Ultrafilters as a double dualIs the non-triviality of the algebraic dual of an infinite-dimensional vector...
Ultrafilters as a double dual
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$begingroup$
Given a set $X$, let $beta X$ denote the set of ultrafilters. The following theorems are known:
$X$ canonically embeds into $beta X$ (by taking principal ultrafilters);- If $X$ is finite, then there are no non-principal ultrafilters, so $beta X = X$.
- If $X$ is infinite, then (assuming choice) we have $|beta X| = 2^{2^{|X|}}$.
These are reminiscent of similar claims that can be made about vector spaces and double duals:
$V$ canonically embeds into $V^{star star}$;- If $V$ is finite-dimensional, then we have $V = V^{star star}$;
- If $V$ is infinite-dimensional, then (assuming choice) we have $dim(V^{star star}) = 2^{2^{dim(V)}}$.
This suggests that the operation of taking the collection of ultrafilters on a set can be viewed as a double iterate of some 'duality' of sets. Can this be made precise: that is to say, is there some notion of a 'dual' of a set $X$, $delta X$, such that the following are true?
- The double dual $delta delta X$ is (canonically isomorphic to) the set $beta X$ of ultrafilters on $X$;
- If $X$ is finite, then $|delta X| = |X|$ (but not canonically so);
- If $X$ is infinite, then (assuming choice) $|delta X| = 2^{|X|}$.
Apart from the tempting analogy between $beta X$ and $V^{star star}$, further evidence for this conjecture is that $beta$ can be given the structure of a monad (the 'ultrafilter monad'), and monads can be obtained from a pair of adjunctions.
ct.category-theory lo.logic gn.general-topology ultrafilters
$endgroup$
add a comment |
$begingroup$
Given a set $X$, let $beta X$ denote the set of ultrafilters. The following theorems are known:
$X$ canonically embeds into $beta X$ (by taking principal ultrafilters);- If $X$ is finite, then there are no non-principal ultrafilters, so $beta X = X$.
- If $X$ is infinite, then (assuming choice) we have $|beta X| = 2^{2^{|X|}}$.
These are reminiscent of similar claims that can be made about vector spaces and double duals:
$V$ canonically embeds into $V^{star star}$;- If $V$ is finite-dimensional, then we have $V = V^{star star}$;
- If $V$ is infinite-dimensional, then (assuming choice) we have $dim(V^{star star}) = 2^{2^{dim(V)}}$.
This suggests that the operation of taking the collection of ultrafilters on a set can be viewed as a double iterate of some 'duality' of sets. Can this be made precise: that is to say, is there some notion of a 'dual' of a set $X$, $delta X$, such that the following are true?
- The double dual $delta delta X$ is (canonically isomorphic to) the set $beta X$ of ultrafilters on $X$;
- If $X$ is finite, then $|delta X| = |X|$ (but not canonically so);
- If $X$ is infinite, then (assuming choice) $|delta X| = 2^{|X|}$.
Apart from the tempting analogy between $beta X$ and $V^{star star}$, further evidence for this conjecture is that $beta$ can be given the structure of a monad (the 'ultrafilter monad'), and monads can be obtained from a pair of adjunctions.
ct.category-theory lo.logic gn.general-topology ultrafilters
$endgroup$
12
$begingroup$
Well, one of the very first things that comes to mind that's sort of in this vein is that $beta X = hom_{text{Bool}}(hom_{text{Set}}(X, 2), 2)$. But if you want to pursue your analogy at a deeper level, try golem.ph.utexas.edu/category/2012/09/…, where both the ultrafilter monad and the double dualization monad are reckoned to be codensity monads induced by the full inclusions of finitary objects.
$endgroup$
– Todd Trimble♦
3 hours ago
2
$begingroup$
Close to Todd's comment, I'd view $beta X$ as $F(X)=mathrm{hom}_{mathrm{Bool}}(mathrm{hom}_{mathrm{Top}}(X,mathbf{Z}/2mathbf{Z}))$. In general, I guess that for a topological space $X$, the map $Xto F(X)$ is the initial object for the category of continuous maps from $X$ to compact Hausdorff totally disconnected topological spaces. A difference with taking biduals is that $F(F(X))=F(X)$ by Stone duality.
$endgroup$
– YCor
2 hours ago
$begingroup$
Well, one major difference is that without choice it is always the case that $X$ embeds into $beta X$, it's just not provable that the embedding is not surjective; whereas $V^*$ might be trivial, let alone $V^{**}$, even though $V$ isn't.
$endgroup$
– Asaf Karagila
2 hours ago
add a comment |
$begingroup$
Given a set $X$, let $beta X$ denote the set of ultrafilters. The following theorems are known:
$X$ canonically embeds into $beta X$ (by taking principal ultrafilters);- If $X$ is finite, then there are no non-principal ultrafilters, so $beta X = X$.
- If $X$ is infinite, then (assuming choice) we have $|beta X| = 2^{2^{|X|}}$.
These are reminiscent of similar claims that can be made about vector spaces and double duals:
$V$ canonically embeds into $V^{star star}$;- If $V$ is finite-dimensional, then we have $V = V^{star star}$;
- If $V$ is infinite-dimensional, then (assuming choice) we have $dim(V^{star star}) = 2^{2^{dim(V)}}$.
This suggests that the operation of taking the collection of ultrafilters on a set can be viewed as a double iterate of some 'duality' of sets. Can this be made precise: that is to say, is there some notion of a 'dual' of a set $X$, $delta X$, such that the following are true?
- The double dual $delta delta X$ is (canonically isomorphic to) the set $beta X$ of ultrafilters on $X$;
- If $X$ is finite, then $|delta X| = |X|$ (but not canonically so);
- If $X$ is infinite, then (assuming choice) $|delta X| = 2^{|X|}$.
Apart from the tempting analogy between $beta X$ and $V^{star star}$, further evidence for this conjecture is that $beta$ can be given the structure of a monad (the 'ultrafilter monad'), and monads can be obtained from a pair of adjunctions.
ct.category-theory lo.logic gn.general-topology ultrafilters
$endgroup$
Given a set $X$, let $beta X$ denote the set of ultrafilters. The following theorems are known:
$X$ canonically embeds into $beta X$ (by taking principal ultrafilters);- If $X$ is finite, then there are no non-principal ultrafilters, so $beta X = X$.
- If $X$ is infinite, then (assuming choice) we have $|beta X| = 2^{2^{|X|}}$.
These are reminiscent of similar claims that can be made about vector spaces and double duals:
$V$ canonically embeds into $V^{star star}$;- If $V$ is finite-dimensional, then we have $V = V^{star star}$;
- If $V$ is infinite-dimensional, then (assuming choice) we have $dim(V^{star star}) = 2^{2^{dim(V)}}$.
This suggests that the operation of taking the collection of ultrafilters on a set can be viewed as a double iterate of some 'duality' of sets. Can this be made precise: that is to say, is there some notion of a 'dual' of a set $X$, $delta X$, such that the following are true?
- The double dual $delta delta X$ is (canonically isomorphic to) the set $beta X$ of ultrafilters on $X$;
- If $X$ is finite, then $|delta X| = |X|$ (but not canonically so);
- If $X$ is infinite, then (assuming choice) $|delta X| = 2^{|X|}$.
Apart from the tempting analogy between $beta X$ and $V^{star star}$, further evidence for this conjecture is that $beta$ can be given the structure of a monad (the 'ultrafilter monad'), and monads can be obtained from a pair of adjunctions.
ct.category-theory lo.logic gn.general-topology ultrafilters
ct.category-theory lo.logic gn.general-topology ultrafilters
edited 2 hours ago
YCor
28.1k483136
28.1k483136
asked 3 hours ago
Adam P. GoucherAdam P. Goucher
6,53322856
6,53322856
12
$begingroup$
Well, one of the very first things that comes to mind that's sort of in this vein is that $beta X = hom_{text{Bool}}(hom_{text{Set}}(X, 2), 2)$. But if you want to pursue your analogy at a deeper level, try golem.ph.utexas.edu/category/2012/09/…, where both the ultrafilter monad and the double dualization monad are reckoned to be codensity monads induced by the full inclusions of finitary objects.
$endgroup$
– Todd Trimble♦
3 hours ago
2
$begingroup$
Close to Todd's comment, I'd view $beta X$ as $F(X)=mathrm{hom}_{mathrm{Bool}}(mathrm{hom}_{mathrm{Top}}(X,mathbf{Z}/2mathbf{Z}))$. In general, I guess that for a topological space $X$, the map $Xto F(X)$ is the initial object for the category of continuous maps from $X$ to compact Hausdorff totally disconnected topological spaces. A difference with taking biduals is that $F(F(X))=F(X)$ by Stone duality.
$endgroup$
– YCor
2 hours ago
$begingroup$
Well, one major difference is that without choice it is always the case that $X$ embeds into $beta X$, it's just not provable that the embedding is not surjective; whereas $V^*$ might be trivial, let alone $V^{**}$, even though $V$ isn't.
$endgroup$
– Asaf Karagila
2 hours ago
add a comment |
12
$begingroup$
Well, one of the very first things that comes to mind that's sort of in this vein is that $beta X = hom_{text{Bool}}(hom_{text{Set}}(X, 2), 2)$. But if you want to pursue your analogy at a deeper level, try golem.ph.utexas.edu/category/2012/09/…, where both the ultrafilter monad and the double dualization monad are reckoned to be codensity monads induced by the full inclusions of finitary objects.
$endgroup$
– Todd Trimble♦
3 hours ago
2
$begingroup$
Close to Todd's comment, I'd view $beta X$ as $F(X)=mathrm{hom}_{mathrm{Bool}}(mathrm{hom}_{mathrm{Top}}(X,mathbf{Z}/2mathbf{Z}))$. In general, I guess that for a topological space $X$, the map $Xto F(X)$ is the initial object for the category of continuous maps from $X$ to compact Hausdorff totally disconnected topological spaces. A difference with taking biduals is that $F(F(X))=F(X)$ by Stone duality.
$endgroup$
– YCor
2 hours ago
$begingroup$
Well, one major difference is that without choice it is always the case that $X$ embeds into $beta X$, it's just not provable that the embedding is not surjective; whereas $V^*$ might be trivial, let alone $V^{**}$, even though $V$ isn't.
$endgroup$
– Asaf Karagila
2 hours ago
12
12
$begingroup$
Well, one of the very first things that comes to mind that's sort of in this vein is that $beta X = hom_{text{Bool}}(hom_{text{Set}}(X, 2), 2)$. But if you want to pursue your analogy at a deeper level, try golem.ph.utexas.edu/category/2012/09/…, where both the ultrafilter monad and the double dualization monad are reckoned to be codensity monads induced by the full inclusions of finitary objects.
$endgroup$
– Todd Trimble♦
3 hours ago
$begingroup$
Well, one of the very first things that comes to mind that's sort of in this vein is that $beta X = hom_{text{Bool}}(hom_{text{Set}}(X, 2), 2)$. But if you want to pursue your analogy at a deeper level, try golem.ph.utexas.edu/category/2012/09/…, where both the ultrafilter monad and the double dualization monad are reckoned to be codensity monads induced by the full inclusions of finitary objects.
$endgroup$
– Todd Trimble♦
3 hours ago
2
2
$begingroup$
Close to Todd's comment, I'd view $beta X$ as $F(X)=mathrm{hom}_{mathrm{Bool}}(mathrm{hom}_{mathrm{Top}}(X,mathbf{Z}/2mathbf{Z}))$. In general, I guess that for a topological space $X$, the map $Xto F(X)$ is the initial object for the category of continuous maps from $X$ to compact Hausdorff totally disconnected topological spaces. A difference with taking biduals is that $F(F(X))=F(X)$ by Stone duality.
$endgroup$
– YCor
2 hours ago
$begingroup$
Close to Todd's comment, I'd view $beta X$ as $F(X)=mathrm{hom}_{mathrm{Bool}}(mathrm{hom}_{mathrm{Top}}(X,mathbf{Z}/2mathbf{Z}))$. In general, I guess that for a topological space $X$, the map $Xto F(X)$ is the initial object for the category of continuous maps from $X$ to compact Hausdorff totally disconnected topological spaces. A difference with taking biduals is that $F(F(X))=F(X)$ by Stone duality.
$endgroup$
– YCor
2 hours ago
$begingroup$
Well, one major difference is that without choice it is always the case that $X$ embeds into $beta X$, it's just not provable that the embedding is not surjective; whereas $V^*$ might be trivial, let alone $V^{**}$, even though $V$ isn't.
$endgroup$
– Asaf Karagila
2 hours ago
$begingroup$
Well, one major difference is that without choice it is always the case that $X$ embeds into $beta X$, it's just not provable that the embedding is not surjective; whereas $V^*$ might be trivial, let alone $V^{**}$, even though $V$ isn't.
$endgroup$
– Asaf Karagila
2 hours ago
add a comment |
1 Answer
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oldest
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$begingroup$
This is a quite standard idea in functional analysis. Let $X$ be any set and let $c_0(X)$ be the space of all functions from $X$ to $mathbb{C}$ which go to zero at infinity. Then the algebra homomorphisms from $c_0(X)$ to $mathbb{C}$ are precisely the point evaluations at elements of $X$, i.e., the spectrum of $c_0(X)$ is naturally identified with $X$.
Going to the second dual we get $l^infty(X)$, the space of all bounded functions from $X$ to $mathbb{C}$, whose spectrum is naturally identified with $beta X$.
$endgroup$
add a comment |
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$begingroup$
This is a quite standard idea in functional analysis. Let $X$ be any set and let $c_0(X)$ be the space of all functions from $X$ to $mathbb{C}$ which go to zero at infinity. Then the algebra homomorphisms from $c_0(X)$ to $mathbb{C}$ are precisely the point evaluations at elements of $X$, i.e., the spectrum of $c_0(X)$ is naturally identified with $X$.
Going to the second dual we get $l^infty(X)$, the space of all bounded functions from $X$ to $mathbb{C}$, whose spectrum is naturally identified with $beta X$.
$endgroup$
add a comment |
$begingroup$
This is a quite standard idea in functional analysis. Let $X$ be any set and let $c_0(X)$ be the space of all functions from $X$ to $mathbb{C}$ which go to zero at infinity. Then the algebra homomorphisms from $c_0(X)$ to $mathbb{C}$ are precisely the point evaluations at elements of $X$, i.e., the spectrum of $c_0(X)$ is naturally identified with $X$.
Going to the second dual we get $l^infty(X)$, the space of all bounded functions from $X$ to $mathbb{C}$, whose spectrum is naturally identified with $beta X$.
$endgroup$
add a comment |
$begingroup$
This is a quite standard idea in functional analysis. Let $X$ be any set and let $c_0(X)$ be the space of all functions from $X$ to $mathbb{C}$ which go to zero at infinity. Then the algebra homomorphisms from $c_0(X)$ to $mathbb{C}$ are precisely the point evaluations at elements of $X$, i.e., the spectrum of $c_0(X)$ is naturally identified with $X$.
Going to the second dual we get $l^infty(X)$, the space of all bounded functions from $X$ to $mathbb{C}$, whose spectrum is naturally identified with $beta X$.
$endgroup$
This is a quite standard idea in functional analysis. Let $X$ be any set and let $c_0(X)$ be the space of all functions from $X$ to $mathbb{C}$ which go to zero at infinity. Then the algebra homomorphisms from $c_0(X)$ to $mathbb{C}$ are precisely the point evaluations at elements of $X$, i.e., the spectrum of $c_0(X)$ is naturally identified with $X$.
Going to the second dual we get $l^infty(X)$, the space of all bounded functions from $X$ to $mathbb{C}$, whose spectrum is naturally identified with $beta X$.
answered 2 hours ago
Nik WeaverNik Weaver
21.3k148130
21.3k148130
add a comment |
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$begingroup$
Well, one of the very first things that comes to mind that's sort of in this vein is that $beta X = hom_{text{Bool}}(hom_{text{Set}}(X, 2), 2)$. But if you want to pursue your analogy at a deeper level, try golem.ph.utexas.edu/category/2012/09/…, where both the ultrafilter monad and the double dualization monad are reckoned to be codensity monads induced by the full inclusions of finitary objects.
$endgroup$
– Todd Trimble♦
3 hours ago
2
$begingroup$
Close to Todd's comment, I'd view $beta X$ as $F(X)=mathrm{hom}_{mathrm{Bool}}(mathrm{hom}_{mathrm{Top}}(X,mathbf{Z}/2mathbf{Z}))$. In general, I guess that for a topological space $X$, the map $Xto F(X)$ is the initial object for the category of continuous maps from $X$ to compact Hausdorff totally disconnected topological spaces. A difference with taking biduals is that $F(F(X))=F(X)$ by Stone duality.
$endgroup$
– YCor
2 hours ago
$begingroup$
Well, one major difference is that without choice it is always the case that $X$ embeds into $beta X$, it's just not provable that the embedding is not surjective; whereas $V^*$ might be trivial, let alone $V^{**}$, even though $V$ isn't.
$endgroup$
– Asaf Karagila
2 hours ago