Examples of transfinite towersIs there such a sufficient condition for “$X$ is a stationary subset of uncountable regular $kappa$” involving limit points?Can $n$ extendible cardinals have trivial algebraic structure?Towers of induced functionsObtaining linear orderings on the classical Laver tables from large cardinalsAre there non-finitely generated algebras of elementary embeddings when one includes compatible $n$-ary operations?What kinds of free algebras can rank-into-rank embeddings produce?The Tall Tale of Terminating Transfinite TowersOn the Number of Parallel Automorphism LinesA linear ordering on the quotient algebras of elementary embeddings?Adjoints to forcing
Examples of transfinite towers
Is there such a sufficient condition for “$X$ is a stationary subset of uncountable regular $kappa$” involving limit points?Can $n$ extendible cardinals have trivial algebraic structure?Towers of induced functionsObtaining linear orderings on the classical Laver tables from large cardinalsAre there non-finitely generated algebras of elementary embeddings when one includes compatible $n$-ary operations?What kinds of free algebras can rank-into-rank embeddings produce?The Tall Tale of Terminating Transfinite TowersOn the Number of Parallel Automorphism LinesA linear ordering on the quotient algebras of elementary embeddings?Adjoints to forcing
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I am looking for examples of constructions for transfinite towers $(X_alpha)_alpha$ generated by structures $X$ where the problem of determining whether the tower $(X_alpha)_alpha$ stops growing is a non-trivial problem or the problem of determining the ordinal in which $X_alpha$ stops growing is a non-trivial problem.
In particular, I want the tower $(X_alpha)_alpha$ to be generated by the following construction. Suppose that for each object $X$, there is a new object $C(X)$ and a morphism $e:Xrightarrow C(X)$. Then define the tower generated by $X$ by letting $X_0=X$, $X_alpha+1=C(X_alpha)$ and
$X_gamma=varinjlim_alpha<lambdaX_alpha$ for limit ordinals $gamma$ where the direct limit is taken in the category that $X$ belongs to.
I want all of the objects $X$ and each $X_alpha$ to be set sized.
Non-Example: The hierarchy of sets $(V_alpha[X])_alpha$ where
$V_0[X]=X,V_alpha+1[X]=P(V_alpha[X])$ and $V_gamma[X]=bigcup_alpha<gammaV_alpha[X]$ does not count as an example of what I am looking for since the tower $(V_alpha[X])_alpha$ never stops growing and therefore whether $(V_alpha[X])_alpha$ terminates is now a trivial mathematics problem.
Example 1: Suppose that $G$ is a group. Let $G_0=G$, and let
$G_alpha+1=mathrmAut(G_alpha)$ and let $G_gamma=varinjlim_alpha<gammaG_alpha$. The transition mapping from $G_alpha$ to $G_alpha+1$ is the mapping $e$ where $e(g)(h)=ghg^-1$. Then $(G_alpha)_alpha$ is the automorphism group tower generated by $G$. The automorphism group tower always terminates. In this case, the mapping $Gmapsto G_alpha$ is not functorial.
Example 2: Frames are the objects that people study in point-free topology. If $L$ is a frame, then let $mathfrakC(L)$ denote the lattice of congruences of the frame $L$. Then $mathfrakC(L)$ is always a frame. Define a mapping $e:LrightarrowmathfrakC(L)$ by letting $(x,y)in e(a)$ if and only if $xvee a=yvee a$. Then the function $e$ is a frame homomorphism.
There are frames $L$ where the congruence tower generated by $L$ never terminates. However, for ordinals $alpha$, it is a difficult open problem to determine whether there is a frame $L$ where $e:L_betarightarrowmathfrakC(L_beta)$ is a surjection if and only if $betageqalpha$ since in all known examples, the congruence tower either terminates before the fourth step or so or it never terminates.
If $L$ is a frame and $e:LrightarrowmathfrakC(L)$ is an isomorphism, then $L$ is a complete Boolean algebra.
Unlike Example 1, Example 2 is functorial.
ct.category-theory set-theory
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|
show 1 more comment
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I am looking for examples of constructions for transfinite towers $(X_alpha)_alpha$ generated by structures $X$ where the problem of determining whether the tower $(X_alpha)_alpha$ stops growing is a non-trivial problem or the problem of determining the ordinal in which $X_alpha$ stops growing is a non-trivial problem.
In particular, I want the tower $(X_alpha)_alpha$ to be generated by the following construction. Suppose that for each object $X$, there is a new object $C(X)$ and a morphism $e:Xrightarrow C(X)$. Then define the tower generated by $X$ by letting $X_0=X$, $X_alpha+1=C(X_alpha)$ and
$X_gamma=varinjlim_alpha<lambdaX_alpha$ for limit ordinals $gamma$ where the direct limit is taken in the category that $X$ belongs to.
I want all of the objects $X$ and each $X_alpha$ to be set sized.
Non-Example: The hierarchy of sets $(V_alpha[X])_alpha$ where
$V_0[X]=X,V_alpha+1[X]=P(V_alpha[X])$ and $V_gamma[X]=bigcup_alpha<gammaV_alpha[X]$ does not count as an example of what I am looking for since the tower $(V_alpha[X])_alpha$ never stops growing and therefore whether $(V_alpha[X])_alpha$ terminates is now a trivial mathematics problem.
Example 1: Suppose that $G$ is a group. Let $G_0=G$, and let
$G_alpha+1=mathrmAut(G_alpha)$ and let $G_gamma=varinjlim_alpha<gammaG_alpha$. The transition mapping from $G_alpha$ to $G_alpha+1$ is the mapping $e$ where $e(g)(h)=ghg^-1$. Then $(G_alpha)_alpha$ is the automorphism group tower generated by $G$. The automorphism group tower always terminates. In this case, the mapping $Gmapsto G_alpha$ is not functorial.
Example 2: Frames are the objects that people study in point-free topology. If $L$ is a frame, then let $mathfrakC(L)$ denote the lattice of congruences of the frame $L$. Then $mathfrakC(L)$ is always a frame. Define a mapping $e:LrightarrowmathfrakC(L)$ by letting $(x,y)in e(a)$ if and only if $xvee a=yvee a$. Then the function $e$ is a frame homomorphism.
There are frames $L$ where the congruence tower generated by $L$ never terminates. However, for ordinals $alpha$, it is a difficult open problem to determine whether there is a frame $L$ where $e:L_betarightarrowmathfrakC(L_beta)$ is a surjection if and only if $betageqalpha$ since in all known examples, the congruence tower either terminates before the fourth step or so or it never terminates.
If $L$ is a frame and $e:LrightarrowmathfrakC(L)$ is an isomorphism, then $L$ is a complete Boolean algebra.
Unlike Example 1, Example 2 is functorial.
ct.category-theory set-theory
$endgroup$
1
$begingroup$
Have you looked into Kelly's "...and so on"?
$endgroup$
– Fosco Loregian
Mar 16 at 20:59
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My questions on Laver tables are better than these questions.
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– Joseph Van Name
Mar 17 at 3:41
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Can you suggest any references for Example 2?
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– Dap
Mar 17 at 15:10
$begingroup$
Example 2 is elaborated int he standard textbook on Frames and Locales - Topology without points by Picado, Jorge and Pultr, Ales (Chapter 4).
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– Joseph Van Name
Mar 17 at 15:13
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I find it very strange and discouraging that these inferior questions about transfinite towers have many times more upvotes than questions about Laver tables.
$endgroup$
– Joseph Van Name
Mar 23 at 2:48
|
show 1 more comment
$begingroup$
I am looking for examples of constructions for transfinite towers $(X_alpha)_alpha$ generated by structures $X$ where the problem of determining whether the tower $(X_alpha)_alpha$ stops growing is a non-trivial problem or the problem of determining the ordinal in which $X_alpha$ stops growing is a non-trivial problem.
In particular, I want the tower $(X_alpha)_alpha$ to be generated by the following construction. Suppose that for each object $X$, there is a new object $C(X)$ and a morphism $e:Xrightarrow C(X)$. Then define the tower generated by $X$ by letting $X_0=X$, $X_alpha+1=C(X_alpha)$ and
$X_gamma=varinjlim_alpha<lambdaX_alpha$ for limit ordinals $gamma$ where the direct limit is taken in the category that $X$ belongs to.
I want all of the objects $X$ and each $X_alpha$ to be set sized.
Non-Example: The hierarchy of sets $(V_alpha[X])_alpha$ where
$V_0[X]=X,V_alpha+1[X]=P(V_alpha[X])$ and $V_gamma[X]=bigcup_alpha<gammaV_alpha[X]$ does not count as an example of what I am looking for since the tower $(V_alpha[X])_alpha$ never stops growing and therefore whether $(V_alpha[X])_alpha$ terminates is now a trivial mathematics problem.
Example 1: Suppose that $G$ is a group. Let $G_0=G$, and let
$G_alpha+1=mathrmAut(G_alpha)$ and let $G_gamma=varinjlim_alpha<gammaG_alpha$. The transition mapping from $G_alpha$ to $G_alpha+1$ is the mapping $e$ where $e(g)(h)=ghg^-1$. Then $(G_alpha)_alpha$ is the automorphism group tower generated by $G$. The automorphism group tower always terminates. In this case, the mapping $Gmapsto G_alpha$ is not functorial.
Example 2: Frames are the objects that people study in point-free topology. If $L$ is a frame, then let $mathfrakC(L)$ denote the lattice of congruences of the frame $L$. Then $mathfrakC(L)$ is always a frame. Define a mapping $e:LrightarrowmathfrakC(L)$ by letting $(x,y)in e(a)$ if and only if $xvee a=yvee a$. Then the function $e$ is a frame homomorphism.
There are frames $L$ where the congruence tower generated by $L$ never terminates. However, for ordinals $alpha$, it is a difficult open problem to determine whether there is a frame $L$ where $e:L_betarightarrowmathfrakC(L_beta)$ is a surjection if and only if $betageqalpha$ since in all known examples, the congruence tower either terminates before the fourth step or so or it never terminates.
If $L$ is a frame and $e:LrightarrowmathfrakC(L)$ is an isomorphism, then $L$ is a complete Boolean algebra.
Unlike Example 1, Example 2 is functorial.
ct.category-theory set-theory
$endgroup$
I am looking for examples of constructions for transfinite towers $(X_alpha)_alpha$ generated by structures $X$ where the problem of determining whether the tower $(X_alpha)_alpha$ stops growing is a non-trivial problem or the problem of determining the ordinal in which $X_alpha$ stops growing is a non-trivial problem.
In particular, I want the tower $(X_alpha)_alpha$ to be generated by the following construction. Suppose that for each object $X$, there is a new object $C(X)$ and a morphism $e:Xrightarrow C(X)$. Then define the tower generated by $X$ by letting $X_0=X$, $X_alpha+1=C(X_alpha)$ and
$X_gamma=varinjlim_alpha<lambdaX_alpha$ for limit ordinals $gamma$ where the direct limit is taken in the category that $X$ belongs to.
I want all of the objects $X$ and each $X_alpha$ to be set sized.
Non-Example: The hierarchy of sets $(V_alpha[X])_alpha$ where
$V_0[X]=X,V_alpha+1[X]=P(V_alpha[X])$ and $V_gamma[X]=bigcup_alpha<gammaV_alpha[X]$ does not count as an example of what I am looking for since the tower $(V_alpha[X])_alpha$ never stops growing and therefore whether $(V_alpha[X])_alpha$ terminates is now a trivial mathematics problem.
Example 1: Suppose that $G$ is a group. Let $G_0=G$, and let
$G_alpha+1=mathrmAut(G_alpha)$ and let $G_gamma=varinjlim_alpha<gammaG_alpha$. The transition mapping from $G_alpha$ to $G_alpha+1$ is the mapping $e$ where $e(g)(h)=ghg^-1$. Then $(G_alpha)_alpha$ is the automorphism group tower generated by $G$. The automorphism group tower always terminates. In this case, the mapping $Gmapsto G_alpha$ is not functorial.
Example 2: Frames are the objects that people study in point-free topology. If $L$ is a frame, then let $mathfrakC(L)$ denote the lattice of congruences of the frame $L$. Then $mathfrakC(L)$ is always a frame. Define a mapping $e:LrightarrowmathfrakC(L)$ by letting $(x,y)in e(a)$ if and only if $xvee a=yvee a$. Then the function $e$ is a frame homomorphism.
There are frames $L$ where the congruence tower generated by $L$ never terminates. However, for ordinals $alpha$, it is a difficult open problem to determine whether there is a frame $L$ where $e:L_betarightarrowmathfrakC(L_beta)$ is a surjection if and only if $betageqalpha$ since in all known examples, the congruence tower either terminates before the fourth step or so or it never terminates.
If $L$ is a frame and $e:LrightarrowmathfrakC(L)$ is an isomorphism, then $L$ is a complete Boolean algebra.
Unlike Example 1, Example 2 is functorial.
ct.category-theory set-theory
ct.category-theory set-theory
asked Mar 16 at 20:19
Joseph Van NameJoseph Van Name
15k34975
15k34975
1
$begingroup$
Have you looked into Kelly's "...and so on"?
$endgroup$
– Fosco Loregian
Mar 16 at 20:59
$begingroup$
My questions on Laver tables are better than these questions.
$endgroup$
– Joseph Van Name
Mar 17 at 3:41
$begingroup$
Can you suggest any references for Example 2?
$endgroup$
– Dap
Mar 17 at 15:10
$begingroup$
Example 2 is elaborated int he standard textbook on Frames and Locales - Topology without points by Picado, Jorge and Pultr, Ales (Chapter 4).
$endgroup$
– Joseph Van Name
Mar 17 at 15:13
$begingroup$
I find it very strange and discouraging that these inferior questions about transfinite towers have many times more upvotes than questions about Laver tables.
$endgroup$
– Joseph Van Name
Mar 23 at 2:48
|
show 1 more comment
1
$begingroup$
Have you looked into Kelly's "...and so on"?
$endgroup$
– Fosco Loregian
Mar 16 at 20:59
$begingroup$
My questions on Laver tables are better than these questions.
$endgroup$
– Joseph Van Name
Mar 17 at 3:41
$begingroup$
Can you suggest any references for Example 2?
$endgroup$
– Dap
Mar 17 at 15:10
$begingroup$
Example 2 is elaborated int he standard textbook on Frames and Locales - Topology without points by Picado, Jorge and Pultr, Ales (Chapter 4).
$endgroup$
– Joseph Van Name
Mar 17 at 15:13
$begingroup$
I find it very strange and discouraging that these inferior questions about transfinite towers have many times more upvotes than questions about Laver tables.
$endgroup$
– Joseph Van Name
Mar 23 at 2:48
1
1
$begingroup$
Have you looked into Kelly's "...and so on"?
$endgroup$
– Fosco Loregian
Mar 16 at 20:59
$begingroup$
Have you looked into Kelly's "...and so on"?
$endgroup$
– Fosco Loregian
Mar 16 at 20:59
$begingroup$
My questions on Laver tables are better than these questions.
$endgroup$
– Joseph Van Name
Mar 17 at 3:41
$begingroup$
My questions on Laver tables are better than these questions.
$endgroup$
– Joseph Van Name
Mar 17 at 3:41
$begingroup$
Can you suggest any references for Example 2?
$endgroup$
– Dap
Mar 17 at 15:10
$begingroup$
Can you suggest any references for Example 2?
$endgroup$
– Dap
Mar 17 at 15:10
$begingroup$
Example 2 is elaborated int he standard textbook on Frames and Locales - Topology without points by Picado, Jorge and Pultr, Ales (Chapter 4).
$endgroup$
– Joseph Van Name
Mar 17 at 15:13
$begingroup$
Example 2 is elaborated int he standard textbook on Frames and Locales - Topology without points by Picado, Jorge and Pultr, Ales (Chapter 4).
$endgroup$
– Joseph Van Name
Mar 17 at 15:13
$begingroup$
I find it very strange and discouraging that these inferior questions about transfinite towers have many times more upvotes than questions about Laver tables.
$endgroup$
– Joseph Van Name
Mar 23 at 2:48
$begingroup$
I find it very strange and discouraging that these inferior questions about transfinite towers have many times more upvotes than questions about Laver tables.
$endgroup$
– Joseph Van Name
Mar 23 at 2:48
|
show 1 more comment
2 Answers
2
active
oldest
votes
$begingroup$
Consider the following construction of sets of ordinals.
$X_0=0$,
$X_alpha+1=$ the closure of $X_alpha$ under $gammamapstogamma+1$ and under countable sums,
$X_alpha=bigcup_beta<alphaX_beta$ for $alpha$ limit.
We can consider in ZF the question of whether this tower stabilizes.
It is definitely consistent that the answer is yes, since in ZFC $omega_1$ is regular, so $X_beta=omega_1$ for all $betageqomega_1$.
The question of whether this tower can be non-stabilizing is much harder, and is equivalent to asking whether it is consistent that all ordinals have countable cofinality. This was shown to be consistent by Gitik in 1980, assuming consistency of a proper class of strongly compact cardinals. Let me note that large cardinals cannot be avoided, because even making $omega_1$ and $omega_2$ singular has consistency at least as high as $0^#$.
$endgroup$
add a comment |
$begingroup$
The following is an example where is it difficult to identify where stabilization occurs. It meets all of your technical criteria but I suppose the word "growing" is not exactly accurate.
A quasitopological group $G$ is a group with a topology $mathcalT$ such that inverse $gmapsto g^-1$ is continuous and multiplication $mu:Gtimes Gto G$ is continuous in each variable. One can "efficiently" give the underlying group of $G$ an actual topological group structure by removing the smallest number of open sets from $mathcalT$ until one arrives at a topological group $tau(G)$. This can be understood precisely in at least two equivalent ways:
- Give $G$ the finest group topology coarser than $mathcalT$, which one can show exists abstractly (the difficulty is with the product topology on $Gtimes G$).
- The inclusion $mathbfTopGrpto mathbfqTopGrp$ of the full subcategory of topological groups into the category of quasitopological groups and continuous homomorphisms has a left adjoint (reflection) $tau:mathbfqTopGrpto mathbfTopGrp$, which exists by the adjoint functor theorem.
The construction of $tau(G)$ from $G$ is purely abstract so to really get your hands on something you can work with (e.g. to prove $G$ and $tau(G)$ in fact have the same open subgroups) you approximate it inductively by a shrinking sequence of quotient topologies.
Let $c(G)$ be the underlying group of $G$ with the quotient topology with respect to multiplication $m:Gtimes Gto c(G)$. While $c(G)$ is not necessarily a topological group, it is a quasitopological group and indeed, one can show that if $c$ is the identity on underlying homomorphisms, $c:mathbfqTopGrptomathbfqTopGrp$ becomes a functor.
It is clear that the identity homomorphism $Gto c(G)$ is continuous and $G=c(G)$ if and only if $G$ is already topological group. Hence one inductively defines $c_0(G)=G$, $c_alpha+1(G)=c_alpha(G)$ and if $alpha$ is a limit ordinal, the topology of $c_alpha(G)$ is the intersection of the topologies of the groups $c_beta(G)$, $beta<alpha$.
By basic set-theoretic arguments, $c_alpha(G)$ stabilizes to the topological group $tau(G)$ and since quotient topologies are used one has $c_gamma(G)=lim_alpha<gammac_alpha(G)$ in $mathbfqTopGrp$.
However, for a given quasitopological group $G$ it is not clear at all when the inductive sequence $c_alpha(G)$ first stabilizes.
Motivation for this example:
- Free topological groups are important in topological group theory and arise in algebraic topology as well. Given a space $X$, one might attempt to create the free topological group on $X$ viewing the free group $F(X)$ as the quotient space of the free topological semigroup $coprod_ngeq 1(Xcup X^-1)^n$ with respect to word reduction; however, without some compactness assumptions, the resulting object $F_q(X)$ is only a quasitopological group. To construct the free topological group one must apply the inductive construction above and this specific use of $c$ is sometimes called the Mal'tsev transfite process. A quote from pg. 5799 of The topology of free topological groups by O. Sipacheva:
Certainly, such an approach to constructing the free group topology
looks very natural. However, it is extremely difficult to trace the
change of the topology at each step or at least understand at what
point the topology stabilizes.
Apparently, the only known results are those where stabilization occurs after the first step.
- The fundamental group $pi_1(X,x)$ with it's natural quotient topology is not a topological group (same for higher homotopy groups), but it is a quasitopological group. There are plenty of interesting things about quasitopological $pi_1$; however, one may turn $pi_1$ naturally into a topological group (reflecting many classical results to the topological group category) by applying $tau$. See this paper for more on topological $pi_1$ and a detailed treatment on the reflection functor $c$. The use of the transfinite sequence is needed in the classification of certain generalized covering maps. I would, personally, like to know when the sequence stabilizes even for simple Peano continua $X$.
Similar constructions are used to build other universal objects in topological algebra.
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Neutrality is one thing; however, I am curious to know how this got a downvote.
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– Jeremy Brazas
Mar 21 at 19:00
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I downvoted this answer because I specifically asked for towers that are increasing in size.
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– Joseph Van Name
Mar 23 at 3:22
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@JosephVanName Had you included a formalization of "growth" here to which my example did not apply, I would not have subjected you to my answer. If you edit the "in particular" paragraph it so that this is the case, I'll happily delete my answer. However, the difference between growing and shrinking for bounded towers seems superficial... if $T_alpha$ is the topology of $c_alpha(G)$, then $X_alpha=P(G)backslash T_alpha$ is a "growing" tower of sets whose stability is difficult to determine.
$endgroup$
– Jeremy Brazas
2 days ago
add a comment |
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2 Answers
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2 Answers
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$begingroup$
Consider the following construction of sets of ordinals.
$X_0=0$,
$X_alpha+1=$ the closure of $X_alpha$ under $gammamapstogamma+1$ and under countable sums,
$X_alpha=bigcup_beta<alphaX_beta$ for $alpha$ limit.
We can consider in ZF the question of whether this tower stabilizes.
It is definitely consistent that the answer is yes, since in ZFC $omega_1$ is regular, so $X_beta=omega_1$ for all $betageqomega_1$.
The question of whether this tower can be non-stabilizing is much harder, and is equivalent to asking whether it is consistent that all ordinals have countable cofinality. This was shown to be consistent by Gitik in 1980, assuming consistency of a proper class of strongly compact cardinals. Let me note that large cardinals cannot be avoided, because even making $omega_1$ and $omega_2$ singular has consistency at least as high as $0^#$.
$endgroup$
add a comment |
$begingroup$
Consider the following construction of sets of ordinals.
$X_0=0$,
$X_alpha+1=$ the closure of $X_alpha$ under $gammamapstogamma+1$ and under countable sums,
$X_alpha=bigcup_beta<alphaX_beta$ for $alpha$ limit.
We can consider in ZF the question of whether this tower stabilizes.
It is definitely consistent that the answer is yes, since in ZFC $omega_1$ is regular, so $X_beta=omega_1$ for all $betageqomega_1$.
The question of whether this tower can be non-stabilizing is much harder, and is equivalent to asking whether it is consistent that all ordinals have countable cofinality. This was shown to be consistent by Gitik in 1980, assuming consistency of a proper class of strongly compact cardinals. Let me note that large cardinals cannot be avoided, because even making $omega_1$ and $omega_2$ singular has consistency at least as high as $0^#$.
$endgroup$
add a comment |
$begingroup$
Consider the following construction of sets of ordinals.
$X_0=0$,
$X_alpha+1=$ the closure of $X_alpha$ under $gammamapstogamma+1$ and under countable sums,
$X_alpha=bigcup_beta<alphaX_beta$ for $alpha$ limit.
We can consider in ZF the question of whether this tower stabilizes.
It is definitely consistent that the answer is yes, since in ZFC $omega_1$ is regular, so $X_beta=omega_1$ for all $betageqomega_1$.
The question of whether this tower can be non-stabilizing is much harder, and is equivalent to asking whether it is consistent that all ordinals have countable cofinality. This was shown to be consistent by Gitik in 1980, assuming consistency of a proper class of strongly compact cardinals. Let me note that large cardinals cannot be avoided, because even making $omega_1$ and $omega_2$ singular has consistency at least as high as $0^#$.
$endgroup$
Consider the following construction of sets of ordinals.
$X_0=0$,
$X_alpha+1=$ the closure of $X_alpha$ under $gammamapstogamma+1$ and under countable sums,
$X_alpha=bigcup_beta<alphaX_beta$ for $alpha$ limit.
We can consider in ZF the question of whether this tower stabilizes.
It is definitely consistent that the answer is yes, since in ZFC $omega_1$ is regular, so $X_beta=omega_1$ for all $betageqomega_1$.
The question of whether this tower can be non-stabilizing is much harder, and is equivalent to asking whether it is consistent that all ordinals have countable cofinality. This was shown to be consistent by Gitik in 1980, assuming consistency of a proper class of strongly compact cardinals. Let me note that large cardinals cannot be avoided, because even making $omega_1$ and $omega_2$ singular has consistency at least as high as $0^#$.
edited Mar 16 at 22:22
answered Mar 16 at 21:47
WojowuWojowu
7,12913055
7,12913055
add a comment |
add a comment |
$begingroup$
The following is an example where is it difficult to identify where stabilization occurs. It meets all of your technical criteria but I suppose the word "growing" is not exactly accurate.
A quasitopological group $G$ is a group with a topology $mathcalT$ such that inverse $gmapsto g^-1$ is continuous and multiplication $mu:Gtimes Gto G$ is continuous in each variable. One can "efficiently" give the underlying group of $G$ an actual topological group structure by removing the smallest number of open sets from $mathcalT$ until one arrives at a topological group $tau(G)$. This can be understood precisely in at least two equivalent ways:
- Give $G$ the finest group topology coarser than $mathcalT$, which one can show exists abstractly (the difficulty is with the product topology on $Gtimes G$).
- The inclusion $mathbfTopGrpto mathbfqTopGrp$ of the full subcategory of topological groups into the category of quasitopological groups and continuous homomorphisms has a left adjoint (reflection) $tau:mathbfqTopGrpto mathbfTopGrp$, which exists by the adjoint functor theorem.
The construction of $tau(G)$ from $G$ is purely abstract so to really get your hands on something you can work with (e.g. to prove $G$ and $tau(G)$ in fact have the same open subgroups) you approximate it inductively by a shrinking sequence of quotient topologies.
Let $c(G)$ be the underlying group of $G$ with the quotient topology with respect to multiplication $m:Gtimes Gto c(G)$. While $c(G)$ is not necessarily a topological group, it is a quasitopological group and indeed, one can show that if $c$ is the identity on underlying homomorphisms, $c:mathbfqTopGrptomathbfqTopGrp$ becomes a functor.
It is clear that the identity homomorphism $Gto c(G)$ is continuous and $G=c(G)$ if and only if $G$ is already topological group. Hence one inductively defines $c_0(G)=G$, $c_alpha+1(G)=c_alpha(G)$ and if $alpha$ is a limit ordinal, the topology of $c_alpha(G)$ is the intersection of the topologies of the groups $c_beta(G)$, $beta<alpha$.
By basic set-theoretic arguments, $c_alpha(G)$ stabilizes to the topological group $tau(G)$ and since quotient topologies are used one has $c_gamma(G)=lim_alpha<gammac_alpha(G)$ in $mathbfqTopGrp$.
However, for a given quasitopological group $G$ it is not clear at all when the inductive sequence $c_alpha(G)$ first stabilizes.
Motivation for this example:
- Free topological groups are important in topological group theory and arise in algebraic topology as well. Given a space $X$, one might attempt to create the free topological group on $X$ viewing the free group $F(X)$ as the quotient space of the free topological semigroup $coprod_ngeq 1(Xcup X^-1)^n$ with respect to word reduction; however, without some compactness assumptions, the resulting object $F_q(X)$ is only a quasitopological group. To construct the free topological group one must apply the inductive construction above and this specific use of $c$ is sometimes called the Mal'tsev transfite process. A quote from pg. 5799 of The topology of free topological groups by O. Sipacheva:
Certainly, such an approach to constructing the free group topology
looks very natural. However, it is extremely difficult to trace the
change of the topology at each step or at least understand at what
point the topology stabilizes.
Apparently, the only known results are those where stabilization occurs after the first step.
- The fundamental group $pi_1(X,x)$ with it's natural quotient topology is not a topological group (same for higher homotopy groups), but it is a quasitopological group. There are plenty of interesting things about quasitopological $pi_1$; however, one may turn $pi_1$ naturally into a topological group (reflecting many classical results to the topological group category) by applying $tau$. See this paper for more on topological $pi_1$ and a detailed treatment on the reflection functor $c$. The use of the transfinite sequence is needed in the classification of certain generalized covering maps. I would, personally, like to know when the sequence stabilizes even for simple Peano continua $X$.
Similar constructions are used to build other universal objects in topological algebra.
$endgroup$
$begingroup$
Neutrality is one thing; however, I am curious to know how this got a downvote.
$endgroup$
– Jeremy Brazas
Mar 21 at 19:00
$begingroup$
I downvoted this answer because I specifically asked for towers that are increasing in size.
$endgroup$
– Joseph Van Name
Mar 23 at 3:22
$begingroup$
@JosephVanName Had you included a formalization of "growth" here to which my example did not apply, I would not have subjected you to my answer. If you edit the "in particular" paragraph it so that this is the case, I'll happily delete my answer. However, the difference between growing and shrinking for bounded towers seems superficial... if $T_alpha$ is the topology of $c_alpha(G)$, then $X_alpha=P(G)backslash T_alpha$ is a "growing" tower of sets whose stability is difficult to determine.
$endgroup$
– Jeremy Brazas
2 days ago
add a comment |
$begingroup$
The following is an example where is it difficult to identify where stabilization occurs. It meets all of your technical criteria but I suppose the word "growing" is not exactly accurate.
A quasitopological group $G$ is a group with a topology $mathcalT$ such that inverse $gmapsto g^-1$ is continuous and multiplication $mu:Gtimes Gto G$ is continuous in each variable. One can "efficiently" give the underlying group of $G$ an actual topological group structure by removing the smallest number of open sets from $mathcalT$ until one arrives at a topological group $tau(G)$. This can be understood precisely in at least two equivalent ways:
- Give $G$ the finest group topology coarser than $mathcalT$, which one can show exists abstractly (the difficulty is with the product topology on $Gtimes G$).
- The inclusion $mathbfTopGrpto mathbfqTopGrp$ of the full subcategory of topological groups into the category of quasitopological groups and continuous homomorphisms has a left adjoint (reflection) $tau:mathbfqTopGrpto mathbfTopGrp$, which exists by the adjoint functor theorem.
The construction of $tau(G)$ from $G$ is purely abstract so to really get your hands on something you can work with (e.g. to prove $G$ and $tau(G)$ in fact have the same open subgroups) you approximate it inductively by a shrinking sequence of quotient topologies.
Let $c(G)$ be the underlying group of $G$ with the quotient topology with respect to multiplication $m:Gtimes Gto c(G)$. While $c(G)$ is not necessarily a topological group, it is a quasitopological group and indeed, one can show that if $c$ is the identity on underlying homomorphisms, $c:mathbfqTopGrptomathbfqTopGrp$ becomes a functor.
It is clear that the identity homomorphism $Gto c(G)$ is continuous and $G=c(G)$ if and only if $G$ is already topological group. Hence one inductively defines $c_0(G)=G$, $c_alpha+1(G)=c_alpha(G)$ and if $alpha$ is a limit ordinal, the topology of $c_alpha(G)$ is the intersection of the topologies of the groups $c_beta(G)$, $beta<alpha$.
By basic set-theoretic arguments, $c_alpha(G)$ stabilizes to the topological group $tau(G)$ and since quotient topologies are used one has $c_gamma(G)=lim_alpha<gammac_alpha(G)$ in $mathbfqTopGrp$.
However, for a given quasitopological group $G$ it is not clear at all when the inductive sequence $c_alpha(G)$ first stabilizes.
Motivation for this example:
- Free topological groups are important in topological group theory and arise in algebraic topology as well. Given a space $X$, one might attempt to create the free topological group on $X$ viewing the free group $F(X)$ as the quotient space of the free topological semigroup $coprod_ngeq 1(Xcup X^-1)^n$ with respect to word reduction; however, without some compactness assumptions, the resulting object $F_q(X)$ is only a quasitopological group. To construct the free topological group one must apply the inductive construction above and this specific use of $c$ is sometimes called the Mal'tsev transfite process. A quote from pg. 5799 of The topology of free topological groups by O. Sipacheva:
Certainly, such an approach to constructing the free group topology
looks very natural. However, it is extremely difficult to trace the
change of the topology at each step or at least understand at what
point the topology stabilizes.
Apparently, the only known results are those where stabilization occurs after the first step.
- The fundamental group $pi_1(X,x)$ with it's natural quotient topology is not a topological group (same for higher homotopy groups), but it is a quasitopological group. There are plenty of interesting things about quasitopological $pi_1$; however, one may turn $pi_1$ naturally into a topological group (reflecting many classical results to the topological group category) by applying $tau$. See this paper for more on topological $pi_1$ and a detailed treatment on the reflection functor $c$. The use of the transfinite sequence is needed in the classification of certain generalized covering maps. I would, personally, like to know when the sequence stabilizes even for simple Peano continua $X$.
Similar constructions are used to build other universal objects in topological algebra.
$endgroup$
$begingroup$
Neutrality is one thing; however, I am curious to know how this got a downvote.
$endgroup$
– Jeremy Brazas
Mar 21 at 19:00
$begingroup$
I downvoted this answer because I specifically asked for towers that are increasing in size.
$endgroup$
– Joseph Van Name
Mar 23 at 3:22
$begingroup$
@JosephVanName Had you included a formalization of "growth" here to which my example did not apply, I would not have subjected you to my answer. If you edit the "in particular" paragraph it so that this is the case, I'll happily delete my answer. However, the difference between growing and shrinking for bounded towers seems superficial... if $T_alpha$ is the topology of $c_alpha(G)$, then $X_alpha=P(G)backslash T_alpha$ is a "growing" tower of sets whose stability is difficult to determine.
$endgroup$
– Jeremy Brazas
2 days ago
add a comment |
$begingroup$
The following is an example where is it difficult to identify where stabilization occurs. It meets all of your technical criteria but I suppose the word "growing" is not exactly accurate.
A quasitopological group $G$ is a group with a topology $mathcalT$ such that inverse $gmapsto g^-1$ is continuous and multiplication $mu:Gtimes Gto G$ is continuous in each variable. One can "efficiently" give the underlying group of $G$ an actual topological group structure by removing the smallest number of open sets from $mathcalT$ until one arrives at a topological group $tau(G)$. This can be understood precisely in at least two equivalent ways:
- Give $G$ the finest group topology coarser than $mathcalT$, which one can show exists abstractly (the difficulty is with the product topology on $Gtimes G$).
- The inclusion $mathbfTopGrpto mathbfqTopGrp$ of the full subcategory of topological groups into the category of quasitopological groups and continuous homomorphisms has a left adjoint (reflection) $tau:mathbfqTopGrpto mathbfTopGrp$, which exists by the adjoint functor theorem.
The construction of $tau(G)$ from $G$ is purely abstract so to really get your hands on something you can work with (e.g. to prove $G$ and $tau(G)$ in fact have the same open subgroups) you approximate it inductively by a shrinking sequence of quotient topologies.
Let $c(G)$ be the underlying group of $G$ with the quotient topology with respect to multiplication $m:Gtimes Gto c(G)$. While $c(G)$ is not necessarily a topological group, it is a quasitopological group and indeed, one can show that if $c$ is the identity on underlying homomorphisms, $c:mathbfqTopGrptomathbfqTopGrp$ becomes a functor.
It is clear that the identity homomorphism $Gto c(G)$ is continuous and $G=c(G)$ if and only if $G$ is already topological group. Hence one inductively defines $c_0(G)=G$, $c_alpha+1(G)=c_alpha(G)$ and if $alpha$ is a limit ordinal, the topology of $c_alpha(G)$ is the intersection of the topologies of the groups $c_beta(G)$, $beta<alpha$.
By basic set-theoretic arguments, $c_alpha(G)$ stabilizes to the topological group $tau(G)$ and since quotient topologies are used one has $c_gamma(G)=lim_alpha<gammac_alpha(G)$ in $mathbfqTopGrp$.
However, for a given quasitopological group $G$ it is not clear at all when the inductive sequence $c_alpha(G)$ first stabilizes.
Motivation for this example:
- Free topological groups are important in topological group theory and arise in algebraic topology as well. Given a space $X$, one might attempt to create the free topological group on $X$ viewing the free group $F(X)$ as the quotient space of the free topological semigroup $coprod_ngeq 1(Xcup X^-1)^n$ with respect to word reduction; however, without some compactness assumptions, the resulting object $F_q(X)$ is only a quasitopological group. To construct the free topological group one must apply the inductive construction above and this specific use of $c$ is sometimes called the Mal'tsev transfite process. A quote from pg. 5799 of The topology of free topological groups by O. Sipacheva:
Certainly, such an approach to constructing the free group topology
looks very natural. However, it is extremely difficult to trace the
change of the topology at each step or at least understand at what
point the topology stabilizes.
Apparently, the only known results are those where stabilization occurs after the first step.
- The fundamental group $pi_1(X,x)$ with it's natural quotient topology is not a topological group (same for higher homotopy groups), but it is a quasitopological group. There are plenty of interesting things about quasitopological $pi_1$; however, one may turn $pi_1$ naturally into a topological group (reflecting many classical results to the topological group category) by applying $tau$. See this paper for more on topological $pi_1$ and a detailed treatment on the reflection functor $c$. The use of the transfinite sequence is needed in the classification of certain generalized covering maps. I would, personally, like to know when the sequence stabilizes even for simple Peano continua $X$.
Similar constructions are used to build other universal objects in topological algebra.
$endgroup$
The following is an example where is it difficult to identify where stabilization occurs. It meets all of your technical criteria but I suppose the word "growing" is not exactly accurate.
A quasitopological group $G$ is a group with a topology $mathcalT$ such that inverse $gmapsto g^-1$ is continuous and multiplication $mu:Gtimes Gto G$ is continuous in each variable. One can "efficiently" give the underlying group of $G$ an actual topological group structure by removing the smallest number of open sets from $mathcalT$ until one arrives at a topological group $tau(G)$. This can be understood precisely in at least two equivalent ways:
- Give $G$ the finest group topology coarser than $mathcalT$, which one can show exists abstractly (the difficulty is with the product topology on $Gtimes G$).
- The inclusion $mathbfTopGrpto mathbfqTopGrp$ of the full subcategory of topological groups into the category of quasitopological groups and continuous homomorphisms has a left adjoint (reflection) $tau:mathbfqTopGrpto mathbfTopGrp$, which exists by the adjoint functor theorem.
The construction of $tau(G)$ from $G$ is purely abstract so to really get your hands on something you can work with (e.g. to prove $G$ and $tau(G)$ in fact have the same open subgroups) you approximate it inductively by a shrinking sequence of quotient topologies.
Let $c(G)$ be the underlying group of $G$ with the quotient topology with respect to multiplication $m:Gtimes Gto c(G)$. While $c(G)$ is not necessarily a topological group, it is a quasitopological group and indeed, one can show that if $c$ is the identity on underlying homomorphisms, $c:mathbfqTopGrptomathbfqTopGrp$ becomes a functor.
It is clear that the identity homomorphism $Gto c(G)$ is continuous and $G=c(G)$ if and only if $G$ is already topological group. Hence one inductively defines $c_0(G)=G$, $c_alpha+1(G)=c_alpha(G)$ and if $alpha$ is a limit ordinal, the topology of $c_alpha(G)$ is the intersection of the topologies of the groups $c_beta(G)$, $beta<alpha$.
By basic set-theoretic arguments, $c_alpha(G)$ stabilizes to the topological group $tau(G)$ and since quotient topologies are used one has $c_gamma(G)=lim_alpha<gammac_alpha(G)$ in $mathbfqTopGrp$.
However, for a given quasitopological group $G$ it is not clear at all when the inductive sequence $c_alpha(G)$ first stabilizes.
Motivation for this example:
- Free topological groups are important in topological group theory and arise in algebraic topology as well. Given a space $X$, one might attempt to create the free topological group on $X$ viewing the free group $F(X)$ as the quotient space of the free topological semigroup $coprod_ngeq 1(Xcup X^-1)^n$ with respect to word reduction; however, without some compactness assumptions, the resulting object $F_q(X)$ is only a quasitopological group. To construct the free topological group one must apply the inductive construction above and this specific use of $c$ is sometimes called the Mal'tsev transfite process. A quote from pg. 5799 of The topology of free topological groups by O. Sipacheva:
Certainly, such an approach to constructing the free group topology
looks very natural. However, it is extremely difficult to trace the
change of the topology at each step or at least understand at what
point the topology stabilizes.
Apparently, the only known results are those where stabilization occurs after the first step.
- The fundamental group $pi_1(X,x)$ with it's natural quotient topology is not a topological group (same for higher homotopy groups), but it is a quasitopological group. There are plenty of interesting things about quasitopological $pi_1$; however, one may turn $pi_1$ naturally into a topological group (reflecting many classical results to the topological group category) by applying $tau$. See this paper for more on topological $pi_1$ and a detailed treatment on the reflection functor $c$. The use of the transfinite sequence is needed in the classification of certain generalized covering maps. I would, personally, like to know when the sequence stabilizes even for simple Peano continua $X$.
Similar constructions are used to build other universal objects in topological algebra.
answered Mar 17 at 17:53
Jeremy BrazasJeremy Brazas
3,90611531
3,90611531
$begingroup$
Neutrality is one thing; however, I am curious to know how this got a downvote.
$endgroup$
– Jeremy Brazas
Mar 21 at 19:00
$begingroup$
I downvoted this answer because I specifically asked for towers that are increasing in size.
$endgroup$
– Joseph Van Name
Mar 23 at 3:22
$begingroup$
@JosephVanName Had you included a formalization of "growth" here to which my example did not apply, I would not have subjected you to my answer. If you edit the "in particular" paragraph it so that this is the case, I'll happily delete my answer. However, the difference between growing and shrinking for bounded towers seems superficial... if $T_alpha$ is the topology of $c_alpha(G)$, then $X_alpha=P(G)backslash T_alpha$ is a "growing" tower of sets whose stability is difficult to determine.
$endgroup$
– Jeremy Brazas
2 days ago
add a comment |
$begingroup$
Neutrality is one thing; however, I am curious to know how this got a downvote.
$endgroup$
– Jeremy Brazas
Mar 21 at 19:00
$begingroup$
I downvoted this answer because I specifically asked for towers that are increasing in size.
$endgroup$
– Joseph Van Name
Mar 23 at 3:22
$begingroup$
@JosephVanName Had you included a formalization of "growth" here to which my example did not apply, I would not have subjected you to my answer. If you edit the "in particular" paragraph it so that this is the case, I'll happily delete my answer. However, the difference between growing and shrinking for bounded towers seems superficial... if $T_alpha$ is the topology of $c_alpha(G)$, then $X_alpha=P(G)backslash T_alpha$ is a "growing" tower of sets whose stability is difficult to determine.
$endgroup$
– Jeremy Brazas
2 days ago
$begingroup$
Neutrality is one thing; however, I am curious to know how this got a downvote.
$endgroup$
– Jeremy Brazas
Mar 21 at 19:00
$begingroup$
Neutrality is one thing; however, I am curious to know how this got a downvote.
$endgroup$
– Jeremy Brazas
Mar 21 at 19:00
$begingroup$
I downvoted this answer because I specifically asked for towers that are increasing in size.
$endgroup$
– Joseph Van Name
Mar 23 at 3:22
$begingroup$
I downvoted this answer because I specifically asked for towers that are increasing in size.
$endgroup$
– Joseph Van Name
Mar 23 at 3:22
$begingroup$
@JosephVanName Had you included a formalization of "growth" here to which my example did not apply, I would not have subjected you to my answer. If you edit the "in particular" paragraph it so that this is the case, I'll happily delete my answer. However, the difference between growing and shrinking for bounded towers seems superficial... if $T_alpha$ is the topology of $c_alpha(G)$, then $X_alpha=P(G)backslash T_alpha$ is a "growing" tower of sets whose stability is difficult to determine.
$endgroup$
– Jeremy Brazas
2 days ago
$begingroup$
@JosephVanName Had you included a formalization of "growth" here to which my example did not apply, I would not have subjected you to my answer. If you edit the "in particular" paragraph it so that this is the case, I'll happily delete my answer. However, the difference between growing and shrinking for bounded towers seems superficial... if $T_alpha$ is the topology of $c_alpha(G)$, then $X_alpha=P(G)backslash T_alpha$ is a "growing" tower of sets whose stability is difficult to determine.
$endgroup$
– Jeremy Brazas
2 days ago
add a comment |
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1
$begingroup$
Have you looked into Kelly's "...and so on"?
$endgroup$
– Fosco Loregian
Mar 16 at 20:59
$begingroup$
My questions on Laver tables are better than these questions.
$endgroup$
– Joseph Van Name
Mar 17 at 3:41
$begingroup$
Can you suggest any references for Example 2?
$endgroup$
– Dap
Mar 17 at 15:10
$begingroup$
Example 2 is elaborated int he standard textbook on Frames and Locales - Topology without points by Picado, Jorge and Pultr, Ales (Chapter 4).
$endgroup$
– Joseph Van Name
Mar 17 at 15:13
$begingroup$
I find it very strange and discouraging that these inferior questions about transfinite towers have many times more upvotes than questions about Laver tables.
$endgroup$
– Joseph Van Name
Mar 23 at 2:48