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: Introduction to the KPT correspondence 2 (of 3).
Lecturer: Lionel Ngyuen Van Thé.
Date: November 16, 2016.
Main Topics: Computing universal minimal flows, , why precompactness is important.
Definitions: Minimal flow, universal flow, Logic action, -equivariant.
Lecture 1 – Lecture 2 – Lecture 3
Last time we looked at how the Ramsey property of a structure ensures that is extremely amenable.
Today we will look at what can be said about the dynamics of when is not Ramsey?
Examples of extremely amenable groups
Last lecture we did not provide many examples of extremely amenable groups, so let us fix that now.
The underlying Ramsey principle here is the classical Ramsey theorem. This was the first known example of an extremely amenable group. Note that it comes seven years before the 2005 KPT paper.
The following examples were shown to be extremely amenable using the 2005 KPT correspondence, although the underlying Ramsey principles were already known.
Theorem (KPT, 2005). The folowing groups are extremely amenable. The needed Ramsey principle is in brackets.
- , where is the Rado-graph. (Nešetřil-Rödl 1977, Abramson-Harrington 1978)
- , where is the random free graph. (Nešetřil-Rödl 1977)
- , where is the rational Urysohn space. (Nešetřil)
- , where is a finite field. (Graham-Rothschild)
- , where is the generic countable atomless Boolean algebra. (Graham-Rothschild)
In order to analyze what happens to when is not Ramsey, we will introduce the notion of a universal minimal flow, which at its heart is a canonical compact object we can associate to a group. The size (both topologically and in terms of cardinality) of a group’s universal minimal flow will be determined by the “amount of Ramsey” that the group has.
Here are two exercises to play around with these concepts.
For a fixed , the object that is universal in the class of minimal -flows will be a canonical object we can associate to , called the universal minimal flow of . To make sense of this, we introduce the concept of universality and flow homomorphism.
Definition. Given -flows and , a flow homomorphism is a map that is continuous and -invariant.
A map is -invariant if we have
These universal objects always exist, although the proof is non-constructive.
Theorem. Let be a topological gorup. There is a minimal -flow that is universal in the sense that for all minimal there is an onto flow homomorphism .
In addition, is unique (up to flow isomorphism). So is called the universal minimal flow of .
Typically will be hard to describe. The following facts show cases where they are easily understood.
- is extremely amenable iff .
- If is compact, then . (Here the action is by left translation.)
Two other examples where is known.
The first known example of a non-trivial metrizable universal minimal flow is the following.
We will compute the universal minimal flow of . The original proof is due to Glasner-Weiss in 2002, but we will present proof that is easier to generalize. You should compare this with their original proof.
Proof. By an earlier exercise, is a minimal flow, so we need “only” show that it is universal. So let and let .
Step 1: Use extreme amenability of a smaller group.
Fix a linear ordering such that .
In this way we have that which is extremely amenable by Pestov’s theorem. Note that . So induces an action . By extreme amenability of , there is a -fixed point .
Step 2: Use uniform spaces to extend the group action.
Now consider the map that sends . Since we have that only depends on . Thus
We also see that
So, in this way we can think of, .
Assume for the moment that can be continuously extended to a map on all of . In this case is a compact subspace of containing (the fixed point), hence . Since is minimal, . So we are done.
Claim. can be continuously extended to a map on all of .
Proof of claim. We would like to show first that is uniformly continuous. What does that even mean in the non-metric setting? How do we capture the interplay between the topology of and the group ?
We can’t assume that has a metric, but it will always have a unique uniformization, which will act like a metric for the purposes of defining uniform continuity.
To extend continuously, if you are familiar with uniform spaces:
- Show that is uniformly continuous when is equipped with (defined in lecture 1).
- Show that is uniformly continuous when is equipped with the natural projection of .
- Show that the identification also holds when is equipped with the natural projection of and is equipped with the uniform structure from .
If you aren’t familiar with uniform spaces, then just pretend that has a metric and do the same as above.
This part shows why this type of argument doesn’t always work.
This proof works directly when you replace by and is replaced by a closed subgroup such that
- is extremely amenable, and
- is precompact (i.e. the completion is compact) when equipped with the projection of .
Question: What does “ is precompact” mean combinatorially? Put another way, what do such look like?
Since we can think of as an expansion of where , where is possibly infinite.
If the parity of is denoted by , then
Here are two exercises to help you understand the interplay of these objects.
A priori, gives the box topology which could be different than the product topology. However, precompactness guarantees that these are the same.
Exercise. Show that is precompact iff generates the product topology on , and every element of has only finitely many expansions in .
That is, is a precompact expansion of , hence the name.
In this case, we write
Recall that is minimal iff there is a flow homomorphism . Now for any minimal flow we take and see that .
Corollary. Under the same assumptions, any minimal subflow of is the universal minimal flow.
In particular, is metrizable.
In practice, computing this requires understanding what the minimal subflows of look like. This amounts to understanding when is minimal.
These are our overarching references
- 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
Here are the references to specific theorems we mentioned. (Mike: I’m missing a couple.)
- Glasner, Weiss. “Minimal actions of the group of permutations of the integers.” 2002.
- Abramson, Harrington. “Models without indiscernibles.” 1978.
- Nešetřil, Rödl. “Partitions of finite relational and set systems.” 1977.