## Daniel Donado – Metric spaces on omega_1 under determinacy

August 24, 2014

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

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.

## 287 Communication in the mathematical sciences – Syllabus

August 24, 2014

Instructor: Andrés E. Caicedo
Fall 2014

Time: MW 1:30-2:45 pm.
Place: Mathematics building, Room 139.

This class satisfies the CID requirement in mathematics.

Contact Information

• Office: 239-A Mathematics building.
• Phone number: (208)-426-1116. (Not very efficient.)
• Office Hours: W 12:00-1:15 pm. (Or by appointment.)
• Email: caicedo@math.boisestate.edu

Textbook

MR1443666
Mount Holyoke College
Laboratories in mathematical experimentation. A bridge to higher mathematics.
Textbooks in Mathematical Sciences. Springer-Verlag, New York, 1997. xx+278 pp.
ISBN: 0-387-94922-4.

Contents (Thanks to Samuel Coskey for suggestions)

The course decription at the Department’s site describes this course as follows:

Integrates mathematics content with the opportunity to develop proof writing and communication skills important in the mathematical sciences. Content is drawn from discrete and foundational math and elementary analysis. Introduction to and engagement with written and verbal communication practices characteristic to mathematical sciences. Introduction to and use of technologies that support communication in the mathematical sciences.

Following up on the theme introduced in Math 187, this course will help you transition from computational to proof-based mathematics. Skill at computations is of course still essential. However, in deeper mathematical study, we ask broader questions, and require that the answers be justified by proofs. In this course we will practice proving theorems, and along the way we will participate in the whole mathematical process.

A key realization is that, in actual mathematical research, we are not told what statement to prove, but must instead ask good questions and investigate them methodically. Even if there is a statement we are interested in, we typically do not know whether it is indeed true. In this class we will get practical experience in discovery, conjecture, and exposition of mathematical truth. We will learn to gather data to inform our conjectures, usually with the aid of a computer program. In the process we will learn to distinguish between evidence and proof, and to use both in support of our statements.

In most classes, you are expected to work individually and you are assessed in a timed environment. In a CID course such as this one, we focus instead on the activities that actually take place in the discipline: collaboration with our peers, writing research papers, attending and giving talks, and so on.  Accordingly, we will take a look at some of the key technologies that mathematicians use to carry out and share their work. There are many mathematical programming languages, but we will use Sage. We will typeset papers in LaTeX, and presentations in Beamer.

LaTeX

Please visit Sharelatex, where you can start practicing right away and work in groups. LaTeX has been the primary tool for the dissemination of mathematics (and many other sciences, take a look at the ArXiv to get an idea of how widely used the program is), and it has been so for almost 35 years, even though it has changed very little in that time. It is important to master the LaTeX system, since the language it provides for expressing mathematics will certainly be the standard for many years to come. MathJax and other technologies are expected to eventually replace LaTeX as the standard, but for the time being, knowing it is essential. for instance, Scott Aaronson lists as the first of his Ten Signs a Claimed Mathematical Breakthrough is Wrong that the authors do not use (La)TeX.

LaTeX is available as free software, and abundant documentation exists. A few useful references are The (not so) short introduction to LaTeX, the NASA guide to LaTeX commands, and The comprehensive LaTeX symbol list. I recommend that you also bookmark and visit frequently the Q&A site on Stack Exchange.

Beamer

Beamer is a LaTeX documentclass designed especially for creating powerpoint-style presentations. The output is a pdf file that you can click through page-by-page while you speak. In the code, you simply enclose each slide within \begin{frame}{My Title} and \end{frame}. Inside, simply use the LaTeX commands that you are used to. The ultimate Beamer reference is the full user guide, but this may contain too much information, and to get started the less intimidating short intro (and its source) may be more useful.

Sage and python

Sage is a computer algebra system and programming language. In the long term, knowing it may be just as important and useful if not more than knowing LaTeX. While you are welcome to use any language you prefer, I recommend Sage because it is free to use online, and has been developed as open source software from the beginning. The recently developed Sage Cloud makes its use even more convenient.

When getting started, check out the guided tour or the introductory video series. There is also a nice web site for the nuts and bolts of Python at learnpython. You can start using Sage very quickly by logging into sagenb.org using your google mail account.