# Moment of Zen

When one is ascending a difficult path uphill, it is a good idea to keep your eyes on the path as you move forward. However, it is not a bad idea to stop sometimes, look back, and appreciate the beauty of the ground you have already covered.

This understanding extends to any difficult task which is completed in a long series of steps.

# What a long strange trip it’s been…

As some of you may have noticed, I don’t use this blog to write about my papers in the “traditional way” math bloggers summarize and explain their recent work. I think my papers are prosaic enough to do that on their own. I do use this blog as an outlet when I have to complain about the arduous toil of being a mathematician (which has an immensely bright light side, of course, so in the big picture I’m quite happy with it).

This morning I woke up to see that my paper about the Bristol model was announced on arXiv. But unbeknownst to the common arXiv follower, this also marks the end of my thesis. The Hebrew University is kind enough to allow you to just stitch a bunch of your papers (along with an added introduction) and call it a thesis. And by “stitch” I mean literally. If they were published, you’re even allowed to use the published .pdf (on the condition that no copyright infringement occurs).

My dissertation is composed of three papers, all of which are on arXiv (links in the “Papers” page of this site):

1. Iterating symmetric extensions;
2. Fodor’s lemma can fail everywhere; and
3. The Bristol model: an abyss called a Cohen real.

Of course, the ideal situation is that all three papers have been accepted for publication, but all three of them are still under review. So it puts me at this odd situation where I will have essentially four sets of referees (one for each paper, and then two additional referees for my thesis), and so the output can end up oddly different between the resulting dissertation and the published papers. But that’s fine.

In any case. Those of you who are interested in reading my thesis can find it in those three papers. I am probably going to post the final thesis online when it will be approved, but the only thing you’re currently missing out is an introduction with some minor historical background and a summary of the three papers. So if you read all three, you don’t really need that introduction anyway.

Good. So what next? I have a few things lined up. More news will follow as reality unfolds itself like a reverse origami.

# Stationary preserving permutations are the identity on a club

This is not something particularly interesting, I think. But it’s a nice exercise in Fodor’s lemma.

Theorem. Suppose that $\kappa$ is regular and uncountable, and $\pi\colon\kappa\to\kappa$ is a bijection mapping stationary sets to stationary sets. Then there is a club $C\subseteq\kappa$ such that $\pi\restriction C=\operatorname{id}$.

Proof. Note that the set $\{\alpha\mid\pi(\alpha)<\alpha\}$ is non-stationary, since otherwise by Fodor's lemma there will be a stationary subset on which $\pi$ is constant and not a bijection. This means that $\{\alpha\mid\alpha\geq\pi(\alpha)\}$ contains a club. The same arguments shows that $\pi^{-1}$ is non-decreasing on a club. But then the intersection of the two clubs is a club on which $\pi$ is the identity. $\square$

This is just something I was thinking about intermittently for the past few years, but now I finally spent enough energy to figure it out. And it’s cute. (Soon I will post more substantial posts, on far more exciting topics! Don’t worry!)

# Got jobs?

Good news! I’m about to finish my dissertation. Hopefully, come summer I will be Dr. Asaf Karagila.

So the next order of business is finding a position for next year. So far nothing came up. But I’m open to hearing from the few readers of my blog if they know about something, or have some offers that might be suitable for me.

Thanks!

# Farewell, Matti

My mentor, teacher, mathematical confidant and generally good friend, Matti Rubin passed away this morning. Many of the readers here know him for his mathematical work, many knew him as a friend as well, or as a teacher.

Matti was a kind teacher, even if sometimes over-pedantic.

Recently he was diagnosed with lung cancer. I spoke with him two weeks ago, shortly before I went to the Arctic Set Theory conference, and he sounded positive. He felt that he might not survive this battle, but he was optimistic about the short time era. I was hoping to talk to him once more and to tell him about a student of mine that he would have surely liked.

Alas, this morning Matti left this world. Apparently there had been a sudden deterioration of his condition, and this morning, tired and hurt he left us here. I take solace in the fact that I have known him. I had learned from him. I have created a minuscule copy of the original in my mind, to remind me where I’m being mathematically dishonest while writing proofs (“This is not obvious, and that is not clear”).

He will be sorely missed by all of us here.

A photo I took from his logic course back in the Spring of 2009. Probably talking about model theory or Turing machines…

If you follow my blog, you probably know that I am a big fan of Michael Stevens from the VSauce channel, who in the recent year or so released several very good videos about mathematics, and about infinity in particular. Not being a trained mathematician, Michael is doing an incredible task.

Non-mathematicians often tend to be Platonists “by default”, so they will assume that every question has an answer and sometimes it’s just that we don’t know that answer. But it’s out there. It’s a fine approach, but it can somewhat fly in the face of independence if you are not trained to think about the difference between true and provable.

This morning, as I was watching the new video of Physics Girl, there was an announcement about a new math channel. So of course I went to look into that channel. It’s fairly new, and there are only a handful of videos, but they already tackled some nice topics. The videos are written and hosted by a Cornell grad student, Kelsey Houston-Edwards. I watched the one about a hierarchy of infinities, and while I was a bit skeptic after the first minute, I was quite happy at the end, when the discussion went from just the fact that the reals are uncountable (although without a proof, and that’s fine, I guess, there are plenty of those on the internet), to a discussion about the continuum hypothesis.

In another video, Kelsey tackles mathematical Platonism and its somewhat-opposite, Formalism. And it’s done well. Kelsey doesn’t lean into one side or another, because at the end of the day, mathematicians—as opposed to mathematical philosophers—do mathematics, and that is their main concern. The philosophy is mostly a spice to add some taste and meaning to your work.

In any case, I enjoyed watching the few videos that I have, and I hope that you will as well. I’m sure that this is not the last that you’ll see me talk about Kelsey and her channel.

I’ve been given the chance to teach the course in axiomatic set theory in Jerusalem this semester. Today I gave my first lecture as a teacher. It went fine, I even covered more than I expected to, which is good, I guess. I am also preparing lecture notes, which I will probably post here when the semester ends. These predicated on some rudimentary understanding in logic and basic set theory, so there might be holes there to people unfamiliar with the basic course (at least the one that I gave with Azriel Levy for the past three years).

Yesterday, however, I spent most of my day thinking about how we—as a collective of set theorists—teach axiomatic set theory. About that usual course: axioms, ordinals, induction, well-founded sets, reflection, $V=L$ and the consistency of $\GCH$ and $\AC$, some basic combinatorics (clubs, Fodor’s lemma, maybe Solovay or even Silver’s theorem). Up to some rudimentary permutation.

Is this the right way to approach axiomatic set theory? This path is not easy to justify. Sure, you can justify things like well-founded sets by arguing that this is how we justify the Axiom of Foundation. And you could argue that this is a rudimentary foray into inner model theory, and that this is important. And you are absolutely right. But on the other hand, I feel that engaging the students should involve more set theory which is “interactive”. Where you obtain actual results, rather than just consistency of axioms, especially axioms which you have very little motivation towards.

I mean, look at how we teach (or learn) about algebraic structures. We don’t spend all semester just with the axioms of groups, or rings, proving things. We also see a lot of examples, and a lot of ways where these structures interact with mathematics. Set theory doesn’t have this luxury, we don’t have natural models to work with and their interactions with mathematics is meta-theoretical, rather than direct as it is the case with groups and rings.

So set theory, in essence, should be taught in a mixture of motivating examples and consistency proofs. I am taking this from my advisor, who is a wonderful teacher, as anyone who ever sat in his lectures could witness. A couple of years ago, Menachem gave a course about stationary tower forcing. In most texts about stationary tower forcing, you spend the first several dozen pages in technical concepts like Completely Jonsson cardinals, and so on. But Menachem started with the motivation: universally Baire sets, and their properties. Once you understand those, stationary tower forcing becomes much easier to digest, because it is with purpose. Last year, and next semester, Menachem is talking about inner models, and again a lot of motivation is given into fine structural considerations, mainly square’ish ones for the basic fine structure of $L$, but also through mice we get a good intuition as to what $K^{DJ}$ is supposed to be.

Right. So the basic axiomatic set theory course. What can we do about that? Well, my initial approach is to take $\ZF$ for starters. Motivate Foundation by talking about induction, and then prove that Foundation adds no new contradictions. After that, we’ll see exactly, but the next step is again motivation for either choice or Reflection principles. In either case, I feel that having motivation interspersed with consistency proofs is key here.

So now, let me ask you, my fellow set theorists, who have taught courses in axiomatic set theory. What is your experience on the matter? What is your take on my approach? This is my first time doing this, and I will definitely be reporting again during the semester and afterwards. But I also want to hear what you have to say on the matter. I will leave the comments open, but also feel free to contact me over email.

# Zornian Functional Analysis or: How I Learned to Stop Worrying and Love the Axiom of Choice

Back in the fall semester of 2015-2016 I had taken a course in functional analysis. One of the reasons I wanted to take that course (other than needing the credits to finish my Ph.D.) is that I was always curious about the functional analytic results related to the axiom of choice, and my functional analysis wasn’t strong enough to sift through these papers.

I was very happy when the professor, Matania Ben-Artzi, allowed me to write a final paper about the usage of the axiom of choice in the course, instead of taking an exam.

I have decided to finally post this paper online. It covers some possible disasters in functional analysis without the axiom of choice, or with “seemingly nice” assumptions (such as automatic continuity). You can find it in the Papers section.

My goal was to make something readable for analysts, rather than to provide a retread of older set theoretic proofs (and some model theoretic proofs). So some things are left with only a reference, and other set theoretic statements are formulated in a rather unusual way. If you are interested, I’d be happy to hear any remarks on this paper, or suggestions for improvements in the comments below or over email. If you know analysts that might be interested to read this, please let them know of the existence of the paper.

For the set theorists the paper can be seen as a nice historical overview of these results, and perhaps it can be of use in other ways.

# In praise of some history

Teaching pure mathematics is not a trivial thing. You have to overcome the several barriers that were constructed by the K12 education that mathematics is a bunch of “fit this problem into that mold”.

I recently had a chat with James Cummings about teaching. He said something that I knew long before, that being a good teacher requires a bit of theatricality. My best teacher from undergrad, Uri Onn, had told me when I started teaching, that being a good teacher is the same as being a good storyteller: you need to be able and mesmerize your audience and keep them on the edge of their seats, wanting more.

My answer to James was something that I had in mind for a while, but never put into words until then. You should know a bit of history of the topic you’re teaching.

If you look at [pure] mathematical education—at least undergrad—it is quite flat. You just have a list of theorems, each extending the previous, building this wonderful structure. But it’s a flat building, it’s a drawing. The theorems come one after the other, after the other… Historically, however, there were many decades (if not centuries) between one theorem to the next. Rolle’s theorem came about the late 17th century, but Lagrange’s theorem came only in the mid-19th century. So in one lecture, we covered some 150 years of mathematical progress. That is amazing, if you can point this out properly to your students. Not to mention the oppositions that people had to infinitesimal calculus in Rolle’s days, which makes it interesting, and contributes to the definitions given by Cauchy as solid foundations for analysis.

Similarly, the history of the Cantor-Bernstein theorem is incredible. As in the history behind’s König’s theorem (about cardinal arithmetic). Those things are amazing, what sort of motivations and mistakes people had made back then, when these fields were fresh.

The more I thought about it, the more I realized that there are two important reasons that one should always spice up their teaching with some historical facts.

1. The first is that mathematical education is flat, as I remarked above. We learn the theorem, one after another. In one lecture, you can cover decades or even centuries of mathematical progress. And it feels dry, it feels like “why are you teaching me all this?” sort of thing. I still remember that as a student, I do.

But with a bit of history, suddenly everything becomes three-dimensional, it becomes something that had actual progress. It shines a light on “there is a notion of mathematical progress”. Something that engineering students, for example, often baffled by.

2. The second reason is that often theorems and motivations were coming from attempts to disprove something. König, if we mentioned him already, proved his lemma as an attempt to prove that the real numbers cannot be well-ordered. Baire, Borel and Lebesgue rejected the axiom of choice because they felt it is preposterous that there are non-measurable sets.

When you explain this to students, you show them that their natural confusion about a topic, especially abstract and confusing topic, is natural. You show them that a lot of smart people made the same mistake before. And while today we know better, their instinctive recoil makes sense. This reinforces the idea that they didn’t misunderstand something, that they are not stupid, and that mathematics is often surprising (at least when dealing with the infinite).

So we have these two reasons. And I think these are excellent reasons for adding some historical references when talking about mathematics. Of course, you shouldn’t put more than a pinch of cumin in your stew, because cumin is not the main part of your dish, it’s just what makes a good meal into a great meal (well, at least good cumin). You shouldn’t talk only about history in a mathematical course. This should be the slight addition that gives taste, flavor and volume to your material.

Historical anecdotes are what turns a flat material into a fleshed form of progress, from one theorem to the other. Use them sparsely, use them well. But use them.

# Constructive proof that large cardinals are consistent

I am not a Platonist, as I keep pointing out. Existence, even not in mathematics, is relative and confusing to begin with, so I don’t pretend to try and understand it in a meaningful way.

However, we have a proof, a constructive proof that large cardinals are consistent. And they exist in an inner model of our universe.

Recall that $0^\#$ exists if and only there exists a non-trivial mouse. Now recall that such mice exist. Vacanati mice.

I’m sorry to all those who claim that inaccessible cardinals are inconsistent. Your claim is that reality is inconsistent. Which might just be the case…

Now you can ask whether or not large cardinals are a human construct. Here we run into a problem, as these non-trivial mice are a human construct themselves…