The following notes are from the Ramsey DocCourse in Prague 2016. The notes are taken by me and I have edited them. In the process I may have introduced some errors; email me or comment below and I will happily fix them.
Title: Topological dynamics and Ramsey classes.
Lecturer: Lionel Ngyuen Van Thé.
Date: November 14, 2016.
Main Topics: Proof of KPT correspondence between extreme amenability and ramsey class.
Definitions: Topological group, , , Polish group, ultrametric, -flow, extreme amenability.
Our main goal is to introduce the KPT correspondence and provide proofs of two main results. The flavour is combinatorial, but the techniques are topological. The KPT correspondence is a powerful bridge between Structural Ramsey Theory and Topological Dynamics.
Here are the main references for these lectures. We will provide other secondary references with each lecture.
- Kechris, Pestov, Todorcevic. “Fraïssé limits, Ramsey theory, and topological dynamics of automorphism groups”. 2005.
- Pestov. “Dynamics of infinite-dimensional groups. The Ramsey-Dvoretzky-Milman phenomenon.”. 2006.
- Ngyuen Van Thé. “More on the Kechris–Pestov–Todorcevic correspondence: Precompact expansions”. 2013
- Ngyuen Van Thé. “A survey on structural Ramsey theory and topological dynamics with the Kechris-Pestov-Todorcevic correspondence in mind”. 2015
Background about topological dynamics
A disclaimer that all spaces discussed will be Hausdorff spaces, so we will not mention it again.
Typically we will be looking at autmorphisms, or isomorphisms, or some other collection of bijections.
Example. Let the collection of all bijections on , together with the topology of pointwise convergence. That is, basic open sets are of the form , where is finite and .
This has some compatible metrics:
- , where .
A metric space is an ultrametric space if
dThis is a strong form of the triangle inequality.
- is left-invariant.
- is right-invariant.
- is neither left-invariant nor right-invariant.
- is a Polish group.
- Compute the completions of and .
- is complete.
- and are ultrametrics.
- The balls of radius give a finite partition of and .
“What is happening today is really about completions; specifically .”
The last exercise is partly why closed subgroups have nice interactions with respect to combinatorics.
The KPT machinery can be transposed into the Polish group setting, but requires continuous Fraïssé theory (which we will learn about in later talks).
A -flow is a continuous action of on a compact space .
A topological group is extremely amenable when every -flow has a fixed point. That is there is a such that we have .
We will not use the notion of amenability here, but to mention it: a group is amenable when every -flow admits an invariant Borel probability measure. So in this way we see that extreme amenability implies amenability.
Flows of this form are very important, and we will investigate them in more detail in the second lecture.
The KPT correspondence
Here is the major correspondence between Ramsey properties and extreme amenability.
- is extremely amenable.
- is a Ramsey class (for structures).
The proof will be self-contained. The right way to think about this might be to use more sophisticated topological notions from functional analysis. We will hint at these at the end of the lecture, then go into more detail in the following lectures.
We use extreme amenability to prove the Ramsey property. We do this by constructing a compact -flow, and then correctly interpreting what a fixed point is.
Let . Fix , . It suffices to show that . So fix a colouring . (You will probably forget about all these things, because we are going to leave them to the side for now. We’ll come back to them though!)
In order to use extreme amenability, we construct a compact space that acts on. Let be the collection of all colourings of . Specifically,
which is compact when given the product topology. acts on by permuting the copies of , specifically . The inverse is only there to ensure that it is an action; it is not mysterious.
Now applying extreme amenability to will be useless. We can already identify fixed points, namely constant colourings. Also, does not know anything about . (Where was our original colouring. Did you forget about it?) So we go to a place that knows about . We instead consider the -flow .
By extreme amenability, this has a -fixed point. So there is a such that we have .
By ultrahomogeneity of , is a constant colouring on . We can see that because for all and all we have . Since there is also an automorphism of that can map to a copy , we have that is constant.
Now we are going to transfer this to knowledge about . Note that is a finite subset of , so the values takes on this set specifies a basic open set in . Since , that means . Namely take to witness this.
Setting we have that is constant, as desired.
To prove that a group is extremely amenable from the Ramsey property we will discretize . We will prove a (discrete) Ramsey-type property in our setting, and a continuous, approximate version (using the discrete version). The continuous Ramsey version will allow us to approximate a fixed point arbitrarily well. By taking a limit, we will get a true fixed point.
First we may assume that the domain of is . Then let be the substructure of supported by the domain . (We used this same trick in Bootcamp 5, but there it was for compactness reasons.)
Since is rigid, the setwise stabilizer is the same as the pointwise stabilizer on . That is
Note that is a closed subgroup of .
The last step used rigidity in the reverse implication.
Observe that is the ball of radius around . Recall that these balls give a finite partition of .
We are now ready to state a discrete Ramsey-type result in this setting.
THEN there is a such that is constant on .
By ultrahomogeneity, there is a such that . (We’ll use this in a moment.)
Since is constant on , it must also be constant on . Since was constant on each equivalence class, this means that is constant on , as desired.
We will now establish a continuous version of this Ramsey property.
There is a such that , is constant up to on . That is,
Use uniform continuity to make sure that is constant on each equivalence class (use the fact about how creates partitions of .)
Then apply the discrete Ramsey to the step function version of . Unwinding what that means about the true will give the desired conclusion.
Now we are in a position to finish the original proof. We wish to show that is extremely amenable. So let be a -flow.
Fix , a finite familiy of functions that are uniformly continuous, bounded. (Note that the domain of these functions is different than the hypothesis of the continuous Ramsey fact. You might also wonder what uniform continuity means in this context. Don’t worry for now; we’ll fix that later.) Let . Define
This is the collection of all approximate fixed points.
This is a closed subset of , and hence compact.
In this way, for a , transfers to , a collection of uniformly continuous, bounded functions from to .
Applying the continous Ramsey fact we see that every is non-empty, and these sets have the finite intersection property (finite nested such have non-empty intersection).
Since they are compact, we know that the full infinite intersection is nonempty. That is there is a
Claim. is a fixed point of .
Once we have this, the proof is finished.
This contradicts the fact that .
Observations and exercises
This proof is not technically difficult, but the picture is hard to see. We’ll give a broader picture in later lectures.
Let us play around with the use of rigidity. It was only used in one part of the proof (find it!).
- If is extremely amenable, then the elements of are rigid.
- If is a Ramsey class, is is extremely amenable? If not, find the correct Ramsey condition to make the proof work without rigidity.
The Ramsey property should be thought of as a natural notion of separation. It says that some functions cannot be separated.
We introduce the concept of uniform structures. Broadly, a uniform structure is weaker than a metric structure, and is the weakest place where the notion of “uniform continuity” still makes sense. This will fix the issue that was present in the proof of where we used uniformly continuous functions from to . We made no assumption about the metrizability of the compact space , but it will turn out that compact spaces always have a unique uniform structure (that agrees with its topology).
These nLab notes provide a good introduction to uniform spaces. (Mike: These notes are better written than I could do without a lot of work. It isn’t essential to understand uniform spaces to understand the arguments being used in these lectures.)