Daniel Donado – Metric spaces on omega_1 under determinacy

Daniel was my first masters student, completing his M.S. in June 2010. I co-advised him together with my friend and colleague Ramiro de la Vega, at the Universidad de los Andes, in Bogota. The following picture is from his Facebook profile.

Daniel Donado

Daniel

Daniel’s thesis, Metric spaces on \omega_1 under the axiom of determinacy (in Spanish), is part of a vastly unexplored field: general topology in the absence of choice.

Work on this area has been mostly about highlighting pathologies, illustrating how vastly different results can be when we keep standard definitions but work in setting where the axiom of choice fails badly. Even in the setting of the real numbers with the standard topology, things may not work as expected: Every set of reals may be Borel (in fact F_{\sigma\sigma}), there may be Borel infinite Dedekind-finite sets, etc. In his PhD thesis at UC Berkeley, Apollo Hogan showed that, instead, we can carry out a systematic and detailed study of general topology if instead of dealing with arbitrary models with absurd failures of choice, we concentrate on settings where the absence of choice is compensated by a rich combinatorial structure. Specifically, Apollo (who was a student at Berkeley at the same time I was there) considered topology under the axiom of determinacy. Daniel’s thesis is a survey of some of the results discovered by Apollo.

Daniel begins by reviewing some of the basic consequences of \mathsf{AD}, the axiom of determinacy. To state \mathsf{AD}, we need to consider certain ideal games between two players, that I will just call I and II. In all these games the format is the same: players I and II alternate with I playing first. In each turn, the corresponding player picks a natural number, repetitions being allowed, and both players having knowledge of all the moves both have made previously. They play for infinitely many rounds. At the end, a sequence of natural numbers \langle n_0,n_1,n_2,\dots\rangle has been produced, with n_0,n_2,n_4,\dots being the numbers picked by player I, and n_1,n_3,\dots being the ones picked by II. We have one of these games for each set A\subseteq\mathbb N^{\mathbb N}, where \mathbb N^{\mathbb N} is the set of all infinite sequences of natural numbers. In the game associated to such an A, player I wins iff the sequence \langle n_0,n_1,n_2,\dots\rangle is in A. Otherwise, player II is the winner.

A strategy \sigma for player I is a function that tells player I what to play each time. Formally, this is just a function from the set of finite sequences of numbers to \mathbb N. A strategy for II is defined similarly. We say that a strategy for I is winning if and only if player I wins the game whenever they play following the strategy. That is, in any such game we have  n_0=\sigma(\langle\rangle), n_2=\sigma(\langle n_1\rangle), n_4=\sigma(\langle n_1,n_3\rangle), etc, and at the end we have that \langle n_0,n_1,n_2,\dots\rangle\in A. Winning strategies for player II are defined similarly. We say that A is determined iff one of the players has a winning strategy.

It is easy to give examples of determined sets A. Using the axiom of choice, we can give examples of undetermined sets, but deep theorems in descriptive set theory indicate that no undetermined set can be particularly simple. For instance, it is a celebrated theorem of Martin that all Borel sets are determined. Here, \mathbb N^{\mathbb N} is made into a topological space by taking the product topology of countably many copies of the discrete set \mathbb N.

The axiom of determinacy is the statement that all A\subseteq \mathbb N^{\mathbb N} are determined. In particular, this statement contradicts the axiom of choice. See here for slides of a talk I gave a few years ago containing a quick introduction to the subject.

The short remainder of this post (after the fold) is by necessity more technical.

Since \mathsf{AD} contradicts choice, it is studied in the context of \mathsf{ZF}. It is not completely incompatible with choice, of course. For instance, it implies countable choice for sets of reals. If determinacy holds, it holds in L(\mathbb R), the constructive closure (in the sense of Gödel) of the set of reals. This is the smallest transitive model of \mathsf{ZF} that contains all the reals and all the ordinals. The relevance of this remark is that the consequences of \mathsf{AD} are much better understood when studied under the assumption that V=L(\mathbb R). For example, Kechris proved that in this case we have not just countable choice for sets of reals, but in fact dependent choice (this is strictly stronger than countable choice for all sets, which in turn is strictly stronger than countable choice for sets of reals). This means that we can carry out a decent theory of analysis under the assumption of determinacy, and we would see no changes at the classical level. On the other hand, the presence of determinacy actually eliminates some of the (perhaps pathological) consequences of choice such as the existence of sets of reals that are not Lebesgue measurable.

One of the remarkable properties of determinacy is that it implies the existence of large cardinals. For example, Solovay proved that \omega_1 and \omega_2 are measurable. This gives us in particular that x^\sharp exists for all reals x. Daniel reviews the theory of sharps from a (very) classical point of view (as EM blueprints). He also verifies that, under determinacy, every subset of \omega_1 is constructible from a real.

With these preliminaries out of the way, Daniel proceeds to examine the theory of metric spaces on ordinals under determinacy. Following Hogan, he shows that any metric space on \omega_1 embeds into the product space \omega_1^{\aleph_0}, where \omega_1 carries the discrete topology. Using this, he shows that any such space is the union of countably many discrete closed sets.

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