}),tau_2)$ is continuous and thus $tau_1 = tau_2$.
– p4sch
Dec 1 at 21:27
I like this, but my only problem is that we haven't covered the open mapping theorem for Frechet spaces. Is there a way to adapt Tito Aliatron's hints into this? I'm looking through the proof of the open mapping theorem for Frechet spaces to see if I can do it too.
– yoshi
Dec 1 at 21:44
1
I don't think that you can prove this much easier. It is a feature coming from the completeness of the space. You may have a look in Rudin's book on functional analysis. There you can find a proof of the open mapping theorem - that is Theorem 2.11. In fact, the idea of the proof is the same as in the case of banach spaces.
– p4sch
Dec 2 at 11:03
add a comment |
The question doesn't exclude that there are different topologies on $C^infty(mathbb{R})$ such that $C^infty(mathbb{R})$ is a Frechet space with this topology. However, since the open mapping theorem is also true for Frechet spaces, we cannot have a finer (or weaker) topology (as the usual one) on $C^infty(mathbb{R})$ in which $C^infty(mathbb{R})$ is also a Frechet space and $f^{(k)}_n rightarrow f^{(k)}$ uniformly on compact sets for each $k in mathbb{N}$.
One can proof that $C^infty(mathbb{R})$ has dimension $2^{aleph_0}$ in the sense of Hamel basis. Thus you can take e.g. the Banach-space $L^1(mathbb{R})$ and transport the norm to $C^infty(mathbb{R})$. With these norm $C^infty(mathbb{R})$ is a Banach space. Since the topology of local convergence of all derivates is not normable, this weird topology is different than the usual one.
The question doesn't exclude that there are different topologies on $C^infty(mathbb{R})$ such that $C^infty(mathbb{R})$ is a Frechet space with this topology. However, since the open mapping theorem is also true for Frechet spaces, we cannot have a finer (or weaker) topology (as the usual one) on $C^infty(mathbb{R})$ in which $C^infty(mathbb{R})$ is also a Frechet space and $f^{(k)}_n rightarrow f^{(k)}$ uniformly on compact sets for each $k in mathbb{N}$.
One can proof that $C^infty(mathbb{R})$ has dimension $2^{aleph_0}$ in the sense of Hamel basis. Thus you can take e.g. the Banach-space $L^1(mathbb{R})$ and transport the norm to $C^infty(mathbb{R})$. With these norm $C^infty(mathbb{R})$ is a Banach space. Since the topology of local convergence of all derivates is not normable, this weird topology is different than the usual one.
edited Dec 1 at 7:17
answered Nov 30 at 20:33
p4sch
4,800217
4,800217
why does "no finer topology" follow from the open mapping theorem?
– yoshi
Dec 1 at 15:22
1
If $tau_1$, $tau_2$ are topologies on $C^infty(mathbb{R})$ with $tau_1 subset tau_2$, i.e. $tau_2$ is finer than $tau_1$, then $mathrm{id} colon (C^infty(mathbb{R}),tau_2) rightarrow (C^infty(mathbb{R}),tau_1)$ the map is continuous. If both topologies are Frechet, then the open mapping theorem already implies that this map is open, i.e. the inverse map $mathrm{id} colon (C^infty(mathbb{R}),tau_1) rightarrow (C^infty(mathbb{R}),tau_2)$ is continuous and thus $tau_1 = tau_2$.
– p4sch
Dec 1 at 21:27
I like this, but my only problem is that we haven't covered the open mapping theorem for Frechet spaces. Is there a way to adapt Tito Aliatron's hints into this? I'm looking through the proof of the open mapping theorem for Frechet spaces to see if I can do it too.
– yoshi
Dec 1 at 21:44
1
I don't think that you can prove this much easier. It is a feature coming from the completeness of the space. You may have a look in Rudin's book on functional analysis. There you can find a proof of the open mapping theorem - that is Theorem 2.11. In fact, the idea of the proof is the same as in the case of banach spaces.
– p4sch
Dec 2 at 11:03
add a comment |
why does "no finer topology" follow from the open mapping theorem?
– yoshi
Dec 1 at 15:22
1
If $tau_1$, $tau_2$ are topologies on $C^infty(mathbb{R})$ with $tau_1 subset tau_2$, i.e. $tau_2$ is finer than $tau_1$, then $mathrm{id} colon (C^infty(mathbb{R}),tau_2) rightarrow (C^infty(mathbb{R}),tau_1)$ the map is continuous. If both topologies are Frechet, then the open mapping theorem already implies that this map is open, i.e. the inverse map $mathrm{id} colon (C^infty(mathbb{R}),tau_1) rightarrow (C^infty(mathbb{R}),tau_2)$ is continuous and thus $tau_1 = tau_2$.
– p4sch
Dec 1 at 21:27
I like this, but my only problem is that we haven't covered the open mapping theorem for Frechet spaces. Is there a way to adapt Tito Aliatron's hints into this? I'm looking through the proof of the open mapping theorem for Frechet spaces to see if I can do it too.
– yoshi
Dec 1 at 21:44
1
I don't think that you can prove this much easier. It is a feature coming from the completeness of the space. You may have a look in Rudin's book on functional analysis. There you can find a proof of the open mapping theorem - that is Theorem 2.11. In fact, the idea of the proof is the same as in the case of banach spaces.
– p4sch
Dec 2 at 11:03
why does "no finer topology" follow from the open mapping theorem?
– yoshi
Dec 1 at 15:22
why does "no finer topology" follow from the open mapping theorem?
– yoshi
Dec 1 at 15:22
1
1
If $tau_1$, $tau_2$ are topologies on $C^infty(mathbb{R})$ with $tau_1 subset tau_2$, i.e. $tau_2$ is finer than $tau_1$, then $mathrm{id} colon (C^infty(mathbb{R}),tau_2) rightarrow (C^infty(mathbb{R}),tau_1)$ the map is continuous. If both topologies are Frechet, then the open mapping theorem already implies that this map is open, i.e. the inverse map $mathrm{id} colon (C^infty(mathbb{R}),tau_1) rightarrow (C^infty(mathbb{R}),tau_2)$ is continuous and thus $tau_1 = tau_2$.
– p4sch
Dec 1 at 21:27
If $tau_1$, $tau_2$ are topologies on $C^infty(mathbb{R})$ with $tau_1 subset tau_2$, i.e. $tau_2$ is finer than $tau_1$, then $mathrm{id} colon (C^infty(mathbb{R}),tau_2) rightarrow (C^infty(mathbb{R}),tau_1)$ the map is continuous. If both topologies are Frechet, then the open mapping theorem already implies that this map is open, i.e. the inverse map $mathrm{id} colon (C^infty(mathbb{R}),tau_1) rightarrow (C^infty(mathbb{R}),tau_2)$ is continuous and thus $tau_1 = tau_2$.
– p4sch
Dec 1 at 21:27
I like this, but my only problem is that we haven't covered the open mapping theorem for Frechet spaces. Is there a way to adapt Tito Aliatron's hints into this? I'm looking through the proof of the open mapping theorem for Frechet spaces to see if I can do it too.
– yoshi
Dec 1 at 21:44
I like this, but my only problem is that we haven't covered the open mapping theorem for Frechet spaces. Is there a way to adapt Tito Aliatron's hints into this? I'm looking through the proof of the open mapping theorem for Frechet spaces to see if I can do it too.
– yoshi
Dec 1 at 21:44
1
1
I don't think that you can prove this much easier. It is a feature coming from the completeness of the space. You may have a look in Rudin's book on functional analysis. There you can find a proof of the open mapping theorem - that is Theorem 2.11. In fact, the idea of the proof is the same as in the case of banach spaces.
– p4sch
Dec 2 at 11:03
I don't think that you can prove this much easier. It is a feature coming from the completeness of the space. You may have a look in Rudin's book on functional analysis. There you can find a proof of the open mapping theorem - that is Theorem 2.11. In fact, the idea of the proof is the same as in the case of banach spaces.
– p4sch
Dec 2 at 11:03
add a comment |
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1
The seminirms are $p_{n,k}(f):= sup_{[-k,k]} |f^{(n)}|$. The key is that any compact set is contained in some interval $[-k,k]$
– Tito Eliatron
Nov 30 at 16:58
I see that, but that's for a particular choice of seminorms. Why should it be true for any arbitrary countable family of seminorms though?
– yoshi
Nov 30 at 17:21
1
For fixed k,n,m, The set ${p_{n,k}(f,0)<m}$ is open, so for your Family of seminorms, you can put a p- Bail inside.
– Tito Eliatron
Nov 30 at 17:31
Can you elaborate on this statement, what is a p-ball and why is it suffient for one p-ball to be inside?
– yoshi
Dec 1 at 15:20