Approximate Ramsey properties and topological dynamics

Speaker: 

Dana Bartosova

Institution: 

University of Sao Paulo

Time: 

Monday, December 8, 2014 - 4:00pm to 5:30pm

Host: 

Location: 

RH 440R

The interplay between structural  Ramsey theory and topological dynamics of automorphism groups has been extensively studied since their connection was established in a paper by Kechris-Pestov-Todorcevic, while earlier works of Pestov, and Glasned and Weiss exhibited the phenomena in special cases. This line of research was extended to metric structures and approximate Ramsey property by Melleray and Tsankov. We establish the approximate Ramsey property for the class of finite-dimensional normed vector spaces and deduce that the group of linear isometries of the universal approximately homogeneous Banach space, the Gurarij space, is extremely amenable, that is, every continuous action on a compact Hausdorff space has a fixed point. Dualizing our ideas, we show that the class of finite-dimensional simplexes with a distinguished extreme point and  affine surjections satisfies the approximate Ramsey property. As a consequence, we find that the universal minimal flow of the group of affine homeomorphisms of the Poulsen simplex is its natural action on the Poulsen simplex. This is a joint work (in progress) with Aleksandra Kwiatkowska (UCLA), Jordi Lopez Abad (ICMAT Madrid and USP) and Brice Mbombo (USP).

 

The existence of an \aleph_{\omega+1} scale for \aleph_{\omega} I

Speaker: 

Geoff Galgon

Institution: 

UCI

Time: 

Monday, November 24, 2014 - 4:00pm to 5:30pm

Host: 

Location: 

RH 440R

We introduce the approachability ideal I[\kappa] for regular \kappa, make some basic observations, and establish a connection with internally approachable models. Using the trichotomy theorem to guarantee the existence of an exact upper bound for a certain sequence, we proceed to prove the existence in ZFC of a scale of length \aleph_{\omega+1} in a reduced product \omega_k for k \in A, an infinite subset of \omega.

 

Todorcevic's proof of Baumgartner Axiom

Speaker: 

Garrett Ervin

Institution: 

UCI

Time: 

Monday, October 20, 2014 - 4:00pm to 5:30pm

Host: 

Location: 

RH 440R

Baumgartner's Axiom postulates that any two $\aleph_1$-dense subsets of the real line are order-isomorphic. A set is $\aleph_1$-dense iff every nonempty open interval intersects the set in $\aleph_1$-many points. We present Todorcevic's argument which shows that Baumgartner's Axiom is a consequence of the Proper Forcing Axiom.

Automorphisms of $P(omega_1)/Fin$

Speaker: 

Paul Larson

Institution: 

Miami University, Oxford, Ohio

Time: 

Monday, October 13, 2014 - 4:00pm to 5:30pm

Host: 

Location: 

RH 440R

It appears to be an open question whether for every regular uncountable regular $\lambda$, every automorphism of $P(\lambda)/fin$ is trivial on a co-countable set. We will show that a small fragment of Martin's Axiom implies that if $\lambda$ is at most the continuum then every automorphism of $P(\lambda)/fin$ which is trivial on sets of cardinality less than $\lambda$ is trivial.
 

An Introduction to Pmax forcing

Speaker: 

Paul Larson

Institution: 

Miami University, Oxford, Ohio

Time: 

Friday, October 10, 2014 - 3:00pm to 5:00pm

Host: 

Location: 

RH 440R

Woodin's $P_{max}$ forcing when applied to a model of Determinacy produces a model which is maximal for sets of countable ordinals. We will briefly introduce $P_{max}$ and its applications and variations, and outline a proof of the maximality of $P_{max}$ extensions.

The isomorphism problem for \kappa-dense sets of reals

Speaker: 

Garrett Ervin

Institution: 

UCI

Time: 

Monday, May 19, 2014 - 4:00pm to 5:30pm

Host: 

Location: 

RH 440R

We discuss the history of Baumgartner's result that all \aleph_1-dense sets of reals can be order-isomorphic, as well as related results of Shelah and Abraham. We'll outline a proof, due to Todorcevic, that is simpler than Baumgartner's original argument. Finally, we present some recent results of Justin Moore concerning the problem of making all \aleph_2-dense sets of reals isomorphic.
 

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